# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 8 The Future of CMCs
### CMC Adoption Roadmap
Furthermore, CMC materials have been used for applications in the hot section of a gas turbine engine for the combustor inner and outer liner, the HPT stage 1 and 2 shrouds, and nozzles during the years 2016 and 2020 [245]. Some of the most common CMC materials used for those types of applications are SiC/SiC, C/SiC, and C/C components produced with melt infiltration process, chemical vapor infiltrations, and pyrolysis. These manufacturing processes allowed for the creation of a denser matrix,  
Figure 20: CMC requirements needed to be met for wider adoption. allowing a relatively uniform distribution of separately coated filament fibers. In addition, new CMC material facilities have been established since 2019, producing SiC fiber CMC parts for jet engines and advanced ceramic coating made from oxide layers [246]. Continuous fiber composite ceramic materials are CMC materials made from high-strength ceramic fibers in a ceramic matrix, where the interface coating between the fibers and the matrix protects the surface of the composite from degradation. The composition methods and the available materials for creating a variety of this type of CMCs are very limited due to the lack of understanding of the relationship of fibers, matrix, and coatings, as well as the properties and application environments in which these materials can operate. Thus, more failure models and mechanisms must be created and explored to optimize CMC materials [247]. Although, prior to the development of life expectancy models, extensive CMC fabrication and testing must be undertaken to have a better understanding of the micromechanics of CMCs when exposed to high-stress and temperature application environments. For the testing of the composite combinations, non-destructive evaluation techniques are required for in-line process control and post-process testing to check for incipient failure and the remaining life of the composite [248]. For composite materials to pass the non-destructive evaluation techniques, the optimization of fabrication processes must be achieved. CFFC materials are highly reliable and can avoid catastrophic failure due to the reproducibility of their fabrication processes and the low cost of fibers and composite matrix materials. More materials should be tested and  
Figure 21: CMC materials, coatings, and manufacturing processes for aero-engine applications Roadmap (the same key as Figure 20 applies). processed to create CFFCs that will meet the aerospace application needs, allowing also for fabrication scale-up and cost reduction. Figure 22 shows that CMCs outperform conventional aero-engine materials, such as metal and polymer matrix composites (MMC, PMC). They also have overall better mechanical and thermal properties than metal, polymer, and natural matrix composites, with the maximum service temperature values being significantly higher (\(\leq\)500 \({}^{\circ}\)C). CMCs that are able to operate between 1000-1500 \({}^{\circ}\)C are used for oxidation protection layer in aero-engine applications. These materials belong mostly to the alumina oxide and aluminosilicate CMC material families. On the other hand, materials that surpass 1500 \({}^{\circ}\)C are mainly used as thermal insulation layers protecting the components used mainly in the hot section of a turbine engine. These CMCs are mainly SiC/SiC, C/C, C/SiC composites as well as alumina oxide matrices (Al2O3/B4C, AlTiO2, Al2O3/TiO). Furthermore, the use of Oxide CMCs in exhaust nozzle applications has shown improvement in component durability against titanium alloys, allowing for lighter weight and equal durability. A great effort has been made regarding manufacturing and design processes for composite materials, where many manufacturing steps (lay-up, vacuum bagging, etc.) have been updated with different manufacturing methods (CVI, PIP, MI, Sol-Gel, etc.) where manual labor cost is avoided, and further cost saving have been achieved [249]. Composite components are being fabricated and designed for affordable and robust manufacturing.