Improvements in or relating to brake discs and clutch discs

This invention relates to carbon fibre-reinforced discs having enhanced factional properties, more particularly to carbon fibre-reinforced brake discs such as brake rotors for use in disc brake systems, and also to carbon fibre-reinforced clutch discs.

Much interest has been shown in the use of carbon fibre-reinforced brake discs and clutch discs, including ceramic discs such as siliconised carbon-carbon fibre composites. These latter discs are of particular interest because of their high strength, their ability to maintain excellent physical and frictional properties at high operating temperatures, and their low weight compared to conventional metal brake discs, for example permitting a weight reduction of 50-60% relative to a standard case iron disc. Such weight reduction is important in improving performance and fuel economy; in the case of automotive brakes it may also improve road holding, handling and comfort of a vehicle by reducing the unsprung weight. Carbon-carbon fibre discs exhibit comparatively poor frictional properties at ambient temperature and so are largely confined to aircraft applications, whereas carbon fibre-reinforced ceramic discs operate efficiently at ambient temperature and are therefore capable of providing occasional light braking, a key requirement of automotive brakes.

Existing commercially available carbon-carbon fibre composite brake discs and clutch discs are usually prepared by a "resin char" method in which the reinforcing carbon fibres and a carbonisable resin such as pitch or a phenolic resin are hot moulded together to approximately the desired shape. The resulting moulded perform is then carbonised, for example by heating to ca. 1000°C under an inert atmosphere or in vacuo, and optionally graphitised, for example by heating to greater than 2000°C. The thus-obtained green bodies may then be shaped- and/or joined together as appropriate, and if desired may then be siliconised by, for example, at least partial immersion in a bath of molten silicon or by a hot isostatic pressure treatment involving encapsulation with excess silicon in an evacuated container which is then subjected to high temperature and isostatic pressure.

Chemical vapour infiltration procedures in which an initial carbon fibre preform is subjected to densification through a series of chemical vapour infiltration steps may be used as an alternative to resin char processing in order to form the carbon-carbon fibre composite. Such procedures permit the generation of composites with greater structural integrity, but involve substantially higher operating costs than resin char processing. A more economical procedure for the manufacture of a siliconised carbon-carbon fibre brake disc and clutch disc in which siliconisation is performed on a carbon fibre preform which has been densified using only a single chemical infiltration step is described in WOA-2007/012865. Siliconised carbon-carbon fibre composite brake discs and clutch discs obtained in such ways may be machined to the desired final dimensions either before or after siliconisation. The former option may be advantageous since the presiliconisation intermediate product is less hard and therefore more readily machinable than the siliconised end product. In practice the friction faces of carbon fibre-reinforced ceramic brake discs and clutch discs such as siliconised carbon-carbon fibre composite discs are commonly subjected to a final machining stage involving wet or dry turning on a lathe. This produces a surface profile derived from the lathe cutting tool which takes the form of a very fine spiral around the turned face. The average roughness measurement R _a for such a surface (an integral of the absolute value of the roughness profile which may be evaluated by dividing the area of the profile by the evaluation length) will typically be 3-4 μm or above. Alternatively the faces may be milled or ground to a comparable level of surface finish.

The present invention is based on our finding that machining the friction faces of carbon fibre-reinforced discs, for example, carbon fibre-reinforced brake discs and clutch discs, to a significantly smoother finish with an average roughness R _a not exceeding 2.5 μm, for example by grinding, has an unexpectedly beneficial effect on their frictional properties. In particular we have found that the smoother friction faces permit higher friction levels to be obtained and/or lead to significantly reduced levels of surface wear and thus to extended product life. Such association of increased friction with increased smoothness is inherently surprising, as is the observation of reduced wear, particularly in the case of ceramic brake discs where

their abrasive characteristics would lead one skilled in the art to expect higher friction inevitably to lead to higher wear.

Thus according to one broad aspect of the invention there is provided a carbon fibre-reinforced disc, in particular a carbon fibre-reinforced brake disc or carbon fibre-reinforced clutch disc, having friction faces with an average roughness R _a not exceeding 2.5 μm, e.g. not exceeding 2.4 μm, 2.3 μm, 2.2 μm or 2.1 μm and preferably not exceeding 2.0 μm.

The surface of a conventional carbon fibre-reinforced disc, when viewed at a high magnification, can be thought of as a 'mountain peak and valley' landscape. In the carbon fibre-reinforced disc of the present invention, this landscape is smoothed off substantially, for example by grinding, removing peaks to leave a smooth 'cratered' surface. This new smoothness can be quantified by measuring the material roughness ratio R _m1 of the material, which should be at least 70% when measured 10 μm below a datum line 5% below the highest peak, more preferably at least 80% when measured 10 μm below a datum line 5% below the highest peak, and more preferably still at least 90% when measured 10 μm below a datum line 5% below the highest peak. The material roughness ratio is the length of material surface, expressed as a percentage of the evaluation length, at a depth below the highest peak, in this case determined at 10 μm below a datum line which is at 5% below the highest peak.

Thus according to another aspect, the present invention can be seen to provide a carbon fibre-reinforced disc having friction faces with an average roughness R _a not exceeding 2.5 μm, more preferably not exceeding 2.0μm, wherein the friction faces have a material roughness ratio R _mr of at least 70%, more preferably at least 90%, when measured 10 μm below a datum line 5% below the highest peak.

