WO2008120046A1

WO2008120046A1 - Method of forming a protective ceramic coating on the surface of metal products

Info

Publication number: WO2008120046A1
Application number: PCT/IB2007/051174
Authority: WO
Original assignee: Gostevs, Vladimirs
Priority date: 2007-04-02
Filing date: 2007-04-02
Publication date: 2008-10-09
Also published as: EA200901050A1; EA012825B1

Abstract

The invention describes a method of forming a ceramic coating on the surface of metal products at a high rate of 4-15 µm per minute owing to optimization of the electrical regimes of anodic-cathodic plasma electrolytic oxidation. The method includes application of short anodic voltage pulses of 5-20 µs duration at an optimal ratio of the durations of anodic and cathodic pulses equal to 0.2-0.4. The oxidation process is controlled by a programmable supply source, matching the amplitudes and durations of voltage pulses with their power. The use of high voltages and current densities in short pulses enables formation of smooth, hard and dense ceramic coatings 5-100 µm thick without increasing the energy consumption over a time of 1-20 min, which meets the requirements of series production. A distinctive feature of the produced coatings is their homogeneity and the complete absence of an outer defect layer, high thickness uniformity of the coating even in complex shapes, including deep grooves and holes.

Description

METHOD OF FORMING A PROTECTIVE CERAMIC COATING ON THE SURFACE

OF METAL PRODUCTS

Technical field

The invention pertains to the electrochemical treatment of the surface of metals and alloys, notably to plasma electrolytic oxidation, and aims at the formation of smooth and uniform ceramic coatings with improved physicomechanical properties.

The claimed method forms wear-, erosion-, corrosion-, heat-resistant and dielectric ally strong coatings on the surface of metal products at a high rate.

The method of forming protective ceramic coatings can be used in aircraft and automotive industries, pump and compressor manufacturing, oil and gas industry, electronics, medicine, manufacturing of sports and household goods.

Prior art

Plasma electrolytic oxidation methods used to form protective coatings in aqueous electrolytes on the surface of metals and alloys are divided, by the polarity of applied current and voltage, into anodic-spark and anodic-cathodic. Anodic-spark methods use direct or unipolar (positive) current, and anodic-cathodic methods make use of bipolar current pulses.

The use of cathodic current pulses, in spite of a greater energy intensity of the process, qualitatively changes the phase composition and properties of the coating. The electron current flowing in the cathodic period induces a significant release of heat and additional heating-up of the formed coating, which contributes to the formation of grains of high- temperature crystalline phases of oxides in them and their fusion, the development of a dense layer of a solid structure. This dense solid coating withstands wear and corrosion better than the coating produced under conditions of unipolar anodic polarization.

A method is known of the oxidation of aluminium alloys using a anodic-cathodic regime (DE 4209733) at a frequency of alternate different-polarity current pulses 10-150 Hz with anodic pulses of 10-15 ms (milliseconds) duration and cathodic pulses of 5-10 ms. The use of anodic-cathodic electrolysis enables the formation of hard, strong and wear-resistant ceramic coatings. Disadvantages of the method are considerable porosity and surface roughness of the coating. Besides, the specifics of the process are such that the formation of the inner solid base layer is preceded and determined by the production of the outer "technological" layer. Therefore, a sufficiently thick coating (120-150 μm) is to be built up for a quality oxide layer to be produced, which is associated with large energy expenses. Herewith, a considerable part (40-50%) of the coating with respect to its thickness is the outer defect layer, which has a relatively loose structure and requires large labour expenses to be removed. The oxidation process is very long, as the coating formation rate does not exceed 1 μm/min.

A method is known of forming a corundum coating on light alloys using an alternating sinusoidal voltage of up to 1000-1800 V (US 2006/0207884). The frequency of the alternate pulses is herewith 50-200 Hz, and the current density is 100-150 A/dm². The use of such high voltages and currents at a large duration of pulses will inevitably lead to potent arc discharges and a deterioration of the coating quality (high porosity and roughness, low adhesion and the considerable outer defect layer). Coatings of this kind are used as thermo-barrier coatings. Besides, the method is characterized by a large energy intensity and low efficiency, as it uses sinusoidal voltage pulses, which are much less efficient than rectangular pulses.

To refine the electrical parameters of the oxidation process and to improve the quality of ceramic coatings, some investigators proposed to increase the fraction of the cathodic pulses in the anodic-cathodic process.

