The response of SiC fibres to vacuum plasma spraying and vacuum hot pressing during the fabrication of titanium matrix composites

Vacuum plasma spraying (VPS) and vacuum hot pressing (VHP) have been used t o fabricate Ti-6Al-4V matrix composite material reinforced longitudinally w ith DERA Sigma C coated SiC 1140+ fibres. VPS of Ti-6Al-4V onto Sigma 1140 SiC fibres caused no fibre/matrix interfacial reaction. During VHP a fibre /matrix reaction occurred, producing a mixture of fine (<50 nm) TiCx (x les s than or equal to 1) adjacent to the fibre coating and coarse-grained (0.3 -0.5 mu m) equiaxed TiCx adjacent to the Ti matrix, A decrease in C concent ration with increasing distance from the C coating is proposed, and is cons istent with the evidence presented. A similar thickness and morphology of r eaction product arose from conventional foil-fibre foil processing, but the matrix coated fibre/hot isostatic pressing process led to a slightly thick er reaction laver. The TiCx reaction product acted as a diffusion barrier, inhibiting further reaction more effectively than in experiments on earlier SiC fibres having a C coating, Surface damage was shown to be a factor in lowering 1140+ SiC fibre failure stress. Surface damage to 1140+ fibres resulted from both VPS and VHP, the former causing a slight reduction in mean ultimate tensile strength (UTS), and a large reduction in the bend strain to failure Weibull modulus. This damage was caused by both fibre winding and by deposition of metal during V PS, giving rise to coating flaws, and is not in itself considered to be a m ajor problem. Surface damage increased after VHP, reducing the mean UTS and tensile Weibull modulus, and the mean bend strain to failure. This damage arose from bending and flattening of the rough monotapes, and from the fibr e/matrix reaction caused by thermal exposure. The level of damage to 1140SiC fibre from VHP was reduced by modification of the process path. Increas ing the VHP temperature and lowering the pressure ramp rate reduced fibre d amage sufficiently to enable a macroscopic composite UTS of 95% of the theo retical maximum to be achieved.