Friction force lubricant viscosity

In conventional-sized machines such as an engine systan, lubricants are often used to reduce the friction force. Liquid lubricants, however, generate capillary force and often cause stiction in micranechanisms. Moreover, even a low-viscosity liquid tends to increase friction force because viscosity drastically iuCTeases when the spacing between solid surfaces becomes narrow and is on the order of nanometers [8]. Therefore, liquid lubricants could cause an increase of the friction force in MEMS. In micromechanisms, one solution for reducing the friction force is molecular boundary lubrication. 

Lubricants are added to lower interfacial frictional forces between individual particles and/or between particles and fonning die surfaces to improve compaction and ejection (i.e. extraction of the pressed compact from the fonning die). Individual particle surfaces can be lubricated by an adsorbed film that produces a smoother surface and/or decreases interiDarticle attraction. Fonning (die) surfaces can be lubricated by coating with a film of low-viscosity liquid such as water or oil. 

All sliding friction forces are dramatically affected by surface contamination. If the surface is covered with a material that prevents the adhesive forces from acting, the coefficient is reduced. If the material is a liquid which has low shear viscosity the condition exists of lubricated sliding where the characteristics of the liquid control the friction rather than the surface friction characteristics of the materials. It is possible by the addition of surface materials that have high adhesion to increase the coefficient of friction. 

When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

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. 

Internal Lubricants. Internal lubricants are substances that have a good affinity to the polymer and reduce its viscosity in the melt state. Moreover, they reduce the heat produced by the action of the friction forces. They also affect... 

Hydrodynamic and elastohydrodynamic lubrication breaks down at low sliding velocities and under excessive loads since the lubricating film is squeezed out of the gap. Ifthe lubricant is completely removed, dry friction would occur, which would imply high friction forces and wear. If a thin layer of lubricant remains adsorbed, friction between surfaces is strongly reduced compared to dry friction. This leads to the so-called boundary lubrication. Friction coefficients under these conditions are typically 100 times higher than those under hydrodynamic lubrication conditions but still substantially smaller than those for dry fnction under UHV conditions. In boundary lubrication, fnction essentially depends on the chemical constitution of the lubrication layer and not on its bulk viscosity. 

Interfacial agents that tend to wet or lubricate the filler surface (titanates, stearates, etc.) tend to reduce viscosity. This may result from attenuated interparticle forces and less tendency to flocculation since polymer molecules may slip between treated filler particles encountering less frictional resistance (see Figure 2.5) 10]. Reduced particle-particle interactions may lead to further flow orientation of fibers and flakes and to a further decrease in viscosity at high shear rates. However, an increase in viscosity may occur when the surface treatment causes strong adhesion of the filler to the polymer. 

Hydrodynamic lubrication is favored by a high sliding velocity, a weak normal force and a high viscosity of the lubricant. In machines, lubricated contacts normally operate in the hydrodynamic regime, minimizing thus friction and wear. 

The primary reason to use lubricants is to reduce friction and wear between two interacting surfaces. Hydrocarbon oils have the proper friction properties to meet these requirements but their low viscosity may cause them to be forced out of the contact region between interfaces. Powders of low MW PIEE may be added to liquid lubricants to provide reserve lubrication in case the liquid phase is forced out. Low-MW PTFE that is used this way is sometimes called an extreme pressure or boundary additive. The type of PTFE used in lubricants may be from either suspension or dispersion polymerization but the small particle size of dispersion-type PTFE is usually preferred to help maintain dispersion in the oil. Many Journal articles and patents have been published that report the performance of lubricating oils with and without the addition of PTFE. For example, Rico et al. [44] provided the results of an extreme pressure wear study of steel balls (Shell four-ball test) with several mineral oils containing four different percentages (1-10%) of PTFE. 

Viscosity. Of the various lubricant properties and specifications, viscosity (also referred to as the body or weight) normally is considered the most important. It is a measure of the force required to overcome fluid friction and allow an oil to flow—that is, the resistance to flow. 

---