4. Atmosphere

The operating environment, which characterizes the contact environment (including, of course, the lubricant), plays a role through several factors:

• temperature, which affects the behaviour of surfaces, the third body ;

• physical properties such as viscosity, thermal conductivity and specific heat, which respectively influence the separation of bodies and the evacuation of heat energy... ;

• pollution (by abrasive products, wear debris, etc.), responsible for abrasion-erosion damage and degradation of lubricant films;

• composition and reactivity (associated with temperature), which govern the physicochemical transformation processes of materials, surfaces and interfaces (table 9 )....