GB2194277A - Composite material of nickel, & carbon fibre - Google Patents

Abstract

A composite for use in the manufacture of internal combustion engine piston rings, and also piston rings manufactured from the material. The composite material comprises carbon fibre in a nickel matrix. The fibre is coated with nickel, for example a nickel phosphorous alloy, and the coated fibre subjected to a temperature of at least 650 DEG C and a pressure of between 3 and 4 Kg/mm<2>.

Description

SPECIFICATION Composite material The present invention concerns composite materials, and in particular, though not exclusively, a material suitable for use in the fabrication of piston rings for internal combustion engines.

To operate satisfactorily over a long period of time a piston ring needs to have a range of optimised properties: (a) low coefficient friction (b) good resistance to welding in sliding conditions (c) good elasticity (d) sufficient flexibility to conform to cylinder bore (e) low mass (f) adequate oxidation, carburization and thermal resistance to resist chemical attack by hot exhaust gases.

From one aspect the present invention consists in a method of manufacturing a composite material, comprising coating carbon fibre with nickel, and subjecting the coated fibre to a temperature of at least 6500C and a pressure of between 3 and 4 Kg/mm2.

From a second aspect the present invention consists in fabricating a piston ring from a composite carbon-nickel material manufactured by the method set out hereinbefore.

The present invention has for an object to provide a material providing the above characteristics.

In order that the present invention may be more readily understood, an embodiment thereof will now be described by way of example.

The composite material proposed by the invention is a suitable metal reinforced with carbon fibre. The carbon fibre consists of a highly oriented chain of hexagonal graphite micro-crystals providing the high strength and elasticity, low mass, low friction and welding potential. The chosen matrix metal for reinforcing the carbon fibre is Nickel. Nickel reacts to only a very limited extent with carbon, it has good resistance to oxidation and corrosion by hot, acidic exhaust gases and can be deposited onto other materials by a range of processes: (a) electrolytic deposition (electro-plating) (b) chemical deposition from solution (electroless plating) (c) chemical deposition from a nickel containing vapour such as, for instance, nickel carbonyl (d) vacuum and plasma deposition (various evaporative, sputtering, ion bombardment techniques).

The material described in this specification has been produced by depositing nickel, in fact a nickel/phosphorus alloy, onto carbon fibre by electroless plating. It is beiieved that any of the other processes which deposit nickel in another or purer form would be equally suitable by appropriate modification of the deposition techniques.

One embodiment of the composite material is fabricated from Courtauld's continuous filament carbon fibre "Grafil" (RTM), Type A-S.

This material contains continuous fibres each of 7 micron diameter. Other grades of fibre are available and, internationally, there are a number of manufacturers of carbon fibre, broadly similar to the above. Whilst there would be small variations in detailed properties, the overall advantages of the composite for piston ring applications are unlikely to be seriously affected by the source of the carbon fibre so that many other types of carbon fibre may be suitable.

In the process continuous lengths of carbon fibre are coated with "electrnless nickel" using a standard bath. The bath comprises: Nickel chloride 30 g./1 Sodium citrate 100 g./1 Ammonium chloride 50 g./1 Sodium hypophosphite 10 g./1 pH 8-10 g./1 Temperature ("F) 190 The pH is adjusted with ammonia, which must be added almost constantly while the bath. is in use. Plating rate is about 0.3 mil/hour, with a bright deposit.

In order to deposit nickel from the electroless solution, the carbon fibre first is 'activated' by gentle, vertical oscillation in a tank of palladium chloride solution for about 10 minutes and then washed first in running water for 15 to 30 minutes, then in demineralised water until ready for the next stage.

The chloride solution used is 1 g./1 Palladium chloride and 1 ml/1 Hydrochloric acid at a temperature of 20 C. It is then transferred to the nickel solution for a time dependent on the thickness of coating required typically 1 hour. Periodically the fibre was oscillated in the solution to separate the fibres and obtain relatively uniform coatings. Electroless solutions have excellent "covering power" and this process produced relatively uniform coatings on the 10,000 fibre yarn. Coating thickness is controlled by weight increase on a 'standard' length (70 cm). The coating produced has an average weight increase of 3.0 to 3.2 gms per length.

