Itsaso Echeverria

Ceramics are used for high temperature applications such as thermal barriers and components in high efficiency engines, due to their refractoriness and sustained mechanical properties at elevated temperatures. Conventional ceramics are inherently brittle with a low fracture toughness compared with metals. To overcome this, ceramic whiskers or continuous fibres are added to ceramic matrices, forming ceramic matrix composites (CMCs). CMCs have the advantages of a higher fracture toughness than monolithic ceramics whilst retaining a lower density compared with metals such as nickel superalloys used in similar applications. They can also have excellent corrosion/oxidation resistance properties in harsh environments. These attributes make them highly suitable as substitutes for metals in high temperature applications and in particular within the aerospace sector.

Currently the processing of CMCs carries a high manufacturing cost in comparison with metal alloys. This is due to the high energy-intensity of the process which limits the use of CMCs to high-end applications. The high cost of the fibres comes from the preparation of the bulk material to be spun, spinning it to form the green fibre and then heat treating it to obtain the ceramic fibre. During the manufacturing process, specialised equipment is required, in addition to high purity, and hence expensive, ceramic powder and long cycles at high temperatures to heat treat the fibres. Furthermore, in order to process ceramic matrix composites a suitable atmosphere is needed during the sintering stage, in particular for non-oxide CMCs for which the presence of oxygen is not favourable. As a result, there is interest in investigating whether processes and techniques that are used with polymer matrix composites (PMCs), can be adapted for use with CMCs to drive costs down. Additionally, the aerospace industry is already familiar with the methods and equipment used with PMCs, hence it will be very beneficial to transfer the knowledge from PMCs to CMCs.