Abstract:
When bearing layers are producing by thermal coating the coating flame causes oxidation of a part of the coating material. To reduce this oxide formation and especially to lower the formation of oxide clusters in the layer a gas stream is projected next to the burner flame with an oxygen content lower than that of air. Preferably a gas with nitrogen ≧99% is used for the gas stream and as cooling gas for the burner. The method disclosed provides a bearing layer for a connecting rod eye. The large connecting rod eye is subjected to pre-spindle trimming, cracking and further spindle trimming to the coating tolerance, sandblasted and plasma coated with an aluminum bronze to produce a microporous layer. At the start of plasma spraying the plasma layer is deposited at a high temperature, which results in good adhesion of the plasma layer to the material of the connecting rod eye and low porosity of the plasma layer. To prevent annealing of the material of the connecting rod eye, the deposit temperature of the plasma layer is reduced during deposition of material which is removed by subsequent machining of the bearing layer. After plasma coating the connecting rod eye is opened by removing the bearing cap, which ruptures the bearing layer. The cap is then remounted and the final bearing surface is formed by fine spindle trimming. To improve the oil retention volume the bearing layer has an annular groove.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP98/04498 filed Jul. 20, 1998, and a continuation-in-part of U.S. application Ser. No. 09/033,874, filed Mar. 3, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to methods for thermal coating of bearing layers and the like. 
     In thermal coating, more particularly in plasma coating, the coating material, such as a metal in the form of powder or rods, is fed to a flame in which it is melted and is then deposited on a substrate. In this process, oxidation of a portion of the coating material can ocurr so that the oxides are incorporated in the coating. The presence of such oxides influences the microhardness of the coating and also produces a porosity in the surface of the coating. This porosity can sometimes be desirable, for example for holding an oil film in a bearing layer or on a bearing surface, but, on the other hand, the porosity can result in insufficient stability of the coating. The porosity can be influenced by the choice of coating material and also by a particle size or particle morphology. The formation of oxides is usually a result of the oxygen content of the ambient air. Only in work under vacuum is the oxygen pumped out so that work can be performed in a controlled atmosphere. As a result, oxide inclusions are eliminated to a very large extent during plasma coating under vacuum. 
     During the coating of interior surfaces, for example cylinder faces and in particular connecting rod eyes, however, the opportunities for process variation are severely limited since the interior surfaces generally are quite cramped, so that process options are relatively limited. A number of additional problems arise in the manufacture of connecting rods in particular. Working under vacuum is not appropriate for connecting rod processing, not least because of the excessively high cost of this process. 
     Conventional connecting rods now in use, especially for internal combustion engines, are so-called cut or cracked connecting rods in which the large connecting rod eye that surrounds the crankshaft is cut or cracked to open it. As a rule, the small connecting rod eye does not need to be opened since it is connected to the piston by a straight bolt. 
     Depending upon the load applied to the bearing, connecting rod eyes are made with a variety of bearing shells providing the friction surface. In particular, supporting shell materials used in bearing shells are as a rule made of C 10 steel according to DIN 17210 or SAE 1010. Depending upon the particular design and application, the bearing shells may be cold hardened. The actual bearing surface layer, which may, for example, be white metal, leaded bronze, light metal, spatter coatings or the like depending upon the expected bearing load, may be applied to the supporting shell material. The bearing shells may be three-component, two-component or solid single component bearing shells. The shells are assembled to the connecting rod eye with an initial stress so that the bearing shells have a satisfactory, firm seat upon assembly. 
     Bearing shells not only constitute a substantial cost factor, but also complicate production and are a potential source of error. For example, the insertion of a bearing shell or bearing shell half may be overlooked in assembly, resulting in considerable engine damage. 
     During coating to produce a bearing in a connecting rod eye, which usually has a diameter in the range of several millimeters to a few centimeters, e.g., &lt;8 cm and more particularly &lt;6 cm, thermal coating is subject to the problem that, depending on the material used, oxides are formed to a relatively large oxide extent so that the overall porosity of a bearing layer can easily be above 3% which can impair the stability of the bearing layer. However, a low percentage of pores in the bearing surface is desirable because the pores provide an oil retention volume. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method for thermal coating of bearing layers which overcomes disadvantages of the prior art. 
     Another object of the invention is to provide a thermal coating method in which oxide formation can be reduced without requiring a controlled atmosphere and which is suitable for interior coating, especially extended cylindrical surfaces and connecting rod eyes. 
     These and other objects of the invention are attained by applying a thermal coating using a flame which is surrounded by a nonflammable gas stream having an oxygen content of less than 18% by volume, preferably ≧10% by volume, desirably ≧5% by volume. 
     In thermal coating of an interior surface in accordance with the invention, a burner produces a plasma flame which is moved over the interior surface to be coated by rotation of the burner. It is also advantageous if the interior surface to be coated has a rotationally symmetrical configuration. The plasma flame is formed by an electrically ignited arc fed with the plasma gas, preferably argon or a mixture of argon, helium, nitrogen and hydrogen, producing a combustion temperature which can in particular exceed 10,000° C., for example 15,000° C. to 30,000° C. The coating material is introduced into the flame, for example by a powder feed or a wire or rod. The coating material is heated in this process, accelerated to speeds such as 400 to 600 m/s, and deposited on the interior surface to form a coating. Depending on the operating conditions and the particular coating material, a partial oxidation of the coating material occurs. The plasma flame is flanked by a nonflammable gas stream in such a way that the gas stream preferably steers or directs the flame. This is particularly advantageous when the flame is moved relatively rapidly, as occurs with rotating flame coating, for example. Typical rotational speeds are in the range of 10-500 RPM, and especially 50-300 RPM. The gas stream can simultaneously serve to cool the burner tip. In accordance with the invention, instead of the usual practice of using air, a gas stream having a reduced oxygen content of less than 18% is used. The oxygen content can be as low as zero, in which case nitrogen, but also noble gases such as argon, can advantageously be used for the gas stream. In principle, an extremely wide variety of inert gases is usable. Reducing the oxygen content results in a reduced inclusion of oxides of the coating material in the coating, which simultaneously reduces the overall porosity of the coating. 
     The process is used to special advantage for coating connecting rod eyes. 