The final coated material contained 50 to 60 vol. % carbon. The various properties required in a piston ring will clearly vary with the carbon fibre content but the material described herein can be used to provide a satisfactory piston ring.

Bars of carbon fibre/nickel made by the above process are then hot-pressed at 600, 650, 700 and 750"C for 1 hour, and at 7000 for 1/4 and 4 hours. These bars were pressed in an 18-8 austenitic steel die assembly using about 3.5 Kg/mm2 on the 45x6.5 mm bar. Approximately 3 gms of coated fibre (ten 45 mm lengths) were placed longitudinally in the die, lightly compacted and placed in the vacuum hot press. Atmospheric pressure was reduced to better than 1O-4 Torr and heating commenced for the appropriate temperature and time.

The vacuum improved during the heat treatment to about 10-5 Torr at the end of the run. The final bar thickness depends upon the degree of compaction but generally is in the range 1.5 to 2.5 mm. The results of these initial bar tests show that bars pressed at 700"C for 1 hour appeared to be the optimum in relation to compaction and time involved.

As a result of these tests piston ring blanks 65.4x58.7x2.5 mm nominal thickness were produced by laying a total length of 1170 mm of coated fibre in a die cavity and hot pressing at 700"C for 1 hour using a pressure of 3.5 Kg/mm2 (2360 Kg total). The coated fibre was produced in the manner just described.

A composite piston ring, final size of 65.06/65.09 ODx58.7 lDx1.98, was decided upon by measurement of the top-ring removed from a test engine. An over-size hotpressed ring was machined to the correct outer diameter, ground to the correct thickness and chamfered at 45"C on its inside upper corner to produce a section corresponding to the original cast-iron ring. It was then slit radially using a 0.3 mm (nominal) slitting wheel, expanded on a former to give a gap of about 8 mm and stress-relieved on this former at 700"C for 1 hour to set the gap.

No serious problems were encountered in machining, grinding or slitting the ring to the required final size 65.06/65.09 ODx58.7 ID x 1.98. The top piston ring was then removed from a Briggs and Stratton Type 130212 agricultural engine. This is a simple single cylinder, 4-stroke engine of 13 cubic inch (200 cc) capacity producing a nominal 5 H.P. at 3600 r.p.m. (max). The top ring was replaced with a closely similar ring produced as described from hot-pressed nickel-phosphorous carbon. The top ring was chosen as it is believed to be subjected to the most severe thermal and mechanical conditions. The engine started immediately when re-assembled and ran satisfactorily for over 106 revolutions with several start/stops. The- composite piston ring was then removed and examined. Apart from a polish on the outer diameter no visible change or damage had occurred to the ring.

Thus a composite, fibre-reinforced metal piston ring has been shown to operate satisfactorily in a 5 H.P. 4-stroke petrol engine. Many potential advantages result from the use of such a material as the properties can be varied considerably by varying the proportion and type of reinforcement.

Claims (11)

1. A method of manufacturing a composite material comprising coating carbon fibre with nickel, and subjecting the coated fibre to a temperature of at least 650"C and a pressure of between 3 and 4 Kg/mm2.

2. A method as claimed in Claim 1, wherein the nickel is deposited in the carbon fibre in the form of an alloy by chemical de position from solution.

3. A method as claimed in Claim 1, wherein the nickel is deposited by electrolytic deposition.

4. A method as claimed in Claim 1 wherein the nickel is deposited from a nickel containing vapour.

5. A method as claimed in Claim 1, wherein the nickel is deposited by vacuum on plasma deposition.

6. A method as claimed in Claim 2, wherein the alloy is a nickel/phosphorous al loy.

7. A method as claimed in Claim 6 wherein the carbon is coated with the nickel/ phosphorous alloy in a bath containing the fol lowing constituents in grams per litre: Nickel Chloride 30 Sodium Citrate 100 Ammonium Chioride 50 Sodium Hypophosphite 10 pH 8-10

8. A method as claimed in Claim 7 wherein the bath is maintained at a tempera ture of 1900F.

9. A method as claimed in Claim 7 or Claim 8 wherein the carbon fibre, prior to coating, is activated by movement in a solu tion of palladium chloride.

10. A method as claimed in any one of Claims 7, 8 or 9 wherein the pressure is maintained for approximately one hour.

11. A piston ring manufactured from a ma terial produced by the method of any one of the preceding claims.