Microarc Oxidation of Carbon-Graphite Materials (Review)

The review is devoted to microarc (or plasma-electrolytic) oxidation (MAO) of carbon-graphite materials with the purpose of protecting their surfaces from oxidation, primarily, high-temperature oxidation. It is found that the formation of protective coatings on graphite under certain conditions occurs in accordance with trends similar to the formation of oxide layers on valve metals. The phenomenological model of the mechanism of formation of MAO coatings on graphite is given. In particular, the results of our own studies are described, as well as the data on the achieved level of characteristics of obtained MAO coatings are given.


INTRODUCTION
Items made of carbon materials are used in a variety of industries. Graphite and materials based on it have a low density, high heat resistance, and good processability. However, carbon is intensively oxidized during their operation in active media, especially at high temperatures, so the problem of protecting carbon-graphite materials is very relevant. One of the most effective ways to solve this problem is to obtain gas-tight, heat-resistant protective coatings with sufficient mechanical and adhesive properties for specific operating conditions.
In [1], the analysis of various approaches to the surface modification of carbon materials (by coating) for their protection against oxidation is given. The authors consider that the most important task is the choice of the coating material, which must have high thermal and heat resistance, maximum resistance to oxidation, film-forming properties inherent to polymers, crack resistance, thermal expansion coefficient values close to those for the base material, the absence of structural transitions under operating conditions, and high adhesion to the base material (in the limiting case, the chemical bond). It was noted that a number of metals and refractory inorganic compounds possess the largest heat resistance associated with melting and flowing temperatures (Table 1), whereas the resistance against oxidation, heat-shock resistance, moisture resistance, and high mechanical strength are the main requirements for the modification of carbon materials.
At present, a number of methods and technologies exist for applying protective coatings on graphite items. In [1], some methods for applying protective coatings on carbon materials are listed, namely: -chemical deposition from the gas (vapor) phase (CVD method), including the deposition of organometallic compounds (MOCVD); -reactive deposition from the gas phase (RCVD method); -plasma sputtering; -chemical modification of the surface; -sol-gel technology of applying coatings from solutions; -electrolytic and electrophoretic deposition from solutions; -high-temperature deposition from salt solutions. Methods of chemical modification by thermal diffusion saturation of the graphite surface by heating in backfill or slip coatings are being widely used, mainly There are examples of the modification of the carbon material surfaces by electrolytic methods. Researchers of the South Russian State Polytechnic University (Novocherkassk) have developed functional composite oxide coatings on the surface of glass-like carbon by the method of nonstationary electrolysis [12]. The deposition was carried out from aqueous solutions of salts under polarization with an asymmetric alternating current. Molybdenum, cobalt, nickel, iron, and vanadium oxides were the main phases of the coating. The appearance of the coating and its phase composition are shown in Fig. 1.
An innovative solution is patented in [13]. The invention relates to the electrochemistry of nanocar-bon clusters, in particular, to the production of a fullerene film deposited on conductive materials (metals, graphite, etc.) in the electrochemical process. The film deposition is carried out on the anode from an anhydrous fullerene solution in a pyridine−acetone mixture at a pyridine-to-acetone ratio of 1 : 4, a temperature of 20-30°C, an electrode potential difference of 6.0-8.0 V, a current density of 1.0-2.0 mA/dm 2 , and a process duration of 30-60 min. This film resistant to dilute solutions of acids and alkalis can be used as a multifunctional barrier layer.
The coatings obtained by the above-listed methods do not always properly satisfy the necessary requirements imposed on them (especially from the standpoint of chemical and morphological homogeneity, and also in the case of processing items with complex configurations), and the methods themselves are often significantly complicated when trying to improve the quality of the protective layers being formed.
In light of what has been said, the method of microarc oxidation (MAO) typically used for the modification of metal surfaces is a promising method for solving the problem of protecting carbon-graphite  materials from high-temperature oxidation and abrasive wear.

MICROARC OXIDATION OF GRAPHITE MATERIALS
A detailed description of the theory, practice, and hardware of the MAO method is given in the monographs [14,15]. The possibility to synthesize nanoceramic layers possessing high adhesion strength with a substrate, controllable corrosion-protective ability and wear resistance, as well as a number of special properties, on the surface of items made of so-called "valve" metals and their alloys by the plasma-electrolytic treatment has been noted.
Carbon materials are not classified as valve materials and, as a rule, are not considered as objects for the modification by MAO. Nevertheless, attempts of plasma-electrolytic oxidation of carbon-graphite materials have been made since the 1980s [16,17].
The main difficulty in processing carbon is connected with the fact that oxidation with the formation of volatile oxides takes place during its anodic polarization in accordance with the following reactions [18,19]: (1) Since carbon does not form protective layers of its own oxides on the surface, carbon-graphite materials can be coated only with electrolyte components by the electrolytic plasma processing.
A typical current−voltage curve (Fig. 2) for the process of MAO on carbon materials or composites on their basis (such as Si-SiC-C and Zr-ZrC-C composites) is similar to the section of the anodic curve corresponding to the transition of a metallic material to the passive state and its being in this state [20].
The authors explain such a pattern of the curve by the facts that the electrolyte boils upon reaching the critical value of the current-voltage product (IU) and its resistance significantly increases in the near-anode region. Then breakdowns of steam bubbles, and plasma and thermal transformations of the electrolyte components (Na 2 SiO 3 , NaAlO 2 , etc.) with the formation of a porous oxide coating on the surface of the carbon (graphite) or composite material (microarc treatment of the working electrode surface) begin. Simultaneously, a sharp decrease in the electric current (by more than 10 times) and its pulsation are observed. An increase in the thickness of the oxide coating in the further course of the microarc electrolysis process basically occurs owing to microbreakdowns of the vapor−gas phase formed in the through pores of the coating. The resulting thickness of the oxide microarc coating on carbon materials may exceed 300 μm. If the process of microarc treatment of graphite materials or composites on their basis is carried out in a potentiostatic mode for a long time, then the microarc discharges self-extinguish. However, solitary microdischarges appear again after the exposure, for example, to a silicate-alkaline electrolyte for the time period, the duration of which is a function of the coating thickness and the concentration of the components. Their emergence is determined by the dissolution of the fluffy precipitates of oxides (SiO 2 , Al 2 O 3 ), which are formed after plasmo-or thermochemical transformation of the corresponding electrolyte polyanions, in alkaline solutions in some of the through pores. It should be noted that the formation of a coating takes place, in the opinion of the authors [21,22], just under the conditions of anodic polarization, and the role of cathodic polarization is reduced to an increase in the number of defects in the coating.
