DE102017209842A1 - Process for coating a surface of a component by thermal spraying - Google Patents

Abstract

The invention relates to a method for coating a surface of a component by means of thermal spraying, wherein a gas stream is generated, sprayed on or melted and transported by the gas flow to the surface of the component and deposited on the surface of the component to form a coating, wherein the specific enthalpy of the gas flow is determined and properties of the coating are controlled by means of a setting of the specific enthalpy.

Description

The invention relates to a method for coating a surface of a component by means of thermal spraying.

Surface coatings on components can be made by thermal spraying techniques. In this case, additional materials, so-called. Spray particles, inside or outside of a spray burner on or melted, accelerated in a gas stream and thrown onto the surface of the component to be coated. The component surface is not melted and only slightly thermally stressed. When hitting the component surface, the spray particles remain adhered to the surface, so that the surface coating can be formed in layers.

The achievable properties of the coating, such. As their adhesion, surface morphology (eg roughness), porosity, density, phase composition, hardness, modulus or thickness, are significantly influenced by the temperature and the speed of the spray particles at the time of their impact on the surface to be coated. The surface condition of the surface to be coated, z. As their purity, activation state or temperature, also exerts a significant influence on the properties of the resulting coating.

Thermal spraying processes are primarily used to apply coatings of metals, oxide-ceramic materials and / or carbidic materials to metallic or non-metallic components.

The energy for heating and accelerating the spray particles may, for. B. be provided by means of a plasma jet. The associated coating method is referred to in this case as plasma spraying.

For plasma spraying, a plasma torch is used which has an anode and at least one cathode. DC voltage creates an arc between anode and cathode. A working gas flowing through the plasma torch is passed through the arc and thereby ionized, so that a very hot, electrically conductive gas of positive ions and electrons (plasma stream) is formed. As a working gas, for example, argon, hydrogen, helium, nitrogen or mixtures of the gases mentioned can be used.

For plasma flow, the spray particles, possibly by means of a carrier gas, fed and thereby on or melted. The plasma stream entrains the spray particles and hurls them at the component to be coated, where they are deposited to form the coating.

To reduce the material wear of the plasma torch and to dissipate the heat flow cooling of the plasma torch is necessary. This can be designed as water cooling.

Another thermal spraying method is the high-speed flame spraying, in which by combustion of a gaseous fuel such. As propane, ethylene, butane, acetylene, hydrogen, or a liquid fuel such. B. diesel or kerosene, in a cooled combustion chamber with downstream expansion nozzle, a gas flow at high speed is formed. Combustion may be either high velocity oxygen fuel (HVOF) or high velocity air fuel (HVAF). The spray particles are either supplied to the combustion chamber or in the region of the expansion nozzle, accelerated, transported in the direction of the component to be coated and deposited on the surface thereof.

To assess the coating process, the order efficiency (DE) is often determined. This corresponds to the ratio of the mass of adhering spray particles on the component to the mass of the injected spray particles and can assume values between 0 and 1.

The electrodes, in particular the cathodes, of a plasma torch are subject to wear (wear) in the course of their use. Such wear usually results in deterioration of the performance of the electrodes.

The wear of the electrodes is u. a. depending on the plasma power used for the coating process. If the required plasma power is high, a faster wear of the electrodes can be observed. This can cause the electrodes to be replaced after a few hours of use.

Due to the wear and deterioration of the performance of the electrodes, the voltage of the arc and thus the plasma power may change. This can reduce the efficiency of the coating process and affect coating properties. This may be reflected in a decrease in the order efficiency.

The wear also changes the geometry of the tip of the cathodes, as material is removed from the cathode tip during the coating process. moreover Also changes the chemical composition of the material at the cathode tip. Furthermore, the anode channel can also erode. This can affect the formation of the arc, which can be determined by changing the voltage.

The changes described may affect the coating to be applied and, for example, lead to changes in the properties of the coating. In other words, there is a risk that the resulting coating does not have the desired properties, eg. B. that no homogeneous coating is obtained on the component.

To the parameters of the plasma torch, z. As the voltage of the arc, and parameters of the coating process, for. As the order efficiency, hold as constant as possible over a certain period of operation and thus to achieve a homogeneous coating on the component, in the prior art, the current and / or the plasma torch supplied power of the working gas, eg. As the flow of gaseous hydrogen changed.

For example, by changing the inflow of working gas, the voltage of the plasma-generating arc can be influenced. By changing the current intensity, the order efficiency can be influenced. In other words, z. B. the order efficiency controlled or regulated, z. B. held constant, by the current level is set.

This can enable the setting or restoration of a specific (predefinable) tension and a certain (specifiable) order efficiency. Both the adjustment of the current intensity and the gas flow to working gas can thus be used to at least approximately adjust parameters of the coating process and thus also properties of the coating and to keep them as constant as possible in the course of a coating process using the same plasma torch.

