GB2529382A

GB2529382A - Lead-free tin or tin-based overlay for a plain bearing

Info

Publication number: GB2529382A
Application number: GB1411055.5A
Authority: GB
Inventors: Yi Zhang; Madan Pal; Elena Banchelli; Ian Kerr
Original assignee: Daido Metal Co Ltd
Current assignee: Daido Metal Co Ltd
Priority date: 2014-06-20
Filing date: 2014-06-20
Publication date: 2016-02-24
Also published as: GB201411055D0

Abstract

A sliding bearing containing a multilayer overlay 11 deposited on the bearing metal layer 13 or optional intermediate layer 12 which contains a layered stack of relatively soft sub-layers 11A interleaved with relatively hard layers 11B. The hard sub layers 11B contain an intermetallic compound of tin (Sn) and at least one other metal element and may contain hard particles (such as PTFE) or soft particles (such as metal oxides) dispersed within. The inter-metallic layers 11 form a composite metal stack from the metal mixtures/alloys and each layer maybe deposited on the previous layer by electroplating. A method of providing such a bearing overlay involves heat treatment.

Description

Lead-free tin or tin-based overlay for a plain bearing

DESCRIPTION

1. Field of the Invention

This invention relates to generally sliding elements such as plain bearings of internal combustion engines, comprising a back layer, a bearing lining layer, an intermediate or anti-diffusion layer and a laminated multilayer overlay consisting of a group of tin-based soft metallic sub-layers separated by hard intermetallic compound sub-layers.

2. Related Art Sliding elements, such as plain bearings of internal combustion engine, often include a metallic copper or aluminium lining alloy bonded to a steel backing. The copper or aluminium alloys provide a strong surface that can withstand the loads subjected on the sliding element in use. Such sliding elements should also exhibit suitable seizure resistance as well as good embedability and conformability, and for this purpose an overlay layer is usually added on top of the lining layer. Historically this layer might well have been a lead or lead alloy. Lead has been proven as a very reliable overlay material combining the properties mentioned above and also providing reasonable fatigue resistance to external loading.

Due to the envronmental considerations shding heahngs manufactured *with lead-free overlay are demanded and requced for the dev&opmmt of new engines. \/arft>us p rernents for cad have been explored. suc:h as tin andt in has ed niloys. Qftsm a rdck& diffuson bamr'r ayer s interposed betwe.en the tin or Un based alkiy oveday and lining layer to prevent the tin from diffusing from overlay into the bearing lining alloy.

Attempts have been made to use conventional pure tin as the overlay material. However conventonal tin is extremely soft wfth typcal Vickers hardness between lHv-iOHv and such a soft material is incapable of withstanding even mild loads in medium and higt speed diesel engines nowadays, US7174637B2 describes a plain bearing structure with an electroplated pure tin overlay having an organic levelling agent in the matrix. Such a structure increases the hardness of the overlay up to 3OHv and consequently the wear resistance and fatigue resistance are improved relative to conventional tin. However the problem for pure tin is that it is prone to diffusion as compared to tin-based alloys. The tin consequently reacts with the entire nickel barrier layer in a very short time leading to the exposure of tin overlay to the underlying lining elements. If this occurs some detrimental intermetallic compounds between tin and the lining elements may form which can subsequently lead to a catastrophic failure of the bearing and engine.

In order to ameliorate the diffusion problem whilst keeping suitable mechanical/tribological properties, various tin based alloy overlays have been developed for plain bearing applications of internal combustion engines. 0E19728777A1 describes a composite multilayer materials for sliding elements, the overlay of which consists of a lead-free alloy comprising tin and copper, wherein the copper content amount is 3-20 wt% and the tin content is 70-97% wt%. The addition of copper increases the overlay hardness, fatigue and wear resistance relative to pure tin and also slows down the diffusion of tin by forming alloys with tin in the overlay. Such a tin-based alloy overlay therefore provides much improved performances at the early stage of engine operation. However the copper in the overlay tends to migrate downwards in the overlay at elevated temperature. Therefore after prolonged engine operation the majority of the copper ends up at the overlay-barrier layer interface. ieavng a copperdepIeted pure tin section the overlay. ThLs s particularly 1fl issue for medium and high speed diesel eng.ines where a relative thick ove day is requred, After the rr per moves away a r&atively thick pure tin:iec on s left in the overlay s.truct.,re which is weak and prone to fatigue attack under eternai loads.

