UDK 621.793:542.943:620.18	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 46(5)439(2012)
STUDY OF THE MICROSTRUCTURE AND OXIDATION BEHAVIOR OF YSZ AND YSZ/AI2O3 TBCs WITH HVOF
BOND COATINGS
ŠTUDIJ MIKROSTRUKTURE IN VEDENJA PRI OKSIDACIJI YSZ IN YSZ/AI2O3 TBC Z HVOF NANESENO ZAŠČITNO PREVLEKO
Abdullah Cahit Karaoglanli1, Garip Erdogan2^ Ya°ar Kahraman3, Ahmet Türk2,
Fatih Üstel2, Ismail Özdemir1
1Department of Metallurgical and Materials Engineering, Bartin University, 74100 Bartin, Turkey 2Department of Metallurgical and Materials Engineering, Sakarya University, 54187 Sakarya, Turkey 3Department of Mechanical Engineering, Sakarya University, 54187 Sakarya, Turkey cahitkaraoglanli@gmail.com, karaoglanli@bartin.edu.tr
Prejem rokopisa - received: 2011-11-11; sprejem za objavo - accepted for publication: 2012-03-27
A significant improvement in efficiency has been achieved by using thermal barrier coatings (TBCs) in gas turbines and diesel engines. A typical TBC is a multilayered coating system that comprises an oxidation-resistant metallic bond coating (BC) and a thermally insulating ceramic top coating (TC). Under service conditions an Al2O3 inter-layer, the thermally grown oxide (TGO), forms in the interface between the bond and the top coating, by a chemical reaction between the metallic aluminum from the BC material and the oxygen that comes from the environment through the pore channels of the TC. The aim of the present study is to describe the TGO formation on metallic bond coats deposited using the high-velocity oxygen fuel (HVOF) spraying technique. Therefore, TBCs that consist of a YSZ top (ZrO2 + 8 % Y2O3) and YSZ-Al2O3 double-layer systems with CoNiCrAlY bond coats were deposited on Inconel 718 super-alloy substrates. The bond coats were applied via HVOF, with the ceramic top coats being applied by atmospheric plasma spraying (APS) as well. The oxidation behaviors of the TBC systems were investigated. The oxidation tests were performed at 1000 °C in an air atmosphere for (8, 24, 50) h. The formation and growth of the TGO layers and the microstructural changes during the oxidation tests were scrutinized systematically. The results indicate that the TBC coating with the YSZ-Al2O3 double layer had a higher oxidation resistance and a lower TGO layer growth than that of the traditional TBC system. Likewise, the initial state of the porosity plays a critical role in enhancing or limiting the growth of the TGO scale in the TBC.
Keywords: thermal barrier coatings (TBCs), oxidation behavior, thermally grown oxide (TGO), high-velocity oxygen fuel (HVOF), atmospheric plasma spraying (APS)
Doseženo je bilo občutno izboljšanje učinkovitosti plinskih turbin in dieselskih motorjev z uporabo toplotnih zaščitnih prevlek (TBC). Značilni TBC je večslojni varovalni sistem, ki vključuje prevleko, odporno proti oksidaciji, s kovinsko vezjo (BC) in toplotno izolativno keramično vrhnjo plastjo (TC). Pri obratovalnih razmerah vmesni sloj Al2O3 omogoča nastanek oksidov (TGO) na stiku med vezivnim in vrhnjim slojem, s kemijsko reakcijo kovinskega aluminija iz BC-materiala in kisika, ki iz okolice prodira skozi pore TC. Namen te študije je opisati nastanek TGO na kovinskem vezivu, nanesenem z nabrizgavanjem s kisikovim plamenom z veliko hitrostjo (HVOF). TBC, ki sestoji na vrhu iz YSZ (ZrO2 + 8 % Y2O3) ter z dvoslojnim sistemom YSZ-Al2O3 z vezivno plastjo CoNiCrAlY so bili naneseni na podlago iz superzlitine Inconel 718. Vezivna plast je bila nanesena z HVOF, keramični vrhnji sloj pa z atmosfersko plazmo (APS). Preiskovane so bile značilnosti oksidacije TBS-sistema. Preizkusi oksidacije so bili izvršeni na zraku pri 1000 °C, za (8, 24 in 50) h. Med preizkusi oksidacije so bili sistematično preiskovani nastanek in rast TGO-plasti ter spremembe v mikrostrukturi. Rezultati kažejo, da ima TBC-prevleka YSZ-Al2O3 z dvojnim slojem boljšo odpornost proti oksidaciji in manjšo rast TGO-plasti v primerjavi z navadnim TBC-sistemom. Videti je, da ima začetna poroznost ključno vlogo pri pospeševanju ali zaviranju rasti TGO-plasti na TBC.
