WO2019234086A1

WO2019234086A1 - Method for producing heat insulation layers with vertical cracks

Info

Publication number: WO2019234086A1
Application number: PCT/EP2019/064617
Authority: WO
Inventors: Benno Gries; Béla-Ralf Schmidt
Original assignee: Höganäs Ab (Publ)
Priority date: 2018-06-05
Filing date: 2019-06-05
Publication date: 2019-12-12
Also published as: TW202003881A; DE102018208815A1

Abstract

The present invention relates to thermally sprayed heat insulation layers which have vertical cracks, a method for the production thereof, and a component coated with such a heat insulation layer.

Description

Method for producing heat insulation layers with vertical cracks

The present invention relates to thermally sprayed heat layer insulation layers, which have vertical cracks, a method for the production thereof, and a component coated with such a heat insulation layer.

Heat insulation layers are used in industry to reduce the material temperature in high-temperature applications. One example are new and retrofitted gas turbines in which, depending on the requirements, the coating is applied within a range of between a few tenths of a millimetre and millimetres on the blades as well as on other components in the so-called hot air path of turbines such as thermal shields and combustion chamber segments.

In order to be able to compensate the different temperature expansion coefficients of substrate and coating, thermally sprayed heat insulation layers, based on zirconium oxide for example, can be produced so that they have vertical cracks. Enlarging and reducing the crack width of the vertical cracks makes it possible to compensate the stresses occurring due to differing expansion characteristics. Such heat insulation layers are characterised by high temperature shock resistance and high erosion resistance.

In the production of such layers a spray powder based on zirconium oxide (with 6 to 9% Y2O3 stabilised or partially stabilised at room temperature in the tetragonal phase) is generally processed to become a coating using a plasma spray system.

Such a method is described in US 5, 073,433. Here the lateral crack concentration (given in cracks per length, 0.78 to 7.8 cracks/mm) , their length (at least 4 mil = 100 ym) and the possible presence of unwanted horizontal cracks are stated as the most important assessment criteria for the coatings disclosed. In the view of the inventors of this disclosure, tensile stresses occur during the coating process described there, which in turn are caused by volume shrinkage of the liquefied and solidifying erstwhile spray powder applied as a molten material. These tensile stresses cause cracks when the tensile strength of the coating material is exceeded. However, the question which suggests itself to the person skilled in the art as to why these cracks are propagated across the layers and do not begin anew in each layer remains unresolved. The feed rate of the spray powder during coating is either not mentioned or is given as 50 g/min. The nominal energy consumption of the plasma spray system remains open, as no voltage is given. The maximum layer thickness per transition lies at 0.34 mil corresponding to 8.6 ym. This layer thickness is achieved by means of surface speeds of 1.16 to 5.1 m/s .

Surface speeds of over 2 m/s are no longer achievable in thermal spraying because of the usual speeds of the robotic arms which move the spraying system. They can therefore only be achieved with rotating, radially symmetric parts. Turbine components with this symmetry are however hardly ever found, so that no typical turbine components such as blades or guide vanes can be coated at high surface speeds. Property right also describes unwanted horizontal cracks which emanate from the vertical cracks. These cracks can lead to eruptions of the layer, for example at corners. Spray powders of the "molten/broken" type were used.

EP 2 038 448 B1 describes a method in which layer thicknesses of 2.5 to 10.2 ym are obtained. In addition to the spraying yield these are the result of an unnamed surface speed and of feed rates of up to 80 g/min (cf . Figure 3 of EP 2 038 448) . The energy consumption of the plasma spray system is between 11.7 and 14.8 kW. The spraying distance is not disclosed.

The phenomenon of unwanted horizontal cracking is described in the publication as "bricking". The concentration of vertical cracks is 0.2 to 7.9 cracks/mm in the lateral direction, i.e. parallel to the substrate surface. The relative thickness of the coatings is greater than 88% of the theoretical thickness. This document illustrates the significance of the less desirable horizontal cracks as an additional quality criterion apart from the concentration of the desirable vertical cracks. The type of spray powder used is not clearly identified in all the embodiments examples.

