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Enhanced cerium migration in ceria-stabilised zirconia
ELSEVIER
Surface and Coatings Technology 94-95 (1997) 112- 117
Enhanced cerium migration in ceria-stabilised zirconia
Abstract The metal components inside jet engines which are exposed to hot combustion gases often must be thermally insulated with thermal barrier coatings. Most thermal barrier coatings consist of a porous zirconia layer deposited over a NiCoCrAlY layer. However, the zirconia layer can spa11 during thermal cycling unless it is stabilised to prevent the zirconia changing between its Letragonal and monoclinic phases. We have smdicd thermal barrier coatings in which the zirconia was stabilised by alloying with 2.5 wt.% yttria and 25 wt.% ceria. The spatial distributions of the cerium in the zirconia layer before and after heat treatment were studied using electron microscopy (backscattered electron imaging and X-ray analysis). The different phases present were identified by X-ray diffraction. We found that the cerium distribution in the newly-formed zirconia layer was essentially uniform. However, heating at 1200°C could cause the formation of ceriumrich zones providedthe coating was not separated from the NiCoCrAlY layer and the substrate before heating. This implied an interaction between the zirconia and the underlying metal. Experiments in the presence of oxygen getters and experiments in vacuum both suggest that cerium migration in ceria-stabiliscd zirconia is enhanced at high temperatures (1 1OO’C) under reducing conditions. 0 1997 Elsevier Science S.A. Keywords: Zirconia; Cerium; Thermal barrier coatings; Diffusion
1. Introductiou
Thermal barrier coatings are often used in gas turbine engines to thermally insulate metallic components from the hot combustion gases [ 1.21, The coatings, in combination with internal air cooling, reduce the surface tcmperature of the underlying metal by up to 150°C [2] and thereby improve the coatedcomponent’s durability. The increasein durability can also be traded for an increasein the temperature of the combustion gasesand thereby an increase in engine performance. A suitablecoating material must not only be a good thermal insulator, but also tolerate the high temperatures (llOO°C) and oxidising atmosphere inside the gas turbine engine. Coatings oi many ceramics meet these criteria, but
most break away during thermal cycling, as seenwhen the engine is turned on and off. Plasma-sprayedzirconia survives and is therefore used. One reason it survives is because it is unusually compliant [ 1,3]: its effective Young’s modulusis only 1 to 50 GPa [3,4] which is much * Corresponding author. Tel.: +61 3 9626731 1; fiuc: +61 3 96267087; e-mail: john.lhornton~dbru.def~nce.gov.au
0257-8973J97c517.00 0 1997 Elsevier Science S.A. All rights reserved PII SO257-S972(97)00487-8
smaller than the 380 GPa for alumina 141,a more typical ceramic [4]. For most substratesit is also necessaryto first deposit an oxidation resistantlayer 1.51.This lower layer is composedof a plasma-sprayedMCrAlY alloy, where M is Ni, Co or Fe. It is known as a bond coat. Typically the zirconia layer is 0.3 mm thick, and the MCrAIY layer 0.2 mm thick. There are many descriptionsof the plasma-spraymethod of depositing coatings [6]. Basically, it involves feeding a powder of the selectedmaterial into a plasmajet, which is directed towardsthe substrate.Before reachingthe substrate the powder particles are melted by the heat from the plasma and so they stick to the substrateon impact. At temperaturesabove 2400°C pure zirconia has a cubic phasestructure. On cooling below this temperatureit forms a tetragonal phaseand then at 1100°C a monoclinic phase. Thus pure zirconia is unsuitablefor thermal barrier coatings becausethe changeto the monoclinic phaseis accompanied by an expansion,which may lead to the disintegrationof the zirconia layer during deposition or while in service [5,7,8]. The disintegration may be prevented by alloying with either calcia, magnesiaor yttria prior to deposition (these additions are commonly called stabilisers). During deposition
when the molten particles of modified zirconia strike the substrate, they are quenched into me&stable cubic or tetragonal phases depending on the amount of stabiliser added. From X-ray diffraction [5,8] and EXAFS [9] evidence it appears that the stabiliser’s metal ions are incorporated into the zirconia lattice and substitute for the zirconium ions. The metastable phases are very stable, but eventually deteriorate at high temperatures because of diffusion of these metal ions. For example, heating at 1400°C is required to cause the formation of the monoclinic phase in Zr0+3.6 mol% YOt,5 [8]. Zirconia coatings stabilised with magnesia and calcia w-ere found to be less durable [5,7,10] and have therefore been superseded by ZrOz-8.6 11101%YO1.j. For example, precipitation of magnesia from Zr07-24 wt.% MgO coatings was observed at 950°C [5]. The precipitation would also have increased the thermal conductivity, reducing the effectiveness of the coating. Zirconia thermal barrier coatings are now essentially limited to ‘clean fuel’ engines because the commonly used stabilisers (magnesia and yttria) react with the sulfur and vanadium contaminants of low-quality fuel [ 11,121. Ceria is not affected by sulfur or vanadium and has therefore been tested as an alternative stabiliser [ 11,121. Furthermore. zirconia stabilised with a combination of ceria and yttria is now commercially available. However, the maximum temperature that it can withstand has still to be established. In the present work the plasma sprayed ceria plus yttria-stabilised zirconia was studied to assess its suitability for gas turbine applications. Preliminary experiments indicated that cerium migration occurred in coatings on metallic substrates, but not in free-standing pieces of zirconia. The following series of experiments were aimed at elucidating this phenomenon.
