SLIDING COMPONENT, MATERIAL AND METHOD

Abstract

A sliding component may include an overlay. The overlay may include graphene platelets functionalised with at least one of O functional groups and F functional groups within a matrix of at least one of a polymeric material and a plastics material.

Claims

1. A sliding component comprising an overlay including graphene platelets functionalised with at least one of O functional groups and F functional groups within a matrix of at least one of a polymeric material and a plastics material.

2. The sliding component according to claim 1, wherein the functionalised graphene platelets includes partially-functionalised graphene platelets.

3. The sliding component according to claim 1, wherein the overlay includes 0.01 wt % to 4 wt % functionalised graphene platelets.

4. The sliding component according to claim 3, wherein the overlay includes 0.1 wt % to 2 wt % functionalised graphene platelets.

5. The sliding component according to claim 1, wherein the functionalised graphene platelets have a maximal planar dimension of 20 m or less.

6. The sliding component according to claim 5, wherein the functionalised graphene platelets have a thickness of less than 50 nm.

7. The sliding component according to claim 1, wherein each platelet of the functionalised graphene platelets has layers, and the functionalised graphene platelets have an average of 20 layers per platelet or less.

8. The sliding component according to claim 1, wherein each platelet of the functionalised graphene platelets has layers, and the functionalised graphene platelets have an average of 4 layers per platelet or less.

9. The sliding component according to claim 1, wherein each platelet of the functionalised graphene platelets has layers, and the functionalised graphene platelets have an average of 5 to 10 layers per platelet.

10. The sliding component according to claim 1, wherein each platelet of the functionalised graphene platelets has layers, and the functionalised graphene platelets have an average of at least 11 layers per platelet.

11. The sliding component according to claim 1, wherein the at least one of the polymeric material and the plastics material includes the polymeric material, and wherein the polymeric material is one of a polyamide imide resin, an acrylate resin, an epoxy resin, a fluoropolymer, and a polybenzimidazole.

12. An overlay material comprising a matrix of polymeric material, and, within the matrix, graphene platelets functionalised with at least one of O functional groups and F functional groups.

13. A graphene platelet filler for an overlay material, comprising graphene platelets functionalised with at least one of O functional groups and F functional groups.

14. A method of forming an overlay for a sliding component, comprising: forming a dispersion via mixing a polymeric material with graphene platelets functionalised with at least one of O functional groups and F functional groups; and depositing the dispersion onto a substrate.

15. An engine comprising a sliding component including an overlay having a matrix of at least one of a polymeric material and a plastics material, the overlay including functionalised graphene platelets dispersed within the matrix, wherein the functionalised graphene platelets are functionalised with at least one of O functional groups and F functional groups.

16. The sliding component according to claim 1, wherein: the functionalised graphene platelets functionalised with the O functional groups are graphene platelets covalently bonded with at least one oxygen functional group; and the functionalised graphene platelets functionalised with the F functional groups are graphene platelets covalently bonded with at least one fluorine functional group.

17. The sliding component according to claim 16, wherein the functionalised graphene platelets includes a subset of graphene platelets functionalised with both O functional groups and F functional groups.

18. The sliding component according to claim 1, wherein the functionalised graphene platelets have a laminar shape oriented substantially parallel to a sliding surface of the overlay and are dispersed throughout the matrix substantially evenly.

19. The sliding component according to claim 1, wherein the overlay is a multi-layer overlay including at least one first layer and at least one second layer, and wherein the functionalised graphene platelets are dispersed only within the at least one first layer.

20. The sliding component according to claim 2, wherein each platelet of the functionalised graphene platelets includes an average of 1 to 20 atomic graphene layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

[0052] FIG. 1A shows a three-quarter view of a semi-cylindrical bearing shell, which is an embodiment of a sliding component embodying the present invention;

[0053] FIG. 1B shows a cross-sectional view through part of the bearing shell of FIG. 1A.

DETAILED DESCRIPTION

[0054] FIG. 1A schematically illustrates a semi-cylindrical bearing shell 100, which is also commonly referred to as a half bearing, for a main bearing assembly of an internal combustion engine for retaining a cylindrical journal of a crankshaft. FIG. 1B illustrates a cross-sectional view through a part of the bearing shell.

[0055] The bearing shell 100 has a layered construction incorporating a substrate comprising a steel backing 102 and a lining layer 104 comprising a layer of copper-tin bronze. An overlay 106 is formed by spray coating onto the lining layer of the substrate.

[0056] The backing 102 provides strength and resistance to deformation of the bearing shell 100, when it is assembled in a main-bearing housing.

