Physico-chemical characterization of coatings and thin films with tribological applications

When two solid bodies are in contact, a force or torque must be exerted to overcome the resistance to relative movement: this is friction, and more precisely, so-called static friction, which opposes this relative movement. As the relative movement of the two bodies continues, friction evolves (dynamic friction), causing damage to their surfaces: scratching, change of appearance, loss or gain of material (wear or transfer), physico-chemical transformation. To control friction and limit wear, transfer and surface degradation, we can lubricate the contact, i.e. interpose a fluid or solid body that can be easily sheared, and/or coat the bodies with a film of submicron or micron thickness. Tribology is the study of these phenomena of friction, lubrication and wear, which concern machine components as well as wheel/rail and tire/road surface contacts, braking and the shaping of materials. In this article, examples are given to illustrate operating modes. They concern classic solutions or future solutions which are currently the subject of research. It should be noted, however, that the objectives of a given application may be multiple: it may be necessary to control friction while at the same time limiting wear and damage. Examples will be treated accordingly. The aim is to understand the link between tribology, which is generally a mechanical approach to materials, and physical chemistry. Note that the thickness of coatings is adapted to the application. The advantages of the coated solution over the bare substrate will be demonstrated. This requires first and foremost that the coating adhere well to the substrate. As a preamble, we therefore discuss the problem of its adhesion to the substrate.