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Microstructure and thermal properties of nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings by air plasma spraying
Available online at www.sciencedirect.com
Scripta Materialia 66 (2012) 109–112 www.elsevier.com/locate/scriptamat
Microstructure and thermal properties of nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings by air plasma spraying A. Rauf,⇑ Q. Yu, L. Jin and C. Zhou⇑ School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China Received 12 August 2011; accepted 11 October 2011 Available online 19 October 2011
Nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings were developed using the air plasma spraying technique. Scanning and transmission electron microscopy studies revealed that the coatings are characterized by a bimodal microstructure consisting of melted zones, nano-zones, splats, nano-pores and micro-cracks, which are typical features of nanostructured plasma-sprayed coatings. These coatings are tetragonal in phase, with a grain size of 30–60 nm. The thermal conductivity achieved by these coatings is lower than that of nanostructured and traditional yttria-stabilized zirconia coatings. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Plasma sprayed; Thermal conductivity; La2O3; YSZ
Thermal barrier coatings (TBC) are widely used to increase the life and efficiency of gas turbine engines [1–7]. They typically have a duplex-type configuration with a metallic bond coat and a ceramic topcoat. The bond coat serves to protect the substrates from oxidation and corrosion attacks, thereby improving the bonding between the ceramic topcoat and the substrate [8–11]. A TBC should have low thermal conductivity [12–15], high temperature phase stability and resistance to sintering in order to maintain the pores present in the coating. A high thermal expansion coefficient is also necessary to minimize the thermal expansion mismatch between the coating and the metallic substrate. It should also be thermodynamically stable with alumina. Plasma-sprayed yttria-stabilized zirconia (YSZ) with tetragonal prime (t0 ) structure is one of the most commonly used TBC because of its low thermal conductivity and phase stability at operating temperatures <1200 °C. However to increase the efficiency, ceramic materials with lower thermal conductivity which can operate at higher working temperatures for long periods are required. There is a lot of research going onto achieve this goal.
⇑ Corresponding
authors. Tel.: +86 10 82338622; e-mail addresses: [email protected]; [email protected]
Recent studies have shown that nanostructured YSZ coatings [16–24] exhibit lower thermal conductivity, higher coefficient of thermal expansion and improved mechanical properties. Similarly, the doping [23,25–29] of YSZ coatings with small amounts of La2O3 [30–34] has resulted in reduced thermal conductivity and resistance to sintering. Zhu et al. have developed a multicomponent defect clustering approach and doped YSZ with multiple dopants [27,28]. However, reports on La2O3 addition in nanostructured YSZ coatings produced by the plasma-spraying technique are limited. The aim of the present work is to study the combined effect of nanostructure and lanthana doping on the microstructural and thermal properties of YSZ developed by the air plasma spraying technique. Nano-sized powder of 5LaYSZ 5 wt.% La2O3 (5.4 wt.% Y2O3–ZrO2) was plasma sprayed on stainless steel substrates. The finely dispersed nano particles must be agglomerated to a size of 30–100 lm before being plasma sprayed. The nano particles were agglomerated by the spray-drying technique. Spray-drying suspensions were prepared by mixing nano-La2O3 and nano-YSZ powders with 2 wt.% acaciagum binder and 1 wt.% ammonium citrate. Then the suspension was ball-milled for more than 24 h. The suspension was spray-dried, and the granules were collected and sieved. The controlled operational parameters were set at an air inlet
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.10.017
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A. Rauf et al. / Scripta Materialia 66 (2012) 109–112