[0057] The overlay 106 is configured to provide a running surface (or sliding surface) facing a cooperating moving part in a bearing assembly. In use, within an assembled bearing, the overlay 106 of the bearing shell 100 and a journaled shaft mutually cooperate, with an intervening film of lubricating oil (preferably providing hydrodynamic lubrication during normal running). The overlay 106 is particularly suited to accommodate small misalignments between the bearing surface and the shaft journal (conformability) and is able to receive and embed dirt particles circulating in the lubricating oil supply, so as to prevent scoring or damage to the journal surface by the debris (dirt embedability). The overlay 106 also provides suitable tribological properties between the bearing 100 and the shaft journal, if a failure of the intervening oil film should occur.

[0058] The overlay 106 comprises a matrix of polyamide imide (PAI) polymeric material, throughout which 2 wt % of O (oxygen) functionalised graphene platelets 108 are distributed (wt % proportions are specified with respect to the content of the formed overlay, after it has been cured). The overlay 106 also comprises further filler materials (not illustrated) distributed throughout the matrix of the plastic polymer material, as is known in the art.

[0059] The functionalised platelets 108 are small sheets of graphene having an average number of atomic layers from 1 to approximately 20 atomic layers. The functionalised platelets have a particularly-high surface area for bonding to the matrix material, and thereby reinforcing the composite overlay.

[0060] The platelets have an average thickness of less than 50 nm.

[0061] The platelets have a maximal planar dimension (i.e. the largest dimension in the plane of the platelet) of 10 m, which advantageously provides particularly enhanced strength in the composite overlay.

[0062] The functionalised platelets 108 are partially functionalised with O functional groups (i.e. only a proportion of the active sites on the outer surface of each platelet are occupied by an oxygen functional group). Advantageously, partial functionalisation provides good platelet dispersion, whilst also providing good bonding performance to the matrix material.

[0063] The use of O functionalised platelets provides significant advantages, including the following. The addition of O functionalised platelets advantageously enhances the seizure performance of the overlay compared to known overlays comprising COOH and/or NH.sub.2 functionalised platelets. The O functionalised platelets provide improved seizure performance, by providing an enhanced lubrication function, if exposed at the bearing surface. Exposed O functionalised platelets, at the bearing surface, increase lubricious properties of the free surface, reducing friction of the overlay. This is important in the event that the journaled shaft contacts the bearing surface, for example when the bearing is not fully supplied with lubrication oil, which can occur when an engine starts and before the lubrication oil has risen to working pressure. Overlays containing O functionalised platelets may also advantageously achieve more consistent seizure performance than that demonstrated by the prior art.

[0064] The O functionalised platelets may also strengthen the polymer matrix by hydrogen bonding to functional groups in the PAI matrix. Further, the functionalised platelets may enhance the thermal conductivity of the composite layer, enabling enhanced thermal dissipation through the composite layer. Yet further, the functionalised platelets may improve fatigue performance, obstructing propagation of fractures in the composite overlay, and may reduce material wear of the overlay.

[0065] In particular, the inventors have found that the plastic polymer-based composite layer comprising O functionalised platelets may achieve enhanced fatigue resistance and wear resistance compared with the COOH or NH.sub.2 functionalised graphene overlays of WO2016008770, whilst still permitting good embedability of any particulate carried in the oil that lubricates the bearing, in use.

[0066] Specifically, tests of overlays containing O functionalised platelets achieved an average reduction of 19% in the quantity of material removed from the overlay by wear, and produced a statistically very-significant improvement over the prior-art overlays. Overlays containing F functionalised platelets achieved a smaller reduction in wear of 5%.

[0067] The platelet may alternatively, or in addition, be functionalised with F (fluorine) functional groups. The inventors have found that the use of F functionalised platelets may provide significant advantages over the prior art, similar to those described above in relation to O functionalised platelets.

[0068] As shown in the experimental results discussed below, F functionalised platelets achieve greatly-improved seizure performance compared to the prior art. In particular, while COOH and NH.sub.2 functionalised platelets have been found to provide a 20 MPa (17%) increase in seizure load compared to a platelet-free PAI overlay, O and F functionalised platelets have been found to generate a 40 MPa (33%) increase in seizure load.

[0069] The F functionalised platelets may advantageously inhibit seizure of the sliding component by providing an enhanced lubrication function, if F functionalised platelets are exposed at the bearing surface. In the same way as described above in relation to O functionalised platelets, F functional groups may also have the advantageous effect of strengthening the polymer matrix. Overlays containing F functionalised platelets may particularly advantageously achieve more consistent seizure performance than that demonstrated by the prior art.

