EP3463678B1 - Coating method - Google Patents

Description

The invention relates to a coating method for coating a curved surface, in particular a concave inner surface of a bore or cylinder wall. Thermal spraying processes such as plasma spraying processes or high-speed spraying processes (HWOF) as well as the corresponding thermal spraying devices such as plasma spraying devices, so-called plasma torches, are generally used to coat parts subject to high thermal or mechanical stresses by using a suitable material, e.g. a ceramic or a metal alloy the arc generated in the plasma torch is melted and applied to the surface to be coated by means of a gas flow. As long as the surface to be coated is easily accessible from the outside or has no curved surfaces, it can be coated with a conventional thermal spray device. However, if, for example, inner walls of bores or tubular geometries are to be coated on the inside, certain problems arise. If a wall of such a geometry is used with a conventional thermal spraying device, for example with a plasma spraying device with a reference coated on its longitudinal axis mainly axially emerging plasma jet, this is highly inefficient, since only a negligible part of the molten coating material is effectively applied to the wall located radially with respect to the longitudinal axis of the plasma spraying device.

This problem arises in technical application, in particular in the thermal coating of cylinder liners of internal combustion engines, corresponding coatings being applied by various thermal spraying methods in the prior art. This is particularly common nowadays, but not only, in engines for motor vehicles, airplanes, boats and ships of all types.

It is common today to use plasma spraying devices with a rotating plasma torch to coat the concave inner surfaces of the cylinders, or it is also possible to rotate the liner itself. In these special plasma spraying devices, a coating jet emerges from the plasma torch either perpendicular to the axis of rotation of the plasma torch or at a certain angle of inclination to the axis of rotation and is, for example, with the aid of a pressurized gas stream, which is often from an inert gas or an inert gas such as nitrogen, or else can simply be formed by air, hurled onto the cylindrical concave surface to form the desired surface layer. Coating processes and plasma spraying devices that use thermal spray powder as the starting material for the coating have proven particularly useful in practice. Such a rotating plasma spraying device and corresponding plasma spraying methods are already in the US, for example EP0601968 A1 disclosed. State-of-the-art devices, such as the SM-F210 burners from Oerlikon Metco, have long been very successful in use and are firmly established on the market. Solutions that use spray wires in rotating burners are also known, for example in the WO 2008/037514 shown.

The corresponding cylinder running surfaces are usually activated by various methods before the thermal coating, e.g. by corundum blasting, chill casting, high pressure water jets, various laser processes or by other activation processes known per se. Most often, substrates made of light metal alloys based on Al or Mg, but also those based on iron or steel are pretreated and then coated. The activation of the surfaces guarantees in particular better adhesion of the thermally sprayed coatings.

There are also special application examples where multilayer systems appear to be advantageous, which are sprayed on successively from different coating materials, or which consist of the same material but are applied using different spraying parameters, so that the applied layer has very special chemical, physical, topological or get other properties that can change, for example, over the layer thickness.

These and a multitude of other innovative measures, which are now well known to the person skilled in the art, have been able to successively improve the layer properties, in particular also of cylinder inner coatings, to this day.

It has been shown, however, that different tread materials also place different demands on the methods with which the coatings are applied.

It has been found that, for example, ceramic layer materials, such as the applicant's proven layer material F6399 (Cr ₂ O ₃ ), are much more difficult to process than a metallic layer material such as XPT512 (a low-alloy carbon steel). This is particularly reflected in an often lower one Layer application rate and in the resulting longer process time again.

It is therefore customary in the prior art, at least for plasma coating with powdered coating materials, to limit the rotation of the burner to a maximum value, the maximum conveying rate of the powder also having to be correspondingly limited at the same time. The aforementioned limitation of the rotational frequency of the plasma torch unit naturally also applies to the Applicant's RotaPlasma ™ unit, which is a tool manipulator with which an APS internal torch is set in rotation in order to apply the powdery material inside a cylinder bore. The limitation of the rotation frequency to around 200 rpm does not only apply to the RotaPlasma ™ unit, but is also an order of magnitude a limitation of the rotation frequency, as is also observed in the state of the art when using other rotating plasma torches that work with powdery materials .

This limitation of the rotational frequency has previously been considered necessary in order to prevent excessive internal stresses in the sprayed layers, which can lead to harmful cracks or other damage to the sprayed layer. For example, what can lead to fatal consequences in the case of coating a cylinder liner of an internal combustion engine, which is of course well known to the person skilled in the art.

