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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 disul 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 disul de
lubricants necessary for processes involving extreme temperatures or extreme contact pressures.
Other compounds that are useful solid lubricants include boron nitride, polytetra uoroethylene
(PTFE), talc, calcium uoride, cerium uoride, and tungsten disul de. Any one of these compounds
may be more suitable than graphite or MoS
2
for speci c applications. Boron nitride and PTFE are
discussed along with graphite and molybdenum disul 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
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 lm should be present in the direction of sliding
to provide reduced coef 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 lm on the substrate for a suf cient period necessary for the lubrication require-
ments. The force of adhesion should exceed that of the sheer forces applied to the 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 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 lm (see Figures 6.2 and 6.3).
4. Orientation. The particles used must be oriented in a manner that parallels the ow of the
stress forces and provide the maximum opportunity for a reduction in the coef 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 sta
y
s in there
Spot welds
No welds
FIGURE 6.1 Contact stresses on bearing points of mating surfaces cause a squeeze-out.
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Solid Lubricants as Friction Modifi ers 175
5. Plastic ow. The lubricant should not undergo plastic deformation when loads are applied
directly perpendicular to the direction of motion. The solid should be able to withstand
the intimate contact between the mating surfaces so that a continuous lm of lubrication is
maintained.
This chapter attempts to guide the formulator toward making successful choices in solid lubricants.
It brie y summarizes the physical and chemical properties of the solid lubricant and discusses the
merits of each type of major lubricant as well as the recommended application. The information will
assist in understanding the chemistry of the lubricant and its general mechanism of lubrication.
6.2 SOLID LUBRICANT PROPERTIES
6.2.1 G
RAPHITE
Graphite is most effective for applications involving high-temperature and high load-carrying
situations. These capabilities make graphite the solid lubricant of choice for forging processes. Solid
lubricants such as MoS
2
will oxidize too rapidly to be of any value at the typical hot-forging tem-
perature range of 760–1200°C, although MoS
2
has a greater lubrication capability than graphite.
FIGURE 6.2 Surface asperities.
FIGURE 6.3 Burnished lubricant.
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176 Lubricant Additives: Chemistry and Applications
Why is graphite such a good lubricant? The answer lies in the platelet, lamellar structure of the
graphite crystallite (see Figure 6.4). Graphite is structurally composed of planes of polycyclic car-
bon atoms that are hexagonal in orientation. Short bond lengths between each carbon atom within
the plane are the result of strong covalent bonds (see Figure 6.4).
Weaker van der Waals forces hold together a number of planes to create the lattice structure.
The d-spacing bond distance of carbon atoms between planes is longer and, therefore, weaker
than the bond distance between carbon atoms within the planes. As a force is applied perpen-
dicular to the crystallite, a strong resistance is applied against the force. This high yield strength
provides the load-carrying capacity for the lubricant. Concurrent with the force applied perpen-
dicular to the substrate is a sliding force applied parallel to the direction of sliding. The weak
bond between the planes allows for easy shearing of the planes in the direction of the force. This
creates a cleaving of the planes and results in friction reduction. The lamellar motion of graphite
cleavage can be illustrated by the concept of a hand applying a force on a deck of playing cards as
shown in Figure 6.5. Forces applied perpendicular to the deck are resisted by the stack’s thickness
d−Spacing
3.354 Å
C
A
B
1.415 Å
FIGURE 6.4 Structure of graphite.
FIGURE 6.5 Representation of lamellar lubrication.
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Solid Lubricants as Friction Modifi ers 177
and yield strength. Yet, a far easier force is required to rupture the stack when the force is applied
parallel along the face of the deck, resulting in the shearing of the cards.
The effects of the lamellar structure of graphite can be observed when sliding conditions are
applied onto metal surfaces. Coef cient of friction data can be generated by various bench test methods
for measuring the lubricity of sliding conditions. In comparison to unlubricated or oil-lubricated metal
surfaces, graphite provides excellent lubricity [4]. This is summarized in Table 6.1.
6.2.1.1 Sources of Graphite
There are many types and sources of graphite. These sources in uence the properties of the graphite,
which affects the performance of the end product that uses graphite. Graphite is characterized by
two main groupings: natural and synthetic.