The application of protective microarc coatings to graphite materials after the formation of a preceding dielectric film or organic polymer layer [16] is most economical and expedient. In this case, the process is started immediately from the stage of microarc oxidation.
Determining the conditions for such a modification, the authors of [19] called the process "microarc electrolysis" (MAE) and formulated its key principles. Their studies showed that the anodic dissolution of carbon materials increases with an increase in the set anode potential for only a relatively small range of voltages. As follows from the anodic polarization curve obtained on graphite (Fig. 3), the strength of the transmitted current and, correspondingly, the rate of carbon dissolution initially increase (Fig. 3a) and then decrease significantly when the critical potential U cr1 is   (Fig. 3b). A similar phenomenon has been observed for the first time on a carbon electrode during electrolysis from the melt of salts and associated with the release of gas bubbles on the anode surface, which creates an additional resistivity by screening a part of the surface. The number of such bubbles increases with an increase in the anode potential, which leads to a further decrease in the amperage. The bubbles merge at a potential of U cr2 and form a continuous gaseous dielectric layer (Fig. 3c). A further increase in the anode potential leads to an electric breakdown of this layer. An electric arc with a temperature of about 3000°C arises at the breakdown point, in the action zone of which a coating is formed. As soon as the coating formed at this point, the electrical resistance of this section becomes larger than the resistance of neighboring zones, and the arc is displaced toward them (Fig. 3d). The process ends up with the formation of a continuous coating layer (Fig. 3e). As follows from the analysis of Fig. 3, the initial stages of the process, at which the gaseous dielectric layer is formed, are the most energy-intensive steps (positions shown in Figs. 3a and 3b). These steps can be avoided if a layer of some dielectric material is preapplied to the surface of the treated items. In this case, the process begins with the breakdown of the applied layer, and curve 1 is converted into curve 2 (Fig. 3).
Thus, the first condition for implementing the MAE process on a carbon material is the creation of a dielectric layer on its surface, the subsequent breakdown of which gives rise to the generation of a hightemperature electric arc.
The second necessary condition is the correct choice of the electrolyte. In contrast to the known electrochemical methods for the creation of metal coatings, in which the deposition of the coating material occurs at the cathode (electrolysis), the coated object in this case is the anode. In aqueous solutions, the salts dissociate with the formation of a cation, i.e., the metal ion, and an anion, i.e., the acid residue. Under the influence of the electric field, the anion moves to the anode and serves as a material for the formation of a coating. Consequently, it should contain the atom of an element that will determine the properties of the future coating. These requirements are met by water-soluble salts of metal-bearing acids: NaAlO 2 , NaTiO 3 , K 2 SiO 3 , KCrO 4 , Na 4 V 2 O 7 , NaMoO 4 , etc. After hydrolysis, the acid residue approaches the anode under the influence of the electric field at the breakdown point (see Fig. 3c), in which an electric contact takes place, accesses the zone of the operation of the arc, and undergoes thermal dissociation there. The atoms formed as a result (or ions, depending on the dissociation energy value, E d ) interact with oxygen to form an oxide. It can be concluded from this that the energy transferred to the anion by the arc (and, correspondingly, the arc temperature) should be sufficient for the thermal dissociation of the anion, but not excessively large in order to not thermodynamically impede the oxide formation (to not exceed the E d value of synthesized oxides). As a result, there is the possibility to produce coatings containing Аl 2 O 3 , TiO 2 , SiО 2 , Сr 2 О 3 , VO, and МoО 2 .
The third condition for implementing the MAE process is the correct choice of the form of the transmitted current. Attempts to conduct the process in a direct current were unsuccessful, since an arc burning in one spot strongly heats up this particular region, reducing the electrical resistance of the oxide, which prevents the arc from shifting to neighboring areas. This was managed by obtaining a coating using a rectified pulse current. The arc burns in this case only when a certain effective voltage value is reached, and the presence of pauses allows the elimination of these obstacles and permits the formation of a protective coating on the anode surface. The best results were obtained by using a pulsed current with adjustable values of the arc burning time, τ a , and the pause, τ p . The MAE method makes it possible to obtain sintered and fused gas-tight coatings with specified properties and can be accomplished in galvanic and potentiostatic modes, in which the power sources serve as a source of the electrical current and stabilized voltage, respectively. The type of the selected mode significantly affects the electrical characteristics of the process and the properties of the obtained coatings (Fig. 4). The total electrical resistance of the circuit in the MAE process can be expressed by the following equation: R total = R int + R ext + R c + R el (R int is the internal resistance of the power supply; R ext is the external resistance of devices and electrical connections; R el is the ohmic resistance of the electrolyte bath; and R c is the resistance of the coating). The authors imply that the first three terms at a given composition, concentration, and temperature of the electrolyte, and given sizes of the bath and the sample remain constant throughout the process of the MAO coating formation. Under these conditions, R total depends on R c . When operated in galvanostatic mode, the electrical current density remains constant in time. Under these conditions, an increase in the coating thickness during its formation and an increase in the electrical resistance, R c , lead to an increase in the R total value and, hence, the voltage at the electrodes when a direct current is used. At the same time, the consumed specific power of the process also increases. In the case of using the potentiostatic mode, the constant voltage is the constant value. Moreover, an increase in the coating thickness and related R c and R total values leads to a decrease in the current flowing in the circuit and, accordingly, the specific power consumption of the process. Such a change in the electrical characteristics of the MAE process has a crucial effect on the formation of coatings and their properties.
As already noted, the MAE process can be accomplished on carbon materials only by using electrical currents with different degrees of pulsation, which is necessary for moving the discharge. However, the role of pulsation is not limited by this, since the pulse duration (τ pulse ) has a significant effect on the parameters of the obtained coatings. During the τ pulse time, the region of the treated surface with a deposited coating, on which the discharge flashes, heats up to a certain temperature, which is higher the longer is the τ pulse time. This region is cooled during pauses with a length of τ c . At a fixed τ c value, the temperature of the formed oxide decreases by nearly the same value, since it proceeds under the same conditions. Consequently, an increase in the τ pulse value causes an increase in the temperature of the oxide at the time of applying the next pulse. The temperature of the oxide determines its electrical resistance, namely, the lower the temperature, the larger the electrical resistance of the oxide and the greater the power required for its breakdown. Such a change in the power caused by an increase in the τ pulse duration significantly affects the properties (density, mass, and thickness) of the coatings produced.
