UNIVERSITY OF CINCINNATI Date:__7/30/07_________________ I, __ MUNISH GUPTA_____________________________________, hereby submit this work as part of the requirements for the degree of: DOCTORATE OF PHILOSOPHY (Ph.D) in: MATERIALS SCIENCE AND ENGINEERING It is entitled: LOW-PRESSURE AND ATMOSPHERIC PRESSURE PLASMA POLYMERIZED SILICA-LIKE FILMS AS PRIMERS FOR ADHESIVE BONDING OF ALUMINUM This work and its defense approved by: Chair: __Dr. F. JAMES BOERIO ___ ______ __Dr. GREGORY BEAUCAGE __ ___ __ __Dr. RODNEY ROSEMAN _____ ___ __Dr. JUDE IROH _ _____________ _______________________________ LOW-PRESSURE AND ATMOSPHERIC PRESSURE PLASMA POLYMERIZED SILICA-LIKE FILMS AS PRIMERS FOR ADHESIVE BONDING OF ALUMINUM A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTORATE OF PHILOSOPHY (Ph.D) in the Department of Chemical and Material Engineering of the College of Engineering 2007 by Munish Gupta M.S., University of Cincinnati, 2005 B.E., Punjab Technical University, India, 2000 Committee Chair: Dr. F. James Boerio i ABSTRACT Plasma processes, including plasma etching and plasma polymerization, were investigated for the pretreatment of aluminum prior to structural adhesive bonding. Since native oxides of aluminum are unstable in the presence of moisture at elevated temperature, surface engineering processes must usually be applied to aluminum prior to adhesive bonding to produce oxides that are stable. Plasma processes are attractive for surface engineering since they take place in the gas phase and do not produce effluents that are difficult to dispose off. Reactive species that are generated in plasmas have relatively short lifetimes and form inert products. The objective of this work was to develop plasma etching and plasma polymerization as environmentally compatible processes for surface engineering of aluminum. Plasma polymerized silica-like films of thickness less than 200 nm were deposited on pretreated aluminum substrates using hexamethyldisiloxane (HMDSO) as the "monomer" and oxygen as a "co-reactant" in low-pressure RF-powered (13.6 MHz) reactor. Recently, plasma deposition at atmospheric pressure has become a promising technology because they do not require vacuum systems, can be applied to large objects with complex shapes, and adapted easily for continuous processing. Therefore, atmospheric pressure plasma processes were investigated and compared with their more traditional counterparts, low-pressure plasmas. Molecular structure and morphology of the plasma polymerized films were determined using surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). ii The effectiveness of plasma etching and plasma polymerization as surface engineering processes for aluminum were probed by determining the initial strength and durability of aluminum/epoxy lap joints prepared from substrates that were plasma pretreated, coated with silica-like film, and then bonded together using a 1-part epoxy adhesive. The durability of joints was evaluated using a “stressed durability test” which involved applying a static load to joints, exposing them to a cyclically varying, corrosive environment, and determining the number of cycles required to produce failure. Atmospheric pressure plasma polymerized HMDSO films exhibit RAIR spectra with prominent features similar to those observed for the low-pressure plasma films. These films had less than 7% carbon, revealing the films to be silica-like in nature. Durability results show that reducing plasma pretreatment of aluminum substrates was better compared to oxygen plasma pretreatment. Joints prepared from aluminum substrates that were acid etched, and then primed with silica-like film had exceptional durability. Durability of these joints was related to the acid etching, which formed a uniform and dense aluminum oxide structure with low magnesium content and high surface topography, and to the primer film which prevented hydration of the oxide. Also, joints prepared from substrates that were atmospheric pressure plasma pretreated exhibited better durability compared to similar joints prepared from substrates that were pretreated in low-pressure reactor. These results show that the atmospheric pressure plasma pretreatment have potential as pretreatment processes that can be applied to metals such as aluminum prior to finishing operations. iii ACKNOWLEDGEMENTS I would like to sincerely thank my advisor Dr. F. James Boerio for his guidance, insight and support throughout the course of this work. He has always encouraged creative thinking and independence. Many members of Dr. Boerio’s research group also deserve recognition for their assistance and companionship. In