Thermal Cycling Setup for Testing Thermal Barrier Coatings

Abstract

A thermal-cycling method for testing the life time of ceramic coatings is described. The test rig is optimized to test thermal barrier coatings for gas-turbine applications. The coatings consist of a ceramic layer on a metallic bond coat and metallic substrate. The test experiment is performed with a thermal gradient through the sample, corresponding (in the case of the most extensively studied samples) to temperatures of 1050±1400 C at the surface of the ceramic layer and 950± 1100 C at the ceramic±metal interface. To characterize the instrument some technical details, the temperature and stress profile of standard samples, the reproducibility, and the basic sample analysis are discussed.Thermal barrier coatings (TBC) are frequently used in thermally highly loaded parts of gas turbines as turbine blades and vanes. [1±3] Despite their frequent use there is a need for further development of TBCs, and research is being done at a large number of public and industrial places.However, the community still has not developed a standard test method, which would have the advantage of the comparability of results. Certainly it is difficult to find a testing method that combines the requirements of testing all relevant loading cases. The main ideas range from isothermal or cyclic oxidation of samples in a furnace [4±7] to thermal mechanical fatigue (TMF) tests, [8] which combine thermal and mechanical loading. Also the durability against corrosion or impurities has been investigated. [9,10] For high-temperature applications, the resistance to cyclic thermal loading is at least as important as the cyclic mechanical loading test for the lower temperature regime. [11] Especially in the case of TBCs in aeronautic applications, severe and frequent cycles are to be expected. For example, at the end of (hypothetically) the same life span, the aeronautic turbine has endured more cycles by a factor of 200. [12] From an economic point of view and to fulfill the demand of optimal technical reliability it is necessary to explore and improve the life time of all components. This leads to the possibility of extending the operating conditions to higher working temperatures and therefore a thermodynamically more efficient regime.In this communication, an instrument for thermal-cycling tests is presented, which simulates the turbine operating conditions and allows a wide range of variations in temperature, sample geometry, and composition to systematically study the influence of several relevant parameters.In the testing facility samples are periodically heated by a natural gas/oxygen burner and cooled in a current of air. During heating the reverse side of the samples is cooled by compressed air to maintain a temperature gradient of typically 0.4±1.0 K/lm across the sample. Such high heat fluxes are possible because disc-shaped samples are used, which allow good heat transfer at both the front and the back face. After the high temperature phase, the burner (mounted on an airdriven slide) is removed and the sample is cooled from both faces...

Development of YSZ Thermal Barrier Coatings Using Axial Suspension Plasma Spraying

“…In the aim of getting reliable lifetime results, two specimens were subjected to thermal cycling tests for each kind of microstructure. The detailed information about the test facility and used samples were given in [33]. Thermal cycling lifetime was defined as the number of thermal cycles that a sample survives before the appearance of visible spallation (25 mm 2 ).…”

Atmospheric Plasma Spraying of Single Phase Lanthanum Zirconate Thermal Barrier Coatings with Optimized Porosity

“…Details on the thermal cycling test setup can be found in [16]. The maximum surface temperature applied in this work was approximately 1400 • C, while the bondcoat temperatures were kept at approximately 1070 • C. Dwell times were 5 min for heating and 2 min for cooling.…”

The Effect of Samples Geometry and Thermal Cycling Test Type on the Thermal Shock Behaviour of Plasma Sprayed TBCs

“…[12] During the cycles, the plane tensile stresses were generated due to the thermal expansion difference between the coating and substrate. [13][14][15] Horizontal cracks occurred near the top coat/ bond coat interface with increase in…”

“…The cycles were carried out for 5 min heating and 2 min cooling, and it was repeated until failure occurred. Cycling was stopped when a visible spallation of the coating occured [12][13][14] Characterization Coated samples were impregnated with epoxy and then sectioned, ground and polished. The surface planar section and cross-section of the coatings were examined macroscopically by using scanning electron microscopy (SEM).…”