Mechanical properties enhancing mechanism of $Al_2O_3-ZrO_2$ two phase ceramic composites was studied for several compositions of different $ZrO_2/Al_2O_3$ ratio.
The optimum sintering behavior of $Al_2O_3-ZrO_2$ composite was obtained at 1670℃ for 2hr in air with dopped Magnesia. In the case of $Al_2O_3-ZrO_2$ (pure) system which contained above 10 vol% $ZrO_2$ in $Al_2O_3$ matrix, nearly all of the dispersed $ZrO_2$ phase has transformed from tetragonal to monoclinic phase during cooling. But, in $Al_2O_3-ZrO_2$ (+3mol% $Y_2O_3$) system, all of the dispersed phase has retained in the form of tetragonal phase. Absorption ratio increased with monoclinic $ZrO_2$ content in both system.
This study showed that this absorption ratio increasing resulted from microcrack formation by the expansion of $ZrO_2$ during the tetragonal →monoclinic transformation.
Microstructural analyses of $Al_2O_3-ZrO_2$ (pure) composites indicated that preexisting microcrack due to larger $ZrO_2$ particle at grainboundary extended along Alumina grainboundaries within process zone. Microcracks also nucleated when very small $ZrO_2$ particles at the grainboundaries transformed to monoclinic phase at near of main crack tip. These type of microcracks could contribute to the toughening achieved via the phase transformation of the $ZrO_2$ by creating additional crack surface area during crack propagation. Microstructural analyses also showed that the average grain size and abnormal grain size of $Al_2O_3$ were decreased with increasing $ZrO_2$ vol% in $Al_2O_3$ matrix.
As a result, it could be concluded as follows.
In $Al_2O_3-ZrO_2$ (pure) System,
1. Microcrack nucreation (Stress-Induced Microcracking) and extension was effective mechanism for absorption of fracture energy.
2. More regular distribution and smaller grain size of $Al_2O_3$ due to $ZrO_2$ particles mainly contributed to maintain the strength and hardness.
