EP2330228B1 - Spray material, thermal spray layer and cylinder with a thermal spray layer - Google Patents

Description

The invention relates to a spray material for the thermal coating of a substrate, in particular for the thermal coating of a tread of a cylinder of a reciprocating internal combustion engine, a thermal spray coating, and a cylinder with a thermal spray coating according to the preamble of the independent claim of the respective category.

The EP 0 167 723 A1 describes a spray material that has ZnO in the range of 1% to 8% of the weight of the spray material.

The EP 1 340 834 A2 suggests a cylinder tread layer for reciprocating engines having embedded FeO and Fe30 4 crystals to form solid lubricants.

The US 2007/099015 A1 discloses a cylinder liner coating material comprising stainless steel, iron and titanium oxides.

The US 4 256 811 A describes a spray material that has ZnO in the range of 1% to 20% of the weight of the spray material.

The DE 10 2004 025672 A1 refers to a spray material that has a Mo metal phase.

Thermal spray applied coatings have long been known for a variety of applications. For example, surfaces of oil-lubricated cylinder liners in vehicle engines have been coated by plasma spraying for some time, among other things, the layer being significantly reduced in friction coefficient between piston rings and cylinder wall, thereby significantly reducing the wear of piston rings and cylinders , which leads to an increase in the mileage of the engine, an extension of the maintenance intervals, for example when changing the oil and not least to a significant increase in engine performance.

This is achieved by different measures in the prior art. For example, such layers may be lubricated for oil Containers of dry lubricants in a base matrix, wherein in the base matrix in addition pores of a predeterminable size can be provided, which act as oil pockets and thus significantly reduce the friction between piston rings and cylinder wall together with the relatively soft embedded dry lubricants. The basic matrix itself, which in particular contains the dry lubricants and the pores among other constituents, is composed of a hard matrix material which guarantees a long service life of the cylinder surfaces and piston rings. Such a modern high-performance cylinder tread is for example in the EP 1 340 834 described in detail.

Other typical applications for thermal spraying surfaces include coating turbine parts with anti-wear and thermal barrier coatings, components lubricated by oil-lubricated bearings, e.g. the coating of crank bearings or other workpieces subjected to particular physical, chemical or thermal stresses. Depending on which purpose the layer has to fulfill, very specific materials are used, usually in the form of spray powders or sprayed wires, which have the necessary specific properties and composition in order to generate the required properties of the surface layer to be sprayed.

With larger production volumes, the powder material price plays a decisive role in terms of the cost effectiveness of the coating, in particular in the coating of cylinder surfaces by means of the plasma spraying process APS, especially in the case of the coating of larger engines (truck diesel, for example).

The production costs of the powder are, on the one hand, dependent on the raw material prices and, on the other hand, on the processing effort required to process the raw materials into a usable material which is suitable for carrying out the chosen process.

In the known atomizing of metallic materials (by means of gas or water), the energy costs can be influenced practically only by a better powder yield down. The specification of particle size distribution plays a crucial role here. Under the best of conditions, the production costs of metallic powders of a quality such as e.g. for internal coatings of cylinders for internal combustion engines, nowadays hardly lower under atomization at less than 10 US $ per kg. It is therefore to be expected that there are certain limits to further cost reduction.

On the other hand, the demands on the performance of the spray materials with time are even greater. In particular, the tribological properties of the coatings at elevated temperatures are becoming increasingly important, since the effect of the lubricant decreases significantly with increasing wall temperature. Tribological solutions that are applicable up to a wall temperature of 350 ° C, are in principle possible. The anti-scuffing properties of the coating materials play a decisive role here.

As a favorable production method, in particular of ceramic or non-metallic powders for thermal spraying, grinding and sieving generally also apply, even in the case of larger quantities in the case of ceramic spray materials of metallic oxides. In the case of certain materials minerals in the powder can be processed without additional melting.

The iron-titanate FeTiO ₃ , which is also known as ilmenite, was previously considered as a potential material for cylinder surfaces. Ilmenite is formed from approximately 53% TiO ₂ and 47% FeO, and crystallizes in a hexagonal crystal system. The hardness of ilmenite crystals is approx. 650 HV, ie in the layers values of 400 to 500 HV are possible with optimized parameters.

