SLIDING COMPONENT AND METHOD

Abstract

An overlay of a sliding component, such as a sliding component for an engine, may provide a bearing surface against a steel journal, for example. The overlay may include intermetallic particles disposed in a matrix including tin (Sn). The matrix may be formed by electroplating. Examples of intermetallic particles include, but are not limited to, aluminides and nickel aluminides. The matrix may include an electroplated matrix of tin and/or a tin alloy.

Claims

1. A sliding component, comprising: an overlay including intermetallic particles disposed in an electroplated matrix comprising Sn.

2. A sliding component according to claim 1, wherein the intermetallic particles include an aluminide.

3. A sliding component according to claim 1, wherein the intermetallic particles include a nickel aluminide.

4. A sliding component according to claim 1, wherein the intermetallic particles include Ni.sub.3Al.

5. A sliding component according to claim 1, wherein the electroplated matrix of Sn has a columnar grain structure.

6. A sliding component according to claim 5, wherein the columnar grains structure has columnar grains perpendicular to a surface of the overlay.

7. A sliding component according to claim 6, wherein an aspect ratio of the grains, evaluated as a ratio of a mean grain size perpendicular to the surface of the overlay to a mean grain size parallel to the surface of the overlay, is greater than 1.

8. A sliding component according to claim 1, wherein the electroplated matrix of Sn is bright electroplated Sn.

9. A sliding component according to claim 5, wherein the intermetallic particles do not disrupt the columnar grain structure of the electroplated matrix of Sn.

10. A sliding component according to claim 1, wherein the electroplated matrix consists of Sn, apart from incidental impurities.

11. A sliding component according to claim 1, wherein the electroplated matrix is Pb-free.

12. A sliding component according to claim 1, wherein the overlay has a thickness between 10 and 20 micrometres.

13. A sliding component according to claim 1, wherein the intermetallic particles have an average size between 1 and 10 micrometres.

14. A sliding component according to claim 1, wherein the intermetallic particles have an aspect ratio of less than 2.

15. A sliding component according to claim 1, wherein the intermetallic particles have a zeta potential of less than −50 mV in a solution used for electroplating the matrix.

16. A sliding component according to claim 1, wherein the intermetallic particles are atomised particles.

17. A sliding component according to claim 1, wherein the intermetallic particles constitute between 0.1 and 1.0 wt % of the overlay.

18. A method for forming an overlay of a sliding component, comprising: mixing particles of an intermetallic compound with an electroplating solution including at least one of Sn and an Sn alloy; and co-depositing the intermetallic compound and the at least one of Sn and the Sn alloy onto a substrate via electrodeposition to provide an electroplated matrix of the at least one of Sn and the Sn alloy containing intermetallic particles.

19. A method according to claim 18, wherein the intermetallic particles include an aluminide.

20. A method according to claim 18, wherein the intermetallic particles include a nickel aluminide.

21. A method according to claim 18, wherein the intermetallic particles include Ni.sub.3Al.

22. A method according to claim 18, wherein co-depositing the intermetallic compound and the at least one of Sn and the Sn alloy includes deposition of the intermetallic particles without disrupting a structure of the electroplated matrix of the at least one of Sn and the Sn alloy.

23. A method according to claim 18, wherein the electroplated matrix consists of Sn, apart from incidental impurities.

24. A method according to claim 18, wherein the electroplated matrix is Pb-free.

25. A method according to claim 18, wherein the intermetallic particles have an average size between 1 and 5 micrometres.

26. A method according to claim 18, wherein the intermetallic particles have an aspect ratio of less than 2 and preferably are equiaxed.

27. A method according to claim 18, further comprising forming the intermetallic particles by atomisation.

28. A method according to claim 18, wherein the intermetallic particles have a zeta potential of less than −50 mV in the electroplating solution.

