Sliding component and method

Abstract

A sliding component may include an overlay comprising a polymeric material and a metal oxide. The metal oxide may have a thermal conductivity of greater than about 1.5 Wm.sup.1K.sup.1, and a Mohs hardness of between about 5 and about 7. The sliding component may be a sliding component for an engine, such as a bearing, a bearing shell, a bush, a thrust washer, a journal bearing or the like.

Claims

1. A sliding component comprising: an overlay, the overlay including: a polymeric material; and a metal oxide; wherein the metal oxide has a thermal conductivity of greater than 1.5 Wm.sup.1K.sup.1, and a Mohs hardness of between 5 and 7; and wherein the metal oxide is dispersed throughout the overlay in the form of particles.

2. A sliding component according to claim 1, wherein the metal oxide includes one or more of cerium oxide, tin oxide, titanium dioxide, and zirconium dioxide.

3. A sliding component according to claim 1, wherein the overlay further includes a dispersant.

4. A sliding component according to claim 1, the metal oxide in the overlay includes metal oxide particles having an average particle size of between 0.5 m and 10 m.

5. A sliding component according to claim 1, wherein the metal oxide in the overlay is between 0.5 wt % and 8 wt % metal oxide.

6. A sliding component according to claim 1, wherein the polymeric material is a polyamide imide (PAI).

7. A sliding component according to claim 1, wherein the overlay has a thickness between 3 m and 12 m.

8. A sliding component according to claim 1, wherein the metal oxide includes metal oxide particles having non-spherical shape, said metal oxide particles having an average aspect ratio of between 1:2 and 1:4.

9. A sliding component according to claim 1, wherein the metal oxide includes metal oxide particles having a spherical shape, said metal oxide particles having an average particle size of between 0.5 and 2 m.

10. A sliding component according to claim 1, further comprising a substrate and a lining layer disposed between the substrate and the overlay, wherein the lining layer includes a copper-based material or an aluminium-based material.

11. A sliding component according to claim 1, wherein the metal oxide includes cerium oxide.

12. A sliding component according to claim 1, wherein the overlay is a multi-layered overlay including at least three distinct layers, the at least three distinct layers each having a different composition from one another.

13. A method of forming an overlay of a sliding component, comprising: mixing a polymeric material with particles of a metal oxide having a thermal conductivity of greater than 1.5 Wm.sup.1K.sup.1, and a Mohs hardness of between 5 and 7, to form a dispersion, wherein the metal oxide is added in a quantity that forms between 0.5 wt % and 8 wt % of the dispersion; and depositing the dispersion onto a substrate.

14. A method according to claim 13, wherein the metal oxide includes one or more of cerium oxide, tin oxide, titanium dioxide, and zirconium dioxide.

15. A method according to claim 13, further comprising adding a dispersant into a mixture of the polymeric material and the metal oxide.

16. A method according to claim 13, wherein the polymeric material includes polyamide imide (PAI).

17. A method according to claim 13, wherein depositing the dispersion includes spraying the dispersion onto the substrate.

18. A sliding component comprising: a substrate; a lining layer disposed on the substrate; and an overlay disposed over the lining layer, the overlay including a polyamide imide polymeric material and metal oxide particles dispersed in the polyamide imide polymeric material, wherein said metal oxide particles have a thermal conductivity of greater than 1.5 Wm.sup.1K.sup.1 and a Mohs hardness of between 5 and 7, wherein said metal oxide particles include spherical cerium oxide particles, wherein the metal oxide in the overlay is between 0.5 wt % and 8 wt % metal oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

(2) FIG. 1 is a three-quarter view of a semi-cylindrical bearing shell, incorporating an overlay embodying the invention; and

(3) FIG. 2 is a graph showing volume loss of material (wear) in tests of seven different overlays, including six overlays embodying the invention.

DETAILED DESCRIPTION

(4) FIG. 1 shows a half bearing, or semi-cylindrical bearing shell 2 for a main bearing assembly of an internal combustion engine, for retaining a cylindrical journal of a crankshaft. This half bearing comprises a semi-cylindrical bearing shell. The bearing shell has a layered construction incorporating a steel backing 4. The backing is coated with or bonded to a lining layer 6 comprising a layer 8 of copper-tin bronze and a nickel diffusion barrier, or interlayer, 10. An overlay 12 is formed by spray coating onto the interlayer.

