The present invention refers to a method for thermally coating the cylinder barrels of a cylinder block of a combustion engine as well as to an apparatus for thermally coating the cylinder barrels of a cylinder block of a combustion engine.
In manufacturing combustion engines, nowadays, more and more cylinder blocks made of a light-alloy material are used. In the past, cylinder sleeves made of cast iron have been inserted into those light alloy cylinder blocks for providing an adequate cylinder barrel for the pistons. However, recently, it becomes more and more usual to provide the cylinder barrels of light alloy cylinder blocks with an iron-containing coating material. For applying such a coating material, usually and preferably, rotary plasma guns are used.
Generally, problems may arise in the course of plasma spray coating of a substrate due to the fact that not all particles molten in the plasma jet reliably adhere to the surface of the substrate. Even if this problem is of no meaning at all in many applications, one has to take care that no loose coating particles are left in the cylinder block once the coating operation has been terminated. Up to know, this problem usually was solved by applying cover members to protect exposed locations of the cylinder block. A disadvantage in using such cover members may be seen in the fact that heat accumulations may occur, leading to a high thermal stress of the cylinder block. Moreover, thereby, undesired particle inclusions in the applied coating layer may be generated because environmental dust and not-molten or already cooled-down particles are entrained by the coating jet, causing the above mentioned undesired inclusions.
1. Prior Art
U.S. Pat. No. 5,573,814 discloses a method of masking one or more extremities of a cylinder bore from internal thermal spraying. For this purpose, an inflatable mask member is provided that is pressed against one end, usually the lower end, of the cylinder bore. The mask member comprises a suction conduit for extracting gases from the cylinder bore.
2. Objects of the Invention
Therefore, it is an object of the invention to provide an easily applicable method for thermally coating the cylinder barrels of a cylinder block of a combustion engine in which the quality of the coating applied to the cylinder barrel can be improved.
It is a further object of the present invention to provide an easily applicable method for thermally coating the cylinder barrels of a cylinder block of a combustion engine in which the cylinder block is protected from contamination during the coating operation.
It is a still further object of the present invention to provide an apparatus for thermally coating the cylinder barrels of a cylinder block of a combustion engine by means of which the method of the invention can be easily and quickly performed.
To meet these and other objects, according to a first aspect, the present invention provides a method for thermally coating the cylinder barrels of a cylinder block of a combustion engine in which a plasma spraying gun having a preferably rotary plasma spraying head is moved into the interior of the cylinder bore of the cylinder whose barrel is to be coated. Before the coating operation is initiated, a flow of air is created through said cylinder bore whose barrel is to be coated. The flow of air has a flow velocity of between 7 m/s and 12 m/s and is maintained during the entire coating operation.
According to a second aspect, the present invention provides an apparatus for thermally coating the cylinder barrels of a cylinder block of a combustion engine. The apparatus comprises a plasma spraying gun having at least one preferably rotary plasma spraying head and a mechanism for moving the plasma spraying head or heads into the interior of the cylinder bore of the cylinder whose barrel is to be coated. An air extraction member has a plurality of extraction collars and a plurality of at least partially flexible conduits for connecting the extraction collars with an extraction fan. Moreover, means are provided for varying the cross section of the conduits.
Surprisingly, using the method and the apparatus of the invention it was found that the quality of the applied coating is improved in different regards and that, simultaneously, the cylinder block is also protected from contamination. In particular, if the air is extracted from the interior of the cylinder bore whose barrel is to be coated with a flow velocity of between 7 m/s and 12 m/s, the coating particles not adhering to the barrel surface are reliably removed and the content of oxygen bound in the applied coating is kept in a region in which both the tribologic properties of the coating and the machining properties thereof are optimized. Moreover, due to the air flowing through the cylinder bores, the cylinder block is kept cooler than usual. Finally, using the method according the invention, the contamination of the plasma gun and its rotary head, respectively, is reduced.