MICROSTRUCTURAL TURBINE BLADES ANALYSES AFTER ENDURANCE TEST OF AIRCRAFT ENGINE

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1 MICROSTRUCTURAL TURBINE BLADES ANALYSES AFTER ENDURANCE TEST OF AIRCRAFT ENGINE Jaroslav TRNÍK a, Ján KAFRÍK a, Ondřej DVOŘÁČEK b a Faculty of Industrial Technologies, University of Alexander Dubček in Trenčín, I. Krasku 491/30, Púchov, Slovakia, trnikjaro@gmail.com, kafrikjan@gmail.com b PBS Velká Bíteš, a.s., Vlkovská 279, Velká Bíteš, Czech Republic, odvoracek@gmail.com Abstract Nickel-base superalloys are used in the aircraft turbines because of their superior properties and performance at high temperatures. Superalloys in such environments are limited from the view of their lowered resistance to the oxidation and corrosion impacts. This problem has been solved using protective coatings deposited on the surface of such engine components as turbine blades and they act as a diffusion barrier by creating thermodynamically stable oxide layer. There are other sources of loads imposed on turbine blades and vanes; centrifugal forces due to rotation, creep, low cycle fatigue and time varying thermo-mechanical fatigue due to sequential engine start-ups and shutdowns. Except oxidation and corrosion attack, the life of the coatings can be reduced by cracking caused by thermal and mechanical cycling due to poor mechanical behaviour of the coatings. The focus of the paper is on the study of thermomechanical fatigue behaviour of turbine rotor blades made from polycrystalline IN 713 LC with diffusion aluminide coatings under service conditions. This test revealed a few of fatigue cracks in the upper part of the layer but they were stopped by inner diffusion zone of the coating. Keywords: superalloys, aluminide coatings, turbine blades, thermal and mechanical fatigue. 1. INTRODUCTION Present aircraft engine are developed with accent to improve operating parameters, mainly performance and thrust of engines. This can be ensured by increasing of gas temperature. Turbine blades work under extreme conditions and a complex state of stress. Turbine blades and other parts of jet engines are often exposed to oxidizing and corroding atmosphere during their operation. Turbine blades resist against high temperatures, increased temperature gradients and high stresses. The interaction among hot combustion gases causes oxidation of the surface layer and hot corrosion and micro cracking of the coating. Gas flow formed in combustion chamber in aircraft engine causes mechanical and thermal strains, which are responsible for start of crack and crack growth in engine parts. The factors limiting the lifetime of turbine blades are the quality of the aluminide coating and microstructure of the superalloy, depending on the service parametersthe temperature and the duration of service [1]. Therefore is necessary to protect nickel-base superalloys, from which are engine parts made, by diffusion coatings. These coatings deposited on superalloys represent the barrier between basic material called substrate and external aggressive environment [2]. The coatings markedly increase lifetime, reliability and function of individual engine parts and thus contribute to the flying safety [3]. Change of flight mode, start or stop of aircraft engine generate thermal and mechanical strain too, which damage engine components [4]. This type of degradation mode is called thermo-mechanical fatigue (TMF). TMF is characterized as long-time creation of external mechanical and thermal strains as well as residual stresses from different thermal expansion between superalloy material and coating [5].

2 2. FUNDAMENTAL OF TEST EXPERIMENT TJ 100 (Fig. 1) is compact designed small turbine engine with perfect weight/thrust ratio, than is especially characterized by low fuel consumption and low exhausted gas production (Fig. 2). This type of engine is used for sports ultralight planes, gliders with additional engine or other pilotless jets. Fig. 1 Small turbine engine TJ 100 Fig. 2 Performance characteristics of TJ Endurance time test of turbine engine This test confirms function ability, reliability and applicability of engine for flight operations. After endurance test damages or excessive wear of main parts are not accepted in comparison with control before test. In process of test are engine modes changed from maximum performance to idle mode (Tab. 1). Total time of test took 55 hours, because test involves 50 cycles and each cycle hold 66 minute of operation (Tab 2). Tab. 1 Operation modes of engine TJ 100 Operation mode Parameter Unit Max. Take off Max. permanent Idle run Revolution % ~50 Thrust N < 160 Fumes temp. C < 800 < 750 < 500 Power generator W Tab. 2 One cycle composition of time test Pt Holding time Operating condition 1. 1 min Engine start and idle min Maximum take-off mode (100% thrust) 3. 5 min Maximum permanent mode (97% of revolutions) 4. 1 min Idle run (50% of revolutions) 5. 5 min Maximum soaring mode (100% thrust) 6. 1 min Idle run min Maximum permanent mode (97% of revolutions) 8. 1 min Idle run 9. Overall 10 min 6 cycles of acceleration and deceleration min Idle + engine stop +cooling, than 12 min break