(5) The wear-test results of a number of bearings of the type shown in FIG. 1, coated in a variety of overlays embodying the present invention, are shown in FIG. 2.

(6) According to a first embodiment, the overlay material is formed by adding polyamide imide (PAI) to a solvent together with metal oxide particles, dispersion agent and fillers, and mixing under high shear. Any fillers not capable of withstanding high shear mixing are then added with levellers and more solvent, and the mixture is mixed at a slow speed until all of the components are incorporated and evenly mixed.

(7) Prior to application of the mixture, or dispersion, onto a substrate, the viscosity of the mixture is adjusted to spraying viscosity by adding appropriate thinners.

(8) The skilled person will understand that a variety of solvents, fillers, levellers, and thinners are appropriate for use in the method of the present invention. For example, appropriate solvents, fillers, levellers, and thinners may include those used to form the polymer-based overlays of the prior art.

(9) According to an alternative second embodiment of the method, the overlay material is formed by mixing together metal oxide particles, dispersant and thinner, and adding the resulting mixture to a pre-prepared PAI-based coating material. The combined mixture is then mixed together under low shear until dispersed.

Example 1: Formulation of a cerium oxide (8 wt %, 2 micron) polymer Coating

(10) A first exemplary method of forming an overlay comprises the following steps. Polyamide imide (PAI) is added to a mixing vessel along with a first portion of N-Ethylpyrrolidone (NEP)/butyl-acetate solvent blend, together with a polymeric dispersant, micronised PTFE, and cerium oxide (CeO.sub.2) particles of a 2 micron average size. The quantity of cerium oxide added is equal to about 8 wt % of the total weight of the final dispersion. A leveller and approximately 24 wt % of aluminium flake are also added. Mixing is continued until the additives are fully dispersed in the mixture to form a dispersion of overlay material.

(11) The viscosity of the overlay material is adjusted with a blend of thinners to achieve a viscosity that is appropriate for spraying or screen-printing.

(12) An overlay formed by this first exemplary method comprises the following materials:

(13) TABLE-US-00001 Wt % to Name solids Polyamide imide (PAI) 54 Cerium oxide 8 Polyethylene terephthalate 9 (PTFE) Aluminium flake 24 Other* 5 *Leveller, dispersant, other fillers

Example 2: Formulation of a Tin(IV)Oxide (0.5 wt %, 0.5 Micron) Polymer Coating

(14) A second exemplary method of forming an overlay comprises the following steps.

(15) Polyamide imide (PAI) is added to a mixing vessel along with a first portion of N-Ethylpyrrolidone (NEP)/butyl-acetate solvent blend, together with a polymeric dispersant, micronised PTFE, and tin oxide (SnO.sub.2), particles of a 0.5 micron average size. The quantity of tin oxide added is equal to about 0.5 wt % of the total weight of the final dispersion. A leveller and approximately 26 wt % of aluminium flake are also added. Mixing is continued until the additives are fully dispersed in the mixture to form a dispersion of overlay material.

(16) The viscosity of the overlay material is adjusted with a blend of thinners to achieve a viscosity that is appropriate for spraying or screen-printing.

(17) An overlay formed by this second exemplary method comprises the following materials:

(18) TABLE-US-00002 Wt % to Name solids Polyamide imide (PAI) 58 Tin(IV)oxide 0.5 Polyethylene terephthalate 9 (PTFE) Aluminium flake 26 Other* 6.5 *Leveller, dispersant, other fillers

(19) The substrate is preferably pre-heated before application of the overlay material. This may help to prevent sagging of the overlay material relative to the substrate. The substrate may be pre-heated to a surface temperature of between about 30 and about 100 degrees C., preferably between about 40 and about 85 degrees C.

(20) Optionally, before the overlay material is applied to the substrate, the overlay material may be pre-heated. The overlay material may be pre-heated to between 40 and 70 degrees, preferably between 40 and 60 degrees C. This may be done only prior to application of the first layer of the overlay material, or prior to the application of more than one, or each one, of layers of the overlay material.

(21) Matching the temperature of the overlay material to the temperature of the substrate before application of the overlay material and/or maintaining the overlay material at a constant temperature during application of the overlay material to the substrate may help to ensure uniformity of the overlay material on the substrate. It may also help to maintain a constant viscosity of the overlay material and thereby ensure that the characteristics of the process of applying each layer of the overlay material and the resultant thickness of the applied layer of the overlay material is consistent and repeatable.

