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Microstructure, phase stability and thermal conductivity of plasma sprayed Yb2O3, Y2O3 co-stabilized ZrO2 coatings
Solid State Sciences 13 (2011) 513e519
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Microstructure, phase stability and thermal conductivity of plasma sprayed Yb2O3, Y2O3 co-stabilized ZrO2 coatings Huaifei Liu a, Songlin Li a, *, Qilian Li b, Yongming Li a, Wuxi Zhou a a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China National Key Laboratory of Science and Technology on Power Beam Process, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2010 Received in revised form 23 November 2010 Accepted 26 November 2010 Available online 25 December 2010
Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) coatings and conventional 4 mol.% Y2O3eZrO2 (4YSZ) coating were deposited by air plasma spraying (APS). The coatings show typical characteristics of APS: lamellar structure and columnar crystallite growth in each lamella. The average grain size of the as-sprayed coating is about 270 nm. XRD results reveal that all the as-sprayed YbYSZ coatings exhibit 100 mol.% nontransformable t0 phase, and 4YSZ coating is composed of 99.15 mol.% t0 and 0.85 mol.% m phase. After aging at 1300 C for 100 h, all YbYSZ coatings still keep single t0 phase, whereas the monoclinic phase in 4YSZ reaches 14 mol.%. After the heat treatment of 1400 C/100 h, monoclinic phase in 1.15 mol.% Yb2O3 e4 mol.% Y2O3eZrO2 (YbYSZ1) and 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3) are of 2.47 mol.% and 0.55 mol.%, whereas that in 4YSZ increases to 43.3 mol.%. 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) has the best phase stability, in which no m phase is detected. Thermo-physical results reveal that the thermal conductivity of YbYSZ1 (2.03e1.62 W/mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.70 e1.40 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), and YbYSZ2 has the lowest value. YbYSZ can be explored as a favorite material for TBCs application. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Thermal barrier coating Yb2O3eY2O3eZrO2 Phase stability Thermal conductivity
1. Introduction Thermal barrier coatings (TBCs) have been widely used as insulators in hot section components of gas turbines and diesel engines to enhance operating temperature and prolong the lifetime of Ni-based super-alloy substrates [1e3]. The most commonly and typically used TBCs is yttria-stabilized zirconia with a composition of 6e8 wt% (3.5e4.5 mol.%)Y2O3eZrO2 (YSZ) [4e6]. However, YSZ coating can not be used for long-term above 1200 C due to the catastrophic phase transformation of tetragonal phase and high sinterability which leads to an increase in thermal conductivity and enhances spallation of the coating [7,8]. With the increase of inlet temperature in turbines, alternative TBCs possessing improved phase stability, better sintering-resistance and reduced thermal conductivity are urgently needed [9]. In recent years, the rare earth-oxides (Yb2O3, Sc2O3, Sm2O3, Nd2O3, Er2O3, La2O3, Y2O3 et al.) as co-doptants of ZrO2 have been widely investigated [10e16]. Among the various compositions, Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) seems promising. Huang et al. [17]. reported that YbYSZ provides higher phase stability and lower
* Corresponding author. Tel.: þ86 731 8830614. E-mail address: [email protected] (S. Li). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.043
thermal conductivity than single Y2O3-doped ZrO2. In this paper, YbYSZ coatings with different stabilizer contents were prepared by air plasma spraying (APS). The phase stability at 1300 C and 1400 C were examined and the effect of Yb2O3 or Y2O3 content on phase stability and thermal conductivities were investigated, in order to evaluate the possibility of using YbYSZ as TBCs materials. The data were compared with those of conventional 4 mol.% Y2O3doped ZrO2 (4YSZ) prepared by a similar technique. The surface and fractured cross-sectional structures of the as-sprayed coating were identified as well.
2. Experimental procedure 2.1. Materials and plasma spraying process 1.15 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ1) coating was deposited by air plasma spraying (APS). The powders for plasma spraying were obtained by agglomerating the initial powder into spherical micrometer-size granules (20e100 mm) by spray-drying. The morphologies of the powders are shown in Fig. 1. The flowability is 40 s/50 g and the apparent density is 2.06 g/cm3. To evaluate the effect of stabilizers on phase stability and thermal conductivity of 1.15 mol.% Yb2O3-4 mol.% Y2O3-ZrO2(YbYSZ1),
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H. Liu et al. / Solid State Sciences 13 (2011) 513e519 Table 2 Parameters of plasma spraying YbYSZ and 4YSZ coatings. Parameter
YbYSZ(4YSZ)
Current (A) Voltage (V) Primary gas (Ar, L/min) Secondary gas (H2, L/min) Carrier gas flow rate (L/min) Powder feed rate (g/min) Spray distance (mm)
650 70 35 11 3 40 70
both the heating rate and cooling rate of 10 C/min. The heat treatments were of 1300 C/100 h and 1400 C/100 h. The phase structures were examined by X-ray diffraction technique and the measured condition was the same as mentioned above. The mole percents of monoclinic phase Mm, tetragonal phase Mt’=t and cubic phase Mc were obtained from the most common equation [18]: Fig. 1. Morphologies of YbYSZ1 pow.
2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2), 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3) and conventional 4 mol.%ZrO2 (4YSZ) coatings were prepared under identical conditions. The chemical compositions were listed in Table 1. It can be seen that the content of Yb2O3 in YbYSZ2 is higher than that in YbYSZ1. Comparing YbYSZ3 with YbYSZ2, the total percents of stabilizers (Yb2O3 þ Y2O3) are equal, whereas the percent of Yb2O3 in (Yb2O3 þ Y2O3) decreases. For thermal conductivity measurement, disk-shaped samples with dimensions of 4 10 1 mm are required. In this work, an aluminium plate machined with grooves (4 10 1 mm) was used as substrate and YbYSZ powders were plasma sprayed onto substrate until the grooves were deposited entirely, then the diskshaped coatings were detached from aluminium substrate using 50/50NaOHeH2O solution. The plasma spray parameters are summarized in Table 2. 2.2. Characterizations of as-sprayed coatings Phase compositions of the as-sprayed coatings were examined by X-ray diffraction technique (D/MAX 2550). The radiation source used was Ni filtered with Cu Ka (k ¼ 0.1542 nm) radiation produced at 40 kV and 250 mA. The scan was performed between 20 and 80 with a step of 0.02 . The microstructures of the coatings were identified by SEM (Nova Nano SEM230). Before measurement, the coating samples were cold-mounted in vacuum in polymer, polished and coated with a layer of gold so as to prevent charging effects due to the non-conductivity nature of zirconia-based ceramics.
