GB2534120A

GB2534120A - Bismuth-based composite coating for overlay applications in plain bearings

Info

Publication number: GB2534120A
Application number: GB1421180.9A
Authority: GB
Inventors: Tudela Ignacio; Zhang Yi; Pal Madan; Cobley Andrew; Kerr Ian
Original assignee: Daido Industrial Bearings Europe Ltd
Current assignee: Daido Industrial Bearings Europe Ltd
Priority date: 2014-11-28
Filing date: 2014-11-28
Publication date: 2016-07-20
Also published as: GB201421180D0

Abstract

A plain bearing 1 with an overlay 2 of a composite coating comprising a bismuth or Bi-based alloy matrix and one or more soft particles 5 with a Mohs hardness of ≤ 5 dispersed within the matrix. A further aspect relates to a method of manufacturing the coating and overlay 2 via electro-deposition or electroless plating from a bath in which the particles are dispersed by ultrasound or megasound and potentially a further agitation method. The ultrasound or megasound and optional further agitation method may be applied during or before the electrodeposition stage. The soft particle 5 may be PTFE, a metal fluoride or sulphate, graphite, hexagonal boron nitride, phyllosilicate or lead or zinc oxide. The coating 2 may also contain hard particles with a Mohs hardness of ≥5.5, such as metal oxides, borides, carbides, nitrides and silicides, diamond, CNTs, or graphene. The soft lubricant particles 5 and optional hard particles may be between 1 nm and 5 µm in size, and take up 0.0001 to 30 % of the volume of the composite coating. The bearing 1 may have a bronze-based or aluminium-based alloy lining material 3, and possibly an interlayer between 5 nm and 15 µm thick comprising Ni, Cu, Ag, Cr, Co. Fe, Mn, Au, Zn or alloys thereof located between the bearing lining 3 and composite coating. The composite coating 2 forming part of the plain bearing 1 may be between 1 and 50 µm thick.

Description

Bismuth-based composite coating for overlay applications in plain bearings This invention relates to Bi-based composite coatings including soft particles, for use as overlay coatings in plain bearings to improve bearing performance and reliability, and a process to produce such composite coatings.

The emerging trends in modern internal combustion engines are to achieve maximum fuel efficiencies and reduce carbon footprints by reducing mechanical and parasitic power losses by using low friction components, thinner oil films, low viscosity lubricants, engine downsizing, turbo-boosting, stop-start cycles and using alternative combustion cycles. However, using any or a combination of these technologies leads to arduous operating conditions for engine components causing higher fatigue loads, higher operating temperatures, greater asperity contact and hence more incidences of boundary lubrication type metal-to-metal contact leading to accelerated wear and seizure of the components.

In internal combustion engines, plain bearings are used as main-bearings and big-end bearings to support the crankshaft in an engine crankcase and to connect the connecting-rods with the crankshaft. The plain bearing bears the load from the combustion of fuel and the inertial loads form the rotation of the crankshaft as the reciprocating motion of the pistons is converted into rotary motion by the linking mechanism between piston, connecting-rod, and crankshaft.

In modern engines, the bearing specific loads and the intensity of fatigue loads are increasing as more and more power is obtained from the ever smaller and lighter engines. There is also a trend to use thinner viscosity lubricants to achieve extra efficiency from engines. Consequently, the sliding bearings are running in mixed and boundary lubrication regimes for a major part of an engine operating cycle with sub-micron level minimum oil film thickness. To deliver an efficient and reliable performance in such operating conditions the sliding bearing material has to have good anti-seizure, fatigue, and wear resistance properties. In other words, to deliver low friction and good seizure resistance the bearing material needs to be soft and lubricous, and to deliver good fatigue and wear resistance the bearing material needs to be strong and tough. However, obtaining an ideal bearing material with contradictory hard and soft material properties is difficult and to some extent only achievable through composite or multilayer material structures. So certain contemporary sliding bearings are made of several layers of composite or alloy bearing material where each layer is customised to deliver specific soft or hard properties.

One plain bearing used in highly loaded engines is made of at least three or more layers of alloy/composite materials, commonly known as tri-metal bearing. The bearing typically has a steel backing to give strength to the slide bearing and allowing a proper interference fit in a housing. On top of the steel backing there is either a bronze-based alloy or an aluminium-based alloy lining. The bronze-based alloy is too hard to be a good bearing material on its own: it has good load capacity, wear resistance and cavitation resistance, but it lacks the necessary compatibility, conformability and embedability (all the properties from the softer spectrum of a bearing material). The aluminium-based alloy, although it is a good bearing material and exhibits good conformability and embeddability, may not be enough for some high load applications where the aluminium-based alloy would be prone to fatigue.

