THE MICROSTRUCTURE OF EXPERIMENTAL SIC FIBER-REINFORCED YTTRIUM MAGNESIUM ALUMINOSILICATE (SICF-YMAS) MATERIALS

Two experimental SiC fibre-reinforced yttrium magnesium aluminosilicat e (SiCf-YMAS)-type ceramic-matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscop y (TEM), high-resolution electron microscopy (HREM) and energy-dispers ive X-ray spectroscopy (EDS). The distribution of the fibres inside th e composite as well as the average diameter of fibres have been determ ined by image analysis. The microstructure of the YMAS matrices has be en characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and beta-yttrium silicate (Y2Si2O7) TWO minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre-matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (2030 nm) has been imaged in both composites at the fibre-matrix interface. It crystallizes along the matrix interface and grows inside the fibre, fo rming a diffuse interphase. The carbon layer is believed to be the con sequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.