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Thermal diffusivity of plasma sprayed monolithic coating of alumina–3 wt.% titania produced with nanostructured powder
Surface & Coatings Technology 195 (2005) 85 – 90 www.elsevier.com/locate/surfcoat
Thermal diffusivity of plasma sprayed monolithic coating of alumina–3 wt.% titania produced with nanostructured powder Xinhua Lin*, Yi Zeng, Xuebin Zheng, Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 8 December 2003; accepted in revised form 23 August 2004
Abstract Nanostructured and conventional alumina–3 wt.% titania monolithic coatings were deposited by air plasma spraying (APS). The thermal diffusivity was measured by the laser flash technique. The thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating was higher compared with that of the corresponding conventional coating at temperature ranging from 200 to 1000 8C. For the nanostructured coating, there was no difference in the thermal diffusivity between during heating and cooling. However, the thermal diffusivities of the conventional coating were higher during cooling than those during heating. SEM and TEM examination showed that the nanostructured coating contained equiaxed grains with sizes from 150 to 800 nm besides splat lamellae. In the nanostructured coating, most of columnar grains in splat lamellae were less than 200 nm. Splat lamellae of the nanostructured coating bonded well each other and their thickness ranged from 0.4 to 1 Am. The decrease of thermal diffusivity of the nanostructured coating was attributed to the increase of grain boundaries and defective crystal structure. The stability of thermal diffusivity of the nanostructured coating was considered to relate to the absence of narrow long microcracks between splat lamellae. Nanostructured and conventional alumina–3 wt.% titania monolithic coatings were deposited by air plasma spraying (APS). The thermal diffusivity was measured by the laser flash technique. The thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating was higher compared with that of the corresponding conventional coating at temperature ranging from 200 to 1000 8C. For the nanostructured coating, there was no difference in the thermal diffusivity between during heating and cooling. However, the thermal diffusivities of the conventional coating were higher during cooling than those during heating. SEM and TEM examination showed that the nanostructured coating contained equiaxed grains with sizes from 150 to 800 nm besides splat lamellae. In the nanostructured coating, most of columnar grains in splat lamellae were less than 200 nm. Splat lamellae of the nanostructured coating bonded well each other and their thickness ranged from 0.4 to 1 Am. The decrease of thermal diffusivity of the nanostructured coating was attributed to the increase of grain boundaries and defective crystal structure. The stability of thermal diffusivity of the nanostructured coating was considered to relate to the absence of narrow long micro-cracks between splat lamellae. D 2004 Published by Elsevier B.V. Keywords: Al2O3–TiO2 coating; Nanostructured; Thermal diffusivity; Micro-cracks
1. Introduction The thermal conductivity of materials is closely related to the microstructure, such as grain size, pores and impurities. Grain boundaries play an important role in controlling thermal transport in polycrystalline materials, particularly
* Corresponding author. Tel.: +86 21 524 12990; fax: +86 21 524 13903. E-mail address: [email protected] (X. Lin). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.08.193
when grain size is reduced to the nanometer scale. Many studies showed that thermal conductivity exhibited a reduction with the reduction of grain size at low temperatures [1–3]. Wu et al. [1] observed the strong decrease in thermal conductivity of small grain Ag compared to larger grain material. In room temperature, the decrease of yttriastablized zirconia (YSZ) coatings was attributed to the decrease of grain size [3]. Pores, filled with gas of a lower thermal conductivity than the solid phase, help to block the heat flow. It is well known that the higher porosity results in the lower thermal conductivity. However, the size, shape
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and distribution of pore also play important roles in determining the thermal conductivity [4–6]. Impurity is another factor influencing thermal conductivity. Dos Santos et al. [7] reported that the thermal conductivity of Al2O3 was decreased by adding Nb due to the increase of lattice vibration scattering. Due to the relative abundance and low cost, alumina is one of the first materials used for plasma spraying. Plasmasprayed alumina-based coatings have been widely used to protect surfaces against various types of adverse environment involving high temperatures, wear and corrosion. Compare to sintered Al2O3, the thermal conductivity of thermal sprayed coatings is reduced by up to approximately a factor due to the porosity of the coating and the thermal resistance at the splat interfaces [4], which makes them also serve as thermal barrier coatings (TBCs) at high temperature [4,8,9]. Thermal conductivity is one of the significant parameters for evaluating TBCs. From this point of view, the thermal diffusivity of plasma-sprayed nanostructured alumina–3 wt.% titania coating was examined in this report and was compared with that of the corresponding conventional coating.