In order to manufacture a plain bearing with not only good load capacity, wear resistance and cavitation resistance, but also good compatibility, conformability and embeddability, an overlay is deposited over the bearing lining to act as a sacrificial running-in layer which confers low friction, compatibility, conformability and embedability to bronze-based linings, and increased fatigue strength to aluminium-based linings. Occasionally, an interlayer may be required between the overlay and the bronze / aluminium-based alloy lining to serve as a bonding layer and / or to act as a diffusion barrier.

Overlay coatings in plain bearings, particularly when bronze-based lining materials are used, conventionally have been made of soft Pb-based alloys. However, as lead constitutes a serious hazard for the environment due to its highly poisonous nature, different Pb-free approaches have been evaluated so as to develop novel coatings to be employed as overlays in plain bearings.

Electroplated Bi and Bi-based alloys have been proposed to be used instead of conventional Pb-based alloys, despite the brittle nature of these materials and their inferior anti-seizure properties. US patent 6309759 describes a Bi or Bi alloy coating acting as an overlay in plain bearings where the relative ratio of the X-ray diffraction intensity I (hkl) of planes other than the (012) planes is from 0.2 to 5 times as high as the ratio of the X-ray diffraction intensity I(012), and the relative ratio of the X-ray diffraction intensity 1(-,ki) of three or more planes other than (012) planes ranges from 0.5 to 2 times as high as the ratio of the X-ray diffraction intensity 1(012).

GB patent application 2400420 also describes a Bi or Bi alloy coating for overlay applications in plain bearings with a modified crystal configuration where Bi crystals with a Miller index (202) represents at least the 30% of the Bi crystals in the deposit and the X-ray diffracted intensity 1(202) of the (202) plane assumes a maximum value when compared with the X-ray diffracted intensity of the other planes.

US patent application publication 2007/0269147 claims a Bi or Bi alloy coating where the X-ray diffraction intensity 1(012) plane is the greatest in comparison with the X-ray diffraction intensities of the other planes, wherein the X-ray diffraction intensity of the plane with the second largest I(nk!) is a maximum of 10% of 1(012)* DE patent application 102007035497 describes an overlay system for plain bearings consisting of a Bi coating acting as the sliding surface of the bearing and a SnNi interlayer electrodeposited between the bronze lining and the Bi coating.

US patent application publication 2008/0152942 claims a Zn alloy coating for overlay applications in plain bearings where the Zn alloy coating contains 1 to 49 % by weight of Bi as the additional alloy element. GB patent application 2438977 relates to an overlay system for plain bearings consisting of a Bi 'wearing layer' and a CuBi alloy or a AgBi alloy 'anti-friction layer' between the Bi wearing layer and the lining material, with the optional addition of an interlayer between the anti-friction layer and the lining material.

GB patent application 2492673 describes a Bi or Bi alloy coating acting as an overlay in plain bearings with an improved grain structure consisting of Bi crystals with a low aspect ratio. GB patent application 2502033 relates to a Bi or Bi alloy coatings for overlay applications in plain bearings where the Bi-based coating has different planar regions with different X-ray diffraction intensities for each of the (hkl) planes present in the sample.

The incorporation of particles to Bi and Bi alloy overlays in plain bearings has also been addressed in order to improve the brittle nature of such coatings and its anti-seizure properties. In this sense, GB patent application 2379449 improves the wear resistance of Bi-based overlays by incorporating 0.05 to 25 % by volume of hard particles of the group of borides, silicides, oxides, nitrides, carbides and intermetallic compounds with a maximum particle size of 5 pm. GB patent application 2438977 also mentioned the addition of hard particles into a CuBi/AgBi alloy anti-friction layer, where the particles exhibit a particle size of between 10 and 100 nm.

The incorporation of soft particles into metal coatings, either on their own or combined with hard particles, has already been addressed in the past for different bearing applications, e.g. DE patent application 102009019601 for Cu-based coatings, US patent 6077815 and DE patent application 102010040469 for Sn-based coatings, and DE patent applications 102010011083 and 102011013881 for Nibased coatings.

In the present invention, the inventors have independently and successfully developed a Bi or Bi alloy composite coatings including soft particles. The composite coating consists of a Bi or Bi alloy metal matrix with soft lubricant particles finely dispersed within said metal matrix. Such a novel Bi-based composite coating shows a significant improvement in ductility, especially for thick coatings (thickness > 15 pm), compared with current state of the art, resulting in a further enhancement in wear resistance and anti-seizure properties, due to the presence of the soft particles. In those cases where Bi-based composite coatings may operate under very harsh conditions, the addition of finely dispersed hard particles into the Bi-based composite coatings with soft particles may result in a further enhancement of the fatigue strength.