The disadvantage, however, that not only one parameter of the plasma torch or coating process is affected by changing the current and / or the gas flow, but several, z. T. interdependent parameters.

For example, by increasing the gas flow, not only the voltage is affected, but also the flow rate of the resulting gas flow is changed. As a result, although the plasma power can be kept constant, the coating rate, i. H. the achievable per unit time layer thickness of the coating changes. If the current intensity is increased to keep the application efficiency constant in order to compensate for a voltage drop due to wear of the electrodes, this can lead to overheating of the spray particles, which ultimately affects the porosity and microstructure of the resulting coating.

In other words, the change of the current intensity and / or the gas inflow may lead to a change in the temperature and the velocity profile of the plasma, which u. a. brings a non-reproducible heating of the spray particles with it. The quality of the coating and repeatability of the coating process can not be guaranteed.

Object of the present invention is therefore to provide a way with the specifiable properties of a thermal spraying, z. B. by plasma spraying or high-speed flame spraying, applied to a component coating can be achieved. For example, consistent properties over a certain period of operation, for example within a coating process, can be guaranteed.

It is also intended to specify a possibility with which it is possible to react to a wear of the electrodes of a plasma torch in a plasma spraying process such that the properties of the coating do not change or only insignificantly change during the course of a coating process.

It would also be desirable to reproducible conditions for obtaining specifiable coating properties during a coating process, the z. B. on another identical device for thermal spraying, for example, on a structurally identical plasma torch, are transferable to identify and determine.

This object is solved by the subject matter of the independent claim. The dependent claims include embodiments of this inventive solution.

The invention is based on the idea that coating by thermal spraying, z. Example by means of plasma spraying or high-velocity flame spraying, taking into account the specific enthalpy of the gas stream generated, ie the energy contained in the gas stream (especially heat energy), based on the mass of the gas stream to perform. For example, the specific enthalpy can be set, ie z. B. changed or kept constant to parameters of the coating process, for. B. the To control order efficiency, and / or coating properties. In other words, there is a direct relationship between specific enthalpy and coating properties. The specific enthalpy can also serve as a manipulated variable in a regulation of the coating properties (controlled variable).

The use of the specific enthalpy as a reference variable for the coating process is a significantly more meaningful parameter than the parameters previously used for process control, such. B. tension or order efficiency. In addition, the monitoring of the specific enthalpy during the coating process may advantageously be sufficient, while according to the prior art, a large number of parameters had to be monitored.

For example, in the case of a plasma spraying process, the specific enthalpy of the generated plasma stream can be utilized.

High-speed flame spraying can take into account the specific enthalpy of gas-fueled gas flow.

This also makes it possible to take into account the cooling performance of the cooling device of the plasma torch during plasma spraying or the combustion chamber during high-speed flame spraying and their effects on the coating method and the resulting coating.

According to the invention, it is therefore provided that, in a method for coating a surface of a component by means of thermal spraying, in which a gas flow is generated, spray particles are melted or melted and transported by means of the gas flow to the surface of the component and on the surface of the component to form a coating are determined, the specific enthalpy of the gas flow determined and properties of the coating are controlled by means of a setting of the specific enthalpy.

The specific enthalpy can be understood as a collective size, which can be varied in order to achieve predefinable coating properties. You can in turn be adjusted by the enthalpy influencing parameters such. B. Amperage of a plasma torch during plasma spraying, to be changed.

In various embodiments, the thermal spraying can be carried out as plasma spraying by using a plasma burner cooled by a cooling device from a working gas, a plasma stream is generated as a gas stream.

In further embodiments, the thermal spraying may be carried out as high speed flame spraying by burning a fuel composition in a combustor cooled by a cooling device to produce a high velocity gas flow.

In general, the enthalpy from the balance between introduced energy, z. As electrical energy in plasma spraying or internal energy of the fuels in high-velocity flame spraying, and emitted energy, eg. B. by cooling the device for thermal spraying based on the cooling capacity of the cooling device, can be determined. At constant gas flow, the specific enthalpy can then be determined by determining the ratio of energy to mass of the gas.

In the case of high velocity flame spraying, the energy produced may be derived from the combustion enthalpy of the combustion reaction. In the case of plasma spraying, the energy introduced is the output of the plasma torch (the product of the current and the voltage of the arc). In both cases, the energy emitted by cooling can be calorimetrically determined or calculated from the product of the mass flow rate of the cooling medium, the specific heat capacity of the cooling medium and the temperature difference between the cooling medium and the gas flow.

