Braking the mould - braking systems using waste carbon fibre
A programme to extend the life of braking systems using waste carbon fibre has come to an end. Dr Houzheng Wu, Senior Lecturer in materials from Loughborough University, UK, outlines the improvements made.
Carbon Fibre Reinforced Ceramic Composites (CFRCC) have undergone rapid development in brake disc applications, due to their combination of thermal and mechanical properties, with less than one third the density of cast iron.
A research project, funded by the Technology Strategy Board (TSB) and The Engineering and Physical Sciences Research Council (EPSRC), aimed to develop an optimised process for manufacturing CFRCCs with high temperature and extended life for aircraft, automotive, rail and industrial friction applications. The project, known as ReBrake, was based on designed-for-purpose raw materials and efficient recycling of polymer-based carbon fibre (CF) waste, mainly from aerospace where variability of material is limited.
The consortium consisted of UK-based industrial partners Surface Transforms plc, Federal Mogul Friction Products Ltd, Meggitt Aircraft Braking Systems, AP Racing Ltd, Twiflex Ltd, Advanced Composites Group Ltd, Faiveley Transport Ltd, and academic partner, Loughborough University.
The introduction of lower cost and more durable lightweight ceramic brakes would ensure lower polluting braking systems. This would have a major environmental impact in the automotive industry through reduced fuel consumption and lower toxic emissions, as, with decreased wear, there is little carbon dust, and fewer pads and rotors to recycle. Vehicle safety is also improved, with less ‘fade’ compared to cast iron discs.
For rail transport, the environmental impact is potentially greater. At a weight saving of five tonnes, CO2 emissions would be reduced by 1.1kg for every kilometre a high-speed train travels.
In aircraft, a lower wear material could lead to weight savings through thinner brake discs and reduced heat sink length. The landings per overhaul could increase by 50-100%. The technology would also enable a shift from hydraulic brakes towards electrically actuated brakes, and the ability to develop aircraft with improved payload to totalaircraft-weight ratio.
Industrial braking and clutch systems, and ancillary equipment, would also benefit from lighter weight and greater energy dissipation for paper and steel manufacture, metal forming, mining, marine, forestry, petrochemical, ceramic and dynamometry.
Proof in the process
A team at Loughborough University, UK, had developed a process for converting CF waste to CF composites via polymer impregnation and pyrolysis. This compared well, on the small scale, to the chemical vapour impregnation (CVI) technique that Surface Transforms has successfully developed.
Until recently carbon waste most often ended up as landfill, but now processes have been developed elsewhere that can recover the carbon fibres by removing the polymeric materials either by chemically dissolving or burning off at high temperature.
In this study, an alternative was investigated for re-using CFRP waste. The carbon derived from the polymeric material is maintained by controlling the charring process, and the carbon fibre-carbon (CF/C) is achieved with only minimum CO2 emissions generated. A further stage had to be developed to increase the amount of pyrolysed carbon from waste to achieve that required for the CF/C preform. With the appropriate CF/C preform, a silicon melt infiltration was used to produce the CFRCC.
The resulting CFRCC achieved mechanical properties comparable to commercial types, but with lower cost for the raw material. The strength achieved ensures that the composites can accommodate the loading conditions of typical rotors in road vehicle brakes.
The hardness, Young’s modulus, and fracture toughness of the constituents in the composites made from CF waste have been studied with nano-indentation, and gave results comparable with those made from virgin carbon fibres through the CVI process. Composites discs with an outer diameter of 50mm were successfully manufactured in the laboratory, as shown in the image above, right, which indicated the potential for commercial exploitation.
Friction performance of CFRCC rotors made from CF wastes was evaluated using a small laboratory-scale dynamometer. The coefficient of friction is between 0.33-0.52 when tested with pad materials developed by Federal Mogul, Michigan, USA, which meets the requirements of road-vehicle brakes. The influence of carbonaceous materials on the silicon carbide matrix, through the silicon melt infiltration route, was systematically studied.
On the small scale, it was found that carbon, pyrolysed from resins, is comparable to the more expensive CVI process. The use of pyrolysed resin provides an opportunity to generate a fine silicon carbide matrix, due possibly to its more porous structure. A fine ceramic microstructure could improve the resistance to micro-scale damage and the compliance of the silicon carbide surface, which has been identified in this project as one of the major factors that determines the stability of the friction surface (see image left).
Results also show that pyrolysed carbon from different carbonaceous sources can protect the CF filaments, which is essential during siliconisation, when silicon carbide is produced. The pyrolysed carbon provides a weak interface between the CF reinforcement and matrix to give the required damage tolerance.
The carbon preforms from carbon waste have been fully densified at ST, and were successful on the small scale, however, at full scale, the carbon waste, with 50-60% CF, is too dense to successfully infiltrate either with carbon or silicon, and delamination occurs.
With further development, CFRCC from carbon fibre reinforced polymer wastes should be a viable route and the produced composites can achieve comparable performance with those from virgin materials when the processing conditions are optimised. Surface Transforms, Ellesmere Port, UK, has successfully produced its first ever ceramic rail disc, leading to further collaboration with Faiveley and Federal Mogul. Surface Transforms has also produced its first thin full-scale clutch plate, and is working with AP Racing to optimise the product.
Dr Houzheng Wu, Department of Materials, Loughborough University, Loughborough, LE11 3TU, UK. Tel: +44 (0)1509 223342. Email: firstname.lastname@example.org This piece replaces an earlier article on the work in the August 2010 issue, providing a more accurate representation of the research.