, Brno, Czech Republic, EU

Transcription

1 STRUCTURAL STABILITY OF PLASMA SPRAYED THERMAL BARRIER COATINGS AFTER ISOTHERMAL EXPOSURE AT 1050 C David JECH a,b, Ladislav ČELKO a.b, Lenka KLAKURKOVÁ a,b, Simona HUTAŘOVÁ a,b, Martin JULIŠ a,b, Jiří ŠVEJCAR a,b a Brno University of Technology, Central European Institute of Technology, Centre for Advanced Materials, Research Program Structure and Phase Analysis, Technická 3058/10, Brno, Czech Republic, EU, jechdavid@gmail.com b Brno University of Technology, Faculty of Mechanical Engineering, Institute of Materials Science and Engineering, Technická 2896/2, Brno, Czech Republic, EU Abstract In present work, the influence of isothermal heat treatment on structural changes of conventional thermal barrier coatings (TBCs) was studied. The as metallic bond coat (BC) in thickness of 200 µm and 8 wt. % yttria stabilized zirconia (YSZ) as ceramic top coat (TC) in thicknesses of 200, 500 and 1000 µm were produced by means of atmospheric plasma spraying (APS). These coatings were sprayed onto the surface of newly developed fine-grained cast polycrystalline nickel-based superalloy Inconel 713LC. Asprepared samples were exposed to the temperature of 1050 C for hours in ambient atmosphere. In comparison with the initial state, where discontinual islands of alumina were found on the bond coat top coat interface, the uniform and continuous oxide layer, also termed thermally grown oxide (TGO), was observed there after the isothermal heat treatment. The growth of TGO layer in dependence on dwell time was also found on the expense of depletion of aluminium from the bond coat. Structural changes of bond coat promoted by above mentioned mechanism and growth of TGO were evaluated by means of electron microscopy and image analysis techniques. Keywords: Thermal Barrier Coatings, Thermally Grown Oxide, Isothermal Heat Treatment, Atmospheric Plasma Spraying,, Structural Stability 1. INTRODUCTION Nickel-based superalloys were developed for high temperature applications mainly in aircraft and power industries. Degradation of extremely loaded components caused by high temperature oxidation is the most common cause of its failure. Thermal barrier coatings provide protection to the superalloys, because of they are not able to resist to extremely hard working conditions, including high temperature oxidation, temperature gradients, high pressures and multiaxial stresses [1, 2]. Yttria stabilized zirconia top coatings have excellent thermal shock resistance in combination with low thermal conductivity, but with relatively high coefficient of thermal expansion. The metallic M-CrAlY bond coatings (M = Ni and/or Co) provide a rough surface for better mechanical bonding of the ceramic top coat and simultaneously protect superalloy substrates against high temperature oxidation and also compensate differences in coefficients of thermal expansion between top coat and substrate [3]. Under the high temperature exposure, and in contact with ambient atmosphere, on the metallic bond coating surface the continuous TGO layer between BC and TC interface is going to be formed. This TGO layer also serves as a diffusion barrier and protects the substrate alloy against oxidation. TGO is thermodynamically stable, slowly growing and adherent to the metallic BC. Generally consist of protective α-al 2 O 3, which inhibits an additional reaction with active oxygen ions and other species. On the other hand, the growth of this layer leads to reduction of Al activity in BC and other oxides, such as chromia (Cr,Al) 2 O 3, Ni(Cr,Al) 2 O 4 spinel and

