LT4456B - Method for formation of composite coatings - Google Patents

Abstract

A method is disclosed to obtain a universal porous coating applied to a hard substrate (i.e. metallic or ceramic) with high adhesion strength to the substrate, with outstanding thermo-mechanical characteristics and with good resistance to thermal or mechanical action. The porous coating incorporates a tricomponent powder mixture which is deposited on the substrate by plasma spraying. The mixture includes aluminum and/or titanium oxide powder and glass powder as the coating's microstructure forming materials; aluminum and/or titanium powder as metal binder and aluminum and/or titanium hydroxide powder as a material assuring the formation of coating's microstructure. Plasma spraying includes passing the powder through the plasma beam, simultaneously melting of the metal oxide, partially alloying with glass powder surface, and preventing the complete thermal decomposition of hydroxide particles, and includes directing the powder components into separate plasma beam zones - the hydroxide to the colder zone, and the metal to the hotter zone. The aluminum particles reach the substrate first and develop the adhesion of the sprayed layer to the substrate, by using fractionating the powder with the higher mass metal and glass particles reaching the surface of the sprayed substrate first.

Description

BACKGROUND OF THE INVENTION The present invention relates to methods of producing composite coatings by molding molten materials on a substrate and can be used in various applications in mechanical engineering, energy, metallurgy, etc. to protect individual parts and products from corrosion, gas erosion and heat and to give them new properties. Composite coatings can be used as substrates for other coatings, for example polymeric coatings, as well as for soaking in various mixtures, including catalysis.

There are known methods for obtaining protective coatings on a metal or ceramic substrate by thermal spraying of aluminum compounds. Aluminum-based alumina is most widely used as a powder for spraying composite coatings (see Metallic and Ceramic Coatings. Production, High Temperature Properties and Application. M. G. Hocking, V. Vasantasree and P. S. Sidky. Materials Department, Imperial College, London 1990). As a rule, ceramic coatings are subjected to thermal spraying on a pre-deposited metal substrate.

A known method of depositing an aluminum sublayer which provides high anodic characteristics and high resistance to erosion and corrosion (U.S. Patent No. 4,238,233, IPC C 22 C 21/10, 1981). The coating of the Al - Zn - Mg - Mn system with the addition of indium, bismuth or tin is applied by cladding on the inner surface of the pipes and provides their cathodic protection.

Known method of forming wear-resistant coatings (see former USSR Authorization No. 2026890, TPK C 25 D, 1992), comprising deposition on a substrate of a fusible Zn - Cu - Al - Ni - B alloy followed by a based on a layer of aluminum compounds containing Al, Cu, Mg, Mn (similar to alloy D 16), which is then oxidized by microwave in an alkaline electrolyte. The method allows to increase the adhesion of the coating to the substrate and at the same time increases its resistance to wear.

Known method of plasma spraying of ceramic coatings (see former USSR Authorization No. 2021388, TPK C 25 D 11/02, 1994), which is applied by plasma spraying on a metal substrate (nickel elastic nickel alloy with chromium and aluminum). 0.2 mm thick, then applied a ceramic layer using various powder mixtures: partially stabilized zirconia, alumina, titanium oxide, chromium oxide, heated to 150-200 ° C, which is applied on a preheated to 1300 ° C of the product surface. In addition, a monolayer of ceramic particles at a temperature of 20 to 80 ° C is first sprayed onto the substrate. The method allows to significantly increase the product's resistance to thermal fatigue.

The techniques listed above are quite complex, requiring labor-intensive and additional labor-intensive operations: application of the substrate, oxidation or constant temperature control of the substrate and coating.

A known method of obtaining a multilayer coating (see former USSR Authorization No. 2049827, IPC C 23 C 4/00, 1995), comprising spraying a layer in an inert gas with a dissociated hydrogen additive. In this case, Al - Ni composite powders, which form hydrated forms of alumina in the presence of an inert gas containing hydrogen ions, are sprayed as a sublayer. The base coat is sprayed using an aluminum or nickel composite powder or a mixture thereof and a chemically inert additive with a layered structure such as boron nitride or carbon. The resulting layer contains aluminum, hydrated forms of alumina, and boron nitride or carbon additives. The latter are solid lubricants and provide resistance to wear. The coating thus prepared is applied to the polymer layer. Hydrated forms of aluminum oxides formed during the spraying process increase the corrosion resistance of the product.

