Powder Metallurgy and Metal Ceramics, Vol. 39, Nos. 9-10, 2000

PROTECTIVE COATINGS ON HEAT-RESISTANT NICKEL ALLOYS (REVIEW) I. A. Podchernyaeva, A. D. Panasyuk, M. A. Teplenko, and V. I. Podol’skii UDC 621.762 The literature on materials for protective coatings on heat-resistant nickel alloys and methods for their production is reviewed in order to generalize the results and determine the principal directions for solution of the problem under consideration. It is shown that a promising approach is the development of layered composite ceramic coatings which, thanks to a graded variation of properties, are able to provide an optimal combination of adherence, mechanical strength, and corrosion and heat resistance. The methods of physical vapor deposition, plasma spraying, and electron-beam deposition remain the principal industrial processes for coating deposition. Keywords: heat-resistant coatings, nickel alloys, aluminides, ceramic layers, deposition methods, gas corrosion. Nickel and cobalt heat-resistance alloys (HRA) are widely used in rocket, space, and aviation technology, principally as materials for the hot sections of gas turbines (GT) such as the combustion chamber, and turbine vanes and blades. The most effective way to increase the efficiency of a gas turbine is to increase the turbine entrance temperature of the gas. The permissible temperature for reliable and sustained operation of stationary turbines of substantial power is 1000-1100°C [1], and this tends to steadily increase. Increased working temperatures cause rapid oxidation of HRA. This gives rise to the problem of protecting the critical parts of GT from oxidation and corrosion by the deposition of protective coatings on their surfaces. The necessity for such coatings appeared in the 1950’s in the manufacture of aircraft engines, when it became obvious that the material compositions required to improve high-temperature strength were incompatible with those needed to attain an optimal degree of protection against attack by the high-temperature aggressive surrounding atmosphere. Among the many requirements imposed on the high-temperature sections of GT, resistance to abrasive wear and multicyclic fatigue and corrosion and a suitable level of mechanical properties stand out. Analysis of the best domestic and foreign HRA indicates that no material yet exists which can guarantee the reliable operation of GT blades for long times at high stresses and temperatures. For this reason a continual search goes on for new coating compositions which are resistant to prolonged high-temperature oxidation and sulfide-vanadium corrosion. It should be noted that in the industrially advanced countries high priority is given to the development of new coating materials and methods for depositing these on HRA. In the present work information in the literature on materials and methods for the deposition of protective coatings on HRA is analyzed. The objective of this study is to generalize the results and determine the most promising approaches to solution of the problem under consideration. Methods of Coating Deposition Methods for the deposition of protective coatings on HRA can be separated into two basic groups [2]: thermal diffusion, based on processes leading to a change in the composition and structure of the surface layer of the HRA as a result of its contact and reaction with alloying chemical elements; and non-diffusional, based on processes in which an external (overlay) coating is deposited on the surface with little interdiffusion of elements  only that necessary to guarantee adherence. Thermal diffusion processes include surface saturation from a liquid (slag method) or solid (powder) phase. The method of saturation from a powder (pack cementation) is widely used at the present time. The latest development in this field Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 9-10(415), pp. 12-27, September-October, 2000. Original article submitted May 11, 1999.

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1068-1302/00/0910-0434$25.00

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2001 Plenum Publishing Corporation

