Lubricating Media

Solid Film Lubricants: A Practical Guide
Mike Johnson, Noria Corporation


It is widely believed that extreme conditions are uncommon; however
nearly every
manufacturing plant has at least one application in which the operating
conditions could be characterized as extreme from a lubrication perspective.
Common extremes could include high and low shaft speeds, high and low
temperatures, high pressures, concentrated atmospheric and process
contaminants, and inaccessibility.
Mineral oil-based fluid lubricants (oil and grease materials) function properly
where the designed surface areas and shaft speeds allow for the effective
formation of an oil film, as long
as the machine operating temperature envelope falls between -20°C and
100°C
(-4°F to 212°F). The only absolute limits that apply for fluid lubricants,
regardless of the base oil type, are conditions that cause a change in the
state of the fluid that prohibits fluid film formation. Fortunately, that is
not the end of the story.
Various materials that protect interacting surfaces after the fluid film is
lost have been either
discovered or created. These materials may be applied to a surface in the
form of an additive to a fluid lubricant, or in a pure form, and may also be
added or alloyed into the surface when the component is being manufactured.

The more common types of materials include the following:

Molybdenum disulfide (MoS2) –also known as moly
Polytetrafluoroethylene (PTFE) – also known as Teflon®
Graphite
Boron nitride
Talc
Calcium fluoride
Cerium fluoride
Tungsten disulfide

These materials are characterized as dry film or solid film lubricants.
Moly, graphite and Teflon are the most commonly recognized by
practitioners of machinery lubrication. Molybdenum
and graphite are agents that are extracted from mined ore. Teflon
was created by DuPont Chemical Company and is manufactured by
various companies for many purposes.

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.)

Graphite has excellent lubricating properties, as long as moisture vapor

is available, and will function as a lubricant up to approximately 1450°F,

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

does not function well as a lubricant in a hard vacuum and is therefore

seldom used in deep-space applications.
Graphite blends and pure graphite dry film lubricant systems are commonly

used in applications such as hot and cold forming, wire drawing and billet

coatings; on high-speed cutting tools; as a mold release for die cast, plastic

and rubber mold applications; cylinder head and exhaust bolts; ammunition

and armament applications; automotive engine and many common

industrial applications.

Figure 3. Graphite Ore


Figure 4. Teflon Molecular Structure

Long Chain Fluorocarbon - Teflon®
The linear long chain molecule “polytetrafluoroethylene”, was accidentally

discovered by Dr. Roy Plunkett on April 6, 1938, at DuPont’s Jackson

Laboratory in New Jersey. Plunkett was working with gases related to

Freon® refrigerants (also known as chlorofluorocarbons), another

DuPont product.
Upon checking a frozen, compressed sample of tetrafluoroethylene, he

discovered that the sample had polymerized spontaneously into a white,

waxy solid to form polytetrafluoroethylene (PTFE). The chemical formula

is [C2F4] N. PTFE is a saturated aliphatic fluorocarbon.

Figure 5

Figure 6

PTFE does not have mechanical occlusion characteristics similar to

graphite or molybdenum. In fact, Teflon itself resists wetting, and the

surfaces coated with these materials likewise resist wetting.

For surfaces to bond with PTFE and the variety of other long chain

fluorocarbons, it is necessary for them to be properly prepared through

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

trademark Teflon®, it has become a household name associated with

its use as a coating on cookware and as a soil and stain repellant for

fabrics and textile products. It does not absorb water, and is unaffected

by acids, bases and solvents normal to industry at temperatures less

than 500°F.

Figure 7

Various fillers can be added to PTFE to enhance certain characteristics,

such as glass fiber (high wear resistance, good electrical, low friction),

graphite (low friction, excellent chemical resistance, high creep resistance),

carbon fiber (high wear resistance, high load resistance,

high bend resistance), glass fiber and molybdenum disulfide

(high wear resistance, low friction, high creep resistance), and bronze

(high wear resistance, heat dissipation). In some industries, such
as the bearing pad industry, filled PTFE has become the standard, where

various percentages of glass fiber are added to the base PTFE resin to

create an extremely tough, weatherproof, interface material that can be

cut or stamped into configurations to match the dimensions of the opposing

surfaces.7 PTFE is licensed to many manufacturing firms for a variety of

material types.
Solid film lubricants offer protection beyond the normal properties of

most mineral and synthetic oil-based fluid lubricants. Conditions that

warrant the use of these agents in a pure form,
or as an additive, include extremes of temperature, pressure, chemical

and environmental contamination. Some agents have a strong affinity

for metallic surfaces, and will adhere to those surfaces through loose

covalent forces. These may be applied directly as a topical coating or

indirectly in the form of an additive to a fluid lubricant. Some agents have

no natural attractiveness to metallic surfaces, and therefore must be

bonded to the surface through specialized treatment.
The solid lubricating materials tend to have upper temperature ranges

well above the surface-protecting capabilities of mineral and most

synthetic base stocks. Fluorinated hydrocarbons are stable in liquid or

solid form to roughly 600°F, but will begin to degrade and may produce

noxious fumes at that temperature.
Graphite and molybdenum can operate in a similar temperature range,

and molybdenum disulfide can also function in a vacuum without losing its

slippery property.