A method is known of oxidizing products in the anodic-cathodic regime at current densities of 15.5-45.8 A/dm and the ratio of the cathodic and anodic current components equal to 1.36-1.92 (RU 2081212). This makes it possible to raise the microhardness of coatings and increase their thickness.

A method is known where a device providing for the anodic-cathodic regime of oxidizing aluminium alloys is described, in which two pulses (main and additional) of negative current follow each pulse of positive current (WO 00/05493). The additional pulse makes it possible to increase the mean value of cathodic current 20-40% as compared with the mean value of anodic current. At this ratio of the currents, the oxidation process is more stable, the porosity of the produced coating is decreased, the rate of its formation is increased.

A method is known of producing oxide-silicate corrosion-resistant coatings, mainly on magnesium alloys (US 2006/0201815). In this method, bipolar voltage pulses of +/-250 V (the shape of the pulses is not specified) are supplied to the electrodes (a workpiece and the counter-electrode). Herewith, durations of the pulses are regulated, and the minimal duration of a pulse is 30 ms. The duration of the anodic pulse should be less than that of the cathodic pulse (the ratio of the anodic and cathodic pulses' durations in the method is not specified). A disadvantage of this method is a low rate of treatment owing to the use of longer pulses of lower voltages. As it follows from the examples given in the disclosure of the method, the oxidation rate for magnesium alloys is 0.1-1.0 μm per minute.

A method is also known of oxidizing aluminium alloys, where the anodic-cathodic regime, which lasts for 5-30 seconds, is alternated with the supply of only cathodic pulses for 1-10 seconds (WO 99/31303). Herewith, the current density during the cathodic regime is 5- 25% of the density of anodic and cathodic currents during the anodic-cathodic regime. The produced oxide-ceramic coatings are characterized by an increased density (porosity is decreased threefold) and microhardness, a larger thickness uniformity.

However, the above listed known methods have the same essential disadvantages. As the alternating current of commercial frequency with the anodic and cathodic half -periods of 2-50 ms duration is used in oxidation, the coating formation rate does not exceed 1 μm/min. The coatings have a relatively thick outer defect layer and a surface roughness R_Ά 5-8 μm. Formation of coatings in the known methods requires considerable energy expenses, which impedes their commercial use.

To improve the protective properties of coatings and increase the performance of the oxidation process, other investigators proposed high-frequency pulsed regimes of anodic- cathodic electrolysis with small-duration pulses.

A method is known of the electrolytic oxidation of metals in a pulsed anodic-cathodic regime with the duration of pulses 100-300 μs (microseconds) and the same pause between them (RU 2077612). Small durations of the pulses and considerable densities of anodic and cathodic currents (up to 800 A/dm ) provide for the formation of a dense fine crystalline coating of high microhardness and small surface roughness. A disadvantage of the method is its low performance (up to 1.5 μm/min). This is due to the fact that the process is run at a frequency of 50 Hz, that is the short anodic pulse, pause and cathodic pulse of equal durations are followed with an unjustifiably long pause of 19.1-19.7 ms.

A method is known of the oxidation of metal surfaces, where a device controlled by the regimes of anodic-cathodic electrolysis using a processor is described (WO 01/81658). Bipolar triangular voltage pulses of 300-600 V and recurrence frequency of 70-400 Hz are supplied to the electrodes. Regulation of the currents and powers in the anodic and cathodic pulses is performed at the expense of a programmable change of the angles (edges) and amplitudes of the triangular voltage pulses. The low-angled leading edge and steep trailing edge of the voltage pulse correspond to a gradual increment of the anodic current and a sharp rise of the cathodic current in the pulse (and vice versa). The use of voltage pulses of an increased recurrence frequency, which are regulated in the process of anodic-cathodic electrolysis, provides for an increase of the coating formation rates up to 2 μm/min and the production of dense and solid ceramic coatings. A disadvantage of the known method is the use of triangular voltage pulses, which are far less efficient than rectangular pulses. The use of triangular voltage pulses is explained by an insufficiently high recurrence frequency of the pulses and, therefore, their insufficiently large duration. The triangular peaky shape of the pulse artificially reduces the duration of its upper part, where the amplitude values of voltages and currents are the largest, which makes it possible to avoid destructive arc discharges. The ceramic coating is formed double-layered with the outer defect layer and increased surface roughness, and the rise of the treatment rate is obviously insufficient.

