EP3052670A1

EP3052670A1 - Sintered molybdenum carbide-based spray powder

Info

Publication number: EP3052670A1
Application number: EP14793791.6A
Authority: EP
Inventors: Benno Gries
Original assignee: HC Starck GmbH
Current assignee: Hoganas Germany GmbH
Priority date: 2013-10-02
Filing date: 2014-10-01
Publication date: 2016-08-10
Also published as: DE102013220040A1; CA2925066A1; TW201536451A; ZA201602071B; WO2015049309A1; JP2016540883A; US20160243616A1; US9919358B2; BR112016006803A2; RU2016117128A; RU2016117128A3

Abstract

The invention concerns a sintered spray powder based on a metal matrix and molybdenum carbide, a method for the production thereof and the use of the spray powder for coating components, in particular rotating and moving components. The invention also describes a method of applying a coating using the spray powder according to the invention and a component coated therewith.

Description

Sintered spray powder based on molybdenum carbide

The present invention relates to a sintered spray powder obtainable using molybdenum carbides, a process for its production and the use of the spray powder for coating components, especially moving components. Furthermore, the invention describes a method for applying a coating using the spray powder according to the invention and a component coated therewith.

Spray powders are used to produce coatings on substrates by means of "thermal spraying." In this process, powdered particles are injected into a combustion or plasma flame which is directed onto a (mostly metallic) substrate which is to be coated the flame completely or partially, collide on the substrate, solidify there and form in the form of solidified "splats" the coating. In the case of so-called cold gas spraying, on the other hand, the particles only melt on impact on the substrate to be coated as a result of the released kinetic energy. By thermal spraying coatings of several μι can be made up to several mm layer thickness.

A common application of wettable powders is the production of wear-resistant coatings. Both the layers and the powders are typically cermet powders, which are characterized by the fact that they contain hard materials (this is the ceramic component, "cer-"), most commonly carbides such as tungsten -, Chromium and more rarely other carbides, and on the other a metallic component as a metallic matrix ("-met"), which consists of metals such as cobalt, nickel and their alloys with chromium, more rarely also iron-containing alloys. Thus, such spray powders and spray coatings produced therefrom are classical composites. Such spray powders are also known to the person skilled in the art as "agglomerated / sintered" spray powders, ie in the production process first agglomerated (also referred to as pelletized), and then the agglomerate is thermally sintered in itself, so that the agglomerates are mechanically required for thermal spraying Get stability, but also such wettable powders, which are produced by sintering powder mixtures or compacts, followed by a comminuting step, meet the necessary requirements. This type of spray powders are familiar to those skilled in the art as "sintered / crushed". The two aforementioned types of wettable powders are typified by the standard DI N EN 1274: 2005, for example. Both powder classes can also be described as "sintered spray powders".

Sintered / crushed spray powders are prepared analogously to agglomerated / sintered powders, with the difference that the powder components are not necessarily wet mixed in dispersion, but can be dry-mixed and optionally tabletted or compacted into moldings. The following sintering is carried out analogously, but compact, solid sintered bodies are obtained, which must be converted by mechanical force into powder form again. The powders thus obtained are of irregular form and are characterized on the surface by fracture processes. These spray powders are significantly less fluid, which is disadvantageous for a constant application rate during thermal spraying. Coatings can - analogous to solid materials - be characterized by empirically determinable material properties. These include hardness (eg Vickers, Brinell, Rockwell and Knoop hardness), wear resistance (eg according to ASTM G65), cavitation resistance and friction behavior, but also the corrosion behavior in various media, as well as the density, in particular the true density. For coatings which are cermets, the material properties are determined by the proportion and the degree of distribution of the metallic and the ceramic or hard material phase. For this purpose, the basic relationships are familiar to the expert. One of these relationships is the Hall Petch Law. This establishes the relationship between the degree of dispersion of the ceramic phase and different material properties. It follows that the ceramic or hard phase should be dispersed as finely as possible in the metallic phase, if high strength and high hardness should be achieved. For this purpose, the metallic phase must be as complete as possible ("Contiguity"). This means that it forms a complete three-dimensional network, in the mesh of which the particles of hard material are embedded and thus separated from one another.

Advantageous for some applications is a low true density of coatings with cermets, especially in moving, in particular rotating and / or flying components. The geometric density of a coating is close to the true density, which is calculated from the volume-weighted proportions of the components (eg, the hard materials, the metallic matrix and any oxidation products) and their true densities. The true density can be determined, for example, on completely dense coatings after they have been removed by means of the Archimedes method. The true density of powdered coating materials can be determined as pure density, for example as skeletal density, by means of pycnometry, in particular by means of helium pycnometry (DIN 66137), with "completely" open-pore powders having the measured values very close to those of the true density. The true density value of single-phase powders or bodies is identical to the X-ray density under ideal conditions.

