he use of ceramic matrix composites is growing in the commercial and aerospace sectors.
Hot structures in aerospace applications are designed to maintain mission functionality in high-temperature environments of greater than 3000°F. Examples of such structures include wing leading edges or control surfaces for atmospheric reentry vehicles, as well as propulsion valve components for maneuvering at high velocities.
These applications have traditionally utilized ceramic or metallic materials, such as the silica ceramic used for the tiled thermal protection system on a U.S. space shuttle or rhenium/molybdenum alloys used in propulsion components. More recently, an extensive focus has been placed on the development of ceramic matrix composite (CMC) materials, which provide a more robust performance than traditional ceramics and are less dense than metals. Both of these attributes are highly desirable in aerospace applications.
The aerospace industry has proven the performance quality of carbon-fiber composites through decades of use by taking advantage of the high ultimate strength combined with low density of these materials (relative to metals). The strength of carbon-fiber composites typically increases at elevated temperatures, making them well-suited for use in extreme operating environments.
Carbon-fiber-reinforced silicon carbide, a CMC material, takes advantage of the strength of carbon fiber and offers several additional advantages. The use of fiber reinforcement with the ceramic material maintains the ceramic material’s inherent material hardness and temperature resistance while improving damage tolerance and resistance to extreme thermal shock.
Figure 1. CMC design considerations.
The nature of the fiber-reinforced composite structure allows for significant opportunities to engineer a material design to yield the desired performance. Material properties to consider in taking advantage of a CMC design are:
Each of these material properties can be impacted by the design of the composite material system. Skillful engineering is required to manage the performance trade-off resulting from each design decision. Composite design considerations are included in Figure 1.
The intended application informs fiber material selection, which must in turn be compatible with the ceramic matrix. Carbon fiber is suitable for carbon and carbide matrices (e.g., silicon carbide, SiC), and oxide fibers are compatible with oxide matrices (e.g., silicon dioxide, SiO2; aluminum oxide, Al2O3; and alumina silicate, x Al2O3 y SiO2). Oxides are more desirable for their insulating and dielectric properties, but carbides are more applicable in extreme temperature environments. Once the material family is selected, several additional decisions must be made to define the fiber.
Carbon fiber preforms provide many options related to fiber composition, size or denier, and in situ performance. Carbon fibers are available as either pitch- or polyacrylonitrile-based (PAN) materials. Pitch fibers typically offer higher density and thermal and electrical conductivity than their PAN counterparts. PAN fibers, however, are often more amenable to the preform fabrication process. In addition, the production of PAN fiber is not impacted by the sometimes-uncertain availability of pitch material.
Typical carbon fiber tow sizes range from 1000 to 12,000 filaments per fiber end. These sizes may be increased by bundling fiber ends together, or they may be decreased via a process known as stretch-breaking, which allows more versatility and options in preform design. Finally, the fiber grade (or modulus) impacts the inherent mechanical strength properties of the fiber. Carbon fibers are typically supplied in standard-, intermediate-, and high-modulus grades that result from the temperature used in the fiber manufacturing process, among other factors.
Figure 2. 3- and 4-D preform architectures.
Preform architecture selection is based on required performance, component design complexity and cost. Fabric-based laminate constructions are often utilized for large-area aerospace applications such as a fuselage or aeroshell. For near net-shape applications like these, tape-wrapping is also used. For bulk material applications where performance is not paramount, the press-molding of chopped fiber lengths is another common approach.
These constructions are cost effective but do not provide continuous fiber reinforcement through the thickness of the preform. Tri-axially braided constructions can produce thin-walled near-net-shape preforms. Though braided designs are laminate in nature-without reinforcement through the thickness of the component wall-the high fiber volume and combination of axial and helical fiber result in a very high-performing composite.
Extreme performance applications, such as control surfaces or propulsion components, benefit from more highly engineered preforms. Three- and four-directionally (3- and 4-D) reinforced woven preforms offer the greatest potential mechanical performance and can be tailored through fiber spacing and preferential fiber loading to focus specifically on the requirements of the use environment.
These 3- and 4-D architectures feature two or three directions of fiber reinforcement in-plane and additional fiber through the thickness or “Z” fiber direction. 3-D preforms are produced in an orthogonal Cartesian or cylindrical, polar coordinate design, whereas 4-D architectures are produced in a hexagonal array. Figure 2 illustrates these design options.
