Mechanical Properties of AIN/hBN Ceramic Composites

AIN-BN ceramic composites were fabricated and their mechanical properties were investigated. The relative density of hot-pressed composites decreased with increasing BN content, but over 99% could be obtained with 30 vol% BN in AIN. YAG was formed in the composites and monolithic AIN as a second phase by the reaction between Y2O3, added as sintering aid, and Al2O3. As expected, Vickers hardness and Young's modulus decreased with increasing BN content. The three-point flexural strength also showed similar behavior decreasing from 500 MPa of monolith down to 250 MPa by the addition of 30 vol% BN. However, interestingly, the standard deviation of the strength values decreased significantly as BN was added to AIN. As a result, the Weibull modulus of the AIN-30 vol% BN composite was 21.3, which was extremely high. Fractography and crack path studies revealed that BN platelets induced grain pull-out and crack bridgin in a bigger scale during crack propagation. Consequently, fracture toughness increased as more BN was added, reaching 4.5 MPa root m at 40 vol% BN.