JOURNAL OF MATERIALS SCIENCE 25 (1990) 3229-3235 Solid particle erosion of SiC-AlaOC ceramics S. GOCHNOUR, J. D. BRIGHT, D. K. SHETTY Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112, USA. R. A. CUTLER Ceramatec, Inc., 2425 South 900 West, Salt Lake City, Utah 84119, USA Erosion rates of SiC-AI20C ceramics, with AI20C content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec -~ and impacting the target at normal incidence. The target ceramics varied in hard- ness from 27.1 GPa for SiC-5wt% AI20C to 10.SGPa for SiC-75wt% AI20C, but the fracture toughness was essentially independent of composition (K~c ~ 3.5 MPa ml/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models. 1. Introduction Because of its high hardness and chemical inertness, SiC is increasingly used in applications that demand wear and corrosion resistance. In fluidized-bed com- bustors, for example, SiC tubes are being considered to replace high-temperature alloys to resist the severe conditions of erosion and corrosion at high tempera- tures [l]. As a result of this interest in wear appli- cations, a number of studies have been directed to examine the performance of SiC in specific wear situ- ations. Routbort and colleagues [2-5], Wiederhorn and Hockey [6] and Wada and Watanabe [7] studied solid particle erosion of SiC ceramics. Emphasis in the majority of these studies has been on evaluating the particle velocity and size dependence of the steady- state erosion rates and comparing them to the predic- tions of the elastic-plastic indentation fracture models of erosion [8,9]. The particle velocity and size exponents measured for reaction-bonded SiC were close to the theoretical expectations, while these exponents were anomalously low for hot-pressed SiC [2-6]. Evans et al. [8] and Wiederhorn and Hockey [6] examined the influence of hardness and fracture toughness of various target ceramics on their relative erosion rates in solid particle impact. In both of these studies, the dependence of the measured erosion rates on a combined fracture toughness and hardness par- ameter was greater than the theoretical expectations based on the indentation fracture models. The effects of target hardness and fracture toughness were, how- ever, not separated. More recently, Wada and Wat- anabe [7] conducted erosion tests on SiC with different impacting particles of varying hardness. The erosion rate of the same target SiC increased very significantly with an increase in the hardness of the impacting particles relative to the hardness of the target SiC. Jackson et al. [10] have sintered SiC at temperatures between 1850 and 1950~ using a transient liquid 0022-2461/90 $03.00 + .12 9 1990 Chapman and Hall Ltd. phase produced by the carbothermal reduction of A1203 by A14C ~ . The resulting ceramic was fine grained (average grain size less than 5 ~m) and consisted of SiC (starting polymorphs) and A12OC as the two major phases and minor amounts of A1203 and WC. The properties of the hot-pressed ceramics varied with the amount of A12OC, but at an optimum composition of about 5 to 10wt% A12OC, the strength (at = 660MPa), hardness (H = 27.1GPa) and fracture toughness (Kit = 3.1 MPam ~/2) obtained were com- parable or superior to the corresponding properties of commercial grades of sintered SiC [10]. The present paper summarizes the results of an investigation of the erosion behaviour of the above SiC-A12OC ceramics under solid particle impact. The primary objective of this study was to examine the influence of a systematic variation of the microstruc- tures and properties of a class of ceramics within the same generic family on their erosion response. A second objective was to compare the erosion response of SiC-A12OC ceramics of optimum composition with the response of several commercial sintered and hot-pressed grades of SiC. 2. Materials and test procedures 2.1. SiC-AI20C ceramics The processing of the SiC-A12OC ceramics via liquid- phase sintering has been described by Jackson et al. [10]. Ceramics with A12OC content varying from 5 to 75 wt % were fabricated and characterized with respect to a number of properties. Some of these properties relevant to erosion are listed in Table I. Fracture toughness of the ceramics was relatively invariant with composition, but all the other properties such as den- sity, hardness, bend strength, elastic modulus and coefficient of thermal expansion, varied systematically with the AI2OC content. Figs la to c show the microstructures of the cer- 3229