Mechanical behavior of mullite-zirconia composites

In this work, mechanical properties of mullite-zirconia composites synthesised through reaction sintering of Algerian kaolin, α-Al₂O₃, and ZrO₂ were characterized. Phases present and their transformations were characterized using x-ray diffraction. Hardness H and fracture toughness K_IC were measured by Vickers indentation using a Zwick microhardness tester. The flexural strength was measured through three point bending test using an Instron Universal Testing Machine. It was found that the increase of ZrO₂ content (from 0 to 32wt.%) decreased the microhardness of the composites from 14 to 10.8 GPa. However, the increase of ZrO₂ content (from 0 to 24wt.%) increased the flexural strength of the composites from 142 to 390 MPa then decreased it with further increase of ZrO₂ content. Also, the fracture toughness increased from 1.8 to 2.9 MPa.m^1/2 with the increase of ZrO₂ content from 0 to 32 wt.%; and the rate of the increase decreased at higher fractions of ZrO₂ content. The average linear coefficient of thermal expansion (within the range 50 to 1450°C) for samples containing 0 and 16 wt.% ZrO₂ sintered at 1600°C for 2 hours was 4.7 x10^-6 K^-1 and 5.2 x 10^-6 K^-1 respectively.