Mm Im ð111Þ þ Im ð111Þ ¼ 0:82 Mt’=t;c It’=t;c ð111Þ
(1)
Mc Ic ð400Þ ¼ 0:88 It’=t ð400Þ þ It’=t ð004Þ Mt’=t
(2)
Where Im ð111Þ and Im ð111Þ are the integral net intensities for monoclinic phase reflected from ð111Þ and ð111Þ peaks; and It’=t;c ð111Þ is the intensity for tetragonal or cubic phase reflected from ð111Þ plane. Ic ð400Þ is the intensity for cubic phase reflected from (400) plane. It’=t ð004Þ and It’=t ð400Þ are the intensities for tetragonal phase reflected from (004) and (400) plane, respectively. 2.4. Thermal conductivity measurement The measurements of thermo-physical properties were carried out on the samples with 10 mm in diameter and 1 mm in thickness. Before the measurements of specific heat capacity and thermal diffusivity, the test faces of the specimen were coated with a layer of gold. Thermal conductivity l was calculated from:
l ¼ aCp r
(3)
Where r is the density (kg/m ), CP is the specific heat capacity (J/kg$K), and a is the thermal diffusivity (m2/s). CP was measured by laser radiation method [12]; a was measured using laser flash method as reported in literature [17]. r was measured by Archimedes’ method. Since the samples were not fully dense, the measured thermal conductivity value was modified for the thermal conductivity (l0) of the fully dense material using eq. (4) [19]: 3
4f 3
2.3. Phase stability tests
l=l0 ¼ 1
For the phase stability assessment, the as-sprayed YbYSZ and 4YSZ coatings were heat treated at different temperatures with
Where 4 is the residual porosity inside the sintered sample which was determined by the expression f ¼ 1 r=rt , where rt is the theoretical density.
Table 1 Chemical compositions of YbYSZ and 4YSZ coatings. Coating number
Composition (mol. %) ZrO2
Y2O3
Yb2O3
YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
94.85 93.28 93.28 95.48
4.00 4.00 4.85 4.00
1.15 2.72 1.87 e
a b
Total percenta Yb2O3 þ Y2O3
Stabilizer percentb Yb2O3
5.15 6.72 6.72 4.00
22.33 40.48 27.83 0
Total mol.% of stabilizing oxide. Mol.% of Yb2O3 in stabilizing oxides (Yb2O3 þ Y2O3).
(4)
3. Results and discussion 3.1. Microstructures and phase compositions of the as-sprayed coatings The micrographs of cross section and fractured cross section are shown in Fig. 2. It reveals that the coating exhibits typical characteristics of plasma sprayed depositions: lamellar structure and
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Fig. 2. Micrographs of YbYSZ1 as-sprayed coating. (a) Cross section (b), (c) fractured cross section.
columnar crystallite growth in each lamella. The columns grow in a direction perpendicular to the substrate, as heat is extracted through the substrate and the solid/liquid interface moves vertically. The interlamellar pores existed between layers of splats provide
Fig. 3. SEM micrographs of the as-sprayed YbYSZ1 coating surface with various magnification. (a) 1000 (b) 30000 (c) 100000.
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Fig. 4. XRD patterns of as-sprayed YbYSZ and 4YSZ coatings.
significant reduction in through-thickness thermal conductivity. However, these are also the regions resulting in delamination and failure during thermal cycling due to the poorly bonded layers. Fig. 3 shows the micrographs of the as-sprayed coating surface. The sample shows fully particle-melted morphology with homogeneous grain size distribution. The micro-cracks existing within splats which are called mud-cracks result from the release of quenching stresses during deposition. For most ceramics, the large quenching stresses can not be retained by splat during deposition and cooling which leads to the form of micro-cracks (Fig. 3 (b)). This kind of cracks can provide a small degree of in-plane compliance [5]. To obtain reliable grain size, ten sighting field as Fig. 3(c) shown are photographed and Image Tool Software is used to calculate. The average grain size is about 270 nm in the as-sprayed coating. No secondary phase particles are observed, indicating that Yb2O3 and Y2O3 have been dissolved in ZrO2 crystal completely and have stabilized ZrO2 by substituting Yb3þ and Y3þ for Zr4þ, although the feed particles experience melting, droplet impacting, spreading and splat solidification during deposition. This is accorded with the XRD result (Fig. 4). There are no diffraction peaks of Yb2O3 or Y2O3 observed in all coatings. The phase compositions, cell parameters and tetragonalities are listed in Table 3. According to literature [20], the nontransformable t phase can be transformable tetragonal t0 phase and p ffiffiffi distinguished by the tetragonality ‘c= 2a ’. The ratio tends to be 1.010 which reaches a superior value for t0 phase while it is superior to 1.010 for t phase. It can be seen that YbYSZ1 shows single tetragonal phase with the cell parameters pffiffiffi of a ¼ 3.60145, b ¼ 3.60145, c ¼ 5.09368. The tetragonality ‘c= 2a ’ equals to 1.0002 (below than 1.010), so the tetragonal phase in YbYSZ1 is t0 phase. YbYSZ2, YbYSZ3 coatings also compose of 100% t0 phase. The cell volume of YbYSZ2 increases in comparison with YbYSZ1 which is
Fig. 5. XRD patterns of the coatings after heat treated at 1300 C for 100 h. (a) Low angle region (b) High-angle region.
