Microwave hybrid sintering of Al2O3 and Al2O3-ZrO2 composites, and effects of ZrO2 crystal structure on mechanical properties, thermal properties, and sintering kinetics

Structural ceramics such as Al2O3 and Al2O3-ZrO2 composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO2 crystal structures on Al2O3-ZrO2 composites, especially on the sintering kinetics, when fabricated by microwave sintering. The microwave hybrid sintering of Al2O3 and Al2O3-ZrO2 composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an alpha-alumina matrix, and the mechanical and thermal properties were studied. It was found that Al2O3 experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO2 was added. Al2O3-c-ZrO2 also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 +/- 11 kJ/mol for pure Al2O3, 319 +/- 71 kJ/mol for Al2O3-c-ZrO2, and 342 +/- 77 kJ/mol for Al2O3-t-ZrO2. These values indicated that the activation energy was increased by the addition of either type of ZrO2, with the highest value shown by Al2O3-t-ZrO2.