US 5,830,586 describes a production method and the resultant coatings with vertical cracks. In addition these coatings have a microscopic substructure ("columnar") . The layer thickness is given as 7.6 ym, the feed rate only as 23 g/min. Surface speeds are not disclosed, but this invention also was implemented by means of rotating a cylindrical component, so that presumably it could not have been achieved using a robot. The commercially available 7MB plasma spray system was used. Crack concentrations are not given. The substrate reaches very high temperatures, which is a disadvantage for transformable metallic materials. The spray powder used was of the "plasma- densified" type.

US 2016/0348226 A1 describes the production of heat insulation layers with vertical cracks using commercially available plasma spray systems from Oerlikon-Metco, for example 9MB, F4, SinplexPro™ or TriplexPro™. The layers are of 5 to 25% porosity. The application does not describe why, in contrast to the known methods, the vertical crack formation can occur at such high porosities, and at what settings (plasma parameters) this effect is obtainable. Equally little information is given on plasma parameters (setting parameters of the plasma spray system) which result in the stated plasma enthalpies and particle velocities, nor are the layer thicknesses disclosed. Disclosed are only the plasma enthalpies resulting from the plasma parameters, the relationship between the plasma gas flows as well as feed rates and particle velocities together with particle temperatures across a very broad range in each case, however no implementation example from which the person skilled in the art could learn the necessary combination. Moreover, by their very nature layer thicknesses of less than 88% of the theoretical thickness are not achievable with porosities of 5 to 12%. Plasma-densified spray powders were used (produced by the "HOSP™ Process", the acronym standing for "Hollow Oxide Spherical Powder") . Powders from any other manufacturing process may be used as an alternative. The substrate is typically pre-heated to 500°C, which is very high for transformable materials and can generally also lead to scaling .

Common to all the methods is therefore the fact that they work with maximum layer thicknesses of 10 ym.

The layer thickness is determined by the following variables: the feed rate of the spray powder, surface speed, thickness of the layer, spraying yield and line spacing. The mathematical relationship can be determined using the following formula:

Spraying Yield Feed Rate

Layer Thickness - - -—

{Line Spacing Surface Speed Thickness )

At the same surface speed an increase in spray powder feed and a higher spraying yield result in an increase in layer thickness. On the other hand, an increase in surface speed with the same feed rate and spraying yield produces a reduction in layer thickness. Since the achievable surface speed (relative speed between spraying system and workpiece to be coated) is limited by the speed of movement of robotic systems (typically a maximum of 2 m/s, double that can also be achieved by opposing movement when using a robotic system each for the spraying system and workpiece) , where work with a small layer thickness and high feed rates must either be carried out at a reduced spray powder feed rate or non-axially symmetric turbine parts must be rapidly rotated, which results in irregular material application and uneven layer properties due to shadowing and the transition from concave to convex surfaces. However, low spray powder feed rates also mean low productivity.

The object of the present invention is therefore to provide a heat insulation layer which overcomes the aforementioned disadvantages of prior art. A further object of the present invention is to provide a method for producing the heat insulation layer according to the invention, in which high spray powder feed rates and surface speeds can be achieved by means of commercially obtainable plasma spraying systems or robots, enabling the production of heat insulation layers with controlled crack properties.

It was surprising to find that the object is achieved by a heat insulation layer in which the individual layers have a layer thickness of at least 15 ym and which has a concentration of vertical cracks of at least 0.1 cracks/mm.

A first subject matter of the present invention is therefore a thermally sprayed heat insulation layer, wherein the heat insulation layer consists of one or more layers and the individual layers each have a layer thickness of at least 15 ym and the heat insulation layer has a concentration of vertical cracks of at least 0.1 cracks/mm. "Layer" and "layer thickness" for the purposes of this invention relate to the vertical measurement of the amount of spray powder deposited on the surface of the substrate to be coated during one spray transition. By spray transition is meant the one-off depositing of the spray powder on the substrate in one direction, horizontally or vertically, by the spraying system, wherein the spray transition can be repeated until the desired layer thickness is obtained. The final layer thickness of the heat insulation layer is determined from the sum of the thickness of the individual layers.

The concentration of the vertical cracks in the heat insulation layer relates to the number of cracks per mm of the heat insulation layer which run vertically relative to the surface of the substrate. By vertically running cracks are meant those which show a deviation from the normal to the surface of the substrate of less than 45°, preferably less than 20°, particularly less than 10°.

