Dry Film Lubrication
Dry film lubricants are solid materials that provide low frictional
resistance between surfaces when applied directly to interacting surfaces.
Each material has different properties. Crystalline lattice (lamella)
structure materials, such as molybdenum disulfide, tungsten disulfide
and graphite, are widely used as agents in lubricants and as stand-alone
lubricants. These materials are used independently or in combination with
other agents and metals (lead, copper) to achieve the desired results.
Lamella lubricating powders have low shear forces between their
crystalline
lattice layers that minimize resistance between sliding surfaces.
Figure 1. Crystal Structure of MoS2
(Reference: Dynamic Coating, Inc.)
As seen in Figure 1, these materials have structured layers that form
and interact against other structure layers.
Most dry lubrication film materials work well in dry environments and
are
excellent supplemental or boundary layer materials in fluid systems.
The long chain fluorocarbon molecules, such as polytetrafluoroethylene,
tend to have wetting angles that promote release and prevent sticking,
as well as a variety of other attractive
characteristics for high-temperature operation. This article addresses the
most frequently used dry film lubricating agents.
General Dry Lubricant Properties
Each dry lubricating material has different properties.
Molybdenum disulfide, graphite and tungsten disulfide are oilioscopic.
Their structure is unable to tolerate detergents. These layer lattice
lamella structures are analogous to stacks
of nonadherent plates which, with slight tangential loading, slip out of
place. It is comparable to walking across a room full of flat slippery plates.
The lamella materials have good load-bearing capacity in sliding and
rolling mode. Graphite has high-temperature capability and functions
well in radiation atmospheres. MoS2 and WS2 function well in hard
vacuum and tolerate higher
loads better than graphite.
Molybdenum Disulfide (MoS2)
Molybdenum was not discovered until the latter part of the 18th century,
and does not occur in the metallic form in nature. Despite this, its
predominant mineral - molybdenite - was used in ancient times
(a 14th-century Japanese sword has been found to contain molybdenum)
but would have been indistinguishable from other similar materials such
as lead, galena and graphite. Collectively, these substances were known
by the Greek word “molybdos”, which means lead-like.
Molybdenum was first positively identified in 1778 by a Swedish scientist,
Carl Wilhelm Scheele. Shortly thereafter, in 1782, Peter Jacob Hjelm
reduced molybdenite oxide with carbon to obtain a dark metallic powder
which he named “molybdenum”.
Molybdenum remained a laboratory curiosity throughout most of the 19th
century until the technology for the extraction of commercial quantities
became practical. In 1891, the French
company Schneider and Co. first used molybdenum as an alloying element
in the production of armor plates. It was quickly noted that molybdenum
was an effective replacement for tungsten in numerous steel alloying
applications because its density is only slightly more than half that of
tungsten.
MoS2 occurs naturally in the form of thin solid veins within granite. It
is mined and highly refined to achieve purity suitable for lubricants.
MoS2 has a hexagonal crystalline structure
as shown in Figure 1. The intrinsic property of easy shear occurs at the
interface between the sulfur molecules. The interaction between layers
provides an effect similar to what a person may experience if trying to
maneuver across a floor completely covered with brand new playing cards.
Each playing card slides easily with respect to each individual layer.
As the total surface resistance is reduced or redistributed among many
other interacting surfaces, the net effect is reduced total surface friction
and resistance.
Because there is no vapor present between lattice plates, MoS2 is
effective in high-vacuum conditions, where graphite will not work.
The particle size and film thickness are important
parameters that should be matched to the surface roughness of the
lubricated component. Particle size selection is much larger for rough
cut surfaces, such as hobbed open gears, than for highly finished surfaces,
such as those found on bearings. Improperly matched particle sizes may
result in excessive wear by abrasion caused by impurities in the MoS2.
The temperature limitation of MoS2 at 400°C (752°F) is imposed by
oxidation. MoS2 oxidizes slowly in atmospheres up to 600°F. In a dry,
oxygen-free atmosphere it can function as a lubricant up to 1300°F.
The oxidation products of MoS2 are molybdenum trioxide (MoO3)
and sulfur dioxide. MoS3 is hydroscopic and causes many of the friction
problems in standard atmosphere. MoO3 is a preferred form of the
metal used as an additive for various other metals, which is its primary
use.
The issue of where molybdenum disulfide should be used, versus graphite
or tungsten disulfide, is generally best addressed by a lubrication
engineer. For most commercial applications,
these are relatively simple judgments. In aerospace applications
where unique environments and exotic materials are employed,
these questions often take substantial research to provide the best answers.
The low friction coefficients of MoS2 often exceed that of graphite.
Graphite
Graphite as a lubricant dates to antiquity. It was first referenced in the
mid-1500s as being used as pencils. Graphite is a soft, crystalline form
of carbon. It is gray to black, opaque, has a metallic luster, and is flexible
but not elastic. Graphite occurs naturally in metamorphic rocks such as
marble, schist and gneiss. It exhibits the properties of a metal and a
nonmetal, which makes it suitable for many industrial applications.
The metallic properties include thermal and electrical conductivity.
The nonmetallic properties include inertness, high thermal resistance and
lubricity. Some of the major end uses of graphite are in high-temperature
lubricants, brushes for electrical motors, friction materials, and battery
and fuel cells.4
Graphite is a layer lattice lamella crystal structure where the bonds
between the carbon atoms in the crystal structure of the layer are stronger
than the carbon bonds between layers.
Graphite is comprised of carbon and water vapor. Each carbon atom is
bonded to three other surrounding carbon atoms. The flat rings of carbon
atoms are bonded into hexagonal structures, as shown in Figure 2.
The hexagonal carbon structure is referred to as a benzene ring.
These plates exist in layers, which are not covalently connected to the
surrounding layers.
Figure 2. Graphite
Lamella Lattice Structure
(Reference: Dynamic Coating, Inc.) and as a release and antiseize up to about 2400°F. The oxidation product
is CO2. Due to the requirement for entrained moisture vapor, graphite
seldom used in deep-space applications.
Graphite blends and pure graphite dry film lubricant systems are commonly
industrial applications.
graphite or molybdenum. In fact, Teflon itself resists wetting, and the
surfaces coated with these materials likewise resist wetting.
surface roughing or pickling.
PTFE is inert to virtually all chemicals and is considered the most slippery
material in existence. The coefficient of static and dynamic friction is
nearly equal to the level of wet ice on wet ice. As DuPont registered
fabrics and textile products. It does not absorb water, and is unaffected
than 500°F.
Figure 3. Graphite Ore
Figure 4. Teflon Molecular Structure
Figure 5 
Figure 6 
Figure 7 
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