CA2886280A1

CA2886280A1 - Method for coating by thermal spraying with an inclined particle jet

Info

Publication number: CA2886280A1
Application number: CA2886280A
Authority: CA
Inventors: Ralph Reiche; Oliver Stier
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2012-09-28
Filing date: 2013-09-17
Publication date: 2014-04-03
Anticipated expiration: 2033-09-17
Also published as: EP2885441A2; US20150240364A1; WO2014048790A3; EP2885441B1; WO2014048790A2; DE102012217685A1; CA2886280C

Abstract

The invention relates to a method for coating a substrate (11) by thermal spraying, in particular cold-gas spraying. Here, a particle jet (13) is oriented at an inclination to the surface (14) of the substrate (11) to form a spraying angle (a). In order to increase the possible spraying angle (a) according to the invention, it is provided that the surface (14) is applied with a directional patterning, for example comprising trenches (17) and ridges (18). The orientation of this patterning is inclined by an angle of inclination (ß), specifically in the direction of the spraying angle (a). As a result, the adhesion of striking particles (12) is improved and it is possible to achieve larger spraying angles (a).

Description

Description Method for coating by thermal spraying with an inclined particle jet The invention relates to a method for coating by thermal spraying. Herein, a particle jet is directed onto a substrate to be coated and the particles of the particle jet are deposited onto the substrate. In that context, the axis of the particle jet is inclined to the surface normal of the untreated substrate forming a spray angle a.
EP 1 534 877 B1 discloses that the deposition of particles during thermal spraying can be improved by a laser being guided along therewith on the workpiece surface which is to be coated.
This laser can warm the substrate at the point of incidence of the particle jet, whereby the incident particles adhere better to the substrate. However, this improvement in adhesion works only in the case of a particle jet which is oriented approximately perpendicular to the surface of the substrate to be coated. In the case of such a particle jet, the spray angle is approximately 00. The spray angle is defined by a non-oriented angle between the axis of symmetry of the particle jet (in the following denoted simply as the jet axis) and the surface normal at the point of incidence of the particle jet.
In the following, the spray angle will be denoted as a.
The particle jet is normally set with a spray angle a = 00. Any orientation of the jet axis which deviates therefrom results in a positive value of the spray angle a. Depending on the basic conditions such as processed particles, surface material and spray parameters, there is a permissible interval for the spray angle, below which the adhesion of the deposited particles reaches maximum values. The zero angle may or may not be included ' 2012P21104W0US =
in this interval. The set of all permissible spray angles thus produces a space between two conical surfaces, whose tips meet at the point of incidence of the particle jet. If the zero angle is included in the interval, only one cone oriented in the described manner is required to describe the spray angle interval.
In particular in the case of cold gas spraying, optimum results can be obtained only in a comparatively small spray angle interval since, as the spray angle a increases, the component of the kinetic energy of the particles in the direction of the surface normal is ever-decreasing. It is thus possible, in the case of components having complex shapes, for certain regions of the component to not be coated by the thermal spraying, in particular cold gas spraying since, for geometric reasons, taking into account the spray angle interval, no particle jet can be oriented onto the surface.
Cold gas spraying is a method known per se, in which particles provided for the coating are accelerated by means of a convergent-divergent nozzle, preferably to supersonic speeds, in order that these adhere to the surface to be coated by virtue of the kinetic energy imparted to them. In this process, the kinetic energy of the particles is used and leads to a plastic deformation of the latter, wherein upon impact the coating particles are melted only at their surface. For that reason, this method is termed cold gas spraying in comparison with other thermal spraying methods because it is carried out at comparatively low temperatures at which the coating particles remain largely solid. Preferably, for cold gas spraying, which is also termed kinetic spraying, a cold gas spraying apparatus is used which has a gas heating device for heating a gas. A stagnation chamber is connected to the gas heating device, wherein the outlet of this chamber is connected with the convergent-divergent nozzle, preferably a de Laval nozzle. Convergent-divergent nozzles have a =

