Oxidation and Hot Corrosion Study on Multi Layer Thermal Barrier Coating

Transcription

1 Oxidation and Hot Corrosion Study on Multi Layer Thermal Barrier Coating 1 K. Vijaya Bhaskar Reddy, 2 Shashi Bhushan Arya, 3 Gosipathala Sreedhar 1 Department of Mechanical Engineering, Chaitanya Bharathi Institute of Technology, Department of Mechanical Engineering, Hyderabad-75 2 National institute of Technology Karnataka, (NITK), Surathkal, Department of Metallurgical & Materials Engineering, Mangalore-25 3 Department of Metallurgical & Materials Engineering, IIT Bombay, Mumbai-76 Abstract : The failure of TBC depends on many factors such as oxidation of metallic bond coat and thermal expansion co-efficient mismatch between bond coat and top coat play an important role. A multilayer coating which consists of TiO 2 stabilized Al 2 O 3 (TSA) coating as an intermediate layer between NiCr bond coat and Yttria stabilized Zirconia (YSZ) ceramic top coat was developed using air plasma spray (APS) process. Oxidation and hot corrosion studies were carried out at 900 C for 600 h and at the same temperature for 65hrs respectively. Periodical observation was done at each 50hrs of interval for oxidation study. Hot corrosion test was done in mixed environment of 50 wt. % Na 2 SO wt. % V 2 O % NaCl. The results show that during oxidation, a thermally grown oxide formed at the interface of NiCr bond coat and TiO 2 stabilized Al 2 O 3 coating and the crack was developed after 600 hrs of study at the interface of coating. YSZ tends to destabilize in the venadate medium by forming YVO 4 spinels and monoclinic ZrO 2 during hot corrosion test. This also caused cracks at the surface. The salt penetrated through this cracks and oxidation becomes more severe. Key words: APS, TBC, NiCr, TSA, YSZ, oxidation, hot corrosion, molten salts. I. INTRODUCTION Thermal barrier coatings (TBCs) are used to provide thermal insulation to the hot section components of gas turbines in order to protect them from thermal degradation and also to enhance the operating temperature of the engine. A typical TBC consists of a MCrAlY bond coat applied on to a substrate such as Inconel alloy followed by the application of a top coat of yittria partially stabilized zirconia. The former provides high temperature oxidation and corrosion resistance, while the latter offers thermal insulation [1 5]. The life of the TBCs depends on several factors. Among them the resistance of the bond coat to high temperature corrosion such as oxidation and hot corrosion is very important. During high temperature exposure of TBCs, oxides form at the top coat and bond coat interface which is called as thermally grown oxide (TGO). As the oxides grow (which depends on many factors [6 8]), they cause compressive residual stresses leading to TBC failure near ceramic topcoat/tgo/bond coat interfaces and/or within the TGO [9 12].This causes spallation. In addition, a coefficient of thermal expansion mismatch between bond coat and ceramic topcoat can exasperate the coating failure during thermal cycles [13-14]. It is known that the former exhibits a higher coefficient of thermal expansion than the latter. To address the last aspect, namely, the role of the coefficient of thermal expansion mismatch, functionally graded thermal barrier coatings (FGTBCs) are being developed [13 25]. To match the coefficient of thermal expansion of the bond coat with that of the ceramic top coat, bond coats with different proportions of YSZ are applied. This involves mixing different proportions of YSZ with the bond coat powder and applying them in a graded or discrete manner on to the substrate. Such bond coats have increasing proportions of YSZ when moving from a substrate/bond coat interface towards the bond coat/ceramic top coat interface. So, graded TBCs can effectively reduce the sharp discontinuity in thermal expansion coefficients between the bond coat and ceramic top coat. Nevertheless, the presence of oxides such as YSZ or Al 2 O3 in the bond coat can affect