2. Experimental Two kinds of alumina–3 wt.% titania coating were deposited by atmospheric plasma spraying system (A2000, Sulzer-Metco, Switzerland) with the nanostructured and conventional fused and crushed powders, respectively. The nanostructured powder was obtained by agglomerating the mixture of nano-sized alumina and rutile particles, which has been described in a separate report. The nanostructured powders exhibited a porous spherical morphology (see Fig. 1). To obtain the free standing samples used in this study, coatings about 3.5 mm thickness were deposited on the aluminium substrate and, then, were removed from the substrate. The coating prepared with the nanostructured
Fig. 1. The typical morphology of the nanostructured Al2O3–3 wt.% TiO2 powders.
powder in the following studies is referred to as nanostructured coating. For the measurement of thermal diffusivity, samples with the dimensions f 10.21.6 mm were prepared. The thermal diffusivity was determined using the laser flash technique. Heat flow was parallel to the direction of plasma spraying. The laser flash technique involved heating one side of the sample with a laser pulse of short duration and measuring the temperature rise on the other side with an infrared detector. Thermal diffusivity was determined from the time required to reach one-half of the peak temperature and a transient heat conduction analysis of a multi-layer body. Measurements were carried out in a vacuum chamber from 200 to 1000 8C at the intervals of approximately 100 8C. Differential thermal analysis was also conducted on the Netgsch 402ES-3 thermal analyser in air atmosphere. The temperature range was from room temperature to 1250 8C and the heating rate was about 10 8C/min. Microstructures were characterized by scanning electron microscope (EPMA-8705QH2, Shimadzu, Japan) and transmission electron microscope (JEM-200CX, JEOL, Japan). Ten of polished cross-section micrographs were used to calculate the porosity by image analysis and point-counting techniques. The normalized bulk density was also measured with the water displacement technique.
3. Results and discussion 3.1. Thermal diffusivity measurement Fig. 2 presents the thermal diffusivity of the as-sprayed coatings from 200 to 1000 8C. Thermal diffusivities of both conventional and nanostructured Al2O3–3 wt.% TiO2 coatings decreased with the increase in temperature up to 900 8C. This is explained by greater mutual scattering of the vibrational waves through Umklapp processes at higher temperatures, which shortens the mean free path [10]. However, the lower value of thermal diffusivity was obtained for the nanostructured Al2O3–3 wt.% TiO2 coating at the same temperature, which decreased by 5– 13% compared to the conventional coating. When the temperature was further increased to 1000 8C, the thermal diffusivity of the nanostructured coating decreased while the reverse trend occurred for the conventional coating, namely, thermal diffusivity increased with temperature from 900 to 1000 8C. It was also found in Fig. 2 that the magnitudes of thermal diffusivity of the conventional coating during cooling were higher than those during heating at the equivalent temperature. For the nanostructured Al2O3–3 wt.% TiO2 coating, there was nearly no difference in thermal diffusivity between during heating and during cooling. That is to say, the thermal diffusivity of nanostructured Al2O3–3 wt.% TiO2 coating was stable below 1000 8C.
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Fig. 2. Thermal diffusivity of the as-sprayed Al2O3–3 wt.% TiO2 coatings: (n) conventional; ( ) nanostructured.
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3.2. SEM and TEM analysis The back-scattered electron micrographs of cross-section of the as-sprayed coatings are shown in Fig. 3. It can be seen that in the nanostructured Al2O3–3 wt.% TiO2 coating, titanium element was distributed more homogeneously compared with that in the conventional coating. According to image analysis, it was calculated that the porosity of the nanostructured coating was about 8%, which was similar to that of the conventional coating. In addition, it was obtained from the water displacement technique that the normalized bulk densities were 3.42 and 3.44 g/cm3 for the nanostructured and conventional coatings. It further confirmed that both nanostructured and conventional coatings contained the similar porosity. Fig. 4 shows the typical TEM micrographs of crosssection of the nanostructured Al2O3–3 wt.% TiO2 coating. From Fig. 4a, it can be seen that the splat lamellae ranged from 0.4 to 1 Am and consisted of columnar grains (G),
Fig. 4. TEM micrographs of cross-section of the nanostructured Al2O3–3 wt.% TiO2 coating.
