GB2272959A

GB2272959A - Hard wearing surfaces for piston grooves

Info

Publication number: GB2272959A
Application number: GB9323865A
Authority: GB
Inventors: David Raymond Eastham; Charan Preet Singh Johal
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1992-11-28
Filing date: 1993-11-19
Publication date: 1994-06-01
Anticipated expiration: 2013-11-19
Also published as: GB9224953D0; GB9323865D0; GB2272959B

Abstract

A surface of a piston ring groove of a piston made of aluminium or of aluminium alloy is provided with a hard wearing surface having a hardness in the range 300 to 750 Hv by applying a coating of cobalt by electroless plating. The coating may contain phosphorus and may be heat-treated after application.

Description

HARD WEARING SURFACES FOR PISTONS This invention is concerned with the provision of hard wearing surfaces for pistons.

The pistons of internal combustion engines are often made of aluminium or an alloy thereof which is a relatively soft material compared to the iron or steel used to make the piston rings which seal the gap between the piston and the wall of its cylinder. The piston rings are received in grooves in the piston and, because of the differential in hardness, the surfaces of such grooves are prone to wear.

This problem is particularly acute in the case of the top ring groove, i.e. the groove nearest to the crown of the piston which borders the combustion chamber of the engine.

It has long been recognised that it is desirable to provide the top ring groove with one or more hard wearing surfaces to reduce the problem of wear. It is known, for example, to provide an insert of harder material, e.g. cast iron, which is cast into the piston so that it provides one or more surfaces of the top ring groove. The insert may be porous so that it is infiltrated by the metal forming the bulk of the piston or the insert may be pre-treated to improve its adherence to aluminium or an alloy thereof, e.g. the insert may be placed in a bath of molten aluminium to form thereon a layer which includes aluminium-iron intermetallic compounds. The use of an insert, while generally successful in reducing wear, has the disadvantages that the manufacturing process is made considerably more complex, the insert may become detached in service, and the weight of the piston is increased.

Other techniques for providing a hard wearing surface for pistons in the ring groove regions involve local alloying of the basic piston alloy, and surface treatments such as anodising. These techniques have not proved as successful as use of an insert. Anodising, for example, produces a rough surface which is generally unacceptable.

It has been suggested (see JP 4,119,265) that piston parts should be given a coating of a cobalt-tungstenphosphorus alloy by electroless plating to achieve a coating of hardness above 800 Hv which after heat treatment becomes 900 to 1200 Hv. Such a coating is considered to be too hard for piston ring grooves and likely to damage cooperating parts.

It is an object of the present invention to provide a method of providing a hard wearing surface as aforesaid which is simpler than using an insert as mentioned above.

The invention provides a method of providing at least one surface of a piston ring groove of a piston made of aluminium or aluminium alloy with a hard wearing surface having a hardness in the range 300 to 750 Hv, the method comprising applying a coating of cobalt or cobalt and phosphorus to said surface by electroless plating.

A method according to the invention is simpler than use of an insert as no positioning of an insert in a mould is involved. Furthermore, a uniform coating with an accurate thickness can be achieved reducing machining of the piston and scrap rates. The coating may be applied to all surfaces of a piston ring groove, e.g. the top ring groove. The coating may also be applied to several ring grooves and surfaces between the grooves.

Preferably, the phosphorus content of the coating is between 1% and 10% by weight, e.g. about 4%.

In order to increase its hardness, the coating may be heat-treated at a temperature between 1500C and 5000C.

This heat treatment may be for at least 30 minutes.

However, no specific heat treatment is necessary in some cases or the elevated temperatures at which the piston operates may have the effect of a heat treatment.

Preferably, a method according to the invention comprises immersing the surface to be coated in an aqueous solution of cobalt ions, catalyst, reducing agent, complexing agent and bath stabiliser. The thickness of the coating is found to be substantially directly dependent on the immersion time and may be between 6 and 40 microns. A thickness of about 6 microns was found to produce less wear than one of 18 microns when heat treated at 400or. Heat treatment at 1500C results in both coating thicknesses having substantially the same wear. The ring groove area and the crown of the piston may be coated by immersing only the top portion of the piston (with the piston inverted) or, if desired, regions not to be coated may be masked.

The aqueous solution is preferably maintained at a temperature above 850C, e.g. between 900 and 950C, as the deposit rate is good at these temperatures.

In order to achieve a smooth coating, the pH value of the aqueous solution is preferably between 8.5 and 10.5.

It was found that use of ammonium hydroxide as a bath stabiliser gave smoother deposits than when sodium hydroxide was used.