Ključne besede: termični varovalni sloj (TBC), vedenje pri oksidaciji, termična rast oksida (TGO), kisikov plamen z veliko hitrostjo (HVOF), atmosfersko plazemsko nabrizgavanje (APS)
bond coat is deposited conventionally by LPPS, HVOF, 1 INTRODUCTION	plasma and also cold gas dynamic spray: a method re-
, ,	,	... cently preferred to avoid complex oxide formation as
Many attempts have been made to understand the wgll5-8
role of TGO, formed at the interface between the bond
coat and the top coat during elevated-temperature service	Generally, the forming of a dense, homogeneous,
conditions, which strictly governs the lifetime of the	«-Al2O3 oxide scale is preferred as it is relatively stable,
TBC. The thickness, the roughness of the TGO, the	chemically and thermally, which means the degradation
adherence quality of the bond coat to the substrate, the	of the a-Al2O3 is negligible and also has a low ion
type and shape of the oxides present in the vicinity of the	diffusivity, which causes a slow growth rate and prevents
TGO during oxidation are the main issues in controlling	further oxidation911. Oxidation-based damage, which is
the degradation of the TBC1-4. Likewise, the oxidation	the result of stresses developed at the interface of the top
behavior of the TBC is strongly linked to the bond coat	coat and the bond coat during TGO growth, is a common
properties, which affect the durability of the TBC. The	failure of TBC since these stresses result in the
spallation-induced failure of the topcoat. In order to retard or avoid such a failure in the TBC, i.e., better oxidation resistance, any methods employed should facilitate the slow growth of the TGO scale, which favors good adherence of the TGO12-14. To achieve this, apart from employing several kinds of methods to deposit the bond coat, a thermal barrier thin film deposited by EB-PVD, CVD, etc. was employed over the bond coat in order to inhibit the formation of undesirable mixed oxides as they have a fast growth rate leading to accelerated TBC failure15-17.
In addition, the introduction of alumina powders to the YSZ might possibly reduce the inward diffusion of oxygen from the topcoat and thus make it difficult to have rapid growth of the TGO. It was claimed in studies that AlaOs present in the topcoat exhibited better oxidation resistance compared to the YSZ without AlaOs powders1819. In this work, the YSZ topcoat with a conventional composition and with YSZ/AlaOs double-layer systems were applied on Inconel 718 super-alloy substrates to investigate and compare their oxidation behaviors. The oxidation results for the traditional TBC that has the YSZ system was compared with the YSZ/AlaOs double-layer TBC system. The microstruc-tural differences and the formation and growth of the TGO layers in the isothermal oxidation resistance of these TBC systems are discussed.
