Refractory ceramic matrix composites (CMCs) are of considerable interest for a range of high-temperature and severe environment applications, including space and defense, and have potential for commercial and industrial applications that impose extreme thermal and environmental demands on materials. Among the more mature CMCs are silicon carbide fiber-reinforced silicon carbide matrix (SiCf/SiC) CMCs which, after decades of development, are being commercially introduced for aircraft engine component applications. These SiCf/SiC CMCs are limited to use at temperatures considerably less than 1,480ºC (2,700ºF), which constitutes a dramatic gain over conventional superalloy use temperatures.

The success of the current generation of CMCs has led to an Air Force-sponsored effort to push the use temperature to 1,480ºC (2,700ºF) or above. Parallel developments are underway to advance capabilities to “ultra-high” temperatures—significantly greater than 1,650ºC (3,000ºF).

Improving Capabilities

A range of benefits is driving the motivation for the use of these materials:

•  Gains in efficiency (e.g., enhanced fuel economy) traceable to the high-temperature operation of aircraft engines with reduced requirements for cooling air to maintain acceptable component operating temperatures

•  Weight reduction of components as a result of the relatively low density of the composites

•  Enabling selected applications in which conventional materials temperature constraints preclude their use in the operating environment.^1-3

A number of monolithic ceramic materials, while providing high-temperature capability and potentially acceptable operating environment resistance, lack the requisite toughness to meet design and use requirements.

Continuous fiber-reinforced refractory CMCs address many of these limitations. Typical fiber reinforcements include a range of non-oxide ceramic fibers (such as SiC), oxide ceramic fibers (generally alumina or silica-alumina based) and carbon fibers. For temperatures above those achievable with SiC matrices, ceramic matrix materials include a number of options, including materials from the family comprising hafnium-zirconium-silicon-carbide (Hf-Zr-Si-C) and their respective nitrides and borides.¹

For ultra-high temperatures, oxide fibers are limited in applicability predominately due to low elevated-temperature strength and poor creep resistance. SiC ceramic fibers are also limited to < 1,480ºC (2,700ºF) as previously noted, leaving carbon fibers as the primary commercially available fibers of interest for ultra-high temperatures.

Manufacturing Effective Composites

Accordingly, to address extreme temperature demands, work has been focused on carbon-fiber-reinforced refractory CMCs including, but not limited to: C/ZrC, C/HfC, C/Zr-Si-C, and C/Hf-Si-C. Various processes, each with limitations and benefits, have been used for producing such composites, including chemical vapor infiltration (CVI), pre-ceramic polymer infiltration and pyrolysis (PIP), and melt infiltration (MI) of the matrices into the reinforcement fiber preforms.

CVI provides high-purity matrices. Precursors for CVI densification are relatively costly and process times are lengthy, leading to high composite cost. In addition, even with extended infiltration times, considerable residual porosity remains within the completed CVI composites.

Individual PIP cycles are faster and typically less costly than CVI cycles; however, achieving acceptable density requires multiple iterations of the PIP process, which leads to extended process times. PIP precursors, especially for SiC matrices, are commercially available but can be costly. Also, the pyrolysis step must be performed in a controlled atmosphere to achieve the desired matrix composition (comprising a mixture of Si-O-C-N depending on details of the precursor and the processing conditions).

MI processing is rapid, requiring only a few hours to achieve nearly full density, and starting materials require only the availability of the respective metal or alloy (e.g., Zr metal for ZrC CMCs), which is generally cost- effective. However, the MI process does expose the fiber preforms to temperatures at or above the melting temperature of the respective alloy. The process thus calls for high-temperature process equipment and requires that the reinforcement fibers be protected from the melt during the MI process. In addition, not all of the metal reacts in situ during infiltration such that the matrix comprises a mix of the metal carbide and the unreacted metal.

