BACKGROUND OF THE INVENTION Typical vehicle components for application of the invention are e. g. brake rotors, such a rotor having a disc-shaped portion comprising working face portions, said working face portions being exerted to friction wear during braking.

Prior art components are available, which have disc-shaped portions of the same substantially metallic material as the main part of the component, e g aluminium, an aluminium alloy or a cast iron alloy, the working face or surface also being of the same material. In order to extend the lifetime of such components, attempts have been made to coat the working face with still more wear resistant materials. Such attempts, e g as described in US Patent No. 4,290, 510 and US Patent No. 4,180, 622, have employed very hard ceramic materials. Such coatings, however, are associated with brittleness and/or structural changes and, thus, cracking and/or loss of wear resistant properties to a severe extent.

Therefore, an object of the present invention is to provide a vehicle component with a coating, which is ductile and prevents cracking and does not undergo structural changes during use.

A further object of the present invention is to provide components with an improved wear resistance.

Yet another object of the present invention is to improve the protection of the coating against undesired damages and thereby further extending the useful life of the vehicle component.

SUMMARY OF THE INVENTION The object of the present invention is obtained by a wear resistant, coated vehicle component according to the accompanying claim 1. Some embodiments are specified in the corresponding dependent claims.

Furthermore the invention relates to a vehicle comprising at least one such wear resistant coated vehicle component.

Advantages and achievements of the present invention according to the claims are described below.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described by way of example with reference to the accompanying diagrammatic drawing in which: Fig. 1 schematically shows an axial section of a brake disc according to a first embodiment of a wear resistant coated vehicle component according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS In the following description and claims all percentages given for elements in the alloys specified are given as percentage by weight.

In Fig. 1,1 designates a brake disc component according to the invention, said brake disc having a disc-shaped part 2 with a main extension plane substantially perpendicular to the axial direction 3 of the brake disc component. The disc-shaped part 2 comprises at least one, but preferably two working faces 4 arranged and adapted to be exerted to relative movement friction wear, normally by means of a vehicle brake pad (not shown) being applied against the working face and pressed against said working face in a braking action, said working face rotating.

According to the invention the working face is formed by a wear resistant top coating 5 made substantially of molybdenum, a typical composition comprising more than about 99 % molybdenum, the balance being normal impurities, including iron when applicable.

The thickness of the top coating 5 is about 100 to 600 um, preferably about 500 p. m.

Further a bond coating 6 is in the shown embodiment applied to the base material of the component to bond the top coating to the base material. Different bond coating compositions may be imagined. According to an embodiment the bond coating comprises a nickel-chromium-aluminium-yttrium alloy. According to another embodiment the bond coating comprises an iron-chromium-aluminium-nickel- yttrium alloy. According to yet another embodiment the bond coating comprises a cobalt-chromium-aluminium-yttrium alloy.

The base material in the above referenced brake disc embodiment may be e. g. an aluminium alloy, e. g. the US standard alloy A 356 or the Swedish standard alloy SIS 4245 comprising, mainly, 7 % silicon and the balance aluminium, or a cast iron alloy, e. g. a standard grey cast iron alloy comprising about 3-3,7 % carbon, 2,1- 2,6 % silicon, 0,5-0, 8 manganese, the balance substantially iron.

According to an embodiment the thickness of the bond coating is about 0-100 um, preferably about 50 pm. According to certain embodiments, especially using cast

iron as a base material, the top coating may be applied without a bond coating. Tests with Molybdenum directly applied to the base material have in some circumstances given a longer lifetime for the vehicle component than a corresponding vehicle component with a bond coating.

Further, according to preferred embodiments both the top coating and, when applicable, the bond coating are applied by a thermal spray process. However, other known processes for applying the coating/-s may be used.

Furthermore, it is essential to the invention that the top coating and, when applicable, the bond coating are finished or terminated on a component surface or face 8, which is not identical to the working surface of the component. According to one embodiment, the top coating and the bond coating end substantially perpendicular to the working face plane, e. g. as can be seen in Fig. 1 where the respective coatings are finished on the cylindrical peripheral surface 9 of the disc- shaped part 2 of the brake disc. This is in order not to expose the end face 10 of the coatings to corrosion etc on the working face or at the peripheral edge 11 of the working face. Such exposure may damage at least parts of the coatings due to high working temperatures etc of the coatings.

It is of course also within the invention to end the top coating and bond coating on another surface on a vehicle component other than a surface substantially perpendicular to the working face plane. In an embodiment where a working face is adjacent to a chamfered edge surface forming an angle to the working face, it may be suitable to end the coatings on the chamfered edge surface adjacent to the working surface.

Described here as an example is a brake disc. However, coatings according to the invention may be applied to other vehicle components, e. g. components being made of an aluminium alloy or a cast iron alloy as a base material and preferably being manufactured by an initial casting process. Examples of such components where the

technical features according to the claimed invention are particularly advantageous are, besides brake discs, clutch components, crankshafts and flywheels. The technical features according to the claimed are however advantageous also for other vehicle components such as cast components in valve systems, gear box components, spring components and gear-wheel components.

Example 1 A brake disc cast from an Al Si7 aluminium alloy was, as far as the two working faces were concerned, coated with a standard bond coating and a top coating of 99 % molybdenum. The coatings were applied by a plasma spray process. The thickness of the bond coating was about 50 u. m and the thickness of the top coating was about 310 lem.

Retardation tests showed that the friction properties of the top coating using ordinary vehicle brake pads corresponded substantially to those of the base material.

Further optimisation may be made.

Example 2 A brake disc cast from a standard grey cast iron alloy comprising about 3,5 % carbon, 2,0 % silicon, 0,4 % manganese, 0,4 % copper, the balance iron and normal impurities was, as far as the two working faces were concerned, coated with a top coating of 99 % molybdenum. The coating was applied by a plasma spray process.

The thickness of the coating was about 300 urn. Results comparable to those described in Example 1 were obtained.

Molybdenum has a thermal expansivity, which is smaller than the one of e. g. the intended base materials aluminium, aluminium alloys and cast iron alloys for components to be coated. This means that compressive stresses appear in the base material of the working face when the temperature is elevated during use of the

component since the molybdenum coating restricts the expansion of the base material. This, in turn, restricts the formation of cracks in the working face base material. Further, the molybdenum top coating has a thermally isolating influence.

The top coating has proven to be especially beneficial and applicable to a vehicle component, especially if it is a brake disc, a flywheel, a rotatable clutch component if the top coating has a surface roughness Rz ranging between 10 and 60 um, where Rz is the Average Maximum Height of the surface profile. It is also advantageous if the top coating has a surface roughness Rk ranging between 2 and 15 urn, where Rk is the Core Roughness Depth; a surface roughness Rpk ranging between 0.5 and 10.0 u. m, where Rpk is the Reduced Peak Height; a surface roughness Ryk ranging between 5 and 20 um., where Ryk is the Reduced Valley Depth and a surface roughness Ra ranging between 2 and 10 um, where Ra is the Arithmetical Mean Deviation. Rz, Rk, Rpk, Rvk and Ra are all known surface roughness parameters known to a person skilled in the art and are therefore not described more in detail.

Above the invention has been described in association with examples and different embodiments. Of course further embodiments and minor changes and additions may be imagined without leaving the basic inventive idea.