Solid Lubricants as Friction Modifi ers 187
axle components might be expected. Along with friction reduction, there should be a corresponding
increase in fuel effi ciency for motor vehicles. Various studies seem to support that conclusion.
One report claims that in fl eet trials conducted according to EPA 55/45 fuel economy testing with
reference motor oils fortifi ed with either MoS 2 or graphite, both in a colloidal dispersion, the fuel
economy was improved by 4.5% [12]. In another fuel economy study using a fl eet of taxicabs, the
use of 2% colloidal graphite or colloidal MoS 2 in low-viscosity-formulated engine oils and rear axle
lubricants improved the fuel economy by 2.5% [13].
The friction-reducing infl uence of colloidal graphite in oil is illustrated in one study by a
dynamometer evaluation conducted on a 2.3 L engine [14]. The study indicates that graphite prop-
erly dispersed in an appropriate liquid lubricant will considerably reduce friction with the subse-
quent benefi t of fuel economy savings.
Solid lubricants are also applied as bonded fi lms for certain applications. For example,
applications requiring a permanent or semipermanent lubricating fi lm would require a bonded fi lm.
Bonded coatings are commonly formulated with MoS 2 or PTFE. One example would be for self-
lubricating composites that require high-temperature stability, such as for what may be needed for
engine piston ring protection [15]. Other examples that benefi t from a bonded lubricant include
fasteners, chains, and reciprocating mechanisms that require a persistent lubricating fi lm. For these
applications, PTFE stands out due to its low coeffi cient of friction. This is summarized in Table 6.8
by comparative coeffi cient of friction data for PTFE, graphite, and MoS 2 , which are bonded onto
cold-rolled steel substrates.
In assessing the lubrication potential for dispersed solid lubricants, some type of bench testing
is utilized to characterize the apparent lubrication performance of the material. The most typical
lubrication tests are Shell 4-Ball Wear method, Shell 4-Ball EP method, Falex Pin–Vee method, Plint
Reciprocating method, Incline Plane method, and FZG Gear Lubrication method. In many cases,
custom lubrication tests are developed for the specifi c application to be considered. When conducting
bench testing for lubricant performance, correlation is best achieved when the mode of contact and
conditions of the application are closely replicated by the bench test. The confi guration of the contact
points for the application is matched with a similar mode of contact for the bench test.
For an illustration of laboratory lubrication assessments, see Table 6.9 [16] to compare the
empirical performance of the four solid lubricants dispersed in an oil carrier. The lubricants were
tested according to two common methods of lubrication evaluation.
In this example, the dispersion of MoS 2 and PTFE provides effective load bearing, wear resis-
tance, and coeffi cient of friction reduction when evaluated by a point-to-point contact (4-ball) and
line-to-point contact (Falex Pin–Vee). Interpretation of any bench test result must be done carefully
to ensure the validity of extrapolating the test performance to the actual application.
What criteria should be considered for an application when selecting the preferred or optimal
solid lubricant? First, consider the service temperature for the application. This dictates which solid
TABLE 6.8
Coeffi cient of Friction for Bonded fi lms
Coeffi cient of Frictiona
MoS 2 0.23
Graphite 0.15
PTFE 0.07
a Evaluated at room temperature, ASTM D4918.
Source: Watari, K., Huang, H.J., Turiyama, M., Osuka,
A., Yamamoto, O., U.S. Patent 5,985,802,
11/16/99.