HIGH-TEMPERATURE STRENGTH OF SIC-ALN SOLID-SOLUTIONS AND COMPOSITES AS EVALUATED BY SMALL PUNCH TESTS

Fully dense compacts of SiC-AlN solid solution and composite were fabricated by the combination of reaction synthesis and hot isostatic pressing (HIP). Their strength and deformation behavior were investigated from room temperature to 1773 K by a Modified Small Punch (MSP) test which has been found to be suitable for the mechanical evaluation of brittle materials. The SiC-AlN solid solution and composite samples showed higher strength than bath of the HIPed SiC (with 0.5 mass%B4C additives) and AlN compacts from room temperature to 1473 K. A decrease in the strength was then observed above 1473 K for the solid solutions containing 5, 10 and 25 mol%AlN, while the SiC, AlN and SiC-AlN composites maintained the room temperature strength up to 1773 K. Nonlinear deformation occurred at 1773 K in the solid solution samples with the composition of SiC-25 mol%AlN, being accompanied by a drastic drop in strength. Considering the fact that single-phase SiC-AlN solid solutions have a fine microstructure of uniform grain size, the nonlinear deformation is due to grain boundary sliding. On the other hand, the SiC-50, 70 mol%AlN composites maintained their high strengths up to 1773 K, probably because coarser grains with AlN-rich composition in the fine-grained matrix effectively suppressed the grain boundary sliding. For the single-phase SiC-AlN solid solutions, moderate grain growth is needed to achieve high strength at elevated temperature.