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Nanoindentation and residual stress measurements of yttria-stablized zirconia composite coatings produced by electrophoretic deposition
Thin Solid Films 494 (2006) 223 – 227 www.elsevier.com/locate/tsf
Nanoindentation and residual stress measurements of yttria-stablized zirconia composite coatings produced by electrophoretic deposition X.-J. Lu, X. Wang, P. Xiao * Materials Science Centre, University of Manchester, Manchester M1 7HS, UK Available online 28 September 2005
Abstract Yttria stabilized zirconia (YSZ)/Al2O3 composite coatings were prepared by an electrophoretic deposition (EPD) method. The mechanical properties of the coatings were determined by nanoindentation and residual stresses were measured by Cr3+ fluorescence spectroscopy. It was found that the nanoindentation was sensitive to the presence of sub-micrometer pores. In order to determine the macroscopic mechanical properties, a penetration depth of >1000 nm was required due to the porous and inhomogeneous nature of the materials. Both microstress and mechanical properties exhibited a gradient across the coating thickness, which can be attributed to the density gradient caused by constrained sintering of the coatings. It has been confirmed that the mechanical properties determined by nanoindentation are not affected by the external applied stress. AFM examination confirmed there was no impression pile-up during indentation. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanoindentation; Stress; Coating; Hardness; Young’s modulus
1. Introduction Electrophoretic deposition (EPD) as a method for fabrication of ceramic coatings [1 – 5] has attracted increasing attention due to its advantages over alternative coating techniques: the capability of producing thin and thick coatings on complex shaped substrates, the simplicity of the process, low cost equipment and a high deposition rate. Two steps are involved in EPD coating methods: (1) charged particles suspended in a liquid migrate towards an electrode under an external electrical field and deposit on the metal substrate, forming green particulate coatings; (2) the green coatings are sintered in order to consolidate the coating and improve the coating/substrate adhesion. Unlike bulk ceramic bodies, the sintering of ceramic coatings on metal substrates is retarded by the constraint of metal substrates. As a consequence, coatings tend to form cracks or spall off during sintering, and a nonuniform density across the film thickness has been observed in EPD coatings [6]. The mechanical properties of coatings, such as stiffness and hardness, are essential in evaluating the coating performance. Various techniques have been developed for evaluating the * Corresponding author. E-mail address: [email protected] (P. Xiao). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.327
mechanical properties of coatings, among which, high-resolution, depth-sensing indentation technique has been successful in the examination of thermal barrier coatings [7 –9]. This method provides more information about the microstructure impact on the measured value [9]. It allows determination of mechanical properties locally on small structural features in the materials due to its small measurement length scale (micrometer or nanometer) and quasi-nondestructive nature. By nanoindentation, the mechanical properties in the coating plane have been observed to be significantly different from those in the normal direction [7,8]. Under many circumstances, the mechanical properties, especially the elastic modulus, are related to residual stresses in the coating. Residual stresses can also be related to coating failure [10 – 13]. For a ceramic coating prepared by EPD, the residual stress can arise from shrinkage during sintering and from thermal mismatch between coating and substrate on cooling from the sintering temperature. Cr3+ fluorescence spectroscopy has been employed to measure the stresses in alumina-containing ceramics [14 – 18]. This technique is based on an optical system and is able to probe microstructures at micrometer length scale with a resolution of ¨2 Am. It can be used to measure the local stress in ceramic coatings, which is difficult to achieve by using other conventional techniques [6].
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In this work, it was found that the mechanical properties and residual stresses in localized positions could be determined by nanoindentation and fluorescence spectroscopy. Both mechanical properties and residual stresses exhibit a gradient across the coating thickness. Mechanical properties determined by indentation have been found to be stress-independent. 2. Experimental details 2.1. Sample preparation The procedure for preparation of YSZ/Al2O3 coatings has been described in detail in a previous paper [6]; here only a brief account is given: 38 g YSZ powder (HSY-8, Daiichi Kigenso Kagaku Kogyo, Japan) and 2 g Al powder (< 6 Am, Alpoco, UK) were attrition-milled for 8 h in 200 ml acetylacetone (Aldrich, UK). After milling, the suspension was diluted 10-fold in acetylacetone for the EPD process. EPD was carried out by applying an electric field between a platinum electrode (as anode) and a substrate (cathode). The substrates were Fecralloy plates of 25 mm 10 mm 1 mm (Fe –22% Cr –5% Al– 1% Y –1% Zr, Goodfellow, UK). The green coatings were dried at room temperature for 24 h before they were sintered in air at 1250 -C for 4 h. For SEM and nanoindentation examinations, coatings were polished down to 1 Am roughness on both the top surface and the cross section. The microstructures of samples were examined by scanning electron microscope (XL30, Philip). The coating thickness is ¨200 Am.
