Page 179 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 165

Table 4.1. Coefficient of wear in order of magnitude for seal face
materials

Sliding material
Wear
rotating stationary coefficient K

carbon-graphite cast iron 10 ~ 6
(resin filled)
carbon-graphite ceramic 10 ~ 7
(resin filled) (85% A1 2O 3)
carbo-graphite ceramic 10" 7
(babbitt filled) (85% A1 2O 3)
carbon-graphite tungsten carbide 10" 8
(bronze filled) (6% cobalt)
tungsten carbide tungsten carbide 10" 8
(6% cobalt)
silicon carbide silicon carbide 10" 9
(converted carbon) (converted carbon)

an expression of the limit of mild adhesive wear. Table 4.2 gives the PV
limitations for frequently used seal face materials.
Physical and chemical bonds can cause adhesion between surfaces; thus
transfer films are formed that are basic to friction and wear processes. With
relative motion, shear occurs in the direction of sliding along the weakest
shear plane in the surface region. With carbon-graphite materials, graphite
usually contributes to that weak shear plane. Inherent in this process is the
development of a transfer film of carbon on the mating surface. The surface
of that transfer film can be expected to be graphitic and highly oriented with
the basal plane essentially parallel to the direction of shear. The example of
transfer films of graphite is analogous to the development of highly
orientated films with solid polymeric materials, especially PTFE. The
surface chemistry of the base material as well as its surface topography
influences the formation of transfer films.
The surfaces of sealing interfaces are usually very smooth. The lack of
roughness is a fortuitous result of a manufacturing process aimed at
providing physical conformance of the mating surfaces to minimize the
potential gap for leakage flow. It is very clear that for many seal
applications a matte type surface texture of the type obtained by lapping,
hard or fine abrasive blasting and ion bombardment provides a good
physical base for achieving the mechanical adherence of a transfer film.
Abrasive wear is a condition of wear in seals, that frequently limits the life
of the seals. Many abrasive wear problems for seals result from the
operating environment. For example, road dust or sand enters the sealing
gap and the particles may move freely to abrade both interfaced surfaces by
a lapping action; that is the surfaces are subjected to three-body abrasive
wear. Alternatively, the particles become partially embedded in one of the
surfaces and can then act as a cutting tool, shearing metal from the mating
surface by a two-body wear mechanism. Abrading particles can also come