29. A method according to claim 18, wherein the intermetallic particles constitute between 0.1 and 1.0 wt % of the overlay.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0031] FIG. 1 is a three-quarter view of a semi-cylindrical bearing shell, incorporating an overlay embodying the invention;

[0032] FIG. 2 is a metallurgical cross section of a bearing incorporating an overlay comprising intermetallic particles in an electroplated tin matrix;

[0033] FIG. 3 is a scanning electron microscope (SEM) image of a surface of the overlay of FIG. 2;

[0034] FIG. 4 is a high contrast SEM image of a cross section of an electrodeposited overlay of tin containing Ni.sub.3Al particles, showing that the grain structure of the electrodeposited tin is not affected or disrupted by the incorporation of the particles; and

[0035] FIG. 5 is a graph showing volume loss of material (wear) in tests of two different overlays, including one overlay embodying the invention.

DETAILED DESCRIPTION

[0036] 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 electroplating onto the interlayer.

[0037] The overlay comprises particles of Ni.sub.3Al in a tin matrix, formed by electroplating onto the interlayer. The interlayer is arranged as a cathode in a bath containing an electroplating electrolyte and an anode, and a cathodic bias (i.e. a negative bias) is applied to the cathode relative to the anode. The cathodic bias drives positively-charged metallic ions, such as tin, towards the cathode, and deposits the metallic ions onto the cathode surface in known manner. The anode is preferably formed of a material corresponding to the metallic layer that is to be deposited. For example, when depositing a layer of pure tin (apart from incidental impurities), it is preferable to use a pure tin anode. In other embodiments, the overlay matrix may be of a tin alloy, in which case the anode may be of an appropriate alloy composition.

[0038] The electrolyte may additionally comprise performance-enhancing additives, such as brighteners and anti-foaming agents. The deposited tin or tin alloy layer may therefore contain incidental impurities, as the skilled person would appreciate. The chemical composition and pH of the electrolyte are maintained during deposition by replenishment of the chemicals consumed from the electrolyte. The electrolyte may be maintained at a temperature of 20 to 30° C. Advantageously, the electroplating conditions may follow conventional practice.

[0039] Ni.sub.3Al particles are mixed with the electrolyte either before or during the electroplating process. More particles may be added during electroplating if required to replace particles that have been incorporated into the overlay. The particles are preferably formed by atomisation, are generally spherical in shape, and are between 1 and 4 micrometres in diameter. In preferred embodiments the particles may be between 1 and 10 micrometres in diameter, or preferably between 2 and 5 micrometres in diameter, and particularly preferably between 2 and 3 micrometres in diameter. Particles outside these ranges may be present, but particles greater than the thickness of the desired overlay should be avoided, as they may protrude unacceptably from the overlay, and particles much smaller than 1 micrometre may be difficult to handle in an industrial process. For example, small particles may be pyrophoric.

[0040] The Ni.sub.3Al particles do not react with the electrolyte, and are maintained suspended in, or mixed throughout, the electrolyte in order to ensure that they are incorporated into the overlay as it is formed. To do this, the electrolyte should be mixed or agitated. Magnetic mixing or agitation should be avoided, as the nickel in the intermetallic particles is magnetic.

[0041] A first exemplary electrolyte is a lead-free, tin methanesulfonic acid (MSA) electrolyte (tin ions in methanesulfonic acid) comprising a solution of: [0042] 30 to 60 g/l tin, although concentrations of 15 to 80 g/l may be used; [0043] 100 to 200 g/l methanesulfonic acid; [0044] 3 to 6 ml/l brightener (35 to 50 wt % 2-isopropoxyethanol, and 5 to 10 wt % 4-15 phenylbut-3-en-2-one); [0045] 40-80 ml/l starter (20 to 25 wt % 2-naptholpolyglycolether, 1 to 2.5 wt % 1,2-dihydroxybenzene, and 1 to 2.5 wt % methacrylic acid); [0046] Ni.sub.3Al particles 1 g/l; and [0047] balance to 1 l of deionised water.