is the use, in certain cases, of chemical deposition from a vapor phase (CVD). An analysis of the various deposition methods is given in [3]. The process which has obtained widest application in the manufacture of aircraft gas turbines is diffusional saturation of the surface of nickel and cobalt alloys with aluminum. For nickel-based alloys formation of the phases Ni3Al, NiAl, and Ni2Al3 is of interest, and for cobalt and iron alloys the formation of CoAl and FeAl2. When these intermetallics are oxidized, high-temperature corrosion-resistant aluminates form on the surface which allow operating temperatures to be increased to 1800°C. Pack cementation is a modification of the vapor deposition method in which the component to be treated is placed in a container with a reactive powder mixture which produces vapor of the desired composition. The mixture (“pack”) includes Al or other elements (for example Cr, Si) halides which serve as chemical activators, and an inert filler such as Al2O3. Upon heating in an inert atmosphere the metallic powder (pack) reacts with the activator to form vapors which interact with surface of the treated part and enrich it in aluminum. The reaction is controlled by pack composition and temperature; coating morphology is determined by time at the given temperature, and subsequent heat treatment of the coated part. Coatings of TiN, CrN, TiCN, and AlTiN can be produced by the method of physical vapor deposition (PVD) [4]; nitride coatings with hardnesses up to 800 HV have been obtained [5]. In the chemical vapor deposition (CVD) method the vapors of a given composition, prepared at an independent stage of the process, are introduced into a chamber containing the specimens and react with their surfaces. The principal advantage of this method is that it allows internal surfaces to be coated. The vapors can be drawn through internal channels by suction and homogeneous coatings of good quality can be obtained even when their geometries are very complex. Another advantage of the CVD method is flexibility in obtaining required vapor-phase compositions. Both methods (pack cementation and chemical vapor deposition) are used for the deposition of not only aluminum, but also other metals such as chromium, silicon, and titanium. Non-diffusional methods for the deposition of external (overlay) coatings are distinguished from diffusional in that they do not require the formation of a diffusion zone at the interface with the substrate in order to obtain a coating with the desired composition and structure. An adherent layer of material with the composition required to form a protective oxide film is deposited on the HRA. The coating is deposited by any method in which interdiffusion occurs only to the degree necessary for adherence to the substrate. At the present time vacuum methods (electron beam vaporization) and gas-thermal spraying [6] are the most widely used of these methods for the deposition of coatings. Vacuum processes for coating deposition stimulated by a plasma which is a source of ionized high-energy particles, have been extensively developed. Activated radioactive vaporization, ionic plating, beam deposition of ionized clusters, electric-arc vaporization, and deposition with ion bombardment belong to this group of processes [7]. In the method of electron beam physical vapor deposition (EBPVD) a target of defined composition is bombarded by a focussed electron beam in vacuum. The treated parts are situated in the cloud of metal vapors which condense on the preheated substrate. According to the Semens company, no less than a two-fold increase in the lifetime of protective coatings on blades operating in GT at 1350-1500°C can be obtained by the use of this method, compared with those obtained by spray deposition. The method provides close control of coating thickness [8]. The EBPVD process usually leads to a coating structure which is oriented perpendicular to the substrate surface. In this case adjacent columnar crystals in the deposited layer are frequently separated by voids. Further heat treatment of the surface is used to heal these defects and assure good coatingsubstrate adherence. Gas-thermal* methods of coating deposition include plasma, high-speed flame, and detonation spraying. The plasma spraying process consists in injection of the coating material, usually in the form of powder of given composition, into a hightemperature gas plasma stream formed in a plasma gun. The powder particles melt in this stream and are propelled toward the substrate. The drops of molten metal deform upon impact with the substrate and flow over the surface to form a layered structure (Fig. 1). A new development in the plasma spraying method is the use of low-pressure vacuum chambers [9, 10]. For many recent coatings which contain chemically active elments such as aluminum and chromium (for example MeCrAlY coatings), low-pressure plasma spraying minimizes the formation of oxide defects in the freshly deposited coating. Higher particle velocities are another advantage of this process.

*Borisov [7] has made considerable contribution in the technology designing for gas-thermal deposition of coatings in Ukraine.

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Fig. 1. Microstructure of a TiCN − NiCrBSi coating obtained by the method of high-velocity gas-flame spraying.