A known method of forming ceramic coatings on metals and alloys using a pulsed anodic- cathodic regime with the current-pulse frequency of more than 500 Hz (preferably, 1-10 kHz) and pulse duration of 20-1000 μs (WO 03/83181) is the closest to the claimed method by its technical essence and achieved results. The pulsed regime of electrolysis with relatively short current pulses makes it possible to considerably expand the working range of voltages and currents, which leads to an increase of the treatment rate and an improvement of the protective properties of ceramic coatings.

However, a disadvantage of the method is a limitation of the frequency of the pulses and, therefore, a reduction of their duration for the reason that in the method the frequency of electric pulses should coincide with the frequency of acoustic vibrations (audible frequency range), which are generated in the electrolyte using special aerohydroacoustic mixing devices. This does not make it possible to perform oxidation using maximally possible voltages and currents in the pulses, which decreases the treatment rate. Besides, the known method fails to provide for an increase of the fraction of the cathodic pulse (amplitude or duration) as compared with the anodic pulse. All this leads to the preservation of the double-layered structure of the oxide coating, when the thickness of the outer defect layer is up to 14% of the total coating thickness, and to the relatively high surface roughness up to R_Ά 2.1 μm. In this case, for instance, in fabrication of the contact surfaces of friction pairs, an additional finishing machining is required.

Disclosure of the invention

The main task of the present invention is to increase the manufacturability of the method of forming a ceramic coating on metal products and to solve problems, which prevent the use of plasma electrolytic oxidation methods in series production, notably, to increase the treatment rate, decrease labour intensity and energy intensity of the process.

Another task of the invention is to increase the quality of ceramic coatings at the expense of improving their physicomechanical characteristics and, respectively, improving the operational properties of coatings, such as wear resistance and erosion resistance, corrosion resistance and dielectric strength.

The set tasks are solved by that in the method of forming protective ceramic coatings on the surface of metal products a product is immersed as an electrode together with a counter-electrode into an aqueous electrolyte solution, and alternate bipolar voltage pulses of rectangular shape are supplied to the electrodes using a power supply source. A novel feature is that the duration of anodic voltage pulses is 5-20 μs, and the ratio of the durations of anodic T_Ά and cathodic T_c voltage pulses is equal to 0.2-0.4; herewith, the programmable power supply source matches the durations and amplitudes of the voltage pulses with their power in such a way that the oxidation process is run in a stable soft spark regime, without the emergence of arc discharges.

The expected technical result, which would provide for the realization of the claimed invention, is as follows:

- an increase of the rate of forming a ceramic coating up to 4-15 μm/min and a reduction of the time of applying a coating 5-100 μm thick down to 1-20 min without increasing the specific energy expenses;

- formation of smooth coatings with surface roughness R_Ά 0.3-0.6 μm without any outer defect layer, which would exclude the requirement of additional finishing machining of the surface;

- formation of hard (HV 800-2300) and dense (porosity down to 2%) fine crystalline coatings strongly adherent to the base;

- formation of structurally and compositionally homogeneous uniform- thickness coatings on complex- shaped surfaces of products with the possibility of applying coatings in such hard-to-reach places as deep holes and narrow grooves without using special electrodes.

The method is realized in plasma electrolytic oxidation of products from valve metals and their alloys, on which ceramic coatings possess an asymmetric conductance of semiconductors. These are predominantly aluminium, magnesium, titanium, tantalum, zirconium, beryllium and niobium.

The use, in oxidation, of pulsed regimes with very short-time voltage pulses limiting the lifetimes of discharges makes it possible to reduce significantly the power in the pulses (the product of current and voltage), therewith preserving the regime of spark discharges without the emergence of coating-destructing arc discharges. Higher powers and, respectively, temperatures in the discharge channels and simultaneously a more rapid cooling and hardening of the molten mass due to decreased microvolumes provide for the intensive formation of coatings with high crystallinity already in relatively thin layers.

The durations of anodic voltage pulses are set within the range of 5-20 μs. During the anodic pulses, spark discharges are initiated and burn, using which discharges the required plasma chemical processes are performed and a ceramic coating builds up. The claimed durations are optimal for the disclosed process. An increase of pulse duration over 20 μs at a set level of voltages would lead to arc discharges and deteriorate the properties of the coatings, and a decrease of the level of voltages would evoke a reduction of the treatment rate. A decrease of pulse duration down to less than 5 μs requires a significant increase of the set levels of voltages in the pulse (over 2000 V), which would lead to a significantly more sophisticated and more costly power supply source and, naturally, would affect the oxidation process on the whole.