Therefore, already in the UA 74,987 proposed an ilmenite spray material for the formation of corrosion-resistant coatings by thermal spraying. In the WO 2004/106711 Applicants suggest ilmenite, in part in combination with other metal-ceramic materials and / or oxides, as a spray material for coating cylinder surfaces of turbocharged engines. However, these coatings are not designed for the increasing tribological requirements at high or strongly fluctuating temperature loads, but improve mainly the hardness or corrosion resistance of the coated surfaces.

Based on this prior art, the applicant has earlier in the PCT / EP2009 / 058565 proposed a significantly improved iron-based spray material for thermal coating of cylinder treads of reciprocating internal combustion engines with FeTiO ₃ as the base material. The improved spray material according to PCT / EP2009 / 058565 comprises at least a first solid lubricant of a sulfide and a second solid lubricant of a fluoride.

By this invention it was demonstrated for the first time that spray materials based on iron titanate, ie based on the so-called ilmenite with the chemical formula FeTiO ₃ , are particularly well suited for the thermal coating of internal combustion engine components, if the ilmenite at least one sulfide and a fluoride Solid lubricant is added. The layers produced therewith are characterized in particular by an excellent resistance to adhesion wear. The addition of solid lubricants, in addition to a sulfide and a fluoride in particular, for example, additionally relate to a nitride, which allows, inter alia, a significant increase in the wall temperature of the cylinder surfaces in the operating state, so that these layers are particularly well suited for use in adiabatic engines ,

By the simultaneous use of at least one sulfide and a fluoride in the spray material of PCT / EP2009 / 058565 It was possible to ensure that the thermally sprayed layers had comparably good tribological properties in different temperature ranges.

The tribological performance of the iron titanate FeTiO ₃ layers (ilmenite) can be significantly improved by the targeted addition of solid lubricants. The properties of these lubricants are based inter alia on the special crystal structure and the low propensity for chemical bonds or reactions with metallic and ceramic materials. The specific class of solid lubricants is selected according to the invention according to the expected temperature loads. In the case of internal cylinder coatings in internal combustion engines, the highest wall temperature, for example in the contact zone of the cylinder running surface / piston ring, is advantageously considered for this purpose.

The solid lubricants based on sulfides, for example MoS ₂ and / or WS ₂ can be used in an oxidizing atmosphere without problems up to a temperature of 350 ° C. In the case of shock loads in internal combustion engines, however, hot contact points, for example, between the cylinder surface and piston rings can form, wherein the local temperature can be significantly higher than 350 ° C. Therefore, according to PCT / EP2009 / 058565 In addition, at least one other type of solid lubricants used, which has an increased temperature resistance and at the same time is chemically resistant under the aggressive chemical conditions in the combustion chamber and the adhesion and hardness of the coating additionally positively influenced.

This teaches the PCT / EP2009 / 058565 In addition, in addition to the fluorides and solid lubricants based on nitrogen, such as hexagonal BN or CrN can be particularly advantageous, since these functions as a solid lubricant up to the highest temperatures of 950 ° C, even under oxidizing conditions meet, with such high temperatures, for example, often occur only locally in cylinders of internal combustion engines.

In the EP 1 790 752 A1 a thermal spray material is proposed with a very high zinc content of at least 70% zinc, but only under certain low pressure conditions of less than 100mbar, preferably even only between 1 mbar and 10mbar gas pressure in a process chamber while maintaining very large spray distances of at least 400mm to the substrate can be sprayed. The spray material of the EP 1 790 752 A1 and the spraying method also proposed therein to replace in the field of corrosion protection the valid as environmentally harmful galvanizing. Therefore, the zinc content must be at least 70%, so that a sufficient effect of the zinc coating against corrosion is achieved. Due to the high vapor pressure of zinc, the spray material of the EP 1 790 752 A1 However, only successfully successfully sprayed in combination with the also proposed in this document low-pressure method, which of course requires the use of a closed process chamber in which the necessary low pressure conditions are adjustable. In this case, the process chamber must have a sufficient size, so that a minimum spray distance to the substrate to be coated of at least 400mm is adjustable. In addition, not only the pressure in the process chamber itself plays an important role, but it must also be set a pressure ratio of about 1 to 40 between a pressure in the interior of the coating jet and the actual gas pressure of the gas atmosphere in the process chamber. That is, the pressure within the coating jet must be greater than the pressure of the gas atmosphere in the process chamber. This choice of printing parameters is also referred to in the art as "under-expanded condition". It is an essential insight of EP 1 790 752 A1 in that spray materials containing a material with a comparatively high vapor pressure, such as Example zinc, with which in the EP 1 790 752 A1 described method must be injected if it is to be prevented that the material with the high vapor pressure evaporated to a great extent during thermal spraying and thus no longer or not sufficiently in the sprayed layer is present.

