Influence of matrix cracking on mechanical properties of a ceramic matrix composite

The first apparent evidence of damage in ceramic matrix composites occurs via matrix cracking when these composites are subjected to stresses beyond the elastic limit. The presence of these matrix cracks may have several consequences to the mechanical response of composites both at room and elevated temperatures. For example, composites containing matrix cracks may show reduced modulus and enhanced susceptibility to damage by fatigue and oxidation at elevated temperatures. In addition, many composites have inherrent matrix cracks/ porosity produced either because of processing limitations (chemical vapour infiltration (CVI), solgel) or as a result of a mismatch in fibre/matrix thermal expansion. Therefore, an understanding of the influence of matrix cracks on the mechanical response of composites is important for designing composites and for assessing their performance under service conditions. In this study, a fully dense and crack-free zircon composite reinforced with SiC whiskers and SiC continuous fibres was loaded in three-point flexure up to the matrix cracking stress, then unloaded. Subsequently, these samples were repeatedly loaded and unloaded below the original matrix cracking stress to study the influence of matrix cracking on the elastic modulus of the composite. The objective of this letter is to report some of the initial observations of such mechanical testing on the mechanical response of a zircon composite reinforced with SiC fibres and whiskers. Some of these observations on matrix cracking behaviour are related to predictions of the steady state and nonsteady state micromechanical models for matrix cracking. This study was performed on a zircon matrix composite reinforced with SiC whiskers and uniaxially aligned SiC continuous filaments. Silicon carbide whiskers were Tateho SCW-1 and SiC monofilaments were SCS-6 type obtained from Textron Corporation. All of the composites tested in this study were processed in a manner described in a previous study [1]. As-fabricated composites were fully dense with an insignificant (< 1%) porosity, and contained about 20 vol % SiC whiskers in the zircon matrix phase and about 25% SiC monofilaments in the entire composite volume (including the zircon matrix, SiC whiskers and SiC filaments). The matrix phase containing zircon and SiC whiskers had an average grain size of about 3/~m, and X-ray diffraction (XRD) analysis of the composite revealed zircon and SiC as major matrix phases with a small but detectable amount of the free-zirconia phase. The zirconia phase was not detected in the starting zircon powder. Therefore, it must have been produced because of the decomposition of zircon [2] at the high processing temperature of 1610 °C. Average mechanical and fibre-matrix interracial properties of as-fabricated composites were measured by three-point flexure and fibre pushout tests, respectively. The details of these tests were also given elsewhere [1]. Typically, composite samples with dimensions of 3.2 cm length, 0.8 cm width and 0.15 cm thickness were individually ground to a finish of 60/~m and then tested in three-point flexure with lower support pins 2.54 cm apart (span-tothickness ratio: 17). All of the tests were performed at a crosshead rate of 0.0127 cmmin -1 to generate load-deflection data. A number of average mechanical properties were determined from these loaddeflection data. These included elastic modulus (E), first matrix cracking strength (Ocr), ultimate strength (~ru) and work-of-fracture (WOF), as described previously [1]. At least four samples of each type were tested to measure the average mechanical properties, which are summarized in Table I. Fibrematrix interracial shear strength was measured by a fibre pushout test [3, 4], and at least 10 fibres in each sample were pushed to calculate an average value of the interfacial shear strength. Average interfacial shear strength was measured during the first push as well as upon re-push (second push) of the already pushed fibres. These results are also summarized in Table I. In this study, a zircon composite reinforced with SiC whiskers and uniaxially aligned SCS-6 monofila-