The use of a pulse current in the MAE process makes it technically possible to use cathodic polarization in the treatment of carbon material surfaces. At the moment when the cathode pulse is applied, a breakdown of the coating takes place, and a discharge is initiated at the breakdown point. However, an additional mass of oxide is not formed in this case, contrary to the discharge that arises when the anode pulse is applied. Cations, which are, as a rule, alkali metal ions subliming at high temperatures (about 3000°C), appear in the discharge region. This is a positive result, in principle, since the presence of such low-meltingpoint components in the protective coating would be undesirable. Thus, the supply of the cathode pulse should not directly affect the mass of the precipitated oxide. Nevertheless, such an influence, and a rather substantial influence, is seen (Fig. 5).
Perhaps, at the time of breakdown the cathode pulse uncovers a portion of the carbon material surface at the breakdown spot. Since the cathode current itself cannot form oxides, this region remains uncovered until the next anode pulse is supplied, the energy of which is no longer consumed for an electrical breakdown. Thus, the specific power consumption, W, seems to be, even in the potentiostatic mode of the MAE process, regardless of the process time, all the time at the level where it was in the initial stage when the energy consumption is still high. This makes it possible to maintain the energy of the anode process at a higher level and, accordingly, to produce a larger amount of oxide per unit time. Moreover, the higher the voltage of the cathode pulse, the greater the number of discharges initiated during its operation and the larger the total surface of the uncovered substrate. Hence, the power consumption at the time when the next anode impulse is applied, and the amount of the oxide produced during its operation should be higher. Consequently, the use of cathode pulses can significantly increase (under certain conditions, by 2.5-3.5 times) the mass and thickness of the resulting layers without directly influencing the amount of the formed material of the MAO coating. However, the functions of the cathodic polarization are not limited to the described effect. Since metal cations coming from the salts contained in the electrolyte composition are supplied to the discharge zone after applying the cathode pulse, the energy portion (in the range 273-336 kJ/mol) consumed for the thermal dissociation of the salt upon applying the anode pulse is used to increase the temperature of the discharge zone. Consequently, using a cathode pulse, a discharge with the temperature higher than in the case of the unipolar anode mode can be obtained, by expending less power, at the same time. In turn, this makes it possible to use the cathode pulse as a tool for obtaining fused glass-like wearresistant coatings with high values of the density and adhesive strength.
The above principles make it possible to explain the possibility of the micro-arc processing of carbongraphite materials and the formation of protective coatings on them. Since natural carbon oxides cannot form the dielectric layer necessary for generating the coating-forming microdischarges, the implementa-tion of the MAO modes acceptable from the point of view of current efficiency and energy costs is possible only after the preliminary application of dielectric films onto the graphite surface. In such cases, the formation of coatings begins with the stage of arcing or microarc oxidation (depending on the thickness and quality of the preliminary film), and different voltammetric modes can be set [17,18].
In practice, it is possible to arrange the accomplishment of the stage of microarc discharges during the MAO processing of carbon graphite materials by the following methods: -mechanical application of a preliminary dielectric film from organic polymers (BF-2, BF-6, and other glues, varnishes, etc.); -formation of the starting layer by electrophoretic incorporation of fine-dispersed refractory powders during the MAO process in electrolyte suspensions; -use of microarc electrolysis to synthesize protective coating from electrolyte components; -microarc oxidation of the valve metal layer applied onto the surface of the carbon graphite material.
When forming a preliminary film of organic polymers on graphite, it should be borne in mind that the breakdown voltage of the film must be higher than the voltage of the emergence and existence of microarc discharges. In this case, electric breakdowns appear in the sample, which give rise to local burnouts of the film and the formation of a coating from the electrolyte material by microarc discharges in the burnt-out spots. Solutions containing the and/or anions are used as electrolytes. Finely dispersed powders (Аl 2 O 3 , SiO 2 , ZrO 2 , etc.) can also be added to the electrolyte, which serve as a main material for the coating formation. The value of the breakdown voltage of the film depends on its thickness and quality. If the thickness and/or quality do not allow one to achieve the voltage of the emergence of microarc discharges, then the film exfoliates under the influence of the electrical current and the coating is not formed. To reproduce the set of starting conditions, in particular, the voltage of the breakdown and the beginning of the process, it is necessary to control the thickness and uniformity of the polymer film both over the surface of the sample and during the transition from one sample to another [23].
The data on the mass yield per unit of consumed energy in terms of the thickness, porosity, and adhesion strength of silicate oxide coatings obtained on graphite in an anodic microplasma process are given in [18,22]. A preliminary dielectric film was formed from an organic polymer (BF-6 glue).
A power source providing a full-wave rectified voltage (a positive pulse repetition frequency of 100 Hz) with capacitors as a filter limiting its pulsation to a level of 3.5-4% was used. The power supply allows  one to set and maintain the galvanostatic (over the mean current) mode. The voltage (amplitude voltage) increases automatically with an increase in the resistance in conjunction with an increase in the thickness of the oxide dielectric layer. When the voltage setpoint (490, 520, 550 V) was reached, the process was interrupted, and this voltage was conditionally considered final. Aqueous solutions served as electrolytes, for the preparation of which soluble liquid sodium glass was used (modulus, μ = хSiO 2 /yNa 2 O = 3.0; density, ρ = 1.44 g/cm 3 ). Samples of a rectangular shape with a size of 25 × 20 × 10 mm were prepared from fine-pored graphite MPG-7. The preliminary film of the organic polymer (BF-6 glue) was applied mechanically, dried at room temperature until solidification, and then placed in a drying oven at a temperature of 80°C for polymerization.