In CMCs, a fiber interface is essential to achieving the desired mechanical performance. The interface is needed to provide limited slip between the fibers and the matrices, resulting in pseudo-ductile behavior of the composite and higher toughness than that afforded by the monolithic ceramic matrix, as shown in Figure 1. For MI CMCs, the interface may also need to protect the fibers from degradation or consumption by the incoming molten metal. Selected interfaces may also serve to promote wetting and infiltration of the molten metal into the preform porosity.

Clearly, ultra-high-temperature-capability refractory MI CMCs are complex and evolving materials. Research and development has advanced MI refractory CMC processing and demonstrated such materials in potential application environments.

MI Refractory CMC Processing

MI processing of refractory CMCs follows a process sequence as depicted schematically in Figure 2. In the first step of the process, a suitable fiber preform is procured or prepared. Depending on the details of the application and properties requirements, the preform may consist of woven or braided fabric layers in a two-dimensional (2D) fiber architecture, a pierced fabric or other three-dimensional (3D) architecture, or a possible multi-D architecture, etc. Fiber type and architecture are keys to achieving desired properties, which are relatively predictable based on composites models such as discussed by Kainer.²

Suitable fiber interface coatings are applied onto the fibers in the second step (or may be coated onto the fibers prior to laying up the preform). Such interfaces serve various needs, including promoting wetting and infiltration, preventing damage to the fibers by the incoming melt and/or providing slip and improved mechanical behavior. The preform is then partially densified with reactive carbon using proprietary processing. The amount, type and distribution of carbon are controlled to optimize reaction with incoming molten metal.³ The target is typically to achieve a high degree of conversion to the carbide such that the matrix exhibits predominately ceramic behavior.

The matrix is then melt infiltrated using the selected metal or alloy to achieve the desired matrix chemistry. Preferably, the preform is wetted, and infiltration is achieved without application of pressure, although pressure infiltration is also feasible. During the infiltration step, reaction with the previously added reactive carbon occurs, such that the resultant matrix constitutes the selected metal carbide, for a carbide matrix CMC. In one typical example, reactive carbon is added to a carbon fiber preform and zirconium metal is infiltrated to create a Cf/ZrC CMC.

It should be noted that the fiber preform may be prepared as a simple geometric structure (e.g., a block or billet) from which desired parts can be excised. Alternatively, and often preferably, the preform will be produced as either a net- or near-net shape to minimize the need for machining, to improve on infiltration and to minimize waste. A near-net-shape fiber preform for an axisymmetric component, in this case a CMC rocket engine combustion chamber, is shown in Figure 3.

For certain MI composites, ultraviolet chemical vapor deposition (UVCVD) can be and often is used for the deposition of the fiber interface coatings prior to infiltration with the molten metal. The UVCVD process provides for uniform coating of a number of interface materials onto the individual filaments within the fiber tows, as shown in Figure 4.

To date, using variants of the above-described process, a number of ultra-high-temperature-capability CMC formulations have been produced, including ZrC, HfC, Zr-Si-C and Hf-Si-C ceramic matrices. One example of such a composite is shown in Figure 5, in which the nearly fully dense matrix has surrounded and partially infiltrated the carbon fiber bundles, or tows.

The selection of the matrix material is based on the application needs. For example, HfC matrices provide preferred performance at extreme temperatures in oxidizing environments. In such applications, the outer surface of the HfC converts to its respective oxide (HfO₂) which, under very high temperature conditions, is tenacious and relatively protective. However, HfC is relatively dense. For slightly lower temperatures in oxidizing environments, a matrix such as ZrC is preferred. The ZrC has a lower density than HfC, and the outer ZrO₂ that forms during service has a service temperature approaching that of HfO₂. For oxidizing environments that do not require the ultimate temperature capability of HfC or ZrC, matrices with some added Si (which forms SiC) have enhanced intermediate temperature oxidation resistance and have lower density than HfC or ZrC without the added SiC.

An added benefit of the MI process is that the melt infiltrates readily and quickly into even, thick cross-sections, resulting in high-density, thick structures without the need for lengthy infiltration times. One such example, a nominally 2.5 cm (1 in.) thick panel, is shown in Figure 6. Other structures have been infiltrated to thicknesses of 6.5 cm (2.5 in.) while achieving low porosity in the matrix.

