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Ni3Al composites determined by the nanoindentation technique
Applied Surface Science 189 (2002) 72±77
Nanohardness and elastic modulus at the interface of TiCx/Ni3Al composites determined by the nanoindentation technique Wenshen Huaa,*, Xingfang Wua, Dianhong Shenb, Hua Lub, M. Polakc a
Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, PR China State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China c Department of Material and Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel
b
Received 15 August 2001; accepted 3 December 2001
Abstract The mechanical properties at the interfaces of TiCx/Ni3Al composites of four different C/Ti ratios were investigated using the nanoindentation technique. We ®nd that the nanohardness and the elastic modulus from the phase TiCx to the phase Ni3Al are a gradient distribution. As x increased from 0.6 to 0.9, the elastic modulus at the interface decreased. The interfacial nanohardness reached its peak value at x equal to 0.7. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanoindentation; Nanohardness; Elastic modulus; Composite; Interface
1. Introduction As is well known, the fracture resistance of the brittle ceramic TiC can be improved signi®cantly by incorporating ductile metal reinforcements. In this present work, the ordered intermetallic compound nickel aluminide (Ni3Al) was selected to fabricate TiCx/Ni3Al composites using a simple pressureless melt-in®ltration process. The choice of Ni3Al as a binder was based on its exceptional high temperature strength and its wettability with the titanium carbide. In principle, this composite is a candidate for high temperature applications up to about 1000 8C [1±3]. The strength of the phase boundary is crucial to loadbearing composite materials. The conventional tensile experiment was designed to analyze the toughing *
Corresponding author.
mechanism qualitatively by observing the crack extension and the fracture surfaces. But the nanoindentation technique [4±9], which developed during the past several years, has made analysis more precise by combining scanning force microscopy (SFM) with the mechanical properties microprobe (MPM) to locate the indentation sites and image the indentation imprints. As a powerful tool to be developed for testing mechanical properties on the nanometer scale, this approach can give the elastic modulus and nanohardness directly. Therefore, it has seen more and more applications in ®elds such as semiconductors, biomaterials, nanostructured materials, and surface science [10±15]. In this work, the mechanical properties at nanosized interfaces of TiCx/Ni3Al composites of different C/Ti ratios were investigated using this nanoindentation technique.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 1 0 5 0 - 9
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
73
2. Experimental procedure
2.2. Experiment details
2.1. Preparation of specimens
The nanoindentation experiments were performed at room temperature using an atomic force microscope, NanoscopeTM (Digital Instruments, Santa Barbara, CA) with the standard head replaced by a Triboscope1 indenter system (Hysitron, Minneapolis, MN). Fused silica was used to calibrate the reduced elastic modulus and to de®ne the tip area function for indentation depth between 50 and 600 nm. Loading was done as follows. A maximum load of 4000 mN was applied. The indentation was made by driving the indenter at a constant loading rate of 2000 mN/s into the material and holding it at this rate for 2 s. The indenter was then withdrawn from the surface at a constant unloading rate of 2000 mN/s. A three-sided pyramid Berkovich diamond indenter was used for indentation and scanning imaging. The displacement (penetration depth) of the indenter was continuously monitored and a load±time history of the indentation recorded. According to the method of Oliver and Pharr [9], the indentation load±displacement data were analyzed to determine the reduced elastic modulus (Er), and the
Nonstoichiometric titanium carbide compacts TiCx with C/Ti atomic ratios x equal to 0.6, 0.7, 0.8, 0.9 were obtained by mixing in advance stoichiometric TiC powder with Ti metal powder in appropriate ratios prior to sintering. Starting particle sizes ranged from 2 to 10 mm. The mixed powders were dry-pressed then sintered in vacuum (10 3 Pa) at 1800 8C for 2 h. The preformed TiCx compacts were placed in an alumina dish with an appropriate amount of the nickel aluminide (Ni3Al) powder on the top surface of the compacts. The furnace was maintained under a dynamic vacuum of about 10 3 Pa and the melt-in®ltration was carried out at a temperature of 1450 8C. The samples were held at this temperature for half an hour allowing suf®cient time for in®ltration. After cooling, specimens were cut by electric sparking along the in®ltration direction, then mechanically ground, and ®nally careful polished with 0.5 mm diamond paste, then examined with an optical microscope to ensure the surface as smooth as possible and the interface as clear as possible.
