CN111373068A

CN111373068A - Method for repairing single crystal material

Info

Publication number: CN111373068A
Application number: CN201880062701.4A
Authority: CN
Inventors: T.卡尔夫豪斯; R.瓦森; O.吉永
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2017-10-26
Filing date: 2018-10-04
Publication date: 2020-07-03
Also published as: WO2019080951A1; EP3701060A1; DE102017009948A1; ES2886226T3; EP3701060B1; PL3701060T3; US20200216942A1

Abstract

The invention relates to a method for coating the surface of a monocrystalline substrate of a component comprising a monocrystalline alloy with a coating material, wherein the surface to be coated is polished and the substrate is then transferred into a vacuum chamber. Where the entire substrate is heated from the outside to a temperature at least corresponding to half the melting temperature of the substrate (in c), but below the melting temperature of the substrate. The coating material is then applied to the surface to be coated in powder form by means of vacuum plasma spraying, wherein a powder having an average particle diameter of 10 to 100 μm is used. The working pressure is between 1 and 200 mbar and an argon/hydrogen mixture is used as working gas. This results in at least one region directly at the interface between the coating material and the polished substrate surface, which has the same single-crystal orientation as the substrate lying therebelow.

Description

Method for repairing single crystal material

The invention relates to the field of metals and alloys, in particular to the field of nickel alloys. In particular, the invention relates to a method for repairing monocrystalline materials which are often used for components which are subjected to high temperatures, such as blades of stationary gas turbines or aircraft turbines.

Background

It is known from the literature that the production of single-crystal blades for stationary gas turbines and/or aircraft turbines by directional solidification and special casting processes is very expensive and complicated. The microstructure produced in the component is oriented in the direction of the axial stress. The blades are generally exposed to high thermal loads during operation and also to the corrosive atmosphere, as a result of which they wear out strongly.

Thus, for example, there is great interest in repairing, rather than remanufacturing, single crystal turbine blades. For this purpose, repair methods are also known from the prior art, wherein there are distinguished methods for repairing, for example, turbine blades by thermal spraying methods, repair methods which use welding and laser cladding, and finally methods in which the ceramic material is epitaxially grown during the thermal spraying method.

Thus, by Kazuhoro et al^[1]A method is known with which defective turbine blades can be repaired. However, here no epitaxy occurs on the monocrystalline substrate, so that a polycrystalline microstructure is produced as a result, which usually does not have the mechanical properties of the original substrate.

Furthermore, US 5,732,467 a1 describes a method for repairing cracks in the outer surface of a component having a superalloy with a directionally oriented microstructure. The method described therein coats and seals the outer surface of a directionally solidified and single crystal structure by coating the defective region using a high velocity oxy-fuel process (also referred to herein as HVOF) and then hot isostatic pressing the corresponding component part. Here, the repaired region is intended to be formed without cracks, without adversely affecting the single-crystal microstructure of the remaining component parts. However, in this case too, polycrystalline microstructures are produced in the repair region, which have the disadvantages described above.

Furthermore, Boris Rottwinkel et al are known from the prior art^[2]In order to repair cracks under the tip region of monocrystalline components, such as turbine blades, a breaking point is provided for saving time and material, in order to eliminate the damaged region concerned first. The breaking point must be suitable in order to be able to be welded and in order to be able to simultaneously achieve the orientation direction of the newly applied material with the same orientation as the remaining material. For this reason, a temperature gradient that promotes the directional orientation (oriientierung sausterichtung) is required. The laser beam deposition described here is in principle a suitable method for welding such fracture points accordingly, because of its special process parameters, such as a small local energy input and a controlled material input. However, the challenge for this method is to achieve a perfectly monocrystalline, crack-free region, since only polycrystalline regions can usually be produced already by means of a small, unstable energy distribution.

By Henderson et al^[3]Automated welding methods for the industrial manufacture of gas turbines are known. The welding of highly alloyed nickel alloys is very complicated and often can only be satisfactorily applied with great difficulty. In welding attempts to repair impellers, special alloy wires are used for filling, for example. This is followed by a standard heating and ageing procedure, but in which microcracks occur.

