Effect of Grain Boundary Composition on High-Temperature Mechanical Properties of Hot-Pressed Silicon Carbide Sintered with Yttria

In materials with microscopic grain sizes, the grain boundary composition plays a decisive role in determining the resulting high-temperature oxidation, creep and slow-crack-growth behaviour. Therefore, it is crucial that the relationship between composition and properties is known in order to tailor the material to meet specific requirements. In the present study, the chemical composition in the system silicon carbide with Y2O3 as a sintering aid was varied. The resultant powders were hot pressed and tested in four-point bending at room temperature as well as at 1500 °C. The room-temperature strength and the fracture toughness (ICL method) were determined. The dynamic fatigue strength was measured at a stress rate of 0.05 MPds and an initial stress of 50 MPa. Hotpressed and tempered (100 h in air at 1500 °C) bending bars with either natural flaws or a single edge notch were tested. The fracture surfaces were examined and an attempt was made to correlate the observations with results of the mechanical tests. Differences were found between the Y2O3-rich and the SiO2-rich materials, independent of the flaw type. At 1500 °C, the SiO2-rich material exhibited slow crack growth, whereas the Y2O3-rich material exhibited typical fracture features such as a mirror, mist and hackle. The Y2O3-rich specimens failed at higher stresses than the SiO2-rich specimens. Additionally, for notched specimens, the dynamic fatigue strengths were higher at 1500 °C than at room temperature and at both temperatures after tempering. It is assumed that processes involving blunting of the crack tip are responsible for the observed behaviour.