Thermal Spraying of Polymer-Ceramic Composite Coatings with Multiple Size

Scales of Reinforcements

A Thesis

Submitted to the Faculty

of

Drexel University

by

Varun Gupta

in partial fulfillment of the

requirements for the degree

of

Master of Science in Materials Engineering

April, 2006 i

© Copyright 2006

Varun Gupta. All Rights Reserved. ii

DEDICATION

To my parents

iii

ACKNOWLEDGEMENTS

I would like to express my gratitude to my primary advisor, Dr. Richard Knight, for

his guidance, constant support and advice in many aspects of my graduate studies and research work but especially for the invaluable insights into thermal spray technology and a great influence on my professional development. I would like to thank my co-advisor,

Dr. Richard Cairncross, for his always constant motivation, understanding, trust and

support given throughout the course of my graduate studies.

I deeply appreciate the friendship, assistance and practical help of Mr. Dustin Doss and Mr. Milan Ivosevic in working with me on this project, especially for the hands-on introduction to thermal spraying. I would also like to extend my thanks to Ms. Dee

Breger for all her help during the SEM analysis and to Mr. Kishore Kumar Tenneti for

assistance with the TGA analysis during the course of this project.

I would like to give my special thanks to Mr. Ranjan Dash, Ms. Maria Pia Rossi, Mr.

Davide Mattia, Mr. Brandon McWilliams and Mr. Stephen Niezgoda for their constant motivation and help.

I also wish to thank all my friends, colleagues, faculty and staff members in the

Department of Materials Science and Engineering who gave me the opportunity to learn

from their advice and who have made my stay here at Drexel unforgettable.

I would also like to thanks Dr. Thomas E. Twardowski for his valuable contribution

during the project.

Above all, my deepest gratitude goes to my parents and my sister for their continued moral support and endurance in helping me in accomplishing my goal. iv

This work was made possible with support from National Science Foundation (NSF) under the Grant Number: DMI-0209319. Any opinions, findings, and conclusions or recommendations expressed in this thesis are those of the author and do not necessarily reflect the views of NSF. v

TABLE OF CONTENTS

LIST OF TABLES viii

LIST OF FIGURES ix

LIST OF EQUATIONS xiv

LIST OF ABBREVIATIONS xv

ABSTRACT xvii

CHAPTER 1: INTRODUCTION 1

CHAPTER 2: BACKGROUND AND LITERATURE REVIEW 5

2.1. Particulate Reinforced Polymer Matrix Composites 5

2.2. Multi-scale Particulate Reinforcement 10

2.3. Thermal Spray Processing 12

2.3.1. Thermal Spray “Family” Tree 13

2.3.2. High Velocity Oxy-Fuel Combustion Spray 16

2.3.3. Thermal Spraying of Polymers and Polymer-Matrix Composites 18

CHAPTER 3: EXPERIMENTAL PROCEDURE 28

3.1. Materials Selection 29

3.1.1. Substrate Material 29

3.1.2. Feedstock Powders 29

3.1.2.1. Matrix Material: Pure Nylon-11 29

3.1.2.2. Reinforcing Phase: Ceramic Powders 30

3.2. Characterization of Feedstock Materials 31

3.2.1. Density 31 vi

3.2.2. Powder Feed Rate Calibration 32

3.3. Production of Composite Feedstock Powders 33

3.4. Coating Procedure 36

3.4.1. HVOF Spray System 36

3.4.2. Spray Setup and Substrate Preheating 37

3.5. Characterization Techniques: Powders and Sprayed Coatings 39

3.5.1. Morphology and Coating Microstructure 39

3.5.2. Elemental Analysis 40

3.5.3. Particle Size Distribution (PSD) 40

3.5.4 Thermal Properties 41

3.5.4.1. Ashing and Thermo Gravimetric Analysis (TGA) 41

3.5.4.2. Differential Scanning Calorimetry (DSC) 41

3.5.5. Scratch Testing 42

CHAPTER 4: RESULTS-FEEDTSOCK MATERIALS 44

4.1. Pure Nylon-11 Powder 44

4.2. Reinforcements: Ceramic Powders 45

4.3. Effect of Ball-milling on Particle Size Distribution 48

4.4. Morphology of the Ball-milled Composite Powders 50

4.5. Ashing and Thermo Gravimetric Analysis of Feedstock Powders 54

4.6. Measurements of Melting Point: Differential Scanning Calorimetry (DSC) 57

CHAPTER 5: RESULTS-SPRAYED COATINGS 58

5.1. Coating Production 58

5.1.1. HVOF Sprayed Pure Nylon-11 Coatings 58 vii

5.1.2. Composite Coating Formation 59

5.1.3. Nano and Multi-scale Reinforced Coatings 62

5.2. Coating Microstructure and Elemental Analysis 63

5.2.1. Nano-scale (7 nm silica) Reinforced Coatings 63

5.2.2. Multi-scale Coatings 65

5.3. Ashing and Thermo Gravimetric Analysis of Composite Coatings 69

5.4. Differential Scanning Calorimetry (DSC) 71

5.5. X-ray Microtomography of Coatings 72

5.6. X-ray Diffraction of Coatings 73

5.7. Mechanical Properties: Scratch Resistance 75

CHAPTER 6: SPLATTING OF NYLON-11 PARTICLES 79

6.1. Splat Tests 79

6.2. Splatting of Nylon-11 on Smooth Surfaces 80

6.3. Splatting of Nylon-11 on Non-smooth Surfaces 82

6.4. Modeling Predictions 84

6.5. Experimental Results 85

CHAPTER 7: SUMMARY AND CONCLUSIONS 90

Suggestions for future work 93

APPENDIX A 94

Powder Feed Rate Calibration 94

LIST OF REFERENCES 96 viii

LIST OF TABLES

Table 2.1: HVOF process and deposit characteristics [modified from Smith, 1992]...... 17

Table 2.2: Thermally sprayed polymers and polymer composites57 [modified from Petrovicova et al., 2002]...... 24

Table 3.1. Composition of blended polymer/ceramic powders prepared for ball-milling.34

Table 3.2. HVOF spray parameters used for the deposition of pure Nylon-11 and polymer-ceramic composite coatings...... 37

Table 4.1. Reinforcement content in composite powders as calculated from ashing and TGA...... 55

Table 5.1. Reinforcement content in composite coatings as determined from ashing and TGA...... 69

Table 5.2. Comparison of reinforcement content in composite powders and sprayed coatings, as determined by TGA...... 71

Table 7.1: Summary of the key results of ashing, TGA and scratch resistance for the composite powders and sprayed coatings...... 92

ix

LIST OF FIGURES

Figure 2.1: Ashby diagram showing the strength vs. density for various engineered materials18, 19...... 6

Figure 2.2: Stress-strain curve for a particle-reinforced polymeric material. The experimental observations show increasing yield stress, tensile strength and strain to failure32...... 9

Figure 2.3: Schematic of load transfer between multi-scale reinforcing particles within a polymer matrix composite (a) a likely composite structure with varying ceramic sizes and (b) the transmission of load in compression or impact moderated by nano-size particles and micron-size particles...... 12

Figure 2.4: Overview of the major thermal spray coating processes [extended and modified by Knight, 2002; from Smith, 1992]...... 14

Figure 2.5: Temperature-Velocity envelope of thermal spray processes9...... 15

Figure 2.6: Schematic of coating deposition, depicting the major microstructural features found in thermal spray coatings50...... 15

Figure 2.7: Schematic of the Jet-Kote II® high velocity oxy-fuel (HVOF) thermal spray gun, including main stages of a feedstock particle transport53...... 18

Figure 2.8: Predicted velocities of Nylon-11 particles in an HVOF jet53 and effect of particle diameter and speed on the calculated degree of melting of Nylon-11 in an HVOF jet57...... 21

Figure 2.9: General influence of molecular weight on (a) polymer properties and (b) viscosity60, 61...... 22

Figure 3.1: Molecular structure of polyamide Nylon-1182...... 30

Figure 3.2: Schematic of a pressurized volumetric powder feeder...... 32

Figure 3.3: Schematic showing the embedding of hard ceramic particles into the surface of a polymer particle during ball-milling, resulting in core-shell structure...... 33 x

Figure 3.4: Ball-milling Process (a) Schematic (side view) and (b) Norton Ball-mill used in this study...... 35

Figure 3.5: Schematic of the Jet Kote II® high velocity combustion spray system50...... 36

Figure 3.6: Spray setup with Jet-Kote II® high velocity oxy-fuel (HVOF) in operation, showing the deposition of coatings onto a substrate...... 38

Figure 3.7: Scratch testing set up (a) BYK Gardner SG-8101 balance beam scrape adhesion and mar tester (ASTM D 5178-9137) and (b) schematic of a typical scratch profile...... 43

Figure 3.8: Scratch test on a standard glass sample with a scratch depth of 10 µm to ensure repeatability of the results...... 43

Figure 4.1: SEM images showing the angular morphology of pure Nylon-11 (D60) feedstock powder: (a) at low magnification (200X); and (b) at high magnification (1,000X)...... 45

Figure 4.2: SEM images of (a) as-received agglomerated silica (SiO2) powder from Degussa Corporation with mean particle size of 15 μm and (b) as-received alumina (Al2O3) powder from AGSCO Corporation with mean particle size of 5 μm...... 46

Figure 4.3: SEM images of as-received silica (SiO2) powder from Degussa Corporation, [(a, b) Sip 50S and (c, d) Sip 320] and as-received alumina (Al2O3) powder from AGSCO Corporation (e, f)...... 47

Figure 4.4: Particle size distribution of polyamide Nylon-11 powder...... 49

Figure 4.5: Feedstock particle size distributions of Nylon-11 (before ball milling) and ball-milled Nylon-11 + 10 Vol. % 7 nm and multi-scale (MS-1) silica reinforced composite powders...... 49

Figure 4.6: SEM micrographs of Nylon-11 (60 μm) + 10 Vol. % multi-scale (MS-1) silica: (a) Vee blended, and (b) after ball-milling...... 50

Figure 4.7: SEM micrographs of Nylon-11 (60 μm) + 10 Vol. % multi-scale (MS-1, 2, 3) reinforcements after dry ball-milling...... 51

Figure 4.8: SEM micrographs of a single particle of Nylon-11 + 10 Vol. % silica powder (MS-1) after ball-milling showing: (a) spherical or “onion-skin”, and (b) flaking of outer layer...... 52 xi

Figure 4.9: SEM images, EDS dot map and spectrum of the cross-section of a ball-milled Nylon-11/silica reinforced multi-scale (MS-1) composite particle indicating the presence of a silica rich outer layer/silica shell (a) a secondary electron image, (b) a backscattered electron image (BSE), (c) EDS dot map (Si) and (d) EDS spectrum...... 53

Figure 4.10: EDS dot maps of the cross-section of a ball-milled Nylon-11 silica reinforced multi-scale (a) MS-2 and (b) MS-3 composite particles indicating the presence of a silica rich outer layer/silica shell...... 54

Figure 4.11: TGA thermograms showing curves for pure Nylon-11, Nylon-11 + 10 Vol. % of 7 nm and multi-scale (MS-1) composite feedstock...... 56

Figure 4.12: DSC thermograms of pure Nylon-11, ball-milled Nylon-11 + 7 nm and multi-scale (MS-1) powders...... 57

Figure 5.1: Optical micrograph of a cross-section of an HVOF sprayed pure Nylon-11 coating...... 59

Figure 5.2: Schematic of coating build up (a) in-flight composite particles consisting of ceramic particle shells embedded into the surface of polymer particles (b) formation of thermally sprayed coatings from overlapping composite particles (c) a thermally sprayed multi-scale polymer/ceramic coating microstructure...... 60

Figure 5.3 SEM images of (a) the microstructure of a HVOF deposited coating and (b) showing the number of passes (6) which appears consistent with the schematic shown in the section 5.1.2...... 62

Figure 5.4: BSE-SEM images showing the microstructure of HVOF sprayed (a) nano- scale (7 nm) and (b) multi-scale (MS-1) composite coatings on steel substrates...... 63

Figure 5.5: EDS spectrum (e) indicating the presence of different elements and SEM- BSE (a) and SEM-EDS dot maps (b) silicon, (c) carbon and (d) oxygen, confirming the presence of silica in or around the splat boundaries of 7nm silica reinforced Nylon-11 composite coating...... 64

Figure 5.6: SEM-BSE (a) and SEM-EDS (b) image of a cross-section of an HVOF sprayed multi-scale (MS-3) reinforced coating showing the two phases present. The inset is a high magnification image that corresponds to Figure 5.8a for EDS analysis...... 66

Figure 5.7: SEM-BSE image and EDS elemental dot maps of the cross-section of an HVOF sprayed multi-scale reinforced coating (MS-3) confirming the presence of a micron-scale silica particle of 3 μm in diameter. (a) SEM-BSE image, (b) Silicon and (c) Carbon EDS dot maps and (d) EDS spectrum with distinct elemental peaks...... 66 xii

Figure 5.8: Chemical microanalysis of an HVOF sprayed multi-scale reinforced coating (MS-3) confirming the distribution of the ceramic phase within the polymer matrix by EDS dot maps (a) SEM-BSE shows the elemental contrast between nylon and silica, (b) silicon (blue), (c) carbon (red), and (d) oxygen (yellow) EDS dot maps confirming the spatial distribution of elements and (e) EDS spectrum with distinct elemental peaks. .... 67

Figure 5.9: EDS spectrum and dot maps of silica and alumina reinforced Nylon-11 multi- scale composite coating (MS-4): (a) SEM-BSE shows the elemental contrast between nylon and silica/alumina, EDS dot maps (b) silicon (blue), (c) aluminum (purple), (d) carbon (red), (e) oxygen (yellow) and EDS spectrum (f) with distinct elemental peaks.. 68

Figure 5.10: TGA thermograms showing the amount of ceramic loading remaining in the polymer ceramic composite coatings after HVOF spraying...... 70

Figure 5.11: DSC thermograms showing the melting points of the HVOF sprayed pure Nylon-11 and composite coatings...... 72

Figure 5.12: X-ray Microtomography images showing the distribution of silica-rich (red) regions and Nylon-11 rich (grey) regions in the multi-scale (MS-1) coating. Top left image is a BSE-SEM image of MS-1 coating...... 73

Figure 5.13: X-ray Microtomography images showing the distribution of porosity-rich regions in the multi-scale (MS-1) coating...... 73

Figure 5.14: X-ray diffraction patterns of pure Nylon-11, 7 nm and multi-scale silica reinforced coatings...... 74

Figure 5.15: Optical micrographs of HVOF sprayed coatings after scratch testing at a load of 0.5 kg: (a) scratch profile of pure Nylon-11 coating and (b) multi-scale (MS-1) polymer ceramic composite coating...... 75

Figure 5.16 Scratch test performance of pure Nylon-11 and composite coatings comparing the scratch depths of nano-scale, micron-scale and multi-scale coatings as a function of applied loads of 1, 1.5 and 2 kg...... 76

