**6. Thermal barrier ceramic coatings of variable thickness**

One of the directions of future investigations are improvement of the designs and rise of durability of ceramic coatings and the development of advanced technologies for their depositing. The blades and turbine components of gas turbine engines have a ceramic coating of uniform thickness. The disadvantages of the part design are the increased thickness of the ceramic coating that maintains or increases the unevenness of temperature distribution and thermal stresses in the metal blade. In addition, in the increased ceramic coating mass of the turbine blades will arise the increased stresses under the influence of centrifugal forces. The factors can lead to lower life of a coating and blade. On a turbine parts is possible to put ceramic coatings of variable thickness. The coatings of variable thickness allow to raise the durability of a turbine part due to increase of strength of a covering and reduction thermal stresses in part metal. The durability raises due to variable thickness of a coating and due to uniformity of temperature distribution in a junction of a coating with metal of a part and due to performance of a coating of the maximal thickness in zones of the maximal temperatures and the minimal thickness in zones of the minimal temperatures on a surface of a coating. In result the temperature drops and thermal stresseson a profile and height of a part (blade) are reduced. Also the probably of occurrence of defects, cracks and chipping of ceramics in the areas of stress concentration decreases and the durability of a coating and blade increases in view of influence of centrifugal forces and

Investigations of Thermal Barrier Coatings for Turbine Parts 165

TBC. It was experimentally shown that when using such a heating method, the temperature of the outer ceramic coating surface exceeds the temperature of the metallic bond coat by 50- 70°C. Thus, a design-experiment method of high-frequency induction heating and thermophysical measurements in thermal cycling tests of blades and other cooled parts with a TBC has been developed taking into account the electrophysical and thermophysical properties of their materials. The results of thermophysical measurements, design-experiment studies of the nonstationary thermal state of the parts with coatings with the use of a thermal vision system, and thermal cycling tests of blades and models of flame tubes with thermal barrier ceramic coatings are presented. The generalized dependence of the temperature gradient across the ceramic coating thickness on the frequency of the electric current was obtained using multivariant calculations. The test rig with induction heating at a frequency 440 kHz was used also for making the comparison evaluation of the influence of various thermal ceramic barrier coatings (APS and EB techniques) on thermocyclic strength of the parts with a TBC (to select material and coating for the part and mature the repair technology). Gas-flame heating is considered to be preferable when investigating the gas-turbine engine parts with a TBC for thermofatigue in the special cases when both the convective and radiant components of thermal flow are of great importance. The small-size rig with gas-flame flow made it possible to conduct the comparison investigations with the purpose of evaluating the efficiency of thermal protection of the ceramic deposited thermal barrier coatings on APS and EB techniques. The developed design-experiment method was introduced in bench tests of turbine blades of gas turbine engines. The use of the developed method for high-frequency induction heating in thermal cycling tests of blades and models with thermal barrier ceramic coatings made it possible to reduce the duration of tests and their cost and to obtain the experimental evaluation of the service life of ceramic coatings with due regard for their nonstationary thermal and thermostressed states. The results of successful tests were used in operation of aircraft engines. At designing columnar ceramic coverings it is necessary to consider that their allowable thickness depending on conditions of influence of the centrifugal forces on the basis of carried out investigations. The further investigations will be carried out in the following directions: comparison of the results of the evaluations of the thermal cyclic fatigue life of the cooled parts during gas and high-frequency induction heating, improvement of the technique developed for determining the thermal conductivity of ceramic coatings

deposited by the electron-beam technique and design of new types of coatings.

in carrying out of the investigations of a thermal ceramic barrier coatings.

This work has been performed at CIAM. I is very thankful to Dr. N.G. Bychkov for his help

Stringer, J.; Streiff, R.; Krutenat, R. & Gaillet, M. (1989) The Reactive Element Effect in High-

Malashenko, I.S.; Vashchilo, N.P.; Belotserkovsky, V.A. & Yakovchuk, K.Y. (1997) Effect of

Miller, R, A.; Garlick, R.G. & Smialek, J.L. (1983). Phase Distribution in Plasma-Sprayed

Zirconia-Yttria. *Am. Ceram. Soc. Bull.,* Vol.63, No.12, pp. 1355-1358

Temperature Corrosion, *High Temperature Corrosion.* No.*2,* pp. 129-137. Elsevier

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**8. Acknowledgment** 

Science Publishers

*Electrometali,* Vol.1, pp. 24-33

**9. References** 

variables thermal stresses. The examples of designs of turbine parts of with thermal barrier ceramic coatings of variable thickness are considered below. The turbine blade and flame tube of a combustion chamber of gas turbine engine with coating variable thicknessare presented in Fig. 38 and Fig. 39 (Lepeshkin, 2005).

Fig. 38. Turbine blade with thermal barrier ceramic coating of variable thickness:*1*- blade,*2*  coating

Fig. 39. Flame tube of a combustion chamber with thermal barrier ceramic coating of variable thickness:*1*- flame tube, *2 -* coating