The incorporation of finely dispersed particles into the coating is achieved by using different agitation techniques, but preferably by the use of agitation by ultrasound or megasound, or a combination of ultrasonic or megasonic agitation with one or more other known agitation method. In the case of using highly hydrophobic particles that are difficult to wet (and therefore typically the use of surfactants is required), the use of ultrasound or megasound significantly improves the dispersion of particles with very little, if any, addition of surfactant, reducing the concentration of the surfactant required to wet the particles, and therefore minimizing the disadvantages associated with the incorporation of surfactants into the electroplating baths.

According to one aspect of the present invention, there are provided Bi-based composite coatings with finely dispersed particles in the coating as set forth in the claims hereinafter. In accordance with another aspect of the invention there are provided methods of forming such coatings as set forth in the claims hereinafter.

According to one aspect of the invention, a Bi-based composite coating comprises or consists of a matrix made of Bi or a Bi-based alloy and soft particles finely dispersed in the metal matrix. By "soft" it is intended that the Mohs hardness should typically be The metal coating may be deposited from an electroplating bath in which the particles are dispersed by using ultrasonic or megasonic agitation or optionally one or more further agitation techniques. The use of ultrasound/megasound permits the substantially uniform distribution of the particles throughout the composite coating, which consequently refines and modifies the microstructure of the coating.

The composite coating thereby formed in accordance with the invention preferably has a thickness of 1 to 50 pm, more preferably 2 to 40 pm, and in typical tribological coatings as per the invention the thickness is optimally 3 to 25 pm thick.

The soft particles dispersed in the Bi-based composite coating may be one or more materials selected from a group of tribological materials/solid lubricants such as PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal sulphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, zinc oxide or lead oxide. If required, one or more types of hard particles (typically particles with Mohs hardness 5.5) and preferably from a group of metal oxides, borides, carbides, nitrides, sulphates and silicides, diamond and other hard carbonaceous particles, may also be incorporated into the Bi-based composite coatings in addition to the soft particles. Both soft and hard particles each have a nominal size of 1 nm to 5 pm, preferably 250 nm to 3 pm.

The coating may be deposited on a variety of substrates, including but not limited to bronze, brass, white metal, aluminium alloys, steel and other bearing materials.

The substrate may be configured as a portion of a plain bearing and the coating may be applied to form an overlay of a bearing. For example, the substrate may be a bearing half shell. Thus, the method of the invention may further comprise coating the bearing surfaces of two bearing half shells, which are then assembled into a housing to form a complete plain journal bearing.

Following is a description by way of example only and with reference to the figures of the drawings of one mode for putting the present invention into effect.

In the drawings: Figure 1 is an optical micro-section image of a Bi composite coating with hBN particles in accordance with an embodiment of the invention.

Figure 2 is an optical micro-section image of a Bi composite coating with hBN particles in accordance with current state-of-the-art.

Figure 3 is an optical micro-section image of a Bi composite coating with WS2 particles in accordance with an embodiment of the invention.

Figure 4 is a schematic diagram of a bending ductility test performed on thin steel strips coated with any coating of interest.

Figure 5 displays data obtained during tribological scratch tests performed on different Bi-based coatings.

Figure 6 displays data obtained during tribological wear tests performed on different Bi-based coatings.

Figure 7 displays data obtained during tribological seizure tests performed on different Bi-based coatings.

Figure 8 is a schematic diagram of an electroplating process using ultrasound/megasound according to the invention.

Figure 9 is a schematic cross-sectional representation of a plane bearing provided with a coating in accordance with the present invention.

Figure 1 displays a Bi composite coating 10 electroplated under ultrasound with fine hBN particles 11 dispersed evenly in a Bi matrix 12. The substrate 13 is a bearing material, but could equally be another metal or alloy.

Figure 2 displays a Bi composite coating 20 electroplated in absence of ultrasound with hBN particles and large agglomerates 21 dispersed poorly in a Bi matrix 22. The substrate 23 is a bearing material, but could equally be another metal or alloy.

Figure 3 displays a Bi composite coating 30 electroplated under ultrasound with WS2 particles 31 dispersed evenly in a Bi matrix 32. The substrate 33 is a bearing material, but could equally be another metal or alloy.

