RU2169800C1 - Method of production of oxide composite coat on aluminium and its alloys - Google Patents

Abstract

FIELD: electrodeposition; methods of production of composite coats by anode oxidizing of aluminum and its alloys. SUBSTANCE: coat is produced by oxidizing in electrolyte containing ultra-dispersed diamonds, 0.001 to 0.120 mkm in size in the amount of 0.05 to 56 g/l. EFFECT: enhanced hardness and wear resistance; improved antifriction properties at low consumption of composite material; facilitated technology. 3 tbl, 14 ex

Description

 The invention relates to electroplating and more precisely to methods for producing a composite coating by anodic oxidation of aluminum and its alloys.

 Anodic oxidation (anodizing) is the main method of protection against corrosion, increasing strength and wear resistance, giving the surface dielectric properties to parts made of aluminum and its alloys.

 Anodic oxidation of parts made of aluminum and its alloys is exclusively widely used in various engineering, aircraft, electrical and other engineering fields.

 Since the oxidation process proceeds due to the matrix metal (part), a necessary condition for film growth is the appearance of pores as a result of the interaction of the oxide film with an electrolyte (most often with acid) and the flow of current.

The oxide coating consists of two layers: a porous thick outer layer and an inner thin layer (barrier layer). When anodic oxidation, for example, in H ₂ SO ₄ , the pore volume is about 30% of the volume of the oxide film.

The inorganic oxide composite coating (hereinafter referred to as the composite coating) of aluminum and its alloys (hereinafter referred to as aluminum) is a nonmetallic matrix (a porous film mainly from Al ₂ O ₃ ) - a kind of framework filled with particles of salts and metal oxides held in its pores , metals, non-metals. Filling of the film is carried out due to adsorption, physico-chemical and chemical forces. Such coatings have improved physical and mechanical characteristics, wear resistance, increased anti-corrosion and electrophysical properties, improved color gamut.

State of the art
Aluminum oxidation is carried out, as a rule, in an acidic environment, sometimes in an alkaline environment. Moreover, 2 main options are used:
1) first, the oxide film itself is obtained, and then it is filled at an elevated temperature with various substances originally dissolved in one form or another in water;
2) oxidation in the active medium occurs simultaneously with the filling of the film, the latter comes from water-soluble metal compounds or organic compounds, and in the film they already exist in the form of insoluble compounds.

The most common and environmentally least harmful is oxidation in sulfuric acid (the concentration of the latter is on average 200 g / l, i _a = 0.8 - 2.0 A / dm ² , voltage - up to 24 V, t = 10-20 ^o C, τ = 15-60 min) [Reference "Electroplating coatings in mechanical engineering" edited by MA Shluger, Moscow, Engineering Engineering, 1985, vol. 2, pp. 60-63; Pat. USA N 5277788, cl. C 25 D 11/12, publ. January 11, 94].

Instead of sulfuric acid, other acids are also used: phosphoric, phosphorous, oxalic [Pat. USA N 5277788, cl. C 25 D 11/12, publ. 01/11/94], a mixture of phosphoric and sulfuric acids (t ~ 27 ^o C, voltage - up to 15-20 V, time - 15 min) [German Application N 4243164, class. C 25 D 11/08, publ. 06/23/94], an aqueous solution of sulfosalicylic or naphthalenedisulfonic acid with sulfuric acid [Application of Japan N 5063559, cl. C 25 D 11/22, publ. 09/10/93 g], a solution of sulfuric acid with 8-hydroxyquinoline [Japanese Application N 5071678, cl. C 25 D 11/08, publ. 10/07/93 g], phosphoric acid in an aprotic solvent [US Pat. U.S. N 5211832, CL C 25 D 11/08, publ. 04/22/92].

After anodic oxidation according to the first embodiment, the parts are washed in water, and then the oxide film is filled in hot water (t = 90-95 ^o C, τ = 20-25 min, pH 4.5-6.5 - the film is hydrated, as a result its volume increases, the pore diameter decreases and the protective properties increase), in a solution of potassium or sodium dichromate in an acidic or alkaline medium [Handbook "Electroplating in Mechanical Engineering" edited by MA Shluger, Moscow, Engineering Engineering, 1985, v. 2, p. 60].

