Field of the invention [0001] The present invention is related to machine parts, and in particular to technical solutions, which relate to construction of machines and devices in respect of lubrication.

State of the art [0002] Biologically decomposable .• lubricants are natural oils of plant sources, e.g. cucurbitaceous oil or sunflower oil, or synthetic lubricants like e.g. saturated or non-saturated esters. The most useful seem to be natural lubricants, which may be obtained at low cost from regenerative sources and which moreover excel in satisfactory lubricating characteristics, which can be better than the corresponding characteristics of synthetic or mineral oils. The main obstacle for using such lubricants is the low oxidation stability thereof. When the temperature is increased (typically above 750C, at 900C practically unusable for long-term operation) , a rapid degradation occurs which leads to quality loss, said lubricants not meeting the application requirements anymore. [0003] Currently, machines need to meet high requirements during operation, so that usually such lubricants can not be used. On the other hand, the ecological aspects also become more and more important, and in certain countries the legislation already requires that use of at least a prescribed portion of biologically decomposable lubricants is recommended or compulsory. In such circumstances the users of such lubricants must be aware of a relatively short operation period, since the lubricant need to be exchanged after 1.000 - 2.000 hours of operation due to degradation. The main reasons for degradation are operation temperature, since natural lubricants are unable to operate above 90°C, and a short- time operation period, which results in direct (more lubricant is needed) and indirect expenses (decrease of the operation capacity due to exchange of lubricant) . It is possible to extend the operation period under normal operation conditions, e.g. up to approximately 3.000 hours, by using synthetic biologically decomposable esters with better oxidation stability. However, such lubricants are very expensive (up to 10 times more expensive) , while their lubricating characteristics are worse. [0004] Generally speaking, when the oxidation stability of the lubricant is better, the lubricant is able to operate at higher temperature, but is more expensive and has worse lubrication characteristics. This in particularly relates to saturated esters (up to 1100C) , while non- saturated esters (up to 900C) are ranged between natural oils and saturated esters. [0005] Some other influences to the tribologic system have to be taken into account, like e.g. increase of friction and wear, which normally will lead to additional increasing of temperature. It is therefore obvious that any solution, which would enable use of biologically decomposable lubricants at higher temperature, e.g. 80 - 900C (which is the operation temperature of the majority of gears and other construction parts connected herewith, like e.g. bearings, guides or various hydraulic components) , could reduce the costs of lubrication including the costs related to exchange of the lubricant (maintenance) . Such solution could also accelerate the use of regenerative and environment-friendly sources, and in addition, the technical and tribologic characteristics may be improved. [0006] Use of biologically decomposable lubricants is also a measure to prevent environmental pollution. In CA 2223326 it has been suggested, that the lubricant should be an oil of plant sources on the basis of fatty acids, in which additives like hydroxy fatty acids and fluid waxes of plant sources could be used. Such lubricant is then decomposable without any serious obstacles by means of bacteria. However use thereof is very restricted, since the durability of the lubricant is too short to enable normal operation of machines and appliances. [0007] During operation of corresponding machine parts under so-called boundary friction conditions, said parts are at least temporarily exposed to direct mutual contact resulting in a relatively high pressure between each co-operating surfaces, generating a relatively high quantity of heat. This leads to oxidation of the lubricant, and consequently, to a decrease of the lubricating capability of the lubricant. [0008] It is furthermore known that friction between sliding surfaces may be reduced, when metallic surfaces are provided with layers consisting of hard diamond-like coatings on the basis of carbon, more in particular so- called DLC-coatings (diamond-like carbon) . Such coatings are especially useful in dry friction conditions, like e.g. in computer memory disc bearings. In addition, such DLC coatings have been used (e.g. KR 2001110609 A) for heavy- duty mechanical or hydraulic appliances either to enhance the efficiency or to minimise dimensions of the complete appliance due to reduction of dimensions of certain parts thereof. However, in such a case the tribologic requirements are extremely high, so that even the most efficient mineral lubricants can not ensure sufficiently efficient lubricating without taking further measures. Also, each exchange of the lubricant results in a certain quantity of waste lubricants, which then must be transformed by means of expensive processing into environmentally friendly substances in order to avoid environmental pollution. [0009] In addition, such DLC-coatings were also applied in certain appliances like e.g. refrigeration systems (US 5,947,710) , where due to presence of certain substances normal lubrication conditions between surfaces by means of classic mineral lubricants is impossible.