temperature of 220 °C; the air outlet temperature was set at 140 °C, while the temperature inside the chamber was 180 °C. The temperature of the substrate was 300–400 °C during the plasma spraying. A thermocouple was placed in the back of the substrate to measure the temperature. The coatings detached from the stainless steel substrate were 1 mm thick. The plasma-spraying parameters are given in Table 1. The phases present in the powder and the coatings were studied using a D/max 2200pc X-ray diffractometer (Cu Ka radiation; Rigaku, Tokyo, Japan). The microstructure and coating morphology of the nanostructured coating were observed by scanning electron microscopy (SEM) using an S-3500 instrument (Hitachi, Tokyo, Japan) and by transmission electron microscopy (TEM). The density of the coating was measured using Archimedes’ principle. The thermal diffusivity was measured at different temperatures using the laser flash technique (LFA427, NET-ZSCH). Three values were taken at each temperature, and the average of these values was used. To evaluate the thermal diffusivity, the solution proposed originally by Parker et al. [35] consists of the following relation a ¼ 0:1388
L2 t1=2
ð1Þ
where L is the thickness of the sample (cm), and t1/2 is the half time (s), which is the time required for the rear surface to reach half the maximum temperature rise. The thermal conductivity of each sample was calculated using the following equation: kðT Þ ¼ aðT ÞqC p ðT Þ
ð2Þ
where k stands for thermal conductivity, a is thermal diffusivity, Cp is heat capacity, and the density q of the coating was measured using Archimedes’ principle. Density was assumed to be constant at all temperatures. Figure 1 shows the XRD pattern of as-sprayed 5LaYSZ coatings. This pattern reveals that the nanostructured 5LaYSZ coatings consist mainly of t-ZrO2. No peak of La2O3 phase was observed. These results showed that La3+ is in solid solution with ZrO2, leading to the stabilization of t-ZrO2. The non-transformable t phase in ZrO2 existed as the non-equilibrium t0 phase, which was formed as a result of quenching of the droplet after impacting on the substrate during plasma spraying.
Figure 1. XRD pattern of 5LaYSZ.
No m-ZrO2, La2Zr2O7 or La2O3 phases were detected after plasma spraying. The mean grain size of 5LaYSZ coating was estimated using the Scherrer equation [36,37]: 0:9k ð3Þ D cos h where D is the dimension of the crystallite (average), Bp (2h) is the line broadening at half the maximum intensity (FWHM), k (0.154 nm) is the wavelength of the X-rays, and h denotes the Bragg diffraction angle. Instrumental broadening in the measurement of peak broadening was taken into consideration, and Gaussian correction was applied by comparing the FWHM of X-ray reflection between the sample and the single crystalline Si standard to obtain the true crystal broadening: Bp ð2hÞ ¼
B2p ð2hÞ ¼ B2h ð2hÞ B2f ð2hÞ
ð4Þ
where Bp(2h) is the true half maximum width, Bh (2h) and Bf (2h) are the half maximum widths of the sample and the single crystalline Si standard, respectively. Corresponding to Figure 1, the average grain size of the as-sprayed 5LaYSZ coating calculated by Eq. (3) is 38 nm. SEM study of the fractured cross-section of 5LaYSZ coating revealed that these coatings are characterized by a bimodal microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks, as shown in Figure 2.
Table 1. Plasma-spraying parameters. Power (kW) Current (A) Voltage (V) Primary gas (Ar) (slpm)a Secondary gas (H2) (slpm)a Carrier gas (Ar) (slpm)a Spraying distance (mm) Feed rate (g min1) Injector internal diameter (mm) Injector position to the torch axis (mm) Injector position to the nozzle exit (mm) a
splm: standard liters per minute.
36 600 60 60 15 4.5 80 25 1.5 6 8.5
Figure 2. SEM micrographs showing the melted zones, nano-zones, splats and micro-cracks.
A. Rauf et al. / Scripta Materialia 66 (2012) 109–112
111
transfer through the material during the measurement of thermal diffusivity may lead to this apparent increase [12]. The variation in thermal conductivity of the 5LaYSZ coatings at different temperatures is plotted in Figure 4b. It can be seen that the thermal conductivity at room temperature was 1.037 W m K1, and it decreased to 0.839 W m K1 at 900 °C. An apparent increase in thermal conductivity is observed at 1200 °C, for the same reason, i.e., at high temperatures radiative heat transfer takes place through the material during the measurement of thermal conductivity, as in the case of thermal diffusivity [12]. The phonon mean path can be given by
Figure 3. TEM micrographs showing nano-grains of 5LaYSZ coatings.
