WO1992008909A1

WO1992008909A1 - Friction members

Info

Publication number: WO1992008909A1
Application number: PCT/GB1991/001992
Authority: WO
Inventors: John Anthony Mudway; John Campbell Watson; Alfred James Taylor; David Andrew Hubbard
Original assignee: Lucas Industries Public Limited Company
Priority date: 1990-11-14
Filing date: 1991-11-13
Publication date: 1992-05-29
Also published as: GB9024702D0; AU8867591A

Abstract

A friction member such as a brake disc has a plasma sprayed surface layer of ceramic material of which at least 50 % by weight is ceramic oxide, particularly alumina. The surface layer, which may be bonded to the material forming the bulk of the friction member by means of a bonding layer, is for use in spreading frictional heat generated when another friction member rubs against a part of said rubbing surface so that the heat transmitted to the bulk of the friction member under the surface layer is controlled to reduce thermal fatigue cracking of the bulk of the friction material.

Description

FRICTION MEMBERS

This invention relates to friction members, for example brake members, clutch members and other members which are subjected to frictional heating in service.

In the case of friction members which take the form of brake discs, such discs are usually formed of cast iron or steel, although there have been proposals to form them of aluminium alloy and other materials. As a result of intensive investigations into the mechanism of heat dissipation in friction members, we have established that one of the primary causes of premature failure of brake discs is as a result of the intensive thermal cycling which takes place under braking conditions. When a friction pad is applied to the brake disc with the latter rotating at speed, each portion of the brake disc contacted by the friction pad at any instant is frictionally heated very rapidly. As that portion continues to rotate, it is subjected to cooling during the remainder of its cycle of rotation until it once again is contacted by the friction pad, whereupon it is subjected to rapid re-heating, and so on until the friction pad is released or until the brake disc no longer rotates relative to the friction pad. This severe thermal cycling eventually leads to thermal fatigue cracking which renders the brake functionally unacceptable and compromises safety. The thermal fatigue cracking arises as a result of the material of construction of the brake disc being incapable of resisting the surface stresses which arise from the thermal flux at the interface between the disc and the friction pad. Although a serious problem in itself, the effect of this thermal cycling is exacerbated because the thermal flow is not uniform, thus giving rise to local "hot spots". The large thermal gradients which are created at the surface resulting in high stresses are in addition to the mechanical stresses caused by rotation and the mechanical rubbing of the friction elements.

US Patent No. 4290510 describes a brake disc or rotor which is formed from aluminium or other lightweight metal to achieve a weight saving, the rotor having a wear resistant coating thereon in order to improve the abrasion resistance of the otherwise low abrasion- resistant aluminium. The wear resistant coating includes about 30 to 70% by volume of a ceramic particulate material of which stated examples are the metal carbides, ie chromium carbide, tungsten carbide, molybdenum carbide, silicon carbide; the nitrides, ie silicon nitride, boron nitride; and the oxides ie aluminates and zirconia, the remainder being binder metal. The preferred ceramics are stated to be the metal carbides and the metal nitrides which are stable materials not readily fluxed by metals or metal oxides such as iron or ferrous oxide. Also, a preference is expressed for the metal carbides on the basis that they have the best heat transfer characteristics of the ceramic materials which aids in dissipating heat from the coating to the rotor. Thus, there is no teaching in US 4290510 to control the rate of heat transfer to the bulk of the brake disc in order to overcome the above-mentioned thermal fatigue cracking problems.

US Patent No. 4715486 discloses brake discs wherein wear of the brake disc is reduced by applying thereto a wear- reducing coating comprising a substantial proportion of tungsten carbide, the object being to provide a virtually abrasion-free brake disc. Thus, US Patent No. 4715486 is also not concerned with controlling the rate of heat transfer to the bulk of the brake disc to reduce thermal fatigue cracking problems. WO89/09889 discloses an asbestos free disc brake device for automobiles and similar vehicles wherein the brake disc is a cast iron body whose opposite lateral faces are cast together with comparatively thin wearing linings having a metallic surface layer with considerably increased wearability. The metallic surface layer is provided by the so-called "Osprey" process and, in a particular embodiment, is said to contain aluminium oxide as its main component. However, as far as we are aware, it is not possible to use the "Osprey" process for spraying a 100% ceramic material. The use of the thin surface layer is said to be for ensuring great hardness and wearability, and a thickness in the range of 50 to 100 μm is disclosed for the surface layer. There is however, no disclosure or recognition of the use of the surface layer for controlling the transmission of frictionally generated heat to the bulk of the brake disc to reduce thermal fatigue cracking.

