Microstructural evolution during thermal shock process and the residual strength of Si3N4 ceramics

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 degrees 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 degrees 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 degrees 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 degrees C. Moreover, the bonding Y and Si can be oxidized into two types of Y2Si2O7 crystals that improve the thermal shock performance of Si3N4.

Automated summary

The thermal shock resistance of silicon nitride was studied using a water quenching method, and the differences in residual strength after thermal shock were analyzed. The results showed that the surface layer of the samples changed with increasing heat treatment temperature, forming a darker subsurface layer, in which more small pores were found and played a role in dissipating thermal stress. However, this subsurface layer disappeared during natural cooling, leading to the formation of large cracks on the surface layer.