Natural graphite is derived from mining operations worldwide. The ore is processed to recover
the usable graphite. Varying quality of the graphite will be evident from the ore quality and the
postmining processing of the ore. High-purity natural graphite will normally be highly lubricating
and resistant to oxidation. This is due to the high degree of crystal structure and graphitization
usually associated with naturally derived graphite.
Natural graphite of lesser quality is also available. A lower total carbon content and a lower
degree of graphitization characterize the lesser quality. The end product is graphite that is more
amorphous in nature, with a higher content of ash components, which are mostly oxides of silicon
and iron. Lubrication functionality decreases as crystallinity and graphitization decrease. Lubrica-
tion functionality also decreases as total ash content of the graphite increases.
Commercially available natural graphite is provided in a variety of grades. The suitability of
the grades depends on the intended application and economic constraints. Table 6.2 characterizes
examples of commercially available natural graphite.
Selecting the type of natural graphite to use is based on the degree of lubrication required for
the application, the particle size of the graphite necessary for the application, and the economic
constraint. For situations where the lubrication demand is severe, a high-carbon crystalline ake
or crystalline vein graphite is desired. The high degree of crystallinity and graphitization provides
superior lubrication. A more economical alternative is the lower-carbon-content ake graphite.
TABLE 6.1
Coeffi cients of Friction Provided by Graphite Films
Test Method Graphite Film Unlubricated Metal Mineral Oil on Metal
Three-ball slider 0.09–0.12 0.16–0.18 0.15–0.17
Bowden–Leben machine 0.07–0.10 0.40 0.17–0.22
TABLE 6.2
Natural Graphite
Amorphous Crystalline Flake 1 Crystalline Flake 2
% Carbon –85.0 90–95 96–98
% Sulfur –0.30 0.15–0.20 0.10–0.70
% SiO
2
6.0–7.0 0.20–0.30 0.05–0.2
% Ash 10–15 7–10 2.0–3.0
Mesh −325 −325 −325
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178 Lubricant Additives: Chemistry and Applications
For most situations, these types of graphite perform adequately in lubricating conditions that do
not require the purity and lubricity of higher-quality crystalline graphite. For occasions where only
minor lubricity is needed and perhaps a more thermally insulating coating is required, amorphous
graphite would be chosen. Amorphous graphite is also the least expensive of the commercially
available natural graphite grades. Combining amorphous and crystalline graphite can also be done
to modify the amount of lubrication to suit the requirements of the application.
Synthetic graphite is an alternative source for lubricating graphite. Synthetic graphite is char-
acterized as primary or secondary grade (see Table 6.3). Primary grade is derived synthetically
from production within an electric furnace, utilizing calcined petroleum coke as well as very high
temperatures and pressures to produce the graphite. The result is usually a product of high purity
and can approach the quality of natural graphite ake in terms of percent graphitization and lubrica-
tion capability.
Secondary synthetic graphite is derived from primary graphite that has been used for the
fabrication of electrodes. This type of graphite is usually less lubricating than natural or primary
grades of graphite because of its lesser degree of crystallinity and graphitization and the presence
of binding agents and surface oxides that do not contribute to lubrication. Secondary synthetic
graphite is perfectly capable of lubricating effectively for many applications that can afford a lesser
degree of lubricity. The chief bene t in using secondary synthetic graphite is the cost, with the
secondary graphite costing signi cantly less than primary-grade synthetic graphite or high-purity
natural graphite.
6.2.1.2 Lubrication
Appropriate-quality graphite is able to meet the ve criteria for an effective solid lubricant. Graphite
possesses the necessary yield strength for successful lubrication. It is able to adhere suf ciently to
metal surfaces due to its af nity to metal and its packing within and above the microstructure of
the surface. Graphite has a burnishing capacity desirable for lubrication mechanisms that require
a “memory” effect. Proper orientation of graphite particles is achieved by the natural tendency
for the graphite crystal to orient itself parallel to the substrate and in the direction of lowest shear.