attributed to the larger diameter of Yb3þ (0.086 nm) than Zr4þ (0.079), and larger percent of Yb2O3 in YbYSZ2 than that in YbYSZ1 as the contents of Y2O3 are identical. 0.9 mol.% monoclinic phase is found in 4YSZ coating, calculated by eq. (1). 3.2. High temperature phase stability Fig. 5 shows the XRD patterns of the coatings after heat treated at 1300 C for 100 h. The XRD pattern in the low angle diffracted region (22 –40 ), as shown in Fig. 5(a), is used for the identification of m phase, and that in the high angle diffracted region (70 –78 )
Table 3 Phase compositons, cell parameters, volumes and tetragonalities of as-sprayed coatings. Coating sample
Phase composition
_ Cell parameter ðAÞ
YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
100 mol.% t0 100 mol.% t0 100 mol.% t0 99.15 mol.% t0 0.85mol.% m
a a a a a
¼ ¼ ¼ ¼ ¼
b ¼ 3.60145; b ¼ 3.60504; b ¼ 3.61070; b ¼ 3.58560; 5.30431; b ¼
c ¼ 5.09368 c ¼ 5.11891 c ¼ 5.12666 c ¼ 5.10792 5.19697; c ¼ 5.15352
3 Cell volume ðA_ Þ
66.0673 66.5270 66.8371 65.6701 142.0637
Tetragonality pffiffiffi c= 2a 1.0002 1.0042 1.0041 1.0075
H. Liu et al. / Solid State Sciences 13 (2011) 513e519
(Fig. 5(b)) is used for distinguishing between tetragonal phase and cubic phase. It can be seen that the diffraction intensities of monoclinic ð111Þ and (111) increases in 4YSZ and the cubic (200) peak replaces the initial tetragonal (002) and (200) peaks. In the high-angle region, cubic (400) peak appears between tetragonal (004) and (400) peaks. All these indicate that partial tetragonal phase has transformed to monoclinic phase in 4YSZ during 1300 C/100 h annealing. The amounts of monoclinic, cubic and tetragonal phase are of 14 mol.%, pffiffiffi33.1 mol.% and 52.9 mol.%, respectively. The tetragonality c= 2a equals to 1.0046, so the tetragonal phase is t0 phase. No evidence of monoclinic phase is found in YbYSZ1w3 coatings from (111) diffraction region, and (400) region with only tetragonal (004) and (400) peaks existing, indicating that no phase transformation occurs in YbYSZ1w3. The tetragonalities of YbYSZ1, YbYSZ2 and YbYSZ3 are valued 1.0060, 1.0012, 1.0071, respectively, so the tetragonal phases are all of t0 phases. Fig. 6 shows the diffraction patterns of the coatings after heat treated at 1400 C for 100 h. It can be seen that the final phase compositions in 4YSZ are of cubic phase and monoclinic phase. Since the composition point corresponding to 4 mol.% Y2O3 (7wt.%
Fig. 6. XRD patterns of the coatings after heat treated at 1400 C for 100 h. (b) Low angle region (b) High-angle region.
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Table 4 Phase compositions, cell parameters and tetragonalities of the coatings after heat treated at 1400 C for 100 h. Coating Phase sample composition YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
97.53 mol.% t0 2.47 mol.% m 100 mol.% t0 99.45mol.% t0 0.55 mol.% m 56.7 mol.% c 43.3 mol.% m
_ Cell parameter ðAÞ a a a a a a a
¼ ¼ ¼ ¼ ¼ ¼ ¼
b ¼ 3.608; c ¼ 5.11165 5.3129; b ¼ 5.2125; c ¼ 5.1471 b ¼ 3.6042; c ¼ 5.10881 b ¼ 3.60981; c ¼ 5.10676 5.3129; b ¼ 5.2125; c ¼ 5.1471 b ¼ c ¼ 5.13907 5.37124; b ¼ 5.21317; c ¼ 5.1309
Tetragonality pffiffiffi c= 2a 1.0020 1.0024 1.0005 e
Y2O3 ) locates near the center of the mixed tetragonal phase and cubic phase in ZrO2eY2O3 phase diagram, the result of 43.3 mol.% monoclinic phase is inconsistent with the almost complete transformation of t0 /t þ c/m þ c. For YbYSZ system, the monoclinic phase in YbYSZ1 and YbYSZ3 are of 2.47 mol.% and 0.55 mol.%, respectively. YbYSZ2 is still composed of 100 mol.% t0 phase, exhibiting higher resistance to destabilization of t0 phase. The phase compositions, cell parameters and tetragonalities of the coatings after heat treated at 1400 C for 100 h are listed in Table 4. The above results show that the addition of Yb2O3 significantly improved the phase stability of 4YSZ. The phase stability of 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) is superior to that of 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3), indicating that the percent of Yb2O3 in (Yb2O3 þ Y2O3) also plays an important role in determining the phase stability. YbYSZ2 exhibits excellent phase stability with no m phase appearance after annealed at 1400 C for 100 h. According to kinetic consideration, the partitioning of t0 phase to equilibrium t and c phases during high-temperature annealing is diffusion controlled, requiring long-range cation diffusion. Therefore the t0 phase stability of YbYSZ3 should be expected to increase in comparison with YbYSZ2 due to the larger ionic radius of Y3þ(0.0893) than Yb3þ(0.0858 nm). However, Nevertheless Rebollo et al. [21] pointed out that the t0 phase stability depends not only on the diffusion kinetics but also on the driving force for partitioning of the t0 phase which is scaled as the width of (t þ c) field. Thermodynamic calculations indicate that the width of t þ c phase
Fig. 7. Thermal conductivity versus temperatures of YbYSZ and 4YSZ coatings corrected by eq. (4).
