BEARING MATERIAL AND SOLID LUBRICANT

Abstract

A bearing material may include a matrix of polyamide-imide polymer material, and a solid lubricant particulate. The solid lubricant particulate may have a median particle size of less than 1 micrometre.

Claims

1. A bearing material, comprising: a matrix of polyamide-imide polymer material; and a solid lubricant particulate; wherein the solid lubricant particulate has a median particle size of less than 1 micrometre.

2. The bearing material according to claim 1, wherein the solid lubricant particulate comprises a fluoropolymer.

3. The bearing material according to claim 2, wherein the fluoropolymer comprises PTFE.

4. The bearing material according to claim 1, wherein the solid lubricant particulate comprises at least one of melamine cyanurate, molybdenum disulphide, tungsten disulphide, hexagonal boron nitride, metal sulphides with layered structures, and graphite.

5. The bearing material according to claim 1, wherein the solid lubricant particulate has a median particle size of between about 100 nanometres and about 800 nanometres.

6. The bearing material according to claim 5, wherein the solid lubricant has a median particle size of about 300 nanometres.

7. The bearing material according to claim 1, comprising between about 5 wt % and about 15 wt % of the solid lubricant particulate.

8. The bearing material according to claim 1, further comprising a metallic particulate.

9. The bearing material according to claim 8, wherein the metallic particulate comprises aluminium flakes.

10. The bearing material according to claim 8, comprising between about 15 wt % and about 35 wt % of the metallic particulate.

11. The bearing material according to claim 1, further comprising at least one of a dispersant, an adhesion agent, and a leveller.

12. The bearing material according to claim 1, comprising: between 8 wt % and 12 wt % of the solid lubricant particulate, the solid particulate comprising PTFE particulate having a median particle size of about 300 nanometres; and the bearing material further comprising between 24 wt % and 28 wt % of aluminium flakes, and between 3 wt % and 5 wt % of silane.

13. A bearing element comprising: a substrate; and an overlay layer on the substrate; wherein the overlay layer includes the bearing material according to claim 1.

14. The bearing element according to claim 13, wherein the bearing material is provided as an overlay layer, the overlay layer having a thickness of between 3 μm and 18 μm.

15. The bearing element according to claim 13, further comprising an intermediate layer between the substrate and the overlay layer.

16. The bearing element according to claim 15, wherein the intermediate layer comprises at least one of a copper-based material, and an aluminium-based material.

17. A method of forming an overlay layer of bearing material on a substrate, the method comprising: mixing polyamide-imide polymer material with solid lubricant particulate, and at least one solvent to form a pre-formulation; applying the pre-formulation to a substrate; and curing the pre-formulation to form an overlay layer of bearing material; wherein the solid lubricant particulate in the overlay layer of bearing material has a median particle size of less than 1 micrometre.

18. The method according to claim 17, wherein the step of applying the pre-formulation to the substrate comprises at least one of spraying and screen printing.

19. The bearing material according to claim 1, including between 24 wt % and 28 wt % of aluminium flakes.

20. The bearing material according to claim 1, including between 3 wt % and 5 wt % of silane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] The invention will further be described by way of example only with reference to the accompanying drawing, in which:—

[0087] FIG. 1 shows a perspective view of a bearing element according to preferred embodiments of the present invention.

[0088] FIG. 2 is a graph showing surface roughness of the exposed surface of four different bearing materials, one of which is according to the present invention.

[0089] FIG. 3 is a micrograph of a bearing material according to the prior art.

[0090] FIG. 4 is a micrograph of a bearing material according to the present invention.

DETAILED DESCRIPTION

[0091] FIG. 1 schematically illustrates a bearing element, the bearing element is a semi-cylindrical bearing shell 100, which is also commonly referred to as a half bearing or a half shell, for a main bearing assembly of an internal combustion engine for retaining a cylindrical journal of a crankshaft.

[0092] The bearing shell 100 has a layered construction incorporating a substrate comprising a steel substrate 102 and intermediate or lining layer 104 comprising a layer of copper-tin bronze material. An overlay 106 of a bearing material is disposed on top of the intermediate layer 104.

[0093] Overlay layer 106 is formed from a bearing material comprising a polymer matrix of polyamide-imide polymer material with solid lubricant particulate dispersed within the polymeric matrix.

[0094] The bearing material comprises about 10 wt % solid lubricant particulate. The solid lubricant particulate has a median particle size of about 300 nanometres. The solid lubricant particulate is PTFE particulate.

[0095] The bearing material further comprises metallic particulate. The bearing material comprises about 26 wt % metallic particulate. The metallic particulate comprises aluminium flakes. The aluminium flakes have a length of less than 10 μm in a longest dimension and less than 1 μm in a dimension perpendicular to a longest dimension.

[0096] The solid lubricant particulate, and metallic particulate are distributed substantially homogeneously throughout the polymer matrix.

[0097] The bearing material also comprises a leveller, the relative quantity of which is set out below.

[0098] An adhesion agent is also added to the pre-formulation shortly before the pre-formulation is applied to the intermediate layer.

[0099] The overlay layer comprising the bearing material has a thickness of about 10 μm.

[0100] The bearing element shown in FIG. 1 is formed by the following method.

