Effect of heat treatment on the microstructure and properties of dense AlN sintered with Y2O3 additions

The secondary phase constitution in two sintered AlN ceramics (1.8% and 4.2% Y2O3 additions) was studied as a function of heat treatment temperatures between 1,750 and 1,900 C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such ad density, thermal conductivity (K), and lattice constants, and on the mechanical properties in three-point bending, was also investigated. Y3Al5O12 was found to getter dissolved oxygen from the AlN lattice below 1,850 C, but evaporated at 1,850 C and above. Y4Al2O9 appeared to sublimate below 1,850 C in the atmosphere used in this study. Depending on the secondary phase constitution, heat treatment affected thermal conductivity favorably or adversely. Occasionally, samples with similar lattice oxygen contents were found to have different thermal conductivities, suggesting that factors besides dissolved oxygen can also influence K. Lattice parameter measurements indicated that, within the small range of lattice oxygen concentrations in the AlN samples studied, the c-axis was more sensitive than the a-axis to oxygen content