GB2535997A

GB2535997A - Composite coating for a plain bearing of an internal combustion engine and method of deposition

Info

Publication number: GB2535997A
Application number: GB1503345.9A
Authority: GB
Inventors: Kerr Ian; Zhang Yi; Banchelli Elena; Verbickas Rolandas; Pal Madan; Tudela Ignacio
Original assignee: Daido Metal Co Ltd
Current assignee: Daido Metal Co Ltd
Priority date: 2015-02-27
Filing date: 2015-02-27
Publication date: 2016-09-07
Also published as: GB201503345D0

Abstract

A method of electroplating a tin composite overlay coating is described. The method comprises plating a tin or tin-based matrix with embedded particles for application in bearings of internal combustion engines. A method of forming a particulate composite tin or tin-based alloy overlay on a sliding bearing. The tin composite overlay of a bearing is pulse plated from a bath containing particulates. The bath with particles is can be treated by the combination of ultrasonic and mechanical agitation. A thin layer of tin may also be applied, between the tin composition overlay and the substrate to improve the bonding. The sliding bearing also has a backing layer upon which is bonded a bearing layer, an interlayer layer may be provided on the bearing layer and the particulate composite overlay is formed on the interlayer, where the interlayer comprises tin and has a thickness of 2.5 microns or less. The composite overlay comprises particles dispersed in tin or a tin-based alloy matrix, and where the particles have an average particle size in the matrix of no more than 1.0 micron and the overlay has a thickness of 5-50µm. The content of particles in the overlay matrix is from 1.5 to 5.0 wt%.

Description

COMPOSITE COATING FOR A PLAIN BEARING OF AN INTERNAL

COMBUSTION ENGINE AND METHOD FOR DEPOSITION THEREOF

DESCRIPTION

1. Field of the Invention

This invention relates to generally a method of electroplating a tin composite overlay coating comprising a tin or tin-based matrix with embedded particles as a second phase or phases, for application in plain bearings of internal combustion engines.

2. Related Art The plain bearings of internal combustion engines 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 plain bearing in use. The sliding elements involved should 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.

Electroplated tin-based overlays have been proven to be a highly effective coating material for use in large medium-speed diesel engines where their excellent sliding properties and high corrosion resistance coupled with reasonable mechanical strength make them extremely effective. However, environmental pressures driving towards lower NOx combustion and higher fuel efficiency are set to raise firing loads above the capability of many current tin based materials in the field. At the same time the proliferation in the use of gas-fired engines, driven by the projected future availability of cheap gas from fracking, is also driving up firing pressures. It would therefore be desirable to provide a tin-based overlay coating with enhanced fatigue and wear resistance which satisfies these performance demands of current engine developments.

Composite coating is known to be an effective way to increase the mechanical strength of the bearing material, through the mechanism of precipitation strengthening. By a careful selection and arrangement of the strengthening particles the movement of dislocations under external forces can be impeded and as a result the mechanical strength of the material is enhanced.

GB 2217347A describes a bearing overlay material with a metal matrix and amorphous carbon contained in the structure. The purpose of such a structure, according to the inventor, is to eliminate the need of nickel interlayer therefore avoid the problems of scuffing and seizure.

GB2253412A describes an electroplated or sputtered composite coating for bearing applications. The coating comprises a soft metallic matrix with incorporated hard and nonmetallic material as second phase in the range of 0.05 up to 2 wt%. The Vickers hardness of the second phase is from 300 to in excess of 600. By the addition of relatively hard particles the wear resistance of the overlay is considerably improved.

A laminated bearing material having improved resistance to abrasion, hardness and wear resistance is described in US6077815. The inventor claimed an electrodeposited tin-based overlay having hard particles embedded in the matrix. The composite overlay not only exhibits enhanced resistance to wear but also excludes the need for a diffusion barrier layer, as the hard particles act as a barrier which precludes diffusion of tin from overlay.

