Nanoscratch behavior of multilayered films using molecular dynamics

DOI: 10.1007/s00339-007-4351-8 Appl. Phys. A 90, 753–758 (2008)

Materials Science & Processing

Applied Physics A

Nanoscratch behavior of multi-layered films

using molecular dynamics

1_{Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632,} Taiwan, R.O.C.

2_{Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan, R.O.C.} 3_{Department of Multimedia Design, National Formosa University, Yunlin 632, Taiwan, R.O.C.}

4_{Department of Product Design and Engineering, Middlesex University, London N14 4YZ, UK}

Received: 13 August 2007/Accepted: 29 October 2007 Published online: 8 December 2007 • © Springer-Verlag 2007

ABSTRACTMolecular dynamics simulations are performed to study the plastic deformation, stress and chip formation of scratched multi-layered films. The results showed that stick– slip and work-hardening behaviors were observed during the scratching process. There was a pile-up of amorphous dis-ordered debris atoms and shear rupture ahead of the probe and a clear side-flow on the lateral sides of the probe when the probe moved forward. Both the plastic energy and the adhesion in-creased with an increase in the scratching depth. The glide band of the interface was on the{111}
110 slip system with a max-imum width of the glide band of about 1 nm. The strain energy stored in the deformed structure caused a higher stress region in the material in front of the tool edge, with a maximum stress of about 10 GPa. In addition, the mechanical response and thermal softness phenomenon are discussed.

PACS02.70.Ns; 46.55.+d; 47.11.Mn; 91.55.Ax; 62.40.+i

1 Introduction

Multi-layered film nanostructures have received a lot of attention for applications such as in high-density storage systems, magnetic media, protective coatings and mi-crosystem technologies [1–3]. The Al/Ni multi-layered thin film is one of the practiced coatings in production and has been used as protective coatings for various device compo-nents due to its good chemical stability and high melting temperature [4]. However, there are many multi-layered film products with mechanical weakness, which dominates the performance of the materials. It is necessary to investigate the processing and mechanical properties of multi-layered films for improvement of their reliability.

Many experimental techniques such as nanoscratch [5, 6] and nanoindentation [7, 8] have been performed to study the processing and mechanical properties of thin films. However, the experimental tests cause difficulty in under-standing the atomic-scale sliding friction but reveal the in situ mechanical and processing response of nanomaterials. These difficulties within experimental methodology can, in

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general, be easily analyzed by using molecular dynamics (MD) simulation.

Molecular dynamics simulation is a useful method for accurate investigation and it plays an important role in aca-demic and industrial research [9–11]. MD simulation can be effective in simulating the atomistic behavior of deformation, offering insights into microscopic behaviors and microscopic fracture processes. Recently, MD simulation of indentation and scratch processes has been widely applied to investi-gate friction and fracture of thin films [12, 13]. Kizler and Schmauder [14] studied the deformation mechanism of ultra-hard carbide layer systems under nanoindentation with the aid of MD analysis. Mulliah et al. [15] investigated the stick– slip phenomenon of a pyramidal diamond tip indenting and scratching a silver surface on the atomic scale by MD analysis. Thus, this study uses MD simulations to evaluate the pro-cessing and mechanical properties of multi-layered films at different temperatures and depths. The effect of the interface of layered films is discussed. In addition, the scratch mechan-ism, stress distribution and plastic energy of the films are also investigated.

2 Simulation method

Figure 1 shows a molecular dynamics simulation model for nanoscratch and includes a diamond probe exert-ing a force on the surface of the Al/Ni multi-layered specimen during processing. The probe was assumed as a rigid tool with 19 465diamond atoms and the conical angle of the probe was 60◦ with a spherical radius of 4 nm. The specimen was ini-tially assumed to have a well-defined atomic layered structure before thermal equilibration. The surface was (001) for all the numerical experiments in this paper. The specimen size in the x, y and z directions was 11.95, 11.95 and 4.66 nm, respec-tively. Al, Ni, Al/Ni/Al and Al/Ni/Al/Ni/Al multi-layered samples were the same size in the transverse direction and also thickness. For Al/Ni/Al and Al/Ni/Al/Ni/Al samples, each thickness of Al and Ni layers was the same on the Al layer sub-strate. The periodic boundary conditions of the specimen were used in the transverse directions, and the bottom two layers of substrate atoms were fixed in space [16]. To study the effect of thermal behavior and plastic deformation of a specimen, temperatures ranging from 300 to 750 K were employed.

754 Applied Physics A – Materials Science & Processing

FIGURE 1 An example of the simulation model of nanoscratch

The equations of motion were solved using the Gear fifth-order predictor corrector algorithm [16] with a time step of 10−15s. For each temperature the specimen was firstly equi-librated for a period of 105fs, which was much longer than what was needed for the system to reach equilibrium. Dur-ing the scratchDur-ing process, the probe was moved along the y direction before the specimen with a distance of 3 nm. The cutting speed of 300 m/s was employed purposely to reduce computation time. The force acting on an individual atom was obtained by summing the forces contributed by the sur-rounding atoms. The force exerted on atom i resulting from the interaction of all the other atoms can be derived from the potential functions. The tight-binding second-moment ap-proximation (TB-SMA) potential function [17] was adopted to simulate the interatomic energy of Ni–Ni, Al–Al and Al–Ni atoms. The Morse potential function was also used to calcu-late atomic interaction forces of C–Al and C–Ni atoms. The potential parameters between different materials were calcu-lated using the mixing rules.