Figure 4 is a schematic representation of a bending ductility test performed on a thin steel strip 41 coated with a Bi-based coating 42 which is resting over a shaft 43 of a certain diameter. The strip is bended 1802 by applying a constant load on both ends of the strip, as indicated by both arrows in the diagram.

Figure 5 is a graph depicting the evolution of the coefficient of friction vs scratch testing time for different coatings: IE5 is a Bi/WS2 composite coating prepared in accordance with an embodiment of the invention, while CE5 is a Bi coating prepared in accordance with current state-of-the-art.

Figure 6 shows wear of different coatings after being tested under wear testing conditions: IE6 is a Bi/WS2 composite coating prepared in accordance with an embodiment of the invention, while CE6 is a Bi coating prepared in accordance with current state-of-the-art.

Figure 7 shows the maximum load applied on different coatings without observing seizure failure under seizure testing conditions: 1E7 is a Bi/WS2 composite coating prepared in accordance with an embodiment of the invention, while CE6 is a Bi coating prepared in accordance with current state-of-the-art.

Figure 8 is a schematic cross-sectional representation of an electroplating process using ultrasonic/megasonic and fluid agitation to deposit Bi-based composite coatings in accordance with the invention. The electroplating tank 101 is filled with an electrolyte solution 102. The specimen to be coated 103 is connected to the cathode of a rectifier 104. A counter-electrode 105 is connected to the rectifier 104 and is set as an anode. An ultrasonic/megasonic transducer 106 with its independent controller box 107 is attached either inside or outside of the plating tank 101 to generate ultrasonic/megasonic waves 111. These waves can be used to disperse particles 108 in the plating electrolyte 102 when electrodepositing Bi-based composite coatings in accordance with the invention. A perforated pipe 109 is connected to a circulatory fluid pump 110, which is used for generating fluid agitation in the electrolyte 102.

Figure 9 shows more generally a plain bearing composite material 1 consisting of a backing substrate 4 (i.e. steel), a lining material 3 (i.e. bronze-based or aluminium-based alloy) and an overlay 2, where the overlay 2 is an ultrasound/megasoundassisted electrodeposited composite coating consisting of a metal matrix (i.e. Bi or Bi alloy) and uniformly and well-dispersed soft lubricant particles 5.

PROCESS OPERATING PARAMETERS

Typical steps to produce a composite coating of the type described hereinabove are as listed below.

1. Particles are dispersed in the plating electrolyte by ultrasound/megasound prior to the electroplating process. The ultrasonic/megasonic transducers operate at a frequency between 10 and 3000 kHz and a power of up to 5 W/cm3 for a period greater than 0.1 seconds. Ultrasonic/megasonic irradiation can be either continuous or discontinuous (as a pulsed signal). The frequency and power of the acoustic field influences the dispersion and the undesired formation of agglomerated particles.

2. The components to be plated are degreased to remove any traces of grease or oil.

3. The components are then mounted on a plating jig and pre-treated to assure a good bonding of the coating to the substrate.

4. The components are then immersed in the electrolyte used for the electrodeposition of the coating. The plating process of the coating is carried out at a current density of up to 100 A/dm2. The current density and the plating time determine the thickness of the composite coating. Four different agitation conditions could be used in this step: i) ultrasonic/megasonic agitation, ii) combination of ultrasonic/megasonic and other agitation methods, iii) any agitation method or combination not involving ultrasound/megasound, and iv) no agitation (still).

Between stages 3 and 4, an optional strike layer/interlayer (as disclosed in GB patent application 2379449 the contents of which are incorporated herein by reference) is provided, consisting in the present invention of Ni, Cu, Ag, Cr, Co, Fe, Mn, Au, Zn or alloys of these metals. This may be deposited to act as a bonding/diffusion barrier layer. This optional interlayer could also contain hard and/or soft particles dispersed as described in stage 1 and be electroplated as described in stage 4.

The cross-section of different Bi-based composite coatings containing different types of soft particles electrodeposited on Cu substrates with and without ultrasound was examined with an optical microscope in order to establish and confirm the improvement in terms of particle incorporation and homogeneous distribution achieved in Bi-based composite coatings when ultrasound is used during plating. Bibased composite coatings containing different types of soft particles were again ultrasonically electrodeposited on steel substrates in order to perform ductility tests to establish the improvement in terms of ductility when compared with pure Bi coatings and Bi composite coatings containing hard particles prepared according to current state of the art. The Bi-based composite coating exhibiting the highest ductility was again electrodeposited on bronze samples for further wear and seizure tests in order to establish the improvement achieved in terms of wear and seizure resistance when compared with Bi coatings prepared according to current state of the art.

