**4.2 Investigations of thermal barrier properties of ceramic coatings with the use of gas-flame heating**

For maintenance of competitiveness of aircraft engines it is necessary to raise a gas temperature over 1700 K in front of the turbine. Thus serviceability of details of a hightemperature gas can keep only at perfection of their heat-protection. Thus the serviceability of details of a high-temperature gas probably keep only at perfection of their heatprotection.It is known, that in world practice the ceramic heat-protective coatings on basis ZrО2 are widely used. At the same time the data on heat conductivity and thermal conductivity and efficiency of a heat-protection of details with help thermal barrier ceramic coatings at their heating in a gas flow are rather limited. The characteristics of heat conductivity thermal barrier ceramic coatings have received at use of various known laboratory methods are inconsistent. Basically the preference is given the thermal barrier ceramic coatings have deposited on a plasma technology. At use of laser pulse heating it has been received that heat conductivity of plasma coatings approximately in 3 times is lower than at the coatings have deposited on the electron beam technology. The laser pulse method is inexpedient to use for determination of the temperature in part transparentceramic coatings as the part of a beam flow warms up directly a metal on which it is deposited coating. The protective thin metal screen with thickness 10-15 m deposited by researchers on surface of coating on the side of the laser at heating, itself starts to let out a beam flow. In real conditions the turbine blades and walls of combustion chambers are heated up by a gas flow. In the given work the developed technique by an objective estimation of efficiency of a heat-protective of metal with the help of coatings of plasma andelectron beam technology is resulted at gas-flame heating of object on the developed rig (Bychkov, 2008). The essence of the given original technique protected by the patent RU will be that through the demountable specimen (collected from two halfs) the high-temperature gas flow (Fig. 23) is passed.

The investigations for evaluating the efficiency of thermal protection of materials of the turbine blades and parts with use TBC (received on electron beam technology and plasma technology) against the convective and radiant components of the high-temperature gas flow were conducted. In this case, it is recommended to conduct the tests at the small-size

assumptions in the mathematical model used for calculating thermal diffusivity and heat capacity. These errors are related to the finite pulse duration and its spatial inhomogeneity, to heat losses (due to irradiation, mainly), and to violation of pulse absorption conditions in the thin surface layer. These errors may be avoided by using certain corrections (Clark & Taylor, 1975). For the TC-3000H unit, pulse duration and spatial inhomogeneity errors determined according to the Sinku-Riko Company recommendations are unessential (less than 1%). Heat losses in the experiment result in a quick temperature rise to its maximum and then a sharply defined smooth temperature decrease. The main cause that gives rise to measurement errors is radiation heat exchange, whose effect rises simultaneously with a temperature rise. The errors caused by radiation may account for 30%. To meet the requirements of pulse absorption in the thin surface layer, the ceramic samples, which are partially transparent, were coated with a thin layer (10 to 12 m) of the NiAl intermetallic compound (20% Al). This layer ensured steady

**4.2 Investigations of thermal barrier properties of ceramic coatings with the use of** 

For maintenance of competitiveness of aircraft engines it is necessary to raise a gas temperature over 1700 K in front of the turbine. Thus serviceability of details of a hightemperature gas can keep only at perfection of their heat-protection. Thus the serviceability of details of a high-temperature gas probably keep only at perfection of their heatprotection.It is known, that in world practice the ceramic heat-protective coatings on basis ZrО2 are widely used. At the same time the data on heat conductivity and thermal conductivity and efficiency of a heat-protection of details with help thermal barrier ceramic coatings at their heating in a gas flow are rather limited. The characteristics of heat conductivity thermal barrier ceramic coatings have received at use of various known laboratory methods are inconsistent. Basically the preference is given the thermal barrier ceramic coatings have deposited on a plasma technology. At use of laser pulse heating it has been received that heat conductivity of plasma coatings approximately in 3 times is lower than at the coatings have deposited on the electron beam technology. The laser pulse method is inexpedient to use for determination of the temperature in part transparentceramic coatings as the part of a beam flow warms up directly a metal on which it is deposited coating. The protective thin metal screen with thickness 10-15 m deposited by researchers on surface of coating on the side of the laser at heating, itself starts to let out a beam flow. In real conditions the turbine blades and walls of combustion chambers are heated up by a gas flow. In the given work the developed technique by an objective estimation of efficiency of a heat-protective of metal with the help of coatings of plasma andelectron beam technology is resulted at gas-flame heating of object on the developed rig (Bychkov, 2008). The essence of the given original technique protected by the patent RU will be that through the demountable specimen (collected from two halfs) the high-temperature

