Phase stability, hardness and oxidation behaviour of spark plasma sintered ZrB2-SiC-Si3N4 composites

Despite significant efforts to develop ultra-high temperature ceramics, the phase stability together with high hardness and oxidation resistance remains to be addressed in ZrB2-SiC based ceramics. ZrB2-20 vol% SiC (ZS20) ceramics with varying amounts of Si3N4 (2.5, 5 and 10 vol%) were processed by multi stage Spark Plasma Sintering (SPS) over a range of temperature (1800-1900 degrees C) for 3 min under 50 MPa. All the ZS20-Si3N4 composites could be densified to more than 98% theoretical density (rho th) after SPS at 1900 degrees C. The XRD, SEM-EDS analysis of the ZS20-Si3N4 composites revealed the presence of reaction product phases (ZrO2, BN, ZrN) along with SiC and ZrB2 major phases. Sintering reactions were proposed to explain the existence of such new phases and extinction of Si3N4. Thermo-Cale software was also used to further confirm the formation of these new phases in the ZS20-Si3N4 samples. The hardness of ZS20-Si3N4 composites varied between 25.50 and 30.56 GPa, in particular, ZrB2-20 vol% SiC-5 vol% Si(3)N(4 )measured with the maximum hardness. In fact, it is the highest ever reported hardness for the ZrB2 composites. Considering oxidation resistance, the weight gain of ZrB2-20 vol%SiC composites decreased (from 13.84 to 9.84 mg/cm(2)) and oxide layer thickness increased (64-128 mu m) after oxidation at 1500 degrees C for 10 h in air. The cross sectional microstructure of oxidized ZS20-Si3N4 composites consists of thick dense outer layer of SiO2, intermediate (ZrO2-SiO2) layer and unreacted bulk. The formation of dense SiO(2 )layer and absence of SiC depleted layer from the oxidized samples are signatures of improved oxidation resistance of Si3N4 reinforced ZrB2-20 vol% SiC