US2013/0330572A1 describes an overlay comprising tin or tin alloy matrix with 0.2-6wt% solid lubricant particles having Mohs hardness no greater than 3. The overlay may contain additionally 0.2-4wt% hard particles with Mohs hardness of no less than 8. The invented overlay shows an improved sliding capacity with the coefficient of friction reducing from 0.1-0.2 to 0.05. The improved lubricity in turn provides good wear resistance and low seizing tendency.

GB2356026 describes a tin based overlay containing inorganic particles. The novelty resides in that the content of particles varies step-wise or successfully in a direction of overlay thickness so that the central portion of the overlay is high in particle and the surface portion is relatively low in particle. Such an arrangement provides a structure combining good bonding strength, high wear resistance as well as improved seizure performance.

The prior art above all focus on electroplated composite overlay for plain bearing applications under conditions of direct current plating. However there are technical limitations associated with direct current plating in terms of overlay particle content, particle agglomeration and particle distribution that are important in determining the final coating properties. During composite plating the positively charged metal ions are absorbed onto the surface of the particles. The particles that are charged in this way then move together with the ions in solution to the cathode under the electrical field. When the metal ions reach the electrode surface they start nucleation under certain kinetic conditions and at the same time the particles are embedded into the coating. For composite plating it is apparent that the number of metal ions arriving at cathode surface per unit time is critical because it decides the overlay particle content and distribution. Under direct current plating the deposition over-potential, which is the driving force for metal ion movement, is less aggressive in order to avoid the formation and growth of dendrites in the overlay. Consequently the number of metal ions reaching the cathode surface per unit time is limited, which in turn limits the number of particles that can be co-deposited at the same time. On the other hand, because the metal ions arriving and depositing at the cathode surface per unit time is limited under direct current plating each grain has sufficient time to grow. Therefore the grain size tends to be big and as a result of the number of grain boundaries is reduced. Because the reinforcement particles normally reside in the grain boundary areas during crystallisation, less grain boundary means the particles have more chance to become agglomerated and thus non-uniformly distributed in the overlay structure. In order to fully exploit the benefit of composite plating the direct current plating conditions have to be modified to optimise the particle content and particle distribution.

Pulse plating has been studied as an effective way to generate much higher deposition over potential without creating the problem of dendrite growth. GB2497520A describes a composite coating with hard particulates embedded in the metallic matrix by applying a repeated cycle of pulse waveform comprising a high cathodic current followed by a low cathodic current, a zero cathodic current or an anodic current. However 20-200g per litre hard particulate has to be used in the plating bath according to the invention. A typical plating tank for medium speed engine bearing is several hundred to a few thousand litres that means tens and hundreds of kilograms of particles would have to be added into the plating tank. Such a heavy presence of particles is impractical in reality as the amount of particles puts a great burden on the pumping system and may significantly shorten the life of the pump and in some cases lead to sudden, frequent and unexpected breakdown of production. If that occurred the manufacturing cost would be increased considerably. Furthermore high particle concentration may cause premature overlay bonding failure due to the heavy particle presence around the plain surface during plating. This often forms big chunks of particle agglomerations in the area of substrate-overlay interface and can cause bonding failures.

3. Objective of the invention It is an objective of the present invention to provide a method of electroplating a tin composite overlay for a plain bearing of an internal combustion engine exhibiting higher fatigue resistance and enhanced wear resistance to satisfy the demand of future engine development.

It is also an objective of the present invention to overcome the disadvantages of the prior art. Therefore the invention tin composite overlay should exhibit a structure with improved particle content, optimised particle distribution and enhanced bonding strength from a plating bath containing low particle concentration.

4. Summary of the invention

One or more objects of the invention is/are achieved in accordance with the present invention by a method of forming a particulate composite tin or tin-based alloy overlay on a sliding bearing, and sliding bearings, in particular for plain bearings, obtainable by said method, as set forth in the claims hereinafter. The tin composite overlay of a plain bearing is pulse plated from a bath containing low particulate concentration. Prior to plating the bath with particles is preferably treated by the combination of ultrasonic and mechanical agitations. A thin layer of tin is also preferably applied between the tin composition overlay and the substrate to improve the bonding.