However, it should be noted that the coating obtained by the described technology is quite expensive due to the large number of intermediate operations and expensive materials used for spraying, such as aluminum-nickel composite powder. In addition, according to the technology described, the coating may not have a sufficiently high resistance to wear and porosity, which renders it unsuitable for use as a base for soaking in various mixtures, e.g. neuniversal.

Known catalytic carrier intermediate aluminum hydroxide-bayerite phases and? - a phase-based method of chemical reaction on the surface of a ceramic matrix in solutions of aluminate, silicate and sodium sulfate at pH 10.5 - 11.5 followed by reduction of the silicon-containing bayerite from the solution (European Patent EP 537871,1994).

The main advantage of this method is the high integrity and uniformity of the deposited layer, the disadvantage is that it does not achieve reliable rotation of the catalytic carrier and the ceramic substrate.

A method of depositing a catalytic carrier on steel or aluminum sheets is also known (Japanese Patent No. 56-078450,1981). The process comprises spraying at high temperature an aqueous suspension containing a glass slag, an oxide metal catalyst, as well as silica and / or alumina.

It is understood that the introduction of glass slag into the spray formulation is to develop a free surface and to form a specific macrostructure which, together with the microstructure of the catalytic coating, must provide an optimal combination of catalytic properties under changing conditions. However, experience has shown that the adhesion of such a composition to the substrate when sprayed with an aqueous suspension is low and does not guarantee reliable operation under adverse conditions.

A known method of depositing plasma coatings (see former USSR Ref. No. 1528810, TPK C 23 C 4/04, 1989) using powder blends: alumina and metallic titanium (15-60%) as a metal binder. The method allows to obtain coatings that are resistant to abrasion. The open porosity of the coating is 8-11%, the cohesive resistance is 9-10 MPa. However, using such a coating as a universal one, i.e. suitable for versatile applications, including when used as a substrate for other coatings, has a problem of resisting adhesion of the substrate to the substrate on the one hand and to the coating which will be applied to the surface of the substrate on the other. To solve this problem, the interlayer (sublayer) must first have a thermal expansion coefficient (shear) as close as possible to the substrate, and secondly be maximally integrated into the substrate structure, i.e. the atoms of the coating material must form metal bonds with the atoms of the base material. If the first condition can be met quite easily by selecting a material for plastic bonding, a metal close to that of the sheet. For the base material (in this case titanium), the condition of high degree of integration is fulfilled only if alloy type compounds are formed at the boundary between the sublayer and the base. In the case of plasma spraying, similar conditions can usually only be achieved by high-temperature annealing after spraying and the temperature range during annealing must ensure effective mutual diffusion of the sublayer and substrate atoms. As these temperatures are relatively high (above 1000 ° C), significant degradation of this coating is usually observed, which is characterized by anomalous grain growth, high liquidation and significant oxidation at its periphery. Such annealing is not contemplated by the present invention, but would, if contemplated, have the undesirable consequences described above.

In order to ensure good adhesion of the substrate to the coating applied to the substrate, the substrate must have a highly developed surface with a relative free surface area of at least 10 m ² / g (meaning that one gram of coating has a total surface area of 10 m ² ). , which should ensure good adhesion of the spray coating. However, recalculation of the porosity (about 10%) found in the invention to the relative free surface area shows that it does not exceed 1 m ² / g in this case, which is clearly not sufficient.

Thus, using the present invention, it is not possible to obtain a coating which provides high adhesion to the substrate and, at the same time, high adhesion to the various coatings applied to its surface.

The object of the present invention is to obtain a highly porous universal porous coating with high thermal-mechanical properties, as well as mechanical and thermal effects, on a solid substrate (e.g., metal or ceramic) by means of thermal spraying. In order to obtain such a porous coating, the sprayable layer must be dense (on the side adjacent to the substrate) to provide high adhesion to the substrate and high thermo-mechanical properties of the coating, and at the same time have a high porosity (on its surface). to provide special properties, including catalytic properties. This contradiction can be resolved by choosing the specific chemical composition of the spray powder and the original spray technology.

