BACKGROUND ART The concept of tribology was coined during the 1960s in the Jost Report which was published by the Ministry for Education and Science in The United Kingdom. Tribology is defined as the science of cooperating surfaces in relative movement, with related matters i.e. friction, wear and lubrication. Tribological elements may be ball bearings and gear flanks, as well as sliding bearings and movable seals. Since the present invention relates to sliding bearings and movable seals, only this type will be considered in the body of this specification.

Cooperating surfaces in relative sliding movement occur in most machines, in all conceivable environments and under the most varying conditions. The demands on tribological elements, i.e. sliding bearings and seals, vary in response to the environment and under what conditions the bearing or seal is to operate. Such demands may be divided into mechanical demands and temperature-related demands. In, for example, piston pumps of the type employed in homogenizers, the seal in the contact surface is exposed to temperatures of 200-300°C. The wear on such piston seals which must meet high standards of sealing efficiency is considerable, at the same time as the seal itself must be able to operate in a hygienic environment. The function of tribological elements often demarcates the very limits of the performance of the entire machine. For example, the function of a lubricated bearing depends upon the ambient temperature and the bearing may fail if the machine is run at too high a temperature. Seals often have a considerably shorter service life than the machines in which they are fitted.

The critical property of bearings and movable seals is resistance to wear, often defined as material loss per wear distance. However, in closer examination, a number of causal contexts and wear models occur, these being well-known to a person skilled in the art, for which reason only those central to the present invention will be described.

It is a well-known fact that wear increases with load and the sliding distance. Further, the wear of metals within a certain load distance is relatively slight. The metal surfaces become rather uniform. On larger loads, the wear increases rapidly up to a hundred or a thousand times more, and the metal surfaces become roughened and uneven.

Three characteristic types of wear of cooperating sliding surfaces in a machine can be discerned. The first is self-inhibiting abrasion or running-in.

This type occurs when new machines are started up, quite considerable amounts of material (debris) being worn off from the sliding surfaces when these are worn into a good fit. The Ra value of the surface falls by up to 30 per cent. The Ra value of a surface is defined as mean surface deviation according to ISO 1302. A second type of wear is so-called uniform wear.

Each respective surface loses a relatively constant amount of material per sliding distance. Both surfaces may lose different amounts of material if they are made of different materials. The third type of wear is accelerated wear, i.e. the wear negatively affects one or both of the surfaces in such a manner that the negatively affected surface leads to increased wear, and so on.

Sliding bearings and movable seals, which are often manufactured of polymer materials, normally display only uniform wear, followed by accelerated wear, since bearings and seals of polymer material are normally not hard enough to bring about abrasion of the counter surface. Sliding bearings and movable seals occurring on the market for simpler applications are, despite the lack of wearing-in properties, normally manufactured of polymer materials, because of the superior capability of the material to adapt its configuration to the counter surface. The slight movements - at right angles to the direction of movement - which occur in all tribological contexts lead to elastic deformation of the surfaces, but if, in such instance, the modulus of elasticity is low, as in polymer materials, the load is kept at the low level which is conditional on uniform wear.

In more extreme applications, as in, for example, piston seals in an internal combustion engine, use is made of thin metal rings of complex geometry in order to be able to benefit from the superior resistance to wear obtained on low loading and with hard surfaces. However, metal ring seals or gaskets suffer from the drawback that they are expensive to manufacture and are difficult to assemble in place. Nor are they suitable for applications within the food industry.

Another method of reducing wear is to actively separate the surfaces wholly or partly from one another by a film of lubricant applied between the surfaces. The problem here however is, on the one hand, that the film must be generated between the surfaces and kept there, and, on the other hand, the surfaces must be formed in such a manner that they can withstand wear when the film is incomplete. Otherwise, in the event of, for example, overloading, accelerated wear may easily be initiated.

A specifically produced piston seal strives to combine a thin, flexible metal ring which functions as a primary seal with a combined elastic suspension and secondary seal of a polymer material. The problem is however that the metal ring cannot be ground so thin that its resistance to deformation will be comparable with a corresponding polymer seal. In addition, the choice of material for the ring is limited to hard steel types (cemented carbide) with the result that this seal cannot be employed in an oxidizing environment such as, for example, in food applications.

