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POWMOD: Molecular dynamics simulations of surface contact in powders

C. Yong and W. Smith
CSE Department, CCLRC Daresbury Laboratory

K. Kendall
Department of Chemical Engineering, University of Birmingham

L.V. Woodcock
Department of Chemistry, UMIST

The study of two surfaces at close contact is crucially important for the understanding of friction. It occurs whenever two moving bodies come into contact and is the principle cause of wear and tear of machinery parts. The underlying atomic mechanisms that drive the macroscopically observed frictional phenomena are still poorly understood. The coefficients of friction that are introduced to describe contact mechanics in many theoretical treatments and computer simulations of powdered materials are, at best, phenomenologically derived and without sound scientific justification. Therefore, it is of considerable interest to study friction at the atomistic level where many of these effects originate.

Experimentally, surface-sensitive techniques such as atomic force microscopy [1] have begun to shed light on micro-contact responses between an atomic-scale tip-like body with a surface. Such studies are particularly relevant to friction since it is believed that a surface is rarely atomically flat and two body contacts usually occur in a number of discrete areas whereby a raised projection of one body makes contact with the surface of the other. In reality, contact interactions are very complex and difficult to characterise. This is because results are usually complicated by undesirable factors that are invariably present in experiments. For example, surface defects and contaminants such as adsorbed layers of hydrocarbons and water molecules. These factors together with the underlying surface molecular interactions give rise to the overall observed frictional phenomena between contacting bodies.

It is clearly desirable to isolate and investigate the individual factors in order to characterise friction properly and gain a better understanding of the contact mechanics of granular materials. To this end we have used the powerful DL_POLY [2] molecular dynamics software suite, written in Daresbury Laboratory, to obtain a mechanistic view of surface contacts on an atomic scale.

Since the primary requirement for friction to occur is for two bodies to come into contact with each other, we started by looking at simple commensurate contact between two materials of similar type - a surface slab and an isolated material block (probe) of finite cross sectional area. Different cross sectional areas of the probe were considered to represent the roughness of a raised projection of a surface in contact with a flat surface. To monitor the contact behaviour, the average normal force, Fz, experienced by the probe was measured. This was obtained while bringing the probe towards the surface at a rate of 5 m/s, a typical rate found in granular flows under normal conditions. The probe's rigid-support plane was advanced by 0.5 and the whole system was allowed to equilibrate for 0.5 ps. This was followed by data sampling and averaging over subsequent 0.5 ps. After that the whole procedure was repeated.

Figure 1: Forces arising during approach and withdrawal of MgO probe

We began by looking at the (001) MgO system, a simple ceramic oxide material. It was found that probes of all sizes gave rise to a characteristic 'jump' in the value of Fz at a certain critical distance where the probe was attracted towards the surface slab. A typical profile is shown in Figure 1 for the 44 (001) MgO probe. The magnitude of this 'jump' is also found to vary according to the area of the probe and can be characterised by a logarithmic exponent of q = 0.12, Figure 2. Interestingly, similar calculations for (001) NaCl systems also yielded a similar value, as shown in Figure 2. In addition, we have also repeated the calculations by withdrawing the MgO probes at the same rate as in the incoming cases. In all cases hysteresis was observed, followed by a similar 'jump', though at larger values of Fz than before. The Fz profiles show a series of complex saw tooth patterns, shown as a dotted line in Figure 1. From the atomic configurations, it was found that probes undergo a necking process, with progressive reduction of cross sectional area. This involves plastic flow with systematic atomic dislocation as the probes are drawn away from the surface.

Figure 2: Power law scaling in frictional forces

The work on MgO systems has been accepted for publication [3]. In addition, we have also looked at a more complex systems such as (110) TiO2 and also NaCl[4]. We found that the contact process is rather different, whereby bridging atoms were formed during the initial contact processes. The NaCl and TiO2 results are in preparation for publication. We have also begun to look at the sliding behaviour of the probes at various distances from the surface slabs.

References

[1] G. Binnig, C. F. Quate and Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930.

[2] T.R. Forester and W. Smith, The DL_POLY Simulation Package J. Molec. Graphics 14 (1996) 136.

[3] C. W. Yong, W. Smith and K. Kendall, J. Mater. Chem. 12 (2002) 593

[4] C. W. Yong, W. Smith and K. Kendall, J. Mater. Chem. 12 (2002) 2807

 
For more information about the work of the Computational Chemistry Group please contact Paul Sherwood p.sherwood@dl.ac.uk.

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