It has been shown that this danger does not only exist, but to a particular extent when using ceramic coating materials, and therefore leads to the fact that, above all, such ceramic coating materials can only be applied with very low delivery rates and, in connection therewith, relatively low rotation rates of the plasma torch if coatings of sufficient quality are to be produced. This fact alone means that ceramic materials, especially on an industrial scale Do not allow coatings on cylinder inner surfaces to be produced sufficiently economically.

But even if the coatings are applied with very low rotation rates of the plasma torch and with correspondingly low powder feed rates, residual stresses can still be so high that cracks or other damage to the applied layers still occur, which are tolerable within certain limits, but of course not desirable are there, for example only slightly pronounced cracks, of course, ultimately negatively affect the quality of the coatings. This plays a crucial role, particularly in the case of cylinder coatings for internal combustion engines, because, last but not least, legislators are also placing ever higher demands on environmental standards and fuel consumption, which are fundamentally easier to achieve with coatings of higher quality. Coatings of inferior quality naturally also lead to shorter downtimes in operation, thus shortening the maintenance intervals and, overall, lead to a shorter service life and ultimately to higher operating costs for the engines equipped with them.

The object of the invention is therefore to provide a plasma coating method for coating a curved surface, in particular a concave inner surface of a bore or tube wall, in particular an inner wall of a tread of a cylinder bore or a cylinder liner for internal combustion engines, with which the Disadvantages known from the prior art are avoided and in particular the application of plasma coatings is significantly improved by means of a powdery spray material, so that the layers produced have massively reduced residual stresses in comparison with the prior art, so that there are significantly fewer or no cracks or other damage , and the coatings at the same time more efficient, faster and cheaper than can be applied with the methods known from the prior art.

The object of the invention solving these objects is characterized by the features of independent claim 1. The respective dependent claims relate to particularly advantageous embodiments of the invention.

The invention thus relates to a coating method for coating a curved surface, in particular a concave inner surface of a bore or cylinder wall, by means of a powdery coating material using a thermal spraying device, in the form of a plasma spraying device or an HVOF spraying device. In this case, a burner, in particular a plasma burner, is provided on a burner shaft of the thermal spraying device for generating a coating jet from the powdery coating material, in particular by means of an arc, and the burner is rotated about a shaft axis of the burner shaft with a predetermined rotation frequency, the coating jet for applying a coating the curved surface is directed at least partially radially away from the shaft axis towards the curved surface. According to the invention, a higher rotation frequency of the burner is selected in relation to a basic rotation frequency of the burner, and the delivery rate of the powdery coating material is changed in accordance with a predetermined scheme such that the delivery rate is adapted to the higher rotation frequency of the burner.

As already mentioned at the beginning, tread materials, such as the applicant's known F6399 (Cr ₂ O ₃ ), are distinguished by their ceramic material properties. Compared to metallic Layer materials, such as XPT512 (low-alloy carbon steel) are usually more difficult to process the ceramic materials in terms of process technology. This is particularly reflected in the often lower layer application rate and the resulting longer process time.

In particular, this problem was addressed seriously for the first time by the present invention and ultimately solved. So far, the maximum speed of the plasma torch, such as that of a RotaPlasma ™ unit is limited to approx. 200 rpm, which also limited the maximum conveying rate of the powdered coating materials. The limitation was necessary if you did not want to risk high internal stresses in the layers. This danger exists in particular in the case of ceramic materials and leads to the fact that these can generally only be applied at very low delivery rates, which questions the economic viability of such ceramic coatings.

Contrary to all previous assumptions of the experts, it has now been recognized for the first time by the present invention that an increase in the rotational frequency of the plasma torch, for example up to 800 rpm or even higher, with a simultaneous suitable increase in the conveying rate of the powdery coating material in the coating process, the layer properties drastically improve. The essential finding of the invention is therefore that, contrary to all previous assumptions, an increase in the frequency of rotation of the plasma torch does not automatically lead to a deterioration in the layer properties if only the conveying rate of the powdery layer material is suitably adjusted. The spray tests carried out by the inventors have clearly shown that the increase in the relative speed between the powder jet and the surface to be coated (as a result of the higher speed) has a positive influence on the layer quality. This can be observed in particular with the ceramic layers. Thereby In addition to improved layer properties, the coating times can of course also be drastically reduced. A reduction of the coating times for the coating of a cylinder running surface of a cylinder by a factor of 2 to 3 or even more can be achieved without problems with the method according to the invention.

