A COATING METHOD, A THERMAL COATING AND A CYLINDER HAVING A THERMAL COATING

Abstract

The invention relates to a coating method for coating a curved surface (1), in particular a concave inner surface (1) of a bore wall or a cylinder wall (2), by means of a powdery coating material (3) by using a thermal spraying device, in particular a plasma spraying device (4) or a HVOF spraying device. A gun (6) is provided on a gun shaft (5) of the thermal spraying device (4) for generating a coating jet (7) from the powdery coating material (3) by means of an arc and the gun (6) is rotated about a shaft axis (A) of the gun shaft (5) at a predetermined rotation frequency (N), wherein the coating jet (7) for applying a coating (8) to the curved surface (1) is directed at least partially radially away from the shaft axis (A) towards the curved surface (1). According to the invention, a higher rotation frequency (N) of the gun (6) is selected with respect to a base rotation frequency (N0) of the gun (6) and the conveying rate (F) of the powdery coating material (3) is changed according to a predetermined scheme in such a way that the conveying rate (F) is adapted to the higher rotation frequency (N) of the gun (6). The invention further relates to a thermal coating (8) and to a coated cylinder.

Claims

1. A coating method for coating a curved surface (1), in particular a concave inner surface (1) of a bore wall or cylinder wall (2), by means of a powdery coating material (3) by using a thermal spraying device (4), in particular a plasma spraying device (4) or a HVOF spraying device, wherein a gun (6) is provided on a gun shaft (5) of the thermal spraying device (4) for generating a coating jet (7) from the powdery coating material (3) by means of an arc, and the gun (6) is rotated about a shaft axis (A) of the gun shaft (5) at a predetermined rotation frequency (N), wherein the coating jet (7) for applying a coating (8) to the curved surface (1) is directed at least partially radially away from the shaft axis (A) towards the curved surface (1), characterized in that a higher rotation frequency (N) of the gun (6) is selected with respect to a base rotation frequency (N.sub.0) of the gun (6) and the conveying rate (F) of the powdery coating material (3) is changed according to a predetermined scheme in such a way that the conveying rate (F) is adapted to the higher rotation frequency (N) of the gun (6).

2. A coating method according to claim 1, wherein the powdery coating material (3) is conveyed to the gun (6) at a predetermined conveying rate (F) in such a way and the conveying rate (F) is adapted to the rotation frequency (N) of the gun (6) such that at a higher rotation frequency (N) of the gun (6), a higher conveying rate (F) of the powdery coating material (3) is also selected.

3. A coating method according to claim 1, wherein the base rotation frequency (N.sub.0) of the gun (6) and a base conveying rate (F.sub.0) corresponding to the base rotation frequency (N.sub.0is predetermined for conveying the powdery coating material (3).

4. A coating method according to claim 3, wherein the base rotation frequency (N.sub.0) and the base conveying rate (F.sub.0) corresponding to the base rotation frequency (N.sub.0) is selected depending on the coating material used (3).

5. A coating method according to claim 3, wherein the rotation frequency (N) is selected to be greater than the base rotation frequency (N.sub.0) by a predetermined rotation factor (FM.sub.N) according to N==FM.sub.NN.sub.0 and at the same time the conveying rate (F) is selected to be greater than the base conveying rate (F.sub.0) by a predetermined conveying factor (FM.sub.F) according to F=FM.sub.FF.sub.0.

6. A coating method according to claim 5, wherein the conveying factor (FM.sub.F) is selected equal to the rotation factor (FM.sub.N).

7. A coating method according to claim 5, wherein a layer thickness (D) of the coating (8) is determined by the selection of a factor ratio (FV) according to FV=FM.sub.N/FM.sub.F.

8. A coating method according to claim 5, wherein a layer characteristic of the coating (8), in particular a hardness, a microhardness, a porosity, a yield strength, an elasticity, an adhesive strength or another layer characteristic of the coating (8), is determined by a suitable selection of the rotation factor (FM.sub.N) and/or by a suitable selection of the conveying factor (FM.sub.F), in particular by a suitable selection of the factor ratio (FV) according to FV=FM.sub.N/FM.sub.F.

9. a coating method according to claim 1, wherein the rotation frequency (N) is greater than 200 rpm, preferably greater than 400 rpm or greater than 600 rpm, especially equal to or greater than 800 rpm.

10. a coating method according to claim 1, wherein the conveying rate (F) is greater than 25 g/min, preferably greater than 50 g/min or greater than 50 g/min, especially equal to or greater than 100 g/min.

11. A coating method according to claim 1, wherein the coating material (3) is a ceramic coating material (3), in particular TiO.sub.2 or CrO.sub.3 and/or wherein the coating material (3) is a metallic coating material (3), in particular a low-alloy steel, especially Fe-1.4Cr-1.4Mn1.2C.

12. A coating method according to claim 1, wherein said multilayer coating (8) consisting of the same or different coating material (3) is applied and/or wherein the multilayer coating (8) has the same or different layer characteristics, in particular hardness, microhardness, porosity, yield strength, elasticity or adhesive strength.

13. A thermal coating (8) on an inner surface (1) of a cylinder wall (2), in particular on a cylinder running surface of a cylinder of an internal combustion engine, applied by a coating method according to claim 1,

14. A cylinder for an internal combustion engine having a thermal coating (8) according to claim 13 applied to the cylinder running surface of the cylinder by means of the coating method.

Description

[0041] In the following the invention is explained exemplarily with reference to plasma spraying processes, It is obvious that the invention is not limited to plasma spraying processes but can be carried out with any suitable thermal spraying process, e.g. a HVOF process.