Figure 5.17 Scratch test performance of pure Nylon-11 and composite coatings: (a) Coating scratch depth vs. load for pure Nylon-11, 7 nm and multi-scale coatings and (b) percentage reduction in scratch depth relative to a pure Nylon-11 coating...... 78

Figure 6.1: Optical images of Nylon-11 splats on a glass slide (a) preheated to 150 °C and (b) at room temperature (28 °C)...... 80 xiii

Figure 6.2: Cross-sections of predicted three-dimensional spreading splats for 30, 60, 90 and 120 µm diameter particles. Colors indicate the internal temperature distribution84. . 81

Figure 6.3: Velocity field inside a spreading 90 µm diameter particle; left-hand side: velocity magnitude, right-hand side: velocity vectors86...... 82

Figure 6.4: Cross-section of four steel substrates: (a) polished with ~1 µm alumina suspension, (b) grit blasted with #120 grit, (c) grit blasted with #50 grit, (d) grit blasted with #12 grit. Right image shows optical interferometric scan of # 120 grit blasted 86 surface using Al2O3 grit at an angle of 45° with an air pressure 0.55 MPa ...... 83

Figure 6.5: Cross-sections of predicted three-dimensional spreading splats for a 90 µm diameter Nylon-11 particle on four different surface roughnesses (Ra) 86...... 85

Figure 6.6: Nylon-11 splats deposited during a single pass over steel substrates: (a) polished with ~1 μm alumina suspension, (b) grit blasted with #120 grit, (c) grit blasted with #50 grit, (d) grit blasted with #12 grit86...... 87

Figure 6.7: Nylon -11 splat on GB120 glass substrate86...... 88

Figure 6.8: Cross-sections of Nylon-11 splats on polished steel substrates...... 89

Figure 7.1: Powder feed rate calibration for pure Nylon-11 (60 μm) feedstock powder. Three measurements were made on each powder rpm (1-5) and the mean reported in each case...... 94

Figure 7.2: Powder feed rate calibration for the Nylon-11 + 10 Vol. % 7 nm silica feedstock composite powder, three measurements were made on each powder rpm (1-5) and the mean reported in each case...... 95

Figure 7.3: Powder feed rate calibration for the Nylon-11 + 10 Vol. % multi-scale (MS-1) silica feedstock powders. Three measurements were made on each powder rpm (1-5) and the mean reported in each case...... 95

xiv

LIST OF EQUATIONS

m ρ = (g/cm3) 3.1...... 31 v

XX =+ 1 3.2 ...... 35 Nylon−11 SiO2

Nylon−11.φρ Nylon−11 X = 3.3 ...... 35 Nylon−11 . + .φρφρ Nylon−11 Nylon−11 2 SiOSiO 2

xv

LIST OF ABBREVIATIONS

BSE Back Scattered Electron

CTE Coefficient of Thermal Expansion

CVD Chemical Vapor Deposition

DSC Differential Scanning Calorimetry

EDS Energy Dispersive Spectroscopy

HVOF High Velocity Oxy-Fuel

MAPP Methylacetylene-Propadiene

MS Material System

NSF National Science Foundation

PMC Polymer Matrix Composite

PSD Particle Size Distribution

PVD Physical Vapor Deposition

RI Refractive Index

RRI Relative Refractive Index xvi

SEM Scanning Electron Microscopy

TS Thermal Spray

TGA Thermo Gravimetric Analysis

VOC Volatile Organic Compound

XRD X-ray Diffraction

xvii

ABSTRACT Thermal Spraying of Polymer-Ceramic Composite Coatings with Multiple Size Scales of Reinforcements Varun Gupta Advisor: Richard Knight, Ph.D. Co-Advisor: Richard A. Cairncross, Ph.D.

Thermal spraying is a solvent-less and low-VOC technique for producing polymer and polymer composite protective coatings. The high velocity oxy-fuel (HVOF) combustion spray process, a part of the thermal spray family tree, has been demonstrated as a viable approach for producing nano-scale and multi-scale reinforced semi-crystalline polymer composite coatings by controlling both the particle dwell time and substrate temperature. HVOF sprayed polymer matrix composites incorporating nominal 10 Vol.

% of multiple size ceramic reinforcements ranging from 7 nm to 15 µm were studied to bridge the nano and conventional micron size scale regimes. The goal of this research project was to improve the scratch and wear resistance of thermally-sprayed polymer coatings by incorporating multiple scales of ceramic reinforcements.

The polymer and ceramic powders were dry ball-milled to produce the composite powders. Dry ball-milling polymer and ceramic particles together resulted in a core-shell powder morphology with ceramic-rich shells around polymer-rich cores. The morphology of the composite powders, particle size distribution and elemental phases present were characterized by scanning electron microscopy (SEM), particle size analysis and energy dispersive spectroscopy (EDS). Ashing & thermo gravimetric analysis (TGA) was used to confirm the ~10 Vol. % loading of ceramic reinforcement in the composite feedstock powders.

The microstructure of the HVOF sprayed composite coatings was a cellular lamellar structure with ceramic reinforcements agglomerated at splat boundaries. EDS xviii analysis confirmed the concentration of ceramic reinforcements at splat boundaries in sprayed coatings. The effect of particle size on dispersion and distribution, and the influence of substrate temperature on coating adhesion, were investigated.

Microstructural characterization was used to analyze the dispersion and distribution of the ceramic reinforcements within the polymer matrix. Changes in crystallinity, as determined by TGA and differential scanning calorimetry (DSC), were correlated to nano/multi scale coating microstructures, reinforcement loadings and processing parameter variations. The amount of ceramic reinforcement incorporated within the sprayed coatings was studied by ashing and TGA, which indicated a ~50% loss of reinforcement during spraying.

The HVOF sprayed coatings were characterized for mechanical properties such as scratch resistance. Multi-scale composite coatings exhibited improved scratch resistance over HVOF sprayed pure Nylon-11 and single scale (nano-scale) reinforced composite coatings. Multi-scale ceramic reinforcements reduced scratch depths by as much as 50% relative to pure polymer coatings, and by up to 20% compared to single-scale reinforcements. Improvement in the mechanical properties was likely due to mechanical reinforcement/load transfer provided by the ceramic particles.

1

CHAPTER 1: INTRODUCTION

Corrosion and wear damage of materials is one of today’s largest industrial concerns, as failure of materials for infrastructure can lead to problems such as loss of human lives as well as millions of dollars in monetary loss; therefore, protection of infrastructure against corrosion and wear is one of the most significant challenges of today’s industry-driven society1-3. The protection offered by coatings contributes to this endeavor by prolonging the useful lifetime of many products such as bridges, pipelines and other essential components of our infrastructure. It is often not economical to protect these items without coatings, because corrosive environments such as weather, sea water and corrosive gases would damage them too quickly4.

The value (sales in dollars) of U.S. shipments of paints and allied coating materials in the year 2000 was over $17 billion5. It has been estimated that the value of the products protected by these coatings annually was $2 trillion, and in many cases, these products are not marketed at all in uncoated form. The cost of the coating material is around 1% of the total value of the products5. Furthermore, the cost of applying coatings varies greatly depending on the process used and the end use, but at least in some cases the application cost may be about the same as the cost of materials such as paints. The coatings may cost an estimated average of 2% of the value of the products they protect, rendering this protective mechanism extremely cost effective.

Various technologies, including painting, electroplating, welding, fluidized beds, chemical vapor deposition (CVD), physical vapor deposition (PVD) and electrostatic spraying are available for the deposition of coatings. Traditionally, painting involves volatile organic solvents and, therefore, is not an environmentally sound option. Fluidized 2 beds, although effective in coating large surfaces, are fixed in location. In the welding technique, a coating is melted onto a substrate surface by a gas flame, electric or plasma arc welding processes6, 7. The high temperatures of this process, therefore, make it impossible to coat flammable or low melting temperature materials, limiting it to metallic substrates only. Other methods, such as CVD, require the use of volatile precursors and vacuum chambers, and PVD is generally used in small objects and cannot be used to coat structural items such as bridges.

Thermal spray (TS) is a process that could solve some of the problems of other coating techniques mentioned above. Thermal spray is a generic term used to describe a group of processes, including flame spraying, plasma spraying, high-velocity oxy-fuel

(HVOF) and cold spray that can be used to apply a variety of different coating materials to different surfaces to provide corrosion, wear and thermal protection8. There are various advantages to using thermal spray. To begin with, many thermal spray processes enable polymer processing without the use of volatile organic compounds (VOCs)9, making the process more environmentally friendly and more compliant with regulations than other competing coating techniques. The amendment to the 1990 Clean Air Act resulted in restrictions being placed on paints and solvents containing volatile organic compounds10, limiting most industrial paints to a maximum VOC content of 350 g/l. In addition, thermal spray processing offers the ability to coat large objects in the field. However, most importantly, thermal spray enables the application of polymer coatings with high melt viscosity. This is especially relevant to the processing of polymers and polymer- matrix composites with high reinforcement volume-fractions (>5 Vol. %) suitable for corrosion and/or wear resistance applications. In these cases, processing requires either 3 large amounts of solvent (20 to 60 Vol. %)11, high processing temperatures or in-situ polymerization12 because of the high viscosity of composite melts.

Thermal spray of coatings for corrosion and wear resistance can be used in many industries, such as shipping, aeronautical, materials processing and mining, where there are requirements for components such as valves, pumps, pipework, turbine and helicopter blades, extrusion dies and powder mixers to perform in aggressive environments which are erosive and corrosive13. Coatings that act as barriers between the corrosive attack and the component surface can protect surfaces subjected to such aggressive environments.

The selection of a suitable material for coating and the process, however, is a challenging task.

Polymers are desirable materials for coatings because they can be used to apply relatively thin (25 μm) as well as thick (5 mm) coatings onto a wide variety of materials; furthermore, they are resistant to corrosive environments14. Thermally sprayed polymer coatings have been used to coat steel under very cold atmospheric conditions, where painting is not practical15. Furthermore, recycled plastics have been successfully used to create thermally sprayed polymer coatings16, proving the versatility and convenience of this technique. It is not a surprise, then, that there is a growing interest in the thermal spray of polymer and polymer composites for infrastructure-protection applications.

Thermally sprayed polymer coatings can be further improved by incorporating reinforcements to the polymers to produce composites that can be used not just for corrosion resistance but also for wear resistance. The particulates, either metallic or ceramic materials, act as barriers to failure, resulting in an improvement in the mechanical properties of the coatings. The wear, abrasion and corrosion resistance of 4 components thus can be increased for protective composite coatings, rendering this technology attractive to industry.

This work focuses on the development and understanding of thermal spraying of multi-scale polymer (Nylon-11)-ceramic (silica and alumina) composite coatings. To begin with, composite powders were prepared for spraying by utilizing a coating concept called “Multi-scale”. The multi-scale coating concept in this thesis is defined as a composite coating incorporating multiple size scales of ceramic reinforcements (i.e. 7 nm

– 15 µm) in the polymer matrix. The use of high velocity oxy-fuel (HVOF) combustion thermal spray process to produce coatings, and the resultant characterization of the coatings, are discussed. The main objectives of this work were:

1. Evaluation of the applicability and viability of the HVOF process to

deposit multi-scale polymer-ceramic composite coatings.

2. Optimization of the processing parameters for the deposition of pure

polymer and multi-scale composite coatings.

3. Investigation of the distribution and dispersion of reinforcements within

the polymer matrix.

4. Evaluation of the amount of ceramic loading that is incorporated during

the processing of feedstock powders and retained during the spraying of

coatings.

5. Correlation of the reinforcement loading to the mechanical properties of

sprayed coatings. 5

CHAPTER 2: BACKGROUND AND LITERATURE REVIEW

In order to understand the spraying of particulate reinforced polymer matrix composites (PMCs), it was first necessary to become familiar with their properties and their current and potential applications to develop advanced coating systems. The background information on thermal spray processes used during this work is also presented in this chapter. The chapter ends with a discussion of information on the thermal spraying of polymers and polymer composite coatings from literature.

2.1. Particulate Reinforced Polymer Matrix Composites

Composites can be defined as macroscopic, multiphase materials with a distinct interface between their constituent phases17. This definition is usually restricted to synthetic materials that contain a reinforcing phase (fibers or particles) supported by a binding material or matrix. Composites were developed for applications requiring affordable materials with unusual combinations of properties that could not be met by a single homogeneous polymer, metal or ceramic. This is especially true for the high strength-to-weight ratio materials used in aerospace, underwater and transportation applications, as shown in Figure 2.1. As shown in Figure 2.1, the axes are Young's modulus and density. The logarithmic scales span a range so as to include all materials.

When data for a given material class such as composites are plotted on these axes, it is found that they occupy a field which can be enclosed in a ‘balloon’. The strength to weight ratio of composites falls between polymers and ceramics. 6

Figure 2.1: Ashby diagram showing the strength vs. density for various engineered materials18, 19.

Composites can be divided into two classes: fiber-reinforced composites and particulate-reinforced composites. The focus of the current work is on particulate- reinforced composites.

Particulate-filled polymer composites are materials of widespread use (automobile tires). The traditional approach to enhance the mechanical properties of polymers is to add a second phase in the form of particles of a material with a higher modulus, such as a metal or a ceramic. By combining two or more dissimilar materials, a new material exhibiting the desired combination of properties can be engineered. Fillers and reinforcements of various dimensions are frequently added to polymers in order to make them stiffer. Other improvements in properties such as heat resistance, dimensional stability, fracture toughness and optical properties can also be targeted. Properties such as yield stress and elastic modulus can be increased through the addition of micron scale 7 particles of ceramics and glasses such as small mineral particles, metal particles such as aluminum, and amorphous materials, including polymers and carbon black. The most important effect of adding particulate reinforcements is the improvement of many polymer physical properties, including thermal and electrical conductivity, UV stability, flammability20, mechanical properties , wear and corrosion performance21.

Over the past twenty years, several composites have been produced by incorporating micron-scale reinforcements into a polymer matrix. The particle diameter is typically on the order of a few microns (10-6 m). Typically, the particles comprise between 20 percent and 40 percent (by volume) of the composite. In this type of composite, the particles carry a major portion of the load, and the purpose of the matrix is to hold the particles together. Examples of micron-scale particle reinforcements would be the addition of carbon black to automobile tires, and cermets.

Recently, scientific and industrial interest has been focused on nanocomposites, i.e., materials in which the secondary phase is 100 nanometers (10-9 m) or less in size22.

Such composites exhibited enhanced performance compared to traditional ones (micron- scale). In particular, a great deal of attention has been devoted to bi-phase or multi-phase systems where inorganic nanoscale particles are added to the polymer. Such reinforcements, if well dispersed in the matrix, offer a larger specific surface area than conventional reinforcements; thus, the interfacial interactions (Van der waals, electrostatic and hydrogen bonding) between the matrix and the particles are enhanced, leading to an even greater improvement in the properties of the material23. It has also been reported that even at very low reinforcement volume contents, such as 1–5 vol.%, a considerable improvement in the mechanical and tribological properties can be 8 achieved24. In particular, several authors have reported that ceramic (silica, alumina,

24, 25 CaSiO3) nanoparticles can effectively reinforce bulk polymers such as epoxy resins .