convergent part section and a divergent part section which are connected by a nozzle throat. The convergent-divergent nozzle generates, on the outlet side, a powder jet in the form of a gas stream with high-speed - preferably supersonic - particles therein.
The object of the invention is to specify a method for coating by thermal spraying, in particular a method for cold gas spraying, by means of which it is possible to coat even hard-to-reach regions of a component, with a satisfactory coat result.
This object is achieved according to the invention with the method mentioned in the introduction in that, prior to the thermal spraying, the substrate undergoes pretreatment in which the surface is structured. In that context, structure elements of the structured surface are produced obliquely on the surface such that the structure elements are oriented towards the inclined axis of the particle jet, that is to say the jet axis.
Producing the structure elements obliquely within the sense of the invention therefore means, in other words, that the finished structure elements do not stand upright, but their principal geometric orientation deviates from the surface normal. If, for example, a structure element is any raised portion, the orientation of the latter would for example be determined taking into account the center line of the structure element. This line is then not oriented parallel to the surface normal but forms an angle of inclination p thereto, wherein the inclined orientation of the structure elements is such that these are oriented towards the inclined jet axis. In other words, the orientation of the named center line of a structure element will be at least similar to that of the jet axis.
It is also provided according to the invention that the spatial extent of the structure elements is smaller, at least in the plane of the spray angle a, than the average diameter of the particles. The plane of the spray angle a is to be defined as a plane containing both arms of the spray angle a. If one considers the spatial extent of the structure elements in this plane, the geometry of the structure elements - as seen in the section of this plane - should be smaller than the size of the particles.
It has namely been found that particles which are incident on a surface structured in this manner can, in spite of a spray angle a, convert a larger proportion of their kinetic energy into deformation than particles which are incident on the unstructured, i.e. smooth, surface at the same spray angle a.
In other words, on account of the geometry of the structure elements, the surface to be coated presents greater adhesion resistance to the incident particles, at least in the direction of the spray angle. This has the advantageous effect that the surface can still be reliably coated at higher spray angles, especially in the direction of the spray angle a. It is thus for example possible to coat components with undercuts by means of thermal spraying, because those regions of the component which are hard to reach on account of the undercuts are made accessible for the spray jet.
It is provided according to one advantageous embodiment of the invention that the angle of inclination p of the oblique orientation of the structure elements largely corresponds to the spray angle a. In other words, the angle of inclination of the oblique orientation of the structure elements is determined so as to achieve an optimal improvement of the spray angle a.
From a manufacturing aspect, it is particularly advantageous in that context if the entire hard-to-reach region of the component is provided with structure elements which have at least largely the same orientation. For the purpose of coating this region, the particle jet is then also oriented such that when coating the region the jet is displaced at least essentially parallel. This can also be achieved if the jet-generating device (for example the spray nozzle of an apparatus for cold gas spraying) is at a large distance from the substrate with respect to the region to be coated and, for the purpose of coating this region, is pivoted by for example plus/minus 50. The resulting angular offsets are then relatively small.
The structuring of the surface of the substrate, consisting of the structure elements, can also be termed a microscopic striation. The binding mechanism of the particles on the surface can also be described more simply in that, when the particles are incident on the "peaks" of the striation, the latter pierce the former and thus improve adhesion. This can be compared with the function of a metal file, which also has a striation. This striation usually has the effect that the file must be guided over the workpiece in a specific direction in order to guarantee optimal chip removal. This is only possible if the striation has an angle of inclination of the oblique orientation of the individual structure elements. This analogy clarifies the function of the microscopic striation.
Microscopic within the sense of the invention means that the extent of the structure elements, as already explained, must be smaller than that of the particles in order that the particles can act like the workpiece under the file, so to speak.
The surface normal of the substrate is defined by the surface of the untreated substrate. This means that, after treating the substrate by generating the structure elements, other local surface normals can result. However, of relevance are the surface normals which are to be determined on account of the superordinate structure of the substrate. The structuring, which is small in comparison, thus generates a surface which is much more finely structured, such that after structuring the substrate the surface normal could only be determined by averaging over multiple structure elements.

