Structure and properties transformation of heat-resistant coating in the process of high-temperature cyclic tests of the turbine blade

Metallurgy and Material Science

Metal science and thermal processing of metals and alloys


Аuthors

Il’inkova T. A.1*, Il'inkov A. V.1**, Klimkin Y. O.1***, Zhivushkin A. A.2****, Budinovskii S. A.3*****

1. Kazan National Research Technical University named after A.N. Tupolev, KNRTU-KAI, 10, Karl Marks str., Kazan, 420111, Russia
2. UEC-Klimov, 11, Kantemirovskaya Str., Saint-Petersburg, 194100, Russia
3. All-Russian Scientific Research Institute of Aviation Materials. State Research Center of the Russian Federation, 17, Radio str., Moscow, 105005, Russia

*e-mail: pochta20006@bk.ru
**e-mail: ailinkov@mail.ru
***e-mail: yurakl@mail.ru
****e-mail: azhivush@gmail.com
*****e-mail: admin@viam.ru

Abstract

Thermal-cycle tests of uncooled working blades of the second stage of the new generation helicopter gas turbine engine turbine were conducted, and changes in the composition, structure and micromechanical properties of the heat-resistant coating were studied.

The blades are made of the new VZhL-21 poly-crystalline casting alloy. The heat-resistant coating was applied employing the MAP-2 installation according to the serial technology by successive applying of the condensed layer of the Ni-20Co-20Cr-12Al-Ti-Y composition (inner layer) and diffusion layer of the Al-5Si-B composition (outer layer).

Both condensed and diffusion layers were being applied in vacuum at the specified parameters of the arc current and bias voltage at the products for 200-220 and 60-65 minutes respectively. After this, vacuum thermal processing of the blades was performed at the temperature of 1000 °C for 240 min to complete the coating structure and phase composition formation.

Comparative tests of blades with and without coating were conducted under identical conditions on a special test bench by a technique that ensures the thermal cycle reproducibility while multiple repetitions. The principle of operation of the experimental setup consisted in the ohmic heating of the test blade with direct electric current, varying according to a given algorithm. The thermal cycle selected for the blades testing was calculated based on an engine test: heating to 480 °С (120 s exposure at this temperature), temperature raising to 770 °С (150 s exposure). Further, cooling to 480 °С (120 s exposure), and cooling to room temperature. After the predefined running time, the blades were being removed from the test and subjected to microstructural and micro-chemical studies of the coating state on the JSM6460-LV scanning electron microscope with the INCA ENERGY 300 energy dispersive attachment, as well as micromechanical measurements on the Shimadzu DUH-211 dynamic ultramicrotester (Japan) using Berkovich indenter. The results of the studies revealed that the coating microstructure on all tested blades had not undergone significant changes compared to the initial one.

In the process of the thermal running time of 500-800 cycles, there is an aluminum diffusion from the coating surface to the contact bound of both coating zones and further to the blade surface. With the running time increase up to 1350 thermal cycles, aluminum diffuses deeper into the blade metal. The character of chromium diffusion seems to be more complicated. Chromium concentration changes insignificantly on the coating surface. However, in the place of the contact of both zones the chrome concentration reduces drastically at running time of 500 cycles and stays at the attained level up to the maximum running time of 1350 cycles. Finally, the “coating-blade” contact zone significantly enriches with chrome.

The creep of the coating material remains at approximately the same level up to 800 thermal cycles, and then increases sharply, while the share of the plastic component of the mechanical work on deforming the coating material starts increasing sharply somewhat earlier, beginning from 500 cycles.

Thus, the performed comprehensive study allows predicting the coating protective functions preserving for no less than 500 thermal cycles.

Keywords:

turbine blade, two-layer heat-resistant coating, thermal fatigue, chrome and aluminum diffusion, microstructure, micromechanical properties

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