MI Refractory CMC Properties and Behavior

As previously noted and described by Kainer and others, the properties and behavior of the MI CMCs are largely predictable based on constituent level models that analyze and predict composite behavior based on the fiber reinforcement type and fiber architecture (fiber orientation and relative fiber loading) and matrix properties.² Properties of selected CMC materials of interest are presented for reference in Table 1.

The Zr, Hf and Si carbides provide high-temperature capability (as discussed) but are subject to the limitations of their respective oxides (also noted in the table) resulting from high-temperature oxidizing environment use. Iridium (Ir), which is known to provide ultra-high-temperature oxidation protection, and rhenium (Re), which provides ultra-high-temperature structural capability, are included particularly to show the lower density achievable with the CMC while maintaining high-temperature performance capability. Although not shown in the table, the density of carbon fibers is about 2 g/cm³. The incorporation of the carbon fiber reinforcement additionally contributes to lowering the composite density relative to that of monolithic materials.

The presence of residual metal in the MI CMCs, while lowering the peak use temperature relative to that of the pure ceramic matrix, imparts some metallic behavior and added toughness to the matrix. This has been demonstrated by high-speed particle impact testing. Both high-speed water drop and nylon bead impact testing have been successfully performed, showing the excellent impact resistance of the MI CMC.

As a result of the ultra-high-temperature capabilities exhibited by these materials, they have potential for and have been demonstrated to be effective in a number of extremely demanding applications. MI CMC rocket engine combustion chambers have been successfully tested at temperatures in excess of 2,200ºC (4,000ºF) in an environment that is oxidizing with respect to the ZrC chamber material. Following the testing, the chamber was cross-sectioned, revealing the formation of a tenacious surface oxide at the chamber inner wall and showing that the oxide only penetrated about one laminate layer deep into the composite structure.

Conclusions

Melt infiltration processing provides a rapid, cost-effective alternative for the fabrication of ultra-high-temperature-capability CMCs. By control of the fiber reinforcement type and fiber architecture and the matrix chemistry, MI refractory CMCs have been produced with a range of properties suited to a number of demanding applications, including potential space, defense, commercial and industrial applications.

Control of the carbide content in the matrices is particularly well-suited to achieve a range of matrix characteristics from predominately or solely ceramic in nature (i.e., for a fully carbide matrix) to a mix of ceramic and metallic behavior. The use of refractory ceramic matrices that form self-protective and tenacious oxide surface layers during service provides for materials that are durable in ultra-high-temperature oxidizing environment applications. Work on these and related materials is continuing in a number of related programs.  

References

1.         Bull, J., White, M.J., and Kaufman, L., U.S. Patent 5,750,450, “Ablation Resistant Zirconium and Hafnium Ceramics,” May 12, 1998.

2.         Kainer, K.U., “Basics of Metal Matrix Composites,” in Metal Matrix Composites: Custom-Made Materials for Automotive and Aerospace Engineering, K.U. Kainer, ed., Wiley, Weinheim, Germany, 2006.

3.         Margiotta J.C., et al, “Formation of Dense Silicon Carbide by Liquid Silicon Infiltration of Carbon with Engineered Structure,” J. Mater. Res.23, 5, 2008, pp. 1237-1248.

4.         “Engineering Property Data on Selected Ceramics, Volume III: Single Oxides,” MCIC-HB-07, Volume III, Metals and Ceramics Information Center, Battelle Columbus Laboratories, Columbus, Ohio, July 1981.

5.         “Engineering Property Data on Selected Ceramics, Volume II: Carbides,” MCIC-HB-07, Volume II, Metals and Ceramics Information Center, Battelle Columbus Laboratories, Columbus, Ohio, August 1979.

 6.         Materials Engineering Materials Selector 1993, Penton Publishing, December 1992.