Influence of polyacrylic acid on rheology of SiC suspension and mechanical properties of densified SiC

Addition of polyacrylic acid (PAA) to a 30 vol% SiC suspension with Al2O3 (2.5 mass% to SiC) and Y2O3 (2.5 mass% to SiC) decreased the viscosity of the suspension at pH 5.0. The prepared suspension was consolidated by filtration through a gypsum mold. The green compact was hot-pressed to 98.7-99.3% theoretical density at 1800 °C under a pressure of 39 MPa for 2 h in an Ar flow. The sintering mechanism was discussed based on the analysis of the shrinkage curve of SiC compacts during the hot-pressing. The grain size of hot-pressed SiC decreased by PAA addition in the suspension. PAA addition improved the Weibull modulus of flexural strength and fracture toughness of SiC at a small expense of average strength. The measured mechanical properties were as follows: average strength 647 MPa, Weibull modulus 6.6, fracture toughness 5.0 MPa m1/2 and Vickers hardness 24.5 GPa for the SiC hot-pressed without PAA, and average strength 615 MPa, Weibull modulus 8.2, fracture toughness 5.8 MPa m1/2 and Vickers hardness 24.8 GPa for the SiC hot-pressed with PAA.