It has been demonstrated that 7 nm silica reinforcements in semicrystalline Nylon 6 increased the yield stress by 30% and Young’s modulus by 170% relative to pure polymers and polymer composites26. Similar results were obtained when nanosized reinforcements were added to polypropylene, which resulted in an increase of 100% in the dynamic storage modulus27.

Previous research has also shown that finer particles can lead to improvements in mechanical properties, as shown in Figure 2.2. Mitsuishi et al. (1985) and Massam and

Pinnavaia (1998) showed that the yield stress increased with particles of smaller dimensions (1 µm) in polypropylene filled with calcium carbonate and epoxy-clay nanocomposites. Other studies demonstrated that a decrease in reinforcement particle size resulted in an increase in yield stress in polyester filled with glass spheres28. The tensile strength in solid and hollow-glass filled rigid polyurethane composites29 and in amine- epoxy-layered silicate nanocomposites30 has been shown to increase for composites with nanoscale reinforcement sizes. Also, the strain to failure in polypropylene reinforced with glass beads31 showed the same trend. 9

Figure 2.2: Stress-strain curve for a particle-reinforced polymeric material. The experimental observations show increasing yield stress, tensile strength and strain to failure32.

Messersmith and Giannelis reported that the dynamic storage modulus of layered mica-type nanocomposites containing 4 Vol. % silicate (Na-Montmorillionite) in an epoxy matrix increased by 58% in the region below the glass transition temperature and by 450% in the rubbery region. They also found that the permeability of water in poly(ε- caprolactone) decreased by an order of magnitude with the addition of 4.8 Vol. % of mica-type silicate33. Yano et al. showed a 60% decrease in permeability for polyimide composites with as little as 2% mica content, while the thermal expansion coefficient was reduced by 25% as compared to the bulk polymer34. Also, Petrovicova et al. showed improvements of up to 35% in scratch resistance and 67% increase in wear resistance for

Nylon-11 coatings filled with nominal 15 Vol. % contents of 7 nm silica or carbon black

(6 nm), respectively, relative to unfilled polymer coatings35.

A possible explanation for the experimental results above is a size effect. There could be a possible relation between the size scales ranging from nanoscale to micron scale on the properties of particulate reinforced composites. Several experiments have 10 indirectly shown that there is a size-scale effect by measuring yield stresses for thin polymer films36 or interfacial shear strength in composites with a polymer matrix that were higher than the bulk values37-39. These findings promoted understanding that could be used in materials development, such as control of the size distribution of reinforcing particles and scaling on the basis of reinforcement size. Further insight into the size-scale dependence, however, is necessary.

2.2. Multi-scale Particulate Reinforcement

The addition of micron sized inorganic particles to reinforce polymeric materials can be traced back to the early years (1960s) of the composite industry. The design of conventional composites focused on maximizing the interfacial interactions between the polymer matrix and the reinforcement to ensure the best possible binding between the two and, consequently, optimize the distribution of the load along the matrix and the reinforcement40. This is commonly achieved by using smaller reinforcement particles to increase the surface area available for interaction (van-der Waals, Hydrogen bonds), also increasing the surface energy and promoting binding to the matrix.

With the emergence of materials synthesis methods (fuming, sol-gel) that can produce nanometer sized reinforcements41, 42, and large increases in specific surface area

(>400 m2/g), polymers reinforced with nanoscale particles can show improved properties such as coefficient of thermal expansion, UV stability and resistance to flammability20.

However, experimental evidence suggested that a simple extrapolation of conventional composites properties cannot be used to predict the behavior of nanocomposites43-45, and the origin of the differences between conventional and nanocomposites is still not clear. 11

One of the challenges in using nano-particulates in composites involves obtaining optimal dispersion into the matrix. Due to the Van der Waals forces between nano- particles, they tend to agglomerate and the improvements on the properties of composites may not be as expected.

Through the use of nano-scale reinforcements, in conjunction with macro-scale particles, materials may be developed with improved properties and performance characteristics. By combining the two approaches, a methodology known as “Multi- scale” is being investigated. The “Multi-scale” concept is defined as multiple size scales of reinforcements present within the matrix of a composite. Multiple scales of reinforcement are likely to exhibit improvements in reinforcing properties beyond those exhibited individually by either nanoscopic or microscopic particles.

The focus of the current work involves bridging between the nanometer and conventional (micrometer) scales of reinforcement through the use of “multi-scale” particulate reinforced polymer matrix composites. In order for particles to serve as reinforcements, the load must be transferred from the matrix to the reinforcement particles and back again. Due to the small size of the nanoscopic particles, a very small fraction of the total load borne by the material is transferred to individual particles.

Furthermore, the specific surface area of nanoparticles is extremely large (> 400 m2/g), which allows the load carried by each particle to be distributed over a large interface.

Thus, each individual particle carries a very small part of the total load. It should be possible to use nano particles to mediate load transfer between the microscopic particles and the matrix. The load is transferred to nearby reinforcing particles through the polymer matrix. Mechanically, the best direct transfer occurs to other large particles and 12 to smaller particles. These particles then transfer the load to other nearby particles until the load has been completely redistributed and dissipated, as shown in Figure 2.3.

(a)

(b) Figure 2.3: Schematic of load transfer between multi-scale reinforcing particles within a polymer matrix composite (a) a likely composite structure with varying ceramic sizes and (b) the transmission of load in compression or impact moderated by nano-size particles and micron-size particles.

2.3. Thermal Spray Processing

In thermal spray, metals, ceramics and composites are commonly used as the raw materials to produce coatings for a wide range of applications such as wear, corrosion and thermal protection8, 46-48. Thermal spray, in general, is a family of processes in which 13 a material in powder, wire or rod form is heated, accelerated and propelled by a high temperature jet through a confining nozzle towards a surface49. The individual molten or softened droplets forming splats impact, spread, cool, and solidify to form a new lamellar surface, known as the coating.

2.3.1. Thermal Spray “Family” Tree

The “family” of thermal spray processes is typically divided into four major categories, which are combustion spray (flame- powder/wire/rod; high velocity oxy-fuel), wire-arc or arc-spray, plasma spray (air, vacuum or inert atmosphere plasma spray; RF induction) and cold spray. The sources of energy that the majority of thermal spray processes use to melt or soften materials include combustion flames, hot gases, electric arcs and plasma jets. The resulting molten, or nearly molten, droplets/particles are accelerated in the process gas stream to velocities in the range of 80 to more than 1000 m/s and are propelled towards the surface to be coated. Upon impacting the surface, they spread, solidify and consolidate to form a coating. Figure 2.4 shows the various thermal spray processes grouped according to their energy source (combustion, electrical, gas- dynamic), feedstock (powder, wire or rod) and surrounding environment (air, low pressure, vacuum, inert gas, or underwater). Equipment capability and material requirements further determine the selection of a particular thermal spray process for a given application. 14

Figure .: Overview of the major thermal spray coating processes [extended and modified by Knight, 2002; from Smith, 1992].

In all cases, the high particle temperatures and/or velocities (Figure 2.5) result in significant particle deformation on impact upon a surface, producing thin layers of overlapping “splats.” Impacted droplets from all positions in the jet, with generally different degrees of melting or oxidation, deposit simultaneously to form a continuous coating containing melted and unmelted particles, oxides and debris. Figure 2.6 illustrates coating formation and the major microstructural features typically found in thermal spray coatings. Oxide inclusions in metallic coatings are present as a dark phase and are produced by either particle/atmosphere interactions or from overheating of the coating during or after deposition. Unmelted or re-solidified particles are a major source of porosity, as shown in the grey regions of Figure 2.6. Solid particles do not flow well on impact and can create voids in their shadow which are not filled by the next arriving particle, thus resulting in porosity in the coating. 15

Figure 2.5: Temperature-Velocity envelope of thermal spray processes9.

Figure 2.6: Schematic of coating deposition, depicting the major microstructural features found in thermal spray coatings50.

The HVOF combustion spray process was used as the primary tool for the development of the pure Nylon-11 and multi-scale coatings studied in this work. HVOF 16 can potentially reduce in-flight polymer degradation due to the shorter particle dwell time

(0.3-1.5 ms) and higher particle velocities (up to 1000 m/s). Also, the key advantages of

HVOF over non thermal spray coating processes include solventless deposition of high melt viscosity polymers and polymer-ceramic composites with reinforcement loadings greater than 5 Vol. %35. HVOF systems are also suitable for on-site applications and coatings are not restricted by the size of the part being coated. In addition, HVOF has the ability to coat large objects without the need for post-deposition melt processing such as oven curing.

2.3.2. High Velocity Oxy-Fuel Combustion Spray

The thermal spray method used in this work was the High Velocity Oxy-Fuel

(HVOF) process, as shown in Figure 2.7. The HVOF combustion spray process was invented in 1958 at Union Carbide (now Praxair Surface Technologies, Inc.), and commercialized in 1974 by Browning51. Traditionally, HVOF has been used to deposit metals and cermets, but not polymers or polymer matrix composites. In HVOF, a gas jet with a high velocity is generated by burning an oxygen-fuel mixture internally in a combustion chamber. High jet speeds (up to 1500 m/s), high jet temperatures (~2500 °C) and high particle velocities, (up to 1000 m/s) are characteristics of the HVOF process.

High particle velocities provide higher momentum and kinetic energy for improved splatting, with a corresponding increase in coating density and coating adhesion.

The HVOF process (including the high velocity air-fuel process – HVAF), has been one of the fastest growing combustion spray processes. The gun schematic shown in

Figure 2.7 illustrates the basic HVOF features with the different stages of particle 17 transport involved during coating deposition. HVOF includes an internal combustion chamber, a water cooling system, particle injection into a barrel with high-pressure combustion gases and a rapid supersonic expansion of the combusting gases. Particle heat transfer is increased; while dwell times (0.3 to 1.5 ms) are reduced by this design compared to other thermal spray processes (e.g. 2 to 4 ms for powder flame spray process). Typical process characteristics of the HVOF process and relevant features are listed in Table 2.1. A high velocity gas jet is generated by burning the oxy-fuel mixture internally under pressure. Fuels used in combination with oxygen include hydrogen, propylene (C3H6), propane (C3H8), MAPP and kerosene. Feedstock powder (typically -45 to +10 μm in size) with carrier gas (Ar, N2 or Air) is fed into the nozzle, where the particles are entrained into the high-pressure combustion gases. HVOF’s gas velocity, higher than other conventional combustion (flame spray, twin wire arc), has been shown to increase coating density52.

Table 2.1: HVOF process and deposit characteristics [modified from Smith, 1992].

PROCESS CHARACTERISTICS Jet Temperature Generally >2500 °C Jet Speed Typically >1000 m/s Gas Flow Rate 400-1100 slm Particle Speed 200-1000 m/s Powder Feed Rate 2-50 g/min (depends on the feedstock density)

DEPOSIT CHARACTERISTICS Density >95 % (due to high particle velocity) Bond Strength ~5-80 MPa (ASTM 0633-01) Microstructures Fine oxide dispersion Oxide content Comparable to air plasma spray 18

Figure 2.7: Schematic of the Jet-Kote II® high velocity oxy-fuel (HVOF) thermal spray gun, including main stages of a feedstock particle transport53.

The higher velocity of the particles (up to 1000 m/s) in HVOF in comparison to those velocities in flame spray or plasma spray processes (ranging between 100 to 800 m/s), provides higher momentum and kinetic energy for better splatting, which can result in higher coating density (>95%) and adhesion as well as finer coating oxide inclusion dispersion. In the case of polymers, HVOF can also potentially reduce thermal decomposition and material degradation due to shorter dwell times (0.3 -1.5 ms) and lower temperatures compared to other thermal spray processes such as flame spray (2-4 ms) and plasma spray35.

2.3.3. Thermal Spraying of Polymers and Polymer-Matrix Composites

Thermal spraying of polymers is receiving increased attention in diverse industrial and military sectors9. The ability to apply thin (about 0.13 mm, or 0.005 in.) and thick (up 19 to 6.4 mm, or 0.25 in.) coatings of polymers onto a variety of metals, ceramics, and composites of complex geometries provides solutions to component and on-site or factory manufacture.

Polymer spraying is a one-step process which acts as both the primer and the sealer with no additional cure times, unlike the traditional three-coat painting processes

(spraying, drying and curing). Polymer coatings can be repaired by re-melting (for thermoplastics) and applying additional material to the on-site location54. Environmental restrictions on the use of volatile organic compounds create the need for environmentally sound processing alternatives; thermal spray, and HVOF in particular, enables polymer processing without the use of volatile organic compounds (VOCs). Most importantly, thermal spray has the ability to apply polymer coatings with high melt viscosity because the materials are processed in the powder form, particularly in the case of high molecular weight polymers and polymer/ceramic composites with high reinforcement contents (up to 15 Vol. %).

In the case of HVOF, thermally sprayed polymer coatings are generally manufactured by the melting of polymer powder in a combustion flame; however, plasma can also be used55, 56. The polymer particulates are propelled through the flame where, once molten or softened, they impact on the substrate; and well-heated particles will deform and solidify, forming an interconnected network of splats – the unit process in thermal spray. The thickness of the coating is governed by the number of repeated passes of the spray gun across the substrate and the powder feed rate.

The significantly higher particle speeds of plasma and HVOF spray compared to flame spray provide better splatting with corresponding increases in coating density. On 20 the other hand, the higher particle speeds in the HVOF and plasma spray processes are often responsible for insufficient melting of the polymeric particles due to the short dwell times (< 5 ms) and low thermal conductivity of polymer particles (~0.2 W/mK). The cores of particles may remain unmelted, while the outer layers may even be over heated or degraded. A prediction of the effects of particle size and speed on the degree of melting during HVOF spraying of Nylon-11 was developed by Petrovicova et al. 2000 and shown in Figure 2.8b. It was predicted that smaller (30 μm) particles would be melted to a higher degree than larger (60 μm) particles, while a lower particle velocity

(900 m/s vs. 1400 m/s) would result in a higher degree of melting by providing longer particle residence time within the thermal jet. More recent research by Ivosevic et al.

2005, predicted the velocity profiles of particles at different distance from the gun accounting for variations in gas temperature, gas velocity and particle velocity. Figure

2.8a shows the predicted temperature profiles within Nylon-11 particles immediately prior to impact on a substrate. In general, the predicted velocity of polymer particles with smaller particle sizes (15 µm) was higher than that of coarser (60 µm) particles when they exit the gun. The predicted particle velocity was 700 m/s for 60 µm particles at a

100 mm spray distance, a significantly higher value than the experimentally measured particle velocity (410 - 450 m/s)53. This indicated that the gas flow and particle acceleration models used require further optimization to provide improved agreement with experimental measurement. 21

Figure 2.8: Predicted velocities of Nylon-11 particles in an HVOF jet53 and effect of particle diameter and speed on the calculated degree of melting of Nylon-11 in an HVOF jet57.

Polymer powders are identified by their chemistry, morphology, molecular weight distribution or melt-flow index and particle-size distribution. Major polymer coating limitations include low scratch resistance, poor adhesion to metallic substrates and high gas permeability. The drawbacks of using polymers as coatings have been dealt with by using blends and modified grafted polymers58, high performance polymers (Nylon® and

Teflon®) and polymer composites59.

