Improvement toughness of SiC ceramic by adding Cr2O3 and annealing process

In this research, the effect of different amounts of Cr2O3 (2.5, 5, 7.5, and 10 wt.%) and sintering temperature (1850, 1900, and 1950 °C) on the sinterability and mechanical properties of liquid-phase sintered SiC-matrix composites was studied. First, raw materials were ground for 3 h using a planetary mill whose rotational speed was 180 rpm. The process of pressing the samples was completed using uniaxial pressing with the applied pressure of 90 MPa. Finally, the samples were sintered under an argon atmosphere at various temperatures for 1.5 h. In the end, the best sintered sample was annealed at 2000°C for 2 h. The phases, microstructure, and chemical composition of the samples were studied using X-ray diffraction analysis (XRD) and field emission scanning electron microscopy (FESEM), respectively. The results suggested that the composite that contained 5 wt.% Cr2O3 and that was sintered at 1900 °C exhibited the best properties. The relative density, microhardness, elastic modulus, flexural strength, indentation fracture resistance, and brittleness index were 97.45%, 27.50 GPa, 395 GPa, 549 MPa, 6.2 MPa m1/2, and 282.58 ×10−6 m−1 respectively. At 1850 °C and 1950 °C, the best mechanical properties were acquired for the samples containing 5 wt.% Cr2O3. According to microscopic images, the formation of elongated grains and the activation of crack deflection as well as crack bridging mechanisms were the most significant mechanisms enhancing the toughness of these composites. The results showed that the sintered sample at 1900 °C, containing 5% additive and annealed at 2000 °C for 2 h, reached the highest fracture toughness (6.92 MPa m1/2). The microscopic images showed that the matrix grains of the as-annealed samples were elongated. There were signs of transgranular fracture at the cross-section of the samples.