A feature of the present invention is the experimentally found optimal ratio of the durations of anodic and cathodic voltage pulses TJT_C equal to 0.2-0.4, at which the highest coating formation rate is achieved at the best quality of the ceramic layer.

It is just at the duration of the anodic voltage pulses 2.5-5 times shorter than that of the cathodic pulses that the rate of 4-15 μm/min of applying ceramic coatings is achieved depending on the material of the base; this is 1.5-2 times higher than in the prototype method.

At a ratio of the durations of anodic and cathodic voltage pulses TJT_C less than 0.2, it has no noticeable effect on the quality of the ceramic coating, but the coating application rate noticeably decreases. At a ratio of TJ T_c larger than 0.4, the quality of the coating is deteriorated, craters and pittings on the surface appear, the roughness of the treated surface increases.

The necessity of the cathodic pulse of notably this duration is explained by several reasons. As a potent anodic pulse is passed, electrochemical plasma reactions occur in pores of the coating, said reactions leading to concentration changes of electrolyte being in the pores and in the near-electrode zone (pH is changed, reaction products accumulate). To equalize the concentration changes in the near-electrode regions, a relatively long cathodic pulse is required before the next anodic pulse. Herewith, intensive mixing of the electrolyte is contributed to by the formation of hydrogen microbubbles on the surface of the electrodes.

The cathodic voltage pulses of certain duration and amplitude are the determining factor in the formation of a quality ceramic coating with high physicomechanical characteristics.

During the cathodic pulse of certain duration, the formed coating is additionally heated up by the heat from the passage of potent electron current. This makes it possible to expand the working range of voltages and currents significantly, and to maintain the stable burning of spark discharges during the anodic pulses. Besides, an additional transition of less stable oxide phases to stable high-temperature phases occurs during the cathodic pulses, the structure and phase composition of ceramic coatings changes. The microhardness of the coatings reaches 800-2300 HV depending on the material of the base metal.

Also, during the cathodic pulse of certain duration the crystalline grains of the oxide phases are fused into a dense solid coating. Conditions are created for healing the breakthrough channels, the through porosity almost vanishes. Open porosity does not exceed 2%. All these essentially affect the increase of wear resistance and corrosion resistance of the coatings. The protective properties of such coatings are improved severalfold.

Small durations of pulses lead to oxide formations of small size and ceramic coatings of ultrafine crystalline structure (size of crystals is 10-100 nm), small pores and low surface roughness. The roughness of the treated surface does not exceed R_Ά 0.3-0.6 μm depending on the initial surface roughness and coating thickness. Herewith, the outer defect layer is totally absent, due to which there is no need for any additional machining. Thus, the problem of finishing machining (for instance, diamond grinding) is lifted.

Short-time pulses provide for uniform distribution of high-density current along the entire surface of the treated product. A longer (than anodic) cathodic pulse additionally contributes to the redistribution of current along the surface, which leads to the formation of uniform-thickness coatings even on complex- shaped surfaces. Thus, the problem is being solved of applying ceramic coatings in hard-to-reach shielded places, such as deep holes and narrow grooves, without special additional electrodes, the use of which is often difficult.

The pulsed power supply source feeds alternate bipolar voltage pulses of rectangular shape to the electrodes and provides for a programmable variation of their amplitude and duration, as well as controls and regulates the pulse power during plasma electrolytic oxidation, thus ensuring the most favourable conditions for spark discharge burning over the time of the entire coating formation process.

The invention makes use of special regimes of plasma electrolytic oxidation with a severe forced voltage rise both in the anodic and cathodic pulses. Such regimes enable a significant increase of the density of pulse plasma at a relatively low level of energy in the load.

The amplitude values of voltages in the anodic and cathodic pulses are set autonomously depending on the treated material of the base and the set thickness of the formed coatings. For anodic pulses, the amplitude values are within the range of 400-1800 V, and for cathodic pulses, within 200-900 V.

As the thickness and, respectively, resistance of the ceramic layer go up, the amplitudes of the voltage pulses should be increased to maintain the spark process. However, an increase of the voltage amplitude leads to an increase of the current density in the pulses and, correspondingly, the pulse power. In this case, to exclude the possible emergence of destructive spark discharges, the durations of anodic and, correspondingly, cathodic pulses are decreased. A decrease of the duration of the pulses restricts the burn time of electric discharges, decreasing their energy, and makes it possible to maintain a soft spark regime.