     Thus, according to the invention, a bearing shell is no longer inserted into a connecting rod eye, either the large eye, or alternatively into both connecting rod eyes. Instead, a bearing layer is applied directly to the connecting rod eye by thermal spraying such as by plasma spraying. Specifically, the connecting rods made in this way are used in an internal combustion engine to connect the crankshaft to the pistons. To increase the adhesive pull strength of the bearing layer, the connecting rod eye to be coated can be roughened preferably by abrasive sandblasting to be carried with sands of successively different particle-size distributions. To increase the quantity of residual oil on the bearing layer, the layer can have a groove and/or is microporous. 
     In accordance with the present invention, machining of the connecting rod bore is advantageously carried out prior to application of the bearing layer by machining the eye to an actual dimension which lies within the tolerance range of a nominal eye dimension. Machining in this context is done in particular by cutting processes, for example by spindle boring. It is especially advantageous when this machining is done in the manufacturing process in such a way that the eye of the connecting rod has a size which is still within the tolerance range of the nominal size when the bearing material is sprayed on. This means that no process step which changes the actual size of the connecting rod bore to a size outside the tolerance range occurs after machining and before thermal spraying of the coating. Process steps which can still take place between machining and spraying of the bearing material include, for example, roughening of the bore surface, broaching of the connecting rod cheeks, etc. 
     It is particularly advantageous for the connecting rod eye to be opened by fracturing, in which the process steps of notching and breaking are customarily used. As already mentioned, the machining in accordance with the invention takes place after the fracturing process. Alternatively, the machining of the connecting rod eye surface can take place before the fracturing process or opening of the connecting rod eye but it is especially advantageous if machining also takes place after opening of the eye. 
     Machining of the connecting rod eye prior to opening, in particular by fracturing, has the advantage that a clean fracture can be produced, which results in only minor deformation so that the connecting rod eye can in certain circumstances be coated without additional machining. Machining after opening of the connecting rod eye is especially advantageous in that the bearing layer can be sprayed onto a connecting rod eye with a diameter having a known, tight tolerance so that a very thin coating is possible. To this end, it is advantageous to machine to a tolerance of 120 μm, preferably 75 μm, and more desirably 50 μm. Thus, it is possible in accordance with the invention to apply the layer of bearing material with an average thickness prior to subsequent machining of 100 μm to 600 μm, more preferably 150 μm to 400 μm, while layer thicknesses in the lower half of the specified ranges are possible in an established production process. These values are approximately 50-200 μm below the layer thicknesses that would be necessary without the step in accordance with the invention of machining the connecting rod bore diameter. 
     In the method according to the invention, the applied bearing layer can be reduced to its working thickness by removing only a small amount of material, so that the typical result is an average removal of 100 μm to 400 μm, and desirably 150 μm to 250 μm. 
     Accordingly, bearing layers with an average working thickness of 30 μm to 200 μm and more particularly 60 μm to 150 μm after additional machining of the bearing layer can advantageously be produced with the method according to the invention. Since the bearing layer contributes little to the strength of the connecting rod eye, the method according to the invention which produces an especially thin bearing layer has, in addition to the above-mentioned cost advantage, the advantage that larger cross-sectional thickness of the connecting rod eye material than was previously attainable for the same outside dimensions of the connecting rod eye material. In the region of the threaded joint for the connecting rod cap, for example, it is possible to provide an overall increase in thickness of approximately 2.4 mm on each side, which is more than with any conventional plain bearing. 
     In the method according to the invention, the following steps are used with particular advantage: 
     (a) preliminary spindle boring of the connecting rod eye in the connecting rod blank; 
     (b) cracking of the connecting rod eye by notching and breaking; 
     (c) subsequent spindle boring of the reassembled connecting rod eye to the coating dimension; 
     (d) plasma coating of the connecting rod eye; and 
     (e) finish spindle boring of the plasma coating to the bearing surface dimension. 
     This combination of manufacturing steps achieves optimum strength of the connecting rod eye with extremely low manufacturing costs. The method according to the invention is in principle applicable to cut connecting rods as well. However, the cutting of the connecting rod results in a cost disadvantage in comparison with cracking. 
     It is advantageous for the bearing material to be deposited on the substrate at different temperatures during the course of the thermal spraying, more particularly plasma spraying. The different temperatures can be achieved by a wide variety of parameter variations in which one or more parameters can be changed. The parameters to be changed in particular in order to achieve a temperature increase are an increase in the current or voltage at the burner, a reduction in the supply of cooling gas or carrier gas, an increase in the supply of fuel gas such as hydrogen, for example, a reduction in the mass flow of the bearing material to be supplied, a reduction in the particle size of the bearing material, or a change in the composition of the bearing material. To reduce the deposition temperature one or more of those parameters can be changed in the opposite direction. 
     It is especially advantageous to reduce the application temperature of the bearing material during the course of thermal spraying. The application temperature can in principle be increased again at a later time, but it is preferable for it to remain reduced. If the surface of the thermally sprayed bearing layer is to be subsequently removed, it is especially advantageous when the part of the bearing layer that is to be removed is sprayed at a low temperature, in which case it is desirable for the removal to extend into a region of the bearing layer that was deposited at a higher temperature. With this method it is especially advantageous that the temperature can be reduced during deposition to such a degree that the region of the bearing layer that is to be removed contains a relatively low-grade portion of the layer which is removed by subsequent machining of the bearing layer. 
     In the method according to the invention, the initial temperature at which the bearing material is applied to the connecting rod eye can be raised so that the processing temperature is high enough that it would cause annealing of the connecting rod material if all of the necessary bearing layer material were applied at that temperature. In this way, a particularly high bonding strength of the bearing layer to the substrate of the connecting rod eye and a desirable reduced porosity in the bearing layer can be achieved. 
     The various particle-size distribution curves that are advantageously used according to the invention are also known as mesh size classes, where according to the invention the classes from 16 mesh to 230 mesh i.e., 16, 18, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 100, 120, 140, 170, 200 and 230 in particular are used. The mesh count specifies the number of holes per square inch, i.e., per 6.45 cm 2  in the screen used. For example, the individual particle-size distribution curves, including tolerances and sieve wire sizes, can be obtained from the Handbook of Chemistry and Physics, 64th Edition, 1983/1984, CRC Press Inc., Florida, p. F-114. 