The sample placed in the cell as an anode was supplied with a voltage increasing in time. At a certain value of the rising voltage (it is determined basically by the thickness of the preliminary film and under the experimental conditions corresponded to a level of 300-350 V), the film breaks down and microdischarges emerge. As a result, the film burns out and an oxide coating is formed from the material of the electrolyte instead of it. The process was terminated at different voltage values (chosen according to the plan and conditionally called "final voltage"), which did not exceed the voltages leading to the emergence of arc discharges or destruction of the coating. Variable experimental factors were set at the following levels: the current density (j) at 11, 15.5, and 20 A/dm 2 ; the final voltage (U) at 490, 520, and 550 V; and the liquid glass concentration (C) at 7, 11, and 15%.
The effect of the thickness of the preliminary film on the specific mass yield of the coating per unit of consumed energy was studied. It is established that this factor is influential ( Table 2). The yield of the mass of the SiO 2 coating sharply increases with an increase in the preliminary film thickness. Therefore, the strict control over the thickness of the preliminary film during its preparation is required not only from one sample to another, but also throughout the sample surface. The difference in the local film thicknesses was within the range 7-10 μm.
The experimental data on the effect of the final voltage on the specific mass yield of the coating ( Figs. 6a and 6b) show that the dependences are close to linear with approximately equal variations of random discrepancies in the semilogarithmic coordinates. The model of the effect of the final voltage and the concentration of silicate on the mass yield in such coordinates is quite simple and has the following form: where Y is the numerical value of the specific mass yield of the coating (measured in units of mg/dm 2 per 1 W/h); U and C are the values of the final voltage (in V) and the concentration of the liquid glass (in %), respectively.
The calculation errors are randomly distributed and do not exceed 0.04 logarithmic units; the standard deviation value is σ = 0.021.
The dependences of the thickness of silicate coatings on the final voltage of the process are shown in Fig. 7. The influence of the process factors on the layer thickness is more complicated than on the specific yield of the substance. The analysis of the numerical data and dependences constructed using the experimental data makes it possible to make an assumption on the importance of their interactions and quadratic effects. The data processing by the least-squares method in accordance with the relevant model leads to the following equation: The randomly distributed calculation errors do not exceed 40 μm; the standard deviation is σ = 18 μm.
The results of the measurements of the coating adhesion showed that the influence of the liquid glass concentration and the current density in the studied intervals is within the measurement error, but there is yet a weak dependence on the value of the final voltage of the process ( Table 3). As seen from the averaged data, the adhesion strength of the coating at a final voltage of 490 V is higher than at 520 V, and its value is retained at 550 V.
The measured values of the coating porosity were widely scattered, but there was a tendency for it to decrease with an increase in both the final voltage and the concentration of the liquid glass in the electrolyte. A joint analysis of these data and the data on the adhesion showed that the adhesion strength at a large ( ) porosity (800-1700 cm -2 ) is somewhat higher than at a small porosity (30-50 cm -2 ). As a result of the studies performed in [18,22], a regime was found for carrying out an anode microarc galvanostatic process in the range of controlled variable parameters (a current density of 11-20 A/dm 2 ; a final voltage 1 of 490-550 V; the electrolytes are aqueous solutions of liquid glass with a concentration in the range 7-15%), which allows one, first, to synthesize protective composite (the presence of amorphous and fine-crystalline phases) SiO 2 coatings on graphite with a preliminarily deposited polymer film and, second, to estimate the thickness of the formed coatings and their specific mass yield by means of the regression models established using a factor analysis.
In [23], the results of a study of the effect of lowfrequency current pulsations arising in the stages of arcing and emerging microarc discharges in the electrolyte-coating-substrate system on the parameters of the MAO process and on the characteristics of coatings deposited on graphite. An electrical current source of full-wave rectification (the repetition frequency of positive pulses is 100 Hz) was used for car-  rying out the experiments. The coating was formed in a 10% solution of liquid technical glass with a modulus of μ = 3.63 and a density of ρ = 1.47 g/cm 3 . The temperature of the electrolyte was maintained in the range 20-25°C. A film of BF-6 glue was applied on the graphite samples with a surface area of 0.15 dm 2 and dried at 70°C for 1 h. The coating formed in this electrolyte was mainly silicon oxide. A comparison of the behavior of the voltage with time, oscillographic observations of the current pulses, and visual observations of the samples during the coating process made it possible to identify three sections on the formation curve (sections I, II, and III in Fig. 8).
The first section corresponds to the prebreakdown state of the polymer film. During this time, the voltage in the electrolytic bath increases linearly until reaching the value of the electrical breakdown of the polymer film and the emergence of microarc discharges U md (section I in Fig. 8).
In the second section, the polymer film is replaced by an oxide coating (section II in Fig. 8). This process on the formation curve is manifested in the form of significant voltage oscillations. They occur at the time of an electric breakdown of the oxide and during the formation of a coating by microarc discharges in the spots with the burnt-out oxide film. Owing to the high temperature of discharges, the film burns out not only in the discharge zone but also in its vicinity, thereby increasing the area of an electrical contact of the electrolyte with the surface of "naked" graphite. For this reason, an avalanche-like increase in the current and the release of the nearly entire applied power in these sections take place, which contributes to further combustion of the film and an increase in the number of microdischarges and their cross-sectional area. Subsequent to the burnt film on graphite exhibiting electronic conductivity, a coating is formed from the material of the electrolyte (SiO 2 ). The coating thickness increases until the voltage of its electric breakdown exceeds the breakdown voltage on the film. The discharges are extinguished at this local section of graphite, and at the next time point they appear on another section with a lower resistance. Thus, a complete replacement of the polymer film with an oxide coating occurs, and the voltage in the electrolyte bath increases in line with the coating resistance (thickness).
The third section is the area of an increase in the thickness of the coating (section III in Fig. 8). In this section of the formation curve, the amplitude of the voltage oscillations decreases significantly. The electric breakdown and the emergence of a microarc discharge occur in the coating and given that it does not collapse in the vicinity of the discharge (does not burn), there is no avalanche-like increase in the current, as in the case of the polymer film. The diameter of the breakdown channel in the coating is small, and therefore the discharge is localized and has a high energy density that accelerates the transfer of matter from the electrolyte to the discharge ignition region, which gives rise to a rapid increase in the electrical resistance of the coating in this local spot of graphite.
The boundary between the second and third sections on the formation curve is conditional, since a decrease in the voltage oscillations occurs in the case of an electric breakdown of not only the coating but also the polymer film regions, the dimensions of which are so small that they do not cause an avalanche-like increase in the electrical current.