which were perpendicular to the surface of previously deposited materials. Splat lamellae bonded well each other and narrow long micro-cracks were seldom between them. SAD analysis showed that the columnar grains were gAl2O3. It has been general accepted that molten Al2O3 powder cooled rapidly to form the columnar g-Al2O3 grains during plasma spraying [11]. Equiaxed grains (E) were observed in the areas adjacent to splat lamellae, which ranged from 150 to 800 nm in size. As for the equiaxed grains, the spectrum of area diffraction belonging to aAl2O3 was observed. This implied that equiaxed grains originated from unmelted/partially melted nanostructured powders. In the high magnification of micrograph (Fig. 5b), the equiaxed grains piled more densely compared with those in the feedstock due to sintering during plasma spraying. In the planar section perpendicular to the substrate surface,
Fig. 3. The typical cross-section micrographs of the as-sprayed Al2O3–3 wt.% TiO2 coatings: (a) conventional; (b) nanostructured.
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grain growth across the lamellae interfaces occurred, suggesting solidification of deposited material is not complete prior to the impact of a subsequent molten droplet. In addition, narrow long micro-cracks were often observed at the splat boundaries, as indicated with an arrow. 3.3. Effect of microstructure on thermal diffusivity
Fig. 5. TEM micrograph of planar section of the nanostructured Al2O3–3 wt.% TiO2 coating.
another kind of equiaxed grain was observed, as shown in Fig. 5. These grains were less than 200 nm in size and their boundaries were more distinct. SAD detected that the finer equiaxed grains took the modification of g-Al2O3. The microstructure represented a cross-section through a number of columnar grains in a single lamella. Fig. 6 shows the typical TEM micrographs of the conventional coating Al2O3–3 wt.% TiO2 coating without thermal treatment. The microstructure of lamellae was composed of columnar grains with the diameter of about 0.7 Am in the conventional coating, which were coarser than those in the nanostructured coating. In some areas, the interfaces between splat lamellae were not obvious and
Fig. 6. The typical TEM micrograph of the conventional Al2O3–3 wt.% TiO2 coating.
In oxides ceramics, heat is predominantly carried by lattice vibration and radiation contribution is insignificant at temperatures b1500 8C [12]. Phonon scattering at grain boundaries can restrain the heat flow [13]. With the increase of the concentration of grain boundaries, phonon scattering is enhanced and the thermal conductivity of material decreases [3]. In the nanostructured Al2O3–3 wt.% TiO2 coating, the presence of equiaxed grains increased the number of grain boundaries. In addition, splat lamellae in the conventional coating were thicker than those in the nanostructured coating (Figs. 4a and 6). This indicated there were more interfaces between splat lamellae in the nanostructured coating with the same thickness. When the melted powders impinged with a curved or irregular surface, columnar grains consisting of splat lamellae exhibited a radial arrangement, as shown in Fig. 4a. The finer columnar grains in the nanostructured coating also increased the interfaces crossed by heat flow path. As a result, phonon scattering on grain boundaries contributed to the decrease of thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating. TiO2 was prone to react with Al2O3 to form solid solution g-Al2O3d TiO2 during plasma spraying due to the higher contact surface area between nano-sized alumina and titania particles and Ti ions were likely in the g-Al2O3 lattice [14,15]. EDS spectrum also showed that g-Al2O3 grains contained Ti element in the nanostructured Al2O3–3 wt.% TiO2 coating. It has been well known that defective crystal structures result in greater scattering of the lattice vibration and shorter phonon mean free path, which is
Fig. 7. Result of the differential thermal analysis (DTA) measurement for the conventional Al2O3–3 wt.% TiO2 coating.
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material, V is the volume fraction of ellipsoidal pores, b is the major axis of the ellipsoid, and a is the minor axis. Though the formation of intermittent necking has no influence on the porosity, it results in the drastic decrease of the ratio of b to a and, accordingly, the thermal conductivity increases. Thus, it was considered that the phenomenon that thermal diffusivity increase during cooling was not apparent for the nanostructured coating was related to the lack of narrow long cracks (Fig. 4a).