The investigations for evaluating the efficiency of thermal protection of materials of the turbine blades and parts with use TBC (received on electron beam technology and plasma technology) against the convective and radiant components of the high-temperature gas flow were conducted. In this case, it is recommended to conduct the tests at the small-size

surface optical parameters of the samples as well.

**gas-flame heating** 

gas flow (Fig. 23) is passed.

Fig. 23. Sketch of specimen: *1* – specimen half with coating, *2* - specimen half without coating, *3* – coating, *4, 5* – thermocouples, *6* – axis of specimen (flame), *7* – burner

rig and use the small-size specimens whose surfaces during tests are accessible for inspecting the thermal state both by the contact and contactless methods. This rig in particular is usable effectively for conducting the comparison thermal barrier propertiesand thermocycles tests of various coatings. The rate of change of the temperature in a thermocycle reaches 100 °C/s. For performing these investigations, a test rig has been developed with gas-flame heating of model specimens. The gas generator is a water electrolysis device equipped with a control system; it provides the variable flammable gas flow. Hydrogen has a high combustion temperature and this fact ensures high-speed heating of the specimens. This test rig has a system for providing enrichment of the flammable gas with different fuels. This makes it possible to attain the required gas composition. While testing, the burner is installed fixed, however the attachment allows its position to be adjusted. The hollow specimen is of an axisymmetrical form. Before the test, the burner is installed in a way ensuring coincidence of the specimen axis with the flame torch axis in the process of heating. While investigating the efficiency of influence of ceramic coating on specimen temperature state, the unit with specimens was fixed. A special specimen construction was developed for these tests. The hollow specimen was cut longitudinally in two equal portions. The ceramic coating under investigation was applied on one half of the specimen, the other half remained uncoated. The thermocouples ХА by diameter of 0,2 mm weld on an external surface of halfs of a compound specimen (Fig. 23) and are connected to recording computer system. The half of a specimen is protected from products of combustion by a coating it is warmed up less, than unprotected half. The difference of temperatures *t* of protected wall with coating and unprotected wall characterizes the efficiency and thermal conducting of the thermal barrier coatings. Heat insulating material was placed between them to exclude the influence of heat transfer through the contacting edges of the specimen halves. While heating, temperature was measured on the outer (opposite to flame torch) specimen side. Conditions for heating the inner surfaces of both specimen halves by flame were the same, but with a difference in heat protection efficiency the outer surface of the specimen with a TBC had a lower temperature than the surface without a TBC. The after of lighting of a combustible gas the heating of an internal walls of both halfs of model begins. The difference of temperatures on lateral side grows until the heat transfer from a hot surface of a wall to cold surface is less, than a heat-conducting from an external surface in an environment. At absence of the organized cooling lateral side of a wall the maximal difference of temperatures *t*max outside of both of halfs corresponds to a gradient of temperatures on TBC under these conditions. In experiment *t*max it is reached at temperature of a cold wall 600 °C. The results of investigations are presented on Fig. 24 and 25.

