**3. Investigations of ceramic thermal barrier coatings of parts with the use of HF induction heating**

## **3.1 HF induction heating of ceramic thermal barrier coatings of parts**

At present, the cyclic fatigue life of thermal barrier coatings in the course of their development has been studied using radiant heating with a low rate (less than 20 K/s), which does not correspond to actual operating conditions. At such low heating rates, thermal stresses are almost completely absent and the main damage factor is the oxidation of a sublayer, which leads to spalling of the coating. Actually, these processes are heat resistance tests at variable temperatures. Under real conditions, the rate of change in the

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

**3. Investigations of ceramic thermal barrier coatings of parts with the use of** 

At present, the cyclic fatigue life of thermal barrier coatings in the course of their development has been studied using radiant heating with a low rate (less than 20 K/s), which does not correspond to actual operating conditions. At such low heating rates, thermal stresses are almost completely absent and the main damage factor is the oxidation of a sublayer, which leads to spalling of the coating. Actually, these processes are heat resistance tests at variable temperatures. Under real conditions, the rate of change in the

**3.1 HF induction heating of ceramic thermal barrier coatings of parts** 

rig.

**HF induction heating** 

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.

### **3.2 Technique and results of investigations**

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-

Investigations of Thermal Barrier Coatings for Turbine Parts 141

in the ceramic coating was 1.8 x 105 W/cm2 (these specific heating powers were obtained from the solution of the electromagnetic problem). The minimum and maximum heating temperatures of the metal surface of the part in the thermal cycle were equal to 350 and 900 °C, respectively. The mathematical simulation of the thermal state of the ceramic coatings takes into account the specific features of the electrophysical properties of zirconia. In particular, an increase in the temperature results in an increase in the permittivity, the dielectric loss tangent, and the electrical conductivity (Kuvaldin & Lepeshkin, 2006). On the whole, the ceramic coating in the course of the test thermal cycle was heated by means of both the heat transfer from the metal of the part and the dielectric heating. The computational scheme for a fragment of the cooled part with the thermal barrier coating is

shown in Fig. 10.

Fig. 9. Parameters

 and *tg*

depending on temperature

Fig. 10. Schematic diagram of a fragment of the cooled part with the thermal barrier ceramic coating: *(1)* ceramic coating, *(2)* metal of the workpiece, *(3)* refractory metal layer, and *(4)* 

direction of the flow of cooling air in the hole. Designation: *d* is the hole diameter

peratures in a metal-ceramic part are set in computational and experimental studies. By using multivariant calculations, it is necessary to obtain a generalized dependence of the temperature gradient across the ceramic coating thickness on the frequency of the electric current.