U52006/0263625A1, US649203982 and US635791761 describe an additional hard layer being applied in the tn alloy overIay structure to further enhance the properties of the bearing. the hard layer is created either on the outmost surface of the overlay or along the overlay, diffusion b arriei layer interlace These hard hyers morove the hard properties of the original tin based alloy overlay in different ways but none of them solves the thick and weak pure tin section issue associated with the migration of alloy elements in tin-based alloy overlays under long term engine operation.

A further approach has been made in US6312579B1 which creates a multilayer material with alternating hard-soft layers to enhance the overlay properties. It describes a lead-free multilayer overplate on the base lining member including at least two distinct lead-tree layers electroplated at different current densities from same bath to provide multiple lead-free overplate layers with different deposit characteristics. No further details were provided but the migration of alloying elements in the tin based alloy overlay at high temperature, either towards the overlay-barrier layer interface, for example copper in the tin matrix, or homogeneously throughout the overlay, for example silver in the tin matrix, still remains unsolved. Therefore the multilayer structure may disappear in a short time at engine operation temperature and the desirable properties as the result of which the multilayer structure may not last long. Furthermore, as the multilayer material is created by electroplating from the same bath the variety and hence the properties of the multilayer overlay structure is therefore limited.

3. Objective of the invention It is the objective of the present invention to provide a lead-free tin based multilayer overlay for a sliding element, for example a plain bearing of internal combustion engine which exhibits high loading capacity, good wear and seizure resistance and other necessary properties for medium and high speed diesel engine applications.

It aiso an obec:tive of i.he present vention to overcome certain ch.sadvantages of the her art whereby the I vented.muhiayer overiay hucture remams stahkt and does not dissipate at engine temperature, so that the desirable. properties associated with the structure may be kept duhng prolonged engine operation,

4. Summary of the hivention

One or more objects of the invention is/are achieved in accordance with the present invention by a siidin beadng, in parflcular for plam heahns, as set forth in the doims hereinafter, The beanng typcaUy exhibits a hack iayer, a bearing metal ayer of a copper alloy or an aluminium alloy, an intermediate layer and an overlay, wherein the overlay comprises a group of tin or tin based soft metallic sub-layers separated by hard intermetaltic compound sub-layers.

Thus in accordance with one aspect of the present invention there is provided a sliding bearing comprising a backing layer, a bearing metal layer, an optional intermediate layer and an overlay deposited on the bearing metal layer or intermediate layer when present, which overlay comprises a stack of relatively soft sub-layers interleaved with one or more relatively hard sub-layers, wherein the relatively soft sub-layers comprise tin or tin-based alloy and the relative hard sub-layers comprise one or more intermetallic compounds.

According to the present invention the hard intermetallic compound sub-layers separate the overlay into two or more thinner sub-layers whereby a laminated multilayer overlay structure is created. The laminated overlay structure significantly improves the fatigue performance of the original overlay. This is achieved by the much reduced accumulation of plastic deformation under external loading associated with the shorter progression route of a thinner layer and consequently the multilayer is less prone to fatigue crack initiation.

According to the present invention the hard intermetallic compound sub-layers also create a series of soft-hard interfaces which further enhance the fatigue resistance of the overlay.

