High loads, friction

Scott and Robbins [123] found that high-load friction is independent of hair fiber diameter, in agreement with theory and with the results of Fishman et al. [128]. However, others [129] have reported a slight increase in friction with wool fiber diameter. For low-load hair on hair friction, this variable would be difficult to test because as fiber diameter increases, the load also changes appreciably. 

Scott and Robbins [123] found that temperature changes from 75 to 110°F produced virtually no changes in the high-load frictional coefficient. Other temperature ranges have not been reported. 

Conditioner sets are an interesting category because some of these products increase hair body and at the same time make the hair easier to comb. Such effects probably arise because these products reduce high-load frictional forces when the hair is being combed wet (or even when dry), and upon drying, they increase the low-load friction and or cohesive forces between hbers. Certain hairsprays behave in this same manner. Increases to the apparent hber diameter are also possible for conditioner sets and hairsprays when they are combed thoroughly and distributed well throughout the hair. 

In many cases of traditional tribology, friction and wear are regarded as the results of surface failure of bulk materials, the solid surface has severe wear loss under high load. Therefore, the mechanical properties of bulk material are important in traditional friction and wear. However, in microscale friction and wear, the applied load on the interactional surface is light and the contact area is also under millimeter or even micrometer scale, such as the slider of the magnetic head whose mass is less than 10 mg and the size is in micrometer scale. Under this situation, the physical and chemical properties of the interactional surface are more important than the mechanical properties of bulk material. Figure 1 shows the general differences between macro and micro scale friction and wear. 

It is observed that indentations made with low loads on an indenter are smaller than expected from the sizes made with high loads. Thus the apparent hardness of a specimen increases as the indentation size decreases. This is known as the indentation size effect (ISE). It has been given a variety of interpretations, but the most simple is that it is associated with friction at the interface between the indenter and the specimen (Li et al., 1993). 

Mathur and Klinzing (1981) have developed a frictional term to account for the solids pressure drop which is meant for high loading systems (pL= 10 to 50). Their expression for / . is... 

In his original studies Amontons found a coefficient of friction of 0.3. Meanwhile it has become clear that friction coefficients can assume a whole range of values. With metals, a clear difference exists between clean metal surfaces, oxidized metal surfaces, and metal surfaces with adsorbed gas. Clean metals have coefficients of friction of 3-7. With oxidation, the value decreases to 0.6-1.0. A consequence is that the coefficient of friction can depend on the load. For small loads, friction is determined by the oxide coating. At high loads the microcontacts penetrate the oxide coating, the bare metals come into contact, and the coefficient of friction increases. 

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. 

Figure 11.11 Different lubrication situations in a journal bearing. Left At low velocities and high loads, boundary lubrication with a high friction coefficient dominates. The shaft climbs the journal on the right side. Right At high speeds and low loads hydrodynamic lubrication leads to much lower friction. The build up of the hydrodynamic wedge moves the shaft to the upper left.

Esters stick to the surface better than mineral oils. Since ester groups are polar, they form physical bonds with metal surfaces. At high loads, esters will tend to form chemisorbed films. Under extreme boundary conditions, esters tend to break down to form acids which leads to wear protection and friction reduction (Randles, 1999). These acids readily react with freshly exposed metal surfaces to form metal carboxylates tribofilms. 

Although petroleum products had been used earlier, their use only became important from the middle of the nineteenth century. They then slowly revolutionised lubrication because of their effectiveness, stability, availability and cheapness, and because of the wide range of viscosity grades which could be easily produced. Vegetable oils and animal fats continued to be used as alternatives, especially where there was a need for high load-carrying capacity or low friction, but otherwise little effort was made to find other types of lubricant for many years. 

Apart from its low-friction properties, the other attribute of molybdenum disulphide which is important in lubrication is its very high load-carrying capacity. Having said that, it is then impossible to give a specific value for the load-carrying capacity, because it depends entirely on the form and conditions in which it is used. 

For effective solid lubrication, it is not enough to have a material with low internal or external friction. It is also necessary for it to form films with sufficient adhesion to a substrate, and internal cohesion, to withstand rubbing under high loads. Molybdenum disulphide has this ability to a very high degree. It can be made to adhere readily and firmly to a substrate, forming a strong, cohesive film. 

In summary, the friction is at its lowest for fully ordered surface films in dry air or vacuum at high load and highest for randomly-oriented films in the presence of water vapour or certain other vapours at low load. 

---