     It has been determined by thermally coating a connecting rod eye that the usable bearing layers, which are made of aluminum bronze for example, have a high residual layer stress which in particular is increased by the fact that the material is deposited with a low porosity. Consequently, it is clear that measures to increase the bonding strength of the bearing layer are necessary. This is advantageously achieved in accordance with the invention in that, in addition to a high initial bearing layer application temperature, the connecting rod eye is sandblasted with successively different grain sizes including at least one fine and at least one coarse grain size distribution. 
     Operationally reliable connecting rod bearings require a wear-resistant design and construction in order to transmit the bearing forces reliably and at permissible operating temperatures. Wear resistance is always provided when the sliding surfaces are separated from each other by a lubricating film such as an oil film that is capable of bearing a load. Such a lubricating film is maintained in friction bearings by a slightly eccentric shaft mounting. With this arrangement, the rotating shaft has a pumping effect that feeds the lubricant such as motor oil into the eccentric bearing slot, and oil pressure is built up at the convergent surfaces of the bearing slot. In other words, the lubricant is pressed into the narrowest cross section of the space between the bearing surfaces. This results in a condition called “interfacial lubrication” in which there is interfacial friction of the bearing material on the journal when the rotary motion of the shaft begins or is very slow. An increase in rotation speed causes the oil film to assist in supporting the journal even though a coherent oil film has not yet been built up. This is a condition called “mixed friction,” i.e., simultaneous interfacial and floating friction. This condition exists chiefly when an engine is being started and stopped. An additional increase in the speed of rotation causes the development of the hydrodynamically supporting lubricating film layer having a thickness equal to half of the bearing clearance resulting in a condition called “floating friction.” With floating friction the bearing clearance is usually equal to about 15 μm to 60 μm. 
     When the surface of a connecting rod eye such as the large eye is plasma coated with an appropriate bearing material having micropores and/or a groove according to the invention, a high oil retention volume is obtained within the connecting rod bearing. As a result, friction and hence wear of the parts moving against each other, in particular during interfacial lubrication and mixed friction, are reduced. The oil retention volume is distinctly improved by the provision of at least one groove in the connecting rod bearing surface, the groove or grooves being preferably in the form of circumferential grooves. For an additional increase in oil retention capacity, the grooves are left at least largely unfinished, i.e., with their raw surface structure as produced. 
     In every bearing condition, regardless of the rotation speed and oil pressure, the microporous but pressure-stable structure of the sliding bearing surface, which preferably is machined, and the unfinished groove having a very rough surface structure cause a certain portion of oil to be stored in the bearing. This makes it possible for the bearing to pass through the conditions of interfacial and mixed friction more quickly even at a low crankshaft speed and hence reach the condition of nearly wear-free hydrodynamic lubrication rapidly. In other words, engine bearing performance characteristics during starting and slow-down of the engine are substantially improved so that higher bearing loads are possible with bearings having the same dimensions. 
     The invention has the following advantages: The bearing shells which were formerly customary in connecting rods to provide the sliding bearing surface are eliminated since, according to the invention, the bearing layer is applied directly to the connecting rod eye surface and not to an extra element inserted in the connecting rod eye. Hence, the necessity for assembly of bearing shells in the eye is also eliminated. Elimination of the bearing shells and of machining of the sliding bearing shell surface according to the invention results in a reduction of dimensional tolerance variations. There are three tolerances in conventional connecting rod bearings, the first tolerance being that of the crankshaft dimensions, while the second tolerance is determined by the dimensions of the bearing shells providing the sliding layer in the connecting rod and the third tolerance is determined by the dimensions of the connecting rod eye into which the bearing shell is inserted. As a result of coating and finishing of the bearing layer directly on the connecting rod eye surface according to the invention, the third tolerance is eliminated. In addition, according to the invention a greater thickness of connecting rod material is provided in the bolt region surrounding the eye since the bearing layer applied according to the invention is thinner than a bearing shell. This permits higher loads to be applied to the bearing of a connecting rod having the same external dimensions. The dimension of the third tolerance, i.e., the dimension of the connecting rod eye, may be sized very roughly in the present invention, since the eye is covered by the subsequent coating which is partially removed during formation of the sliding surface by, for example, fine spindling, to provide the second tolerance dimension. 
     In accordance with the invention, the bearing layer is advantageously sprayed on in such a manner that, after any necessary removal of bearing layer material, the layer has a certain desired porosity, at least at the surface. This porosity is achieved by forming the bearing layer with oxide inclusions which are removed during additional machining of the layer surface to form free micropores. 
     The porosity of the bearing surface area is preferably between about 0.2% and about 7% and desirably between about 0.5% and about 4%. In addition, the micropores are preferably not interconnected, so that the pore volume of the micropores is constituted predominantly by closed pores. These micropores, which form a hydrodynamic micropressure chamber lubricating system, are opened up by, for example, the cutting operation of surface trimming, for example fine spindling of the bearing surface. As a result, the pores in the sliding bearing surface act as oil retention chambers so that, during startup or slowdown of the engine and at the beginning or termination of rotation of the crankshaft, a sufficient volume of oil is still available, presumably by adhesion of the motor oil in the micropores, for sliding film formation, i.e., floating friction, even though the oil pressure in the friction bearing has dropped or has not yet been built up. The duration of mixed friction operation during startup and slowdown is substantially shortened by the micropressure chamber system provided by the invention. The emergency operating characteristics of the bearing are also substantially improved without requiring the introduction of additional sliding bearing materials such as lead, tin or nickel alloys or the like. Advantageously, both the total pore area and the pore volume are determined during thermal spraying of the bearing materials according to the bearing load to be applied to the sliding surface. The major portion of the pore volume is constituted by pores of a size preferably in the range from 0.2 μm to 250 μm, desirably in a range from 1 μm to 50 μm. 