In a preferred embodiment of the present invention the individual layers of the heat insulation layer each have a thickness of 15 ym to 100 ym, preferably 20 ym to 80 ym, particularly preferably 25 ym to 60 ym.

The heat insulation layer according to the invention preferably has a concentration of vertical cracks of 0.1 to 5 cracks/mm, particularly 0.2 to 3 cracks/mm. In this manner the stresses in the heat insulation layer which could, for example, occur during rapid heating or cooling of the substrate can be compensated without the heat insulation layer spalling and thus damaging the substrate.

The number of cracks as well as their length can be determined, for example, by imaging methods. The cracks in the heat insulation layer according to the invention preferably have a length of at least 100 ym, particularly a length of 100 to 500 ym. The vertical cracks may extend over several layers in the heat insulation layer according to the invention.

In a preferred embodiment the heat insulation layer according to the invention has one or more heat insulation materials, wherein the heat insulation materials have an intrinsic thermal conductivity of 3 W/mK or less, determined at 1000°C. By intrinsic thermal conductivity in the context of the present invention is meant that of a single crystal.

The heat insulation material preferably consists of oxides with an intrinsic thermal conductivity of 3 W/mK or less. In a particularly preferred embodiment the heat insulation material is zirconium oxide (ZrCy) stabilised at room temperature, wherein this is preferably selected from the group comprising zirconium oxide stabilised at room temperature in the cubic phase, zirconium oxide stabilised at room temperature in the tetragonal phase, pyrochlores, perowskites and blends thereof. The heat insulation material preferably has a density of 4 to 8 g/cm³.

In principle any spray powder and any heat insulation material can be used to produce the heat insulation layer according to the invention. It has been demonstrated, however, that the best results in respect of spray powder yield are achieved if the spray powder has a nominal particle size of which the lower nominal limit is between 5 and 75 ym and the upper nominal limit is between 22 and 220 ym. Particularly suitable spray powders preferably have a nominal particle size of 45/22 ym, 22/5 ym, 75/45 ym, 125/11 ym or 150/75 ym in accordance with DIN EN 1274:2004.

Depending on the melting point of the heat insulation material and the nominal particle size of the corresponding spray powder, the person skilled in the art is in a position to adjust the plasma parameters to achieve the necessary fusion rate of the powder. Coarser powders and higher melting points require an increase in plasma enthalpy, which is most simply achieved by increasing the flow. Coarser powders also permit a reduction in the carrier gas in order to arrive at the same fusion rate. The person skilled in the art is familiar with how the carrier gas for a selected spray powder should be adjusted so that as many particles as possible are injected into the interior of the plasma plume. Moreover, the person skilled in the art is familiar with how the electrical power and the plasma gases should be adjusted so that the temperature of the particles at the end of the plasma plume exceeds the melting temperature of the heat insulation layer material. For example, commercially available plasma diagnostic systems may be used here, as, for example, supplied by Accuraspray. The person skilled in the art is further familiar with the fact that in principle coarser spray powders of the same powder type require higher plasma enthalpy and/or lower feed rates.

In a preferred embodiment the heat insulation layer according to the invention has a proportion of at least 90% by weight of the heat insulation material with reference to the total weight of the heat insulation layer.

In order to adjust the properties of the heat insulation layer to individual requirements, the heat insulation layer in a preferred embodiment may contain other components such as porosification agents, wherein these preferably do not exceed 10% by weight of the heat insulation layer with reference to the total weight of the heat insulation layer. It was surprising to find that the presence of a porosification agent prevents developing cracks ending in unfilled pores and failing to propagate during the solidification process, which means that the mechanical stability of the heat insulation layer can be impaired. Porosification agents are preferably materials which can be thermally decomposed after coating and in this way leave unfilled pores behind. The porosification agent preferably consists of organic macromolecules such as organic polymers, which are preferably used in powder form. In this way porous heat insulation layers which have vertical cracks at the same time can be achieved for the purposes of the invention, since the filled pores do not represent final stops for crack propagation during the coating process, as would be the case with unfilled pores.