of penetration depth. The load was held at 70% of a maximum load for 60 s to correct for the thermal drift. The tip geometry and machine compliance were calibrated prior to testing. Hardness and Young’s modulus were determined using the Oliver and Pharr analysis method [20]. The morphology of the indentation impression was examined by scanning electron microscope and atomic force microscope (AFM, Veeco Instruments Inc.). Both supported and freestanding coatings were examined by nanoindentation and fluorescence. Freestanding coating samples were prepared by chemical dissolution of the metal substrates in hydrochloric acid. 3. Results and discussion 3.1. Load– displacement curve The microstructure of PSZ/Al2O3 EPD coatings is shown in Fig. 1. It can be seen that coatings were porous and many small crack-like pores are present on the top surface (Fig. 1a). From the cross-section of the coatings (Fig. 1b), a layered structure appeared to have been formed, which was caused by the orientation of plate-like aluminium particles during EPD, which transformed completely to Al2O3 plate-like inclusions (¨2 Am thickness and ¨15 Am length) during sintering of the coating [21]. A typical indentation load – displacement curve for a PSZ/ Al2O3 EPD coating is shown in Fig. 2. A number of kinks are
2.2. Stress measurements by Cr3+ fluorescence spectroscopy The instrumentation for Cr3+ fluorescence measurements consisted of a modified optical microscope (Nikon), a laser system (He – Ne, 632.8 nm wave length, 15 mW in power), a double spectrometer (Glen, Creston) and a charge coupled device camera (CCD, Wright instruments, peltier cooled). A 50 objective lens and 2 s collecting time were used for all the measurements. The R-line spectra were recorded and analyzed by commercial software (Renishaw Wire). The spectra were fitted by Gaussian-Lorentzian functions. At least 20 measurements were made to get an average value for each datum. The R1 line shift has been found to have a nonlinear dependence on strain, whereas the R2 line shift has a fairly good linearity under compression regime [19]. For this reason, the stress calculation in this work was based on the R2 line shift. 2.3. Nanoindentation Nanoindentation experiments were performed employing a nano-indenterR XP (Nano Instruments, MTS systems corporation, USA). Indentation was made using a Berkovich indenter calibrated with a standard silica specimen by running a standard continuous stiffness measurement (CSM). The CSM continuously measures the stiffness and allows the hardness and elastic modulus to be determined as a continuous function
Fig. 1. SEM secondary electron micrograph (a) top surface, (b) cross-section of PSZ/Al2O3 EPD coatings, schematic of indentation impression with a penetration depth of 2000 nm being also shown.
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Fig. 3 rendering an increasing trend. Therefore the huge noise in small depth regime and the uncertainty prevent any correlation to the indentation size effect. Fig. 3 indicates that a displacement of >1000 nm would give measured mechanical properties independent of contact area. Material effects scale with the extent of deformed volume or area, known as indentation size effect [8]. To determine the macroscopic mechanical properties of a material, the equivalent contact radius for an indenter should be approximately one order of magnitude larger than the characteristic length scale [23,24]. In this work, the grain size in the coatings was measured as about 400 nm by SEM observation. An indentation depth of 1000 nm would result in an equivalent contact diameter of about 5 Am. This appears to agree with the requirements for the macroscopic mechanical property measurement. The residual stress was measured by Cr3+ fluorescence spectroscopy. The residual stress of freestanding YSZ/Al2O3 coatings measured by fluorescence spectroscopy is due to the thermal mismatch between YSZ and Al2O3. It is termed microthermal mismatch stress [6]. The measurement positions for residual stress and nanoindentation on the cross-section and top surface are schematically shown as an inset in Fig. 4, where the distance between measurement points was at least 30 Am. Twenty measurement points were made along each parallel line in order to correlate the micro-stress and mechanical properties with the distance from coating/substrate interface. The microthermal mismatch stress as a function of distance from the interface is shown in Fig. 4 and the hardness and Young’s modulus of the coatings as a function of distance from interface are shown in Fig. 5, where the hardness and Young’s modulus measured on the top surface are also given. A striking feature of Fig. 5 is the large data error bars. The large scatter of data can be attributed to the inhomogeneity of the coating microstructure. A penetration depth of 2000 nm results in an indentation area of about f10 Am (refer to Fig. 1a), which appears to be large enough to cover the microstructural features on the top surface, but not on the cross-section (refer to Fig. 1b). Therefore, the data scatter from the top surface was significantly smaller than that from the cross-section.