In order to optimise the strength of the disc the carbon fibre content thereof may advantageously comprise arrays of fibres laid at 90° to each other. Such arrays may for example be cut to shape from a continuous sheet or cylinder of carbon fibre fabric such as a woven fabric or non- woven felt comprising alternating layers of carbon fibres laid at 0° and 90°, and then subjected to densification, e.g. by resin

char, chemical vapour infiltration or wetting monomer infiltration, and optionally to subsequent ceramifi cation. Other orientations of fibres may also be useful in certain applications, particularly automotive applications, for example, 15°, 30°, 45° and 60°, or any multiples of these. In some applications it may be desirable to lay the fibres at varying and/or non-regular orientations, and in others a random orientation may provide benefits.

Carbon fibre-reinforced ceramic discs may be obtained in per se known manner, for example by infiltrating a carbon-carbon fibre composite disc with one or more molten carbide-forming metals, e.g. selected from members of Groups III- VI of the Mendeleef Periodic Table of Elements (CAS version). Representative metals for this purpose thus include Group IIIA metals such as boron, Group IVA metals such as silicon, Group IVB metals such as titanium, zirconium or hafnium, Group VB metals such as vanadium or tantalum, Group VIB metals such as molybdenum or tungsten, or mixtures of any of the foregoing. The use of siliconised carbon- carbon fibre composites is preferred.

Brake discs and clutch discs of the invention are advantageously characterised by friction faces which have an average roughness R _a not exceeding 2.5 μm and a material roughness ratio R _m1 - of at least 70% when measured 10 μm below a datum line 5% below the highest peak, this parameter being the length of material surface below the measurement line expressed as a percentage of the evaluation length.

Commercially available surface roughness testers such as the Surftest SJ-301 instrument available from Mitutoyo Corporation may be employed to determine the R _a and Rmr parameter values. In the interests of consistency it may be desirable to conduct measurements at two, three or more separate areas of each disc face, both in line with and across the predominant carbon fibre direction at the face.

The invention further provides a method of manufacturing a carbon fibre- reinforced brake disc and clutch disc as hereinbefore defined in which the friction faces of a preformed disc are machined to achieve the required surface finish. Machining of the faces to a smoothness below the hereinbefore defined R _a limit and preferably also above the hereinbefore defined R _nU - limit may, for example, be effected by grinding, e.g. using a diamond embedded grinding disc. It may be

advantageous to employ a wet grinding process using a coolant solution, particularly where a ceramic disc is to be processed.

If desired the grinding procedure may be preceded by coarser machining, e.g. on a lathe, to remove rougher surface irregularities such as may, for example, be introduced during ceramifϊcation. The grinding procedure may optionally be carried out in two or more stages using successively finer grinding wheels, in order to achieve an optimum level of surface finish, and may if desired by followed by further treatment such as honing. It will be appreciated that other machining procedures which produce the required surface finish may equally well be employed.

Whilst there are generalised references in the prior art to grinding of the friction faces as a finishing step in the manufacturing of carbon fibre-reinforced ceramic brake discs (see, for example, US Patents Nos. 6,527,092 and 7,045,207), there is no suggestion in such art of grinding to achieve a surface finish with the characteristics hereinbefore defined. Moreover, there is no suggestion that such grinding or other machining to an exceptionally smooth surface may lead to the unexpectedly enhanced frictional properties which we have observed.

Thus a further aspect of the invention may be defined as use of machining as a means for enhancing the frictional properties of a carbon fibre-reinforced brake disc or clutch disc, wherein the friction faces of the disc are machined to an average roughness R _a not exceeding 2.5 μm, e.g. not exceeding 2.4 μm, 2.3 μm, 2.2 μm or 2.1 μm and preferably not exceeding 2.0 μm, and preferably also to a material roughness ratio R _mr of at least 70%, e.g. at least 80% and more preferably at least 90%, when measured 10 μm below a datum line 5% below the highest peak. The machining may conveniently be effected by grinding and the disc advantageously comprises a siliconised carbon-carbon fibre composite orother carbon fibre-reinforced ceramic disc. The enhanced frictional properties preferably comprise one or both of increased friction levels and reduced wear.

The present invention embraces, inter alia, both automotive and aircraft brake discs, in particular brake discs for performance cars and for motorcycles, a,s well as clutch discs for performance automotive applications such as performance cars and motorcycles. In testing of an automotive disc/pad system on a

dynamometer we have found that a "smooth" siliconised disc in accordance with the invention bedded in more rapidly than a conventional disc finished by turning on a lathe. It also achieved higher friction levels which were maintained as testing was continued long beyond normal bedding. Significantly lower wear was recorded for the smooth disc relative to the turned disc despite this higher friction.

In a test of an aircraft brake system comprising a three disc stack with one rotor and two stators, conventionally finished siliconised discs with an average roughness R ₃ of 3-4 μm exhibited wear rates quantified as 1160 equivalent landings. Corresponding siliconised discs in accordance.with the invention having R _a < 2 μm exhibited essentially equivalent frictional behaviour but very substantially reduced wear rates corresponding to 2762 equivalent landings.