For this reason alone, pure zinc comes as a spray material additive for thermal spraying, which are not carried out in a process chamber under low pressure atmosphere, so for example for that internal coating of cylinders with rotating spray guns for the expert out of the question. In addition, layers of pure zinc do not have the necessary mechanical strength or temperature resistance for applications as a cylinder surface.

The object of the invention is to provide a new spray material in the form of a powder material or in the form of a sprayed wire, in particular spray filler wire for thermal coating of a substrate, with which using conventional spray methods, but preferably not necessary under ambient atmosphere, that is preferably not under a reduced Gas pressure thermally sprayed layers can be produced, which have excellent tribological properties at the same time in different temperature ranges, so that the powder material is particularly suitable for forming friction-optimized running surfaces on cylinders of reciprocating internal combustion engine, which are operated even under changing load conditions. The surface layers formed with the spray material should also be sufficiently resistant to corrosion and have excellent hardness and at the same time be easily machinable, especially when honing the sprayed layers.

Furthermore, it is an object of the invention, a corresponding thermal spray coating and a cylinder for a reciprocating internal combustion engine coated with a thermal Spray coating made of a spray material of the present invention to propose.

The objects of the invention solving these objects are characterized by the features of the independent claim of the respective category.

The respective dependent claims relate to particularly advantageous embodiments of the invention.

The invention thus relates to a spray material for the thermal coating of a substrate, in particular for the thermal coating of a running surface of a cylinder of a reciprocating internal combustion engine. According to the invention, the spray material comprises a solid lubricant of ZnO, wherein the volume fraction of ZnO in the spray material is in the range of 0.1% to 15% of the volume of the spray material.

In particular, the invention relates to a spray material for thermal coating of a substrate, in particular for thermal coating of a tread of a cylinder of a reciprocating internal combustion engine, wherein the spray material comprises a solid lubricant of ZnO, and the volume fraction of ZnO in the spray material in the range of 0.1% to 15% the volume of the spray material is. According to the invention, the spray material additionally contains one or more elements of the elements consisting of the group of elements C, Cr, Ti, O, Mn, Mo, Fe, S, W, B, Ba, Ca and F.

In one specific embodiment, the spray material contains one or more materials from the material group consisting of the materials MoS ₂ , WS ₂ , BN, CrN, CaF ₂ , BaF ₂ , TiO ₂ , FeTiO ₃ , Fe 1 Cl 1 Cr 1 Mn, Fe 13 Cr ₂ Mo 0.5 C, XPT-512, alpha-Fe, iron carbide, wustite and magnetite.

By means of the present invention it can thus be demonstrated for the first time that spray materials containing ZnO are particularly well suited for the thermal coating of internal combustion engine components, if Zn is not used in pure form but bound as ZnO in the spray material and the volume fraction of ZnO in the spray material is in the range of 0.1% to about 15% of the volume of the spray material.