In a preferred embodiment the heat insulation layer according to the invention has a porosity of 2 to 8%, preferably 2.5 to 6%, wherein the percentages refer to an area percentage without cracks. Porosity can be determined, for example, using an optical microscope with subsequent image processing. It was surprising to find that with heat insulation layers of a porosity within the claimed range, optimal protection of the substrate can be achieved without coating having a negative impact during operation of the substrate.

The heat insulation layer according to the invention is particularly suitable for applications in the high temperature range, for example for coating turbine blades and combustion chamber components. The layer thickness of the heat insulation layer applied should be as thin as possible in order to prevent other mechanical properties, for example the rotation characteristics of the coated substrate, from being altered by the heat insulation layer. In a preferred embodiment the heat insulation layer according to the invention therefore has a layer thickness of 100 to 10, 000 ym, preferably 150 to 8,000 ym.

A further subject matter of the present invention relates to a method for producing the heat insulation layer according to the invention. The method is characterised by the spray powder being applied at a surface speed not exceeding 4 m/s, preferably at a surface speed of 0.5 to 4 m/s, particularly preferably 0.8 to 1.2 m/s.

It was surprising to find that a surface speed in the range claimed permits automated application, for example by means of robots, without requiring a limitation of the layer thickness. An embodiment of the method according to the invention, in which the creation of the layer is affected using at least one robot, is therefore preferred.

In a preferred embodiment of the method according to the invention the application of the coating is carried out at a feed rate of 25 to 250 g/min, preferably 30 to 220 g/min. The line spacing is preferably 0.5 to 7 mm, particularly 1 to 6 mm.

The spraying yield (also called "deposition efficacy" in English) of the method according to the invention is preferably 40 to 80%.

By way of example Table 1 shows an overview of suitable parameters which can be used in the method according to the invention and which were calculated using the formula given above. The person skilled in the art is familiar with the fact that the desired layer thicknesses can be achieved from the parameters of line spacing, surface speed and feed rate using the aforementioned formula, even with heat insulation materials of varying thickness or with varying spraying yield.

Table 1

The method according to the invention is characterised by the fact that the spray powders used need be subject to no special requirements. The thermal spray powder used in a preferred embodiment of the method is therefore a spray powder of the "agglomerated/sintered" type. A spray powder of the "molten/broken" type is a preferred alternative. The use of a spray powder of the "sintered/broken" type is another preferred alternative.

The spray powder in a particularly preferred embodiment is a spray powder of the HOSP™ (Hollow Oxide Spherical Powder) type. Optimum fusion rates of the thermal spray powder can be obtained by using such a powder in the method according to the invention in combination with the parameters claimed.

It was surprising to find that the quality of the coating deposited can be improved by selecting powders of suitable particle sizes. The spray powder used in a preferred embodiment therefore has a nominal particle size with a lower nominal limit of 5 to 75 ym and an upper nominal limit of 22 to 220 ym in accordance with DIN EN 1274:2004.

It has proved advantageous to avoid undesirable heating of the substrate to be coated while the coating is being produced, for example to avoid thermal transformation of the material. An embodiment of the method according to the invention is therefore preferred in which the back of the substrate to be coated is cooled during the spraying process. Particularly preferably the temperature of the substrate to be coated is less than 100°C during the spraying process, measured at the back. "At the back" in the context of the present invention designates the surface of the substrate to be coated facing away from the surface to be coated. A further subject matter of the present invention is a method for coating components, which incorporates the application of a heat insulation layer according to the invention to the component by means of thermal spraying.

A further subject matter of the present invention is a component comprising a heat insulation layer according to the invention, as well as a component which is coated using the method according to the invention. The component according to the invention is particularly suited to applications in the high-temperature range. An embodiment in which the component is a turbine component is therefore preferred.

The present invention will now be explained in more detail using the following examples, wherein these are not to be understood as restricting the inventive concept.

Examples :

Example 1

Coatings were produced using the "F4" plasma spraying system from Oerlikon-Metco on Inconel 625 sheet metal. The plasma spraying system was moved using a robotic arm in x-y direction, whereas the sheet metal substrate was fixed. The back of the substrate was intensively cooled using compressed air. The temperature measured at the back of the substrate was always below 100°C throughout the coating process. The substrate was not preheated prior to coating.

