**2.1 Ceramic thermal barrier coatings**

In recent years, works on introduction and practical use of thermal barrier ceramic coatings on parts of a high-temperature system of gas turbine engines have been carried out especially actively. The protection of the material of the part against the heat flux with heatbarrier coatings is most effective when the ceramic coatings used are based on ZrO2 [2-4, 8]. The heat-protective effect of the thermal barrier ceramic coating reaches 100-120 °C under operating conditions. The heat-protective effect – decrease of metal temperature is function of thickness and heat conductivity of a thermal barrier ceramic coatings and thermal flows in a wall of a protected detail. The values of thermal flows on workers turbine GTE blades are in a range from 1,0 up to (2÷2,5)×106 W/(mК) in works [1, 2, 3]. In some cases the thermal flow makes 3×106 W/(mК) and more [4]. In the given work with the use of system ANSYS the calculated investigations of influence of the specified factors on decrease of metal temperature of cooled blades were carried out and the estimations of the heatprotective effect of a thermal barrier ceramic coatings for cooled details have been obtained by author. The results of calculated investigations are presented in Fig. 1 and Fig. 2. The values of decrease of metal temperature on a surface of cooled GTE blades depending on thermal flows at thickness *h =* 0,14 mm of ceramic coatings and different heat conductivity coatings are shown in Fig 1. The values of decrease of metal temperature on a surface of cooled GTE blades depending on thickness of ceramic coatings at gas thermal flow *q* = 1,8106 W/m2 and different heat conductivity of coatings in Fig 2. However, the questions regarding the thermal cyclic fatigue life are very problematic because the fracture strength of these coatings under tension is very low and thermal cycling usually leads to the appearance of alternating thermal stresses. Moreover, during operation of turbine blades, oxygen from an oxidizing medium (air, fuel combustion products) penetrates into the "ceramicsmetal" interface. The penetration of oxygen through the ceramic layer results in the oxidation of the sublayer. The formation of oxides gives rise to additional stresses and decreases the adhesion of the ceramic layer. Therefore, the above factors must be taken into account in the design of coatings. The efficiency of thermal protection of coatings and their thermal fatigue resistance depend not only on the thermophysical properties (Fig.1 and Fig.2)but also on the technique used for depositing of the coating. Among numerous techniques currently employed for depositing of the coatings, the electron-beam technique provides the best thermal protection with a high thermal fatigue resistance.

### **2.2 Technique for depositing of ceramic thermal barrier coatings**

Development of thermal barrier coatings applied to cooled blades is one of the trends for improving gas turbines. Unlike aluminide protective coatings, the ceramic coatings not only protect blade surfaces from high-temperature oxidation and corrosion but also prevent base material softening at high temperatures. Thermal barrier coating application allows the reduction of the blade temperature and the significant increase in its service life. Under both

the case where care is taken to restrict the passage of heat flow through the wall of the part. The heat flow from the gas to the wall of the base material of the part can be considerably reduced by means of either using a well-organized protective cooling without ejection or depositing thermal barrier coatings on the surface of the most strongly heated regions of the

In recent years, works on introduction and practical use of thermal barrier ceramic coatings on parts of a high-temperature system of gas turbine engines have been carried out especially actively. The protection of the material of the part against the heat flux with heatbarrier coatings is most effective when the ceramic coatings used are based on ZrO2 [2-4, 8]. The heat-protective effect of the thermal barrier ceramic coating reaches 100-120 °C under operating conditions. The heat-protective effect – decrease of metal temperature is function of thickness and heat conductivity of a thermal barrier ceramic coatings and thermal flows in a wall of a protected detail. The values of thermal flows on workers turbine GTE blades are in a range from 1,0 up to (2÷2,5)×106 W/(mК) in works [1, 2, 3]. In some cases the thermal flow makes 3×106 W/(mК) and more [4]. In the given work with the use of system ANSYS the calculated investigations of influence of the specified factors on decrease of metal temperature of cooled blades were carried out and the estimations of the heatprotective effect of a thermal barrier ceramic coatings for cooled details have been obtained by author. The results of calculated investigations are presented in Fig. 1 and Fig. 2. The values of decrease of metal temperature on a surface of cooled GTE blades depending on thermal flows at thickness *h =* 0,14 mm of ceramic coatings and different heat conductivity coatings are shown in Fig 1. The values of decrease of metal temperature on a surface of cooled GTE blades depending on thickness of ceramic coatings at gas thermal flow *q* = 1,8106 W/m2 and different heat conductivity of coatings in Fig 2. However, the questions regarding the thermal cyclic fatigue life are very problematic because the fracture strength of these coatings under tension is very low and thermal cycling usually leads to the appearance of alternating thermal stresses. Moreover, during operation of turbine blades, oxygen from an oxidizing medium (air, fuel combustion products) penetrates into the "ceramicsmetal" interface. The penetration of oxygen through the ceramic layer results in the oxidation of the sublayer. The formation of oxides gives rise to additional stresses and decreases the adhesion of the ceramic layer. Therefore, the above factors must be taken into account in the design of coatings. The efficiency of thermal protection of coatings and their thermal fatigue resistance depend not only on the thermophysical properties (Fig.1 and Fig.2)but also on the technique used for depositing of the coating. Among numerous techniques currently employed for depositing of the coatings, the electron-beam technique