According to one aspect of the present invention the combination of pulse plating and agitation using ultrasonic and mechanical methods is capable of producing a tin composite overlay with high particle content and uniform particle distribution even from a bath having low particle concentration.

According to another aspect of the present invention a thin interlayer of tin is preferably present between the tin composition overlay and the substrate. The purpose of tin is to further improve the bonding strength. The tin bond layer is typically manufactured by an electroplating method from either the same bath or a separate second bath.

According to a further aspect of the present invention the pulse current permits the generation of very negative deposition over-potential whereby the number of metal ions arriving at the bearing surface per unit time is significantly increased. As a result the overlay particle content is considerably improved in comparison with the overlay plated from the same bath under direct current conditions.

According to the present invention the plating bath with particles is treated by the combination of ultrasonic and mechanical agitation prior to plating. The ultrasound breaks down the particle clusters and avoids agglomeration thus making the particles easy to be dispersed. On the other hand the mechanical agitation mixes the particles with plating solutions hence further enhances the dispersion. One of the key factors in deciding the particle content in the overlay is how effectively the particles are dispersed in the solution rather than how many particles are present in the plating bath. The combination of ultrasonic and mechanical agitation makes the particles well mixed and dispersed in the plating bath which consequently makes effective composite plating possible from a bath with relatively low particle concentration.

According to the present invention the bath may contain 2-20g/I particles preferably 5-10g/I. Such a concentration is beneficial from the production point of view as it significantly reduces the burden on the pumping system therefore makes mass production realistic. On the other hand the low particle concentration has no adverse effect on the inclusion of particles in the overlay under the invented conditions with the preferred range of 1.5wt%-5.0wt% particle content being consistently achieved.

The low particle concentration also avoid the potential particle agglomeration in the overlay-substrate interface thus improves the bonding strength.

To achieve a tin composite overlay with high particle content and uniform particle distribution from a bath with low particle concentration the combination of pulse plating and agitation using ultrasonic and mechanical methods has to be applied.

The pulse contains a repeat cycle of square waveform where each waveform comprises a cathodic peak current density followed by a period of zero current. The peak current density is at least two times of the average current density to facilitate the co-deposition of particles. The length of individual square waveform is 20-1000ms preferably 30-120ms. Any length less than 20ms distorts the shape of the waveform whilst more than 1000ms compromises the benefits of pulse plating.

Prior to plating the combination of ultrasonic and mechanical agitation is applied to treat the bath to facilitate the deposition of tin composite overlay at low particle concentration. The combination of ultrasonic and mechanical agitation not only mixes/incorporates the particles into plating solution but also breaks down the particle agglomeration and therefore provides a much better dispersion in comparison with conventional mechanical method.

A thin tin additional layer may be applied between the tin composite overlay and the substrate to improve the bonding strength. This tin bond layer is typically manufactured by electroplating from either the same bath or a separate second bath. The thickness of the tin bond layer is preferably no more than 2.5 microns, most preferably less than 2.0 microns.

The matrix of tin composite overlay is applied by electroplating 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 particles deposited in the tin-based composite matrix of the overlay are formed of a material phase which is different from the matrix in which it is dispersed. Preferably the material is present as one or more type of hard particles. These are typically non-metallic, covalently-bonded materials. So the particles may be 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 crystal structures. The particle size of particles provided for deposition should be no more than 5 microns but is typically considerably less. The particles deposited will typically be at the smaller end of the particle distribution.

Alternatively, but typically in addition to the above-mentioned hard particles, the tin composite overlay 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, titanium oxide, zinc oxide and lead oxide, and other particles that may present hexagonal crystal structures. The soft particle size is no more than 5 microns, but typically considerably less.

The thickness of the tin composite overlay is advantageously from 5 p.m to 50 p.m, preferably 15 km to 35 pm.

A barrier layer may be present between the overlay (or interlayer when present) and the bearing lining metal. The barrier layer acts as a diffusion barrier and has a thickness of between 1prn to 5pm. The barrier layer when present typically comprises a layer of nickel or nickel-based alloy.

5. Brief Description of the Drawings

Figures 1 shows an SEM cross-section image of tin composite overlay of a sliding bearing according to the present invention.