[0101] Polyamide-imide polymer material is mixed with PTFE particulate, aluminium flakes, a dispersant, and a leveller. A solvent is added to the mixture to form a pre-formulation which is capable of being applied to a substrate. The solvent comprises a mixture of n-butyl acetate and n-ethyl pyrrolidone.

[0102] The relative quantities, given in weight percentages, of the components of the finished bearing material, following the drying and curing steps, are set out below in Table 1.

TABLE-US-00001 TABLE 1 wt % in Bearing Component Material Polyamide-imide Balance Aluminium flakes 24 to 28 PTFE particulate  8 to 12 Adhesion agent (silane) 3 to 5

[0103] The method further comprises providing a steel substrate 102 having an intermediate layer 104 provided on its surface. The pre-formulation is then applied to the intermediate layer using spraying. The applied pre-formulation is then dried to remove the solvent. The dried pre-formulation is then cured using a thermal curing process at a temperature and for a duration to achieve a desired degree of cross-linking of the polyamide-imide polymer matrix. The pre-formulation is cured to form an overlay layer of bearing material having the composition set out in Table 1.

[0104] FIG. 2 is a graph showing the relative surface roughness for four different bearing materials, Coating A, Coating B, Coating C, and Coating D. The surface roughness shown on the vertical axis refers to the arithmetical mean roughness value (Ra) in micrometres. Ra is the arithmetical mean of the absolute values of the profile deviations from the mean line of the roughness profile. Ra was measured in accordance with EN ISO 4287. Each of the four different bearing materials have a composition as set out in Table 1. The solid lubricant PTFE particulate in Coating A, Coating B, and Coating C has a median particle size of between 2 micrometres and 4 micrometres and are not according to the present invention. The solid lubricant PTFE particulate in Coating D has a median particle size of about 300 nanometres and is according to the present invention. The weight fraction of solid lubricant PTFE particulate in each example is the same.

[0105] As can be seen from the graph in FIG. 2, the surface roughness of the sample according to the present invention, Coating D, has a markedly lower surface roughness compared to the samples with larger solid lubricant PTFE particulate. As set out above, it is thought that this reduced surface roughness is as a result of more, but smaller, point contacts of solid lubricant particles exposed on the surface of the bearing material. A lower surface roughness may advantageously be associated with improved fatigue resistance.

[0106] In addition, several scanning electron micrographs of the surface of each of Coating A to Coating D were obtained from an untested bearing. From the micrographs, the solid PTFE particulate solid lubricant could be identified. Using image analysis software, the average total percentage area of the solid PTFE particulate in the micrographs was determined along with the average total number of individual PTFE particles visible in each micrograph. These results are set out in Table 2 (below).

TABLE-US-00002 TABLE 2 Coating A Coating B Coating C Coating D Average 4.2 percent 5.5 percent 4.9 percent 17.7 percent % area of PTFE particles Average 846.49 840.84 1811.4 5874.48 number of Individual PTFE particles

[0107] As can be seen from Table 2, the coating according to the present invention having a median particle size of about 300 nanometres (Coating D) exhibits the greatest total area of PTFE particles. Indeed, the coating according to the present invention surprisingly exhibits over three times the area of PTFE particles compared to the closest coating of the prior art (Coating B) despite containing the same weight fraction of PTFE particulate. As set out above, an increased area of PTFE particles exposed on the surface of the coating improves the lubrication properties of the solid lubricant leading to improved seizure and fatigue resistance.

[0108] Furthermore, Table 2 also shows that the coating according to the present invention having a median particle size of about 300 nanometres (Coating D) exhibits the greatest total number of individual PTFE particles. Indeed, the coating according to the present invention surprisingly exhibits over three times more individual PTFE particles compared to the closest coating of the prior art (Coating C) despite containing the same weight fraction of PTFE particulate. As set out above, the increased number of individual PTFE solid lubricant particles leads to a greater number of point contacts of solid lubricant particles exposed on the surface of the bearing material, for a given weight percentage of solid lubricant particulate. Again, this has been found to improve the lubrication properties of the solid lubricant.

[0109] FIG. 3 shows a 500× micrograph obtained using a scanning electron microscope of the surface of bearing material sample following a rig test, Coating A, comprising solid lubricant PTFE particulate having a median particle size of between 2 micrometres and 4 micrometres. The white scale bar has a length of 300 micrometres.

[0110] FIG. 4 shows a 500× micrograph obtained using a scanning electron microscope of the surface of bearing material sample following a rig test, Coating D, comprising solid lubricant PTFE particulate having a median particle size of about 300 nanometres. As with FIG. 3, the white scale bar in FIG. 4 has a length of 300 micrometres.

[0111] As can be seen from the micrographs, the sample containing smaller solid lubricant particulate (FIG. 4) has a considerably finer structure than the sample containing larger solid lubricant particulate. The contrast shown in the micrographs identifies the fatigue cracks initiated by the solid lubricant particulate. The fatigue cracks shown in FIG. 4 are shallower and extend in a greater number of directions than the cracks in FIG. 3. The finer, shallower, multidirectional morphology of the cracks in FIG. 4 are associated with improved fatigue resistance compared to the cracks in FIG. 3.

[0112] Accordingly, it was shown that bearings including a bearing material according to the present invention exhibit superior wear resistance compared to those of the prior art.

[0113] Although described herein and illustrated in the drawing 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.