4. Conclusions
Fig. 8. TEM micrograph of the conventional Al2O3–3 wt.% TiO2 coating after heat treatment under the same condition as that during measuring the thermal diffusivity.
responsible for the low thermal conductivity of YSZ [3]. Thus, the decrease in thermal diffusivity of the nanostructured coating is also relative to that of the continuity of crystal lattice which was destroyed due to the existence of Ti in g-Al2O3 grains. It was always observed that heat treatment at high temperatures resulted in the increase of thermal diffusivity of plasma-sprayed thermal barrier coatings [4, 16]. As shown in Fig. 4, the similar phenomenon occurred that the conventional Al2O3–3 wt.% TiO2 coating exhibited an increase in thermal diffusivity during cooling compared to during heating. TDA analysis revealed that there was an exothermic peak at 1100 8C (see Fig. 7), which corresponded to the aAl2O3 formation [17]. This indicated no phase-transformation for the conventional coating at temperatures less than 1000 8C. In addition, normalized bulk density nearly had no variations during measuring. The increase of thermal diffusivity during cooling was not related to the change of density and phase. A large population of narrow long microcracks existed between splat lamellae in the conventional monolithic coating (see Fig. 6). It has reported that during heat treatment, the formation of intermittent particle necking across these narrow micro-cracks results in the increase of the thermal diffusivity of plasma sprayed coatings [16]. In our experiment, the similar phenomenon that part of narrows cracks between splat lamellae were interrupted was also observed in the conventional coating after thermal treatment under the condition identical to that during measuring the thermal diffusivity, as shown with an arrow in Fig. 8. According to Hasselman model [18], thermal conductivity of the micro-cracked material is relative to both porosity and shape of pore as follows: k=k0 ¼ ð1 þ ð2V =pÞðb=aÞÞ1
ð1Þ
where k is the thermal conductivity of the micro-cracked material, k 0 is the thermal conductivity of the uncracked
In this paper, the influence of microstructure on thermal diffusivity of the nanostructured and conventional Al2O3–3 wt.% TiO2 monolithic coatings was investigated. The thermal diffusivity of the nanostructured coating was lower than that of the conventional coating. The decrease of thermal diffusivity was relative to the existence of equiaxed grains and finer columnar grains consisting of splat lamellae in the nanostructured coating. The defective crystal structure resulting from the solid solution of Ti in g-Al2O3 and the decrease in thickness of splat lamellae also contributed to the decrease in thermal conductivity. During cooling, the thermal diffusivity of the nanostructured coating nearly had no variation compared to during heating, while for the conventional coating, the thermal diffusivity increased by about 5% during cooling. It was considered that the formation of intermittent particle necking across microcracks between lamellae resulted in the increase of thermal diffusivity of the conventional coating during cooling.
Acknowledgements The authors gratefully acknowledge Engineer Xiaming Zhou and Yefang Zhang for the preparation of samples. The authors also thank Dr. Huang Chen and Dr. Weichang Xue for stimulating discussions.
References [1] B. Wu, Y. Du, H. Tang, P. Du, X. Qin, L. Zhang, J. Phys. Chem. Solids 57 (1996) 1211. [2] S. Fayette, David S. Smith, C. Martin, J. Eur. Ceram. Soc. 20 (2000) 297. [3] H.S. Yang, G.R. Bai, L.J. Thompson, J.A. Eastman, Acta Mater. 50 (2002) 2309. [4] R. Dutton, R. Wheeler, K.S. Ravichandran, K. An, J. Therm. Spray Technol. 9 (2000) 204. [5] I. Sevostianov, M. Kachanov, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 297 (2001) 235. [6] G.F. Hurley, F.D. Gac, Ceram. Bull. 58 (1979) 509. [7] W.N. dos Santos, P.I. Paulin Filho, R. Taylor, J. Eur. Ceram. Soc. 18 (1998) 807. [8] B.R. Marple, J. Voyer, P. Be´chard, J. Eur. Ceram. Soc. 21 (2001) 861. [9] S. Sharafat, A. Kobayashi, V. Ogden, N.M. Ghoniem, Vacuum 59 (2000) 185.
