CHEMICAL MODIFICATIONS OF POLYMER-DERIVED SILICON CARBIDE FIBERS TO ENHANCE THERMOMECHANICAL STABILITY BY GUANG JIN CHOI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1993 To My Family ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to Dr. Christopher D. Batich, my supervisor, for his support, guidance and advice during the course of this study. I am also grateful to the members of my supervisory committee, Drs. Tim Anderson, Anthony Brennan, Robert DeHoff, Eugene Goldberg and Jack Mecholsky, for their sincere discussions and their advices. I would like to thank Dr. Michael Sacks for his constant technical guidance and instrumental help. Special thanks are given to Dr. William Toreki who provided me with plenty of useful advice during the years of our collaboration. Deep appreciation is also expressed to many people who helped with my data; Dr. Augusto Morrone for TEM analysis, Mr. Gary Scheiffele for technical assistance, the technical personnel at MAIC, and to the students in Dr. Sacks group, as well as in our research group, for providing a friendly atmosphere. My deepest appreciation is given to Jungran, Jieun, Geunsu and other family members, for their unlimited understanding and sacrifice in supporting me during this study. I would like to acknowledge the financial support of Defense Advanced Research Projects Agency under contract nos. MDA 972-88-J-1006 and N00014-91-J-4075 throughout my graduate study at UF. TABLE OF CONTENTS page ACKNOWLEDGEMENT iii LIST OF TABLES vi LIST OF FIGURES ix LIST OF ABBREVIATIONS xiv ABSTRACT xvii CHAPTERS 1. INTRODUCTION 1 2. LITERATURE SURVEY 8 Ceramic Matrix Composites 8 Mechanical Behavior of Ceramic Fibers 14 Evolution of SiC Fibers 18 Nicalon and UF SiC Fibers 22 Polymer Precursors for SiC Materials 26 Characterization of SiC Materials 33 Preparation of Si and SiC Particles 43 Carbides and Silicides 47 Densification Behavior of SiC 48 Boron Precursors 59 3. EXPERIMENTAL PROCEDURE 67 Preliminary Study 1: Polymerization and Copolymerization 67 Preliminary Study 2: Carbide Formation Reactions 71 Preliminary Study 3: Decaborane High-dope Study 73 Fibers from PCS-based Polymers 76 iv Fibers from PCS-based Polymers with Decaborane 82 Fibers from PCS-based Polymers with Si Particles 87 Fibers from PCS-based Polymers with Si Particles and Decaborane 92 Other Fibers 94 Fibers from PCS-SX Polymers 94 Fibers from PCS-SZ Polymers with SiC Particles 94 Characterization 96 4. RESULTS AND DISCUSSION 102 Preliminary Study 1 : Polymerization and Copolymerization 102 Preliminary Study 2: Carbide Formation Reactions 105 Preliminary Study 3: Decaborane High-dope Study 120 Fibers from PCS-based Polymers 136 Fibers from PCS-based Polymers with Decaborane 146 Fibers from PCS-based Polymers with Si Particles 183 Fibers from PCS-based Polymers with Si Particles and Decaborane 206 Fibers from PCS-SX Polymers 225 Fibers from PCS-SZ Polymers with SiC Particles 230 5. CONCLUSIONS 236 6. FUTURE WORK/SUGGESTIONS 240 REFERENCES 242 BIOGRAPHICAL SKETCH 260 v LIST OF TABLES page 1.1 Properties of Commercial Ceramic Fibers 2 1.2 Properties of Silicon Carbide Materials 4 2. 1 Potential Markets of Ceramic Composite Materials 10 2.2 Research Programs for Ceramic Composite Materials in USA 13 2.3 Summarized Properties of Selected Carbide Materials 50 2.4 Properties of Potential Silicide Materials 54 2.5 List of Potential Boron Precursors 63 3.1 List of Studied Chemical Reagents 68 3.2 Properties of Dow Corning Polycarbosilane 69 3.3 List of Silicide Particles for Carbide Reaction Study 72 3.4 Polymer Compositions in Carbide Reaction Study 72 3.5 Polymer Compositions in Decaborane High-dope Study 77 3.6 Summarized Compositions and Processing of CP Fiber Batches (PCS-base polymer) 79 3.7 Summarized Compositions and Processing of CPD Fiber Batches (PCS-based polymer + decaborane) 83 3.8 Summarized Compositions and Processing of CS Fiber Batches (PCS-based polymer -1- Si particles) 88 3.9 Summarized Compositions and Processing of CSD Fiber Batches (PCS-based polymer + Si particles + decaborane) 93 vi 1 3.10 Summarized Compositions and Processing of CY Fiber Batches (DC PCS-SX polymer with/without decaborane) 95 4.13.11 Summarized Compositions and Processing of CX Fiber Batches (DC PCS-SZ polymer + SiC particles) 95 Summarized Results of Ceramic Yield Measurements: after 1000°C Pyrolysis . 