In addition, the coatings applied by the method according to the invention are of significantly better quality than the coatings known from the prior art, particularly in the upper and lower edge regions of an internally coated cylinder. In this regard, for example, cylinder top surfaces of cylinders for internal combustion engines repeatedly had problems with the quality of the applied coating at the upper and lower ends of the cylinders. Since thermal spraying at these edge areas e.g. turbulence in the coating jet and / or other negative effects can occur, these edge areas were often of significantly poorer quality, e.g. in terms of porosity, hardness, adhesive strength, etc., than the rest of the cylinder tread further inside the cylinder. This deficiency is also essentially completely eliminated by the present invention, so that the invention enables coatings of consistently high quality to be produced even at the edge regions of a cylinder.

In an embodiment which is particularly preferred in practice, the powdered coating material is conveyed to the plasma torch at a predetermined feed rate and the feed rate is adapted to the rotation frequency of the plasma torch in such a way that a higher feed rate of the powdered coating material is selected when the plasma torch is rotated at a higher rate. This means that the conveying rate of the powdered coating material is preferably also increased if the speed of rotation of the plasma torch is increased. As a result, for example, despite a shorter processing time by the plasma torch, that is, despite a faster rotation of the plasma torch similar or the same layer thicknesses are generated as with a lower rotational frequency of the plasma torch. The selection of the higher rotation frequency and / or the adaptation of the delivery rate to the higher rotation frequency can take place before the start of a coating cycle, for example before the powdery coating material is supplied, so that no adjustment of the rotation frequency and / or delivery rate is necessary during a coating cycle. A coating pass can be understood here to mean the application of a layer with one or more layers of the powdery coating material and / or a further powdery coating material.

In practice, a plasma torch to be used, such as the RotaPlasma ™ unit, often defines a base rotation frequency of the plasma torch for technical reasons, as well as a base feed rate corresponding to the base rotation frequency for conveying the powdered coating material, and is therefore predetermined. The base rotation frequency of a plasma torch and the base delivery rate corresponding to the base rotation frequency in practice are very often not only dependent on the plasma torch unit actually used, but are also determined by the coating material used or the geometry of the bore. Therefore, the basic rotation frequency and the basic delivery rate for a specific coating process must also be selected in many cases depending on the spray material.

The basic rotation frequency and the basic delivery rate are thus nothing other than the rotation frequency and the delivery rate with which the prior art has been used as standard.

In practice, the rotation frequency is usually greater than the basic rotation frequency by a predetermined rotation factor according to N = FM _N x N ₀ is selected in order to achieve a better coating and a shorter coating time, with the delivery rate being particularly preferably selected to be greater than the basic delivery rate by a predetermined delivery factor according to F = FM _F x F ₀ .

In particular, if an unchanged layer thickness of the coating is to be achieved despite the faster rotation of the plasma torch, the delivery factor can be selected to be the same as the rotation factor. The person skilled in the art understands that a layer thickness of the coating through a suitable choice of a factor ratio according to FV = FM _N / FM _{F is} the layer thickness, but also another layer property of the coating, in particular a hardness, a microhardness, a porosity, a yield point, an elasticity , Adhesive strength or another layer property of the coating by a suitable choice of the rotation factor and / or by a suitable choice of the conveying factor, in particular by a suitable choice of the factor ratio according to FV = FM _N / FM _F as required. The factor ratio FV can be in the range 0.5 F FV 10 10, preferably in the range 0.75 F FV 8 8, particularly preferably in the range 1 F FV 4 4. The factor ratio FV can also be FV = 4 or FV = 3 or FV = 2 or FV = 1.

In practice, an increased rotational frequency of a powder plasma torch is understood to mean, for example, a rotational frequency greater than 200 rpm, preferably greater than 400 rpm or greater than 600 rpm, in particular equal to or greater than 800 rpm. An increased delivery rate is understood to mean, for example, a delivery rate of greater than 25 g / min, preferably greater than 50 g / min or greater than 50 g / min, in particular equal to or greater than 100 g / min. The above-mentioned increased rotation frequencies and delivery rates are particularly typical for plasma torch units of the RotaPlasma ™ type. But they can also be universally understood for other powder plasma torch units, because Technically reasonable application rates are mainly determined by the properties of the substrate and the spray materials used, in particular ceramic or metallic or non-ceramic spray materials, and only depend in the second place on the special type of rotating plasma torch.

In particular when coating cylinder treads for cylinders of internal combustion engines, a ceramic coating material, in particular TiO ₂ or Cr ₂ O ₃ and / or a metallic coating material, in particular a low-alloy steel, in particular Fe-1.4Cr, is preferably used as the coating material -1.4Mn1.2C or another coating material advantageously used.

