# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 4 Oxide CMCs
### Alumina Matrix CMCs
Alumina and titanium carbide is a new combination, and thus a very small amount of experimental research has been reported. Experiments have been created using TiC in CMCs in the range of 15-30%. More specifically, Al\({}_{2}\)O\({}_{3}\)-TiC composites with various amounts of TiC, from 16 to 30 vol.%, were pressureless-sintered using 1 wt% Al as an additive. As the volume fraction of TiC grew, the hardness and toughness gradually increased as well. As the TiC volume percentage rose from 0.20 to 0.23, the electrical conductivity increased along with it and grew by about 10 orders of magnitude. The best wear resistance was found in a sample with a TiC volume fraction of 0.23, which is close to its saturation point [53]. Some newly developed fabrication techniques include the cementation process, where the provided alumina oxide, titanium particles, and activated carbon fine precursor are brought together to react upon sintering, creating a functionally graded-cemented layer. This technique offers lower synthesis costs without losing any of the mechanical and thermal strength of the final composite [54]. Such CMCs are mostly used as a slurry mixture and for manufacturing wear-resistant tools, protective coating, and cutting tools, due to their good chemical inertness and thermal conductivity as it is considered prominent reinforcement material [55]. Further experimental research on such CMCs focused on aero-engine applications demonstrated that the material cracks were healed when the composite was exposed to a high-velocity gas mixture at a temperature of 1000 \({}^{\circ}\)C and a low oxygen partial pressure [56]. Using sintering methods, researchers proved that the in-situ synthesis of TiC creates composites with high density, exhibiting enhanced toughness in comparison with Al\({}_{2}\)O\({}_{3}\) oxide ceramics [57]. Further investigations have been conducted on the manufacturability of such CMCs, focusing on mechanical milling, cementation process, and pressure-less sintering, but the results varied without having a consistently successful outcome [58]. Another important use of the in-situ synthesis of TiC particles is for healing surface cracks in the alumina matrix. Literature suggests the use of 15-30% volume percentage of TiC in temperatures ranging from 400 to 800 \({}^{\circ}\)C at a certain time. The results showed that the composite obtained maximum strength when it contained a 30% volume percentage of TiC [56]. Furthermore, surface cracks were healed when the composite was exposed to a combustion chamber with a high-velocity exhaust gas mixture at a temperature of 1000 \({}^{\circ}\)C and a low oxygen partial pressure [52]. * Alumina and Titanium Dioxide  
Titanium dioxide (TiO\({}_{2}\)) particulates are mainly produced in the form of white powder pigment due to their brightness and very high refractive index. This means that a low level of pigment is required to achieve a white opaque coating. One of the major advantages of TiO\({}_{2}\) is its resistance to discoloration under ultraviolet light. It is mainly used in coatings, plastics, fibers, and pharmaceuticals. In mildly reducing atmospheres, TiO\({}_{2}\) loses oxygen and turns into a semiconductor [59]. The combination of alumina and TiO\({}_{2}\) particulate reinforcement allows for highly reactive powder with low thermal expansion, thermal conductivity, and high thermal shock resistance [60]. The mechanical properties of such CMCs are similar to those of alumina and titanium carbide ones and comparatively higher than all the other oxide CMCs [61]. Typical applications for such CMCs are crucibles, nozzles, tubes, thermocouples, and coatings (for thermal corrosion and wear protection) [62]. Researchers have studied the characterization of Al\({}_{2}\)O\({}_{3}\) coatings with the addition of TiO\({}_{2}\), as well as the effects of TiO\({}_{2}\) in addition to alumina base plasma sprayed coatings concluding that titanium dioxide lowered the microhardness of the alumina coating [18].