Due to the favorable crystallographic and physical properties (decomposition point of ZnO approx. 1975 ° C, density of ZnO approx. 5.6 g / cm ³ ), the material zinc oxide ZnO shows a real potential for use as solid lubricant, especially in combination with thermal spray coatings. In particular, the hexagonal crystal structure (wurtzite), the relatively low hardness (Mohs 4,5 corresponds to about 350HV) and the high vapor pressure of the zinc oxides are of particular importance. For the production of thermal spray coatings, the solid lubricant ZnO is mixed or agglomerated, for example, with the powder XPT-512 (low-alloyed carbon steel). For the effectiveness of the lubrication effect, for example in the application as a cylinder coating, the particle size should preferably be from a few micrometers up to 15 micrometers. Thus, a microstructure of the layer of alpha-Fe forms with fine iron carbides, Wustite FeO, Magnetite Fe ₃ O ₄ , and according to the invention of zinc oxide ZnO. The amount of ZnO in the spray material is conveniently between 4% and 10% volume% in many applications, and in some cases may be slightly lower or higher. In practice, optimization tests, for example by means of friction tests and engine test series, for determining the optimum amount of ZnO for the specific application will usually be necessary. The same procedure can also be used with the corrosion resistant (13 weight% Cr steel) material. Also, ceramic layers can be changed or improved by the addition of ZnO, for example in the case of iron titanate FeTiO ₃ (ilmenite). In particular, with the ceramic materials is by the Addition of ZnO significantly improves the machinability during honing. In addition, the addition of zinc oxides reduces the risk of dreaded scuffing in case of insufficient lubrication and corresponding elevated local temperatures.

The use of ZnO as an additive for thermal spray materials is also important from an economic point of view, since zinc oxide is automatically obtained as a waste product in the industrial production of brass (in foundries in the production of semi-finished products) and thus very cost-effectively as a raw material for the production of the inventive spray material is available.

When melting brass alloys (for example, copper alloyed with 30 to 40% by weight of zinc), a large amount of zinc vapor is formed due to the high vapor pressure of zinc. These vapors react with the oxygen in the air and thus form particles of zinc oxide, which are usually retained in a filter for environmental reasons alone. The use of zinc oxide from the filter residues thus makes sense both for economic and environmental reasons. The often inevitable contamination of zinc oxides with copper plays only a minor role for the properties of the solid lubricants based on zinc oxide and can be accepted, so that no elaborate cleaning is necessary for further use. Thus, essentially only sieving to the desired particle size is necessary as preparatory operations, wherein a known air sieve method can be used with particular advantage.

In the following Table 1, some particularly preferred embodiments of spray powders according to the invention and thermal spray coatings produced therefrom are specified in more detail by way of example. The specified microhardness applies to thermal spray coatings used in experiments with a F210 plasma torch from Sulzer Metco were applied. These experimental results apply to optimized parameters Ar / H2. <u> Table 1: </ u> Typical spray powder material with addition of zinc oxides for the production of cylinder surface. Based material Vol.% Of ZnO Particle size ZnO [micrometer] Layer hardness HV 0.3 Carbon Steel Fe 1C 1Cr 1Mn 5 or 10 2 to 15 350 - 500 Corrosion Resistant Steel Fe 13Cr 2Mo 0.5C 10 5 to 20 350 - 500 Iron titanate FeTiO3 12 5 to 20 400 - 600 Titanium oxide (rutile) TiO2 10 2 to 15 550 - 850

Table 2 lists further particularly preferred spray materials of the present invention, while at the same time giving preferred application examples in the field of automotive engineering for different types of engines and types of loads. <u> Table 2: </ u> Typical application examples of spraying materials according to the invention with the solid lubricant ZnO in layers of cylinder running surface of reciprocating internal combustion engines. of engine Layer material Load type Typical application OTTO engines 4-stroke Fe 1C 1Cr 1Mn + 5 vol.% ZnO Higher speeds Regular power Water cooling Sports car with automat OTTO engines 4-stroke Fe 1C 1Cr 1Mn + 10% by volume ZnO Higher speeds Changing power Water cooling Racing engines Engines for hybrid vehicles Diesel engine 2-4 stroke Iron titanate FeTiO3 + 12% by volume ZnO Regular speeds Regular power Ship diesel power generator Diesel engine 4-stroke Fe 13Cr 2Mo 0.5C + 10 Vol.% ZnO Strong change. Power and speeds. Truck and car OTTO engines 4-stroke Titanium oxide (rutile) TiO2 + 10% by volume ZnO Very high speeds, up to over 20,000 rpm. Strong change. Power and speeds. Water cooled. Racing engines for extreme conditions

As can be clearly seen in particular from Table 2, there is a relationship between the amount of ZnO contained in the spray material or in the thermally sprayed layer and the requirements of these layers in the operating state of the internal combustion engine. In particular, when very high thermal loads occur relatively high zinc oxide concentrations have been found to be particularly advantageous. High load can mean that the machines are operated at high or strongly changing speeds. Examples are racing engines for extreme conditions and / or for operation under strongly changing speeds or under strongly changing performances. ZnO concentration of about 10% by volume has been found to be advantageous in the specific examples given here.