PARTICLE INCORPORATION AND DISTRIBUTION IN BISMUTH COATINGS

The procedure followed to produce invention example 1E1 consisting of a Bi composite coating with hBN particles is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. Addition of hBN particles to the MSA-based Bi plating bath. The resulting solution was treated with ultrasound and mechanical stirring for 30 minutes to achieve an optimum dispersion with finely dispersed particles. The ultrasonic frequency was 35 kHz and the ultrasonic power was 0.2 W/cm3. The concentration of the particles was 15 g/L.

3. The substrates to be plated were cleaned by vapour-degreasing in a hydrocarbon cleaner for 10 minutes.

4. The substrates were then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze lining surface was applied at a current density of 3 A/drri2 for 2 minutes.

5. Electrodeposition of the Bi/hBN composite coating on the Cu substrate at a current density of 4.5 A/dm2 during 20 minutes under mechanical stirring.

Ultrasound was also used during the plating process at a frequency of 35 kHz and an ultrasonic power of 0.2 W/cm3.

The procedure followed to produce invention example 1E2 consisting of a Bi composite coating with WS2 particles is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. Addition of WS2 particles to the MSA-based Bi plating bath. The resulting solution was treated with ultrasound and mechanical stirring for 30 minutes to achieve an optimum dispersion with finely dispersed particles. The ultrasonic frequency was 35 kHz and the ultrasonic power was 0.2 W/cm3. The concentration of the particles was 15 g/L.

4. The substrates were then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze lining surface was applied at a current density of 3 A/dm2 for 2 minutes.

5. Electrodeposition of the Bi/WS2 composite coating on the Cu substrate at a current density of 4.5 A/dm2 during 20 minutes under mechanical stirring. Ultrasound was also used during the plating process at a frequency of 35 kHz and an ultrasonic power of 0.2 W/cm3.

The procedure followed to produce comparative example CE1 consisting of a Bi composite coating with hBN particles is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

4. The substrates were then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze lining surface is applied at a current density of 3 A/dm2 for 2 minutes.

The procedure followed to produce comparative example CE2 consisting of a Bi composite coating with WS2 particles is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. Addition of WS2 particles to the MSA-based Bi plating bath. The resulting solution was treated with ultrasound and mechanical stirring for 30 minutes to achieve an optimum dispersion with finely dispersed particles. The ultrasonic frequency was 35 kHz and the ultrasonic power was 0.2 W/cma. The concentration of the particles was 15 g/L.

5. Electrodeposition of the Bi/WS2 composite coating on the Cu substrate at a current density of 4.5 A/dm2 during 20 minutes under mechanical stirring.

Figures 1 and 2 display cross-section images of IE1 and CE1, respectively, which are in both cases Bi composite coatings containing hBN particles. hBN particles in IE1 were successfully incorporated and evenly distributed within the Bi matrix, whereas hBN particles and large agglomerates in CE1 were poorly incorporated into the coating and unevenly distributed.

Figure 3 displays a cross-section image of 1E2, which is a Bi composite coating electrodeposited under ultrasound containing WS2 particles. Whereas WS2 particles in 1E2 were successfully incorporated and evenly distributed within the Bi matrix, no acceptable coating was produced for CE2 (and hence no cross-section images of CE5 are included in the present invention), as a thick layer of a compacted sludgy WS2 deposit completely covered the surface of the substrate when plating was conducted in absence of ultrasound. Large aggregates of WS2 particles were electrophoretically deposited over the surface of the Cu substrate, inhibiting to some extent the electrodeposition of Bi, especially when the whole cathode area was blocked with the sludgy deposit. This sludgy deposit of agglomerated WS2 would be greatly damaged when prepared the samples to be examined with the microscope, and hence the lack of cross-section images of CE2 in the present invention.

The results obtained for 1E1 and CE1 prove the improvement in terms of particle de-agglomeration, incorporation and distribution that can be achieved when ultrasound is used during the electroplating process, while the results observed for 1E2 and CE2 demonstrate how effective and critical the use of ultrasound/megasound is, compared with current state-of-the-art, in order to prevent massive particle agglomeration and blocking of the surface.

DUCTILITY TESTS

To compare how ductile different Bi-based coatings were, bending tests were performed on thick (25 pm) Bi-based composite coatings according to the present invention which were compared with Bi-based coatings with the same thickness in accordance with current state-of-the-art. In a ductility bending test, a thin flat steel strip (100x10x0.5mm) coated with the coating to be tested and resting over a shaft with a certain diameter is bended 180° by applying a constant normal force on both ends of the strip, as shown in Figure 4. Once the strip is bended, the surface of the coating is thoroughly checked to detect any cracks that may have formed during the bending process. The apparition of cracks indicates poor ductility of the coating.

