Effect of MgO additions on the phase stability and degradation ability in ZrO2-Al2O3 composite systems

The present study investigates the effect of MgO additions in the ZrO2-Al2O3 composite system. An in situ citrate-nitrate sol-gel process was used to carry out four different concentrations of MgO additions in the ZrO2-Al2O3 composite system and the results were compared with the pure ZrO2-Al2O3 composite system synthesized by the similar process. The characterization results confirmed the formation of pure tetragonal zirconia (t-ZrO2) phase for all the investigated compositions at 900 °C. Pure ZrO2-Al2O3 composite powders resulted in the degradation to yield monoclinic zirconia (m-ZrO2) at 1100 °C whereas the MgO containing ZrO2-Al2O3 composite powders restricted the formation of m-ZrO2 till 1200 °C. The reaction between MgO and Al2O3 to form magnesium aluminate (MgAlO4) started to occur from 1000 °C and its quantitative yield was dependent on the two important factors namely the MgO content and heat treatment temperatures. The complete formation of MgAlO4 at 1200 °C resulted in the onset of ZrO2 degradation to yield m-ZrO2 at 1200 °C.