the latter's oxidation and hot corrosion behavior. But the review of the literature shows that the published work on FGTBCs is mostly devoted to examining thermal shock behavior of TBCs (for example [24-25]) And the literature on the oxidation and hot corrosion behavior of these oxide dispersed bond coats is sparse. Therefore further studies are needed to understand the oxidation and hot corrosion behavior of oxide dispersed bond coats to get reliable corrosion resistant coatings in many environments. In this work the oxidation and hot corrosion behavior of YSZ dispersed bond coats is studied in terms of their resistance to hot corrosion which is one of the important requirements of any bond coat. Hot corrosion occurs when low quality fuels containing impurities such as Na and V are used. These elements form Na 2 SO 4 and V 2 O 5 24

2 salts respectively on the surface of the turbine blades and cause corrosive attack. In the present study, stainless steel 316L is used as the substrate material, this material having high yield strength, tensile and creep-rupture properties at temperatures up to 600 C. Which display exceptionally high yield, tensile and creep-rupture properties at higher temperatures. This alloy has been used for jet engines and high-speed airframe parts such as wheels, buckets, spacers, high temperature bolts and fasteners. As the main aim of this work is to examine, how oxide dispersion in a bond coat can affect the hot corrosion behavior of the NiCr bond coat, and TSA a two layer of bond coat with a predetermined proportion of the YSZ was coated, using the air plasma spray technique and its behavior was examined. Oxidation test at 900 C for 600hrs and Hot corrosion test were conducted in a 50 wt. % Na 2 SO wt. % V 2 O % NaCl mixture environment at 900 C for 65 hrs II. EXPERIMENTAL WORK 2.1. Substrate material Austenitic stainless steel used as a substrate in the present study and the chemical composition of Type 316L SS is given in table 1. The specimen was used of 1 cm 1 cm 0.6 cm dimensions. Table.1 Chemical composition of stainless steel 316L (wt.%) Table. 2 Coating Powders 2.2 Coatings development A 40 kw Metco Thermal Plasma Spray unit was used to apply the coatings. In each type of coating, the thickness of the coating was different. Argon and hydrogen were used as primary and secondary gases respectively. The coating parameters such as arc current, powder feed rate and spray distance were varied for each coating composition. These parameters were adjusted based on the data sheet on coating parameters produced by suppliers of these powders (Sulzer Metco). The arc current for NiCr is 450 A and for YSZ, it is 650 A. The spray distance used for NiCr is 150 mm, 130 mm for alumina titania and for YSZ it is 120 mm. To accommodate these differences in spraying parameters adjustments were made. The rationale for varying the above parameters is as follows. Coating parameters generally depend on two factors namely the melting temperatures and the particle sizes of YSZ, TSA and NiCr powders used for the coating. High melting temperature and large particle size need high arc current to adequately melt the particles before they are deposited on to the substrate. Accordingly, it can be seen that YSZ needs high arc current than NiCr as the former exhibits much higher melting temperature than the latter. For this reason, when NiCr, TSA is blended with YSZ, the arc current needs to be enhanced. As the proportion of YSZ in the NiCr increases the arc current also needs to be increased. In the same manner, the spray distance affects the solidification of the melt, before it reaches the substrate. High melting solids solidify quickly and hence low spray distance is maintained. With this, the time of flight of the molten particles is reduced and therefore solidification before deposition is minimized. 