Figures 2 shows an SEM cross-section image of tin composite overlay of a sliding bearing according to the prior art.

Figures 3 shows an SEM cross-section image of tin composite overlay of a sliding bearing according to another prior art.

Figure 4 shows the results of particles size distribution in the plating bath between conventional mechanical agitation and the combination of ultrasonic and mechanical agitations according to the present invention.

Figure 5 shows the wear resistance of the tin composite overlay according to the prior art and present invention in relation to an existing benchmark tin alloy overlay.

6. Detailed description of the drawing

Table 1 below shows the overlay particle content of tin composite overlays according to the present invention and prior art, and their fatigue strength assessments. In the table MEC means mechanical, 'US' means ultrasonic. 'N' means not present and 'n/a' means not applicable.

Table 1 Material + particles Concentration Agitation Tin inter-layer (urn) Plating Particulate wt% Sapphire of particles in Conditions fatigue rate the bath, g/I Current LIOMPa 50MPa mode, ton-tott Samples per 1 Sn+T102 40 MEC N Direct 0.3 1 1 Comparative current

Example A

2 Sn+Si3N4 40 MEC N Direct 0.1 1 1 current 3 Sn+A1203 10 MEC N Direct 0 1 1 current 4 Sn+Ti02 10 MEC + US N Direct 0.1 1 1 current Samples per 5 Sn+A1203 40 Mechanical N Pulse 3.3 5 2 Comparative current, Example B 10ms-20ms 6 Sn+T102 40 Mechanical N Pulse 3.0 5 2 current, 10ms-40ms Samples per 7 Sn+Ti02 10 Mechanical N Pulse 1.1 3 1 Comparative current, Example C lOms-90ms 8 Sn+Si3N4 10 Mechanical N Pulse 0.8 2 1 current, 10ms-20ms Samples of 9 Sn+Ti02 10 MEC + US N Pulse 3.2 5 3 Invention current, 10ms-40ms Sn+T102 10 MEG + US 3.0 Pulse 3.2 5 3 current, 10ms-40ms Preferred 11 Sn+T102 10 MEG + US 1.0 Pulse 3.2 5 5 Samples of current, Invention 10ms-40ms 12 Sn+Ti02 20 MEG + US 2.0 Pulse 4.0 5 5 current, 10ms-40ms 13 Sn+Ti02 5 MEG + US 0.5 Pulse 2.7 5 5 current, 1Orns-90ms Benchmark 14 SinCu 2 1 sample In Table 1 prior art comparative example A relates to tin composite overlays plated under direct current condition without a tin bond layer between the overlay and substrate.

Comparative example B relates to tin composite overlays plated under pulse current conditions without the tin bond layer, but from the baths having a high concentration of particles.

Comparative example C and relates to tin composite overlays plated under pulse current condition without the tin bond layer, but from the baths having a low concentration of particles.

Manufacture method of the tin composite overlays In the following tables the manufacturing conditions for deposition of the respective comparative and invention overlays are summarised.

Sample 1 (comparative example A): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density n/a Waveform & Duty cycle n/a Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 2 (comparative example A): Tin composite Electrolyte methanesulfonic acid tin bath overlay Si3N4 Si3N14 powder with mean size of 1.0 microns, S.G.=3.20g/cm3 Mean current density 2.0 A/dm 2 Peak current density n/a Waveform & Duty cycle n/a Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 3 (comparative example A): Tin composite Electrolyte methanesulfonic acid tin bath overlay A1203 A1203 powder with mean size of 0.5 microns, S.G.=3.96g/cm3 Mean current density 2.0 A/dm 2 Peak current density n/a Waveform & Duty cycle n/a Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 4 (comparative example A): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm2 Peak current density n/a Waveform & Duty cycle n/a Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 5 (comparative example B): Tin composite Electrolyte methanesulfonic acid tin bath overlay A1203 A1203 powder with mean size of 0.5 microns, S.G.=3.96g/cm3 Mean current density 2.0 A/dm 2 Peak current density 6.0 A/dm 2 Waveform & Duty cycle Square waveform, 33% duty cycle Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 6 (comparative example B): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 10.0 A/dm 2 Waveform & Duty cycle Square waveform, 20% duty cycle Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 7 (comparative example C): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 20.0 A/dm 2 Waveform & Duty cycle Square waveform, 10% duty cycle Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 8 (comparative example C): Tin composite Electrolyte methanesulfonic acid tin bath overlay Si3N4 Si31\14 powder with mean size of 1.0 microns, S.G.=3.20g/cm3 Mean current density 2.0 A/dm 2 Peak current density 6.0 A/dm 2 Waveform & Duty cycle Square waveform, 33% duty cycle Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 9 (invention example C): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 10.0 A/dm' Waveform 8t Duty cycle Square waveform, 20% duty cycle Plating time 20mins Temp. 25°C Tin bond layer Electrolyte n/a Mean current density Plating time Temp.

