# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 4 Oxide CMCs
### Alumina Matrix CMCs
In addition to that, an increase in density, hardness, flexural strength, and fracture toughness was also accomplished, making it a suitable base material for the creation of a highly usable CMC [39]. Silicon Nitride (Si3N4) is used in CMCs in particulate form with high mechanical strength and wear resistance. It has high dielectric properties, and as such, it is a good energy insulator. Moreover, silicon nitride has been developed and used in structural applications, such as valves, gas turbine components, and turbocharger rotors [16; 17]. Despite its high-temperature resistance and strength, when exposed to high mechanical and thermal conditions, it fractures. This is the biggest challenge in engineering applications of this type of ceramics. Introducing silicon nitride in alumina matrix (Al2O3/Si3N4) increases overall mechanical properties by a great margin [40; 41]. Additionally, this class of CMCs can be manufactured with relatively low-cost processes [6]. Literature indicates combinations of Si3N4 and Al2O3 are used in consolidation. The discovery of an enlarged B -Si3N4 structure known as the B -phase was first reported independently by various researchers [42; 43]. Recently, it was discovered that this B phase is a solid solution of AIN-Al2O3 in B -SiN4 instead of what was previously believed to be a solid solution of Al2O3 to B -Si3N4. When Si3N4 and Al2O3 are hot pressed or pleasureless sintered, dense solids are created that are principally composed of b and one or more minor phases. SiAION is the name given to this kind of material today. Candidates for high-temperature structural applications include SiAION, Si\({}_{3}\)N\({}_{4}\), and SIC [44]. Further investigations demonstrate the thorough mixing and contact of the Si\({}_{3}\)N\({}_{4}\) and Al\({}_{2}\)O\({}_{3}\) powders that was ensured by intense stirred-ball-mill blending of the starting powders [44]. Powder mixtures of different Al\({}_{2}\)O\({}_{3}\)/Si\({}_{3}\)N\({}_{4}\) mole ratios were pressure sintered in a vacuum at temperatures ranging from 1100 to 1700 \({}^{\circ}\)C (2010 to 3090 \({}^{\circ}\)F) for studying the kinetics of reaction and densification. During heating and at equilibrium temperature, press ram movement resulting from the application of constant pressure (a measure of powder compaction) was recorded. In order to explain the production and densification of SiAION as a function of temperature and the Si\({}_{3}\)N\({}_{4}\):Al\({}_{2}\)O\({}_{3}\) mole ratio, powder compaction rates, and end-point densities were coupled with X-ray diffraction and metallographic investigations [45,46]. Further development of the material systems (thermal stability, non-oxidizing interface, and matrix), low-cost manufacturing processes, and non-destructive evaluation techniques are investigated [47,48]. Literature reports experimental studies where Al\({}_{2}\)O\({}_{3}\)/Si\({}_{3}\)N\(4\) is proposed for hybrid bearing applications, where the inner and outer races are created from steel, and the balls are manufactured from such CMC. Components with increased compressive strength (12.5% increase) are reported [49]. The functional graded material of Si\({}_{3}\)N\({}_{4}\)-Al\({}_{2}\)O\({}_{3}\) is developed by using a polypoid functional gradient and is successfully manufactured with the powder stacking method, resulting in a crack-free joining of the two materials. For the powder stacking method to be successful, the process includes the stacking of 20 layers of polyploids with a thickness of 500 \(\upmu\)m each to reduce the thermal residual stress resulting in a smooth gradient [50]. Titanium carbide (TiC) is a very hard refractory ceramic material. It has been tested to have the highest specific strength and modulus, high melting point, and electrical and thermal conductivity. Due to its high mechanical and physical properties, TiC has been used as reinforcement in titanium matrix composites [51]. Several researchers have described the ability of TiC particles to heal surface cracks in the alumina matrix [52].