Microstructural evolution during thermal shock process and the residual strength of Si ₃ N ₄ ceramics

Abstract Thermal shock resistance of silicon nitride was investigated from aspects of residual strength and microstructure, using a water quenching method. The residual strengths after 800 and 1000°C thermal shock polarized as higher ones and much lower ones, and reasons for the huge disparity are explored. With heat treatment temperature getting higher, the inner small pores rush to and aggregate in the surface layer of the samples. When the heat treatment reaches 1400°C, a darker subsurface layer is observed, which is caused by the loss of most Al and Y elements. Moreover, many more small pores are found in this layer, acting as the dissipation sources, they protect the material strength by releasing the intense thermal stress. But this subsurface layer disappears during the natural cooling down to 600°C as Al and Y uniformly redistributed in extended oxidation, then huge cracks form on the surface layer undergoing much smaller thermal stress from 600 to 0°C. Moreover, the bonding Y and Si can be oxidized into two types of Y ₂ Si ₂ O ₇ crystals that improve the thermal shock performance of Si ₃ N ₄ .