2 EXPERIMENTAL METHODS
2.1 Materials and coating-deposition methods
The Inconel 718 Ni-based super-alloy, in disc-shaped coupons, was used as the substrate. Co38Ni32.5Cr21Al8Yo,5, ZrO2-8 % Y2O3 and AlaOs powder were used as the starting materials. A Microtrack S3500 laser particle-size analyzer was used to determine the powder size distribution. The mean diameters were determined to be
Table 1: Spraying parameters for deposition of the coatings Tabela 1: Parametri nabrizgavanja pri nanašanju prevlek
HVOF CoNiCrAlY Bond Coatings	APS YSZ Top Coatings	APS Al2O3 Over Top Coatings
Combustion medium	Voltage (Plasma)	Voltage (Plasma)
O2 (880 L/min) and kerosene (25 L/h)	70 V	70 V
Powder Carrier Gas	Current (Plasma)	Current (Plasma)
Argon (15 L/min)	650 A	600 A
Powder Feed Rate	Carrier Gas	Carrier Gas
50 g/min	3 nlpm	2.5 nlpm
Powder feed gas flow	H2 (Plasma)	H2 (Plasma)
12 L/min	13 L/min	13 L/min
Stand-off distance	Argon (Plasma)	Argon (Plasma)
330 mm	45 L/min	45 L/min
	Spraying Distance	Spraying Distance
	100 mm	120 mm
	Traverse Speed	Traverse Speed
	300 mm/s	300 mm/s
d50 = 33 pm, 38.52 pm, 33.36 pm for the CoNiCrAlY, ZrO2-8 % Y2O3, and Al2O3 powders, respectively. Half of the TBC samples consisted of a CoNiCrAlY BC and a ZrO2-8 % Y2O3 TC and the other half of the TBC samples were composed of a CoNiCrAlY BC, a ZrO2-8 % Y2O3 TC and an Al2O3 top coat over the YSZ. The HVOF technique was used to produce bond coats and the ceramic top coatings were produced by the APS method using a fully automated MultiCoat System from Sulzer Metco. All the spraying parameters are shown in Table 1.
2.2 Microstructural Characterization
The microstructures of the TBC systems were investigated by scanning electron microscopy (SEM, Tescan VEGA II, SBU Bruker EDX, Czech Republic). The porosity of the bond coatings was measured using optical image-analysis software (Olympus a4i). The coating microhardness (HV0.3) was determined using a microhardness tester (Shimadzu, Japan) with a load of 300 g for 15 s from the bond coats and the top coats. The oxidation tests of the TBC system produced were conducted by means of a high-temperature furnace (Nabertherm, Germany) with an air atmosphere.
3 RESULTS AND DISCUSSION
3.1 Microstructure of the powders and the coatings
Two types of TBC were prepared using the HVOF method to produce bond coats that included CoNiCrAlY. The APS method was used to produce ceramic top coats, which included traditional YSZ and a double layer of YSZ and Al2O3 in which the Al2O3 was a top coat over the YSZ in a second system. The thickness of the bond and the top coats of both systems were about 100 pm and 300 pm, respectively. The YSZ top coat used in the first system was 300 pm. The YSZ and Al2O3 ceramic top coatings used in the second system were both 150 pm. The type, components, thicknesses and spray systems of the coating layers are shown in Table 2.
Table 2: Type, components, thicknesses and spray systems of the coating layers
Tabela 2: Vrsta, komponente, debelina in sistem za nanašanje prevlek
Type of TBC system	Component	Thickness of layers, pm	Spray system
1	CoNiCrAlY	100	HVOF
	YSZ	300	APS
	CoNiCrAlY	100	HVOF
2	YSZ	150	APS
	Al2O3	150	APS
Figure 1 shows the morphology of the as-received ZrO2+Y2O3, Al2O3 and CoNiCrAlY powders. As can be seen from this figure, the CoNiCrAlY powder has a spherical morphology, while the Al2O3 is angular. Figures 2a and b show the cross-sectional microstructure of
traditional TBC and Al2O3-YSZ gradient double layer TBC system. The HVOF-CoNiCrAlY bond coats have relatively less porosity and cracks in the TBCs.