 The filling of the oxide film is also carried out in a solution of aluminum chromate and trivalent chromium ions - chromium is released in the pores of the film [Japanese Application No. 5-7476, cl. C 25 D 11/08, publ. January 28, 93]; in a solution of lithium salts [Pat. U.S. N 5411607, CL C 25 D 11/18, publ. November 10, 1993]; in a solution of salts of Nickel, cobalt, zinc, iron, tin, copper and / or silver [Application of Japan N 5079757, cl. C 25 D 11/22, publ. November 4, 1993]; in a solution containing metavanadate ion, and then in a solution containing metal ions, coprecipitating with metavanadate ions to form a sparingly soluble compound in the pores of the porous layer [PCT Application (WO) 9615296, cl. C 25 D 11/24, publ. May 23, 1996].

Sometimes, rather complex compositions are used to fill the porous film:
- 10-25 g / l of sulfate or nitrate of the required metal: 25-30 g / l of boric acid; 0.3-0.5 g / l of sulfuric or nitric acid at a temperature of 5-20 ^o C and with an alternating voltage of 10-30 V [Application of Japan N 5-14033, cl. C 25 D 11/22, publ. 02.24.93 g.];
- complex organophosphorus [Pat. USA N 5277788, cl. C 25 D 11/12, publ. January 11, 1994];
- solution containing potassium and / or ammonium hexaflortitanate and water-soluble zinc compounds [Japanese Application No. 2-15192, cl. C 25 D 11/18, publ. January 18, 1990].

 For thick-layer anodizing, in addition to sulfuric acid, a combined solution of sulfuric and oxalic acids is used. In addition to sulfuric acid, solutions of oxalic acid or an aqueous solution of oxalic, sulfuric and sulfosalicylic acids are used for electrical insulating anodization [Reference "Electroplating coatings in mechanical engineering" edited by M. А. Schluger. Moscow, "Engineering", 1985, vol. 2., p. 62].

 Less commonly used for anodizing alkaline baths: sodium polyborate solution [Application PCT (WO) 9517536, cl. C 25 D 11/06, publ. June 29, 1995); solution with a concentration of 2 N. alkali metal and alkaline earth metal carbonate containing an alkali metal halide [Application of Japan N 5-8278, cl. C 25 D 11/04, publ. 02/01/93 g]; a solution of alum in an organic polar solvent with a sparingly soluble salt of an alkaline earth metal, aluminum, zinc and / or lead [Application of Japan N 6031472, cl. C 25 D 11/04, publ. April 27, 94].

 The prototype of the invention is the method described in RF patent N 2078449, C 25 D 11/08, 11/18, publ. 04/27/97. In this method, an oxide coating is applied to aluminum and its alloys in electrolytes, which are solutions of sulfuric, sulfosalicylic or oxalic acid, in which particles of polytetrafluoroethylene (fluoroplast) are suspended using an alternating asymmetric current.

 Particles of fluoroplastic, when an alternating asymmetric current is applied, enter the pores of the coating formed during the evolution of hydrogen, thereby filling the porous film and giving it improved properties.

The disadvantages of the prototype are:
1) insufficiently high microhardness and wear resistance due to the introduction of soft particles of polytetrafluoroethylene into micropores;
2) a limited number (3 pcs.) Of electrolytes in which the effect of filling a porous film with polytetrafluoroethylene particles is achieved;
3) the use of only a specific current load - alternating asymmetric current.

Disclosure of Invention
The basis of this invention is to create such a method for producing an anodic composite coating of Al and its alloys based on a porous film, which would make it possible to obtain coatings with increased hardness, wear resistance, high antifriction properties at a low consumption of composite material by simple technology.

 This problem is solved by the fact that the proposed method of producing an oxide composite coating on aluminum and its alloys, including electrochemical oxidation in an electrolyte containing solid particles, while ultrafine diamonds (UDD) with a size of 0.001-0.120 microns in the amount of 0.05 are used as particles -56.0 g / l.

The best embodiment of the invention
As electrolytes of anodic oxidation, as a rule, aqueous solutions of sulfuric, chromic, and oxalic acids, their mixtures, an alkaline solution of sodium polyborate are used.

 The ultrafine diamonds used in this method or otherwise cluster diamonds are particles that are close in shape to spherical or oval, without sharp edges (non-abrasive). Such diamonds form sedimentation and coagulation stable systems in electrolytes both at a working concentration of components and at high (in electrolyte concentrates).

 Currently, UDD synthesis is most often carried out by undermining specially prepared charges from TNT-hexogen mixed compositions in explosive chambers filled with a non-oxidizing medium. The resulting diamond charge (a mixture of diamonds with non-diamond forms of carbon) is subjected to chemical cleaning, the most advanced of which is the processing of the diamond charge in nitric acid at high temperatures and pressure, followed by washing.