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[10] S. Fayette, D.S. Smith, A. Smith, C. Martin, J. Eur. Ceram. Soc. 20 (2000) 297. [11] R. Mcpherson, J. Mater. Sci. 15 (1980) 3141. [12] K.S. Ravichandran, K. An, R.E. Dutton, S.L. Semiatin, J. Am. Ceram. Soc. 82 (1999) 673. [13] C.-W. Nan, R. Birringer, Phys. Rev., B 57 (1998) 8264. [14] B. Normand, V. Fervel, C. Coddet, V. Nikitine, Surf. Coat. Technol. 123 (2000) 278.
[15] E.H. Jordan, M. Gell, Y.H. Sohn, D. Goberman, L. Shaw, S. Jiang, M. Wang, T.D. Xiao, Y. Wang, P. Strutt, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 301 (2001) 80. [16] R.E. Taylor, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 245 (1998) 160. [17] J. IIavsky, C.C. Berndt, H. Herman, P. Chraska, J. Dubsky, J. Therm. Spray Technol. 6 (1997) 439. [18] D.P.H. Hasselman, J. Compos. Mater. 12 (1978) 403.
Thermal diffusivity of plasma sprayed monolithic coating of alumina–3 wt.% titania produced with nanostructured powder Xinhua Lin*, Yi Zeng, Xuebin Zheng, Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China Received 8 December 2003; accepted in revised form 23 August 2004
Abstract Nanostructured and conventional alumina–3 wt.% titania monolithic coatings were deposited by air plasma spraying (APS). The thermal diffusivity was measured by the laser flash technique. The thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating was higher compared with that of the corresponding conventional coating at temperature ranging from 200 to 1000 8C. For the nanostructured coating, there was no difference in the thermal diffusivity between during heating and cooling. However, the thermal diffusivities of the conventional coating were higher during cooling than those during heating. SEM and TEM examination showed that the nanostructured coating contained equiaxed grains with sizes from 150 to 800 nm besides splat lamellae. In the nanostructured coating, most of columnar grains in splat lamellae were less than 200 nm. Splat lamellae of the nanostructured coating bonded well each other and their thickness ranged from 0.4 to 1 Am. The decrease of thermal diffusivity of the nanostructured coating was attributed to the increase of grain boundaries and defective crystal structure. The stability of thermal diffusivity of the nanostructured coating was considered to relate to the absence of narrow long microcracks between splat lamellae. Nanostructured and conventional alumina–3 wt.% titania monolithic coatings were deposited by air plasma spraying (APS). The thermal diffusivity was measured by the laser flash technique. The thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating was higher compared with that of the corresponding conventional coating at temperature ranging from 200 to 1000 8C. For the nanostructured coating, there was no difference in the thermal diffusivity between during heating and cooling. However, the thermal diffusivities of the conventional coating were higher during cooling than those during heating. SEM and TEM examination showed that the nanostructured coating contained equiaxed grains with sizes from 150 to 800 nm besides splat lamellae. In the nanostructured coating, most of columnar grains in splat lamellae were less than 200 nm. Splat lamellae of the nanostructured coating bonded well each other and their thickness ranged from 0.4 to 1 Am. The decrease of thermal diffusivity of the nanostructured coating was attributed to the increase of grain boundaries and defective crystal structure. The stability of thermal diffusivity of the nanostructured coating was considered to relate to the absence of narrow long micro-cracks between splat lamellae. D 2004 Published by Elsevier B.V. Keywords: Al2O3–TiO2 coating; Nanostructured; Thermal diffusivity; Micro-cracks
1. Introduction The thermal conductivity of materials is closely related to the microstructure, such as grain size, pores and impurities. Grain boundaries play an important role in controlling thermal transport in polycrystalline materials, particularly
* Corresponding author. Tel.: +86 21 524 12990; fax: +86 21 524 13903. E-mail address: [email protected] (X. Lin). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.08.193
when grain size is reduced to the nanometer scale. Many studies showed that thermal conductivity exhibited a reduction with the reduction of grain size at low temperatures [1–3]. Wu et al. [1] observed the strong decrease in thermal conductivity of small grain Ag compared to larger grain material. In room temperature, the decrease of yttriastablized zirconia (YSZ) coatings was attributed to the decrease of grain size [3]. Pores, filled with gas of a lower thermal conductivity than the solid phase, help to block the heat flow. It is well known that the higher porosity results in the lower thermal conductivity. However, the size, shape
