Effect of processing parameters and MgO doping on properties of pressureless sintered alumina ceramics

The aim of the present study is to investigate the pressureless sintering of coarse alumina powder. The impact of various processing parameters on the densification and mechanical properties of sintered alumina ceramics has been examined. Specifically, the study focuses on parameters such as sintering temperature (1300–1500°C), holding time at sintering temperature (0, 4, 8 and 12 h), MgO doping and the starting powder grain size. The overall study showed an improvement in the densification of sintered samples, with increasing sintering temperature and time. However, it was observed that when the holding time was extended to 12 h, a decline in densification occurred specifically within the temperature range of 1450–1500°C. This decline might be attributed to the occurrence of excessive grain growth. It is worth noting that during this interval of sintering temperature (≤1500°C), where densification was still ongoing (90% relative density), the hardness of the sintered samples remained relatively stable. Nevertheless, compressive strength, flexural strength and fracture toughness improved with an increase in sintering temperature and time. The MgO-doped samples showed enhanced densification compared to the undoped ones, indicating the grain growth inhibition effect of MgO. Furthermore, a noticeable improvement was observed in both densification and mechanical properties when the particle size of the initial alumina powder was reduced from 87.34 to 19.6 µm.