One further configuration of the invention provides that the plane of the spray angle a and the plane of the angle of inclination p of the oblique orientations of the structure elements are at an angle y of at most 25 to one another. In this context, manufacturing considerations play a role. On one hand, there are production-induced tolerances when producing the structure elements. In addition, the geometry of the substrate to be coated can be designed such that an ideal orientation of the structure elements, i.e. an orientation with an angle y of 0 , would lead to more complex production. In this case, it is possible to choose an orientation of the structure elements which, in parts of the hard-to-coat regions of the component, deviates to a greater or lesser extent - but never by more than 25 - from the ideal orientation.
Particularly preferably, it is also possible to establish an upper limit of 15 or 10 for the angle y. This is then advantageous if, on account of the processed particles, the chosen thermal coating method or the substrate material, a tighter tolerance for the angle y is required in order to still achieve the required coating results.
It is of particular advantage if trenches are produced as structure element. Advantageously, these trenches are particularly easy to create in manufacturing terms. They may be created for example by means of a laser, by moving the latter over the surface of the workpiece. The trenches can then run parallel to one another in order to make best use of the available surface area of the substrate. Depending on the geometry of the substrate to be coated, the trenches can be either straight or curved. The latter is also important for optimizing the angle y.
Particularly advantageously, the trenches have, at least locally, planes of symmetry which are oriented during production such that they are oriented towards the inclined axis of the particle jet during the thermal spraying. This produces, so to speak, the required orientation to the particle jet. It is to be noted in this context that not only the trenches but in particular the ridges remaining between the trenches are responsible for the resulting effect of an improvement in adhesion. The structuring of the surface is thus influenced not only by the trenches produced but also by the structure elements in the form of ridges therebetween.
Another advantageous possibility provides that the structure elements are produced as holes. Advantageously, holes can also be produced simply. These can for example be produced by punctiform laser shots to the surface of the substrate to be coated. Advantageously, these holes have axes of symmetry which are oriented obliquely during production such that they are oriented towards the inclined axes of the particle jet during the thermal spraying. The axes of symmetry of the holes thus simultaneously represent the center lines of the holes, wherein, when using a laser, these can in particular be directly influenced by the orientation of the latter. Moreover, this also holds for the production of trenches by means of the laser. In the case of producing holes, it is also worth mentioning that the uprights remaining between the holes also constitute oriented structures which, as structure elements, participate in the improvement of adhesion of the surface.
It is particularly advantageous if cold gas spraying is used as thermal spraying. This is advantageously well-suited to the relatively cost-effective production of relatively thick coats.
Particularly if the coat is produced in multilayer fashion, i.e. coating by alternating multiple times between production of structure elements and production of layers of the desired coating, it is possible to produce relatively thick coats. In each case, the deposited layers in turn serve for the production of the next layer, wherein in the case of hard-to-reach regions of the substrate, of course, the layer must always once again be structured in the manner according to the invention as a surface to be coated. Otherwise, subsequent layers would not adhere to the preceding layers.
In order to be able to coat workpieces with hard-to-reach regions, it is advantageous if the substrate is produced in the regions which are inaccessible to the particle jet while observing the maximum permissible spray angle in the case of the untreated substrate, such that an increase, connected thereto, of the permissible spray angle a results in the region being able to be coated with the thermal spraying method. This is the case if the maximum permissible spray angle a increases specifically in that direction whence the particle jet is incident on account of the geometric specifications of the substrate to be coated.
The structure elements may advantageously be generated with a laser beam, by means of an erosion method, by chip removal, by means of a shaping method or chemically, depending on which method is best suited to the relevant substrate.
If, for example, the surface structuring is carried out by means of an erosion process, the structure is copied by an erosion electrode onto the surface to be coated. To that end, it is necessary to provide the electrode with the desired structure prior to copying. The surface to be coated of the substrate can then be structured using one or more electrodes.