Studies have shown that the soft-hard interface is very effective in blocking the propagation of plastic deformations as well as terminating the fatigue crack progression should it eventually occur. Therefore the chancec of crack ntiatic,n and overlay de!.amination under externE ioad.aremnimired Accordina. to aspects of the present nvMWofl the ntermetaiic compound suh-ayers are thermallystable alloys. Hence at engine operation temperature the movement of the alloying elements in the tin matrix is confined in a relatively thin sub-layer which consequently eliminates the problems associated with the formation of thick pure tin sections as highhghted in the prior art, The muftayer overlay structure is therefore stabiiised which maker.. it suitabP2 for cfrng term engine operatou wfthout, a detenoration of the desired petormance cha actedstics.

According to the present invention the intermetallic compounds of which the sub-layers are made provide a hard but very good bearing material. It offers superior wear and seizure protections particularly when the overlay is worn through to the intermetallic compound sub-layer. The intermetallic sub-layer also has very low internal stresses so it possesses a low risk of stress-induced cracking in the overlay system.

The tin and tin based sub-layers may be applied by any conventional coating methods with preferred compositions as follows: tin, bright tin, tin-copper, tin-silver, tin-gold, tin-antimony, tin-cobalt, tin-nickel, tin-bismuth, tin-indium, tin-iron, tin-zinc and tin-manganese or the combination of any two of them.

The tin and tin based sub-layers may also include one or more soft particles selected from the group consisting of PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal sulphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, zinc oxide and lead oxide.

The tin and tin based sub-layers may also include one or more hard particles selected from the group consisting of metal oxides, borides, carbides, nitrides, sulphates and suicides, diamond, carbon nanotubes, graphene and other hard carbonaceous particles, and other particles that may present cubic structures.

The tin or tin based sub-layers in one oveday structure can he same or different n c:ornnosfton the thickness of tri ond tin based wh iavers is advantareousk' from I urn to C: pm pre er y 5 pm to 1.0 urn.. .A thin mart in or Un based sub-i ave rtvp icaiiy does not.

provide the required conformability and embeddability, whilst a thicker suh4ayer may compromise the fatigue strength and the benefits of the original multilayer structure.. The number of tin and tin-alloy based sub-layers is defined as k, where k»=2.

The intermetallic compound sub-layer may be created by appiyng a metallic layer using any conventonal coatng roethud then followed by thermal prc'c.essutg to perint migration of atoms to form ntermetathr grains. Preferred metalhc: ayers are made from one material selected from the group consisting of nickel, cobalt, iron, copper, zinc, silver, gold or alloys thereof. The heat treatment is particularly suitable to form a beneficial intermetallic compound layer with preferred compositions as follows: nickel-tin, cobalt-tin, iron-tin, copper-tin, zinc-tin, silver-tin and gold-tin or the combination of any two of them.

The intermetallic compound sub-layer may also include one or more soft particles selected from the group consisting of PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal suiphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, titanium oxide, zinc oxide and lead oxide, and other particles that may present hexagonal structures.

The intermetallic compound sub-layer may also include one or more hard particles selected from the group consisting of metal oxides, borides, carbides, nitrides, sulphates and silicides, diamond, carbon nanotubes, graphene and other hard carbonaceous particles, and other particles that may present cubic structures.

The thickness of each intermetallic compound sub-layer is advantageously from 0.1 jim to 8 pm, preferably 2 pm to 5 jim. A thicker intermetallic layer typically offers no further benefits in hard properties and tends to deteriorate the conformability and embedability of the overlay as a whole, and also increases the manufacturing costs. The number of intermetallic compound sub-layers is preferably k-i, where k»=2.

The hardness of the tin and tin based sub-layers is advantageously from lOHv to35Hv.

preferahiy i2Hv to 2OHv, A softer iayer provides inadenuate loading ciapabLlitv wbik a harder one exhibits a loss.of the soft pr' perhes, The hardness of intermetallic compound sub-layer is advantageously no less than 200Hv, preferably no less than 360Hv.

The ratio between the hardness of the ntermetaIlic compound sub-layer arid the hardness of th tin or tin based sub-layer is defined as (r). r Is reater than 6, advantageously greater than 15 and more preferably greater than 30 to achieve a beneficial hard-soft interface effect.

