FIELD OF THE INVENTION The present invention relates to a low resistivity material with improved wear performance for electrical current transfer and methods for preparing same.

In a particular non-limiting aspect, the invention relates to a copper-graphite composite material prepared by a powder metallurgy (P/M) route which shows improved electrical conductivity compared with conventional copper-graphite composite materials, while maintaining higher density than other similarly prepared materials. It also relates to devices and systems including such composites.

BACKGROUND OF THE INVENTION Carbon composite materials for use in applications such as brushes and contact materials in light rail systems are known. The preparation of these materials may be via P/M techniques. However, currently available materials tend to exhibit either low conductivity or cause excessive wear of counterpart components.

The present invention seeks to provide materials and methods of preparing same which are directed to ameliorating these difficulties significantly.

DISCLOSURE OF THE INVENTION According to one aspect of the present invention, there is provided a copper-graphite composite material comprising a copper network matrix having a plurality of pores therethrough, at least some of the pores containing graphite.

According to another aspect of the present invention, there is provided a copper-graphite composite material having an IACS value of at least about 40% more preferably, 45%, and a density of at least about G. Og/cm'.

Preferably, the composite materials have a density in the range of from about 6.3 to 7. 6/cm3.

The microstructure of the composite material prepared in accordance with the invention has pores and contains graphite islands in a copper network matrix.

The following explanation of the way in which the invention provides improved performance is offered as a likely mechanism. The invention is not dependent on, nor is it limited by the explanation.

The composite materials according to the invention advantageously exhibit a self lubricating function resulting from the formation of a transfer graphite layer onto the surface of a counterpart component. The self-lubricating function of the copper-graphite composite material effectively protects the counterpart, and thus extends the lifetime of the counterpart. This may advantageously be effective in protecting and extending the lifetime of, for example, railway electrical power transmission systems. More particularly, it is estimated that the lifetime in such an application may be extended by as much as three times relative to currently used materials.

Thus the invention provides in one aspect a material which can be mounted on a pantograph for a railway train such as a pole shoe which includes a copper-graphite composite as hereinafter described as an electrical contact for receiving power from overhead power lines. It also includes power transmission systems using such a composite.

In a preferred embodiment the IACS value of the composite material is at least 60%.

As will be understood by a person skilled in the art, the IACS percentage is the standard conductivity (resistivity) used to judge a material's property of conduction based on the International Annealed Copper Standards (IACS).

According to the invention the materials may be prepared by mixing and compacting copper and graphite powders under certain conditions, and then sintering the compacted materials. The various steps of the process may suitably be carried out under non-oxidising conditions, such as under a reducing atmosphere.

According to a further aspect of the invention there is provided a method of preparing a copper-graphite composite material comprising the steps of :

mixing a copper powder and a graphite powder under conditions to substantially prevent oxidation of the copper powder; and pressing the mixture under conditions which result in the formation of a copper network matrix having a plurality of pores therethrough at least some of the pores containing graphite, wherein the copper powder has a varying particle size of no greater than about 1 O, um, and wherein the graphite powder has a particle size of no greater than about seul.

The conditions may include compacting the well mixed powders using a pressure in the range of from about 500 to about 1600 Mpa. They may also include sintering the compacted powder in the form of compacts at a temperature in the range of from 960°C to 1100°C for a predetermined period under an atmosphere of H, and N2 Alternatively the process may include any other process of heating and pressing such as, for example hot isostatic pressing (hipping), isolated hot pressing (IHP) or vacuum sintering.

The compaction of the copper and graphite powders following the mixing step is preferably performed by either two-direcdonal compacting or dynamic compacting.

When two-directional compacting is employed, a compressing pressure of from about 500 to about 1600 Mpa is applied preferably for a period of from about 5-10 Minutes. The alternative to this is dynamic compacting. When dynamic compacting is employed, the shock frequency is preferably in the range of from about 150 to 250 Hz. Such a shock frequency will achieve a similar result to the application of a constant pressure as described above for the two-directional compacting method.

The copper powder used is advantageously of commercial grade purity or better, and is preferably of about 99.9% purity. The varied particle size of the copper powder facilitates the optimisation of the"particle size effect"on mixing of the copper and graphite powders. For example, copper powder may be used at sizes of 10 micrometers (about 600 mesh) and and 400 mesh. Preferably, the particle size of the copper powder ranges between about 5 micrometers and about 150 mesh.

The copper powder is advantageously such that oxides and thinly oxidised films are not present on the particle surfaces. As such, in a preferred embodiment the copper powder, prior to mixing with the graphite powder, is cleaned and annealed in a controlled atmosphere which is reducing, such as a mixture of hydrogen and nitrogen. Other suitable reducing atmospheres may include carbon monoxide, hydrogen, water reformed natural gas, reducing endothermic or exothermic natural gas mixtures and/or mixtures of these with less reactive gases such as nitrogen.

Preferably, this is conducted at a temperature of from about 600°C to about 850°C.

