Rolls-Royce is drawn towards composite materials because of their high specific strength and stiffness. Of the three groups, the polymer matrix composites (PMCs) currently account for almost all of the composite materials used in the gas turbine, despite their relatively low temperature capability of typically less than 150°C. Metal matrix composites (MMCs) are attractive for application at intermediate temperatures while ceramic matrix composites (CMCs) offer exciting possibilities for very high temperature applications where loads are modest.
If all these classes of composite materials achieve their full potential then the gas turbine engine 20 years from now will have significantly higher composite content than today’s. PMCs could constitute much of the nacelle, fan system, shaft support structures, casings and some stators. The intermediate and high pressure compressor rotors will be MMC while the combustor can, nozzles and some of the rear structure will exploit the advantages of CMCs.
PMCs have been used in the gas turbine since the early 1960s. In fact, glass reinforced epoxy constituted 40 per cent by volume of the RB162, a 1960s lift engine,including the compressor stators and rotor blades. Interest in carbon fibre composites was stimulated by their successful application for fan blades on some Rolls-Royce Conway engines for VC10 airliners.
Further expansion of these materials into the core of the engine and components critical to its successful operation will require a greater maturity from both the materials and the associated design methodologies. The higher temperatures will demand the development of a high temperature matrix system capable of being economically moulded into complex geometries. Fire and damage tolerance are also barriers to be overcome, yet the rewards of replacing heavier expensive metal fabrications are large.
MMCs extend the potential temperature range for the application of composite materials. Currently the most promising MMC system for gas turbine application is the titanium MMC (TiMMC). The high performance of the continuous, silicon carbide fibres means that when they are embedded in a Ti-6Al-4V matrix, the overall system can yield a 50 per cent increase in strength, a two-fold improvement in stiffness and a reduction in density compared with the parent material.
TiMMCs offer an alternative to conventional titanium alloys in the compressor section of the engine. The application which is seen to yield the greatest benefit is a natural development beyond the titanium blisk to the integrally bladed ring or bling. Here the bore of the disc can be eliminated, as the hoop stress can be born by the fibre reinforced rim alone, resulting in significant weight savings of up to 70 per cent over a conventional disc and blade arrangement and 40 per cent over a conventional titanium blisk design.
The first TiMMC bling application will be in a military vehicle where weight is critical. However, costs remain an issue and there are a number of projects underway to look at reducing fibre and manufacturing costs.
CMCs offer potentially significant temperature advantages over metals, together with a density typically one third that of nickel. They exhibit very high levels of specific stiffness compared to the competitor superalloys, but this must be weighed against a lack of ductility and the consequently low defect tolerance. Whilst a CMC does retain its specific strength to higher temperatures than metals, in absolute terms the useable strength is low. Due to these mechanical limitations, CMCs have to-date only been considered for niche applications as high temperature components at low structural loads, where their temperature capability can be exploited in order to avoid the need for cooling of metal components. This can yield a cost benefit for the overall engine even when the use of the CMC is not justified on a component for component basis.
Two systems have emerged as having significant potential: SiC/SiC and oxide/oxide.