GB2452779A

GB2452779A - Carbon fibre reinforced brake discs

Info

Publication number: GB2452779A
Application number: GB0718103A
Authority: GB
Inventors: Kevin Johnson; David Andrew Hubbard; Andrew John Marson; Geoffrey Shawn Whitfield
Original assignee: Surface Transforms PLC
Current assignee: Surface Transforms PLC
Priority date: 2007-09-17
Filing date: 2007-09-17
Publication date: 2009-03-18
Also published as: JP2010539423A; EP2200952A1; WO2009037437A1; GB0718103D0

Abstract

The frictional properties of carbon fibre-reinforced brake discs, in particular ceramic discs such as siliconised carbon-carbon fibre composites, are enhanced by machining the friction faces to an average roughness Ra not exceeding 2.5 pm, preferably not exceeding 2.0 pm, and preferably also to a material roughness ratio Rmr of at least 90% when measured 10 pm below a datum line 5% below the highest peak. The enhanced frictional properties may comprise one or both of increased friction levels and reduced wear.

Description

Improvements in or relating to brake discs This invention relates to brake rotors for use in disc brake systems, more particularly to carbon fibre-reinforced brake discs having enhanced frictional properties.

Much interest has been shown in the use of carbon fibre-reinforced brake 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 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 in which siliconisation is performed on a carbon fibre preform which has been densified using only a single chemical infiltration step is described in WO-A-2007/O 12865.

Siliconised carbon-carbon fibre composite brake 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 then the siliconised end product.

In practice the friction faces of carbon fibre-reinforced ceramic brake 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 Ra 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 j.tm 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 brake discs to a significantly smoother finish with an average roughness Ra not exceeding 2.5.tm, 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 aspect of the invention there is provided a carbon fibre-reinforced brake disc having friction faces with an average roughness Ra not exceeding 2.5 J1m, e.g. not exceeding 2.4 pm, 2.3 tm, 2.2.tm or 2.1 im and preferably not exceeding 2.0 pm.

In order to optimise the strength of the disc the carbon fibre content thereof may advantageously comprise arrays of fibres laid at 900 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 00 and 90°, and then subjected to densification, e.g. by resin char, chemical vapour infiltration or wetting monomer infiltration, and optionally to subsequent ceraniification.

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 Ill-VI of the Mendeleef Periodic Table of Elements (CAS version). Representative metals for this purpose thus include Group lilA 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 of the invention are advantageously also characterised by friction faces which have a material roughness ratio Rmr of at least 90% when measured 10 tm 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 Surfiest SJ-301 instrument available from Mitutoyo Corporation may be employed to determine the Ra 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 as herejnbefore 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 Ra limit and preferably also above the hereinbefore defined R 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 ceramification. 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 embodiment 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, wherein the friction faces of the disc are machined to an average roughness Ra not exceeding 2.5 tm, e.g. not exceeding 2.4.tm, 2.3 m, 2.2 tm or 2.1 j.im and preferably not exceeding 2.0 j.tm, and preferably also to a material roughness ratio Rmr of at least 90% when measured 10 im 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 or other 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 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 Ra of 3-4 tm exhibited wear rates quantified as 1160 equivalent landings.

Corresponding siliconised discs in accordance with the invention having Ra < 2.tm exhibited essentially equivalent frictional behaviour but very substantially reduced wear rates corresponding to 2762 equivalent landings.

Claims

CLAIMS: 1. A carbon fibre-reinforced brake disc having friction faces with an average roughness Ra not exceeding 2.5 tm.
2. A brake disc as claimed in claim 1 wherein the friction faces have an average roughness Ra not exceeding 2.0 tm.
3. A brake disc as claimed in either claim 1 or claim 2 which comprises arrays of carbon fibres laid at about 90° to each other.
4. A ceramic brake disc as claimed in any of claims 1 to 3.
5. A brake disc as claimed in claim 4 which comprises a siliconised carbon-carbon fibre composite.
6. A brake disc as claimed in any one of claims I to 5 wherein the friction faces have a material roughness ratio Rmr of at least 90% when measured 10 tm below a datum line 5% below the highest peak.
7. An automotive brake disc as claimed in any one of claims 1 to 6.
8. An aircraft brake disc as claimed in any one of claims I to 6.
9. A method of manufacturing a brake disc as defined in any one of claims 1 to 8 in which the friction faces of a preformed carbon fibre-reinforced disc are machined to achieve the required surface finish.
10. A method as claimed in claim 9 wherein the machining is effected by grinding.
11. A method as claimed in claim 10 wherein the grinding is effected using a diamond embedded grinding disc.
12. Use of machining as a means for enhancing the frictional properties of a carbon fibre-reinforced brake disc, wherein the friction faces of the disc are machined to an average roughness R3 not exceeding 2.5 pm.
13. Use as claimed in claim 12 wherein the friction faces are machined to an average roughness Ra not exceeding 2.0 pm.
14. Use as claimed in either claim 12 or claim 13 wherein the disc comprises arrays of carbon fibres laid at about 900 to each other.
15. Use as claimed in any of claims 12 to 14 wherein the disc is a carbon fibre-reinforced ceramic brake disc.
16. Use as claimed in claim 15 wherein the disc comprises a siliconised carbon-carbon fibre composite.
17. Use as claimed in any one of claims 12 to 16 wherein the friction faces are machined to a material roughness ratio R of at least 90% when measured 10 pm below a datum line 5% below the highest peak.
18. Use as claimed in any one of claims 12 to 17 wherein the disc is machined by grinding.
19. Use as claimed in any one of claims 12 to 18 wherein the disc is an automotive brake disc.
20. Use as claimed in any one of claims 12 to 18 wherein the disc is an aircraft brake disc.
21. Use as claimed in any one of claims 12 to 20 wherein the enhanced friction properties comprise at least one of increased friction levels and reduced wear.