2. Methods and materials The thermal barrier coatings were deposited onto a 1-mm thick Hastelloy X sheet. This nickel-based superalloy was chosen because it is representative of the alloys used for the static hot components of gas turbine engines. The thermal barrier coating consisted of a 2-mm thick coating of ceria (25 wt.%) plus yttria (2.5 wt.%). stabilised zirconia (referred to here as CYZ) on top of a 0.2~mm thick layer of Ni (74 wt.%), Cr (17.5 wt.%), Co (2.5 wt.%), Al (5.5 wt.%) Y203 (0.5 wt.%) bond coat. The coatings were applied using a Metco 9MB gun with an argon and hydrogen gas mixture. Also, it must be noted that the bond coat powder used throughout this work was chosen because it was particularly coarse and therefore produced a porous coating with a large internal surface area. Because of this large surface area large volumes of oxide scale are produced during heating at 1100°C in air (0.03 mm thick). In comparison, a dense coating would produce much less scale (0 003 mm thick). The samples were heated at 1200°C for 125 h in air. Preliminary experiments indicated that significant cerium
migration occurred only when the ceramic was in contact with metal substrate. In order to accelerate this effect 1 x 2 x 0.1 cm samples, with 2-mm thick CYZ and a 0.2 mm-thick bond coat, were wrapped in 0.15-mm thick nickel sheet. These samples are referred to as mounted CYZ. Control samples of free-standing zirconia were obtained by breaking off sections of the coating. The sections were 2 mm thick because failure aImost always occurred only slightly above the ceramic to bond coat interface. These samples are referred to as free-standing CYZ and they were not wrapped in nickel sheet. Prior to and following heat treatment, X-ray diffraction patterns were taken of the zirconia. Following heat treatment the samples were also examined using a scanning electron microscope. The most useful pictures were obtained using backscattered electrons to highlight areas of high cerium concentration (the number of backscattered electrons is greater for elements of higher atomic number). The range of elements present at selected spots was determined by energy dispersive X-ray analysis (EDAX). Assprayed CYZ was also examined by EDAX. The CYZ was observed to change colour in reducing conditions. These changes were mapped out by: 1. Heating free-standing CYZ to lOOO”C, in a 1O:l by volume, nitrogen and carbon monoxide atmosphere for 48-h. This material we called reduced CYZ; it had turned brown. 2. Heating reduced CYZ to 800°C in air for 1 h. This material we called partially-reduced CYZ; it had turned yellow. 3. Heating free-standing CYZ to 1100°C in air for 1 week. This material we called oxidised CYZ; it had turned white. Thus there was a general trend from brown, through yellow to white which seems to be linked to the degree of reduction. The mounted CYZ material had turned brown after heat treatment.
3. Results The ratios of areas of the largest cerium to zirconia peaks in the EDAX spectra from the top surface of the samples are shown in Table 1. The spectra were accumulated over large (0.2 x 0.2 mm) areas. A comparison of these ratios indicates the difference in surface cerium concentration. The surface of as-sprayed CYZ (not shown) displayed the extensive cracking common to plasma sprayed ceramic layers. The corresponding X-ray diffraction pattern is shown in Figs. 1 and 2 as the dashed line, with the peaks marked 16. ICDD file 38-1437 for tetragonal Zr0,&ea,i602 provides a good fit [ 131. The surface of the free-standing CYZ after the heat treatment is shown in Fig. 3. The heat treatment has removed the cracking and produced a sponge-like microstructure inter-
114 Table
J. Thornton
et rri. / Sur&re
and Contings
1
Technology
10000
A qualitative guide to cc&m concentration the peak areas from the EDAX spectra
obtained
by taking
CeiZr
Sample
As-sprayed Heat-treated Heat-treated
CYZ free-standing CYZ mounted CYZ
Ihe ratio of
of As-Sprayed CYZ and CM after heating
I
’I
I
-
Heated
------
As-Sprayed
I
16
30
35 2 Theta
Fig. 1. A comparison
Polished
Between Surface
As-Sprayed
of Mounted
CYZ and the and
Heated
CYZ
218
60
25
Comparison
0.23 0.42
El0
2% 4cco-
1
(I 997) 11_7- I I7
peak area ratio
spersed with some large (2 pm) crystals, for example the one labelled C. Heat treatment did not change the X-ray diffraction pattern of the free-standing CYZ, although itb EDAX-derived ceria to zirconia ratio did increase (Table I), The surface of a heated mounted CYZ sample is shown in Fig. 3. The surface is covered with crystals. The elements present in the different regions were determined by EDAX. The bright crystals in the picture are cerium-rich zirconia (CeO) as is the shapeless patch (SPj. The dark crystals (NO) were identified as a nickel oxide. These oxide crystals possibly flaked off the nickel sheet in which the sample was wrapped. Fig. 5 shows the top of a heated mounted CYZ sample in cross-section. This area was the closest to the nickel foil wrapping. The cerium-rich surface is clearly seen (CeO), because it is brighter than the bulk of the layer when imaged with backscattered electrons. Similar cerium-rich regions were found where the CYZ had been in contact with the bond coat. There were also smaller cerium-rich regions within the bulk, some of which can be seen in Fig. 5. There were no such regions evident, at this magnification, in the cross-section of the as-sprayed material (not shown). It was also noted that the heated mounted CYZ was far less porous. indicating that sintering had taken place. The EDAX results in Table 1 also show that the surface of heated mounted CYZ is much richer in cerium than either the as-sprayed or the heated free-standing CYZ. The X-ray diffraction pattern of the heated mounted CYZ is also different from those of both the as-sprayed and the heated free-standing CYZ, as shown in Fig. 1. A new phase Comparison Mounted
94-95
40
55
45
(degrees)
of part of the: X-ray
diffraction
patterns
from as-
sprayedCYZ (dashedline) andmountedCYZ (full line). Thr labelson the peaks refer to rhc following phases: 16 is Zr,, 8JCeo,,602; 60 is ZroJCeO,&; and BCO is an oxide of Ni and possibly Cr.
