The effect of a weak W/SiC interface on the strength of Sigma silicon carbide monofilament

Fractography studies have shown that the strength-determining flaws in sili con carbide monofilaments are generally at the core/silicon carbide interfa ce or in the vicinity of the outside, carbon-based coating. In tungsten-cor ed monofilaments like DERA Sigma, the W/SiC flaws primarily determine the s trength, Fracture is accompanied by brittle failure or. the tungsten. The c rack propagates simultaneously outwards through the silicon carbide, inward s through the tungsten and also around the W/SiC interface before being def lected into the tungsten or out through the silicon carbide. Experiments de positing boundary layers between the tungsten and silicon carbide have resu lted in significantly different fracture behaviour. The tungsten fails in a ductile manner and the strength-determining flaws are located predominantl y at the outside surface of the silicon carbide, This behaviour is discusse d in terms of models proposed by E, Martin and W. Curtin. It is thought tha t the work will ultimately lead to a significantly stronger, tungsten-based monofilament.