The invention relates to a bearing part and to a thermal spray method for producing a layer system on a bearing part, in particular a connecting rod eye of a connecting rod for a reciprocating piston combustion engine.
Bearing devices of all kinds, e.g., plain bearings and roller element bearings, represent the predominant portion of bearings used in engineering. Bearings or bearing devices are to be understood in the following as all bearing parts which define the function of the bearing, e.g., bearing parts rolling off or sliding off on one another or supported in one another, and which cooperate in supporting contact.
As a rule, wear phenomena, particularly at the bearing running surfaces, are associated with the friction arising at the bearing running surfaces cooperating in bearing contact. Plain bearings are used in an almost countless number of embodiments in this respect. For example, plain bearings are used in reciprocating piston combustion engines as crankshaft bearings, big end bearings, thrust bearings, crosshead pivot bearings, and propeller shaft bearings or in many other applications.
A plain bearing and a method of manufacturing a plain bearing are disclosed in WO 00/23718, for example, wherein a coating composed of white metal is alloyed to a carrier part composed of an iron material. The forming of an alloy, however, requires the presence of liquid alloy components so that, accordingly, a large amount of heat is released in the coating process, whereby not only the white metal melts, but also a metal bath composed of molten base material is produced at the upper side of the carrier part at the running surface side. The melts of white metal and iron material thus produced can alloy with one another, with FeSn2 arising to a large amount. Accordingly, a comparatively thick connection zone largely composed of FeSn2 arises in this respect. This connection zone admittedly produces a good metallurgic connection between the base material and the coating, but FeSn2 represents a very brittle material so that a formation of cracks and brittle fracture can already occur at smaller loads on the known bearing arrangement. In addition, there is the fact that, in the event of an unfavorable cooling, a conversion of the iron material close to the coating to Martensite can take place which is likewise very brittle, whereby the aforementioned disadvantage is further intensified.
The consequence is that, with a bearing of the previously described kind, a correspondingly short service life results as a consequence of the high brittleness and the small stretchability within a comparatively thick intermediate region between the steel of the base material and the white metal bearing layer.
To avoid this, it is also known to apply a coating composed of a bearing metal such as white metal to the carrier part in a centrifugal casting process. In this respect, the formation of a metal bath composed of molten base material is admittedly suppressed. On the solidification of the white metal applied in the centrifugal casting process, a separation of the alloy components can, however, occur in that needle-shaped crystals composed of Cu6Sn5 are first precipitated and then cubic crystals composed of SbSn and finally the remaining tin-rich matrix solidifies.
The density of Cu6Sn5 is larger, that of SbSn smaller than the density of the matrix which is liquid longer. The Cu6Sn5 crystals accordingly migrate radially outwardly and in so doing weaken the region at which the white metal coating adjoins the base material, which can likewise have an unfavorable effect on the service life.
To avoid these problems and related problems, a plain bearing and a method of manufacturing a plain bearing is proposed in WO 2007/131 742 A1 in which a relatively thin connection zone containing FeSn2, which is at most 10 μm thick, is formed between the pure white metal layer and the base material containing iron.
This solution, however, only represents a compromise since the FeSn2 layer is admittedly relatively thin, but is actually still present so that the problem of the brittle fracture and of crack formation is still not conclusively solved. The problem of brittle fracture and of crack formation for plain bearings which are exposed to enormously high loads, for example crankshaft bearings of internal combustion engines, is in particular still present because the deformation capability of the plain bearings in accordance with WO 2007/131 742 A1 is not sufficient by a large amount.
To avoid these problems in principle and to prevent the replacement of the complete bearing in the event of wear of the bearing surfaces, bearing shells are, inter alia, also used in plain bearings which are inserted into the bearing and which can be replaced after their wear, i.e., so-called split plain bearings with bearing shells are used. In part, a combined support in plain bearings and in roller element bearings is also used. The shafts are, as a rule, in this respect forged or sintered from forged steels or spherulitic graphite cast iron, the counterparts from heat treatable steels or are in part also manufactured from annealed cast iron. The bearing shells are usually realized as so-called two-layer bearings or three-layer bearings.
The constructive structure of the plain bearing is, however, thereby made substantially more complex and, in addition, a cost-intensive maintenance process becomes necessary with the exchange of the bearing shells. In addition, the manufacture of such bearing shells is relatively complex and/or expensive. With high operational performance rates of the machine, the bearing shells moreover wear prematurely so much that the changing of the bearing shells becomes necessary, with the changing of the bearing shells in such machines such as were mentioned above being associated with high costs.
However, with other types of bearings, e.g., with ball bearings, the premature wear of the bearing parts, that is for example of the balls of the ball bearings, of an axle to be supported by the ball bearing, a cage for the balls of the ball bearings or other bearing parts of the ball bearing, is a basic problem which causes a substantial effort and/or costs for repairs and maintenance and is ultimately associated with substantial costs.
A spray powder for manufacturing a bearing layer on a bearing part via a thermal coating process has therefore been proposed to solve these and further problems in EP 1 637 623 B1, said spray powder containing up to 30% zinc, up to 10% tin, up to 3% silicon, up to 7% aluminum, up to 2% iron, up to 4% manganese, up to 3% cobalt, with the remaining difference to 100% being copper. All figures in this respect are in percent by weight.
A spray powder is thus provided by EP 1 637 623 B1 with which a surface layer containing copper is applied to a bearing part via a thermal coating process so that the problems with the white metal coatings, which are applied via centrifugal coasting, for example, are avoided in the same way as the problems with the complex split bearings with bearing shells.
The corresponding components of the spray powder in accordance with EP 1 637 623 B1 are in this respect, on the one hand, iron, cobalt, manganese and silicon which are deposited as intermetallic phases or compounds in the sprayed layer on cooling in the form of a hard phase so that isolated regions arise in the layer which form spatially isolated hard phases of intermetallic phases or intermetallic compounds of iron, cobalt, manganese and silicon. The hard phases form relatively hard regions, that is, regions with high hardness, in the otherwise soft base matrix containing copper which is substantially made up of copper, aluminum and zinc and thus form a copper-aluminum-zinc base matrix, which form a soft base matrix containing copper in relationship to the enclosed regions of the hard phases.
Although the bearing layers formed from the spray powder in accordance with EP 1 637 623 B1 have shown exceptional mechanical properties, it has unfortunately been found, however, that the zinc portion can have the result that zinc abrasion can enter into the lubrication oil lubricating the bearings, at least for specific applications, which can, under certain circumstances, dependent on the chemical composition of the lubrication oil used, result in a type of zinc poisoning of the lubrication oil, that is which can negatively influence its properties. In addition, the manufacture of the spray powder in accordance with EP 1 637 623 B1 is complex and thus expensive due to its relatively complicated chemical composition.
A further disadvantage of the bearings in accordance with EP 1 637 623 B1 is that, depending on the application and and the specific bearing type, the running-in behavior of the bearing layer is not ideal. Since the coating of EP 1 637 623 B1 is actually configured so that it is particularly resistant to mechanical wear and thus has a long life, this coating can as a rule not ensure that, for example, the counterparts of a bearing provided with this coating are ideally matched to one another or are run in ideally geometrically or with respect to friction after a running-in time, which normally characterizes a good running-in behavior of a bearing layer.