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Table 5 Thermal conductivity of YbYSZ and 4YSZ coatings before and after correction. Sample
rt (g/cm3)
r (g/cm3)
Thermal conductivity (W/mK)
4YSZ
6.03
5.53
YbYSZ1
6.11
5.63
YbYSZ2
6.18
5.70
YbYSZ3
6.08
5.65
l l0 l l0 l l0 l l0
30 C
200 C
400 C
600 C
800 C
1000 C
1200 C
2.04 2.29 1.82 2.03 1.36 1.52 1.54 1.7
2.01 2.26 1.74 1.94 1.34 1.49 1.49 1.65
1.95 2.19 1.68 1.88 1.29 1.44 1.43 1.58
1.88 2.11 1.64 1.83 1.24 1.38 1.37 1.51
1.83 2.06 1.56 1.74 1.17 1.31 1.31 1.45
1.81 2.04 1.50 1.68 1.09 1.22 1.29 1.42
1.74 1.96 1.45 1.62 1.07 1.19 1.27 1.40
field scaled as the radius of the rare-earth cation [22], so small Yb3þ cation provides less driving force than Y3þ for t0 partitioning. Therefore it can be speculated that for YbYSZ system the latter acts more on phase stability than the former. 3.3. Thermal conductivity Fig. 7 shows the thermal conductivity versus temperatures of YbYSZ1w3 and 4YSZ coatings. The data were corrected for porosity according to eq. (4), so they represent the thermal conductivity of fully dense sample. It can be seen that all the thermal conductivities decrease gradually with temperature rising. Table 5 lists the thermal conductivity data of YbYSZ and 4YSZ coatings before and after correction by eq. (4). Thermal conductivities of YbYSZ1 (2.03e1.62 W/ mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.7e1.4 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), and YbYSZ2 has the lowest data, indicating that the stabilizer type and content play an important role in determining thermal conductivity. It is well known that the substitutional solid solution is formed by the substitution of Zr4þ cation by trivalent rare-earth cation (Yb3þ, Y3þ). As the substitution of two Zr4þ cations with two Yb3þ or Y3þ cations, one oxygen vacancy is produced in order to maintain the electroneutrality of the lattice, so the more content of stabilizers (Yb2O3, Y2O3), the more oxygen vacancies are introduced. According to thermal conductivity theory, the scattering effect from each point defect is associated with type of the point defect (oxygen vacancy or substitutional defects) and characteristics of the substitutional cations (mass, ionic radius, valence). All these defects can increase scattering centers of phonons and thereby reduce the thermal conductivity. According to literature [23], the thermal conductivity l is proportional to phonon mean free path l, while l is proportional to (6M2)1 and (6R2)1, where 6M and 6R are the differences of masses and ionic radius between tervalent dopant cations and host cation. In this paper, Yb3þ and Y3þ have the same valences, so the more codopants are added, the more oxygen vacancies are introduced. This explains that YbYSZ1w3 have lower thermal conductivities than 4YSZ, while YbYSZ2 and YbYSZ3 both have lower thermal conductivities than YbYSZ1. Additionally, the atomic masses of Yb (173.04) is much greater than those of Y (88.91) and Zr (91.22), so the addition of Yb2O3 leads to a more significant reduction on thermal conductivity than Y2O3, although the ionic radius difference between Yb3þ(0.086 nm) and Zr4þ(0.079) is a little smaller than that between Y3þ (0.089 nm) and Zr4þ. This is consistent with the result that YbYSZ2 has lower thermal conductivity than YbYSZ3, which attributes to the higher percentage of Yb2O3 in (Yb2O3 þ Y2O3) for YbYSZ2. Except for porosity and stabilizers, the shape of pores also has an important influence on thermal conductivity, especially for the micro-crack/porous materials. Thermal conductivity relates to both porosity and shape of pores as follows [24]:
K=K0 ¼ ð1 þ ð2V=pÞðb=aÞÞ1
(5)
Where K is the thermal conductivity of the micro-cracked material; K0 is the thermal conductivity of the un-cracked material. V is the volume fraction of ellipsoidal pores. b is the major axis of the ellipsoid and a is the minor axis. However, the corrected equation of thermal conductivity accounted for these two factors is not clear. More work is needed to be concerned on this. There are many investigations on the thermal conductivity of earth-oxide co-doped zirconia TBCs. Huang et al. [17] found that the thermal conductivity of 7 wt%Y2O3eZrO2 (3.99e4.51 W/mK) was decreased differently by adding Ta2O5 (2.57e0.71 W/mK), Nb2O5 (3.13e0.78 W/mK), Sc2O3 (2.35e3.5 W/mK), Yb2O3 (2.30e1.9 W/ mK) and CeO2 (3.23e3.64 W/mK) during the measuring temperature range 22e800 C. Matsumoto et al. [9] lowered the thermal conductivity of 7.1 wt%Y2O3eZrO2 (1.93 W/mK) by doping with 5 mol % La2O3 (1.2 W/mK). Recently, Matsumtoto et al. [25] also found that the thermal conductivity of La2O3, HfO2 co-doped 4 mol.% Y2O3eZrO2 is about 0.5 W/mK, which is much lower than that of 4 mol.% Y2O3eZrO2. All the studies prove that the thermal conductivity of two or more than two oxides co-doped ZrO2 is lower than that of the single Y2O3 stabilized ZrO2.
4. Conclusions Three types of Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) coatings and conventional 4 mol.% Y2O3-doped ZrO2 (4YSZ) coating were prepared by air plasma spraying. YbYSZ provide remarkable opportunities for developing novel TBCs due to their improved phase stability and reduced thermal conductivity in comparison with those of the stateof-art 4YSZ. In particular, 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) keeps 100% non-transformable t0 phase even after heat treated at 1400 C for 100 h and small amount of monoclinic phase are formed in 1.15 mol.% Yb2O3-4 mol.% Y2O3eZrO2 (YbYSZ1) and 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3), 2.47 mol.% and 0.55 mol.%, respectively, whereas that in 4YSZ reaches 43.3 mol.%, corresponding almost completely phase transformation of tetragonal phase. The thermal conductivities of YbYSZ1 (2.03e1.62 W/mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.7e1.4 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), among which YbYSZ2 has the lowest data, because of the more introducing vacancies and substitutional defects in YbYSZ and the larger atomic mass difference between Yb and Zr than that between Yand Zr. YbYSZ can be explored as a novel prospective candidate material for TBCs applications.
Acknowledgements This work was supported by Program for New Century Excellent Talents in University (NCET-2006), PR China and Project supported by Hunan Provincial Natural Science Foundation of China (10JJ2037) which are gratefully acknowledged.