     The usual sequence of steps for the production of a bearing layer according to the invention is as follows: First the surface of the substrate to be coated, for example the large connecting rod eye, is cleaned, and especially freed from grease. This is carried out by applying superheated steam, for example. Then, the surface of the substrate is sandblasted, for example with Al 2 O 3  powder. SiO 2  and SiC are also suitable. The discharge pressure is preferably about 3 to 8 bar and desirably about 4 to 6 bar, and the sandblasting is preferably carried out with successively different Al 2 O 3  grain size ranges. In this connection, an increasing grain size is advantageously used, i.e., sandblasting is done first with a finer grain and subsequently with a coarser grain. Most preferably, at least three different grain size ranges are used. In this case, the grain sizes usually used are in a 15 to 250 mesh range, with 80 mesh and smaller, preferably 100 mesh to 230 mesh for the fine grain. For the medium-size grain, a size of 100 mesh or greater, preferably up to 40 mesh and desirably 80 mesh to 45 mesh, is used. For the coarse grain, a grain size of 45 mesh or greater and in particular 30 mesh or greater, preferably up to 16 mesh, i.e., 600 μm to 1.18 mm standard screen size, is used. 
     Using grains of different size ranges in sandblasting results in good surface roughness, even in the region of a notch, for example a breaking notch or a groove, producing a surface structure with an average peal-to-valley height R a  of approx. 5 μm to 10 μm and in particular 6.5 μm to 8 μm in the smooth region, with an R z  of approx. 35 μm to 60 μm and in particular 42 μm to 54 μm. Such roughnesses produced by sandblasting result in especially good adhesive strength of the thermally sprayed layer on the connecting rod. 
     Sandblasting is followed by plasma coating, for example using an AlCuFe (aluminum bronze) alloy powder. The thermally sprayed bearing layer preferably is produced with progressively increasing porosity, a layer having low porosity, for example, ≦2%, in particular ≦1%, being produced first. Alternatively, or in addition, other temperature reducing procedures can be used during thermal spraying. For the first layer, a powder grain size of 38 μm (400 mesh), for example, is suitable. This first layer is produced with a thickness of approx. 100 μm to 300 μm, in particular 200 μm to 250 μm. Then a second layer having a porosity of approx. 2% to 6%, in particular 2.5% to 4%, is produced on the first layer by using a powder grain size of approx. 63 μm (230 mesh), for example. In this case, the powder grain size is chosen so that at least 40% by weight, preferably at least 50% by weight, of the powder is smaller than or equal to the specified screen size, and preferably at least 70% by weight, and desirably at least 80%, by weight falls within the next standard screen size, and preferably at least 90% by weight falls within twice the specified screen size, i.e., half the mesh size. 
     The layer thickness is preferably in the range from 150 μm to 800 μm, desirably in the range from 200 μm to 500 μm. When only one layer is formed, it is preferably in the range from 100 μm to 600 μm, desirably in the range from 200 μm to 400 μm, thick. 
     It has been shown in accordance with the invention that the residual layer stress increases with decreasing porosity, giving rise to the danger of separation of the layer from the connecting rod eye. The present invention overcomes this in particular by special sandblasting steps, which produce an especially high bonding strength of the layer on the connecting rod eye. In addition, separation of the layer resulting from excessive residual layer stress is prevented in accordance with the invention by increasing the deposition temperature of the layer. This can be produced, for example, by increasing the voltage or current to a plasma burner. In this way, the layer is distributed better on the surface of the connecting rod eye, and better adhesion is achieved. This happens advantageously only to the extent that the applied layer has a certain small porosity as is described above. On the other hand, increasing the application temperature also brings with it the risk of annealing the substrate, in other words the substrate can be overheated. This is particularly problematic with ferrous materials. This problem in turn is counteracted according to the invention by depositing only a lower layer at the high application temperature. Further layers are then deposited at lower temperatures, and also with a higher porosity. If such higher porosity is unsuitable for the desired bearing material that does not present a problem if this more porous layer is removed by subsequent machining according to the invention. In this case, the more porous layer or the layer applied at lower temperatures merely serves to build up the substrate for subsequent mechanical machining. 
     Application of the bearing layers preferably is effected in one operation, i.e., the coating sequence is not interrupted. To accomplish this, automatic powder and/or parameter adaptation is effected in order to obtain different porosities in the layers. 
     The lower bearing layer preferably covers the out-of-roundness which is typically about 30 μm to 150 μm that is produced for example by the cracking of the connecting rod upon breaking open of the bearing shell, and, to obtain a sufficient thickness for later finishing, the lower layer may be covered with one or more additional and, particularly, more porous, layers that can be applied to the bearing material of the connecting rod in a similar fashion. The upper layer or layers in turn can be removed without any major problem all the way down to the first slightly porous layer to produce the actual sliding bearing surface. This procedure results in an adjustment of the internal stress of the bearing layer, slight annealing of the connecting rod and high adhesive strengths of the sliding bearing layer, which commonly are ≧20 N/mm 2  and particular ≧25 N/mm 2 . Adhesive strengths above 28 N/mm 2  are possible with this procedure. A layer hardness of, for example, approx. 185 HV 0.3  can be obtained using aluminum bronze for the bearing layer. 
     For more highly loaded sliding bearing layers in the connecting rod, particularly the large connecting rod eye, as are found especially in diesel engines, it is advantageous to increase the residual oil volume within the sliding bearing layer. This is effected according to the invention by increasing the oil retention volume at engine standstill or at an engine speed below idling speed by the providing one or more oil grooves in the sliding bearing layer. In this way, the region of mixed friction is traversed in a shorter time upon engine startup or slowdown. Such shorter time is a result of longer maintenance or more rapid formation of the sliding film, which results from adhesion of the motor oil in the micropores and the grooves or grooves in the surface facing the crankshaft. The introduction of at least one groove in the bearing layer, in particular in a bearing with a somewhat porous surface, results in the lubricating film being maintained for a longer time even though the oil pressure of the oil pump has already dropped or is just being built up. The effect of grooves is especially advantageous when the grooves extend predominantly circumferentially, i.e., completely around the inner bearing surface. In this way each groove is self-contained, i.e., is an endless groove and, in particular, an annular endless groove. The groove advantageously is V-shaped or trapezoidal in cross section, the sides of the groove extending at an angle to each other of preferably 30° to 80° and desirably 45° to 60°. A groove depth in the range of 0.2 mm to 1 mm and preferably 0.4 mm to 0.6 mm is suitable. The number of grooves depends mainly on the width of the friction bearing, the supporting portion of the bearing, and the required additional oil retention volume. The specified groove shape and depth preferably refer to the groove dimensions before application of a thermally sprayed coating. Plasma spraying is desirably used for applying the thermally sprayed coating. 