For many polymeric coating needs, including wear and corrosion resistance, a higher molecular weight typically offers the desired increase in elastic modulus and yield stress, as shown in Figure 2.9a. However, the degree of intermolecular attraction between polymer chains increases significantly when the molecular weight exceeds a critical level

(Mc) (Figure 2.9b), resulting in a rapid increase in melt polymer viscosity. Processing of such materials, therefore, requires a temperature significantly higher than the melting temperature. The key advantage of using thermal spraying for the deposition of high molecular weight polymers is based on the particle consolidation nature of thermal spray 22 processes using powdered feedstock materials and the combination of temperature and time, which enables the processing of polymers with high melt viscosities.

Figure 2.9: General influence of molecular weight on (a) polymer properties and (b) viscosity60, 61.

The jet temperature and velocity, the spray distance and torch traverse speed have been shown to have a major effect on the structure and properties of thermally sprayed polymeric coatings62 because the high temperature jet, in conjunction with the slow torch traverse speed, may overheat the polymer coating. On the other hand, the low thermal conductivity of polymers (~0.2 W/mK) results in a substantial buildup of temperature at the surface of the coating and large temperature gradients can be developed through the thickness63, 64. In addition, the large difference in coefficient of thermal expansion (CTE) between metallic substrates and polymeric coatings may also lead to adhesive failure due to residual stresses generated during the process. Coatings on thin substrates heat up substantially as a result of repeated exposure to the flame, while there is little temperature build up in coatings on thick metal substrates which act as a large heat sink. It was shown by Bao et al. in 2005 that the temperature gradient was much steeper in thick polymer 23 coatings than in thin ones, but the average coating temperature was the same owing to the similar heat capacities of the components that were coated with the polymer coatings.

However, sufficient substrate preheating can prevent crack nucleation and alleviate thermal expansion mismatch problems59. Furthermore, a porous transition layer can also be used to absorb recrystallization and/or cooling shrinkage of polymeric coatings65.

Polymers that have been sprayed using common thermal spray processes are summarized in Table 2.2. The flame spray process was the first technique used for the deposition of polymers because of its simplicity and low cost; it is still fairly widely used for the industrial application of polymeric coatings14, 57, 66. Although Table 2.2 summarizes the different polymers that have been studied from the vast choice of polymers currently available, only a small number of them have been used as coatings and, more specifically, literature regarding the behavior of polymeric particles during thermal spray is scarce because a majority of the work is still in the developmental stages62, 67.

24

Table 2.2: Thermally sprayed polymers and polymer composites57 [modified from Petrovicova et al., 2002].

Polymeric Material FLAME HVOF PLASM A Bismaleimide-phenolic resin # Cyanate ester thermosets #

Ethylene-methacrylic (EMAA) copolymer/Al2O3 # Epoxy or epoxy-Nylon blend filled with Cu or Cu/Ni # Epoxy enamels #

Epoxy filled with TiO2 # Epoxy thermosets # Ethylene vinylene acetate (EVA) # Ethylene-acrylic acid (EAA) copolymer # # Ethylene-methyl methacrylate (EMAA) copolymer # Ethyltetrafluoroethylene (ETFE) # Liquid-crystalline polymers (LCP) # Nylon-11/glass or Al2O3 or SiO2 or carbon black # # # Polyethylene (PE) # # PE modified with methacrylic acid #

PE (ultrahigh molecular weight)/WC-Co or MgZrO3 # PE/Al2O3 # Phenolic thermosets # Polyamides (PA) – Nylons # # Polyarylene sulfide (PAS) # Polyaryletherketone (PAEK) #

Polycarbonate (PC) and PC/ SiO2 # # Polyester (PES) # # PES or polyurethane filled with TiO2, SiO2 and Al2O3 # Polyester-epoxy resin # # Polyether block amide copolymer # Polyether-amide (PEA) copolymer # Polyetheretherketone (PEEK) # # # PEEK/ Al2O3 # Polyethylene terephthalate (PET) # # # Polyethylene-polypropylene copolymer # Polymethylmethacrylate (PMMA) # Polyphenylene sulfide (PPS) # # # PPS/ Al2O3 # Polypropylene (PP) # Polysulphone # Polytetrafluoroethylene (PTFE) and its copolymers # # Polyvinylidene fluoride (PVDF) # Post-consumer commingled polymers (PCCP) # PVDF-hexafluoropropylene (HFP) copolymer #

PVDF/WC-Co or MgZrO3 # Polyimide (PMR)/WC-Co # Urethane # 25

Polymer composite coatings are one of the growing application areas for both combustion and plasma spraying. The addition of a reinforcing phase (ceramic, organic or metallic) results in polymer coatings with improved mechanical properties, such as elastic modulus and yield stress as well as other protective and barrier properties, including chemical resistance and oxidation protection67, 68. The presence of reinforcing particles often improves powder flowability during thermal spraying and reduces shrinkage due to the significantly lower CTE (0.4–20 x10-6 °C-1) of the reinforcements relative to the polymer matrix material (80–300 x10-6 °C-1)20. The presence of reinforcing particles in composites may also help to reduce polymer degradation, and consequently, porosity69.

One of the most important applications of polymer and polymer composite coatings is wear, abrasion and corrosion resistance. Li et al. filled PTFE with nanoparticles of ZnO and found that the wear resistance was improved by nearly two orders of magnitude, with a maximum wear resistance at ZnO concentrations of 15

Vol.%70. It has also been shown that the addition of glass caused a reduction in the sliding wear rate by almost an order of magnitude55. Wang et al. filled polyetheretherketone (PEEK) with various weight fractions of SiC, Si3N4, SiO2, and

ZrO2. The addition of these particulates at loadings of less than 10 Wt. % improved the wear resistance and reduced the friction coefficient71-75. Similarly, Petrovicova et al. demonstrated that Nylon-11 reinforced with nanoscale (7 nm) silica increased the sliding wear resistance by 48 % in comparison to the unfilled pure Nylon-11 coatings67. Avella et al. filled polymethylmethacrylate (PMMA) with nanoscale CaCO3. The abrasion

76 resistance improved by a factor of two through the addition of 3 Wt. % of CaCO3 . 26

Schwartz and Bahadur showed that polyphenylene sulfide (PPS) filled with alumina nanoparticles may lead to good dispersion of the reinforcement particles in the

PPS matrix77. Achieving a uniform dispersion of the particulate reinforcements within the polymer matrix is an important technical challenge in the creation of nano and multi- scale composites because of the high viscosity of composite melts. Processing requires either large amounts of solvent (20 to 60 Vol. %), high processing temperatures or in-situ polymerization. Reinforcement volume, particle geometry and size have the most significant effect on the viscosity of filled polymer melts.

Recent restrictions on the use of volatile organic compounds (VOC’s) by the

Environmental Protection Agency (EPA) also create a need for environmentally acceptable alternatives to solvents for the application of organic coatings and paints.

Another important technical challenge in the use of polymer composites is the processing of particle reinforced polymer composites with high loading content (>15 Vol. %).

Thermal spray techniques, specifically High Velocity Oxy Fuel, are excellent solutions for overcoming these processing limitations.

The variations in particle size, density and morphology, in combination with the non-uniform velocity distributions of thermal spray jets, can result in considerable segregation when dissimilar materials are co-sprayed78. Another problem is that nano- sized particles cannot be fed through a thermal spray system, as size scales in the nano regime leads to agglomeration. One approach is melt/mold compounding, which might produce homogenous particles and a more homogeneous reinforcement distribution within a sprayed coating compared to dry blending, but it is not always feasible and could be associated with processing difficulties related to rapid increases in melt viscosity of 27 the liquid polymers when the reinforcement material is introduced in amounts larger than

~15 Vol. %. Another approach considered for minimizing material segregation and proper feeding of particles has been dry ball-milling. Dry ball-milling can be used to produce polymer/nano-sized ceramic composite powders that can be thermally sprayed as a “composite” feedstock, with the ceramic phase mechanically embedded into the polymer component57, 67. However, some thermosetting polymers (cured epoxy) are relatively hard and brittle, therefore ball-milling may not result in any significant embedding of the reinforcement material into polymer particles.

The use of thermal spray methods to process polymer and polymer composites present a large number of potentially new applications for polymer coatings. New and promising applications are currently being investigated, such as composite coatings for erosion protection of carbon fiber reinforced composites78, 79. A number of other applications that are looked at include magnetic polymer composites (using ferrite materials as reinforcement) for use in magnetic guide strips, automotive components and electromechanical devices57. In this work, the use of ceramic reinforcements for increasing the wear and corrosion resistance of thermally sprayed polymer composites was explored. 28

CHAPTER 3: EXPERIMENTAL PROCEDURE

The as–received Nylon-11 and ceramic reinforcements were characterized for morphology, shape and particle size distribution. The powders, consisting of Nylon-11 and 10 Vol. % of ceramic reinforcements, were ball-milled to produce composite feedstock materials. In order to get a clear picture of the distribution and dispersion of ceramic reinforcements within the Nylon-11 matrix, cross-sectional views of the particles of the composite powders were obtained. Backscatter electron imaging (BSE) using scanning electron microscopy (SEM) was used to analyze cross-sectional images, as this technique gives the elemental contrast between the different phases based on the difference in atomic numbers of the elements. In addition, energy dispersive spectroscopy

(EDS) was used to obtain confirmation of the presence of the ceramic reinforcements.

The composite powders were used to produce semi-crystalline coatings by the

HVOF combustion spray process. The coatings were sprayed onto steel and aluminum substrates that were grit blasted to roughen the surface, since in thermal spray mechanical interlocking is the primary bonding mechanism of the coatings to the substrate. The microstructural characterization, along with the distribution of ceramic phases within the

Nylon-11 polymer matrix, was analyzed by SEM-BSE, and further confirmation of the ceramic phases present in the coatings was obtained by EDS.

The amount of reinforcement present in the composite powders and the HVOF sprayed coating was evaluated by ashing and confirmed by thermo gravimetric analysis

(TGA). Furthermore, the melting point and crystallinity of the composite powders and sprayed coatings were obtained by Differential Scanning Calorimetry (DSC). 29

The scratch performance of the multi-scale coatings in comparison to the pure

Nylon-11 coatings was studied at different loads using a balance beam scrape adhesion and mar tester.

3.1. Materials Selection

3.1.1. Substrate Material

1018 steel and Al 6061-T6 were chosen as substrate materials due to their low cost and availability. The substrate had dimensions of 25.4 x 75.2 x 3 mm (1 x 3 x 0.125 in). The substrates were roughened with a Trinco 24/BP2 grit blasting system with a grit size of 1500 µm prior to spraying.

3.1.2. Feedstock Powders

3.1.2.1. Matrix Material: Pure Nylon-11

Thermoplastic polyamide Nylon-11 was selected as the matrix material for the proposed multi-scale coatings because it is a widely used industrial material for producing semi-crystalline coatings, has a high chemical resistance and fairly broad temperature processing window, i.e. a large difference between the melting (~183 °C) and degradation temperatures (360 to 550 °C). The glass transition temperature, Tg, is typically in the range 45 to 53 °C, varying slightly as a function of crystallinity and molecular weight. Also, Nylon-11 has been used as a coating because of its favorable combination of properties, including a low coefficient of friction (~0.25), low density

(1.01 g/cm3) and flexibility at temperatures as low as -40 °C80, 81.

Mechanically crushed and cyro grounded, semi-crystalline polyamide, designated as Nylon-11 (D60) “French Natural ES” (donated by Arkema, Inc., King of Prussia, PA), 30 was used as the matrix material to produce nano and multi-scale coatings. The as- received powder had a mean particle size of 60 µm and a corresponding particle size distribution of 10 to 180 µm. Previous research studies67 successfully used the same material for the production of nano reinforced semi-crystalline composite coatings, confirming that Nylon-11 was a suitable matrix material.

The molecular structure of Nylon-11 is shown in Figure 3.1. Nylons are known as polyamides because their backbones contain amine (N—H) groups. These amide groups are extremely polar, and hydrogen bond with each other. The backbone of Nylon is regular and symmetrical, so Nylon is often semi-crystalline. Nylons are classified according to the number of carbon atoms in the repeat units of the polymer chain. For example, the structure shown in Figure 3.1 is Nylon-11 since each repeat unit in the polymer chain is 11 carbon atoms long.

O

H2 H2 H2 H2 H2 C C C C C C N C C C C C H2 H2 H2 H2 H2 H

Figure 3.1: Molecular structure of polyamide Nylon-1182.

3.1.2.2. Reinforcing Phase: Ceramic Powders

Fumed and precipitated silica was used as a ceramic reinforcement because of its desirable reinforcing properties, including hardness (~6 GPa) and high surface area (~400 m2/g). In addition, silica is one of the most widely used and readily available ceramic reinforcements. Silica powders with nominal particle sizes of 7, 12, 20 40, 60 nm, 10 and

15 µm, designated as Aerosil 300, 200, 90 and A90, Sip 50s and Sip 320 by the 31 manufacturer, (donated by Degussa Corporation) were used to produce the composite powders.

Similarly, alumina powders of various sizes were obtained from AGSCO

Corporation. Alumina with particle sizes of 300 nm, 1, 3 and 5 μm with hardness (~20

GPa) were used as the second reinforcing material.

3.2. Characterization of Feedstock Materials

3.2.1. Density

The densities of the Nylon-11 and ceramic reinforcements were measured in order to calculate the volume fractions of these materials in the composite powders prior to dry ball-milling. In order to confirm the densities values of the powders provided by the manufacturers, the densities were measured using a liquid displacement technique

(Archimedean method), using ethanol as the liquid medium. The bulk densities of these materials were calculated based on 10 measurements using Equation 3.1.

m ρ = (g/cm3)...... 3.1 v

The bulk densities of the Nylon-11, silica and alumina powders were found to be

1.01 g/cm3, 2.2 g/cm3 and 3.67 g/cm3 respectively, which were in agreement with the values provided by the manufacturers (Arkema, Inc., Degussa Corporation and AGSCO

Corporation).

32

3.2.2. Powder Feed Rate Calibration

Powder feed calibrations (g/min vs. feeder revolutions per meter (RPM)) were carried out for the Nylon-11 and composite powders using a Praxair Model 1207 volumetric powder feeder. A volumetric powder feeder consists of a rotating slotted wheel used for collecting small portions of powder into the slots as they pass over the powder pick-up area. Metered volumes of powder then pass over a hole through which carrier-gas flows, as shown in Figure 3.2.

Figure 3.2: Schematic of a pressurized volumetric powder feeder.

Powders were fed at various RPM settings for a fixed time, and the mass of powder fed was collected in a plastic jar and weighed. Multiple tests (five) were conducted under the conditions listed below:

• Sampling time: 300 s (i.e. 5 min). 33

• Carrier gas: Ar.

• Pressure: 120 psi manifold pressure/80 psi console pressure.

• Carrier gas flow rate: 50 scfh.

• Feeder Speed: 1-6 rpm.