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Microstructure, phase stability and thermal conductivity of plasma sprayed Yb2O3, Y2O3 co-stabilized ZrO2 coatings Huaifei Liu a, Songlin Li a, *, Qilian Li b, Yongming Li a, Wuxi Zhou a a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China National Key Laboratory of Science and Technology on Power Beam Process, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2010 Received in revised form 23 November 2010 Accepted 26 November 2010 Available online 25 December 2010
Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) coatings and conventional 4 mol.% Y2O3eZrO2 (4YSZ) coating were deposited by air plasma spraying (APS). The coatings show typical characteristics of APS: lamellar structure and columnar crystallite growth in each lamella. The average grain size of the as-sprayed coating is about 270 nm. XRD results reveal that all the as-sprayed YbYSZ coatings exhibit 100 mol.% nontransformable t0 phase, and 4YSZ coating is composed of 99.15 mol.% t0 and 0.85 mol.% m phase. After aging at 1300 C for 100 h, all YbYSZ coatings still keep single t0 phase, whereas the monoclinic phase in 4YSZ reaches 14 mol.%. After the heat treatment of 1400 C/100 h, monoclinic phase in 1.15 mol.% Yb2O3 e4 mol.% Y2O3eZrO2 (YbYSZ1) and 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3) are of 2.47 mol.% and 0.55 mol.%, whereas that in 4YSZ increases to 43.3 mol.%. 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) has the best phase stability, in which no m phase is detected. Thermo-physical results reveal that the thermal conductivity of YbYSZ1 (2.03e1.62 W/mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.70 e1.40 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), and YbYSZ2 has the lowest value. YbYSZ can be explored as a favorite material for TBCs application. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Thermal barrier coating Yb2O3eY2O3eZrO2 Phase stability Thermal conductivity
1. Introduction Thermal barrier coatings (TBCs) have been widely used as insulators in hot section components of gas turbines and diesel engines to enhance operating temperature and prolong the lifetime of Ni-based super-alloy substrates [1e3]. The most commonly and typically used TBCs is yttria-stabilized zirconia with a composition of 6e8 wt% (3.5e4.5 mol.%)Y2O3eZrO2 (YSZ) [4e6]. However, YSZ coating can not be used for long-term above 1200 C due to the catastrophic phase transformation of tetragonal phase and high sinterability which leads to an increase in thermal conductivity and enhances spallation of the coating [7,8]. With the increase of inlet temperature in turbines, alternative TBCs possessing improved phase stability, better sintering-resistance and reduced thermal conductivity are urgently needed [9]. In recent years, the rare earth-oxides (Yb2O3, Sc2O3, Sm2O3, Nd2O3, Er2O3, La2O3, Y2O3 et al.) as co-doptants of ZrO2 have been widely investigated [10e16]. Among the various compositions, Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) seems promising. Huang et al. [17]. reported that YbYSZ provides higher phase stability and lower
* Corresponding author. Tel.: þ86 731 8830614. E-mail address: [email protected] (S. Li). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.11.043
thermal conductivity than single Y2O3-doped ZrO2. In this paper, YbYSZ coatings with different stabilizer contents were prepared by air plasma spraying (APS). The phase stability at 1300 C and 1400 C were examined and the effect of Yb2O3 or Y2O3 content on phase stability and thermal conductivities were investigated, in order to evaluate the possibility of using YbYSZ as TBCs materials. The data were compared with those of conventional 4 mol.% Y2O3doped ZrO2 (4YSZ) prepared by a similar technique. The surface and fractured cross-sectional structures of the as-sprayed coating were identified as well.
2. Experimental procedure 2.1. Materials and plasma spraying process 1.15 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ1) coating was deposited by air plasma spraying (APS). The powders for plasma spraying were obtained by agglomerating the initial powder into spherical micrometer-size granules (20e100 mm) by spray-drying. The morphologies of the powders are shown in Fig. 1. The flowability is 40 s/50 g and the apparent density is 2.06 g/cm3. To evaluate the effect of stabilizers on phase stability and thermal conductivity of 1.15 mol.% Yb2O3-4 mol.% Y2O3-ZrO2(YbYSZ1),
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H. Liu et al. / Solid State Sciences 13 (2011) 513e519 Table 2 Parameters of plasma spraying YbYSZ and 4YSZ coatings. Parameter
YbYSZ(4YSZ)
Current (A) Voltage (V) Primary gas (Ar, L/min) Secondary gas (H2, L/min) Carrier gas flow rate (L/min) Powder feed rate (g/min) Spray distance (mm)
650 70 35 11 3 40 70
both the heating rate and cooling rate of 10 C/min. The heat treatments were of 1300 C/100 h and 1400 C/100 h. The phase structures were examined by X-ray diffraction technique and the measured condition was the same as mentioned above. The mole percents of monoclinic phase Mm, tetragonal phase Mt’=t and cubic phase Mc were obtained from the most common equation [18]: Fig. 1. Morphologies of YbYSZ1 pow.
2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2), 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3) and conventional 4 mol.%ZrO2 (4YSZ) coatings were prepared under identical conditions. The chemical compositions were listed in Table 1. It can be seen that the content of Yb2O3 in YbYSZ2 is higher than that in YbYSZ1. Comparing YbYSZ3 with YbYSZ2, the total percents of stabilizers (Yb2O3 þ Y2O3) are equal, whereas the percent of Yb2O3 in (Yb2O3 þ Y2O3) decreases. For thermal conductivity measurement, disk-shaped samples with dimensions of 4 10 1 mm are required. In this work, an aluminium plate machined with grooves (4 10 1 mm) was used as substrate and YbYSZ powders were plasma sprayed onto substrate until the grooves were deposited entirely, then the diskshaped coatings were detached from aluminium substrate using 50/50NaOHeH2O solution. The plasma spray parameters are summarized in Table 2. 2.2. Characterizations of as-sprayed coatings Phase compositions of the as-sprayed coatings were examined by X-ray diffraction technique (D/MAX 2550). The radiation source used was Ni filtered with Cu Ka (k ¼ 0.1542 nm) radiation produced at 40 kV and 250 mA. The scan was performed between 20 and 80 with a step of 0.02 . The microstructures of the coatings were identified by SEM (Nova Nano SEM230). Before measurement, the coating samples were cold-mounted in vacuum in polymer, polished and coated with a layer of gold so as to prevent charging effects due to the non-conductivity nature of zirconia-based ceramics.
