Microstructure development for optimum fracture toughness of YSZ added Al2O3 composites

Abstract

Alumina (Al2O3) based ceramics are widely used for making machine parts, high-temperature components, wear-resistant material and biological ceramic due to their high hardness, strength, wear-resistance, and high chemical and thermal stability. However, low fracture toughness (KIC ~ 3.0 MPa.m1/2) inhibit its usage in other potential applications where high toughness is a pre-requisite (e.g. ceramic engine). In an attempt to improve the strength, particularly the fracture toughness, two step sintering process was used along with addition of 3 mol% Yttria stabilized Zirconia (3YSZ) in an attempt to make composite with enhanced mechanical properties. Optimization of sintering profile was initially carried out based on the results obtained from dilatometry and DSC-TG analysis. Both traditional and two-step sintering methods were used in this investigation. The benefit of this two-step sintering approach is to get higher mechanical properties at low sintering temperature. Composites of different composition were sintered at the same sintering cycle to determine the optimum composition. X-ray fluorescence (XRF) was used to make compositional analysis and X-ray Diffraction (XRD) was used to check the phase purity and crystallinity for both raw powders and sintered samples. Microstructure was investigated by using FESEM. Resultant microstructure was correlated to hardness, fracture toughness, and percentage theoretical density (Dth). The maximum fracture toughness and hardness obtained were 5.51 MPa.m1/2 and 10.96 GPa, respectively, for the Al2O3-30 wt% 3YSZ composite. Micro-structural analysis revealed that higher numbers of elongated grains formed with increasing additions of 3YSZ. These elongated grains, along with phase transformation toughening of 3YSZ, are believed to influence the enhancement of fracture toughness. Density of the best-performing sintered sample was found to be 96.12% Dth and the average grain size was 0.35 µm. The thermal expansion coefficient was measured as 6.5×10-6 K-1.