Fig. 1. The backscattering SEM micrographs of the four TiCx/Ni3Al composites.
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W. Hua et al. / Applied Surface Science 189 (2002) 72±77
nanohardness (H) by the followings. p p Pmax Er p s; H A 2 A Obviously, from the formula above, in addition to the load on the indenter (P) other parameters such as the stiffness of contact between the indenter and the test material (s), and the projected area of contact (A), are required for getting the Er and H values. The stiffness of the contact between the indenter and the test material can be obtained by determining the slope of the initial portion of the unloading curve, s dp=dh. The projected area of contact A was determined by the indenter tip geometry. For the threesided pyramidal Berkovich indenter, the projected contact area was given by A 24:5h2c. Where the contact depth (hc) was calculated with the following
formula hc hmax 0:75 pmax =s. The elastic modulus of the tested material can be calculated from the following equation: 1 1 u2i 1 u2s Er Ei Es where Ei and ui are Young's elastic modulus and Poisson's ratio of the indenter; and Es and us are those of the materials being tested. For a diamond indenter, Ei 1141 GPa and ui 0:07. 3. Results and discussion As shown in the backscattering SEM images of Fig. 1, the black titanium carbide particles are bound together by Ni3Al. According to the image analysis,
Fig. 2. AFM images of nanoindentation imprints.
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
75
Fig. 3. Load±displacement curves obtained by nanoindentation on the four composites.
the areal fractions of the in®ltrated Ni3Al in the four TiCx/Ni3Al composites are 22.98, 29.87, 19.38 and 24.34, respectively. Fig. 2 shows the images of the nanoindentation imprints and their positions, where the numbers 1±3 stand for the order of the indentation points. It is seen that the imprint size is obviously different due to the different properties of the two phases. The white zone around the indentation imprint representing the hunch revealed the range of the strain zone. There must be an appropriate space between indentations in case of interfering each other. The load±displacement curves for each indentation site are shown in Fig. 3. Some of the curves are almost overlapped, which re¯ected the homogeneous of the same phase. The pop-in phenomena (in Fig. 2, x 0:7, the fourth curve) revealed the holes existed under the surface. Corresponding to Figs. 2 and 3, Fig. 4 gives site-speci®c values for the reduced elastic
modulus and the nanohardness obtained from each indentation curve. Because the spacings between indentations are nearly equal, the reduced elastic modulus and the nanohardness show a gradient distribution across the interface, most likely a parabolic distribution. Obviously, this distribution, which is similar to that in the FGM (functionally gradient materials), is very bene®cial to the structure materials for dissipation or decreasing the stress concentration at the interfaces [3]. The reduced elastic modulus at the interface as shown in Fig. 5(b) is reduced as x changes from 0.6 to 0.9, but the interfacial nanohardness in Fig. 5(a) has its peak value at x equal to 0.7. Titanium carbide, according to the phase diagram, exists over a wide range of composition (C/Ti atomic ratio with the range 0.5±1). Its chemical, physical and mechanical properties are composition-dependent. But in this study the main reason for the results above can be attributed to
76
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
Fig. 4. Reduced elastic modulus and nanohardness of different positions.
Fig. 5. Nanohardness (a) and reduced elastic modulus (b) as the function of x for different TiCx/Ni3Al composites.
the difference of in®ltrated Ni3Al content, the bound strength between the two phases, and the diffusion of Ti during the in®ltration process from TiCx to Ni3Al. Some new harder phases such as the Heusler type Ni2AlTi alloy may be formed in the TiC0.7/Ni3Al composite during solidi®cation contributing to its higher nanohardness. A more in-depth study will be presented in the future.
4. Conclusions 1. The mechanical properties at the interface of TiCx/ Ni3Al composites were found to have a gradient distribution, approximately a parabolas distribution. 2. The elastic modulus decreased as the C/Ti ratio was increased from 0.6 to 0.9.
