The document discusses lubricants used in vacuum applications. It begins by explaining the differences between wet and dry lubricants. Wet lubricants stay wet on surfaces while dry lubricants dry as they are applied. Greases adhere better than oils and last longer, while oils are preferred where lubricant needs to be circulated. It then compares characteristics of dry and wet lubricants. Dry lubricants have negligible vapour pressure and surface migration while wet lubricants require seals. Dry lubricants have a shorter life than wet lubricants. Various solid lubricants used in vacuum applications are described, including soft metals, lamellar solids like molybdenum disulfide and tungsten disulfide, polymers

1. AND THEIR
REQUIRED CHARACTERISTICS
Argha Saha
IIT Kharagpur

2.  Lubricants in vacuum applications include wet and dry lubricant types, greases
and oils.
 So-called “wet” lubricants tend to stay wet on the surface to which they are
applied, while dry lubricants go on wet but dry as they are applied.
 In general solid particulates do not stick to dry lubricants but they do not tend
to last as long as wet lubricants and as such need to be reapplied.
 By contrast, greases adhere better than oils and tend to last longer. Oil is
preferred where the lubricant needs to be circulated.
INTRODUCTION

3. Dry Lubricants Wet Lubricants
 Negligible vapour pressure  Finite vapour pressure
 Wide operating temperature.  Viscosity, creep and vapour
pressure all temperature
dependent.
 Negligible surface migration  Seals are obviously required
for wet lubricants.
 Short life in air (Though it depends on
type and matrix) than wet lubricants and
as such are needed to be reapplied.
 Insensitive to air or vacuum.
 Debris causes frictional noise.  Low frictional noise.
 Friction speed independent.  Friction speed dependent.
 Life is determined by lubricant wear.  Life is determined by
lubricant degradation.
 Poor thermal characteristics.  High thermal conductance.
 Electrically conductive  Electrically insulating.
COMPARISON

4.  The major disadvantage of conventional liquid lubricants is that they have
relatively high vapor pressures (= 1.3 x 10-4 Pa at room temperature) and
surface diffusion coefficients (= 1 x 10-8 cm2/s) with low surface tensions (in
the order of 18 – 30 dyne/cm) and can volatilize or creep away from areas of
mechanical contact resulting in high friction, wear or mechanical seizure.
 In addition, their volatility can cause issue with achieving proper vacuum
levels and/or depositing on component part surfaces.
 The presence of other gaseous species in a vacuum environment (e.g., water
vapor, oxygen, carbonaceous gases) can cause the force of adhesion between
metal surfaces joined by liquid lubricants to be so strong that the joined areas
can only be separated by fracture.
 The major disadvantage of solid lubricants is a (relatively) short life (in
relation to liquids or greases). Once a solid lubricant is no longer in the contact
zone, failure can be abrupt (compared to wet lubricants) without prior signs of
degradation.
COMPARISON

5. Lubricant
Type
Examples
 Soft Metal Au, Ag, Al, Ba, Cu, In, Ni
 Lameller
solids
Molybdenum Disulfide (MoS2), Tungsten Disulfide (WS2),
Molybdenum di Selenide (MoSe2)
 Polymer PTFE (Teflon), FEP (fluorinated ethylene propylene), Polyacetal,
Polymide, Phenolic and epoxy resins, PEEK (polyether ether
ketone).
 Soft Solids Oxides (Cd, Co, Zn), Sulfides (Bi, Cd), Flurides (Ca, Li, Ba, Y )
Solid Lubricants, available for
vacuum
 In simplest terms, dry film lubricants are solids that are intended to provide low
frictional resistance between surfaces (due to their layered lamella structures)
and can be applied directly to the surface by rubbing.
 For most vacuum applications, crystalline materials, such as molybdenum
disulfide, tungsten disulfide and graphite, are common as stand-alone lubricants.
 These materials can be used independently or in combination with other
lubricants or soft metals to achieve the desired results

