Disc brakes comprising a caliper and a disc have been widely adopted, particularly for automotive applications. The disc is squeezed during braking between pads of friction material, mounted in the caliper; and since the disc has a high resistance to compression, and since much of the disc is exposed and so can cool more quickly than for instance the drum of a drum brake, a disc brake is less prone than the drum brake to brake"fade"i. e. to partial loss of braking at high disc and/or pad temperatures, as may occur following intensive brake usage over a short period. It will also be of advantage to. form other components of this material able to withstand similar conditions of use. Such components can be parts used in automotive, aerospace, railways aeronautic, machine tool, construction and like equipment.

It is one object of this invention to provide a component formed of a C-based composite, which component is able to withstand stresses induced in use. e. g. applied frictional and abrasive forces.

Heretofore, in the field of carbon brake discs, such discs have utilized some type of reinforcement, invariably carbon filaments in some form such as cloth, chopped fibre, woven and the like. The primary purpose of such filaments is to impart high mechanical proE) erties such as strength and rigidltyDto. the discs sathat they can withstand the forces encountered during braking in high performance applications.

Such applications are typically but not exclusively automotive and aerospace, for example for use in the braking systems of high performance sports cars and the like.

The fabrication of laminated brake discs containing carbon cloth or filament layers is commonly referred to in the art as a carbon/carbon (C/C) composite. (The term "carbon"is used in the generic sense and can mean either amorphous carbon or crystalline graphite). Additionally, brake discs have been made utilizing a bulk carbonaceous layer such as bulk graphite in order to reduce the long fabrication time of the C/C composite laminate and to reduce cost. Of course, brake discs have also utilized combinations of the C/C composite and bulk graphite layers. Furthermore discs have been produced from Carbon performs that have been infiltrated with materials other than Carbon, for example Silicon, these discs are sometimes referred to as"Carbon ceramic"discs.

In operation these brakes can reach temperatures in excess of 1000°C. Such high temperatures lead to oxidation of the disc (the threshold for oxidation of Carbon is approximately 1010°C) and resulting high wear rates of both the friction and non- friction areas.

Various patents teach the coating of the non-friction regions of the disc to reduce oxidation and loss of mass from those areas. For example in GB 1544994 it is taught that coatings can be applied to the"non-wear"surface of C/C discs in order to provide an oxidation limiting coating. Significantly none of these patents teaches the application of a coating to the friction or wear surface. Presumably this is because CDISC-INT-01 Description Page 2, this surface is critical to the operation of the C/C brake and hence it is important that it is free to form a friction couple with the brake pad (or in the case of a disc stack the neighbouring disc). Patent EP 0 54 196 teaches the application of a ceramic coating to a metallic substrate, preferably an aluminium BIC, the intention being to improve the temperature capability of the aluminium disc by the formation of a thermal barrier.

The present invention teaches the application of a coating to the surface of a C-based brake disc with the intention of protecting the surface of the disc from oxidation and increasing the wear resistance of the disc. Thus the life of the disc is extended and the cost of the, component is significantly reduced. The coating also provides one half of the friction-eouple formed between disc and pad that previously would have been formed between the C-based substrate and the pad.

The resultant disc brake possesses the properties of 1) a friction coefficient that can be altered to suit the application 2) low weight due to the low density of the C/C composite 3) strength and stiffness at temperature such that the integrity of the disc is retained under severe braking conditions 4) high wear resistance All of these properties are highly desirable in a brake disc.

In another aspect the invention teaches the application of a coating to other types of carbon based brake discs. In this respect it is possible to use alternative materials to the C/C composites used in uncoated brake discs as in the presence of a coating there is no longer the requirement for the carbon based material to have the friction characteristics of a brake material.

According to the invention in one aspect there is provided a brake disc or like <BR> <BR> component formedof CIC composite, the component. having a protective coating on at least one surface thereof characterized in that the protective coating is a ceramic coating : selected to provide a combination of properties selected from wearresistance thermal capacity, thermal conductivity, friction performance, oxidation resistance and has a thickness of about 0. 2 millimetres.

As indicated, one particular component is a brake disc which in use can be worn down by a brake pad during use and in this case the protective coating is selected to protect the surface against an applied abrasive force. In another aspect the applied abrasive force is the passage of hot gases over the combustors and turbine blades in a jet engine. In addition or alternatively the coating may be selected to provide corrosion resistance, hardness, wear resistance, compatibility or other properties. Depending on the intended use, the coating may be selected to have thermal properties, e. g. conductivity.

The coating can be applied by any method and can be a combination of several coatings and layers incorporating interlayers designed for example to provide high oxidation and/or abrasion resistance and/or bonding of the upper layer to the CDISC-INT-01 Description Page 3 substrate. Preferably, the protective coating is applied by electroplating, plasma deposition, magnetron sputtering, dipping or impregnation or a like technique.

The coating is chosen to be compatible with the component, e. g. to have a rate of linear heat expansion substantially the same as that of the composite of the brake disc component to avoid the risk of debonding at service temperature.

A brake disc of the invention can be used with a wide range of brake pads. For certain applications it is possible to use commercially available brake pads without modification. The discs may be used at the front and/or rear of a vehicle, including a racing vehicle.

The invention will be further described by way of example with reference to the accompanying diagrammatic drawing in which : Figure 1 is a cross-sectional view of a typical disc for use in a disc brake; and Figure 2 is an enlarged section of a circled portion of Figure 1.

The vehicle brake disc 10 is of generally a"floating"form, with a mounting bell portion 13 adapted to be secured to rotate with a vehicle wheel or axle (11) ; in alternative embodiments the disc can be mounted to rotate with the vehicle propshaft and may be ventilated. The bell and disc are held together in known manner, e. g. by bolts and nuts, not shown. Friction pads A, B are movable into engagement with opwsed side faces lOa, I vfdisc 10 to effect braking of the disc. During such engagement if the disc is rotating, high temperatures are generated at the discs sumaches, which may have therefore a frictionally induced temperature exceeding 1000°C.

The disc 10 comprises a body portion 12, formed of a C/C composite with a coating applied by a plasma spray tschmque. The density of the composite is about 1. 81 gm/cm3.

The plasma sprayed coating 20 is applied to a depth sufficient to protect the underlying composite, typically about 0.2 millimetre. In this instance the coating was alumina based. An objective of this particular embodiment of the invention is to provide enhanced wear resistance to the C/C composite. Another objective of this particular embodiment is to provide a friction couple with the brake pad that is similar to that obtained when using the C/C composite without the coating in the same application.

A brake disc was prepared according to the invention suitable for application to a Formula 1 racing car. Carbon-carbon composite was used for the substrate and a coating applied consisting of predominantly Alumina by the process of plasma spraying. The disc was tested on a purpose built dynamometer situated at the CDISC-INT-01 Description Page 4, European Friction Industries plant in Bristol, United Kingdom. Organic brake pads were used to provide the friction interface.

Amongst other tests the discs were subjected to a simulation of a lap of the Grand Prix Circuit in Montreal Canada applying torque and speed typical of a Formula One car.

The disc generally performed well in these tests producing friction coefficients up to 0. 7 dependent on conditions. A temperature trace taken from one of the tests is shown in figure 3 where the disc is compared with a standard C/C disc and pads.