particular, Dr. Jennifer Chase and Pablo Rosales for their mentoring in the initial months of my graduate work. I would also like to thank Dr. Jude O Iroh, Dr. Gregory Beaucage and Dr. Rodney D. Roseman for taking their time to review this thesis and serving in my defense committee. I would like to acknowledge Dr. John Siemon for doing stressed durability tests and Alcoa Aluminum Co. for providing Al-6022 sheets. I would also like to gratefully acknowledge Dr. R. Giles Dillingham, of Brighton Technologies Group for great scientific advice and for salt fog testing. I would like to show my appreciation to Sumeet Bhargava, Advanced Materials Characterization Center for his help in performing the SEM experiments. My thanks and gratitude also goes to Michael Starr for performing the AFM experiments. I acknowledge the Environmental Protection Agency for their financial support under contract R-829579O1-O. I would also like to show my appreciation to my friends without their help and encouragement this work would not have been finished on time. Finally, I would like to thank my family for their love and support. iv TABLE OF CONTENTS ABSTRACT ii ACKNOWLEDGMENTS iv TABLE OF CONTENTS v LIST OF TABLES viii LIST OF FIGURES xi LIST OF ABBREVIATIONS xxiv I. INTRODUCTION 1 A. Aluminum and its Corrosion 1 B. Adhesive Bonding 2 C. Adhesion Mechanisms 4 1. Electrostatic Theory 4 2. Mechanical Interlocking Theory 4 3. Diffusion Theory 5 4. Adsorption Theory 5 5. Wettability and Adhesion 5 D. Durability of Aluminum/Epoxy Joints 6 E. Aluminum Pretreatment for Durable Adhesive Bonding 8 1. Anodizing 9 2. Inorganic Corrosion Inhibitors 10 3. Silanes 12 F. Mechanical Strength of Adhesive Joints 13 G. Plasma Processing 15 1. Background 15 2. Plasma Pretreatment of Aluminum Substrates 16 3. Plasma Polymerization 19 H. Plasma Polymerized Hexamethyldisiloxane Films 21 I. Atmospheric Pressure Plasmas 26 II. OBJECTIVES 29 III. EXPERIMENTAL 30 A. Substrate Preparation 30 1. Aluminum Substrates 30 a. Solvent cleaning 30 b. Acid etch pretreatment 30 2. Ferrotype Substrates 30 B. Plasma Processing 31 1. Low-pressure Plasma Reactor 31 a. Plasma treatment 32 b. Plasma polymerized films 32 2. Atmospheric Pressure Plasma 33 a. Plasma pretreatment 33 b. Plasma polymerized films 34 C. Silane Coupling Agent 34 D. Analytical Techniques 35 1. Fourier Transform Infrared Spectroscopy (FTIR) 35 v 2. Ellipsometry 38 3. X-ray Photoelectron Spectroscopy (XPS) 38 a. Depth profiling experiments 40 b. Oxide layer thickness 41 c. Hydroxyl groups on metal oxide films 42 4. Environmental Scanning Electron Microscopy & Energy Dispersive 43 X-ray Spectroscopy (ESEM/EDS) 5. Atomic Force Microscopy (AFM) 44 6. Contact Angle 44 E. Mechanical Testing 45 1. Lap Joint Preparation 45 2. Initial Strength and Durability Testing 45 F. Salt Spray (Salt Fog) Test 46 IV. RESULTS AND DISCUSSION 48 A. Characterization of Low-pressure Plasma Polymerized Films 48 1. Infrared Characterization 48 a. Monomer 48 b. Siloxane-like film 48 c. Low-hydroxyl silica-like film 49 d. High- hydroxyl silica-like films 50 2. Ellipsometry 50 3. XPS Characterization 51 B. Effect of Plasma Pretreatment Time on Aluminum Substrates 56 1. Mechanical Testing of Lap Joints 56 2. XPS Characterization of Aluminum Substrates 57 3. Discussion of Results 61 C. Effect of Plasma Polymerized Silica-like Films 61 1. Mechanical Testing of Lap Joints 62 2. Effect of Annealing on Plasma Polymerized Silica-like Films 64 D. Effect of Aluminum Substrate Pretreatment 65 1. Mechanical Testing of Lap Joints 65 2. XPS Characterization of Aluminum Substrates 66 3. SEM Characterization of Aluminum Substrates 73 4. AFM Characterization of Aluminum Substrates 75 5. SEM Characterization of Plasma Polymerized HMDSO Films 76 6. AFM Characterization of Plasma Polymerized HMDSO Film 79 7. Discussion of Results 80 E. Atmospheric Pressure Plasma Pretreatment of Aluminum Substrates 83 1. XPS Characterization of Aluminum Substrates 83 2. SEM Characterization of Aluminum Substrates 87 3. AFM Characterization of Aluminum Substrates 88 F. Characterization of Atmospheric Pressure Plasma Polymerized HMDSO 89 Films 1. Infrared Characterization 90 a. Air-HMDSO plasma polymerized films 90 b. Nitrogen-HMDSO plasma polymerized films 92 vi 2. Ellipsometry 93 3. XPS Characterization 94 4. SEM Characterization 98 5. AFM Characterization 101 6. B117 Salt Fog Test 101 7. Mechanical Testing of Lap Joints 104 G. Atmospheric Pressure versus Low-pressure Plasma Polymerized 105 HMDSO Films V. CONCLUSIONS 110 VI. FUTURE WORK 114 VII. REFERENCES 115 vii LIST OF TABLES Table 1. Reaction parameters used for plasma pretreatment and plasma