Lubricant Additives

174 Lubricant Additives: Chemistry and Applications

Solid lubricants also assist applications where the sliding surfaces are of a rough texture or sur-
face topography. Under this circumstance, the solid lubricant is more capable than liquid lubricants
for covering the surface asperity of the mating surfaces. A typical application is a reciprocating
motion that requires lubrication to minimize wear. Another application for solid lubricants is for
cases where chemically active lubricant additives have not been found for a particular surface, such
as polymers or ceramics. In this case, a solid lubricant would function to provide the necessary
protection to the mating surfaces, which would normally occur due to the reaction of a liquid com-
ponent with the surface [2].
Graphite and molybdenum disulfi de (MoS 2 ) are the predominant materials used as solid
lubricants. These pigments are effective load-bearing lubricant additives due to their lamellar
structure. Because of the solid and crystalline nature of these pigments, graphite and MoS 2 exhibit
favorable tolerance to high-temperature and oxidizing atmosphere environments, whereas liquid
lubricants typically will not survive. This characteristic makes graphite and molybdenum disulfi de
lubricants necessary for processes involving extreme temperatures or extreme contact pressures.
Other compounds that are useful solid lubricants include boron nitride, polytetrafl uoroethylene
(PTFE), talc, calcium fl uoride, cerium fl uoride, and tungsten disulfi de. Any one of these compounds
may be more suitable than graphite or MoS 2 for specifi c applications. Boron nitride and PTFE are
discussed along with graphite and molybdenum disulfi de in this chapter.
What are the basic requirements for an effective solid lubricant? Five properties must be met in
a favorable way [3].

1. Yield strength. This refers to the force required to break through the lubricant or deform its
   fi lm. There should be high yield strength to forces applied perpendicular to the lubricant.
   This will provide the required boundary lubrication and protection to loads between the
   mating surfaces. Low yield strength of the fi lm should be present in the direction of sliding
   to provide reduced coeffi cient of friction. This dependency on directional application of
   forces is considered an anisotropic property.


2. Adhesion to substrate. The lubricant must be formulated in a manner that maintains the
   lubricant fi lm on the substrate for a suffi cient period necessary for the lubrication require-
   ments. The force of adhesion should exceed that of the sheer forces applied to the fi lm. Any
   premature adhesion failure will result in a nonprotective condition between the two sliding
   surfaces that require lubrication.


3. Cohesion. Individual particles in the fi lm of solid lubricant should be capable of building a
   layer thick enough to protect the high asperities of the surface and to provide a “reservoir” of
   lubricant for replenishment during consumption of the solid fi lm (see Figures 6.2 and 6.3).


4. Orientation. The particles used must be oriented in a manner that parallels the fl ow of the
   stress forces and provide the maximum opportunity for a reduction in the coeffi cient of
   friction. For this to occur, it is necessary for the dimensions of the particles to be greatest
   in the direction of low shear.

Liquid lubricant is squeezed out

Solid lubricant stays in there

Spot welds

No welds

FIGURE 6.1 Contact stresses on bearing points of mating surfaces cause a squeeze-out.