GB2500487A - Tribological coatings and methods of forming - Google Patents

Abstract

A method of forming a tribological coating 13 on a substrate 12 comprises depositing layers of a relatively soft solid lubricant material 14b, 15b, 16b, 17b and layers of a relatively hard material 14a, 15a, 16a, 17a, wherein at least the soft material is deposited so as to provide a plurality of discontinuous planar layers interposed between layers of the hard material, permitting underlying and overlying layers of the hard material to interlace. One or both of the materials are preferably deposited by PVD or CVD, most preferably by plasma-assisted PVD. The soft solid lubricant material may comprise a fluoro-polymer such as PTFE or may comprise MoS2. The substrate may be configured as a portion of a plain bearing. A further method of forming a tribological coating on a substrate comprises depositing layers of a relatively soft solid lubricant material 14b, 15b, 16b, 17b and layers of a relatively hard material 14a, 15a, 16a, 17a that is or comprises aluminium, aluminium alloy, silver alloy, Ag-Sn alloy or Sn-Ag alloy. Also disclosed are tribological coatings applied to substrates.

Description

Tribological Coating The present invention claims priority from GB1 204927.6 filed 20 March 2012, the contents of which are hereby incorporated into this application by reference.

The invention relates to the field of low friction and wear resistant coatings, and in particular to tribological coatings which may be used in mechanical systems having sliding surfaces and in particular in the manufacture of plain bearings.

In mechanical systems the relative motion between surfaces results in friction and wear, causing loss of efficiency and ultimately damage or failure of the mechanical system in which the wear takes place. For example, plain bearing failure will result in seizure of the bearing. Tribologists have used coatings and lubricants to overcome issues of wear and friction in mechanical systems. However, recent legislation and market trends have created limitations in terms of what type of coating materials and lubricants can be used for a mechanical system.

One particular example is the automotive industry where EU End-of-Vehicle life directive (2000/53/EC) bans use of toxic and heavy materials from vehicles. One specific component of internal combustion engines affected by this legislation is bearings which historically relied upon using lead. Lead is an excellent tribological material but is of course toxic to life and thus harmful to the environment. Other emerging trends in automotive markets are to achieve maximum fuel efficiencies and reduce carbon footprints by reducing parasitic power losses from internal combustion engines.

There is an increasing trend towards smaller engines with higher power outputs.

Such engines require bearings that can withstand a much higher fatigue load. There is also a requirement to use thinner lubricants, resulting in more metal-metal contact between the bearing and the shaft. It is beneficial therefore if at the same time as increasing the bearing load the coefficient of friction between the shaft and the bearing can be reduced.

Engine manufacturers are adopting many radical ways to achieve maximum fuel efficiencies and reduce pollutant emission, for example, using low friction components, thinner oil films, low viscosity lubricants, engine downsizing, stop-start cycles and using alternative combustion cycles. Using any or a combination of these techniques leads to an arduous operating environment for engine components resulting in higher fatigue loads, higher temperatures, greater asperity contact and hence more incidences of boundary lubrication failure.

These operational complexities require high strength and tribologically efficient coatings for engine components to make them more efficient and reliable.

For highly loaded engines, it is common to use electroplated overlay bearings.

Conventionally, lead based PbSnCu overlay coatings have been used with bimetal and trimetal bearings for high specific load applications such as big-end bearings.

More recently, new lead-free electroplated overlay coatings have been developed in response to the legislation. A lead-free AgBi coating disclosed in US-A-7629058 has been successfully used in many highly loaded engine bearing applications. One of the main disadvantages associated with all electroplated coatings is that they are produced from aqueous chemical solutions and some of the chemicals used in the production process are hazardous to humans and environment.

US-A-2011/044572 describes a diamond-like-carbon (DLC) coating for lubricated engine components. The DLC coating is deposited at temperature less than 200t using plasma assisted chemical vapour deposition process (PACVD) to provide a coating hardness of around 300Hv.

WO-A-20061 20017, DE-A-102005006719, and WO-A-2009046476 describe PVD (sputter) deposited AISn, AISnCu, AISnSi, AIBi, AIBiCu coatings for application in engine bearings. The disadvantages with all these coatings are that they lack the necessary compatibility and embeddability to compensate for any misalignment in the operating components, or foreign particles in the lubricating oil. This lack of compatibility and embeddability in these coatings may lead to a premature failure of mechanical components.

DE-A-1 02008037871 describes a multilayer coating structure formed by deposition of several layers by PVD and PACVD processes; first a Ti, Ni or Cr bonding layer is deposited onto a substrate, then a metal doped DLC layer of hardness over 3OGPa (1GFa = approx. 98Hv) is deposited over the bonding layer, then an interlayer of Cr or Ni is deposited over the DLC layer and finally a running-in layer of AISn alloy or any other solid lubricant layer with hardness ranging from 10 to 400Hv is deposited over the interlayer.

US-A-2007/065067, WO-A-2007/033709, WO-A-2006/1 20015, J P-A-2006 336674 also describe other types of multilayer coatings where a running-in layer of soft metal alloy or a thermoplastic resin with solid lubricant is deposited over the sputter-deposited AISn, AISnCu, AISnSi, AIBi, AIBiCu hard coatings. The comparatively soft running-in layer in these multilayer coatings provides reasonable compatibility and embedability to compensate for any misalignment or foreign particles in the lubricant.

The layer typically do not last for the full life time of the component because the running-in layer coating is thin and soft and wears out during the first few hours of the engine operation. Other disadvantages of such multilayer coatings are that the production process is very complicated and expensive which involves using more than one manufacturing process. Also, the boundaries formed between different layers of a rnultilayer coating increases the possibility of fatigue or delamination failure during operation.