     Preferably after the formation of one or more grooves in the surface of the eye, the surface is coated with the bearing layer, in particular with the composite bearing layer having variable porosity which is described above. In subsequent finishing of the bearing surface the grooves preferably are finished only slightly or not at all since the rough plasma-applied layer structure produces especially good oil retention in the grooves. 
     In the application of the bearing layer, a metal powder is thermally sprayed and metal alloys are preferably used. In addition, a mixture of a variety of metals and preferably metal alloys is used. Such a mix may, for example, be a mixture of aluminum and tin, the mixture advantageously being obtained by mixing of the individual metal components in powder form. 
     The selection of the metal powder to be thermally sprayed depends, among other things, on the specific spray parameters and can easily be determined by a person skilled in the art by a series of tests. Advantageously bronzes, especially aluminum bronze (aluminum/tin), but alternatively copper bronze as well as metal-soft material layers and/or metal-solid lubricant layers, are employed as bearing materials. The soft materials used are, for example, soft metals, such as lead, which are distributed in a harder metal, such as an alloyed aluminum, for example, aluminum/copper/magnesium/chromium. However, other soft materials, such as, for example, fluorocarbon polymers, e.g., polytetrafluorethylene, may alternatively be used. Compounds such as molybdenum disulfide, boron nitrite and graphite, for example, are suitable as solid lubricants. 
     The bearing material is preferably sprayed in excess to produce a layer thicker than the desired thickness which is then mechanically finished. Honing is used for finishing, fine spindling in particular being suitable. In honing, 20 μm to 300 μm and preferably 50 μm to 200 μm of the bearing material preferably are removed. In fine spindling, the excess to be removed advantageously amounts to 50 μm to 1,000 μm and preferably 100 μm to 500 μm. 
     After removal of the excess, the finished bearing layer preferably has a thickness of 150 μm to 800 μm and desirably 200 μm to 500 μm. Such a layer is considerably thinner than conventional bearing shells, which have a thickness in the range of about 2.5 mm. This means that either the connecting rod material surrounding the eye can be thicker, permitting higher loading, or that the material surrounding the eye can have the same thickness and the connecting rod can be produced with a lower weight. 
     According to the invention, the connecting rod eye, which is coated with the bearing material by thermal spraying, is preferably opened only after application of the bearing material. This procedure is especially preferred when the connecting rod eye is broken open by cracking. In this process, the inside of the connecting rod eye is provided with a notch at each of the desired rupture locations. The notches preferably are made by a laser, for example an FK laser which is moved through the connecting rod eye at an angle to the bearing surface of approx. 45°. The laser power used is preferably 5 kW to 10 kW. The notch advantageously has a width of 0.3 mm to 0.8 mm and a depth of 0.2 mm to 0.7 mm. The rupture location usually is roughly centered in the connecting rod eye. Alternatively, the notch may also be produced by eroding or by a broach, for example, by ramming. 
     In notching and breaking the connecting rod eye, the sequence of operations preferably is effected so that first the connecting rod eye is notched, for example, with a tool, a laser or by eroding, then the bearing material is applied by thermal spraying, and subsequently, breaking is effected. This procedure eliminates or minimizes the slot that would otherwise exist at the rupture location or between the individual bearing parts. At a high engine load such a slot promotes detachment of the oil film. Better lubricating properties are thus obtained by notching before coating of the bearing material. Under some circumstances, breaking may take place before coating of the bearing material, which is broken again after coating, preferably without notching. 
     When the connecting rod eye is opened by being cut, this preferably takes place before application of the bearing layer by thermal spraying. In this procedure, the connecting rod eye is cut apart and the resulting surfaces of the remaining connecting rod and the connecting rod cap are individually broached smooth. The parts are then reassembled, provided with bolt bores and threads and bolted together. Preferably, a notch is also introduced at the parting plane of the surfaces in the connecting rod eye after it has been bolted back together again and before coating the surface of the eye with the bearing material. The connecting rod eye is preferably broken open again to break the bearing layer. If this break is too uneven, the bearing layer itself must be notched. 
     The honing or spindling treatment of the bearing layer described above is effected especially advantageously only after breaking of the connecting rod eye. In this way, any burrs produced upon rupture of the bearing layer at the rupture edge are removed simultaneously with removal of excess bearing material. 
     In the connecting rod produced according to the invention, an oil channel may if ,desired be introduced into the connecting rod eye. The oil channel is preferably bored into the connecting rod eye only after application of the bearing layer and in particular after its finishing. In this connection, the oil channel may alternatively be bored through the connecting rod to the facing connecting rod eye. 
     According to the invention, the large planar side surfaces of the connecting rod are preferably finished only after application of the bearing layer. In this case the finishing is preferably carried out by grinding of the planar surfaces. 
     In the method according to the invention, coating is preferably preceded by a step in which the connecting rod eye is roughened. This roughening is preferably effected by sandblasting, but blasting with a liquid under high pressure is alternatively possible. In this connection, the material of the eye such as C 70 steel, is preferably roughened to produce an average peak-to-valley height R a  of 4 to 30 μm, desirably 8 μm to 12 μm. Such average peak-to-valley height values produce especially good adhesion of the bearing material to the material of the connecting rod eye. 
     For advantageous finishing of the bearing layer in the connecting rod eye, at least one of the plane side surfaces of the connecting rod eye is covered with a template that has an opening in the region of the eye. The template opening should be approximately the same size as the eye, so that, on the one hand, the coating of the bearing surface is not affected by the presence of the template and, on the other hand, coating of the bearing material on the planar side surfaces in the region of the connecting rod eye is largely avoided. If only one planar surface of a connecting rod is covered by the template, the other planar surface preferably lies on a pallet that also has an opening like that of the template in the region of the connecting rod eye. 
