DE102011119087B3 - Method for producing a chromium protective layer and its use - Google Patents

Abstract

A process for producing a gas-tight and crack-free chromium protective layer for substrates of iron and nickel and titanium-based alloys by plasma spraying, wherein the chromium content in the finished layer is at least 70 wt.% And a wettable powder is selected from three components, a first component of fine-grained Chromium powder, a second one of fine-grained powder of a nickel-based alloy, and a third of coarse-grained cristobalite or quartz powder as a carrier for the first and second components.

Description

The invention relates to a method for producing a chromium protective layer according to the preamble of patent claim 1 and to the use of a plasma-sprayed protective layer.

State of the art

• 1. galvanische Chrom-Schichten
• 2. PVD- und CVD-Chrom-Schichten
• 3. Chrom-Schichten, die durch Hochtemperatur-Diffusion entstehen

Chromium is one of the most important metals for coatings. Its very high corrosion resistance against many aggressive media in a wide temperature range is comparable to that of precious metals. Depending on how they are made, the properties of chrome coatings vary widely. Three types of chromium-based coatings are known:

• 1. galvanic chrome layers
• 2. PVD and CVD chrome layers
• 3. Chromium layers, which are formed by high-temperature diffusion

Galvanic chromium layers are the oldest and most widely used chromium-based layers. The first description of the electrolytic deposition of chromium from 1843 is from AC Becquerel. In 1854, RW Bunsen described the chromium deposition from hot chromium (III) chloride solution with anodes of carbon and cathodes of platinum. Chromium deposition in the chrome bath was invented by Erik Liebreich ( DE398054A and DE448526A ). After that, the galvanic bath is based on CrO ₃ and H ₂ SO ₄ . To date, almost all chromium layers are produced by this process. Here, layers of pure chromium with thicknesses of <1 .mu.m to about 300 .mu.m are applied to very different substrates (metals, glasses, ceramics, plastics and even wood). Depending on the layer thickness, this is referred to as decorative chrome plating (layers <5 μm) or hard chrome (layer thicknesses: 10-200 μm). The peculiarities of galvanically deposited chromium layers consist of very high hardness and brittleness, relatively weak adhesion to the substrate and a fine crack network with layer thicknesses> 5 μm. These features and the fact that chromium has a low coefficient of thermal expansion - well below that of the most important metallic substrates - significantly limits the use of electroplated chromium layers. Because of their fine cracks, these chromium layers are basically permeable to gaseous and liquid media, their mechanical strength is relatively low due to poor adhesion and high brittleness, and the maximum allowable operating temperature is less than 500 ° C, although chromium as a compact metal temperatures over 1100 ° C in air can withstand.

PVD and CVD chromium layers are obtained by deposition from the gas phase in a vacuum oven. A distinction is made between a purely physical separation of chromium vapor (physical vapor deposition, PVD short) and a deposition by means of a chemical reaction between chromium-containing gas and substrate (chemical vapor deposition, CVD short). Because of this chemical reaction, the CVD chromium layers generally have higher adhesion than PVD chromium layers. However, the CVD process requires significantly higher temperatures of 800-1000 ° C compared to 200-500 ° C for the PVD process. Both methods allow the application of dense thin layers of pure chromium or chromium nitride (CrN). In comparison with galvanic chromium layers, the PVD and especially the CVD-chromium layers have a very good adhesion to the substrate, but are much more expensive compared to galvanic layers and therefore only limited usable for large-area parts. In addition, the maximum layer thickness is only about 10 microns.

The coating of steels known by the term "thermal chromium plating" by means of a thermochemical chromium diffusion at temperatures of 1000-1200 ° C. comprises two different process variants which, however, lead in principle to the same results: the known DE1905717A ) Chromium diffusion from solid phase z. B. chrome powder, and the known gas chromating ( EP0043742 A1 ) from a gas phase z. B. CrCl ₃ . In both processes, chromium diffuses into a steel surface to a depth of about 50 μm, forming a protective layer. In this diffusion layer, a chromium concentration of max. 50% achieved, with high corrosion resistance and high hardness of the steel surface. In fact, since the diffusion chromium layers are not pure chrome layers such as electroplated or PVD chrome layers, they also have very different properties such as good adhesion, good mechanical strength, and a coefficient of thermal expansion close to that of steel. These properties allow the use of diffusion chrome coated parts at temperatures higher than 800 ° C. Compared to pure chrome layers, however, the high-temperature corrosion resistance of the diffusion chromium layers is far below that for compact metallic chromium. Even with wet-chemical corrosion resistance, the layers of pure chromium are superior. Despite comparably favorable properties of diffusion chromium layers, these are only used to a very limited extent because of their high complexity.

