Carbon-carbon composites in clutches
It is common to see composite materials in a range of applications in motorsport. Typically glass- and carbon fibre composites reinforced within a polyester or epoxy matrix not only lend themselves to the manufacture of complex shapes but also allow engineers to incorporate both isotropic and anisotropic mechanical properties into a single structure. As with all composite materials, the constituents' properties and the positioning of both the reinforcement and matrix material will determine the overall composite material's properties.
So how do we engineer a composite to have the necessary mechanical thermal and frictional properties required for a clutch plate material?
Carbon-carbon composites are the answer. Carbon exists in a number of different forms and the term 'carbon/carbon' relates to both the reinforcement and the matrix material - both being carbon, although different forms of it. This composite consists of a fibrous carbon material within a graphite matrix. As with fibre-reinforced polymer composites, directional strength can be determined by selecting the direction of the fibres. Because of the multi-directional loading to which the clutch plate is subjected, a randomly orientated fibre is used.
The material combines the stiffness and strength of polymer-reinforced composites with the excellent thermal properties of the graphite material. Because of this, the carbon fibre-reinforced graphite composite exhibits a low coefficient of thermal expansion (and therefore low deformation) and the excellent frictional properties required of a clutch plate material. And the relatively low density of the material means it also exhibits an extremely low inertial weight. In addition, depending on the frictional and wear rate properties required, the porosity can be controlled by the composite constituent properties and the manufacturing process.
Unlike glass- and carbon-reinforced polymers, the manufacturing process is slightly more complex. Carbon-carbon composites are made using various methods, to extract the graphite from a natural carbon product through carbonisation (charring). In the early development of carbon-carbon technology, Chemical Vapour Infiltration (CVI) and Liquid Phase Infiltration (LPI) techniques were used to fabricate the composites; in some cases they still are. The CVI process involves the infiltration and pyrolysis (thermochemical decomposition in the absence of oxygen) of a vapour that deposits carbides on the fibrous carbon surface. Further heat treatment forms the crystalline graphite from the deposited carbon.
LPI is a similar process that involves the infiltration of the composite with a carbon-rich resin or similar; the pyrolysis process then extracts the carbon from the resin, and further heat treatment or carburisation forms the graphite from the carbon. This process is then repeated to achieve the required density and porosity of the composite.
Because of this lengthy and complex process, the Across Corporation has patented a process that uses a pre-formed carbon fibre yarn that aims to cut the number of processes required in the manufacture of carbon-carbon composites - the CVI process consumes large amounts of energy - in order to reduce cost and throughput times, and increase quality.
Typically exhibiting much greater longevity than that of sintered clutches, combined with the increased durability at elevated temperatures and an extremely low inertial weight, it is easy to see the importance of this aerospace-derived material in performance automotive applications.
Fig. 1 - Carbonetic Paddle clutch exhibiting the carbon-carbon friction material
Written by Chris Thwaites