     The connecting rod eyes of several individual connecting rods are preferably coated in one operation according to the invention. To this end, a plurality of connecting rods, preferably two to ten and desirably four to eight are positioned against each other so that the connecting rod eyes to be coated form a continuous cylindrical opening. For this purpose, a holder into which the connecting rods can be inserted with their eyes aligned may be provided on a special pallet. The connecting rods which are coated at the same time in this process preferably are kept as a group so that they can subsequently be installed in the same internal combustion engine. All connecting rods for the same internal combustion engine preferably are coated together by such superposition. When this is not structurally possible because of the large number of cylinders, for example twelve cylinders, at least the connecting rods of one row of cylinders, for example, six cylinders in a V-12 engine, are superimposed for coating. This procedure ensures that connecting rods of like quality are installed in the same internal combustion engine. 
     In an especially preferred method, a gas stream is passed through the connecting rod eye during thermal spraying, particularly, when a plurality of superimposed connecting rods is coated. Air that is conditioned and purified provides a particularly suitable gas stream. The gas stream should desirably be free of grease and moisture and, insofar as possible, having a temperature maintained within a range of approx. 20° C. The gas stream preferably has a flow velocity, i.e., a downward flow rate, of about 3 m/s to 15 m/s and desirably 5 m/s to 8 m/s. Any overspray produced during thermal spraying is blown off by the gas stream. 
     Thermal spraying of the bearing material is preferably effected by using a rotating spray nozzle that is driven in a rotating manner through the connecting rod eye preferably starting from a location above the connecting rod eye. An especially uniform coating in a connecting rod eye is obtained by using such a spray nozzle. In coating the connecting rod eye according to the invention, the spray nozzle is advanced through the eye at a rate of preferably 0.5 mm/s to 20 mm/s, desirably 2 mm/s to 8 mm/s. 
     It is especially preferred that a plurality of layers, in particular four to thirty layers of the bearing material are applied to the connecting rod eye when the bearing layer is formed in the eye. The successive layers preferably are applied in opposite directions, which is effective to improve the layer quality. This is carried out by operating the spray nozzle to coat the surface of the connecting rod eye while running in and then running out while rotating that spray nozzle preferably in the same direction of rotation. 
     The connecting rods are coated with the bearing material in mass production according to the invention. In this case it is advantageous if at least a few connecting rods in each series are measured. In this connection, at least the peak-to-valley height R a  and/or the uniformity of distribution of the bearing material when a mixture is used, should be measured. Especially preferably, measuring of the connecting rod is effected in a nondestructive manner. 
     The features and procedures mentioned above and in the following description are equally applicable to the method and to the connecting rods made according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a front view, partly in section, showing a connecting rod; 
     FIG. 2 is a side view showing laser notching of the large eye of the connecting rod shown in FIG. 1; 
     FIG. 3 is a cross-sectional view showing the bearing layer thickness of a thermally sprayed bearing layer in the large connecting rod eye; 
     FIG. 4 is a perspective view showing a connecting rod cap with a crankshaft bearing pin and microporous bearing layer; 
     FIG. 5 is a view similar to FIG. 4 showing a connecting rod cap with a single circumferential oil groove; and 
     FIG. 6 is a view similar to FIG. 5 showing a connecting rod cap with a plurality of circumferential oil grooves. 
     FIG. 7 is a schematic cross-sectional view showing a plasma burner in a connecting rod eye; and 
     FIG. 8 includes front and top views of the plasma burner shown in FIG.  7 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Only some of the steps of the method described hereinafter are necessarily linked. Basically, individual steps may be omitted, supplemented, carried out alternatively and/or replaced by others. 
     Production Sequence for Cracked Connecting Rod 
     The eyes of connecting rods such as shown in FIG. 1 have heretofore been conventionally provided with bearing shells, made, for example, of C 70 steel, on an assembly line. This is followed by rough-grinding of the side faces  2 . Then the large and small connecting rod eyes  3  and  4  are premachined to the desired dimension. In addition, bolt holes for the connecting rod cap  9  are machined in the side faces  2  to produce bores  5  and threads  6 . 
     To prepare the large eye  3  for cracking, an FK laser  7  is driven through the eye  3  at an angle of 45° burning a notch  8  having a width of approx. 0.5 mm and a depth of approx. 0.3 to 0.5 mm on both sides and in the center, respectively. The notch may alternatively be produced by a broach. 
     After the notches  8  have been made, the large connecting rod eye  3  is plasma coated, as described below, in accordance with the invention to produce a bearing layer having a thickness  15 , shown in FIG.  3 . After plasma deposition of the bearing layer in the large eye  3 , the eye, together with the bearing layer, is cracked using a breaking device which applies a breaking force of about 100 kN. The region of the resulting rupture is cleaned by blowing it with compressed air and the broken-off connecting rod bearing cap  9  is reassembled to the connecting rod with bolts  10  using a specified torque. The small connecting rod eye is also completed by insertion of a bushing  11 . After that the planar surfaces  12  are finish-ground. 
     The large eye  3 , and optionally alternatively the small eye  4 , are then machined to produce a desired dimension  16 , shown in FIG. 3, by fine boring or fine spindling. Subsequently, the connecting rod is subjected to thorough cleaning, measured and classified. 
     Production Sequence for Cut Connecting Rod 
     The production sequence for a cut connecting rod is substantially the same as the sequence described above, but the connecting rod eye is separated into two parts by cutting it open after broaching of the side faces, the head faces and bolt supports. After the eye is cut open, the surfaces at the parting plane on the connecting rod and the connecting rod cap are individually broached. This is followed by a wash step, after which the small eye is premachined and finish-machined. The holes for the cap bolt are then introduced into the side faces by machining of bores and threads. The surfaces of the connecting rod and connecting rod cap at the parting plane are finish-ground and washed again and the cap is mounted by bolts on the connecting rod. The bearing surface is again notched along the split with an FK laser and then the large eye is coated with the bearing layer which subsequently is broken again. 
     Individual production steps, such as, for example, pressing of the bearing shell  11  into the small eye  4 , may take place at various points in the sequence, for example, before plasma coating. 
