MMCs & CMCs
Over the last decade, significant advances have been made in metal matrix composites (MMCs) and ceramic matrix composites (CMCs). These advances have occurred in both the synthesis of the ceramic reinforcements—including SiC, Al2O3, and alumino-silicate in powder, fiber and/or whisker forms—and in the methods used to form the ceramic reinforced composites. Two of the more commonly used composite systems are aluminum (Al) MMCs and alumina (Al2O3) CMCs. Within these two systems, aluminum MMCs allow significant weight reduction in automotive and military components, while alumina CMCs offer improved toughness and wear resistance required in high-performance cutting tool inserts (see Figure 1).
MMCs have entered the marketplace in lightweight brake rotors; brake drums and driveshafts; high-efficiency diesel pistons; and various recreational applications, including bicycle frames, golf clubs and horseshoes. Applications under development include lightweight tank track shoes, missile structures and advanced engine components. CMCs have entered the marketplace in cutting tool inserts for machining nickel based superalloys, precision dies for forming aluminum beverage cans, and advanced armor. CMC applications under development include various automotive and aerospace components.
The success of these materials is typically the result of a lengthy development cycle, which includes component design and modeling; matrix selection; reinforcement selection, including placement in the matrix; and method of composite fabrication.
Development CycleComponent Design and Modeling
MMCs typically replace monolithic materials, which can have quite different material properties. This forces engineers to redesign the components rather then performing a one-to-one geometric substitution. For Al MMCs replacing cast iron or forged steel components where weight reduction is desired, the component must be redesigned based on the reduced strength of aluminum compared to steel. Although the reinforced Al MMC possesses higher strength, wear resistance and stiffness than the base aluminum alloy, the ductility is decreased. This makes design of a reduced weight Al MMC component critical if the component is to perform the same as or better than its steel counterpart.
CMCs typically replace other ceramic materials with the intention of increasing the component’s fracture toughness. Alumina reinforced with silicon carbide whiskers (SiCw) replaces tungsten carbide (WC) cutting tool inserts for machining Ni-based superalloys such as Inconel, Waspoloy and Hastelloy. Alumina/SiCw CMCs generally are not used for machining ferrous-based alloys because of a reaction between the SiCw and iron at the high cutting temperatures.
Design considerations for the alumina/SiCw are related to how much SiCw reinforcement is required, and how to densify the composite without forming a strong bond between the SiCw and the matrix (which is a prerequisite to increase the CMC fracture toughness).
Matrix selection is important in both MMCs and CMCs to ensure proper interface between the matrix material and the ceramic reinforcement, as well as to optimize composite properties. In Al MMCs, the matrix should bond strongly with the reinforcement but should not be chemically affected by adverse reactions. Proper matrix selection will promote part formability by wrought or casting processes.
In CMCs, the matrix should not bond strongly with the reinforcement; if it does, fracture toughness and the resulting strength enhancements will not be realized. Matrix powder reactivity and sintering additives must be carefully controlled to avoid reaction with the reinforcement. In SiCw/ Al2O3 CMCs, boron nitride or carbon coatings are sometimes applied to the SiCw to prevent reaction with the matrix and allow debonding and energy dissipation to occur, which promotes fracture toughness.
Proper ceramic reinforcement selection is critical to achieve the required composite properties. Several reinforcement materials can be found in particle, fiber, chopped or milled fiber, and whisker forms. Typical particle reinforcements include SiO2, Al2O3 and SiC particles that are used to reinforce the entire matrix material. These materials are relatively low in cost and are available in several particle sizes.
The next group of materials is often referred to as “white fibers” and includes alumino-silicate fibers and alumina fibers. These materials are available in both fiber and chopped or milled fiber form, depending on whether anisotropic (exhibiting different values when measured in different directions) or isotropic (exhibiting the same values when measured in different directions) properties are desired in the composite. These medium-performance materials cost more than particles but offer engineers the ability to selectively reinforce the matrix by forming the fiber into a porous preform shape.
Another reinforcement is SiC whiskers (see Figure 2)—single crystal acicular shaped particles that possess the highest strength and modulus of all reinforcements. This is currently the highest-cost reinforcement, but it offers engineers the optimal strength, modulus, fatigue resistance and wear at a given reinforcement level. The use of preforms and selective reinforcement can provide a cost effective solution when using SiC whiskers.
Introduction of Reinforcement in Matrix
Reinforcement materials can be introduced into the matrix in different ways, including blending of the reinforcement throughout the matrix material prior to consolidation, or adding shaped forms—called preforms—before consolidation.
In the blending approach, reinforcement particles are uniformly dispersed in the matrix by stirring in molten aluminum for the manufacture of Al MMCs. The particles are slurried with alumina and spray dried for the manufacture of Al2O3 CMCs.
In the preform approach used for Al MMCs, reinforcements, typically in the form of fibers, chopped fibers or whiskers, are blended with low and high temperature binders and formed into the desired selective reinforcement shape or preform using vacuum forming, pressing or injection molding forming techniques (see Figure 3). Vacuum forming is the most common method for manufacturing simple shaped preforms, such as the plates/disks, rings or cylinders used in the manufacture of Al MMCs for pistons and cylinder liners. Pressing of plastic or granulated reinforcements is currently being developed to make more complex preform shapes required for new applications. Injection molding has also been used to some extent to make very complex preform shapes, but preform density is limited based on the need to maintain a flowable plastic body, which is then heated or cooled to provide adequate green strength for removal from the die without distortion.
Various methods are used to form composites, depending on the final part geometry and property requirements.