The ceramic top coats contain porosity and some crack-like discontinuities during the spraying process. A smaller amount of porosity for the BC of both TBC systems was measured to be approximately 1.0 %. On the other hand, like for the top coat the porosity values were not significantly different from each other. The porosity level of the top coat was found to be approximately 5.0 % in both TBC systems. But as is clear from Figure 2, the size and distribution of the porosity in the TC of the two-layer YSZ/Al2O3 TBC are quite different from the traditional values.
3.2 Oxidation tests
The TBC specimens were subjected to oxidation tests. These oxidation tests were carried out in an air atmosphere at 1000 °C for (8, 24 and 50) h. Typical SEM microstructures of the whole TBC-systems are shown in Figures 3 and 4.
As shown in these figures, the TGO was formed at the ceramic/bond-coat interface due to oxygen penetration through the ceramic layer. Various formations of
Figure 1: SEM micrographs of the morphology of: a) ZrO2-8 % Y2O3, b) Al2O3 and c) CoNiCrAlY powders
Slika 1: SEM-posnetek morfologije: a) ZrO2-8 % Y2O3, b) Al2O3 in c) CoNiCrAlY-prahov
Figure 2: SEM micrographs of as-sprayed thermal barrier coatings: a) APS YSZ with HVOF bond coat and b) YSZ/Al2O3 with HVOF bond coat system
Slika 2: SEM-posnetek nabrizganega sloja toplotne prevleke: a) APS YSZ z HVOF vezivno plastjo in b) YSZ/Al2O3 z HVOF vezivno plastjo
Figure 3: Cross-sectional microstructures at the bond coat/ceramic layer interface for YSZ top coats with CoNiCrAlY coatings after oxidation at 1000 °C for (8, 24, 50) h
Slika 3: Mikrostruktura prereza vezivne plasti/keramične plasti z vrhnjo plastjo YSZ za CoNiCrAlY-prevleko po oksidaciji (8, 24 in 50) h na 1000 °C
Figure 4: Cross-sectional microstructures at the bond coat/ceramic layer interface for YSZ/Al2O3 top coats with CoNiCrAlY coatings after oxidation at 1000 °C for (8, 24, 50) h
Slika 4: Mikrostruktura prereza vezivne plasti/kerami~ne plasti z vrhnjo plastjo YSZ/Al2O3 za CoNiCrAlY-prevleko po oksidaciji (8, 24 in 50) h na 1000 °C
discontinuities between the bond layer and the ceramic top layer can be clearly seen in Figures 3 and 4.
When the oxidation properties of the two different TBC systems are compared, it was clear that the oxidation in the conventional TBC system (CoNiCrAlY bond layer with YSZ top coat) developed faster than that of the YSZ/Al2O3 double-layer TBC system, and the TGO growth rate in the former system was found to be higher. As seen from the interface microstructures given in Figure 5, the TGO structure in the YSZ/Al2O3 double-layer TBC system is more uniform and is mainly composed of Al2O3, which was confirmed by an EDX analysis. In the conventional YSZ system, due to the increasing oxidation process, complex oxides developed in the TGO layer and affected the growth behaviors of the TGO. This was caused by the prevention of the oxygen penetration to the bond coat from the surface due to the Al2O3 layer and hence a slowing down of the oxygen attack in YSZ/ Al2O3 coating systems. As a result, the decrease of the Al2O3 content in the TGO layer caused by bond-coat oxidation is delayed, the degradation of the uniform structure is retarded and in this way an increase in the volume of the TGO occurs at
Figure 5: Microstructures at the bond coat/ceramic-layer interface after oxidation at 1000 °C for 50 h; a) TGO in traditional YSZ coating; b) TGO in YSZ/Al2O3 coating
Slika 5: Mikrostruktura stika vezivne plasti/kerami~ne plasti po oksidaciji 50 h na 1000 °C; a) TGO in navadna YSZ-plast; b) TGO in YSZ/Al2O3-plast
a lower rate. Similar results showing an increase in the oxidation resistance of the coatings due to the Al2O3 layer depending on the temperature and the time exist in the literature18 20-25.