By using continuous stiffness measurement (CSM) mode, nano-indenterR XP allows the hardness and elastic modulus to be determined as a function of penetration depth. The measured hardness and Young’s modulus were found to depend on the penetration depth. As shown in Fig. 3, within the range of < 600 nm of displacement, both the hardness and the Young’s modulus decreased sharply as displacement (i.e., contact area) increased. When the indentation contact area was small enough, i.e. the displacement was ¨100 nm, the hardness and Young’s modulus were measured as ¨10 GPa and ¨120 GPa, respectively, which are commensurate with the values of the fully dense counterpart [22]. A similar observation was made by Thompson and Clyne on plasma sprayed TBCs [7]. It’s worthy to mention that there is no fixed trend identified for the first 100-nm indentation depth from different measurements on EPD PSZ/Al2O3 coatings even though the curve showed in
Fig. 3. Measured hardness and Young’s modulus as a function of penetration depth.
Fig. 2. Typical load – displacement curve for YSZ/Al2O3 EPD coatings, with the inset being the load – displacement curve for dense YSZ/Al2O3 bulk samples.
present in the curve. These kinks may be due to the presence of pores or the presence of two different phases in the ZrO2/Al2O3 composite coatings. The presence of pores might cause microcollapse underneath the indenter, which would lead to a sudden change in the strain – stress response. Similarly, when the indenter approaches a phase with different elastic properties, the slope in the load –displacement curve would also be affected. A PSZ/Al2O3 composite bulk sample without metal substrate was sintered at 1450 -C and achieved >95 wt.% theoretical density. This sample was examined by indentation and its load – displacement curve (inset in Fig. 2) was found to be fairly smooth and kink-free. This implies that the kinks in Fig. 2 must be mainly due to the presence of the pores. When coatings sintered at different temperatures were examined further by nanoindentation, it was found that the low temperature sintered samples tended to generate the load – displacement curve with kinks, in comparison with the curves generated from the high temperature sintered ones. Therefore, the load – displacement curve is sensitive to the presence of pores at sub-micrometer length scale.
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Fig. 4. Micro-thermal mismatch stress across the thickness, with an inset being the schematic of measurement positions for nanoindentation and fluorescence spectroscopy measurements.
Despite large scatter in data, both hardness and Young’s modulus exhibited an increase as the measurement was moved from a near interface position to a near top surface position. Whether the coating was supported or freestanding did not seem to have an effect on the measured mechanical properties. It is noteworthy that the mechanical properties, especially Young’s modulus, measured on the top surface of the coating, were significantly higher than those measured on the cross-section. It is interesting to note that the micro-thermal mismatch stress (Fig. 4) presented a similar trend to the hardness and Young’s modulus as a function of distance from interface (Fig. 5), while the standard deviation of the microstress was slightly smaller than that of mechanical properties. Our previous work [6] found that the constrained sintering of EPD coatings led to a density gradient across the coating thickness. The region near the top surface is better sintered than that near the interface. It was also found that this micro-thermal mismatch stress can be related to local density of the materials: a high density results in
Fig. 5. Hardness and Young’s modulus as a function of distance from the interface.
