EFFECT OF HEAT-TREATMENT ON THE MICROSTRUCTURE AND PROPERTIES OF DENSE ALN SINTERED WITH Y2O3 ADDITIONS

The secondary phase constitution in two sintered AIN ceramics (1.8% and 4.2% Y2O3 additions) was studied as a function of heat treatment temperatures between 1750 degrees and 1900 degrees C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such as density, thermal conductivity (K), and lattice constants, and on the mechanical properties in three-point bending, was also investigated. Y3Al2O12 was found to getter dissolved oxygen from the AlN lattice below 1859 degrees C, but evaporated at 1850 degrees C and above. Y4Al2O9 appeared to sublimate below 1850 degrees 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 AIN samples studied, the c-axis was more sensitive than the a axis to oxygen content.