For the necessary polishing ability of coatings to achieve very low roughness, as is necessary in tribologically stressed layers, the hard materials present in the coating must have a sufficiently good distribution in the metallic matrix and be of small size. It follows that thus also the metallic matrix should have a web width, which is of the same order of magnitude, which is also necessary for the polishing ability. A small web width of the metallic matrix leads to low elongation at break in cermet powders, which improves the polishing ability.

As the web width of the metallic matrix, the mean distance between adjacent hard material particles in the coating is defined, which is filled with the metallic matrix. The larger this web width, the greater the maximum absolute elongation at break and the larger the deformed areas and thus the roughness of the polishing process. This makes it clear why thermal spraying of powder mixtures (so-called "blends") is not advantageous: because of the turbulence in the flame, the powders used must, inter alia, have a certain minimum size, which is typically between an average particle size of 15 and 100 μm However, it does require that the coating represent a heterogeneous texture ("patchy landscape") of the powder types used, with the result that the matrix and hard material are not distributed on the m scale, with negative consequences for the polishability Typical examples of a blend of agglomerated / sintered Mo / Mo ₂ C with an alloy powder can be found in the patent EP 0701 005 Bl. Obtained coatings with a lamellar structure, resulting from the use of NiCrFeBSi alloy powder as a metallic matrix, which contains no hard materials and therefore the hard material described -free metallic lamellae are produced e material advantages that would result from a high degree of dispersion of the metallic phase in the hard material, not feasible by means of a blend.

For the mixed friction area according to Stribeck, the chemical state of the surface is important. Soft oxides are advantageous as surface species, the z. B. can be detected by surface analytical methods. These are advantageously soft layer lattice oxides such as B ₂ 0 ₃ , W0 ₃ or Mo0 ₃ and their hydrate acids. These have, inter alia, a strong, positive influence on the so-called breakaway torque after prolonged non-activity of the friction pair, as may occur especially in hydraulic piston rods or piston rings. A coating used in the prior art is electroplated hard chrome. A disadvantage is the highly polluting production of hexavalent chromium, which is classified as carcinogenic. Advantageous is the very low coefficient of friction (μ). Also disadvantageous are tensile stresses and resulting cracks, which do not effect effective corrosion protection of the substrate. In addition, the tensioned coating represents a weakening of the substrate with respect to its mechanical strength (fatigue). The cracks also transport hydraulic oil into the environment during the extension of a piston rod, the poisonous one Contains components such as ethyleneamine. Hard chrome has virtually no elongation at break and is therefore easy to polish (down to 0.1 μι average roughness), but behaves brittle under mechanical shock. The wear resistance is rather moderate due to lack of hard materials. The geometric density is comparatively low at about 7 g / cm ³ . It is thus below the true density of metallic chromium (7, 19 g / cm ³ ). The reason for this are pores and cracks.

Ni- or Co-CrFeBSi-based melts (compositions see, for example, DIN EN 1274: 2005, Table 2) are distinguished by extraordinarily dense, ie low-porous, layers. After melting of the initially porous sprayed layer, very hard but also very brittle CrB precipitates are present. Melting materials show a very low coefficient of friction, presumably because of the boron trioxide present on the surface, which is known to have good properties as a solid lubricant. Furthermore, the melts show very good polishing behavior, but are less resistant to wear (similar to hard chrome) because of the very low elongation at break. Therefore, they are often processed in admixture with other hard-containing spray powders (eg., WCCo 88/12 or 83/17), or even with metallic molybdenum, which in turn often contains Mo ₂ C precipitates, or The latter coatings - often with a third component such as CrC-NiCr - are state of the art, for example, on piston rings in internal combustion engines, but they do not represent a uniform distribution of the hard phases in the range of less than 10 μm. These different materials are then present in the layer as regions approximately in the size of the spray powders used (which typically have a band of 45-10 μm as the specified grain), so that the coating is present in the coating when exposed to foreign bodies in the micrometer range, behaves in accordance with their local composition e Therefore, they are not particularly advantageous where penetration of foreign bodies into the tribological friction pairing is to be expected. The true density of pure melt melts is on the order of about 8 g / cm ³ , in admixture with other wettable powders but slightly above, depending on which other spray powders were added.