The following is a spray diagram of a ternary powder mixture consisting of aluminum hydroxide powder, silica glass powder and aluminum powder.

In this model composition, the metallic aluminum acts as, first, a metallic sublayer that provides high adhesion to the substrate of the spray layer and, second, a metal bond to fix the porous surface particles of the spray layer.

Silicate glass powder ensures the macrostructure of the spray coating. When spraying the silicate glass powder, it is necessary to ensure partial adhesion of the surface particles to ensure reliable adhesion, on the one hand, to the aluminum spray and, on the other, to the aluminum hydroxide powder deposited on the surface of the composite layer formed of molten aluminum and glass.

Aluminum hydroxide powder acts as a material to provide a highly developed, porous surface ceramic coating on an alumina basis (i.e., providing a coating microstructure). This task can be accomplished by thermal decomposition of aluminum hydroxide (520 - 600 ° C) in the following reaction:

Al (OH) ₃ = Al ₂ O ₃ + 3H ₂ O (I)

In order to obtain such a porous coating, the thermal decomposition reaction of the aluminum hydroxide must take place already on the surface of the sprayed layer and the hydroxide particles must be solidly fixed in the layer. This can be achieved when plasma spraying avoids the dissociation of hydroxide particles in the plasma stream (temperature 5000 - 8000 ° C). Therefore, in order to increase the affinity with aluminum, it is necessary to ensure partial overlap of the hydroxide particles with the surface.

Thus, during the plasma spraying process of the model composition, the transition of the powder through the plasma stream must ensure simultaneous melting of the metal aluminum powder, partial melting of the glass particles from the surface and prevention of complete thermal decomposition of the aluminum hydroxide particles.

This is achieved by directing the powder to different areas of the plasma stream - aluminum hydroxide is directed to a colder zone and metal aluminum is directed to a hotter zone.

In addition, a priority transition of the aluminum particles to the surface of the substrate should be ensured during the spraying process to ensure a resilient adhesion of the sprayed layer to the substrate. This is achieved by selecting the fractional composition of the sprayed powder. For example, aluminum and silicate glass particles tend to have a higher mass and, therefore, gain faster velocity through the plasma stream, which in turn ensures their earlier transition to the surface of the sprayed substrate.

In this way, during the spraying process, a composite layer is formed which consists of aluminum melted in the plasma stream and formed an elastic sublayer bond on the substrate surface. This layer contains glass particles that have partially melted on the surface, resulting in a strong adhesion to the layer. Finally, particles of aluminum hydroxide, which is not broken through the plasma flow, are introduced into the surface of this layer.

The coating thus obtained is not a finished product because, firstly, it is not porous and, secondly, its phase composition does not meet the requirements for special coatings (so, for example, for catalytic coating, its surface should exclusively have γ - and Θ - modifications). aluminum oxides) and, thirdly, its residual thermal stress (as a result of the plasma spraying process) is quite high.

The required physico - chemical properties of the sprayed composite coating are obtained by heat treatment in an oxidative environment, mainly air.

Together, aluminum oxide is formed on the surface of the aluminum substrate by the reaction:

Al + 3 O ₂ = 2 Al ₂ O ₃ (II)

On the surface of the glass / aluminum border (ie glass particles embedded in the aluminum substrate) 520 - 580 ° C (Χημμη οτθκJia, AA Απλθη, K3fl. "”Ημηη", JlerotHrpaflCKoe otflejierote, JlHKHrp oxide, JleHKHrp), reduction of silicon from its oxide by reaction:

SiO ₂ + 4AI = 3 Si + 2 Al ₂ O ₃ (III)

Reaction (III) is of autocatalytic character. The occurrence of crystallization centers of alumina occurs in microparticles (^ fapyai'iOHHafr CBapna cTeKJia m KepaMMKH c MeTaJiJiaMH, B.A. EanHH, H3fl. MainHHOCToeHHe, MocKBa, 1986), which are on the surface of glass particles (what is plasmin). A specific feature of this process is the removal of thermal stresses in the glass particles and the healing of the micro-cracks during the reaction (III).

