Coating processes – Spray coating utilizing flame or plasma heat
Reexamination Certificate
2000-01-28
2002-04-30
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
C427S455000, C427S456000, C427S236000, C427S357000, C427S367000, C029S888090, C029S888091
Reexamination Certificate
active
06379754
ABSTRACT:
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., <8 cm and more particularly <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
Schlegel Udo
Vogelsang Reinhard
Baker & Botts LLP
Bareford Katherine A.
Volkswagen AG
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