Fig. 6. Macroscopic residual stress and hardness as a function of substrate thickness.
a high micro-stress in the freestanding coatings [6]. Therefore, the stress gradient in Fig. 4 actually can be attributed to a density variation across the thickness. For the same reason, the variation in Young’s modulus and hardness (Fig. 5) can also be attributed to the variation of local density, if the influence of macroscopic residual stress on mechanical property measurement can be excluded. The significantly higher mechanical properties obtained on the top surface as shown in Fig. 5 might be due to a nearly constraint-free sintering of the materials at the top surface. 3.3. Effect of macroscopic residual stress on measured mechanical properties Previous research found that the mechanical properties of materials measured by indentation could be affected by the residual stress [25 –29]. Tensile stress could decrease the hardness, while the compressive stress could increase the hardness. The hardness of aluminium films was found to increase as tensile stress decreased [28]. In this work, the supported coatings gave similar mechanical properties to those of the freestanding coatings as shown in Fig. 5. This implies that the macroscopic residual stress did not have significant influence on the measured mechanical properties. As the supported coatings had a compressive stress arising from thermal mismatch between the substrate and the
Fig. 7. AFM image of indentation impression of a cross-sectional coating.
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coating, while the freestanding coatings did not, the supported coatings should have shown a higher measured hardness than the freestanding coating if there had been a residual stress effect. In order to look further into the effect of the macroscopic residual stress, the substrate of PSZ/Al2O3 coatings was thinned gradually by grinding. The thinning of the substrate would result in a partial relaxation of the macroscopic stress caused by thermal mismatch between coating and substrate. The macroscopic residual stress can be calculated by [6]: free Dm sup R2 Dm R2 th r ð1Þ ¯ macro ¼ 5:06 1 þ Bp sup free where Dm R2 is the R2 line shift of supported coatings, Dm R2 is the R2 line shift of freestanding coatings, B p is the load transfer coefficient which was calculated to be 0.573 for YSZ/Al2O3 EPD coatings [6]. As shown in Fig. 6, the macroscopic residual stress increased with increasing thickness of substrates, while the measured hardness was fairly constant. Therefore we can conclude that the measured mechanical properties of the PSZ/ Al2O3 coating by nanoindentation were not affected by the macroscopic residual stress. It is known that impression pile-up affects the hardness measurement by indentation [27,29]. Impression made on a PSZ/Al2O3 coating was examined by AFM, for which the image is shown in Fig. 7. Based on the AFM examination, no pile-up was found around the indent. For this reason, the mechanical properties of the coating determined by indentation should not be affected by the macroscopic residual stress.
4. Conclusion (1) Nanoindentation is sensitive to the presence of submicrometer pores in the materials. Due to the porous and inhomogeneous nature of the PSZ/Al2O3 EPD coatings, a penetration depth larger than 1000 nm was required in order to determine the global mechanical properties by indentation. (2) Mechanical properties were found to present a gradient across the coating thickness, which is similar to the gradient of micro-thermal mismatch stress. The gradient in mechanical properties and microstress can be attributed to the density gradient caused by constrained coating sintering.
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(3) Mechanical properties measured by nanoindentation were found to be stress-independent. AFM confirmed there was no impression pile-up. Acknowledgment The authors would like to thank Prof. R. Young for the use of the spectroscopy instrument in his laboratory. This research is supported by EPSRC under Contract No. GR/R32215. References [1] Z. Wang, J. Shemilt, P. Xiao, J. Eur. Ceram. Soc. 22 (2002) 183. [2] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251. [3] C. Kaya, F. Kaya, B. Su, B. Thomas, A.R. Boccaccini, Surf. Coat. Technol. 191 (2004) 303. [4] S.Y. Ng, A.R. Boccaccini, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 116 (2004) 208. [5] B. Ferrari, I. Santacruz, M.I. Nieto, R. Moreno, J. Eur. Ceram. Soc. 24 (2004) 3073. [6] X. Wang, P. Xiao, Acta Mater. 52 (2004) 2591. [7] J.A. Thompson, T.W. Clyne, Acta Mater. 49 (2001) 1565. [8] J. Malzbender, R.W. Steinbrech, J. Mater. Res. 18 (2003) 1975. [9] E. Lugscheider, K. Bobzin, A. Barwulf, A. Etzkorn, Surf. Coat. Technol. 138 (2001) 9. [10] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505. [11] X. Bi, H. Xu, S. Gong, Surf. Coat. Technol. 130 (2000) 122. [12] X. Chen, J.W. Hutchinson, M.Y. He, A.G. Evans, Acta Mater. 51 (2003) 2017. [13] T. Tomimatsu, S. Zhu, Y. Kagawa, Acta Mater. 