The TEM micrograph in Figure 3 reveals that the average size of the grains is between 30 and 60 nm. These features are typical of nanostructured plasmasprayed coatings, and the presence of nano-zones, nano-pores, voids and micro-cracks enhances phonon scattering, thereby reducing the thermal conductivity. The variation in thermal diffusivity values of YSZ, nano-YSZ and 5LaYSZ from room temperature to 1200 °C are shown in Figure 4a. It can be seen that the thermal diffusivity of lanthana-doped nanostructured 5LaYSZ is lower than both nano-YSZ and conventional YSZ. The room temperature value of thermal diffusivity was 0.415 mm2 s1, and it decreased as the temperature increased until 900 °C, where the value of thermal diffusivity was 0.245 mm2 s1. It increased at 1200 °C to 0.281 mm2 s1. At high temperatures, radiative heat
Figure 4. (a) Variation in thermal diffusivity with temperature. (b) Variation in thermal conductivity with temperature.
1 1 1 1 ¼ þ þ l li lp lb
ð5Þ
where phonon mean paths due to intrinsic conductivity, point defect scattering and grain-boundary scattering are li, lp and lb, respectively. In nanostructured coatings, grain-boundary scattering also has a significant effect on phonon mean path, which can be described by [40] 1 T c2 ¼ lb 20T m a
ð6Þ
where Tm is the absolute melting temperature, a is the lattice constant, and c is the Gruneisen constant. Using the above relation, lb calculated for single crystal of YSZ is 25 nm. The grain size of 5LaYSZ is 30–60 nm, which is comparable to phonon mean free path due to grain boundary scattering lb. The small grain size results in lower thermal conductivity due boundary thermal resistance promoted by phonon scattering at grain boundaries [41]. The effect of point defect scattering, such as substituting atoms on the phonon mean path, can be described by [38,42] 2 1 a3 4 DM ¼ x c lp 4pv4 M
ð7Þ
2 1 2ca3 x4 2 2 DR ¼ J c lp R 4pv4
ð8Þ
where a3 denotes volume per atom, v is the transverse wave speed, x is the phonon frequency, c is the defect concentration per atom, J is a constant, c is the Gruneisen parameter, M is the average mass of the substituted atoms, and DM and DR are the average atomic mass difference and ionic radii difference between substituted (Zr) and substituting (Y, La) atoms. The atomic masses of Zr, Y and La are 91.2, 88.9 and 138.9, respectively, the atomic mass difference (|DM|) between the solute (La) and host atom (Zr) is 47.2, while the (|DM|) between Y and Zr is 2.3. Similarly the ionic radii difference (|DR|) between Y3+ and Zr4+ is 0.0179 nm, while the difference (|DR|) between La3+ and Zr4+ ionic radii is 0.032 nm. Higher |DM| and |DR| contribute to more effective phonon scattering by La solute cations in 5LaYSZ than that of YSZ. The lower thermal conductivity values obtained for 5LaYSZ is due to both the La2O3 doping and the
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nanostructure of the coating. Substituting Zr4+ with La3+ creates additional oxygen vacancies to maintain the neutrality of the lattice, which can result in enhanced phonon scattering [31,38]. Similarly, strains are generated due to lanthana doping, as lattice distortion occurs because the ionic radius of La3+ is larger than that of Zr4+. Atomic vibrational waves can be scattered by the interfaces between YSZ and lanthana, thereby limiting the phonon mean free path [34]. Thermal conductivity decreases for nanostructured coatings as grain boundary contribution to phonon scattering is increased [21]. In nanostructured coatings, the splats are thinner, the grains are smaller, and nanopores, micropores and inter-splat pores are present, which increases the number of interfaces, thereby enhancing phonon scattering and, as result, reducing the thermal