The overall thickness of the laminated multilayer overlay is advantageously from 5 pm to 50 pm, preferably 15 pm to 35 pm for medium and high speed diesel engine applications.

5. Brief Description of the Drawings

Figures la and lb show a schematic cross-section through a preferred embodiment of a sliding bearing according to the present invention, respectively before and after a heat treatment is applied.

Figures 2a and 2b show a schematic cross-section of an alternative preferred embodiment of a sliding bearing according to the present invention, respectively before and after a heat treatment is applied.

Figure 3 is an electron photomicrograph which shows an SEM cross-section image of the laminated structure of multilayer material obtained as described in Figure 1.

Figure 4 shows the seizure resistance of nickel-tin intermetallic compound layer according to the present invention in relation to a benchmark tin-copper layer.

Figure 5 shows the wear resistance of nickel-tin intermetallic compound layer according to th.e present invention n relator to a benchmark tm-copper layer.

able I Jiow:; the Sanphirc fatigue test rews zf tha cnn ftflayec overay according to the )Thient nvention having a tin coppe 3/nickel-tn/tin-copper3 muftilayc r 3tructw e and benchmark materials, Table 2.shows the coefficient of friction of nickel-tin intermetallic compound layer according to the present invention in relation to other known bearing rnatedais measured by scratch tester.. a

6. Detailed description of the drawings

Figure la shows a cross-section of a preferred embodiment of a sliding bearing according to the present invention. The bearing comprises a strong backing 14, a lining metal layer 13 bonded to the backing, an intermediate layer or diffusion barrier layer 12, and an overlay layer 11. The backing layer 14 may be steel or any other suitable materials such as bronze or aluminium alloy. The lining metal layer 13 niay be any suitable materials but in practical use is either copper based alloy or aluminium based alloy. The intermediate layer or diffusion barrier layer 12 is designed to prevent tin from diffusing into lining alloy in case of the copper based lining. The intermediate layer 12 is generally deposited by electroplating method and may comprise a layer of nickel or nickel based or any other suitable materials.

The thickness of the intermediate layer 12 generally lies in the range between him to 5j.xm.

The multilayer overlay 11 comprises three layers with two tin based sub-layers ha with reinforcing alloy element lic separated by a hard intermetallic compound sub-layer lib.

The multilayer structure is manufactured by a combination of conventional coating methods such as PVD or electro-deposition and thermal processing. The overall thickness of the overlay is in the range between Sjim to SOpm.

Figure lb shows that after 500hr engine operation, typically between 100°C to 150°C, the laminated multilayer structure remains intact, but the alloying element lic has migrated to form planer intermetallic compound sub-layers (hld÷llb).

Figure 2a shows a cross-section of an alternative preferred embodiment of a sliding bearing accorthng to the present invention. In the embodiment of the igure / the muftilayer overlay comprises o seven layers with four Un based st.thlayers 21'a wfth reinforcing alloy element 2k separated by three hard intermetaflic compound subayer 2.Ib, Ta fflUftHa%'er structure is again manufactured by the combination of conventional coating methods and thermal processing.

The overall thickness of the overlay is in the range between Sum to bOurn. Figure 2 also shows that after 500hr engine operation, typically between 100°C to 150°C, the larnuiated mult[Iaye r structure ernans intact de5pite the nigration of the alloying element 2k to form ntcrmeta.Uir compound sub1avers (21d+2thL Figure 3 shows an SEM cross-sectional photomicrograph image of the laminated structure of the multilayer material obtained as described in Figure lb. The multilayer overlay comprises three distinct sub-layers with a hard intermetallic compound sub-layer 31b sandwiched by two tin based sub-layers 31a. The overall thickness of the overlay is approximately 2Spm and this comprises a top tin-based sub-layer and a bottom tin-based sub-layer of approximately ten microns each. An intermetallic compound sub-layer of approximately five microns depth is formed between the two tin-based sub-layers. The hardness is about lSHv for both the top and the bottom tin based sub-layers. The hardness of the intermetallic compound sub-layer on the other hand is 5SOHv, The hardness ratio therefore is approximately 36-37. rfl m

Z rn rn rd o - rn ___-z 171 If) o -to Z m Lfl m In LI) If) if) m r-.l 3 Z If) If) If 1 Li)

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C C ° ti) vl In I 1-.