Fiber Interface Coating
In fiber-reinforced ceramic materials, the fiber interface coating provides an essential compliance layer between the fiber and the brittle ceramic or glassy matrix. The coating both protects the fiber from being damaged by reaction with the matrix and allows the fiber to strain independently from the ceramic. This boundary between fiber and matrix is key to realizing the performance potential of the combined composite material system design characteristics.
Pyrolytic carbon (PyC) coating is utilized for many carbon-fiber applications, while boron nitride (BN) is typically used with silicon carbide fiber in aerospace applications. Other coating compositions may be selected for different fiber/matrix systems.
Chemical vapor infiltration and deposition (CVI and CVD, respectively) are extremely effective at providing uniform micron-level coating thicknesses on each fiber filament within a preform. These methods can be costly and may result in undesirable gradients as part thickness increases.
For larger carbon fiber CMCs, a partial carbon matrix provides sufficient protection for the fiber with an increased starting density. This method is a useful means of tuning the relative composition of carbon and ceramic in the final composite. The ratio of carbon to ceramic matrix will influence the material’s performance and allow for an optimal balance of capability for the intended application.
Following the preform interface coating process, the matrix is introduced to the composite. CVD, melt-infiltration (MI), and pre-ceramic polymer infiltration and pyrolysis (PIP) are three manufacturing methods that are commonly applied in the aerospace industry. In order to retain structural integrity at high temperatures and resist oxidative erosion, the matrix between the reinforcing fibers should be ceramic, glass or glass-ceramic.
The amorphous glass and crystalline ceramic phases each offer performance benefits and drawbacks. For example, an amorphous matrix requires lower maximum process temperatures with shorter cycle times and results in a less porous composite. However, a coarse crystalline ceramic will be more resistant to oxidation.
A well-balanced glass-ceramic can be designed to provide the benefits of both phases. The glass-ceramic is more resistant to impact as it is less susceptible to the cracking of glass matrix or the grain boundary cleaving of the crystalline form. Silicon carbide can exist in all three morphs, as controlled by processing conditions. Pre-ceramic polymers and slurries lend process options to allow for homogeneous or graded matrices.
Figure 3. CMC fabrication process.
The majority of bulk material properties are determined by the preform, interface coating and ceramic matrix densification processes. The densification process is often directed to a targeted minimum density and maximum porosity level with the intent of achieving desirable performance. Often, relevant material properties and characteristics are quantified after the completion of ceramic densification in the manufacturing process. The exacting standards in aerospace manufacturing may require extensive testing to ensure that performance requirements are met prior to final component fabrication. Figure 3 outlines the CMC fabrication process flow.
After the bulk properties of the material have been verified, the final steps toward component production are performed. Machining is required to yield finished hardware for those preform designs, which produce large blocks or billets of material. Other preform constructions may require little to no machining, as they are processed in a form resembling their final design.
Environmental Barrier Coating
The application of an environmental barrier coating (EBC) to the surface of the composite provides yet another opportunity to influence the behavior of the material system in its operating environment. An EBC may be applied to create a higher-density, lower-porosity ceramic surface while maintaining a less-dense, less-conductive bulk composite.
While an EBC can provide a different performance from the bulk composite, it is advisable to select EBC/matrix systems that are process-compatible and thermally well-matched in order to avoid separation or spallation at use temperatures. The method of applying the coating (e.g., CVD or slurry coating) is also crucial to achieving thickness requirements and adherence of the EBC to the substrate in order to retain the performance benefit. A surface coating may also be ground or polished to a finer surface finish, which is critical to some applications.
CMC material use is growing within the commercial and aerospace sectors. High-performance brake rotors are valued for their capability to withstand high temperatures and friction. Propulsion valve components that provide steering in rocket motor systems are being designed and developed to function in environments that are both hot and moisture-rich. Carbon-fiber reinforced CMCs are being actively employed as an alternative to traditional monolithic materials in applications requiring high strength combined with reduced mass and thermal conductivity.
With myriad design and fabrication options available to the CMC manufacturer, understanding the component operating requirements must be coupled with skillful composite engineering to take advantage of this material system’s versatility. Hot structure applications benefit from the mechanical strength, oxidation resistance and damage tolerance of these lightweight materials. As advances are made in the development of new matrix precursors, performance in more demanding operational environments will become achievable.