U52005-A-003225 describes a sputtered AISnCu overlay in which the constituents of the coating vary from the substrate to the outer most layer of the coating. The targets used for depositing the coating are (A) Al 99.5%, (B) pure Cu, and (C) pure Sn. By adjusting the input power to each target, the following multilayer coating was deposited; first a pure Cu layer serving as diffusion barrier, followed by a layer of Al-Cu5 w/w%, then Al-Sn20 w/w% layer, and then Sn-Cue w/w% layer. The major disadvantages with such a coating are; the complexity of the production process which involves simultaneous control of input power to many targets, and the cost of the pure targets which makes the coating very expensive to produce.

DE-A-1 02009019601 describes a Cu-based overlay with soft particles embedded in the overlay which claims excellent sliding properties and high strength at elevated temperatures with high corrosion resistance, low hardness and low seizure tendency.

The Cu/Cu alloy overlay is deposited from an alkaline non-cyanide electroplating solution with complexant and wetting agent to suspend solid lubricant particles of Mohs hardness less than 2 and size of less than 10Mm. Finally, a running-in layer of indium, zinc, tin, or their alloys can be deposited by PVD or CVD process. The disadvantages with this type of multilayer coating is that the production process uses two completely different production processes, electroplating and sputtering, which makes the coating complex and expensive to produce. Also, the boundaries formed between different layers of the coating while switching between different manufacturing processes increases the possibility of fatigue or delamination failure during operation.

US patent 5,268,215 (to Keem et al) discloses a composite, multilayer thin film solid lubricant structure in which comprises solid lubricant layers of MoS2 interleaved with relatively thin interlayers of gold, silver or nickel. The interlayers are intended to interrupt propagation of growth defects in the MoS2 deposition process. Typical solid lubricant layer thickness is up to 2000 angstroms about 100 to 200 angstroms (10 to 20 nm) and interlayer thickness is about 2 to 5 angstroms. Ion beam deposition (IBD) is used to lay down the layers. This structure is primarily concerned with improving the physical properties of a bulk solid lubricant layer.

It is known to provide self-lubricating bearings comprising a steel substrate upon which is sintered a porous bronze layer which is coated and impregnated with RIFE polymer to form a continuous layer of RIFE on the upper surface.

It remains a requirement for future fuel-efficient engine to provide new coatings for bearings combining low friction and high strength. Ihese will be required across all applications; from downsized three cylinder range extender engines for hybrid passenger cars to large four stroke marine and power generation engines.

Factors which influence the specification of the coatings include: 1) Ihe use of higher firing pressure, different fuels (e.g. biogas), and different combustion patterns (e.g. HCCI) in order to reduce CO2 and NO emissions.

This will result in higher peak firing loads that require higher fatigue strength from the bearings. i.e. bearings should be stronger.

2) Ihe use of lower viscosity lubricants to reduce parasitic losses from oil pumping and oil shear. This reduces CO2 emissions but will result in much thinner oil films and more bearing/shaft contact that require more seizure resistance and lower running friction from the bearings. i.e. bearings should be softer and provide better inherent lubrication.

Tackling both seizure/friction and fatigue resistance in the same material has been hitherto problematic using conventional engineering materials, composite alloys and coatings.

The present inventors have chosen to seek a solution to the continuing problems of improved composite materials in replicating the naturally occurring composites. One such example is mother of pearl which combines low friction and high strength in form of a natural intercalation compound. Thus a biomimetic approach may be adopted to develop a synthetically engineered composite material which mimics the intercalated structure of naturally occurring materials by using at least two or more materials to combine both; the softness and lubricity -to give low friction and good seizure resistance on the one hand, and strength and toughness, -to give high fatigue and wear resistance on the other.

According to the present invention there are provided tribological coatings and methods of forming such coatings as set forth in the statements of invention below and/or the claims hereinafter.

According to one aspect of the present invention there is provided a method of forming a tribological coating on a substrate, comprising depositing layers of a first a relatively soft, solid lubricant material and depositing layers of a second relatively hard material, thereby to accumulate a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed within (i.e. between) layers of the second material. By interleaving the layers areas of solid lubricant and areas of hard bearing material is distributed through the thickness of the coating.

In a preferred aspect, at least the first soft material is deposited as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material.

In another preferred aspect of the invention the hard material is also deposited so as to provide a plurality of discontinuous layers thereof, thereby to form a coating structure in which the first and second materials are intercalated (or interspersed) with one another. This provides a particularly effective tribological coating which combines the strength and toughness of the hard material along with much of the tribological performance of the solid lubricant.

The soft material is preferably deposited in alternating layers with the hard material.

The soft material may be deposited in layers having a lower frequency of occurrence than the layers of hard material. Thus an occasional, preferably regularly occurring, transverse layer or seam of lubricant material may be provided in a matrix of hard material.

One or both of the materials may be deposited by physical and/or chemical vapour deposition. The preferred deposition method is plasma-assisted physical vapour deposition, for example as described in more detail hereinafter.

One or more (preferably all) of the layers of each material may be laid down to a thickness which corresponds to a mono-layer of the material up to a thickness of 30 pm. A deposition of each layer hereinbefore mentioned may comprise several individual sub-layers to be applied, or may be applied as a single deposition step.

Most preferably some or all of the layers have a thickness which corresponds to a mono-layer of the material up to a thickness of 10 nm. Nanoscale layers may be readily applied by plasma assisted PVD and can provide excellent interlayer bonding and structural integrity. A sufficiently thinly applied layer will automatically provide heterogeneous coverage, that is to say areas which are missed, or alternatively islands of material coverage in otherwise uncovered areas.