[0048] A second exemplary electrolyte is a lead-free, tin sulfuric acid electrolyte (tin ions in sulphuric acid) comprising a solution of: [0049] 10 to 50 g/l tin; [0050] 170 to 190 g/l sulphuric acid; [0051] 2 to 6 ml/l brightener; [0052] 10 to 30 g/l make-up; [0053] 2 to 6 ml/l starter; [0054] Ni.sub.3Al particles 1 g/l; and [0055] balance to 1 l of deionised water.

[0056] The overlay is advantageously deposited by direct current (DC) plating, as in a conventional plating process. Electroplating is carried out in galvanostatic mode at a cathode current density between 1.5 and 2.0 Adm.sup.−2; the electroplating voltage is therefore dictated by the current density and the resistivity of the electrolyte. The intermetallic particles are co-deposited with the metallic ions, which form a metallic matrix in which the particles are distributed. The particles may be suspended in the electrolyte with a concentration of approximately 2 g/l and preferably 1 to 20 g/l. Ultrasonic and/or mechanical stirring or agitation is used to maintain the particulate in suspension during deposition.

[0057] FIGS. 2, 3 and 4 are images of the overlay deposited as described above.

[0058] FIG. 2 is a light-microscopy image of a transverse section through the half bearing of FIG. 1, showing the overlay 12, the lining layer 6 and the backing 4. The section shows the intermetallic particles 14 (indicated by arrows in the drawing) incorporated into the tin matrix of the overlay. Atomic absorption analysis shows that the overlay contains 0.2 wt % Ni.sub.3Al.

[0059] FIG. 3 is a scanning-electron microscopy (SEM) image of the bearing surface of the overlay, after electroplating. Intermetallic particles 14 incorporated at and just beneath the surface of the overlay can be seen. This image does not show intermetallic particles that are fully incorporated deeper beneath the surface of the overlay.

[0060] FIG. 4 is a high-contrast SEM image of a section through the overlay 12, at a higher magnification than the section in FIG. 2. This section shows that the thickness of the overlay is about 12 micrometres, and shows the location within the thickness of the overlay of a spherical intermetallic particle 14 of about 4 micrometres diameter. The columnar grain structure of the tin matrix of the overlay can also be seen from the vertical texturing of the imaged surface, through the thickness of the overlay and, importantly, it can be seen that the columnar grain structure is not disrupted by the incorporated intermetallic particle. The columnar grain structure can be seen in the region of the tin electroplate immediately above the incorporated particle and to either side of the incorporated particle, in exactly the same way as in the region between the particle and the lining layer (or substrate) beneath the particle. This is very different from the skilled person's experience in the prior art of seeking to incorporate hard ceramic particles into electroplated tin, where the particles disrupted and prevented the formation of a columnar grain structure.

[0061] Advantageously, therefore, the overlay illustrated in FIGS. 2, 3 and 4 is a bright tin layer.

[0062] In the overlay, the columnar grains have an aspect ratio defined by the length of the grains in the direction perpendicular to the surface of the substrate (i.e. in the growth direction during electroplating) relative to the width of the grains in the direction parallel to the surface of the substrate. As shown in FIG. 4, the grain structure of the electroplated layer is columnar (with column-like grains extending away from the substrate) and the grains have an aspect ratio of approximately 10:1. Advantageously, grains having higher aspect ratios (e.g. columnar grains) have higher strength, and higher load carrying capacity, by comparison to the equiaxed grains of dull tin electroplate.

[0063] The improved performance of embodiments of the invention have been demonstrated by accelerated wear testing, as illustrated in FIG. 5. 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.

[0064] Two types of bearings were tested under the same conditions, termed bearings A and B in FIG. 5. Type A was a bearing having an electroplated overlay of bright tin. Type B was a bearing comprising an overlay embodying the invention, as described above, having a electroplated overlay of bright tin the same as that of Type A except that it contained 0.2 wt % Ni.sub.3Al particles. Under the same wear-test conditions, the Type A bearing lost 2.2 mm.sup.3, and the Type B bearing lost only 1.9 mm.sup.3. The bearing embodying the invention therefore displayed significantly better wear resistance than the reference bearing.

[0065] 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.