Depending on the need or application, a coating applied by the method according to the invention can also be applied in a manner known per se in the form of a multilayer coating, which can consist of the same or different coating material, the multilayer coating then having the same or different layer properties, in particular hardness, May have microhardness, porosity, yield strength, elasticity or adhesive strength.

The invention further relates to a thermal coating on an inner surface of a cylinder wall, in particular on a cylinder surface of a cylinder of an internal combustion engine, applied by a coating method according to the invention, and a cylinder for an internal combustion engine with a thermal coating applied by means of a coating method according to the invention.

Fig. 1

schematisch ein Ausführungsbeispiel eines erfindungsgemässen Beschichtungsverfahrens am Beispiel einer Zylinderlauffläche;

Fig. 2

ein schematisches Diagramm zur Erläuterung des Zusammenhangs zwischen Rotationsfrequenz und Förderrate;

Fig. 3a

zeichnerische Darstellung eines Schnittes durch eine Beschichtung aus TiO₂ gespritzt bei 200 U/min;

Fig. 3b

zeichnerische Darstellung eines Schnittes durch eine Beschichtung aus TiO₂ gespritzt bei 400 U/min;

Fig. 3c

zeichnerische Darstellung eines Schnittes durch eine Beschichtung aus TiO₂ gespritzt bei 600 U/min;

Fig. 3d

zeichnerische Darstellung eines Schnittes durch eine Beschichtung aus TiO₂ gespritzt bei 800 U/min;

The invention is explained in more detail below with reference to the drawing. In a schematic representation:

Fig. 1

schematically an embodiment of a coating method according to the invention using the example of a cylinder running surface;

Fig. 2

a schematic diagram for explaining the relationship between rotation frequency and delivery rate;

Fig. 3a

graphic representation of a section through a coating of TiO ₂ sprayed at 200 rpm;

Fig. 3b

graphic representation of a section through a coating of TiO ₂ sprayed at 400 rpm;

Fig. 3c

graphic representation of a section through a coating of TiO ₂ sprayed at 600 rpm;

Fig. 3d

graphic representation of a section through a coating of TiO ₂ sprayed at 800 rpm;

In the following we will explain the invention using the example of plasma spraying methods. It goes without saying that the invention is not limited to plasma spraying processes, but with any suitable thermal spraying process, e.g. can be carried out with an HVOF procedure.

The Fig. 1 shows a schematic representation of the implementation of a simple embodiment of the inventive method using the example of coating a cylinder surface of a cylinder of a car engine.

In the method according to the invention illustrated with the aid of Fig. 1 a coating 8 is just being applied to a curved surface 1, which here is the concave cylinder running surface of a cylinder of a passenger car.

In a manner known per se, a torch shaft 5 of the plasma spraying device 4 according to FIG. Fig. 1 a plasma torch 6 for generating a coating jet 7 from a powdery coating material 3 by means of an arc is provided, the plasma torch 6 for coating the curved surface 1 being arranged rotatably about a shaft axis A of the burner shaft 5. In the specific example of the Fig. 1 rotates the burner shaft 3 at the rotation frequency N, as indicated by the arrow N. Here, the coating jet 7 for applying the coating 8 to the curved surface 1, that is to say here to the cylinder running surface of the cylinder, is directed essentially radially away from the shaft axis A towards the curved surface 1, so that the surface 1 is as effective as possible with the coating material 3 is applied. With regard to a basic rotation frequency N ₀ (see Fig. 2 ) of the plasma torch 6 a higher rotation frequency N of the plasma torch 6 is selected and the conveying rate F of the powdery coating material 3 is according to one in Fig. 1 The predetermined scheme, not shown, has been changed such that the delivery rate F is suitably adapted to the higher rotational frequency N of the plasma torch 6. The basic rotation frequency of the plasma torch 6 is approximately 200 rpm at the in Fig. 1 used special plasma spraying device 4, which here for example comprises a RotaPlasma ™ unit.

In particular, the method according to Fig. 1 the powdery coating material 3 is conveyed to the plasma torch 6 at a predetermined conveying rate F, and the conveying rate F is adapted to the rotation frequency N of the plasma torch 6 in such a way that it matches the rotation frequency N of the plasma torch 6, which is greater than its basic rotation frequency N ₀ , a higher delivery rate F of the powder coating material 3 is also selected. That is, the delivery rate F is higher than the basic delivery rate F ₀ .

