Licensed t sell

Potential higher yields and lower costs from a new method of producing fibres for composite materials will stimulate interest among manufacturers and users in a variety of markets. Sue Stuckey investigates the long and the short of the fibre debate

British inventor Gerry Curran reckons he is about to become a very rich man. He is poised to sell licences on patents he holds relating to the processing of fibres used to strengthen modern, high-performance metal and ceramic composite materials. These are starting to replace conventional materials on account of low weight, performance at high temperature and resistance to wear.

His development is being hailed as a commercial breakthrough for the industry, which has been hampered by the high material costs associated with existing fibre production methods. The technique, which can be used to make continuous or short lengths of the fibre, produces far higher yields than achieved elsewhere.

This is expected to collapse the price of a high performance continuous fibre reinforced ceramic material to around £1,000/kg compared with up to £6,000/kg now. The quality, weight and long-term integrity of the fibre are also said to exceed current processing capabilities.

City backer KPMG Corporate Finance in London is poised to issue licences. Andy Hales, the partner dealing with the issue, expects interest to be strong. `Potentially this is an outrageous piece of technology,’ says Hales. Last week invitations were sent to the chief executives of the top 100 firms on the advanced materials scene. The US and Japan are expected to be hot contenders with companies such as 3M, General Electric and United Technologies being strong bidders.

Interest is also expected from companies who use, rather than produce, the materials. At the more demanding end of the spectrum, aeroengine maker Rolls-Royce is the only serious UK customer for continuous fibre reinforced metal matrix composite materials (MMCs).

It is evaluating silicon carbide fibres from Textron in the US and Defence Research Agency offshoot DRA Sigma in Farnborough. They are the only two producers of a fibre which has a protective coating, considered in some quarters to be essential for high temperature operation above 1100 degreesC. Coatings are a hot topic in academic circles.

By 2000, Rolls-Royce expects to have a demonstrator engine which it will show off as a successor to the RB199 and EJ200 engines on the Tornado and the Eurofighter jets. Key parts in the engine compressor will be made from a titanium alloy reinforced with silicon carbide, a high strength man-made ceramic. Starting with the load-bearing, rotating compressor rings to which blades are attached, work will progress to bladed disc drums and later to shafts and casings. Titanium MMCs could account for 5% of an engine’s weight. The material is already used in superstructures where the stress loads are less arduous as, for example, the ventral fin on the US F-116 fighter aircraft.

Components made from reinforced composites are roughly half the weight of the monolithic (unreinforced) material. The titanium composite is half as strong again as the unreinforced alloy and twice as stiff. These properties make a compelling argument for its use under high load conditions where there is a need to reduce weight or improve fuel economy.

But unless material and processing costs come down, which means volumes going up, widespread applications are unlikely. Phil Doorbar, team leader of Rolls-Royce’s MMC group, says: `We are looking to get current costs down by an order of magnitude before MMCs are used in commercial engines.’

Cost considerations

Rolls-Royce is working closely with Sigma on the processing side. High temperature protection of the silicon carbide fibres is a problem, adding hugely to the time and cost of making parts. According to the National Physical Laboratory at Teddington, parts are so costly that test pieces rather than actual components are frequently used to evaluate key mechanical properties. NPL is supported by the DTI to work with industry to develop standardised test methods.

Dr Bryan Roebuck, project leader at NPL’s Centre for Materials Measurement, says: `The thing people are most nervous about is cost. Some of the long fibre composites are very expensive and this is what worries them most of all.’

He says there is much greater interest in cheaper short-fibre, discontinuous or particle reinforced composites (DMMCs or PMMCs) which NPL has identified in a report to the Government as vital to the competitiveness of UK industry. The market for PMMCs in western Europe is expected to reach 10,000 tonnes by 2000 and be worth up to £500m compared with 200 tonnes now. That is against a figure for general steels in the UK of 16m tonnes today, worth around £3.5bn.

Honda leads the field with the commercial use of PMMCs. Starting this year, the engine cylinder liners on all Prelude cars will be made from a silicon carbide particulate reinforced aluminium alloy. Apart from being lighter, the material, which replaces cast iron, has good thermal, mechanical and wear properties. It is also easy to process to near net shape using normal casting methods. Drawbacks identified by NPL are a lack of large-scale production capacities as well as machining, joining and recycling problems, and the lack of reliable test data on failure modes.