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
3. The nanohardness at the interface of the TiC0.7/ Ni3A1 composite exhibits a peak value.
Acknowledgements The authors would like to acknowledge the ®nancial support by the Ministry of Sciences and Technology of China. References [1] R. Subramanian, J.H. Schneibel, K.B. Alexander, K.P. Plucknett, Scripta Mater. 35 (5) (1996) 583. [2] C.T. Liu, J.O. Stiegler, Science 226 (1984) 636. [3] L.M. Zhang, R.Z. Ruan, M. Oomori, T. Hirai, J. Mater. Sci. Lett. 14 (1995) 1620.
77
[4] K. Miyahara, N. Nagashima, T. Ohmura, S. Matsuoka, NanoStruct. Mater. 12 (1999) 1049. [5] J. Woirgard, J.-C. Dargenton, C. Tromas, V. Audurier, Surf. Coat. Technol. 100±101 (1998) 103. [6] D. Tabor, Phil. Mag. A 147 (5) (1996) 1207. [7] N.X. Randall, C. Julia-Schmutz, J.M. Soro, Surf. Coat. Technol. 108±109 (1998) 489. [8] J.B. Pethica, R. Hutchings, W.C. Oliver, Phil. Mag. A 48 (4) (1983) 593. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [10] S. Habelitz, S.J. Marshall, G.W. Marshall Jr., M. Balooch, Achives Oral Biol. 46 (2001) 173. [11] L. Boudoukha, F. Halitim, S. Paletto, G. Fantozzi, Ceram. Int. 24 (1998) 189. [12] Y. Ding, Y. Zhang, D.O. Northwood, A.T. Alpas, Surf. Coat. Technol. 94±95 (1997) 483. [13] F.K. Mante, G.R. Baran, B. Lucas, Biomaterials 20 (1999) 1051. [14] S.V. Hainsworth, T.F. Page, J. Mater. Sci. 29 (1994) 5529. [15] M.J. Mayo, R.W. Siegel, A. Narayanasamy, W.D. Nix, J. Mater. Res. 5 (5) (1990) 1073.
Nanohardness and elastic modulus at the interface of TiCx/Ni3Al composites determined by the nanoindentation technique Wenshen Huaa,*, Xingfang Wua, Dianhong Shenb, Hua Lub, M. Polakc a
Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, PR China State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China c Department of Material and Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel
b
Received 15 August 2001; accepted 3 December 2001
Abstract The mechanical properties at the interfaces of TiCx/Ni3Al composites of four different C/Ti ratios were investigated using the nanoindentation technique. We ®nd that the nanohardness and the elastic modulus from the phase TiCx to the phase Ni3Al are a gradient distribution. As x increased from 0.6 to 0.9, the elastic modulus at the interface decreased. The interfacial nanohardness reached its peak value at x equal to 0.7. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanoindentation; Nanohardness; Elastic modulus; Composite; Interface
1. Introduction As is well known, the fracture resistance of the brittle ceramic TiC can be improved signi®cantly by incorporating ductile metal reinforcements. In this present work, the ordered intermetallic compound nickel aluminide (Ni3Al) was selected to fabricate TiCx/Ni3Al composites using a simple pressureless melt-in®ltration process. The choice of Ni3Al as a binder was based on its exceptional high temperature strength and its wettability with the titanium carbide. In principle, this composite is a candidate for high temperature applications up to about 1000 8C [1±3]. The strength of the phase boundary is crucial to loadbearing composite materials. The conventional tensile experiment was designed to analyze the toughing *
Corresponding author.