6.  A grease is a semisolid liquid that consists of a liquid lubricant (oil) and a thickener.
 The oil does the lubricating while the thickener is used to hold the oil in place and
provide a resistance to flow. The consistency of grease varies: it may be so hard that it
could be cut with a knife or soft enough to pour.
 The characteristics of the grease are largely determined by the thickener. If the thickener
is hydrophobic, the grease will be unaffected by water; if the thickener has high-
temperature stability, the grease will function at high temperatures, etc.
 Usually greases are thickened by soaps. A soap is a metallic element reacted with a fat or
fatty acid. Metallic elements used to make soaps include: lithium, calcium, sodium,
aluminum, and barium.
 In addition to soaps, lubricating additives such as PTFE graphite, and lead are sometimes
used as thickeners.
 Additives are often added to greases to provide anti-oxidation, -rust, and -corrosion,
improved load carrying ability, etc.
GREASE Lubrication

7. Commonly
Used
GREASES for
Vacuum
Application

8. In the near vacuum environment, the absolute pressure is roughly 10-7 torr
and the liquid lubricant evaporates at a significant rate.
For a given liquid film, the evaporation rate can be estimated using the
Langmuir’s expression,-
Revp =
𝑑𝑚
𝑑𝑡
=
𝑃
17.14
∗
𝑀0
.
5
𝑇0
.
5
R – Evaporation Rate. T – Temperature (K).
P – Saturation Pressure. M – Molecular weight (mm of Hg).
Evaporation Rate
Roundtree and Todd used this equation assuming T= 323 K and M = 2000 to
calculate the rate of evaporation of various lubricants.

9. Some
Lubrican
ts

10.  Graphite is one of the oldest and most
common types of dry lubricants. It is a soft,
crystalline form of carbon and is grey to
black in colour.
 Graphite is a layer lattice lamella crystal
structure, which allows the surfaces to
slide easily relative to one another. 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
(Shown beside). The flat rings of carbon
atoms are bonded into a hexagonal
structure.
Graphite
Positional Relationship Between
Planes in Graphite Lamellar
Lattice Structure

11.  Graphite is considered to have excellent lubricating properties where moisture (water
vapor) is available since its shear strength is dependent on absorbed gases and at low
pressure (vacuum) levels, these gases desorb and the coefficient of friction
dramatically rises.
 Graphite will function as a lubricant up to approximately 790°C, and as a release and
anti-seize up to about 1315°C. The oxidation product is carbon dioxide. Due to the
requirement for entrained moisture vapor, graphite does not function well as a
lubricant in hard (10-4 Torr and lower) vacuum levels.
 Its metallic properties include good thermal and electrical conductivity, while its non-
metallic properties include chemical inertness, high thermal resistance and lubricity.
Graphite

12. Molybdenum Disulfide
 This is particularly so if small amounts of sulfur are available to react with iron and
provide a sulfide layer, which is compatible with MoS2 in maintaining the lubricating
film.
 Molybdenum disulfide, MoS2, is a blue-gray, crystalline inorganic powder, which is
used in many vacuum applications as a dry lubricant. It can also be used as a high
temperature protective coating having a melting point of around 2000ºC and a density
of 5.31 g/cc.
 In addition, molybdenum-disulfide is an additive in greases, dispersions, friction
materials and bonded coatings. Molybdenum-sulfur complexes may be used in
suspension but are more commonly dissolved in lubricating oils at concentrations of a
few percent.
 Molybdenum-disulfide is the most common natural form of molybdenum, is extracted
from the ore and then purified for direct use in lubrication since it has the durability to
withstand heat and pressure.

13. A number of unique properties distinguish molybdenum disulfide from other solid
lubricants. These include:
 A low coefficient of friction (0.03-0.06) which, unlike graphite, is inherent and not a
result of absorbed films or gases;
 A strong affinity for metallic surfaces;
 Film forming structure; yield strength as high as 3450 MPa (500 x 103 psi);
 Stability in the presence of most solvents;
 Effective lubricating properties from cryogenic temperatures to about 350oC in air
(1200oC in inert or vacuum conditions);
 Molybdenum disulfide will perform as a lubricant in high vacuum where graphite
fails.
 Molybdenum disulfide is very sensitive to water vapor and if stored in a humid
environment will oxidize readily (forming MoO3) destroying its lubricity
characteristics.
 The ability of molybdenum disulfide to function as a lubricant is intimately related to
its layer structure
Molybdenum Disulfide