The anisotropic characteristic of graphite lends itself well to its lubricating capability and friction
reduction property. The planar orientation of the graphite particles on the substrate takes advantage
of the anisotropic property. Proper orientation allows the lamellar functionality of graphite where
easy shear is achieved along the crystal plane when sliding forces are put along the length of the
particles. The high yield strength in graphite is maintained in the direction perpendicular to the
direction of shear force, providing for the load-carrying capability.
Graphite is best suited for lubrication in a regular atmosphere. Water vapor is a necessary com-
ponent for graphite lubrication. The role that adsorbed water vapor plays in the lubricating proper-
ties of graphite has been studied [5]. It is theorized that water vapor helps to reduce the surface
energy of the graphite crystallite. The adsorption of a water monolayer onto the planar surface of
TABLE 6.3
Synthetic Graphite
Typical Values
Primary Secondary
% Carbon 99.9 99.9
% Sulfur Trace 0.01
% SiO
2
0.02 0.05
% Ash 0.1 0.1
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Solid Lubricants as Friction Modifi ers 179
the graphite likely reduces the bonding energy between the hexagonal planes of the graphite to a
level that is lower than the adhesion energy between a substrate and the graphite crystal. This allows
for lamellar displacement of the graphite crystals when shear forces are applied to the graphite lm.
The result is a reduction of friction and corresponding lubrication. Because water vapor is a require-
ment for lubrication, graphite is usually not effective as a lubricant in a vacuum atmosphere.
The lubricating ability of graphite as a function of temperature is very good. Graphite is
able to withstand continuous temperatures of up to 450°C in an oxidizing atmosphere and still
provide effective lubrication. The oxidation stability of graphite depends on the quality of the
graphite, the particle size, and the presence of any contaminants that might accelerate the oxi-
dation. Graphite will also function at much higher temperatures on an intermittent basis. Peak
oxidation temperatures are typically near 675°C. For these instances, modifying the composition
of the graphite mixture may be necessary as a way to control its rate of oxidation.
The thermal conductivity of graphite is generally low. For example, primary-grade synthetic
graphite has a conductivity of ~1.3 W/mK at 40°C. Amorphous graphite is even less conducting and
is sometimes considered for providing some degree of thermal insulation for speci c applications.
6.2.2 MOLYBDENUM DISULFIDE
Molybdenum disul de is the second signi cant solid lubricant widely used in industry. It has been
used since the early nineteenth century for lubrication applications. MoS
2
, also known as molybde-
nite, is a mined material found in thin veins within granite. Lubricating-grade MoS
2
is highly re ned
by various methods to achieve a purity suitable for lubricants [6]. This purity usually exceeds 98%.
MoS
2
is commercially available in a variety of particle size ranges. Table 6.4 lists basic properties
for molybdenum disul de. The low friction of MoS
2
is an intrinsic property related to its crystal
structure, whereas graphite requires the adsorption of water to behave as an effective lubricant.
Molybdenum disul de achieves its lubricating ability with a mechanism similar to graphite. Just
like graphite, MoS
2
has a hexagonal crystal lattice structure.
Sandwiches of planar hexagonal Mo atoms are interspersed between two layers of sulfur
atoms. Similar to graphite, the bond strength between the hexagonal planes between the sulfur
atoms are weak van der Waal-type bonds when compared to the strong covalent bond between
molybdenum and sulfur atoms within the hexagonal crystal. Orientation of the MoS
2
crystallites
is important if effective friction reduction is to be achieved. MoS
2
has anisotropic properties that
are comparable to graphite. When a force is applied parallel along the hexagonal planes, the weak
bond strengths between the planes allow for easy shearing of the crystal, resulting in a lamellar
TABLE 6.4
Characteristics of Hexagonal Molybdenum
Disulfi de
Property Value
Bulk hardness 1.0–1.5 Ʊ
Coef cient of friction 0.10–0.15
Color Blue-gray to black
Electrical conductivity Semiconductor
Luster Metallic
Melting point >1800°C
Molecular weight 160.08
Service temperature Up to 700°F
Speci c gravity 4.80–5.0
Thermal conductivity 0.13 W/mK at 40°C
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180 Lubricant Additives: Chemistry and Applications
mechanism of lubrication. At the same time, the crystal structure and strong interplanar bond
forces of MoS
2
allow for high load carrying against forces applied perpendicular to the plane of
the crystal. This is necessary for the prevention of metal on metal contact for high-load applica-
tions such as gearbox lubrication.