o i _*-.__. 25
s p ' ---___* 30
.35
40
2 Theta
1 45
,. .--'
/: ____r___ 1 I 50 55
(degrees)
Fig. 2. A comparison of part of the X-ray diffraction patterns from mounted CYZ after polishing back the surface ifull line), and as-sprayed CYZ idashed line). The labels are as defined in Fig. I.
has appeared, which is best fitted by ICDD file 38-1439 for Zr0.ACe0.602 [14] (peaks marked 60 in Fig. 1). The new phase with higher Ce content is further evidence for the cerium enrichment. Fig. 2 shows the X-ray diffraction pattern of the material after polishing back the surface with Sic paper. This uncovers the tetragonal Zro.s4 Ceo.1602, indicating that the Zr~,~Ceo.602 (the cerium-rich phase) was a surface layer. This supports the cross-sectional evidence.
4. Discussion Zirconia is an odd material, because anion diffusion is about six orders of magnitude faster than cation diffusion [7,14]. Cation diffusion, including that of the stabiliser ions, is expected to be negligible at 1200°C. Consequently, the formation of stabiliser-rich regions is not expected below 1200°C. This is confirmed by our results for CYZ and those published in the literature for ceria-stabilised zirconia [15], provided the ceramic coating is removed from its substrate before heating. However, our results also show that if a CYZ coating on its bond coat and substrate is heated at lZOO”C, cerium migration does take place. This was demonstrated by the formation of a cerium-rich surface layer on the mounted CYZ sample. Oxidation is the most obvious change to bond coat and substrate during heating. This suggests that cerium migration occurs at high temperatures and when the CYZ is in contact with an oxygen getter. Perhaps the CYZ is reduced. Further evidence for cerium migration when the CYZ is reduced is shown in Fig. 6, which shows the X-ray diffraction patterns from a 0.3-mm thin CYZ layer on a substrate after heating at 1100°C in a evacuated sealed quartz tube. The CYZ has decomposed into Zro,slCeo.1602, Zr0.4Ce0.602 and monoclinic zirconia, the peaks of which are labelled 16, 60 and M, respectively. When another sample was heated identically, but in air, it did not decompose. The sealing of the sample in the evacuated tube meant that any reduction of the zirconia by the bond coat was not compensated by the
J. Tirorrzton
et al. / Surfacr
and
Coniiirgs
Technology
94-95
(1997)
112-117
115
Fig. 3. A backscattered electron image of free-standing CYZ after heat treatment at 1200°C for 125 h. See text for explanation of annotation.
of oxygen from the air. Phaseseparation was thus able to take place at 1100°C rather than 1200°C becauseof the greater reduction. The thinner CYZ alsomeant there was less zirconia for the bond coat to reduce. Earlier heating experiments in air on 0.3~mm thin CYZ also showed evidence of the formation of a cerium-rich phase at 1100°C
supply
1161. On first sight, reduction of the zirconia by the bond coat
seemsunlikely because cerium and zirconium are more electro-positive than any of the atoms in the substrateor bond coat. However, partial reduction of the Ce” ions to Ce3+may be possibleby the aluminium or chromium in the bond coat. Unfortunately the free energy data for CeYZr is not readrly available, but the Gibb’s free energy of formation for the following reactionsat 1200°C [ 171indicatesthat partial reduction is likely:
Fig. 4. A backscattered electron image of mounted CYZ after heating at 12OOT for 115 h. See text for explanation of annotation.
116
Fig. 5. A backscattercd electron arcas are rich in cerium.
image of a polished
cross-section
of mounted
6Ce02 + 2&l = 3Ce203+ A1,03
AC” = - 250 kJ
(1)
6Ce02 + 2Cr = 3Cez03 + Crz03
AC” = - 230 kJ
(2)
Theseare of the sameorder as the free energy of formation of Alz03 from the elements (680 kJ) [17]. Therefore the heating of CYZ on an aluminium- or chromium-containing bond coat is expected to result in the formation of an aluminium and chromium oxide layer on the bond coat, the reduction of the Ce’” ions in the CYZ to Ce3*ions, and the production of a correspondingnumber of oxygen vacancies in the lattice. Furthermore, the pale yellow or off-white colour of the as-sprayedCYZ suggestthat it is also partially reduced (the hydrogen in the plasmagas was probably the reducing agent here). Therefore the reduction already has a head-start on fully-oxidised CYZ. Also, the porosity of the bond coat employed meansthat a larger than usual amount of aluminium and chromium will be in intimate contact with the CYZ. Thus there was a large area capable of taking oxygen from the CYZ. Partial reduction may not directly accelerate the cerium migration; however, it doesremove many oxygen ions from the lattice, and oxygen ions are the biggest in the CYZ lattice (the respective radii are: 0.132 nm for O’-, 0.087 nm for Zr”, 0. 106 nm for Y3+, 0.102 nm for Ce”’ and 0.118 nm for Ce”*) [ 181.Therefore it is feasiblethat removing oxygen ions from the lattice gives the cations (cerium and zirconium) more freedom, and thus reducesthe activation energy for cation migration. The disintegration-prone equilibrium mixtures of cerium-rich and cerium-poor material are therefore obtained in accessibletimes (1 week) and
CYZ after heating
at 1300°C for 125 h. This shows the top surface. The bright
temperatures (11OO’C to 1200°C). Another side-effect of the cation migration might be accelerated sintering. The much lower porosity seen in the heated mounted CYZ cross-sectioncomparedto as-sprayedcross-sectionsuggest that sintering hasoccurred. Sintering will make the coating stiffer and therefore more prone to spalling. Although the calculated free energiesimply that aluminium and chromium can partially reducethe Ce’+ ions in the CYZ to Ce”+ ions, they also show that nickel cannot. However, when a sampleis heatedin air, the oxygen taken from the CYZ by the aluminium and chromium can be replenComparison 2500
16
of Thin
CYZ on Substrate
Heated in Air or Vacuum -Air -----Vacuum
s 2000 5
i
-1 4 6 EE 1500 22 =
k
16
IOOO-
25
30
35
40
45
50
55
2 Theta(degrees) Fig. 6. A comparison of part of the X-ray diffraction patterns from a 0.3mm thin CYZ coating after it had been heated for 1 week at IlOO’C in a vacuum (dashed line) and air (full line) while it was still on its bond coat and substrate. See text for explanation of annotation.