-C -C o 0

O C-

jir H

C

I,,. [IIh. L -. C.. C In C I C: A n 7 7) , -v -d) .- -.1! -F I-. I r -. Li »= -,-.-rnrn-.-a m E ctot S tiC-.) C. t *.-.) C L t.L i E i E E E.5 E * E Il TI I -H (3 o e-c) ci) J lit it I I 0 I" -1-4

VI I C II) 0 C)

Table 1 above shows the results of the fatigue strength tests conducted on the bearings having the overlay specified according to the present invention. The tests were carried out on a Sapphire fatigue test rig under specific loads ranging from 3OMPa to 7OMPa for 20 hours for each load. After the test the bearings were taken out, cleaned, visually examined and rated from between 1 and 5. So S represents the bearing having no indication of any fatigue cracks on the surface whilst 1 is the lowest fatigue rating indicating the bearing is damaged severely by the fatigue test load.

Two groups of bearing overlays according to the present invention were tested. The first group has a tin-copper3/nickel-tin/tin-copper3 multilayer structure but one of them) T-1 was tested as manufactured, and the other one, 1-2 was heated treated at engine operation temperature for 500 hours before testing, The second group has a tin-silver3/cobalt-tin/tin-silver3 multilayer structure and again one of them, T-3 was tested as manufactured. The other one, 1-4 was heated treated at engine operation temperature for 500 hours before testing. The thickness arrangements of tin-copper3/nickel-tin/tin-copper3 multilayer and tin-silver3/cobalt-tin/tin-silver3 multilayer are lOpm/Spm/lOpm with the overall thickness being approximately 25pm.

Two comparison overlays, C-i and C-2, comprise a 25Mm tin-copper monolayer and a 25pm tin-silver monolayer with identical copper and silver content (3wt%) were also tested under the identical Sapphire test conditions. The other two comparison samples, C-3 and C-4, are muitilayer overlays comprisng tincopper sub-layers plated from the same electroiyte with diferent. ccppec ccnl.et m 3w1% and I 2wt%. as described in the prier ai., As may be senn in Fiue 5, the fatig.ue strength of the tn..copper/nftke!-tm/tin.-copper:4 muitilaver oueriay structure Li and 1-2. according to the present invention k signihc:antiy greater than the comparison materials C-*1, C--3 and 04. which represent the prior art bearing overlays.

Similarly the fatigue strength of tin-silver3/cobalt-tin/tin-silver3 multilayer overlay structure 1-3 and 1-4 according to the present invention i5 higher than the comparison materials C-?.

The superior fatigue strength is thought to be achieved by the fact that the muftilayer structure according to the present invention introduces a thermaiq stabe intemnietahc; compound sublayer in the structure which not only strengthens the as-manufactured structure but also minimises the movement of the alloy elements in the tin based sub-layer.

Accordingly the multilayer structure remains even after prolonged engine operation at high temperature. In contrast, though the comparison material C-3 has improved fatigue strength to a certain extent relative to C-i because of the multilayer structure after plating, the improvement quickly disappears after SOOhrs engine running due to the movement of the alloying element. This subsequently leads to the disappearance of the multilayer structure, for which refer to C-4.

