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AFM investigations of the morphology features and local mechanical properties of HTS YBCO thin films
Physica C 408–410 (2004) 846–847 www.elsevier.com/locate/physc
AFM investigations of the morphology features and local mechanical properties of HTS YBCO thin films Yakov M. Soifer, Armen Verdyan *, Igor Lapsker, Jacob Azoulay Sciences Department, Holon Academic Institute of Technology, 52 Golomb Str., P.O. Box 305, Holon 58102, Israel
Abstract In the paper presented here the application of the atomic force microscope (AFM) is considered for evaluation of hardness and Young’s modulus of high Tc superconducting YBCO thin films of different thickness (from 0.05 to 1 lm) grown on unbuffered SrTiO3 (film I) and on sapphire with a buffer layer of CeO2 (film II). The best film features a transition temperature Tc of 90 K, critical current density Jc ðH ¼ 0Þ of 3 · 107 A/cm2 at 4.2 K and 2 · 106 A/cm2 at 77 K. The relationship between mechanical properties and microstructure of these films was investigated. It was found that all the films comprised well-defined Cu-rich precipitates of different size and with different density on their surface. For both type of films the hardness was measured to be in the range of 12–18 GPa. The Young’s modulus of the films was about 180–200 GPa. The nanoindentation and nanoscratching measurements showed that the mechanical strength of the films studied was determined mainly by mechanical failure and surface defects (secondary phases). 2004 Elsevier B.V. All rights reserved. Keywords: AFM; HTS thin films; Mechanical properties
YBCO thin films of different thickness (0,05 to 1 lm) on different substrates (ceramics––sapphire, MgO, SrTiO3 and metallic––Ni, Ag) were prepared by thermal evaporation and magnetron sputtering. The best quality films featuring a transition temperature Tc of 90 K, critical current density Jc ðH ¼ 0Þ of 3 · 107 A/cm2 at 4.2 K and 2 · 106 A/cm2 at 77K were obtained mainly by magnetron sputtering on a pure SrTiO3 (film I) and on sapphire buffered with CeO2 layer (film II). In this article, the study of morphological features and local mechanical properties of these two films is reported. Structure and surface morphology was measured and analyzed by X-ray diffraction, AFM and scanning electron microscopy techniques. The mechanical properties such as hardness, Young’s modulus, scratch resistance of thin YBCO films were evaluated by the depth-sensing indentation and nanoscratching methods with AFM technique (DI Dimen*
Corresponding author. Tel.: +972-502-6609; fax: +972-5026619. E-mail address: [email protected] (A. Verdyan).
sion 3100 equipped with a diamond tip mounted to the end of the metal cantilever). The indentation cantilever had a spring constant of 297 N/m, a resonant frequency of 78 kHz and could be used to image indented or scratched surfaces. Hardness ðH Þ and Young’s modulus ðEÞ were calculated from a load–displacement curve using Oliver and Pharr [1] approach. P H¼ ð1Þ A pffiffiffi p S 1 1 m2ind 1 m2 pffiffiffi ; ¼ þ ð2Þ Er ¼ Eind E 2 E A r where P is the load, A is the projected contact area at that load and S is the elastic constant stiffness. Er is the reduced elastic modulus, E is the elastic modulus and m is the Poisson’s ratio for the test material, Eind and mind are the elastic modulus and Poisson’s ratio of the indenter respectively. Indentations were made at loads ranging from 10 to 500 lN. However, for thin films, hardness alone is insufficient to predict their mechanical properties. Therefore scratch resistance test was carried out where a scratch hardness and critical failure parameters
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.217
Y.M. Soifer et al. / Physica C 408–410 (2004) 846–847
847
were evaluated. The scratch hardness Hs which is a measure of a deformation made normal to the scratch direction, is determined by: Hs ¼
8P pw2
ð3Þ
where P is the normal load and w the width of the scratch [2]. Scratching was made at different loads from 10 to 500 lN by the diamond tip at scratching rate from 0.05 to 10 mm/min and scratch length from 0.5 to 5 lm. All the measurements were carried out at room temperature. The SEM and AFM study has revealed that the YBCO films compromised secondary phase of CuO in form of semi- spherical shape particles of different size. Similar particles in YBCO films were reported previously [3]. The characteristic size of these particles varies from 100 to 500 nm for film I and from 25 to 300 nm for film II. The typical AFM image of the YBCO films is given in Fig. 1. The CuO particles are very high (up to 200 nm, which is comparable to the film thickness) and they cover almost 15–20% of the film surface and yet the current density is very high. The hardness values for both types of films are high and vary from 12 to 18 GPa and depend strongly on the local surface roughness. The Young’s modulus values calculated from the load–distance curves were measured to be 180 GPa for film I and 200 GPa for film II. The range difference in Young’s modulus values is quite conceivable considering the different morphology features and phase composition of film I and film II. Nanoscratch testings carried out at different normal forces show that scratch scar becomes visible only at loads exceeding 30–40 lN. This value can be considered as a critical load at which clear groove is formed, accompanied by formation of material pile-up at the sides of the scratch. This suggests that initial film damage that occurs at the critical load is due to ploughing associated with cutting and plastic deformation. The scratch resistance behavior of film II is illustrated in Fig. 2, where the scratch was made at load 100 lN. Despite exceeding of the critical load, no visible cracks are observed, however, debris (chips) are already seen in addition to material pile-up at the sides of the scratch. At load of 100 lN the depth of the scratch is
Fig. 2. Section analysis of the scratch made at P ¼ 100 lN in film I.