MoS
2
scores well in the other criteria for an effective solid lubricant. It forms a strong cohesive
lm, that is smoother than the surface of the substrate on which it is bonded. MoS
2
lm has suf-
ciently high adhesion to most metal substrates, which it successfully burnishes onto the wearing
surfaces, thus minimizing metal wear and prolonging friction reduction. This characteristic is an
exception, however, with titanium and aluminum substrates due to the presence of an oxide layer on
the metal surface, which tends to reduce the tenacity of the MoS
2
lm.
The lubrication performance of MoS
2
often exceeds that of graphite. It is most effective for
high load-carrying lubrication when temperatures are <400°C. Another advantage of MoS
2
is that
it lubricates in dry, vacuum-type environments, whereas graphite does not. This is due to the intrin-
sic lubrication property of MoS
2
. On the contrary, the lubricating ability of MoS
2
deteriorates in
the presence of moisture because of oxidation of MoS
2
to MoO
3
. The temperature limitation of
MoS
2
is due to similar decomposition issues of the material as that experienced with moisture. As
MoS
2
continues to oxidize, MoO
3
content increases, which induces abrasive behavior and increases
coef cient of friction for the surfaces to be lubricated.
The effectiveness of MoS
2
improves as contact forces increase on the lubricated surface.
Burnished surfaces exhibit coef cient of friction reduction as a function of increasing contact forces
[7]. In contrast, graphite does not necessarily exhibit this behavior. The frictional property of MoS
2
systems has been reported to be generally better than graphite in many instances, up to the service
temperature limitations for the lubricant.
The particle size and lm thickness of MoS
2
will affect lubrication. Generally, the particle size
should be matched to the surface roughness of the substrate and the type of lubrication process
considered. Too large a particle distribution may result in excessive wear and lm reduction as
mechanical abrasion is experienced. Too ne a particle size may result in accelerated oxidation in
normal atmospheres as the high surface area of the particles promotes the rate of oxidation.
6.2.3 BORON NITRIDE
Boron nitride is a ceramic lubricant with interesting and unique properties. Its use as a solid lubricant
is typically for niche applications when performance expectations render graphite or molybdenum
disul de unacceptable. The most interesting lubricant feature of boron nitride is its high-temperature
resistance. Boron nitride has a service temperature of 1200°C in an oxidizing atmosphere, which
makes it desirable for applications that require lubrication at very high service temperatures. Graph-
ite and molybdenum disul de cannot approach such higher service temperatures and still remain
intact. Boron nitride also has a high thermal conductivity property, making it an excellent choice for
lubricant applications that require rapid heat removal.
A reaction process generates boron nitride. Boric oxide and urea are reacted at temperatures
from 800 to 2000°C to create the ceramic material. Two chemical structures are available: cubic and
hexagonal boron nitride. As one might expect, the hexagonal boron nitride is the lubricating version.
Cubic boron nitride is a very hard substance used as an abrasive and cutting tool component.
Cubic boron nitride does not have any lubrication value. The hexagonal version of boron nitride is
analogous to graphite and molybdenum disul de. The structure consists of hexagonal rings of boron
and nitrogen, which are connected to each other, forming a stack of planar hexagonal rings. As with
graphite, boron nitride exhibits a platelet structure.
The bond strength within the rings is strong. The planes are stacked and held together by
weaker bond forces. Similar to graphite and molybdenum disul de, this allows for easy shearing
of the planes when a force is applied parallel to the plane. The ease of shear provides the expected
friction reduction and resulting lubrication. Concurrently, the high bond strength between boron
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Solid Lubricants as Friction Modifi ers 181
and nitrogen within the hexagonal rings provides the high load-carrying capability that is necessary
to maintain metal–metal separation of the substrates. Similar to MoS
2
, boron nitride has intrinsic
lubrication properties. Boron nitride effectively lubricates in a dry as well as a wet atmosphere.