The coating thereby formed in accordance with the invention preferably has a thickness of 0.1 to 100 pm, preferably 0.1 to 30 pm. Typical tribological coatings of the invention are preferably 5 to 15 pm thick.

The relatively hard material is preferably a metal or metal alloy. The metal alloy may be an aluminium alloy or pure aluminium. Other suitable metals and alloys include, without limitation, silver, silver alloys, tin, tin alloys, Ag-Sn alloy (i.e. a silver based alloy with minor amounts of tin) or Sn-Ag alloys (tin based alloys with minor amounts of silver).

In some embodiments the relatively soft material solid lubricant comprises a polymer, such as a fluoro-polymer, in particular PTFE. The solid lubricant is not necessarily a polymer, and could for example be a metal sulphide, preferably molybdenum sulphide (MoS2).

The soft material is preferably disposed as layers of platelets which extend generally transversely in the coating.

The coating may comprises one or more further deposited materials, preferably deposited in layers thereof. Thus further hard materials may be deposited, or further lubricant materials.

An intermediate layer may be deposited on the substrate before the coating is applied, whereby the coating is applied to the intermediate layer lying on the substrate. The intermediate layer (or interlayer) may be used to aid in adhesion between the substrate and coating, orto provide chemical compatibility between the coating and substrate (e.g. to avoid a corrosion potential or to provide diffusion barrier). One example is a NiCr alloy, but the material selection will depend upon the substrate and coating materials and the intended operational conditions and environment of the bearing.

The substrate upon which the coating is provided preferably comprises a metal, but could include fibre or particulate composite materials or ceramic materials. Typical metals upon which a tribological coating may be applied include bearing materials such as bronzes, brasses, white metals, aluminium alloys, steels and alloy steels.

The substrate metal may be lead free bronze, leaded bronze, aluminium alloy bimetal, steel or a known bearing material.

Naturally, the substrate may be configured as a portion of a plain bearing and the coating is applied to form the exposed bearing surface. For example, the substrate may be a bearing half piece. Thus the method of the invention may further comprise coating bearing surfaces of two bearing half pieces, which are then assembled into a housing to form a whole plain bearing.

In yet another aspect of the invention there is provided a tribologically coated substrate obtainable by the invention methods hereinbefore described.

Similarly there may be provided in accordance with the invention a tribologically coated plain bearing obtainable by the methods or a tribological coating obtainable by the said methods.

Thus in accordance with the invention there is provided a coating for a substrate comprising a plurality of layers of a first relatively hard solid material with a plurality of layers of a second relatively soft solid lubricant material. The layers may be continuous in some embodiments, but in others are discontinuous. For example, the soft material may be dispersed therein in discontinuous layers. The coating is preferably a tribological coating which is to say that it has low friction and highly wear resistant properties. When both the hard and soft materials are deposited as discontinuous layers they may be termed "intercalated", by which the applicant means that the materials making up the coating interpenetrate (or interlace) each other to form an interconnected network of each material within the other material.

This structure may be contrasted with a conventional dual phase alloy in which a single material has one or more minor phases dispersed within a matrix of another phase. It may also be contrasted with a particulate composite in which a phase of one material may be dispersed in a matrix of another different material.

In a preferred arrangement the coating comprises a plurality of discontinuous layers of each respective material laid down one upon the other to produce the intercalated structure. The discontinuities in each layer permit the laying down of an upper layer directly onto exposed regions of both the underlying layer and exposed regions of the layer or substrate below.

The respective hard and soft materials are typically disposed in the coating as generally planar portions or platelets which extend generally transversely in the coating.

The coating may comprise one or more further intercalated materials. Thus the coating may be made up of two or more relatively hard materials or two or more relatively soft materials. The materials may be mixed at an intra-layer level (i.e. two or more materials per layer, or layer by layer with a discrete material for each layer).

The relatively hard material is typically a metal or metal alloy. The metal may be selected from one or more of a group of metals such as Al, Cu, Sn, Ag, Bi, Sb, In, Ni, Cr, Co, Fe, Sc, Ti, V, Mn, Pt, Au, Si, W, Ta, Zr, Zn, Pb, Mo, Y, Nb, Cd, Ca, Mg or alloys of these metals, and combinations thereof. The relatively soft material may be one or more materials selected from the group of tribological materials / solid lubricants such as fluorinated polymers, metal sulphides (preferably MoS2), metal fluorides, metal sulphates, graphite, C, hBN, phyllosilicates, titanium, zinc or lead oxides. The materials listed above are not necessarily limiting and no doubt other suitable materials will occur to persons skilled in the art.

In a specific embodiment the metal alloy is pure aluminium.

In another preferred embodiment the metal alloy is an aluminium alloy and in particular an aluminium alloyed with copper and magnesium. In a specific embodiment the metal is A195-Cu4-Mgl.

As previously mentioned in preferred embodiments the soft material is a high-surface energy polymer such as a fluoro-polymer and in one specific embodiment PTFE. It is possible to use copolymers or mixtures of polymers.

The present invention in one aspect thus provides an intercalated multilayer coating for mechanical systems. The coating may be made of many alternating and discontinuous nano-scale layers of relatively hard phase and relatively soft material applied by means of a plasma assisted physical vapour deposition (PA-PVD) process. Such coatings have surprisingly been found to have superior mechanical, tribological and metallurgical properties.

The current invention in some embodiments is an intercalated multilayer coating with nanoscale discontinuous layers of a solid lubricant relatively soft phase and a relatively hard phase. The soft phase may be a soft metal or a fluorinated polymer of a solid lubricant or a mixture of these. The hard phase may be a structural metal such as an aluminium or copper alloy or a composite consisting of metal and beneficial intermetallic/ceramic phases or precipitates. The hard and soft phases are arranged to give an intercalated structure analogous to the mother of pearl, bone or a shell. This structure allows the new coating to simultaneously give higher strength than existing hard sleeve bearing materials and higher seizure resistance than existing soft sleeve bearing materials.

