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, fi lm control is easily achieved through spray, dip, or fl 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 infl 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
infl uence the application of the lubricant onto the substrate in a manner that creates an inferior
and ineffective fi 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.