For large values of current pulsations (more than 60%), forced interruption of discharge glows takes place (this regime can be equated with the pulse regime). During the pulse, the voltage increases and reaches the breakdown potential, after which microarc discharges forming the coating emerge. When passing through the amplitude value, the voltage is reduced to the potential of the extinction of discharges; therefore, the discharges glow only during the pulse and in the interval ranging from the breakdown voltage to the voltage of the extinction of discharges. If the extent of current pulsations causes the extinction of discharges, then the time of the glow of discharges is somewhat less than the pulse duration. The value of current pulsations affects the change of the resistance of the graphite-coating-electrolyte electrochemical circuit during both the pulse and the entire period of the oxide layer formation. Under the conditions of forced interruption of discharges, the period from the time of the extinction of discharges to the time of their appearance increases with an increase in the current pulsation amplitude. In this connection, the coating cooling time increases and the conditions for the appearance of microarc discharges change accordingly. In the mode of forced interruption of the glow of discharges, the coating is formed as a densely sintered fine powder. At 100% current pulsations, the roughness of the coating was 36 μm on average at a mean total coating thickness of 360 μm. If the current pulsation is such that the decreasing voltage after passing through the amplitude value does not reach the discharge voltage, then the discharges extinguish spontaneously with an increase in the electrical resistivity of the coating (once the breakdown voltage becomes higher or equal to the formation voltage on account of this). In this case, the glowing time of an individual discharge is longer than the duration of the voltage pulse, and the discharge passes through all phases of its development. The long glow of discharges at one place heats up the coating to a greater depth, and the abrupt cooling of the channel from the electrolyte side gives rise to the formation of a large number of closed pores. This makes it possible to assume that the coating is in a boiling state at the time of the extinction of discharges. At 1% current pulsations, a fused glass-like coating having the developed through and closed porosity is formed. The roughness of the coating increases by a factor of 4-5. The substance yield with respect to the current weakly depends on the current pulsation, but the time of reaching the final voltage (600 V) increases more than 5 times with a decrease in the pulsation amplitude from 100 to 1%. One of the reasons for an increase in the time of the glow of discharges is that the stage of the arc discharges at current pulsations that do not cause their extinction is reached earlier than the selected final voltage during the coating formation. The power and lifetime of a separate arc discharge increase significantly, which affects, in turn, the morphology of the coating and the parameters of the process. The coatings formed at different current pulsations have not only different macrostructures but also different masses per unit surface area at an equal breakdown potential (600 V). Thus, the coating porosity and roughness, as well as the time of reaching the final voltage, substantially increase with a decrease in the current pulsation [23].
The authors of the patent [24] proposed the extra addition of powdered refractory filler with a particle size of 50-70 μm in an amount of 40-75% to the organic polymer when depositing the preliminary film. It was suggested to use the BF-6 and 88 glues or the zapon varnish as a substrate for the dielectric coating and chrome magnesite, zircon, or periclase powders as refractory filler. The components are thoroughly mixed and applied uniformly onto the item under treatment with a brush, after which the item is dried for 12 h at room temperature.
The coated and dried item is placed in an electrolytic bath and is connected to the positive pole of the power source. The negative pole is connected to the body of the bath. The electrolyte is a 40% aqueous solution of sodium silicate Na 2 SiO 3 (TU 6-15-433-75). The electrodes are supplied with a voltage of U = 650 V from the power supply, and the item undergoes the MAO processing for a time period of τ = 10-30 min. The spinel-type oxide coatings formed on carbon materials to protect items operating in contact with molten metals have demonstrated the possibility of multiple thermal loadings up to a temperature of 1600°C (Table 4).
In order to simplify the MAO processing of carbon materials, to reduce the gas permeability of the coating, and to increase its homogeneity, it was suggested in [25] to anodize sodium silicate in two steps: first, at a voltage of 100-250 V for 2-5 min, and then at a voltage of 300-700 V until a coating of the required thickness is obtained.
In the stage of preliminary anodizing at a voltage of 100-250 V, a dielectric continuous gas layer of carbon oxides is formed on the surface of the material as a result of anodic dissolution of the surface of the carbon material. The resulting gas bubbles are retained on the surface due to the arising electrostatic forces. The formed gas layer performs functions similar to those that would be performed by an organic dielectric layer preapplied to the surface of the carbon material, thereby eliminating the preparatory operation of applying an organic dielectric coating to the surface of the anodized material.
The anodization was carried out in an electrolyte containing 40 g/L of sodium silicate. After applying a preliminary voltage of 100-250 V, the current density increased to 160 A/dm 2 and then spontaneously decreased. A decrease in the current density to 10 A/dm 2 indicates that a continuous gas layer has formed. With a further increase in the voltage to operating values, an electric breakdown of the formed gas layer and the anodization by a known mechanism occur.
A carbon material (TU 48-4807-65-85) having a starting thermal conductivity coefficient of 2.7 W/(m K) and a specific heat of 0.68 kJ/(kg K) was subjected to the anodization. The applied protective coating increased the specific heat to 0.84 kJ/(kg K) and lowered the thermal conductivity coefficient to 1.8 W/(m K) ( Table 5).
The gas permeability of the coating was evaluated at an overpressure of 5 atm. As follows from the data given in Table 4, the proposed method makes it possible, in comparison with the known method, to simplify the process by eliminating the preparatory operation of applying a dielectric coating and also to obtain a coating with a lower gas permeability and higher homogeneity.
Based on the general principles and the mechanism of MAO in metals, a method for the plasma electrolytic treatment of graphite was suggested in [26], which is called by the authors "the micro-spark oxidation process" (MSO). A preliminary dielectric (barrier) film was formed on graphite from n-type oxides (valve metal oxides, such as Ta 2 O 5 , Nb 2 O 5 , ZrO 2 , TiO 2 , Al 2 O 3 , etc.), which ensures the blocking of the anodization current and the appearance of breakdowns at high voltages initiating plasma−chemical reactions at the anode.
