Metal-matrix composites

Thursday, August 11, 2011

Tags :  advanced-materials

advanced-metalsThere are a wide range of metals used in the modern race engine. Where regulations are sufficiently liberal, we may find an engine containing everything from aluminium, magnesium and steel to titanium and tungsten. In many ways the materials behave very differently but in others their specific properties, especially specific modulus (elastic modulus divided by density), are very similar. For example, a typical aluminium alloy has a modulus of 70 GPa and a density of 2.7 g/cc, giving a specific stiffness of 25.9 GPa/(g/cc). If we repeat the exercise with steel, magnesium and titanium, we find very similar answers.

There are certainly some special alloys that can improve on this, but very little that would come close to breaking the FIA-imposed limit of 40 GPa/(g/cc) for metallic materials in Formula One.

If there were no such 'glass ceiling' imposed by regulation, and we wished to maximise specific stiffness, we would probably need to turn to metal-matrix composites (MMCs). As the name suggests, these are materials where a reinforcement material is held within a metallic matrix. Most such materials are based on finely divided discontinuous reinforcement using a number of ceramic materials, although ceramics are not exclusively used as the reinforcement or filler.

A number of these materials are commercially available, with at least one commercially available aluminium MMC exceeding 48 GPa/(g/cc). Where the mass of a component is determined by considerations of stiffness, such materials are capable of realising much lower mass components.

The strength levels of materials can be significantly raised, too. For example, there is an aluminium MMC with an ultimate tensile strength of 1300 MPa (188 ksi) - more than double that of an exotic 'conventional' aluminium alloy, and three or four times that of a typical material. This level of strength is exceptional, even for an aluminium MMC.

However, we should not assume that improvements in specific stiffness and strength are the only gains to be had. MMCs can also offer improvements in other properties - they are used to improve thermal conductivity, even for materials where the conductivity of the matrix is already good, for example in copper or aluminium. While it is common for people to think of ceramics as good thermal insulators, this is a generalisation that does not hold true; for example, the thermal conductivity of aluminium can be improved by adding certain ceramics.

So, where can we find MMC materials used in current race engines? Aluminium MMCs have found use in pistons, con rods, and valve spring retainers. An MMC aluminium connecting rod has been covered in these Monitor articles in the past, with the rod further distinguishing itself by running with neither a bearing shell nor a roller bearing. MMC steel has been used in piston pins, and MMC titanium is thought to have been used in at least one Formula One engine for the same purpose in the past. Prototype MMC inlet valves were tested some years ago, but were not successful.

If Formula One and other big-budget race series such as NASCAR had less restrictive material regulations, we would see more use of MMCs, and their use would possibly become more widespread in other series as a consequence.

Fig. 1 - MMC pistons would perhaps be more widely used if motorsport material regulations were more flexible (Courtesy of AMC-MMC)

Written by Wayne Ward

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