Table 2

Material Coefficient of Friction Nickel 0.66

________________ _________________

Comparison materials Silver 0.16 Silver alloy 0.30 Test material NiSn 0.17 Table 2 shows the coefficient of friction of nickel-tin intermetallic compound overlay layers according to the present invention in relation to other known hearing materials measured by a scrtch tester. Of the listed becing maeriais nc k& s known to have a poor frictional prooerties and s therefore extremely prone to siz.urr-attack once it is exposed w a 5teei shaft counterpart, On the other hand despite of the high cost, siiver is a known hearing material offering low coeffident of friction and hence very high seizure resistance to the rotating steel shaft counterpart. The nickel-tin intermetallic compound sub-layer exhibits a coefficient of friction comparable to pure silver and therefore has similar seizure resistance if it is exposed to the rotating shaft counterpart.

Figure 4 shows the seizure resistance of the nickel-tin intermetatlic compound overlay layer according to the present invention relative to a benchmark tiri-copper3 layer. The maximum toad fDr tin-copper3 layer to seizure is averagely 53-S4MPa. By contrast the nickel-tin intermetallic compound layer did not fail even at 56MPa, which is the limit of the test rig.

The nickel-tin intermetallic layer therefore offers similar or better seizure protection in relation to the known seizure-friendly materials, for example tin-copper layers or silver layers, once the intermetallic layer is exposed to a rotating steel shaft, which corresponds to a worn sliding bearing.

Figure 5 shows the wear resistance of the nickel-tin intermetallic compound overlay layer according to the present invention in relation to a benchmark tin-copper layer. The wear is measured as the reduction of thickness before and after the test on the test piece, then calculated as an average wear. As shown in the figure the average wear of the nickel-tin intermetallic compound layer according to the present invention is less than 30% that of the comparison tin-copper3 layer. Thus the wear is significantly reduced once the tin-copper3 layer is worn out and the shaft runs on the nickel-tin intermetallic compound layer.

Claims

Claims 1. A sliding bearing comprising a backing layer, a bearing metal layer, an optionat intermediate layer and an overlay deposited on the bearing metal layer or intermediate layer when present, which overlay comprises a stack of relatively soft sub-layers interleaved with one or more relatively hard sub-layers, wherein the relatively soft sub-layers comprise tin or tin-based alloy and the relative hard sub-layers comprise one or more intermetallic compounds.
2. The sliding bearing of claim 1 wherein each hard sub-layer comprises an intermetallic compound of tin with at least one other metal element, and preferably comprises an intermetallic compound of at least one of: nickel-tin, cobalt-tin, iron-tin, copper-tin, zinc-tin, silver-tin and gold-tin alloys or a combination of any two of them.
3. The sliding bearing of claim 1 or claim 2 wherein the hard sub-layer has dispersed therein one or more soft particles selected from the group consisting of: PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal suiphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, titanium oxide, zinc oxide and lead oxide, and other particles that may present hexagonal structures.
4. The sliding bearing of any of the preceding claims wherein the hard sub-layer has cbspersed therein ona or more hard particles selected from the group cons.sting of meta oxides. normes carthdes, rltrides, sulphates and sitiddes, diamond, carbon.rinotues,grapherc. oh other hard carbonaceous pzni ides, and other particles that may present cubic structures.
5. The sflding bearing of any of the preceding claims wherein there i.a plurality of hard sub-iayers in the oveday each sub-layer comprising the same intermetaiiic composition.
6. The sliding bearing of any of claims 1 to 4 wherein there is a plurality of hard sublayers in the overlay, at least one of which sub-layers comprises a different intermetallic composition to the others.
7. The sliding bearing of any of the preceding claims wherein the thickness of each hard sub-layer is from 0.1 pm to 8 pm, preferably 2 pm to 5 pm.
8. The sliding bearing of any of the preceding claims wherein the hardness of each hard sub-layer is no less than 200Hv, preferably no less than 36OHv.
9. The sliding bearing of any of the preceding claims wherein each tin or tin-based alloy soft sub-layer comprises at least one of: tin, bright tin, tin-copper, tin-silver, tin-gold, tin-antimony, tin-cobalt, tin-nickel, tin-bismuth, tin-indium, tin-iron, tin-zinc and tin-manganese or a combination of any two of them.
10. The sliding bearing of any of the preceding claims wherein one or more of the tin or tin-based alloy soft sub-layers also includes a dispersion of soft particles selected from the group consisting of: PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal sulphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, titanium oxide, zinc oxide and lead oxide, and other particles that may present hexagonal structures, and combinations thereof.
11. The sliding bearing of any of the preceding claims wherein one or more of the tin or tin-based alloy soft sub-layers as claimed in dam 1 further includes a dispersion of hard particles ekected from the group conskting of metai oxIdes, bondes, carhides, nitrdes suphates and silicdes, diamond, carbon nanntue5, gruohe e and other hard Donaceous narudes, and other partiues that may present cubr: strLc:tures, and combinations thereof.
12. The sliding bearing of any of the preceding claims wherein the tin or tin-based sub-layers the overay have the same composition.
13. The sliding bearing of any of the preceding claims wherein at least one of the tin or tin-based alloy sub-layers has a different composition of tin or tin based-alloy from one or more of the others.
14. The sliding bearing of any of the preceding claims wherein the thickness of the tin or tin-based alloy sub-layers is from 1 pm to 30 pm, preferably 5 pm to 10..tm.
15. The sliding bearing of any of the preceding claims wherein the hardness of the tin or tin-based alloy sub-layers is from SHy to 3SHv, preferably l2Hv to 2OHv.
16. The sliding bearing of any of the preceding claims wherein the ratio of the hardness of the hard sub-layers to the hardness of the soft tin or tin-based alloy sub-layers is greater than 6, preferably greater than 15 and more preferably greater than 30.
17. The sliding bearing of any of the preceding claims wherein the bearing metal layer is either a copper-based alloy or an aluminium-based alloy.
18. The sliding bearing of any the preceding claims wherein the intermediate layer is present and comprises a layer of nickel or nickel-based alloy.
19. The sliding bearing of any of the preceding claims wherein the intermediate layer is present and is deposited by electroplating.