Investigations of Thermal Barrier Coatings for Turbine Parts 155

60-70 °C and *t*мах = 100-110 С. By he received results it is possible to estimate the thermal conductivity EB ceramic coatings which on the average in 1,6 times is lower than at APS coatings. Thus the received results of experimental estimation of the thermal conductivity and decrease of wall temperature of heat-resistant materials after deposited TBC of ZrO2 + 8%Y2O3 by thickness about 120 m show that at gas-flame heating of models the investigated EB coating of columnar structure protects metal is better than the tested APS coating. The developed original method of the experimental determination of thermal conductivity and estimation of efficiency of the thermal protection of details with thermal barrier ceramic coatings at gas-flame heating provides the reception of more exact data about thermophysic properties of ceramics

**5. Calculated investigations of stress state of columnar structure of thermal** 

The most effective protection of a detail material against a thermal flow occurs in case of use ofelectron beam method for depositing of ceramic coatings ZrO2 (Tamarin & Kachanov, 2008). With the help of the specified method the ceramic coating of column structure on a surface of a metal sublayer (heat resisting coating) of working turbine blade is formed. The specified ceramic barrier coating is generated as columns (Fig. 26), are directed perpendicularly surface on which it is deposited. The columns of the ceramic coating possess low heat conductivity and provides the required durability at thermal cycles. The

**barrier ceramic coatings with view of influence of centrifugal forces** 

under operating conditions of turbine details of aviation engines.

Fig. 26. Columnar thermal barrier ceramic coating (EB technique)

strength characteristics of ceramics are very low.

Fig. 24. The temperature on an external (cold) surface specimen: *1* – without coating, *2* – with ceramic coating (APS technique)

Fig. 25. The temperature on an external (cold) surface specimen: *1* – without coating, *2* – with ceramic coating (EB technique)

The models with ceramic TBC ZrO2 + 8%Y2O3 deposited on plasma technology were made of Ni-alloy. Other models with TBC of columnar structure deposited on electron beam technology were made of other Ni-alloy. Unprotected halfs of each model were made of the same material as halfs with TBC. The tests on each model were repeated some times for maintenance of reliability of determination of heat-protective efficiency. At retesting the model was unwrapped about the axis on 180 °. The difference of temperatures at repeated measurements did not exceed 10 °С. The maximal temperature on the "cold" side of a wall made 900 °С. The temperature of a gas stream made 1773 K. The experimental investigations have shown that the efficiency of decrease of metal temperature at gas-flame heating after deposited TBC by thickness = 120 m on plasma and electron beam technologies make correspondingly *t*мах =

Fig. 24. The temperature on an external (cold) surface specimen: *1* – without coating, *2* –

Fig. 25. The temperature on an external (cold) surface specimen: *1* – without coating, *2* –

The models with ceramic TBC ZrO2 + 8%Y2O3 deposited on plasma technology were made of Ni-alloy. Other models with TBC of columnar structure deposited on electron beam technology were made of other Ni-alloy. Unprotected halfs of each model were made of the same material as halfs with TBC. The tests on each model were repeated some times for maintenance of reliability of determination of heat-protective efficiency. At retesting the model was unwrapped about the axis on 180 °. The difference of temperatures at repeated measurements did not exceed 10 °С. The maximal temperature on the "cold" side of a wall made 900 °С. The temperature of a gas stream made 1773 K. The experimental investigations have shown that the efficiency of decrease of metal temperature at gas-flame heating after deposited TBC by thickness = 120 m on plasma and electron beam technologies make correspondingly *t*мах =

with ceramic coating (APS technique)

with ceramic coating (EB technique)

60-70 °C and *t*мах = 100-110 С. By he received results it is possible to estimate the thermal conductivity EB ceramic coatings which on the average in 1,6 times is lower than at APS coatings. Thus the received results of experimental estimation of the thermal conductivity and decrease of wall temperature of heat-resistant materials after deposited TBC of ZrO2 + 8%Y2O3 by thickness about 120 m show that at gas-flame heating of models the investigated EB coating of columnar structure protects metal is better than the tested APS coating. The developed original method of the experimental determination of thermal conductivity and estimation of efficiency of the thermal protection of details with thermal barrier ceramic coatings at gas-flame heating provides the reception of more exact data about thermophysic properties of ceramics under operating conditions of turbine details of aviation engines.