From the point of view of morphology, UDDs are a powder with a specific surface area of 150-450 m ² / g and a pore volume of 0.3 - 1.5 cm ³ / g (in the dry state). In suspension, UDD aggregates can have a size of up to 50 nm (0.05 μm), subject to special treatment. The average size of individual diamond crystals is 4-6 nm (0.004 - 0.006 μm), usually their size falls within the range from 1 to 120 nm (0.001-0.120 μm).

 UDDs have a classical cubic (diamond) crystal lattice with large surface defects, which leads to a significant surface energy of such crystals. The excess surface energy of UDD particles is compensated by the formation of numerous surface groups, forming a shell ("fringe") on the surface of hydroxyl, carbonyl, carboxyl, nitrile, quinoid, and other groups chemically bonded to the crystal, representing various stable combinations of carbon with other elements of explosives used - oxygen, nitrogen and hydrogen.

 UDD microcrystallites cannot exist without such a shell under ordinary conditions - this is an integral part of cluster nanodiamonds, which largely determines their properties.

 T. about. , UDD combine a paradoxical beginning - a combination of one of the most inert and solid substances in nature - diamond (core) with a sufficiently chemically active shell in the form of various functional groups that can participate in various chemical reactions. In addition, such diamond crystals, despite the compensation of part of unpaired electrons due to the formation of surface functional groups, still have a rather large excess of them on the surface, i.e. each diamond crystal is, in essence, a multiple radical.

 All these many heterogeneous properties determine their unusual behavior in various processes, including the anodic oxidation of aluminum.

 Indeed, attempts at introducing solid, insoluble ultrafine particles of silicon dioxide, metal nitrides, and silicon, inert materials, into the oxidation electrolytes did not lead to a positive change in the properties of the surface oxide layer.

 On the contrary, UDDs with a negative charge in the electrolyte, when EMF is applied, rush to the anode (aluminum and its alloys) and penetrate into the pores formed during oxidation of the surface, holding there not only mechanically, but also with the help of Van der Waals and others physical and chemical forces. In this case, the filling is so dense that the weight gain of the oxide film increases by a factor of 2–3.5 (with the same thickness, without and using UDD). The wear resistance of such a film increases by 10-13 times, the corrosion resistance and electrical insulation significantly increase.

T. about. , the use of cluster diamonds to obtain anodic oxide films on aluminum and alloys leads to the simultaneous filling of films with insoluble UDD directly during the electrolysis process and a significant improvement in the properties of the resulting non-metallic inorganic composite coating, namely:
- cluster diamonds form stable dispersions in oxidation electrolytes;
- small mass (low inertia) of diamond clusters provides effective mass transfer of diamond particles to the oxidized surface, this allows you to work at high current densities;
- cluster diamonds due to their high physicochemical activity provide deep penetration into the pores of the oxide film and the dense packing of their particles, resulting in the formation of a highly dispersed structure of the composite coating with increased microhardness, wear resistance;
- filling of the UDD oxide film leads to an increase in adhesion to the metal substrate and film cohesion;
- improving the quality of the coating is achieved, including with a relatively small, and most importantly, regulated diamond content in the coating of 0.2 - 10 wt.%, which makes the process economical (UDD content can be brought up to 70%);
- composite oxide-diamond coating has a high corrosion resistance;
- effective mass transfer of diamonds to the porous anode film and inside it ensures uniform filling of the film with them, including on equipotential surfaces.

 The content of cluster diamonds in oxidation electrolytes is 0.5 - 56.0 g / l. An increase in the UDD content above 56 g / l leads to a strong thickening and structuring of electrolytes, which complicates gas evolution, convection of the electrolyte, current flow, and uniformity of filling of the oxide film with diamonds. With a decrease in the content of cluster diamonds in the electrolyte below 0.05 g / l, the quality of the coating is significantly reduced. The content of cluster diamonds in the electrolyte is set in accordance with the required thickness and filling and, accordingly, qualitative and quantitative indicators of the coating. So, for products with an oxide film thickness of 3 to 10 μm, electrolytes with a UDD content of 15-56 g / l are more effective. Such products include, as a rule, small products with fairly sharp edges.

 The high concentration of cluster diamonds in the electrolyte ensures a decrease in the intensity of electric fields on the sharp edges of the products and a decrease in their embrittlement.

The filling of such films, depending on the temperature of electrolysis, current loads, of course, can be different. However, the weight gain of the film is usually maintained in the range of 1.5-2.0 times, their wear resistance increases by 7-10 times. Microhardness is at the level of 800 - 1200 kg / mm ² . Corrosion resistance and electrical insulation sharply increase.