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and distribution of pore also play important roles in determining the thermal conductivity [4–6]. Impurity is another factor influencing thermal conductivity. Dos Santos et al. [7] reported that the thermal conductivity of Al2O3 was decreased by adding Nb due to the increase of lattice vibration scattering. Due to the relative abundance and low cost, alumina is one of the first materials used for plasma spraying. Plasmasprayed alumina-based coatings have been widely used to protect surfaces against various types of adverse environment involving high temperatures, wear and corrosion. Compare to sintered Al2O3, the thermal conductivity of thermal sprayed coatings is reduced by up to approximately a factor due to the porosity of the coating and the thermal resistance at the splat interfaces [4], which makes them also serve as thermal barrier coatings (TBCs) at high temperature [4,8,9]. Thermal conductivity is one of the significant parameters for evaluating TBCs. From this point of view, the thermal diffusivity of plasma-sprayed nanostructured alumina–3 wt.% titania coating was examined in this report and was compared with that of the corresponding conventional coating.
2. Experimental Two kinds of alumina–3 wt.% titania coating were deposited by atmospheric plasma spraying system (A2000, Sulzer-Metco, Switzerland) with the nanostructured and conventional fused and crushed powders, respectively. The nanostructured powder was obtained by agglomerating the mixture of nano-sized alumina and rutile particles, which has been described in a separate report. The nanostructured powders exhibited a porous spherical morphology (see Fig. 1). To obtain the free standing samples used in this study, coatings about 3.5 mm thickness were deposited on the aluminium substrate and, then, were removed from the substrate. The coating prepared with the nanostructured
Fig. 1. The typical morphology of the nanostructured Al2O3–3 wt.% TiO2 powders.
powder in the following studies is referred to as nanostructured coating. For the measurement of thermal diffusivity, samples with the dimensions f 10.21.6 mm were prepared. The thermal diffusivity was determined using the laser flash technique. Heat flow was parallel to the direction of plasma spraying. The laser flash technique involved heating one side of the sample with a laser pulse of short duration and measuring the temperature rise on the other side with an infrared detector. Thermal diffusivity was determined from the time required to reach one-half of the peak temperature and a transient heat conduction analysis of a multi-layer body. Measurements were carried out in a vacuum chamber from 200 to 1000 8C at the intervals of approximately 100 8C. Differential thermal analysis was also conducted on the Netgsch 402ES-3 thermal analyser in air atmosphere. The temperature range was from room temperature to 1250 8C and the heating rate was about 10 8C/min. Microstructures were characterized by scanning electron microscope (EPMA-8705QH2, Shimadzu, Japan) and transmission electron microscope (JEM-200CX, JEOL, Japan). Ten of polished cross-section micrographs were used to calculate the porosity by image analysis and point-counting techniques. The normalized bulk density was also measured with the water displacement technique.
3. Results and discussion 3.1. Thermal diffusivity measurement Fig. 2 presents the thermal diffusivity of the as-sprayed coatings from 200 to 1000 8C. Thermal diffusivities of both conventional and nanostructured Al2O3–3 wt.% TiO2 coatings decreased with the increase in temperature up to 900 8C. This is explained by greater mutual scattering of the vibrational waves through Umklapp processes at higher temperatures, which shortens the mean free path [10]. However, the lower value of thermal diffusivity was obtained for the nanostructured Al2O3–3 wt.% TiO2 coating at the same temperature, which decreased by 5– 13% compared to the conventional coating. When the temperature was further increased to 1000 8C, the thermal diffusivity of the nanostructured coating decreased while the reverse trend occurred for the conventional coating, namely, thermal diffusivity increased with temperature from 900 to 1000 8C. It was also found in Fig. 2 that the magnitudes of thermal diffusivity of the conventional coating during cooling were higher than those during heating at the equivalent temperature. For the nanostructured Al2O3–3 wt.% TiO2 coating, there was nearly no difference in thermal diffusivity between during heating and during cooling. That is to say, the thermal diffusivity of nanostructured Al2O3–3 wt.% TiO2 coating was stable below 1000 8C.
