Depending on the relative speed of the contact area and the lubrication condition, the friction displays non-linear behaviour, which can be attributed to characteristic mechanisms of action. A distinction is made between three different friction conditions (Figure R 23):
- Solid dry friction (a) arises due to adhesion and deformation in the case of direct contact between solid bodies. Solid dry friction is characterised by high frictional forces, which are not dependent on speed, and a high level of wear, μ > 0.5;
- Mixed friction (b) occurs in the transitional area between solid and liquid friction, μ = 0.1 to 0.2. Mixed friction is characterised by the simultaneous presence of areas with lubricating film and touching solids, which causes a frictional force that decreases as relative speed increases.
- Liquid friction (c) arises due to the build-up of a stable lubricating film between the sliding bodies. The dynamic pressure created in the wedge-shaped gap due to the viscosity of the lubricant separates the sliding bodies from one another, producing a low frictional force that rises as the relative speed increases and a low level of wear, μ = 0.01 to 0.05.
Figure R 23: Friction conditions: a) Solid dry friction, b) Mixed friction, c) Hydrodynamic friction