alpha SiC-beta SiC-graphene composites

In this study, the mechanical properties of the pressureless sintered samples of alpha-SiC based composite containing 0-3 wt.% graphene and 0-15 wt.% nano beta-SiC were investigated. Simultaneous usage of nano beta-SiC and graphene and transformation of beta-SiC (3C) to alpha-SiC (6H/4H) resulted in elongation of secondary alpha-SiC grains, which significantly improved the mechanical properties (e.g. fracture toughness) of SiC ceramics. According to the results, the highest Relative density of 99.04%, Young's modulus of 537.76 GPa and fracture toughness of 5.73 MPa x m(1/2) were obtained in the sample containing 5 wt.% nano beta-SiC and 1 wt.% graphene (5B1G). In addition, two methods of measuring bending strength including three-point bending tests and biaxial tests (piston-on-three-ball) were compared. Strip-shaped specimens were prepared for three-point bending test and disc-shaped specimens were prepared for biaxial bending test. Each bending test was evaluated using a universal testing machine. The results showed that the biaxial bending strength is less than the three-point bending strength. Also, the maximum three-point bending strength of 582.01 MPa and biaxial bending of 441.56 MPa were obtained in 5 wt.% Nano beta-SiC and 1 wt.% Graphene samples (5B1G). Studies have shown that in addition to the many advantages of using the biaxial bending method, the results have a very similar trend to the three-point bending strength. Also, the most-increased hardnesses of 28.03 GPa and 29.97 GPa were seen in the sample containing 5 wt.% nano beta-SiC (5B) with forces of 10 N and 1 N, respectively. One of the effective mechanisms in improving the fracture toughness of alpha-SiC ceramics is crack deflection/bridging. Also, the difference in thermal expansion of the alpha-SiC matrix and the reinforcements, leading to the creation of residual stresses between the matrix grains and the reinforcement, is effective in improving the mechanical properties (e.g. strength and fracture toughness).