51 (2003) 2397. [14] A. Selcuk, A. Atkinson, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 335 (2002) 147. [15] D.M. Lipkin, D.R. Clarke, J. Appl. Phys. 77 (1995) 1855. [16] K.W. Schlichting, K. Vaidyanathan, Y.H. Sohn, E.H. Jordan, M. Gell, N.P. Padture, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 291 (2000) 68. [17] Q. Ma, D.R. Clarke, J. Am. Ceram. Soc. 77 (1994) 298. [18] D.M. Lipkin, D.R. Clarke, Oxid. Met. 45 (1996) 267. [19] X.A. Shen, Y.M. Gupta, Phys. Rev., B 48 (1993) 2929. [20] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [21] Y. Yuan, X. Wang, P. Xiao, J. Eur. Ceram. Soc. 24 (2004) 2233. [22] R.G. Wellman, A. Dyer, J.R. Nicholls, Surf. Coat. Technol. 176 (2004) 253. [23] A.E. Giannakopoulos, Thin Solid Films 332 (1998) 172. [24] T. Nakamura, G. Quina, C.C. Berndt, J. Am. Ceram. Soc. 83 (2000) 578. [25] G. Sines, R. Carlson, ASTM Bull. 180 (1952) 35. [26] T.R. Simes, S.G. Mellor, D.A. Hills, J. Strain Anal. 19 (1984) 135. [27] T.Y. Tsui, W.C. Oliver, G.M. Pharr, J. Mater. Res. 11 (1996) 752. [28] W.R. LaFontaine, C.A. Paszkiet, M.A. Korhonen, C. Li, J. Mater. Res. 6 (1991) 2084. [29] W.C. Oliver, G.M. Pharr, J. Mater. Res. 19 (2004) 3.
Nanoindentation and residual stress measurements of yttria-stablized zirconia composite coatings produced by electrophoretic deposition X.-J. Lu, X. Wang, P. Xiao * Materials Science Centre, University of Manchester, Manchester M1 7HS, UK Available online 28 September 2005
Abstract Yttria stabilized zirconia (YSZ)/Al2O3 composite coatings were prepared by an electrophoretic deposition (EPD) method. The mechanical properties of the coatings were determined by nanoindentation and residual stresses were measured by Cr3+ fluorescence spectroscopy. It was found that the nanoindentation was sensitive to the presence of sub-micrometer pores. In order to determine the macroscopic mechanical properties, a penetration depth of >1000 nm was required due to the porous and inhomogeneous nature of the materials. Both microstress and mechanical properties exhibited a gradient across the coating thickness, which can be attributed to the density gradient caused by constrained sintering of the coatings. It has been confirmed that the mechanical properties determined by nanoindentation are not affected by the external applied stress. AFM examination confirmed there was no impression pile-up during indentation. D 2005 Elsevier B.V. All rights reserved. Keywords: Nanoindentation; Stress; Coating; Hardness; Young’s modulus
1. Introduction Electrophoretic deposition (EPD) as a method for fabrication of ceramic coatings [1 – 5] has attracted increasing attention due to its advantages over alternative coating techniques: the capability of producing thin and thick coatings on complex shaped substrates, the simplicity of the process, low cost equipment and a high deposition rate. Two steps are involved in EPD coating methods: (1) charged particles suspended in a liquid migrate towards an electrode under an external electrical field and deposit on the metal substrate, forming green particulate coatings; (2) the green coatings are sintered in order to consolidate the coating and improve the coating/substrate adhesion. Unlike bulk ceramic bodies, the sintering of ceramic coatings on metal substrates is retarded by the constraint of metal substrates. As a consequence, coatings tend to form cracks or spall off during sintering, and a nonuniform density across the film thickness has been observed in EPD coatings [6]. The mechanical properties of coatings, such as stiffness and hardness, are essential in evaluating the coating performance. Various techniques have been developed for evaluating the * Corresponding author. E-mail address: [email protected] (P. Xiao). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.327
mechanical properties of coatings, among which, high-resolution, depth-sensing indentation technique has been successful in the examination of thermal barrier coatings [7 –9]. This method provides more information about the microstructure impact on the measured value [9]. It allows determination of mechanical properties locally on small structural features in the materials due to its small measurement length scale (micrometer or nanometer) and quasi-nondestructive nature. By nanoindentation, the mechanical properties in the coating plane have been observed to be significantly different from those in the normal direction [7,8]. Under many circumstances, the mechanical properties, especially the elastic modulus, are related to residual stresses in the coating. Residual stresses can also be related to coating failure [10 – 13]. For a ceramic coating prepared by EPD, the residual stress can arise from shrinkage during sintering and from thermal mismatch between coating and substrate on cooling from the sintering temperature. Cr3+ fluorescence spectroscopy has been employed to measure the stresses in alumina-containing ceramics [14 – 18]. This technique is based on an optical system and is able to probe microstructures at micrometer length scale with a resolution of ¨2 Am. It can be used to measure the local stress in ceramic coatings, which is difficult to achieve by using other conventional techniques [6].