However, high loads can also occur at relatively even and / or low speeds, for example, in large engines for ships or generators for generating electrical energy, in which not infrequently several thousand horsepower per cylinder can be generated.

In this case, by suitable choice of the base material, eg Fe 1C 1Cr 1Mn, FeTiO ₃ (Illmenite) etc., and / or by the addition of further materials such as Mo, Mn, titanium oxide or other materials known per se, the layers can be further adapted to specific requirements such as temperature fluctuations, Dry Attacks by acids, corrosion, oxidation, etc. are optimally adapted. The above table 2 also provides information about these possibilities.

In particular, the tribological performance of the inventive layers can be significantly improved by the targeted addition of solid lubricants. The properties of these lubricants are based inter alia on the special crystal structure and the low propensity for chemical bonds or reactions with metallic and ceramic materials. The specific class of solid lubricants is selected according to the invention according to the expected different types of stress. In the case of internal cylinder coatings in internal combustion engines, for example, the highest wall temperature, for example, in the contact zone cylinder surface / piston ring is considered.

For example, solid lubricants based on sulfide can be added. For example, MoS ₂ and / or WS ₂ can be used in an oxidizing atmosphere without problems up to a temperature of 350 ° C. In the case of shock loads in internal combustion engines, however, hot contact points, for example, between the cylinder surface and piston rings can form, wherein the local temperature can be significantly higher than 350 ° C. Therefore, additionally at least one other type of solid lubricants can be used, which has an increased temperature resistance and at the same time is chemically resistant under the aggressive chemical conditions in the combustion chamber and the adhesion and hardness of the coating additionally positively influenced.

In addition to the sulfides and fluorides are also solid lubricants based on nitrogen, for example, hexagonal BN or CrN particularly advantageous in question, as these functions as a solid lubricant up to the highest temperatures of 950 ° C, even under oxidizing conditions meet such high temperatures, for example, often occur only locally in cylinders of internal combustion engines.

In the special case of adiabatic diesel engines even higher local contact temperatures are to be expected. Certain fluorine-based solid lubricants are capable of reliably ensuring lubrication even under these critical conditions. For example, calcium fluorides CaF ₂ and barium fluoride BaF ₂ can reliably ensure lubrication even at locally occurring temperatures of more than 1200 ° C. In this case, the eutectic formed from 62% by weight of BaF ₂ and 38% by weight of CaF ₂ has proven to be particularly effective, which ensures significantly improved lubrication already from 500 ° C.

The thermally sprayed layers are advantageously post-processed in a manner known per se by diamond honing after the thermal spraying.

In a preferred embodiment of the present invention, the volume fraction of ZnO in the spray material is in the range of 0.5% to 12%, preferably in the range of 4% to 12% of the volume of the spray material.

In this case, the spray material according to the invention comprises a carbon steel, in particular an atomized carbon steel, a chromium steel, in particular a ferritic and / or martensitic chromium steel and / or TiO ₂ , and / or Mn and / or Mo or further advantageous components. In particular, in order to obtain a sufficient hardness of a base matrix of the inventive thermal spray coatings, the spray material may comprise a ceramic material. The ceramic material is particularly preferred except for impurities FeTiO ₃ .

Depending on the thermal spraying method used, depending on the structure, which is a thermally sprayed layer depending on the particular application The ZnO in the spray material may be present as ZnO powder with a predefinable particle size and / or the spray material may be formed by agglomeration and / or mixing with the ZnO powder.

As a preferred range for the particle size of the ZnO powder thereby a particle size in the range between 1μm and 25μm, preferably in the range between 5μm and 15μm has been found to be particularly advantageous.

