Grain growth of beta silicon nitride in silicon nitride ceramics.

Abstract

The grain growth mechanism and kinetics of anisotropic $\beta$-Si$\sb3$N$\sb4$ grain growth in silicon nitride ceramics was studied using a disintegration method. The disintegration method can provide a micrograph corresponding to a three-dimensional microstructure of silicon nitride ceramics. The grain size of the elongated $\beta$-Si$\sb3$N$\sb4$ can be determined by measuring the grain length and width of each grain separately. The results show that the distributions of the length and width follow log-normal distribution. The grain growth behavior of $\beta$-Si$\sb3$N$\sb4$ grains follows the empirical growth law D$\sp{\rm n}$ $-$ D$\sb{\rm o}\sp{\rm n}$ = kt with the exponents equaling 3 and 5 for length (001) and width (210) directions, respectively. Activation energies for grain growth were 686 KJ/mol for length and 772 KJ/mol for width. These differences in rate constants and exponents for length and width directions are responsible for the anisotropy of $\beta$-Si$\sb3$N$\sb4$ during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with increasing sintering temperature and time. The microstructural development of the silicon nitride ceramics is a function of the particle size of the starting powder rather than the initial $\beta$-Si$\sb3$N$\sb4$ contents. The results show that the larger the particle size of the starting powder, the larger the grain length and width but the lower the aspect ratio of elongated $\beta$-Si$\sb3$N$\sb4$ grain.