**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

Investigations of Thermal Barrier Coatings for Turbine Parts 139

temperature of parts lies in the range 100-200 K/s. In this case, there arise cyclic thermal stresses and deformations of the base material and coating, which are accompanied by the appearance of alternating stresses. The results of tests for thermal fatigue of parts with thermal barrier coatings can differ significantly from the results of tests for cyclic heat resistance, which have been obtained by developers at a low rate of change in temperature. Therefore, in the design of thermal barrier coatings, it is necessary to investigate their heat resistance together with a protected material under the conditions providing high rates of heating and cooling. The tests performed in a gas-dynamic flow are expansive and require a long time. The high-frequency induction heating is significantly lower in cost and requires a shorter time. The process of high-frequency heating involves not only induction heating of conductive materials but also heating of dielectrics, including ceramic materials. The dynamics of heating of the coating and the base material depends on the electrophysical and thermophysical properties of the material, its volume, the cooling conditions, the rate of heating of the object, the dielectric properties of the ceramic coating, and the frequency of the electric current used for heating. The calculated simulation of the heating conditions for parts with thermal barrier ceramic coatings has not been adequately developed as compared to thermal calculations of the parts operating in a gas dynamic flow. More reliable data on the temperature state of parts with thermal barrier ceramic coatings during their heating in a high-frequency electromagnetic field and on their heat resistance can be obtained from experimental investigations. In order to create prerequisites that are necessary for the development of computational methods used for determining the thermal and thermostressed states of parts with thermal barrier coatings in the course of their heating in a high-frequency electromagnetic field and for the experimental evaluation of the thermal cyclic fatigue life of these parts, in this work we set the problem of the development of a technique for high-frequency heating and thermophysical measurements in tests of blades and models of other parts with thermal barrier coatings based on zirconia. The develop of a design-experiment method is necessary for modeling of high-frequency induction heating and determination of fatigue and thermophysical measurements in thermal cycling tests of blades of gas turbine engines, to perform experimental investigations on the determination of the temperature state of blades and models with zirconia thermal barrier coatings with the use of a thermal vision system during highfrequency heating of parts with ceramic coatings, to determine the ratio between the processes of high-frequency and dielectric heatings, to obtain a generalized dependence of the temperature gradient across the ceramic coating thickness on the frequency of the electric current from multivariant calculations, and to compare the thermal cyclic fatigue

lives of parts with a thermal barrier coating and without it.

The design-experiment method involves complex interrelated physical processes (such as heating of metal and ceramic materials in a high-frequency electromagnetic field, dielectric heating of the ceramic material, and interactions of nonstationary fields of temperatures and thermal stresses in a metal-ceramic part with cooling holes) and takes into account the electrophysical and thermophysical properties of the materials in thermal cyclic tests [Kuvaldin &Lepeshkin, 2006). New tasks on the determination of the ratio between the processes of high-frequency and dielectric heatings and on the identification of the dielectric heating effect and its influence on the distributions of heat fluxes and tem-

**3.2 Technique and results of investigations** 

current, the coating temperature is lower than the base material temperature. Radiant heating of parts is of certain use when conducting the thermocyclic tests for specimens with a TBC. In so doing the surface is heated at a high rate, however, because of radiation focusing during test of a part (or a part model) it is difficult to simulate the required temperature field. Additionally, the heaters have a low cyclic lifetime. Evidently, induction heating with the use of high-frequency currents in the surface of a part is of greatest use to heat parts and models of parts when conducting tests for thermomechanical fatigue. Such a method may be used to test both standard specimens and engine parts. When it is used, the surface part heating realized under service conditions is well simulated. In so doing, heat releases directly in the part. There is no need to use expensive heating equipment, and the equipment used features of high durability. The mechanical loading device can be used in the rig with inductor heating. It provides the possibility of conducting thermomechanical fatigue tests of turbine blades. In so doing, with the use of a special inductor the temperature field is simulated for the blade section under the service conditions of which the strength margin is minimum and with the use of a suitable loading device, the centrifugal load is simulated in this section. It is worth noting that induction heating is only effective for testing of metallic alloys. For tests of parts made of ceramic materials, it is recommended in a number of papers to use dielectric heating (in Mega Hertz frequency range) or heating with the use of a susceptor. In the latter case, it is not possible to provide suitable heat-up rates for the temperature of the part. As conducted investigations showed that when using currents of more than 400 kHz to heat a metallic part with a TBC, both heating of metal located under the external layer coating and the effective heating of the dielectric (TBC) take place. Correlation of heat shared depends on the thermophysical properties of the base and coating materials and the frequency at which heating is performed, and a number of other factors. The experiments showed that the ceramic ZrO2 based thermal barrier coatings on specimens and parts made of high-temperature nickelbased alloys are effectively heated at frequencies between 0.4 and 2.0 MHz. Use of a higher frequency requires a complicated rig design. Consequently, it seems that in spite of a lack of data concerning the absence of a knowledge of the influence of induction heating on mechanical properties of the materials under investigation, this method of heating can be successfully used for tests of specimens and engine parts (primarily for comparative tests for selection of coatings and materials, design solution, manufacture and repair of engine parts with a TBC by production processes). The cost of the tests conducted with the use of highfrequency heating is by an order lower than the cost of the tests conducted on a gasdynamic rig.