conductivity [39]. Nanostructured 5LaYSZ coatings were developed using the air plasma spraying technique. The as-sprayed 5LaYSZ coating with grain size ranging between 30 and 60 nm was characterized by a typical bimodal microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and microcracks. It was completely tetragonal, and no m-ZrO2 or La2O3 phase was detected. This reduction in thermal conductivity was due to the combined effect of nanostructure and lanthana doping. [1] C.G. Levi, Curr. Opin. Solid State Mater. Sci. 8 (2004) 77. [2] R. Vaßen, M.O. Jarligo, T. Steinke, D.E. Mack, D. Sto¨ver, Surf. Coat. Technol. 205 (2010) 938. [3] D. Sto¨ver, G. Pracht, H. Lehmann, et al., J. Therm. Spray. Technol. 13 (2004) 76. [4] D. Zhu, R.A. Miller, Thermal Barrier Coatings for Advanced Gas Turbine and Diesel Engines, 1999, NASA. [5] S. Choi, D. Zhu, R. Miller, Int. J. Appl. Ceram. Technol. 1 (2004) 330. [6] S. Choi, D. Zhu, R. Miller, J. Am. Ceram. Soc. 88 (2005) 2859. [7] A. Evans, D. Clarke, C. Levi, J. Eur. Ceram. Soc. 28 (2008) 1405. [8] J.A. Haynes, M.K. Ferber, W.D. Porter, J. Therm. Spray. Technol. 9 (2000) 38. [9] W.J. Brindley, J. Therm. Spray. Technol. 6 (1997) 85. [10] V. Tolpygo, D. Clarke, Surf. Coat. Technol. 200 (2005) 1276. [11] L.Y. Ni, C. Liu, H. Huang, C.G. Zhou, J. Therm. Spray. Technol. (2011) 2. [12] D. Clarke, Surf. Coat. Technol. 163–164 (2003) 67. [13] D. Clarke, S. Phillpot, Materials Today 8 (2005) 22. [14] D. Zhu, R. Miller, Int. J. Appl. Ceram. Technol. 1 (2004) 86.
[15] D. Zhu, R.A. Miller, J. Therm. Spray. Technol. 9 (2000) 175. [16] R.S. Lima, A. Kucuk, C.C. Berndt, Surf. Coat. Technol. 135 (2001) 166. [17] C. Zhou, N. Wang, H. Xu, Mater. Sci. Eng. A 452–453 (2007) 569. [18] M. Gell, Mater. Sci. Eng. A 204 (1995) 246. [19] R.S. Lima, A. Kucuk, C.C. Berndt, Mater. Sci. Eng. A 313 (2001) 75. [20] R.S. Lima, A. Kucuk, C.C. Berndt, Mater. Sci. Eng. A 337 (2002) 224. [21] C. Zhou, N. Wang, Z. Wang, S. Gong, H. Xu, Scripta Mater. 51 (2004) 945. [22] J. Wu, H. Guo, L. Zhou, L. Wang, S. Gong, J. Therm. Spray. Technol. 19 (2010) 1186. [23] Q. Yu, A. Rauf, C. Zhou, J. Therm. Spray. Technol. 19 (2010) 1294. [24] R.S. Lima, B.R. Marple, J. Therm. Spray. Technol. 16 (2007) 40. [25] S. Tsipas, J. Eur. Ceram. Soc. 30 (2010) 61. [26] M. Matsumoto, J. Ceram. Soc. Jpn. 115 (2007) 118. [27] D. Zhu, J.A. Nesbitt, C.A. Barrett, T.R. Mccue, R.A. Miller, J. Therm. Spray. Technol. 13 (2004) 84. [28] D. Zhu, Y.L. Chen, R.A. Miller, Defect Clustering and Nanophase Structure Characterization of Multicomponent Rare Earth-Oxide-Doped Zirconia-Yttria Thermal Barrier Coatings, 2004, NASA. [29] T.A. Schaedler, R.M. Leckie, S. Kra¨mer, A.G. Evans, C.G. Levi, J. Am. Ceram. Soc. 90 (2007) 3896. [30] M. Matsumoto, H. Takayama, D. Yokoe, et al., Scripta Mater. 54 (2006) 2035. [31] M. Matsumoto, K. Aoyama, H. Matsubara, et al., Surf. Coat. Technol. 194 (2005) 31. [32] M. Matsumoto, T. Kato, N. Yamaguchi, et al., Surf. Coat. Technol. 203 (2009) 2835. [33] Y. Liu, Y.F. Gao, S.Y. Tao, et al., J. Therm. Spray. Technol. 17 (2008) 603. [34] Y. Liu, Y.F. Gao, S.Y. Tao, et al., Surf. Coat. Technol. 203 (2009) 1014. [35] W.J. Parker, R.J. Jenkins, C.P. Butler, et al., J. Appl. Phys. 32 (1961) 1679. [36] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polystalline and Amorphous Materials, John Wiley, London, 1954. [37] L.L. Shaw, D. Goberman, Surf. Coat. Technol. 130 (2000) 1. [38] P.G. Klemens, Physica B 263–264 (1999) 102. [39] H. Zhou, J. Wang, B.-de Sun, J. Coat. Tech. Res. 6 (2009) 383. [40] G. Soyez, J.A. Eastman, L.J. Thompson, et al., Appl. Phys. Lett. 77 (2000) 1155. [41] X. Guo, Solid State Ionics 96 (1997) 247. [42] H. Lehmann, D. Pizter, G. Prach, R. Vassen, D. Stover, J. Am. Ceram. Soc. 8 (2003) 1338.
