Asperity contacts indentations

As the simplest test to simulate multiple asperity contacts, misaligned dual-pin/disk experiments were conducted [12]. Two diamond pyramidal indenters were slid repeatedly along close tracks on pure aluminium, as shown schematically in Fig. 1. Figure 2 shows the main part of the dual-pin/disk rig. A disk specimen is mounted on a table driven at slow speeds by an AC servomotor via harmonic gears. Two pins were supported by loading arms, and pressed against the disk. The arms were pivoted on position-... 

Molecular dynamics (MD) permits the nature of contact formation, indentation, and adhesion to be examined on the nanometer scale. These are computer experiments in which the equations of motion of each constituent particle are considered. The evolution of the system of interacting particles can thus be tracked with high spatial and temporal resolution. As computer speeds increase, so do the number of constituent particles that can be considered within realistic time frames. To enable experimental comparison, many MD simulations take the form of a tip-substrate geometry correspoudiug to scauniug probe methods of iuvestigatiug siugle-asperity coutacts (see Sectiou III.A). 

Abrasion occurs when one material is in contact with a harder material. Surface asperities of the harder material cut, plough, or indent characteristic scratches or grooves into the softer material (two-body abrasion). Abrasion can also be caused by hard particles that are trapped in between two surfaces (three-body abrasion). Irregular patterns of small indentations are formed. Contamination in the lubricants can significantly contribute to this type of abrasion. 

The fact that this model considers elastic and inelastic fracture aperture or closure has already been discussed in the literature. For instance, Renner et al (2000) investigated the behaviour of fractured argillaceous rocks including permeability variations induced by changes in confining pressures. In this work crack dimensions and permeability are correlated by means a model that takes into account elastic crack closure and crack closure controlled by inelastic processes. This later is explained by asperity indentation when rough crack walls contact each other. 

When two metals come into contact under a given load, asperities are pressed against one another and undergo plastic flow and creep the area of contact increases with temperature and contact time, as in a hot hardness mutual indentation. Then, when the stress has dropped sufficiently the area of contact predominantly increases by surface or boundary diffusion as in sintering experiments. As these various mechanisms are thermally activated, the increase of area of contact with temperature can be written ... 

If Pasp exceeds the elastic limit of the material, the asperities undergo plastic deformation. As a consequence, the real contact area increases until the value of Fn/Aj becomes equal to the elastic limit of the material. This is the same situation as in a hardness test the application of a given normal force produces an indentation whose area characterizes the hardness H of the material. We can therefore write ... 

Fig. 6.39. Images of dislocation nucleation in a bubble raft model of a single crystal subjected to surface indentation. In the case of the smallest surface roughness (a), dislocation nucleation occurs at the peaks of the asperities where they contact the indenter. At the intermediate scale (b), dislocation nucleation occurs at the reentrant corners at the bases of the asparities. Finally, for the largest scale asperity (c), the stress level near the stress concentrations has been reduced geometrically, and dislocations nucleated homogeneously at an interior point where the shear stress is the largest. Each bubble in this raft is 1 mm in diameter and it represents an atom which is approximately 0.3 nm in diameter. Reproduced with permission from Gouldstone et al. (2001).

In principle, the coefficient of friction is independent of the surface area of the two materials in contact. In the case of polymers the actual contact area is difficult to determine, because asperities of hard surfaces can indent the polymer while those of the polymer deform until the contact area is sufficient to support the applied forces. 

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