Solid-particle erosion of hot-pressed silicon carbide and SiC-TiB2 composite

Abstract

The erosion behaviour of SiC has been studied for hot-pressed materials [1–3]. An anomalous particle size dependence [1], a low velocity exponent of less than 2 [1, 2] and fracture other than by lateral cracking have been reported in the erosion of hotpressed, monolithic SiC [3]. Most of these works were performed on commercial SiC of limited compositions and hot-pressing conditions were not reported. Compared to monolithic SiC, much less information is available on the erosion of composite materials. The erosion behaviour of composites has been reported only for SiC–SiC whiskers [4] and the SiC–Al2O3 system [5]. The objective of the present work is to investigate and compare the erosion behaviour of monolithic SiC and SiC–TiB2 particulate composite, both hot-pressed with 5 vol % Al2O3. These materials are chosen because SiC is toughened by the addition of TiB2 and the erosion rate is greatly affected by the fracture toughness [6, 7]. Two hot-pressed SiC materials were used for this work. One, designated SA, was monolithic SiC containing 5 vol % Al2O3, and the other, designated SBA, was SiC composite containing 5 vol % Al2O3 and 14 vol % TiB2. Specimens were produced from commercial powders of mean particle size of less than 0.9 μm. The combined powders were vibromilled in ethanol using SiC balls with a diameter of 8 mm and hot-pressed in graphite dies lined with graphite foil at 2000 to 2050 8C at 30 to 38 MPa under vaccum. In all cases the density of the hotpressed pieces exceeded 97% of theoretical. After hot pressing, specimens were cut and mechanically ground to dimensions of 20.0 mm by 20.0 mm by 50 mm. Erosion tests were conducted at room temperature using an air blast type apparatus. The erodent used was commercial angular SiC, average particle size of 50, 100 and 150 μm. Erodent velocities were 40, 70 and 100 m sy1 and angles of impact were 30, 60 and 908. The specimens were weighed to an accuracy of 10y5 g, subjected to erosion, cleaned with an air blast and then reweighed. Steady-state erosion rates were determined as the mass lost from the target per mass of impacting particle. Fig. 1 presents the angular dependence of the steady-state erosion rate of the materials SA and SBA for particles 100 μm in diameter eroding at a rate of 40, 70 and 100 m sy1. The shape of the curve is representative of both materials obtained for all particle sizes and velocities. Maximum erosion occurs for α ˆ 608, unlike the previous results on hot-pressed SiC [1–3]. The shift of peak in erosion rate indicates that the erosion of these materials occurs in a more ductile manner [8] compared to materials showing peak erosion for normal incidence. The erosion rate is higher for SA under more test conditions compared to the composite SBA, though the difference in erosion rate is not large under some test conditions. A higher erosion rate is obtained for SA in 13 conditions out of 18 test conditions, as shown in Fig. 1. Thus, a higher erosion rate is obtained for SA, having lower fracture toughness under more test conditions, compared to SBA. The fracture toughnesses of the materials measured by the single-edge pre-cracked beam (SEPB) method are 4.0 and 4.8 MPa m1=2 for SA and SBA, respectively [9]. Toughening observed for SBA is due to crack deflection caused by the interaction of the crack front with a residual stress field that surrounds the TiB2 particles [10]. The origin of this stress field is the difference in thermal expansion and elastic moduli between SiC and TiB2. The effect of fracture toughness on the erosion rate found in this work is roughly consistent with the theoretical predictions