In order to repair damage to monocrystalline materials, such as occur, for example, in blades or airfoils of gas turbines, a welding method is likewise proposed^[3]. In the case of repair by Laser beam deposition (Laser metal Forming (LMF) or Laser Cladding), it is in principle possible to produce monocrystalline structures on monocrystalline substrates. The method is characterized by a minimum of heat input into the component during the build,thereby preventing further cracking or recrystallization of the single crystal material.

Furthermore, by means of this method, the orientation of the starting material of the single crystal can be maintained until the interface is crossed into the newly applied material. The optimized process parameters can furthermore lead to a uniform epitaxy on the monocrystalline substrate, for example wherein the ratio between the temperature gradient in the welding zone and the solidification speed is above a threshold value for the material dependence.

However, until now, cracks cannot be repaired in a targeted manner. In repairing larger areas, increased stresses are typically generated due to thermal expansion. Furthermore, the document does not provide results with respect to repairing by this method the area in which the cooling hole or the cooling duct extends. A complex solidification system is formed by cooling holes-similar to those in the indentations. Directional crystalline solidification is generally only performed when the heat flow is constant and not disturbed. However, in the presence of cooling holes, this constant heat flow is often disturbed, resulting in cracks forming in this region and/or undesirable polycrystallinity. Thus, repair in such areas below the turbine tip is generally not possible with these methods.

Therefore, with the existing repair methods, it has not been possible to restore the microstructure of the single crystal base material at each region of the part to be repaired (e.g., turbine blade). This means that although the blades can be repaired, they do not generally have as good mechanical properties as the new blades.

In the field of ceramic processing, Shu-Wie Yao et al^[4]Subject matter of spraying molten TiO in plasma₂Study of the epitaxis during coagulation. It has been found that many parameters, such as the application temperature, crystallographic orientation and supercooling of the melt, have a significant effect on the epitaxial growth. In particular, the temperature of the melt determines whether heterogeneous nucleation or epitaxy occurs. This document shows that directional solidification has also been observed in plasma spraying.

Objects and solutions

The object of the invention is to provide a repair method for monocrystalline materials, in particular for monocrystalline blades for stationary gas turbines and/or aircraft turbines, in which the added material has largely the same microstructure and crystal orientation as the material to be repaired.

The object of the invention is achieved by a method for repairing monocrystalline materials according to the main claim.

Advantageous embodiments of the method result from the claims which refer to it.

Subject matter of the invention

It has been found within the scope of the present invention that epitaxy can be produced on a monocrystalline material (substrate) by means of vacuum plasma spraying.

Within the scope of the invention, the material to be repaired, hereinafter referred to as substrate material, generally comprises a metal alloy, in particular a nickel-based alloy or a cobalt-based alloy.

The term plasma spraying is understood to mean coating processes which are carried out by means of plasma and are not based on plasma polymerization.

Vacuum plasma spraying is understood to mean, in contrast to atmospheric plasma spraying, a coating process which is carried out in a vacuum chamber at a pressure of 1 to 200 mbar in order to avoid oxidation of the coating material by oxygen in the air.

Most preferably, the same material as the material constituting the base material is used as the coating material. Since the components to be repaired (for example, stationary gas turbine blades and/or aircraft turbine blades) are generally materials which are subjected to high temperatures, all metal superalloys or superalloys are considered as coating materials.

Currently, known superalloys include primarily solid and high strength nickel-based or cobalt-based alloys. As superalloys are generally meant metallic materials of complex composition (iron, nickel, platinum, chromium or cobalt based with the addition of the elements Co, Ni, Fe, Cr, Mo, W, Re, Ru, Ta, Nb, Al, Ti, Mn, Zr, C and B) for high temperature applications. Most of them are non-oxidizing and high temperature resistant. Their preparation can be carried out by melt metallurgy and powder metallurgy.