When viewing the plasma torch as ohmic resistance heating can be assumed at a constant specific enthalpy of the plasma flow of a constant temperature and a constant velocity profile of the plasma, so that the spray particles are reproducibly heated and accelerated and ultimately reproducible coating properties result. The specific enthalpy can be used to control or control the properties of the coating.

Even in the case of high-speed flame spraying, the specific enthalpy can be used for process control in order to obtain predefinable and possibly constant coating properties. In addition, the specific enthalpy can also be used to easily and inexpensively adjust the desired coating properties in another device for high-speed flame spraying.

According to various embodiments can be provided in an embodiment of the thermal spraying as plasma spraying, the specific enthalpy by varying the plasma power and / or variation of the mass flow rate of the gas stream and / or variation of the composition of the working gas, z. B. the hydrogen content of the working gas to adjust.

For this purpose, the specific enthalpy of the plasma power and the mass flow rate of the gas stream and the composition of the working gas can be determined.

According to further embodiments, the plasma power can be adjusted by varying the current intensity and / or voltage of the plasma torch and / or the cooling capacity of the cooling device.

mit

P_Plasma

Plasmaleistung

U

Spannung

I

Stromstärke

m

Massendurchsatz des Kühlmediums

c

spezifische Wärmekapazität des Kühlmediums

ΔT

Temperaturdifferenz zwischen Kühlmedium und Plasmabrenner

wobei (m·c·ΔT) die Kühlleistung und U I die Ausgangsleistung ist.For this purpose, the plasma power can be determined from the current intensity and voltage of the plasma torch and the cooling capacity of the cooling device. The following applies: $$P_{Plasma}=U\cdot I-\left(\dot{m}\cdot c\cdot \mathrm{\Delta }T\right)$$

With

_Plasma

plasma power

U

tension

I

amperage

m

Mass flow rate of the cooling medium

c

specific heat capacity of the cooling medium

.DELTA.T

Temperature difference between cooling medium and plasma torch

where (m · c · ΔT) is the cooling power and UI is the output power.

Investigations showed that further influences, such. B. heat losses due to heat radiation are negligible. If necessary, correction factors can be included in the determination of the specific enthalpy.

To maintain consistent coating properties despite electrode wear during a plasma spraying coating operation, i. H. a specific time period in which the coating process is carried out, the specific enthalpy can be kept constant, for example by changing the constant mass flow rate of the gas stream or changing the mass flow rate of the gas stream at constant current or correspondingly changing the two parameters current intensity and mass flow rate of the gas stream , It is thus possible to keep the specific enthalpy constant over the lifetime of the electrodes. The process control of the coating process can be significantly improved.

In addition, the specific enthalpy can also be used to build with identical plasma torches, z. B. due to electrode wear have a different voltage to achieve substantially the same coating properties. For example, the parameters of the plasma torch and the coating process can be adjusted so that a certain specific enthalpy results.

As a result, the time required for setting parameters of the plasma torch and the coating method can also be reduced since the same coating properties can be achieved with the same specific enthalpy. Elaborate and expensive tests of the parameters can thus be avoided or at least reduced to a minimum.

According to various embodiments, in an embodiment of thermal spraying, high-speed flame spraying may be provided to adjust the specific enthalpy by varying the cooling capacity of the cooling device and / or varying the mass flow rate of the fuel and / or varying the fuel composition.

The specific enthalpy can be determined from the combustion enthalpy of the fuel composition, the cooling capacity of the cooling device and the mass flow rate of the fuel.

In other words, the specific enthalpy can be influenced and adjusted to the desired size by changing the cooling capacity, the mass flow rate of the fuel or the fuel composition, which has an effect on the combustion enthalpy.

According to further embodiments, the specific enthalpy can be kept constant. Due to the direct relationship between specific enthalpy and coating properties, homogeneous coating properties can be achieved.

According to further embodiments, the adhesion, roughness, porosity, density, phase composition, hardness, modulus of elasticity and / or layer thickness can be controlled as the property of the coating. Among other things, these properties are directly related to the specific enthalpy of the gas stream.

According to further embodiments, the surface of a gas turbine component, for. B. one Guide or blade of a gas turbine to be coated.

Such components are exposed to high, in particular thermal loads. They are therefore usually coated with an oxidation protection layer and / or corrosion protection layer and / or thermal barrier layer. In order to enable a high effectiveness of these coatings, a particularly homogeneous coating is necessary. For example, even the smallest imperfections in the coating can lead to rapid oxidation and / or corrosion or impairment due to excessive thermal load. The invention enables the creation of such particularly homogeneous coatings.

