EP2006410B1 - Thermal sprayed protective layer for metallic substrates - Google Patents

Description

The invention relates to protective layers for metals or metallic alloys which can be used at high temperatures and in aggressive gaseous, liquid and solid media. More particularly, the present invention relates to a thermally sprayed, gas-tight protective layer for metallic substrates, in particular those based on Fe, Ni, Al, Mg and / or Ti, wherein the spray powder for this purpose comprises at least two components, of which the first is a silicate Mineral or rock and the second is a metal powder and / or another silicate mineral or rock.

Emailes are known as non-metallic protective layers for various metals and alloys (see [1]: A. Petzold, H. Pöschmann, enamel and enamel technique, Wiley-VCH; Edition 2, (1992 )). These protective layers have good adhesion to the substrate and reliably protect the metallic base materials up to approx. 400 ° C against many aggressive media. Industrially, silicate glasses with a relatively low SiO ₂ content and a high content of alkali oxides are used as enamel for steels and cast iron (see [1]). Typical enamels for white enamelling of sheet steel consist of a base and a top enamel and have the following compositions: basic email ceiling mail material Proportion of (%) material Proportion of (%) SiO ₂ 47-53 SiO ₂ 56 Al ₂ O ₃ 4-6 Al ₂ O ₃ 7 B ₂ O ₃ 17-19 B ₂ O ₃ 7 Na ₂ O + K ₂ O 15-18 Na ₂ O + K ₂ O 22.5 TiO ₂ 2-8 CaO 7 CaO + MgO rest F 0.5

Special enamels for aluminum, copper alloys, stainless steels, titanium and other metals usually have even less SiO ₂ and more alkalis than enamel for steel and cast iron.

A high alkali content negatively affects the corrosion resistance of the silicate enamels against water and acids, but is absolutely necessary for the enamelling process: on the one hand to keep the melting temperature low and on the other hand to achieve a high thermal expansion coefficient - adapted to the respective substrate. Due to enamelling processes, enamels must have a melting point (liquidus temperature TL) below 850 ° C and aluminum enamel even below 550 ° C (see [1]). Low melting temperatures and high necessary coefficients of thermal expansion make an enamel insert of known acid resistant glasses, e.g. Silica glass, borosilicate glasses, E-glass, acid-resistant porcelain stains and others impossible.

Also known ceramic layers of refractory corrosion-resistant materials, which are applied to metallic substrates by means of thermal spraying (flame spraying, high-velocity flame spraying (HVOF), plasma spraying) or PVD or CVD method. For example, yttrium-stabilized zirconia (YSZ) can be produced both by thermal spraying [ UK 2100621 A ; US 4,377,371 ; WO 91/05888 ; US 5,169,689 ] as well as PVD [ US 4,321,310 ; US 4,321,311 ; US 4,401,697 ; US 4,405,659 ; WO 92/0598 ] are applied to substrates made of steel and nickel-based alloys. A difference in the coefficients of thermal expansion of the layer and the substrate is compensated in YSZ layers by a porous structure with a crack network. Thanks to this property, these layers are thermoshock resistant. However, they do not guarantee protection against oxidation and corrosion and can only be used as pure thermal barrier coatings at temperatures up to 1200 ° C. A second important disadvantage of YSZ layers is weak adhesion to the substrate. Together with a low mechanical strength (due to cracks and pores) this means poor erosion resistance.

Other known ceramic layers such as TiN, TiC, CrC, CrN, DLC, etc., which are produced by PVD / CVD method, have low thermal expansion coefficients and therefore can not be operated at high temperatures; in fact, when the temperature increases, the layer tears because a metallic substrate expands much more than the layer. For this reason, these very thin layers with layer thicknesses of less than 5 μm, mainly at room temperature, serve as wear and corrosion protection.

Other protective coatings, which are used as thermal insulation for high-temperature applications, are from the DE 19852285 C1 and the EP 1141437 B1 known. Unlike YSZ, these glass-metal / ceramic layers are free of pores and cracks, and as a result are gas-tight. The adhesion to the metallic substrate is also essential better than in the case of YSZ layers, because the metallic surface is wetted by the glass portion of the layer. The aforementioned generic layers are also resistant to thermal shock, because the coefficients of thermal expansion of the layer, possibly a. Present metallic intermediate layer and substrate are approximated or adapted to each other. A metal content improves the mechanical properties of the layer. The adaptation of the thermal expansion coefficients is possible by a variation of the glass composition and / or the metal-glass or ceramic / glass ratio.