 Electrolytes with a concentration of UDD 0.05 - 15 g / l can effectively harden the surfaces of large-sized parts with an oxide film thickness of 5 to 100 microns. Filling the film provides its weight gain of 2 - 3.5 times, wear resistance increases by 9-13 times.

In addition to the concentration of UDD in the electrolyte, the temperature regime is of great importance for regulating the film thickness and its filling. Moreover, with decreasing temperature, ceteris paribus, the increase in film thickness increases, and the relative weight gain of the films increases. However, at low temperatures (less than 0 ^o C) the concentration of UDD should not be too high (more than 30 g / l), because significantly increases the viscosity of the electrolyte. The latter circumstance worsens the natural thermal convection and uniformity of filling of the UDD film. In general, the presence of UDD in the electrolyte does not affect the temperature regime adopted in a particular technological process and electrolyte. The presence of UDD does not affect the film growth rate, but only ensures its filling during electrolysis.

The nature of the dependence of the relative weight gain of the anode film, reflecting the degree of its filling on 4 parameters: the concentration of the main component of the electrolyte (H ₂ SO ₄ ), the anode current density, the concentration of UDD and temperature, is shown in table 1. The most common electrolyte was used for anodic oxidation - an aqueous solution of sulfuric acid.

The anode current density in oxidation electrolytes in the presence of UDD is also set to the extent necessary to achieve the required film thickness and its filling in UDD. The specific value i _a is within the usual range.

The anode current density, standardly used for a specific type of electrolyte and type of alloy, is 0.5 - 2.0 A / dm ² .

As a rule, with free convection in the electrolyte, the current density is set from 0.2 to 1.5 A / dm ² , and with forced supply of electrolyte to the film-forming zone and with mechanical activation of the UDD suspension, 1.5 - 4.5 A / dm ² .

 The complex of properties of the oxide-diamond coating obtained by the proposed method, and the simplicity of the process make this method competitive with any of the known methods for producing filled oxide films.

 Oxide-diamond coating has wide potential for use in the hardening processes of the external elements of aircraft, ships, cars, subjected to intense abrasive and corrosive effects of internal friction pairs, one or both of which are made of aluminum or its alloys, soles of irons, the inside of frying pans, pistons of cars , hydraulic cylinders, other details of machines and mechanisms.

 Table 2 shows the data on abrasion resistance of samples from Al-9, D-16 alloy.

Abrasion was carried out on a friction machine by the method of reciprocating movements under a load of 200 g / cm ² . Wear resistance was determined by the loss of mass of the sample after abrasion for 10 hours.

 Samples obtained from electrolytes with UDD with a content of 10 g / l or more did not practically change their appearance and the weight loss was minimal (from 20 g / l there was practically no weight loss).

 The method of obtaining composite oxide-diamond coatings is simple in technological design and is carried out as follows.

 A 5-20% aqueous cluster diamond colloid is introduced into the anodic oxidation electrolyte prepared by known methods. The content of cluster diamond in the electrolyte is 0.05-56.0 g / L. The workpiece, which serves as the anode, is immersed in a bath with an electrolyte pre-cooled or heated to a predetermined temperature. Cathodes are made of lead.

 The bath is made of a material resistant to the action of an electrolyte, for example, titanium, glass, ceramic or plastic. Before starting work, the electrolyte is stirred mechanically or by purging with gas, for example, compressed air. Further forced mixing is not carried out. The stability of the suspension of UDD in the electrolyte is ensured by their properties, as well as due to the release of gases at the anode and cathode and due to thermal convection of the electrolyte.

 The thickness of the anode-oxide film with UDD is set depending on the purpose of the product, usually in the range from 3 to 100 microns.

 A preferred method for preparing an electrolyte is to prepare a concentrate which, when diluted with water, forms an electrolyte of a working concentration. The concentrate allows you to simplify the transportation and storage of electrolytes with cluster diamond.

 During the operation of the bath with UDD, an analysis and the necessary adjustment of its composition are carried out. For applying anodic-oxide diamond-containing films on parts of complex shapes, cathodes and screens manufactured by known methods are used. The preparation of products for anodic oxidation, namely, machining, degreasing, etching, removal of old oxide films, is also performed by known methods.

 Specific examples are provided for a better understanding of the present invention. The following electrolyte compositions do not exclude the possibility of introducing various additives into them, for example, to reduce the surface tension of the electrolyte or to reduce evaporation.

