# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 6 Mechanical and Thermal Properties of CMCs  
CMCs have a wide range of applications in many different sectors. The main aspect of selecting a material for an application focuses mostly on the mechanical and thermal properties of the material. Composite materials, and more specifically CMCs, are materials that can withstand very high temperatures and mechanical loads due to the nature of their base material matrix and reinforcement combination. Therefore, the aero-engine industry has started in the past years to implement CMC parts in both hot and cold sections of an aero-engine, as shown in Figure 6.  
Until now, the CMCs used for aero-engine applications are limited to aluminosilicate or alumina/mullite CMCs from the oxide-CMCs and SiC/SiC, C/C, and C/SiC from the non-oxide-CMCs. The barriers associated with such limited use of CMCs include high ceramic matrix development costs, risks with scale-up and material capability uncertainty, and risks of ceramic fillers and reinforcements robustness, which leads to high costs for material qualification and material availability [181]. Furthermore, the aerospace industry has set strict material requirements and specifications that include the material's mechanical, physical, thermal, and chemical properties, which advocate that for a material to be used, it needs to have high strength, stiffness, fatigue durability, damage tolerance, low density, high thermal stability, high corrosion, and oxide resistance [182]. More specifically,  
Figure 6: Representation of CMC component parts in a gas turbine engine. the most important aspects of mechanical and thermal results are the material's elastic limit and stiffness from mechanical properties and thermal expansion coefficient and thermal conductivity. The figures below demonstrate the comparison of the already classified oxide and non-oxide CMC materials from Sections 4 and 5, respectively, according to the aerospace requirements for mechanical and thermal properties. The output of this comparison allows for drawing conclusions on the differences between the properties of the used CMCs in aerospace applications. The mechanical and thermal properties included in this analysis are compressive strength, specific stiffness, fatigue resistance, fracture toughness, maximum temperature, thermal conductivity, and thermal expansion coefficient. All these attributes play a role in the aero-engine material selection. For comparison, the properties of conventional superalloys used in aerospace applications are included as well. The values of the properties are sourced from the reviewed literature. Figures 7 and 8 compare the compressive strength and specific stiffness of the different CMC materials. Compressive strength allows for an estimation of the load a solid material can handle before fracture, and the specific stiffness allows for a weight to ratio comparison of the solid materials. Although such mechanical properties are not as important for aero-engines as for air structures, it is important to position CMCs compared with the competition. Both properties offer an estimation of the level at which the materials can withstand the maximum load with the lowest density. Figure 8 shows silicon carbide matrix composites outperform the rest of the CMCs and are at the same level as the conventional aero-engine superalloys used. With regards to the specific stiffness, all CMCs outperform the conventional superalloys, with titanium carbide matrix CMCs managing to have an almost eightfold higher performance than conventional superalloys. Figures 9 and 10 compare the fatigue resistance and fracture toughness of the available oxide and non-oxide materials. Fatigue resistance shows the highest stress the materials can withstand for a specific amount of working cycle without breaking and fracture toughness allows for an estimation of how fast cracks propagate and become dangerous. Alumina matrix oxide CMCs outperform the rest of the CMCs and are at the same level as the conventional superalloys used in aero-engines.