     Construction of the Plasma Burner 
     A burner  51  shown in FIGS. 6 and 7 which is rotatably mounted so that its flame  52  moves in a circle, is used for interior coating of hollow spaces. The axis of rotation is chosen here so that the flame  52  has an optimal separation from the connecting rod eye  3 . The burner  51  is a plasma burner which has an annular anode  54  surrounding a cathode  53 , between which a discharge  155  takes place. Moreover, the burner  51  has discharge openings  55  arranged between the cathode  53  and the anode  54  through which a plasma gas flows, for example hydrogen with argon as a carrier gas. The plasma gas is ignited by the discharge  55  and forms the flame  52  into which a metal powder  56  together with argon as a carrier gas is introduced through a powder feed device  57 . A temperature of approximately 20,000° C. prevails in the flame  52 . The metal powder is deposited as the coated layer  15 . In order to coat the entire connecting rod eye  3 , of which several may be stacked one upon another, the burner  51  is additionally movable up and down. Preferably, the burner  51  moves through the connecting rod eye  3  multiple times in a helical manner. 
     Furthermore, gas discharge openings  58  are provided in the burner  51  through which a gas  60  flows to cool the burner  51  and direct the flame  52 , which would be excessively deflected by the rotational speed of the burner  51  which is approximately 200 RPM in the absence of this gas stream. 
     The flow of gas  60 , which has an oxygen content lower than that of air, through the discharge openings  58 , reduces the formation of oxides in the metal powder  56 , and corres-pondingly in the coating  15 . The use of &gt;99% pure nitrogen reduces oxide formation in the coating by approximately 50%. In this way a reduction in the oxide clusters in the coating is accomplished, so that a better distribution of pores produced by the oxides is obtained. Moreover, negative effects of microturbulences in the flame  52  during coating within the eye are sharply reduced. However, at the same time the microhardness of the coating  16  is slightly reduced by the reduced proportion of oxides. Visual examination of the overall porosity shows no significant differences with the exception of the reduction of oxide clusters in oxide-rich zones mentioned above. 
     Production Sequence for Plasma Coating of the Large Connecting Rod Eye 
     For plasma coating, the connecting rods are washed and the large eye of the connecting rod is degreased with superheated steam, then dried so as to be essentially free from residual moisture. The connecting rods so pretreated are stacked over one another in sets of four to eight, so that the large eyes are concentric with a corresponding opening in a special pallet. The connecting rods are aligned and fixed on the pallet through the premachined small eye of the connecting rod and/or the side faces. The loaded pallets pass through a preparatory zone into a sandblasting unit, in which the large eye is roughed by sandblasting to an average peak-to-valley height R a  of about 8 μm to 12 μm. Then, the connecting rods are moved into a cleaning station and the sandblasted surface blown off or blown out with compressed air. Lastly, the pretreated connecting rods move into a plasma station in which the large eye is coated with an aluminum bronze by a rotating plasma burner to produce a layer thickness  15  of about 0.5 mm as shown in FIG.  3 . The coated connecting rods then go into a cooling zone, after which the cooled rods are removed from the pallet and delivered for the subsequent processing described above. 
     Connecting rods produced according to the invention have the advantage that they contain no bearing shell in the large eye. Consequently, assembly of the bearing shell itself, as well as the introduction of holding grooves or deburring of the bearing shells, is also eliminated. As a result, the formation of a bearing layer by plasma coating is competitive in terms of cost. In addition, reliability of assembly is increased in plasma-coated connecting rods, since bearing shells cannot be left out during assembly. 
     The connecting rods according to the invention have a greater thickness of the connecting rod material in the region of the bearing cap mounting since the plasma layer is only about 0.3 mm thick after finishing, whereas a bearing shell is 1.5 mm thick. Providing a greater thickness of connecting rod material permits a high connecting rod load for higher-loaded engines. Alternatively, maintaining the same thickness of connecting rod material permits a saving of weight. In addition, there is no slot between two bearing shell halves at which the oil film begins to detach in conventional bearings. The connecting rods according to the invention thus have improved lubricating properties. 
     Application Sequence 
     As described for the production sequences described above, cleaning of the surface to be coated and provided with radial grooves may be effected with superheated steam. In this way, practically 100% freedom from grease is obtained. Sandblasting of the substrate surface which is, for example, C 70 steel, is effected with Al 2 O 3  providing at about 4 to 6 bar blasting pressure and using successively different grain sizes. For example, a first sandblasting operation for the eroded rupture notch in a cracked connecting rod or a split connecting rod is carried out using a grain diameter of 0.063 mm to 0.15 mm (230 to 100 mesh), a second sandblasting operation for the substrate surface and the circumferential groove is carried out with a grain diameter of 0.18 mm to 0.35 mm (80 to 45 mesh), and a third sandblasting operation for the substrate surface and the circumferential groove is carried out with grains having a diameter of 0.6 mm to 1.1 mm (30 to 16 mesh). This is followed by plasma coating with an aluminum-copper-iron alloy, for example, an aluminum bronze containing preferably 5% to 15% aluminum, 1% to 5% iron, 1% to 4% cobalt and 0.5% to 4% manganese, and desirably 9% to 12% aluminum, approx. 2% manganese, approx. 2% to 2.5% cobalt and approx. 3% to 4% iron. Initial plasma coating is effected to a layer thickness of about 200 μm to 250 μm with a powder grain size for the coating material having an average of about 38 μm and, using the application parameters which are dependent upon the burner employed in each instance, a coating porosity of ≦1% is set. Coating is continued without interruption with a somewhat coarser powder grain having a diameter of approx. 65 μm, and a layer thickness of about 200 μm to 250 μm is formed. For this layer, the application parameters are set so that a porosity in the range of about 1.5% to 3.5% is obtained. 
     This kind of coating can also be provided for bearings without circumferential grooves. 
     Production Sequence for Cut Connecting Rods with Circumferential Grooves 
     The entire production sequence for cut connecting rods is described below but, as already mentioned, individual steps may be omitted, supplemented or replaced by other steps, depending upon production planning. 
     After delivery of the rough connecting rods, they are put on the assembly line where preliminary grinding and broaching of the planar side surfaces  12  takes place. Then the side faces  2 , head faces and bolt supports are broached and the large connecting rod eye is separated. This is followed by broaching of the surfaces of the parting plane in the large eye  3 , the connecting rod and the cap  9  being worked individually. After the parts are washed, the small eye  4  is preworked and finish-machined and the bores and thread are cut for the connecting rod cap bolts  10 . The bearing bushing  11  is inserted into the small eye  4 , pressed on and set. The parting plane surfaces of the connecting rod and the cap  9  are finish-ground and the whole is washed once again. 