provides the best thermal protection with a high thermal fatigue resistance.

Development of thermal barrier coatings applied to cooled blades is one of the trends for improving gas turbines. Unlike aluminide protective coatings, the ceramic coatings not only protect blade surfaces from high-temperature oxidation and corrosion but also prevent base material softening at high temperatures. Thermal barrier coating application allows the reduction of the blade temperature and the significant increase in its service life. Under both

**2.2 Technique for depositing of ceramic thermal barrier coatings** 

part.

**2.1 Ceramic thermal barrier coatings** 

Fig. 1. Values of decrease of metal temperature *t* on a surface of cooled GTE blades depending on thermal flows *q* at thickness *h =* 0,14 mm of ceramic coatings ZrO2 and different heat conductivity: *1* - 1,5 W/(mК); *2* - 0,8 W/(mК)

Fig. 2. Values of decrease of metal temperature *t* on a surface of cooled GTE blades depending on thickness *h* of ceramic coatings ZrO2 at gas thermal flow *q* = 1,8106 W/m2 and different heat conductivity of coatings: *1* - 1,5 W/(mК);*2* - 0,8 W/(mК)

Investigations of Thermal Barrier Coatings for Turbine Parts 133

thermal stresses in ceramics, which in turn results in ceramic layer spalling from the surface. To reduce thermal stresses, various technological procedures are used. In the ceramic layer deposited by the APS technique, special heat treatment is used to form a network of microcracks that break the ceramics into isolated fragments (Ruckle & Duvall, 1984). In the ceramic layer deposited by the EB technique, some specific columnar structure is formed that is readily fragmentizing when tensile stresses arise in (Strangman, 1982). The point crucial to success in the development of TBCs lies in obtaining the required adhesive strength of the ceramic coating/heat-resistant bond coat, providing for holding of the ceramics on the blade surface during all the blade service life. As a rule, in aircraft engine manufacturing, the technique of plasma deposition is used for nozzle vanes; in aircraft engine turbine blades, the EB technique is considered to be preferable. This is due to the fact that the following properties can be rendered ramie layer. The specific columnar structure, with the crystallites oriented perpendicular to the surface, forms in the ceramic vapor-deposited coating. In the case of tensile stresses, the ceramic layer is readily fragmentizing, thus reducing ceramic tearing stress during thermal cycling. In the temperature range of 850 to 950 °C, which is below the blade heating

temperature at ceramic layer deposition, compressive stresses arise in it.

Fig. 3. Thermal barrier coating system: *1* – ceramic coating ZrO2-8%Y2O3, 2 – bond coating

MCrAlY, *3* - superalloy

steady-state and transient conditions, the application of ceramic TBC can diminish temperature gradients over the blade surfaces as well as reduce thermal stresses in them. A typical design of a TBC is presented in Fig. 3. The ceramic coating deposited directly on the superalloy surface does not show the required service life. Penetration of oxygen through the ceramic layer to the superalloy surface results in its quick oxidation and in spallation of the ceramic layer. That is why, as a rule, a TBC consists of at least two layers. An inner aluminide heat-resistant bond coat may be formed by different techniques. It may be either a diffusion or an overlay coating, depending on the requirements of its physical-mechanical properties and protection targets. The requirements of bond coat properties and protective coatings properties are much the same, yet the bond coat should meet some special requirements. First of all, it must be highly heat resistant; the oxides formed on its surface should have high adhesion to both the bond coat and the outer ceramic layer. When choosing a bond coat composition, one should pay special attention to its yttrium content as well as to the contents of the other elements, which guarantee high oxide adhesion to the surface and reactive element effect (Stringer, 1989). It is of special importance for bond coats deposited by the electron beam technique, because their yttrium contents depend on the yttrium content of the liquid bath and vary within wide limits (Malashenko et al., 1997). In this case, the required yttrium content of 0.2 to 0.3% is guaranteed by different technological procedures, such as direct yttrium addition to the liquid bath (Tamarin & Kachanov, 2008). Under these conditions, it is noteworthy that high yttrium contents of the liquid bath cause slag formation on its surface, thus resulting in occurrence of microdrops. These microdrops on the bond coat surface may provoke defects in the ceramic coating (layer).