Within the scope of the present invention it has been found that the polycrystallinity of the thermally sprayed metal layer can be suppressed by spraying an alloy of at least the same type as the alloy that the monocrystalline substrate material has onto the heated and polished substrate surface under strongly reduced pressure and in an argon atmosphere. Within the scope of the present invention, the term "same type" is understood to mean that the proportions/compositions of the alloying elements of the substrate and the layer differ only slightly and that they have almost the same microstructure after the heat treatment.

It has been found that a small solidification rate is advantageous for directional single crystal growth of the applied material. The solidification rate within the applied layer generally decreases with increasing substrate temperature.

According to the invention, the temperature of the substrate is so high that the solidification speed of the molten powder particles is greatly reduced, but not so high as to reach the melting temperature of the substrate. Typically, the substrate temperature is set for this purpose between 700 ℃ and a temperature just below the melting temperature of the substrate used, i.e. for example 50 ℃ below the melting temperature of the substrate.

In this process, the coagulation speed is disadvantageously not accurately measured, but it should preferably be less than 100 mm/s.

Under the stated conditions, nucleation does not take place at any point within the applied layer, but advantageously directly at the substrate surface, where it is oriented in the predefined orientation of the single crystal of the substrate. Thus, the applied layer can be oriented on the substrate.

Preferably, the heating of the area of the substrate to be repaired is carried out by a meandering motion of the plasma burner on the surface of the substrate without transporting the powder.

Heating of the entire substrate is additionally performed. Here, the substrate can be heated in different ways: electrically, inductively or by electromagnetic radiation. Advantageously, the entire substrate is heated to at least 700 ℃, advantageously to at least 800 ℃, preferably even to about 1100 ℃ depending on the alloy.

It is important in the method according to the invention that during the application of the thermal spray coating, the substrate itself is heated, but not to a temperature at which the substrate melts. In the repair process, therefore, the powder melted in the plasma theoretically strikes the solid, polished substrate surface, nucleates there (nukliert) and can therefore advantageously solidify with the same crystallographic orientation. In practice, however, depending on the implementation of the method, it cannot be excluded that the surface of the substrate is locally melted by a few μm.

This method step is clearly different from the repair methods known to date (for example welding methods by means of a laser), in which the substrates themselves are usually also melted together at least at the surface to be repaired.

It is important for the composition of the plasma gas that it has hydrogen. The hydrogen gas creates reducing conditions that generally inhibit oxidation of the substrate material during the heating process. In this regard, a suitable argon-containing plasma gas may have a minimum of 5 NLPM and a maximum of 25 NLMP hydrogen under 50 NLPM argon. NLPM represents standard liters per minute and refers to the gas flow rate under standard conditions (T273.15K). This then corresponds to a concentration range of 10 to 50% by volume of hydrogen in the plasma gas argon.

In order to carry out the method, a vacuum plasma spraying apparatus with a powder delivery system and a means for heating the substrate (component part) to a temperature of about 700 ℃ up to 1300 ℃ are preferably required. The area of the component to be repaired should preferably be polished.

The repair process of defective components usually starts with the removal of the bond coat and top coat of the thermal barrier by hydrofluoric acid, also known as stripping (in english), as long as such bond coat and top coat are present on the substrate material.

In the next step, critical damage is identified and typically removed, ground and polished by cutting methods.

Sanding may be performed, for example, with sandpaper of the following grit: 320. 640, 1200 and 4000.

The subsequent polishing can be carried out with a diamond suspension on a soft cloth, wherein for example a suspension of diamond particles having an average particle size of about 3 μm is used first, followed by a suspension of diamond particles having an average particle size of about 1 μm. Optical microscopy is suitable for inspecting polished substrate surfaces. Here, the treated substrate surface should be free of scratches.

Masking of the undamaged areas of the substrate is performed in the next step.

The removed area can now be reconstructed by the method according to the invention. In this case, layer thicknesses of approximately 10 μm up to several mm can be achieved. The layer thickness at one pass/spraying of the plasma burner can be set individually and derived from the robot speed in relation to the powder transport rate. The entire layer thickness is usually achieved by multiple passes/sprays.