Sample 10 (invention example C): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 10.0 A/dm' Waveform & Duty cycle Square waveform, 20% duty cycle Plating time 17mins Temp. 25°C Tin bond layer Electrolyte methanesulfonic acid tin bath Mean current density 2.0 A/dm 2 Plating time 3mins Temp. 25°C Sample 11 (in accordance with a preferred aspect of the present invention): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 10.0 A/dm2 Waveform & Duty cycle Square waveform, 20% duty cycle Plating time 19mins Temp. 25°C Tin bond layer Electrolyte methanesulfonic acid tin bath Mean current density 2.0 A/dm 2 Plating time 1mins Temp. 25°C Sample 12 (preferred present invention): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 10.0 A/dm2 Waveform & Duty cycle Square waveform, 20% duty cycle Plating time 18mins Temp. 25°C Tin bond layer Electrolyte methanesulfonic acid tin bath Mean current density 2.0 A/dm 2 Plating time 2mins Temp. 25°C Sample 13 (preferred present invention): Tin composite Electrolyte methanesulfonic acid tin bath overlay TiO2 TiO2 powder with mean size of 0.5 microns, S.G.=3.85g/cm3 Mean current density 2.0 A/dm 2 Peak current density 20.0 A/dm 2 Waveform & Duty cycle Square waveform, 10% duty cycle Plating time 19.5mins Temp. 25°C Tin bond layer Electrolyte methanesulfonic acid tin bath Mean current density 2.0 A/dm 2 Plating time 0.5mins Temp. 25°C The overlays in accordance with a particularly preferred aspect of the present invention are those in which the tin composite overlay was plated under pulse current condition, with tin bond layer present, and derived from plating baths having relatively low concentrations of particles. Prior to plating, the baths are typically treated by the combination of ultrasound and mechanical agitations.

The measurements of TiO2 content in the matrix were carried out after plating and analysed using Wavelength X-ray Fluorescence (WXRF) under vacuum conditions. In total five measurements were recorded on each sample and then calculated to give an average. It is clear from table 1 (and consistent with observations by the inventors) that achieving an overlay with higher than 1.5 wt% TiO2 content from a bath containing less than 20g/I TiO2 particle concentration was essentially impossible (Prior art A) unless a combination of pulse plating and agitation using ultrasonic and mechanical methods was applied.

The fatigue assessments shown in table 1 were carried out on Sapphire fatigue test rig under specific loads of 40 MPa and 50 MPa for 20 hours each load. After the test the bearings were taken out, cleaned, visually examined and rated from between land 5. 5 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.

The results suggests that the present invention material per examples 9 and 10, but especially 11-13 have much improved fatigue resistance than benchmark material 14 and prior art A materials 1-4. It is not difficult to understand the difference as 1-4 were produced under direct current conditions therefore the overlay particle content was very low, and the tin matrix was not significantly strengthened.

It was not possible to differentiate the present invention samples 11-13 from prior art B materials 5 and 6 until the load reached SO MPa. At SO MPa S and 6 showed inferior fatigue resistance in comparison to samples 11-13 although all of them were of similar overlay particle content. The difference in performance is thought to be down to the enhanced bonding strength of samples 11-13 through the application of a thin tin bond layer and reduced particle concentration in the bath. Furthermore although the prior art samples 5 and 6 showed a slight improvement in fatigue in comparison with invention overlay material sample 10, the heavy presence of particles in the plating bath (40g/1) made them unrealistic prospects for use in commercial mass production.

