GB2509164A - Sliding bearings and methods of forming - Google Patents

Abstract

A sliding bearing 201 has a tin-based overlay layer 205 comprising up to 7 weight % of bismuth. The overlay layer may consist of tin and bismuth, apart from incidental impurities. The sliding bearing preferably comprises at least 0.25 weight % bismuth and may be a bush, a bearing shell or a thrust washer. Also disclosed is a method of forming a sliding bearing comprising providing a substrate as a cathode in an electrolyte comprising tin and bismuth ions and depositing a tin-based overlay layer comprising up to 7 weight % bismuth with a cathodic bias. The layer may be deposited with a DC cathodic bias or with a cyclically pulsed cathodic bias.

Description

SLIDING BEARING

The present nvention relates to sliding bearings having a tin layer or tin-based alloy layer, and more particularly, but not exclusvely, to sliding bearings for connecting rod bearings and crankshaft main bearings.

BACKGROUND

Known engne sliding bearings comprise a strong backng layer, typically of steel, a copper based lining and a metal overlay layer, whioh provides the running surface for the rotatably held journal (e.g. crankshaft journal or pin). Such sliding bearings comprise bearing shells, thrust washers for bearing shells, and bearing bushes.

Pairs of hollow semi-cylindrical bearing shells are typically assembled in the big end of a connecting rod to connect onto a crankshaft pin, and in the housing (engine block) to support the crankshaft. In the case of crankshaft main bearings, it is known to provide a bearing shell with a thrust washer that is generally semi-annular, annular or circular. Bearing bushes are hollow cylindrical bearing shells, that may be solid sleeve bushes, splt bushes (n whch a strip is formed into a cylinder with butt-jointed ends) or clinch bushes (like split bushes, additionally provided with mutual engagement features on the ends of the strip).

Overlay coatings are used to improve the running characteristics of sldng bearings. The overlay coating s deposited on the surface of the harder bearing alloy and endows the finshed bearing with properties which include conformability and the abilty to embed dirt particles, and so prevent scoring of a shaft journal by particles of debris carried in the lubricating oil. Although overlay materials in their bulk form are relatively weak, they have the ability, when applied as a thin layer to another harder bearing alloy, to increase the fatigue strength of a bearing embodying that harder and intrnsically stronger bearing alloy.

This is due to the conformability of the overlay material, which is able to deform slightly to accommodate slight misalignments, especially in new engines during the "running n' phase, and so distribute the load more evenly across the bearing surface area.

It is known to use overlay coatings that are relatively soft metals or alloys having a hardness in the region of about 2OMHK (Knoop micro-hardness measurement), which are deposited on another harder bearing alloy, at a thickness in the range from about 10 to 30Mm. Overlay materials of the type under consideration are usually applied by electro-deposition from aqueous plating solutions. For example, WO2004007809 discloses a known tin-based overlay of this type, in which an organic levelling agent (e.g. nonylphenolpolyglycolether or pyrocatechol) is ncluded within the electrolyte to enhance the smoothness of the deposited overlay, and the levelling agent becomes incorporated in the deposited overlay at a low level.

When tin or tin-alloy overlay layers are deposited upon a harder bearing alloy such as a copper-based alloy layer, for example, a problem exists that under engine operating conditons the tin tends to diffuse out of the overlay into the underlying bearing alloy. This is addressed by coating the surface of the underlying, harder bearing alloy with a thin diffusion barrier of a suitable metal, e.g. about 1-3pm of nickel.

An exemplary known bearing shell has a hollow generally semi-cylindrical steel backing layer, a copper-based alloy lining layer, a nickel diffusion barrier layer, and a pure tin overlay on the inner surface.

It is desirable to provide increased wear resistance and to improve the fatigue strength of tin overlay layers. A particular challenge to the performance of sliding bearings is provided by the configuration of vehicle engines to save fuel by usng a stop-start operation, in which the engine is stopped each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehcle's journey. Engines configured for stop-start operation may restart approximately one hundred fimes more frequently than conventionally configured engines. The particular problem that an engine configured for stop-start operation presents arises because engine bearngs are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearings when the engine starts, leading to particularly signficant wear during the start-up phase.

