Solid Lubricants as Friction Modifi ers 191
prepared with a dry-fi lm lubricant. When developing bonded-fi lm lubricants, consider the formula-
tion of effective binders and bonding agents so that the solid lubricant can function as intended.
For metalworking applications at elevated temperatures, the operating temperature will
determine which solid lubricant can be used. All the solid lubricants mentioned would be suitable
for temperatures up to 260°C. Above that temperature, PTFE will be eliminated from consider-
ation due to its decomposition. MoS 2 will be suitable for applications up to 400°C in an oxidizing
environment. Above that temperature, decomposition of MoS 2 will occur. Both graphite and boron
nitride will lubricate effectively above an operating temperature of 400°C. Graphite is the predomi-
nant lubricant used for plastic deformation at elevated temperatures.
The use of graphite is common and preferred for what is considered warm- and hot-forging
situations. The forging process is considered warm forging when billet temperatures are up to
950°C. The process is considered hot forging when billet temperatures exceed 950°C. In both cases,
oxidation of graphite will occur. But the rate of oxidation depends on temperature and is regulated
by the formulation and characteristics of graphite. Graphite quality, contaminants, crystallite size,
and particle size will infl uence the rate of oxidation. The components of the fi nished formulation
also play a role in controlling the oxidation rate of graphite, allowing it to survive for an appropriate
length of time necessary for lubricating the process.
The type and quality of graphite play an important role in performance. Its consideration is
the fi rst step in a selection process. The fi rst choice is to choose between natural and synthetic
graphites. Often the choice is dictated by the degree of graphite quality suitable for the application.
For instances where average lubrication is required, natural graphite of lesser quality can be used.
More demanding lubrication will require the use of high-purity synthetic or natural graphite.
Selection of the particle size of graphite will vary depending on the intentions for the job. Par-
ticle size should be matched to the type of metal movement expected from the process, the surface
roughness of the die and part, and the degree of stability required for the formulated lubricant. If a
large particle distribution is desired, then concern about physical stability of the lubricant must be
addressed. Rapid settling and hard packing of graphite could occur due to the large particle size if
countermeasures are not taken. This would create handling costs and product inconsistency for the
end user.
For most circumstances, high-quality graphite should be used so as to minimize performance
inconsistency. The quality and characteristic of graphite can affect the lubricating performance.
Table 6.12 illustrates a lubricity comparison of standard formulations produced with differ-
ent graphites. In this example, the application is warm forging of steel. Actual forging of a steel
billet generates lubrication data where the spike height is determined using preset forging press
parameters (see Figure 6.12). A greater spike height and lower coeffi cient of friction suggest better
lubrication from the coating.
O n c e t h e t y p e o f g r a p h it e t o b e u s e d i s s e l e c t e d , t h e n t h e c o s t o f t h e p owd e r n e e d s t o b e c o n s i d e r e d
versus the benefi t derived from its use. In general, high-purity natural or primary synthetic graphite
will be costlier than secondary synthetic graphite. However, the performance benefi t of using the
higher-cost material may justify its selection for the application. Benefi ts normally associated with
TABLE 6.12
Graphite Infl uence on Forging Lubrication (800°C
Forging Temperature)
Graphite Spike Height (mm) Coeffi cient of Friction
A 1.5 0.05
B 1.3 0.08
C 1.1 0.10