180 Lubricant Additives: Chemistry and Applications
mechanism of lubrication. At the same time, the crystal structure and strong interplanar bond
forces of MoS 2 allow for high load carrying against forces applied perpendicular to the plane of
the crystal. This is necessary for the prevention of metal on metal contact for high-load applica-
tions such as gearbox lubrication.
MoS 2 scores well in the other criteria for an effective solid lubricant. It forms a strong cohesive
fi lm, that is smoother than the surface of the substrate on which it is bonded. MoS 2 fi lm has suf-
fi ciently high adhesion to most metal substrates, which it successfully burnishes onto the wearing
surfaces, thus minimizing metal wear and prolonging friction reduction. This characteristic is an
exception, however, with titanium and aluminum substrates due to the presence of an oxide layer on
the metal surface, which tends to reduce the tenacity of the MoS 2 fi lm.
The lubrication performance of MoS 2 often exceeds that of graphite. It is most effective for
high load-carrying lubrication when temperatures are <400°C. Another advantage of MoS 2 is that
it lubricates in dry, vacuum-type environments, whereas graphite does not. This is due to the intrin-
sic lubrication property of MoS 2. On the contrary, the lubricating ability of MoS 2 deteriorates in
the presence of moisture because of oxidation of MoS 2 to MoO 3. The temperature limitation of
MoS 2 is due to similar decomposition issues of the material as that experienced with moisture. As
MoS 2 continues to oxidize, MoO 3 content increases, which induces abrasive behavior and increases
coeffi cient of friction for the surfaces to be lubricated.
The effectiveness of MoS 2 improves as contact forces increase on the lubricated surface.
Burnished surfaces exhibit coeffi cient of friction reduction as a function of increasing contact forces
[7]. In contrast, graphite does not necessarily exhibit this behavior. The frictional property of MoS 2
systems has been reported to be generally better than graphite in many instances, up to the service
temperature limitations for the lubricant.
The particle size and fi lm thickness of MoS 2 will affect lubrication. Generally, the particle size
should be matched to the surface roughness of the substrate and the type of lubrication process
considered. Too large a particle distribution may result in excessive wear and fi lm reduction as
mechanical abrasion is experienced. Too fi ne a particle size may result in accelerated oxidation in
normal atmospheres as the high surface area of the particles promotes the rate of oxidation.
6.2.3 BORON NITRIDE
Boron nitride is a ceramic lubricant with interesting and unique properties. Its use as a solid lubricant
is typically for niche applications when performance expectations render graphite or molybdenum
disulfi de unacceptable. The most interesting lubricant feature of boron nitride is its high-temperature
resistance. Boron nitride has a service temperature of 1200°C in an oxidizing atmosphere, which
makes it desirable for applications that require lubrication at very high service temperatures. Graph-
ite and molybdenum disulfi de cannot approach such higher service temperatures and still remain
intact. Boron nitride also has a high thermal conductivity property, making it an excellent choice for
lubricant applications that require rapid heat removal.
A reaction process generates boron nitride. Boric oxide and urea are reacted at temperatures
from 800 to 2000°C to create the ceramic material. Two chemical structures are available: cubic and
hexagonal boron nitride. As one might expect, the hexagonal boron nitride is the lubricating version.
Cubic boron nitride is a very hard substance used as an abrasive and cutting tool component.
Cubic boron nitride does not have any lubrication value. The hexagonal version of boron nitride is
analogous to graphite and molybdenum disulfi de. The structure consists of hexagonal rings of boron
and nitrogen, which are connected to each other, forming a stack of planar hexagonal rings. As with
graphite, boron nitride exhibits a platelet structure.
The bond strength within the rings is strong. The planes are stacked and held together by
weaker bond forces. Similar to graphite and molybdenum disulfi de, this allows for easy shearing
of the planes when a force is applied parallel to the plane. The ease of shear provides the expected
friction reduction and resulting lubrication. Concurrently, the high bond strength between boron