For Al MMCs, particulate reinforced parts can be formed by both wrought and casting processes, and the entire part is reinforced. For fiber or whisker reinforced parts, pressure or vacuum/pressure casting processes are used. Lower-cost squeeze casting is becoming a more commonly used method of fabrication. In this method, reinforcement preforms are placed in the steel squeeze cast die where they are desired in the final part, the die closes, and molten aluminum is pumped in and pressurized during a controlled solidification process. Selectively reinforced MMCs made with this process exhibit high tensile and fatigue strengths based on a fine grain microstructure and limited microporosity.
For Al2O3 CMCs, hot pressing is typically used to obtain full densification at low temperature and low levels of sintering aid. This minimizes the reaction that can occur between the matrix and the reinforcement. Sinter-hipping—where the part is pressureless sintered to closed porosity then gas hipped to full density—has also been successfully used in some applications.
MMC PropertiesProperties for MMCs depend on the matrix metal and the volume percent of composite reinforcement, which are optimized for specific applications. As shown in Figure 4, Al MMCs using different volume percentages of SiCw generally indicate significant improvement in mechanical properties over a base alloy, such as A356 T6.
Fatigue testing suggests that A356 reinforced with SiC whiskers may be useful for applications requiring higher performance aerospace alloys (see Figure 5). Wear test results (Figure 6) show dramatic reductions in wear depths when proper volume percent of SiC whiskers in aluminum are used, compared to forged steel and austempered ductile iron (ADI).
CMC PropertiesAs stated previously, one of the more common CMCs uses alumina (aluminum oxide or Al2O3) as the matrix. Alumina possesses chemical inertness and hardness, but it is brittle and does not have sufficient toughness and thermal shock resistance required for milling and turning operations. By adding SiC whiskers, the properties of alumina can be enhanced, resulting in a material ideally suited for high-speed machining processes. Figure 7 shows the result of reinforcement with SiCw, nearly doubling flexural strength and fracture toughness. Normalized to Al2O3, resistance to thermal shock also doubles when SiC whiskers are introduced.
Paced by availability of affordable SiC whiskers, practical uses for MMCs are emerging for a variety of applications where lower weight and increased strength or durability are critical factors for improved operational performance.
Military tank track shoes – Under sponsorship of the U.S. Army Tank-automotive and Armaments Command (TACOM), Al MMC track shoes are being developed for military land and amphibious tracked vehicles. Using selective reinforcement SiC whisker technology (see Figure 8), weight savings of up to 25% are being realized over the existing forged steel track shoes, which weigh nearly three tons per set. Testing to date indicates that Al MMC wear in the SiCw reinforced areas is comparable to steel. The testing also suggests the potential for increasing shoe bushing life by up to 200%. This is attributable to the improved heat dissipation characteristics of the Al MMC track (see Figure 9).
Aerospace structures – Lightweight, near-net shape Al MMCs are being developed for critical missile support structures to replace higher-cost, heavily machined titanium. Al MMC SiCw’s low CTE and high modulus make it an acceptable alternative to titanium for use in the demanding aerodynamic environment.
Electronic substrates – SiC particulate aluminum MMCs are being used in electronic substrate applications, where the MMC serves as the heat sink for a silicon device. The MMC has a coefficient of thermal expansion (CTE) closer to that of silicon, reducing the stresses that lead to device cracking or debonding, and offers high thermal conductivity for enhanced heat dissipation.
Pistons – Use of Al MMC in the dome region (see Figure 10) enables operation at higher cylinder firing pressures, particularly important in advanced high performance gas and diesel engines. The ability to selectively reinforce areas such as the ring belt can reduce wear rate and noxious emissions with no penalty in reciprocating mass.
Cylinder liners – Selective reinforcement of the cylinder bore for cast aluminum engines would provide superior thermal conductivity and durability performance compared to traditional cast iron liners. Blending of SiCw with a secondary reinforcement of nickel powder or graphite could also improve sliding characteristics.
Disk brake rotors – SiC particulate has been used for homogeneous reinforcement of the rotor. Activities are ongoing to further improve wear characteristics using fiber and whisker preform technology.
Driveshafts and torque tubes – Use of MMCs for driveshafts and other components responsible for transfer of torque enable them to be lighter and stiffer. MMC driveshafts also reduce noise resulting from vibration.
Recreational and sports – Bicycles, golf clubs and racing horseshoes are among the “recreational and sports” applications for MMCs. Applications for these categories are driven by their light weight and wear resistance features.
The intrinsic hardness and thermal dissapative properties of CMCs are revolutionizing machining operations, enabling faster cutting and longer tool life. These same properties are spawning new CMC applications where heat, friction and fracture toughness are often limiting factors. CMC applications include:
Cutting tools – The largest application for CMCs is SiCw reinforced alumina cutting tool inserts used to machine nickel based alloys. Specifics of this application were discussed earlier under CMC properties.
Wear parts – Alumina and silicon nitride reinforced with SiCw have also been used for punches and dies used in the manufacture of aluminum beverage cans. Such composites must have high strength, high fracture toughness, good wear resistance and the proper friction coefficient against the aluminum sheet.
Engine components – Alumina, silicon nitride, sialon and silicon carbide reinforced with SiCw have been investigated for various engine components, including turbocharger rotors, valves, pistons, piston liners and camshaft lifters. Such composites must have high strength, high fracture toughness and good wear resistance, and be designed to be compatible with the metal counterparts at cold and operating conditions.
Armor – CMC armor has been produced with numerous combinations of materials to obtain a system that exhibits the desired response under specific threats. Such systems have been successfully demonstrated on military aircraft and ground systems.