2.3 Oxidation test Oxidation behavior of NiCr/TSA/YSZ coating is studied in terms of their resistance to oxidation which is one of the important requirements of any bond coat. Oxidation occurs when presence of oxygen at high temperatures on the surface of the turbine blades and cause oxidation attack. In the present study, stainless steel 316L is used as the substrate material. Air Plasma Spray coated Specimens of 1 cm 1 cm 0.6 cm were cut from a sheet by using high speed disc cutter. The edges of the specimens were ground to eliminate edge effects. Thermal spray coated samples are kept in furnace with the help of ceramic boat in normal atmospheric condition and maintain the furnace temperature of at 900 C about 600 hrs period. Periodically temperature checking done by using thermo couples. The samples were collected from furnace every each 50 hrs. These samples were studied by using XRD SEM, EDS. 2.4 Hot Corrosion Test Hot corrosion behavior of multi-layer coating is studied in terms of their resistance to hot corrosion which is one of the important requirements of any bond coat. Hot corrosion tests were conducted in the Heatron muffle type furnace in 50 wt. % Na 2 SO wt. % V 2 O %NaCl mixed environment at 900 C for 65 hrs. Air plasma spray coated samples are kept in furnace keeping the sample in ceramic boat in mixed environment were prepared and the same was dissolved in distilled water to obtain a 50wt. % salt solution. The above salt solution was applied onto the surface of the specimen in the following manner. Firstly, specimen was weighed and pre-heated at 200 C for a period of 10 min, as preheating provided good adhesion of the salt to the specimens. Then the solution was applied onto the specimens using a camel hair brush. This salt coated specimen was kept in a furnace and heated at 200 C for 15min to remove the moisture from the salt coating. Thereafter, each salt coated specimen was weighed individually to calculate the amount of the salt deposited per unit area. The above procedure provided 3 10 mg/cm 2 of the salt over the specimen. Before the 25

3 commencement of hot corrosion test each salt-coated specimen was kept in a crucible and the total weight of the crucible and specimen was determined. Then go for X-ray and SEM (EDS) studies were conducted for these samples. 2.5 X-ray diffraction studies X- Ray studies were conducted before and after subjecting to oxidation & Hot Corrosion, samples were examined using X-ray diffraction. The scan rate was used 2 degree per min. The X-ray diffractograms were obtained using Cu K α radiation. With the obtained X-ray diffractograms with respect to angle various phases were identified using the JCPDS-PCPDFWIN powder diffraction software package. 2.6 Metallographic Sample Preparation In order to view the microstructure of multi-layer coating, the specimen was cut across the cross-section using precision disc cutter. To minimize the cutting damage to the surface layer, a low cutting load, moderate cutting speed (200 rpm) and a slow feed rate were used. All samples were mounted in a coldmounting epoxy resin with reinforcing mineral filler, which created a gradual transition in hardness across the epoxy-specimen interface. Polishing of specimens was done using standard polishing procedure with 400, 600, 800 and 1000 grit size emery papers (Silicon Carbide) followed by diamond polishing. 2.7 Microscopy studies (SEM and EDS) Surface and cross-sectional morphology of the oxidized and hot corrosion specimens were examined using (Model-JEOL-JSM-6380LA) Scanning Electron Microscope (SEM). SEM images were taken in secondary and back scattered electron micrographs; Energy Dispersive Spectroscopy (EDS operated at kv) analysis also conducted. The specimens exposed to hot corrosion were washed with running distilled water before they were examined by SEM. To obtain the cross-section, the specimens were first mounted in an epoxy resin. Then sectioned with disc cutter and subjected to mirror polishing using diamond paste (1 µm). III. RESULTS & DISCUSSION 3.1. (a) Oxidation Kinetics The Oxidation kinetics for three layers of TBC (NiCr/Titania stabilized Alumina/YSZ (multi-layer