Abstract
The indentation behaviour and erosion properties of two commercially available ceramic materials, a silicon carbide (Hexoloy SA) and silicon carbide-titanium diboride composite (Hexoloy ST) have been investigated over a range of experimental conditions. The microstructure of both materials has been examined using reflected light and scanning electron microscopy. Hexoloy SA is a single phase material with a grain size typically ranging from 4 to 8 mum, while Hexoloy ST is a two phase particulate composite, containing about 16 vol% of discrete titanium diboride particles in a silicon carbide matrix, with a more uniform grain size. The materials have both been shown to have weak grain boundaries. The silicon carbide-titanium diboride interface is weak and this is believed to be due to tensile residual stresses arising from the mismatch in coefficients of thermal expansion of the two phases. Vickers indentation testing indicated that both materials have similar hardness values, but that the composite was significantly tougher than the monolithic material. Sub-surface crack profiles have been examined with a particular regard to radial and lateral cracking. It was found that the scale of lateral cracking was not directly proportional to the length of radial cracks in these materials. Indeed, lateral cracks were not seen when the radial/median system was fully formed, but only when it was partially formed. This is an important observation since one of the fundamental assumptions of two models of erosion is that radial and lateral length are directly proportional. Another important finding of the indentation study was that lateral cracking occurred to a greater extent in the composite than in the monolithic materials at low loads, indicating that wear of the composite may be relatively more extensive for the smaller erodent sizes. Erosion testing has been performed using a gas blast apparatus. Different sizes of silica and silicon carbide erodent have been used for tests from room temperature to 1000°C. With the silica erodent, material loss progressed by small scale cracking. The mechanisms of material removal involved grain boundary cracking in the monolithic material and grain boundary cracking and cracking along the particuiate-matrix interface in the composite. For the silicon carbide erodent, lateral cracking has been shown to be the dominant mechanism of material removal. In the monolithic SIC the lateral cracking scales with erodent size, while in the composite the TiB2 particles inhibit growth of the laterals generated by the largest erodent, but proved to be detrimental when using the smallest erodent. This observation was consistent with the observations from quasi-static Indentation. The presence of an easily removed oxide on the surface of the TiB2 particles has led to an increase in the erosion rate of the composite at temperatures greater than 800 °C for the silica erodent. At lower temperatures both materials behaved similarly. When using the silicon carbide erodent, increasing the temperature resulted in an increase in the erosion rate for both materials although at the lower temperatures, the composite was more erosion resistant than the monolithic material. As the temperature increased, the erosion rates converged, suggesting that the toughening mechanisms of the composite were decreasing in effectiveness. Thus, it has been shown that the presence of TiB2 particles can lead to increased or decreased erosion resistance relative to the monolithic material, depending on the precise erosion conditions. In general, the composite has the lower wear rate at lower temperatures and larger erodent sizes. Also, it has been shown that cracking due to quasi-static indentation using a sharp indenter is consistent with the damage produced by hard, sharp erodent particles at room temperature.