It will be readily understood by those skilled in the art that the temperature for cleaning and annealing xvill depend substantially on the particle size of the copper powder.

The copper powder may also have been treated to remove unwanted impurities. A magnetic separation step may be used for this purpose. Alternatively or additionally, lighter non-magnetic materials may be removed by processes such as electrostatic or centrifugal separation.

The graphite powder should preferably have a particle size of no greater than about Sum and preferably has a particle size in the range of from about 1pm to about 2pm.

In a preferred embodiment the graphite powder is electro-grade quality.

As is the case in known P/M processes, other metallurgical powders may be included as additives. These may include, for example, Zn, Nions, and Si. (Note: the Si additive may be in the form of a silicate.) As described above, the mixing of the copper and graphite powders is performed under conditions to prevent oxidation of the copper powder. Preferably, the powder mixing is performed at a relatively slow speed, such as about 150 rpm in a conventional mill As discussed above, the compacting of the mixed powder is advantageously performed by a two-directional compacting method or a dynamic compacting method. The upper compression pressure of about 1600 Mpa, which may be used in accordance with the present invention is substantially higher than that conventionally used in P/M techniques. This is generally about 690 Mpa. It is worth noting that the pressure here is defined as load/cross sectional area of the compacting die.

The sintering temperature of the sintering step may be in the range of from about 960°C to about 1100°C. The holding time in the furnace will depend on the furnace facilites as would be readily understood by those skilled in the art. The reducing atmosphere used in the sintering step preferably consists of 10% H2 and 90% N, and provides an exothermic atmosphere in the furnace.

It will be understood that the above process is provided for exemplification only as a preferred method of forming the composite materials of the invention. Other methods may also be employed provided that these produce a composite material having the advantageous characteristics as described herein.

BRIEF DESCRIPTION OF THE ILLUSTRATION Figure 1 illustrates the typical microstructure of a copper-graphite composite material prepared in accordance with the invention. As can be seen from the figure, the copper phase has dispersed therein a number of large pores enclosing graphite islands. As mentioned above, this network of copper with graphite dispersed throughout advantageously provides a substantial improvement in the electrical conductivity of the composite material, and also advantageously supplies graphite which forms a lubricating carbonaceous film between the moving parts, i. e. the current collectors (contact material) and the electrical contact xvires.

Particular embodiments of the present invention will now be described with reference to the following examples. The examples are provided for exemplification only and should not be construed as limiting on the invention in any way.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following table provides more detailed information on particular embodiments of the composite material prepared in accordance with the invention in terms of chemical composition, physical properties, heat capacities, electrical properties and tribological properties.

The samples in Table 1 were prepared in accordance with the methods described hereinbefore, ie the copper powder having a range of particle sizes of 10p,. 40,150, 220 and 400 mesh was cleaned by electrostatic and magnetic separation. It was then annealed in a reducing atmosphere of 10% hydrogen and 90% nitrogen. The

annealed copper powder was mixed with other powder components, of which the graphite powder had a particle size range of lu. to 2µm. The mixture was compacted using a txvo-directional compacting or dynamic compacting approach and the compacted mixture in the form of a compact was sintered for about two hours in a reducing atmosphere at 10% hydrogen and 90% nitrogen. The sintering temperature was in the range 960°C to 1100°C.

1) Nominal chemical compositions are given in Table 1.

Table 1

Sample Weight proportion in initial mixture (%) | C. G. C. M. No. Cu Graphite Zinc MoS2 Silicate suggested name 7.0-0.50.5CuGMnSi192.0 2 89. 0 10. 0--1. 0 CuGSi 3 87. 0 11. 0 1. 5 0. 5 CuG1 2ZnSi 4 85. 0 15. 0---CuG 5 82. 0 17. 5--0. 5 CuGSi 6 68. 0 27. 0 2. 0 1. 5 1. 0 CuGZnMoSi 7 80. 0 15. 0-5. 0-CuGlSMo 16.5-5.00.5CuG16Mo5Si878.0 15.0-10.0-CuG15Mo10975.0 1070. 023. 52. 54. 0-CuG23Mo4Zn 2) Some measured physical properties are given in Table 2: Table 2 Sample Density Max Operation Melting Coefficient of Thermal No. (gr/cm3) Temperature point thermal expansion conductivity (°C) (°C) (W/m-°K) 1 6. 984 450 1085 17. 30 366 450108516.3435427.049 3 7. 545 470 1085 16. 02 346 4 6. 372 470 1085 15. 82 338 480108515.7232656.656 6 6. 163 480 1100 10. 65 271 480110014.4831877.119 8 6. 837 480 1100 13. 50 310 480110012.2429996.370 10 6. 300 480 1100 11. 56 278

suggested temperature beyond which it is estimated that the properties deteriorate rapidly.

3) The heat capacities* of CGCM are given in Table 3, as calculated from thermodynamic data.

