EP0640189A1

EP0640189A1 - Plain bearing materials

Info

Publication number: EP0640189A1
Application number: EP93908019A
Authority: EP
Inventors: Philip Ferdani
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1992-05-13
Filing date: 1993-04-08
Publication date: 1995-03-01
Also published as: GB2266935A; WO1993023682A1; JPH07506657A; GB9210239D0; GB2266935B

Abstract

A plain bearing material comprises a supporting layer, and a surface layer overlying the supporting layer to provide a low friction surface. The material of the surface layer comprises particles of a low friction polymeric material, e.g. polytetrafluoroethylene, and particles of structured carbon black. The supporting layer comprises a porous structure which is impregnated by the material of the surface layer to bond the surface layer to the supporting layer. The particles of structured carbon black contact one another to form an electrically-conductive network in the low friction material.

Description

PLAIN BEARING MATERIALS

This invention is concerned with plain bearing materials and with methods of producing such materials.

For many years, plain bearing materials of the type described in US Patent Specification No. 2,689,380 have been in common use. Such materials are used for making journal bearings, thrust washers, bearing bushes etc. Such bearing materials comprise a backing layer, e.g. of steel, a supporting layer joined to the backing layer, and a surface layer overlying the supporting layer and bonded thereto. The surface layer provides a low friction surface of the plain bearing material and is formed from a material which comprises a low friction polymeric material, for example polytetrafluoroethylene, and a filler or several fillers, such as lead, zinc, bronze, calcium fluoride, or glass fibre. The supporting layer comprises a porous structure of a sintered metallic material, e.g. bronze, formed on the backing layer and bonded thereto. The porous structure of the supporting layer is impregnated by the material of the surface layer to bond the surface layer to the supporting layer. The surface layer is typically about 25 microns thick.

Plain bearing materials of this known type are not electrically conductive in a direction normal to the surface layer thereof. Although the backing layer and the sintered metallic material are good electrical conductors and the surface layer may comprise a filler which is a good electrical conductor, it is found that such a plain bearing material has an electrical resistance in the direction mentioned which exceeds 10⁸ ohms. This lack of electrical conductivity is acceptable for many uses of the material as electrical conduction through a plain bearing is not a usual requirement. However, there are circumstances where such electrical conduction is advantageous. For example, the plain bearing material may be used for making bushes used in the hinges of the doors of vehicles. These bushes may electrically insulate the door, which is often made of metal, from the remainder of the vehicle body, which is also often made of metal. In this case, it is possible for users of the vehicle to receive unpleasant electro-static shocks from the door. Another possibility is for arcing to occur between the door and the remainder of the vehicle body during electro-static paint spraying of the vehicle, with detrimental effects on the finished paintwork.

It is an object of the present invention to provide a plain bearing material which is electrically-conductive in the direction mentioned and which substantially retains the tribological properties of the aforementioned known plain bearing materials.

The invention provides a plain bearing material comprising a supporting layer, and a surface layer overlying the supporting layer to provide a low friction surface of the material, the material of the surface layer comprising a low friction polymeric material, and the supporting layer comprising a porous structure which is impregnated by the material of the surface layer to bond the surface layer to the supporting layer, characterised in that the material of the surface layer also comprises particles of structured carbon black which contact one another to form an electrically-conductive network.

A plain bearing material according to the invention has good tribological properties and can have an electrical resistance below 100 ohms or, in some circumstance below 10 ohms. This is in contrast to the aforementioned resistances above 10⁸ ohms. Attempts to achieve similar reductions in resistance by adding powdered metals or granite, instead of structured carbon black, in quantities which do not substantially reduce the tribological properties of the material failed to reduce the resistance below 10⁸ ohms. It is believed that this failure is due to the metal or graphite particles not forming an electrically-conductive network in the low friction material.

Preferably, the structured carbon black forms between 2.5% and 20% by volume of the material of the surface layer. At 20%, it was found that electrical resistances between 10 and 1 ohm could be achieved. The 20% level appears, however, to approximate to an optimum level for achieving lowest resistance so that higher percentages are not expected to give significantly lower resistances. Satisfactory resistance can be achieved when the structured carbon black forms between 8% and 12%, preferably about 10%, by volume.

The structured carbon black can be obtained from Akzo Chemie and is marketed under the name "Ketjenblack" (Registered Trade Mark) . The structure of such carbon blacks can be defined by two different measurements. Firstly, the pore volume can be measured by the absorption of dibutylphthalate (DBP) which gives information about the void volume between the carbon black particles when they form aggregates. It is preferred that structured carbon black used in the invention has a pore volume of at least 150 (DBP) cm³/100 gm. Secondly, the surface area including the surface area accessible through pores in the structure can be measured by a nitrogen adsorption technique (known as BET-N₂) . It is preferred that the structured carbon black used in the invention has a surface area of at least 250 m /gm and, more preferably, of at least 850 (BET-N₂) m²/gm. As in known plain bearing materials, the supporting layer used in the invention may comprise a sintered metallic material, e.g. bronze, a woven mesh of wires, or an expanded mesh, i.e. a metal sheet formed with many small discrete slits in a repetitive pattern, the sheet being stretched so that the slits open and the sheet takes on the appearance of a mesh.

As is conventional, the supporting layer of a plain bearing material according to the invention may join the surface layer to a backing layer, e.g. of steel.