The developed intercalated multilayer coating in accordance with the invention has several potential advantages. Instead of a conventional metallic alloy structure, the new intercalated multilayer coating may comprise an intimate nano-scale mixture of metallic and solid lubricant phases in form of alternating and discontinuous layers.

This gives the coating the strength and ductility of a metal with the lubricious properties of a solid lubricant, but in a single composite structure.

No existing prior art coating provides a combination of good mechanical, tribological and metallurgical properties in a single coating structure. However, by intercalating at least two or more materials the developed intercalated multilayer coating has the potential to achieve good mechanical, tribological and metallurgical properties in a single coating matrix.

The developed intercalated multilayer coating is in commercial embodiments made of lead-free materials making it suitable for use in the automotive industry. The coating will have many applications, for example, as high strength sleeve bearing materials in highly loaded engines that will be required in the future, especially in downsized automotive engines, to large gas fired industrial engines. Existing state-of-the-art coatings are not expected to be able to withstand the combined requirements for increased load and lower viscosity lubricants that will be needed to achieve high fuel efficiencies and to minimise 002 emissions from the future engines. However, by combining good mechanical, tribological and metallurgical properties the developed intercalated coating will be suitably applicable for future low emission engines. The developed intercalated multilayer coating can be deposited by a PVD process which is not harmful to humans and environment, as compared to the commonly used chemical electroplating processes in forming tribological coatings.

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

In the drawings:-Figure 1 is a schematic cross sectional representation of a plasma-assisted PVD deposition apparatus used in preparing the coatings in accordance with the invention.

Figure 2 is a perspective view of a schematic representation of a substrate coated with an intercalated multilayer coating in accordance with an embodiment of the invention.

Figure 3 is a similar view to that of figure 2 which shows an alternative configuration in which an interlayer is disposed between the substrate and the intercalated multilayer coating.

Figure 4 is an SEM image of a multilayer coating on a substrate, sectioned to show alternating nanoscale continuous layers of Al and FIFE, in accordance with the invention.

Figure 5 is an SEM image of a similar multilayer coating substrate section with much thinner alternating nanoscale layers of Al and PTFE in accordance with the invention.

Figure 6 shows a SFEG image of an intercalated multilayer coating (with substrate not shown) made up of interlaced nanoscale compounds of Al alloy and PIFE, in accordance with the invention.

Figure 7 shows the Ring-on-Disc tribometer configuration used for testing the friction and wear performance of developed coating.

Figure 8 shows the friction results of the intercalated multilayer coatings of the present invention and a conventional Al-Sn20 sputter coating.

Figure 9 shows the wear results of the intercalated multilayer coatings of the present invention and a conventional Al-Sn20 sputter coating.

Figure 10 shows the maximum load without seizure occurring for examples of the invention and comparative examples.

I

Figure 2 shows a low friction and high wear resistant structure 10 in accordance with one embodiment of the invention. A planar rectilinear substrate 12 is a plate of iron, but could equally another metal, alloy or polymer material. A multilayer coating 13 is applied to the substrate. A first coating layer 14 provided on the substrate upper surface is made up of a relatively hard material (a) intercalated with a relatively soft, solid lubricant material (b). Further layers 15, 16, 17 are shown, each having a similar structure of intercalated hard and soft materials. In practice many more layers will be applied to build up a desired coating thickness.

To form the above intercalated multilayer coating structure on the substrate in the present embodiment plasma assisted physical vapour deposition (PA-PVD) or chemical vapour deposition (PA-CVD) is used. In this embodiment the hard material is pure aluminium and the soft lubricant material is PTFE. The deposition of FIFE by PA-PVD is known and has been described for example by Nicholls and Lawson (in ii Fluoropolymers 1: Synthesis, Topics in Applied Chemistry, 2002, III, pp313-320, the disclosure of which is incorporated herein in full by reference).

A first layer (14-a) of aluminium alloy is deposited on the substrate by PA-PVD with insufficient coverage to form a complete layer. Thus regions of the substrate surface remain exposed, with discrete islands of deposited material dispersed over the substrate surface. The islands will typically be very thin, down to mono-layer thickness (i.e. single atom thickness). Then a second discontinuous layer (14-b) of PTFE material is deposited over the substrate and first layer by PA-PVD. Further layers (15-a, 15-b, 16-a, 16-b, 17-a, 17-b etc) are applied, each of which (in this embodiment) is discontinuous, to produce the intercalated/interlaced network of aluminium and PTFE.

Figure-3 shows another embodiment 18 which is a variation of that described above.

In this case with a continuous interlayer 19 is provided on the substrate and under the multilayer intercalated coating 13. The interlayer serves to aid bonding between the multilayer coating and the substrate. In the specific embodiment described in more detail below the interlayer is a conventional NiCr alloy, applied by PVD sputtering.

The present PA-PVD methodology is set out in the following. A magnetron physical vapour deposition (PVD) coater is employed to deposit an intercalated coatings composed of Al and PTFE. The schematic configuration of the coater 20 is shown in Figure 1. The specimens 21 to be coated are mounted on an underside of a worktable 22 which is disposed in an upper region of a coating chamber 23. The worktable is suspended from the depending rotor 24 of a stepper motor (not shown).

The speed of this motor dictates the rotation speed of the worktable. In one lower corner region 25 of the chamber an aluminium target is provided by a magnetron 26 which serves as a source of aluminium plasma 30. In another corner region 27 a further magnetron 28 serves as a PTFE target or source of PTFE-forming plasma 31.