GB-A-2191156 discloses a wheel rim for a two-wheeled vehicle, eg a bicycle or a motor bicycle, having a spray- coated layer on the region thereof against which a brake shoe is contacted in use. The spray coated layer is a metal and/or ceramic layer and is provided for the purpose of mitigating the problem of loss of braking efficiency under wet conditions. The coating layer is also said to be wear resistant. Examples of suitable metals for the coating layer are stated to be Al, Ti, Mo, Ta, Fe, Si, W, Co, Ni, Zn and Y, and alloys thereof. Examples of suitable ceramics for the coating layer are stated to be the oxides, nitrides, carbides or borides of these metals, with Al₂0₃, gray alumina, Ti0₂, Cr₂0₃, Si0₂, Zr0₂, Y₂0₃, MgO and CaO being specifically mentioned as suitable oxides. There is therefore no disclosure or recognition of the use of the coating layer for controlling the transmission of frictionally generated heat to the bulk of the wheel rim.

The present invention is concerned with friction members which are mainly intended for use in situations where the friction member may be required to handle a relatively high dissipation of frictionally generated heat in service, eg in brakes for heavy and relatively heavy vehicles, such as trains, aircraft, trucks, vans and motor cars.

An object of the present invention is to provide for proper thermal management of the brake disc whereby to achieve a controlled transfer of frictionally-generated heat to the bulk of the friction member in order to obviate or mitigate the above mentioned problem of thermal fatigue cracking.

According to the present invention, there is provided a friction member having a ceramic surface layer defining a rubbing surface, characterized in that the ceramic surface layer is formed of at least 50% by weight ceramic oxide(s), and in that the ceramic surface layer is for use in spreading frictional heat generated when another friction member rubs against a part of said rubbing surface so that the heat transmitted to the bulk of the friction member under the surface layer is controlled to reduce thermal fatigue cracking of the bulk of the friction member.

The surface layer preferably has a combination of thermal conductivity and heat capacity properties at elevated temperatures such as to produce the required control of heat transfer to the bulk of the friction member so as to complement the properties of the bulk material at least under the most arduous conditions anticipated to be experienced by the friction member in service. Typically, the surface layer has a thermal conductivity below 40 Wπf K^'1. The surface layer should be capable of maintaining the relevant properties at the elevated temperatures which are encountered in service. In the case of very heavy duty braking applications, the elevated temperatures may reach 1000°C, and even 1500°C. The surface layer will normally have a melting temperature which is higher than that of the material forming the bulk of the friction member. However, depending upon the intended application, this is not necessarily an essential requirement. For example, the material forming the bulk of the friction member may have a relatively high melting temperature but still be prone to thermal fatigue cracking. Thus, it is possible for the surface layer to have the same or even a lower melting temperature than the bulk material, although of course it should have a sufficiently high melting temperature to be able to withstand the temperatures encountered in service.

The surface layer can have a thermal fatigue resistance which is higher than that of the bulk material. However, this is also not necessarily an essential requirement but depends upon the intended end use. It is possible for the surface layer to develop cracks in service provided that the cracks do not propagate to the bulk material and provided that, in spite of cracks appearing in the surface layer, no spalling of the latter takes place.

The thickness of the surface layer is chosen to suit the particular circumstances of use, but is generally 100 μm to 750 μm, preferably 100 μm to 500 μm. In the case where the surface layer is a plasma sprayed coating, the preferred coating thickness is about 150 to 300 μm and is most preferably about 200 to 250 μm.

To reduce the risk of spalling in service, it is preferred to provide a bonding layer between the surface layer and the material forming the bulk of the friction member, the bonding layer being arranged to be compatible with the two materials with which it is in contact. Typically, for a cast iron or steel body and a surface layer of alumina, the bonding layer is formed of a nickel alloy, eg a nickel alloy containing about 95% nickel and about 5% aluminium.

By using a surface layer as in the present invention, it is possible to control the rate of flow of heat to the body material by spreading the heat laterally within the surface layer before it reaches the bulk material, thereby minimising the risk of localised overheating of the body material in service. It is therefore possible to reduce the thermal gradients which occur in the body material and this reduces the risk of thermal fatigue cracking.

The term "ceramic oxide(s)" includes one or more mixed oxide ceramics as well as simple oxide ceramics. The preferred ceramic oxide(s) is/are metal oxide ceramics, the term "metal" in the context of light duty braking applications including silicon.

More preferably, material forming the surface layer is selected from ceramic oxides based on the metals of Groups IIIA, IIIB, IVA, IVB, IIA, VB and VIB of the Periodic Table of Elements including oxide mixtures and mixed oxides based on any of such metals. Amongst these, particularly preferred are the oxides of the metals of Groups IIIA, IVA, IVB and VIB including oxide mixtures and mixed oxides based on any such metals. Even more preferred are alumina, chromium sesquioxide, zirconia, zirconates, titanates, aluminates, silicates and mixtures thereof. Of these, alumina and chromium sesquioxide, with alumina being most preferred. The surface layer is preferably composed of 100% ceramic oxide(s) .

For compatibility between the surface layer and the body material particularly in cases where there is no intervening bonding layer, it is preferred for these to have thermal expansion coefficients over the temperature range encountered in service (and particularly at the upper end of such range) which are matched so as to act to minimise the stresses therebetween under the conditions where maximum stress would otherwise be expected to occur. Typically, these thermal expansion coefficients are matched to within almost 20%.