[0042] FIG. 1 shows in a schematic representation the execution of a simple embodiment of the method according to the invention by using the example of coating a cylinder running surface of a cylinder of a passenger car engine.

[0043] In the method according to the invention represented by FIG. 1, a coating 8 is currently being applied to a curved surface 1, which here is the concave cylinder running surface of a cylinder of a passenger car.

[0044] In a manner known per se, a plasma gun 6 is provided on a gun shaft 5 of the plasma spraying device 4 for generating a coating jet 7 from a powdery coating material 3 by means of an arc in accordance with FIG. 1, wherein the plasma gun 6 is arranged rotatably about a shaft axis A of the gun shaft 5 for coating the curved surface 1. In the special example of FIG. 1, the gun shaft 3 rotates at the rotation frequency N, as indicated by the arrow N. The coating jet 7 for applying the coating 8 to the curved surface 1, i.e. here to the cylinder running surface of the cylinder, is directed substantially radially away from the shaft axis A towards the curved surface 1, so that the surface 1 is applied as effectively as possible with the coating material 3. A higher rotation frequency N of the plasma gun 6 was selected with respect to a base rotation frequency N.sub.0 (see FIG. 2) of the plasma gun 6 and the conveying rate F of the powdery coating material 3 was changed according to a predetermined scheme not shown in FIG. 1 in such a way that the conveying rate F is suitably adapted to the higher rotation frequency N of the plasma gun 6. The base rotation frequency of the plasma gun 6 is approx. 200 rpm for the special plasma spraying unit 4 used in FIG. 1, which here for example comprises a RotaPlasma unit.

[0045] In particular, in the method described in FIG. 1 the powdery coating material 3 is conveyed to the plasma gun 6 at a predetermined conveying rate F and the conveying rate F is adapted to the rotation frequency N of the plasma gun 6 in such a way that a higher conveying rate F of the powdery coating material 3 is also selected in correspondence with the rotation frequency N of the plasma gun 6, which is greater than its base rotation frequency N.sub.0. This means that the conveying rate F is higher than the base conveying rate F.sub.0.

[0046] A schematic diagram illustrating the relationship between the rotation frequency N and the conveying rate F is illustrated in FIG. 2. The conveying rate F is plotted on the vertical ordinate axis and the rotation frequency N is plotted on the horizontal abscissa. The plotted curve shows a special example of how the parameter pair (conveying rate F/rotation frequency N) could be selected appropriately for a given plasma spraying device 4 and a powder coating material 3 to be used. The plotted coordinate (F.sub.0/N.sub.0) corresponds to a parameter pair, as it has been used so far in the state of the art, while the parameter (FM.sub.FF.sub.0/FM.sub.NN.sub.0) corresponds to a special parameter pair (F.sub.1/N.sub.1), which is used for coating in a spraying process according to the invention, e.g. as described in FIG. 1.

[0047] It is obvious that the course of the curve in FIG. 2 is to be understood purely schematically. In practice, the curve shown in FIG. 2 will very often be a straight line, for example, so that the rotation frequency N and the conveying rate F are always changed with the same factor, so that the same layer thicknesses C of the coating 8 are always achieved even at different rotation frequencies N.

[0048] In principle, it is of course also possible to select a parameter pair (N/F) that lies above or below a curve according to FIG. 2. In doing so, it can be achieved, for example, that a smaller or larger layer thickness D is achieved at a different rotation frequency F and/or other parameters of the coating 8, such as in particular a hardness, a microhardness, a porosity, a yield strength, an elasticity, an adhesive strength or another layer characteristic of the coating 8, are determined by a suitable selection of the rotation factor FM.sub.N and/or by a suitable selection of the conveying factor FM.sub.F, in particular by a suitable selection of the factor ratio FV according to FV=FM.sub.N/FM.sub.F.

[0049] Finally, FIGS. 3a to 3d each show a graphic representation of a section through four coatings of TiO.sub.2, which each were sprayed at different rotation frequencies N and correspondingly adapted different conveying rates F.

[0050] FIG. 3a shows a coating 8, which were sprayed onto a cylinder wall 2 by a method according to the state of the art using a RotaPlasma plasma spraying device 4. Here, the conventional parameters were selected with a rotation frequency of N=200 rpm and a conveying rate of F=25 g/min. As can be clearly seen, the coating 8 has fine cracks R, which were previously considered tolerable, but fundamentally undesirable. In addition to the cracks R, fine pores P are also visible in all coatings of FIGS. 3a to 3d, which pores are usually desired or even specifically introduced with a predetermined porosity.

[0051] The coating 8 according to FIG. 3b was sprayed with a double rotation frequency of N=400 rpm and a double conveying rate of F=50 g/min compared to the state of the art according to FIG. 3a. As can be clearly seen, the formation of cracks R in the coating 8 has reduced. The quality of the coating has therefore already improved considerably.

[0052] The coating 8 according to FIG. 3c was sprayed with the threefold rotation frequency of N=600 rpm and a threefold conveying rate of F=75 g/min compared to the state of the art according to FIG. 3a. Here there are practically no more cracks R to be found in the coating 8. The quality of the coating has therefore improved even further.

[0053] The coating 8 according to FIG. 3d was finally sprayed with the fourfold rotation frequency of N=800 rpm and a fourfold conveying rate of F=100 g/min compared to the state of the art according to FIG. 3a. Here there are no more cracks R at all to be found in the coating 8. The quality of the coating has therefore improved even further and is to be regarded as ideal for practical use.

[0054] It is clear that the invention is not limited to the embodiments described and, in particular, that all suitable combinations of the embodiments depicted are covered by the invention.