There are two main types of metal matrix composite (MMC), powder reinforced, and fibre reinforced. Powder MMCs (PMMCs) will be used in the near future, as they are the only price/performance relevant source of MMCs at the moment.
Molten Metal Mixing
Molten metal mixing is the lowest cost route currently available for fabricating PMMCs. This produces an ingot, usually aluminium or magnesium, via standard foundry techniques. This may be further processed by rolling, extrusion, forging and drawing.
Aluminium alloys containing more than 7% silicon can be used with silicon carbide particulates (10-25 µm) up to 30%, while below 7% silicon, a larger aluminium oxide particulate must be used.
Norsk Hydro of Norway currently has a capacity to produce 150 tonnes pa of MMCs by this molten metal mixing. Alcan has invested in a 12,000 tonne pa plant to produce silicon carbide reinforced aluminium foundry alloys, wrought extruded sections, draw rod and tube products. Sub-contracted forging is also available. Lanxide, which originally developed liquid metal infiltration of ceramic foams, also has a cast product similar to Norsk Hydro's product, from liquid metal mixing. All of these products cost approximately US$3 per kilogram.
An alternative technique for fabricating PMMCs involves thoroughly mixing aluminium alloy powder with the particulate, compressing it into a billet and forging or extruding to shape. The advantage of this route is that smaller particulates can be used (2.5 µm) while large particulates will allow greater particulate concentrations up to 70%. Aerospace Composites (formerly BP Composites) are the only exponents of this technology, and supply a wide range of aluminium alloys, including aluminium-lithium(4%) alloys.
Two other techniques are being used to produce PMMCs, spray casting and in situ processing. Spray casting is a modification of the mothballed Osprey process, in which the atomised liquid metal and ceramic particulates are co-sprayed onto an initial mandrel. This process allows a larger range of aluminium alloys to be used, including aluminium-lithium(4%) alloys. In the technique of in situ processing, two alloys containing elements that react are melted together. This method has been used to produce aluminium alloys reinforced by titanium diboride, and steels reinforced by titanium carbide.
Short or Chopped Fibre MMCs
The production of short or chopped fibre MMCs involves sintering the fibres into the required component, then infiltrating with liquid metal. The only real improvement in properties is wear resistance, and a reduced coefficient of expansion.
Continuous Fibre Reinforced MMCs
Continuous fibre reinforced MMCs have the advantage of linear anisotropic properties, but certain problems restrict their widespread use at present. There are only two commercial suppliers, and the fibres are monolithic silicon carbide rather than a sophisticated ceramic. The cost of the continuous fibres is high, and the reactivity of these fibres, both during manufacturing, and in operational use, can create problems.
However, for aerospace applications, silicon carbide coated with titanium diboride would be of considerable use if it could be fabricated in the required diameter of 10-15 µm, and at a cost of approximately US$200 per kilogram, commensurate to that of titanium (US$925/kg) plus processing costs. Silicon carbide monofilaments are fabricated at present by a chemical vapour deposition process, but they are an order of magnitude too large, (100-140 µm diameter) with a small bend radius, and are too expensive, at approx US$1350/kg. Manufacture using a single pass process, rather than the multi-step chemical vapour deposition, might improve filament diameter and bring down cost. This method is currently used by both the SCS family and Sigma monofilaments or the monolithic Japanese fibre.
Complications with MMC Processing
Any of the production routes for MMCs can suffer from problems. In the case of silicon carbide reinforced aluminium, overheating can produce a viscous mess consisting of Al3C4 and AlSi2. These compounds form as result of the interaction of the silicon carbide and the aluminium and some free carbon from non-stoichiometric SiC. The production of a low-cost fibre with an integral interface between the reinforcement material the matrix would help to bring MMCs into wider use. One option is a bicomponent extrusion process for ceramic fibres, which can produce fibres with a sheath that protects the inner core from reacting with the metal matrix.
There are several near-term applications in Europe for MMCs. In the automotive field, virtually all the European automotive manufacturers are evaluating MMC brake discs and drums, including BMW, Rover, Daimler Benz, Renault, Peugeot, Volvo, Ford and General Motors. They are concentrating mainly on PMMCs at present, due to cost considerations. The high thermal conductivity of aluminium PMMCs gives much better cooling than cast iron, which runs at 500°C. However, these components will only be introduced onto new models.