The programmable power supply source performs the set rise of the amplitude values of voltage. Herewith, the final values of voltage can exceed the initial values 1.5-2 times. During the oxidation, the power source controls and performs regulation in the set regime of the magnitude of the pulse powers, changing them by decreasing the durations of the pulses, preventing the spark stage of discharges from occurring. Herewith, the ratio 0.2-0.4 of the durations of anodic T_Ά and cathodic T_c pulses is preserved. The range of the pulse frequency is herewith within the limits of 10-30 kHz.

Thus, the maintenance of the maximally possible pulse power by the programmable power source at the expense of an optimal ratio of high amplitude values of voltage in the pulses and their small duration makes it possible to achieve a rate of forming the ceramic coatings 4-15 μm/min depending on the material of the base, which exceeds 1.5-4 times the coating formation rate in the known methods.

In spite of the relatively high voltages and current densities used in the claimed method, the specific energy intensity of the oxidation process itself is lower than in the known oxidation methods. This is due to a considerably increased coating application rate and a significantly shorter oxidation time.

Method of forming a protective ceramic coating on the surface of metal products is explained in attached drawing. Figure 1 presents a diagram of the temporal dependence of the parameters of voltage pulses (positive and negative) fed to the electrodes in the power supply - electrolytic bath circuit during the oxidation process.

The claimed method provides for the formation of high-quality ceramic coatings. Metal products with such protective coatings acquire exceptional operational characteristics and longevity.

The new method makes it possible to form hard, dense, smooth- surface ceramic coatings of 5 up to 100 μm thick over a relatively short time (1-20 min). Herewith, as electron microscopy showed, any coating within this broad range of thicknesses was characterized by a high homogeneity with respect to structure and composition.

The main feature of these coatings is their high uniformity of thickness along the entire surface of a treated product and the complete absence of the outer defect layer.

The roughness of the coating surface does not exceed R_Ά 0.3-0.6 μm depending on the thickness of the coating and the initial roughness of the surface prior to oxidation. The low roughness of the surface and high accuracy of the linear size of products (after oxidation, the geometric size practically does not change, as the coating grows into the metal) make it possible to use oxidized products in friction units without any additional labour-intensive machining.

In the known oxidation methods, sufficiently thick layers (60-150 μm) have to be formed to ensure a relatively high microhardness and density of ceramic coatings. A time of 1-3 hours is required to apply such coatings. The formed coatings have a complex structure; the outer loose highly porous defect layer makes about half of the total thickness of the coating. Removal of this layer requires significant labour and energy expenses.

The structure of ceramic coatings formed by the claimed method consists of ultrafine crystals of oxides 10-100 μm in size. This structure is characterized by a high microhardness and simultaneously an increased strength. The microhardness of ceramic coatings is HV 800-2300, depending on the grade of the coated base alloy.

Such hard coatings possess a high resistance to abrasive wear. Besides, the ultrafine crystalline structure of the coatings increases the resistance to abrasive action even more. Fine densely packed crystals resist micro- and macrodestructions at the dynamic action of abrasive particles better.

The wear resistance of ceramic coatings during the lubricated friction exceeds that of hardened steels 3-10 times. In hydroabrasive action, new ceramic coatings withstand wear 3-5 times better than nickel- alloyed irons (Ni resists).

And, finally, ceramic coatings protect products from light alloys from high-temperature gas erosion 2-4 times better than high-alloy steels.

Oxide-ceramic coatings are inert with respect to most aggressive media. However, penetration of such media via through pores of the ceramic layer can lead to corrosion breakdown of the base alloy and exfoliation of the coating.

The technical result of the invention is also the formation of dense ceramic coatings with minimal open porosity and practically absent through porosity at the expense of the healing of the pores during the oxidation process. Open surface porosity of new coatings does not exceed 2%, and micropores are 0.01-1.0 μm in diameter. These ceramic coatings decrease the corrosion rate 10-100 times as compared with unoxidized light alloys.

The small pore size and the absence of through porosity lead to an increase of the electric strength of new ceramic coatings up to 40-50 V/μm, which is two times as high as the electric strength of ceramic coatings formed in the known oxidation methods.

Thus, the claimed method provides for the formation of high-quality ceramic coatings with high protective properties, which is the technical result of the method.

The invention is illustrated by examples of embodiments of the method. All examples made use of cylindrical specimens 20 mm in diameter and 50 mm in length with the through axial hole 7 mm in diameter. The parameters of quality of the coatings (thickness, microhardness, porosity) were measured on cross microsections. In oxidation, the optimal concentrations of electrolytes were chosen with the aim to achieve a required level of voltages in electrolysis and to obtain the stable passivation of the surface.