In the US, PMMCs look likely to help the Clinton administration reach its goal of the 80mpg car. In December, research group USCAR will unveil three concept cars. The target is a 40% reduction in overall body weight at a cost not exceeding $1 for every pound of weight saved.

The £750m UK motor racing industry is a big customer for PMMCs with Grand Prix motor bikes and the Lotus Elise sports car using the material in disc brake rotors.

Metal Composite Technology of Towcester makes brake discs for 125cc and 250cc bikes using a silicon carbide reinforced aluminium/silicon alloy from US producer Duralcan. Parts are made using standard gravity casting methods. But John White, MCT technical manager, says the high cost of the diamond tools used to finish machine the parts is an issue.

Sports goods, bicycles and aeroengines are suitable applications for MMCs, but the market is tiny compared with brake drums and discs for cars and trains, which are expected to account for more than half of all tonnages by 2000.

Two German materials processors, Knorr Bremse in Munich and Bergerische-Stahl Industrie of Dusseldorf, are using the Duralcan alloy in trials on the German ICE high speed train. Each has supplied a set of brake discs to one bogie and these have been running successfully for more than 1,000km. On a full train set the weight saving over cast iron is 13 tonnes.

Knorr Bremse will also supply production quantity discs to trains on the Copenhagen S-Bahn railway while most of the European railway operators are showing interest in the PMMC technology. For example, Munchen U-Bahn has identified fuel cost savings of DM600,000 (£250,000) a year if it were to replace cast iron discs with aluminium-ceramic discs.

Railway industry consultant Colin Baker is talking to one UK wheel producer about the potential to replace the outer steel rim on aluminium alloy wheels with an aluminium-ceramic composite supplied by Hydro Aluminium of Norway. The material’s high wear, low friction properties are expected to improve wheel traction and reduce fuel costs.

But while the processing costs of PMMCs are relatively low because they use existing methods and machinery, the same is not true of continuous fibre MMCs. And this is a problem for high temperature users such as Rolls-Royce and materials suppliers.

Like Textron in the US, Sigma uses chemical vapour deposition (CVD) to produce ceramic fibres and a second CVD process to coat them with titanium diboride which acts as a protective barrier between the fibre and the matrix. The problem with uncoated fibres is that they start to decompose as temperatures rise and this is an issue especially for carbon matrix composites operating above 1400 degreesC.

Other fibre producers in the US and Japan provide uncoated fibres which leads to criticism over the long-term stability of their products. Some scientists, however, will argue their case. Professor Tony Kelly of the materials science and metallurgy department of Churchill College Cambridge, and this year’s president of the Institute of Materials, is one. His solution, shared by companies such as Lanxide in the US and SEP in France, is to take advantage of adversity.

Most materials are unstable in air at high temperatures – they oxidise, some more readily than others. During oxidation, especially at higher temperatures, they become reactive, unstable and hostile. That is a problem for the fibres and host material. Oxides, however, have no tendency to combine with oxygen and can survive temperatures up to 2000 degreesC. Composites made from oxides would be inherently more stable.

For very high temperatures GEC-Alsthom Research Centre at Stafford claims to lead the field with a ceramic composite based on `new fibres combined with a new matrix’. Dr Andrew Hyde, head of glass and ceramics, says his group made a breakthrough just before Christmas, nearly hitting the magic 2000 degreesC mark with a composite capable of long service periods up to 30,000 hours, destined for military applications.

Sigma says it has overcome the temperature processing problem for titanium MMCs using a layering technique in which a wrap of continuous fibres is laid between thin sheets of the alloy to form a foil/fibre stack typically between 6 and 10-ply, up to 2mm thick. That is placed in a stainless steel former compressed at pressures of around 100MPa into the shape of the desired component.

Sigma metallurgist Stuart Godfrey will not divulge the exact pressures and processing times, but explains that the key advantage of this method is that the titanium alloy becomes superplastic, allowing it to take up the shape of the former. Close control of the hot isostatically pressed method prevents the alloy from becoming molten, thus preventing it from becoming reactive.

Curran says his method, in which the fibre and sheath are extruded in a single bicomponent process (see diagram), provides more effective protection for the fibre up to 1700 degreesC. He says it allows more careful control over the coating which acts as a mechanical interface with the matrix that helps determine its behaviour. He claims his fibres – typically 8 to 16 microns thick – are stiffer, stronger and lighter than other fibres. For cost reasons, Textron and Sigma, for example, produce `fat’ fibres around 100 microns thick.