Surface treatments commonly used as
alternatives to surface film depositions include thermal
diffusion, ion implantation, and chemical conversion
coatings.
(a) Thermal diffusion: This is a process
that introduces foreign atoms into a surface for various
purposes such as increasing wear-resistance by increasing
surface hardness; producing low shear strength to inhibit
scuffing or seizure; and in combination with these to
enhance corrosion-resistance.
(b) Ion implantation: This is a recently
developed method that bombards a surface with ions to
increase hardness, which improves wear- and
fatigue-resistance.
(c) Chemical conversion coatings:
Frequently, solid lubricants will not adhere to the
protected metal surface. A conversion coating is a porous
nonlubricating film applied to the base metal to enable
adherence of the solid lubricant. The conversion coating by
itself is not a suitable lubricant.
(4) Polymers: Polymers are used as thin
films, as self-lubricating materials, and as binders for
lamellar solids. Films are produced by a process combining
spraying and sintering. Alternatively, a coating can be
produced by bonding the polymer with a resin. Sputtering can
also be used to produce films. The most common polymer used
for solid lubrication is PTFE The main advantages of PTFE
are low friction coefficient, wide application range of -200
to 250°C (-328 to 418°F), and lack of chemical reactivity.
Disadvantages include lower load-carrying capacity and
endurance limits than other alternatives. Low thermal
conductivity limits use to low speed sliding applications
where MoS2 is not satisfactory. Common
applications include antistick coatings and self-lubricating
composites.