In another embodiment which is very important in practice, a particle of the ZnO powder may also be mixed and / or agglomerated with a metal powder and / or a ceramic powder and / or a particle of the ZnO powder may be mixed with a powder of a low-alloyed carbon steel and / or agglomerated.

Of course, it is also possible that a particle of ZnO powder completely or partially enclosed by a metal-containing powder, that is completely or partially covered, which the skilled person also referred to as "cladding".

It goes without saying that mixtures of the abovementioned powder preparations are also possible for very specific applications.

In further embodiments which are particularly important in practice, a particle of the ZnO powder is mixed and / or agglomerated and / or coated with a powder of a corrosion-resistant chromium steel and / or with a ceramic powder of FeTiO ₃ .

Particularly preferably, a thermal spray coating is produced from a spray material of the present invention by a thermal plasma spraying process or a flame spraying process, in particular by a high-speed flame spraying process (HVOF process), the thermal spraying material preferably being used as powder, but also in the form of a sprayed wire, can be present in particular in the form of a cored wire.

Finally, as already mentioned several times, the invention finally also relates to a cylinder for a reciprocating internal combustion engine which is coated with a thermal spray coating made of a spray material of the present invention.

Claims (14)

1. A spray material for thermal coating of a substrate, in particular for thermal coating of a running surface of a cylinder of a reciprocating piston combustion engine, wherein the spray material includes a solid lubricant of ZnO, and the volume fraction of Zn0 in the spray material is in the range from 0.1 % to 15% of the volume of the spray material, wherein the spray material additionally includes one or more elements of the group of elements consisting of C, Cr, Ti, O, Mn, Mo, Fe, S, W, B, Ba, Ca, F
   characterized in that
   the spray material includes a carbon steel.
2. A spray material in accordance with claim 1, wherein the spray material includes one or more materials of the group of materials consisting of the materials MoS₂, WS₂, BN, CrN, CaF₂, BaF₂, TiO₂, FeTiO₃, Fe1C1Cr1Mn, Fe13Cr2MoO.5C, XPT-512, alpha-Fe, iron carbide, wustite and Magnetite.
3. A spray material in accordance with claim 1 or 2, wherein the volume fraction of ZnO in the spray material is in the range from 0.5% to 12%, preferably in the range from 4% to 12% of the volume of the spray material.
4. A spray material in accordance with any one of the preceding claims, wherein the spray material includes a gas atomized carbon steel.
5. A spray material in accordance with any one of the preceding claims, wherein the spray material includes a chrome steel.
6. A spray material in accordance with any one of the preceding claims, wherein the spray material includes a ceramic material.
7. A spray material in accordance with claim 6, wherein the ceramic material except for contaminants is FeTiO₃.
8. A spray material in accordance with any one of the preceding claims, wherein the ZnO in the spray material is present as ZnO powder having a presettable particle size and/or the spray material is formed by agglomeration and/or by mixing and/or by cladding with the ZnO powder.
9. A spray material in accordance with claim 8, wherein the particle size of the ZnO powder is in the range from 1µm to 25µm, preferably between 5µm and 15µm.
10. A spray material in accordance with claim 8 or 9, wherein a particle of the ZnO powder is mixed with a metal powder and/or a ceramic powder and/or is formed by agglomeration and/or by means of cladding, and/or wherein a particle of the ZnO powder is mixed with a low alloy carbon steel and/or is agglomerated.
11. A spray material in accordance with any one of claims 8 to 10, wherein a particle of the ZnO powder is mixed with a powder of a corrosion resistant chrome steel, in particular a ferritic chrome steel and/or a martensitic chrome steel and/or is formed by agglomeration and/or by means of cladding.
12. A spray material in accordance with any one of claims 8 to 11, wherein a particle of the ZnO powder is mixed with a ceramic powder of FeTiO₃ and/or is formed by agglomeration and/or by means of cladding.
13. A thermal spray layer consisting of a spray material in accordance with any one of claims 1 to 12.
14. A cylinder for a reciprocating piston combustion engine coated with a thermal spray layer in accordance with claim 13.