Mechanical and Microstructural Change in 0–90° SiC/MAS-L Composites after Thermal Ageing: Vacuum, Ar and CO Atmosphere

To determine the effects of an oxidizing environment on SiC (ex-PCS) fibres, 0–90° cross-plyed (2D) SiC/MAS-L composites have been heat-treated under vacuum, argon and CO in the 900–1200°C temperature range. The mechanical behaviour of the material was assessed by room temperature tensile tests and correlated to the fibre, matrix and interfacial zone changes. Transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and electron probe microanalysis (EPMA) were used to characterize the fibre and fibre/matrix interface. The degree of crystallization of the matrix was obtained using X-ray diffraction (XRD). The fibre/matrix bond strength is characterized by push-out tests on thin slices. Up to 1000°C under vacuum, the material was quite stable with a better fibre/matrix bond strength leading to an increase in the composite ultimate tensile strength. As soon as the temperature was raised to near 1100°C, the crystallization of the matrix, largely influenced by the atmosphere nature, resulted in a drop in the mechanical properties of the composite. The fibre/matrix interfacial zone as well as the fibres were modified by insitu phenomena such as diffusion of Si, Al or Mg species coming from the matrix, or by reaction with the CO atmosphere.