J. Thornron
er al. / S~~fnce rind Comings
ished from the air. The degree of reduction achieved will be a balance between the oxygen supply and the reduction. Wrapping the sample in nickel will adjust this balance in favour of reduction, because the nickel oxidises and reduces the concentration of oxygen in the air surrounding the sample. The acceleration of cation diffusion in zirconia has been noticed before. Indeed: acceleration was necessary to determine the phase diagram [19], where the addition of compounds called mineralisers (LiCI) were deliberately introduced to cause enhanced diffusion. Possibly, the mechanism here was also reduction. Furthermore, the formation of monoclinic zirconia at the anodic face of tetragonal zirconia solid electrolytes [20] (as used in fuel cells and oxygen pumps) may also be due to cation diffusion brought about by a reduction of the zirconia. Oxygen ions are removed from the anodic face of these electrolytes by hydrogen to form water.
5. Summary
The diffusion of cerium ions in CYZ is accelerated by heating when it is in contact with oxygen getters, and the diffusion is further enhanced by heating in a vacuum. It is proposed that this is due to the removal of oxygen ions from the lattice which gives the cerium ions, and the other cations, more freedom (when a cation moves from a lattice site to a similar one it must move through the space between; anions occupy much of this space, so removing an anion will give the cation more freedom to move between cation lattice sites). This diffusion eventually results in the formation of a phase prone to disintegration on thermal cycling, and perhaps also enhances the sintering of the coating. Both results can cause early failure of the-coating. Consequently, to use CYZ as a coating, in parts of an engine where the gases are reducing and the coating temperature exceeds 1100°C may lead to premature failure of the coating. Furthermore, CYZ should only be used on the most oxidation-resistant substrates or bond coats.
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94-95
(1997)
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117
Acknowledgements
The authors are grateful to Dr. 1. Drennan, Dr. M. Paterson and Ms. J. Cocking with whom one of us, J.T., had many interesting discussions; to Mr. J. Shenton for his help with the electron microscope; and to Dr. Y.C. Tsui for his help with the vacuum experiment. References
[II
A. Bennett, F.C. Toriz and A.B. Thakker, SW$ Coat. Techno!., 32 (1987) 359. 121R.A. Miller, .Sur$ Cotrr. Techrmi., 30 (1987) 1. [31 T.W. Clyne and Y.C. Tsui, in B. Ilschner (cd.), Proc. of 3rd Znr. Sump. on Srructurol and Functionai Gradient Mmerinls (FGM ‘941, PPUR, Lausanne, 199-l. fj.1 W.J. Lackey, D.P. Stinton, G.A. Cemy, A.C. Schaffhauser and L.L. Fehrenbacher, At/i,. Crrorn. ?,4orer., 2 (1987) 21. PI A. Bennett, &f&?r. Sci. Tecivrol., -7 (1986) 257. I61 J.H. Glare and D.E. Crawmer, ASI Memls Hmdbook 9th Edn., Vol. 5, 1982, p. 361. PI J.D. Cawley, Overvim’ of Zirconia Jvith Respect to Gas Turbine Engines, NASA Technical Paper 2286, 1984. 181R.A. Miller, J.L. Smialek and R.G. Garlick, in Adwnces in Cerarnics: Scie~rce rind Technology ofZirconin, Vol. 3, American Ceramic Society Publ., 1981, p. 241. PI P. Li and L-W. Chcn, Phys. Rev. B, 48 (1993) 10074. I101 S. Stecura and C.H. Leibert, US Pmenr No. 4,0X,705 (25 October 1977). 1111P.A. Siemens and D.W. McKee, US Pateirf No. 4,328,X5 (4 May 1982). [I21 R.L. Jones, Mater. High Temp., 9 (1991) 228. Cemre for Diffraction Dnta, PA, USA, 1995. [I31 JCPDS-lmertunionni and A. Dominguez-Rodriguez, II‘+1 A. Bavon-Leon, M. Jimenez-Melendo Acra Metal/. Mcirer., 40 (1992) 2717. I151 J.R. Brandon and R. Taylor, Sur$ Cocir. Technol., 46 (1991) 91. I161 3. Thornton and A. Majumdar, in A. Ohmori (ed.), Proc. 14fh fnf. Thermal Spray Cor2frrence, High Temperature Society of Japan Publ., 1995, p. 1075. 62nd edition, pp D-34, CRC Press, 198 1. [l71 CRC Hrwdbook, [I81 C.J. Smithells, Smirhel2s Merals Reference Book, 6dr eciifion, Butterworth, 1983. section 4.5.1. I191 E. Tani, M. Yoshimura and S. Somiya, J. Am. Ceram. Sot., 66 (1983) 506. [20] S.P.S. Badwal and N. Nardella, App1. Piiys., A49 (1989) 13.