Very high quality coatings are those based on tungsten carbide, such as WCCo 83/17 or WC-CoCr 86/10/4. Due to the presence of tungstic acid or tungsten trioxide as a solid lubricant on the surface of the coating, the friction behavior is favorable. The wear resistance is high, the layers can be produced without pore under suitable conditions, that is, the density of the coating is close to the true density, and have a low elongation at break. The polishing ability is very good due to the finely divided metallic matrix (Co or CoCr, alloyed with W). In particular, can be produced under internal compressive stress layers, which is essential for the fatigue strength of the substrate under mechanical cycling. A disadvantage is the very high true density of these coating materials and the resulting high geometric densities, typically up to about 14 g / cm ³ , which in comparison to hard chrome slightly higher coefficient of friction and the high raw material costs for tungsten. The high geometric densities of rotating and flying components lead to increased energy consumption due to the increased moment of inertia or the larger flying weight. Another alternative is Cr and chromium carbide-containing alloys, especially those based on iron and nickel, and cermet spray powder such as CrC-NiCr 75/25. This is common that during thermal spraying chromium oxide (Cr ₂ 0 ₃ ) is formed. This oxide is harder than metallic friction partners and dreads them, but has low coefficients of friction compared to metallic materials. Furthermore, these oxide precipitates are predetermined breaking points of the ductile metallic matrix and reduce their elongation at break, so they are not a priori harmful. However, it lacks the self-lubricating effect by soft oxides, which can be essential in the field of mixed friction. The true density is comparatively low and is about 7.3 g / cm ³ . The wear resistance of these coatings is comparatively low and not sufficient for many applications. It is therefore an object of the present invention to provide a coating which overcomes the disadvantages of the prior art. In particular, it should be a composite material with a density of less than 10 g / cm ³ of true density, which has finely divided hard materials with an average of at most 10 μι size with favorable friction in a gmaistegigen and finely divided metallic matrix, coupled with a low true density.

The present invention therefore relates to a sintered spray powder which comprises the following components: a) 5 to 50 wt .-% metallic matrix, based on the total weight of the spray powder, wherein the matrix 0 to 20 wt .-% molybdenum, preferably above 0 wt .-% to 20 wt .-%, in particular 0, 1 to 20 wt .-%, based on the total weight of the metallic matrix; b) 50 to 95% by weight of hard materials, based on the total weight of the spray powder, consisting or comprising at least 70% by weight

Molybdenum carbide based on the total weight of the hard material, wherein the average diameter of the molybdenum carbide in the sintered spray powder <10 μιη, in particular <5 m, is; and c) optional wear-modifying oxides. The mean diameter of the molybdenum carbide was determined according to the ASTM B330 standard ("FSSS" Fisher Sub Sieve Sizer).

The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each.

Suitable wear-modifying oxides in the context of the present invention are those which are sufficiently stable under the sintering conditions of the spray powder and are not reduced. These oxides are sufficiently hard due to their high thermodynamic stability and have the advantage of having low coefficients of friction compared to metallic systems. Preferably, the wear-modifying oxides are selected from the group consisting of Al ₂ 0 ₃ , Y ₂ 0 ₃ and oxides of the 4th subgroup of the Periodic Table. Farther The oxides are preferably provided as powders with mean particle sizes between 10 nm and 10 μm.

In a preferred embodiment, the spray powder according to the invention comprises wear-modifying oxides, the amount of wear-reducing oxides being between 0 and 10% by weight, preferably between 1 and 8% by weight, based on the total weight of the spray powder.

The percentages by weight add up to 100% by weight.

The spray powder according to the invention is sintered, particularly preferably agglomerated and sintered. Such wettable powders are also referred to as agglomerated / sintered.

The powders according to the invention of the sintered / broken type are furthermore favorable, but in total the powders of the agglomerated / sintered type, as shown in DIN EN 1274: 2005, are preferred. The basis of the hard material consists of fine-grained molybdenum carbides, preferably MoC and Mo ₂ C. In the context of the present invention, "base" means that at least 70% by weight of the corresponding substance is present, based on the total weight of the hard material. The remaining maximum of 30 wt .-% hard materials may be other carbides, preferably chromium and iron carbides because of their non-volatile and brittle oxides, or preferably tungsten carbide and boron carbide, the soft surface oxides have been found to be advantageous. Furthermore, other carbides from the 4th to 6th subgroup of the periodic table can be used. The choice of suitable carbides will be made by the person skilled in the art on the basis of the surface state of the carbides and the intended application of the coating.

The spray powder contains 5 to 50 wt .-% metallic matrix, and thus 95 to 50 wt .-% of hard materials, of which molybdenum carbides constitute at least 70 wt .-%. The spray powder thus contains 95 to 35 wt .-% molybdenum carbides, which are fine-grained (<10 .mu.m according to ASTM B330, measured on the powder used for spray powder production). The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each.

