Page 318 - The Tribology Handbook

e5 Solid lubricants and coatings

Contaminate product - Food machinery, electrical contacts
Undesirable Maintenance difficult - Inaccessibility, storage problems
SOLID LUBRICANTS Hostile environments - Corrosive gases, dirt and dust
REQUIRED WHEN
FLUIDS ARE: High temperatures - Metalworking, missiles
Cryogenic temperatures - Missiles, refrigeration plant
I Ineffective Radiation - Reactors, space
I Spacefvacuum - . Satellites, X-ray equipment
Fretting conditions }
I Extreme pressures 1 General, often used with oils

A TYPES OF SOLID LUBRICANT

Materials are required which form a coherent film of low shear strength between two sliding surfaces
Lamellar solids

Special features
MoS, 350 Stable to > 1150°C in vacuum
WS, 400 Oxidative stability > MoS,
Graphite 500 Ineffective in vacuum/dry gases
TaS, 550 Low electrical resistivity
CaF,/BaF, 1000 Ineffective below 300 "C
Polymers
UHMWPE 100 Exceptionally low wear
FEP 2 75 1 Chemically inert; useful at
PTFE 210 cryogenic temperatures
Polyimides 300 Friction > PTFE
Polyurethanes 150 I Useful for abrasion resistance but
Nylon 11 loo friction relatively high.
Soft metals

Pb, Au, Ag, Sn, In Useful in vacuum.
Oxides
MOO,, PbO/SiO,, B,O,/PbS Effective only at high temperatures
Miscellaneous
AsSbS,, Sb(SbS,), Ce,(MoS,) Oil & grease additives.
Zn,P,O,/Ca(OH), White lubricant additive for grease.
Plasma-sprayed coatings
Ag/Ni-Cr/CaF,/Glass Wide temperature range lubricant formulations,
Ag/Cr,C,/Ni-AI/BaF,-CaF, 20-1 000 "C.

B METHODS OF USE

General
Powder - Rubbed on to surfaces to form a 'burnished film', 0.1-10 pm thick.
See subsection C.
Dispersion with resin in volatile fluids - Sprayed on to surfaces and cured to form a 'bonded coating', 5-25 pm thick.
See subsection D.
Dispersion in non-volatile fluids - Directly as a lubricating medium, or as an additive to oils and greases.
See subsection E.
Specialised
As lubricating additives to metal, carbon and polymer bearing materials.
As proprietary coatings produced by vacuum deposition, plasma spraying,
particle impingement, or electrophoresis.
C5.1