Alumina Ceramics (I.E., Alumina-Celsian Composites) Due to Its High Temperature Stability and Low Thermal Expansion
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T-4685 A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Material Science). Golden, Colorado Date :?o Mardi 1715 Signed: ·~ £~ Richard E. Chinn Approved: ,;jfl;~ ij ~MichaelJ.Haun Thesis Advisor Golden, Colorado Date ~'!,o, /C/95' .7 Professor and Head, Department of Metallurgical and Materials Engineering 11 T-4685 ABSTRACT Monoclinic strontium aluminosilicate with the stoichiometric celsian composition (SrA'2Si20s) is a potential matrix material for fiber-reinforced composites. High processing temperatures (>2000°C) are required to form glass from the stoichiometric composition. Heat treatment of the glass initially crystallizes an undesirable metastable hexagonal phase (hexacelsian), which after further heat treatment transforms to the stable monoclinic form (celsian). Some properties of celsian have been reported, such as modulus of rupture, thermal expansion and density, but many others have not, such as hardness and dielectric properties. Furthermore, celsian has promise as an intergranular phase in high-alumina ceramics (i.e., alumina-celsian composites) due to its high temperature stability and low thermal expansion. However, neither a process to make these composites nor their properties have been previously investigated. The use of transient glass-phase processing, TGPP, to lower the glass melting and subsequent heat treatment temperatures to produce celsian was investigated in this thesis. A non-stoichiometric, low-melting, alumina-deficient composition of SrC03, Alz03 and SiOz with a glass transition temperature more than 600 K below the crystallization temperature of crystalline stoichiometric celsian was melted and prepared into glass powder. This composition was modified to form a second composition with the addition of 1.8 mole% B203 to control the wetting, densification and crystallization behavior. The research was divided into three areas of investigation. In the first investigation, the two glass powders were mixed with enough u-AI203 powder such that the net compositions 111 T-4685 were those of stoichiometric celsian, then dry-pressed and sintered in vanous time temperature combinations. In the second investigation, the two glass powders were mixed with fine a.-Al203 powder in a 1:24 mass ratio (96% alumina), dry-pressed in pellet form and sintered. A third ceramic with the same net composition (without B203) was also prepared from crystalline raw materials for comparison. Compositions in the third investigation were similar to those of the second, except that a coarser a.-A1203 powder was used and bars were also fabricated for strength and toughness measurements. The results of the first investigation were that dry-pressed glass-alumina pellets containing B203 in the glass, sintered at 1100°C for 10 hours, densified by viscous-phase sintering, completely dissolved the a-Al203 and crystallized to celsian. Alumina-glass pellets without B203 in the glass, sintered at 1I00°C for 10 hours and 1640°C for 4 hours, also densified and crystallized to virtually 100% celsian, albeit by a more tortuous reaction path. Results of the second investigation were that all the lesser component transformed to celsian in each composition, and all three were slightly harder and more dense than published values for 96% alumina. Each composition had significant grain growth from the fine a-Al203 powder. The result of the third investigation was that mullite formed preferentially to celsian in all three compositions; yet alumina-celsian composites appear to have strong potential for commercialization based on their mechanical, microstructural and dielectric properties. These alumina-celsian compositions have potential for high-temperature applications that are currently limited to more expensive grades of alumina, due to a highly crystallized intergranular phase in 96% alumina. IV T-4685 TABLE OF CONTENTS Page ABSTRACT ................................................................................................................ iii LIST OF TABLES ...................................................................................................... vi LIST OF FIGURES ..................................................................................................... viii ACKNOWLEDGMENTS ........................................................................................... xi Chapter 1: RESEARCH OBJECTIVES AND LITERATURE REVIEW ..................... 1 1.1 Introduction and Research Objectives ........................................................ 1 1.2 Literature Review ...................................................................................... 5 Chapter 2: EXPERIMENTAL BACKGROUND ......................................................... 10 2.1 Celsian Structure Model ............................................................................ 10 2.2 ASTM Procedures ..................................................................................... 10 2.3 Glass Powder Preparation ......................................................................... 14 2.4 X-Ray Diffraction (XRD) .......................................................................... 28 2.5 Density Measurements ............................................................................... 32 2.6 Ceramography ........................................................................................... 33 2.6.1 Ceramographic Preparation ......................................................... 34 2.6.2 Ceramographic Examination and Evaluation ............................... 37 2.7 Knoop Hardness ........................................................................................ 39 2. 8 Thermal Analysis ....................................................................................... 41 V T-4685 2.8.1 Differential Thermal Analysis (DTA) ........................................... 41 2.8.2 Thermogravimetric Analysis (TGA) ............................................ 42 2.9 Sintering and Densification of Ceramics ..................................................... 43 2.10 Four-Point Bending Fixture ..................................................................... 46 2.11 Indentation Strength in Bending (ISB) ..................................................... 48 2.12 Dielectric Properties ................................................................................ 53 Chapter 3: QUANTITATIVE X-RAY DIFFRACTION OF MONOCLINIC SrAl2Si208 AND HEXAGONAL SrSi03 ................................................................... 57 3 .1 Introduction .............................................................................................. 57 3.2 Experimental Procedure ............................................................................ 58 3 .3 Results and Discussion .............................................................................. 63 3 .4 Summary ................................................................................................... 73 Chapter 4: LOW-TEMPERATURE TRANSIENT GLASS-PHASE PROCESSING OF MONOCLINIC SrAl2Si208 ......................................................... 75 4.1 Introduction .............................................................................................. 75 4.2 Experimental Procedure ............................................................................ 76 4.3 Results and Discussion .............................................................................. 76 4.4 Time-Temperature Modification ofTGPP ................................................. 88 4.5 Summary ................................................................................................... 93 Chapter 5: DEVELOPMENT OF ALUMINA-CELSIAN COMPOSITES ................... 96 5 .1 Introduction .............................................................................................. 96 5.2 Experimental Procedure ............................................................................ 97 5.3 Results and Discussion .............................................................................. 100 5.4 Summary ................................................................................................... 110 VI T-4685 Chapter 6: MICROSTRUCTURES AND PROPERTIES OF 96% ALUMINA WITH INTERGRANULAR MONOCLINIC SrAl2Si20g ........................................... 111 6.1 Introduction .............................................................................................. 111 6.2 Experimental Procedure ............................................................................ 112 6.3 Results and Discussion .............................................................................. 116 6.4 Summary ................................................................................................... 136 Chapter 7: CONCLUSIONS AND FUTURE WORK .................................................. 138 7.1 Conclusions ............................................................................................... 138 7.2 Future Work ............................................................................................. 140 REFERENCES CITED ............................................................................................... 141 APPENDICES ............................................................................................................ 149 A: Reference Intensity Calculations ................................................................