It should be taken into consideration that TBCs are usually applied to the blades of hightemperature turbines. The blades of such turbines feature directionally solidified or single-crystal structures, thin walls, and high cooling efficiency. Under service conditions, high thermal stresses and strains arise in these blades, especially in their surface layers. That is why thermomechanical fatigue characteristics are as important in choosing a bond coat composition as its heat resistance. During thermal cycling, the bond coat should not experience considerable plastic strain. For example, the effect of a "rippled" blade surface (Fig. 4) always entails spallation of the ceramic layer. The outer zirconium oxide/yttrium oxide (ZrO2-Y2O3) system base ceramic layer can-be applied by two techniques (Fig 4 and Fig 5): air plasma spraying of powders (APS-technique) or vapor condensation at electron beam evaporation of ceramic pellets (EB-technique). For this system, ceramic coating service life depends on Y2O3 content. The ZrO2-(6 to 9%) Y2O3 compositions are usually applied, because they have demonstrated maximum service lives in the tests carried out (Miller, 1983 &Stecura, 1986). However, one should bear in mind the fact that the coating service life depends not only on its chemical composition but also on its structure and adhesive strength at the ceramic layer/bond coat interface, which depends on deposition technique. For coatings deposited by different techniques, the optimal chemical compositions may be other than that stated previously. The ceramic layer deposition technique determines such characteristics as ceramic layer structure and adhesive strength, its corresponding service life, thermal stresses in the ceramic layer, and its surface roughness. The main difficulty in designing TBCs for turbine blades lies in the combination of the ceramics on the blade surface and the superalloy that they are made of. At heating-up/cooling-down cycling, considerable difference between the ceramics and superalloy expansion coefficients~ 5.0 10-6 1/°C causes the generation of high

steady-state and transient conditions, the application of ceramic TBC can diminish temperature gradients over the blade surfaces as well as reduce thermal stresses in them. A typical design of a TBC is presented in Fig. 3. The ceramic coating deposited directly on the superalloy surface does not show the required service life. Penetration of oxygen through the ceramic layer to the superalloy surface results in its quick oxidation and in spallation of the ceramic layer. That is why, as a rule, a TBC consists of at least two layers. An inner aluminide heat-resistant bond coat may be formed by different techniques. It may be either a diffusion or an overlay coating, depending on the requirements of its physical-mechanical properties and protection targets. The requirements of bond coat properties and protective coatings properties are much the same, yet the bond coat should meet some special requirements. First of all, it must be highly heat resistant; the oxides formed on its surface should have high adhesion to both the bond coat and the outer ceramic layer. When choosing a bond coat composition, one should pay special attention to its yttrium content as well as to the contents of the other elements, which guarantee high oxide adhesion to the surface and reactive element effect (Stringer, 1989). It is of special importance for bond coats deposited by the electron beam technique, because their yttrium contents depend on the yttrium content of the liquid bath and vary within wide limits (Malashenko et al., 1997). In this case, the required yttrium content of 0.2 to 0.3% is guaranteed by different technological procedures, such as direct yttrium addition to the liquid bath (Tamarin & Kachanov, 2008). Under these conditions, it is noteworthy that high yttrium contents of the liquid bath cause slag formation on its surface, thus resulting in occurrence of microdrops. These microdrops

on the bond coat surface may provoke defects in the ceramic coating (layer).