With the ever increasing demands placed on bearings there has been a demand for these relatively soft overlay alloys to have improved wear resstance whilst at least maintaining exsting levels of fatigue resistance, cavitation resistance and corrosion resistance.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the invention, there s provided a sliding bearing having a tin-based overlay layer comprising up to 7%wt of bismuth.

According to a second aspect of the invention, there is a method of formng a slding bearing, the method comprising providing a substrate as a cathode in an electro-plating electrolyte comprisng tin and bismuth ions, and depositing a tin-based overlay layer comprising up to 7%wt of bismuth with a cathodic bias.

According to a third aspect of the invention, there is a use of up to 7%wt bismuth in a tin-based alloy overlay layer to increase the hardness of the overlay layer.

Advantageously the inclusion of the bismuth in the tin-based overlay material may increase hardness of the overlay layer, may improve wear resistance and may improve fatigue resistance.

The bsmuth may be incorporated into the tin-based overlay with a high level of uniformty, by forming a solid solution (i.e. a binary alloy) with the tin. Advantageously, uniform incorporation of the bismuth into the tin strengthens the entre overlay, which may provide superior performance relative to the inclusion of hard particulate as a reinforcement within a tin matrix.

The upper lmit of 7%wt bismuth limits the extent to which the melting point of the tin-bismuth alloy is lowered below that of tin, and avoids brittleness, which would reduce the fatigue resistance of the overlay layer, if the level of bismuth present were too high.

A tin-based layer may comprise at least 80%wt of tin, and preferably at least 90%wt.

The sliding bearing may comprise at least 0.25%wt bismuth. Alternatively, the sliding bearing may oomprise at least 1%wt bismuth. The %wt of bsmuth is greater than the level of any incidental impurities.

The overlay layer consists of tin and bismuth, apart from incidental impurites. The overlay layer may incorporate a low level of additives present in the electrolyte to mprove deposition performance, e.g. brightener, grain refiner, ant-foaming agent and/or a levelling agent (e.g. less than 0.1%wt) as an incidental impurity. The incidental impurities are less than 1%wt of the overlay layer: and preferably less than 0.5%wt.

The sliding bearing may be a bush, a bearing shell (e.g. a semi-cylindrical half-bearing shell, or a cylindrical bearing shell) or a thrust washer.

The overlay layer may be deposited with a DC cathodic bias.

The overlay layer may be deposited with a cyclically pulsed cathodic bias. Advantageously, pulse plating the overlay layer may increase the proportion of bismuth deposited relative to the proportion of tin deposited, compared with deposition with a non-pulsed cathodic bias.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the inventon are further described hereinafter with reference to the accompanying drawings, in which: * Figure 1 shows a schematic illustration of a hollow cylindrical bush; and * Figure 2 shows a schematic illustration of a hollow semi-cylindrcal bearing shell.

DETAILED DESCRIPTION

In the described embodiments, like features have been identified with like numerals, albeit in some cases having increments of nteger multiples of 100. For example, in different figures, 101 and 201 have been used to indicate a sliding bearing.

Figure 1 illustrates a clinched bush 101 (a type of hollow generally cylindrical sliding bearing) having a steel backng layer 102, a copper-based alloy lining layer 103, a nickel diffusion barrierlO4, and a tin-bismuth overlay layer 105 havng 3%wt bismuth. In this case, the closed shape of the bush 101 is maintained by the clinch 106 (which is an optional feature absent from split bushes and solid sleeve bushes).

Figure 2 illustrates a half-bearing shell 201 (a hollow generally semi-cylindrical bearing shelhaving a steel backing layer 202, a copper-based alloy Hnng layer 203, a nickel diffusion barrier 204, and a tin-bismuth overlay layer 205 having 3%wt bismuth. Bearing shells are commonly used in big-end connecting rod bearings and crankshaft main bearings.

In the latter case, the bearing shell may be provided with a thrust washer (not shown), whch may also have a tin-bismuth overlay layer.

The figures are schematic, and in an exemplary embodiment, the steel backng layer is 1.5mm thck, the copper-based alloy lining layer (e.g. copper-tin bronze) is 0.3mm thick, the nickel interlayer is 0.005mm thick, and the tin-bsmuth overlay is 0.014mm thick.

Advantageously, the inclusion of 3%wt bismuth is found to increase the hardness of the tin-based overlay layer by 40%.