Mm Im ð111Þ þ Im ð111Þ ¼ 0:82 Mt’=t;c It’=t;c ð111Þ
(1)
Mc Ic ð400Þ ¼ 0:88 It’=t ð400Þ þ It’=t ð004Þ Mt’=t
(2)
Where Im ð111Þ and Im ð111Þ are the integral net intensities for monoclinic phase reflected from ð111Þ and ð111Þ peaks; and It’=t;c ð111Þ is the intensity for tetragonal or cubic phase reflected from ð111Þ plane. Ic ð400Þ is the intensity for cubic phase reflected from (400) plane. It’=t ð004Þ and It’=t ð400Þ are the intensities for tetragonal phase reflected from (004) and (400) plane, respectively. 2.4. Thermal conductivity measurement The measurements of thermo-physical properties were carried out on the samples with 10 mm in diameter and 1 mm in thickness. Before the measurements of specific heat capacity and thermal diffusivity, the test faces of the specimen were coated with a layer of gold. Thermal conductivity l was calculated from:
l ¼ aCp r
(3)
Where r is the density (kg/m ), CP is the specific heat capacity (J/kg$K), and a is the thermal diffusivity (m2/s). CP was measured by laser radiation method [12]; a was measured using laser flash method as reported in literature [17]. r was measured by Archimedes’ method. Since the samples were not fully dense, the measured thermal conductivity value was modified for the thermal conductivity (l0) of the fully dense material using eq. (4) [19]: 3
4f 3
2.3. Phase stability tests
l=l0 ¼ 1
For the phase stability assessment, the as-sprayed YbYSZ and 4YSZ coatings were heat treated at different temperatures with
Where 4 is the residual porosity inside the sintered sample which was determined by the expression f ¼ 1 r=rt , where rt is the theoretical density.
Table 1 Chemical compositions of YbYSZ and 4YSZ coatings. Coating number
Composition (mol. %) ZrO2
Y2O3
Yb2O3
YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
94.85 93.28 93.28 95.48
4.00 4.00 4.85 4.00
1.15 2.72 1.87 e
a b
Total percenta Yb2O3 þ Y2O3
Stabilizer percentb Yb2O3
5.15 6.72 6.72 4.00
22.33 40.48 27.83 0
Total mol.% of stabilizing oxide. Mol.% of Yb2O3 in stabilizing oxides (Yb2O3 þ Y2O3).
(4)
3. Results and discussion 3.1. Microstructures and phase compositions of the as-sprayed coatings The micrographs of cross section and fractured cross section are shown in Fig. 2. It reveals that the coating exhibits typical characteristics of plasma sprayed depositions: lamellar structure and
H. Liu et al. / Solid State Sciences 13 (2011) 513e519
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Fig. 2. Micrographs of YbYSZ1 as-sprayed coating. (a) Cross section (b), (c) fractured cross section.
columnar crystallite growth in each lamella. The columns grow in a direction perpendicular to the substrate, as heat is extracted through the substrate and the solid/liquid interface moves vertically. The interlamellar pores existed between layers of splats provide
Fig. 3. SEM micrographs of the as-sprayed YbYSZ1 coating surface with various magnification. (a) 1000 (b) 30000 (c) 100000.
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Fig. 4. XRD patterns of as-sprayed YbYSZ and 4YSZ coatings.
significant reduction in through-thickness thermal conductivity. However, these are also the regions resulting in delamination and failure during thermal cycling due to the poorly bonded layers. Fig. 3 shows the micrographs of the as-sprayed coating surface. The sample shows fully particle-melted morphology with homogeneous grain size distribution. The micro-cracks existing within splats which are called mud-cracks result from the release of quenching stresses during deposition. For most ceramics, the large quenching stresses can not be retained by splat during deposition and cooling which leads to the form of micro-cracks (Fig. 3 (b)). This kind of cracks can provide a small degree of in-plane compliance [5]. To obtain reliable grain size, ten sighting field as Fig. 3(c) shown are photographed and Image Tool Software is used to calculate. The average grain size is about 270 nm in the as-sprayed coating. No secondary phase particles are observed, indicating that Yb2O3 and Y2O3 have been dissolved in ZrO2 crystal completely and have stabilized ZrO2 by substituting Yb3þ and Y3þ for Zr4þ, although the feed particles experience melting, droplet impacting, spreading and splat solidification during deposition. This is accorded with the XRD result (Fig. 4). There are no diffraction peaks of Yb2O3 or Y2O3 observed in all coatings. The phase compositions, cell parameters and tetragonalities are listed in Table 3. According to literature [20], the nontransformable t phase can be transformable tetragonal t0 phase and p ffiffiffi distinguished by the tetragonality ‘c= 2a ’. The ratio tends to be 1.010 which reaches a superior value for t0 phase while it is superior to 1.010 for t phase. It can be seen that YbYSZ1 shows single tetragonal phase with the cell parameters pffiffiffi of a ¼ 3.60145, b ¼ 3.60145, c ¼ 5.09368. The tetragonality ‘c= 2a ’ equals to 1.0002 (below than 1.010), so the tetragonal phase in YbYSZ1 is t0 phase. YbYSZ2, YbYSZ3 coatings also compose of 100% t0 phase. The cell volume of YbYSZ2 increases in comparison with YbYSZ1 which is
Fig. 5. XRD patterns of the coatings after heat treated at 1300 C for 100 h. (a) Low angle region (b) High-angle region.