Figure 6 indicates that the thickness of the TGO layer increased with increasing exposure time for both
Figure 6: TGO thickness measurements as a function of oxidation time at 1000 °C, respectively (• YSZ top coatings with CoNiCrAlY bond coatings, ■ YSZ/Al2O3 top coatings with CoNiCrAlY bond coatings)
Slika 6: Debelina TGO v odvisnosti od ~asa oksidacije pri 1000 °C (• vrhnja plast YSZ z vezivno plastjo CoNiCrAlY, ■ vrhnja plast YSZ/Al2O3 z vezivno plastjo CoNiCrAlY)
kinds of TBC specimens. The specimens with the YSZ top coatings with CoNiCrAlY bond coatings show a higher rate of TGO thickness growth than the samples with YSZ/Al2O3 top coatings with CoNiCrAlY bond coatings. This difference could be attributed to the initial porosity state of the as-sprayed TBC samples and/or the Al2O3 layer acting as a diffusion barrier due to its low diffusion coeficient for oxygen ions. This effect has been observed in many other studies20-25.
Therefore, the greater the increase of the TGO thickness of the specimen with traditional YSZ top coatings with CoNiCrAlY bond coatings at the same exposure time compared to the specimen YSZ/Al2O3 top coatings with HVOF-BC could be attributed to this mechanism. After increasing the exposure time the TGO layer thickness increased to higher values.
3.3 Mechanical properties
The microhardness value for the bond and top coats were taken from the average value of all the measurement points. Figures 7 and 8 present the Vickers micro-hardness measurements before and after the oxidation tests for the bond and the top layers. The bottom and top limit lines in the graph show the maximum and minimum hardness values. The microhardness of the substrate Inconel 718 super-alloy was in the range 310-340 HV.
The mean values of the bond and top-coat micro-hardness changed before and after the oxidation tests with increasing time. The microhardness of the bond coats decreased with increasing time at 1000 °C for traditional and two-layered coatings. The decline in microhardness of the bond coatings was possibly linked to the decrease in the density of the bond coats. The
Figure 7: Microhardness values of bond coats for two kinds of TBCs, respectively (0 CoNiCrAlY bond coats with YSZ top layer, • CoNiCrAlY bond coats with YSZ/Al2O3 top layer) Slika 7: Vrednosti mikrotrdote vezivnih plasti za dve vrsti TBC, (0 vezivna plast CoNiCrAlY z vrhnjo plastjo YSZ, • vezivna plast CoNiCrAlY z vrhnjo plastjo YSZ/Al2O3)
Figure 8: Microhardness values of top coats for two kinds of TBCs, respectively (□ YSZ top coats with CoNiCrAlY bond layer, • YSZ top coats with YSZ/Al2O3 top layer and CoNiCrAlY bond layer, 0 Al2O3 top coats with YSZ/Al2O3 top layer and CoNiCrAlY bond layer)
Slika 8: Vrednosti mikrotrdote za dve vrsti TBC, (□ vrhnja plast YSZ z vezivno plastjo CoNiCrAlY, • vrhnja plast YSZ z vrhnjo plastjo YSZ/Al2O3 in vezivno plastjo CoNiCrAlY, 0 vrhnja plast Al2O3 z vrhnjo plastjo YSZ/Al2O3 in vezivno plastjo CoNiCrAlY)
decrease of the microhardness in the HVOF bond coats is related to the thermal relaxation of the residual stress present in the as-sprayed coating due to the high temperature. The fact that the decrease in microhardness after 8 h is higher while the decrease after 24 h is lower and no change is observed after 50 h should support this theory, i.e., thermal relaxation occurs very quickly, and since almost no change in microhardness is observed after 8 h it can be concluded that this initial change is due to thermal relaxation.
The microhardness of the top coats increased with increasing time at 1000 °C for the traditional and two-layered ceramic coatings. According to the literature, the situation mentioned above is caused by the decreasing density of the porosity and the increasing density of the ceramic top coating depending on increasing time26.
