Sintering and mechanical properties of Zr-doped Al2O3

Abstract

Alumina based structural ceramics have assumed a status of high technological importance and are widely known for their various engineering and scientific applications. The objective of the proposed work was to fabricate Zr-doped Al2O3 ceramics where Al2O3 constituted the major phase and Zr-rich secondary phase (ZrO2) was formed in-situ at the grain boundaries with the use of a Zr-containing molecular precursor. Dopant (ZrO2) concentrations pertaining to 2000 wt. ppm (~ 830 cat. ppm of Zr) and 5000 wt. ppm (~ 2070 cat. ppm of Zr) were used in the current study. For convenience and clarity, the first group was designated as AZ2 and the latter was designated as AZ5. The overall approach was to coat the matrix phase (Al2O3) with the aforesaid precursor so that ZrO2 microphase could be formed in-situ with a homogeneous distribution at the grain boundaries of Al2O3. The first phase of the investigation was to ascertain the influence of Zr-doping on the densification behaviour and activation energy of Al2O3. A Visual basic application (VBA) master sintering curve (MSC) macro programme was used for the estimation of apparent activation energies of the undoped and Zr-doped Al2O3 samples. The validity of the MSCs was then verified by a few experimental runs. The apparent activation energies of undoped Al2O3, AZ2 and AZ5 were found to be 529, 670 and 790 kJ/mol, respectively. Subsequently, a two-step sintering technique was adopted to densify the undoped and Zr-doped Al2O3 samples, and the influence of Zr-doping on the density, grain size and mechanical properties of the Al2O3 was studied. No major change in Vickers microhardness values was observed in the undoped and the doped samples indicating a negligible dependence of microhardness on the grain size of Al2O3. However, a significant improvement in the strength values was observed in the doped samples despite the use of extremely low concentrations of Zr. The improvement in the strength values of the doped samples was attributed to the combined effect of microstructural refinement and grain-boundary strengthening mechanism.