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Fig. 2. Thermal diffusivity of the as-sprayed Al2O3–3 wt.% TiO2 coatings: (n) conventional; ( ) nanostructured.
!
3.2. SEM and TEM analysis The back-scattered electron micrographs of cross-section of the as-sprayed coatings are shown in Fig. 3. It can be seen that in the nanostructured Al2O3–3 wt.% TiO2 coating, titanium element was distributed more homogeneously compared with that in the conventional coating. According to image analysis, it was calculated that the porosity of the nanostructured coating was about 8%, which was similar to that of the conventional coating. In addition, it was obtained from the water displacement technique that the normalized bulk densities were 3.42 and 3.44 g/cm3 for the nanostructured and conventional coatings. It further confirmed that both nanostructured and conventional coatings contained the similar porosity. Fig. 4 shows the typical TEM micrographs of crosssection of the nanostructured Al2O3–3 wt.% TiO2 coating. From Fig. 4a, it can be seen that the splat lamellae ranged from 0.4 to 1 Am and consisted of columnar grains (G),
Fig. 4. TEM micrographs of cross-section of the nanostructured Al2O3–3 wt.% TiO2 coating.
which were perpendicular to the surface of previously deposited materials. Splat lamellae bonded well each other and narrow long micro-cracks were seldom between them. SAD analysis showed that the columnar grains were gAl2O3. It has been general accepted that molten Al2O3 powder cooled rapidly to form the columnar g-Al2O3 grains during plasma spraying [11]. Equiaxed grains (E) were observed in the areas adjacent to splat lamellae, which ranged from 150 to 800 nm in size. As for the equiaxed grains, the spectrum of area diffraction belonging to aAl2O3 was observed. This implied that equiaxed grains originated from unmelted/partially melted nanostructured powders. In the high magnification of micrograph (Fig. 5b), the equiaxed grains piled more densely compared with those in the feedstock due to sintering during plasma spraying. In the planar section perpendicular to the substrate surface,
Fig. 3. The typical cross-section micrographs of the as-sprayed Al2O3–3 wt.% TiO2 coatings: (a) conventional; (b) nanostructured.
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grain growth across the lamellae interfaces occurred, suggesting solidification of deposited material is not complete prior to the impact of a subsequent molten droplet. In addition, narrow long micro-cracks were often observed at the splat boundaries, as indicated with an arrow. 3.3. Effect of microstructure on thermal diffusivity
Fig. 5. TEM micrograph of planar section of the nanostructured Al2O3–3 wt.% TiO2 coating.
another kind of equiaxed grain was observed, as shown in Fig. 5. These grains were less than 200 nm in size and their boundaries were more distinct. SAD detected that the finer equiaxed grains took the modification of g-Al2O3. The microstructure represented a cross-section through a number of columnar grains in a single lamella. Fig. 6 shows the typical TEM micrographs of the conventional coating Al2O3–3 wt.% TiO2 coating without thermal treatment. The microstructure of lamellae was composed of columnar grains with the diameter of about 0.7 Am in the conventional coating, which were coarser than those in the nanostructured coating. In some areas, the interfaces between splat lamellae were not obvious and
Fig. 6. The typical TEM micrograph of the conventional Al2O3–3 wt.% TiO2 coating.
In oxides ceramics, heat is predominantly carried by lattice vibration and radiation contribution is insignificant at temperatures b1500 8C [12]. Phonon scattering at grain boundaries can restrain the heat flow [13]. With the increase of the concentration of grain boundaries, phonon scattering is enhanced and the thermal conductivity of material decreases [3]. In the nanostructured Al2O3–3 wt.% TiO2 coating, the presence of equiaxed grains increased the number of grain boundaries. In addition, splat lamellae in the conventional coating were thicker than those in the nanostructured coating (Figs. 4a and 6). This indicated there were more interfaces between splat lamellae in the nanostructured coating with the same thickness. When the melted powders impinged with a curved or irregular surface, columnar grains consisting of splat lamellae exhibited a radial arrangement, as shown in Fig. 4a. The finer columnar grains in the nanostructured coating also increased the interfaces crossed by heat flow path. As a result, phonon scattering on grain boundaries contributed to the decrease of thermal diffusivity of the nanostructured Al2O3–3 wt.% TiO2 coating. TiO2 was prone to react with Al2O3 to form solid solution g-Al2O3d TiO2 during plasma spraying due to the higher contact surface area between nano-sized alumina and titania particles and Ti ions were likely in the g-Al2O3 lattice [14,15]. EDS spectrum also showed that g-Al2O3 grains contained Ti element in the nanostructured Al2O3–3 wt.% TiO2 coating. It has been well known that defective crystal structures result in greater scattering of the lattice vibration and shorter phonon mean free path, which is
Fig. 7. Result of the differential thermal analysis (DTA) measurement for the conventional Al2O3–3 wt.% TiO2 coating.