It should be taken into consideration that TBCs are usually applied to the blades of hightemperature turbines. The blades of such turbines feature directionally solidified or single-crystal structures, thin walls, and high cooling efficiency. Under service conditions, high thermal stresses and strains arise in these blades, especially in their surface layers. That is why thermomechanical fatigue characteristics are as important in choosing a bond coat composition as its heat resistance. During thermal cycling, the bond coat should not experience considerable plastic strain. For example, the effect of a "rippled" blade surface (Fig. 4) always entails spallation of the ceramic layer. The outer zirconium oxide/yttrium oxide (ZrO2-Y2O3) system base ceramic layer can-be applied by two techniques (Fig 4 and Fig 5): air plasma spraying of powders (APS-technique) or vapor condensation at electron beam evaporation of ceramic pellets (EB-technique). For this system, ceramic coating service life depends on Y2O3 content. The ZrO2-(6 to 9%) Y2O3 compositions are usually applied, because they have demonstrated maximum service lives in the tests carried out (Miller, 1983 &Stecura, 1986). However, one should bear in mind the fact that the coating service life depends not only on its chemical composition but also on its structure and adhesive strength at the ceramic layer/bond coat interface, which depends on deposition technique. For coatings deposited by different techniques, the optimal chemical compositions may be other than that stated previously. The ceramic layer deposition technique determines such characteristics as ceramic layer structure and adhesive strength, its corresponding service life, thermal stresses in the ceramic layer, and its surface roughness. The main difficulty in designing TBCs for turbine blades lies in the combination of the ceramics on the blade surface and the superalloy that they are made of. At heating-up/cooling-down cycling, considerable difference between the ceramics and superalloy expansion coefficients~ 5.0 10-6 1/°C causes the generation of high thermal stresses in ceramics, which in turn results in ceramic layer spalling from the surface. To reduce thermal stresses, various technological procedures are used. In the ceramic layer deposited by the APS technique, special heat treatment is used to form a network of microcracks that break the ceramics into isolated fragments (Ruckle & Duvall, 1984). In the ceramic layer deposited by the EB technique, some specific columnar structure is formed that is readily fragmentizing when tensile stresses arise in (Strangman, 1982). The point crucial to success in the development of TBCs lies in obtaining the required adhesive strength of the ceramic coating/heat-resistant bond coat, providing for holding of the ceramics on the blade surface during all the blade service life. As a rule, in aircraft engine manufacturing, the technique of plasma deposition is used for nozzle vanes; in aircraft engine turbine blades, the EB technique is considered to be preferable. This is due to the fact that the following properties can be rendered ramie layer. The specific columnar structure, with the crystallites oriented perpendicular to the surface, forms in the ceramic vapor-deposited coating. In the case of tensile stresses, the ceramic layer is readily fragmentizing, thus reducing ceramic tearing stress during thermal cycling. In the temperature range of 850 to 950 °C, which is below the blade heating temperature at ceramic layer deposition, compressive stresses arise in it.

Fig. 3. Thermal barrier coating system: *1* – ceramic coating ZrO2-8%Y2O3, 2 – bond coating MCrAlY, *3* - superalloy

Investigations of Thermal Barrier Coatings for Turbine Parts 135

Their generation is due to the different values of the ceramic and superalloy thermal expansion coefficients. These stresses do not relax on subsequent process annealing and under service conditions. The adhesive strength of the ceramic coating is controlled by physical-chemical reactions occurring between the ceramics and the metallic bond coat. As-deposited ceramic coating adhesive strength is above 50-100 MPa. The surface roughness of the ceramic coatings does not exceed 1.5 m after their deposition. The need for maintaining the parameters of condensation and ceramic coating crystallite growth at a steady level requires a certain layout of relative positions of the blades, the vapor generator, and the EB guns for blade heating. As is shown in (Schulz, 1997), substrate rotation speeds have the same effect as temperatures. This behavior is caused by the effect of rotation on the time of growing crystal presence in the zones with different vapor density. The higher the temperature and rotation rate, the larger the diameter of an individual crystallite of the condensing ceramics (Fig. 6). A deposited ceramic coatings at the temperatures *t*3 = 850-950 C, *t*2 = 0,85 *t*3, *t*1 = 0,7 *t*3 and rotational speeds are shows on the

Fig. 6. Influence substrate and rotational speed on columnar microstructure of the deposited

Using different rotation speeds, structural characteristics of the ceramic coating can be governed. From the experience of ceramic coating deposition and taking into consideration intricate blade profiles and a need for simultaneous coating deposition on several blades, the best results can be achieved by combining blade revolution around the evaporator and rotation about then-axes. An illustration of blade arrangement and their revolution/rotation

thermal barrier ceramic coating(EB technique)

is given in Fig. 7.

Fig. 6.