The mean particle diameter of the molybdenum carbide in the sintered spray powder is preferably less than 10 μm, preferably 0.5 to 6.0 μm, in particular 0.5 to 4.0 μm, particularly preferably 0.5 to 2.0 μm, 1.0 to 6.0 pm or 1.0 to 4.0 pm, determined according to ASTM E112. The improvement of the wear resistance is at the expense of ductility and vice versa; Accordingly, the preferred range depends on the appropriate application, depending on whether a higher wear resistance or higher ductility is required. The range of 1.0 to 6.0 pm is an optimum range for most applications as a special compromise between these two properties. Since the determination of the particle sizes in the powder used for spray powder production takes place by a different method (ASTM B330) than the determination of particle sizes in the sintered spray powder (ASTM E112), the particle sizes thus obtained can not be compared directly. Usually, however, a particle growth is observed in the course of sintering, so that the actual particle sizes in the sintered spray powder are greater than those in the powder used for spray powder production. Overall, it has been shown that the finer the molybdenum carbide powder used (that is, the smaller the particle size of the molybdenum carbide powder used in accordance with ASTM B330), the better the distribution of the metallic matrix and its mean web width resulting in the spray powder. Particle diameter or diameter in the context of the present invention designates the maximum extent of a particle, namely the dimension from an edge of the particle to the edge of the particle farthest from this. By a particle size of less than 10 pm, a favorable application rate of the powder during spraying and a better adhesion is achieved. Due to the better adhesion, the overspray is minimized and thus a health hazard is reduced.

It was found that with less than 5 wt .-% of metallic matrix, based on the total weight of the spray powder, the content of metallic matrix not sufficient to ensure the metallic properties of the composite material. At more than 50 wt .-%, the wear resistance decreases so that the wear-resistant cermet character of the composite is no longer present. Furthermore, the elongation at break increases to the extent that this is at the expense of polishing ability.

The elongation at break of the sprayed layer can be reduced to such an extent by the presence of embrittling elements, in particular of boron and / or silicon, that undesirable cracking may occur upon cooling after thermal spraying. On the other hand, in terms of polishing ability, some level of these elements may be beneficial.

Therefore, an embodiment is preferred in which boron is present in an amount of at most 1.4% by weight, preferably from 0.001 to 1.0% by weight, based on the total weight of the metallic matrix.

Further preferred is an embodiment in which silicon is present in an amount of at most 2.4 wt .-%, preferably from 0.001 to 2.0 wt .-%, based on the total weight of the metallic matrix.

The content of boron and silicon in the spray powder according to the invention makes it possible to adjust, for example, together with the content of refractory metals, whether and which quantities of refractory metal borides and silicides can be eliminated. These also have favorable tribological properties. Furthermore, the contents of boron, silicon and refractory metal can be determined according to the respective requirements by the principle of the solubility product. Refractory metal in the context of the present invention are to be understood as meaning, in particular, the high-melting, base metals of the fourth, fifth and sixth subgroups, in particular titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and Tungsten, in particular molybdenum. Preferably, the melting point of these metals is above 1772 ° C.

It has been shown that the use of molybdenum carbide, especially in the aerospace industry can be beneficial. Therefore, an embodiment is preferred in which the molybdenum carbide has the structure MoC or Mo ₂ C, preferably Mo ₂ C.

The properties of the spray powder and consequently the properties of the subsequent coating can be influenced, for example, by the addition of further carbides. Accordingly, an embodiment is preferred in which the hard material comprises further carbides, preferably carbides selected from the group consisting of tungsten carbide, chromium carbides and boron carbide. Particularly preferred are chromium carbides and boron carbide. Further preferably, the carbide is a carbide of a metal selected from the metals of the 4th, 5th and 6th subgroup of the periodic table.

In a preferred embodiment of the present invention, the metallic matrix contains at least 60% by weight, preferably 70 to 90% by weight, of a metal selected from the group consisting of iron, cobalt and nickel, the amounts being based on the Total weight of the metallic matrix refer. These metals wet the carbides and thus improve the internal cohesion of the composite material in the spray powder after sintering and in the sprayed layer. The percentages by weight (% by weight) with respect to the powders and blends in the present invention add up to 100% by weight each.

Further preferably, the metallic matrix comprises elements which reduce the elongation at break of the metallic matrix and act to harden. Preferably, these elongation at break and solidifying elements are selected from the group consisting of molybdenum, tungsten, boron, silicon, chromium, niobium and manganese and combinations / mixtures thereof. Preferably, the amount of elongation at break and solidifying elements in the metallic matrix is less than 40% by weight, preferably 5 to 20% by weight, based on the total weight of the metallic matrix. The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each.

In a preferred embodiment, the metallic matrix comprises nickel in an amount of 50% to 95% by weight, preferably 60% to 85% by weight, based on the total weight of the metallic matrix. The presence of nickel can lead to the formation of intermetallic compounds, whereby the metallic matrix is also solidified.

The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each. The metallic matrix preferably comprises cobalt in an amount of from 10 to 90% by weight, preferably from 20 to 90% by weight, in particular from 50 to 90% by weight, based on the total weight of the metallic matrix.

The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each. Further preferred is an embodiment of the present invention in which the metallic matrix comprises iron in an amount of 10 to 90 wt .-%, preferably 20 to 60 wt .-%, in particular 20 to 50 wt .-%, based on the total weight the metallic matrix.

