Types of Friction

 In metal fabricating, various materials, in combination with
different types of lubricants, or in the absence of the same, will generate three basic types
of friction:
• Static, or dry friction—created between two metallic surfaces with no lubricant added.
The friction mechanism depends on the physical properties of the two materials in contact.
A metallic lubricant (for example, lead, zinc, tin, or copper) may improve this condition.
• Boundary friction––where two surfaces are separated by a layer of nonmetallic lubricant
a few molecules thin. The shear strength of the lubricating material is low, resulting in
low friction.
• Hydrodynamic friction—where two surfaces are totally separated by a viscous lubricant
of hydrodynamic qualities. In such a case, friction depends strictly on the properties of
the lubricant.
• Combined friction—or a mixture of the above conditions. This type of friction is the most
frequently encountered in metal-forming processes.
Out of all metal-forming processes, only a few do not require any surface treatment or
coating when it comes to friction. These are: Open-die-forming, spreading, some bending
operations, and extrusion of easily deformable materials. All other metal forming depends
on the use of proper lubricants. Even die forging requires a surface treatment of raw material;
in this case for the protection of the die itself.