Metal Matrix Composites in Industry

A composite material is a mixture of two or more separate phases, which have been intimately bonded together at a scale that is sufficiently fine that the result can be considered a material with properties of its own.

In their most common form, metal matrix composites are combinations of a metal with a ceramic (“cermets” was a term used about 40 years ago to designate metal/ceramic composites of high ceramic content). Other combinations nonetheless exist. Metallic reinforcements are used in several materials that are metal matrix composites by the present definition: tungsten reinforced copper, tungsten heavy alloys, and copper-niobium superconductors are examples. Metal foams can also be viewed as metal matrix composites according to this definition (the second phase is a gas).

By the definition adopted here (Section 1.1), all metal matrix composites are produced by artificially combining reinforcements with a metallic phase, “artificially” meaning that the two phases preexist the material before being combined. “Making a composite” is therefore the act of combining these two preexisting phases, matrix and reinforcement, into a new material, the composite (as illustrated in Figure 1.1). This we call “primary processing” of the composite. It is distinct from “secondary processing” where the composite is deformed, shaped, hardened, machined, coated, or joined (with itself or with another material) after it is created. Preliminary processing steps may also exist upstream of primary processing; these we call “pre-processing”. This of course includes making the reinforcement and the matrix, but may also comprise steps such as arranging these in various configurations. Examples include packing short fibres held together with a silica binder into a preform for subsequent infiltration, or creating powdered matrix cloth using an organic binder before solid-state consolidation with monofilaments.

The following methods were employed for data collection in building the present survey of the metal matrix composites industry:

3M has developed a high performance, high strength, metal matrix composite material reinforced with the 3M’ Nextel’ Ceramic Oxide Fiber 610. This non-magnetic, low-density material has the strength of steel at a fraction of the weight and has excellent performance at high temperature. Products have been manufactured using traditional aluminum alloys, such as 354, 356, and 6061, and also pure Al and Al-2%Cu. Application and desired performance determine the alloy that is used, as well as the vol. % of Nextel fibres. Common pressure infiltration technologies can be used to manufacture the part, including pressure, squeeze, and die casting. After infiltration, heat treatment can be used to harden the alloy. A final machining is done to ensure precision surfaces.

This survey covers 117 companies active in the metal matrix composite industry at the end of 2001 (20 more previously active), producing some 157 materials and 73 products. One can thus conclude without ambiguity that metal matrix composites have entered the engineering world, even within the rather narrow scope of the present survey (given in Section 2.1: these numbers would for example be significantly higher if cemented carbides and diamond tool materials were included). Metal matrix composites have thus reached a first level of industrial maturity.