The resulting powder feed rates were used with the HVOF system to produce nano- scale and multi-scale polymer composite coatings. The results of calibration are summarized in Appendix A.

3.3. Production of Composite Feedstock Powders

Ball-milling of powders was investigated with the expectation that the hard ceramic particles would adhere to, or be embedded into, the softer Nylon-11 polymer matrix. Dry ball-milling produced polymer-ceramic composite feedstock powders suitable for thermal spraying with the ceramic phase mechanically embedded into the polymer component. The composite powders had a core-shell morphology with the core of the polymer particles surrounded by a thin shell of ceramic-rich nylon, as shown in

Figure 3.3.

Figure 3.3: Schematic showing the embedding of hard ceramic particles into the surface of a polymer particle during ball-milling, resulting in core-shell structure.

34

300 g batches of the Nylon-11 were dry ball-milled with 10 Vol. % of ceramic reinforcement for 48 hours at 60 rpm in a Norton Ball Mill (Figure 3.4b) using Zirconia

(ZrO2) milling media. Equal nominal volumes (~500 ml) of ZrO2 balls (diameter 10-20 mm) and Nylon-11/ceramic reinforcements were used throughout the ball-milling

3 process. Owing to the difference in densities of the materials (ρNylon-11 = 1.01 g/cm , ρsilica

3 3 = 2.2 g/cm and ρalumina = 3.67 g/cm ), the mixtures were first Vee-blended for 10 minutes prior to ball-milling in order to homogenize the materials and improve the effectiveness of subsequent ball-milling. Table 3.1 summarizes the powders that were weighed and

Vee blended prior to ball-milling with 10 Vol. % of reinforcement content.

Table 3.1. Composition of blended polymer/ceramic powders prepared for ball-milling.

Polymer Matrix Reinforcement Content Materials System (90 Vol. %) (10 Vol. %) Nylon-11 Nylon-11 - 7 nm (nano-scale) Nylon-11 Silica (7 nm) Silica (7, 12, 20, 40 nm + 3, 10, 15 MS-1 Nylon-11 µm) MS-2 Nylon-11 Silica (7, 12, 40 nm + 3, 15 µm) MS-3 Nylon-11 Silica (7, 12, 20 40 nm + 10 µm) Silica (7, 12, 20, 40 nm) + Alumina MS-4 Nylon-11 (300 nm, 1µm, 5µm) 35

(a) (b)

Figure 3.4: Ball-milling Process (a) Schematic (side view) and (b) Norton Ball-mill used in this study.

The percentage of ceramic reinforcements within the total mixture was determined on a volumetric basis. The multi-scale powders contained equal amount by mass of each size-scale powder as listed in table 3.1, i.e., the mass that would yield the desired Vol. % of ceramic reinforcement (10 Vol. % in this case) was divided by the number of reinforcement components. Density (ρ) of materials were determined experimentally for all volume fraction (φ) calculations. Selected volumetric ratios of the

Nylon-11 and ceramic reinforcement mixtures were prepared using the mass fractions (X) of the materials calculated according to Equations 3.2 and 3.3.

XX =+ 1 ...... 3.2 Nylon−11 SiO2

Nylon−11.φρ Nylon−11 X = ...... 3.3 Nylon−11 . + .φρφρ Nylon−11 Nylon−11 2 SiOSiO 2 36

Where X , X are the mass fractions of Nylon-11 and ceramic reinforcement SiO2 −11Nylon

(SiO2 in this case) φ , φ are the volume fractions of Nylon-11 and ceramic SiO2 −11Nylon reinforcements and ρ , ρ are the densities of Nylon-11 and ceramic SiO2 −11Nylon reinforcement.

3.4. Coating Procedure

3.4.1. HVOF Spray System

A Stellite Coatings, Inc., Jet Kote II® HVOF high velocity combustion spray system, was used to spray the pure Nylon-11 and composite feedstock powders, as shown in Figure 3.5. The detailed HVOF processing parameters used to spray all the composite powders are summarized in Table 3.2.

Supersonic flow shock diamonds

Powder injector

Carrier Gas

Combustion chamber Cooling water in Coating

Cooling Substrate Oxygen Fuel water out

Figure 3.5: Schematic of the Jet Kote II® high velocity combustion spray system50.

The system consisted of a spray gun supported by a control console, a water cooling system, a volumetric powder feeder, hydrogen and oxygen gas supply manifolds and connecting hoses. The combustion and resulting supersonic jet produced noise levels in excess of 130 dB(A), so all spray operations were carried out in an isolated acoustic 37 room. A semi automatic X-Y traverse manipulator (traverse speed range 0 – 0.23 m/s) was used to mount and move the Jet Kote II® spray gun.

The gun comprised of an internal combustion chamber located perpendicular to a water-cooled copper nozzle. A 150 mm (6 in) long by 8 mm (5/16 in) ∅ nozzle was used to spray the polymers and polymer composites. A hydrogen pilot flame was used to ignite the main HVOF jet. The flow rates of the two main jet gases (hydrogen and oxygen), the pilot gas (hydrogen) and the powder career gas (argon) were controlled from the console.

The powders were fed using a Praxair Model 1207 volumetric powder feeder. All coatings were sprayed using the spray parameters summarized in Table 3.2.

Table 3.2. HVOF spray parameters used for the deposition of pure Nylon-11 and polymer-ceramic composite coatings.

HVOF Spray Parameter Value Spray distance (m) 0.125

Powder feed rate (g/min) 5 – 15 Carrier gas Ar Carrier gas flow rate (m3/s) 0.5 x 10-4

H2:O2 molar ratio 0.4- 0.5 3 -3 -3 H2 and O2 flow rates (m /s) 2.8 x 10 / 5.6 x 10 Surface speed (m/s) 0.11 Substrate temperature (°C) 140 – 150

3.4.2. Spray Setup and Substrate Preheating

Spraying of the pure Nylon-11 and multi-scale composite coating structures required reliability and repeatability. Figure 3.6 shows the setup evaluated for its suitability for deposition of polymer and polymer composites. It consisted of an aluminum frame on which the substrate was placed in the line of sight of the HVOF gun. 38

Coatings were sprayed with the HVOF torch mounted on an X-Y manipulator, using raster or ladder scanning of the spray pattern across the surface of the substrate, with each

“pass” overlapping the previous one by approximately half the spray pattern diameter to ensure complete coverage. Once through, this sequence was termed a "cycle." Multiple cycles, typically 6-8, were repeated until the desired coating thickness, 100-800 µm, was obtained.

Figure 3.6: Spray setup with Jet-Kote II® high velocity oxy-fuel (HVOF) in operation, showing the deposition of coatings onto a substrate.

Based on the stoichiometry (H2:O2) and combustion reaction of hydrogen and oxygen it was expected that the maximum temperature of the HVOF gun (> 2400 °C) should have been sufficient to generate substrate front face temperatures in the range of

140-150 °C. Substrates were externally preheated to ~140 °C by traversing the HVOF jet over the substrate surface prior to coating deposition. The temperature was measured using a hand-held type K thermocouple probe. A ~25 µm (0.001 in) single layer of 39 coating was first applied to the heated substrate. After application of the first layer, the substrate temperature was kept constant by forced air cooling (pressure 40 psi) from the rear until subsequent spraying was completed.

3.5. Characterization Techniques: Powders and Sprayed Coatings

Several characterization techniques were used to characterize the feedstock powders and HVOF sprayed coatings. A number of techniques, including optical microscopy, scanning electron microscopy (SEM), SEM-energy dispersive spectroscopy

(EDS), Horiba laser scattering particle size analysis, thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC), x-ray diffraction (XRD), x-ray microtomography and scratch testing were used.

3.5.1. Morphology and Coating Microstructure

An Olympus PMG-3 optical metallograph and an FEI XL-30 field emission environmental scanning electron microscope (ESEM) were used to characterize the feedstock morphology, coating microstructures and reinforcement distribution. Standard metallographic techniques (sectioning, mounting and polishing) were used for preparing and revealing the coating microstructures for analysis. Samples were mounted in a slow curing epoxy (HUDSON® HE-40) and then polished using a Struers Abrapol™ automated polishing and grinding machine. Polishing was carried out in several steps using 200, 320, 400, 600, 800 and 1200 grit SiC papers at 150 rpm for 120 s each. Final polishing was carried out using 3 and 1 μm diamond cloths for 160 s at 300 rpm.

40

3.5.2. Elemental Analysis

The elemental composition of the powders and coatings was characterized by

Energy Dispersive Spectroscopy (EDS) using an EDAX attached to the ESEM (FEI XL-

30); in addition, a Siemens D-500 X-Ray Diffractometer was used for x-ray diffraction analysis of the samples.

The distribution of the ceramic reinforcements within the composite coatings was also evaluated by X-ray microtomography using a SkyScan-1172 desk-top micro-CT system. Multiple x-ray “shadow” transmission images of the coatings from different angular views were obtained. From these shadow images, cross-sectional images of the object were reconstructed, creating a complete 3-D representation of internal microstructure.

3.5.3. Particle Size Distribution (PSD)

A Horiba, LA-910 laser scattering particle size analyzer was used to analyze the particle size distributions of the pure Nylon-11 and other ball-milled composite feedstock powders. The Horiba LA 910 measures particle sizes ranging from 0.02 µm to 1000 µm.

The Horiba unit uses an optical system with multiple light sources and a large-diameter lens with low optical aberration, which enables the Horiba LA-910 to measure particle size distributions across the entire range. The Relative Refractive Index (RRI) values were entered in the program in order to calculate the particle size distribution. RRI is the primary variable to be entered into the software for Horiba's LA-series particle size analyzer. This is obtained by dividing the refractive index of the particle material by the refractive index of the dispersion medium. The particle material was Nylon-11 (R.I. 1.55) and dispersion medium was ethyl alcohol (R.I. 1.37). 41

3.5.4 Thermal Properties

Thermal properties were also characterized in order to compare the thermal properties of the composite feedstock powders and HVOF sprayed coatings.

3.5.4.1. Ashing and Thermo Gravimetric Analysis (TGA)

Ashing and Thermo Gravimetric Analysis (TGA) were carried out using a Perkin

Elmer thermal analysis TGA-7 to determine the amount of ceramic reinforcing phase incorporated in the ball-milled composite powders and sprayed coatings. Powders and sprayed coatings were heated in a furnace from room temperature (27 °C) to 900 °C in air at a rate of 10 °C per minute and the polymer in the samples degraded, leaving silica and carbon. The amount of residue was weighed to calculate the percentage of the reinforcements present in the powders and sprayed coatings.

3.5.4.2. Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) was carried out using a Perkin-Elmer

DSC-7 to determine the melting points of the powders and coatings. In this procedure, two aluminum pans were used, one with the sample and one that served as a reference.

The aluminum pans were heated in air gradually and the heat of reaction was measured as the difference in heat input required by the aluminum pan with the sample and the reference pan. 5 mg samples were weighed and three experiments were carried out over the temperature range 30 to 240 °C at a heating rate of 10 °C/min. To ensure accurate results, the system was calibrated using a standard indium sample with a well-defined 42 melting temperature (156.6 °C) and heat of fusion (28.45 J/g). The percent difference between the reference sample and published values was about 0.5%.

3.5.5. Scratch Testing

Scratch testing of sprayed coatings were performed using a BYK Gardner SG-

8101 balance beam scrape adhesion and mar tester according to ASTM standard D 5178-

9137 using applied loads of 0.5 to 2 kg, as shown in Figure 3.7a. Scratch profiles and scratch depths were characterized using a Hommelwerke model Dektak II stylus-tracing profilometer with a stylus tip radius of 5 µm and an associated data acquisition system. A schematic of a typical scratch profile is shown below in Figure 3.7b. The system was calibrated on a standard glass sample with a scratch depth of 10 µm as shown in the

Figure 3.8.

43

(a) (b)

Figure 3.7: Scratch testing set up (a) BYK Gardner SG-8101 balance beam scrape adhesion and mar tester (ASTM D 5178-9137) and (b) schematic of a typical scratch profile.

Figure 3.8: Scratch test on a standard glass sample with a scratch depth of 10 µm to ensure repeatability of the results. 44

CHAPTER 4: RESULTS-FEEDTSOCK MATERIALS

Experimental results and discussion related to powder characteristic, particle size distribution and the effects of the ball-milling process on powder morphology are presented. In this chapter the influence of dispersion, distribution and size of the reinforcing phase on Nylon-11 was studied. These subjects were necessary for better understanding of the powder behavior during thermal spraying. The effect of ball-milling was analyzed by looking at the microstructure and particle size distributions of the composite powders and comparing these to the original powders. The amount of ceramic reinforcements incorporated during ball-milling was estimated by ashing, and further confirmation was derived from TGA. The thermal properties of the powders were further evaluated by DSC.

4.1. Pure Nylon-11 Powder

Figure 4.1 shows SEM images of the morphology of the “as received” Nylon-11

(D60) feedstock powder used to produce the HVOF coatings. The powder comprised particles ranging in sizes from 5 to 180 μm, as shown in Figure 4.1a., consistent with the

HORIBA particle size measurement discussed in section 4.3. The particles had an irregular and angular morphology (Figure 4.1 a, b) consistent with the mechanical crushing and cyro-grinding techniques used to produce the powder. Although mechanical crushing and cyro-grinding of Nylon-11 is an effective way to produce polyamide powders, the resulting angular and blocky powder morphology is likely not optimal for thermal spray deposition. Thermal spraying of polymers with an angular feedstock morphology may be expected to result in an increased level of in-flight polymer 45 degradation because of thermally “overexposed” sharp edges and corners. Particle morphology strongly influences the powder feeding characteristics, including transport of the particles from the powder feeder to the spray gun. From this perspective, the ideal particle morphology is spherical as the particles can tumble over each other and improve flowability. Although spherical morphology is preferred, many thermal spray powders are, however, angular and are still used in thermal spraying.

(a) (b)

Figure 4.1: SEM images showing the angular morphology of pure Nylon-11 (D60) feedstock powder: (a) at low magnification (200X); and (b) at high magnification (1,000X).

4.2. Reinforcements: Ceramic Powders

Fumed and precipitated silica powders (Degussa Corporation) with hydrophilic surface chemistry and with particle sizes ranging from 7 nm to 15 µm were selected as the reinforcements for the multi-scale coatings. Hydrophilic silica was shown previously to have a higher interfacial attraction to Nylon-11, making it better suited for use as reinforcement in this application67. Figure 4.2a contains an SEM image of the silica particles with a mean particle size of 15 μm. The silica particles had a rounded, almost 46 spheroidal, morphology with some agglomeration characteristic of the fuming and precipitation processes used to produce them.

Similarly, alumina (AGSCO Corporation) with particle sizes ranging from 300 nm to 15 μm was used as other reinforcing material. Alumina was explored because it is commercially available and because it was previously shown to be a suitable material for reinforcement of polymer composites. Alumina particles with a mean particle size of 5

μm are shown in Figure 4.2b. In comparison to the silica particles, which were spherical and rounded, the alumina particles had a plate-like morphology, which allowed comparison of the differences in their mechanical embedding during ball-milling and in thermal spraying of composites using particles of different morphologies. Additional examples of ceramic reinforcements are shown in Figure 4.3.