Seizure Resistance

[0070] The improved performance of embodiments of the invention has been demonstrated by seizure testing. These tests were carried out using a test rig in which a constantly-increasing lateral (downward) load is applied to steel journal rotating within a test bearing. Lubrication is applied to the test bearing but a groove in the journal prevents hydrodynamic running, in order to enable an accelerated test under challenging lubrication conditions. The torque required to rotate the journal is measured, and the temperature of the bearing is measured. A test continues until either a maximum lateral load (of 200 MPa) is applied to the bearing, or the bearing fails (seizes) because the measured torque exceeds a predetermined maximum torque, or because the measured temperature exceeds a predetermined maximum temperature. If either the threshold (cut-off) value of torque or temperature is reached, the test is automatically stopped. This seizure test provides a repeatable set of accelerated-wear conditions for comparing different overlays. To ensure statistical robustness at least six of each type of bearing is tested.

[0071] The seizure test measures the average seizure load (Mpa) at which the overlay seizes, as well as the failure mode, i.e. whether the seizure was due to an increase in the measured torque or temperature.

[0072] Five types of bearings were tested under the same conditions, termed bearings A to E.

[0073] Bearing A was a bearing having a PAI-based overlay according to the prior art, in which the overlay contained no functionalised graphene platelets.

[0074] The overlays of Bearings B to E comprised graphene platelets at 2 wt % of the PAI overlay, so that Bearings B to E differed only according to the functional groups used in the platelets of their overlays.

[0075] In Bearing B the graphene platelets were functionalised with NH.sub.2 functional groups as described in WO2016008770.

[0076] In Bearing C the graphene platelets were functionalised with COOH functional groups as described in WO2016008770.

[0077] In Bearing D the graphene platelets were functionalised with O functional groups.

[0078] In Bearing E the graphene platelets were functionalised with F functional groups.

[0079] Six samples of each of the bearings were tested under the same seizure-test conditions and the following results were obtained:

[0080] Bearing A exhibited an average seizure load (downward load applied to the test journal just before bearing seizure) of 116.5 Mpa with a standard deviation of 12 MPa;

[0081] Bearing B exhibited an average seizure load of 142 MPa with a standard deviation of 14 MPa;

[0082] Bearing C exhibited an average seizure load of 142 MPa with a standard deviation of 20 MPa;

[0083] Bearing D exhibited an average seizure load of 160 MPa with a standard deviation of 6 MPa; and

[0084] Bearing E exhibited an average seizure load of 155 MPa with a standard deviation of 5 Mpa.

[0085] Analysis of the seizure modes of Bearings A to E also showed that:

[0086] Bearing A failed due to an increase in measured torque beyond a predetermined torque failure threshold (bearing seizure) in each of the six tests;

[0087] Bearing B failed due to an increase in measured torque beyond a predetermined torque failure threshold (bearing seizure) in each of the six tests;

[0088] Bearing C failed due to an increase in measured torque beyond a predetermined torque failure threshold (bearing seizure) in four of the six tests, and in the other two tests reached the upper temperature limit of the test (200 C.) without seizing;

[0089] Bearing D failed due to an increase in measured torque beyond a predetermined torque failure threshold (bearing seizure) in two of the six tests and in the other four tests reached the upper temperature limit of the test (200 C.) without seizing; and

[0090] Bearing E reached the upper temperature limit of the test (200 C.) without seizing in each of the six tests.

[0091] The scuff rating of the bearings was also tested. The scuff rating determines seizure performance by analysing recovery events experienced by the bearings before failure. As the load on the bearing is increased, miniature seizure events can occur, from which a bearing may be able to recover. A recovery event may occur where low melting point constituents in the overlay melt and allow the bearing friction to reduce so that the bearing continues to function without fully seizing. These recovery events can be characterised by a rise in overlay temperature followed by a drop back down again. The scuff rating of an overlay is measured as the average number of recovery events that the overlay can experience before failure.

[0092] Traditional metallic bearing overlays typically demonstrate multiple recovery events, and so achieve relatively high scuff ratings (such as 3 to 5 scuffing events before failure for bimetal or leaded electroplated bearings). Conventional polymer overlays, however, tend to seize without demonstrating these recovery events.

[0093] Scuff ratings of more than zero may be desirable as a recovery event may serve as a warning prior to final seizure of the bearing. This may advantageously allow the engine to be safely shut down in advance of catastrophic failure.

[0094] The scuff ratings of six samples of each of Bearings A to E were measured and the following results were obtained:

[0095] Bearing A exhibited a scuff rating (average number of recovery events before failure due to an increase in measured torque beyond a predetermined torque failure threshold) of 0.5;

[0096] Bearing B exhibited a scuff rating of 0.5;

[0097] Bearing C exhibited a scuff rating of 1.5;

[0098] Bearing D exhibited a scuff rating of 1; and

[0099] Bearing E exhibited a scuff rating of 2.2.

