# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 4 Oxide CMCs
### Alumina Matrix CMCs  
Alumina is a mixture of aluminum and oxygen with high thermal conductivity and is widely used as an electrical insulator. It has high hardness, high compressive strength, and good chemical and thermal stability [7,24]. However, its application as a structural material has been limited by its low fracture toughness and strength. This is attributed to the fact that cracks easily propagate, and therefore, they might fail unpredictably during service [25, 26, 27, 28]. Literature indicates a small amount of developed ceramic oxide interphases composed with the alumina matrix, while the largest number of studies are experimental, conducted with the use of metal oxides (lanthanum vanadate, calcium tungstate, etc. ), which have been proven to present processing challenges [29,30]. Alumina matrices are used predominantly for O-CMC applications due to their high mechanical and thermal properties and low density. Their chemical stability is a factor that allows them to be combined with other available oxide fibers enabling better overall temperature capability of the composite  
Figure 4: Oxide CMC material categories. material. Some of the most used alumina-based CMCs include boron carbide, silicon nitride, titanium carbide, and titanium dioxide as secondary composite materials (Figure 4). In the following paragraphs, these composite systems will be discussed in more detail. Boron carbide has high strength and hardness, and it is the third hardest material after diamond and boron nitride. Furthermore, it also exhibits low density, neutron absorption capability, and acceptable chemical stability values. These properties allow the boron carbide to ionize radiation [31]. The combination of alumina matrix with boron carbide creates a high-temperature and high-pressure-resistant composite CMC material which can be used in high structural applications [31; 32]. The literature indicates that 10-40% vol of B4C whiskers have been tested and showcased the higher density results. B4C between 5 and 15 vol% showed that it could assist in the sintering of the alumina densification by methods other than grain development inhibition. The composites with 10-20 vol% B4C whiskers have enhanced fracture toughness of up to 6.2 MPa.m\({}^{1/2}\)[33]. Due to their high hardness and fracture toughness, composites made of aluminum oxide (Al2O3) and boron carbide (B4C) have been suggested for use in high-temperature applications and as cutting tools. The composite Al2O3 and B4C coatings were made using the air plasma spraying technique. On ordinary carbon steel substrates, plasma was used to spray three different Al2O3:B4C composition ratios at 90:10, 80:20, and 70:30 by weight. Using scanning electron microscopy (SEM), microhardness tests, X-ray diffraction (XRD), and the flash diffusivity method, the impact of B4C content on the microstructure, hardness, porosity, and thermal diffusivity of the coatings was investigated. The plasma spray's characteristics were refined to attain a theoretical density of about 90% [34]. Sintering is one of the most used manufacturing techniques for boron carbide [35]. On the other hand, sintering boron carbide in high densities is challenging to accomplish, but literature shows that if sintering methods are executed correctly, they offer high overall thermal properties [36]. In recent years, hot pressing of sintering additives has also been successful, although it required high temperatures to keep a fully dense boron carbide body [37]. Experimental studies have been presented combining various metallic phases in the boron carbide matrix, but the results could not be attained at the required temperatures without fracturing [38]. On the other hand, when boron carbide was tested with oxide matrix materials as sintering aids (non-reactive sintering), better results were achieved due to their high chemical stability [39]. Alumina oxide proved to increase the sinterability of boron carbide at a higher rate.