Also preferred is an embodiment in which the metallic matrix comprises molybdenum in an amount of 2 to 15 wt .-%, preferably 5 to 10 wt .-%, based on the total weight of the metallic matrix.

Further preferred is an embodiment in which the provision of the components of the metallic matrix takes place exclusively or partly by one or more alloy powders. Here, the Schmalstegigkeit the metallic matrix can be ensured in the spray powder and in the coating, for example, by intensive grinding with the carbides.

Many components, especially those in the aerospace industry, are exposed to extreme conditions, such as large temperature fluctuations and erosive wear. To make matters worse, that due to the field of application stringent requirements are placed on the weight of the components and thus on the geometric and thus the true density of the materials used. It has become common practice to provide heavily stressed components with coatings that protect the components from external influences and thus contribute to a longer life of the components.

Therefore, another object of the present invention is the use of the spray powder for surface coating according to the invention.

The sintered spray powder according to the invention is particularly suitable for use in thermal processes. Consequently, an embodiment is preferred in which the surface coating is carried out by thermal spraying.

For the application of a coating by means of thermal spraying methods, a number of methods are available to the person skilled in the art, the selection taking place in accordance with the requirements of the coating, such as, for example, its thickness. The powders of the invention may then optionally be adjusted according to the required processing parameters. The surface coating is preferably carried out by means of a thermal spraying method which is selected from the group consisting of flame spraying, plasma spraying, high-velocity air-fuel (HVAF) spraying and HVOF (high-velocity oxygen fuel) spraying. As already stated, the spray powder according to the invention is distinguished by its comparatively low true density and is therefore particularly suitable for the coating of components which have a low weight, while at the same time extreme conditions such as high temperatures, large temperature fluctuations, weathers and / or exposed to particle erosion, but at the same time have to have a high wear resistance. Above all, the requirements, placed on moving parts, in particular rotating and flying parts are particularly high due to the additional mechanical stress. In addition, a reduction in the flying weight requires a reduction in the fuel requirement or an increase in the so-called "payload", for example in the aviation industry.

Therefore, the spray powder according to the invention is preferably used for coating components, especially for moving, in particular rotating components, preferably selected from the group consisting of fan blades, compressor blades, hydraulic piston rods, suspension parts and guide rails.

Especially in the aerospace industry, reducing weight without compromising on stability and thus safety is an important aspect in the development of new technologies, which must be balanced above all from an economic and ecological point of view. Therefore, an embodiment of the present invention is preferred in which the spray powder according to the invention is used for coating aircraft components.

Another object of the present invention is a process for the preparation of the spray powder according to the invention. The method comprises the following steps: a) providing a mixture comprising i) hard materials, comprising or consisting of molybdenum carbide, wherein the mean particle diameter of the molybdenum carbide <10 μιη, in particular <5 μηι, determined according to ASTM B330, and ii) a or more matrix metal powders wherein the matrix metal powder (s)

0 to 20% by weight of molybdenum, based on the total weight of the matrix powder (s); and iii) optional wear-modifying oxides, wherein the proportion of oxides is between 0 and 10% by weight, preferably between 1 and 8% by weight, based on the total weight of the sprayed powder; and b) sintering the mixture to obtain a sintered powder, preferably a sintered powder of the agglomerated / sintered type.

The percentages by weight (% by weight) of the powders and blends in the present invention add up to 100% by weight each. Matrix metal powder in the sense of the present invention refers to metal powders which are suitable for the formation of the metallic matrix according to the invention.

The wear-modifying oxides are preferably selected from the group consisting of Al ₂ 0 ₃ , Y ₂ 0 ₃ and oxides of the 4th subgroup of the Periodic Table. Due to the fine particle size of the hard materials, the desired narrowstability of the matrix lamellae, which form between the particles, can be adjusted in a controlled manner. It has been shown that the smaller the particle size of the hard materials used, the greater is their specific surface area, which leads to a lower film thickness and thus to a lower web width of the wetting metal matrix.

It has proved to be particularly advantageous if the powders used are present during the production process as a mixture in the form of a dispersion in a liquid. Therefore, an embodiment of the process is preferred in which the provision of the mixture by a dispersion in which the components i), ii) and iii) are carried out. Suitable liquids are, for example, water, alcohols, ketones or hydrocarbons, without being limited to them by the exemplary list.

It has also been shown that the powders according to the invention develop their advantageous properties above all when present as agglomerates. Consequently, a preferred embodiment is characterized in that an agglomeration step takes place between step a) and b) of the process according to the invention. The agglomeration can be done, for example, by spray drying.

An embodiment in which a temporary organic binder is added to the mixture from step a) before the agglomerating step is particularly preferred becomes . The organic binder can be, for example, paraffin wax, polyvinyl alcohol, cellulose derivatives, polyethyleneimine and similar long-chain organic auxiliaries, which are removed from the mixture in the course of the further process, for example during sintering, for example by evaporation or decomposition.