It was found that some piston samples required the aluminium or aluminium alloy to be activated before immersion in the aqueous solution. This can be achieved by immersing in zincate solution.

The method may comprise dipping the crown region of the piston into the aqueous solution while keeping the skirt region of the piston out of the solution.

The invention also provides a piston made of aluminium or aluminium alloy having a hard wearing coating having a hardness in the range 300 to 750 Hv on at least one surface of a piston ring groove thereof, the coating comprising cobalt deposited by electroless plating.

There now follows a detailed description of an illustrative example of a method according to the invention.

The illustrative example is a method of providing all the surfaces of a piston ring groove of a piston made of aluminium alloy with a hard wearing surface in the form of a coating of cobalt. The method also applies the coating to other areas of the piston. The groove in question is the top ring groove of the piston.

In the illustrative method, an aqueous solution containing cobalt ions is prepared using the following formulation: Cobalt chloride heptahydrate 30g/L Sodium hypophosphite 20g/L (reducing agent) Sodium citrate 20g/L (complexing agent) Ammonium chloride 50g/L (catalyst) Ammonium hydroxide Sufficient to bring pH to 9 to 10. (bath stabiliser) In this solution, cobalt ions were catalytically reduced to the metal in alkaline solutions by the hypophosphite ions which were thereby oxidised to orthophosphate ions.

The aluminium alloy surface was activated with zincate solution following vapour degreasing, alkali cleaning and acid desmutting. The activation involved dipping the piston into a zincate-bondal dip at 20 to 250C for 45 seconds.

The ring groove region and the crown of the piston were then immersed in the aqueous solution for 90 minutes with the temperature of the solution being maintained above 900C.

The piston was removed from the solution and rinsed and dried. The area of the piston immersed was found to have a coating thereon including on the radial faces of the ring groove. The coating had, therefore, been applied by electroless plating.

The coating was of smooth appearance and was very uniform in thickness including over the corners at the top and bottom of the ring groove. The coating was 18-20 microns in thickness on the complete section of the ring groove and was made up of cobalt with 1.5% by weight of phosphorus.

The coating was then subjected to a heat-treatment at 4500C for 90 minutes. This heat-treatment increased the hardness of the coating from an as-deposited value of 360 + 20 Hv. to 710 + 30 Hv. This is attributed to a transformation of the amorphous cobalt-phosphorus alloy to a dispersion of Co2P in a cobalt matrix.

The piston was then subjected to wear tests and found to produce less wear than conventional pistons with inserts at engine operating temperatures. The weight loss at 250 0C for conventional pistons in a 4 hour test was 6.9mg whereas that for the piston of the illustrative example was 4.4mg.

This test was under a load of 1.35kg and with no lubrication.

Claims

CLAINS

1 A method of providing at least one surface of a piston ring groove of a piston made of aluminium or aluminium alloy with a hard wearing surface having a hardness in the range 300 to 750 Hv, the method comprising applying a coating of cobalt or cobalt and phosphorus to said surface by electroless plating.

2 A method according to Claim 1, wherein the coating has a phosphorus content between 1% and 10% by weight.

3 A method according to either of Claims 1 and 2, wherein the coating is heat-treated at a temperature of between 150 and 5000C.

4 A method according to Claim 3, wherein the heat treatment is for at least 30 minutes.

5 A method according to any one of Claims 1 to 4, wherein the method comprises immersing said surface in an aqueous solution of cobalt ions, catalyst, reducing agent, complexing agent and bath stabiliser.

6 A method according to Claim 5, wherein the solution is maintained at a temperature above 850C.

7 A method according to either one of Claims 5 and 6, wherein the pH level of the solution is between 8.5 and 10.5.

8 A method according to any one of Claims 5 to 7, wherein the surface of the aluminium or aluminium alloy is activated before immersion in said aqueous solution.

9 A method according to any one of Claims 5 to 8, wherein the method comprises dipping the crown region of the piston into the aqueous solution while keeping the skirt region of the piston out of the solution.

10 A method of providing at least one surface of a piston ring groove of a piston made of aluminium or aluminium alloy with a hard wearing surface having a hardness in the range 300 to 750 Hv substantially as hereinbefore described with reference to the illustrative example.

11 A piston made of aluminium or aluminium alloy having a hard wearing coating on at least one surface of a piston ring groove thereof, the coating having a hardness in the range 300 to 750 Hv comprising cobalt deposited by electroless plating.

12 A piston according to Claim 11, wherein the coating contains between 1 and 10% by weight of phosphorus.

13 A piston according to either one of Claims 11 and 12, wherein the coating is between 6 and 40 microns in thickness.