coating) were studied at 900 C for duration of 500 hrs. The oxidation data for all the specimens obtained as weight gain as a function of time graph has shown in fig.1. The oxidation kinetics follows the parabola curve as shown in fig.1 Hence, the oxidation assumed to be diffusion controlled process in this case. There are two parabolic curves observed due to spallation at that particular point (350 hrs) and a new surface being exposed to oxidation and leads to formation of another parabola curve. Fig.1 Weight gain vs. time plot for coated specimen exposed to oxidation at 900 C for 500 hrs The kinetic behavior of the coating was further examined and results found as. (weight gain/area) 2 with time shown in fig. 2 On curve fitting, a least square fit is observed and it is found to be good for this relationship and so it can be said that the oxidation kinetics of the coating follow the equation [26] as given as:. 26 W A = (K pt) C o (1) Where, W/A is weight gain per unit area mg/cm 2 at time T (hrs), K p is parabolic rate constant (mg 2 cm 4 s -1 ), and Co is constant. The parabolic rate constant (K p ) calculated from the fitted curve and the multi-layer coating exhibits a k p value about mg 2 cm -4 s -1 Fig.2 (Weight gain/area) 2 vs. time plots for coated specimens exposed to oxidation at 900 C for 500hrs 3.1. (b) X Ray Diffraction Analysis The nature of phases formed during oxidation test can influence the oxidation tendency of the coating. Furthermore, it can also reveal the stability of multilayer coating. XRD analysis was carried out as received condition and exposed to 900 C for 500 hrs of multilayer coating and shown fig.3&4. The presence of tetragonal YSZ phase observed in as received condition of multilayer coating (shown in figure.4.3). Further, The multi-layer coating after subjecting to oxidation exhibit the presence of stable YSZ and Cr 2 O 3 oxide phases and found that a decrease

4 of peak intensity and peak broadening. This shows that YSZ stable during oxidation process at that temperature but NiCr bond coat develops Cr 2 O 3, NiO (due to air entrapment through ceramic coatings) as TGO and it brings instability for the coating. (Leyens, et al. 1999) 3.1 (c) Cross sectional morphology The cross sectional morphology of the multi-layer coating subjected to oxidation was examined to identify the nature of the attack. Typical cross sectional morphologies observed and shown in fig.5 (a-f). The coating as received condition shown in Fig. 5 (a) and it is found that the presence of multi layers of bond coat, intermediate Titania stabilized alumina (TSA) and YSZ top coat. The samples were undergone isothermal oxidation test at different time period. analysis reveals that the TGO formed is purely nickel oxide and chrome oxide as shown in table 4. A crack has been developed between bond coat and intermediate layer of TSA after 600hrs of oxidation test due to mismatch co-efficient of thermal expansion between NiCr bond coat and intermediate coatings (Tawancy et al. 1999) shown in fig. 5(f). Table 4. Energy dispersive spectroscopy study on TGO showing the composition of the sample after subjecting to oxidation at 900 C for 500hrs. Fig.3 XRD patterns obtain multi-layer coating (As Sprayed Condition) Fig.4 XRD patterns of multi-layer coating obtained after exposed to oxidation at 900 C for 500 hrs. And fig. 5(b), shown a formation of oxides that is thermally grown oxides (TGO) between bond coat and TSA coating after 200 hrs. Bond coat is forming as a chromium oxide (Cr 2 O 3 ) and possibility of nickel oxides (NiO, Ni 2 O 3 ) between bond coat and intermediate TSA. The TGO growth increasing with respective of time shown in fig. 5(c) at 300 hrs and further it is observed that at 400 hrs the Cr 2 O 3 spinels clearly observed, shown in fig. 5 (d). A formation of pores observed even after 300hrs [shown in fig. 5(c)] but pores sizes and shapes increased at 500hrs. Thermally grown oxide (TGO) analysis was done by EDS analysis of the multi-layer coated sample after subjecting to oxidation at 900 C for 500 hrs the 27