attributed to the larger diameter of Yb3þ (0.086 nm) than Zr4þ (0.079), and larger percent of Yb2O3 in YbYSZ2 than that in YbYSZ1 as the contents of Y2O3 are identical. 0.9 mol.% monoclinic phase is found in 4YSZ coating, calculated by eq. (1). 3.2. High temperature phase stability Fig. 5 shows the XRD patterns of the coatings after heat treated at 1300 C for 100 h. The XRD pattern in the low angle diffracted region (22 –40 ), as shown in Fig. 5(a), is used for the identification of m phase, and that in the high angle diffracted region (70 –78 )
Table 3 Phase compositons, cell parameters, volumes and tetragonalities of as-sprayed coatings. Coating sample
Phase composition
_ Cell parameter ðAÞ
YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
100 mol.% t0 100 mol.% t0 100 mol.% t0 99.15 mol.% t0 0.85mol.% m
a a a a a
¼ ¼ ¼ ¼ ¼
b ¼ 3.60145; b ¼ 3.60504; b ¼ 3.61070; b ¼ 3.58560; 5.30431; b ¼
c ¼ 5.09368 c ¼ 5.11891 c ¼ 5.12666 c ¼ 5.10792 5.19697; c ¼ 5.15352
3 Cell volume ðA_ Þ
66.0673 66.5270 66.8371 65.6701 142.0637
Tetragonality pffiffiffi c= 2a 1.0002 1.0042 1.0041 1.0075
H. Liu et al. / Solid State Sciences 13 (2011) 513e519
(Fig. 5(b)) is used for distinguishing between tetragonal phase and cubic phase. It can be seen that the diffraction intensities of monoclinic ð111Þ and (111) increases in 4YSZ and the cubic (200) peak replaces the initial tetragonal (002) and (200) peaks. In the high-angle region, cubic (400) peak appears between tetragonal (004) and (400) peaks. All these indicate that partial tetragonal phase has transformed to monoclinic phase in 4YSZ during 1300 C/100 h annealing. The amounts of monoclinic, cubic and tetragonal phase are of 14 mol.%, pffiffiffi33.1 mol.% and 52.9 mol.%, respectively. The tetragonality c= 2a equals to 1.0046, so the tetragonal phase is t0 phase. No evidence of monoclinic phase is found in YbYSZ1w3 coatings from (111) diffraction region, and (400) region with only tetragonal (004) and (400) peaks existing, indicating that no phase transformation occurs in YbYSZ1w3. The tetragonalities of YbYSZ1, YbYSZ2 and YbYSZ3 are valued 1.0060, 1.0012, 1.0071, respectively, so the tetragonal phases are all of t0 phases. Fig. 6 shows the diffraction patterns of the coatings after heat treated at 1400 C for 100 h. It can be seen that the final phase compositions in 4YSZ are of cubic phase and monoclinic phase. Since the composition point corresponding to 4 mol.% Y2O3 (7wt.%
Fig. 6. XRD patterns of the coatings after heat treated at 1400 C for 100 h. (b) Low angle region (b) High-angle region.
517
Table 4 Phase compositions, cell parameters and tetragonalities of the coatings after heat treated at 1400 C for 100 h. Coating Phase sample composition YbYSZ1 YbYSZ2 YbYSZ3 4YSZ
97.53 mol.% t0 2.47 mol.% m 100 mol.% t0 99.45mol.% t0 0.55 mol.% m 56.7 mol.% c 43.3 mol.% m
_ Cell parameter ðAÞ a a a a a a a
¼ ¼ ¼ ¼ ¼ ¼ ¼
b ¼ 3.608; c ¼ 5.11165 5.3129; b ¼ 5.2125; c ¼ 5.1471 b ¼ 3.6042; c ¼ 5.10881 b ¼ 3.60981; c ¼ 5.10676 5.3129; b ¼ 5.2125; c ¼ 5.1471 b ¼ c ¼ 5.13907 5.37124; b ¼ 5.21317; c ¼ 5.1309
Tetragonality pffiffiffi c= 2a 1.0020 1.0024 1.0005 e
Y2O3 ) locates near the center of the mixed tetragonal phase and cubic phase in ZrO2eY2O3 phase diagram, the result of 43.3 mol.% monoclinic phase is inconsistent with the almost complete transformation of t0 /t þ c/m þ c. For YbYSZ system, the monoclinic phase in YbYSZ1 and YbYSZ3 are of 2.47 mol.% and 0.55 mol.%, respectively. YbYSZ2 is still composed of 100 mol.% t0 phase, exhibiting higher resistance to destabilization of t0 phase. The phase compositions, cell parameters and tetragonalities of the coatings after heat treated at 1400 C for 100 h are listed in Table 4. The above results show that the addition of Yb2O3 significantly improved the phase stability of 4YSZ. The phase stability of 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) is superior to that of 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3), indicating that the percent of Yb2O3 in (Yb2O3 þ Y2O3) also plays an important role in determining the phase stability. YbYSZ2 exhibits excellent phase stability with no m phase appearance after annealed at 1400 C for 100 h. According to kinetic consideration, the partitioning of t0 phase to equilibrium t and c phases during high-temperature annealing is diffusion controlled, requiring long-range cation diffusion. Therefore the t0 phase stability of YbYSZ3 should be expected to increase in comparison with YbYSZ2 due to the larger ionic radius of Y3þ(0.0893) than Yb3þ(0.0858 nm). However, Nevertheless Rebollo et al. [21] pointed out that the t0 phase stability depends not only on the diffusion kinetics but also on the driving force for partitioning of the t0 phase which is scaled as the width of (t þ c) field. Thermodynamic calculations indicate that the width of t þ c phase
Fig. 7. Thermal conductivity versus temperatures of YbYSZ and 4YSZ coatings corrected by eq. (4).
