Improving strength by controlling segregation in liquid phase sintered aluminium nitride ceramics

SEM investigations on liquid phase sintered AlN ceramics showed segregation of the liquid phase. The liquid phase concentrated in clusters of some tens of micrometers in diameter. Crack deflection was observed in the vicinity of the clusters indicating high thermal stresses which could impair strength and reliability of the ceramics. Therefore, a thermodynamic model based on interface and grain boundary energies was established. With it the parameters were identified which caused segregation of the liquid phase during sintering. Due to the high contrast in thermal conductivity between liquid phase and AlN, wetting properties of the liquid phase could be determined indirectly by measuring the thermal conductivity of the ceramics at temperatures between 1600 and 1850 °C. In situ measurements of shrinkage and thermal conductivity were performed using a Thermooptical measuring device (TOM). With it a sintering process was developed which led to a homogeneous microstructure without segregation. An increase of flexural strength by 25% and of Weibull modulus higher than 80% was achieved.