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material, V is the volume fraction of ellipsoidal pores, b is the major axis of the ellipsoid, and a is the minor axis. Though the formation of intermittent necking has no influence on the porosity, it results in the drastic decrease of the ratio of b to a and, accordingly, the thermal conductivity increases. Thus, it was considered that the phenomenon that thermal diffusivity increase during cooling was not apparent for the nanostructured coating was related to the lack of narrow long cracks (Fig. 4a).
4. Conclusions
Fig. 8. TEM micrograph of the conventional Al2O3–3 wt.% TiO2 coating after heat treatment under the same condition as that during measuring the thermal diffusivity.
responsible for the low thermal conductivity of YSZ [3]. Thus, the decrease in thermal diffusivity of the nanostructured coating is also relative to that of the continuity of crystal lattice which was destroyed due to the existence of Ti in g-Al2O3 grains. It was always observed that heat treatment at high temperatures resulted in the increase of thermal diffusivity of plasma-sprayed thermal barrier coatings [4, 16]. As shown in Fig. 4, the similar phenomenon occurred that the conventional Al2O3–3 wt.% TiO2 coating exhibited an increase in thermal diffusivity during cooling compared to during heating. TDA analysis revealed that there was an exothermic peak at 1100 8C (see Fig. 7), which corresponded to the aAl2O3 formation [17]. This indicated no phase-transformation for the conventional coating at temperatures less than 1000 8C. In addition, normalized bulk density nearly had no variations during measuring. The increase of thermal diffusivity during cooling was not related to the change of density and phase. A large population of narrow long microcracks existed between splat lamellae in the conventional monolithic coating (see Fig. 6). It has reported that during heat treatment, the formation of intermittent particle necking across these narrow micro-cracks results in the increase of the thermal diffusivity of plasma sprayed coatings [16]. In our experiment, the similar phenomenon that part of narrows cracks between splat lamellae were interrupted was also observed in the conventional coating after thermal treatment under the condition identical to that during measuring the thermal diffusivity, as shown with an arrow in Fig. 8. According to Hasselman model [18], thermal conductivity of the micro-cracked material is relative to both porosity and shape of pore as follows: k=k0 ¼ ð1 þ ð2V =pÞðb=aÞÞ1
ð1Þ
where k is the thermal conductivity of the micro-cracked material, k 0 is the thermal conductivity of the uncracked
In this paper, the influence of microstructure on thermal diffusivity of the nanostructured and conventional Al2O3–3 wt.% TiO2 monolithic coatings was investigated. The thermal diffusivity of the nanostructured coating was lower than that of the conventional coating. The decrease of thermal diffusivity was relative to the existence of equiaxed grains and finer columnar grains consisting of splat lamellae in the nanostructured coating. The defective crystal structure resulting from the solid solution of Ti in g-Al2O3 and the decrease in thickness of splat lamellae also contributed to the decrease in thermal conductivity. During cooling, the thermal diffusivity of the nanostructured coating nearly had no variation compared to during heating, while for the conventional coating, the thermal diffusivity increased by about 5% during cooling. It was considered that the formation of intermittent particle necking across microcracks between lamellae resulted in the increase of thermal diffusivity of the conventional coating during cooling.
Acknowledgements The authors gratefully acknowledge Engineer Xiaming Zhou and Yefang Zhang for the preparation of samples. The authors also thank Dr. Huang Chen and Dr. Weichang Xue for stimulating discussions.
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