Lubricant Additives

Solid Lubricants as Friction Modifi ers 189

and formulation that utilizes the lubricant. The effectiveness of the fi nal formulation may prove that
a costlier solid lubricant is more cost-effective in use. Table 6.10 attempts to rate the effectiveness
of the solid lubricants for various criteria of application.

6.4.2 LUBRICATION FOR PLASTIC DEFORMATION OF METALS

Lubrication requirements for assisting metal deformation operations such as forging and metal
drawing are far more demanding than those for wear lubrication. The metal movement process cre-
ates very fast metal fl ow and rapid new surface generation. This creates a demand for a lubricant to
fl ow with the metal, remain adhered to the surface, maintain suffi cient fi lm cohesion to “meter” out
the lubricant with the advancing metal, and interact rapidly with the newly formed metal surface.
Metal-forming operations are inherently high-load and high-stress processes, which put a signifi -
cant demand on protective lubrication.
Most applications are conducted at an elevated temperature region. Under this circumstance,
conventional liquid lubricants fail to withstand the stresses for the application. Solid lubricants

FIGURE 6.11 Orientation of solid lubricant particles in the direction of motion.

TABLE 6.10

Solid Lubricant Selection Comparison and Rating

Criteria Graphite MoS 2 PTFE Boron Nitride

Normal atmosphere 1 1 1 1

Vacuum atmosphere 3 1 1 1

Ambient temperature 1 1 1 1

Continuous service

temperature to 260°C in air

111 1

Continuous service

temperature to 400°C in air

111 1

Continuous service

temperature to 450°C in air

2 3 N/A 1

Burnishing capability 1 1 3 2

Hydrolytic stability 1 2 1 1

Thermal conductivity 2 3 3 1

Load-carrying lubrication 2 1 1 2

Friction reduction 2 2 1 3

Dispersability 1 1 3 2

Color Black Gray White White

Relative cost 1 2 2 3

Note: 1 = best, 2 = good, 3 = ok