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In this work, it was found that the mechanical properties and residual stresses in localized positions could be determined by nanoindentation and fluorescence spectroscopy. Both mechanical properties and residual stresses exhibit a gradient across the coating thickness. Mechanical properties determined by indentation have been found to be stress-independent. 2. Experimental details 2.1. Sample preparation The procedure for preparation of YSZ/Al2O3 coatings has been described in detail in a previous paper [6]; here only a brief account is given: 38 g YSZ powder (HSY-8, Daiichi Kigenso Kagaku Kogyo, Japan) and 2 g Al powder (< 6 Am, Alpoco, UK) were attrition-milled for 8 h in 200 ml acetylacetone (Aldrich, UK). After milling, the suspension was diluted 10-fold in acetylacetone for the EPD process. EPD was carried out by applying an electric field between a platinum electrode (as anode) and a substrate (cathode). The substrates were Fecralloy plates of 25 mm 10 mm 1 mm (Fe –22% Cr –5% Al– 1% Y –1% Zr, Goodfellow, UK). The green coatings were dried at room temperature for 24 h before they were sintered in air at 1250 -C for 4 h. For SEM and nanoindentation examinations, coatings were polished down to 1 Am roughness on both the top surface and the cross section. The microstructures of samples were examined by scanning electron microscope (XL30, Philip). The coating thickness is ¨200 Am.
of penetration depth. The load was held at 70% of a maximum load for 60 s to correct for the thermal drift. The tip geometry and machine compliance were calibrated prior to testing. Hardness and Young’s modulus were determined using the Oliver and Pharr analysis method [20]. The morphology of the indentation impression was examined by scanning electron microscope and atomic force microscope (AFM, Veeco Instruments Inc.). Both supported and freestanding coatings were examined by nanoindentation and fluorescence. Freestanding coating samples were prepared by chemical dissolution of the metal substrates in hydrochloric acid. 3. Results and discussion 3.1. Load– displacement curve The microstructure of PSZ/Al2O3 EPD coatings is shown in Fig. 1. It can be seen that coatings were porous and many small crack-like pores are present on the top surface (Fig. 1a). From the cross-section of the coatings (Fig. 1b), a layered structure appeared to have been formed, which was caused by the orientation of plate-like aluminium particles during EPD, which transformed completely to Al2O3 plate-like inclusions (¨2 Am thickness and ¨15 Am length) during sintering of the coating [21]. A typical indentation load – displacement curve for a PSZ/ Al2O3 EPD coating is shown in Fig. 2. A number of kinks are
2.2. Stress measurements by Cr3+ fluorescence spectroscopy The instrumentation for Cr3+ fluorescence measurements consisted of a modified optical microscope (Nikon), a laser system (He – Ne, 632.8 nm wave length, 15 mW in power), a double spectrometer (Glen, Creston) and a charge coupled device camera (CCD, Wright instruments, peltier cooled). A 50 objective lens and 2 s collecting time were used for all the measurements. The R-line spectra were recorded and analyzed by commercial software (Renishaw Wire). The spectra were fitted by Gaussian-Lorentzian functions. At least 20 measurements were made to get an average value for each datum. The R1 line shift has been found to have a nonlinear dependence on strain, whereas the R2 line shift has a fairly good linearity under compression regime [19]. For this reason, the stress calculation in this work was based on the R2 line shift. 2.3. Nanoindentation Nanoindentation experiments were performed employing a nano-indenterR XP (Nano Instruments, MTS systems corporation, USA). Indentation was made using a Berkovich indenter calibrated with a standard silica specimen by running a standard continuous stiffness measurement (CSM). The CSM continuously measures the stiffness and allows the hardness and elastic modulus to be determined as a continuous function
Fig. 1. SEM secondary electron micrograph (a) top surface, (b) cross-section of PSZ/Al2O3 EPD coatings, schematic of indentation impression with a penetration depth of 2000 nm being also shown.