The invention also provides a method of producing a plain bearing material comprising forming an aqueous dispersion containing particles of a low friction polymeric material and particles of structured carbon black; partially drying the dispersion to form a workable mush; applying the mush to a surface of a porous material; and subjecting the mush to heat and pressure to form it into a low friction surface layer on the porous material and also to force it into the pores of porous material so that the material of the surface layer impregnates the porous material and bonds the surface layer thereto.

There now follow detailed descriptions of examples which are illustrative of the invention.

In the illustrative examples, three samples of plain bearing . materials were prepared differing only in the quantity of structured carbon black used. In each case, an aqueous dispersion was formed containing particles of polytetrafluoroethylene, particles of lead, and particles of structured carbon black, as well as a small quantity of toluene as a lubricant and a small quantity of aluminium nitrate as a flocculant. An aqueous dispersion containing the above-mentioned materials, except for the structured carbon black, was first formed (this dispersion is the same as that formed in producing known plain bearing materials) and an aqueous dispersion of the structured carbon black was added to this dispersion. The dispersions were then thoroughly mixed by agitation.

Next, the mixed dispersion was flash heated to partially dry it to form a workable mush of low friction material.

Next, the mush was applied to a surface of a sintered bronze layer which had been sintered on to a surface of a steel sheet. As is conventional, the mush was applied with a spoon.

Finally, the mush was subjected to heat and pressure to form it into a low friction surface layer on the sintered bronze so that the low friction material impregnated the bronze and bonded the surface layer thereto. Specifically, the pressure was applied by a roller to give smooth surface layer about 25 microns thick.

In all three samples, 770 gms of polytetrafluoroethylene and 1000 gms of lead was used. In all three samples, the structured carbon black was

Ketjenblack EC 310 having a surface area (BET-N₂) of 950 m²/gm and a pore volume of 360 cm³/100 gms

In sample 1, 177 gms of structure carbon black was used giving a total weight of low friction material of 1947 gms. The polytetrafluoroethylene represented 64% by volume (39.5% by weight), the lead 16.5% by volume (51.4% by weight), and the structured carbon black 19.5% by volume (9.1% by weight). The electrical resistance of sample 1 was found to fall in the range between 1 ohm and 10 ohms. (measured with 1 sq.cm electrodes on opposite sides of a sample 1.5mm thick). In sample 2, 88.5 gms of structured carbon black was used giving a total weight of low friction material of 1858.5 gms. The polytetrafluoroethylene represented 70.8% by volume (41.4% by weight), the lead 18.3% by volume (53.8% by weight) , and the structured carbon black 10.9% by volume (4.8% by weight) . The electrical resistance was found to lie in the range between 10 and 100 ohms with 20 ohms being a typical value.

In sample 3, 45 gms of structured carbon black was used giving a total weight of low friction material of 1815 gms. The polytetrafluoroethylene represented 74.9% by volume (42.4% by weight), the lead 19.3% by volume (55.1% by weight) and the structured carbon black 5.8% by volume (2.5% by weight) . The electrical resistance was found to lie in the range between 10³ and 10⁴ ohms with a typical value of 2000 ohms.

The much lower electrical resistance of the samples, in comparison with conventional materials, is considered to be due to the particles of structured carbon black in the surface layer and impregnating the supporting layer contacting one another to form an electrically-conductive network in the low friction material providing a large number of electrical current paths through the plain bearing material.

Claims

A plain bearing material comprising a supporting layer and a surface layer overlying the supporting layer to provide a low friction surface of the material, the material of the surface layer comprising a low friction polymeric material, and the supporting layer comprising a porous structure which is impregnated by the material of the surface layer to bond the surface layer to the supporting layer, characterised in that the material of the surface layer also comprises particles of structured carbon black which contact one another to form an electrically-conductive network.

A plain bearing material according to claim 1, characterised in that the structured carbon black forms between 2.5% and 20% by volume of the material of the surface layer.

A plain bearing material according to claim 2, characterised in that the structured carbon black forms between 8% and 12% by volume of the material of the surface layer.

A plain bearing material according to any one of claims 1 to 3, characterised in that the structured carbon black has a pore volume of at least 150 (DBP) cm³/100 gms.

A plain bearing material according to any one of claims 1 to 4, characterised in that the structured carbon black has a surface area of at least 250 (BET- N₂) m²/gm.

A plain bearing material according to any one of claims 1 to 5, characterised in that the supporting layer comprises a sintered metallic material providing the porous structure.

A plain bearing material according to any one of claims 1 to 5, characterised in that the supporting layer comprises a woven mesh providing the porous structure.

A plain bearing material according to any one of claims 1 to 5, characterised in that the supporting layer comprises an expanded mesh providing the porous structure.

A plain bearing material according to any one of claims 1 to 8, characterised in that the supporting layer joins the surface layer to a backing layer.

A method of producing a plain bearing material comprising forming an aqueous dispersion containing particles of a low friction polymeric materials and particles of structured carbon black; partially drying the dispersion to form a workable mush; applying the mush to a surface of a porous material; and subjecting the mush to heat and pressure to form it into a low friction surface layer on the porous material and also to force it into the pores of porous material so that the material of the surface layer impregnates the porous material and bonds the surface layer thereto.