(a) (b)

Figure 4.2: SEM images of (a) as-received agglomerated silica (SiO2) powder from Degussa Corporation with mean particle size of 15 μm and (b) as-received alumina (Al2O3) powder from AGSCO Corporation with mean particle size of 5 μm.

47

(a) (b)

(c) (d)

(e) (f)

Figure 4.3: SEM images of as-received silica (SiO2) powder from Degussa Corporation, [(a, b) Sip 50S and (c, d) Sip 320] and as-received alumina (Al2O3) powder from AGSCO Corporation (e, f).

48

4.3. Effect of Ball-milling on Particle Size Distribution

The particle size distribution of pure Nylon-11 was another important factor affecting the powder feeding characteristics. The as-received Nylon-11 powder had a mean particle size of about 60 µm and a corresponding particle size range of 5 to 180 µm, as shown in Figure 4.4. Figure 4.5 shows the particle size distributions measured by

Horiba particle size analysis before and after ball-milling of feedstock powders as discussed in section 3.5.3. After the ball-milling process, the composite powders reinforced with 7 nm and multi-scale (MS 1, 2, 3, and 4, as shown in table 3.1) ceramic reinforcements exhibited similar particle size distributions, with a mean at approximately

60 µm. Although there was no change in the mean particle size (location of the peak centered ~60 μm) of the powders, there was reduction in the number or frequency of larger particles. The number of large Nylon-11 particles (50 to 100 μm) observed was less in the composite powders than in the pure Nylon-11 powders; for example there was a 30 % reduction in the number of 40-100 μm particles in MS-1 composite powder. The ceramic balls in the ball-milling process imparted sufficient mechanical energy to cause embedding of the silica/alumina into the Nylon-11 matrix. The repeated impact of ceramic balls on the polymeric particles during ball-milling may have caused the break up of the coarser particles into finer particles, thus reducing the number of coarse particles. In addition, there was ~40 % increase in the number of finer particles (10-20

μm), which may also have been due to the breaking of coarser particles during ball- milling; furthermore, finer particle sizes can also be caused by the presence of smaller ceramic reinforcements (7 nm to 15 μm). 49

Figure 4.4: Particle size distribution of polyamide Nylon-11 powder.

Figure 4.5: Feedstock particle size distributions of Nylon-11 (before ball milling) and ball-milled Nylon-11 + 10 Vol. % 7 nm and multi-scale (MS-1) silica reinforced composite powders. 50

4.4. Morphology of the Ball-milled Composite Powders

Nylon-11 powder was ball-milled with 10 Vol. % of the ceramic phases to produce multi-scale composite powders. The SEM image of the Nylon-11 particles mixed with silica particles prior to ball-milling using a Vee-blender, is shown in Figure 4.6a. In comparison to the Vee-blending, the morphology of the ball-milled Nylon-11/ceramic powders (Figure 4.6b) exhibited rounded shapes with no segregation of either powder.

During the ball-milling process, the reinforcing particles were embedded into the Nylon-

11 matrix and a composite powder formed. Additional examples of the morphology of ball-milled powders are shown in Figure 4.7.

(a) (b)

Figure 4.6: SEM micrographs of Nylon-11 (60 μm) + 10 Vol. % multi-scale (MS-1) silica: (a) Vee blended, and (b) after ball-milling.

51

(a) (b)

(c) (d)

Figure 4.7: SEM micrographs of Nylon-11 (60 μm) + 10 Vol. % multi-scale (MS-1, 2, 3) reinforcements after dry ball-milling.

The morphology of the ball-milled powders appeared mostly spherical in SEM analysis, with some flakes on the particle surfaces. Loosely bounded flakes surrounded the spherical particles, as shown in Figure 4.8a. These observations, indicating an “onion- skin” morphology of the particles, consisted of a nylon core with an outer silica-rich layer. There was some evidence of embrittlement, likely attributed to high localized SiO2 loading and/or excessive ball-milling time. Figure 4.8a shows a particle with a partially de-bonded flake from the surface of a single Nylon-11 particle, which was apparently from the outer silica-rich layer. The presence of the silica-rich regions in the flake was verified by EDS (section 4.4). 52

(a) (b)

Figure 4.8: SEM micrographs of a single particle of Nylon-11 + 10 Vol. % silica powder (MS-1) after ball-milling showing: (a) spherical or “onion-skin”, and (b) flaking of outer layer.

Figure 4.9a shows the cross-section of a single ball-milled composite particle and

Figure 4.9b shows a backscattered electron SEM image indicating the presence of two distinct phases within the composite powder. Nylon appears darker due to the presence of carbon, which has a lower atomic number than silicon, which in turn appears brighter.

The presence of a silica-rich outer layer in the ball-milled composite powders was confirmed using SEM-Energy Dispersive Spectroscopy (EDS), as shown in Figure 4.9c and 4.10. The presence of the reinforcing phase as a silica-rich shell on the surface of the

Nylon-11 particles after ball-milling can be seen in an EDS elemental dot map (Figure

4.9c). Figure 4.9d shows an EDS spectrum which shows the characteristic Si peak, confirming the presence of silica (Si). The thickness of the silica shell was 6-9 % of the core of the Nylon-11 particle (~60 μm) and the average silica shell thickness was measured to be around 4 µm. Ten measurements were made on each of the five different images using the analysis software of the ESEM, and an average value of 4 µm (± 1) was determined. 53

(a) (b)

(c) (d)

Figure 4.9: SEM images, EDS dot map and spectrum of the cross-section of a ball-milled Nylon-11/silica reinforced multi-scale (MS-1) composite particle indicating the presence of a silica rich outer layer/silica shell (a) a secondary electron image, (b) a backscattered electron image (BSE), (c) EDS dot map (Si) and (d) EDS spectrum.

54

(a) (b)

Figure 4.10: EDS dot maps of the cross-section of a ball-milled Nylon-11 silica reinforced multi-scale (a) MS-2 and (b) MS-3 composite particles indicating the presence of a silica rich outer layer/silica shell.

4.5. Ashing and Thermo Gravimetric Analysis of Feedstock Powders

To determine the actual amount of ceramic reinforcement present in the ball- milled powders, pure Nylon-11 and composite powders were heated in air in a furnace from room temperature (25 °C) to 700 °C at a heating rate of 10 °C/min. 5g batches of pure Nylon-11 and ball-milled composite powders were placed in a silica crucible and completely ashed. Nominal 10 Vol. % of ceramic reinforcement was added to the Nylon-

11 powder during the ball-milling. The degradation temperature of Nylon-11 is in the range of 360 - 550 °C, while the ceramic reinforcement (silica) has a melting point of

1710 °C. When the composite powders were heated, the polymer degraded to carbon and hydrocarbons, leaving a residue of silica. After the polymer was completely ashed, the amount of reinforcement present in the ball-milled powders was calculated by weighing.

Table 4.1 shows the actual amount of reinforcement (Vol. %) measured to be in the various composite feedstock powders after ball-milling.

55

Table 4.1. Reinforcement content in composite powders as calculated from ashing and TGA.

Material TGA Ashing Composition System (Vol. %) (Vol. %) Pure Nylon-11 Pure Nylon-11 >0.2 (residue) >0.2 (residue) 7 nm Nylon-11 + 10 Vol. % 7 nm silica 9.6 9.0 Nylon-11 + 10 Vol. % silica (7, MS-1 9.6 8.8 12, 20, 40 nm + 3, 10, 15 µm) Nylon-11 + 10 Vol. % silica (7, MS-2 9.6 9.2 12, 40 nm + 3, 15 µm) Nylon-11 + 10 Vol. % silica (7, MS-3 9.7 9.2 12, 20 40 nm + 10 µm) Nylon-11 + 10 Vol. % silica (7, MS-4 12, 20, 40 nm) + alumina (300 9.4 9.0 nm, 1µm, 5µm)

Figure 4.11 shows the loss in mass of the pure Nylon-11 and two composite powders during TGA. TGA experiments were carried out with a few milligrams (~5 mg) of sample over a temperature range of 150 to 900 °C. When the composite powders were subjected to TGA, the polymer degraded, leaving the silica and carbon as residue which was confirmed by EDS. The degradation begins at 300 °C and progresses over the range of 360 - 600 °C and is completed by about 650 °C. After the polymer was completely burned off, the amount of reinforcement present in the ball-milled powders was estimated from Figure 4.11 and reported in Table 4.1. The mass of the residue at 900 °C was regarded as the actual silica content. Results indicated that the loading of silica incorporated in the ball-milled Nylon-11 feedstock powders was close to 10 Vol. %, with some loss of silica likely during ball-milling and subsequent handling. 56

Figure 4.11: TGA thermograms showing curves for pure Nylon-11, Nylon-11 + 10 Vol. % of 7 nm and multi-scale (MS-1) composite feedstock.

The TGA results in Figure 4.11 indicated significant mass loss of the composite powders began at lower temperature than degradation of the pure Nylon-11. The decomposition temperature of the composite powders decreased as a result of the incorporation of silica. The shell of the silica particles surrounding the polyamide matrix accelerates the initial mass loss at a lower temperature (350-450 °C), however, at higher temperatures (700-800 °C) a stable system was formed. Previous thermogravimetric research work and experiments showed that metal oxides decrease the thermo-oxidative stability of Nylon83. The difference in the thermal transport behavior between the ceramic and the polymer may be a possible cause of the observed differences in the onset of decomposition of the composite powders, but the true mechanism is not completely understood. 57

4.6. Measurements of Melting Point: Differential Scanning Calorimetry (DSC)

Figure 4.12 shows the DSC thermograms of pure Nylon-11 powder, ball-milled

Nylon-11 + 7 nm silica and multi-scale (MS-1) powders. Nylon-11 melts over a range of temperatures of 180 °C to 190 °C. There was no significant difference in the melting points of the pure Nylon-11 and ball-milled powders.

Figure 4.12: DSC thermograms of pure Nylon-11, ball-milled Nylon-11 + 7 nm and multi-scale (MS-1) powders. 58

CHAPTER 5: RESULTS-SPRAYED COATINGS

The dispersion and distribution of ceramic reinforcements in the composite powders was essential to obtain the optimal distribution in the HVOF coatings. In addition, characterization of the thermal properties of the nylon and composite powders was necessary, as they influence the final properties of coatings. After the powders were characterized, the first step in the production of composite coatings was the spraying of pure Nylon-11 coatings. It was crucial to understand the behavior of the pure Nylon-11 during HVOF spraying in order to develop spray parameters for the multi-scale polymer- ceramic composite coatings.

5.1. Coating Production

5.1.1. HVOF Sprayed Pure Nylon-11 Coatings

Spraying of pure Nylon-11 coatings was an important first step in the development of multi-scale coatings, since the Nylon-11 was the matrix material for the multi-scale composites; in general, spraying of polymers has proven to be a challenging task, since polymer degradation during spraying can occur. Parameters and techniques for the HVOF spraying of pure Nylon-11 and composite coatings were previously developed

35, and little or no additional development was required. A series of ten spray trials and process parameter variations was carried out to fine-tune the parameters. The corresponding horizontal gun speed (dx/dt) was ~ 0.1 m/s, with a low powder feed rate (2 rpm or 6 g/min). The Fuel (H2):Oxygen ratio used to deposit the pure Nylon-11 material was 0.5 with Hydrogen/Oxygen flow rates of 2.8 x 10-3 and 5.6 x 10-3 m3/s. Using this technique, a 500-700 µm thick deposit of pure Nylon-11 was successfully sprayed onto a 59 grit-blasted steel substrate. Spraying of the pure Nylon-11 coatings indicated that substrate temperature was a key parameter affecting both deposition and coating build- up. Deposition and build-up of the Nylon-11 coating were only successful when the substrate was preheated to temperatures in the range of 150-170 °C; otherwise, little or no build-up was obtained. The HVOF jet was used to preheat the substrate, and the temperature was measured by using a hand-held type K thermocouple probe to make sure that the temperature was maintained in the range of 150-170 °C. Figure 5.1 shows a typical microstructure of a pure Nylon-11 coating.

Coating

Substrate

Figure 5.1: Optical micrograph of a cross-section of an HVOF sprayed pure Nylon-11 coating.

5.1.2. Composite Coating Formation

As discussed in section 3.3, the ball-milled particles had a core-shell morphology.

These powders were fed into the HVOF system using a volumetric feeder, where they were heated, partially melted, and propelled with a high velocity, toward a flat grit- blasted, roughened steel substrate, as shown in Figure 5.2a. When deposited onto the substrate, these splats or particles formed a lamellar structure (Figure 5.2b). The structure 60 of the coating consisted of a network of overlapping layers of deformed composite particles, with the silica-rich outer layer separated by regions of pure polymer, as shown in Figure 5.2c.

Figure 5.2: Schematic of coating build up (a) in-flight composite particles consisting of ceramic particle shells embedded into the surface of polymer particles (b) formation of thermally sprayed coatings from overlapping composite particles (c) a thermally sprayed multi-scale polymer/ceramic coating microstructure.

61

After deposition, the composite particles may bond to one another to form an interconnected network. The resulting coating microstructure depended strongly on the degree of splatting and melting of the polymer phase at the time of impact. Poorly melted particles can trap porosity or lead to large, unmelted, polymer-rich zones. These particles do not flow well on impact and can create voids in their shadow which are not filled by the next arriving particle, thus forming porosity in the coating.

A typical microstructure of an HVOF sprayed multi-scale coating is shown in

Figure 5.3a. The individual spray passes were visible with overlapping spray passes, as shown in 5.3b. The backscattered electron images showed that the coatings exhibited an overlapped network of reinforcing phase within the polymer matrix. Distinct stratification was visible between passes and where the HVOF spray steps overlapped. The layer by layer and particle by particle deposition resulted in a network of ceramic reinforcement within the polymer matrix. Good substrate-to-coating conformation was observed, indicating good mechanical adhesion of the coating to the substrate. In general, the coatings were found to be dense and adherent, with some spherical voids/pores ~20 µm in diameter. While it has been hypothesized that this porosity may have been produced by water vapor or other gas evolution, the true cause of the porosity is still unknown. The porosity of the coating was measured by optical image analysis using a background subtraction by 2-D rolling ball averaging, was found to be 1.1%.

62

(a) (b)

Figure 5.3 SEM images of (a) the microstructure of a HVOF deposited coating and (b) showing the number of passes (6) which appears consistent with the schematic shown in the section 5.1.2.

5.1.3. Nano and Multi-scale Reinforced Coatings

A series of ten spray trials and process parameter development runs was carried out in order to deposit reproducible nano and multi-scale reinforced composite coatings.

Additionally, several trials were done to prevent the degradation of the polymer within the composite. The SEM images of the microstructures of the coatings sprayed onto grit blasted steel substrates are shown in Figure 5.4a and b, where the spray patterns and layers can be clearly seen. The adherent and thick (up to 800 µm) coatings were sprayed with good layer-to-layer coherence and coating/substrate interfaces. The inset in Figure

5.4a and b are high magnification images indicating the distribution of the reinforcing phase in the polymer matrix. The confirmation of the elemental composition and distribution in these images will be discussed in section 5.2.