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3.2. Hardness and Young’s modulus
Fig. 3 rendering an increasing trend. Therefore the huge noise in small depth regime and the uncertainty prevent any correlation to the indentation size effect. Fig. 3 indicates that a displacement of >1000 nm would give measured mechanical properties independent of contact area. Material effects scale with the extent of deformed volume or area, known as indentation size effect [8]. To determine the macroscopic mechanical properties of a material, the equivalent contact radius for an indenter should be approximately one order of magnitude larger than the characteristic length scale [23,24]. In this work, the grain size in the coatings was measured as about 400 nm by SEM observation. An indentation depth of 1000 nm would result in an equivalent contact diameter of about 5 Am. This appears to agree with the requirements for the macroscopic mechanical property measurement. The residual stress was measured by Cr3+ fluorescence spectroscopy. The residual stress of freestanding YSZ/Al2O3 coatings measured by fluorescence spectroscopy is due to the thermal mismatch between YSZ and Al2O3. It is termed microthermal mismatch stress [6]. The measurement positions for residual stress and nanoindentation on the cross-section and top surface are schematically shown as an inset in Fig. 4, where the distance between measurement points was at least 30 Am. Twenty measurement points were made along each parallel line in order to correlate the micro-stress and mechanical properties with the distance from coating/substrate interface. The microthermal mismatch stress as a function of distance from the interface is shown in Fig. 4 and the hardness and Young’s modulus of the coatings as a function of distance from interface are shown in Fig. 5, where the hardness and Young’s modulus measured on the top surface are also given. A striking feature of Fig. 5 is the large data error bars. The large scatter of data can be attributed to the inhomogeneity of the coating microstructure. A penetration depth of 2000 nm results in an indentation area of about f10 Am (refer to Fig. 1a), which appears to be large enough to cover the microstructural features on the top surface, but not on the cross-section (refer to Fig. 1b). Therefore, the data scatter from the top surface was significantly smaller than that from the cross-section.
By using continuous stiffness measurement (CSM) mode, nano-indenterR XP allows the hardness and elastic modulus to be determined as a function of penetration depth. The measured hardness and Young’s modulus were found to depend on the penetration depth. As shown in Fig. 3, within the range of < 600 nm of displacement, both the hardness and the Young’s modulus decreased sharply as displacement (i.e., contact area) increased. When the indentation contact area was small enough, i.e. the displacement was ¨100 nm, the hardness and Young’s modulus were measured as ¨10 GPa and ¨120 GPa, respectively, which are commensurate with the values of the fully dense counterpart [22]. A similar observation was made by Thompson and Clyne on plasma sprayed TBCs [7]. It’s worthy to mention that there is no fixed trend identified for the first 100-nm indentation depth from different measurements on EPD PSZ/Al2O3 coatings even though the curve showed in
Fig. 3. Measured hardness and Young’s modulus as a function of penetration depth.
Fig. 2. Typical load – displacement curve for YSZ/Al2O3 EPD coatings, with the inset being the load – displacement curve for dense YSZ/Al2O3 bulk samples.
present in the curve. These kinks may be due to the presence of pores or the presence of two different phases in the ZrO2/Al2O3 composite coatings. The presence of pores might cause microcollapse underneath the indenter, which would lead to a sudden change in the strain – stress response. Similarly, when the indenter approaches a phase with different elastic properties, the slope in the load –displacement curve would also be affected. A PSZ/Al2O3 composite bulk sample without metal substrate was sintered at 1450 -C and achieved >95 wt.% theoretical density. This sample was examined by indentation and its load – displacement curve (inset in Fig. 2) was found to be fairly smooth and kink-free. This implies that the kinks in Fig. 2 must be mainly due to the presence of the pores. When coatings sintered at different temperatures were examined further by nanoindentation, it was found that the low temperature sintered samples tended to generate the load – displacement curve with kinks, in comparison with the curves generated from the high temperature sintered ones. Therefore, the load – displacement curve is sensitive to the presence of pores at sub-micrometer length scale.