The procedure followed to produce invention example 1E3 consisting of a Bi composite coating with hBN particles on a steel strip is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

3. The strip to be plated was cleaned by vapour-degreasing in a hydrocarbon cleaner for 10 minutes.

4. The strip was then mounted in a plating jig and anodic acid-etching pretreatment of the steel strip surface was applied at a current density of 3 A/dm2 for 2 minutes.

5. Electrodeposition of the Ni/hBN composite coating on the steel strip at a current density of 4.5 A/dm2 during 25 minutes under mechanical stirring. Ultrasound was also used during the plating process at a frequency of 35 kHz and an ultrasonic power of 0.2 W/crna.

The procedure followed to produce invention example 1E4 consisting of a Bi composite coating with WS2 particles on a steel strip is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

4. The strip was then mounted in a plating jig and anodic acid-etching pre-treatment of the steel strip surface was applied at a current density of 3 A/dm2 for 2 minutes.

5. Electrodeposition of the Ni/WS2 composite coating on the steel strip at a current density of 4.5 A/dm2 during 25 minutes under mechanical stirring.

The procedure followed to produce comparative example CE3 consisting of a Bi coating on a steel strip is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. The strip to be plated was cleaned by vapour-degreasing in a hydrocarbon cleaner for 10 minutes.

3. The strip was then mounted in a plating jig and anodic acid-etching pretreatment of the steel strip surface was applied at a current density of 3 A/dm2 for 2 minutes.

4. Electrodeposition of the Bi coating on the steel strip at a current density of 4.5 A/dm2 during 25 minutes under mechanical stirring.

The procedure followed to produce comparative example CE4 consisting of a Bi composite coating with Sal% particles is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. Addition of S3N4 particles to the MSA-based Bi plating bath. The resulting solution was treated with ultrasound and mechanical stirring for 30 minutes to achieve an optimum dispersion with finely dispersed particles. The ultrasonic frequency was 35 kHz and the ultrasonic power was 0.2 W/cma. The concentration of the particles was 40 g/L.

5. Electrodeposition of the Ni/S31\14 composite coating on the steel strip at a current density of 4.5 A/dm2 during 25 minutes under mechanical stirring.

Table 1 shows the results of the ductility bending tests performed on 1E3, 1E4, CE3 and CE4 using various shafts with different diameters 0. The results show that the Bi-based coatings deposited in 1E3 and 1E4 samples exhibited a far more ductile behaviour than the Bi coatings. In terms of ductility, the samples would be organised as follows (most ductile to most brittle): 1E4 > 1E3 » CE4 » CE1. These results demonstrated the significant improvement in terms of ductility achieved when soft particles are successfully incorporated into Bi-based coatings.

Table 1

0 (mm) CE3 CE4 1E3 1E4

PASS PASS PASS PASS

19 FAILED PASS PASS PASS (few cracks on surface)

FAILED PASS PASS PASS

(few cracks on surface) 12 FAILED FAILED PASS PASS (several cracks on surface) (few cracks on surface) 9 FAILED FAILED PASS PASS (several cracks on surface) (several cracks on surface)

FAILED FAILED FAILED PASS

(several cracks on surface) (several cracks on surface) (few cracks on surface) 3 FAILED FAILED FAILED PASS (several cracks on surface) (several cracks on surface) (several cracks on the surface)

TRIBOLOGICAL SCRATCH TESTS

Lubricated scratch tests were performed on the coating showing the highest ductility to observe the effect that the incorporation of soft particles has on the coefficient of friction of Bi coating. The tests were performed on 15 pm coatings, as coatings in accordance to current-state-of-the-art would present bonding/brittleness issues during the test when using thicker coatings due to low ductility. The conditions of the test were: * Sliding distance: 10 mm * Sliding speed: 10 mm/min * Load: 0-200 N (progressive) * Ball diameter: 6.3 mm * Ball material: JIS SUJ2 -ISO 100Cr6 * Lubricant: SAE 10 oil The procedure followed to produce invention example 1E5 consisting of a Bi composite coating with WS2 particles on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

3. The substrate to be plated was cleaned by vapour-degreasing in a hydrocarbon cleaner for 10 minutes.

4. The substrate was then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze substrate surface was applied at a current density of 3 A/dm2 for 2 minutes.

5. Electrodeposition of the Ni/WS2 composite coating on the bronze substrate at a current density of 4.5 A/dm2 during 15 minutes under mechanical stirring. Ultrasound was also used during the plating process at a frequency of 35 kHz and an ultrasonic power of 0.2 W/crna.