63

(b) (a)

Figure 5.4: BSE-SEM images showing the microstructure of HVOF sprayed (a) nano- scale (7 nm) and (b) multi-scale (MS-1) composite coatings on steel substrates.

5.2. Coating Microstructure and Elemental Analysis

HVOF sprayed nano and multi-scale reinforced coatings exhibited good conformation to the substrate surface, as indicated by the presence of distinct interfaces.

This may also indicate good substrate adhesion and inter-layer cohesion. The dispersion and distribution of the reinforcement within the polymer matrix was characterized by

SEM and elemental analysis was carried out by SEM-EDS.

5.2.1. Nano-scale (7 nm silica) Reinforced Coatings

The presence of the silica within the coatings was confirmed by the EDS dot maps

(Figure 5.5b, c and d) and the EDS spectrum (Figure 5.5e). Figure 5.5a shows the cross- section of a 7 nm silica reinforced Nylon-11 coating at a high magnification. After spraying the silica phase embedded in the surface of the Nylon-11 particles during ball milling remained agglomerated at the splat boundaries, which were sharply visible as the white regions in the Figure 5.5a. EDS elemental dot maps and the spectrum confirm the elemental composition and distribution of silica within the nanoscale reinforced coatings. 64

BSE

(a) (b)

(c) (d)

(e)

Figure 5.5: EDS spectrum (e) indicating the presence of different elements and SEM- BSE (a) and SEM-EDS dot maps (b) silicon, (c) carbon and (d) oxygen, confirming the presence of silica in or around the splat boundaries of 7nm silica reinforced Nylon-11 composite coating. 65

5.2.2. Multi-scale Coatings

Figure 5.6a shows an SEM image of an HVOF sprayed multi-scale (MS-1) coating indicating the presence of two distinct phases. The image shows that the ceramic phase was distributed at or around the splat boundaries (white regions) and was well dispersed throughout the polymer matrix in a series of interconnected lamellar sheets.

The splat boundaries in coatings with the multi-scale reinforcements were very clearly defined as a result of the ceramic-rich shells around the Nylon-11 cores of the feedstock powder, whereas in the nanoscale coatings the splat boundaries were much narrower due the smaller size (7 nm) of the silica particles. Evidence of the presence of one of the size scales is shown in Figure 5.7. Figure 5.7 shows a 3 μm particle of silica in a multi-scale coating, as confirmed by EDS elemental dot maps. The powders reinforced with multiple-scales of ceramic particles showed high levels of incorporation (~10 Vol. %); consequently, the splat boundaries in these coatings appeared broader, with gaps where two Nylon-11 particles come together. Figure 5.8 shows EDS dot maps and an EDS spectrum of the multi-scale coating, confirming the presence of silica in the coatings.

Figure 5.8b shows an EDS dot map of Si where the blue color confirmed that the bright regions in Figure 5.8a were the silica-rich regions. Figure 5.8a, b and c correspond to other elements present in the SiO2 and Nylon-11 matrix, such as red for Carbon (C) and yellow for Oxygen (O2). Colors were set arbitrarily by the EDAX software to distinguish between elements. 66

(a) (b)

Figure 5.6: SEM-BSE (a) and SEM-EDS (b) image of a cross-section of an HVOF sprayed multi-scale (MS-3) reinforced coating showing the two phases present. The inset is a high magnification image that corresponds to Figure 5.8a for EDS analysis.

(a) (b)

(c) (d) Figure 5.7: SEM-BSE image and EDS elemental dot maps of the cross-section of an HVOF sprayed multi-scale reinforced coating (MS-3) confirming the presence of a micron-scale silica particle of 3 μm in diameter. (a) SEM-BSE image, (b) Silicon and (c) Carbon EDS dot maps and (d) EDS spectrum with distinct elemental peaks. 67

(a) (b)

C O

(c) (d)

(e) Figure 5.8: Chemical microanalysis of an HVOF sprayed multi-scale reinforced coating (MS-3) confirming the distribution of the ceramic phase within the polymer matrix by EDS dot maps (a) SEM-BSE shows the elemental contrast between nylon and silica, (b) silicon (blue), (c) carbon (red), and (d) oxygen (yellow) EDS dot maps confirming the spatial distribution of elements and (e) EDS spectrum with distinct elemental peaks. 68

The multi-scale (MS-4) coatings with Nylon-11 as the matrix material and silica/alumina as the ceramic reinforcements showed similar microstructural features.

MS-4 contained silica with nanoscale particles (7, 12, 20, 40 nm) and alumina with micron-scale particles (0.3, 1, 5 μm). Figure 5.9 shows the EDS dot maps for the constituent elements of this coating system. The presence of the alumina and silica ceramic phases was confirmed by the EDS spectrum, where peaks of Al and Si were detected. The dot maps indicated that the silica and alumina were present in and around the splat boundaries, as in the nanoscale and multi-scale (MS-3) coatings.

(a) (b) (c)

(d) (e) (f)

Figure 5.9: EDS spectrum and dot maps of silica and alumina reinforced Nylon-11 multi- scale composite coating (MS-4): (a) SEM-BSE shows the elemental contrast between nylon and silica/alumina, EDS dot maps (b) silicon (blue), (c) aluminum (purple), (d) carbon (red), (e) oxygen (yellow) and EDS spectrum (f) with distinct elemental peaks.

69

5.3. Ashing and Thermo Gravimetric Analysis of Composite Coatings

Table 5.1 shows the amount of ceramic reinforcement present in the nano and multi-scale sprayed coatings. Coatings were peeled off from the substrate and 5 grams of each coating was placed in a silica crucible. The coatings were heated in air in a furnace from room temperature (25 °C) to 700 °C to completely ash the sample to estimate the actual amount of ceramic reinforcement present in the sprayed coatings. The amount of reinforcement present in the coatings was found to be ~5 Vol. %, indicating significant loss of ceramic reinforcement during spraying.

Table 5.1. Reinforcement content in composite coatings as determined from ashing and TGA.

Material Composition TGA Ashing System (Vol. %) (Vol. %) Pure Nylon-11 Pure Nylon-11 >0.1 >0.1 (residue) (residue) 7 nm Nylon-11 + 10 Vol. % 7 nm silica 5 5.0 MS-1 Nylon-11 + 10 Vol. % silica (7, 12, 20, 5 4.8 40 nm + 3, 10, 15 µm) MS-2 Nylon-11 + 10 Vol. % silica (7, 12, 40 5 4.6 nm + 3, 15 µm) MS-3 Nylon-11 + 10 Vol. % silica (7, 12, 20 5.1 5.0 40 nm + 10 µm) MS-4 Nylon-11 + 10 Vol. % silica (7, 12, 20, 4.8 4.6 40 nm) + alumina (300 nm, 1µm, 5µm)

The TGA analysis (Figure 5.10a) results from the composite coatings also showed the amount of silica and/or alumina residue at 900 °C. Figure 5.10b shows the weight percentage of polymer on a silica-free basis, which is useful in comparing degradation of just the Nylon fraction of the composite. 70

Similar to ashing, the TGA results of coatings also showed that the actual amount of silica present in the multi-scale coatings was around 5 Vol. %, when nominal 10 Vol.

% was ball-milled with the Nylon-11 as reported in Table 5.2. The composite coatings had ~50 % lower filler contents than the starting powders in all cases. The loss of silica can be attributed to the fact that the composite particles are carried by the high velocity moving jet and are impacted at the substrate by a high kinetic energy. The shear forces within the HVOF jet, together with the high variations in the temperature and velocity may have caused some silica rich fragments (flakes) to debond from the spherical particles in flight (morphologies of the particles are shown in section 4.4).

Figure 5.10: TGA thermograms showing the amount of ceramic loading remaining in the polymer ceramic composite coatings after HVOF spraying.

A comparison of the results of ashing and TGA showed similar outcomes, with reinforcement contents in the composite coatings close to nominal 5 Vol. % of ceramic loading, as shown in Table 5.2. However, ashing showed that there was less ceramic loading in the powders possibly due to amount of sample used in ashing (5 g) in 71 comparison to TGA (mg). Also, TGA is a more controlled process with well defined heating rates and automated temperature control.

Table 5.2. Comparison of reinforcement content in composite powders and sprayed coatings, as determined by TGA.

Material Reinforcement Content Reinforcement Content System (Powder Vol. %) (Coating Vol. %) Pure Nylon-11 >0.2 (residue) >0.1 (residue) 7 nm 9.6 5 MS-1 9.6 5 MS-2 9.6 5 MS-3 9.7 5.1 MS-4 9.4 4.8

5.4. Differential Scanning Calorimetry (DSC)

Figure 5.11 shows the DSC thermograms of the pure HVOF sprayed Nylon-11,

Nylon-11 + 7 nm and multi-scale reinforced coatings. The melting temperature of all the coatings was 180-190 °C which falls in the melting point range of the Nylon-11 powder.

There was no significant change in the melting point of the coatings in comparison to the starting powders used to deposit these coatings.

72

Figure 5.11: DSC thermograms showing the melting points of the HVOF sprayed pure Nylon-11 and composite coatings.

5.5. X-ray Microtomography of Coatings

X-ray images were collected to further confirm the distribution of silica within the

Nylon-11 matrix of the multi-scale coatings. The red regions in Figure 5.12 indicated the presence of silica-rich regions and the grey regions represented the nylon-rich regions.

The images gave a qualitative indication of the amount of silica present in the multi-scale coatings, as the x-ray cross-section of silica is much higher than that of Nylon-11. The porosity in the multi-scale coatings was estimated from the images collected. Figure 5.13 shows the distribution of the pores in the multi-scale coatings. The estimated porosity from image analysis was around 1.1% ± 0.1. 73

Figure 5.12: X-ray Microtomography images showing the distribution of silica-rich (red) regions and Nylon-11 rich (grey) regions in the multi-scale (MS-1) coating. Top left image is a BSE-SEM image of MS-1 coating.

Figure 5.13: X-ray Microtomography images showing the distribution of porosity-rich regions in the multi-scale (MS-1) coating.

5.6. X-ray Diffraction of Coatings

Figure 5.14 shows X-ray diffraction patterns for selected HVOF sprayed coatings.

The molecular structure of Nylon-11 comprises two stable crystalline forms, the triclinic-

α-form and the monoclinic-γ-form80. The most stable form is the triclinic α-form which exhibits characteristic twin XRD peaks at 2θ values of 21° and 23.4° that corresponding to the (100) and (010, 110) reflections, respectively, and a short broad peak around 15°. 74

The diffraction pattern for the 7 nm silica reinforced Nylon-11 coating exhibited a primarily α character, however, the peaks were not as distinct and the (010, 110) reflection was shifted to a slightly higher angle, indicating increasing perfection in the crystal structure. The multi-scale reinforced coatings exhibited less shifting of the (010,

110) peaks, and reduction in the intensity of the (100) reflection, as shown in Figure 5.14.

These changes indicated a loss of α-character and increase of the δ and δ’ pseudo- hexagonal phases (δ and δ’ Nylon-11 are meta-stable crystal structures). The onset of the

δ phase was an indication of an increase in the spacing between the hydrogen bonded sheets of the Nylon-11 chains (Chen et al., 1991). In addition, the shift in peaks could also be attributed to the tilt in the position of the sample and sample holder or due to the residual stress present in the coating.

Figure 5.14: X-ray diffraction patterns of pure Nylon-11, 7 nm and multi-scale silica reinforced coatings.

75

5.7. Mechanical Properties: Scratch Resistance

Pure Nylon-11 and multi-scale coatings were scratch tested using loads of 0.5 to 2 kg. Figure 5.15a and b shows a comparison in the scratch depth of the tested coatings with a load of 0.5 kg. Ten scratch depth measurements were carried out on each scratch on three similar coating samples, and the mean reported in each case. The multi-scale coating showed an improvement in scratch resistance, which was evident by a decrease in the depth of the scratches in comparison to the pure Nylon-11 coating. Figure 5.16 shows the comparison of the scratch depth of the nano-scale and micron-scale coatings with respect to the multi-scale and pure Nylon-11 coatings.

(a) (b)

Figure 5.15: Optical micrographs of HVOF sprayed coatings after scratch testing at a load of 0.5 kg: (a) scratch profile of pure Nylon-11 coating and (b) multi-scale (MS-1) polymer ceramic composite coating.

76

Figure 5.16 Scratch test performance of pure Nylon-11 and composite coatings comparing the scratch depths of nano-scale, micron-scale and multi-scale coatings as a function of applied loads of 1, 1.5 and 2 kg.

All the samples from Table 3.1 were scratch tested and the results were compared to pure Nylon-11 and coatings reinforced with nanoscale particles in order to estimate the performance of the multi-scale concept. Figure 5.17a shows scratch depth as a function of loads ranging from 0.5 to 2 kg.

While nanoscale reinforcements have been shown to increase the scratch resistance of the composites, multi-scale reinforcements provided further improvement.

There was a 40 to 50 % (0.5, 1 kg) and ~28 % (1.5, 2 kg) improvement in scratch resistance for silica reinforced multi-scale composite coatings. Figure 5.17b gives the 77 percentage improvements in scratch resistance with respect to pure Nylon-11 coatings.

The improvement likely resulted from mechanical reinforcement associated with dispersion of the load-bearing silica and their distribution within the Nylon-11 matrix. In particular, two factors may play a role in this case. First, micron scale particles inhibit agglomeration of nanoparticles as indicated in the EDS dot maps. Secondly, they may act as a bridge between the nanoscale reinforcements and polymer matrix leading to distribution of load within the coating system.

78

(a)

(b)

Figure 5.17 Scratch test performance of pure Nylon-11 and composite coatings: (a) Coating scratch depth vs. load for pure Nylon-11, 7 nm and multi-scale coatings and (b) percentage reduction in scratch depth relative to a pure Nylon-11 coating. 79

CHAPTER 6: SPLATTING OF NYLON-11 PARTICLES

The goal of this segment of the project was to develop a model of HVOF sprayed polymer particles impacting onto an arbitrary rough substrate. A particle splatting model on non-smooth surfaces would provide a better understanding of how the geometrical irregularities of a surface affect the splatting behavior and final splat morphology. The mathematical models used to generate the predictions presented in this chapter were developed by Milan Ivosevic and the experimental (SEM) results were produced by

Varun Gupta. The results of modeling are presented in Milan Ivosevics’s PhD thesis84 and several publications53, 85, 86. This chapter presents comparisons between experimental observations from this research and numerical predictions from Milan Ivosevic's research.

6.1. Splat Tests

Substrate preheating has been shown to enhance the deposition behavior and coating formation during thermal spraying of polymers and polymer composites14, 62, 78.

Nylon-11 splats sprayed onto glass slides with, and without, substrate preheating, are shown in Figures 6.1a and b, respectively. Nylon-11 sprayed onto a preheated substrate

(150 °C) resulted in well-melted and deformed splats (Figure 6.1a) while only partially melted splats were observed on a cool glass slide (28 °C), as shown in Figures 6.1b. Splat tests indicated that external substrate preheating could enhance the deposition behavior during the HVOF spraying of Nylon-11 materials.