OBJECTS OF THE INVENTION One object of the present invention is to realise a tribological element which is usable as sliding bearing and for movable seals where it is possible to benefit from the resistance to wear of a hard surface without incurring, on undesired movement and defects, the high loads which the deformation of a hard element entails.

A further object of the present invention is to make it possible to use an optional - including inferiorly wear resistant - polymer material for sliding bearings and movable seals, for example sterilisable materials which are approved for applications in the food industry.

Yet a further object of the present invention is to realise a tribological element which is applicable for sliding bearings and movable seals, manufactured from a polymer material but which displays superior wearing-in properties also against hard surfaces and which, thereby, displays superior wear resistance.

Still a further object of the present invention is to realise a tribological element for sliding bearings and movable seals which combines the demands on easy operation and installations as in bearings and seals of polymer material with the performance which is to be found in corresponding elements of harder materials.

SOLUTION These and other objects have been attained according to the present invention in that the tribological element of the type described by way of introduction has been given the characterizing feature that the above- mentioned wear surface is coated with a metal or ceramic layer.

Preferred embodiments have further been given the characterizing features as set forth in the appended subclaims.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS One preferred embodiment of the present invention will now be described in greater detail hereinbelow, with reference to the accompanying Drawings, in which: Fig. 1 shows, partly in section and on a magnified scale, an optional tribological element and a counter surface; Fig. 2a shows, partly in section, the load extent of two objects: Fig. 2b shows, partly in section, the load extent of two other objects; Fig. 3 shows, partly in section, and on a magnified scale, the proportions of a tribological element according to the present invention; Fig. 4 shows, partly in section, a tribological element according to the present invention; Fig. 5a shows, partly in section, a tribological element, inner design; and Fig. 5b shows, partly in section, a tribological element, outer design.

The Drawings show only those details essential to an understanding of the present invention. The placing of the parts and details in their context, well-known to a person skilled in the art, has been omitted.

DESCRIPTION OF PREFERRED EMBODIMENT Fig. 1 shows, in considerable magnification, a part of an optional tribological element 1 which may be of the sliding bearing or movable seal type. The tribological element is disposed to cooperate with a counter surface 2. The counter surface 2 may constitute the surface 2 of a machine part 3 such as, for example, a shaft. The tribological element 1 is characterized in that a relative movement takes place between the element 1 and the counter surface 2, which is illustrated in Fig. 1 by the arrow 4, at the same time as a loading is transferred, this loading being illustrated by the

arrow 5.

The element 1 and the counter surface 2 are wholly or partly discrete and separated from one another by a space 6 which contains surrounding medium as well as debris from the element 1 and the surface 2. The surface of the element 1 and the counter surface 2, which are shown greatly magnified, have a profile after manufacture consisting of surface peaks 7, so- called asperities, and surface depressions 8. The difference in depth H constitutes the difference between surface peak 7 and surface depression 8 which corresponds to the Ra value x 4 of the surface.

Continuously, or on specific operational occasions, contact points 9 occur between the asperities 7 on each respective surface. The size of the contact point 9, the number of points 9, the impression into counter surface, bridge formations and other mechanisms, together with the transferred load 5, are all decisive for the size of the wear occurring at the contact point 9. The combination of these factors also determines the type of wear, i.e. whether the wear is self-inhibiting, uniform or accelerating.

Figs. 2a-b illustrate a load extent 10 of circular cross section for a tribological element 1 and a counter machine part 3. The load, illustrated by the arrow 5, is the same in both cases. The examples apply in principle also to planar surfaces where, for example, oblique abutment or configurational defect gives a corresponding picture. Fig. 2b shows the parts 1, 3 manufactured of a material with a high modulus of elasticity, i.e. with high hardness which gives slight deformation. The load extent 10 displays a very small contact surface and the surface pressure Pb will be high. In Fig. 2a, one or both of the parts 1, 3 are manufactured from a polymer material of low modulus of elasticity. The material in the one or both of the parts 1, 3 makes for considerable deformation which displays a large contact surface and the surface pressure Pb will be low. Since lower surface pressure means less wear, the case illustrated by Fig. 2a is that to be sought for.