The inventive method for producing the wettable powders according to the invention comprises a method step in which the mixture is sintered. The sintering of the mixture is preferably carried out at temperatures of 800 ° C to 1500 ° C, preferably from 900 ° C to 1300 ° C. As already stated, sintering is carried out after a preceding agglomeration step to produce agglomerated / sintered powders. On the other hand, in order to produce sintered / crushed powders, the sintered body obtained by sintering is subsequently crushed (broken up).

It may happen that the hard materials used, for example molybdenum carbide, are oxidized during sintering. Preference is therefore given to an embodiment in which the sintering of the mixture or agglomerates takes place under non-oxidizing conditions, preferably in the presence of hydrogen and / or inert gases and / or reduced pressure. The sintering can be carried out in the presence of hydrogen and / or inert gases. Likewise, sintering may be in the presence of hydrogen and / or reduced pressure. Furthermore, it is possible to carry out the sintering in the presence of inert gases and / or under reduced pressure. Inert gases in the context of the invention, for example, noble gases or nitrogen are to be understood. In a particularly preferred embodiment, the sintering can additionally be carried out in the presence of carbon, in order to further counteract possible oxidation reactions of the molybdenum carbide by its entropy properties.

In order to realize the smallest possible particle size distribution, it has proved to be advantageous to remove unwanted coarse and fine fractions of the sintered powder. Therefore, an embodiment is preferred in which the method comprises an additional classification step which takes place after sintering and / or optionally already after agglomeration. In particular, the use of alloy powders has proved to be advantageous in the production of the spray powders according to the invention. Consequently, an embodiment is preferred in which an alloy powder is used as the matrix material. Another object of the present invention is a method for producing a coated component, wherein the method comprises applying a coating by thermal spraying of the spray powder according to the invention.

Furthermore, an object of the present invention is a coated component, which is obtainable according to the inventive method. In this case, the method comprises applying a coating by thermal spraying of the spray powder according to the invention, as described in the present invention.

The present invention will be further described by the following examples.

Examples

As the matrix metal powder, for example, cobalt powder "efp" or "hmp" from Umicore (Belgium), nickel powder "T255" from Vale (Great Britain) or carbonyl iron powder "CM" from BASF (Germany) can be used. The additives, which reduce the elongation at break as elongation at break or solidifying elements, consist of fine-grained metal or alloy powders, such as commercially available molybdenum powders, atomized alloys such as NiCr 80/20, or powdered ferroalloys such as ferrochrome, ferromanganese, nickel egg, ferrosilicon, ferroboron or nickel boron.

Example:

From 70 kg of a molybdenum carbide (Mo ₂ C 160, HC Starck GmbH, Goslar) with a mean particle size of 1.6 μι (ASTM B330) as a hard material, and 25 kg of nickel metal powder 255 (Vale-Inco, UK) and 5 kg Molybdenum metal powder (average particle size 2.5pm, determined according to ASTM B330, HC Starck GmbH, Goslar), an agglomerated / sintered spray powder was prepared by dispersing these powders together in liquid and agglomerating them after addition of polyvinyl alcohol by means of spray drying. After eliminating unwanted coarse and fines, the sintering was carried out at 1152 ° C under hydrogen in the presence of carbon. As a result, an agglomerated / sintered spray powder was obtained, which had the desired nominal particle size band of 45/15 μηι after further classification (see 3.3 in DIN EN 1274). The resulting agglomerated / sintered spray powder had the following properties: Chemical composition (in percent by weight):

Carbon: 4.27% by weight

Nickel: 24.9% by weight

Oxygen: 0.36 wt%

Average particle diameter of the sintered agglomerates according to laser diffraction (determined according to ASTM B822, for example by means of Microtrac S3000): 33 μm

Hall Flow (ASTM B212): 18 sec / 50 g (1/10 inch funnel)

Apparent density (ASTM B212): 3.87 g / cm ³

Pyknometric Density (He): 9.02 g / cm ³ X-ray diffraction shows reflections of Mo ₂ C (nominal carbon content: 5.88% by weight) and a cubic face centered Ni phase due to molybdenum alloyed in it has a shift of the main reflex by about 1 °.

With the aid of the known true densities (Mo ₂ C: 9, 18 g / cm ³ , Ni: 8.9 g / cm ³ , Mo: 10.2 g / cm ³ ), a true weight is calculated for the composite material by weight Density of 9.15 g / cm ³ . The pyknometrically determined skeletal density of the powder is only slightly below the calculated true density, probably due to closed porosity and surface oxides or hydroxides. Figure 1 shows an electron-optical recording of Pulveranschliffs invention (backscattered electrons). Light gray, the molybdenum carbide is recognizable, which has an average particle size of about 5 pm. The optical evaluation to determine the particle size is based on the limitation by the dark gray NiMo phase and grain boundaries, which represent the former surface of the molybdenum carbide used for the preparation of powder particles.