Example 1 A specimen from aluminium alloy 6061 was oxidized for 20 min in a phosphate-silicate electrolyte with pH 10.5. Bipolar alternate voltage pulses with anodic pulse duration of 20-7 μs, cathodic pulse duration of 80-27 μs and pulse frequency of 10-30 kHz were supplied to the bath. The current density was 80-40 A/dm², and the final voltage (amplitude) was: anodic, 1800 V; cathodic, 900 V. The thickness of the ceramic coating was: in the middle of the cylinder's generatrix, 100 μm; at the ends, 102 μm; in the middle of the hole, 63 μm. The roughness of the oxidized surface was R_Ά 0.6 μm; microhardness of the coating, 2300 HV; porosity, 2%. The specific energy expenses were 0.12 kW-h/μm-dm .

Example 2

A specimen from magnesium alloy AZ 31 was oxidized for 2 min in a phosphate- aluminate electrolyte with pH 12.0. Bipolar alternate voltage pulses with anodic pulse duration of 12-8 μs, cathodic pulse duration of 38-26 μs and pulse frequency of 20-30 kHz were fed to the bath. The current density was 20-12 A/dm², and the final voltage (amplitude) was: anodic, 550 V; cathodic, 250 V. The thickness of the ceramic coating was: in the middle of the cylinder's generatrix, 29 μm; at the ends, 30 μm; in the middle of the hole, 19 μm. The roughness of the oxidized surface was R_Ά 0.3 μm; microhardness of the coating, 800 HV; porosity, 2%. The specific energy expenses were 0.008 kW-h/μm-dm .

Example 3

A specimen from titanium alloy Ti A16 V4 was oxidized for 5 min in a phosphate-borate electrolyte with pH 9.3. Bipolar alternate voltage pulses with anodic pulse duration of 20-9 μs, cathodic pulse duration of 55-25 μs and pulse frequency of 15-30 kHz were fed to the bath. Current density was 30-15 A/dm , and the final voltage (amplitude) was: anodic, 500 V; cathodic, 200 V. The thickness of the ceramic coating was: in the middle of the cylinder's generatrix, 30 μm; at the ends, 31 μm; in the middle of the hole, 17 μm. The roughness of the oxidized surface was R_Ά 0.4 μm; microhardness of the coating, 900 HV; porosity, 1.5%. The specific energy expenses were 0.025 kW-h/μm-dm².

Claims

1. A method of forming a protective ceramic coating on the surface of metal products by oxidation, which method includes immersion of a product as an electrode together with a counter-electrode into an aqueous electrolyte solution, supply of alternate bipolar- voltage pulses of rectangular shape to the electrodes using a power supply source, characterized in that the duration of anodic voltage pulses is 5-20 μs, and the ratio of the durations of anodic and cathodic voltage pulses TJT_C is equal to 0.2-0.4; herewith, the durations and amplitudes of voltage pulses are matched with their power to ensure that the oxidation process is run in the spark- discharge regime.

2. The method as per claim 1, characterized in that the protective ceramic coating is formed on the surface of metals: Al, Mg, Ti, Ta, Zr, Be, Nb and their alloys.

3. The method as per claim 1, characterized in that the formation of the coating by oxidation is performed at a rate of 4-15 μm/min at an independent setting of anodic voltage pulses within the range of 400-1800 V and cathodic voltage pulses within the range of 200-900 V.

4. The method as per claim 1, characterized in that the process of forming the coating by oxidation is run at a pulse frequency within the range of 10-30 kHz.

5. The method as per claim 1, characterized in that the limitation of power of the pulses to maintain the spark regime of oxidation with the thickness of the coating increasing and the amplitude of voltage pulses rising is performed by decreasing the duration of pulses.

6. A protective ceramic coating, formed on the surface of metal products by the method of any of claims 1-5, having a thickness of 5-100 μm and characterized by: (a) a thickness homogeneity and uniformity, absence of an outer defect layer and small surface roughness R_Ά 0.3-0.6 μm;

(b) a ultrafine crystalline structure consisting of crystals of oxides 10-100 nm in size and a high microhardness HV 800-2300 providing for a high wear resistance;

(c) a high density and low surface porosity down to 2% at a pore size from 0.01 up to 1.0 μm providing for a high corrosion resistance.