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

Abstract

Aluminum nitride (AlN) ceramics exhibiting different secondary phase morphologies were fabricated using pressureless sintering at varying temperatures ranging from 1700-1850 °C, with a 5 mass% Y2O3 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 Dq obtained by multifractal analysis increased below 1800°C and decreased above 1800°C. Higher values of Dq 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 Dq. The sample sintered at 1800 °C had the highest Dq 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 Dq may be an indicator of the microstructure that achieves both thermal conductivity and mechanical properties.