Metal matrix composites

Metal matrix composites (MMCs) were developed in two successive waves from 1960-1965. Major research efforts were carried out in the USA and France in the 60s around a boron monofilament fiber, without any real subsequent industrial development. This metal-metal composite was penalized by the very high cost of the fiber. At that time, the applications envisaged were exclusively oriented towards the aerospace industry. In the 1980s, the availability of numerous new ceramic fibers revived research in this field, with more encouraging prospects for industrial development. Examples of industrial applications were developed in the automotive sector, at the initiative of Japanese industry.

These materials are still evolving, and the properties that can be expected from them are only partially known; they are far superior to those of metal alloys. In the case of metal-reinforced composites, these properties are very often accompanied by a reduction in ductility compared with metal alloys. This characteristic needs to be taken into account in part sizing rules, and in many cases a new part design will be required. However, the addition of several types of reinforcement (of complementary sizes and properties) can alleviate this problem, inducing an increase in several physical properties that would be impossible with the addition of a single type of reinforcement.

Manufacturing processes can be divided into three main categories, allowing us to modulate reinforcement levels and distribution, and to control the nature of interfaces or interphases. There is no ideal method for processing metal matrix composites, but rather a need to be able to choose the method to suit the microstructure and properties required. That's why it's important to be familiar with the full range of forming techniques available for these materials.

The use of a metal matrix in a composite offers several advantages over organic matrices:

• improved temperature resistance, extending the limits of use in hypersonic structures and engine environments;

• better physical properties (e.g. mechanical, thermal, electrical) intrinsic to the matrix, enabling localized or unidirectional reinforcement;

• better resistance to ageing and fire.

On the other hand, manufacturing technologies for metal matrix composites are generally more complex than for organic matrix composites. During manufacture, good cohesion must be ensured at the interfaces between matrix and reinforcement, without degrading the latter. The chemical fiber-matrix bond must be achieved with a matrix in liquid, solid or semi-liquid phase, without degrading the intrinsic properties...