2009-02-20

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.

9 comments:

  1. hi,
    Rubber cable and silicon cables are the well known cable in all over the country because of its quality and it is mostly us in factories and offices..
    regards,
    udaygroup

    ReplyDelete
  2. UDAY Group brings state-of-the art marketing and distribution system for many manufacturing companies. We are manufacturing Electronic and Electro-mechanical components like PTFE/TEFLON CABLE Manufacturers, UNINYVIN CABLE/NYVIN CABLE Manufacturers etc.


    for more details
    http://www.udaygroup.com/

    ReplyDelete
  3. UDAY Group brings state-of-the art marketing and distribution system for many manufacturing companies. We are manufacturing Electronic and Electro-mechanical components like PTFE/TEFLON CABLE Manufacturers, UNINYVIN CABLE/NYVIN CABLE Manufacturers etc.


    for more details
    www.udaygroup.com

    ReplyDelete
  4. copper wire drawing lubricant supplier

    As a specialty lubricants supplier in India, we supply our specialty lubricants and automatic lubricators to various industries. We also offer free technical service to automotive industries, steel industries, cement industries, textile industries, bearing industries, etc. At times we as also known as copper wire drawing lubricant supplier since our lubricants fulfill all the needs pertaining to the application of wire drawing.

    ReplyDelete
  5. Hi,
    Thanks for the great details about wires & stainless steel, i have bookmarked your blog for the future references.

    We are leading manufacturers of PTFE Wire, Teflon Wire, Stainless Steel Wires, MIG Wire Welding, TIG Wire.

    ReplyDelete

 
Add to Technorati Favorites Subscribe with Bloglines
Custom Search