It is very resistant to oxidation, more so than either graphite or MoS
2
, and maintains its lubricating
properties up to its service temperature limit.
Commercial grades are available in a variety of purities and particle sizes. These varieties
in uence the degree of lubrication provided by boron nitride since particle size affects the degree
of adhesion to substrate, burnishing ability, and particle orientation within a substrate. Impurities
such as boric oxide content need to be considered with respect to the lubrication capability of boron
nitride powder since this will in uence the ability of the powder to reduce the coef cient of friction
for an application. The variation in grades will also in uence the thermal conductivity properties
and ease of suspension in a liquid carrier. Table 6.5 summarizes typical properties for hexagonal
boron nitride.
6.2.4 POLYTETRAFLUOROETHYLENE
Polytetra uoro-ethylene (PTFE) has been in use as a lubricant since the early 1940s. Structurally, the
polymer is a repeating chain of substituted ethylene with four uorine atoms on each ethylene unit:
⫺⫺(CF CF )
22
ᎏ
n
(6.1)
Contrary to the other lubricants discussed, PTFE does not have a layered lattice structure. The
lubrication properties are at least partially the result of its high softening point. As frictional
heat begins to increase from sliding contact, the polymer maintains its durability and is able to
lubricate.
Various grades are produced and applied to speci c applications as a result of the properties
imparted by the grade. For example, molecular weight and particle size are two characteristics that
can alter the performance of the polymer as a lubricant.
The critical characteristic of PTFE—the one it is widely known for [8]—is the outstandingly
low coef cient of friction imparted by the molecule. PTFE has one of the smallest coef cients
of static and dynamic friction than any other solid lubricant. Values as low as 0.04 for sliding
conditions have been reported for various combinations of PTFE lms on substrates [9]. The low-
friction property is attributed to the smooth molecular pro le of the polymer chains, which orient
in a manner that facilitates easy sliding and slip. It is postulated that the PTFE polymer results
in rod-shaped macromolecules, which can slip along each other, similar to lamellar structures.
TABLE 6.5
Hexagonal Boron Nitride
Property Value
Coef cient of friction 0.2–0.7
Color White
Crystal structure Hexagonal
Density 2.2–2.3 g/cm
3
Dielectric constant 4.0–4.2
Dielectric strength ∼35 kV/mm
Molecular weight 24.83
Service temperature 1200°C (Oxidizing atmosphere)
Thermal conductivity ∼55 W/mK
Size (grades) 1–10 µm
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182 Lubricant Additives: Chemistry and Applications
Its chemical inertness makes it useful in cryogenic to moderate operating temperatures and in a
variety of atmospheres and environments. Operating temperatures are limited to ∼260°C due to
the decomposition of the polymer.
One consideration in using PTFE is the cold weld property of the material. This could eliminate
its use for some applications where extreme pressure is encountered. Such pressure may result in the
destruction of the polymer particle and in the lubrication failure, as the PTFE congeals and fails to
remain intact on the rubbing surface.
PTFE nds many uses in bonded lm lubrication at ambient temperature. These applications
include fasteners, threading compounds, and chain lubrication and engine oil treatments. PTFE is
widely used as an additive in lubricating greases and oils, for both industrial and consumer applica-
tions (see Table 6.6 for basic properties).
Although dif cult to accomplish due to the low surface energy of PTFE, colloidal dispersions of
PTFE in oil or water can be produced. This is useful for applications requiring the stable suspension
of PTFE particles in the lubricating medium such as for crankcase oil or hydraulic oil. The nature
and feedstock of the PTFE in uence the ability to create a stable, un occulated dispersion, which
is necessary for effective lubrication.
6.3 PREPARATION FOR LUBRICANT APPLICATION
For a lubricant to be effective, the solid has to be applied in a manner that provides an effective
interface between the mating substrates that require wear protection or lubrication. Dry-powder
lubrication can be used, but it is limited in its scope of application. In other words, the dry powder
can be sprinkled onto the load-bearing substrate. By a combination of the rubbing action from slid-
ing and the natural adhesion properties of the solid lubricant, some measure of attachment to the
substrate will occur by burnishing to provide lubricating protection [10]. MoS
2
seems to function
particularly well from this manner of application, as it has an effective burnishing capability.