Of the known chromium layers described above, only diffusion chromium layers are suitable for use at high temperatures above 800 ° C. Because of their relatively low chromium content of max. However, 50% do not achieve the desired resistance of pure chromium layers. With regard to the layer thickness, all known chromium layers are insufficient, because the maximum layer thickness of dense crack-free chromium layers is limited to approximately 10 μm.

Out EP 2 006 410 A2 Furthermore, a thermally sprayed protective layer for metallic substrates is known, wherein the spray powder 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.

Then describes DE 693 13 456 T2 a ceramic composite coating material, wherein the applied metal layer may include quartz glass.

WO 2003/031 672 A1 Finally, a spray powder composed of ceramic particles, including quartz, and a metal powder composed of Ni, Cr, Fe and Si is disclosed.

• – der Dauereinsatz an Luft soll bis 1000°C möglich sein
• – die Schicht soll beständig gegenüber Thermoschock sein
• – der Chromgehalt der Schicht soll mindestens 70% betragen
• – eine gute Haftung zum Substrat soll gewährleistet sein
• – hohe Gasdichtigkeit der Schicht soll durch entsprechende Rissfreiheit gegeben sein
• – es sollen Schichtdicken bis ca. 1 mm möglich sein

The aim of the present invention is to use the advantages of the metal chromium as a material for protective coatings against high-temperature corrosion without taking its disadvantages into account. The intended improvement is intended to refer to the following properties of chromium layers:

• - Continuous use in air should be possible up to 1000 ° C
• - The layer should be resistant to thermal shock
• - The chromium content of the layer should be at least 70%
• - Good adhesion to the substrate should be guaranteed
• - High gas-tightness of the layer should be given by appropriate freedom from cracks
• - Layer thicknesses up to approx. 1 mm should be possible

In particular, the object of the present invention comprises finding a solution in order to be able to use fine-grained chromium powders without disadvantages, ie to be able to produce sufficient adhesion despite increased oxidation of the fine chromium particles; to get a high kinetic energy and thus non-porous and firm microstructure despite fine chrome particles as well as to achieve a good flowability of the powder;
it is also a matter of finding additives for the chromium powder which reduce the brittleness of the layer and increase its thermal expansion coefficient;
Finally, it is a question of developing a method for heat treatment of the coated substrates, which leads to a solid metallurgical bond between layer and substrate.

This object is achieved according to the characterizing part of claim 1 by the combined use of the plasma spraying process with inventive powder mixture and optionally a subsequent diffusion heat treatment.

The application of the chromium layers by thermal spraying according to claim 1 allows low cost even for large parts. In addition, it allows the application of fairly thick layers, which, using known techniques such. B. by electroplating, PVD, CVD, gas chromating and inchromating would be unthinkable.

Due to the high melting point of chromium of approx. 1900 ° C, plasma spraying is optimally selected. It allows under the process conditions of claim 1, the melting of the chromium powder, but prevents its burning (oxidation) in the flame.

Accordingly, the inventive method provides to bring the chromium particles to melt and to accelerate against a substrate. It also involves the production of an oxidation protection for both the flying powder and the surface of the substrate under the flame. Since in plasma spraying the temperature of the particles in the plasma is mainly determined by their size, the chromium particles should be sufficiently fine-grained to ensure their melting. On the other hand, the use of fine-grained chromium powder means that it is very susceptible to oxidation because of its large surface area. The resulting particles of chromium oxide Cr ₂ O ₃ can not be reduced in the plasma but are melted and accelerated together with chromium particles against the substrate. The disadvantage here is that the chromium oxide hinders the adhesion between the metallic substrate and chromium, since no metallurgical compound can form. Another disadvantage of fine-grained chromium powder consists in a low kinetic energy of the small particles in the flame, with the result that in insufficient adhesion, a layer structure is formed, which is neither pore-free nor sufficiently solid.