A variant of the vacuum-plasma method is the intensified plasma process in which coating deposition is carried out in an evacuated chamber with the use of three independently controlled energy sources. After bombardment with aluminum and silicon nitrides a dense, highly adherent coating with good structure and improved service properties was formed on the surface of the component [11]. The adaptability of the vacuum-plasma method to the production of complex alloyed coatings, and also the high ductility, structural stability, and absence of porosity in the coatings, distinguish this from other methods [12]. The high adherence of the coating to the substrate when this is ionically cleaned before deposition [9], or when the coating is laser treated afterward [13], should also be mentioned. Laser melting of gas-thermal coatings leads to higher adherence and lower porosity which retard the diffusion of oxygen into the coating, and, therefore, increase its heat resistance. A shortcoming of the laser process is increased roughness of the coating surface: at a thickness of 50 µm or higher Ra ≥ 1.25-2.5 µm [9]. This is connected with the presence of microdrops in the coating material plasma. The principal disadvantage of gas-thermal spraying and vacuum processes compared to diffusional methods is that they cannot be used to coat the internal surfaces of cooled blades. Not one of the known non-diffusional methods is able to deposit a protective coating in the internal channels of a component. To obtain complete protection use is made of hybrid coatings consisting of overlays on the external surface of the component, and aluminides deposited from a vapor phase on the internal. Such coatings consist of two or more layers of different compositions deposited by the same, or different, methods. Their use avoids complications related to undesired coating − substrate interdiffusion, and thus overcomes limitations on the use of overlay coatings. For example, the resistance of CoCrAlY coatings to hot corrosion can be increased by enriching only the surface layers with silicon. This excludes the mutual diffusion of silicon from the coating and nickel from the substrate [2]. Frequently the structure of turbine blades is such that the working temperature of different portions are substantially different, and they are subject to corrosional attack by dissimilar mechanisms. The use of hybrid coatings with layers of differing composition can provide protection from different types of corrosion. Other non-diffusional methods of coating deposition are ion implantation [14], vacuum-arc spraying [9], and gasflame metallization [15]. Ion implantation has the advantage that it can alloy a surface with practically any elements. By varying the regime of ion implantation it is possible to vary the state of the surface layer within broad limits. With the method of gas-flame metallization, described in [15], the metallized layer can be simultaneously fused with the substrate. For this type of metallization it is useful to use so-called self-fluxing alloys based on Ni(Cr)BSi, in which cobalt and (or) iron can substitute for nickel; these alloys have relatively low working temperatures (1030-1120°C), and when deposited have a flux-like deoxidising effect which aids in the formation of a good bond with the substrate. In [16] a method of generating coating material is considered, in which a vacuum arc is created in the vapors of a consumable cathode. The degree of ionization of the plasma reaches tens of percents. Electrostatic acceleration of the ions to several tens of kiloelectron-volts for substrate cleaning, and several tens of electron-volts for condensation, produces highly adherent coatings, eliminating the need for additional diffusion annealing such as used in electron-beam or magnetron methods. Reaction sintering in a pack of controlled composition has been developed as an inexpensive alternative to the deposition of overlay coatings of the MeCrAlY type [17]. Reaction-sintered coatings are obtained by a slip-heating method: 436