Samples 9 and 10 of the invention also achieved high overlay particle content from the baths with low particle concentrations. Samples 9 and 10 showed excellent fatigue performance at 40MPa but were not as good as preferred invention samples 11-13 at 50 MPa. The difference in performance again is thought to be down to the enhanced bonding strength of samples 11-13 through the application of a thin tin bond layer. As can be seen in table 1 sample 9 had no tin bond layer and the tin bond layer of sample 10 was too thick to function properly as a bond layer.

Figure 1 shows the SEM cross-section image of Sn+Ti02 overlay according to the present invention. The overlay corresponds to sample 11 of Table 1. Figures 2 show an SEM cross-section image of Sn+Ti02 overlay according to the prior art. The overlay corresponds to sample 6 of Table 1. Figures 3 show an SEM cross-section image of Sn+Ti02 overlay according to another prior art. The overlay corresponds to sample 1 of Table 1.

TiO2 particles can be clearly seen in Figure 1 and Figure 2 as both samples (according to WXRF measurement) have around 3.0 wt% particles in the overlay structure. The microstructure of figure land figure 2 are similar but the particles in figure tare visibly smaller than those of figure 2. The combination of ultrasonic and mechanical agitation is effective to reduce the particle size in figure 1. While small particles act to strengthen the structure, large particles (such as those that can also be observed around the area of the overlay-substrate interface) act as stress raisers which eventually weaken the bonding strength and therefore the fatigue strength is reduced. It should be noted that sample 6 in figure 2 achieved high particle content from a bath containing 40 g/I of particles whilst the sample 11 in figure 1 is from a bath with particle concentration of only 10g/I.

In comparison with figure land figure 2 there is no particle visible in the structure of figure 3. As the samples of figure 3 were produced under direct current conditions, even when the particles concentration was increased to 40 g/I the overlay particle content was still very low.

Figure 4 shows the results of particles size distribution in the plating bath as between conventional mechanical agitation and the combination of ultrasonic and mechanical agitation according to the present invention. A Mastersizer 2000 (manufactured by Malvern) was used for the measurement. The Mastersizer 2000 uses the technique of laser diffraction to measure the size of particles. It does this by measuring the intensity of light scattered as a laser beam passes through a dispersed particulate sample. This data is then analysed to calculate the size of the particles that created the scattering pattern. The results in figure 4 indicate that the combination of ultrasonic and mechanical agitation is more effective in breaking down the particle agglomeration and therefore improves the dispersion of particles and favours the co-deposition of particles with tin matrix. As shown, the particle sizes after the treatment of ultrasonic and mechanical agitations moves to the left side of the graph, representing a much finer particle distribution than conventional mechanical agitation. So by applying ultrasound the extent of agglomeration was reduced thereby increasing the percentage of smaller particles in the electrolyte (as shown in figure 4). In the deposition process only those smaller particles (typically less than 1.0micron) are incorporated into the overlay successfully.

Figure 5 shows the wear resistance of the tin composite overlays according to the prior art and present invention respectively in relation to an existing benchmark tin alloy overlay. The wear was measured as the reduction of thickness before and after the test on the test piece and then calculated as an average wear. The average wear of present invention sample 11 as shown in the figure is less than 25% of that benchmark material per sample 14 and prior art material sample 1. In comparison with prior art sample 6, the wear is also reduced considerably. This is due to the improved bonding by the application of a tin bond layer between the substrate and overlay and the reduced TiO2 particles size in the overlay matrix.

