# Ceramic Matrix Composites for Aero Engine Applications--A Review
## 7 Manufacturing Processes for CMCs
### Selecting the Most Appropriate Manufacturing System  
According to the literature presented in this section, the main fabrication processes used for CMCs include CVI, polymer infiltration, pyrolysis, and hot press sintering techniques. Polymer infiltration and pyrolysis are the most commonly used ones. The main aim is to be able to control the thermal and mechanical properties of the fabricated CMC, but several factors, such as the distribution of the final phase formed and the processing route applied from the synthesis of the composite, are very important aspects. These aspects can be manipulated from the correct choice of composite fabrication processes. Every process has different positive and negative attributes, as shown in Figure 16.  
A major criterion is the price per unit volume of the ceramic matrix and fiber used, as well as the environmental impact of the material (going all the way back to the extraction of the constituting materials, their processing, and delivery to the CMC fabrication plant). This environmental impact can be quantified through the use of embodied energy, which accounts for the energy consumed by all manufacturing processes used. As can be seen in Figure 17, aluminosilicate CMCs have the highest embodied energy as they have some of the more complex material compositions. This indicates that the manufacturing process used to compose this type of CMCs (sol-gel infiltration, sintering) requires a lot of energy and time, increasing the final cost of the production of aluminosilicate-based applications. On the other hand, carbon matrix, alumina oxide, and silicon carbide CMCs have lower embodiedenergy, meaning that these materials are easier to be produced at larger production rates with lower overall costs and faster time scales. This is because their process materials have fewer complex compositions as they include smaller amounts of fibers and matrix materials.  
It must be mentioned that manufacturing processes differ in processing time and processing stages before finalizing the finished material. As a result, the process runs at a higher cost and with longer times than other processes. For example, manufacturing processes that have more complex process technology and complex composite preparation, such as CVI and EPD, are more expensive to run. Although, as mentioned before, the more complex the material composition, the more embodied energy and electrical resistivity are  
Figure 16: Fabrication procedure steps for CMCs.  
Figure 17: Embodied energy and price per unit volume of CMCs.  
spent. Hence it has been found that infiltration processes and, more specifically, PIP, CVI, and MI are less cost-efficient and more time-consuming, although their final composite outcome has very good mechanical and thermal properties [233]. This could also be justified by conducting a comparison of the price per unit volume against the price of the available oxide and non-oxide CMCs, which includes both the material production and manufacturing cost, allowing for an estimation of the final manufacturing procedure's cost outcome. Aluminosilicates are the most expensive composites to produce from both oxide and non-oxide CMCs. On the other hand, alumina oxide composites are much more cost-effective to produce from the oxide CMCs allowing a wider range of materials submerged into the final matrix without having an excessive price increase in manufacturing processes. Finally, carbon matrix CMCs have the lowest production and manufacturing cost as they require the least amount of energy while being cost-effective to extract and purchase.