Finally, the decomposition of aluminum hydroxide in the temperature range 480-660 ° C during the heat treatment process results in the formation of alumina with a highly developed micro-porous surface according to reaction (I).

In this way, the thermal treatment of the sprayed composite layer in the oxidation environment results in the necessary chemical and phase composition on the surface, while forming a specific macro- and micro-surface layer with a high adhesive backing.

In practice, the object is achieved by using a powder blend containing aluminum and / or titanium as the starting material for plasma spraying in the known process for preparing a composite coating, which comprises applying to the substrate a porous coating comprising a metal oxide and a metal binder. ,% by weight of aluminum and / or titanium hydroxides, glass powder and metallic aluminum and / or titanium in the form of metal bonds in the following composition:

aluminum and / or titanium metal oxides of aluminum and / or titanium glass powder aluminum and / or titanium hydroxides

The dispersion of the powder must be:

aluminum and titanium oxides aluminum and titanium hydroxide glass powder aluminum and titanium

-7 0.1 -12

2-45 remaining amount less 30 microns

- less than 20 microns

- less than 30 microns

- less than 50 microns

During the plasma spraying process, aluminum and / or titanium powders directed to the "hot" plasma flow zone, i.e. through the annular gap between the plasmotron anode and the anode body, it melts completely and is transported to the substrate where it forms a plastic interlayer on the surface of the injected substrate. The glass powder and the oxides of aluminum and / or titanium are directed to the 'cold plasma flow zone, i.e. on the plasmotron nozzle at a distance (mm) from the empirical formula:

or the titanium hydroxide decomposes according to reaction (I) to form aluminum and / or titanium oxide and water vapor.

Due to the fact that the oxidation of aluminum on the surface of glass particles occurs in the range of 520 - 580 ° C, mainly in the range of 560 - 580 ° C, and the decomposition of aluminum hydroxide occurs in the range of 480 - 660 ° C, including γ - modification alumina. In the temperature range of ° C and oa - modification alumina - in the range of 570 - 660 ° C, separate heat treatment modes calculated for the formation of oxides of modification a - and γ are most optimal.

In view of the above, the heat treatment must be carried out in the range 480 - 660 ° C and the temperature treatment must be carried out for 2 to 20 hours. Increasing this speed limit leads to the formation of a defective structure of the surface layer and, as a consequence, a decrease in its porosity

An example.

The composite coating was sprayed on a substrate made of 40 microns thick steel (Χ15 grade 5). An aluminum hydroxide powder formulation with the following composition by weight was used:

aluminum oxide, dispersion less than 10 microns 3 metallic aluminum, dispersion less than 30 microns 5 silicate glass powder, dispersion less than 30 microns 7 aluminum hydroxide, dispersion less than 10 microns residual.

The plasmidron PN-V1 with separate powder component feed and the APR-403 current source were used for spraying. Atmospheric air was used as plasma-forming gas. Spraying process parameters: voltage - 200 V, current -170 A, nozzle cut distance to base -100 mm.

The thickness of the applied coating was 30 microns. The coating adhesion resistance after application was 25 kg / mm ² .

The resulting composite coating was heat treated in a SNOL furnace at 560 ° C for 2 hours and cooled in air.

The radiographic analysis was performed with the DRON - 3 device. The coating structure after heat treatment consists of γ - modification alumina (base) with a small amount of δ - (less than 3%), θ - and ξ modification (trace).

The free surface size was measured with a special sorbtometer. The free surface size of the coating after heat treatment was 65 m ² / g.

Other performance examples and the resulting coating characteristics are given in Table 1.

High-speed plasma spraying is carried out in the air environment using air as the plasma-forming gas. The coating components are fed to various plasma flow points. The spraying is followed by a heat treatment in the temperature range 480 - 660 ° C.

During the spraying process, as the plasma flow passes through, the metal aluminum particles melt, the glass powder particles melt on the surface, the aluminum and titanium hydroxide particles either do not decompose or partially decompose to the intermediate phases, while the alumina and titanium oxide particles only heat up. As the particles are transported on the substrate, they are distributed by weight and, as a result, the aluminum particles form a continuous, very thin layer (less than 5 m) on the substrate surface, which is trapped by glass and oxide particles.