The plasma parameters were as follows: current 470A, voltage 64V, hydrogen 6 1/min, Ar 40 1/min, nozzle: 6 mm, injector 1.8 mm diameter, carrier gas 3.8 1/min (Ar) , spraying distance 90 mm, surface speed 0.833 m/s, line spacing 4 mm. Use was made of a commercial spray powder with 8% by weight Y2O3, 90% by weight ZrCy and 2% by weight HfCy (available commercially under the designation Amperit^® 831.054 from H.C. Starck Surface Technology and Ceramic Powders GmbH, Germany) . This spray powder has a nominal grain size distribution of 45/10 ym, was manufactured by plasma spheroidisation (sometimes also known by the synonym "plasma densification") , and is a so-called "HOSP™" powder. Heat insulation materials of this nominal composition are also called "7YSZ" or "8YSZ".

Using the carrier gas the plasma spray powder was injected into the plasma plume, which in turn was generated by the plasma spray system. Using a robotic arm the plasma spray system was moved in a vertical direction to the longitudinal axis of the plasma plume in order to produce the surface speed and relative velocity.

The layer thickness was determined on the basis of the given formula using the established spraying yield and a spray material density of 6.0 g/cm³.

The feed rate of the spray powder was varied and hence the resultant layer thickness. The results are summarised in Table 2, wherein the trial numbers 01, 04 and 05 designate heat insulation layers according to the invention, while trial numbers 02 and 03 represent comparative trials.

Table 2

The crack concentration was determined on a metallographic transverse section of the coating. Evaluation was carried out over a horizontal width of 10 mm, wherein only vertical cracks with a length of at least 100 ym were counted. On the basis of the examples provided it is clear to see that with small layer thicknesses corresponding to those of prior art, no vertical cracks or only few could be produced.

The layer thicknesses were between 470 and 560 ym.

Example 2

Example 1 was repeated, with the following parameters having been altered: current 600A, voltage 67V, spraying distance 80 mm, hydrogen 10 1/min

This alteration caused the plasma enthalpy to increase and hence the fusion rate of the spray powder or the temperature of the molten particles. The shorter spraying distance also caused an increase in the proportion of solidification taking place following deposition, as less solidification took place in mid-air and more after deposition.

The results are summarised in Table 3, wherein the trial numbers 01, 04 and 05 designate heat insulation layers according to the invention, while trial numbers 02 and 03 represent comparative trials.

Table 3

As can be seen from the trial data, this alteration in the plasma parameters causes a higher crack concentration and wider cracks, as due to the greater solidification of the molten powder the volume shrinkage during solidification, and therefore the tensile stresses in a lateral direction, increase in the layer. The concentration of vertical cracks and their width also increase (see Figures 2 and 3) . However, the dependence on layer thickness remains.

The layer thicknesses were between 475 and 576 ym.

Example 3

Example 2 was repeated, but with a molten/broken 7YSZ (Amperit^® 825.001, nominal particle size distribution 45/10 ym, commercially available from H.C. Starck Surface Technology and Ceramic Powders GmbH, Germany) .

The results are summarised in Table 4, wherein the trial numbers 01, 04 and 05 designate heat insulation layers according to the invention, while trial numbers 02 and 03 represent comparative trials.

Table 4

The crack concentration is less than in Example 1 or 2 under the same conditions. The porosity is also higher and the cracks often not continuous. The reason for this is the compact "molten/broken" type powder which is more difficult to melt. The processing conditions of Example 2 are better suited to this powder type and this particle size. Alternatively, a finer spray powder or increased plasma enthalpy could be used to achieve the inventive effect.

The layer thicknesses were between 408 and 542 ym.

Example 4

Example 3, trial number 04 (80 g/min) was repeated with the respective powders used, but the layer thickness was achieved by different feed rates and surface speeds. The surface speed was set by the speed of movement of a robotic arm. The results are summarised in the following table: Table 5

As will be seen from Table 5, a lower surface speed with roughly the same layer thickness has a positive effect on the formation of the vertical cracks. Without being bound to a specific theory, the supposition is that this is possibly linked to the solidification speed of the molten material deposited. Here again the difference between the two powders is confirmed.

