AN INVESTIGATION OF THE THERMAL SHOCK BEHAVIOR OF THERMAL BARRIER COATINGS

AN INVESTIGATION OF THE THERMAL SHOCK BEHAVIOR OF THERMAL BARRIER COATINGS ANDERSON.A Research scholar, Sathyabama University, Chennai, India. Email: anderson_ranji@yahoo.co.in, Phone: +91-9940223023 RAMACHANDRAN.S Professor and Head, Department of Mechanical Engineering, Sathyabama University, Chennai, India. Abstract: High temperature thermal cycling fatigue causes the breakdown of Thermal Barrier Coating (TBC) systems. This paper presents the development of thick TBCs, focusing attention on the microstructure of the Yttria Partially Stabilized Zirconia (YPSZ) coating and Zirconia ceria powder (Ce+YPSZ), in relation to its resistance to thermal cycling fatigue. Thick TBCs, were produced by means of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat or Zirconia ceria powder (Ce+YPSZ), both sprayed by Atmospheric Plasma Spray process. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling were evaluated. The limit of thermal fatigue resistance increases in the Zirconia ceria powder (Ce+YPSZ) top coat. Keywords : Atmospheric plasma spray coating, thermal cycling, Thermal Resistance 1. Introduction Thermal Barrier Coatings (TBCs) are applied on gas turbine and aeronautical engine components in order to improve their oxidation resistance and their service life time through a reduction of the service temperature. The TBC systems consist of a duplex structure made up of a metallic MCrAlY (M stands for either Co, Ni or Fe or a combination of these elements) bond coat and yttria-partially stabilized zirconia (YPSZ) ceramic top coat. The bond coat is typically obtained by means of either vacuum plasma spray (VPS), atmospheric plasma spraying (APS) or high-velocity oxy-fuel (HVOF) techniques; the top coat is typically deposited by APS process [1-3]. The bond coat surface, onto which the YPSZ top coat is deposited, has a thin oxide layer that consists mainly of various oxides (NiO, Ni(Cr,Al)2O4, Cr2O3, Y2O3, Al2O3). The presence of this thin oxide layer provides the adhesion (bonding) between the metallic bond coat and the ceramic top coat[4]. However, during engine operation, another oxide layer forms in addition to the native oxide. This second layer, which is composed mainly by alumina, is commonly referred to as the thermally grown oxide (TGO) and it grows slowly during exposure to elevated temperatures[5-7]. TBC systems have a tendency to fail by spalling or debonding processes under cyclic high temperature conditions. The performance of TBCs is affected by thermal expansion mismatch between the ceramic and the metal, thermal stresses generated by the temperature gradients in the TBC, ceramic sintering, phase transformations, corrosive and erosive attack and residual stresses arising from the deposition process [8, 9]. The growth of thermally grown oxides (mainly alumina) between the bond coat and the top coat layers, cause large residual stresses which lead to the spallation of TBC, too [10].The aims of this work were to compare the resistance of the various TBC systems at thermal fatigue. 2. Experimental Procedure 2.1 Thermal shock behavior Stainless steel 304 metal discs (diameter: 25 mm; thickness: 5 mm) were used as substrates for the present study. The bond coat of the samples was obtained by means of Atmospheric Plasma Spray using commercial CoNiCrAlY powder (with a grain size distribution in the range 45 105 μm); the thickness of the bond coat was in the range 10 μm. Furthermore ceramic coating (top coat) was deposited by Atmospheric ISSN : 0975-5462 Vol. 3 No. 11 November 2011 8154