During the subsequent heat treatment of the coating, partial oxidation of the aluminum layer occurs, but the thin aluminum layer underneath the oxide layer retains plastic properties, ie practically has the properties of a conventional plastic layer. In addition, glass particles, as well as aluminum and titanium oxides, form a "skeleton" of the coating, providing its macro-porosity, while the γ-modified alumina microparticles formed during the thermal decomposition of the intermediate hydroxide phases of the spray layer up to 30 70m ² / g).

- 4 examples illustrate variations of plasma spraying of powder compositions on a ceramic substrate made of high temperature silicon ceramic.

- Examples 8 show variants of plasma spraying of powder formulations on a metal substrate, for example on a heat-resistant chromium-aluminum foil Χ155. Spraying is carried out without prior preparation of the foil surface.

In Examples 1,2,5 and 6, the oxide content was within the minimum stated and in Examples 3,4,8,9 and 10 close to the maximum. As shown in the table, the ratio of oxides to hydroxides determines the size of the free surface, which increases from 40 to 50 to 60 to 70 m ² / g, with the proportion of hydroxides varying from 30 to 80 to 90%. In addition, partial or complete replacement of aluminum hydroxide by titanium hydroxides slightly reduces the free surface size, as a rule, by 10-15 units.

As noted earlier, aluminum forms a sublayer on the substrate surface, which is an elastic bond between the substrate and the working layer and provides a high adhesion resistance, which is particularly important for bending effects on the substrate. Due to the fact that the coating when sprayed on a ceramic substrate does not require high bending resistance (because the ceramics are extremely fragile), the proportion of metallic aluminum in the ceramic spray mixture is reduced compared to the metallic substrate (1-3 versus 3-7%). The magnitude of the adhesion resistance of the coating to the ceramic substrate in Examples 1-4 is not less than that of the ceramic substrate itself, and therefore, when tested with the coating, the samples disintegrate without flaking or intensive coating abrasion. In this case, the open porosity controlled by the free surface size is 50 - 70 m ² / g.

Increasing the amount of aluminum in the sprayed metal substrate to 3-7% ensures the adhesion of the sprayed layer to the level of the substrate resistance limits. In the case of tensile and flexural testing, the substrate disintegration occurs in all cases without peeling and intense abrasion. The latter circumstance opens up practically unlimited prospects for conventional machining of a sprayed mechanical substrate. As our research has shown, the substrate can be mechanically cut, perforated, corrugated, cold stamped and so on. All of these operations can be performed without any damage to the coating condition. In order to obtain the required level of properties, the semi-finished product (which may be a part) of the machining result must only be annealed in the air at temperatures between 480 and 660 ° C.

The relative free surface area when sprayed on a metal substrate is 60 70 m ² / g for aluminum oxide coatings (Example 5), 40 - 50 m ² / g for titanium hydroxide (Example 6), for a mixture of hydroxides (Example 7 and 50 - 60 m ² / g This way, by changing the composition of the initial spray formulation, the spraying and subsequent machining modes, it is possible to obtain coatings with the required chemical and, moreover, physico-mechanical properties.

Comparison of the adhesion resistance of the coating with the prototype shows that in this case the adhesion resistance is higher than the order of magnitude.

The coating obtained by this technology can be further used for anti-corrosion coatings based on metals and polymers, anti-friction coatings, paint-lacquer coatings, thermal protection (thermal dissipation) coatings, including screen coatings, catalytic coatings for inorganic and organic synthesis and etc.

Experiments have shown that aluminum oxide and titanium oxide exhibit qualitatively close properties in coatings and can be considered as technical equivalents.