If multiple electrodes are used, these can have different configurations in order to be able to treat different regions of the surface of the substrate at an optimum distance.
A particularly versatile erosion electrode is for example in the form of an elongate ellipsoid of revolution (similar to a cigar or a cylinder) whose surface is knurled circumferentially along the longitudinal extent of the electrode. These striation grooves can then be formed by bringing the workpiece surface close to the erosion tool, wherein the longitudinal axis of the erosion tool is oriented parallel to the direction of the smallest curvature of the workpiece surface at the point of contact with the erosion tool. The erosion tool can then be guided over the surface of the substrate, perpendicular to the longitudinal axis. A
relatively effective structuring of the surface is thus possible.
If a laser machining process is chosen for structuring, the laser can be guided taking into account the accessibility of the workpiece and its geometric configuration. This is in particular possible using CAD data of the substrate. Using a laser has the advantage that it has an axis, similar to the particle jet, such that it can be guided in the same manner as the particle jet will be subsequently. That is to say that, if both the jet-generating device (for example a cold gas spray nozzle) and the laser are moved according to the same movement pattern, then the spray angle a always largely matches the angle of inclination p of the oblique orientation of the structure elements, making it possible to achieve an optimum coating result.
According to the invention, the coating apparatus and the device for generating the structure elements can be used in alternation or simultaneously. Simultaneous use advantageously shortens production times but is possible only if the region of the substrate is sufficiently accessible. Alternating use results in longer production times but harder-to-reach regions of the substrate can be optimally provided with a coating. In order to optimize the production method for a certain component, both simultaneous and alternating production of the structure ' elements and of the coating can be chosen, such that the production times for each substrate can be optimized.
One example for which the method according to the invention can be used is coating what are termed blade drillings for gas turbines. These have three blades which are produced together.
This results in hard-to-reach regions between the blades, which can advantageously be coated with the method according to the invention.
Further details of the invention will be described below with reference to the drawing. Drawing elements which are identical or which correspond to one another are in each case provided with the same reference signs and will be explained more than once only where there are differences between the individual figures, in which:
figure 1 shows an exemplary embodiment of the method according to the invention in a three-dimensional view, showing the angles a and p, figure 2 is a three-dimensional representation of the substrate shown in figure 1, showing the angles a and Y, figure 3 shows an alternative substrate, as an exemplary embodiment with holes, figure 4 shows an exemplary embodiment of the method according to the invention, in which a blade drilling is coated, figure 5 shows various steps of an exemplary embodiment of the method according to the invention, in which a multilayer coat is deposited and figure 6 is the three-dimensional graphic representation of the spray angle interval of the method according to the invention in the form of a modified cone.
Figure 1 shows a substrate 11 which is to be coated with particles 12 by means of cold gas spraying. To that end, a particle jet 13 is directed onto the surface 14 of the substrate 11. However, the particle jet 13 is inclined with respect to the surface normal 15 by an angle a. This angle a is named the spray angle.
In addition, according to the invention and, in order to improve an adhesion of the incident particles 12, the surface 14 is subjected to a structuring process using a laser beam 16.
This involves generating trenches 17 by laser ablation, wherein the laser beam 16 is incident on the surface 14 of the substrate 11 at an angle of inclination p. This means that the trenches 17 and the resulting ridges 18 between the trenches are oriented obliquely. That is to say that they do not project vertically upwards in line with the surface normal 15 but the trenches have a plane of symmetry 19 which is inclined by the angle of inclination p, in line with the orientation of the laser beam 16. In figure 1, in the represented detail of the substrate 11, the spray angle a exactly matches the angle of inclination p. Under these geometric conditions, it is possible, for spray angles a > 0, to achieve an optimum adhesion result of the particles 12 on the surface 14. Figure 1 indicates schematically the manner in which the particle 12 is deformed when it hits the structured surface 14, such that the structure elements formed as trenches 17 and ridges 18 therebetween pierce the surface of the particle. This increases the surface area for adhesion such that the subsequent further plastic deformation (not shown in figure 1) of the particle 12 leads to an effective interlocking between the particle 12 and the substrate 11.