Table 3

Sample No. a bxl0'cxl03 Temp. range (°K) 1.388-0.158298#1.35615.285 2 5. 225 1.437-0.210 298-1. 356 1.43135.207 -0.252 298#1.356 4 5. 214 1.428-0.315 298-1. 356 5 5. 154 1.409-0.368 298-1. 356 6 4. 973 1.379-0.567 298-1. 356 1.47375.211 -0.315 298#1.356 1.458-0.347298#1.35685.165 9 5. 064 1.429-0.378 298-1. 356 10 5. 039 1.415-0.504 298-1. 356 1.500-298#1.356Copper5.410 *Note: Cp = a + bT + cT2 (Cal/°°K mole) 4) Compacting stress and mechanical properties are given in Table 4.

Table 4 CompressModulusofDuctilityPossion'sVickersSampleCompacting No. stress strength elasticity (EL in ratio hardness (Mpa) (Mpa) (Mpa) 1. 5 in) (VH) (5) 1523200872Q03278-80 18578180.2868#862523 17260180.2660#693523 17554200.2060#794523 16739170.3180#905523 10528170.2165#726523 15042150.1570#897523 14035140.1875#828523 9 523 138 32 14 0. 12 86-92 12030120.1358#6710523

Note: Underlined data are estimated values based on calculation of composite materials properties-ASM, Metals Handbook,-Composite Materials.

5) Electrical properties of these materials were measured and are given in<BR> <BR> <BR> <BR> Table 5.

Table 5 OperatingResistivityMax.currentSamplePercentage No. of IACS* (%) Voltage (V) (µ#cm 20°°C) density (amp/mm2) 1 65. 8 600 5. 05 18 2 76. 4 600 4. 74 20 3 59. 0 600 6. 14 16 4 69. 0 600 5. 25 15 5 67. 2 600 5. 39 15 6 46. 8 600 7. 74 14 7 64. 6 600 5. 60 13 8 42. 9 600 8. 24 13 9 6. 19 600 5. 85 12 10 43. 0 600 8. 42 10

Note 1 : The percentage of IACS is the standard conductivity (resistivity) used to judge the material's property of conduction, and is based on the International Annealed Copper Standard (IACS) adopted by IEC in 1913, which states that 1/58 Q mm2/m and the value of 0.017241 Q gm mm'/m and the value of 0.15328 # gm/m2 at 20°C (68°F) are, respectively, the international equivalent of volume and weight resistivity of annealed copper equal to 100% conductivity.

Note 2: Underlined data are estimated values. The current capacity is calculated from the electrical current which can pass through 1 mu'are of material with no damage to that area at maximum operational temperature.

6. Tribological properties Table 6 Sample No Wear Factor Wear Rate (out) Coefficient of Layets (x10-4 X) friction (X) transferring rate** (10''A/m) 1 1.64 9.23 0. 25 3.13 2 0.22-0.269.17 3 1.43 9. 32 0-25 4.19 4 2. 56 14. 60 5 0.20-0.245.87 6 1. 69 10. 50 7 2.97 8 1.30 7. 16 0 18-0. 21 9 9.38 45-90 0. 18 3.59 10 6.23

*Note 1 : The tribological properties were measured under the conditions of normal load-13. 5N, sliding velocity 0.25m/sec and the counterpart metal is pure copper contact wire (after 108 wear cycles).

Note 2: Double underlined data were obtained on undefined metal (copper) surfaces before wear test.

Note 3 (**) : The data for rate transfer was measured using a specially designed testing device.

The following table summarises the relevant properties of other materials containing copper and carbon prepared by conventional P/NI techniques. It is worth noting that the highest conductivity listed in the table is just above 40% IACS, with the majority of these values being significantly below the IACS values of composite materials prepared in accordance with the materials of the invention, 43% IACS being the lowest value in this respect.

Table 7: Commercial materials available for use of electrical contacts Composition Approximate density Electrical conductivity Hardness (%) (g/cm2) (% IACS) (HRB) 30Cu,70Graphite 2. 5 0. 11 80 36Cu,64Graphite 2. 75 3 75 40Cu,60Graphite 2. 75 4 52 50Cu,50Graphite 3. 05 2. 5 35 62Cu,38Graphite 3. 65 3 28 65Cu,35Graphite 3. 15 3 30 75Cu,25Graphite 3. 25 0. 51 21 92Cu, 8Graphite 7. 30 41 40 95Cu, 34386.30 96Cu, 42407.75 2.20.252821Cu,79C 35Cu, 65C 2. 5 2 28 50Cu, 5282.75 65Cu, 35C 3. 5 8 20B 75Cu, 25C 4. 0 21 18 95Cu, 5C 7. 57 40-46 38HR15-T

It is envisaged that the composite materials of the invention may be used as contact brushes for electrical motors, pantographs and pole shoes for light rail applications, power generators and other electrical components such as switches, etc.

Furthermore, the particular method of production described above is advantageously relatively simple and economical.

Throughout this specification and the claims which follow, unless the context requires othervise, the word "comprise", and variations such as"comprises"and

"comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers or steps.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications.

The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.