Comparison of spark plasma and microwave sintering of mullite based composite: Mullite/Ta 2 O 5 reaction

Abstract In this study, mullite ceramic matrix composites (CMCs) reinforced with Ta 2 O 5 particles (10 wt%) were fabricated through spark plasma sintering (SPS) and microwave sintering (MW) methods. The prepared batches were sintered at 1200 °C and 1300 °C in SPS method. The composite was also sintered at 1300 °C by MW method. In SPS, in order to achieve maximum densities, the sintering temperature increased so that the maximum displacements maintain in each sample. By reaching the temperature to 1200 °C in SPSed sample, the punch displacement stopped, while at 1300 °C in the same sample, it occurred at 1300 °C. The results showed that a high enough density was achieved in SPS sintered samples, while the MW sintered sample revealed higher amounts of porosities (23.81 ± 0.08%) and water absorption (12.20 ± 0.08%) together with low bulk density (2.562 ± 0.003 g/cm 3 ). The values of apparent density, porosity, and water absorption of the SPS sintered sample at 1200 °C were obtained as 3.283 ± 0.002 g/cm 3 , 2.38 ± 0.05% and 1.02 ± 0.04%. Also, these values for the SPSed sample at 1300 °C were 3.301 ± 0.001 g/cm 3 , 1.53 ± 0.03% and 0.48 ± 0.05%, respectively. The higher punch displacement, higher bulk density, lower porosity and water absorption of the SPSed sample at 1300 °C might be due to the higher sintering temperature, which causes the reduction of porosities alongside the increase in the probable reactions between the mullite and Ta 2 O 5 phases. The XRD and FESEM investigations revealed the formation of AlTaO 4 phases at mullite/Ta 2 O 5 boundaries. It was also proved that increasing the sintering temperature has a significant effect on AlTaO 4 phase formation. The SPSed sample at 1200 °C obtained the maximum bending strength of 329 ± 31 MPa in comparison to the sample sintered at 1300 °C (228 ± 26 MPa).