Fig. 4. Thermal barrier ceramic coating ZrO2-Y2O3 (APS-technique)

Fig. 5. Thermal barrier ceramic coating ZrO2-Y2O3 (EB-technique)

Fig. 4. Thermal barrier ceramic coating ZrO2-Y2O3 (APS-technique)

Fig. 5. Thermal barrier ceramic coating ZrO2-Y2O3 (EB-technique)

Their generation is due to the different values of the ceramic and superalloy thermal expansion coefficients. These stresses do not relax on subsequent process annealing and under service conditions. The adhesive strength of the ceramic coating is controlled by physical-chemical reactions occurring between the ceramics and the metallic bond coat. As-deposited ceramic coating adhesive strength is above 50-100 MPa. The surface roughness of the ceramic coatings does not exceed 1.5 m after their deposition. The need for maintaining the parameters of condensation and ceramic coating crystallite growth at a steady level requires a certain layout of relative positions of the blades, the vapor generator, and the EB guns for blade heating. As is shown in (Schulz, 1997), substrate rotation speeds have the same effect as temperatures. This behavior is caused by the effect of rotation on the time of growing crystal presence in the zones with different vapor density. The higher the temperature and rotation rate, the larger the diameter of an individual crystallite of the condensing ceramics (Fig. 6). A deposited ceramic coatings at the temperatures *t*3 = 850-950 C, *t*2 = 0,85 *t*3, *t*1 = 0,7 *t*3 and rotational speeds are shows on the Fig. 6.

Fig. 6. Influence substrate and rotational speed on columnar microstructure of the deposited thermal barrier ceramic coating(EB technique)

Using different rotation speeds, structural characteristics of the ceramic coating can be governed. From the experience of ceramic coating deposition and taking into consideration intricate blade profiles and a need for simultaneous coating deposition on several blades, the best results can be achieved by combining blade revolution around the evaporator and rotation about then-axes. An illustration of blade arrangement and their revolution/rotation is given in Fig. 7.

Investigations of Thermal Barrier Coatings for Turbine Parts 137

ceramic layers, it is advantageous to form thin layers 0.2 to 2.0 m thick in the crystallites. Their boundaries ensure effective phonon scattering. Multilayer structure may be formed by plasma discharge to vary the density of the ceramics during deposition. According to the research, the efficiency of thermal conductivity reduction by means of multilayer structure

Fig. 8. Thermal barrier ceramic coating ZrO2-Y2O3 (EB-technique):

**2.3 Methods of heating for investigations of ceramic thermal barrier coatings of parts**  For providing the above-indicated heating conditions, there are various ways of heating such as gasdynamic heating and radiant heating, for example, in a reflective furnace electrical current (AC or DC) or induction heating with the use of high-frequency currents. Gasdynamic (flowing hot gas) heating has been used for more than 50 years. When using this method, a more accurate simulation of the heat exchange conditions from gas flow to the part is realized relevant to the gas-turbine engine. The rigs with gasdynamic heating enable a high heating rate to be provided to the part, to investigate the influence of oxidation in gas flow, but at the same time it is difficult to provide mechanical loading of parts. The cost of tests using such rigs is very high and the bench equipment needs to be frequently repaired or replaced. Alternating current (AC) or direct current (DC) resistance heating is effective for testing solid and tubular specimens. In accordance with this method, there is no need to use expensive and complex equipment. It enables tests to be conducted both at in-phase and at out-of-phase change of temperatures and mechanical loads. This method provides ease for inspection of the specimen surface. At the same time, this method cannot be used for tests of gas-turbine engine parts. The direct passing of electrical current can influence on the mechanical properties of the specimen material. In addition, this method does not enable the actual conditions for part heating in gas flow to be simulated. When a specimen with a thermal barrier coating is heated by direct passing of electrical

of a two-layer model, h – thickness of a coating


may be high.

Fig. 7. Scheme for depositing of ceramic thermal barrier coatings (EB-technique) : 1 – rotor, 2 – blades, 3 – ceramic, 4 – electron beam gun of evaporator, 5 - electron beam gun for blade heating

