Effects of Composite Sintering Additives: Al(OH)3+Y2O3+CaF2 on the Performance of Porous Mullite Oxide-Bonded SiC Ceramics

A composite sintering additive system: Al(OH)3+Y2O3+CaF2 was proposed for porous mullite oxide-bonded SiC ceramics. Small variations of sintering additives have significant influences on the phase composition, pore shape/size, density and flexural strength. Samples sintered at 1550 ℃ for 4 h in the air atmosphere realized both good mullite densification and no detectable cristobalite phase, which was difficult to be achieved at the same time. Besides, the composite sintering additive system also promoted the formation of columnar shape mullite, which acts as a reinforcement. Flexural strength as high as 108 MPa was achieved at an apparent porosity of 40.3 vol%, which is higher than that sintered by SPS technique. Moreover, those additives also act as pore formers determining the shape and size of pores. Around 8.9 µm strip-like, 11.8 µm continuous channel-like and 4.1 µm irregular pores were obtained for Al(OH)3, Al(OH)3-Y2O3 and Al(OH)3-Y2O3-CaF2 added samples, respectively. Corresponding phase evolution, sintering mechanisms and pore formation models were established. This work provides a simple way to modify the phase, pore size/shape, and strength of mullite oxide-bonded porous SiC ceramics by properly selecting sintering additives without any additional pore formers.