Aims of the invention [0010] The main aim of the present invention is therefore to develop an improved tribologic system, which could be applied in various mechanical assemblies of machines and appliances, in particular for lubricating of gears, bearings, guides, shafts and also hydraulic components, and should be based on combining appropriate materials, which are brought in contact, with biologically decomposable lubricants and additives, in particular lubricants on the basis of oils of plant sources or also on the basis of biologically decomposable saturated or non- saturated esters. Such tribologic system should be environmentally friendly or should at least have a reduced impact to the environment, when compared to the existing tribologic systems.

Summary of the invention [0011] The present invention aims to provide a solution to the problem how to modify each corresponding mutually co-operating machine part, in particular those surfaces of such parts, which are exposed to friction and wear, that even in such loading conditions, in which previously for the purposes of decreasing of friction coefficient and preventing of wear various mineral or synthetic lubricants had to be used, a sufficiently efficient lubrication could also be achieved by means of biologically decomposable lubricants. In particular oils of plant sources and/or saturated and/or non-saturated esters, without reduction of efficiency and reliability of the lubrication as such are preferred.

Short description of the drawings [0012] Fig. 1 represents the oil temperature as a function of FZG stages for all the selected material combinations. [0013] Fig. 2 represents the degree of oxidation of the oil (TAN) in function of testing time.

