# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 1 Introduction  
Materials such as ceramic matrix composites (CMCs) have been the focus of research and being tested in different conditions for several decades now. They are known as a subgroup of composite materials and ceramics. Ceramic composites were developed to control and address problems that occurred with other commonly used ceramics, such as silicon carbide, alumina, silicon nitride, aluminum nitride, and zirconia. Such ceramics fractured with ease, revealing scratches and cracks while mechanical and thermo-mechanical loads were applied to them. To increase hardness and fracture toughness, several methods have been presented. As an example, mono-crystalline whiskers or platelets were added to the matrix, allowing for small improvement with the usage of only ceramic cutting tools [1, 2]. In recent years, some improvements have been implemented with the consolidation of multi-strand fibers, which have proven to be crack, elongation, and thermal shock resistant, allowing for the creation of new applications in the aerospace and manufacturing industry [3]. This combination of materials has been adopted in the aerospace industry, making parts more resistant in extreme conditions enhancing the fracture toughness of thecombined material system, adding higher strength and young's modulus in the matrix, as well as being lighter compared with the alternative, more conventional structures [4,5]. According to the literature, continuous-length ceramic fiber with an elastic modulus is the most used type of CMC reinforcement or filler [6]. This allows the total composite stress to increase, enabling the expended energy during crack dissemination to bridge cracks without fracturing, providing the material with maximum tensile strength. This results in CMC materials being preferred over other conventional technical ceramics that exhibit similar low fracture toughness and low thermal shock resistance but more extreme brittle behavior [7,8]. In the aerospace sector, there have been challenges for the optimization of thermal insulation structures and the design and fabrication of non-eroding firing thrusters which can survive high thermal-structural and thermal chemical combustion environments without the use of a cooling system [9]. Parts, such as the inner surface of an exhaust nozzle, where the propellant flow is accelerated to supersonic conditions, are safety critical components, as the highest amount of shear stress, pressure, and heat fluxes are observed. These conditions result in ablation because of the heterogeneous reactions between hot gas and solid walls, leading to a decrease in the engine thrust [9,10]. On that basis, the most common materials used for this type of application are refractory metal carbides, ceramics, fiber-reinforced plastics, and graphite [10,11]. CMCs are potential alternative materials for such applications. Furthermore, literature shows that certain categories of CMCs demonstrate superior performances under such high temperature and pressure operating conditions without, however, having an overall better performance [12,13,14]. In recent years this has been addressed through the adoption of ultra-high temperature ceramic matrix composite (UHTCMC) materials that exhibit preferable performance in high-temperature environments as well [15,16]. Such UHTCMC materials are composed of C or SiC continuous fibers in basic ceramic matrices [17]. The focus of the present research is on the classification of the available oxide and non-oxide CMC materials. A high-level classification is shown in Figure 1. Furthermore, a literature survey of the CMCs used for aero-engine applications is presented and compared to CMCs that have already been used in aero-engine applications regarding their mechanical and thermal applications. For the critical analysis comparison of the discussed materials, Ashby charts have been drawn to help with the material selection. The results of this survey were focused on creating an available classification of the existing oxide and non-oxide CMC materials.