Plasma Spray, using a Yttria Partially Stabilised Zirconia grain size distribution in the range of 45-125 μm. The thickness of the outer coat was in the range 150 μm.the composition of the powder is shown in Table 1. Table 1. Chemical composition of Yttria Partially Stabilised Zirconia powder (YPSZ) compounds Percentage Yttrium oxide (Y 2 O 3 ) 8% Zirconium Oxide (Zr 2 O 3 ) 92% Table 2. Chemical composition of Zirconia Ceria powder (Ce+YPSZ) compounds Percentage Yttrium oxide (Y 2 O 3 ) 8% Zirconium Oxide (Zr 2 O 3 ) 67% Cerium Oxide (CeO 2 ) 25% Furnace cycle tests (ASTM C 385-58) was performed by using a test equipment consisting of an isothermal static air furnace a specimen tray in stainless steel positioned on a vertical elevator, and a circular tube for forced cooling of specimens when elevator is lowered. Each thermal cycle in the Furnace Cycle Test consist of a 5-minute heat up to the steady-state temperature, a 45-minute soak at the steady-state temperature and a 10-minute forced air cool down. The chosen steady-state temperature was 1150 C. According the Original Engine Manufacturer (OEM) specification,the The minimum requested number of thermal cycle to pass the thermal cycling fatigue test is 250 cycle[11]. Metallographic investigation has been performed by means Scanning Electron Microscope in order to determine coatings microstructure. 3. Results and Discussion 3.1 Coating Characterization Figure 1. shows the surface of the obtained Yttria Partially Stabilised Zirconia powder (YPSZ) top coat coatings. Figure 2. shows the surface of the obtained Zirconia Ceria powder (Ce+YPSZ) top coat coatings. The image reveals that the Zirconia Ceria powder (Ce+YPSZ) top coat coatings are densly packed than the Yttria Partially Stabilised Zirconia powder (YPSZ) top coat coatings Fig 1. Scanning Electron Microscope images of YPSZ top coat of the made TBC s ISSN : 0975-5462 Vol. 3 No. 11 November 2011 8155

Fig 2. Scanning Electron Microscope images of Ce+YPSZ top coat of the made TBC s Table 3 shows the level of porosity, measured by image analysis for each type of coating materials. Table 3. Porosity of different coating material Coating Material Porosity % YPSZ 19.88 Ce+YPSZ 10.18 3.2 Thermal Fatigue Behavior of Coated Samples As mentioned previously, according to the applied OEM specification, the minimum requested number of thermal cycle to pass the thermal cycling fatigue test is 250 cycle. The cracking mode is different when YPSZ top coat is produced by either APS or EB-PVD techniques [12] and it may be influenced also by coating thickness. Normally the thicker TBCs provide a greater temperature drop across the coatings. In addition, the increased thickness of the coating will increase the stored elastic strain energy and hence the energy release rate for a crack [13]. Thus, the failure mechanisms that cause spallation of thick TBCs are expected to be different in some degree from those of the traditional thin TBCs. Failure of thin plasma sprayed TBCs occurs in most cases by interface delamination due to different thermomechanical properties of the coating and substrate and oxidation of the bond coat [14 17]. In particular, thick TBCs have a worse thermal shock resistance than thin TBCs.In the Thermal shock test the type of damage is only by peel off. The peel off shows the starting of damage. The coated specimen was planned to keep for 250 cyles. After coating, the specimens were examined visually for appearance and tested in Thermal shock equipment for thermal resistance.the table 4 determines the weight loss of the specimen coated by yitria stabilized zirconia (8% Y 2 O 3 + 92% Zr 2 O 3 ) exposed to the thermal shock test is shown for various exposure cycles. The peel off on a sample started at 145 hours. Initially the weight of the specimen was 14.66 gms. After the 145 hours the 14.20 gms. Initial weight Table 4. The weight of coated sample by YPSZ 145 hours 175 hours 200 hours 14.66 14.20 14.16 14.12 The specimen coated by zirconia Ceria Powder (8% Y 2 O 3 + 67% Zr 2 O 3 +25% CeO 2 ). exposed to the thermal shock test did not peel off till 250 cycles. The surface views of the SS304 steel specimens are given below ISSN : 0975-5462 Vol. 3 No. 11 November 2011 8156

Fig.3 yitria stabilized zirconia 8% Y 2O 3+ 92% Zr 2O 3(before the test) Fig.4 yitria stabilized zirconia 8% Y2O 3+ 92% Zr 2O 3(after the test) Fig 3, Fig 4 represents the yitria stabilized zirconia 8% Y 2 O 3 + 92% Zr 2 O 3 coated sample before and after the test. Fig 5, Fig 6 represents the Zirconia Ceria powder (8% Y 2 O 3 + 67% Zr 2 O 3 +25% CeO 2 ) coated sample before and after the test. Fig.5 Ceria+Yttria Partially Stabilised Zirconia powder (before the test) Fig.6 Zirconia Ceria Powder (8% Y 2O 3+ 67% Zr 2O 3+25% CeO 2) (after the test) 4. Conclusion TBC systems with different chemical composition with an average thickness of 250 µm were made by APS. The TBCs were characterized and their thermal fatigue behavior was tested. Thermal schok test were conducted for Zirconia Ceria powder coated sample (8% Y2O3+ 67% Zr2O3+25% CeO2) and yitria stabilized zirconia 8% Y2O3+ 92% Zr2O3 coated sample. The experimental results revealed that the Zirconia Ceria coated (8% Y2O3+ 67% Zr2O3+25% CeO2) coated sample has much Thermal resistance than yitria stabilized zirconia 8% Y2O3+ 92% Zr2O3 coated sample. ISSN : 0975-5462 Vol. 3 No. 11 November 2011 8157