2 nickel oxide NiO, can be formed in APS produced TBC systems. This complex oxide can also disrupt the TBC durability, because of its relatively fast growth in thickness and related volume changes [4, 5]. A ceramic top coat and a metallic bond coat can be deposited by several different techniques using plasma arc. Low pressure or vacuum plasma spraying (LPPS and VPS) has an advantage in minimized oxidation of particles during deposition. However, due to the comparatively high operating costs and time-consuming (associated with evacuating large production spray cells) is atmospheric plasma spraying technique predominantly used in coatings production [6]. The composition and fine microstructure of TGO has a great influence on TBC durability and therefore the aim of this paper is focused on structural stability of atmospheric plasma sprayed ZrO 2 +Y 2 O 3 thermal barrier coatings after the isothermal exposure at 1050 C for different dwell-times in ambient atmosphere. 2. MATERIAL AND METHODS The conventional TBC system was prepared by atmospheric plasma spraying from commercially available powders (GTV ) and (GTV ) which were purchased from company GTV GmbH. Nominal chemical composition and the average particle size guaranteed by the manufacturer are summarized in Table 1. Tab.1 Nominal chemical composition (wt. %) and the average grain size of used powders [μm] Powder Average size Al Y Zr Cr Ni Co bal bal This type of functional graded coating system was deposited on specimens made of a newly developed finegrained cast polycrystalline nickel-based superalloy Inconel 713LC. Substrate s surface was blasted by angular Al 2 O 3 particles due to formation of mechanical interlocks which ensure better adhesion of metallic bond coat to the substrate surface. The metallic bond coat and ceramic top coat were produced by atmospheric plasma spraying in ambient atmosphere in cooperation with the S.A.M. Metallizing company. The thickness of as-sprayed bond coat for all specimens was about 200 μm. The ceramic top coat was sprayed in thicknesses of 200, 500 and 1000 μm. Structural stability of all systems was studied after isothermal annealing in Heraeus tubular furnace. Specimens were exposed to temperature of 1050 C in ambient atmosphere with dwell ranging hrs. Metallographic samples were made of deformationfree cutting using the apparatus Akutom co. Struers. Subsequently were the samples grounded by sandpapers (from #120 up to #2000) under the intensive water cooling, polished by 3 μm and 1 μm diamond pastes and finally chemically polished using OPCHEM. Polishing dwell and pressing force were reduced to avoid the crack formations and minimize negligible enlargement of pores on less bonded regions/interfaces of the coating system. To microstructural studies were used the light microscope (LM) GX-51 co. Olympus and scanning electron microscope (SEM) Super Ultra 50 co. Zeiss equipped by energy dispersive microanalyzer (EDX). To BC and TC coatings thickness measurements was utilized the image analysis software StreamMotion co. Olympus. 3. RESULTS A DISCUSSION The atmospheric plasma sprayed conventional ZrO 2 +Y 2 O 3 thermal barrier coating system in initial state (Fig. 1) shows a typical inhomogeneity in microstructure which correspond to used APS technology. Yttria stabilized zirconia top coats are deposited continuously and locally irregularly in thickness, see Fig. 1a-c. As can be shown, these irregularities correspond to initial geometry of the underlying bond coat [7]. In its microstructure is able to observe the number of microcracks (especially apparent in the case of thick 1000 μm top coat), cracks at the interfaces of individual splats and closed

3 pores. The microstructure of bond coat deposited onto the surface of newly developed finegrained cast polycrystalline nickel-based superalloy Inconel 713LC substrate seems to be denser and more uniform as in a case of ceramic top coat. It could be related to the thermal expansion and toughness of these materials during rapid cooling [8]. The bond coat consists of remelted flattened particles, also termed splats, an adequate quantity of area porosity (approx. 8%), small amount of unmelted particles and oxide scale formed in between of the splats, see Fig. 1d [2]. a) 200 µm 500 µm b) Substrate Substrate c) 1000 µm Substrate BC/TC interface Fig. 1 Cross section micrographs of YSZ- TBC system with thickness of ceramic top coat of (a) 200 m, (b) 500 m, (c) 1000 m (LM) and (d) detail of as-sprayed metallic bond coat and ceramic top coat interface (SEM). The as-sprayed state is characteristic by the number of segmented not uniform Al 2 O 3 veins present within the bond coat and also on bond coat top coat interface probably as a result of metallic powder oxidation during spraying (Fig. 1d) [9]. In the TBC samples, after isothermal exposure at 1050 C, continuous and uniform oxide layer predominantly consist of Al 2 O 3 thermally grown oxide, was formed along metallic bond coat and ceramic top coat, see Fig. 2. Microstructures of bond coat and top coat after 200 hrs of isothermal exposure result in no significantly changes compare with the bond coat and top coat microstructure after 10 hrs of isothermal exposure, but with except the thickness of growing TGO layer. The figures 2a and c clearly demonstrate the influence of isothermal exposure dwell on TGO growth in thickness. d)

4 a) b) c) d) Fig. 2 Detail micrographs of bond coat top coat interface after isothermal exposure on (a) 1050 C/ 10 hrs top coat thickness of 200 µm, (b) 1050 C/ 10 hrs top coat thickness of 1000µm, (c) 1050 C/ 200 hrs top coat thickness of 200 µm, (d) 1050 C/ 200 hrs top coat thickness of 1000 µm (SEM). Chemical composition of thermally grown oxide layer and adjacent complex oxide layer was studied at several locations as indicated by numbered white point at Fig. 3. Present phases were determined based on this analysis, known stochiometry of ceramic and intermetallic phases and previous results of study on shortterm structural stability of M-CrAlY coating systems [2, 8]. Fig. 3 Detail micrograph of BC-TC interface after 1050 C/ 200 hrs - top coat thickness of 200 m (SEM); marked cross points demonstrate the places of performed EDX analyses