4 CONCLUSIONS
Traditional YSZ and YSZ/ Al2O3 double-layer TBCs were produced using the APS technique and bond coats were deposited using the HVOF spraying technique. The following results were obtained:
During oxidation of the TBCs, the TGO layer was formed along the interface of the BC/TC layer. The thickness of the TGO in the traditional YSZ coating is higher in comparison with the YSZ/Al2O3 coating after oxidation at 1000 °C for different oxidation times. According to the TGO growth in both TBC systems, the TGO thickening became steady state in the YSZ/Al2O3 two-layer system and, on the other hand, the TGO
thickness in the traditional TBC system was still increasing.
The different formations of the discontinuities between the bond and the ceramic top layers were observed. The ceramic top coats contained porosity and some crack-like discontinuities for both kinds of TBC. After a prolonged oxidation time the number of cracks was much larger in the traditional YSZ ceramic top coat with the CoNiCrAlY bond coating system. The initial porosity state of the as-sprayed TBC samples and/or the acting of the Al2O3 layer as a diffusion barrier at high temperatures have a great influence on determining the TGO growth, since they change the penetration behavior of the oxygen from the surface. In the conventional YSZ system, due to the increasing oxidation process, complex oxides developed in the TGO layer and affected the growth behaviors of the TGO. The mean values of the bond and top-coat microhardnesses changed before and after the oxidation tests with increasing time. The microhardness of the bond coats decreased and that of the top coats increased with increasing time at 1000 °C for the traditional and two-layered coatings.
5 REFERENCES
1 A. G. Evans, D. R. Mumma, J. W. Hutchinson, G. H. Meierc, F. S. Pettit, Mechanisms controlling the durability of thermal barrier coatings, Progress in Materials Science, 46 (2001) 5, 505-553 2Y. Li, C. J. Li, Q. Zhang, G. J. Yang, C. X. Li, Influence of TGO Composition on the Thermal Shock Lifetime of Thermal Barrier Coatings with Cold-sprayed MCrAlY Bond Coat, Journal of Thermal Spray Technology, 19 (2010) 1-2, 168-177 3 W. R. Chen, R. Archer, X. Huang, B. R. Marple, TGO Growth and Crack Propagation in a Thermal Barrier Coating, Journal of Thermal Spray Technology, 17 (2008) 5-6, 858-864 4L. Swadzba, G. Moskal, B. Mendala, T. Gancarczyk, Characterisation of air plasma sprayed TBC coating during isothermal oxidation at 1100 °C, Journal of Achievements in Materials and Manufacturing Engineering, 21 (2007) 2, 81-84
5	W. J. Brindley, Properties of Plasma-Sprayed Bond Coats, Journal of Thermal Spray Technology, 6 (1997) 1, 85-90
6	Y. Li, C. J. Li, G. J. Yang, L. K. Xing, Thermal fatigue behavior of thermal barrier coatings with the MCrAlY bond coats by cold spraying and low-pressure plasma spraying, Surface and Coatings Technology, 205 (2010), 2225-2233
7W. O. Soboyejo, P. Mensah, R. Diwan, J. Crowe, S. Akwaboa, High temperature oxidation interfacial growth kinetics in YSZ thermal barrier coatings with bond coatings of NiCoCrAlY with 0.25 % Hf, Materials Science and Engineering A, 528 (2011), 2223-2230 8 S. Saeidi, K. T. Voisey, D. G. McCartney, The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings, Journal of Thermal Spray Technology, 18 (2009) 2, 209-216