These glass-metal / ceramic layers are used as thermal barrier coatings for turbine blades. An advantage over YSZ layers lies in an oxidation protection for the substrate through the gas-tight layer structure. However, these layers are not suitable as a corrosion protection layer. For the glass-metal / ceramic layers according to the prior art, alkaline glasses had to be selected in order to achieve the highest possible coefficient of thermal expansion for adaptation to the substrate. When used as a thermal barrier coatings, this is not critical.

In contrast, it is the object of the present invention to provide a generic, thermally sprayed and gas-tight protective layer for metallic substrates, in particular those based on Fe, Ni, Al, Mg and / or Ti, and a process for their preparation, which - also in high temperatures - provides corrosion protection for the substrate.

For the sake of completeness, the following known powder compositions are mentioned, but which relate to different types of protective layers.

To EP 0455996 A1 Spray powders of a metallic and a ceramic component are known, with only the metallic component in the flame (plasma) melts and ensures a layer formation. The ceramic component remains solid and is "dragged" by the metal. A dense layer with good adhesion to the base material is not achievable.

DE 4038254 A1 describes a glassy protective layer of easily fusible glass. The powder consists of one or two glasses with alkali content> 14%, measured on the sum of Li20 + Na20 + K20. The melting temperatures of the glasses used are below 800 ° C, so that in this case a corrosion protection for high temperatures up to about 1000 ° C can not be achieved.

In the present invention according to claim 1 are thermally sprayed protective layers of the type mentioned, wherein the application of the protective layer on the metallic substrate by flame spraying, high-velocity flame spraying (HVOF) or plasma spraying, especially as corrosion protection against extremely aggressive media normal and especially at high temperatures were developed and which are characterized in that the proportion of silicate mineral or rock in the spray powder has an alkali content of less than 6 weight percent and that the grain size of the at least one silicate component of the spray powder is selected such that in the Application of the protective layer through the partial Entglasen an adjustment of the coefficient of thermal expansion of the protective layer to the substrate is carried out and that the thermal conductivity of the protective layer between 0.8 and 5 W / mK. Alkaline content is to be understood as meaning the proportion by weight of oxides of alkali metals or of alkali metals as such.
These coatings provide protection for metallic base materials against all aqueous salt solutions and acids (excluding HF) in a low temperature range and against many corrosive ashes, molten salts, and corrosive gases in a high temperature range. Since the layers have a low thermal conductivity and can be applied with a large layer thickness, their use is also possible as thermal insulation.
In contrast to the above-mentioned glass-metal / ceramic layers are used for the spray powder of the invention Protective layers not ordinary silicate glasses, but selects mixtures of particularly corrosion-resistant, low-alkali, natural or artificially produced minerals and rocks, which glaze during spraying and immediately in the resulting layer partially devitrify, ie crystallize.

The manufacturing method according to the invention according to claim 8 involves applying the protective layer to the metallic substrate by means of flame spraying, high-velocity flame spraying (HVOF) or plasma spraying and is characterized in that the proportion of silicate mineral or rock in spray powder has an alkali content of less than 6 weight percent, and in the case of the application of the protective layer by controlled partial devitrification of the at least one silicate component of the spray powder, an adaptation of the thermal expansion coefficient of the protective layer to the substrate and with a suitable choice of grain size of the at least one siliceous component of the spray powder, a thermal conductivity of the protective layer between 0.8 and 5 W. / mk is achieved.

The thermal expansion coefficient of the layer is thus adjusted by growing in the layer, new crystalline phases, that it is adapted to the substrate. By targeted crystallization of the silicate components can be - even without a high alkali content in the at least one silicate component to have to accept - to produce a wide range of thermal expansion coefficients. For a controlled crystallization is thus no longer only a suitable choice of mineral materials crucial; rather, in particular, their particle size distribution is of crucial importance. Because a variation of the grain size, the temperatures of the particles in the flame or in the plasma and thus the crystallization behavior in the resulting layer are strongly influenced, which ultimately allows an adjustment of the coefficient of thermal expansion.

The protective layers of the present invention have all the advantages of the already known glass-metal / ceramic layers, because during the layer construction the mineral or rock component is present as glass. This glass contributes to a good wetting of the substrate and the metal particles and thus a good Adhesion to the substrate, can be plastically deformed and forms a perfect non-porous mixture with the possibly existing metal component.

The partial crystallization takes place in the still plastic layer in such a way that no mechanical stresses develop in the protective layer. The decisive advantage of the protective layer according to the invention and of the method according to the invention over glass-metal / ceramic layers and enamels is that in the context of the present invention also low-alkali and thus corrosion-resistant silicates are used, which in the prior art because of a low thermal expansion coefficient and high Melting temperatures were considered unusable for the coating of metals.

As a metal component in the spray powder for the protective layer are in principle all possible metals or metal alloys in question. However, it is preferably a metal powder of a nickel or copper-based alloy.

