Microstructure and thermal shock behavior of sol–gel introduced ZrB₂ reinforced SiBCN matrix

Here we assess the effects of selected methods of introducing ZrB₂ in SiBCN matrix on microstructure and thermal shock properties prepared by spark plasma sintering (SPS). In one approach Zirconium n-propoxide (ZNP), the precursor of zirconia was introduced then reacted with amorphous BN(C) from the matrix, forming ZrB₂ phase, labeled as SZ1; A second approach is to introduce ZNP, boric acid, and furfuryl alcohol (C₅H₆O₂) (FA), the precursor of zirconia, boron oxide, and carbon in SiBCN matrix by sol–gel method to form ZrB₂ through carbon/borothermal reduction, labeled as SZ2. Results show that ZrB₂ particles are distributed homogeneously in SiBCN matrix. For SZ2 system, the grain sizes of ZrB₂ (100–500 nm) are much smaller than SZ1 composites(1–1.5 µm). SZ2 shows a higher diffusion rate (16.5 mm²/s) than SZ1 (12.8 mm²/s) at room temperature, and the high-temperature diffusion rate are very close to each other. After rapid quenching, residual stresses are caused by coefficient of thermal expansion mismatch and temperature gradient within the sample. On the other hand, the oxidized products formed on the surface during high temperature exposure could also contribute to thermal stresses causing spallation and enhancing damage.