first, the components (MeCrY) are sprayed on the part to be treated in the form of a slip, and then the part is placed in an aluminum-containing pack of suitable activity where a controlled reaction with aluminum takes place. The principal steps of this process are: formation of oxides; production of a borosilicate glass phase; liquid-phase sintering. A coating of the NiCoCrAlY-type is deposited by this method (the particles have a γ-phase matrix and also contain a small amount of ε-Co and γ′ [18], as well as NiCrSi). Another technique for the non-diffusional deposition of protective coatings is cladding [2], which requires the preparation of an alloy of the needed composition in the form of a thin sheet of prescribed thickness, and then diffusional welding this to the substrate surface under high temperature and pressure. This method is very promising since any combination of materials can be obtained in the layers, and it is less expensive. However, the difficulty of fabricating thin sheets of corrosion-resistant alloys with low ductility places limits on its wide application. Electron-beam, plasma, and cathodic spraying are the latest and most promising methods for coating gas-turbine blades [2], but the thermal-diffusion method has not been completely abandoned (in particular, the deposition of coatings in the internal cooling channels of blades is possible only by the circulation method). Protective Coating Materials In addition to austenitic steels, heat-resistant (age-hardening-nickel) alloys  Nimonics  are widely used for GT components. The basic “classical” Nimonic is a quaternary Ni − Cr − Ti − Al alloy containing approximately 20% Cr, 1% Al, 2% Ti, remainder Ni [19]. Quenching from 1050-1150°C leads to the formation of γ-solid solution with a FCC lattice (a = 0.357 nm). Contemporary Ni-base heat-resistant alloys are complex multicomponent heterophase systems. The most widely used method for their preparation is casting in a ceramic mold in a vacuum induction furnace. Temperature-time treatment of the melt, including that carried out with the aid of concentrated energy fluxes [20], is used to decrease chemical microinhomogeneity and suppress alloying element segregation in the castings. Directional solidification is used to obtain traditional HRA castings with a columnar structure in which there is few transverse grain boundaries. In this case the weak structural elements (grain boundaries) are directed parallel to the axial stresses developed in the turbine blades during use. The operating temperatures of directionally solidified alloys are approximately 20-30°C higher than those of alloys with an equiaxed structure. Directionally solidified alloys (ZhS6F, ZhS30, ZhS26, ZhS26Y) correspond in properties to the similar foreign alloys MAPM-200, CM-247*, and others [21]. Substantial progress in elevating the heat resistance of directionally solidified alloys has been obtained by adding refractory metals  tantalum and rhenium. Thus, for example, the working temperature of alloy ZhS32 containing 4% Ta and 4% Re is 40°C higher than that of the similar alloy ZhS26 without these elements [21]. The addition of refractory compounds together with metals in order to increase the corrosion resistance of HRA is of great interest. Heat-resistant alloys display an increased tendency to fracture over practically the entire range of temperatures used for heat treatment, welding, and fusion. This is caused by the presence of eutectic, inclusions of brittle and low-melting phases in intergranular spaces and dendrite axes, and also by melting at structural element interfaces (grains, dendrites, crystallites). Therefore, in the processes of welding, soldering, and spraying it is necessary to limit the maximum temperature to which components are heated, taking into account the critical state of the alloy structure. For Ni-base HRA this temperature should not exceed 1240-1260°C [22]. It should be noted that directional solidification does not guarantee the attainment of strictly oriented individual crystals. Furthermore, its use does not completely exclude the formation of transverse grain boundaries in a generally longitudinally oriented structure. This manifests itself as a relative large dispersion of mechanical properties. Further directions for improving the service properties of GT are the use of monocrystalline blade castings [23], and also of new classes of superalloys strengthened with fibers, dispersed oxides, etc. [2]. The limiting temperature for the use of traditional Ni-, Co-, and Fe-based superalloys in high-temperature components is 1050-1200°C. Further progress is connected with the use of more heat-resistant materials  alloys based on Nb, Mo, Ta, W, etc., carbon-carbon composites, and high-temperature ceramics. Nickel-base HRAs contain scarce elements (Ni, Co, Mo, W, etc.) and are expensive [24]. Therefore, it is important to make use of various types of HRA scrap, including GT components discarded after their service life in aviation or space technology has been exhausted. Ceramic materials

*The heat-resistant nickel alloys IN-738, X-40, 23CMSX-3, Rene 80, Rene 125, MAPM-200, and CM-247 are called “superalloys” in the foreign literature.

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Fig. 2. Kinetics of oxidation in air at 1050°C of a nickel-base alloy (1) without a coating, and (2) with an FeAl coating.