2012P21104W0US =
Figure 1 shows how the pretreatment by means of the laser beam 16 and the coating by means of the particle jet 13 are effected simultaneously. However, this means that the laser beam 16 and the particle jet 13 act simultaneously but at different points of the surface 14. Alternatively, it is of course also possible for the structuring of the surface to take place in a special production step as a pretreatment prior to coating.
Figure 2 shows another detail of the substrate. This is chosen such that the trenches 17 run diagonally. The angle of inclination p and the spray angle a are also marked in figure

2. It can also be seen in figure 2 that the particle jet 13 and the surface normal 15, at the point of incidence of the particle jet, give the arms of the spray angle a, wherein these two arms define a plane 20 which stands vertically on the surface 14 of the substrate 11. The figure further shows a plane 21 at right angles to the trenches 17 and through which a surface normal 15 also passes. Marked in this plane is the angle of inclination p, whose two arms lie in this plane 21 (even if the inclination in the trenches 17 cannot be seen in figure 2).
The difference which appears in the case of the cold spraying method according to figure 1 and according to figure 2 is that, in figure 2, the spray angle a and the angle of inclination p do not lie in one plane (the plane of the drawing in figure 1).
Instead, the planes 20, 21 of the angles a and p lie rotated with respect to one another by an angle y. Since both planes 20, 21 stand vertically on the surface 14, the angle y can be measured in the aspect on the surface 14, i.e. looking towards the surface normal 15. The maximum possible angle y of 25 defines in this context the maximum permissible deviation from the state represented in figure 1, in which the spray angle a and the angle of inclination p lie in one and the same plane.
In spite PCl/EP2013/069218 - 13 -of the deviation y, the structured surface 14 can still be advantageous for the coating result, since the effective interlocking mechanism of the incident particle 12, as shown in figure 1, is still effective.
The background for a deviation y between the planes 20, 21 being permitted is that real components have a more complex geometry than the substrates shown in figures 1 and 2. Here, for example for reasons of accessibility (cf. figure 4), it is necessary to deviate from the ideal geometric proportions according to figure 1 in the permissible range of the angle y.
Figure 3 shows a three-dimensional representation of a substrate which, instead of the trenches 17, has holes 22 which can for example be shot with a laser beam (not shown in more detail) into the surface 14 of the substrate 11. This also results in an angle of inclination p as a consequence of the orientation of the laser, wherein the arms of this angle are the surface normal 15 and the axis of symmetry 23 of the holes 22. Between the holes 22 there results a grid 24 which is also inclined by the angle of inclination 8 and which is also part of the adhesion-improving structuring of the surface 14.
Figure 4 shows a blade drilling 25 as the substrate. This is cast as a one-piece component, as represented, and is then assembled with other blade drillings to give a blade ring for a gas turbine. The position with respect to one another of turbine blades 26 of the blade drilling has the effect that problems would arise when coating, for example by cold gas spraying with the particle jet 13 under conditions according to the prior art. In order to illustrate this, the spray angle interval according to the prior art is represented at two points in the form of a cone 27. Taking into account the maximum permissible spray angle, which according to the prior art must lie within the cone, it is possible that a region 28 in the middle of the turbine blade is not coated. This is where the measure according to the invention comes into play. Shown not to scale on the middle turbine blade are the ridges 18 and trenches 17 which run substantially perpendicular to the principal blade curvature of the turbine blade 26 and thus also to the plane of the drawing.
These make it possible for the depicted particle jets 13 to be directed past the adjacent turbine blade and into the region 28, since the spray angle interval is increased thereby (cf.
also figure 6). Complete coating of the turbine blades 26 is thus possible.
Moreover, a method by means of erosion is indicated on one of the blades, which method could serve to produce the surface structuring on the blades. In this case it is a roller which in figure 4 is seen only from the end side. The roller is namely arranged perpendicular to the plane of the drawing, according to the trenches 17 and ridges 18 to be produced, and hence to the principal curvature of the turbine blades. The roller has a profile which matches the trenches 17 and ridges 18 to be produced. This profile is comparable to a sawtooth profile 29.
Figure 5 shows a method in multiple steps, in which a coat 30 to be generated consists of three layers 31a, 31b, 31c. The individual stages of coating the substrate 11 are separated from one another with break lines 32 and the stages are denoted with Roman numerals. These are briefly explained below.
The substrate 11 is made available.
II The surface 14 of the substrate 11 is structured. Trenches 17 and ridges 18 as shown in figure 1 are produced. This is also the case for the following structuring steps IV
and VI.
III The first layer 31a is produced and makes the surface 14a available for the subsequent coating step. It can also be seen that, in this context, the structuring of the surface 14 changes, wherein in this context an interlocking of the sprayed particles (no longer visible in figure 5) with the surface 14 takes place simultaneously. This is also the case for steps V
and VII.
IV The surface 14a again undergoes surface structuring (in the same manner as in step II).
V The next layer 31b is applied. This can be the same coat material as applied in step III or a multilayer coat is produced in which another coat material is applied.
VI The surface 14b resulting from step V again undergoes surface structuring.
VII The last layer 31c of the coat 30 is applied and forms the final surface 14c of the coated component.
Figure 6 shows how the measure according to the invention of a surface treatment prior to coating influences the spray angle interval. As already explained in relation to figure 4, the spray angle interval is formed by the cone 27. The permissible spray angles, or the particle jet which is not shown in more detail in figure 6, lie within the cone.
Figure 6 shows the cone 27 in part by means of dashed lines.
This form of the cone results, according to the prior art, in the case of an untreated surface. However, it can also be seen in figure 6 that the modified cone is not conical but deviates from the conical configuration which is depicted by the dashed line. Namely, on account of the angle of inclination p (cf.
figure 1, not shown in figure 6), there is a preferred orientation for the spray angle interval. In the direction of the angle of inclination, there results a nose-like protuberance. This means that, in this direction, much greater spray angles a (cf. figure 1, not shown in 2012P21104WOUS =
figure 6) are possible. Conversely, however, this leads to a flattening 34 of the cone since in this direction only smaller spray angles are possible. The representation according to figure 6 thus makes clear that, depending on the desired spray geometry, the structuring must be applied to the surface 14 in a targeted manner. It is also clear that, in the case of a deviation from the preferred orientation by the angle y (cf.
figure 2), an increased spray angle interval can still be of advantage, although the effect is not as pronounced.

