# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 6 Mechanical and Thermal Properties of CMCs
CMCs, on the other hand, have significantly lower fracture toughness than conventional superalloys; however, this is expected for any composite material. However, if the quality of the final component is closely monitored to avoid the development of any cracks, then this is not a critical parameter for aero-engine components. Figure 7: Comparison of compressive strength vs. specific stiffness. As indicated by [PERSON] et al. [182], materials used in aero-engines need to control thermomechanical processing conditions to minimize material defects. Figures 11 and 12 compare the thermal expansion coefficient against and thermal conductivity of the available CMCs. Figure 8: (**Left**) Compressive strength and (**right**) specific stiffness of available CMCs and comparison with superalloys. Figure 10: (**Left**) Fatigue strength at \(10^{7}\) cycles and (**right**) fracture toughness of the available CMCs and comparison with superalloys. Figure 9: Comparison of fatigue strength vs. fracture toughness of the available CMCs. oxide and non-oxide CMCs and the conventional superalloys used in aero-engines. Both thermal properties are highly considered for the selection of a material for high-temperature applications (e.g., aero-engine). The thermal expansion coefficient gives an estimation of the dimensional transformation of the materials when exposed to high temperatures and at constant pressure, whereas thermal conductivity shows the ability of a material to conduct and transfer heat. The lower the thermal expansion coefficient and thermal conductivity results, the more attractive material for high-temperature applications becomes. As can be seen from Figure 12, all CMCs outperform superalloys when it comes to the thermal expansion coefficient, indicating that structures composed of CMCs will offer higher thermal stability and rigidity. With regards the thermal conductivity, most CMCs perform at similar levels to conventional superalloys. Aluminosilicate matrix composites present the lowest thermal conductivity coefficient, whereas tungsten carbide matrix CMCs are at the other end of the spectrum. This indicates that the potential selection of tungsten carbide matrix CMCs for components might require additional consideration for cooling the components in order to dissipate the heat better (as an example, this could be achieved by using internal cooling channels). The literature review indicated that aluminosilicate CMCs, and more specifically Nextel 720 woven fabric, exhibit the lowest thermal conductivity and thermal expansion coefficient. Figure 11: Comparison of thermal expansion coefficient vs. thermal conductivity of the available CMCs. Figure 12: (**Left**) Thermal expansion coefficient and (**right**) thermal conductivity of CMCs and comparison with superalloys. Figures 13 and 14 compare the maximum service temperature and thermal shock resistance of the available oxide and non-oxide CMCs with the conventional superalloys. Maximum service temperature allows for an estimation of the highest operating temperature material can be used without failing, and thermal shock resistance shows how much temperature change a material can handle without causing any interphase damage or even catastrophic failure. According to research, materials used in aero-engine applications require high thermal shock resistance due to the instant temperature changes. Aluminosilicate oxide matrices, and more specifically Nextel 720 woven fabric, have better overall results as they can withstand temperatures higher than 1000 \({}^{\circ}\)C and over 700 \({}^{\circ}\)C thermal shock resistance. On the other hand, carbon fiber-reinforced carbon matrix composites have the highest service temperature and the highest thermal shock resistance surpassing the performance of the superalloys used in aero-engines.