possess low thermal conductivity and are 2-3 times lighter than HRAs and steels. However they tend to crack under impact loading, vibration, abrupt temperature changes, and, therefore, are primarily used for immobile components subjected to mainly static loads, for example nozzles. High-temperature coatings for HRA may be defined as metallic, ceramic, or combinations of layers able to prevent or retard the direct reaction of the substrate with a potentially aggressive surrounding medium. An aggressive gas phase decreases the strength and ductility of an alloy as a result of oxidation and corrosion: formation of a low-ductility heterophase oxide film in the initial stages; weakening of grain boundaries with the formation of stress concentrations which facilitate grain-boundary fracture; spalling and removal of oxides by the gas stream. Coatings acquire their protective property by reaction of the material with oxygen in the surrounding medium and the formation of a dense oxide film (scale) which is very adherent to the protected surface, and retards the diffusion of undesired elements such as O, N, and S to the substrate. It follows that coatings must have a high concentration of elements such as Al, Cr, and Si which tend to form a protective scale. The efficacy of aluminide-type thermal diffusion coatings under the action of an aggressive gas atmosphere at high temperatures is determined by the properties of the formed oxide film. The scale which forms on the surface of an unprotected Ni alloy consists of NiO, Cr2O3, TiO2, and the spinel NiCr2O4, and spalls off at high temperatures. When coated, a protective film of oxidation products of the systems Ni − Al, Ni − Al − Cr, Ni − Al − Si, and Ni − Al − B is formed, namely α-Al2O3, NiAl2O4, and NiCr2O4 [25]. Deterioration of the protective properties of a coating in use is produced by coating-substrate interaction, and Al(Cr) impoverishment by the formation (and spalling) of the oxide scale. The service properties of aluminide coating can be improved by alloying with Hf, Y, La, Ce, W, Si, and other elements [26] which retard thermal diffusion and stabilize the oxide film [27]. The chemical potential of elements in the coating and substrate should differ to only a slight extent; otherwise, excess phases, often with unfavorable morphologies, form under the coating and act as stress concentrators and crack initiators. For example, carbides and other compounds which impair the service properties of GT blades form rapidly in the diffusion zone at the interface between a heat-resistant alloy and heat-resistant Ni − Cr − Al − Y coating. The addition of Ta and Y retards diffusion processes and the reaction of elements in the diffusion zone [28]. Powders of specially melted commercial Fe − Al − Si and Fe − Al − Si − REM ferroalloys are promising for the formation of heat-resistant diffusional protective coatings on the blades of stationary gas turbines [29]. A 60 µm-thick coating of finely dispersed NiAl β-phase with a hardness of 750-800 HV is formed by the process of thermodiffusional aluminosiliciding. Figure 2 shows the test results for specimens of Ni-base alloy ChS-70VI coated with the aluminide FeAl [25]. The formation of an ultrafine grain structure is of great importance in heat-resistant coatings for the hot-section components of GT, since it is highly ductile and stable for long times at high temperatures. Among Ni − Cr − Al − Y coatings such a structure is obtained in alloy ZhC6Y by the vacuum-arc method of coating deposition [9]. The average grain size of the coating (0.25 µm in the initial state) increases after annealing in air at 1000°C for 100 h by approximately an order of magnitude, but nevertheless remains small. The stability of the ultrafine structure at high temperatures is connected with the fact that it can recrystallize only by coalescence. Overlay and diffusional aluminide coatings oxidize by basically the same processes. According to the oxidation test results in Table 1, developed NiCoCrAlY coatings are superior to diffusional aluminum. 438

TABLE 1. Oxidation Resistance of Coatings on a Ni-base Alloy Substrate [2] Coating Aluminide Platinum-aluminide NiCoCrAlY

Lifetime* at 1190°C, h 100 250 >1000

*Gas velocity 1.0 M, air atmosphere, one cycle per hour; the lifetime was determined visually and metallographically by the depth of coating deterioration.