5 Fig. 8 (Weight gain/area) 2 vs time plot for coated specimens exposed to mixed environment at 900 C for 65 hrs. Table 5 Hot corrosion kinetics weight gain and square of weight gain as a function of time Fig.5: SEM images of the multi-layer cross-sectional morphologies (a) As sprayed coating condition (b) 200 hrs (c) 300hrs (d) 400hrs (e) 500 hrs (f) 600 hrs of oxidation 3.2. Hot Corrosion Studies 3.2 (a) Hot Corrosion kinetics The hot corrosion kinetics of multi-layer coating were studied in the solution of 50 wt.% Na 2 SO wt.% NaCl + 25wt.% V 2 O 5 (mixed) at 900 C for a duration of 65 hrs. Weight gain as a function of time was recorded for all the specimens and summarized in table 5 and corresponding plot shown in fig. 7. It is observed that the coating kinetics behaves as a parabolic in nature during hot corrosion test. 3.2(b) X-ray Diffraction analysis The nature of phases formed during hot corrosion influence the behavior of coating and it was analyzed by XRD so that it can be correlate with surface morphology. XRD analysis was carried out after exposure to hot corrosion at 900 C for 65hrs shown in fig. 3. As already the presence of the YSZ (tetragonal) phase observed in the multi-layer coating as received condition (ASP). The presence of various types of oxide phases observed such as Cr 2 O 3, NiO, NiCr 2 O 4, NiAl 2 O 4, YVO 4, and monoclinic ZrO 2 on hot corrosion test and shown in fig. 9. YVO 4 and monoclinic ZrO 2 phases formed on the due to disintegration of YSZ. Fig.7 Weight gain vs time plot for coated specimens exposed to mixed environment at 900 C for 65hrs The hot corrosion kinetic behaviour of the coating was further examined and results found as. (Weight gain/area) 2 with time shown in fig. 8. On curve fitting, a least square fit is observed and it is found to be good for this relationship and so it can be said that the oxidation kinetics of the coating follows the equation (Gosipathala et al.2009) as given as: W A = (K pt) C o (1) Fig.9 XRD patterns of multi-layer coating obtained after exposed to hot corrosion at 900 C for 500 hrs. 3.2 (c) Surface Analysis 28

6 Surface morphology of NiCr/TSA/YSZ (multi-layer coating) coating is subjected to hot corrosion was examined to identify the nature of the attack. Typical surface morphology of the coating in the exposed condition is shown in fig. 10 (a). It is noticed that the nature of hot corrosion attack on the coating is not discernible, as the surface has been covered with the salts. Hence, these specimens were washed with running distilled water and then examined SEM images shown in fig.10 (b). A higher magnification of SEM microstructures shown in fig. 11 (a and b) at 1000 X and 6000X after hot corrosion test. A cracks have observed because of de stabilization of Yittria which is reacting with salts and forming YVO 4 further a rod like morphology shown in fig. 11 (b), YVO4 crystals and monoclinic ZrO 2 exhibited at higher magnification (6000X). Fig.11 (a, b) SEM images of surface morphology after exposed to 65hrs hot corrosion test. (a) Monoclinic Zirconia and YVO 4 showing the (b) showing the crack propagation The Cracks have been developed at the surface due to additional internal stresses shown in fig. 11(a) [26] 3.2(d) Cross sectional analysis Fig.10 Surface morphology of coating after exposure to mixed environment at 900 C for 65hrs (washed & unwashedcondition) Fig.12 SEM image of the multi-layer coating cross sectional morphology (a) after 30hrs (b) after 65hrs 29