An upstanding partition wall 29 serves to separate the Al from the FTFE plasmas.

Thus, using this apparatus continuous multilayers of Al or FTFE can be formed, or by varying the rotational speed of the worktable and target power, discontinuous nano-layers can be deposited. It will be appreciated that this approach is not necessarily limited to Al or PTFE, as other materials can be deposited via this approach. In the present specific embodiment an initial coating of Ni-20%Cr is made on the underlying substrate to form an interlayer, as will be explained in more detail below.

Factors that dictate the thickness of the layers deposited include: the time spent by the specimens over the targets (depends upon the rotational speed of the table), the substrate-target distance, the magnetron sputtering power applied to the targets and the coating chamber gas pressure.

After mounting the samples to the worktable, the coating chamber is pumped to a base pressure of at least 10-6 mbar. Argon is the working gas for processing. Prior to deposition, the samples are ion-cleaned by biasing the worktable at -500V for 15 minutes, followed by 30 minutes at a bias of -700V. Thereafter, power to the targets is switched on. The Al target is pulsed-DC sputtered at powers ranging from 30 -200 watts. The PTFE target is RF sputtered at powers ranging from 30-150 watts.

Process Operating Parameters.

Typical steps to produce a nanocomposite intercalated coating of the type described hereinafter are as listed below. Initially the substrate surface is pre-treated chemically: a. Ultrasonic cleaning in acetone -20 mins at 30°C.

b. Ultrasonic cleaning in IPA -20 mins at 30°C.

The sample is immediately loaded into the coating chamber and vacuum-pumped to the aforementioned base pressure. The target-to-substrate working distance is fixed at 5cm. The surface is then ion-etched using Ar ions by applying a negative potential to the worktable, under the following operating conditions: i) Ar pressure 25 mbar (ii) NiCr target power 25W (pulsed DC 220 KHz at 1.1 ps) (iU) 15 mins at -500V, followed by 30 mins at -700V potential.

The deposition of the interlayer coating immediately follows.

For deposition of a 1 pm thick Ni-20%Cr interlayer there is an argon pressure of 15 mbar; the target is sputtered by using a pulsed DC power of 120W (220 KHz at 1.1 ps). The worktable is biased at -75V. The deposition duration is 3hrs and the worktable rotational speed is about 3.3 revs/mm.

A thin layer of aluminium is then deposited using pulsed DC power of 120W (220 KHz @1.1 ps); argon pressure of 15 mbar is established and a worktable bias of OV (i.e. electrically-grounded). The deposition time was 15-20 mins and the worktable rotational speed was about 3.3 revs/mm.

The AI/PTFE coating step was then carried out using an Al target pulsed DC power of 120W (220 KHz @l.ips); a PTFE RF power of 60W (at 13.56 MHz); and an argon process pressure in the chamber of 15 mbar. The deposition time was 6hrs to provide a 2pm-thick coating using a worktable rotational speed of about 3.3 rev/mm.

To build up a thickness of 8pm a coating period of 30 hours was provided. After processing the samples were left in vacuum to cool overnight.

Using the above methodology alternating layers of distributed islands of PTFE and aluminium were deposited on the interlayer (or directly on the substrate when no interlayer is present). The process is repeated to build-up gradually the multilayer structure.

This procedure of building-up of alternating discontinuous layers of aluminium alloy and PTFE was repeated until the desired thickness of the intercalated multilayer coating 13 is achieved. In the present embodiment the thickness of each discontinuous layer is about 0.3 to 20 nm (i.e. about ito 70 atoms) thick. The total thickness of the applied multilayer coating 13 in this example was 8 pm.

In some embodiments it is important that individual layers of the soft material (PTFE) are to some extent discontinuous so as to maintain inter-layer strength, which is derived primarily from the aluminium, whereas the soft material provides the main contribution to tribological properties.

Figure 4 shows a SEM microstructure image of a cross-sectioned coating 111 which is in accordance with the invention but which also provides a structural comparison with other embodiments. In this example, a multilayer coating 113 is deposited over a glass substrate 112 by using a PA-PVD process. The multilayer coating 113 is built-up by depositing alternating nanoscale layers of hard phase 114 and soft phase 115.

As before, the hard phase 114 used for building-up the multilayer coating 113 is pure Al and the soft phase 115 used for building the multilayer coating 113 is a synthetic fluoropolymer (PTFE). The thickness of each alternating layer of hard phase and soft phase ranges from 20 to 500 nm. It is clearly visible from the microstructure shown in Figure-4 that at a thickness between 20 to 500 nm the alternating layers of hard and soft phase are in this case continuous which gives a clearly delineated a multilayer structure. The absence of discontinuous regions generally does not permit one hard layer to structurally penetrate another.

Figure 5 shows a SEM microstructure image of a section through another coating 221. In this configuration, a multilayer coating 223 is deposited over a glass substrate 222 by using a PA-PVD process. The multilayer coating 223 is built-up by depositing alternating nanoscale layers of hard phase 224 and soft phase 225. As above, the hard phase 224 used for building-up the multilayer coating 223 is pure Al. The soft phase 225 used for building the multilayer coating 223 is a synthetic fluoropolymer (PTFE). In this case the thickness of each alternating layer of hard phase and soft phase ranges from 10 to 20 nm. It may be concluded from the microstructure shown in Figure 5 that though the thickness of each alternating layer of the hard and soft phase is much thinner (10 -20 nm) the layers are still continuous to a major extent, giving a generally discrete multilayer structure. There is however evidence of intercalation behaviour, specifically the discontinuous appearance in some of the layers.