• 1 Abhängigkeit der spezifischen Enthalpie und Spannung von der Stromstärke während eines Plasmaspritzverfahrens;
• 2 Abhängigkeit der spezifischen Enthalpie und Spannung vom Wasserstoffgehalt des Arbeitsgases in einem Plasmaspritzverfahren;
• 3 Zusammenhang zwischen Ausgangsleistung, Wasserstoffgehalt des Arbeitsgases und spezifischer Enthalpie in einem Plasmaspritzverfahren.

The invention will be explained in more detail with reference to embodiments. The accompanying drawings show:

• 1 Dependence of specific enthalpy and voltage on current during a plasma spraying process;
• 2 Dependence of the specific enthalpy and voltage on the hydrogen content of the working gas in a plasma spraying process;
• 3 Relationship between output power, hydrogen content of the working gas and specific enthalpy in a plasma spraying process.

In the Examples explained below, reference is made to the accompanying drawings, which form a part of the Examples, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It should be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. It should be understood that the features of the various exemplary embodiments described herein may be combined with each other unless specifically stated otherwise. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

All embodiments relate to methods for coating a surface of a component by means of thermal spraying, the thermal spraying being carried out as plasma spraying. 1 and 2 show the dependence of the specific enthalpy of the plasma current and the voltage on the current ( 1 ) and the hydrogen content in the working gas ( 2 Incidentally, the working gas is made of argon.

It turns out that the specific enthalpy is linearly dependent on the current strength as well as on the hydrogen content (with the exception of a hydrogen content of 0% by weight) and increases with increasing current intensity and increasing hydrogen content.

In contrast, the relationship between current and voltage as well as between hydrogen content and voltage is not linear. The specific enthalpy can be determined at least by means of a variation of the composition of the working gas, i. H. be set in the embodiment by varying the hydrogen content, and the current.

The determination of the specific enthalpy can be carried out directly at the measuring station of the coating device based on the output power (voltage x current), the power loss by cooling (cooling power) and the mass flow rate of the gas stream. The determined value of the specific enthalpy can be displayed directly on the measuring stand on a display device.

Since the voltage changes in the course of a coating operation or during the service life of the coating apparatus and this u. a. On the melting or melting of the spray particles and ultimately affects the coating properties, this voltage change must be counteracted to achieve homogeneous coating properties. According to the invention, the energy content of the plasma stream, d. H. the specific enthalpy of the plasma stream, kept as constant as possible.

Out 3 It can be seen that a certain specific enthalpy can be achieved in different ways, ie by different combinations of the parameters influencing the specific enthalpy. In the illustration according to 3 the values entered for output power, ie the product of voltage and current, and hydrogen content in the working gas (remainder argon) are to be regarded as pairs of corresponding values.

A specific enthalpy of about 14.8 kJ / g can be achieved, for example, by an output of 34.5 kW and a hydrogen content of about 1.2% by weight.

It is clear that the specific enthalpy can be used as a reliable parameter of the coating process to influence the properties of the resulting coating. If homogeneous coating properties are to be achieved, the specific enthalpy should be kept as constant as possible. This can, as it turns out 3 results, done in different ways, for. B. by the output power and / or the hydrogen content in the working gas, that is, the composition of the working gas can be varied. The specific enthalpy of the gas stream can be used as a manipulated variable in a thermal spraying process, for. In a plasma spraying process.

Claims (10)

Method for coating a surface of a component by means of thermal spraying, in which a gas flow is generated, sprayed or melted and transported by means of the gas flow to the surface of the component and deposited on the surface of the component to form a coating, wherein the specific enthalpy of the Gas flow determined and properties of the coating can be controlled by means of a setting of the specific enthalpy. Method according to Claim 1 , wherein the thermal spraying is carried out as a plasma spraying by using a cooled by a cooling device plasma torch from a working gas, a plasma stream is generated as a gas stream. Method according to Claim 2 wherein the specific enthalpy is adjusted by varying the plasma power and / or varying the mass flow rate of the gas stream and / or varying the composition of the working gas. Method according to Claim 3 wherein the plasma power is adjusted by varying the current intensity and / or voltage of the plasma torch and / or the cooling capacity of the cooling device. Method according to Claim 1 wherein the thermal spraying is carried out as high-speed flame spraying by burning a fuel composition in a combustion chamber cooled by a cooling device to generate a high-velocity gas flow. Method according to Claim 5 wherein the specific enthalpy is adjusted by varying the cooling capacity of the cooling device and / or varying the mass flow rate of the fuel and / or varying the fuel composition. Method according to one of the preceding claims, wherein the specific enthalpy is kept constant. Method according to one of the preceding claims, wherein the adhesion, roughness, porosity, density, phase composition, hardness, modulus of elasticity and / or layer thickness is controlled as a property of the coating. Method according to one of the preceding claims, wherein the surface of a gas turbine component is coated. Method according to Claim 9 wherein a guide or blade of a gas turbine is coated.

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