There are some barriers to widespread use including:
• temperature limitations on PMMC discs and drums
• the cost is higher compared to the current cast iron components
• development of compatible brake pad materials is necessary, because the performance is dependent upon brake disc/pad interaction.
Unlike brake discs, the main requirement for brake callipers is stiffness, as distortion of the calliper leads to increased pedal travel and braking efficiency loss. The improved Youngs modulus in MMCs, compared to conventional materials, reduces the flexing of the callipers. MMC callipers are used in racing cars and high performance sports cars, but their high cost has so far precluded their use in family cars.
Automotive pistons benefit from the reinforcement of MMCs in two main areas. The top ring groove for wear resistance preventing loosening of the ring and emission control. Currently a short fibre preform of either alumina or aluminium silicate is used in this area. In a diesel engine, the combustion chamber is in the piston crown, and the inner crown suffers from cracking mainly due to thermal fatigue. The addition of a ceramic fibre preform minimises this problem and increases piston life. Various companies supply ceramic fibre reinforced pistons, including Izumi Saitami and ART Metals in Japan; AE Goetze (T&N), TRW, and Hatch & Kirk (OEM) in the US; Motoren Oberle, Nural/Alcan, Mahle, and Kolbenschmidt in Germany; and GKN and T&N in the UK.
The rotational speed of a prop shaft is controlled by its length, diameter and stiffness. Space restrictions prevent increases in diameter, which is the most efficient method. Renault uses carbon-fibre wound aluminium shafts to increase stiffness. Ford and Dodge vans have a PMMC with 20-30% silicon carbide extruded tube as an efficient way to achieve the same end result.
The largest use for PMMCs in Europe has come from the Finnish government forcing tyre stud manufacturers to reduce the weight of studs by 50%. Since 1992, manufacturers have replaced a tungsten carbide pin with one made from aluminium PMMC with 20% aluminium oxide.
Motorcycle Brake Disks
Motor cycle brake discs from PMMC were originally developed for racing motor cycles. In addition to the weight savings, PMMC discs reduce the gyroscopic effects during braking through corners. However, prior to the year 2000, they are likely to be used only at the top end of the market.
PMMCs are finding application in the railway industry as well. Two German railway brake manufacturers, Bergische Stahl Industrie (BSI) and Knorr Bremse, have been developing PMMC brake discs, and also the brake pad material to match for the most efficient braking. Brake discs weigh 135 kg in cast iron, while aluminium PMMC discs weigh 70 kg, giving a saving of 520 kg per bogie axle, or thirteen tonnes per train. These discs are sand cast from an Al-Si alloy with 20% silicon carbide particulate.
PMMC brake discs are being used on the German Inter-City Express, with a million kilometres under their belts so far. They are also to be used in Copenhagen for the new S-bahn trains, and are under evaluation in France, Switzerland, Sweden and Belgium. This application will become more important as higher speed trains will require further weight reductions.
Both aluminium and magnesium matrix PMMC extruded tubes have been evaluated for bicycle frames. Duralcan and MEL, with Aerospace Composites, are working with bicycle manufacturers on this application, but it is unlikely to become reality before the year 2000.
Golf clubs were developed as drawn tube via the co-spray process, but as this process is now defunct, little is likely to happen in Europe. PMMC golf clubs are fabricated in the US, and a version using continuous ceramic fibres is manufactured in Japan from Nikalon from Nippon Carbon.
The only current programs for aerospace applications of MMCs are for titanium reinforced with silicon carbide. These programs are all run by large consortia, or in conjunction with government research agencies. The components most likely to been seen are blisks and blings or integrally bladed compressor rings, maunufatured from chemical vapour deposition silicon carbide. The fabrication process for these is esoteric and slow. It involves a fibre and foil technique lain up by hand, which adds considerably to the cost.
Another area where MMCs could be applied is electrochemical conversion of gas for domestic central heating or hot water, and also domestic electrical requirements. One kilogram of yttria-stabilised zirconia can produce one kilowatt of electrical power. If used for domestic hot water and central heating with simultaneous electrical power production to run most domestic appliances, this could cut the amount of standby electrical power generation required by the west. This could reduce Europe's greenhouse gas emissions by 15%.