These and other disadvantages of the plasma spraying of pure fine-grained chromium powder are overcome by the inventive method by the composition of the spray powder according to claim 1.

• • Chrompulver < 20 μm (d50 < 10 μm): 30–50 wt.%
• • Pulver einer Nickelbasislegierung, (z. B. 80Ni20Cr) < 20 μm (d50 < 10 μm): 5–10 wt.%
• • Cristobalit- oder Quarzpulver 50–100 μm (d50 = 70–90 μm): Rest

In this case, the powder for plasma spraying is preferably produced by simply dry mixing of three components:

• Chromium powder <20 μm (d50 <10 μm): 30-50 wt.%
• Powder of a nickel-based alloy, (eg 80Ni20Cr) <20 μm (d50 <10 μm): 5-10 wt.%
• • Cristobalite or quartz powder 50-100 μm (d50 = 70-90 μm): balance

In this mixture serves light (2.3 g / cm ³ ) coarse-grained cristobalite powder as a carrier for the heavy fine-grained chromium and Nickelchrompulver: Large cristobalite particles whose volume is more than 70% of the mixture (<30 vol.% Cr + NiCr) covered on the surface with fine chromium and nickel-based particles. These, quite large, round agglomerates make the powder very free-flowing. In the plasma, the agglomerates on the surface are heated so that all metallic particles melt. The large refractory (1720 ° C) cristobalite core remains basically solid. An agglomerate particle consisting of a cristobalite core encased in a molten metallic "crust" gets high kinetic energy in the plasma flame because of its size and weight. During its impact on the substrate, the following happens:
The solid cristobalite core breaks into small pieces that bounce off the substrate and are carried away by the gas flow. Only a fraction of the original amount of cristobalite is "taken in" into the layer, namely about 1-5% of the layer mass. These cristobalite residues then form small uniformly distributed inclusions (<20 μm) in the finished metallic layer. By contrast, almost the entire metallic proportion of the wettable powders "sticks" to the substrate and forms a finely structured dense layer. The large cristobalite particles fulfill another advantageous function: upon impact with the substrate or on inner layers, the hard and brittle grains act in the manner of a sandblasting material which "radiates away" oxide layers (Cr ₂ O ₃ ) immediately during coating. As a result, the adhesion to the substrate and between individual layer layers increases and the layer structure becomes stronger, wherein it contains only minimal amounts of Cr ₂ O ₃ . Since the nickel-based alloy has a much lower melting temperature than chromium, it later solidifies as chromium. This creates fine nickel-based lamellae on the surface of the chrome particles, ie in the finished layer hard chromium particles are surrounded by a fine "mesh" of soft nickel-based alloy. The "net" of soft nickel-based alloy significantly increases the ductility of the layer. Tensions that arise when solidifying molten chrome, no longer lead to cracking, but are removed by the plastic deformation of the nickel-base fins.

• • Chrom: 70–90 wt.%
• • Nickelbasislegierung: 7–25 wt.%
• • Cristobalit: 1–5 wt.% (3–15 vol.%)

The resulting layer has the following composition:

• • Chrome: 70-90 wt.%
• Nickel-based alloy: 7-25 wt.%
• • Cristobalite: 1-5 wt.% (3-15 vol.%)

Even with admixture of quartz powder, the finished layer contains only cristobalite, because quartz is transformed into cristobalite in the plasma.

Since cristobalite has a very high coefficient of thermal expansion of about 50 × 10 ^-6 K ^-1 , a coefficient of thermal expansion of the layer of about 9-10 × 10 ^-6 K ^-1 (compared with about 6.2 × 10 ^-6 K ^-1 for pure chromium). This value is already close to values for some steels, nickel-base alloys and titanium alloys, so that no dangerous stresses can arise in the layer during cooling.

An even better adhesion of the layer to iron and nickel base alloys is obtained by heat treatment of the coated parts. This is done in the oven at temperatures from 900 ° C in air. This heat treatment to about 5 hours leads to a diffusion of chromium from the layer into the substrate up to about 5 microns. As a result of this diffusion, the layer and the substrate are to a certain extent "welded together". At the same time, the heat treatment causes a reduction of residual stresses that are present in the layer after the plasma spraying.

Examples

Example 1.

Use in heavily loaded valves of heavy diesel engines operated by heavy fuel oil: Corrosion protection of nickel-base alloys against aggressive molten ash (sodium vanadate) in combination with exhaust gases containing SO ₂ and temperatures up to approx. 900 ° C.