80

(a) (b)

Figure 6.1: Optical images of Nylon-11 splats on a glass slide (a) preheated to 150 °C and (b) at room temperature (28 °C).

6.2. Splatting of Nylon-11 on Smooth Surfaces

Particle impact velocity, temperature profile neglecting polymer degradation and temperature and shear rate dependent viscosity were all combined together in a three- dimensional deformation model, the results of which are summarized in Figure 6.2. The predicted splat shapes shown in Figure 6.2 exhibited a good qualitative agreement with experimentally observed splat shapes (Figure 6.1b). The larger, 90 and 120 µm diameter particles were spread into “fried egg” splats with large nearly-hemispherical cores in the centers of thin disks, whereas 30 and 60 µm diameter particles generated flattened splats with small elevations in the center. This was consistent with the predicted internal temperature profiles, since both the 90 and 120 µm diameter particles had unmelted cores. In addition, a larger unmelted portion of the 120 µm diameter particle transformed into a larger central “dome” than for a splat arising from a 90 µm diameter particle. Both

30 and 60 µm diameter particles generated rims with breakup and satellites radiating 81 from the splat edge. This was expected since the smaller particles had higher temperatures and lower viscosities at their surfaces.

Figure 6.2: Cross-sections of predicted three-dimensional spreading splats for 30, 60, 90 and 120 µm diameter particles. Colors indicate the internal temperature distribution84.

The velocity field inside a 90 µm diameter particle during spreading on impact with a flat substrate is shown in Figure 6.3. The velocity field vectors shown in the right- hand side of the droplet indicates that the characteristic “fried-egg” splat shape was formed as a low viscosity “skin” flowed around a high viscosity core. The velocity magnitude shown in the left-hand side of the droplet (Figure 6.3) indicated that after impact the droplet developed a leading rim with relative velocity almost double the original impact velocity. The high velocity in the rim was the result of the squeezing flow between the fairly rigid particle core and the rigid substrate. A combination of this high velocity and the no-slip boundary condition at the substrate surface generates shear rates 82 almost three orders of magnitude higher (~109 s-1) than the characteristic shear rates (~106 s-1) at impact. Very high shear rates lead to low viscosity (shear thinning) resulting in the very thin rim (2 – 5 µm) of the splat. The low viscosity in the rim also contributes toward the formation of break-ups and satellites, as predicted in Figure 6.2 for 30 and 60 µm diameter particles.

Figure 6.3: Velocity field inside a spreading 90 µm diameter particle; left-hand side: velocity magnitude, right-hand side: velocity vectors86.

6.3. Splatting of Nylon-11 on Non-smooth Surfaces

The primary bonding mechanism of thermally sprayed coatings is mechanical interlocking between the coating and the mechanically or chemically roughened substrate. Before spraying, substrate surfaces are typically roughened by grit blasting using 100 – 1,500 µm ceramic particles (e.g. Al2O3, SiC) as shown in Figure 6.4. The morphology of the initial splats deposited onto a substrate surface play an important role in the integrity of the coating/substrate interface and the coating’s adhesive strength. A particle splatting model on non-smooth surfaces would help to better understand how the 83 geometrical irregularities of a surface affect the final splat morphology. This includes prototype cases of particle splatting onto surfaces exhibiting different size scale and morphology steps and asperities. Also, a particle splatting model using an arbitrary 3D surface was imported into Flow-3D® from an optical interferometry scan of an actual grit blasted steel substrate surface (Figure 6.4), as typically prepared for the deposition of thermally sprayed polymer coatings.

Figure 6.4: Cross-section of four steel substrates: (a) polished with ~1 µm alumina suspension, (b) grit blasted with #120 grit, (c) grit blasted with #50 grit, (d) grit blasted with #12 grit. Right image shows optical interferometric scan of # 120 grit blasted 86 surface using Al2O3 grit at an angle of 45° with an air pressure 0.55 MPa . 84

6.4. Modeling Predictions

Predicted three-dimensional cross-sections of spreading splats on four different substrate surfaces are shown in Figure 6.3. The predictions exhibited good qualitative agreement with experimentally observed splats shown in Figure 6.4. Larger splat spreading ratios (Dfinal/Dinitial) occurred on smoother surfaces, i.e. larger diameter splats were predicted on an ideally flat surface, as shown in Figure 6.4. Moreover, splat jetting/fingering was promoted as substrate surface roughness increased, with the most prominent jetting occurring on the roughest surface, as indicated by the yellow and red splat fingers in Figure 6.3.

Cross-sections of the predicted three-dimensional 90 µm diameter splat revealed a fully conformed interface between the center of the splat and the underlying substrate topography, while high speed radial jets spread over the substrate asperities. This was believed to be due to the high stagnation pressure below and around the center of the splat, including a flow mainly directed normal to the surface, promoting filling of the surface cavities. On the other hand, after impact the droplet developed a leading edge with a relative velocity higher than the original impact velocity. The high velocity within the rim was the result of the squeezing flow between the relatively viscous particle core and the rigid substrate. This high shear rate flow led to low viscosity due to shear thinning, resulting in the very thin (2-5 µm) splat rim. The low viscosity-high shear rate in the rim and interaction with the substrate roughness contributed to the formation of jets and fingers. These predictions were in agreement with the experimentally observed radial finger morphology shown in Figure 6.4 and 6.5. 85

Figure 6.5: Cross-sections of predicted three-dimensional spreading splats for a 90 µm diameter Nylon-11 particle on four different surface roughnesses (Ra) 86.

6.5. Experimental Results

A range of substrate roughnesses was generated using several grit sizes in a

Trinco 24/BP2 grit blasting system. Polished steel and as-received glass slides represented the smoothest surfaces, while decreasing grit mesh size increased the roughness of the surface. A surface grit blasted with 50 mesh grit is denoted as a “GB50” surface. Grits with mesh sizes of 120, 50, and 12 were used. Grit blasting was carried out at an angle of approximately 45° to the substrate surface, and the entire substrate was passed over at least five times with the grit stream, in order to achieve a nearly uniformly rough surface and complete blasting coverage.

All splat tests were conducted using a Stellite Coatings, Inc. Jet-Kote II® HVOF spray system with Nylon-11 powders having a mean particle size of 60 μm. A single pass of the spray gun was used in the tests in order to capture isolated splats on the substrate. 86

Images of individual splats were obtained using an FEI/Phillips XL30 Field Emission

Environmental Scanning Electron Microscope (FE-ESEM), in variable pressure mode to limit the charging that often occurs in high vacuum mode. Cross-sectional images of splats and substrates were obtained by mounting the samples in an epoxy resin and hardener, and grinding and polishing the resulting sample to a 1 µm finish. An Olympus

PMG-3 optical metallograph was then used to inspect and image the cross-sections of interest. The platinum sputter coating applied to the Nylon-11 splats sprayed onto steel substrates provided optical contrast between the splats and the epoxy. Sputter coating of platinum was performed for a total of 50 minutes, at a current of 40 mA and vacuum level of ~100 Torr.

The morphologies of Nylon-11 splats deposited over one polished and three grit blasted steel substrates during a single spray run are shown in Figure 6.6. Most of the larger splats (> ~100 μm) observed on all four substrates exhibited a characteristic ‘fried egg’ shape with a large, nearly hemispherical, core in the center of a thin disk.

This observation was consistent with the previously reported splat shapes deposited onto a flat glass slide substrate53, however, a preliminary observation, after

SEM analysis of the multiple splat regions, indicated that the final splat diameter decreased as the substrate roughness increased. Statistical analysis and quantification of this observation is in progress. Observations also indicated that an increase in general substrate roughness promoted splat instability, resulting in radial jetting and break-up, and producing more irregularly shaped splat shapes on rougher surfaces. Furthermore, it was observed that fingers on the periphery of the ‘fried egg’ shaped splats solidified and settled on top of rough substrate asperities. This was believed to be due to the high speed 87 squeezing flow of the leading edge of the splat leading rim over the top of the substrate asperities.

(a) (b)

(c) (d)

Figure 6.6: Nylon-11 splats deposited during a single pass over steel substrates: (a) polished with ~1 μm alumina suspension, (b) grit blasted with #120 grit, (c) grit blasted with #50 grit, (d) grit blasted with #12 grit86.

Figure 6.7 shows a single Nylon-11 splat on a GB120 glass substrate. One fingerlike nylon segment has been magnified in order to examine the substrate-splat interface. Although the segment appears to have not filled the valley features of the substrate surface, this top-down SEM image was insufficient evidence to make specific conclusions about the extent of substrate-splat mechanical interlocking. 88

Figure 6.7: Nylon -11 splat on GB120 glass substrate86.

In order to better understand substrate-splat interactions such as the extent of valley feature conformation, splat cross-sections were imaged. Figure 6.8 shows images of splat cross-sections. The small white dots forming an arch shape above the white steel substrate are particles of the platinum sputter coating, applied to provide optical contrast between the splats and the mounting epoxy. While the splat on the left-hand side in

Figure 6.8a appears long and flat, the right-hand splat in Figure 6.8b looks thicker and more rounded. This could be because the former is a cross-section of the “white” part of a large fried-egg, while the latter is a cross-section taken much closer to the center of a smaller, dome-shaped splat.

89

(a) (b)

Figure 6.8: Cross-sections of Nylon-11 splats on polished steel substrates.

Since splat cross-section imaging can yield useful information about both the substrate-splat and splat-splat interactions, improved splat cross-section imaging methods would be helpful. It has been found that by illuminating the cross section at an angle of approximately 45° to the sample, improved contrast was achieved between the substrate, epoxy and splat. This new approach would allow the splats to be viewed without sputter coating. In one instance, the epoxy appeared white, the substrate appeared dark red, and the splats appeared black. These techniques are being developed further and will be used to gather information on the mechanical interlocking between splats, the substrate, and other splats. 90

CHAPTER 7: SUMMARY AND CONCLUSIONS

High Velocity Oxy-Fuel (HVOF) combustion spraying of dry ball-milled Nylon-

11/ceramic composite powders is an effective, economical and environmentally sound method for producing semi-crystalline micron and nano-scale reinforced polymer and polymer composite coatings. HVOF spraying is a solvent-less, low-VOC technique for producing polymer and composite protective coatings. Composite coatings reinforced with nominal 10 Vol. % of multiple scales of ceramic particulate material exhibited improved load transfer between the reinforcing phase and the matrix due to interactions between large and small ceramic particles.

Composite feedstock powders were produced by dry ball-milling Nylon-11 together with 10 vol. % overall ceramic phase loadings. Dry ball-milling polymer and ceramic particles resulted in a core-shell powder morphology with ceramic-particles embedded into a shell surrounding polymer-rich cores. The morphology of the composite powders and elemental phases present were characterized by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results indicated a shell thickness of ~4 µm. Effect of ball-milling on particle size distributions was studied by HORIBA particle size analysis which indicated a ~30% reduction in the number of coarser particles and a ~40% increase in the number of finer particles. Ashing and thermo gravimetric analysis (TGA) was used to confirm the ~10 Vol. % loading of ceramic reinforcement in the composite feedstock powders as summarized in Table 7.1.

The effectiveness of the ball-milling process as a function of reinforcement size was qualitatively evaluated by SEM+EDS microanalysis and by characterizing the behavior of the powders during HVOF spraying. The microstructures of the sprayed 91 coatings were characterized by optical microscopy, SEM, EDS and XRD. The microstructure of the HVOF sprayed composite coatings was a cellular lamellar structure with ceramic reinforcements agglomerated at splat boundaries. The dispersion and distribution of the ceramic reinforcements within the composite coatings were co-related to the HVOF process parameter variations. The reinforcement particles were found to be concentrated at the splat boundaries in the coatings, forming a series of interconnected lamellar sheets with good distribution. EDS analysis confirmed the concentration of the ceramic reinforcements at the splat boundaries in sprayed coatings. The amount of ceramic reinforcement incorporated within the sprayed coatings was studied by ashing and TGA, which indicated a ~50% loss of reinforcement during spraying as listed in

Table 7.1.

The mechanical strength of the HVOF sprayed coatings were characterized for by scratch resistance (Table 7.1). Multi-scale composite coatings exhibited improved scratch resistance over HVOF sprayed pure Nylon-11 and nano-scale reinforced composite coatings. Multi-scale ceramic reinforcements reduced scratch depths by as much as 40 to

50% relative to pure polymer coatings, and by up to 20-30% compared to single-scale reinforcements. 92

Table 7.1: Summary of the key results of ashing, TGA and scratch resistance for the composite powders and sprayed coatings.

93

SUGGESTIONS FOR FUTURE WORK

To be able to understand and make use of the advantages of multi-scale coatings, further investigations should be carried out. The first step would be to do more experimental work with different polymer materials and different reinforcements to further quantify the effect. The underlying mechanisms for multi-scale effects in various polymers have to be examined. Another key aspect to study would be the effect of polymer degradation on the performance of the thermally sprayed coatings. Degradation during and after the thermal spray deposition, should be quantified with the help of Differential Scanning Calorimetry

(DSC) and Spectroscopic techniques (FTIR and Raman).

Mechanical and mathematical models that describe the phenomenon, preferably mechanism based, should also be developed. More experimental work would be necessary to calibrate parameters of the models. Development and use of different and complementary characterization methods to quantify the multi-scale effects would be valuable in the formulation of general models. The models could be implemented with finite element method (FEM) to predict the behavior in arbitrary geometries and for different volume fractions of loading. The mechanical properties of the sprayed coatings should be evaluated by indentation techniques such as nanoindentation. To complement the scratch resistance, pin on disc sliding wear performance of coatings should be evaluated and correlated to dispersion and distribution of the ceramic reinforcements present within the polymer matrix.

These findings promote understanding that could be used in materials development. However, further insight into the damage mechanisms and their size-scale dependence is necessary to fully understand the multi-scale phenomenon. 94

APPENDIX A

Powder Feed Rate Calibration

The powder feed rates of the Nylon-11 and composite feedstock powders were measured three times as a function of different feeder rpm and the results are shown in

Figures 7.1, 7.2 and 7.3. The details of the set up and volumetric powder feeder used are described in section 3.3. The powder feed rates were nominally a linear function of feeder rpm. The composite powder feed rate was approximately 1.25 times higher for a given rpm due to the higher density of composite powder (2.2 vs. 1.04 g/cm3).

Figure 7.1: Powder feed rate calibration for pure Nylon-11 (60 μm) feedstock powder. Three measurements were made on each powder rpm (1-5) and the mean reported in each case. 95

Nylon-11 + 10 vol. 7 nm silica

Figure 7.2: Powder feed rate calibration for the Nylon-11 + 10 Vol. % 7 nm silica feedstock composite powder, three measurements were made on each powder rpm (1-5) and the mean reported in each case.

(MS-1)

Figure 7.3: Powder feed rate calibration for the Nylon-11 + 10 Vol. % multi-scale (MS-1) silica feedstock powders. Three measurements were made on each powder rpm (1-5) and the mean reported in each case. 96

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