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Table 5 Thermal conductivity of YbYSZ and 4YSZ coatings before and after correction. Sample
rt (g/cm3)
r (g/cm3)
Thermal conductivity (W/mK)
4YSZ
6.03
5.53
YbYSZ1
6.11
5.63
YbYSZ2
6.18
5.70
YbYSZ3
6.08
5.65
l l0 l l0 l l0 l l0
30 C
200 C
400 C
600 C
800 C
1000 C
1200 C
2.04 2.29 1.82 2.03 1.36 1.52 1.54 1.7
2.01 2.26 1.74 1.94 1.34 1.49 1.49 1.65
1.95 2.19 1.68 1.88 1.29 1.44 1.43 1.58
1.88 2.11 1.64 1.83 1.24 1.38 1.37 1.51
1.83 2.06 1.56 1.74 1.17 1.31 1.31 1.45
1.81 2.04 1.50 1.68 1.09 1.22 1.29 1.42
1.74 1.96 1.45 1.62 1.07 1.19 1.27 1.40
field scaled as the radius of the rare-earth cation [22], so small Yb3þ cation provides less driving force than Y3þ for t0 partitioning. Therefore it can be speculated that for YbYSZ system the latter acts more on phase stability than the former. 3.3. Thermal conductivity Fig. 7 shows the thermal conductivity versus temperatures of YbYSZ1w3 and 4YSZ coatings. The data were corrected for porosity according to eq. (4), so they represent the thermal conductivity of fully dense sample. It can be seen that all the thermal conductivities decrease gradually with temperature rising. Table 5 lists the thermal conductivity data of YbYSZ and 4YSZ coatings before and after correction by eq. (4). Thermal conductivities of YbYSZ1 (2.03e1.62 W/ mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.7e1.4 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), and YbYSZ2 has the lowest data, indicating that the stabilizer type and content play an important role in determining thermal conductivity. It is well known that the substitutional solid solution is formed by the substitution of Zr4þ cation by trivalent rare-earth cation (Yb3þ, Y3þ). As the substitution of two Zr4þ cations with two Yb3þ or Y3þ cations, one oxygen vacancy is produced in order to maintain the electroneutrality of the lattice, so the more content of stabilizers (Yb2O3, Y2O3), the more oxygen vacancies are introduced. According to thermal conductivity theory, the scattering effect from each point defect is associated with type of the point defect (oxygen vacancy or substitutional defects) and characteristics of the substitutional cations (mass, ionic radius, valence). All these defects can increase scattering centers of phonons and thereby reduce the thermal conductivity. According to literature [23], the thermal conductivity l is proportional to phonon mean free path l, while l is proportional to (6M2)1 and (6R2)1, where 6M and 6R are the differences of masses and ionic radius between tervalent dopant cations and host cation. In this paper, Yb3þ and Y3þ have the same valences, so the more codopants are added, the more oxygen vacancies are introduced. This explains that YbYSZ1w3 have lower thermal conductivities than 4YSZ, while YbYSZ2 and YbYSZ3 both have lower thermal conductivities than YbYSZ1. Additionally, the atomic masses of Yb (173.04) is much greater than those of Y (88.91) and Zr (91.22), so the addition of Yb2O3 leads to a more significant reduction on thermal conductivity than Y2O3, although the ionic radius difference between Yb3þ(0.086 nm) and Zr4þ(0.079) is a little smaller than that between Y3þ (0.089 nm) and Zr4þ. This is consistent with the result that YbYSZ2 has lower thermal conductivity than YbYSZ3, which attributes to the higher percentage of Yb2O3 in (Yb2O3 þ Y2O3) for YbYSZ2. Except for porosity and stabilizers, the shape of pores also has an important influence on thermal conductivity, especially for the micro-crack/porous materials. Thermal conductivity relates to both porosity and shape of pores as follows [24]:
K=K0 ¼ ð1 þ ð2V=pÞðb=aÞÞ1
(5)
Where K is the thermal conductivity of the micro-cracked material; K0 is the thermal conductivity of the un-cracked material. V is the volume fraction of ellipsoidal pores. b is the major axis of the ellipsoid and a is the minor axis. However, the corrected equation of thermal conductivity accounted for these two factors is not clear. More work is needed to be concerned on this. There are many investigations on the thermal conductivity of earth-oxide co-doped zirconia TBCs. Huang et al. [17] found that the thermal conductivity of 7 wt%Y2O3eZrO2 (3.99e4.51 W/mK) was decreased differently by adding Ta2O5 (2.57e0.71 W/mK), Nb2O5 (3.13e0.78 W/mK), Sc2O3 (2.35e3.5 W/mK), Yb2O3 (2.30e1.9 W/ mK) and CeO2 (3.23e3.64 W/mK) during the measuring temperature range 22e800 C. Matsumoto et al. [9] lowered the thermal conductivity of 7.1 wt%Y2O3eZrO2 (1.93 W/mK) by doping with 5 mol % La2O3 (1.2 W/mK). Recently, Matsumtoto et al. [25] also found that the thermal conductivity of La2O3, HfO2 co-doped 4 mol.% Y2O3eZrO2 is about 0.5 W/mK, which is much lower than that of 4 mol.% Y2O3eZrO2. All the studies prove that the thermal conductivity of two or more than two oxides co-doped ZrO2 is lower than that of the single Y2O3 stabilized ZrO2.
4. Conclusions Three types of Yb2O3, Y2O3 co-doped ZrO2 (YbYSZ) coatings and conventional 4 mol.% Y2O3-doped ZrO2 (4YSZ) coating were prepared by air plasma spraying. YbYSZ provide remarkable opportunities for developing novel TBCs due to their improved phase stability and reduced thermal conductivity in comparison with those of the stateof-art 4YSZ. In particular, 2.72 mol.% Yb2O3e4 mol.% Y2O3eZrO2 (YbYSZ2) keeps 100% non-transformable t0 phase even after heat treated at 1400 C for 100 h and small amount of monoclinic phase are formed in 1.15 mol.% Yb2O3-4 mol.% Y2O3eZrO2 (YbYSZ1) and 1.87 mol.% Yb2O3e4.85 mol.% Y2O3eZrO2 (YbYSZ3), 2.47 mol.% and 0.55 mol.%, respectively, whereas that in 4YSZ reaches 43.3 mol.%, corresponding almost completely phase transformation of tetragonal phase. The thermal conductivities of YbYSZ1 (2.03e1.62 W/mK), YbYSZ2 (1.52e1.19 W/mK) and YbYSZ3 (1.7e1.4 W/mK) are obviously lower than that of 4YSZ (2.29e1.96 W/mK), among which YbYSZ2 has the lowest data, because of the more introducing vacancies and substitutional defects in YbYSZ and the larger atomic mass difference between Yb and Zr than that between Yand Zr. YbYSZ can be explored as a novel prospective candidate material for TBCs applications.
Acknowledgements This work was supported by Program for New Century Excellent Talents in University (NCET-2006), PR China and Project supported by Hunan Provincial Natural Science Foundation of China (10JJ2037) which are gratefully acknowledged.
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