Effect of the parameters of bimodal microstructure on the mechanical properties of alumina: A case of sintering regime effects

Abstract

The effect of sintering regimes on the density, microstructure parameters, and mechanical properties of Al₂O₃ and Al₂O₃ + 0.25 % MgO ceramics has been investigated. The ceramics were sintered in three regimes: Regime I – heating at a constant rate (2.5, 5, 10, 20 °C⋅min⁻¹) up to the temperature T = 1650 °C; Regime II – heating with a varied heating rate up to 1565 °C with the duration corresponding to sintering at the heating rate of 10 °C⋅min⁻¹ in Regime I followed by a three-fold decrease in the shrinkage rate; Regime III – two-stage sintering: heating according to Regime II up to the temperature T₁ = 1550 °C, then lowering the temperature down to T₂ = 1300–1500 °C and holding for 3 h at the T₂. The sintering regimes were chosen so that the ceramics had the relative density of 97–99 % and a bimodal distribution of the microstructure parameters. The Al₂O₃ and Al₂O₃ + 0.25 % MgO ceramics obtained in Regimes I–III had a microstructure with abnormally large grains in a fine-grained matrix. The sizes and volume fractions of the large grains depended on the sintering regime. Most abnormally large grains had elongated shapes that leads to deviations in the crack propagation trajectories from the straight line. The optimal parameters of the bimodal microstructure parameters distribution providing enhanced mechanical properties of the ceramics (hardness, indentation fracture toughness, ultimate strength) have been determined.