20, ht slithng bearing of any of thet rireL.eding caims wherein intennedato hyer is present and has a oP between 1pm to:Ltrn 21. The sliding bearing of daim 20 wherein the overall thickness of the overlay is from 5 pm to 50 pm, preferably 15 pm to 35 pm.22, The sliding bearing of any of the preceding claims wherein the or each hard sublayer is initially deposited as a layer of a first metaflic material, and wheroin a heat treatment, step apr4ied to the overla which catnes tin atoms n the soft sub1ayer to migrate to the first material sub-layers, thereby to form one or more hard intermetallic compounds, by which the hard sub-layers are formed.23. The sliding bearing of claim 22 wherein the said first metallic material comprises a material selected from the group consisting of: nickel, cobalt, iron, copper, zinc, silver, gold or the alloys thereof.24. The sliding bearing of claim 22 or 23 wherein the heat treatment step is applied for sufficient time and at a sufficient temperature to ensure that the microstructure becomes thermally stable during use at engine operating temperatures.25. A method of forming an overlay on a sliding bearing comprising: providing a sliding bearing comprising a backing layer, a bearing metal layer, and an optional intermediate layer, depositing sub-layers of tin or a tin based alloy interleaved by one or more sub-layers of another metallic material thereby to form a stack of sub-layers which forms the overlay, applying a heat treatment to the overlay at a temperature which is sufficient to cause migration of tin atoms in one sub-layer towards and into the other metallic material in the other sub-layer, thereby to form in the other sub-layer one or more intermetallic compounds each comprising tin and the other metallic material, so that sub-layers are developed in which the tin or tin-based alloy sub-layers are relatively soft and the sub-layers.comprising intermetallc c:ornpound are relatively hard.16. A m thod o*T orm.ing an ove.ri ay n accor dance wfth c. laim 25 *wh eren the.Sd ing bearing is a sliding bearing in accordance with any of daims i to 24.27, The method of claim 25 or claim 26 wherein the heat treatment temperature is between 80 to 200 06, preferably 100 and 150 "C.