approximately 10–15 nm and the scratch width is 130 nm. From Eq. (3) the scratch hardness Hs is estimated to be 16 GPa which is very close to the hardness value evaluated from the nanoindentation test (see above). This is in good agreement with the predicted correlation between nanoindentation and nanoscratch hardness, thus supporting the analogies between the indentation and scratch processes [4]. The results obtained are of great importance for characterization and understanding the mechanical properties of YBCO films. Analysis of the data such as hardness, Young’s modulus, indent and scratch images show that at room temperature there are only slight differences between the mechanical properties of film I and film II, despite of the significant differences in the surface structure. It is likely due to the fact that in nanoindentation and nanoscratch test mostly the local mechanical properties of YBCO matrix are determined. Inclusions of CuO particle seems to play a more significant role at low temperature when the difference in thermo-mechanical properties between particles, films and substrate can lead to a development of large cracks and exfoliation. The experimental samples (film II) were supplied from Siemens Ag, Corporate Technology, Erlangen, Germany.
References
Fig. 1. D image of YBCO film II surface.
[1] [2] [3] [4]
W. Oliver, G. Pharr, J. Mater. Res. 7 (1992) 1564. B.J. Briscoe, Wear 200 (1996) 137. A.C. Westerheim, J. Appl. Phys. 75 (1994) 393. S. Sundarajan, B. Bhushan, J. Mater. Res. 16 (2001) 437.
AFM investigations of the morphology features and local mechanical properties of HTS YBCO thin films Yakov M. Soifer, Armen Verdyan *, Igor Lapsker, Jacob Azoulay Sciences Department, Holon Academic Institute of Technology, 52 Golomb Str., P.O. Box 305, Holon 58102, Israel
Abstract In the paper presented here the application of the atomic force microscope (AFM) is considered for evaluation of hardness and Young’s modulus of high Tc superconducting YBCO thin films of different thickness (from 0.05 to 1 lm) grown on unbuffered SrTiO3 (film I) and on sapphire with a buffer layer of CeO2 (film II). The best film features a transition temperature Tc of 90 K, critical current density Jc ðH ¼ 0Þ of 3 · 107 A/cm2 at 4.2 K and 2 · 106 A/cm2 at 77 K. The relationship between mechanical properties and microstructure of these films was investigated. It was found that all the films comprised well-defined Cu-rich precipitates of different size and with different density on their surface. For both type of films the hardness was measured to be in the range of 12–18 GPa. The Young’s modulus of the films was about 180–200 GPa. The nanoindentation and nanoscratching measurements showed that the mechanical strength of the films studied was determined mainly by mechanical failure and surface defects (secondary phases). 2004 Elsevier B.V. All rights reserved. Keywords: AFM; HTS thin films; Mechanical properties
YBCO thin films of different thickness (0,05 to 1 lm) on different substrates (ceramics––sapphire, MgO, SrTiO3 and metallic––Ni, Ag) were prepared by thermal evaporation and magnetron sputtering. The best quality films featuring a transition temperature Tc of 90 K, critical current density Jc ðH ¼ 0Þ of 3 · 107 A/cm2 at 4.2 K and 2 · 106 A/cm2 at 77K were obtained mainly by magnetron sputtering on a pure SrTiO3 (film I) and on sapphire buffered with CeO2 layer (film II). In this article, the study of morphological features and local mechanical properties of these two films is reported. Structure and surface morphology was measured and analyzed by X-ray diffraction, AFM and scanning electron microscopy techniques. The mechanical properties such as hardness, Young’s modulus, scratch resistance of thin YBCO films were evaluated by the depth-sensing indentation and nanoscratching methods with AFM technique (DI Dimen*
Corresponding author. Tel.: +972-502-6609; fax: +972-5026619. E-mail address: [email protected] (A. Verdyan).
