# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 5 Non-Oxide CMCs
### Tungsten Carbide CMCs  
Tungsten carbide (WC) is a chemical compound consisting of equal portions of tungsten and carbon atoms [157,158]. The most basic form of this material is fine-grey powder [157]. Its mechanical and thermal properties are equal to titanium carbide (TiC), making it ideal for extremely high temperature and stress applications, although it is very hard to work with it, so it is mostly used for making cutting tools [159,160]. WC is combined with four different material families creating different CMC material combinations. These material families are carbon, cobalt, nickel, and nickel/chrome bonded matrices. * Tungsten Carbide-Carbon CMCs. Tungsten carbide includes ingredients of cemented carbides which are based on WC-Co, which, apart from great hardness, offer high wear resistance and hardness retention when exposed to high temperatures [159]. On the other hand, research has shown that tungsten carbide and generally cemented carbides have low corrosion resistance when exposed to corrosive environments due to the cobalt binder [161]. Hence many tests have taken place by reducing the amount of binder material and replacing it with another filler or removing the binder material completely [161,162]. The binders used to reinforce tungsten apart from cobalt are mainly carbon (C) and nickel (Ni). The most researched cementing material for WC is the WC-Co system. This is because Co allows for very good wettability and adhesion with tungsten carbide, as well as increased strength and toughness [163]. Furthermore, researchers proved that the abrasion resistance and hardness are oppositely proportional to the added percentage of cobalt due to the composition, chemical purity, and micro-structure of the carbides existing in the matrix [164,165]. Studies also demonstrated that the higher the volume fraction of Co reinforcement, the higher the fracture toughness but the lower the hardness and abrasion resistance [165]. The fabrication of this type of composite started in 1957 when other researchers tried to fabricate pure WC by using conventional sintering techniques, but it was found that the densification of the nanocrystalline fiber of WC-Co powder could be treated only in solid-state, meaning that it could not be processed [166,167]. Researchers managed to completely densify pure WC with vacuum or pressurized gas sintering, creating ultra-fine WC starting powder that, with further processing, could achieve a high hardness and wear resistance, avoiding any large amounts of grain growth inhibitor [167]. Furthermore, the literature shows that the reduction of WC grain size in WC-Co hard metals allows for the solubility of WC when Co is in the liquid or solid phase, allowing for improved strength and wear resistance [167]. Further research shows that the abrasion resistance of the nanostructure of this type on CMC is about twice the size of other types of ceramic composite filler/reinforcement, such as ZrO\({}_{2}\) and SiC [168]. The addition of free carbon into the tungsten carbide matrix can upgrade the densification process of WC and reduce or even extinct the creation of a brittle phase composition when exposed to temperatures over 1800 \({}^{\circ}\)C [162]. Studies have also proved that the addition of carbon in WC sinters formed graphite nanolayers on inter-granular boundaries, improving the thermal conductivity of the polycrystals close to double [169]. Although, research showed that large amounts of added carbon in a WC matrix cause abnormal grain growth of tungsten carbide, allowing for the creation of cavities and pores in the sintered products [170]. * Nickel and Nickel/Chrome bonded Tungsten carbide CMCs Nickel is a material cheaper than Co, easily available with good toughening properties, and with lower ductility than WC-Co [171]. In addition, it is used as a binder for tungsten carbide composites to improve corrosion and oxidation resistance. It has high-temperature strength, high temperature, and high wear resistance [172].