Effect of porosity on the thermal shock behaviour of reaction-sintered silicon nitride

Reaction-sintered silicon nitride was investigated to determine the effect of its pore size on thermal stress resistance to fracture initiation. Samples of controlled pore structure were prepared by using an organic component to incorporate pores in the green silicon compact as well as by using silicon starting powders with different particle size. Critical temperature differences ΔT_c after water quenching is discussed in relation to changes in most important variables affecting thermal shock, such as fracture strength, Young's modulus of elasticity and thermal conductivity. The results show that when total porosity as well as other microstructural parameters are held constant, an increase in pore size leads to a decrease in ΔT_c. Moreover, the results indicate that thermal conductivity plays a significant role in the interpretation of the thermal shock behaviour of reaction-sintered Si₃N₄.