Surface and Coatings Technology 94-95 (1997) 112- 117
Enhanced cerium migration in ceria-stabilised zirconia
Abstract The metal components inside jet engines which are exposed to hot combustion gases often must be thermally insulated with thermal barrier coatings. Most thermal barrier coatings consist of a porous zirconia layer deposited over a NiCoCrAlY layer. However, the zirconia layer can spa11 during thermal cycling unless it is stabilised to prevent the zirconia changing between its Letragonal and monoclinic phases. We have smdicd thermal barrier coatings in which the zirconia was stabilised by alloying with 2.5 wt.% yttria and 25 wt.% ceria. The spatial distributions of the cerium in the zirconia layer before and after heat treatment were studied using electron microscopy (backscattered electron imaging and X-ray analysis). The different phases present were identified by X-ray diffraction. We found that the cerium distribution in the newly-formed zirconia layer was essentially uniform. However, heating at 1200°C could cause the formation of ceriumrich zones providedthe coating was not separated from the NiCoCrAlY layer and the substrate before heating. This implied an interaction between the zirconia and the underlying metal. Experiments in the presence of oxygen getters and experiments in vacuum both suggest that cerium migration in ceria-stabiliscd zirconia is enhanced at high temperatures (1 1OO’C) under reducing conditions. 0 1997 Elsevier Science S.A. Keywords: Zirconia; Cerium; Thermal barrier coatings; Diffusion
1. Introductiou
Thermal barrier coatings are often used in gas turbine engines to thermally insulate metallic components from the hot combustion gases [ 1.21, The coatings, in combination with internal air cooling, reduce the surface tcmperature of the underlying metal by up to 150°C [2] and thereby improve the coatedcomponent’s durability. The increasein durability can also be traded for an increasein the temperature of the combustion gasesand thereby an increase in engine performance. A suitablecoating material must not only be a good thermal insulator, but also tolerate the high temperatures (llOO°C) and oxidising atmosphere inside the gas turbine engine. Coatings oi many ceramics meet these criteria, but
most break away during thermal cycling, as seenwhen the engine is turned on and off. Plasma-sprayedzirconia survives and is therefore used. One reason it survives is because it is unusually compliant [ 1,3]: its effective Young’s modulusis only 1 to 50 GPa [3,4] which is much * Corresponding author. Tel.: +61 3 9626731 1; fiuc: +61 3 96267087; e-mail: john.lhornton~dbru.def~nce.gov.au
0257-8973J97c517.00 0 1997 Elsevier Science S.A. All rights reserved PII SO257-S972(97)00487-8
smaller than the 380 GPa for alumina 141,a more typical ceramic [4]. For most substratesit is also necessaryto first deposit an oxidation resistantlayer 1.51.This lower layer is composedof a plasma-sprayedMCrAlY alloy, where M is Ni, Co or Fe. It is known as a bond coat. Typically the zirconia layer is 0.3 mm thick, and the MCrAIY layer 0.2 mm thick. There are many descriptionsof the plasma-spraymethod of depositing coatings [6]. Basically, it involves feeding a powder of the selectedmaterial into a plasmajet, which is directed towardsthe substrate.Before reachingthe substrate the powder particles are melted by the heat from the plasma and so they stick to the substrateon impact. At temperaturesabove 2400°C pure zirconia has a cubic phasestructure. On cooling below this temperatureit forms a tetragonal phaseand then at 1100°C a monoclinic phase. Thus pure zirconia is unsuitablefor thermal barrier coatings becausethe changeto the monoclinic phaseis accompanied by an expansion,which may lead to the disintegrationof the zirconia layer during deposition or while in service [5,7,8]. The disintegration may be prevented by alloying with either calcia, magnesiaor yttria prior to deposition (these additions are commonly called stabilisers). During deposition
when the molten particles of modified zirconia strike the substrate, they are quenched into me&stable cubic or tetragonal phases depending on the amount of stabiliser added. From X-ray diffraction [5,8] and EXAFS [9] evidence it appears that the stabiliser’s metal ions are incorporated into the zirconia lattice and substitute for the zirconium ions. The metastable phases are very stable, but eventually deteriorate at high temperatures because of diffusion of these metal ions. For example, heating at 1400°C is required to cause the formation of the monoclinic phase in Zr0+3.6 mol% YOt,5 [8]. Zirconia coatings stabilised with magnesia and calcia w-ere found to be less durable [5,7,10] and have therefore been superseded by ZrOz-8.6 11101%YO1.j. For example, precipitation of magnesia from Zr07-24 wt.% MgO coatings was observed at 950°C [5]. The precipitation would also have increased the thermal conductivity, reducing the effectiveness of the coating. Zirconia thermal barrier coatings are now essentially limited to ‘clean fuel’ engines because the commonly used stabilisers (magnesia and yttria) react with the sulfur and vanadium contaminants of low-quality fuel [ 11,121. Ceria is not affected by sulfur or vanadium and has therefore been tested as an alternative stabiliser [ 11,121. Furthermore. zirconia stabilised with a combination of ceria and yttria is now commercially available. However, the maximum temperature that it can withstand has still to be established. In the present work the plasma sprayed ceria plus yttria-stabilised zirconia was studied to assess its suitability for gas turbine applications. Preliminary experiments indicated that cerium migration occurred in coatings on metallic substrates, but not in free-standing pieces of zirconia. The following series of experiments were aimed at elucidating this phenomenon.