The potential barrier on the surface of the film is due to the negative charge of the n-type oxide surface, since the polarity of the anodization current during the oxidation of the valve metals corresponds to the reverse current (blocking current) of the current-voltage characteristic of the metal-oxide-electrolyte system similar to the diode structure. The potential barrier on the oxide surface increases with an increase in the anodization voltage until the appearance of electric breakdowns of the film. A similar situation arises in the graphite-oxide-electrolyte system, which makes it possible to carry out the MSO process on the graphite surface and to obtain protective oxide coatings. An oxide barrier film on graphite is formed owing to the transport and deposition of complex anions of the electrolyte containing metal atoms of the valve group under the influence of the electric field. An increase in the anodization voltage prior to the appearance of microarcing causes the occurrence of high-temperature reactions of formation of oxides in the areas of electric breakdowns that ultimately propagate to the entire film surface. The authors considered the possibility of obtaining protective coatings based on aluminum oxide as the most refractory, mechanically strong, and chemically inert material on the surface of graphite of grades S-3 and OSCh-7-4. For this purpose, electrolytes based on aqueous solutions of sodium aluminate in a concentration range from 1 g/L to a limiting solubility of 13 g/L were prepared. The samples of graphite represented various types of complex figures with volumes of no more than 100 cm 3 . The treatment technique was described in detail in [27].
In an aqueous solution of sodium aluminate as a salt of a weak acid, the following complex anions are formed as a result of hydrolysis in an alkaline medium: The anodization process is accompanied by dehydration due to the release of Joule heat and the discharge of the complex anion on the anode with the formation of aluminum oxide, as follows: The MSO processing of graphite was carried out in two stages. The first stage was the formation of a barrier layer by anodizing graphite at a voltage of 100 V and a current density of 10 -2 A/cm 2 , followed by drying (aging) of the sample at a temperature of 60°C for 10 h. When the repeated anodization is carried out in the second stage, the necessary condition for MSO is achieved, namely, the anodizing current decreases with the lapse of time, and microarcing occurs when the voltage rises. At high voltages in the range from 450 V and higher, the reaction of carbon oxidation develops as a result of an increase in the temperature at the anode, and the cleavage of the oxide film is observed. This reaction was suppressed by the intensive cooling of the anode and limiting the anodizing voltage to 420 V. Optimal regimes for the formation of protective coatings are as follows: an anodizing voltage of 380-420 V and a current density of about 10 -1 A/cm 2 . As a result of the MSO processing of graphite, a white porous mechanically strong coating was obtained, the thickness of which was about 100 μm as measured from the chipped spot. According to the X-ray diffraction studies, the coating consisted of the alpha phase of aluminum oxide Al 2 О 3 .
As shown by metallographic and gravimetric studies carried out using a Q-1500D derivatograph and a NU-2E microscope, the coating preserved its protective and mechanical properties after the ten-hour cyclic isothermal annealing of samples at a temperature of 1000°C. Mass losses accompanied by an endothermic effect were observed at the beginning of annealing, which is apparently connected with the dehydration of the coating. Further annealing cycles did not lead to a change in the mass of the sample. Under these conditions, the coating protected graphite from oxidation [26]. Thus, if the MAO treatment of metals is characterized by the growth of the barrier film and a subsequent increase in the thickness of its own oxide, then the deposition of a preliminary oxide film "from the environment" is necessary in the case of graphite. These preliminary dielectric films can consist of either organic polymers or electrolyte elements.
In this connection, our attempt of the one-step formation of the MAO coating on carbon-graphite materials in silicate electrolytes on the basis of liquid glass deserves attention.
Samples with dimensions of 20 × 6 × 8 mm were made of graphite of the MPG-6 grade (fine-grained dense graphite), which has the following characteristics: -a density of not less than 1.65 g/cm 3 ; -a compressive strength of not less than 73.0 MPa; -an ultimate bending strength of not less than 34.3 MPa; -an electric resistivity of no more than 15 μΩ m; -an impurity content of no more than 0.02%. It was revealed that an increased total current density (up to 200 A/dm 2 ) should be maintained at the initial stage of processing (up to 5 min) to ensure the conditions for the implementation of the MAO process, otherwise the formation voltage does not reach the required values (not less than 150 V) (Fig. 9). To continue further, the process was carried out in the anode-cathode mode by using voltage pulses of a trapezoidal shape at the following processing parameters: the current density was controlled in the range 10-200 A/dm 2 ; the voltage value of the anode pulse front was controlled within 200-800 V with a step of 200 V; the ratio of the cathode and anode currents was controlled within the range 0.2-1 with a step of 0.2.
When the voltage of the pulse front is more than 800 V, the probability of the transition of the microarc discharge to the arc discharge is high and carrying out the MAO process in the manual mode is not practical. An increase in the voltage value of the anode pulse front in the entire range of the investigated values leads to a monotonic increase in the thickness of the coatings and to the through porosity (Fig. 10). In addition, there is an observable increase in the tendency to "freeze" electrolyte discharges upon an increase in the voltage of the front of pulses, which is manifested in the following trends: the discharges are most mobile at 200 V and are least mobile at 800 V. This leads to the appearance of burnt-out spots in the coating and, hence, to reducing the quality of the latter.
Carrying out the MAO process at different ratios of the cathode and anode currents has shown that an increase in the I C /I A ratio above 1 is impractical, since the discharge is quenched, and the quality of the MAO coatings is sharply reduced to nearly zero adhesion to the substrate material. A decrease in the ratio of the cathode and anode currents brings the MAO process closer to pure anodizing. However, when the I C /I A ratio is less than 0.4, a sharp increase in the porosity and the formation of particles weakly fixed on the outer surface of the coating is observed (Fig. 11). Most likely, this is due to the fact that the MAO process under these conditions is realized by alternating voltage pulses with "dips" between them-the duty cycle of pulses equals 2 (the process was carried out at a frequency of 50 Hz). Moreover, the discharge region is not heated to temperatures at which the formation of a dense oxide layer is possible. The appearance of the sample before and after the MAO processing is shown in Fig. 12.
The results of the MAO treatment of graphite MPG-6 in a silicate−alkaline electrolyte are shown in Fig. 13. It was found that the concentration of KOH (within the studied range from 2 to 6 g/L) has a significant effect on the parameters of the MAO process and the characteristics of coatings obtained on graphite.
