Solid Lubricants as Friction Modifi ers 181
and nitrogen within the hexagonal rings provides the high load-carrying capability that is necessary
to maintain metal–metal separation of the substrates. Similar to MoS 2 , boron nitride has intrinsic
lubrication properties. Boron nitride effectively lubricates in a dry as well as a wet atmosphere.
It is very resistant to oxidation, more so than either graphite or MoS 2 , and maintains its lubricating
properties up to its service temperature limit.
Commercial grades are available in a variety of purities and particle sizes. These varieties
infl uence the degree of lubrication provided by boron nitride since particle size affects the degree
of adhesion to substrate, burnishing ability, and particle orientation within a substrate. Impurities
such as boric oxide content need to be considered with respect to the lubrication capability of boron
nitride powder since this will infl uence the ability of the powder to reduce the coeffi cient of friction
for an application. The variation in grades will also infl uence the thermal conductivity properties
and ease of suspension in a liquid carrier. Table 6.5 summarizes typical properties for hexagonal
boron nitride.
6.2.4 POLYTETRAFLUOROETHYLENE
Polytetrafl uoro-ethylene (PTFE) has been in use as a lubricant since the early 1940s. Structurally, the
polymer is a repeating chain of substituted ethylene with four fl uorine atoms on each ethylene unit:
(CF^2 CF )^2 n (6.1)
Contrary to the other lubricants discussed, PTFE does not have a layered lattice structure. The
lubrication properties are at least partially the result of its high softening point. As frictional
heat begins to increase from sliding contact, the polymer maintains its durability and is able to
lubricate.
Various grades are produced and applied to specifi c applications as a result of the properties
imparted by the grade. For example, molecular weight and particle size are two characteristics that
can alter the performance of the polymer as a lubricant.
The critical characteristic of PTFE—the one it is widely known for [8]—is the outstandingly
low coeffi cient of friction imparted by the molecule. PTFE has one of the smallest coeffi cients
of static and dynamic friction than any other solid lubricant. Values as low as 0.04 for sliding
conditions have been reported for various combinations of PTFE fi lms on substrates [9]. The low-
friction property is attributed to the smooth molecular profi le of the polymer chains, which orient
in a manner that facilitates easy sliding and slip. It is postulated that the PTFE polymer results
in rod-shaped macromolecules, which can slip along each other, similar to lamellar structures.
TABLE 6.5
Hexagonal Boron Nitride
Property Value
Coeffi cient of friction 0.2–0.7
Color White
Crystal structure Hexagonal
Density 2.2–2.3 g/cm^3
Dielectric constant 4.0–4.2
Dielectric strength ∼35 kV/mm
Molecular weight 24.83
Service temperature 1200°C (Oxidizing atmosphere)
Thermal conductivity ∼55 W/mK
Size (grades) 1 – 10 μm