121 4.2 Summarized XPS Results: Atomic Concentration for Pyrolyzed Disks 132 4.3 Summarized XPS Results: Binding Energy and FWHM 134 4.4 XPS Peak Positions in Literatures 135 4.5 Tensile Properties of CP Fibers (PCS) 137 4.6 Tensile Properties of CPD Fibers (DC PCS-SZ + decaborane): CPD1 to CPD8 149 4.7 Tensile Properties of CPD Fibers (DC PCS-SZ + decaborane): CPD9 to CPD1 151 4.8 Tensile Properties of CPD13 and CPD14 Fibers (DC PCS-SZ + decaborane + DPEA): Effect of Spinning Atmosphere 158 4.9 Tensile Property Comparison: Effect of Decaborane Incorporation 158 4.10 Summarized Properties of Dow Corning New SiC Fibers 162 4.11 Tensile Property Comparison: Effect of Decaborane Content 164 4. 12 Tensile Property Comparison: Effect of Soaking in Decaborane Solution . 167 4.13 Summarized XRD Results: Effect of Decaborane Incorporation on Crystallinity after 1800°C Treatment 178 4.14 Measured Density of SiC Fibers after Pyrolysis and after 1800°C Treatment: Effect of Decaborane Incorporation 182 4.15 Tensile Properties of CS Fibers (DC PCS-SZ + Si): CS1 to CS11 188 vii 4.16 Summarized XRD Results of CP5 (DC PCS-SZ) and CS9 (DC PCS-SZ + Si) Fibers: Effect of Si Particle Incorporation on Crystallinity after Heat-treatments 193 4.17 Tensile Properties of CS Fibers (UF PCS-PSZ + Si): CS12 to CS14 199 4.18 Summarized Results of Neutron Activation Analysis 205 4.19 Measured Density of CS14 Fibers (UF PCS-PSZ + 25 w/o Si) 207 4.20 Summarized Results of BET Analysis of CS Fibers (PCS + Si) 207 4.21 Tensile Properties of CSD Fibers (PCS + Si + decaborane) 210 4.22 Weibull Moduli of Selected CP (PCS), CPD (PCS + decaborane), CS (PCS + Si) and CSD (PCS + Si + decaborane) Fibers 220 4.23 Tensile Properties of CY Fibers (DC PCS-SX with/without decaborane) .... 227 4.24 Tensile Properties of CX Fibers (DC PCS-SZ + SiC) 232 LIST OF FIGURES page 1.1 SiC Fiber Processes of (a) Nicalon and (b) UF Fibers 5 2.1 Schematic Morphology of a Typical Fracture Cross-section of Brittle Materials 15 2.2 A Typical Weibull Plot: UF 127-12 SiC Fibers 19 2.3 Tensile Strengths of UF and Nicalon SiC Fibers: Effect of Heat-treatment . 24 2.4 A Schematic Structure of Branched Polycarbosilane 29 2.5 Melting Temperatures of Binary Refractory Materials 49 2.6 Chemical Structure of Decaborane (B 10 H 14) 64 2.7 Chemical Reaction Tree of Decaborane (B 10 H 14) 66 3.1 Temperature Profiles of 1000°C Pyrolysis 74 3.2 Temperature Profiles of Heat-treatments 75 3.3 SEM Micrographs of Spinneret Holes: (a) 4-hole and (b) 8-hole Spinneret . 81 3.4 Flow Diagram of CS Fiber Processing 90 3.5 Schematic Diagram of Fiber Specimen for Tensile Testing 97 4.1 Composition Diagram of Silazane Polymerization 103 4.2 Composition Diagram of Siloxane Polymerization 104 4.3 Composition Diagram of DC PCS-Silazane Copolymerization 106 IX 4.4 X-ray Diffraction Patterns of PCS-SZ Copolymer: Effect of Heat-treatment Temperature (a) before Pyrolysis, (b) after 1300°C Treatment, (c) after 1400°C Treatment and (d) after 1500°C Treatment 107 4.5 X-ray Diffraction Patterns of a Mixture of PCS-SZ Polymer and Si Particles: Effect of Heat-treatment Temperature (a) before Pyrolysis, (b) after 1300°C Treatment, (c) after 1400°C Treatment and (d) after 1500°C Treatment .... 109 4.6 X-ray Diffraction Patterns of a Mixture of PCS-SZ Polymer and Si Particles: Effect of Temperature Profile during 1400°C Treatment (a) with no hold at 1000°C and (b) with one hour hold at 1000°C Ill 4.7 HT-XRD Peak Intensity Change of a Mixture of PCS-SZ Polymer and Si Particles 112 4.8 X-ray Diffraction Patterns of a Mixture of PCS-SZ Polymer and HfSi 2 : Effect of Heat-treatment Temperature (a) before Pyrolysis, (b) after 1300°C Treatment and (c) after 1500°C Treatment 114 4.9 HT-XRD Peak Intensity Change of a Mixture of PCS-SZ Polymer and HfSi2 Particles 116 4. 10 X-ray Diffraction Patterns of a Mixture of PCS-SZ Polymer and TiSi 2 : Effect of Heat-treatment Temperature (a) before Pyrolysis and (b) after 1500°C Treatment 117 4.11 HT-XRD Peak Intensity Change of a Mixture of PCS-SZ Polymer and TiSi2 Particles 119 4.12 TG/DTA Results of Decaborane to 1000°C: (a) Regular Pyrolysis Temperature Profile and (b) Fast Pyrolysis Temperature Profile 122 4.13 SEM Micrographs of Pyrolyzed Decaborane High-dope Disks: (a) without Decaborane and (b) with Decaborane (15 w/o) 125 4.14 X-ray Diffraction Pattern of As-purchased Decaborane 126 4. 15 X-ray Diffraction Patterns of (a) PCS-SX polymer and (b) Decaborane High-dope PCS-SX Polymer, before and after 1000°C Pyrolysis 127 4. 16 X-ray Diffraction Patterns of (a) PCS-SZ Polymer and (b) Decaborane High-dope PCS-SZ Polymer, before and after 1000°C Pyrolysis 129 x 4.17 XPS Wide Scan Spectrum of Pyrolyzed Disk: Decaborane High-dope PCS-SX Polymer 130 4.18 XPS Wide Scan Spectrum of Pyrolyzed Disk: Decaborane High-dope PCS-SZ Polymer 131 4.