In addition to their excellent oxidation resistance a great advantage of MeCrAlY coatings over diffusional aluminide is their higher melting points, practically independent of the composition and properties of the substrate. Melting in the diffusion zone at a lower temperature than in the coating itself does not occur. While the initial melting temperature of most diffusional aluminide coatings is 1121-1204°C, overlay coatings can be heated to 1288°C without any sign of melting. However, the high melting temperature of overlay coatings is achieved at the price of very poor high-temperature strength, which can lead to cracking by thermal fatigue during cyclic operation. New developments in overlay coatings consist mainly in investigations of additions to improve oxidation resistance. Modified NiCoCrAlY coatings containing, for example, silicon, tantalum, and (or) hafnium possess increased oxidation resistance but, as a rule, decreased ductility. Increased gas temperatures and the use of more aggressive fuels in GT have resulted in a decrease in the protective life of single-layer multicomponent Me − Cr − Al − Y coatings to very short times (300-6000 h). The need to create new protective coatings resistant to high-temperature gas corrosion and abrasive wear has led to another method for improving the service properties of aluminide coatings  the use of graded coatings, consisting of a metallic thermal diffusion layer of the Me − Cr − Al − Y type, and a chemically inert external ceramic layer with low thermal conductivity. The external layer acts as a thermal barrier which protects the base metal from overheating. Zirconium dioxide ZrO2 is most often used for this purpose [30, 31]. The thermal conductivity of zirconium dioxide is very low (2.9 W/m⋅K) and the thermal expansion coefficient quite high (for a ceramics) (10.5⋅10−6 K−1) [32]. However, ZrO2 undergoes a transformation from a monoclinic to a tetragonal structure beginning at 1170°C, accompanied by volume changes which can lead to spalling of the ceramics [2]. Such changes can be excluded by additions of Mg, Sm, CaO, Y2O3, and oxides of rare-earth elements which lower the transformation temperature to room temperature, and stabilize the tetragonal phase. The improvement in service properties obtained by the deposition of an external layer of ZrO2 on an aluminide coating is shown in [30]. The coating was produced by the circulating gas method from a mixture of η-Fe2Al5, α-Zr, and α-Al2O3 powders with additions of Mg, Sm, and Y. It consisted of an outer layer of tetragonal ZrO2, ∼12 µm thick, below which appeared β-phase (Ni, Co)Al) alloyed with chromium, a two-layer diffusion zone, and the substrate alloy. An important feature of this coating was good adherence of the ZrO2 to the β-(Ni, Co)Al phase. However, thermal cycling (to water from 1050°C) produced intense crack formation in the ceramic layer, although the layer was retained on the metallic substrate. The positive effect of zirconium dioxide on the corrosion resistance of a laser-alloyed diffusional aluminide coating was established in [33]. A 50-60 µm thick coating was deposited on a nickel-base heat-resistant alloy in a mixture of Fe − Al − Si − REM powders contained in a liquid-sealed chamber. Next, the surface layer was alloyed by laser melting a slip on the coating, made up of an organic binder with dispersed Al, Si, and ZrO2 powders. A two-layer laser-realloyed zone (ZrO2 ceramic external layer 10-15 µm thick, two-phase β-NiAl + γ-Ni metallic layer) containing an elevated concentration of refractory-metal silicides was formed. Tests showed that laser alloying of the aluminide coating with silicon and zirconium dioxide substantially improved its heat and corrosion resistance thanks to the formation of refractory silicides, and to the good protective properties of the ZrO2 layer which was inert to oxygen and other corrosive elements. A disadvantage of external ZrO2 layers is their low heat resistance.

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Fig. 3. Dependence of specimen lifetime on ceramic layer thickness.

A review of diffusional aluminide coatings is presented in [2]. It is noted that the use of platinum markedly improves the hot-corrosion resistance of aluminide coatings. Other aluminide coatings have been developed in which platinum is replaced by less expensive metals such as palladium or rhodium, but these are less promising than coatings with platinum. Nevertheless, taking cost into account, they, and also various silicides and binary chrom-aluminum diffusional coatings, appear to be more attractive for use in less severe corrosive media. It has been shown that it is possible to extensively vary the properties of HRA with plasma-diffusional coatings [34]. The elevated heat resistance of diffusion coatings containing an external layer of Cr2N is noted, due to the high thermal conductivity, low coefficient of thermal expansion, and low brittleness of this material. Thus, the deposition of graded aluminide coatings with an external ceramic layer is an effective method to improve the service properties of HRA. However, the lack of fundamental research on corrosion mechanisms in these complex systems, and the insufficiency of data on the properties of diffusional coatings with a ceramic external layer hinder the development of this approach. The most frequently used ceramic materials for protective high-temperature coatings are oxides, above all zirconium dioxide. In the low-pressure plasma spraying of ZrO2 on HRA (thickness 40 µm) use is made of a sublayer of NiCrAl alloy (14 mass% Cr, 14 Al, 0.1 Zr, remainder Ni) and an addition of Y2O3 (5-10%) for increased heat resistance [12]. Laser alloying of HRA with particles of Y2O3 (0.5-2%) leads to increased oxidation resistance due to the formation of a thin, strongly attached scale consisting of an outer layer of Cr2O3, α-Al2O3, NiCr2O4, and CoCr2O4, and an inner layer of Al2O3 [35]. A protective coating formed by the electron-beam evaporation and condensation of the ceramics ZrO2⋅8Y2O3 has also been used [36]. The addition of dispersed particles of CeO2 instead of Y2O3 to ZrO2 increases the corrosion resistance of ceramic coatings designated for blade protection in gas turbines using high-sulfur fuel. The positive effect of CeO2 essentially consists in retarding the slagging-off process which occurs on the surface of a ZrO 2 ceramics in a medium containing Na2SO4 [31]. One of the methods to increase the heat and corrosion resistance of ZrO2-based coatings is the application of binary Al2O3 + ZrO2 systems or layered coatings of the type: sublayer NiAl, second layer NiAl − ZrO2 − MgO, external layer ZrO2 − MgO [32]. The thickness of a ZrO2 ceramics layer affects its service properties (Fig. 3). Its durability is 40% higher at the optimal thickness (from 25 to 50-60 µm); if the thickness is greater than 60 µm the coating spalls [37]. The systems SiO2 − Al2O3 [38] and TiO2 − Al2O3 [39] are particularly interesting for the deposition of protective coatings since it is possible during their formation, heat-treatment, and oxidation to obtain the high-temperature corrosionresistant phases mullite and β-tialite, respectively. From this point of view the combination of a high-temperature process for coating formation with a specific chemical transformation can be considered as a method for the synthesis of new coating materials. In spite of the fact that oxides are highly stable in oxidizing media and possess low thermal conductivity and high coefficients of thermal expansion, preference has recently been given to carbonitride ceramics because of their superior