Fig. 3 shows a tribological element 1 according to the present invention in magnification. The tribological element 1 has a wear surface 11 intended to be turned to face towards the counter surface 2 with which the tribological element cooperates. The tribological element 1 consists of a carrier body 12 consisting a polymer material. The body 12 is coated with a thin metal or ceramic layer 13.

The wear surface 11 of the tribological element 1 has a profile depth H

which constitutes the difference between the asperities 7 of the surface 11 and its surface depressions 8. The profile depth H is much less than the thickness Y of the metal or ceramic layer 13. The thickness Y of the layer 13 is in turn much less than the total thickness X of the tribological element 1. The profile depth H is an indirect measure of how large stresses which, on wear, can be introduced in the metal layer 13. Small asperities 7 give a slight profile depth H and, on collision between small asperities 7, the mechanical stress, like the development of frictional heat, will be slight.

Since the metal layer 13 is thin and is dimensioned to be flexible, the order of magnitude of the profile depth H must therefore be so slight that stresses introduced on wear will not lead to deformations, i.e. the profile depth H must be much less than the thickness Y of the metal or ceramic layer 13. By optimising the thickness Y of the metal layer 13 against the modulus - of elasticity of the metal layer 13 and the body 12, the property will be achieved that, within a predetermined deformation range, a critical surface pressure for initiating accelerating wear can never be attained solely because of the deformation inertia of the metal layer 13. As a result, the load dissipating properties of the body 12 will be predominant, and also the wear because of the lower surface pressure will be less than for a corresponding element 1 of solid metal.

The body 12 is dimensioned in response to application requirements and material selection so as to give the desired flexibility and may be an integral part of, for example, a sliding bearing or a seal. The selection of material for the body 12 may be governed by other properties than wear resistance, such as, for example, that the polymer material should be approved for use in connection with foods, and one example of such a material is polyether sulphone (PES).

Metalisation of polymer materials is based on adhesion between two interactive substances, a polymer and a metal, so that the polymer material, the body 12, by intermolecular forces, is coated with a thin layer 13 of a metal. The metal layer 13 consists of but a few or a few tenths of a millimetre.

The metalisation may be put into effect either by an electrochemical process, so-called plating, or by mechanical processes such as, for example, vacuum deposition.

A suitable metal for coating might be, for example, chromium which may also be suitable for applications within the food industry, since a hard

chromed surface with a 0.2 mm coating of chromium is approved for use in connection with foods. After chroming, the metal layer 13 is hard from the outset, but a surface may also be made hard after metalisation, for example aluminium coating which is treated with the TuframB method, which also gives a surface approved for use in connection with foods.

Fig. 4 shows a tribological element 1 according to the present invention, applied as a movable seal in a sealing construction between two machine parts 3 and 14. The tribological element 1 constitutes the seal lip in a U-seal of high plasticity, which seals against, for example, a shaft 3 which may either reciprocate or rotate. The U-seal 1 consists substantially of the body 12 which has a layer 13 of a metal or a ceramic material on that surface which constitutes the wear surface 11 of the element 1 turned to face towards the counter surface 2 on the machine part 3. This application may be a part of a piston pump where the seal 1 seals against a shaft 13 constituting the piston rod of the piston pump.

Figs. 5a and b show two different applications in which a tribological element 1 according to the present invention is employed as a radial bearing for a shaft 3. The radial bearing 1 with its metal layer 13 facing towards the counter surface 2 may be placed as an internal element 1 (Fig. 5a) or as an external element 1 (Fig. 5b).

As will have been apparent from the foregoing description, a design and construction according to the present invention will realise a tribological element 1 which, with its metal layer 13, gives good wear resistance, and its body 12 of optional natural or synthetic polymer which may be integrated in a larger part with several functions.

A tribological element 1 according to the present invention may be employed within the food industry, with its particular requirements where sliding bearings and movable seals must be washable and, on occasion also capable of being sterilized.

The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being possible without departing from the scope of the appended Claims.