The procedure followed to produce comparative example CE5 consisting of a Bi coating on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

2. The substrate to be plated was cleaned by vapour-degreasing in a hydrocarbon cleaner for 10 minutes.

3. The substrate was then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze substrate surface was applied at a current density of 3 A/dm2 for 2 minutes.

4. Electrodeposition of the Bi coating on the bronze substrate at a current density of 4.5 A/dm2 during 15 minutes under mechanical stirring.

Lubricated scratch tests for 1E5 and CE5 are shown in Figure 5. The coefficient of friction measured for 1E5 was significantly lower that for CE5, demonstrating the improvement in terms of coefficient of friction achieved in the Bi coating when soft particles (WS2 in this case) are successfully incorporated into Bi coatings electroplated under ultrasonic/megasonic conditions.

TRIBOLOGICAL WEAR TESTS

Wear tests were also performed on the coating showing the highest ductility to observe the effect that the incorporation of soft particles has on the coefficient of friction of Bi coating. Again, the tests were performed on 15 pm coatings, as coatings in accordance to current-state-of-the-art would present bonding/brittleness issues during the test when using thicker coatings due to low ductility. The coatings were electrodeposited on special bronze samples to be used on a ring (shaft)-on-disk tribometer. The conditions of the test were: * Sample dimensions: outer diameter = 27.2 mm, inner diameter = 22 mm * Load: 1MPa increase every 2 minutes up to 12MPa, then constant at 12 MPa * Sliding speed: 0.05 m/s * Lubricant: VG22 (SAE10) * Inlet lubricant temperature: 60 °C * Lubricant flow rate: 5 ml/min * Sliding Distance: 400 meter * Shaft: case hardened EN36 * Shaft hardness: 500-600 Hv10 * Shaft roughness: 0.3pm (1:1,,a.) The procedure followed to produce invention example 1E6 consisting of a Bi composite coating with WS2 particles on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

5. Electrodeposition of the Ni/WS2 composite coating on the bronze substrate at a current density of 4.5 A/dm2 during 15 minutes under mechanical stirring.

The procedure followed to produce comparative example CE6 consisting of a Bi coating on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

The average wear observed on 1E6 and CE6 samples (after wear tests) is shown in Figure 6. Lower wear was measured on 1E6 samples than on CE6 samples, demonstrating the improvement in terms of wear resistance achieved in the Bi coating when soft particles (WS2 in this case) are successfully incorporated into Bi coatings electroplated under ultrasonic/megasonic conditions.

TRIBOLOGICAL SEIZURE TESTS

Seizure tests were also performed on the coating showing the highest ductility to observe the effect that the incorporation of soft particles has on the coefficient of friction of Bi coating. Again, the tests were performed on 15 pm coatings, as coatings in accordance to current-state-of-the-art would present bonding/brittleness issues during the test when using thicker coatings due to low ductility. The coatings were electrodeposited on special bronze samples to be used on a ring (shaft)-on-disk tribometer. The conditions of the test were: * Sample dimensions: outer diameter = 27.2 mm, inner diameter = 22 mm * Load: 1MPa increase every 5 minutes * Sliding speed: 2 m/s * Lubricant: VG22 (SAE10) * Inlet lubricant temperature: 60 °C * Lubricant flow rate: 20 ml/min * Sliding Distance: until seizure * Shaft: case hardened EN36 * Shaft hardness: 500600 Hv10 * Shaft roughness: 0.5pm (Pima.) The procedure followed to produce invention example 1E7 consisting of a Bi composite coating with WS2 particles on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

4. The substrate was then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze substrate surface was applied at a current density of 3 A/dm' for 2 minutes.

The procedure followed to produce comparative example CE7 consisting of a Bi coating on a bronze substrate is: 1. Preparation of a MSA-based Bi plating bath with temperature of 40 °C.

3. The substrate was then mounted in a plating jig and anodic acid-etching pre-treatment of the bronze substrate surface was applied at a current density of 3 A/drn2 for 2 minutes.

The average maximum load without seizure achieved on 1E7 and CE7 samples is shown in Figure 7. Higher loads without seizure were reached on the 1E7 samples than on the CE7 samples. In addition, the loads without seizure measured were significantly more consistent for 1E7 samples than for CE7 samples. These results demonstrate the improvement in terms of seizure resistance and reliability achieved in the Bi coating when soft particles (WS2 in this case) are successfully incorporated into Bi coatings electroplated under ultrasonic/megasonic conditions.