     After this the cap  9  is tightened on the connecting rod with a specified torque and, if circumferential grooves  32  are provided, the grooves are introduced into the large eye, for example by milling or turning. 
     To facilitate later opening of the plasma coating  15  or  16 , a rupture notch is made in the large connecting rod eye, for example by eroding laser cutting. This is followed by plasma coating of the large eye. 
     If necessary, depending upon the kind of connecting rod, an oil channel may be bored through the connecting rod from the large eye to the small eye. This may be done for gasoline engines and usually is done for diesel internal combustion engines. 
     The plasma layer  15  that has been deposited, which is often already cracked, depending upon plasma coating, due to the internal stress of the plasma layer  15  may be cracked by loosening of the cap bolts  10 , The cap is removed and the area of rupture is cleaned, for example, by blowing. Then the cap  9  is reassembled and the bolts  10  are tightened with the appropriate torque. The large eye is fastened on both sides and, if grooves are provided, the edges of the radial grooves are broken to round them off. 
     After this, the planar side surfaces  12  are finish-ground, the small eye  4  is finish-bored, and the large eye  3  is finish-spindled to produce the actual bearing surface  35  and to expose micropores  33  on the surface of the bearing layer  16 . After an additional wash operation, the connecting rods are measured and classified, during which preferably two tolerance classes are selected. 
     The connecting rods so produced are disassembled on the assembly line and then assembled onto the crankshaft  34  of an internal combustion engine. 
     After plasma coating of the large connecting rod eye and optionally boring of the oil channel, the large eye may be chamfered on both sides and, if grooves are provided, breaking of the radial groove edges may take place. This is then followed by finish-grinding of the plane surfaces, the small eye being finish-bored and the large eye finish-spindled. After washing and measuring and classifying the connecting rods, cracking of the plasma layer by loosening of the cap bolts then takes place, desirably directly on the assembly line. The connecting rod cap is removed and the rupture area is cleaned for example by blowing off. Then, the connecting rod is assembled onto the crankshaft of the internal combustion engine. 
     Advantageously, reversing of the orientation of the connecting rod cap is prevented, for example, by providing a slight displacement of the cap bolt bores, so that the serrations produced in the plasma layer upon cracking will match each other. 
     Production Sequence for Cracked Connecting Rod 
     The rough connecting rod for example of C 70 steel, is placed on the production line and rough ground, the side cheeks being ground plane. Pretreatment of the large and small connecting rod eyes and bolt hole processing for the bearing cap is effected by boring and thread-cutting. If desired, one or more circumferential grooves may be introduced into the large eye, for example by milling or turning depending upon the kind of bearing. Then the large connecting rod eye is notched, for example with an FK laser, which is driven through the eye at 45° with a laser power of approx. 7 kW to produce a notch of approx. 2 mm in width and 0.5 mm in depth centrally in the eye. The notch may alternatively be introduced by a broach or by eroding. The connecting rod eye is cracked by a breaking device with a breaking force of approx. 100 kN. The connecting rod bearing cap is removed and the rupture area cleaned, for example by compressed air. The connecting rod bearing cap is then reassembled and the cap bolts tightened with an appropriate torque. After that, the large connecting rod eye is plasma coated and a bearing bushing is pressed into the small eye. The large eye is chamfered on both sides, the connecting rod cheeks are finish-ground and the large and small eyes are fine-bored or fine-spindled. This is followed by thorough cleaning (washing) and measuring and classification of the connecting rod. The connecting rod is disassembled on the assembly line and assembled on the crankshaft of an internal combustion engine. 
     Cracking of the plasma layer, in particular by loosening of the connecting rod cap bolts, may be effected directly on the assembly line or alternatively at an earlier point in time. After cracking of the plasma layer, it is advisable to clean the connecting rod cap rupture area by blowing. 
     Production Sequence for Plasma Coating 
     The connecting rod is washed and at least the large eye is degreased with superheated steam and then dried to as low a residual moisture content as possible. A plurality of connecting rods such as four to eight rods, is stacked with the large eyes, concentric, on top of each other on a special pallet. Fixing of the connecting rods to the pallet may be effected through the small connecting rod eye or the side faces of the connecting rods. The loaded pallet is moved through a neutral zone into a sandblasting unit and positioned. The large connecting rod eye is sandblasted with the sand classifications described above to an average peal-to-valley height of approx. 6.5 μm to 8 μm. Then the pallet, with the sandblasted connecting rods, moves into a cleaning station in which the blasted surface is blown off or blown out with compressed air. The pallet moves on into the plasma station, in which the large eye is plasma coated first with a slightly porous and then with a somewhat more porous layer of aluminum bronze to a total thickness of 0.5 mm. The coated connecting rods move into a cooling zone and are taken from the pallet for subsequent processing after cooling. 
     The present invention has the advantage that large eye bearing shells and their assembly, as well as the holding grooves and the deburring therefor are eliminated. This increases the reliability of assembly. Plasma coating results in a greater web thickness, i.e., a smaller connecting rod eye diameter, in the region of the bearing cap fastening, since a bearing shell has a thickness of approx. 2.5 mm compared with a plasma layer of less than 0.5 mm. This permits a higher connecting rod loading in highly loaded engines and/or a reduction in weight of the connecting rod. In addition, a slot between two bearing shell halves, which due to the construction may in some cases lead to detachment of the oil film, is eliminated. Better lubricating properties are obtained by plasma coating. At the same time, tolerances are reduced, since one tolerance class is eliminated by plasma coating. Due to smaller manufacturing tolerances, this results in better running of the engine. The use of circumferential grooves in plasma coating allows a higher bearing load and a reduction of mixed friction time to be obtained. The micropressure chamber system, especially in combination with one or more grooves, reduces friction. The grooves may alternatively be coated with some other sliding bearing material, which, after machining of the coating, is present essentially only in the grooves. Such bearing materials may be based on lead, tin or alternatively nickel. This also makes higher bearing loads and/or service lives possible. 
     Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.