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mechanical properties. Attention has been attracted to nitride coatings of the types Ti − BN [40], TiN [41], Si3N4 [2, 42, 43], and AlN [43]. Powders with particle sizes 40-80 µm are used for the plasma spraying of Ti − BN coatings [40]. Particles of size <20 µm consist of titanium nitride, and several phases  titanium, boron nitride, titanium borides and nitride  are present in larger particles (up to 80 µm). The sprayed coatings have a layered structure and consist mainly of two phases  an αsolid solution of nitrogen and boron in titanium (soft matrix hardened with titanium nitrides and borides), and titanium diboride (with inclusions of residual boron nitride). The oxidation of BN begins at temperatures above 700°C [42] and leads to the formation of B2O3. At temperatures ≈1200°C (for graphite-like BN) B2O3 evaporates, which leads to an impairment of the corrosion resistance of boron nitride and makes it possible for oxygen to penetrate into the metallic substrate. The addition of 5-30% SiO2 improves the oxidation resistance of materials based on boron nitride [42]. Gas turbine blades are often subject to gas-abrasive erosion by solids in the combustion products of the fuel. Multilayer TiC − TiN coatings deposited by the vacuum-plasma method can protect the blade surface from high-temperature gas-abrasive wear [41]; the wear resistance of such coatings increases with increase in the number of layers [41, 44]. Tests of heat-resistance showed that multilayer graded coatings whose compositions varied from Ti to TiN displayed the highest number of cycles to destruction (>25) while one- and two-layer coatings, and also plasma-sprayed ZrO2, Al2O3, and Cr2O3 coatings tested for comparison, withstood only ≤8 cycles [45]. Four- and six-layer TiC − TiN coatings are the most wear resistant [41]. Increase of wear resistance with the number of microlayers is due to layer by layer coating deterioration, and the presence of different textures. It is also known that scandium additions increase the microhardness of TiC − TiN coatings from 22-32 to 45 GPa [46]. Coatings of two successively deposited TiAlN layers have been proposed, and the total amount of Ti, Al, and N in each layer is 50-100 at.%, but the second layer contains less titanium and more aluminum than the first [47]. Chromium carbide coatings deposited on alloys ZhS6U and ZhS-26 by laser gas-powder fusion are comparable to heat resistant alloys in wear resistance at 1000°C. At 1200°C the coatings retain their superior properties, while cast alloys can not be used under these conditions [48]. The superior mechanical properties of silicon nitride Si3N4 make it a promising composite-base material for the protection of gas turbine blades. The strength of SiC and Si3N4 at 1400°C is 1.5-2 times higher than that of aluminum oxide ceramics [49]. When silicon nitride is deposited on a Ti alloy it reacts with titanium to form TiN and Ti5Si3 [50]. When a Si3N4-based ceramics is oxidized, in most cases a layer of silicon dioxide forms on its surface  amorphous SiO2 up to 1065°C; α-cristobalite at higher temperatures [42]. When sodium and sulfur are present in the oxidizing medium the protective SiO2 film is destroyed at 1100°C. A Si3N4 ceramic coating deposited on high-chromium steel possessed high hardness (1600-1800 HV) and wear resistance [51]. Reaction-sintered coatings obtained by the slip-roasting method, based on the systems Si − B − Cr − TiB2 − MoSi2 and Si − B − Cr − MoSi2, are used for the protection of heat-resistant nickel-base alloys from gas corrosion at 1100-1150°C [17]. Thanks to the presence of crystalline Si, Cr, MoSi2, TiB2, SiO2, and Cr2O3 in the coating, the mass increase of coated specimens upon oxidation was 6 times less than that of uncoated specimens. Gasothermal silicide coatings of similar compositions from the Si − B − Ti − Mo − Y system are used for heat-resistant molybdenum-base and nickel-base alloys [5254]. Silicides are heat resistant mainly due to the formation of a protective film of SiO2 on their external surfaces [52]. The presence of 10% TiB2 increases the heat resistance of the silicide coating by 1.5 times, thanks to improved self-healing of defects in the oxide film. Additions of TiB2 and TiC to the silicide coating raise the threshold of alloy operating temperatures to 1500°C (diffusional dissipation of titanium diboride occurs at higher temperatures [53]). The corrosion rate in a 10% aqueous solution of Na2CO3 of a composition with added TiB2 is 8 times lower than that of an uncoated specimen, and that of a composition with added TiC 2 times lower [55]. The ductility of the alloyed silicide layer is also improved, which makes it highly resistant to thermal cycling. Such a coating is able to protect non-uniformly heated surfaces (sharp edges, steep temperature gradients, etc.), and provides super-rapid self healing of defects in the initial stages of surface erosion [54]. It differs from known silicide coatings mainly by the formation of a dendritic, cellular, heterophase structure in the form of a framework of the refractory alloyed disilicides (Ti, Mo)Si2, TiSi2, and also TiB2. The spaces within this skeleton are filled with the ductile, relatively low melting eutectic (Si + disilicides), which wets the above phases and plays the role of a healing agent in the structure.