Scripta Materialia 66 (2012) 109–112 www.elsevier.com/locate/scriptamat
Microstructure and thermal properties of nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings by air plasma spraying A. Rauf,⇑ Q. Yu, L. Jin and C. Zhou⇑ School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, People’s Republic of China Received 12 August 2011; accepted 11 October 2011 Available online 19 October 2011
Nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings were developed using the air plasma spraying technique. Scanning and transmission electron microscopy studies revealed that the coatings are characterized by a bimodal microstructure consisting of melted zones, nano-zones, splats, nano-pores and micro-cracks, which are typical features of nanostructured plasma-sprayed coatings. These coatings are tetragonal in phase, with a grain size of 30–60 nm. The thermal conductivity achieved by these coatings is lower than that of nanostructured and traditional yttria-stabilized zirconia coatings. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Plasma sprayed; Thermal conductivity; La2O3; YSZ
Thermal barrier coatings (TBC) are widely used to increase the life and efficiency of gas turbine engines [1–7]. They typically have a duplex-type configuration with a metallic bond coat and a ceramic topcoat. The bond coat serves to protect the substrates from oxidation and corrosion attacks, thereby improving the bonding between the ceramic topcoat and the substrate [8–11]. A TBC should have low thermal conductivity [12–15], high temperature phase stability and resistance to sintering in order to maintain the pores present in the coating. A high thermal expansion coefficient is also necessary to minimize the thermal expansion mismatch between the coating and the metallic substrate. It should also be thermodynamically stable with alumina. Plasma-sprayed yttria-stabilized zirconia (YSZ) with tetragonal prime (t0 ) structure is one of the most commonly used TBC because of its low thermal conductivity and phase stability at operating temperatures <1200 °C. However to increase the efficiency, ceramic materials with lower thermal conductivity which can operate at higher working temperatures for long periods are required. There is a lot of research going onto achieve this goal.