Figure-6 shows an SFEG microstructure image of an intercalated multilayer coating in accordance with a preferred embodiment of the invention 331. Unlike figures 4 and this image is an oblique section through coating layers, with the substrate itself not visible. As before, the intercalated multilayer coating is built-up by depositing alternating nanoscale layers of hard phase 334 and soft phase 335 by using PA-PVD process. The hard phase used for building-up the intercalated multilayer coating is pure aluminium. The soft phase 335 used for building the intercalated multilayer coating is PTFE. In this case the thickness of each alternating layer of hard phase 334 and soft phase 335 ranges between 0.3 nm to 10 nm. It may be established from the microstructure shown in Figure 5 that with this reduced deposition thickness the alternating layers of hard and soft phase are discontinuous which creates an intercalated structure where constituting layers of hard phase and soft phase are interlaced and thus interpenetrate each other.

The intercalated multilayer coating 331 in accordance with the invention mimics the structure of natural composite materials such as the mother of pearl, bone or a shell and thus can be classified as a synthetic biomimetic composite material.

The following examples and comparative examples were prepared:

Example 1

An 8Mm Al-PTFE intercalated multilayer coating of the type shown in Figure 6 deposited on a Pb-free bronze substrate with mild steel backing.

Example 2

An 8Mm Al-MoS2 intercalated multilayer coating of the type shown in Figure 6 deposited on a Pb-free bronze substrate with mild steel backing.

Comparative example 1 A l5pm Al-Sn20 sputtered coating deposited on a Pb-free bronze substrate with mild steel backing.

Comparative example 2 A 5pm diamond like carbon (DLC) coating was deposited by PVD on a Pb-free bronze substrate with mild steel backing.

Comparative example 3 A cast and roll bonded 300pm thick AISnSi alloy with mild steel backing.

Wear and Friction Testing A Ring-on-Disc tribometer as shown in Figure-7 was used to compare the wear and friction performance of intercalated multilayer coatings in accordance with the invention against conventional Al-Sn20 sputter coated substrate per comparative example 1. The tribometer 40 is shown in vertical section in figure 7A. The tribometer is of standard construction and includes a sample holder 41 having a downward-facing square pressure plate 42 which faces an upper end surface of an annular end collar 43 of a rotatable shaft 45. The pressure plate is provided with four circumferentially spaced arcuate strips 44 which serve as sample carriers. The substrate underside of each sample is adhered to the associated sample carrier strip 44 so that the coated substrate surface of each faces the collar 43. In use, the sample holder is urged against the collar 43 upper surface so as to provide a wear/friction testing regime as the shaft is rotated.

Table 1 below shows the test parameters used for measuring the wear and friction performance of test coatings.

Table-i:

Items Conditions Sample size l.D.22 X O.D.27 mm Load 4 MPa constant Velocity 2 m/sec Test Duration 80 mm Lubricant SAE#1 0 Temperature 60 degree C Oil supply rate 150 ml constant bath Shaft material JIS S55C (Carbon steel) Shaft roughness c 0.3pm (Rm) Shaft hardness 500-600 (HV1O) Figure 8 compares the coefficient of friction over time for the conventional substrate/coating combination and for the invention substrate/intercalated coatings.

According to the testing methodology the test was run at constant load and speed.

Figure 9 shows the wear performance results of the intercalated multilayer Al-PTFE and Al-MoS2 coatings against a conventional sputtered Al-Sn20 coating.

During the running-in phase of the Ring-on-Disc test (see figure 8 up to 1800 seconds), the newly developed intercalated multilayer Al-PTFE and Al-MoS2 coatings showed approximately 45 to 60% reduction in average coefficient of friction compared to the conventional AISn2O sputtered coating. The intercalated multilayer Al-PTFE coating maintains its low coefficient of friction performance during normal operation (above 1800 seconds in figure 8), having approximately 40 to 55% lower average coefficient of friction compared to the conventional Al-Sn20 sputtered coating.

Table 2 below summarises the wear and friction performance data set out in figures 8 and 9.

Table-2:

Average coefficient of Total Wear friction amount (pm) Substrate During During Normal Coating Type Type Running-in operation Test-i Test-2 period (0-(i800 -4800 i800sec) sec) Embodiment 1 -Pb-free Intercalated 0.024 0.012 6.75 4.75 Bronze Multilayer Al-PTFE Embodiment 2 -Pb-free Intercalated 0.016 0.009 5.85 7.10 Bronze Multilayer Al-MoS2 Comparative Pb-free Sample 1 -Al-Sn20 0.043 0.020 10.75 9.75 Bronze Sputter Measurements of the total wear amount (figure 9 and table 2) of the tested samples showed that there is 30% -55% less wear in intercalated multilayer Al-PTFE and Al-MoS2 coatings compared to conventional Al-Sn20 sputtered coating. The superior wear and friction performance of the intercalated multilayer Al-RIFE and Al-MoS2 coatings is thought to be the result of a strong yet lubricious nano-intercalated structure obtained by depositing several alternating discontinuous layers of Al and lubricant phases such as PTFE and MoS2. During a Ring-on-Disc friction/wear test, the lubricant phase from the intercalated coatings transfers to the counter surface which helps in reducing the total wear and friction for the full test duration. PIFE and MoS2 are of course known to be good solid lubricant material, but heretofore it has not been possible to incorporate RTFE or MoS2 into a solid matrix to provide a wear resistant and low friction coating. Conventionally, PTFE, Mo52 or other solid lubricants are added as a distinct continuous upper layer on a bearing, rather than as continuous or discontinuous layers interposed between relatively hard material layers as specified in the embodiments of current invention.

Seizure lesting Ihe Ring-on-Disc tribometer shown in Figure-7 was used to compare the seizure performance of claimed intercalated multilayer coatings against three comparative coatings: sputtered Al-Sn20, diamond like carbon (DLC) coating, and AISnSi bearing alloy.

lable 3 below shows the test parameters used for measuring the seizure performance of test coatings.

