ABSTRACT
Friction and wear mitigation, o§en interrelated, are typically accomplished by introducing a shear accommodating layer (e.g., a thin Ÿlm of liquid) between contacting surfaces. When the operating conditions are beyond the liquid realm, attention turns to solids (bulk or coatings). According to the classical theory [8], friction force, F, is a product of the contact area and the shear strength of the solid material, A · τ. ˜us, the friction coe²cient, μ, can be expressed by
m t t t a= +F
L A L P PH
⋅
= = (35.1)
where L is the normal force (load) PH is the mean Hertz pressure τo is the interfacial shear strength, a “velocity accommodation parameter” which is a property of the
interface α represents the pressure dependence of the shear strength
˜e constant “α” is the lowest attainable friction coe²cient for a given friction couple. In principle, a hard material with a so§ skin (e.g., so§ metal or oxide Ÿlm on the metal) ought to provide low friction coe²cient by reducing το and increasing PH (low A). Bowden and Tabor validated their concept by demonstrating that indium metal when applied as a coating on a much harder steel substrate can indeed reduce friction [8,26]. It is also interesting to note from their study that while the friction coe²cients for the unlubricated steel and for the steel lubricated with mineral oil obeyed Amonton’s Ÿrst law of friction (μ is independent of L), the friction coe²cient of indium Ÿlm on steel substrate decreased considerably with increasing normal load (see explanation in Chapter 39). Here, the increased deformation of the underlying steel resulting from an increase in normal load produced only a small increase in the real contact area. ˜us, there is only a slight increase in frictional force as the load is increased, which results in a corresponding decrease in friction coe²cient according to Equation 35.1. For thin layers and so§ metal coatings, the pressure is primarily supported by the substrate and increasing the substrate modulus and hardness will decrease the contact area for a given normal load. ˜us, the ideal scenario for achieving low friction according to the Bowden and Tabor concept is to have an elastically stiµ and hard substrate support the normal load and keep the contact area small, while the surface provides shear accommodation and reduces junction strength, until the substrate begins to yield and plastically deform. ˜ese are necessary but not totally su²cient conditions for most low friction metal-based systems. In addition, compliant transfer Ÿlms (third bodies) that adhere to the counterface or transfer layers that adhere on the wear track, o§en tribochemically, can lower το further and provide long wear life by preventing native metal-metal contact while accommodating interfacial shear [27]. For more details, see Chapter 39.
