Surface asperity

In contrast chemical and electrolytic polishing enables a smooth level surface to be produced without any residual stress being developed in the surface because the surface is removed by dissolution at relatively low chemical potential and at relatively low rates is such a way that metallic surface asperities are preferentially removed. For this to be most effective the solution properties must be optimised and the pretreatment must leave an essentially bare metal surface for attack by the electrolyte. 

FTR is an effective method for film thickness measurement in mixed lubrication. If the strength of incident light is properly adjusted, the resolution of film thickness by the FTR method can be limited within 5 nm. Theoretically, if the typical height of the surface asperities is less than the penetrating depth, the FTR method can be successfully used. In the tests, it is found that if Ra is greater than 0.15 /rm, a con-... 

It should also be remembered that surface asperities might increase the potential for mechanical interlocking of particles, which will influence the aggregation state and ease of dispersion of particles. 

The other type of superhydrophobicity is the Cassie-Baxter form, wherein the liquid does not fully penetrate the surface topography, but instead will sit atop surface asperities and air pockets. For nonwetting surfaces, this is a more likely scenario, since it may be more energetically favorable for the hquid to be in contact with air than a repelhng surface. This type of wetting can be described by... 

Mixed Film Ijibrication. Mixed film lubrication is almost invariably the true state ol affairs when boundary and EP lubrication are encountered, i.c.. an appreciable fraction of the load is carried by the fluid film in the valleys" of the surface w hile the asperities in contact are permitted to carry the halancc of (he load without seizure through ihe beneficent intervention of the boundary or EP lubricant. The very important hreakin process of rubbing surfaces consists in the controlled induction of the number and die size of the surface asperities so that fluid lubrication will prevail for must of ihe lime. 

Depending on the thickness of the lubricating layer, we distinguish between two different lubrication regimes. In hydrodynamic lubrication the lubrication layer is thicker than the maximum height of the surface asperities resulting in a complete separation of the friction partners. In boundary lubrication the lubrication layer is typically only a few molecular layers thick and therefore thinner than the surface roughness. In many practical applications we are between the two extremes, which is referred to as mixed lubrication. 

For a fixed geometry, the friction force depends solely on the viscosity of the lubricant. We could try to decrease the viscosity of the lubricant to reduce friction. There is, however, a limit to this The lubrication film thickness must always be kept higher than the surfaces asperities. Otherwise the surfaces will come into direct contact, resulting in much higher friction. Therefore, it is common to use an oil with a viscosity that is just high enough to maintain a continuous lubrication layer. 

At low sliding velocities and high loads, the lubricating film is squeezed out of the gap. This leads to so-called boundary lubrication. Friction coefficients under these conditions are typically 100 times higher than under hydrodynamic lubrication conditions, but still substantially smaller than for dry friction under UHV conditions. This is due to the fact that the surfaces are still wetted by molecular layers of the lubricant, even under conditions where the local stress is high enough to deform the surface asperities. Under these conditions friction depends more on the chemical constitution of the lubrication layer than on its viscosity. 

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. 

Apart from oxidation of the lubricant and the metal surfaces, there can be complex tribo-chemical reactions. Chemical reactions at the surfaces can be stimulated by different factors. One factor is heating due to friction. This can either be a global effect (elevated mean temperature of surfaces and lubricant) or a localized phenomenon. Especially in situations where mixed or boundary lubrication exists, the direct contact of surface asperities can lead to high flash temperatures. At these hot spots temperatures in excess of 1000°C promote chemical reactions and surface melting. Other factors promoting chemical reactions are ... 

During sliding, surface asperities undergo plastic deformation and/or fracture. The subsurface, up to several micrometers in thickness, also undergoes plastic deformation and strain hardening with microhardness, by as much as a factor of two or higher, than the bulk hardness.75... 

Figure 1.21. Advancing contact angles plotted as a function of the maximum slope of surface asperities for non-wetting (top) and wetting (bottom) systems (Hitchcock et al. 1981) [2],...

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