Thus, for example, a layer thickness of approximately 25 μm is obtained in one pass. The layers can be constructed to be arbitrarily thick depending on the number of passes. However, the application rate in one pass should not be too high, since otherwise increased pore formation would occur disadvantageously. No polishing is required between passes.

According to the invention, it is also possible to apply a plurality of layers, as long as the polishing of the respective surfaces takes place between the application of the layers. This may be necessary, for example, when further repairs are needed after a first repair and inspection of an area. In this regard, the repaired substrate may then be repolished and used for further repair.

Since the applied layers are generally very low in stress due to the high application temperature, there is no physical limit to the maximum layer thickness that can be applied by the method according to the invention. A range of layer thicknesses of a few μm up to about 5 mm can be obtained by this method.

Subsequent post-treatment and optionally restoration of the original component dimensions and heat treatment, for example in the form of solution annealing and precipitation annealing, are carried out.

In a final step, a new insulation layer may then be reapplied and the cooling holes optionally re-drilled as required.

The method according to the invention therefore advantageously offers the possibility of bringing defective and rejected monocrystalline blades into a condition as good as new.

Optimized processThe parameters can be determined here by the person skilled in the art with the aid of some preliminary tests. To this end, depending on the material, a CET model and/or microstructure map may already exist, e.g. made of CMSX-4 ® type^[5](see FIG. 1).

In summary, it can be said that in the method according to the invention it is important to heat the entire substrate or the entire component from the outside first to a temperature just below the melting temperature of the substrate. The temperature to be set here is alloy-specific. Additional further heating by the plasma beam is required in order to suppress oxidation of the surface. The hydrogen contained in the plasma beam creates reducing conditions. The temperature sought should be as high as possible, so temperatures below the melting temperature of 50K are desirable.

The temperature difference from the rest of the substrate/component should be as small as possible, since the area to be repaired should advantageously have as uniform a temperature distribution as possible.

The main difference of the method according to the invention is the additional external heating of the substrate. Only with this method can the sought microstructure be achieved with the outstanding mechanical properties and minimal intrinsic stress of the single crystal alloy. In contrast, it is not sufficient to heat the components only by the energy of the introduced plasma.

In the CET diagram (Columnar to Equipment transformation (CET)), the solidification speed and the influence of the temperature gradient present at this point on the microstructure formed here by the solidified material are shown.

Fig. 2 schematically shows a solidification model of the described method. The particles of molten powder having a velocity v_pImpinging on the heated surface of the sample. Three temperature zones are formed near the surface. The temperature near the substrate is below the melting temperature. Where dendrites and interdendritic regionsThe domain (interdintristche Bereich) has solidified. Located above this is a transition region in which the solidification front is located and dendrites are formed. The interdendritic regions are not yet solidified. In the upper region of the figure, the molten particles impinge on the substrate. Here, the temperature is above the melting temperature. In addition, the figure shows a large dendrite arm spacing (Dendritenermabmstand) λ in the substrate_{1 substrate}Which is formed in the manufacture of the monocrystalline substrate due to the very small solidification speed v and due to the small temperature gradient G (see CET diagram).

Due to the high temperature of the molten powder particles, the temperature gradient G increases and the solidification speed v also increases due to the sought substrate temperature. This results in a reduced dendrite arm spacing λ 1_{Repair layer}。

Detailed description of the invention

The invention is explained in more detail below with the aid of embodiments and some figures, without this being intended to lead to a limitation of the broad scope of protection.

The following exemplary shows CMSX-4^®Directional solidification of the powder on an ERBO-1 substrate. The two alloys are very similar. The exact composition can be learned from the following table. CMSX-4^®Is a registered trademark of Single Crystal (SC) alloys of Cannon-Muskegon, MI (USA). ERBO/1 is the second generation single crystal nickel base superalloy by Doncaster Precision Casting, Bohong (Germany).