Iable-3: Items Conditions Sample size l.D.22 X O.D.27 mm Load 1 MPa/lOmins step Velocity 2 m/sec lest Duration Until failure Lubricant SAE#1 0 Temperature 60 degree C (oil inlet) Oil supply rate 20 mI/mm Shaft material JIS S55C (Carbon steel) Shaft roughness < 0.3pm (Rm) Shaft hardness 500-600 (HV1O) Figure 10 compares the seizure performance of the intercalated multilayer Al-PIFE and Al-MoS2 coatings against a conventional sputtered Al-Sn20 coating, PVD deposited DLC coating, and a roll bonded AISnSi bearing alloy. The seizure of the material is judged when either the friction torque is reached at 490Nm or the temperature of the test sample is reached at 200°C. During test load is increased in steps of 1 MPa every 10 minutes until the seizure criterion is met.

Table 4 below summarises the seizure performance data for tested coatings.

Table-4:

Substrate Maximum seizure load Coating type type without failure (Mpa) Example 1 -Intercalated Pb-free Multilayer Al-PTFE Bronze Example 2 -Intercalated Pb-free Multilayer Al-MoS2 Bronze Comparative example 1 -Al-Pb-free Sn20 Sputter Bronze Comparative example 2 -Pb-free 6 DLC coating Bronze Comparative example 3 -Steel 7 AISnSi alloy The results of the seizure testing conducted shows that the intercalated multilayer Al-PTFE and Al-Mo52 coatings have much superior seizure performance as compared to conventional sputtered Al-Sn20 coatings, PVD deposited DLC coatings, and a roll bonded AISnSi bearing alloy. The superior seizure performance of the intercalated multilayer coatings is thought to be the result of a strong yet lubricious nano-intercalated structure obtained by depositing several alternating discontinuous layers of Al and solid lubricant phases such as PTFE and Mo52. During seizure tests, the lubricant phase from the intercalated coatings transfers to the counter surface which helps in reducing the total wear and friction for the full test duration and prolonging the seizure resistance of these coatings.

The invention relates to the field of low friction and wear resistant coatings, and in particular to tribological coatings which may be used in mechanical systems having sliding surfaces and in particular in the manufacture of plain bearings. According to the invention there is provided a method of forming a tribological coating on a substrate, comprising depositing layers of a first a relatively soft, solid lubricant material and depositing layers of a second relatively hard material, thereby to accumulate a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed between layers of the second material. Preferably at least the first soft material is deposited as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material. The relatively hard material may be, or comprise: aluminium, aluminium alloy, silver alloy, an Ag-Sn alloy or a Sn-Ag alloy.

Claims (51)

