In this project, we have performed a systematic study on the effects of load, speed, and temperature using molecular dynamics simulation of an Ag tip sliding on a Cu substrate. The simulations were performed at sliding velocities of 1, 5, and 10 m/s, average normal stresses of 0, 100, 200, 300, 400, and 500 MPa, and temperatures of 10, 100, 200, 300 and 400 K.
The yx and yz components of the shear stress for the tip and substrate were investigated to get a global picture of the stick-slip properties in the contact area. Next, the energy transfer mechanism was discussed briefly. Then, the average, minimum and maximum values of the shear stress as functions of velocity, load, and temperature were illustrated.
It was found that the average, maximum and absolute value of the minimum shear stress increase with the velocity of the tip; the maximum and absolute value of the minimum shear stress decreased with initial normal stress; and except for one outlying case, the average shear stress increased with temperature. This temperature effect was studied further and it was determined that temperature affects shear stress through three competing mechanisms: thermal activation, surface roughness, and tip-substrate distance. Next, atom diffusion was also investigated as a means of understanding atomic-scale wear during stick-slip under different operational conditions.
Finally, dislocation theory was used to explain the stick-slip. The common neighbor method (CNA) was used to show the crystal structure alternating between face center cubic (FCC) hexagonal close packed (hpc). The displacement and shear stress were measured in the contact area to show why and how the stick-slip happened.
1. Side change of the tip
The size of the tip changed in three directions during simulation because of the thermal expansion.
2. Dislocation analysis
The following code was used to output the percentage of the dislocated atoms for the contact area.
3. Slick-slip illustration by tip layers
The layer was divided into 30 layers in z direction. This code was used to show the movement of the tip for each layer during the sliding.
The CNA was done by this code. The results were exhibited and consistent with those done by IMD.
5. Lengthen the movement of the tip on the substrate
The code was used to change the initial location of the tip atoms to make them move longer on the substrate.
6. Surface roughness
In this part, the code was used to calculate the surface roughness for the substrate free surface and tip free surface.
7. Diffused atoms
The diffused atoms were showed by this code as red atoms.
8. Shear tress distribution
The code was used to show the shear stress distribution in the contact area.
9. CNA from IMD illustration
The code could illustrate the CNA from IMD.
LIST OF REFERENCES
B. Luan, M. O. (2006). Contact of single asperities with varying adhesion: Comparing continuum mechanics to atomistic simulations. Phys. Rev. E74, 026111 , 1-17.
Bacon, D. H. (2001). Introduction to Dislocations.
Binquan Luan, M. O. (2005). The breakdown of continuum models for mechanical contacts. Nature , 929-932.
C.L. Cleveland, W. D. (1999). Melting of gold clusters. PHYSICAL REVIEW B60 , 5065-5077.
E. Gnecco, R. B. (2000). Velocity dependence of atomic friction. PHYSICAL REVIEW LETTERS , 1172-1175.
E. T. Lilleodden, J. Z. (2003). Atomistic simulations of elastic deformation and dislocation nucleation during nanoindentation. Journal of Mechanics and Physics of Solids 51 , 901-920.
Frank, F. C. (1951). Crystal dislocation.-Elementary concepts and definitions. Correspondence , 809-819.
Guangtu Gao, R. J. (2007). Atomic-scale friction on diamond: a comparison of different sliding directions on (001) and (111) surfaces using MD and AFM. Langmuir 23, 5394 , 5394-5405.
Haile, J. M. (1997). Molecular dynamics simulation Elementary methods.
Helio Tsuzuki, P. S. (2007). Stuctural characterization of deformed crystals by analysis of common atomic neighborhood. Computer Physics Communications 177, 518 , 518-523.
Izabela Szlufarska, M. C. (2008). Topical review Recent advances in single asperity nanotribology. J. Phys. D: Appl. Phys. 41, 123001 , 1-39.
J. A. Harrison, C. T. (1992). Molecular-dynamics simulations of atomic-scale friction of diamond surfaces. Phys. Rev. B46 , 9700-9708.
J. Roth, F. G.-R. (2000). A molecular dynamics run with 5.180.116.000 particles. Int. J. Mod. Phys. C11, 317 , 317-322.
J. Stadler, R. M.-R. (1997). IMD: A software package for molecular dynamics studies on parallel computers. Int. J. Mod. Phys. C8 , 1131-1140.
J. Wang, R. H. (2008). Atomistic simulations of the shear strength and sliding mechanisms of copper-niobium interfaces. Acta Materialia 56 , 3109-3119.
J.D. Honeycutt, H. A. (1987). Molecular dynamics study of melting and freezing of small Lennard-Jones clusters. J.Phys. Chem. 91 , 4950-4963.
Juan A. Hurtado, K. K. (1999). Scale effects in friction of single-asperity contacts. I. From concurrent slip to single-dislocation-assisted slip. Proc. R. Soc. Lond A , 3363-3384.
Katsuyoshi Matsushita, H. M. (2005). Atomic scale friction between clean graphite surfaces. Solid State Communications 136 , 51-55.
Kaviany, M. (2008). Heat Transfer Physics. Cambridge: CAMBRIDGE UNIVERSITY PRESS.
Kraska Norbert Lummen, T. K. (2007). Common neighbor analysis for binary atomic systems. Modelling Simul. Mater. Sci. Eng. 15 , 319-334.
M. R. Sorensen, K. W. (1996). Simulations of atomic-scale sliding friction. Phys. Rev. B53 , 2101-2113.
M.H. Cho, S. D. (2005). Atomic scale stick-slip caused by dislocation nucleation and propagation during scratching of Cu substrate with a nanoindenter: a molecular dynamics simulation. Wear 259 , 1392-1399.
Martin D. Perry, J. A. (1995). Universal aspects of the atomic-scale friction of diamond surfaces. J. Phys. Chem. , 9960-9965.
Murray S. Daw, M. I. (1984). Embedded-atom method: Derication and application to impurities, surfaces, and other defects in metals. Phys. Rev. B29, 6443 , 6443- 6453.
P. L. Willams, Y. M. (2006). An embedded-atom potential for the Cu–Ag system. Modelling and Simulation in Materials Science and Engineering14 , 817-833.
Qing Zhang, Y. Q. (2005). Atomic simulations of kinetic friction and its velocity dependence at Al/Al and a-Al2O3 interfaces. Phys. Rev. B72, 045406 , 1-12.
R. D. Arnell, P. B. (1991). Tribology Principles and Design Application. Springer.
Ruan, X. (2009). Microscale Heat Transfer. Classnote.
S. M. Foiles, M. I. (1986). Embedded-atom-method functions for the fcc metals Cu, Ag, Au, Ni, Pd, Pt, and their alloys. Phys. Rev. B33, 7983 , 7983-7991.
S. Maier, Y. S. (2005). Fluctuations and jump dynamics in atomic friction experiments. PHYSICAL REVIEW B 72. 245418 , 1-9.
Vlad A.Ivanov, Y. M. (2008). Dynamics of grain boundary motion coupled to shear deformation: An analytical model and its verifiction by molecular dynamics. Phys. Rev. B 78, 064106 , 1-12.
W. Lee, C. L. (2006). Effect of temperature and strain rate on the shear properties of Ti–6Al–4V alloy. Proc. IMechE Vol. 220 Part C: J. Mechanical Engineering Science , 127-136.
Yifei Mo, K. T. (2009). Friction laws at the nanoscale. Nature, Vol457 , 1116- 1119.
Yue Qi, Y. T. (2002). Friction anisotropy an Ni(100)/(100) interfaces: Molecular dynamics studies. PHYSICAL REVIEW B 66, 085420 , 1-7.