This thesis examines the influence of initial α-SiC contents in the starting α+β-SiC starting powder mixtures on the microstructural development and mechanical properties of liquid-phase sintered silicon carbide ceramics. Five kinds of starting powders with different αto β-SiC ratios using 7wt.% $(60wt.%Y_2O_3+40wt.%Al_2O_3)$ additives were hot-pressed at 1950℃ for 10 minutes and subsequently annealed for 2, 4, and 6 hours at the same temperature in order to study the coarsening behavior of SiC grains and the crystallization of the grain-boundary glass phase. Mechanical properties such as flexural strength and fracture toughness of each annealed samples were also investigated.
The use of hot-pressing enabled densification above 98% theoretical density in all specimens. Average grain size increased with increasing introduction of β-SiC powder in the α+β-SiC mixed starting powders, and with increasing isothermal heat treatment time, and some platelet large-grains were formed when annealed above 4 hours with α+β mixed starting powders. The distribution functions of grain sizes of the SiC materials annealed above 4 hours with α+β-SiC mixed starting powders were typically bimodal. However, the rate of grain growth and formation of platelet large-grains in this study were slower than those of the specimens sintered with $4Y_2O_3-6Al_2O_3$ additives, probably due to the higher liquid formation temperature and viscosity of the liquid formed in this study.
In all images, the grain boundary phases are seen to have a nearly continuous network structure. Isothermal annealing was effective in controlling the grain boundary phase and resulted in the variation of the fracture mode, which affected mechanical properties. When the crystallization of the grain boundary phase leads to an aluminum-rich phase (Y-Al garnet) during the intermediate stage of annealing, flexural strength is improved. On the other hand, fracture toughness can be increased when crystallization of the grain boundary glass is directed to an yttrium-rich phase during extended annealing.
At each powder composition, flexural strength showed maximum in the specimens heat treated for 4 hours, and these maximum strengths decreased with decreasing α-SiC powder content in the α+β mixed starting powder. The maximum and minimum strengths shown in this study were 711 and 425 respectively. Hall-Petch relationship was observed when the strengths of the specimens with identical heat treatment conditions were tracked.
An increase in toughness was generally observed with heat treatment time when specimens of identical starting powder composition were tracked. Toughnesses from specimens heat treated for 6 hours increased to a maximum value of 5 MPaㆍ$\sqrt{M}$ when α-SiC made up 20% of the starting powder mixture.
Microstructural evolution was accompanied by the β-SiC(3C) to α-SiC(6H, 4H, 15R), α-SiC(6H) to α-SiC(4H, 15R) phase transformation and the crystallization of the grain boundary phase.
This thesis demonstrates the feasibility of a new processing technique for obtaining SiC ceramics with bimodal grain size distributions, by introducing similar sized α-SiC into β-SiC starting powder and with the use of $6Y_2O_3+4Al_2O_3$ additive composition. However, the relatively low grain growth and transformation rate of SiC brought about lower fracture toughnesses than those reported by others. This was due to the high liquid formation temperature of additives used in this study. Further improvement in fracture toughness might be obtainable by controlling and optimizing the volume fraction, diameter, and aspect ratio of the large platelet grains in this system.
