Hardness of silicon-nitride and sialon

Hardness characteristics of hot pressed silicon nitride, sintered silicon nitride, reaction sintered sialon and liquid phase sintered sialon have been discussed in terms of porosity, grain size and mechanical properties, viz. Young's modulus, flexural strength and fracture toughness. Hardness decreases with an increase in volume fraction open porosity and grain size, and in most of the materials hardness increases with an increase in the aforesaid mechanical properties. The statistical correlation coefficient is -0.91 between porosity and hardness, and almost equally high between mechanical properties and hardness of most of the materials examined. Among four proposed empirical hardness-porosity relationships, the equation H = H0(1 - f1P + f2P2), where H is hardness, P is volume fraction open porosity, and H0, f1 and f2 are empirical parameters, represents the data of the present work best