(22) The overlay material may be applied to the substrate in one of a number of ways that will be known to the skilled person. A preferred method of applying the overlay material onto the substrate to form a sliding component embodying the present invention is to spray the overlay material in liquid form onto a substrate. The overlay material is preferably applied by a spray lance or nozzle.

(23) After spraying of the overlay material and drying of the overlay material, the overlay material is cured. Curing hardens the polymeric material and causes cross-linking of the polymer chains. Curing gives the overlay material the desired sliding or running properties.

(24) The improved performance of embodiments of the invention have been demonstrated by accelerated wear testing, as illustrated in FIG. 2. These tests were carried out using a test rig in which a single half bearing is arranged facing upwardly, and an electrically-driven revolving eccentric test journal is loaded downwardly into the half bearing. The eccentrically-mounted journal is offset from its axis of rotation by about 1.8 mm, to achieve 3.64 mm TIR (total indicator reading) and is nominally 6.7 mm smaller than the test-bearing bore, to generate a large clearance so that the shaft's eccentricity is accommodated within the half bearing. The bearing is lubricated by a spray bar located above the leading side of the bearing clearance. This wear test provides a repeatable set of conditions for comparing different overlays, and to ensure statistical robustness, at least six of each type of bearing is tested.

(25) The volume loss of material in the overlay is a measurement of overlay wear.

(26) Seven types of bearings were tested under the same conditions, termed bearings A to Gin FIG. 2.

(27) Bearing A was a bearing having an overlay formed by the second embodiment of the method described above, comprising PAI and 4 wt % of spherical CeO.sub.2 particles with an average particle size (d50) of 2.554 microns, a d5 particle size of 0.889 microns, and a d95 particle size of 4.666 microns. Cerium oxide has a thermal conductivity of 12 Wm.sup.1K.sup.1 and a Mohs hardness of 6.

(28) Bearing B was a bearing comprising an overlay formed by the second embodiment of the method described above, comprising PAI and 8 wt % of spherical CeO.sub.2 particles with an average particle size (d50) of 2.554 microns, a d5 particle size of 0.889 microns, and a d95 particle size of 4.666 microns.

(29) Bearing C was a bearing having an overlay formed by the second embodiment of the method described above, comprising PAI and 4 wt % of spherical SnO.sub.2 particles with an average particle size of 0.5 microns. Tin oxide (SnO.sub.2) has a thermal conductivity of 40 Wm.sup.1K.sup.1 and a Mohs hardness of approximately 6.5.

(30) Bearing D was a bearing having an overlay formed by the second embodiment of the method described above, comprising PAI and 4 wt % of spherical SnO.sub.2 particles with a d10 particle size of 0.2-0.3 microns, a d50 particle size of 0.5-0.8 microns, and a d90 particle size of 1.6-1.9 microns.

(31) Bearing E was a bearing having an overlay formed by the first embodiment of the method described above, comprising PAI and 8 wt % of spherical CeO.sub.2 particles with an average particle size (d50) of 2.554 microns, a d5 particle size of 0.889 microns, and a d95 particle size of 4.666 microns.

(32) Bearing F was a bearing having an overlay formed by the first embodiment of the method described above, comprising PAI and 4 wt % of spherical SnO.sub.2 particles with an average particle size of 0.5 microns.

(33) Bearing G was a bearing having a PAI-based overlay according to the prior art, in which the overlay contains no metal oxide particles.

(34) Under the same wear-test conditions, Bearing A lost approximately 0.3 mm.sup.3 of the overlay material due to wear; Bearing B lost 0.25 mm.sup.3; Bearing C lost 0.2 mm.sup.3; Bearing D lost 0.25 mm.sup.3; Bearing E lost 0.7 mm.sup.3; Bearing F lost 0.4 mm.sup.3; while the prior art Bearing G lost approximately 3.6 mm.sup.3. All of the bearings embodying the present invention therefore displayed significantly better wear resistance than the reference bearing of the prior art.

(35) Bearings A, B, C, and D all exhibited a more than 90% reduction in wear compared to the reference bearing, with the worst performing bearing of the invention, Bearing E, still exhibiting more than 80% reduction in wear over the reference bearing. The wear reduction exhibited by all of coatings A to F has also been found to be significantly higher than that displayed by prior art overlays comprising metal oxide particles in the form of iron oxide.

(36) 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.