The tin-bismuth overlay layer may be provided by an electroplating method, in which the bearing shell onto which the overlay layer is deposited is provided as a cathode in a bath of a tin-containing and bsmuth-containing solution as an electroplating electrolyte, with a high purity tin-bismuth anode, or a tin anode may be used with separate additions of a bismuth containing solution.

The electrolyte is a lead-free, tin bismuth methanesulfonic add (MSA) electrolyte (which has tin and bismuth ions in methanesulfonic acid), which may comprise additives that enhance deposition performance, such as brightener and anti-foaming agent, as well as a levelling agent. For example, a typical plating solution producing a tin-bismuth-based overlay on a bearing accordng to the present invention may have a composition as follows: Sn2= 30-40 gIl Bi3= 0.5-5.Og/l SnSO4= 58-68 gIl H2S04 185-210 gIl Cu < 50mg/I Chloride c 2oppm Addtives that enhance deposition performance (e.g. brightener, grain refiner, anti-foaming agent, levelling agent) in the range from 18 to 70 mIll of the solution specified above.

A cathodic bias (i.e. a negative bas is applied to the cathode relative to the anode) creates a cathodic current (i.e. a negative current, with respect to the anode) that drives the positively charged tn and bismuth ions towards the sliding bearing cathode, and deposits the tin and bismuth ions onto the cathode surface.

The rate of deposition ot each metal (e.g. tin and bsmuth ions) is limited by the ionic mobility of the metal ions within the electrolyte, due to the presence for each metal of a depletion region in the electrolyte, against the cathode surface. The tin and bismuth have different ionic mobilities and deposit at different rates. The ionic mobility of the tin is higher than that of the bismuth, which can limit the proportion of bismuth deposited.

The relative deposition rates of tin and bismuth are also affected by the cathodic current density, with lower cathodic current densities increasing the relative proportion of tin deposited.

The cathodic current may be a DO cathodic bias.

Alternafively, to provide an enhanced incorporation of the bismuth, the cathode bias may be cyclically pulsed. During the zero cathodic bias portions of the pulse cycle, the concentrations of both metal ions close to the cathode surface are able to ncrease, leading to a rapd burst of deposition occurring during the cathodic bias portons. The zero cathodic bias portions of the pulse cycle may disproportionately benefit the replenishment of bismuth to the bismuth depletion region in the electrolyte, against the cathode surface. Accordingly, such cyclical pulsing of the cathodc bias may enable the deposition of higher proportions of bismuth in the tin-bismuth overlay than with a DC cathodic bias.

In the present embodiment, the peak cathodic current density is between 0.5 and 5.OA/dm2.

The sliding bearing may be a bearing bush, a bearing shell or a thrust washer for a bearing shell, for insertion into a bearing assembly, e.g. in an internal combustion engine.

The figures provided herein are schematic and not to scale.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the descrption and claims of ths specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the ndefinite article is used, the specification is to be understood as contemplating plurality as well as singularty, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specifcaton (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so dsclosed, may be combined in any combination, except combinations where at least some of such features and/or steps arc mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel onc, or any novol combiriatiDn, of the steps Df any method or process so disclosed.

The reader's attention is directed to all papers and documonts which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims (9)

1. CLAIMS1. A sliding bearing having a tin-based overlay layer comprising up to 7%wt of bismuth.
2. 2. A sliding bearing according to claim 1, wherein the sliding bearing comprises at least Q.25%wt bismuth.
3. 3. A sliding bearing according to claims 1 or 2, wherein the overlay layer consists of tin and bismuth, apart from incdental impurities.
4. 4. A slidng bearing according to any preceding claim, wherein the sliding bearing is a bush, a bearing shell or a thrust washer.
5. 5. A method of forming a sliding bearing, the method comprising providing a substrate as a cathode in an electrolyte comprising tin and bismuth ons, and depositing a tin-based overlay layer comprising up to 7%wt Df bismuth with a cathodic bias.
6. 6. A method according to claim 5, wherein the overlay layer is deposited with a DC cathodic bias.
7. 7. A method according to clam 5, wherein the overlay layer s deposited with a cyclically pulsed cathodic bias.
8. 8. A sliding bearing substantially as hereinbefore described with reference to theaccompanying description and the Egures.
9. 9. A method of manufacturing a sliding bearing substantially as hereinbefore described with reference to the accompanying description and the Figures.