Table 1:

elements [ weight%]	Al	Cr	Co	Hf	Mo	Re	Ti	Ta	W	Ni
											CMSX-4^®Powder of	6.0	6.4	9.5	0.1	0.6	2.9	0.9	8.5	8.1	Balance of
ERBO/1^®Substrate	5.7	6.5	9.6	0.1	0.6	2.9	1.0	6.5	6.4	Balance of

First, substrate samples were made from ERBO-1 plates with dimensions of 32 mm × 20 mm × 2.5.5 mm and with holes of 1.1 mm in diameter and 10 mm in length by means of spark erosion (funkenedodieren) fig. 3 shows the sample geometry used here.

Prior to coating, the substrate samples were ground and polished. Here, the surface was first treated with sandpaper having a grit size of 320, 640, 1200 and finally with a grit size of 4000 in this order.

The subsequent polishing is carried out with the aid of a soft cloth impregnated with a diamond suspension. A cloth having a suspension of diamond particles with an average particle diameter of about 3 μm was first used, and the surface was subjected to circular polishing. Subsequently, another cloth having a suspension of diamond particles with an average particle size of about 1 μm was used, and the surface was repolished.

The surface of the substrate thus treated and polished was examined by optical microscopy. No scratch was detected on the substrate surface.

Subsequently, the polished sample was mounted in a heated sample holder. The technical diagram shows the exact configuration according to fig. 4.

Insulated SiN flat heater 2 with a power of 1000W enables heating of sample 4 up to 1100 ℃ in vacuum, preferably at 1 to 200 mbar. On the heater 2 is a SiC heater plate 3, which provides a more constant temperature of the sample. Heater 2, thermally conductive plate (SiC) 3 and sample 4 were surrounded by prepared insulation 1, 5, which reduces convection. The application of the sprayed layer or layers takes place via openings in an orifice plate (blend) 6. The temperature regulation is carried out by means of a regulator and by measuring the temperature in the sample with a thermocouple. The cable of the thermocouple and the power supply cable of the heater are separately placed in the vacuum chamber by means of a bushing.

The powder conveyor Sulzer Metco powder feeder Twin-120-V was filled with CMSX-4^®A powder having spherical particles with an average geometric particle size of 25-60 μm. The determination of the average particle size is carried out here by means of laser diffraction with the equipment Horiba LA-950V2 from Retsch.

For powders having an average particle diameter of 38.53mm, for example, D₁₀A value of 27.70 μm, D₅₀The value was 39.77 μm, D₉₀The value was 55.27. mu.m.

The powder was stored beforehand for 2 hours at 150 ℃. This step serves to remove water from the powder.

Followed by a coating process according to the present invention. The spray parameters set for this are known from table 2 below.

Table 2:

the experimental number is v-17-061-f4	The user:
		item WDS. internal	Described as 20 x 30 x 2.5 mm
Spray part (Spritzteil) number: RX-samples
		Powder (line 1) CMSX 4/V2	Injection site (line 1) bottom (90 degree)
Powder (wire 2)	Injection site (line 2)
		Powder (wire 3)	Injection position (line 3)
Scraper (line 1) Nl	Powder groove (line 1): 16 x 1.2
		Scraper (wire 2)	Powder groove (line 2): 11 x 0.5
A scraper (wire 3)	Powder groove (line 3): 11X 0.5
		Process pressure (mbar): 60	Splash stream (a):
spray distance (mm): 275	Rotation stage (1/min.):
		robot-speed (mm/s) 440	robot-PRG MHOR 4Y 440X 120
Coating cycle/time 8	0₂Addition (SLPM) 0
		Substrate CMSX-4	Surface treatment: spraying and polishing
Coating temperature (DEG C) 900	Layer thickness (. mu.m) 320
		Note that heated sample holder X + -135R 10=3	Layer weight (g) 0
And (3) diagnosis:	data are reported on the heating by the serpentine (M ä ander) -procedure and direct feeding of the powder

If the heating process is started, the sample heater is first activated. Starting at a temperature of about 300 c, the plasma flame of oerlinkon inc F4-VB facilitates heating of the substrate surface up to a coating temperature of about 900 c.