Based on Fig. 2 is a schematic diagram for explaining the relationship between the rotation frequency N and the delivery rate F is illustrated. The conveying rate F is plotted on the vertical ordinate axis and the rotational frequency N is plotted on the horizontal abscissa. The curve shown shows a specific example of how the parameter pair (conveying rate F / rotational frequency N) is suitably selected for a given plasma spraying device 4 and a powdery coating material 3 to be used could be. The drawn coordinate (F ₀ / N ₀₎ corresponds to a pair of parameters, as it has been USER in the art, while the parameter (FM _F x F ₀ / FM _N x N ₀₎ a special parameter pair (F ₁ / N ₁ ) corresponds with which in a spraying process according to the invention, such as in Fig. 1 described, coated.

It goes without saying that the course of the curve in Fig. 2 is to be understood purely schematically. The curve is very often according to Fig. 2 in practice, for example, be a straight line, so that the rotation frequency N and the delivery rate F are always changed with the same factor, so that the same layer thicknesses D of the coating 8 are always achieved even at different rotation frequencies N.

Of course, it is of course also possible in principle to choose a parameter pair (N / F) that corresponds above or below a curve Fig. 2 lies. In this way it can be achieved, for example, that a different or greater 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, adhesive strength or a another layer property of the coating 8 is determined by a suitable choice of the rotation factor FM _N and / or by a suitable choice of the conveying factor FM _F , in particular by a suitable choice of the factor ratio FV according to FV = FM _N / FM _F.

The 3a to 3d Finally, each show a drawing of a section through four coatings of TiO ₂ , which were each injected at different rotational frequencies N and correspondingly adapted different delivery rates F.

Fig. 3a shows a coating 8 which was sprayed onto a cylinder wall 2 using a method from the prior art using a RotaPlasma ™ plasma spraying device 4. Here the conventional parameters with a rotation frequency of N = 200rpm and a delivery rate of F = 25g / min were chosen. As can clearly be seen, the coating has 8 fine cracks R, which were hitherto considered tolerable but fundamentally not desirable. In addition to the cracks R are in all coatings 3a to 3d fine pores P can also be seen, which are mostly desired or even specifically introduced with a predetermined porosity.

The coating 8 according to Fig. 3b was compared with the state of the art Fig. 3a double rotation frequency of N = 400rpm and a double delivery rate of F = 50g / min. As can clearly be seen, the formation of the cracks R in the coating 8 has been reduced. The quality of the coating has already improved significantly.

The coating 8 according to Fig. 3c was compared with the state of the art Fig. 3a sprayed with three times the rotation frequency of N = 600rpm and a triple delivery rate of F = 75g / min. There are practically no more cracks R in the coating 8. The quality of the coating has thus improved even further.

The coating 8 according to Fig. 3d was finally compared to the state of the art Fig. 3a injected at four times the rotation frequency of N = 800rpm and a fourfold delivery rate of F = 100g / min. There are no cracks R at all in the coating 8 here. The The quality of the coating has thus improved even further and can be regarded as ideal in practice.

It is understood that the invention is not limited to the exemplary embodiments described, but is defined by the appended claims.

Claims (12)

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 the form of 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 (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).
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 or 2, wherein the base rotation frequency (N0) of the gun (6) and a base conveying rate (F0) corresponding to the base rotation frequency (N0) is predetermined for conveying the powdery coating material (3).
4. A coating method according to claim 3, wherein the base rotation frequency (N0) and the base conveying rate (F0) corresponding to the base rotation frequency (N0) is selected depending on the coating material used (3).
5. A coating method according to anyone of the claims 3 or 4, wherein the rotation frequency (N) is selected to be greater than the base rotation frequency (N0) by a predetermined rotation factor (FMN) according to N = FMN x N0 and at the same time the conveying rate (F) is selected to be greater than the base conveying rate (F0) by a predetermined conveying factor (FMF) according to F = FMF x F0.
6. A coating method according to claim 5, wherein the conveying factor (FMF) is selected equal to the rotation factor (FMN).
7. A coating method according to anyone of the claims 5 or 6, wherein a layer thickness (D) of the coating (8) is determined by the selection of a factor ratio (FV) according to FV = FMN /FMF.
8. A coating method according to anyone of the claims 5 to 7, 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 (FMN) and / or by a suitable selection of the conveying factor (FMF), in particular by a suitable selection of the factor ratio (FV) according to FV = FMN / FMF.
9. A coating method according to anyone of the preceding claims, 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 anyone of the preceding claims, 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 anyone of the preceding claims, wherein the coating material (3) is a ceramic coating material (3), in particular TiO₂ or CrO₃ 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 anyone of the preceding claims, 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.

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