14. Molybdenum Disulfide

15.  A layer of molybdenum atoms is
sandwiched between two layers of
sulfur atoms.
 When molybdenum disulfide is
dispersed between two metal surfaces,
a layer binds to each metal surface
through the sulfur atoms. Then the
surface irregularities on the metals are
prevented from coming into contact.
 Sliding contact is between the outer
layers of sulfur atoms, which are only
weakly interacting. As with graphite,
the surfaces are therefore able to slide
easily relative to one another.
Molybdenum Disulfide
Layered structure
of molybdenum
disulfide

16.  A combination of molybdate and water-soluble sulfides can provide both lubrication and
corrosion inhibition in cutting fluids and metal forming materials.
 Oil soluble molybdenum-sulfur compounds, such as thiophosphates and thiocarbamates,
provide engine protection against wear, oxidation and corrosion. Several commercial
manufacturers supply these additives to the lubrication industry.
 A dry film coating of molybdenum disulfide is helpful on bolts used with vacuum (so-
called CF) flanges to avoid breakage issues as the seal force requirements can be such that
high torque is required.
 Molybdenum disulfide has recently attracted a lot of interest for semiconductor device
applications and has potential use for high-temperature electronics.
Molybdenum Disulfide

17.  Tungsten disulfide can be used in almost all
applications where molybdenum disulfide or
graphite are currently employed and is used
extensively by NASA, military, aerospace and
automotive industries.
 Tungsten disulfide (WS2) is often considered as an alternative to Molybdenum
disulfide given its lubricous characteristics.
 It has a coefficient of friction of 0.03 and offers excellent dry lubricity under
conditions of load, vacuum and temperature. It can also be used in high temperature
and high-pressure applications.
 Tungsten disulphide offers temperature
resistance from 188ºC to 1316ºC in vacuum.
The load bearing property of coated film is
extremely high at 400,000 psi and a
coefficient of friction of 0.044 at 20,000 psi
and 0.024 between 200,000 to 400,000 psi.
Tungsten Disulfide
Sulphur atom pairs
align one above the
other

18. Comparison Between WS2 and MoS2

19.  PFPE (Perfluoropolyether, also called Perfluoroalkylether (PFAE)) is a clear colourless
fluorinated synthetic oil that is nonreactive, non- flammable, safe in chemical and
oxygen service, and is long lasting.
 PFPE grease is made by mixing different types of non-soap thickeners with the PFPE
base oil. The base oil is the key component in the lubrication regime. The base oil is a
polymer with a molecular weight range from 3000 to 13000 gm/gm mole, with
viscosities varying from 2 cSt (Centi-Stoke) to 100 cSt at 100°C.
 The chemical structure of the base oil can be straight chain or branched chain. The
physical properties of the base oil depend on the structure of the polymeric chain.
 The thickener is usually Polytetrafluoroethylene (PTFE), or if the application requires
a high thermally stable grease various types of fumed silica can be used.
 This type of oils and oil-based greases are being used with an increasing frequency
in spacecraft systems because of their favourable properties which include a wide
application temperature range , a good viscosity index, and general chemical
inertness.
PFPE

20. PFPE
Chemical structure of PFPE

21. Viscosity-temperature slope (ASTM) as a function
of
kinematic Viscosity at 400C and 1000C for
PFPE

22. Excellent outgassing : PFPE is the best lubricant for the clean room, in the electronics
industry because of it's very low outgassing properties compared to any other lubricants,
and it does not outgas any hydrocarbons. Hydrocarbons will outgas low molecular
weight hydrocarbons which will react with other materials. Ester based lubricants also
react similarly to hydrocarbons. Synthetic hydrocarbons will outgas less than mineral
based lubricant but still can be considered reactive. Silicone lubricants have a strong
desire to migrate and may adversely affect electrical conductivity of electrical contacts.
Bearing cleanliness: It is important to select the proper PFPE for the specific
application. Both PFPE oil and grease lubricants provide a viscous, hydrodynamic film
sufficient to support the load and separate ball from the raceway in bearing applications.
Usually greases are displaced during the initial run in and remain fixed in place during
their life.
The oil in the thickener will bleed into the raceway. In high speed bearings, the oil is
agitated severely producing an oil mist. This also occurs in slow speed bearings but to a
lesser extent. This oil mist can migrate outside the bearing cavity. Therefore, it is
preferable to use an inert oil, like one of the PFPE oils.
Benefits of PFPE