An increase in the concentration of both the electrolyte components reduces the parameters of the MAO process (maximum duration) and the characteristics of the resulting MAO coatings (Fig. 13). Apparently, this is connected with an increase in the electrolyte conductivity. The observed power of electrolyte discharges on the sample surface and the voltage of the MAO processing indirectly confirm this conclusion. In all the cases, the current density was maintained in the range from 160 to 200 A/dm 2 at the initial stage of the processing (from 1 to 3 min) in order to create conditions for the ignition of the electrolyte discharge. The current density was reduced stepwise; the anode amplitude voltage, the limiting value of which was about 1000 V, served as a criterion. Exceeding this value led either to the transition from the microarc discharge to the arc discharge or the triggering of individual protection relays of the capacitors of the technological electrical current source. The discharge disruption is well detected visually and represents a transition from discharges migrating over the sample surface (the normal course of the MAO process) to solitary stationary discharges of high power (Fig. 14). The maximum duration of the MAO process was limited for the same reason.
On the whole, the obtained data made it possible to draw a conclusion about the negative effect of potassium hydroxide when it is present in the composition of the silicate-alkaline electrolyte. An increase in its concentration throughout the entire considered range leads to a decrease in the density of MAO coatings (the through porosity increases) and a decrease in the stability of the MAO process. All the above provides a foundation for carrying out the process of microarc oxidation of graphite in electrolytes on the basis of liquid glass that do not contain KOH.
MODIFICATION OF CARBON FIBERS Carbon materials are widely used in the form of fibers and fiber materials. This is connected with both the specifics of the production of such materials (graphitization of carbon-containing fibers and fabrics) and the need for their use in this particular form (catalytic and reinforcing components). In this case, the problems associated with the need to protect the fibers and with the implementation of the technology of their MAO processing not only persist, but also become more acute owing to an increase in the specific surface area of the processed material.
For the deposition of thin layers comprised of the refractory aluminum, zirconium, and silicon oxides onto carbon fibers and ribbons, it is worthwhile to use the sol-gel method [28].
In the last decade, methods for the formation of coatings on carbon materials by the electrolytic and electrophoretic deposition from solutions containing the components necessary to achieve the prespecified characteristics have been developed. In [29], the data on the electrophoretic and/or electrolytic deposition of aluminum, zirconium, and titanium oxides, hydroxyapatite, and chemically bound ceramics of the CaO-SiO 2 -P 2 O 5 system on carbon fibers that serve as a cathode substrate are given. The authors of [30] describe the cathodic electrophoretic and electrolytic deposition of ZrO 2 and CeO 2 coatings on the substrates composed of stainless steel and graphite.
A detailed study of the electrochemical processes taking place in the treatment of carbon materials has been performed by researchers of the Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences [31][32][33][34][35][36][37][38]. The metal oxide/carbon fiber composite materials were obtained by the method of electrochemical formation of metal oxide in pores of the carbon matrix. The deposition of titanium oxide was carried out using the method of electrodeposition by charging the graphite electrode to specified potentials separated from the immersion potentials by values of -0.1 to -0.9 V and exposing to the attained potential for 1-4 h [32]. Microphotographs of the activated carbon fibers obtained in this way are shown in Fig. 15. They do not show appreciable aggregation of the particles modifying the carbon fiber, even though the amount of precipitated titanium oxide is not less than 12%. This implies a high degree of dispersion of titanium oxide on the carbon fiber surface.
The authors of [31][32][33][34][35][36][37][38] believe that the electrochemical deposition produces a more uniform coating over the entire fiber surface (in contrast to the chemical modification), and the process makes it possible to obtain a modified material with improved properties and controllable morphology and microstructure, since the appearance of the modified surface makes it possible to draw a conclusion about the mesoporous structure of the composite.
The results of studies of the electrochemical deposition of manganese [33] and nickel [34] oxides are shown Figs. 16 and 17, respectively.
The experimental results on the development of chitosan/carbon fiber composites and functional materials on their basis are given in [35][36][37][38]. The natural polysaccharide chitosan, which is a product of the deacetylation of chitin, has unique properties (biocompatibility, biodegradability, nontoxicity, bactericidal activity, etc.). This attracts close attention to its production and practical applications.
The formation of a chitosan film by the electrochemical method depends on the potential of the carbon fiber, and the deposition process is associated with the effect of pH on the solubility of chitosan (Fig. 18). The process was carried out under the conditions of both the cathodic and anodic polarization of the fiber. When polarized into the cathodic region, chitosan precipitates on the surface in an insoluble form, whereas the reprecipitation in the anodic region occurs in the salt form.
An analysis of the microphotographs of the carbon fiber surfaces allowed the authors to estimate the size of chitosan particles embedded in pores of the carbon fiber at the potential of an open circuit and deposited in the form of a film during the anodic polarization in the range from 500 to 2000 nm (Figs. 19b and 19c). The initial surface (Fig. 19a) of the chitosan/carbon material and the surface formed in the region of the anode potentials (Fig. 19c) have a widely developed system of pores with sizes in the range from 100 to 600 nm, in contrast to the surface modified in the absence of external polarization (Fig. 19b). When modified under the conditions of cathodic polarization, the deposited insoluble chitosan film is continuous and homogeneous, and covers the fiber, completely closing the pores of the initial fiber (Fig. 19d).

SPRAYING OF ALUMINUM ON GRAPHITE WITH SUBSEQUENT MAO TREATMENT
To protect the surface of carbon-containing items, it is also possible to use a combination of the two technologies: electric arc metallization with aluminum and the subsequent MAO treatment [39]. Electric arc metallization is one of the most productive and lowcost technologies for the production of coatings. It is implemented by melting the material with an arc and then by flame spraying. The use of aluminum wire for the surface metallization of the item ensures the formation of a coating with a high plasticity and a relatively low porosity. The aluminized surface can be protected by a heat-resistant layer of aluminum oxide obtained as a result of the anodic-spark processes in aqueous electrolytes.
In the described work, samples of electrode graphite in the form of a cylinder (tube) with an external diameter of 5 cm, an inner diameter of 3 cm, and a length of about 65 cm served as a substrate. To apply a layer of commercially pure aluminum on their surface, the gas−thermal spraying with the use of the electric arc sputtering of the aluminum wire was used. This technology provided a sufficiently dense metallized coating with a porosity of no more than 4%. The coating pores are predominantly closed and can affect the subsequent oxidation process.  The metallized sample surface was subjected to microarc oxidation by using a device, whose scheme is given in Fig. 20.