te entel

Free surface area, m ² / g 60-70 40-50 50-60 o IO 1 o 'T 60-70 40-50 50-60 50-60 40-50 50-60 Adhesion resistance kg / mm ² more 40 more 40 more 40 o □ Π3 'cn O CU rs more 25 more 25 i more 25 more 25 more 25 more 25 Coating thickness μ m o ΙΛ  1 And CM co 30-40 (2-5) o m · LO o - m o co „ ¹ 1 OT— cm 33 o co Z? r CO O 'CM 20-30 (1-3) o co O '1 o T- CM 33 o co 3? 1 co o ¹ CM t- 20-30 (1-3) Titanium oxide, masses % 1 2-4 1 CM Τ- Ι o T— t Μ- ι CM 2-4 1 10-12 10-12 Aluminum oxide,% by weight 1-3 1 10-12 1 1-13 • ¹ ' ³ 10-12 1 10-12 Titanium hydroxy das, % by weight 1 the rest quantity t the rest quantity the rest quantity the rest quantity the rest quantity the rest quantity Aluminum hydroxy das, % by weight the rest quantity 1 the rest quantity 1 the rest quantity 1 50 the rest quantity 1 30th Glass powder, masses % 2-5 2-5 2-5 IO 1 CM 5-20 5-20 5-20 i 5-20 5-20 5-20 Aluminum nis, masses % 1-3 1-3 1-3 CO t- 3-7 3-7 3-7 3-7 3-7 3-7 Basis pottery pottery pottery E CO ω stainless steel stainless steel stainless steel stainless steel stainless steel stainless steel o o o. n .55, - CM co to co N co m o

Claims (16)

DEFINITION OF INVENTION 1. A process for the preparation of composite coatings, wherein the initial substrate is coated with a plasma spraying method comprising a porous coating comprising a metal oxide and a metal binder. as a coating macrostructure material, as a metal bonding agent for aluminum and / or titanium powder and as a material to provide a microstructure of the coating in the following composition by weight: metal aluminum and / or titanium 1 to 7 aluminum and t or titanium oxide 0.1 to 12 glass powder 2 to 25 aluminum and / or titanium hydroxide remaining 2. A method according to claim 1, characterized in that all the injectable materials are divided into two groups, mixed and fed separately into different zones of plasma flow. 3. A process according to claims 1 and 2, characterized in that the first group consists of aluminum and / or titanium hydroxide powder, aluminum and / or titanium oxide powder and glass powder. 4. A process according to claims 1, 2 and 3, wherein the first group dispersion has a powder dispersion of: aluminum and titanium oxides aluminum and titanium hydroxides glass powder less 30 microns less 20 microns less 30 microns. 5. A process according to claims 1,2,3 and 4, characterized in that the powder of the first group is mixed in a disintegrator according to the regimes which ensure the mechanical mechanical alloying of the powder components. 6. A method according to claims 1,2,3,4 and 5, characterized in that the first group of powder is fed through an annular gap between the plasmotron anode and the anode body. 7. A process according to claims 1 and 2, characterized in that the second group consists of metallic aluminum and / or titanium powder. 8. A process according to claims 1,2 and 7, wherein the second group of powder has a dispersibility of: metal aluminum and titanium less than 50 microns. 9. A method according to claims 1,2,7 and 8, characterized in that the second group of powders is applied to the nozzle of a plasmotron. 10. A method as claimed in claims 1,2,7,8 and 9, wherein the powder is applied to the nozzle section at a distance λ (millimeters) from the end surface of the plasmotron according to the empirical formula: λ = (0.1 -0.2) W - where W is the power used in the injection process (kilowatts). 11. A method according to claims 1,2,7,8,9 and 10, characterized in that the powder is fed to the tip of the nozzle in a direction opposite to the direction of the outgoing plasma at an angle α (degrees) according to the empirical formula: a = (800-1500) W - where W is the power used in the injection process (kilowatts). 12. A method according to claims 1 and 2, characterized in that, in order to reduce the thermal effect on the substrate, during spraying, a device is provided which blows the end of the plasma stream. 13. The method of claims 1,2 and 12, wherein the distance L (in millimeters) from the point of inflow of the plasma stream to the base on which it is sprayed is determined by an empirical formula: L = (0.50 - 0.75) W - where W is the power used during injection (kilowatts). 14. The method of claim 1, wherein the spray-coated substrate is heat-treated to complete disintegration of the intermediate and metastable phases obtained by the plasma spraying process. 15. A process according to claims 1 and 14, wherein the heat treatment is carried out in an air atmosphere at a temperature in the range 480-660 ° C. 16. A process according to claims 1.14 and 15, characterized in that the heat treatment is carried out at a rate of not more than 10 ° C per hour.