Claims

Claims 1. A method of forming a particulate composite tin or tin-based alloy overlay on a sliding bearing comprising: providing a backing layer having a surface upon which is provided a bearing layer, optionally depositing an interlayer on the surface of the bearing layer and depositing the particulate composite overlay on the bearing layer surface, or the optional interlayer surface, to form a tin-based matrix in which is dispersed particles, wherein the overlay is deposited by electrodeposition from a liquid electrolyte bath which comprises said particles, tin ions and any other metal ions or additives required to form the desired tin-based matrix, characterised in that the electrodeposition is by pulse-plating in which a plating current is applied in repeating cycles in each of which cycles a plating current is applied for a duty period and then a reduced or zero current is applied for a rest period.
2. A method according to claim 1 wherein the electrolyte bath is agitated before and/or during electrodeposition so as to disperse said particles within the electrolyte and help disrupt particulate agglomeration.
3. A method as claimed in claim 2 wherein the agitation comprises a combination of mechanical agitation and ultrasound agitation applied concurrently or alternately.
4. A method as claimed in any of the preceding claims wherein the particulate concentration in the electrolyte bath is from 2 to 20 g/I.
5. A method as claimed in claim 4 wherein the concentration is from 5 to 10 g/I.
6. A method as claimed in any of the preceding claims wherein the pulse current cycles are applied as a square current waveform.
7. A method as claimed in any of the preceding claims wherein the pulse current cycles have a waveform which has a cathodic peak current density followed by a period of zero current.
8. A method as claimed in any of the preceding claims wherein the waveform provides a peak current density which is at least double the average applied current density.
9. A method as claimed in any or the preceding claims wherein the pulse plating cycle has a wavelength of between 20 and 1000 ms, preferably 30-120 ms.
10. A method as claimed in any of the preceding claims wherein the interlayer is a bond layer deposited on the bearing layer and the overlay is deposited on the interlayer surface.
11. A method as claimed in claim 10 wherein the interlayer layer comprises tin, or consists of tin, and no added particles.
12. A method as claimed in claim 10 or 11 wherein the interlayer layer has a thickness of 2.5 microns or less and preferably 2.0 microns or less.
13. A method as claimed in claim 12 wherein the interlayer layer has a thickness of at least 0.1 microns.
14. A method as claimed in any of the preceding claims wherein the particles are selected from one or more of particulate ceramic materials, such as 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; but preferably TiO2 or Si3N4 or A1203.
15. A method as claimed in any of the preceding claims wherein the average size of the particles in the electrolyte bath is no more than 5 microns.
16. A method as claimed in any of the preceding claims wherein the particles are one or more 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, which particles are provided in the electrolyte bath for deposition with the overlay.
17. A method as claimed in claim 14 wherein further, relatively soft, particles are provided which are one or more 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, which particles are provided in the electrolyte bath for deposition with the overlay.
18. A method as claimed in any of the preceding claims wherein a diffusion barrier layer is deposited on the bearing layer before deposition of the overlay or optional interlayer, wherein the diffusion barrier layer preferably comprises nickel or a nickel-based alloy.
19. The method of any of the preceding claims wherein the thickness of the deposited tin composite overlay is from 5 ktrn to 50 p.m, preferably 15 jam to 35 Rm.
20. The method of any of the preceding claims wherein the overlay matrix deposited comprises or consists 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 these.
21. The method of any of the preceding claims wherein the backing layer comprises steel.
22. The method of any of the preceding claims wherein the bearing layer comprises aluminium or an aluminium alloy or copper or a copper alloy.
23. The method of any of the preceding claims wherein the sliding bearing is an internal combustion engine bearing.
24. A particulate composite tin-based overlay on a sliding bearing obtainable by a method according to any of the preceding claims.
25. A sliding bearing comprising a backing layer upon which is bonded a bearing layer, an interlayer layer provided on the bearing layer and a particulate composite overlay formed on the interlayer, wherein the interlayer comprises tin and has a thickness of 2.5 microns or less, the composite overlay comprises particles dispersed in tin or a tin-based alloy matrix, and wherein the particles have an average particle size in the matrix of no more than 1.0 micron and the overlay has a thickness of 5 to 50 microns.
26. The sliding bearing of claim 25 wherein the content of particles in the overlay matrix is from 1.5 to 5.0 wt%.
27. The sliding bearing of claim 25 wherein a diffusion barrier layer is disposed between the bearing layer and the interlayer.