7 The cross sectional analysis has done to know the morphological changes during the hot corrosion test the cross sectional images shown in fig. 12, fig. 12 (a) here the image showing As Sprayed condition i.e. before exposed to hot corrosion test. The formation of TGO was observed after 30hrs exposed to hot corrosion shown in fig. 12 (a). The crack was observed after the 65 hrs of hot corrosion test which is shown in fig. 12 (b) 3.2(e) Mechanisms of Coating Degradation Multi-layer coating subjected to hot corrosion revealed the presence of the YVO 4 phase with focus on the hot corrosion of YSZ have reported the reaction of yittria, from the zirconia solid solution, with vanadiumcontaining compounds leading to the formation of yttrium vanadate (YVO 4 ) [26] ZrO 2 (Y 2 O 3 ) + V 2 O 5 ZrO 2 (monoclinic) + 2YVO 4 (2) The resultant structural destabilization of zirconia by depletion of yittria is followed by phase transformation from tetragonal to monoclinic. Another possible chemical reaction would be the zirconia matrix itself with the molten vanadate ZrO 2 + V 2 O 5 ZrV 2 O 7 (3) Sodium sulphate also playing major role during hot corrosion which is combining with vanadate and forming NaVO 3 and SO 3 V 2 O 5 + Na 2 SO 4 2 (NaVO 3 ) + SO 3 (4) Then, NaVO 3 having a melting point of 610 C reacted with yittria from the zirconia solid solution to form YVO 4 ZrO 2 (Y 2 O 3 ) + 2(NaVO 3 ) ZrO 2 (monoclinic) + 2(YVO 4 ) + Na 2 O (5) It seems that the molten NaVO 3 has increased atom mobility enhancing the depletion of yittria and hence promoted the growth of YVO 4 crystals. In terms of destabilization and consequent phase transformation (Batista et al. 2006). Sodium chloride (NaCl) effect on coating up pressed due to presence of stable YSZ. The destabilization and consequent phase transformation to monoclinic accompanied with volume increase has severely weakened the coating. Furthermore, the formation of the rod likeyvo 4 crystals has introduced additional stresses. SUMMARY OF WORK Oxidation study on multi-layer coating reveals that coating has failed by forming crack, 900 o C at 600hrs. Coating failure occurred at the interface of Bond coat and TGO due to formation metallic oxides. Hot corrosion studies reveal that environmental stability of the coating. Environments like V 2 O 5 and Na 2 SO 4 have an important effect on YSZ coating and found that the formation of YVO 4 spinel s leaving zirconia in the monoclinic form which developed the cracks on the surface. These cracks will enhance the infiltration of the salt into the coating and will raise the corrosion rate or oxidation rate. The effect of NaCl environment was not found. ACKNOWLEDGEMENTS The authors are thankful to VTC surface coating technologies, Visakhapatnam, India, for coating the specimens. REFERENCES: [1] R.A. Miller, Current status of thermal barrier coatings an overview, Surf. Coat. Technol. 30 (1987) [2] R. Taylor, J.R. Brandon, P. Morrel, Microstructure, composition, and property relationships of plasma-sprayed thermal barrier coatings, Surf. Coat. Technol. 50 (1992) [3] D.J. Wortman, B.A. Nagaraj, E.C. Duderstadt, Thermal barrier coatings for gas turbine use, Mater. Sci. Eng. A 121 (1989) [4] B.C. Wu, E. Chang, S.F. Chang, C.H. Chao, Thermal cyclic response of yttriastabilized zirconia/conicraly thermal barrier coatings, Thin Solid Films 172 (1989) [5] M. Gell, E.H. Jordan, K. Vaidyanathan, K. McCarron, B. Barber, Y. Sohn, V.K. Tolpygo, Bond strength, bond stress and spallation mechanisms of thermal barrier coatings, Surf. Coat. Technol (1999) [6] V.K. Tolpygo, D.R. Clarke, Microstructural study of the theta alpha transformation in alumina scales formed on nickel-aluminides, Mater. High Temp. 17 (2000) [7] C.S. Giggins, F.S. Pettit, Oxidation of Ni Cr Al alloys between 1000_ and 1200 _C, J. Electrochem. Soc. 118 (1971) [8] E.Y. Lee, R.R. Biederman, R.D. Sisson Jr, Diffusional interactions and reactions between a partially stabilized zirconia thermal barrier coating and the NiCrAlY bond coat, Mater. Sci. Eng. A 121 (1989) [9] V.K. Tolpygo, D.R. Clarke, K.S. Murphy, Oxidation-induced failure of EB-PVD thermal barrier coatings, Surf. Coat. Technol (2001) [10] Y.H. Sohn, J.H. Kim, E.H. Jordan, M. Gel, Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: II. Evaluation of photostimulated luminescence, Surf. Coat. Technol (2001) [11] R.J. Christensen, D.M. Lipkin, D.R. Clarke, K. Murphy, Nondestructive evaluation of the 30

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