The hydrogen contained in the plasma gas contained in argon (plasma gas: 50 NLPM argon and 9 NLPM hydrogen) provides the reducing conditions here. Thus, the oxygen contained in the argon can be oxidized in a targeted manner without it reacting with the substrate surface and disadvantageously forming an oxide layer.

The parameters selected for coating can be learned in table 2.

Table 2:

argon [ NLPM ]]	50.0±6.1
		Hydrogen gas [ NLPM ]]	9.0±0.6
Sample temperature [ deg.C]	900±10
		Spraying distance [ mm ]]:	275±0.1
Robot-speed [ mm/s ]]	440±5
		Process pressure [ mbar ]]:	60±1
Powder delivery rate based on maximum delivery rate given in%	15±0.5
		Powder delivery Rate (Absolute)	47.7g/min.

After coating, a heat treatment is generally advantageous.

For example, Solution annealing (SHT) may be necessary in order to reduce non-uniformities that may be present in the texture of the coating.

The heat treatment described above can advantageously be carried out with the aid of the pressure of a Hot Isostatic Press (HIP), in the english Hot Isostatic Press. The pores in the tissue are generally reduced by pressure-assisted heat treatment.

The regular arrangement of the γ' precipitates within the γ matrix is usually performed by solution annealing. The gamma' precipitation plays a decisive role for very good mechanical properties in the high temperature range.

The precise temperature schedule for performing the heat treatment of this example is set forth below:

solution annealing, namely under the protective atmosphere, cooling at 1300-1315 ℃ for 6 hours and then at 150-400 ℃/min to about 800 ℃.

Precipitation annealing at 1140 + -10 deg.C for 4 hr and then at 870 + -10 deg.C for 16 hr under a protective atmosphere.

In fig. 5a and 5b, scanning electron micrographs of cross sections of the thus treated samples are shown, showing directional solidification on a monocrystalline substrate. Fig. 5a shows a single crystal substrate with a repair layer sprayed on it. The columnar structure of the grains in the polycrystalline layer is the sign of directional solidification. At the transition between the substrate and the layer, areas appear with a grey coloration similar to the substrate. This means that due to the contrast of crystal orientation in the backscattered electron image of the scanning electron microscope, the same crystal orientation is present for the substrate and the layer in this same color region. Figure 5b shows a higher magnification of this region. No oxide is present in the transition from substrate to layer. This is very important for the nucleation of the molten powder on the substrate. Dark gamma' deposits in the gamma matrix can be identified in the substrate.

Fig. 6a and 6b show scanning electron micrographs of cross sections of the same samples, which were first solution annealed and then precipitation annealed after coating with the above parameters.

Fig. 6a shows the transition range from the monocrystalline substrate to the repair layer. The white dashed line represents the previous interface. By this heat treatment, the crystal grains nucleated on the single crystal substrate grow into the polycrystalline layer at the expense of small crystal grains. At least at the interface, a single crystal structure having the same crystal orientation as the substrate has is formed. The repair layer has only a slightly increased pore density, which disappears by means of pressure-assisted heat treatment by HIP. The smaller black dots indicate Al generated due to slight oxidation of the spray material₂O₃And (4) inclusion.

Fig. 6b shows an enlarged portion. At the previous interface, Al₂O₃The hole edge (Porensaum) indicates it. By precipitation annealing, the γ' precipitates in the γ matrix are reduced in size and they are arranged in cubes. This arrangement provides the best possible mechanical properties of the alloy. In addition to the same crystal orientation contrast, the orientation of the precipitate also shows that the monocrystallinity of the substrate has continued into the repeat layer.

In addition to the studies with scanning probe microscopy, photographs of electron back-scattered Diffraction (EBSD) analysis, english Elektron backsschaften Diffraction, were prepared for these samples (not shown here). Thus, the applied coating can be identified by its red color, which represents the (001) crystal plane in which the substrate material is also oriented. It can thus be shown that in the application according to the invention, the applied spray coating solidifies in the same directional orientation as the monocrystalline substrate material, at least in wide regions.

In the development of a repair method within the scope of the present invention it has been found that the porosity in the spray coating is determined by the application rate caused by the powder transport rate and the robot speed, the porosity of the layer (Pr ö sit ä t) also decreases with decreasing application rate.