[0100] Both O and F functionalised platelets therefore significantly enhanced the scuff rating compared to a PAI overlay containing no graphene platelets. Of all the overlays tested, overlays containing F functionalised platelets (Bearing E) provided by far the highest scuff rating. Fluorine functionalised platelets may therefore be particularly advantageous for increasing the scuff rating of polymer overlays so as to allow the engine containing the overlay to be safely shut down in advance of catastrophic failure.

[0101] Of the five bearing types tested, the overlays embodying the present invention comprising O and F functionalised platelets produced the highest average final seizure loads and the tightest spreads of seizure loads (lowest standard deviation). Additionally, O and F functionalised GNP overlays reached the temperature limit of the test rather than seizing in the majority of cases. The test is stopped when the bearings reach 200 C., as this is well beyond the operational temperature range of a main bearing of an internal combustion engine. For example, the standard material of Bearing A seized on all six tests whereas all six overlays comprising F functionalised GNPs reached the temperature cut-off of the test without seizing.

[0102] Overall, COOH and NH.sub.2 functionalised platelets have been found to provide a 20 MPa (17%) increase in seizure load, while the O and F functionalised platelets of the present invention generate a 40 MPa (33%) increase in seizure load.

Forming an Overlay

[0103] Prior to functionalisation, graphene platelets comprise one or more one-atom-thick planar layers of carbon atoms. Each carbon atom is sp.sup.2 hybridised and forms a bond with each of three neighbouring carbon atoms in a trigonal planar configuration. Once functionalised a layer typically becomes non-planar (e.g. with periodically-distributed functional groups, a layer may become puckered or corrugated), particularly in those regions of the layer to which the functionalisation is attached. Typically, any carbon atom in the lattice which is functionalised will be sp.sup.3 hybridised, forming a bond with each of three neighbouring carbon atoms and one further bond to the functional group (e.g. to the O or F group), and thus adopts a non-planar tetrahedral configuration.

[0104] The functionalised graphene platelets may be formed by plasma functionalisation, in which active sites on the planar surface and/or edges of the platelets are populated with functional groups, providing complete or partial saturation of the available active sites on the outside of the platelets.

[0105] Overlays embodying the invention may conveniently be made using techniques that are described in the prior art for forming overlays comprising fillers in polymer matrices. Such techniques are well known to the skilled person, but exemplary comments are set out below for completeness.

[0106] The overlay 106 is formed by depositing a deposition mixture comprising the polymeric PAI material dissolved in a solvent, in which the O and/or F functionalised platelets (and any other desired overlay fillers or particulates) are suspended. In the illustrated examples, the solvent comprises N-ethyl-2-pyrrolidone (NEP) and/or N-methyl-2-pyrrolidone (NMP), a small proportion of xylene solvent, and water. The solvent system can be employed in various proportions, relative to the plastic polymer and functionalised platelets (and any other suspended solid particulate) in order to achieve a particular desired viscosity of the deposition mixture for coating onto the substrate. Prior to deposition, the functionalised platelets (and any other suspended solid particulate) are preferably maintained in suspension by agitation of the deposition mixture. The solvent system facilitates forming and depositing the mixture, and the proportion of solvent to polymer (and any particulate) in the mixture is chosen to optimise deposition performance.

[0107] The O and/or F functionalisation of the platelets may advantageously enhance the dispersion of the platelets within the deposition mixture, prior to deposition of the polymeric material, by repelling the O and/or F functional groups on nearby platelets, and therefore reducing attraction between the platelets. This reduces agglomeration and promotes more uniform dispersion of the platelets within the deposited overlay.

[0108] The overlay may be deposited onto the substrate by a spray coating, from a spray gun. Alternatively, the overlay may be deposited by screen printing (i.e. through a mask), by a pad-printing process (i.e. an indirect offset-printing process, e.g. in which a silicone pad transfers a patterned layer of the plastic polymer composite material onto the sliding-bearing substrate), or by a transfer rolling process.

[0109] Although the overlay may be deposited in a single deposition step, for greater thicknesses the overlay may be built up by deposition of a succession of sub-layers, with a flash-off stage provided between successive depositions to remove solvent from the sub-layers.

[0110] Curing the deposited overlay induces molecular cross-linking of molecules in the plastic polymer. Curing also removes substantially all solvent from the overlay, including any residual solvent from flashed-off sub-layers.

[0111] The cured overlay 106 may have a thickness of 3 to 14 m, with thicker layers being formed from a succession of sub-layers. For example, an overlay 106 of 8 to 12 m thickness may be built up by the deposition of two or three sub-layers.

[0112] Although described herein and illustrated in the drawings in relation to a half bearing shell, the present invention may equally apply to other sliding engine components, including semi-annular, annular or circular thrust washers, and bushes, and engines comprising such sliding engine components.