9N. Mu, T. Izumi, L. Zhang, B. Gleeson, Compositional Factors Affecting the Oxidation Behavior of Pt-Modified y-Ni+y'-Ni3Al-Based Alloys and Coatings, Materials Science Forum, 239 (2008) 595-598, 239-247 10 P. Richer, M. Yandouzi, L. Beauvais, B. Jodoin, Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying, Surface and Coatings Technology, 204 (2010), 3962-3974
11 Q. Zhang, C. J. Li, C. X. Li, G. J. Yang, S. C. Lui, Study of oxidation behavior of nanostructured NiCrAlY bond coatings deposited by cold spraying, Surface and Coatings Technology, 202 (2008), 3378-3384
12J. A. Thompson, T. W. Clyne, The effect of heat treatment on the stiffness of zirconia top coats in-plasma sprayed TBCs, Acta Mater., 49 (2001), 1565-1575
13 J. A. Haynes, M. K. Ferber, W. D. Porter, Thermal Cycling Behavior of Plasma-Sprayed Thermal Barrier Coatings with Various MCrAIX Bond Coats, Journal of Thermal Spray Technology, 9 (2000) 1, 38-48
14F. H. Yuan, Z. X. Chen, Z. W. Huang, Z. G. Wang, S. J. Zhu, Oxidation behavior of thermal barrier coatings with HVOF and detonation-sprayed NiCrAlY bond coats, Corrosion Science, 50 (2008), 1608-1617
15	J. R. V. Garcia, T. Goto, Thermal barrier coatings produced by chemical vapor deposition, Science and Technology of Advanced Materials, 4 (2003), 397-402
16	F. Pedraza, C. Tuohy, L. Whelan, A. D. Kennedy, High Quality Aluminide and Thermal Barrier Coatings Deposition for New and Service Exposed Parts by CVD Techniques, Materials Science Forum, 461-464 (2004), 305-312
17	M. H. Li, X. F. Sun, S. K. Gong, Z. Y. Zhang, H. R. Guan, Z. Q. Hu, Phase transformation and bond coat oxidation behavior of EB-PVD thermal barrier coating, Surface and Coatings Technology, 176 (2004), 209-214
18	M. Saremi, A. Afrasiabi, A. Kobayashi, Microstructural analysis of YSZ and YSZ/Al2O3 plasma sprayed thermal barrier coatings after high temperature oxidation, Surface and Coatings Technology, 202 (2008), 3233-3238
19	Q. Yu, A. Rauf, C. Zhou, Microstructure and Thermal Properties of Nanostructured 4 % Al2O3-YSZ Coatings Produced by Atmospheric Plasma Spraying, Journal of Thermal Spray Technology, 19 (2010) 6, 1294-1300
20	Kh. G. S. Thomas, U. Dietl, Thermal barrier coatings with improved oxidation resistance, Surface and Coatings Technology, 68/69 (1994), 113-115
21	Q. Yu, A. Rauf, C. Zhou, Microstructure and Thermal Properties of Nanostructured 4 % Al2O3-YSZ Coatings Produced by Atmospheric Plasma Spraying, Journal of Thermal Spray Technology, 19 (2010) 6, 1294-1300
22	C. Ren, Y. D. He, D. R. Wan, Fabrication and Characteristics of YSZ-YSZ/Al2O3 Double-Layer TBC, Oxidation of Metals, 75 (2011), 325-335
23	J. Müller, M. Schierling, E. Zimmermann, D. Neuschütz, Chemical vapor deposition of smooth a- Al2O3 films on nickel base superalloys as difusion barriers, Surface and Coatings Technology, 120-121 (1999), 16-21
24	H. Bolta, F. Koch, J. L. Rodet, D. Karpov, S. Menzel, Al2O3 coatings deposited by filtered vacuum arc-characterization of high temperature properties, Surface and Coatings Technology, 116-119 (1999), 956-962
25	A. C. Karaoglanli, E. Altuncu, I. Ozdemir, A. Turk, F. Ustel, Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions, Surface and Coatings Technology, 205 (2011) 2, 369-S373
26	H. Guo, X. Bi, S. Gong, H. Xu, Microstructure Investigation On Gradient Porous Thermal Barrier Coating Prepared By EB-PVD, Scripta Mater., 44 (2001), 683- 687