The quality of argon in terms of oxygen content should be improved if an oxide layer is formed between the substrate and the repair layer that prevents nucleation. Another reason for the formation of an oxide layer may be an unfavourable robot movement during the spraying process. Preferably, it should be adjusted so that the sample does not leave the range of influence of the plasma burner. If nucleation does not occur at the polished surface of the area to be repaired even if the oxide layer is not present, the temperature of the workpiece to be repaired must be increased.

Documents cited in the present application:

[1] kazuhiro Ogawa and Downon Seo (2011), Repair of Turbine Blades using Cold Spray Technology, Advances in Gas Turbine Technology, Dr. Ernesto Benini (Ed.), InTech, DOI: 10.5772/23623, available from https:// www.intechopen.com/books/Advances-in-Gas-Turbine Technology/Repair-of-Turbine-Blades-using-cold-Turbine-Technology.

[2]Boris Rottwinkel, Luiz Schweitzer, Christian Noelke, Stefan Kaierle,Volker Wesling. Challenges for single-crystal (SX) crack cladding, PhysicsProcedia 56 (2014) 301 - 308.

[3]M. B. Henderson, D. Arrell, R. Larsson, M. Heobel&G. Marchant,Nickel based super-alloy welding practices for industrial gas turbineapplications, Science and Technology of Welding and Joining Volume 9, 2004 -Issue 1.

[4]Shu-Wei Yao, Tao Liu, Chang-Jiu Li, Guan-Jun Yang, Cheng-Xin Li,Epitaxial growth during the rapid solidification of plasma-sprayed moltenTiO₂splat, Acta Materialia 134 (2017) 66e80.

[5]W. Kurz, C. Bezençon, M. Gäumann, Columnar to equiaxed transition insolidification processing, Science and Technology of Advanced Materials 2(2001) 185 - 191.

W. Kurz, D. J. Fisher, Fundamentals in Solidification, Chryst. Res. Tech.(1986) 21 (9), 1176.

[6]Boris Rottwinkel, Christian Nölke, Stefan Kaierle, Volker Wesling,Crack repair of Single crystal turbine blades using laser claddingtechnology,Procedia CIRP(22) 2014, 263, 267, obtained from:

http://www.sciencedirect.com/science/article/pii/S2212827114007732

Claims

1. a method of coating a surface of a monocrystalline substrate of a component part comprising a monocrystalline alloy with a coating material, the method having the steps of:

-polishing the surface to be coated,

-transferring the substrate into a vacuum chamber,

heating the entire substrate to a temperature corresponding to at least half the melting temperature (in C.) of the substrate, but below the melting temperature of the substrate,

applying the coating material in powder form onto the surface to be coated by means of vacuum plasma spraying,

-wherein a powder with an average particle size of 10 to 200 μm is used,

-wherein the pressure is set to 1 to 200 mbar, and

-wherein an argon atmosphere with a hydrogen proportion of 10 to 50% by volume is used as the working gas,

-thereby creating at least one region directly at the interface of the coating material and the polished substrate surface, which region has the same orientation of the single crystal orientation as the substrate lying therebelow.

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

wherein the same material as the substrate is used as the coating material.

3. The method according to one of claims 1 to 2,

wherein a single-crystal nickel-based alloy or cobalt-based alloy is used as the substrate and as the coating material, respectively.

4. The method according to one of claims 1 to 3,

wherein the entire substrate is heated to at least 700 c, advantageously to at least 800 c.

5. The method according to one of claims 1 to 4,

wherein the heating of the entire substrate is performed electrically, inductively or by electromagnetic radiation.

6. The method according to one of claims 1 to 5,

wherein the heating of the substrate surface is carried out by means of a plasma burner without powder supply.

7. The method according to one of claims 1 to 6,

wherein after coating, the coated substrate is subjected to solution annealing and/or precipitation annealing and/or pressure assisted heat treatment.

8. The method according to one of claims 1 to 7, wherein a substrate with at least one cooling hole is used.