⇑ Corresponding
authors. Tel.: +86 10 82338622; e-mail addresses: [email protected]; [email protected]
Recent studies have shown that nanostructured YSZ coatings [16–24] exhibit lower thermal conductivity, higher coefficient of thermal expansion and improved mechanical properties. Similarly, the doping [23,25–29] of YSZ coatings with small amounts of La2O3 [30–34] has resulted in reduced thermal conductivity and resistance to sintering. Zhu et al. have developed a multicomponent defect clustering approach and doped YSZ with multiple dopants [27,28]. However, reports on La2O3 addition in nanostructured YSZ coatings produced by the plasma-spraying technique are limited. The aim of the present work is to study the combined effect of nanostructure and lanthana doping on the microstructural and thermal properties of YSZ developed by the air plasma spraying technique. Nano-sized powder of 5LaYSZ 5 wt.% La2O3 (5.4 wt.% Y2O3–ZrO2) was plasma sprayed on stainless steel substrates. The finely dispersed nano particles must be agglomerated to a size of 30–100 lm before being plasma sprayed. The nano particles were agglomerated by the spray-drying technique. Spray-drying suspensions were prepared by mixing nano-La2O3 and nano-YSZ powders with 2 wt.% acaciagum binder and 1 wt.% ammonium citrate. Then the suspension was ball-milled for more than 24 h. The suspension was spray-dried, and the granules were collected and sieved. The controlled operational parameters were set at an air inlet
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.10.017
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A. Rauf et al. / Scripta Materialia 66 (2012) 109–112
temperature of 220 °C; the air outlet temperature was set at 140 °C, while the temperature inside the chamber was 180 °C. The temperature of the substrate was 300–400 °C during the plasma spraying. A thermocouple was placed in the back of the substrate to measure the temperature. The coatings detached from the stainless steel substrate were 1 mm thick. The plasma-spraying parameters are given in Table 1. The phases present in the powder and the coatings were studied using a D/max 2200pc X-ray diffractometer (Cu Ka radiation; Rigaku, Tokyo, Japan). The microstructure and coating morphology of the nanostructured coating were observed by scanning electron microscopy (SEM) using an S-3500 instrument (Hitachi, Tokyo, Japan) and by transmission electron microscopy (TEM). The density of the coating was measured using Archimedes’ principle. The thermal diffusivity was measured at different temperatures using the laser flash technique (LFA427, NET-ZSCH). Three values were taken at each temperature, and the average of these values was used. To evaluate the thermal diffusivity, the solution proposed originally by Parker et al. [35] consists of the following relation a ¼ 0:1388
L2 t1=2
ð1Þ
where L is the thickness of the sample (cm), and t1/2 is the half time (s), which is the time required for the rear surface to reach half the maximum temperature rise. The thermal conductivity of each sample was calculated using the following equation: kðT Þ ¼ aðT ÞqC p ðT Þ
ð2Þ
where k stands for thermal conductivity, a is thermal diffusivity, Cp is heat capacity, and the density q of the coating was measured using Archimedes’ principle. Density was assumed to be constant at all temperatures. Figure 1 shows the XRD pattern of as-sprayed 5LaYSZ coatings. This pattern reveals that the nanostructured 5LaYSZ coatings consist mainly of t-ZrO2. No peak of La2O3 phase was observed. These results showed that La3+ is in solid solution with ZrO2, leading to the stabilization of t-ZrO2. The non-transformable t phase in ZrO2 existed as the non-equilibrium t0 phase, which was formed as a result of quenching of the droplet after impacting on the substrate during plasma spraying.
Figure 1. XRD pattern of 5LaYSZ.
No m-ZrO2, La2Zr2O7 or La2O3 phases were detected after plasma spraying. The mean grain size of 5LaYSZ coating was estimated using the Scherrer equation [36,37]: 0:9k ð3Þ D cos h where D is the dimension of the crystallite (average), Bp (2h) is the line broadening at half the maximum intensity (FWHM), k (0.154 nm) is the wavelength of the X-rays, and h denotes the Bragg diffraction angle. Instrumental broadening in the measurement of peak broadening was taken into consideration, and Gaussian correction was applied by comparing the FWHM of X-ray reflection between the sample and the single crystalline Si standard to obtain the true crystal broadening: Bp ð2hÞ ¼
B2p ð2hÞ ¼ B2h ð2hÞ B2f ð2hÞ
ð4Þ
where Bp(2h) is the true half maximum width, Bh (2h) and Bf (2h) are the half maximum widths of the sample and the single crystalline Si standard, respectively. Corresponding to Figure 1, the average grain size of the as-sprayed 5LaYSZ coating calculated by Eq. (3) is 38 nm. SEM study of the fractured cross-section of 5LaYSZ coating revealed that these coatings are characterized by a bimodal microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks, as shown in Figure 2.
Table 1. Plasma-spraying parameters. Power (kW) Current (A) Voltage (V) Primary gas (Ar) (slpm)a Secondary gas (H2) (slpm)a Carrier gas (Ar) (slpm)a Spraying distance (mm) Feed rate (g min1) Injector internal diameter (mm) Injector position to the torch axis (mm) Injector position to the nozzle exit (mm) a
splm: standard liters per minute.