Claims

1. A method for coating by thermal spraying, in which a particle jet (13) is directed onto a substrate (11) to be coated and the particles of the particle jet (13) are deposited onto the substrate (11), wherein the axis of the particle jet (13) runs inclined to the surface normal of the untreated substrate (11) forming a spray angle (.alpha.), characterized in that, prior to the thermal spraying, the substrate (11) undergoes pretreatment in which the surface is structured, wherein .cndot. structure elements (17, 18, 22, 24) of the structured surface are produced obliquely on the surface such that the structure elements (17, 18, 22, 24) are oriented towards the inclined axis of the particle jet (13) during the thermal spraying and .cndot. the spatial extent of the structure elements (17, 18, 22, 24) is smaller, at least in the plane of the spray angle (.alpha.), than the average diameter of the particles.

2. The method as claimed in claim 1, characterized in that the angle of inclination (.beta.) of the oblique orientation of the structure elements largely corresponds to the spray angle (.alpha.).

3. The method as claimed in either of the preceding claims, characterized in that the plane of the spray angle (.alpha.) and the plane of the angle of inclination (.beta.) of the oblique orientations of the structure elements are at an angle (.gamma.) of at most 25° to one another.

4. The method as claimed in one of the preceding claims, characterized in that trenches (17) are produced as structure elements.

5. The method as claimed in claim 4, characterized in that the trenches (17) have planes of symmetry (19) which are oriented obliquely during production such that they are oriented towards the inclined axis of the particle jet (13) during the thermal spraying.

6. The method as claimed in one of claims 1 to 3, characterized in that the structure elements are produced as holes (22).

7. The method as claimed in claim 6, characterized in that the holes (22) have axes of symmetry (23) which are oriented obliquely during production such that they are oriented towards the inclined axis of the particle jet (13) during the thermal spraying.

8. The method as claimed in one of the preceding claims, characterized in that a cold gas spraying is used as thermal spraying.

9. The method as claimed in one of the preceding claims, characterized in that .cndot. the substrate (11) has a region (28) which is inaccessible to the particle jet while observing the maximum permissible spray angle in the case of the untreated substrate, and in that .cndot. in this region (28) the structure elements (17, 18, 22, 24) are produced such that an increase, connected thereto, of the maximum permissible spray angle (.alpha.) results in the region (28) being able to be coated with the thermal spraying method.

10. The method as claimed in one of the preceding claims, characterized in that the structure elements (17, 18, 22, 24) are generated with a laser beam (16), by means of an erosion method, by chip removal, by means of a shaping method or chemically.

11. The method as claimed in one of the preceding claims, characterized in that the coating is effected by alternating multiple times between production of structure elements (17, 18, 22, 24) and production of layers of the desired coating.

12. The method as claimed in one of the preceding claims, characterized in that the coated substrate is a blade drilling (25) of a turbomachine, in particular a gas turbine.