3 3. TURBINE BLADES MICROSTRUCTURE ANALYSIS Most parts of engine TJ 100, especially turbine blades was casted from polycrystalline nickel-base superalloy INCONEL 713 LC. Turbine blades were protected by diffusion Al coating using CVD method, concretely out of pack. Stator and rotor blades are localized in stator and rotor wheel of aircraft engine. Blades were investigated in three cross-sections marked on (Fig. 3) and (Fig. 4). Fig. 3 Marked crosssections for analysis of stator blade Fig. 4 Marked crosssections for rotor blade analysis Samples for selected stator and rotor blades were analysed by scanning electron microscopy (SEM) to observe fatigue cracks or other damages. Leading edges (LE) and trailing edges (TE) of blades were analysed too. Closer information about chemical content or phase changes on surface of blades provided energy dispersive x-ray analysis (EDS). 3.1 STATOR BLADE ANALYSIS Surface analysis of stator blade showed ( Fig. 5), that section LE 1 to LE 3 contain no significant defects or damages. In marked blue area wads carried out EDS analysis, which results represents (Fig. 6) and (Tab. 3). Fig. 5 Analysed surface of stator blade Fig. 6 EDS analysis of stator blade surface

4 Tab. 3 Chemical composition of stator blade surface Na-K Al-K Si-K P-K Ca-K Ti-K Cr-K Fe-K Ni-K pt1 1,2 67,7 1,3 16,5 1,6-1,7 1,5 8,4 EDS analysis demonstrated, that surface of stator blade content especially aluminum, which together with oxygen create protective layer Al 2 O 3. Other elements as P, Ca or Na were made by combustion process of engine fuel. On stator blade in section leading edge 1 was found one crack (Fig. 7). Detailed view of crack (Fig. 8) showed that damage has only surface character and doesn t goes to the deep. Fig. 7 Fatigue crack on leading edge of stator blade Fig. 8 Detailed view of crack 3.2 ROTOR BLADE ANALYSIS Fig. 9 represents comparison of trailing edges 1-3. Detailed analysis showed, that surface of trailing edges are without any significant damages, cracks or phase changes. Fig. 9a Detailed view on rotor blade TE 1 Fig. 9b Detailed view on rotor blade TE 2 Fig. 9c Detailed view on rotor blade TE 3

5 Investigation of rotor blade in cut showed, that rotor blade contains no cracks. Average thickness of coating on TE 1 TE 3 was measured 59,5 μm. In marked points 1-9 on cross-section of rotor blade (Fig. 10) was carried out EDS analysis (Fig. 11). Fig. 10 EDS analysis of crosssection on rotor blade Fig. 11 EDS analysis of cross-section of rotor blade EDS analysis showed, that surface contains Al and O which formed Al 2 O 3. Organic elements as C and P were originally from combustion process probably. Content of aluminium is lower in direction into the coating and nickel content increases in direction into base material (Tab 4.). Tab. 4 Chemical content of rotor blade cross-section Na-K Al-K Si-K P-K Ti-K Cr-K Ni-K Nb-L Mo-L pt1 8,0 64,5-14, ,5 - - pt2-88, ,8 10,6 - - pt3-46, ,6 51,5 - - pt4-13,2 0, ,2 23,3-11,2 pt5-7,9 0,3-0,3 64,8 14,7-12,1 pt6-12,4 1,4-2,0 10,0 54,6 9,7 9,8 pt7-43, ,2 51,4 - - pt8-37, ,6 4,6 52,8 3,4 1,0 pt9-24, ,9 23,7 44,6 1,8 4,6 In the end was carried out mapping of chemical elements in cross-section 1 (Fig. 12). Fig. 12 Mapping of chemical elements in cut 1

6 4. CONCLUSION Stator and rotor blades were analysed by SEM to find out fatigue cracks or damages. EDS analysis was used to observe phase changes or chemical composition of blade surface and coatings. Turbine blades underwent 50-hours endurance time test under the thermo-mechanical loading. Stator and rotor blades were deposited by protective aluminide coating in CVD process. Aluminide coating acts as diffusion barrier and protects base material of blades (substrate) from high temperature environment in aircraft engine. One fatigue crack was found on stator blade in section leading edge 1, but this mentioned fatigue crack has only surface character and it was not markedly deep. The coating was ruptured neither by the corrosion process nor oxidation which cause generally the flow of hot gases during the burning of aviation fuel in the combustion chamber. There was not any loss of coating thickness. Rotor blades were without cracks and other damages. From the aspect of the thermo-mechanical fatigue which was simulated by help of the endurance time test, it can be concluded that the protective coating is not degraded in a significant way and from the practical aspect it can be used for determined lifetime and conditions of exploitation. LITERATURE [1] SOZANSKA, M.; CHMIELA, B.; KIANICOVÁ, M.; et al. Degradation of microstructure after service in ZhS6K superalloy with diffusive aluminide coating. 2011, MICROSCOPY XIV Book Series: Solid State Phenomena Volume: 186 Pages: DOI: / [2] BOSE, S. High temperature Coatings. Elsevier Science & Technology Books, p., ISBN [3] ASM Handbook: Failure Analysis and Prevention, Vol. 11, ISBN: [4] TAMARIN, Y. Protective Coatings for Turbine Blades. Ohio: ASM International, p., ISBN [5] HUANG, Z.W., WANG, Z.G., ZHU, S.J., JUAN, F.H., WANG, F.G. Thermomechanical fatigue behavior and life prediction of a cast nickel-based superalloy. Materials Science and Engineering A. 2006, vol. 432, no. 1-2, pp ISSN