190 Lubricant Additives: Chemistry and Applications
are most appropriate for such applications because of their ability to withstand the operating tem-
peratures, orient and adhere to the substrate surface, provide the coeffi cient of friction reduction
necessary to promote metal fl ow, and provide the required load-carrying properties to prevent
metal-on-metal contact. Indeed, most applications that involve plastic deformation of metal will
utilize solid lubricants as either the primary or the secondary lubricant within a formulation.
What application criteria are used for determining the necessity for a solid lubricant? Severity
of metal movement is the most signifi cant factor. In cases where it is judged that metal movement
would be considered extreme, solid lubricants will most likely be required. Application examples
include forward, backward, and extreme lateral extrusion of metals. For example, forging of spindles,
constant velocity (CV) joints, crankshafts, and hubs would fall in this category. For these and simi-
lar cases, liquid lubricant technology falls short of providing the necessary lubrication, coeffi cient
of friction reduction, and die wear protection.
Once it has been determined that a solid lubricant is necessary, the temperature criteria need
to be determined. Metalworking applications done at ambient temperature can utilize MoS 2 as
the solid lubricant. MoS 2 has the best lubrication properties among the four lubricants discussed.
In fact, for applications such as cold forging, MoS 2 is the preferred lubricant because of its ability to
handle the very high load and stress applied onto the part being deformed.
In some cases, application of the MoS 2 is by dry-powder tumbling of the billets. Usually the
billets are phosphated before applying powder to anchor the MoS 2 onto the surface and within
the structure of the phosphate coating. The phosphate coating acts as an anchor for the powder
and allows the lubricant to advance with the metal deformation. Table 6.11 compares forging
performance for bare versus coated steel. Lubrication is improved as press tonnage falls and spike
height of the forged billet increases.
Dry-powder tumbling is an effective application method for some cases. Other situations will
require a more detailed and accurate depositing of MoS 2 fi lm onto the substrate. This requires the
use of a dispersed MoS 2 to provide a controlled coating thickness and particle size distribution
considered appropriate for the job.
There may be instances where MoS 2 is not desirable—for example, environmental concerns or
housekeeping issues. In these instances, PTFE or boron nitride would be appropriate. The white
color of the pigments alleviates concerns regarding cleanliness of using graphite and molybde-
num disulfi de. Situations that require a reduction in emissions and material reactivity would favor
boron nitride since PTFE will decompose at typical warm and hot forging temperatures. Both
would effectively lubricate, with perhaps boron nitride faring better than PTFE for applications
with signifi cant metal fl ow.
PTFE can, however, stand out as a lubricant for cold metal-forming operations involving sheet
stock and bar stock. The low coeffi cient of friction imparted by PTFE will provide the necessary
lubrication to assist metal fl ow in a manner far better than boron nitride and much cleaner than
graphite or molybdenum disulfi de.
All the solid lubricants would be appropriate for bonded-fi lm applications for metal deforma-
tion processes. Bonded fi lms are desirable for sheet metal applications where coil or blank metal is
TABLE 6.11
Cold-Forging Lubrication
Sample Press Tonnage Spike Height (mm)
Bare steel 80.2 10.67
Bare steel + zinc phosphate 79.6 11.11
Bare steel + zinc phosphate + MoS 2 78.4 11.46
Source: Acheson Colloids test data.