mechanism qualitatively by observing the crack extension and the fracture surfaces. But the nanoindentation technique [4±9], which developed during the past several years, has made analysis more precise by combining scanning force microscopy (SFM) with the mechanical properties microprobe (MPM) to locate the indentation sites and image the indentation imprints. As a powerful tool to be developed for testing mechanical properties on the nanometer scale, this approach can give the elastic modulus and nanohardness directly. Therefore, it has seen more and more applications in ®elds such as semiconductors, biomaterials, nanostructured materials, and surface science [10±15]. In this work, the mechanical properties at nanosized interfaces of TiCx/Ni3Al composites of different C/Ti ratios were investigated using this nanoindentation technique.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 1 0 5 0 - 9
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
73
2. Experimental procedure
2.2. Experiment details
2.1. Preparation of specimens
The nanoindentation experiments were performed at room temperature using an atomic force microscope, NanoscopeTM (Digital Instruments, Santa Barbara, CA) with the standard head replaced by a Triboscope1 indenter system (Hysitron, Minneapolis, MN). Fused silica was used to calibrate the reduced elastic modulus and to de®ne the tip area function for indentation depth between 50 and 600 nm. Loading was done as follows. A maximum load of 4000 mN was applied. The indentation was made by driving the indenter at a constant loading rate of 2000 mN/s into the material and holding it at this rate for 2 s. The indenter was then withdrawn from the surface at a constant unloading rate of 2000 mN/s. A three-sided pyramid Berkovich diamond indenter was used for indentation and scanning imaging. The displacement (penetration depth) of the indenter was continuously monitored and a load±time history of the indentation recorded. According to the method of Oliver and Pharr [9], the indentation load±displacement data were analyzed to determine the reduced elastic modulus (Er), and the
Nonstoichiometric titanium carbide compacts TiCx with C/Ti atomic ratios x equal to 0.6, 0.7, 0.8, 0.9 were obtained by mixing in advance stoichiometric TiC powder with Ti metal powder in appropriate ratios prior to sintering. Starting particle sizes ranged from 2 to 10 mm. The mixed powders were dry-pressed then sintered in vacuum (10 3 Pa) at 1800 8C for 2 h. The preformed TiCx compacts were placed in an alumina dish with an appropriate amount of the nickel aluminide (Ni3Al) powder on the top surface of the compacts. The furnace was maintained under a dynamic vacuum of about 10 3 Pa and the melt-in®ltration was carried out at a temperature of 1450 8C. The samples were held at this temperature for half an hour allowing suf®cient time for in®ltration. After cooling, specimens were cut by electric sparking along the in®ltration direction, then mechanically ground, and ®nally careful polished with 0.5 mm diamond paste, then examined with an optical microscope to ensure the surface as smooth as possible and the interface as clear as possible.
Fig. 1. The backscattering SEM micrographs of the four TiCx/Ni3Al composites.
74
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
nanohardness (H) by the followings. p p Pmax Er p s; H A 2 A Obviously, from the formula above, in addition to the load on the indenter (P) other parameters such as the stiffness of contact between the indenter and the test material (s), and the projected area of contact (A), are required for getting the Er and H values. The stiffness of the contact between the indenter and the test material can be obtained by determining the slope of the initial portion of the unloading curve, s dp=dh. The projected area of contact A was determined by the indenter tip geometry. For the threesided pyramidal Berkovich indenter, the projected contact area was given by A 24:5h2c. Where the contact depth (hc) was calculated with the following
formula hc hmax 0:75 pmax =s. The elastic modulus of the tested material can be calculated from the following equation: 1 1 u2i 1 u2s Er Ei Es where Ei and ui are Young's elastic modulus and Poisson's ratio of the indenter; and Es and us are those of the materials being tested. For a diamond indenter, Ei 1141 GPa and ui 0:07. 3. Results and discussion As shown in the backscattering SEM images of Fig. 1, the black titanium carbide particles are bound together by Ni3Al. According to the image analysis,
Fig. 2. AFM images of nanoindentation imprints.
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
75
Fig. 3. Load±displacement curves obtained by nanoindentation on the four composites.
