Non-oxide ceramic composites prepared by sol-gel and spark plasma sintering processes
2017-02-23T02:39:30Z (GMT) by
Ultra High Temperature Ceramics (UHTCs) are the most promising candidates for high temperature applications in extreme environments, such as conditions experienced during re-entry of out space flying objects, for their known high melting points, good chemical and thermal stability and high thermal conductivity. To improve the densification and oxidation resistance of UHTCs, a second phase, such as silicon carbide, is usually added to monolithic UHTCs to form ceramic matrix composites. While UHTCs/composites have many unique advantages for high temperature applications, they are very difficult to densify due to their very strong covalent chemical bonds and very high melting point. Conventional methods for processing UHTCs usually use powders as starting materials. A major issue of the conventional methods is the poor mixing/processing of powders with different densities and sizes, especially for nano-sized powders. Thus more research needs to be focused on improving UHTCs’ densification, microstructure and properties. This thesis studies the development of novel sol-gel techniques for preparation of starting materials and spark plasma sintering (SPS) for densification of UHTCs and composites. The thesis is organized into eight chapters. The First Chapter is an introduction to the thesis structure organization. The Second Chapter is the background of the research and literature review. Chapter Three is a description of the experimental procedures and characterization techniques. In the first result chapter, Chapter Four, silicon nitride ceramic and silicon nitride/titanium nitride composite were fabricated by hot pressing and spark plasma sintering (SPS) respectively to study the effect of the SPS DC current on the densification, microstructure and properties of conductive and non-conductive materials. A sol-gel based approach was investigated to improve the UHTC ceramics/composites with fine microstructures in Chapter Five. Although sol-gel techniques may not be suitable for making large bulk materials, they have unique advantages on modifying or improving the microstructures, properties and surfaces of materials through the solution chemistry approach. The sol-gel processing technique was used to synthesize TiC/SiC nanocomposites and followed by SPS sintering. Dense TiC/SiC composites were fabricated successfully to form a microstructure consisted of nano TiC and SiC grains. Work in Chapter Six used a sol-gel infiltration technique to fabricate dense TiC/SiC nanocomposites from porous TiC scaffolds. In Chapter Seven, zirconium diboride/silicon carbide composites of fine microstructures were successfully fabricated by sol-gel coating of ZrB₂ powders and SPS sintering. Compared to conventional ceramic processing methods, the sol-gel based wet chemistry process has many unique advantages such as homogeneous mixing of starting materials, infiltration of porous green bodies with complex shapes, avoiding the formation of intermediate transit phase/liquid phase in sintering, restricting grain growth and assisting densification by reaction. Chapter Eight presents the conclusions of the thesis and provides some directions for future work.