Detailed description of the invention [0014] For the purposes of this application, the term "assembly of mutually co-operating machine parts" relates to any assembly of at least two mutually co¬ operating machine parts, wherein one of said machine parts is loaded in appropriate manner and at least a temporary abutment along at least one contact surface of another machine part can exist, either by means of sliding, rolling or in any other manner. [0015] Furthermore, the term "biologically decomposable lubricant" means a liquid or a solid, gelatinous lubricant of animal or plant sources, i.e. of natural or biological sources, which is biologically decomposable or degradable. The term also includes biologically decomposable saturated or non-saturated natural or synthetic esters, and excludes both mineral lubricants and synthetic lubricants, which are not decomposable by means of natural processes i.e. by means of micro-organisms, which may be normally found in nature. Preferable biologically decomposable oils are oils of plant sources, e.g. cucurbitaceous oil or sunflower oil or any other similar oil, including waste oils, which were already used e.g. in preparing food. Standard criteria and methods are available to determine biodegradability of a compound. [0016] The present invention proposes the application of a modified hard coating on the basis of diamond-like coatings (DLC) together with a suitable biologically decomposable lubricant, which is either an oil of plant sources or a saturated or non-saturated synthetic ester, or also a combination of these substances, optionally with additives which are preferably biologically decomposable, and which are advantageously not toxic and have no negative impact with respect to the biologic decomposition of the lubricant. Such tribologic system is useful in various circumstances and in various fields of industry, namely for lubricating machine parts like gears, bearings and shafts, or any other construction elements like guides or seats, or for various parts of hydraulic systems like pumps, valves, pistons, for food industry or pharmacy, where when using classic mineral or an other non- decomposable lubricants an environmental pollution might occur during maintenance or exchange of the lubricant. Additives that have a toxic effect and/or are not completely biodegradable can of course be used, provided that the toxic effects and the rest fraction of decomposition are permissibly small. [0017] The invention generally relates to an assembly of mutually co-operating machine parts, which generally consists of at least two parts, one of them being exposed to translational or rotational movement in respect to the other part, and the surfaces thereof, which are faced one towards another, are intended for at least temporary abutment to each other, while between the contact surfaces of said mutually co-operating parts there is a lubricating layer. The previously exposed technical problem is solved according to the invention in such a manner, that at least one of the said parts is equipped with a DLC- coating on its contact surface and that the lubricant is a biologically decomposable lubricant either with included additives on the basis of biologically acceptable substances or without the said additives. [0018] In a preferred embodiment, each contact surface is equipped with a DLC-coating, and the biologically decomposable lubricant is a lubricant of plant sources, a biologically decomposable saturated ester or a mixture of such esters, a biologically decomposable non- saturated ester or a mixture of such non-saturated esters or any combination of these biologically decomposable substances, and optionally comprising other non-toxic i.e. biologically acceptable additives. [0019] A further object of the invention is the use of a biologically decomposable lubricant of plant sources, a biologically decomposable saturated ester or a mixture of such esters, a biologically decomposable non-saturated ester or a mixture of such non-saturated esters or any combination of such biologically decomposable substances, as a lubricant intended for lubrication of machine parts, in which at least one of the contact surfaces of at least one part is provided with a DLC-coating, and preferably, that all the surfaces, which are brought in mutual contact, are provided with DLC-coatings . The temperature in such a modified tribologic system may be decreased compared to a system without DLC coatings under the same operation conditions. [0020] When using the DLC coatings in combination with oil, the reduced friction between/among the contact surfaces results in lower temperature, so that oxidation is also reduced in boundary-friction conditions. Generation of heat is especially critical in boundary-friction conditions, since during the operation the contact surfaces are occasionally brought into direct contact. The lubricant is necessary either due to insufficient relative velocity between the said surfaces or to overheating and consequently reduced viscosity, which makes it impossible for the lubricant to keep said surfaces separate, i.e. to maintain them apart from another. In such circumstances the conditions correspond to friction without full film lubrication, since the lubricating layer is absent, and the contact surfaces are brought into a direct mutual contact. Such circumstances can occur at any time when starting or breaking off operation of the machine parts, as well as when experiencing shocks or vibrations. Machines with reciprocal oscillating movement are periodically exposed to such conditions when starting or finishing of every half- cycle. In such exposed portions of the surfaces, which are brought into a direct contact and where the full lubrication layer is then absent, i.e. on the peaks of each real surface, a tangential force is generated, which hinders movement and represents a portion of the overall friction. [0021] In order to avoid the said phenomena, anti- wear additives have been used in the past, which should prevent establishing a direct mutual contact between two surfaces by means of forming a protective layer on the basis of an appropriate chemical reaction. Such layer is then promptly detached from the surface. However, heat generation remains relatively high, so that any long-term use of biologically decomposable lubricants in such circumstances is impossible. Accordingly, even by using any known additive the said problem cannot be solved in appropriate manner. It is also already known, that the said DLC-coatings, when used in dry lubrication conditions, in the DLC/DLC or DLC/steel contact may show reduced friction when compared with the steel/steel contact. The differences in the friction coefficient may be e.g. from 10% to up to several times, depending on operation conditions and circumferential media. Various analyses have been performed on the basis of models by using various DLC coatings, various oils and various additives. Further testing was executed by means of gears and by using the so-called FZG testing machine. Four combinations of gears and counter- gears were tested, namely steel/steel, steel/DLC, DLC/steel and DLC/DLC. The obtained results are represented by means of a diagram (Fig.l) . Results of measuring the oil in the oil bath, refer to all four combinations. The temperature was measured within the oil bath (reservoir), i.e. at a sufficient distance from the locations on the friction surfaces, where the heat was generated, which shows, that amounts of generated heat and differences between particular materials, which have been brought in contact, are greater. An intensive degradation of oil normally occurs in the area of absence of the lubrication layer, where the temperature increase is maximal . The achieved temperature difference in the said reservoir between the combinations DLC/DLC and steel/steel was approximately 100C, which however in accordance with the knowledge available in the prior art actually represents a real basis for minimising oxidation and correspondingly longer operation period of the lubricant. A further experiment was executed by means of another testing machine and included two systems of axial piston pumps, exemplifying the present invention in a hydraulic system. Testing was performed in parallel by using on the one hand a conventional system steel/steel, and on the other hand a DLC-coating, which was applied on the surfaces of the piston, that slide along the pressing plate. For the purpose of this test, a biologically decomposable oil was used, which was prepared by the applicants. Duration of testing was 2.000 hours, which is in practice the equivalent of 12-months operation of 8 hours daily. Upon measuring of the general acid ratio (the so-called TAN-number; Fig. 2) , which represents a degree of oxidation of the oil, it has been found, that thanks to reduced temperature the oxidation degree of the oil in a system, in which the DLC-coatings have been used, was approximately 50% lower than in a conventional system. As presented above, invention has been exemplified by means of two different systems, namely by means of an axial pump, which is a hydraulic tribologic system, and also by means of a gearbox, which is as a mechanical tribologic system. Contact pressures in a mechanical system are normally higher than in a hydraulic system.