In summary, the present invention relates in one aspect to a plain bearing consisting of a backing substrate (e.g. steel), a lining material (e.g. bronze-based or aluminium-based alloy) and a composite overlay, where the overlay is an ultrasound / megasound-assisted electrodeposited composite coating consisting of a metal matrix (i.e. Bi or Bi alloy) and uniformly and well-dispersed soft lubricant particles.

Claims

CLAIMS 1.
2.
3.
4.
5.
6.
7.A composite coating comprising a metal matrix made of Bi or a Bi-based alloy and one or more types of soft particles with Mohs hardness 55 dispersed within the metal matrix.

A composite coating according to claim 1, wherein the metal matrix is a Bi alloy.

A composite coating according to claim 2, wherein the Bi alloy contains one of more of the following metals: Sn, Cu, Ni, Ag, Cr, Co, Fe, Mn, Au and/or Zn.

A composite coating according to any of the preceding claims, wherein the soft particles incorporated into the metal matrix of the composite coating are selected from the group of following materials; PTFE, fluorinated polymers, metal sulphides, metal fluorides, metal sulphates, graphite and other soft carbonaceous particles, hexagonal boron nitride, phyllosilicates, zinc oxide and lead oxide, and any other particles with Mohs hardness 5 5, and mixtures thereof.

A composite coating according to any of the preceding claims, wherein hard particles with Mohs hardness are additionally included dispersed in the metal matrix.

A composite coating according to claim 5, wherein the hard particles are selected from the group of metal oxides, borides, carbides, nitrides and silicides, diamond, carbon nanotubes, graphene and other hard carbonaceous particles, and any other particles with a Mohs hardness of 5.5, and mixtures thereof.

A composite coating according to any of the preceding claims, wherein the content of soft particles and optional hard particles incorporated into the metal matrix of the composite coating ranges from 0.0001 to 30%vol., preferably from 0.01 to 15 %vol.
8. A composite coating according to any of the preceding claims, wherein the soft particles and optional hard particles incorporated within the metal matrix of the composite coating have a size range of 1 nm to 5 pm, preferably 250 nm to 3 pm.
9. A method of manufacturing a composite coating according to any of the preceding claims, wherein the composite coating is electrodeposited or electroless plated from a bath in which particles are dispersed by the use of agitation by ultrasound or megasound and optionally a further agitation method.
10. A method of manufacturing a composite coating according to claim 9, wherein the ultrasonic or megasonic frequency is between 10 and 3000 kHz, preferably from 15 to 200 kHz, and the ultrasonic or megasonic power is between 0.00001 and 50 W/cm3, preferably from 0.0001 to 5 W/cm3.
11. A method of manufacturing a composite coating according to claims 9 and 10, wherein the ultrasound or megasound and optional further agitation method is applied for a period of time longer than 0.01 seconds and no larger than 60 days, preferably between 20 seconds and 48 hours.
12. A method of manufacturing a composite coating according to claim 11, wherein the ultrasound or megasound or the optional further agitation method is applied for a period of time between 1 minute and 24 hours, preferably between 5 minutes and 4 hours.
13. A method of manufacturing a composite coating according to any of the claims from 9 to 14, wherein ultrasound or megasound and the optional further agitation method is applied before and/or during the electrodeposition stage of the coating process.
14. A method as claimed in any of claims 9 to 13 wherein the composite coating is deposited to a thickness of 1 to 50 pm, preferably 2 to 40 pm, and more preferably 3 to 25 pm thick.
15. A plain bearing with an overlay, wherein the overlay is a Bi-based composite coating according to any of the claims from 1 to 8.
16. A plain bearing with a composite overlay according to claim 15, wherein the overlay is deposited by a method according to any of claims 9 to 14.
17. A plain bearing with a composite overlay according to claim 15 or 16, wherein a bearing lining material over which the composite overlay is deposited is either a bronze-based alloy or an aluminium-based alloy.
18. A plain bearing with a composite overlay according to any of claims 15 to 17, wherein a strike layer or interlayer comprising Ni, Cu, Ag, Cr, Co, Fe, Mn, Au, Zn or alloys or mixtures thereof is deposited between the composite overlay and the bearing lining material.
19. A plain bearing with a composite overlay according to claim 18, wherein the strike layer or interlayer deposited between the composite overlay and the bearing lining material has a thickness between 5 nm to 15 pm, preferably from 100 nm to 5 pm.
20. A plain bearing as claimed in any of the preceding claims wherein the composite coating has a thickness of 1 to 50 pm, preferably 2 to 40 pm, and more preferably 3 to 25 pm thick.