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Fig. 4. Micro-thermal mismatch stress across the thickness, with an inset being the schematic of measurement positions for nanoindentation and fluorescence spectroscopy measurements.
Despite large scatter in data, both hardness and Young’s modulus exhibited an increase as the measurement was moved from a near interface position to a near top surface position. Whether the coating was supported or freestanding did not seem to have an effect on the measured mechanical properties. It is noteworthy that the mechanical properties, especially Young’s modulus, measured on the top surface of the coating, were significantly higher than those measured on the cross-section. It is interesting to note that the micro-thermal mismatch stress (Fig. 4) presented a similar trend to the hardness and Young’s modulus as a function of distance from interface (Fig. 5), while the standard deviation of the microstress was slightly smaller than that of mechanical properties. Our previous work [6] found that the constrained sintering of EPD coatings led to a density gradient across the coating thickness. The region near the top surface is better sintered than that near the interface. It was also found that this micro-thermal mismatch stress can be related to local density of the materials: a high density results in
Fig. 5. Hardness and Young’s modulus as a function of distance from the interface.
Fig. 6. Macroscopic residual stress and hardness as a function of substrate thickness.
a high micro-stress in the freestanding coatings [6]. Therefore, the stress gradient in Fig. 4 actually can be attributed to a density variation across the thickness. For the same reason, the variation in Young’s modulus and hardness (Fig. 5) can also be attributed to the variation of local density, if the influence of macroscopic residual stress on mechanical property measurement can be excluded. The significantly higher mechanical properties obtained on the top surface as shown in Fig. 5 might be due to a nearly constraint-free sintering of the materials at the top surface. 3.3. Effect of macroscopic residual stress on measured mechanical properties Previous research found that the mechanical properties of materials measured by indentation could be affected by the residual stress [25 –29]. Tensile stress could decrease the hardness, while the compressive stress could increase the hardness. The hardness of aluminium films was found to increase as tensile stress decreased [28]. In this work, the supported coatings gave similar mechanical properties to those of the freestanding coatings as shown in Fig. 5. This implies that the macroscopic residual stress did not have significant influence on the measured mechanical properties. As the supported coatings had a compressive stress arising from thermal mismatch between the substrate and the
Fig. 7. AFM image of indentation impression of a cross-sectional coating.
X.-J. Lu et al. / Thin Solid Films 494 (2006) 223 – 227
coating, while the freestanding coatings did not, the supported coatings should have shown a higher measured hardness than the freestanding coating if there had been a residual stress effect. In order to look further into the effect of the macroscopic residual stress, the substrate of PSZ/Al2O3 coatings was thinned gradually by grinding. The thinning of the substrate would result in a partial relaxation of the macroscopic stress caused by thermal mismatch between coating and substrate. The macroscopic residual stress can be calculated by [6]: free Dm sup R2 Dm R2 th r ð1Þ ¯ macro ¼ 5:06 1 þ Bp sup free where Dm R2 is the R2 line shift of supported coatings, Dm R2 is the R2 line shift of freestanding coatings, B p is the load transfer coefficient which was calculated to be 0.573 for YSZ/Al2O3 EPD coatings [6]. As shown in Fig. 6, the macroscopic residual stress increased with increasing thickness of substrates, while the measured hardness was fairly constant. Therefore we can conclude that the measured mechanical properties of the PSZ/ Al2O3 coating by nanoindentation were not affected by the macroscopic residual stress. It is known that impression pile-up affects the hardness measurement by indentation [27,29]. Impression made on a PSZ/Al2O3 coating was examined by AFM, for which the image is shown in Fig. 7. Based on the AFM examination, no pile-up was found around the indent. For this reason, the mechanical properties of the coating determined by indentation should not be affected by the macroscopic residual stress.
4. Conclusion (1) Nanoindentation is sensitive to the presence of submicrometer pores in the materials. Due to the porous and inhomogeneous nature of the PSZ/Al2O3 EPD coatings, a penetration depth larger than 1000 nm was required in order to determine the global mechanical properties by indentation. (2) Mechanical properties were found to present a gradient across the coating thickness, which is similar to the gradient of micro-thermal mismatch stress. The gradient in mechanical properties and microstress can be attributed to the density gradient caused by constrained coating sintering.
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