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Copyright © 2010 American Scientific Publishers All rights reserved
Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol.10, 1–5, 2010
Mechanical Properties of Multilayered Films Using
Different Nanoindenters
Te-Hua Fang
1 ∗, Tong Hong Wang
2, and Jia-Hung Wu
1 1Institute of Mechanical and Electromechanical Engineering,National Formosa University, Yunlin 632, Taiwan
2Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan
The effects of interface, contact hardness, deformation, and adhesion of Al/Ni multilayered films under nanoindentation were investigated using molecular dynamics (MD) simulations. The results show that the indentation force of the sphere indenter is the largest among nanoindentations using sphere, cone, Vickers, or Berkovich type indenters at the same penetration depth. Force increas-ing, relaxation and adhesion took place during loadincreas-ing, holding depth and unloadincreas-ing, respectively. The interface occurred along the {111} 110 slip systems and the maximum width of the glide bands was about 1 nm. The reaction force and plastic energy of the indented films are also discussed.
Keywords:
Nanoindentation, Molecular Dynamics, Nanotribology, Adhesion.1. INTRODUCTION
Multi-layered thin films have been widely used for pro-tecting the structures of electronics devices such as mag-netic media, hard discs, micro-electromechanical systems (MEMS).1–3Among these, Al/Ni films, are popular protec-tive coatings due to their high chemical stability and high melting temperature.4
Nanoindentation is one way to understand the intrinsic behaviors of such materials,5–7 although some nano-scale features like thin films are hard to measure precisely on current test equipment as well as the deviation from envi-ronment, man and other noises.However, such difficulties can be resolved through numerical calculation.Molecu-lar dynamics (MD) is an appropriate numerical method to describe the behavior among atoms which uses Newtonian equations of motion, and Fang and Wu8 and Fang et al.9 have used this method to investigate the nanoindentation and nanoscratch behaviors on Al/Ni multilayered films caused by a conical indenter.
However, different indenter shapes incur different defor-mation behaviors.To further understand the behavior with different popular indenters, such as Vickers, Berkovich, spherical and conic indenters, we therefore conducted the MD analysis.
∗Author to whom correspondence should be addressed.
2. MDMODELING
The MD model in this study consists of a diamond inden-ter lying on a 11.95 × 11.95 × 4.66 nm3 Al/Ni/Al multi-layered film that each layer has a thickness of 1.55 nm, as shown in Figure 1.Due to the ultra hard charac-teristics of diamond, the indenter was assumed rigid for Vickers, Berkovich, conical, and spherical indenters. Detailed geometries of the indenters can be elsewhere.10 The pyramidal indenters generally in use have a tip radius on the order of 50 to 100 nm; nevertheless, we con-structed a sharp tip for both Vickers and Berkovich inden-ters.The periodic boundary conditions and all degrees of freedom were set on the four-side lateral boundaries and at the bottom two layers of the film atoms, respectively. The equations of motion were integrated using the Verlet algorithm11 with a time step of 5 fs per calculation.The tight-binding second-moment approximation (TB-SMA) potential function and the Morse potential function were adopted to simulate the interatomic energies8 9 of the Ni–Ni, Al–Al and Al–Ni atoms and interatomic forces of the C–Al and C–Ni atoms, respectively.
Both loading and unloading of the indenter were con-ducted under a constant speed of 100 m/s at 300 K, and the indenters were impressed into the films to a depth of 1.2 nm. The high speed indentation has been selected for these simulations because it is computationally inex-pensive, and it has been used for several similar studies previously.We expect it to be adequate for the qualitative
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Fang et al. Mechanical Properties of Multilayered Films Using Different Nanoindenters
Vickers Berkovich Cone Sphere
–400 –300 –200 –100 0 100 200 300 400 500 600 700 Force (nN) Indenter –301.0 632.0 –179.7 354.8 –196.7 236.5 –80.1
Max. indentation force during loading Max. adhesion force during unloading
63.5
Fig. 7. Maximum indentation force during loading and maximum adhe-sion force during unloading for different indenters.
Vickers Berkovich Cone Sphere
0 100 200 300 400 500
Plastic energy (aJ)
Indenter
Fig. 8. Plastic energy for different indenters.
the tip approached the topmost atoms, a small indenta-tion was formed due to plastic deformaindenta-tion and the local contact stress exceeded 5.8 GPa.14 Lee et al.15 studied the defect nucleation and evolution in Al single crystal under nanoindentation and confirmed that the nucleation sites of the initial dislocation loops were displaced from the indenting axis below the contact surface.This study shows a similar result which consisted with the incipient plasticity and deformation mechanism.
4. CONCLUSIONS
The behaviors of Al/Ni/Al multilayered films at 300 K under 1.2-nm nanoindentation with Vickers, Berkovich, conical and spherical indenters were studied using MD analysis.It is apparent from the results that the sequences of the least to the most influential indenters to displace and adhesion atoms are Vickers, Berkovich, cone, and sphere. This is as expected, since that sequence is followed by the minimum to the maximum contact area on the films.In addition, the displaced atoms correspond to the geometries of the indenters, and there are maximum indentation forces of 63.5, 236.5, 354.8, and 632.0 N during loading and maximum adhesion forces of −80.1, −196.7, −179.7, and −301.0 nN during unloading as well as plastic energies of 68, 206, 254 and 443 aJ for Vickers, Berkovich, conical, and spherical indenters, respectively.
Acknowledgments: This work was supported in part by National Science Council of Taiwan, under Grant No.NSC 95-2221-E150-066 and NSC 96-2628-E-150-005-MY3.
References and Notes
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Received: 20 July 2008.Accepted: 20 January 2009.