the areal fractions of the in®ltrated Ni3Al in the four TiCx/Ni3Al composites are 22.98, 29.87, 19.38 and 24.34, respectively. Fig. 2 shows the images of the nanoindentation imprints and their positions, where the numbers 1±3 stand for the order of the indentation points. It is seen that the imprint size is obviously different due to the different properties of the two phases. The white zone around the indentation imprint representing the hunch revealed the range of the strain zone. There must be an appropriate space between indentations in case of interfering each other. The load±displacement curves for each indentation site are shown in Fig. 3. Some of the curves are almost overlapped, which re¯ected the homogeneous of the same phase. The pop-in phenomena (in Fig. 2, x 0:7, the fourth curve) revealed the holes existed under the surface. Corresponding to Figs. 2 and 3, Fig. 4 gives site-speci®c values for the reduced elastic
modulus and the nanohardness obtained from each indentation curve. Because the spacings between indentations are nearly equal, the reduced elastic modulus and the nanohardness show a gradient distribution across the interface, most likely a parabolic distribution. Obviously, this distribution, which is similar to that in the FGM (functionally gradient materials), is very bene®cial to the structure materials for dissipation or decreasing the stress concentration at the interfaces [3]. The reduced elastic modulus at the interface as shown in Fig. 5(b) is reduced as x changes from 0.6 to 0.9, but the interfacial nanohardness in Fig. 5(a) has its peak value at x equal to 0.7. Titanium carbide, according to the phase diagram, exists over a wide range of composition (C/Ti atomic ratio with the range 0.5±1). Its chemical, physical and mechanical properties are composition-dependent. But in this study the main reason for the results above can be attributed to
76
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
Fig. 4. Reduced elastic modulus and nanohardness of different positions.
Fig. 5. Nanohardness (a) and reduced elastic modulus (b) as the function of x for different TiCx/Ni3Al composites.
the difference of in®ltrated Ni3Al content, the bound strength between the two phases, and the diffusion of Ti during the in®ltration process from TiCx to Ni3Al. Some new harder phases such as the Heusler type Ni2AlTi alloy may be formed in the TiC0.7/Ni3Al composite during solidi®cation contributing to its higher nanohardness. A more in-depth study will be presented in the future.
4. Conclusions 1. The mechanical properties at the interface of TiCx/ Ni3Al composites were found to have a gradient distribution, approximately a parabolas distribution. 2. The elastic modulus decreased as the C/Ti ratio was increased from 0.6 to 0.9.
W. Hua et al. / Applied Surface Science 189 (2002) 72±77
3. The nanohardness at the interface of the TiC0.7/ Ni3A1 composite exhibits a peak value.
Acknowledgements The authors would like to acknowledge the ®nancial support by the Ministry of Sciences and Technology of China. References [1] R. Subramanian, J.H. Schneibel, K.B. Alexander, K.P. Plucknett, Scripta Mater. 35 (5) (1996) 583. [2] C.T. Liu, J.O. Stiegler, Science 226 (1984) 636. [3] L.M. Zhang, R.Z. Ruan, M. Oomori, T. Hirai, J. Mater. Sci. Lett. 14 (1995) 1620.
77
[4] K. Miyahara, N. Nagashima, T. Ohmura, S. Matsuoka, NanoStruct. Mater. 12 (1999) 1049. [5] J. Woirgard, J.-C. Dargenton, C. Tromas, V. Audurier, Surf. Coat. Technol. 100±101 (1998) 103. [6] D. Tabor, Phil. Mag. A 147 (5) (1996) 1207. [7] N.X. Randall, C. Julia-Schmutz, J.M. Soro, Surf. Coat. Technol. 108±109 (1998) 489. [8] J.B. Pethica, R. Hutchings, W.C. Oliver, Phil. Mag. A 48 (4) (1983) 593. [9] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [10] S. Habelitz, S.J. Marshall, G.W. Marshall Jr., M. Balooch, Achives Oral Biol. 46 (2001) 173. [11] L. Boudoukha, F. Halitim, S. Paletto, G. Fantozzi, Ceram. Int. 24 (1998) 189. [12] Y. Ding, Y. Zhang, D.O. Northwood, A.T. Alpas, Surf. Coat. Technol. 94±95 (1997) 483. [13] F.K. Mante, G.R. Baran, B. Lucas, Biomaterials 20 (1999) 1051. [14] S.V. Hainsworth, T.F. Page, J. Mater. Sci. 29 (1994) 5529. [15] M.J. Mayo, R.W. Siegel, A. Narayanasamy, W.D. Nix, J. Mater. Res. 5 (5) (1990) 1073.