The surface layer can be applied to the body material, or to the bonding layer when provided, by any suitable means, eg by plasma spraying, flame spraying or other suitable thermal spraying techniques, or by a detonation gun process (eg the D-gun process - Union Carbide) . The porosity of the surface layer is generally up to about 15% and preferably about 8 to 15%.

It is to be appreciated that the precise choice of material for the coating layer will depend not only upon the desired performance of friction member concerned, but also upon, inter alia, the nature of the body material, the nature of the other friction element which the friction member is intended to cooperate in use, and the operating environment eg corrosion. Since the surface temperatures which exist on the friction member according to the'present invention are rather higher than those which exist with conventional friction members, it is necessary to take this into account when choosing the friction element with which the friction member intended to cooperate in use. For example, in the case where the friction member is a brake disc, the material of the friction pad (forming the friction element) is chosen to be suitably heat resistant and may even be coated with a surface layer which is the same as or at least compatible with that of the brake disc.

The invention will now be described in further detail in the following example:-

Example

A cast iron brake disc having a thickness of 13mm was used. The cast iron had a heat capacity C_P of 500 JK^" kg^"1, a thermal expansion coefficient of 11 x lO^' ", a thermal conductivity of 75 Wπf'K^"", a Young's modulus of llOGPa, a density of 7.150Mg/m³ as measured at room temperature, and a melting point of 1500K. The brake disc was first degreased and then grit blasted with alumina grit. The cleaned brake disc was then plasma sprayed with a bonding coat of a nickel aluminium alloy (95% nickel 5% aluminium-AM956 ex Plasma Teknik Ltd of Newport, Gwent) to a thickness of 100 μm. A top coat of alumina (1003 grade-ex Plasma Teknik Ltd) was then plasma sprayed onto the bonding coat to a thickness of 300 μm and then diamond ground back to a thickness of 200 μm to produce an alumina coating having a porosity of approximately 10%. The alumina coating had a melting point of 2300K and (as measured at room temperature) a heat capacity of 800 JK^"kg^-1, a thermal expansion coefficient of 9 x 10^" K^" , a thermal conductivity of 29 Wm^" K^" , a Young's modulus of 345 GPa, a thickness of 250 μm, and a density of 3.32 Mg/m³.

The resultant coated disc was subjected to a test using a high temperature FERODO X406 material on a scale friction rig simulating a high speed train thermal input. The treatment involved subjecting the disc to drag braking for 30 seconds followed by a cooling time of 3.5 minutes. The power input to the disc was 6.283 kW. Both the disc and the pad were weighed prior to the test starting and the disc was weighed at convenient intervals during the test. Each pad used was weighed before and after each part of the test to estimate the wear rate in terms of MJ/cc. Similarly, the disc wear rate is estimated in MJ/cc and then a comparison was made with known technology. The weight of the pads and discs were related to wear by their respective densities.

Subjected to the above test, the disc was found to complete 5000 test cycles with very little apparent damage and was thus able to complete substantially more test cycles before failure than conventional uncoated discs.

Whilst in the above test, the pad had a maximum contact area during braking of about 30% of the disc face, the beneficial effects of the present invention can be applied to braking systems where full interface contact is intended.

The invention is applicable not only to friction members in the form of brake discs, but any friction members where a sufficient amount of frictionally generated heat is encountered to give rise to the previously mentioned cproblem of thermat fatigue cracking of the bulk of the friction member, eg heavy duty clutches etc.

The surface layer provided in accordance with the present invention can give a degree of corrosion resistance and can also improve the aesthetic appearance of the friction member.

Claims

1. A friction member having a ceramic surface layer defining a rubbing surface, characterised in that the ceramic surface layer is formed of at least 50% by weight is ceramic oxide(s), and in that the surface layer is for use in spreading frictional heat generated when another friction member rubs against a part of said rubbing surface so that the heat transmitted to the bulk of the friction member under the surface layer is controlled to reduce thermal fatigue cracking of the bulk of the friction material.

2. A friction member as claimed in claim 1, wherein the thickness of the surface layer is 100 μm to 750 μm.

3. A friction member as claimed in claim 1 or 2, wherein the ceramic oxide(s) is/are metal oxide ceramics.

4. A friction member as claimed in claim 1, 2 or 3, wherein the material forming the surface layer is selected from ceramic oxides based on the metals of Groups IIIA, IIIB, IVA, IVB, IIA, VB and VIB of the Periodic Table of Elements including oxide mixtures and mixed oxides based on any such materials.

5. A friction member as claimed in any preceding claim, wherein the material forming surface layer is selected from alumina, chromium sesquioxide, zirconia, zirconates, titanates, aluminates, silicates and mixtures thereof.

6. A friction member as claimed in any preceding claim, wherein the surface layer is a thermally sprayed layer.

7. A friction member as claimed in any preceding claim, wherein the surface layer has a porosity of 8 to 15%.

8. A friction member as claimed in claim 1, 2 or 3, wherein the surface layer is formed of 100% ceramic oxide(s) .

9. A friction member as claimed in any preceding claim, wherein a bonding layer is provided between the surface layer and material forming the bulk of the friction member.