2. Methods and materials The thermal barrier coatings were deposited onto a 1-mm thick Hastelloy X sheet. This nickel-based superalloy was chosen because it is representative of the alloys used for the static hot components of gas turbine engines. The thermal barrier coating consisted of a 2-mm thick coating of ceria (25 wt.%) plus yttria (2.5 wt.%). stabilised zirconia (referred to here as CYZ) on top of a 0.2~mm thick layer of Ni (74 wt.%), Cr (17.5 wt.%), Co (2.5 wt.%), Al (5.5 wt.%) Y203 (0.5 wt.%) bond coat. The coatings were applied using a Metco 9MB gun with an argon and hydrogen gas mixture. Also, it must be noted that the bond coat powder used throughout this work was chosen because it was particularly coarse and therefore produced a porous coating with a large internal surface area. Because of this large surface area large volumes of oxide scale are produced during heating at 1100°C in air (0.03 mm thick). In comparison, a dense coating would produce much less scale (0 003 mm thick). The samples were heated at 1200°C for 125 h in air. Preliminary experiments indicated that significant cerium
migration occurred only when the ceramic was in contact with metal substrate. In order to accelerate this effect 1 x 2 x 0.1 cm samples, with 2-mm thick CYZ and a 0.2 mm-thick bond coat, were wrapped in 0.15-mm thick nickel sheet. These samples are referred to as mounted CYZ. Control samples of free-standing zirconia were obtained by breaking off sections of the coating. The sections were 2 mm thick because failure aImost always occurred only slightly above the ceramic to bond coat interface. These samples are referred to as free-standing CYZ and they were not wrapped in nickel sheet. Prior to and following heat treatment, X-ray diffraction patterns were taken of the zirconia. Following heat treatment the samples were also examined using a scanning electron microscope. The most useful pictures were obtained using backscattered electrons to highlight areas of high cerium concentration (the number of backscattered electrons is greater for elements of higher atomic number). The range of elements present at selected spots was determined by energy dispersive X-ray analysis (EDAX). Assprayed CYZ was also examined by EDAX. The CYZ was observed to change colour in reducing conditions. These changes were mapped out by: 1. Heating free-standing CYZ to lOOO”C, in a 1O:l by volume, nitrogen and carbon monoxide atmosphere for 48-h. This material we called reduced CYZ; it had turned brown. 2. Heating reduced CYZ to 800°C in air for 1 h. This material we called partially-reduced CYZ; it had turned yellow. 3. Heating free-standing CYZ to 1100°C in air for 1 week. This material we called oxidised CYZ; it had turned white. Thus there was a general trend from brown, through yellow to white which seems to be linked to the degree of reduction. The mounted CYZ material had turned brown after heat treatment.
3. Results The ratios of areas of the largest cerium to zirconia peaks in the EDAX spectra from the top surface of the samples are shown in Table 1. The spectra were accumulated over large (0.2 x 0.2 mm) areas. A comparison of these ratios indicates the difference in surface cerium concentration. The surface of as-sprayed CYZ (not shown) displayed the extensive cracking common to plasma sprayed ceramic layers. The corresponding X-ray diffraction pattern is shown in Figs. 1 and 2 as the dashed line, with the peaks marked 16. ICDD file 38-1437 for tetragonal Zr0,&ea,i602 provides a good fit [ 131. The surface of the free-standing CYZ after the heat treatment is shown in Fig. 3. The heat treatment has removed the cracking and produced a sponge-like microstructure inter-
114 Table
J. Thornton
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10000
A qualitative guide to cc&m concentration the peak areas from the EDAX spectra
obtained
by taking
CeiZr
Sample
As-sprayed Heat-treated Heat-treated
CYZ free-standing CYZ mounted CYZ
Ihe ratio of
of As-Sprayed CYZ and CM after heating
I
’I
I
-
Heated
------
As-Sprayed
I
16
30
35 2 Theta
Fig. 1. A comparison
Polished
Between Surface
As-Sprayed
of Mounted
CYZ and the and
Heated
CYZ
218
60
25
Comparison
0.23 0.42
El0
2% 4cco-
1
(I 997) 11_7- I I7
peak area ratio
spersed with some large (2 pm) crystals, for example the one labelled C. Heat treatment did not change the X-ray diffraction pattern of the free-standing CYZ, although itb EDAX-derived ceria to zirconia ratio did increase (Table I), The surface of a heated mounted CYZ sample is shown in Fig. 3. The surface is covered with crystals. The elements present in the different regions were determined by EDAX. The bright crystals in the picture are cerium-rich zirconia (CeO) as is the shapeless patch (SPj. The dark crystals (NO) were identified as a nickel oxide. These oxide crystals possibly flaked off the nickel sheet in which the sample was wrapped. Fig. 5 shows the top of a heated mounted CYZ sample in cross-section. This area was the closest to the nickel foil wrapping. The cerium-rich surface is clearly seen (CeO), because it is brighter than the bulk of the layer when imaged with backscattered electrons. Similar cerium-rich regions were found where the CYZ had been in contact with the bond coat. There were also smaller cerium-rich regions within the bulk, some of which can be seen in Fig. 5. There were no such regions evident, at this magnification, in the cross-section of the as-sprayed material (not shown). It was also noted that the heated mounted CYZ was far less porous. indicating that sintering had taken place. The EDAX results in Table 1 also show that the surface of heated mounted CYZ is much richer in cerium than either the as-sprayed or the heated free-standing CYZ. The X-ray diffraction pattern of the heated mounted CYZ is also different from those of both the as-sprayed and the heated free-standing CYZ, as shown in Fig. 1. A new phase Comparison Mounted
94-95
40
55
45
(degrees)
of part of the: X-ray
diffraction
patterns
from as-
sprayedCYZ (dashedline) andmountedCYZ (full line). Thr labelson the peaks refer to rhc following phases: 16 is Zr,, 8JCeo,,602; 60 is ZroJCeO,&; and BCO is an oxide of Ni and possibly Cr.