 Example 1

200.0 g / l sulfuric acid is dissolved in distilled water and an 8% colloid of cluster diamond with a particle size of 0.001 - 0.120 μm is introduced. The amount of UDD in the electrolyte is 5 g / l. The resulting electrolyte is poured into a bath cooled by a heat carrier. The electrolyte is cooled to a temperature of + 10 ± 1 ^o C. Hang the cathodes in the bath, the workpiece serves as the anode. When processing products of complex shape, special cathodes and screens are used.

 The product is previously thoroughly cleaned, degreased in well-known chemical and (or) electrochemical baths and connected to the anode busbar.

 Use standard sources of direct current with adjustable voltage and current, with variable polarity of the current.

The products are placed in an electrolyte, cooled to an electrolyte temperature, a reverse polarity current of 0.5 A / dm ^{2 is} turned ^on for 30 seconds, then a direct polarity current of 1.5 A / dm ^{2 is} turned ^on . During the oxidation process, the voltage is changed as follows: from 10 to 20 V. The rate of formation of the oxide-diamond film is ~ 0.5 μm / min. Electrolysis time 40 minutes.

The microhardness of the coating, measured by the method of indenting a pyramid of natural diamond with an angle at the apex of 108.9 degrees, was ~ 800 kg / mm ^{2 The} relative weight gain of the coating increased by 2.3 times. The wear resistance of the oxide-diamond film in comparison with the oxide film (without UDD) increased by 5.9 times.

 Examples 2-5.

The diamond oxide film is prepared according to Example 1. The amount of UDD in the electrolyte is 0.05, respectively; 10, 20, 56 g / l. The relative weight gain of the film increased by 1.6, respectively; 2.4; 3.4; 3.7 times. In this case, the average microhardness of the resulting coatings is 700, respectively; 800; 1200; 1500 kg / mm ² . The increase in abrasion resistance is 1.25, respectively; 44.0; more than 100, more than 100 times. At UDD concentrations of 20 g / l or more, wear of the plates during abrasion for 10 hours was practically not detected.

 Examples 6-7.

Coatings are prepared according to Example 1. The amount of UDD in the electrolyte is set at 15 g / l. The temperature of the electrolyte is set respectively 20 and 10 ^o C. In this case, the relative weight gain of the film increased in both cases ~ 3 times. The average microhardness of the resulting coatings is respectively 1000 and 1200 kg / mm ² . The increase in abrasion resistance was 30 and 90 times, respectively.

 Example 8

 50 g / l of chromic anhydride is dissolved in distilled water, 3% aqueous UDD colloid is introduced. The amount of cluster diamond is 4 g / l.

The resulting electrolyte is heated to 40 ± 2 ^o C and stirred for 15 minutes by known methods. All subsequent operations are carried out according to example 1. The anode current density is set to 2.0 A / dm ² , the voltage is adjusted to 40 V in 10 minutes. Electrolysis time 45 minutes. The coating properties are shown in table 3.

 Examples 9-13.

 The content of UDD in the electrolyte is set accordingly to 0; 0.05; 4.0; 15.0; 30.0; and 56.0 g / l. The coating properties are shown in table 2.

Tests of wear resistance of coatings were performed on a friction machine by the method of reciprocating movements under a load of 200 g / cm ² .

The friction surface is 1 cm ² .

 The test time is 10 hours.

 The test specimen is fixed motionless, the moving plate is made of stainless steel, its surface has a 10 class of cleanliness.

 The use of UDD according to these examples leads to a sharp decrease in the abrasive wear of the oxide-diamond film.

 Example 14

40 g / l sodium polyborate of the formula Na ₂ B ₄ O ₇ · 10H ₂ O, pH 9.5 is dissolved in distilled water, a 20% aqueous UDD colloid is introduced. The amount of UDD is 12 g / l.

The resulting electrolyte is heated to 30 ± 2 ^o C and stirred for 15 minutes by known methods. All subsequent operations are performed according to example 1. Anode current density is set to 3.0 A / dm ^{2, the} voltage is adjusted from 20 to 60 V in 15 minutes. The duration of the process is 30 minutes. The weight gain of the film and the wear resistance of the coatings are equivalent to those obtained in example 8.

Industrial applicability
The proposed method for producing composite coatings on aluminum and its alloys can be used in various fields of technology, in particular, in mechanical engineering, shipbuilding, in the manufacture of aircraft and household appliances.

Claims (1)

 A method of producing an oxide composite coating on aluminum and its alloys, including electrochemical oxidation in an electrolyte containing solid particles, characterized in that ultrafine diamonds with a size of 0.001 - 0.120 μm in an amount of 0.05 - 56 g / l are used as solid particles.

Images