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Therefore oxide, carbonitride, and silicon-containing ceramics have a number of specific properties which make them useful for the reliable protection of HRA. Their main disadvantage  brittleness  is removed by the use of additions, or formation of a layered structure. Additions increase ductility, which makes ceramic coatings very promising for the protection of gas turbine blades. Conclusion The development of new coatings for heat-resistant nickel alloys will actively continue. It is probable that investigations directed at the creation of reliable thermal barrier coatings for gas turbine blades will be accelerated. In view of constantly increasing turbine operating temperatures more oxidation resistant coating materials with higher resistance to thermal fatigue are required, and the appearance of large stationary gas turbines using carbonaceous fuel may lead to the creation of entirely new coating types. Metallic coatings with a high concentration of aluminum, such as aluminide and MeCrAlY, provide effective resistance to oxidation at high temperatures. The highest resistance to hot corrosion is provided by high-chromium coatings of the MeCrAlY-type together with aluminides of platinum-group metals. Very high fuel combustion temperatures, which would be destructive to other methods of protection, are maintained in a gas turbine with the help of thermal barrier coatings which prevent overheating of the base material. The common use of zirconium dioxide as a thermal barrier does not solve the problem of low heat resistance of such coatings. Most promising is the development of layered composite ceramic coatings which, thanks to a graded variation of properties, are able to provide an optimal combination of adherence, mechanical strength, and corrosion and heat resistance. The best results might be attained by the use of binary, and even ternary ceramic composites, however information about such coatings and their corrosion mechanisms is limited. New coating processes such as laser melting and plating, ion implantation, or magnetron sputtering are promising developments, but physical deposition from a vapor phase, plasma spraying, and electron-beam deposition remain the principal industrial processes for depositing coatings on heat resistant alloys.

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