The fixture in use revolves in the vapor flow with the speed of ~ 12 rpm. At each fixture revolution the blades additionally revolve once around the fixture axis. The choice of blade rotation conditions depends also on the requirements to the ceramic coating thickness and its spread over the blade surface. The ceramic structure features the pronounced texture of growth perpendicular to the surface (Fig. 5). Some individual ceramics crystals are preferably oriented in a [100] direction. Their diameters are in the range of 0.6 to 1.2 m. They do not vary much along the full crystal lengths. Ceramic coating crystallites should have high cohesive strength and withstand an attack of a high-temperature gas flow. That is why the ceramic evaporation process feature is a requirement to its continuity. Unlike metallic bond coat deposition, in which no process interruption is harmful for the coating quality, any interruption of ceramic coating deposition forms an additional boundary, in the ceramics. The strength of this boundary is much lower than the crystallite strength. Thus, under these conditions, the ceramic coating will never meet the requirements of its properties. In the case of any pause in ceramics evaporation, all the lot of blades being coated are rejected and sent to ceramic layer removal procedure, followed by its redeposition. When ceramic layer deposition is carried to its completion, the blades are removed from the unit and passed to heat treatment. After treatment its color changes from dark gray to white. Two-step annealing does not change ceramic layer structure and phase composition. Check operations in the TBC quality control include a visual inspection to guarantee that its surface is free from ceramics droplets; measurements of ceramic layer thickness in the specified blade zones; and a bend test of a flat check sample on the radius of 3 and 10 mm to assess its adhesive strength. On its bending to the angle of 90°, ceramic coating spallation is prohibitive. Some cracking of the ceramic coating is allowed. The results of thermal conductivity studies for different ceramic coatings formed by the EB technique are presented in (Nichols et al., 2001). On the basis of the studies (Lawson et al., 1996), a two-zone model of a ceramic coating is suggested. Thermal conductivity of a dense, inner ceramic zone that forms at the starting moment of condensation is much lower than thermal conductivity of an outer zone (Fig. 8). This effect is attributed to the presence of numerous boundaries in the dense zone. Therefore, for reducing thermal conductivity of EB

Fig. 7. Scheme for depositing of ceramic thermal barrier coatings (EB-technique) : 1 – rotor, 2 – blades, 3 – ceramic, 4 – electron beam gun of evaporator, 5 - electron beam gun for blade

The fixture in use revolves in the vapor flow with the speed of ~ 12 rpm. At each fixture revolution the blades additionally revolve once around the fixture axis. The choice of blade rotation conditions depends also on the requirements to the ceramic coating thickness and its spread over the blade surface. The ceramic structure features the pronounced texture of growth perpendicular to the surface (Fig. 5). Some individual ceramics crystals are preferably oriented in a [100] direction. Their diameters are in the range of 0.6 to 1.2 m. They do not vary much along the full crystal lengths. Ceramic coating crystallites should have high cohesive strength and withstand an attack of a high-temperature gas flow. That is why the ceramic evaporation process feature is a requirement to its continuity. Unlike metallic bond coat deposition, in which no process interruption is harmful for the coating quality, any interruption of ceramic coating deposition forms an additional boundary, in the ceramics. The strength of this boundary is much lower than the crystallite strength. Thus, under these conditions, the ceramic coating will never meet the requirements of its properties. In the case of any pause in ceramics evaporation, all the lot of blades being coated are rejected and sent to ceramic layer removal procedure, followed by its redeposition. When ceramic layer deposition is carried to its completion, the blades are removed from the unit and passed to heat treatment. After treatment its color changes from dark gray to white. Two-step annealing does not change ceramic layer structure and phase composition. Check operations in the TBC quality control include a visual inspection to guarantee that its surface is free from ceramics droplets; measurements of ceramic layer thickness in the specified blade zones; and a bend test of a flat check sample on the radius of 3 and 10 mm to assess its adhesive strength. On its bending to the angle of 90°, ceramic coating spallation is prohibitive. Some cracking of the ceramic coating is allowed. The results of thermal conductivity studies for different ceramic coatings formed by the EB technique are presented in (Nichols et al., 2001). On the basis of the studies (Lawson et al., 1996), a two-zone model of a ceramic coating is suggested. Thermal conductivity of a dense, inner ceramic zone that forms at the starting moment of condensation is much lower than thermal conductivity of an outer zone (Fig. 8). This effect is attributed to the presence of numerous boundaries in the dense zone. Therefore, for reducing thermal conductivity of EB

heating

ceramic layers, it is advantageous to form thin layers 0.2 to 2.0 m thick in the crystallites. Their boundaries ensure effective phonon scattering. Multilayer structure may be formed by plasma discharge to vary the density of the ceramics during deposition. According to the research, the efficiency of thermal conductivity reduction by means of multilayer structure may be high.

Fig. 8. Thermal barrier ceramic coating ZrO2-Y2O3 (EB-technique): - thermal conductivity of a two-layer model, h – thickness of a coating
