Effect of interface chemistry on the mechanical properties of Si3N4-matrixcomposites

The effect of systematic modifications in the chemistry of the phase-bounda ry film on the macroscopic mechanical properties of Si3N4-matrix composites was investigated. Model composite materials, containing SiC or WC platelet s, were prepared and only the bulk anion composition of the glassy-SiO2 int ergranular phase was varied by adding increasing amounts of fluorine to the material. Detailed material characterizations by high-resolution electron microscopy (HREM) and Raman spectroscopy on both undoped and F-doped compos ites allowed to derive a structural model of the phase-boundary film as wel l as to evaluate the average microscopic stresses acting on it. In addition , high-temperature internal friction measurements provided an estimate of t he grain-boundary relaxation temperature as a function of the F content. No ticeable variations of both elastic modulus and fracture energy of the comp osite were detected upon F addition, which were related to a spontaneous pr ocess of phase-boundary microcracking upon cooling. A threshold of the F-co ntent was found for microcrack formation and its existence is theoretically explained according to a percolation process of non-bridged SiO4-tetrahedr a, which arises from the incorporation of F into the intergranular film net work. (C) 1999 Kluwer Academic Publishers.