References [1] Brandl,W., Toma,D., J. Krüger, J., H.J. Grabke,H.J and Matthäus,G.,1997, The oxidation behaviour of HVOF thermalsprayed MCrAlY coatings, Surf. Coat. Technol., Vol 94 95, p. 21-26 [2] Toma,D., Brandl,W. and Koster,U., 1999, Studies on the transient stage of oxidation of VPS and HVOF sprayed MCrAlY coatings, Surf. Coat. Technol., Vol 120, p. 8-15 [3] Lugscheider,E., Herbst,C. and Zhao,L., 1998 Parameter studies on high- velocity oxy-fuel spraying of MCrAlY coatings, Surf. Coat. Technol., Vol 108 109, p. 16-23 [4] Lee,E.Y. and Sisson,R.D. Jr.,1994 The effect of bond coat oxidation on the failure of thermal barrier coatings, 7 th National Thermal Spray Conference Proc., p. 55 (Boston, Massachusetts) [5] Yaslier,Y. and Alperine,S., 1998 EB-PVD Thermal Barrier Coatings: Comparative Evaluation and Competing Deposition Technologies, (Report 823, AGARD, Thermal Barrier Coatings) [6] Haynes,J.A., M.K. Ferber,M.K. and Porter,W.D., 2000 Thermal Cycling Behavior of Plasma-Sprayed Thermal Barrier Coatings with Various MCrAIX Bond Coats, J. Thermal Spray Technology., Vol 9, p. 38-43 [7] He.Y, K.N. Lee, S. Tewari and R.A. Miller, 2000 Development of Refractory Silicate-Yttria-Stabilized Zirconia Dual-Layer Thermal Barrier Coatings, J. Thermal Spray Technology., Vol 9 (No 1), p. 59-64 [8] Allen Haynes,J., Douglas Rigney,E., Ferber.M.K. and W. D. Porter, 2000 Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAIY bond coats, J. Thermal Spray Technol., Vol 9 (No 1) p. 38-48 [9] Scardi,P., M. Leoni,M. and L. Bertamini,L.,1995 Influence of phase stability on the residual stress in partially stabilized zirconia TBC produced by plasma spray, Surf. Coat. Technol., Vol. 76-77, p. 106-112 [10] Kuroda,S., and Clyne,T.W.,2000 The quenching stress in thermally sprayed coatings, Thin Solid Films, Vol 200, p. 49-66 [11] Scrivani,A. and Rizzi,G., Thermal Fatigue Behaviour of Thick and Porous Thermal Barrier Coatings Systems [12] Evans,A.G., Mumm,D.R., Hutchinson,J.W., Meier,G.H. and Pettit,F.S., 2001 Mechanisms controlling the durability of thermal barrier coatings, Prog. Mater Sci., Vol. 46, p. 505-511 [13] Hutchinson,J.W. and Suo,Z., 1992 Mixed mode cracking in layered materials, Adv. Appl. Mech., Vol 29, p. 63-69 [14] Tzimas,E., Müllejans,H., Peteves,S.D., Bressers.J., and Stamm,W.,2000 Failure of thermal barrier coating systems under cyclic thermomechanical loading, Acta Mater., Vol. 48, p. 4699-4707 [15] Qian,G., Nakamura,T., Berndt,C.C. and S.H. Leigh, 1997, Tensile toughness test and high temperature fracture analysis of thermal barrier coatings, Acta Mater., Vol. 45, p. 1767-1774 [16] De Masi Marcin,J.T., K.D. Sheffler,K.D. and Bose,S., 1990, J. Eng. Gas Turb. Power, Vol 112, p. 521-528 [17] Rabiei,A., Evans,A.G., 2000, Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings, Acta Mater., Vol. 48, p. 3963-3976 ISSN : 0975-5462 Vol. 3 No. 11 November 2011 8158