Finite element modelling of the effect of a functionally graded protective coating for SiC monofilaments on Ti-based composite behaviour

Metal matrix composites consisting of Ti-based alloys reinforced with SiC monofilaments show great promise for aerospace applications. This work is on a novel functionally graded (FG) coating system for SiC monofilaments, used as improved reinforcements for Ti-based matrices. The system is abbreviated as [SiCf/C/(Ti,C)/Ti]. A finite element analysis was carried out to investigate the effect of the coating on thermal residual stresses generated at the fibre/matrix interface due to differences in the thermal expansion coefficient (CTE mismatch) between the various materials within the coating system. These residual stresses are known to affect fracture behaviour of the composite. The aim of the study was to compare a graded system [SiCf/C/(Ti,C)/Ti], and a non-graded multi-layer system [SiCf/C/TiC/Ti]. The role of the carbon layer within the FG coating system and the effect of layer thickness were also investigated. The FG coating was seen to reduce residual stresses generated due to CTE mismatch. In addition correct choice of the coating material was seen to allow tailoring of the residual stress-state at the fibre/matrix interface.