5 After isothermal exposure in air, an oxide layer, which contained 5,04 at% Ni, 10,02 at% Co, 11,59 at% Al, 51,63 at% Cr, 0,71 at% Zr and with oxygen as the balance (Tab. 2), started to form along with TGO layer at the interface between the top coat and bond coat. The complex oxide layer is most probably formed from (Cr,Al) 2 O 3 + (Co,Ni)(Cr,Al) 2 O 4 [9, 10]. Tab. 2 Results from SEM/EDX analysis indicated in Fig. 3 The TGO growth in thickness was measured on all samples using image analysis software and is plotted as a function of isothermal exposure dwell, see Fig. 4. Influence of different top coat thicknesses is predominantly apparent after 200 hrs isothermal exposure, when the thickness of TGO is lowest in the case of top coat thickness of 1000 µm (alumina TGO thickness is of 1,89 µm) in comparison with top coat thickness of 200 µm, where the alumina TGO thickness is of 2,42 µm. There is no clear explanation for this phenomenon and more measurements from different places on the samples must be performed. But it could also be noted that this effect based on the top coat thickness can be related with difference in thermal gradients, where lower temperature at BC-TC interface beneficially affecting slower alumina TGO scale formation [8]. Nevertheless after the 200 hrs of isothermal exposure on high temperature the existence of thermal gradient is not expected. Fig. 4 The thickness of alumina TGO layer as a function of isothermal exposure dwell

6 4. CONCLUSIONS Structural stability of atmospheric plasma sprayed ZrO 2 +Y 2 O 3 thermal barrier coatings after the isothermal exposure at 1050 C with different thicknesses of top coats were investigated. After 10 to 200 hrs dwell in ambient atmosphere and despite of ceramic top coat thicknesses was observed the formation of continuous and uniform thermally grown oxide layer. By means of electron microscopy was determined the chemical composition of TGO, where alumina layer adjacent to the bond coat and moreover another one complex oxide layer, predominantly composed of Cr, Al, Co and Ni was measured. The growth in thickness of alumina TGO layer, measured by image analysis, is strongly affected by the ceramic top coat thickness. ACKNOWLEDGEMENT The authors acknowledge the financial support for this work provided by the projects (GA 107/12/1922 and GA 107/11/2065) of Czech Science Foundation and CEITEC - Central European Institute of Technology (CZ.1.05/1.1.00/ ) from European Regional Development Fund. REFERENCES [1] Seo, D., Ogawa, K., Tanno, M., Shoji, T., Murata, S. Influence of heat exposure time on isothermal degradation of plasma sprayed coatings. Surface and Coatings Technology 201 (2007), [2] Čelko, L., Klakurková, L., Slámečka, K., Pospíšilová, S., Juliš, M., Němec, K., Podrábský, T., Švejcar, J. Structural stability of thermal barrier coatings produced by thermal spraying. In METAL 2012 Conference proceedings.1. Ostrava: TANGER Ltd., s ISBN: [3] Khan Nusair, A., Lu, J. Behavior of air plasma sprayed thermal barrier coatings, subject to intense thermal cycling. Surface and Coatings Technology 166 (2003), [4] Chen, W.R., Wu, X., Marple, B.R., Nagy, D.R., Patniak, P.C. TGO growth behavior in TBCs with APS and HVOF bond coats. Surface and Coatings Technology 202 (2008), [5] Poza, P., Gómez-García, J., Múnez, C.J. TEM analysis of the microstructure of thermal barrier coatings after isothermal oxidation. Acta Materialia 60 (2012) [6] Patterson, T., Leon, A., Jayaraj, B., Liu, J., Sohn, Y.H. Thermal cyclic lifetime and oxidation behavior of air plasma sprayed bond coats for thermal barrier coatings. Surface and Coatings Technology 203 (2008), [7] Dong, Z.L., Khor, K.A., Gu, Y.W. Microstructure formation in plasma-sprayed functionally graded NiCoCrAlY/yttria-stabilized zirconia coatings. Surface and Coatings Technology 114 (1999), [8] Begley M.R, Wadley H.N.G. Delamination resistance of thermal barrier coatings containing embedded ductile layers. Acta Materialia 60 (2012); [9] Chen, W.R., Wu, X., Marple, B.R., Lima, R.S., Patniak, P.C. Pre-oxidation and TGO growth behavior of an airplasma-sprayed thermal barrier coating. Surface and Coatings Technology 202 (2008), [10] Čelko, L., Řičánková, V., Klakurková, L., Podrábský, T., Dvořáček, E., Švejcar, J. Changes in Microstructure of Air Plasma Sprayed MCrAlY Coatings After Short Thermal Exposure in Argon Atmosphere. Acta Physica Polonica, 2011, roč. 120, č. 2, s ISSN: