Friction rough surfaces

Years of development have led to a standardized system for objective evaluation of fabric hand (129). This, the Kawabata evaluation system (KES), consists of four basic testing machines a tensile and shear tester, a bending tester, a compression tester, and a surface tester for measuring friction and surface roughness. To complete the evaluation, fabric weight and thickness are determined. The measurements result in 16 different hand parameters or characteristic values, which have been correlated to appraisals of fabric hand by panels of experts (121). Translation formulas have also been developed based on required levels of each hand property for specific end uses (129). The properties include stiffness, smoothness, and fullness levels as well as the total hand value. In more recent years, abundant research has been documented concerning hand assessment (130—133). 

Therefore, for a rough surface, although it is smooth macro-scopically, the friction coefficient is greatly affected by its surface morphology. For a smooth surface on atomic scale, 0 can be neglected and the friction coefficient is only related to the twist angle of the cantilever. 

In the region of low viscosity the friction is higher on rough surfaces than on smooth ones whilst in the high viscosity region the reverse is the case indicating that viscosity alone is not responsible... 

This is not surprising since at a given speed the coarseness of a track (the average spacing of the asperities) influences the friction only on a logarithmic scale. Also the observed dependence of the friction coefficient on load of soft mbber compounds on smooth surfaces disappears for harder black or silica-filled treads compounds on rough surfaces. 

Antiadherents. Some materials strongly adhere to the metal of the punches and dies. Although not a frictional effect, this results in material preferentially sticking to the punch faces and gives rise to tablets with rough surfaces. This effect, called picking, can also arise in formulations containing excess moisture. 

The flow friction of a channel having subcooled local boiling behaves in a manner similar to the flow friction of a channel having a rough surface. From air-water data obtained at low void fractions (Malnes, 1966), the two-phase flow friction factor without bubble departure was found (Tong, 1968b Thorgerson, 1969)to be... 

The friction coefficient is expected to depend on the normal pressure which is quite high (of order hundreds of kilobars) surface roughness surface homogeneity and humidity (or other environmental factors). As a result, a is not known, so a quantitative model is not possible, but the expected qualitative behavior is clear. 

As a final point, we note that typical surfaces are usually not crystalline but instead are covered by amorphous layers. These layers are much rougher at the atomic scale than the model crystalline surfaces that one would typically use for computational convenience or for fundamental research. The additional roughness at the microscopic level from disorder increases the friction between surfaces considerably, even when they are separated by a boundary lubricant.15 Flowever, no systematic studies have been performed to explore the effect of roughness on boundary-lubricated systems, and only a few attempts have been made to investigate dissipation mechanisms in the amorphous layers under sliding conditions from an atomistic point of view. 

The comparison of continuum and atomistic models by Luan and Robbins demonstrates that the atomic details of this contact can have a significant influence on the calculated friction. However, those calculations did not explore atomically rough surfaces, which are most likely found in real engineering contacts. The effect of roughness has been investigated recently by Qi et al. in a study of the friction at the interface between two Ni(100) surfaces.85 Two models were considered in that work. In the first model, both surfaces were atomically flat i.e., the rms roughness was 0.0 A. In the... 

Figure 21 Friction coefficient for differently oriented Ni(100)/Ni(100) interfaces. Rough surfaces have a 0.8 A rms variation in roughness added to the atomically smooth surfaces. Reproduced with permission from Ref. 85.

Red phosphors are formed either by heating white phosphorus or by exposing white phosphorus to sunlight. It is quite different from the explosive white phosphorus. For instance, when scratched on a surface, the heads of safety matches made of red phosphorus convert back to white phosphorus and ignite due to the heat of the shght friction of the match on a rough surface. Red phosphorus is also used in fireworks, smoke bombs, and pesticides and to make phosphoric acid, electroluminescent paints, and fertilizers. 

Sensitiveness to friction. Nitroglycerine is sensitive to friction. Thus, for instance, an explosion can take place simply as the result of rubbing it in a porcelain mortar with a rough surface. 

The shearing stress, r, exerted by the wind on the ground entails a downwards flux of momentum. In the aerodynamic boundary layer above the surface, the momentum is transferred by the action of eddy diffusion on the velocity gradient. The friction velocity is defined by w = t/pa and is a measure of the intensity of the turbulent transfer. Near to a rough surface, the production of turbulance by mechanical forces... 

Equation (10) was shown to follow from a thermally activated model of Eyring s type, that is a stress-assisted thermally activated process for viscous flow [154]. For rough surfaces the area of true molecular contact is very small and, hence, the adhesion contribution to the total friction force is usually negligible, as manifested by a zero friction at zero load [148]. Yet in the microscopic limit of the SFM experiment employing a comparatively sharp... 

Greenwood JA (1992) Contact of rough surfaces. In Singer IL, Pollock HM (eds) Fundamentals of friction macroscopic and microscopic processes. Kluver Academic Publishers, London, p 37... 

---