The use of free powder has limitations. The lms tend to have a short duration of service since
adhesion is usually insuf cient to provide any longevity for a continuous application. The use of
dry powder also makes it dif cult in many circumstances to accurately apply the lubricant to the
place intended, with the possible exception of tumbling metal billets for achieving a coating over
phosphated substrates.
This can be overcome by the use of bonded lms. Bonded lms will provide a strong adhesion to
the substrate requiring protection. It also allows for a more controlled rate of lm wear, which depends
on the properties of the bonding agent and the lm thickness of the bonded lm. Bonded lms can be
achieved by a number of ways, all by use of secondary additives that promote a durable and longer-
lasting lm. The intended application will dictate the appropriate type of bonding agent. For applica-
tions of continual service, resin and polymer bonding agents are typically used. These include phenolic
resins, acrylics, celluloses, epoxies, polyimides, and silicones. Some of the binders such as epoxies are
TABLE 6.6
Typical Physical Properties of PTFE
Property Value
Coef cient of friction (ASTM D1894) 0.04–0.1
Dielectric constant 2.1–2.4
Hardness 50–60 Shore D
Melting point 327°C
Service temperature Up to 260°C
Speci c gravity 2.15–2.20
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Solid Lubricants as Friction Modifi ers 183
curable at room temperatures. Others such as the phenolic resins require elevated-temperature curing.
Service temperature may be the limiting consideration for the chosen bonding agent.
To overcome service temperature limitations, alternative type bonding agents are also widely
used. Most typical are inorganic salts such as alkali silicates, borates, and phosphates. These types
of salts overcome temperature limitations of organic bonding agents, transferring the burden of
temperature consideration to the solid lubricant. Conversely, the use of inorganic salts as bonding
agents typically does not provide for a coating life that is as durable as an organic bonded coating.
This usually limits the application to those requiring constant replenishment of the lubricant.
To facilitate the application of the solid lubricant, dispersion in a liquid is most commonly
used. The liquid can be a solvent, oil, synthetic oil, or water. Suspension within a liquid allows for
the easy and precise application of the solid lubricant to the intended areas that require protection.
Compared to dry-powder application, lm control is easily achieved through spray, dip, or ow
methods onto the substrate. Environmental cleanliness is also improved since the solid particles are
entrapped within a liquid matrix, preventing the airborne dispersion of the particles. For applica-
tions in which the solid lubricant is a secondary additive in a liquid, proper suspension is critical for
achieving effective lubrication.
A consideration for liquid suspensions is that the shelf life of the lubricant is limited. Because
the particles require suspension within a liquid carrier, eventual sedimentation of the solid lubricant
will occur. This necessitates proper mixing procedures for the handling of the suspension to pro-
vide for consistent lubricant performance within the stated shelf life of the material. Adjustment to
formulations with respect to dispersion and viscosity controls will in uence the time it takes for the
suspension to destabilize. The quality of the suspension will also determine how easily the settled
pigment is redispersed with mild agitation (see Figure 6.6).
To create the suspension, the solid lubricant particles require treatment of the particle surface to
make it amenable to suspension within the carrier liquid. This is similar to paint, where the colorant
is chemically treated to provide the required dispersion characteristics and form what is consid-
ered a colloidal suspension (see Figure 6.7). This treatment is necessary to maximize the available
particles for lubrication and provide the degree of dispersion stability required for the job. Without
such treatment, particle agglomeration and rapid sedimentation will occur. This would negatively
in uence the application of the lubricant onto the substrate in a manner that creates an inferior
and ineffective lm. Wetting agents and suspending agents such as polymeric salts, starches, and
polyacrylics are used to treat the surface of the solid lubricant to render it capable of suspension
within the liquid carrier.
When creating the dispersion, the particle size distribution of the solid lubricant has to be con-
sidered. Small, submicron particles are easier to suspend and retain physical stability than large,
coarse particles. To this end, milling action on the solid lubricant is usually necessary to alter the
size distribution to the desired range of sizes (see Figure 6.8).
FIGURE 6.6 Particle sedimentation.
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