MICROSTRUCTURE AND CREEP CHARACTERISTICS OF EXPERIMENTAL SICF-YMAS COMPOSITES

A unidirectional SiCf-YMAS glass-ceramic composite has been developed by Ceramiques-Composites (Bazet) and ONERA (Establishment of Palaiseau ) in France. The matrix is totally crystalline and consists essentiall y of two main phases, cordierite and yttrium disilicate, with some min or phases, mullite, spinel, zirconium and titanium oxides. Image analy sis methods have been used to characterize the homogeneity of the comp osite plates and to obtain granulo-metric information on the different matrix phases. Different interphase layers formed during the process by reaction between the matrix and the Nicalon NLM 202 fibres have bee n studied by using HREM and EDX. Their chemical composition has been d etermined by stepping the probe (8 nm) across the fibre-matrix interfa ce. Two distinct nanoscale sublayers have been imaged, The sublayer on the matrix side has a light contrast in the TEM. The microstructure o f this layer (approximate to 80 nm) is typical of a turbostratic carbo n, The carbon layer also contains Al, O, Mg and Si. The silicon conten t is low in the carbon layer. The sublayer on the fibre side (approxim ate to 100 nm thick) has a dark contrast in the TEM, Profiles have bee n taken across this sublayer also. Tensile creep tests in air have bee n performed to investigate the tensile creep behaviour at 1223 K. They have been conducted in the 50-200 MPa stress range. Tensile creep res ults indicate that creep rates are of the same order of magnitude as f or other glass-ceramic composites, Optical micrographs and SEM observa tions have revealed the damage in the composite. Changes occurring in the interface region have been studied at a finer scale by TEM and HRE M at the surface of the sample and in the core. These observations ena ble us to explain the mechanical behaviour of the composite observed o n a macroscopic scale.