The spray powder advantageously consists of a total of three components, namely a first and a second silicate mineral or rock and a metal powder. With suitable particle sizes of the three components of the spray powder and by suitable choice of their respective proportions, the glazing and the partial devitrification of the spray powder can be controlled for a protective layer optimally adapted to the respective substrate.

In the spray powder is preferably a proportion of at least 10 weight percent of a silicate component with high purity of silica present, which advantageously exceeds a proportion of 99% in the component.

Protective layers according to the invention can advantageously have a thermal conductivity of between 0.8 and 5 W / mK which is also suitable for heat-insulating purposes and can be applied in a layer thickness of from 100 to 2500 μm. Layer thicknesses of more than 2 mm prove to be particularly advantageous in a protective layer according to the invention, in particular if its heat-insulating property is required.

Incidentally, the present invention relates not only to a protective layer according to the invention but also to an at least two-component spray powder for the production thereof. Moreover, the invention is also directed to the use of the protective layer for protecting parts of the combustion chamber of an internal combustion engine or of a gas turbine serving as substrate against high temperatures, corrosion and erosion. In the case of an internal combustion engine these are in particular valves, pistons and cylinder heads; in gas turbines, this relates in particular to the blades and plates. However, the protective layer according to the invention is also ideal for other machine parts serving as substrates, for example for protecting parts of steam turbines, chemical plants, heat exchangers, etc. effectively against temperature, corrosion and erosion.

In the following the invention will be explained in more detail by means of examples.

example 1

The substrate is made of a steel or a nickel-based alloy. Then an inventive mineral metal spray powder is sprayed by flame spraying, plasma spraying or HVOF. The spraying succeeds on a sandblasted, not preheated substrate without re-melting. The spray powder with a grain size <50 μm is produced by spray-drying with subsequent sintering (850 ° C, inert gas) from the following components: 65% Metal powder of gas-atomized 80Ni20Cr alloy (nickel chrome), grain size <25 μ m; 25% molten and finely ground artificial black basalt, wt%: SiO ₂ -50, CaO-20, Al ₂ O ₃ -15, MgO-8, Fe ₂ O ₃ -7, grit <10 μm ; Alkali content <0.5 wt.% 10% Ground and sieved natural quartz or cristoballite (grain size 25-50 μ m) with a purity of> 99% SiO ₂ .

The mineral-metal layer, which results from this spray powder, is free of pores and cracks and has a thermal expansion coefficient of approx. 12x10 ^-6 K ^-1 at 20 ° C. The thermal conductivity of the layer at 700 ° C is about 3 W / mK. The layer thickness can be varied in the range 100 - 2500 μm . The maximum operating temperature in air is 1200 ° C. The coating is suitable as corrosion protection and thermal insulation for various high-temperature and thermal shock-stressed parts made of steels and nickel-based alloys.

Example 2

The substrate consists of a steel, cast or a nickel-based alloy. Then, an inventive, two-component mineral spray powder is sprayed by flame spraying or plasma spraying. The spraying success on a sandblasted, preheated to about 500 ° C substrate with a re-melting at about 1100 ° C. The spray powder with a grain size <100 μ m is produced by mixing together the following mineral components: 67% melted, ground and sieved (particle size 25 - 50 μ m) artificial white basalt, wt%: SiO ₂ -54, CaO 20 MgO 5, Al ₂ O ₃ -16, Na ₂ O-5; Alkali content ≤ 5 wt.% 33% Ground and sieved (grain size 25-100 μ m) Kristoballit with a purity of> 99% SiO ₂ .

In addition, wt .-% of the following oxides can be added to the spray powder for dyeing the layer: CoO, Cr ₂ O ₃ , TiO ₂ , ZrO ₂ , ZnO and Fe ₂ O ₃ .
A mineral layer, which results from this spray powder, is pore-poor (<3%), free of cracks and has a thermal expansion coefficient at 20 ° C of approx. 11x10 ^-6 K ^-1 . The thermal conductivity of the layer is approx. 1 W / mK at 700 ° C. The layer thickness can be varied in the range 100-600 μm . The maximum operating temperature in air is approx. 1000 ° C. Since the coating contains no metallic component, it is less thermally shock resistant than metal-containing mineral-metal layers. The preferred field of application of the layer is thus corrosion protection, in particular against acids for medium thermally shock-stressed parts.

Example 3

The substrate is made of an aluminum or magnesium alloy. Then a mineral-metal spray powder is sprayed on by plasma spraying or HVOF. The spraying succeeds on a sandblasted, not preheated substrate without re-melting. The spray powder with a grain size of <50 μm is produced by spray-drying with subsequent sintering (620 ° C, inert gas) from the following components: 62% Metal powder made of gas-atomized 90Cu10Sn alloy (tin bronze), grain size <25 μ m; 18% finely ground (grain size <10 μ m) natural black basalt (basalt flour); Alkali content <5 wt.% 20% Ground and sieved (grain size 25-50 μ m) natural quartz or crystallite with a purity of> 99% SiO ₂ .