3 Results and discussion

3.1 Cutting force and stick–slip phenomenon

Figure 2 shows the cutting force–distance curve obtained from the simulated scratching tests of Ni, Al, Al/Ni/Al and Al/Ni/Al/Ni/Al films at a temperature of 300 K and a scratching depth of 2 nm. The initial slope of the curve was increased when the tool began to contact and scratch into the material at distances of about 3–8 nm. The reason for this was due to the probe approach and the inter-action that occurred between the probe and sample atoms. When a diamond tool had scratched a soft metal film, the film material was pushed outward on the sides of the tool and ahead of the scratching direction. The material movement was a result of plastic deformation of the metal surface and even-tually led to a loose debris chip. The chip formation was an undesirable situation around the groove path. When the film atoms accumulated ahead of the tool, the deformation energy required to form the crest of the film in front of the tool in-creased the magnitude of the cutting force. The cutting force is defined as the force exerted in the direction parallel to the scratched surface. The cutting force represents the ability to

FIGURE 2 Cutting force–distance curve obtained from the simulated scratch tests of films at a temperature of 300 K

overcome the bond energy between the tool and the workpiece at atomic scale. The cutting force is related to the scratching resistance which occurs around both the interface of the chip and the tool and the contact area between the scratched mate-rial and the tool. When a sharp probe scratches into a softer material, the edge of the probe tool will bear a high pressure due to the higher scratching resistance. The force increased slightly at distances from 8 to 16 nm. This was a result of work hardening, which is the accumulation of pile-up and dis-locations. The variation of the cutting force was due to the stick–slip phenomenon [12] which occurred between the sam-ple surface atoms and the probe tip during the nanoscratching process. The stick behavior resulted from an increase of adhe-sive effect and accumulation of film atoms. The slip occurred when the chip debris crumpled and the dislocation slipped. The stick–slip phenomenon was also commonly observed in atomic-scale friction experiments.

The average cutting force of Ni, Al, Al/Ni/Al and Al/Ni/Al/Ni/Al multi-layered films was about 1968, 667, 773 and 787 nN, respectively. The cutting force of the pure Ni sample was larger than that of pure Al and the multi-layered films. When the diamond tip had passed the material after dis-tances of larger than 16 nm, the force would decrease rapidly and the film atoms accumulated around and ahead of the di-amond tool and the adhesive effect took place. The adhesive force could be calculated from the interaction force among the tool atoms and the sample atoms. The average adhesive force of Ni, Al, Al/Ni/Al and Al/Ni/Al/Ni/Al films was about 66, 164, 166 and 159 nN, respectively. Based on a previous study [12] there is no clear influence on the magnitude of the cutting force when the cutting velocity varies in a range of 50–200 m/s. But, it should be noted that the possible effects of a high cutting speed will reduce the dislocation slip and plastic deformation during a short interaction time level. The shorter time would allow for less specific cutting energy to be dissipated.

758 Applied Physics A – Materials Science & Processing

The average chip thickness of the steady-state region at temperatures of 300, 450, 600 and 750 K was about 1.2, 1.4, 1.6 and 1.7 nm, respectively. The material had affinity for the tool–specimen interface, with a weaker cohesive energy at a high temperature, and tended to increase debris chip forma-tion. The chip thickness was increased when the scratching occurred at higher temperature.

Figure 10 shows the plastic energy U and adhesion of the steady-state region at temperatures of 300, 450, 600 and 750 K. The average plastic energy of the steady-state re-gion at temperatures of 300, 450, 600 and 750 K was about 3.9 × 10−15, 3.4 × 10−15, 3.1 × 10−15and 3.0 × 10−15J, re-spectively. The plastic energy decreased with an increase in the temperature. This phenomenon was the result of the inter-action force of the specimen atoms becoming weaker as the specimen’s atomic distance increased at a high temperature.

The average adhesive force at temperatures of 300, 450, 600 and 750 K was about 166, 198, 207 and 200 nN, re-spectively. The effect can be analyzed because when the dia-mond tip passed the film atoms more easily they accumulated around the diamond tool at a higher temperature.

4 Conclusions

The nanoscratch response of multi-layered thin films was studied using molecular dynamics simulations. The stick–slip phenomenon and work hardening were un-derstood from an atomic viewpoint based on MD analy-sis. When the scratching depth was less than 0.4 nm, the material had no observed groove or debris chip. The be-havior was similar to the no-wear sliding mechanism in

atomic-scale friction. The material had an affinity for the tool–specimen interface, with a weaker cohesive energy at a high temperature, and tended to easily show the debris chip formation; also, the material had the dislocations and obvious slip lines on the {111}
110 slip system. Further-more, the stress and the thermal softness of the films were discussed.

ACKNOWLEDGEMENTS This work was partially supported by the National Science Council of Taiwan, under Grant Nos. NSC 95-2221-E150-066 and NSC 96-2628-E-150-005-MY3.

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