o i _*-.__. 25
s p ' ---___* 30
.35
40
2 Theta
1 45
,. .--'
/: ____r___ 1 I 50 55
(degrees)
Fig. 2. A comparison of part of the X-ray diffraction patterns from mounted CYZ after polishing back the surface ifull line), and as-sprayed CYZ idashed line). The labels are as defined in Fig. I.
has appeared, which is best fitted by ICDD file 38-1439 for Zr0.ACe0.602 [14] (peaks marked 60 in Fig. 1). The new phase with higher Ce content is further evidence for the cerium enrichment. Fig. 2 shows the X-ray diffraction pattern of the material after polishing back the surface with Sic paper. This uncovers the tetragonal Zro.s4 Ceo.1602, indicating that the Zr~,~Ceo.602 (the cerium-rich phase) was a surface layer. This supports the cross-sectional evidence.
4. Discussion Zirconia is an odd material, because anion diffusion is about six orders of magnitude faster than cation diffusion [7,14]. Cation diffusion, including that of the stabiliser ions, is expected to be negligible at 1200°C. Consequently, the formation of stabiliser-rich regions is not expected below 1200°C. This is confirmed by our results for CYZ and those published in the literature for ceria-stabilised zirconia [15], provided the ceramic coating is removed from its substrate before heating. However, our results also show that if a CYZ coating on its bond coat and substrate is heated at lZOO”C, cerium migration does take place. This was demonstrated by the formation of a cerium-rich surface layer on the mounted CYZ sample. Oxidation is the most obvious change to bond coat and substrate during heating. This suggests that cerium migration occurs at high temperatures and when the CYZ is in contact with an oxygen getter. Perhaps the CYZ is reduced. Further evidence for cerium migration when the CYZ is reduced is shown in Fig. 6, which shows the X-ray diffraction patterns from a 0.3-mm thin CYZ layer on a substrate after heating at 1100°C in a evacuated sealed quartz tube. The CYZ has decomposed into Zro,slCeo.1602, Zr0.4Ce0.602 and monoclinic zirconia, the peaks of which are labelled 16, 60 and M, respectively. When another sample was heated identically, but in air, it did not decompose. The sealing of the sample in the evacuated tube meant that any reduction of the zirconia by the bond coat was not compensated by the
J. Tirorrzton
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Fig. 3. A backscattered electron image of free-standing CYZ after heat treatment at 1200°C for 125 h. See text for explanation of annotation.
of oxygen from the air. Phaseseparation was thus able to take place at 1100°C rather than 1200°C becauseof the greater reduction. The thinner CYZ alsomeant there was less zirconia for the bond coat to reduce. Earlier heating experiments in air on 0.3~mm thin CYZ also showed evidence of the formation of a cerium-rich phase at 1100°C
supply
1161. On first sight, reduction of the zirconia by the bond coat
seemsunlikely because cerium and zirconium are more electro-positive than any of the atoms in the substrateor bond coat. However, partial reduction of the Ce” ions to Ce3+may be possibleby the aluminium or chromium in the bond coat. Unfortunately the free energy data for CeYZr is not readrly available, but the Gibb’s free energy of formation for the following reactionsat 1200°C [ 171indicatesthat partial reduction is likely:
Fig. 4. A backscattered electron image of mounted CYZ after heating at 12OOT for 115 h. See text for explanation of annotation.
116
Fig. 5. A backscattercd electron arcas are rich in cerium.
image of a polished
cross-section
of mounted
6Ce02 + 2&l = 3Ce203+ A1,03
AC” = - 250 kJ
(1)
6Ce02 + 2Cr = 3Cez03 + Crz03
AC” = - 230 kJ
(2)
Theseare of the sameorder as the free energy of formation of Alz03 from the elements (680 kJ) [17]. Therefore the heating of CYZ on an aluminium- or chromium-containing bond coat is expected to result in the formation of an aluminium and chromium oxide layer on the bond coat, the reduction of the Ce’” ions in the CYZ to Ce3*ions, and the production of a correspondingnumber of oxygen vacancies in the lattice. Furthermore, the pale yellow or off-white colour of the as-sprayedCYZ suggestthat it is also partially reduced (the hydrogen in the plasmagas was probably the reducing agent here). Therefore the reduction already has a head-start on fully-oxidised CYZ. Also, the porosity of the bond coat employed meansthat a larger than usual amount of aluminium and chromium will be in intimate contact with the CYZ. Thus there was a large area capable of taking oxygen from the CYZ. Partial reduction may not directly accelerate the cerium migration; however, it doesremove many oxygen ions from the lattice, and oxygen ions are the biggest in the CYZ lattice (the respective radii are: 0.132 nm for O’-, 0.087 nm for Zr”, 0. 106 nm for Y3+, 0.102 nm for Ce”’ and 0.118 nm for Ce”*) [ 181.Therefore it is feasiblethat removing oxygen ions from the lattice gives the cations (cerium and zirconium) more freedom, and thus reducesthe activation energy for cation migration. The disintegration-prone equilibrium mixtures of cerium-rich and cerium-poor material are therefore obtained in accessibletimes (1 week) and
CYZ after heating
at 1300°C for 125 h. This shows the top surface. The bright
temperatures (11OO’C to 1200°C). Another side-effect of the cation migration might be accelerated sintering. The much lower porosity seen in the heated mounted CYZ cross-sectioncomparedto as-sprayedcross-sectionsuggest that sintering hasoccurred. Sintering will make the coating stiffer and therefore more prone to spalling. Although the calculated free energiesimply that aluminium and chromium can partially reducethe Ce’+ ions in the CYZ to Ce”+ ions, they also show that nickel cannot. However, when a sampleis heatedin air, the oxygen taken from the CYZ by the aluminium and chromium can be replenComparison 2500
16
of Thin
CYZ on Substrate
Heated in Air or Vacuum -Air -----Vacuum
s 2000 5
i
-1 4 6 EE 1500 22 =
k
16
IOOO-
25
30
35
40
45
50
55
2 Theta(degrees) Fig. 6. A comparison of part of the X-ray diffraction patterns from a 0.3mm thin CYZ coating after it had been heated for 1 week at IlOO’C in a vacuum (dashed line) and air (full line) while it was still on its bond coat and substrate. See text for explanation of annotation.
