Compatibility of the mechanical properties and thermal conductivity by the formation of a secondary phase in AlN ceramics: Application of multifractal analysis for the microstructure evaluation

Aluminum nitride (AlN) ceramics exhibiting different secondary phase morphologies were fabricated using pressureless sintering at varying temperatures ranging from 1700 to 1850 °C, with a 5 mass% Y 2 O 3 sintering additive. Although the thermal conductivity of the AlN ceramics increased as the sintering temperature increased, the bending strength increased below 1800 °C and decreased above 1800 °C. The morphology of the secondary phase in the AlN ceramics demonstrated granular, reticulate, and linear forms in that order of the low sintering temperature. The value of generalized dimensions D q obtained by multifractal analysis increased below 1800 °C and decreased above 1800 °C. Higher values of D q indicated more complex morphology and higher dispersion, corresponding to the observed morphological changes in secondary phase and the increase or decrease in the value of D q . The sample sintered at 1800 °C had the highest D q value and the reticulate secondary phase, with a thermal conductivity of 160 W m −1 K −1 and a maximum bending strength of 324 MPa. These results suggest that the morphology of the secondary phase affects the bending strength of AlN ceramics and that the value of D q may be an indicator of the microstructure that achieves both thermal conductivity and mechanical properties.