Coatings were produced from the spray powder by means of HVOF spraying (kerosene as fuel, spray gun JP-5000 from Praxair, USA), which had the following properties, depending on the spraying conditions selected:

Order rate: 37 - 45%,

Vickers hardness HV0.3: 920 kg / mm ²

Friction coefficient μ against 100Cr6: 0.85 - 0.87 (pin on disk method)

Wear according to ASTM G65 Method B: 25 mg = 2.8 mm ³ Chemical composition (in% by weight): C: 3.46% by weight, O: 0.15% by weight

According to X-ray diffraction, the sprayed layer consists of Mo ₂ C and a cubic face-centered metallic matrix containing Ni with a very broad main reflection, which is shifted by about 1.2 ° to lower diffraction angles, ie must contain more alloyed Mo than the spray powder. As can be seen by comparing the oxygen content of the spray powder and in the spray coating, the spray powder is self-cleaning, since the oxygen content in the spray coating is lower than that of the spray powder, although oxidation is to be expected during the spraying. One possible explanation would be that volatile MoO ₃ evaporates during thermal spraying. This effect is also to be assumed for WCCo spray materials, whereby W0 ₃ evaporates here.

In the salt corrosion test (ASTM B117), a good resistance of the sprayed layer to common salt was found.

The coefficient of friction is in the usual carburizing frame. FIG. 2 shows a photomicrograph of a section of a pointed layer according to the invention. Clearly visible are the finely dispersed distribution of dark gray molybdenum carbide, a small ridge width of the light gray metallic matrix and an average particle size of molybdenum carbide, which is optically well below 10 pm. The microstructure of the sprayed layer differs considerably in these points from the structures of other systems known from the prior art (cf., for example, EP 0 701 005 B1, FIG. 1 and [0011]).

Comparative Example: Commercial, agglomerated / sintered WC- and chromium-carbide-based spray powders were processed into coatings under the same spraying conditions as described above and the wear results were measured according to ASTM G65. For the purpose of comparison, the mass loss was divided by the true density to directly compare the volume wear rates. An industrial electrolytic hard chrome coating was included. Further, the oxygen content of the layer after peeling was measured.

The results are shown in Table 1, wherein Examples 1 to 3 and 5 are comparative examples and Example 4 is an example according to the invention. Except for hard chrome, all examples are cermets with a high degree of dispersion of the hard materials in the metallic matrix.

Table 1 :

ers

ASTM G65 1.3 - 1.6 3.5 5 - 7 2.8 4.2

(mm ³ )

Oxygen 0.3-0.6 O, 1 -0.3 0.4-0.7 O, 15 about 1.0

(% By weight) ^a) the figures relate to percent by weight of the hard materials and the metallic matrix ^b) geometric density

It can be seen that the two chromium-free agglomerated / sintered spray powders (Examples 2 and 4) produce self-cleaning sprayed coatings due to the absence of Cr and thus of non-volatile chromium oxide and have similar wear rates, but the sprayed layer of molybdenum carbide (Ex ) has the advantage of lower density. Although the chromium carbide sprayed layer has an even lower density, it has insufficient wear resistance.

Although the hardness of the spray coating according to the invention is more in a range comparable to chromium carbide based sprayed coatings (700-900) than tungsten carbide based coatings (1100-1300), the wear rate is more comparable to the latter, considering the hardness expected main influence on the wear is surprising.

Claims

A sintered spray powder comprising a) 5 to 50% by weight of metallic matrix, based on the total weight of the sprayed powder, the metallic matrix containing 0 to 20% by weight of molybdenum, based on the total weight of the metallic matrix; b) 50 to 95 wt .-% hard materials, based on the total weight of the spray powder, comprising or consisting of at least 70% by weight molybdenum carbide, based on the total weight of the hard materials, wherein the average diameter of the molybdenum carbide in the sintered spray powder <10 μι determined according to ASTM E112; and c) optional wear-modifying oxides.

Spray powder according to claim 1, characterized in that boron is present in an amount of at most 1.4 wt .-%, preferably from 0.001 to 1.0 wt .-%, based on the total weight of the metallic matrix.

Spray powder according to one or more of claims 1 and 2, characterized in that silicon is present in an amount of at most 2.4 wt .-%, preferably from 0.001 to 2.0 wt .-%, based on the total weight of the metallic matrix ,

Spray powder according to one or more of claims 1 to 3, characterized in that the molybdenum carbide MoC and / or Mo ₂ C, preferably Mo ₂ C.

Spray powder according to one or more of claims 1 to 4, characterized in that the average particle diameter of the molybdenum carbide in the sintered spray powder is less than 10 μαι, preferably 0.5 to 6.0 μιη, in particular 1.0 to 4.0 μιτι, determined according to ASTM E112.