J. Thornron
er al. / S~~fnce rind Comings
ished from the air. The degree of reduction achieved will be a balance between the oxygen supply and the reduction. Wrapping the sample in nickel will adjust this balance in favour of reduction, because the nickel oxidises and reduces the concentration of oxygen in the air surrounding the sample. The acceleration of cation diffusion in zirconia has been noticed before. Indeed: acceleration was necessary to determine the phase diagram [19], where the addition of compounds called mineralisers (LiCI) were deliberately introduced to cause enhanced diffusion. Possibly, the mechanism here was also reduction. Furthermore, the formation of monoclinic zirconia at the anodic face of tetragonal zirconia solid electrolytes [20] (as used in fuel cells and oxygen pumps) may also be due to cation diffusion brought about by a reduction of the zirconia. Oxygen ions are removed from the anodic face of these electrolytes by hydrogen to form water.
5. Summary
The diffusion of cerium ions in CYZ is accelerated by heating when it is in contact with oxygen getters, and the diffusion is further enhanced by heating in a vacuum. It is proposed that this is due to the removal of oxygen ions from the lattice which gives the cerium ions, and the other cations, more freedom (when a cation moves from a lattice site to a similar one it must move through the space between; anions occupy much of this space, so removing an anion will give the cation more freedom to move between cation lattice sites). This diffusion eventually results in the formation of a phase prone to disintegration on thermal cycling, and perhaps also enhances the sintering of the coating. Both results can cause early failure of the-coating. Consequently, to use CYZ as a coating, in parts of an engine where the gases are reducing and the coating temperature exceeds 1100°C may lead to premature failure of the coating. Furthermore, CYZ should only be used on the most oxidation-resistant substrates or bond coats.
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Acknowledgements
The authors are grateful to Dr. 1. Drennan, Dr. M. Paterson and Ms. J. Cocking with whom one of us, J.T., had many interesting discussions; to Mr. J. Shenton for his help with the electron microscope; and to Dr. Y.C. Tsui for his help with the vacuum experiment. References
[II
A. Bennett, F.C. Toriz and A.B. Thakker, SW$ Coat. Techno!., 32 (1987) 359. 121R.A. Miller, .Sur$ Cotrr. Techrmi., 30 (1987) 1. [31 T.W. Clyne and Y.C. Tsui, in B. Ilschner (cd.), Proc. of 3rd Znr. Sump. on Srructurol and Functionai Gradient Mmerinls (FGM ‘941, PPUR, Lausanne, 199-l. fj.1 W.J. Lackey, D.P. Stinton, G.A. Cemy, A.C. Schaffhauser and L.L. Fehrenbacher, At/i,. Crrorn. ?,4orer., 2 (1987) 21. PI A. Bennett, &f&?r. Sci. Tecivrol., -7 (1986) 257. I61 J.H. Glare and D.E. Crawmer, ASI Memls Hmdbook 9th Edn., Vol. 5, 1982, p. 361. PI J.D. Cawley, Overvim’ of Zirconia Jvith Respect to Gas Turbine Engines, NASA Technical Paper 2286, 1984. 181R.A. Miller, J.L. Smialek and R.G. Garlick, in Adwnces in Cerarnics: Scie~rce rind Technology ofZirconin, Vol. 3, American Ceramic Society Publ., 1981, p. 241. PI P. Li and L-W. Chcn, Phys. Rev. B, 48 (1993) 10074. I101 S. Stecura and C.H. Leibert, US Pmenr No. 4,0X,705 (25 October 1977). 1111P.A. Siemens and D.W. McKee, US Pateirf No. 4,328,X5 (4 May 1982). [I21 R.L. Jones, Mater. High Temp., 9 (1991) 228. Cemre for Diffraction Dnta, PA, USA, 1995. [I31 JCPDS-lmertunionni and A. Dominguez-Rodriguez, II‘+1 A. Bavon-Leon, M. Jimenez-Melendo Acra Metal/. Mcirer., 40 (1992) 2717. I151 J.R. Brandon and R. Taylor, Sur$ Cocir. Technol., 46 (1991) 91. I161 3. Thornton and A. Majumdar, in A. Ohmori (ed.), Proc. 14fh fnf. Thermal Spray Cor2frrence, High Temperature Society of Japan Publ., 1995, p. 1075. 62nd edition, pp D-34, CRC Press, 198 1. [l71 CRC Hrwdbook, [I81 C.J. Smithells, Smirhel2s Merals Reference Book, 6dr eciifion, Butterworth, 1983. section 4.5.1. I191 E. Tani, M. Yoshimura and S. Somiya, J. Am. Ceram. Sot., 66 (1983) 506. [20] S.P.S. Badwal and N. Nardella, App1. Piiys., A49 (1989) 13.