A powder, mixed together from 40 wt.% Chromium <20 .mu.m, 10 wt.% 80Ni20Cr <20 microns and 50 wt.% Cristobalit 50-100 microns, was using Axial-3 plasma spraying the Thermico GmbH with the following parameters on a valve disk Nimonic 80 A sprayed on: Jet: 3/8 '' Electricity: 200 A (burner power: 95 kW) Plasma gas: Argon - 200 L / min, nitrogen - 55 L / min, hydrogen - 12 L / min Powder gas: Nitrogen - 10 L / min Amount of powder: 20 g / min

The coated valve was heat treated at 1020 ° C for one hour in air.

After the coating and the heat treatment, a pore-free and crack-free layer 800 μm thick was produced on the valve disk surface with the following composition: Chrome: about 82 vol.% 80Ni20Cr: about 12 vol.% Cr ₂ O ₃ : about 3 vol.% cristobalite: about 3 vol.%

Example 2.

Use in heavily loaded pipes of waste incineration plants: Corrosion protection for steels against chloride and sulphate ashes in combination with SO ₂ and HCl-containing exhaust gases and temperatures up to approx. 600 ° C.

A powder mixed together from 40 wt.% Chromium <20 .mu.m, 10 wt.% 80Ni20Cr <20 microns and 50 wt.% Cristobalit 50-100 microns, was using Axial-3 plasma spraying the Thermico GmbH with the following parameters on a boiler tube Steel 37 sprayed on: Jet: 3/8 '' Electricity: 200 A (burner power: 95 kW) Plasma gas: Argon - 200 L / min, nitrogen - 55 L / min, hydrogen - 12 L / min Powder gas: Nitrogen - 10 L / min Amount of powder: 20 g / min

The coated tube was heat treated at 900 ° C for five hours in air.

After coating and heat treatment, a 100 μm thick non-porous and crack-free layer with the following composition was formed on the tube surface: Chrome: about 82 vol.% 80Ni20Cr: about 12 vol.% Cr ₂ O ₃ : about 3 vol.% cristobalite: about 3 vol.%

Example 3.

Use in highly stressed titanium valves of racing engines: Oxidation protection for all titanium alloys and titanium aluminides at temperatures up to approx. 800 ° C.

A powder, mixed together from 40 wt.% Chromium <20 .mu.m, 10 wt.% 80Ni20Cr <20 microns and 50 wt.% Cristobalit 50-100 microns, using Axial-3 plasma spraying the Thermico GmbH with the following parameters on a valve disk and Shaft made of Ti6Al2Sn4Zr2Mo sprayed on: Jet: 3/8 '' Electricity: 200 A (burner power: 95 kW) Plasma gas: Argon - 200 L / min, nitrogen - 55 L / min, hydrogen - 12 L / min Powder gas: Nitrogen - 10 L / min Amount of powder: 20 g / min

After coating, a 100 μm thick pore-free and crack-free layer with the following composition was produced on the complete valve surface: Chrome: about 84 vol.% 80Ni20Cr: about 12 vol.% Cr ₂ O ₃ : about 1 vol.% cristobalite: about 3 vol.%

This layer also serves as a wear-resistant overlay on the valve stem.

Claims (6)

A process for producing a gas-tight and crack-free chromium protective layer for substrates of iron and nickel and titanium-based alloys by plasma spraying, wherein the chromium content in the layer is at least 70 wt.% And a spray powder is selected from three components, a first component of fine-grained chromium powder a second one of a fine-grained powder of a nickel-based alloy, and a third of coarse-grained cristobalite or quartz powder as a carrier for the first and second components. The method of claim 1, wherein the second component of the spray powder is mixed to improve the mechanical properties of the protective layer. The method of claim 1, wherein the third component of the spray powder is mixed to increase the thermal expansion coefficient of the protective layer. A method according to claim 1, characterized by a subsequent heat treatment of the protective layer in air at temperatures higher than 900 ° C. Method according to one of the preceding claims, wherein the thickness of the protective layer is selected to 1 mm. Use of a plasma-sprayed protective layer according to one of the preceding claims, on corrosion-endangered substrates of components of internal combustion engines, gas turbines, steam turbines, engine compressors or heat exchangers.