sion 3100 equipped with a diamond tip mounted to the end of the metal cantilever). The indentation cantilever had a spring constant of 297 N/m, a resonant frequency of 78 kHz and could be used to image indented or scratched surfaces. Hardness ðH Þ and Young’s modulus ðEÞ were calculated from a load–displacement curve using Oliver and Pharr [1] approach. P H¼ ð1Þ A pffiffiffi p S 1 1 m2ind 1 m2 pffiffiffi ; ¼ þ ð2Þ Er ¼ Eind E 2 E A r where P is the load, A is the projected contact area at that load and S is the elastic constant stiffness. Er is the reduced elastic modulus, E is the elastic modulus and m is the Poisson’s ratio for the test material, Eind and mind are the elastic modulus and Poisson’s ratio of the indenter respectively. Indentations were made at loads ranging from 10 to 500 lN. However, for thin films, hardness alone is insufficient to predict their mechanical properties. Therefore scratch resistance test was carried out where a scratch hardness and critical failure parameters
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.217
Y.M. Soifer et al. / Physica C 408–410 (2004) 846–847
847
were evaluated. The scratch hardness Hs which is a measure of a deformation made normal to the scratch direction, is determined by: Hs ¼
8P pw2
ð3Þ
where P is the normal load and w the width of the scratch [2]. Scratching was made at different loads from 10 to 500 lN by the diamond tip at scratching rate from 0.05 to 10 mm/min and scratch length from 0.5 to 5 lm. All the measurements were carried out at room temperature. The SEM and AFM study has revealed that the YBCO films compromised secondary phase of CuO in form of semi- spherical shape particles of different size. Similar particles in YBCO films were reported previously [3]. The characteristic size of these particles varies from 100 to 500 nm for film I and from 25 to 300 nm for film II. The typical AFM image of the YBCO films is given in Fig. 1. The CuO particles are very high (up to 200 nm, which is comparable to the film thickness) and they cover almost 15–20% of the film surface and yet the current density is very high. The hardness values for both types of films are high and vary from 12 to 18 GPa and depend strongly on the local surface roughness. The Young’s modulus values calculated from the load–distance curves were measured to be 180 GPa for film I and 200 GPa for film II. The range difference in Young’s modulus values is quite conceivable considering the different morphology features and phase composition of film I and film II. Nanoscratch testings carried out at different normal forces show that scratch scar becomes visible only at loads exceeding 30–40 lN. This value can be considered as a critical load at which clear groove is formed, accompanied by formation of material pile-up at the sides of the scratch. This suggests that initial film damage that occurs at the critical load is due to ploughing associated with cutting and plastic deformation. The scratch resistance behavior of film II is illustrated in Fig. 2, where the scratch was made at load 100 lN. Despite exceeding of the critical load, no visible cracks are observed, however, debris (chips) are already seen in addition to material pile-up at the sides of the scratch. At load of 100 lN the depth of the scratch is
Fig. 2. Section analysis of the scratch made at P ¼ 100 lN in film I.
approximately 10–15 nm and the scratch width is 130 nm. From Eq. (3) the scratch hardness Hs is estimated to be 16 GPa which is very close to the hardness value evaluated from the nanoindentation test (see above). This is in good agreement with the predicted correlation between nanoindentation and nanoscratch hardness, thus supporting the analogies between the indentation and scratch processes [4]. The results obtained are of great importance for characterization and understanding the mechanical properties of YBCO films. Analysis of the data such as hardness, Young’s modulus, indent and scratch images show that at room temperature there are only slight differences between the mechanical properties of film I and film II, despite of the significant differences in the surface structure. It is likely due to the fact that in nanoindentation and nanoscratch test mostly the local mechanical properties of YBCO matrix are determined. Inclusions of CuO particle seems to play a more significant role at low temperature when the difference in thermo-mechanical properties between particles, films and substrate can lead to a development of large cracks and exfoliation. The experimental samples (film II) were supplied from Siemens Ag, Corporate Technology, Erlangen, Germany.
References
Fig. 1. D image of YBCO film II surface.
[1] [2] [3] [4]
W. Oliver, G. Pharr, J. Mater. Res. 7 (1992) 1564. B.J. Briscoe, Wear 200 (1996) 137. A.C. Westerheim, J. Appl. Phys. 75 (1994) 393. S. Sundarajan, B. Bhushan, J. Mater. Res. 16 (2001) 437.