Spray powder according to one or more of claims 1 to 5, characterized in that the hard material comprises further carbides, preferably Carbides selected from the group consisting of tungsten carbide, chromium carbide and boron carbide and carbides of the metals of the 4th, 5th and 6th subgroups of the periodic table.

Spray powder according to one or more of claims 1 to 6, characterized in that the spray powder is agglomerated and sintered.

Spray powder according to one or more of claims 1 to 7, characterized in that the metallic matrix contains at least 60% by weight, preferably 70 to 90% by weight, of a metal selected from the group consisting of iron, cobalt and nickel on the total weight of the metallic matrix.

Spray powder according to one or more of claims 1 to 8, characterized in that the amount of elongation at break and hardening elements is less than 40 wt .-%, preferably 5 to 20 wt .-%, based on the total weight of the metallic matrix.

0. Spray powder according to claim 9, characterized in that the elongation at break and hardening elements are selected from the group consisting of molybdenum, tungsten, boron, silicon, chromium, niobium and manganese and mixtures thereof.

1. Spray powder according to one or more of claims 1 to 10, characterized in that the metallic matrix comprises nickel in an amount of 50 to 95 wt .-%, preferably 60 to 85 wt .-%, each based on the total weight of metallic matrix.

2. Spray powder according to one or more of claims 1 to 11, characterized in that the metallic matrix cobalt in an amount of 10 to 90 wt .-%, preferably from 20 to 90 wt .-%, in particular from 50 to 90 wt. -%, in each case based on the total weight of the metallic matrix.

3. Spray powder according to one or more of claims 1 to 12, characterized in that the metallic matrix iron in an amount of 10 to 90 wt .-%, preferably from 10 to 60 wt .-%, in particular of 20 to 50 wt .-%, in each case based on the total weight of the metallic matrix.

14. Spray powder according to one or more of claims 1 to 13, characterized in that the metallic matrix comprises molybdenum in an amount of 2 to 15 wt .-%, preferably from 5 to 10 wt .-%, each based on the total weight the metallic matrix.

15. Spray powder according to one or more of claims 1 to 14, characterized in that the spray powder wear-modifying oxides in an amount of 0 to 10 wt .-%, preferably from 1 to 8 wt .-% comprises, in each case based on the total weight of the spray powder.

16. Use of a spray powder according to one or more of claims 1 to 15 for surface coating.

17. Use according to claim 16, characterized in that the surface coating is carried out by thermal spraying.

18. Use according to claim 17, characterized in that the thermal spraying process is selected from the group consisting of flame spraying, plasma spraying, HVAF and HVOF spraying.

19. Use of a spray powder according to one or more of claims 1 to 15 for coating components, especially for moving, in particular rotating components, preferably selected from the group consisting of fan blades, compressor blades, hydraulic piston rods, suspension parts and guide rails.

20. Use of a spray powder according to one or more of claims 1 to 15 for coating aircraft components.

21. A process for producing a wettable powder according to one or more of claims 1 to 15, comprising the steps: a) providing a mixture comprising i) hard materials, comprising or consisting of molybdenum carbide, wherein the average particle diameter of the molybdenum carbide <10 μηι, determined according to ASTM B330, and ü) one or more matrix metal powder, wherein the / the matrix metal powder contains 0 to 20 wt .-% molybdenum / based on the total weight of the matrix metal powder (s); and iü) optionally wear-modifying oxides, wherein the proportion of the oxides 0 to 10 wt .-%, each based on the total weight of the spray powder is; and b) sintering the mixture to obtain a sintered powder.

22. The method according to claim 21, characterized in that the provision of the mixture by a dispersion in which the components i), ii) and iii) are carried out. 23. The method according to claim 21 or 22, characterized in that an agglomeration step takes place between step a) and b).

24. Process according to claim 23, characterized in that a temporary organic binder is added to the mixture from step a) before the agglomeration step. 25. The method according to one or more of claims 21 to 24, characterized in that the sintering of the mixture at temperatures of 800 ° C to 1500 ° C, preferably from 900 ° C to 1300 ° C, takes place.

26. The method according to one or more of claims 21 to 25, characterized in that the sintering of the mixture under non-oxidizing conditions, preferably in the presence of hydrogen and / or

Inert gases and / or under reduced pressure takes place.

27. The method according to one or more of claims 21 to 26, characterized in that the method comprises an additional classification step which takes place after sintering and / or optionally after agglomeration.

28. The method according to one or more of claims 21 to 27, characterized in that an alloy powder is used as the matrix metal powder.

29. A method for producing a coated component comprising applying a coating by thermal spraying a spray powder according to one or more of claims 1 to 15.

30. Coated component obtainable according to the method according to claim 29.