**2. Types of lubrication conditions**

In 1902, Stribeck was the first to report the dependence of the coefficient of friction on the shaft speed in bearings. In his work he presented a curve with three different lubrication regimes. This curve was named "Stribeck Curve". Subsequently, the coefficient of friction was presented as a function of the combination of the following parameters: lubricant viscosity, number of bearing shaft revolutions and normal shaft pressure. **Figure 3** schematically illustrates the Stribeck curve where the initiation of various types of lubrication mechanisms as a function of lubricant viscosity, η, slide velocity, ν, and normal pressure, p.

Friction effects are always related to energy dissipation, so they are sometimes considered undesirable in manufacturing processes. In the case of metal forming, metal flow is limited by the contact pressure between die and part. Thus, friction can generate internal and surface defects, in addition to influencing the tensions in the tools, load and energy required. The purpose of the lubricant is to separate the tool and workpiece surfaces, thus shifting the friction conditions from the boundary friction area in the direction of hydrodynamic friction as shown in **Figure 3**.

There are four distinct lubrication conditions that determine the influence of friction on metal forming:


**Figure 3** also shows the separation between the contact surfaces, as friction approaches Hydrodynamic Lubrification, there will be an increasing separation between work material and tools. This is not only in macroscopic conditions but can also occur in microscopic level where the lubricant will be retained in the roughness valleys and will be able to act when the pressure is greater. This will be explained further in the next item.

*The Role of Friction on Metal Forming Processes DOI: http://dx.doi.org/10.5772/intechopen.101387*

**Figure 3.**

*Stribeck diagram for different states of lubrication, with g dynamic viscosity, v sliding speed, p pressure/surface pressure. Source: Klocke [5].*

**Figure 4.**

*Schematic illustration of the components of the coefficient of friction present in metal/metal contact. Source: Folle and Schaeffer [6].*

In addition to the conditions described above, two main factors can occur in the contact between two surfaces, the first is a friction by material adhesion that is caused by micro-welding at the contact interfaces and the second is a penetration friction. In the first case (**Figure 4a**), there is usually the pullout of the material with a lower hardness (workpiece) and this material ends up working as an interface material that promotes scratching of the shaped parts. In the second case (**Figure 4b**), there is a very large approximation between the parts that the roughness ends up being interpenetrated. In this case, it is very likely that material will be pulled out of the workpiece, promoting abrasive wear on the pieces and tools. In either case, a good lubrication condition can resolve.