36 600 60 60 15 4.5 80 25 1.5 6 8.5
Figure 2. SEM micrographs showing the melted zones, nano-zones, splats and micro-cracks.
A. Rauf et al. / Scripta Materialia 66 (2012) 109–112
111
transfer through the material during the measurement of thermal diffusivity may lead to this apparent increase [12]. The variation in thermal conductivity of the 5LaYSZ coatings at different temperatures is plotted in Figure 4b. It can be seen that the thermal conductivity at room temperature was 1.037 W m K1, and it decreased to 0.839 W m K1 at 900 °C. An apparent increase in thermal conductivity is observed at 1200 °C, for the same reason, i.e., at high temperatures radiative heat transfer takes place through the material during the measurement of thermal conductivity, as in the case of thermal diffusivity [12]. The phonon mean path can be given by
Figure 3. TEM micrographs showing nano-grains of 5LaYSZ coatings.
The TEM micrograph in Figure 3 reveals that the average size of the grains is between 30 and 60 nm. These features are typical of nanostructured plasmasprayed coatings, and the presence of nano-zones, nano-pores, voids and micro-cracks enhances phonon scattering, thereby reducing the thermal conductivity. The variation in thermal diffusivity values of YSZ, nano-YSZ and 5LaYSZ from room temperature to 1200 °C are shown in Figure 4a. It can be seen that the thermal diffusivity of lanthana-doped nanostructured 5LaYSZ is lower than both nano-YSZ and conventional YSZ. The room temperature value of thermal diffusivity was 0.415 mm2 s1, and it decreased as the temperature increased until 900 °C, where the value of thermal diffusivity was 0.245 mm2 s1. It increased at 1200 °C to 0.281 mm2 s1. At high temperatures, radiative heat
Figure 4. (a) Variation in thermal diffusivity with temperature. (b) Variation in thermal conductivity with temperature.
1 1 1 1 ¼ þ þ l li lp lb
ð5Þ
where phonon mean paths due to intrinsic conductivity, point defect scattering and grain-boundary scattering are li, lp and lb, respectively. In nanostructured coatings, grain-boundary scattering also has a significant effect on phonon mean path, which can be described by [40] 1 T c2 ¼ lb 20T m a
ð6Þ
where Tm is the absolute melting temperature, a is the lattice constant, and c is the Gruneisen constant. Using the above relation, lb calculated for single crystal of YSZ is 25 nm. The grain size of 5LaYSZ is 30–60 nm, which is comparable to phonon mean free path due to grain boundary scattering lb. The small grain size results in lower thermal conductivity due boundary thermal resistance promoted by phonon scattering at grain boundaries [41]. The effect of point defect scattering, such as substituting atoms on the phonon mean path, can be described by [38,42] 2 1 a3 4 DM ¼ x c lp 4pv4 M
ð7Þ
2 1 2ca3 x4 2 2 DR ¼ J c lp R 4pv4
ð8Þ
where a3 denotes volume per atom, v is the transverse wave speed, x is the phonon frequency, c is the defect concentration per atom, J is a constant, c is the Gruneisen parameter, M is the average mass of the substituted atoms, and DM and DR are the average atomic mass difference and ionic radii difference between substituted (Zr) and substituting (Y, La) atoms. The atomic masses of Zr, Y and La are 91.2, 88.9 and 138.9, respectively, the atomic mass difference (|DM|) between the solute (La) and host atom (Zr) is 47.2, while the (|DM|) between Y and Zr is 2.3. Similarly the ionic radii difference (|DR|) between Y3+ and Zr4+ is 0.0179 nm, while the difference (|DR|) between La3+ and Zr4+ ionic radii is 0.032 nm. Higher |DM| and |DR| contribute to more effective phonon scattering by La solute cations in 5LaYSZ than that of YSZ. The lower thermal conductivity values obtained for 5LaYSZ is due to both the La2O3 doping and the
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