1. Claims 1. A method of forming a tribological coating on a substrate, comprising depositing layers of a first a relatively soft, solid lubricant material and depositing layers of a second relatively hard material, thereby to accumulate a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed between layers of the second material wherein at least the first soft material is deposited as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material.
2. 2. A method of coating as claimed in claim 1 wherein the relatively hard material is, or comprises, a metal or metal alloy, for example: aluminium, aluminium alloy, silver, silver alloy, an Ag-Sn alloy, a Sn-Ag alloy, or an intermetallic compound.
3. 3. A method as claimed in claim 11 wherein the metal alloy is an aluminium alloy.
4. 4. A method of forming a tribological coating on a substrate, comprising depositing layers of a first a relatively soft, solid lubricant material and depositing layers of a second relatively hard material, thereby to accumulate a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed between layers of the second material, wherein the relatively hard material is, or comprises: aluminium, aluminium alloy, silver alloy, an Ag-Sn alloy or a Sn-Ag alloy.
5. 5. A method as claimed in claim 4 wherein the metal alloy is an aluminium alloy.
6. 6. A method as claimed in claim 4 or claim 5 wherein at least the first soft material is deposited as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material.
7. 7. A method as claimed in any of claims 1 to 3 or 6 wherein the hard material is also deposited so as to provide a plurality of discontinuous layers thereof, thereby to form a coating structure in which the first and second materials are intercalated with one another.
8. 8. A method as claimed in any of the preceding claims wherein the soft material is deposited in alternating layers with the hard material.
9. 9. A method as claimed in any of the preceding claims wherein the soft material is deposited in layers having a lower frequency of occurrence than the layers of hard material.
10. 10. A method as claimed in any of the preceding claims wherein one or both of the materials is/are deposited by physical and/or chemical vapour deposition.
11. 11. A method as claimed in claim 10 wherein said deposition method is plasma-assisted physical vapour deposition.
12. 12. A method as claimed in any of the preceding claims wherein some or all of the layers are each laid down to a thickness which corresponds to a mono-layer up to a thickness of 10 pm of each respective material.
13. 13. A method as claimed in any of claims 1 to 11 wherein some or all of the layers are each laid down to a thickness which corresponds to a mono-layer up to a thickness of 10 nm of each respective material.
14. 14. A method as claimed in any of the preceding claims wherein the coating thereby formed has a thickness of 0.1 to 100 pm, preferably ito 30 pm.
15. 15. A method as claimed in any of the preceding claims wherein the relatively soft material is, or comprises, a solid lubricant such as a metal sulphide or tribological material such as a polymer.
16. 16. A method as claimed in claim 15 wherein the solid lubricant comprises a fluoro-polymer such as PTFE.
17. 17. A method as claimed in claim 15 wherein the solid lubricant comprises MoS2.
18. 18. A method as claimed in any of the preceding claims wherein the soft material is disposed as layers of platelets which extend generally transversely in the coating.
19. 19. A method as claimed in any of the preceding claims wherein the coating comprises one or more further deposited materials, preferably deposited in layers thereof.
20. 20. A method as claimed in any of the preceding claims wherein an intermediate layer is deposited on the substrate before the coating is applied, whereby the coating is applied to the intermediate layer lying on the substrate.
21. 21. A method as claimed in any of the preceding claims wherein the substrate comprises a metal.
22. 22. A method as claimed in claim 21 wherein the metal is lead free bronze, leaded bronze, aluminium alloy bimetal, steel or a known bearing material.
23. 23. A method as claimed in any of the preceding claims wherein the substrate is configured as a portion of a plain bearing and the coating is applied to form the exposed bearing surface.
24. 24. A method as claimed in claim 23 wherein the substrate is a bearing half piece and the method further comprises coating two bearing surfaces of said half pieces which are then are assembled into a housing to form a whole plain bearing.
25. 25. A tribologically coated substrate obtainable by the method of any of the preceding claims.
26. 26. A tribologically coated plain bearing obtainable by the method of claim 24.
27. 27. A tribological coating obtainable by the method of any of claims 1 to 22.
28. 28. A tribological coating applied to a substrate, comprising a plurality of layers of a first a relatively soft, solid lubricant material and a plurality of layers of a second relatively hard material, accumulated to provide a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed between layers of the second hard material, wherein at least the first soft material is provided as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material.
29. 29. A coating as claimed in claim 28 wherein the relatively hard material is, or comprises, a metal or metal alloy, for example: aluminium, aluminium alloy, silver, silver alloy, an Ag-Sn alloy, a Sn-Ag alloy, or an intermetallic compound.
30. 30. A coating as claimed in claim 29 wherein the metal alloy is an aluminium alloy.
31. 31. A tribological coating applied to a substrate, comprising a plurality of layers of a first a relatively soft, solid lubricant material and a plurality of layers of a second relatively hard material, accumulated to provide a coating on the substrate, wherein at least the first soft material is deposited so as to provide a plurality of planar layers thereof interposed between layers of the second hard material, wherein the relatively hard material is, or comprises: aluminium, aluminium alloy, silver alloy, an Ag-Sn alloy or a Sn-Ag alloy.
32. 32. A tribological coating applied to a substrate as claimed in claim 31 wherein the metal alloy is an aluminium alloy.
33. 33. A tribological coating applied to a substrate as claimed in claim 31 or 32 wherein at least the first soft material is provided as discontinuous planar layers thereof interposed between the layers of the second material, which discontinuous layers each include a plurality of missed regions, the missed regions permitting underlying layers of hard material to interlace with overlying layers of hard material.
34. 34. A tribological coating applied to a substrate as claimed in any of claims 28 to or 33 wherein the hard material is also provided as a plurality of discontinuous layers thereof, thereby to form a coating structure in which the first and second materials are intercalated with one another.
35. 35. A tribological coating applied to a substrate as claimed in any of claims 28 to 34 wherein the soft material is provided in alternating layers with the hard material.
36. 36. A tribological coating applied to a substrate as claimed in any of claims 28 to wherein the soft material is provided in layers having a lower frequency of occurrence than the layers of hard material.
37. 37. A tribological coating applied to a substrate as claimed in any of claims 28 to 36 wherein one or both of the materials is/are deposited by physical and/or chemical vapour deposition.
38. 38. A tribological coating applied to a substrate as claimed in claim 37 wherein said deposition method is plasma-assisted physical vapour deposition.
39. 39. A tribological coating applied to a substrate as claimed in any of claims 28 to 38 wherein some or all of the layers are each provided with a thickness which corresponds to a mono-layer up to a thickness of 10 pm of each respective material.
40. 40. A tribological coating applied to a substrate as claimed in claim 39 wherein some or all of the layers are each laid down to a thickness which corresponds to a mono-layer up to a thickness of 10 nm of each respective material.
41. 41. A tribological coating applied to a substrate as claimed in any of claims 28 to wherein the accumulated coating has a thickness of 0.1 to 100 pm, preferably ito pm.
42. 42. A tribological coating applied to a substrate as claimed in any of claims 28 to 41 wherein the relatively soft material is a solid lubricant or tribological material, such as a metal sulphide or a polymer.
43. 43. A tribological coating applied to a substrate as claimed in claim 42 wherein the solid lubricant is, or comprises, a fluoro-polymer such as FIFE.
44. 44. A tribological coating applied to a substrate as claimed in claim 42 wherein the solid lubricant is MoS2.
45. 45. A tribological coating applied to a substrate as claimed in any of claims 28 to 44 wherein the soft material is disposed as layers of platelets which extend generally transversely in the coating.
46. 46. A tribological coating applied to a substrate as claimed in any of claims 28 to wherein the coating comprises one or more further materials, preferably deposited in layers thereof.
47. 47. A tribological coating applied to a substrate as claimed in any of claims 28 to 46 wherein an intermediate layer is provided on the substrate between the substrate and the accumulated coating.
48. 48. A tribological coating applied to a substrate as claimed in any of claims 28 to 47 wherein the substrate comprises a metal.
49. 49. A tribological coating applied to a substrate as claimed in claim 48 wherein the substrate metal is lead free bronze, leaded bronze, aluminium alloy bimetal, steel or another known bearing material.
50. 50. A tribological coating applied to a substrate in accordance with any of claims 28 to 49 wherein the substrate is configured as a portion of a plain bearing and the coating is provided at the exposed bearing surface.
51. 51. A plain bearing comprising two bearing half pieces assembled into a housing to form the plain bearing wherein the substrate is provided by one or both of the bearing half pieces and the tribological coating in accordance with any of claims 28 to 49 is provided on one or both of said bearing half pieces, which half pieces serve as the substrates.