SiC/SiC Composites for Aerospace

CMC's (Ceramic Matrix Composites) consist of fibres imbedded in a matrix, bonded by an interphase - either carbon or boron nitride. CMC's have been suggested for use in multiple markets over the years for their high-performance mechanical and thermal properties - amongst other benefits. In aerospace applications they are in directed competition with titanium alloys (which have limited temperature usability) and nickel-based super alloys (which add significant weight to the engines). However to be deployed in service- in particular in aeroengines further research is required. In particular the reliability and predictability of CMC properties are major obstacles to immediate engine implementation - this is because CMC fibre macrostructures demonstrate complexity, and their microstructural toughening mechanisms not fully understood. This project is in collaboration with Rolls Royce and will further develop the understanding of these materials and move them towards market.

Despite being built of brittle ceramic materials, CMC's have great creep resistance and distinct toughening mechanisms (including fibre pull-out) which upgrade their fracture behaviour to quasi-brittle / pseudo-ductile. Fibre pull-out makes use of an interlayer between the fibre and matrix with distinct tribological properties, which enable transfer of any load from one to the other3. Successful load transfer is characterised by high strains and premature 'yields'4. Fibres are able to slide along the interface, providing a source of displacement and therefore absorption of elastic energy5 and lowering of thermal stresses at any crack tip.

Whilst the mechanisms of fibre sliding and toughening are comparatively well understood at room temperature, these materials can develop significant changes in mechanical properties at elevated temperatures. This is due to the reaction of the interlayer with the water vapour present in combustion. This produces a variety of microstructural changes - often the formation of glassy phase and the evolution of gasses based around compounds of boron, nitrogen, oxygen and carbon.

This project will use advanced microscopy based tools to study the degradation of the interlayer as a function of water vapour partial pressure, over stress and cyclic loadings. This will combine high resolution electron microanalysis methods, thermal gravational analysis and secondary ion mass spectrometry. Allowing both the left behind reaction product and the evolved gas to be studied. This is important as the evolved gas can degrade metallic components further back in the engine - and an understanding of this will be vital to build a safety case for these materials. The mechanical properties post exposure will be correlated with the observed microstructures. This will be the first time these techniques have been used together to understand degradation and failure in these materials. In particular no one knows what gasses are evolved and what they may do to other components.

This work fits into the engineering and manufacturing the future themes.