The mineral-metal layer that results from this spray powder is free of pores and cracks and has a coefficient of thermal expansion of approx. 18x10 ^-6 K ^-1 at 20 ° C. The thermal conductivity of the layer is at 400 ° C at about 5 W / mK. The layer thickness can be varied in the range 100 - 2500 μm . The maximum operating temperature of the protective layer in air is 700 ° C - apart from the substrate. The coating is suitable as corrosion protection and thermal insulation for various high thermal shock loaded parts made of aluminum and magnesium alloys.

Example 4

The substrate consists of a titanium alloy. This is followed by plasma spraying or HVOF a mineral-metal spray powder sprayed. The spraying succeeds on a sandblasted, not preheated substrate without re-melting. The spray powder with a grain size <50 μ m is produced by spray drying with subsequent sintering (800 ° C, inert gas) from the following components: 57% Metal powder of gas-atomized 80Ni20Cr alloy (nickel chrome), grain size <25 μ m; 31% finely ground (grain size <10 μ m) natural black basalt (basalt flour) 12% Ground and sieved (grain size 25-50 μ m) of natural spodumene with a purity of> 95% LiAlSi2O6.

A mineral-metal layer, which results from this spray powder, is free of pores and cracks and has a thermal expansion coefficient of approx. 7.5x10 ^-6 K ^-1 at 20 ° C. The alkali content of the mineral components is also here (including Li) at <5 wt.%. The thermal conductivity of the layer is approx. 2 W / mK at 700 ° C. The layer thickness can be varied in the range 100 - 2500 μm . The maximum operating temperature in air is 900 ° C. The coating is suitable as high-temperature corrosion protection and thermal insulation for various highly thermally shock-stressed titanium alloy parts.

Claims (9)

1. A thermally sprayed, gas-tight protective layer for metallic substrates, in particular those on the basis of Fe, Ni, Al, Mg and/or Ti, the protective layer being applied to the metallic substrate by flame spraying, high-velocity oxy-fuel spraying (HVOF) or plasma spraying, and the spray powder therefor comprising at least two components, the first of which is a silicate mineral or rock, and the second of which is a metal powder and/or a further silicate mineral or rock,
   characterised in that
   the portion of silicate mineral or rock in the spray powder has an alkali content of less than 6 per cent by weight, that the grain size of the at least one silicate component of the spray powder is selected such that the coefficient of thermal expansion of the protective layer is adapted to the substrate by the partial devitrification of the protective layer when same is applied, and that the thermal conductivity of the protective layer is between 0.8 and 5 W/mK.
2. The protective layer according to claim 1,
   characterised in that
   the metal powder consists of a nickel- or copper-based alloy.
3. The protective layer according to claim 1 or 2,
   characterised in that
   the at least one silicate component of the spray powder consists of natural or synthetically produced minerals or rocks.
4. The protective layer according to claim 1,
   characterised in that
   the spray powder consists of three components, specifically of a first and a second silicate mineral or rock and of a metal powder.
5. The protective layer according to claim 1,
   characterised in that
   a proportion of at least 10 per cent by weight of the spray powder consists of a third component consisting of silicate mineral or rock having a proportion of > 99 % SiO₂.
6. The protective layer according to claim 1,
   characterised in that
   the protective layer has a layer thickness of 100 - 2500 µm.
7. The protective layer according to claim 6,
   characterised in that
   the protective layer has a layer thickness of more than 2 mm.
8. A method for producing a sprayed, gas-tight protective layer for metallic substrates according to at least one of claims 1 to 7, the spray powder comprising at least two components, the first of which is a silicate mineral or rock, and the second of which is a metal powder and/or a further silicate mineral or rock, the protective layer being applied to the metallic substrate by flame spraying, high-velocity oxy-fuel spraying (HVOF) or plasma spraying, characterised in that
   the portion of silicate mineral or rock in the spray powder has an alkali content of less than 6 per cent by weight, and that the coefficient of thermal expansion of the protective layer is adapted to the substrate by the controlled partial devitrification of the at least one silicate component of the spray powder when the protective layer is applied, and, with suitable selection of the grain size of the at least one silicate component of the spray powder, a thermal conductivity of the protective layer between 0.8 and 5 W/mK is achieved.
9. The use of a protective layer according to any one of claims 1 to 7 for protecting parts of the combustion chamber of an internal combustion engine or of a gas turbine, acting as the substrate, from high temperatures, corrosion and erosion.