23. High Viscosity Index: A wide range of Kinematic Viscosity fluids with a high Viscosity
Index, VI = 350, makes certain PFPE oils most suitable for applications that requires a
small change in viscosity over a wide range of temperature.
Low pour point and vapor pressure: PFPE, Z type, has a straight chain molecular
structure which enables it to flow freely at very low temperature, (freezing point < -1000
F). Also, it has a low vapor pressure at 200 C = < 4 x 10-13 torr. These two properties are the
two most important properties for vacuum application
Fire resistant: The PFPE oils and greases are not combustible under any circumstances
making PFPEs safe to use in various critical applications, where fire resistance is a
requirement.
Low surface tension: Low surface tension, 20 dyn/cm at 200C will ensure that oil will
reach the narrow gaps in any machine it lubricates and also gives the highest oil to surface
affinity.
Extreme Pressure: In the ASTM D-2596, 4-ball Weld Point Test, unadditised PFPE
provides a pass result above 800kg. This property makes the PFPE a good lubricant in any
application where a requirement exists for extreme pressure properties.
Benefits of PFPE

24. Nontoxic and biologically inert: PFPE oil and grease applications are the safest among any
other lubricant application. PFPE's relative non-toxicity and biological inertness makes it a
preferred lubricant in the food and pharmaceutical industries.
Safe operation: General chemical inertness and radiation resistance of the PFPE makes it
the lubricant of choice in chemical and nuclear facilities.
Comparative
Temperature Limits
Benefits of PFPE

25. Characteristics of Commonly used Vacuum
lubricants
Vapour pressure at 20o C of commonly used lubricants and
approximated time of a 10 micro meter thickness film of
lubricant to evaporate in vacuum

26. Characteristics of Commonly used Vacuum
lubricants
Time in year to lose I.0 ml of lubricant per cm2 of outlet area
as function of temperature

27. O- Ring Lubrication
 Lubrication of O-ring seals is extremely important for installation and operation of
dynamic seals as well as proper seating of static seals.
 The general rule for use of lubrication is: The greatest benefit in using a lubricant is
obtained during the initial installation of the O-ring.
 Lubricants are commonly used on O-rings and other elastomeric seals. Using a
suitable grease or oil during assembly helps protect the O-ring from damage by
abrasion, pinching, or cutting. It also helps to seat the O-ring properly, speeds up
assembly operations, and makes automated assembly line procedures possible.
 An additional benefit is the protection that the lubricant provides as a surface film.
Proper lubrication also helps protect some polymers from degradation by atmospheric
elements such as ozone and its presence helps extend the service life of any O-ring. A
lubricant is almost essential in pneumatic applications requiring dynamic service.
 In vacuum applications, appropriate lubricants help reduce the overall leak rate by
filling the microfine inclusions of the gland’s metal surfaces and lowering permeation
rates of the elastomer.

28.  https://vacaero.com/information-resources/the-heat-treat-doctor/1441-dry-
lubricants-for-vacuum-service.html
 Jones, W.R. "Thermal Oxidative degradation reactions of linear perfluoroalkyl
ethers" NSAS, 1983, TM-82834.
 Robert L. Fusaro et al, “Liquid Lubrication for Space Applications”, July 1992,
NASA Technical Memorandum 105198.
 Mahmoud A. Fowzy, “PFPE, A Unique Lubricant for a Unique Application’’,
1998, Castrol Industrial North America Specialty Products Division- (630)241-
4000
 https://www.anchorrubber.com/specialty/parker_o-lube.pdf
REFERENCES

29. Thank You