The device structure includes frame 1, protective casing 2, tank 3 with electrolyte 4, anode-item 5 (tube), feeding mechanism 6, and closed annular cathodes 7. The electrolyte is supplied from the tank by pump 8 to cavities 9 of cathodes 10, and then to the annular gap between the surfaces of the anode-item and the cathodes. The electrolyte flowing through the annular gap prevents the overheating of the anodeitem and enables the operability at high current densities. The mechanism for feeding the tube is simultaneously a current collector providing the current supply to the aluminized surface of the tube. The electric potential is applied from the special transformer through the passages in the protective casing.  Fig. 18. Scheme of the preparation of chitosan/carbon materials by using carbon fiber as an electrode [38]. The device is operated as follows. The supply of the electrolyte to cathodes 7 is switched on, and then item 5 is moved by means of mechanism 6 to the cathodes. At the same time, an electric potential is applied to all the cathodes. The potential can consecutively increase from one cathode to another cathode along the item from 150 to 380 V. This ensures a smooth increase in the thickness of the oxide layer on the surface of the metallized item. The device allows one to carry out the oxidation with interelectrode gaps of 20-30 mm, which reduces the electrical energy losses and provides the formation of an oxide layer up to 150 μm in thickness at a voltage of about 340 V. The optimal temperature of the electrolyte for the implementation of the technology is 50-60°C, which is provided by using the electric energy losses in the interelectrode gap of the electrolytic cell. The dimensions of the parts can exceed 1 m and be of different thicknesses.
An aqueous solution of KOH was used as an electrolyte. The treatment was carried out for 15 min with a smooth increase in the electric potential in the interelectrode gap from 150 to 300 V. The current density in the steady-state oxidation regime was 0.1-0.5 A/cm 2 . The technological equipment ensured the oxidation only of the surface that was washed with the electrolyte, which made it possible to create a protective coating on the local spots of the item.
A metallographic analysis of the samples showed that the obtained oxide layer (more than 120 μm in  thickness) is denser near the aluminum substrate (Fig. 21). It has a high porosity on the surface and consists of a number of fused areas in the form of microcraters and teardrop-like traces of the oxide layer melting. An analysis of the results of the study shows that the microarc process also takes place in pores of the aluminized layer. There are traces of localization of microarcs in the form of melted craters in these pores. Thus, it can be stated that the oxide layer is formed on the entire surface of the sprayed aluminum layer that was in contact with the electrolyte, producing a developed surface inside the aluminized layer.
The X-ray diffraction phase analysis of the coating revealed that the latter mainly consists of the refractory phase of aluminum oxide (α-Al 2 O 3 ) and metallic aluminum. The high-temperature form of the oxide is produced at the oxide-metal interface. The energy spectra (Fig. 22) obtained by the method of elastic resonance on protons in the studies of the Al 2 O 3 coating formed on the surface of metallized graphite show that the coatings contain, in addition to the main element (aluminum), traces of metals that are contained in the electrolyte (potassium) and the counterelectrodecathode (iron).
The density of the upper layer of the coating was 3.74 g/cm 3 , which is close in value to the density of α-Al 2 O 3 (ρ = 3.98 g/cm 3 ). The adhesion of the coating comprised of the oxidized aluminum layer (determined by the method of scribing with a diamond pyramid) was 42 to 67 MPa, and the hardness varied in the range from 1.36 × 10 4 to 1.72 × 10 4 N/mm 2 at different coating sites.
A version of the technology involving the deposition of an aluminum layer with the subsequent MAO treatment is patented in [40]. The method of preparation of protective coatings on the surface of items made of carbon materials includes the microarc anodization in an electrolyte solution at a voltage of 450-550 V and differs in that the aluminum layers, which are successively applied to the item by means of an EM-17 electrometallizing apparatus, are treated. The first layer is 40-50% of the total thickness of the coating and the subsequent layers 50-100% of the thickness of the first layer; before applying an aluminum layer, 5-50% of the anodized surface is mechanically removed with brushes immediately after anodizing or no later than 3-5 days. The microarc anodization is carried out in a solution containing 20-30 g/L of liquid glass and 2-4 g/L of sodium hydroxide under the potentiostatic conditions at a voltage of 450-500 V and a constant electric power maintained by adjusting the growth rate of the anodized surface area, and then in the self-regulating mode with an electric power decreasing to a current density of 1-3 A/dm 2 and a voltage of 500-550 V. The coating is formed until reaching a total thickness value sufficient to effectively exploit the item (200-600 μm). The growth rate of the anodized surface area is controlled by raising the level of the electrolyte in the anodizing bath. The process parameters are given (for the processing of a sample with a surface area of 1 dm 2 in an electrolyte containing 30 g/L of liquid glass and 2 g/L of sodium hydroxide). The dependence of the increase in the ceramic layer thickness on the anodizing time (the electrolyte composition comprises 30 g/L of liquid glass and 2 g/L of sodium hydroxide; the sample surface area is 1 dm 2 ) is shown in Fig. 23. The dependence of the electric power consumption on the thickness of the produced layer (the electrolyte composition comprises 30 g/L of liquid glass and 2 g/L of sodium hydroxide; the sample surface area is 1 dm 2 ) is shown in Fig. 24.

CONCLUSIONS
As a result of the analysis of the literature data and our own data, it can be concluded that the technology of microarc oxidation is feasible and promising for effective protection of the surface of carbon-graphite materials from oxidation, first and foremost, hightemperature oxidation. Under certain conditions listed in the review, the formation of coatings on graphite occurs in accordance with trends that are similar to the formation of oxide layers on valve met-als. The engineering of the carbon material surfaces by means of the MAO processing makes it possible to obtain an actually new class of composite materials with a wide range of controllable properties and specified performance characteristics, which creates the prerequisites for a wider use of carbon graphite in many fields of science and technology.
It should also be noted that the extrapolation of the above approaches and principles can promote the extension of the MAO processing to other conductive materials, metals, and alloys that are not classified as valve metals, such as nickel, chromium, steel alloys of different grades, etc. [13,19,26,30]. The advancement of research and developments in this area can substantially expand the application scope of the MAO modification for processing new types of materials and give impetus to scientific and technological progress in this direction.  Electrical energy consumption, kW h