INTERFACE CHARACTERIZATION AND FRACTURE OF CALCIUM ALUMINOSILICATE GLASS-CERAMIC REINFORCED WITH NICALON FIBERS

An investigation of the structure and properties of a calcium aluminosilicate glass-ceramic reinforced with Nicalon fibres is described. Microstructural analysis of the interface showed that during manufacture of the composite a reaction zone rich in carbon formed between the Nicalon fibre and the anorthite matrix. Tensile strengths were approximately 330 MPa for unidirectional material and around 210 MPa for a (0-degrees/90-degrees)3s composite, little more than half that predicted by the mixtures rule. Flexural strengths were, however, higher than tensile strengths, by a factor 1.5-2.5 depending on lay-up. Studies carried out on specimens heat treated in air for 24 h at temperatures in the range 600-1200-degrees-C showed a progressive change of interface microstructure in the outermost regions of the specimens due to oxidation of the carbon-rich layer; at 1000-degrees-C and above the carbon had disappeared to leave voids and silica-rich bridges between fibre and matrix. These changes affected the strength of the interfacial bond, as measured by an micro-indentation technique, and also the degree of fibre pull-out produced in mechanical tests. Thus as-received material exhibited appreciable pull-out whilst heat-treated samples were characterized by brittle behaviour in the outer (oxidized) regions. Nevertheless, the composites whilst in the unstressed condition appeared to survive these short-term exposures to oxidizing environments. An interfacial shear stress of around 5 MPa was calculated by applying the Aveston, Cooper and Kelly theory to crack spacings measured in our room-temperature deformation experiments, a value which agreed well with the 3-5 MPa obtained by the micro-indentation method.