The layer thicknesses were between 467 and 568 ym.

Example 5

While retaining the procedure described in Example 1, the spraying parameters below were achieved using plasma spray system "F4": current 600A, voltage 67V, argon 35 1/min, hydrogen 12 1/min, spraying distance 100 mm, surface speed 1.25 m/s by rotating a section of pipe, feed rate 80 g/min.

A plasma spray powder of the "agglomerated/sintered" type was used (Amperit^® 827.054, commercially available from H.C. Starck Surface Technology and Ceramic Powders GmbH, Germany) . Unlike molten/broken powders this powder has an internal porosity. This is also an 8YSZ powder. A layer with 0.9 cracks/mm and 5.1% porosity was obtained. The spraying yield was 45%, the layer thickness 22 ym. The layer thickness was 430 ym.

This example shows that the effect according to the invention is caused by the layer thickness and is independent of the type of powder used.

Figure 1 shows a picture of the heat insulation layer according to the invention as described in Example 1, trial number 04. The numbers designate cracks which are longer than 50% of the layer thickness.

Figure 2 shows a picture of the heat insulation layer according to the invention as described in Example 2 trial number 05.

Figure 3 shows a picture of the heat insulation layer according to the invention as described in Example 2, trial number 04. The numbers designate cracks which are longer than 50% of the layer thickness.

Claims

Patent claims:

1. Thermally sprayed heat insulation layer, characterised in that the heat insulation layer consists of one or more layers, wherein the individual layers have a layer thickness of at least 15 ym and wherein the heat insulation layer has a concentration of vertical cracks of at least 0.1 cracks/mm.

2. Heat insulation layer according to claim 1, characterised in that the individual layers each have a thickness of 15 ym to 100 ym, preferably 20 ym to 80 ym, particularly preferably 25 ym to 60 ym.

3. Heat insulation layer according to at least one of claims

1 or 2, characterised in that the heat insulation layer has a concentration of vertical cracks of 0.1 to 5 cracks/mm, preferably 0.2 to 3 cracks/mm.

4. Heat insulation layer according to one or more of the preceding claims, characterised in that the vertical cracks have a length of at least 100 ym, preferably a length of 100 ym to 500 ym.

5. Heat insulation layer according to one or more of the preceding claims, characterised in that the heat insulation layer has one or more heat insulation materials, wherein the heat insulation layers have an intrinsic thermal conductivity of 3 W/Km or less, determined at 1000°C.

6. Heat insulation layer according to one or more of the preceding claims, characterised in that the heat insulation material is zirconium oxide (ZrCy) , wherein the zirconium oxide is preferably selected from the group comprising zirconium oxide stabilised at room temperature in the cubic phase, zirconium oxide stabilised at room temperature in the tetragonal phase, pyrochlores, perowskites or blends thereof.

7. Heat insulation layer according to one or more of the preceding claims, characterised in that the heat insulation layer has a porosity of 2 to 8%, preferably 2.5 to 6% .

8. Method for producing a heat insulation layer according to one or more of claims 1 to 7 by thermal spraying, characterised in that the spray powder is applied at a surface speed not exceeding 4 m/s, preferably at a surface speed of 0.5 to 4 m/s, particularly preferably 0.8 to 2 m/s .

9. Method according to claim 8, characterised in that the spray powder is applied using a robot.

10. Method according to one or more of the preceding claims, characterised in that the spray powder is of the HOSP™

(Hollow Oxide Spherical Powder) type.

11. Method according to one or more of the preceding claims, characterised in that the spray powder has a nominal particle size with a lower nominal limit of 5 to 75 ym and an upper nominal limit of 22 to 220 ym according to DIN EN 1274:2004.

12. Method according to one or more of the preceding claims, characterised in that the back of the substrate to be coated is cooled during the spraying process.

13. Method for coating a component, characterised in that the method comprises the application of a heat insulation layer according to one or more of claims 1 to 7 by means of thermal spraying on the component.

14. Component incorporating a heat insulation layer according to one or more of claims 1 to 7 and/or obtainable with the method according to claim 13.

15. Component according to claim 14, characterised in that the component is a turbine component.