Page 181 - Tribology in Machine Design

Friction, lubrication and wear in lower kinematic pairs 167

pressure. Also, the ambient temperature level is important since rates of
chemical reaction approximately double with each 10 °C temperature
increase.
The chemistry of the process fluid or environment is very important in
the selection of seal materials. Consideration must be given to both the
normal corrosion reactions and the possibility of corrosive wear. Some
surface reaction is essential to many useful lubrication processes in forming
films that inhibit adhesive wear. However, excessive active chemical
reactions are the basis for corrosive or chemical wear. It is important to
remember, however, that air is perhaps the most influential chemical agent
in the lubrication process and normal passive films on metals and the
adsorbates on many materials, are a' basic key to surface phenomena,
critical to lubrication and wear.
Pitting or fatigue wear and blistering are commonly described pheno-
mena in the wear of seal materials that can be, but are not necessarily,
related. Carbon has interatomic bonding energies so high that grain growth
or migration of crystal defects is virtually impossible to obtain. Accord-
ingly, one would expect manufactured carbon and graphite elements to
have excellent fatigue endurance. Pitting is usually associated with fatigue
but may have other causes on sealing interfaces. For example, oxidative
erosion on carbons can cause a pitted appearance. Cavitation erosion in
fluid systems can produce a similar appearance. Carbon blistering may
produce surface voids on larger parts. Usually blistering is attributed to the
subsurface porosity being filled with a sealed liquid and subsequently
vaporized by frictional heating. The vapour pressure thus created lifts
surface particles to form blisters. Thermal stress cracks in the surface may
be the origin for blisters with the liquids filling such cracks. In addition, the
hydraulic wedge hypothesis suggested for other mechanical components
might also be operative in seals. In that case, the surface loading forces may
deform and close the entrance to surface cracks, also causing bulk
deformation of adjacent solid material so as to create a hydraulic pressure
that further propagates the liquid-filled void or crack. The blister pheno-
mena is of primary concern with carbon seal materials, but no single
approach to the problem has provided an adequate solution.
Impact wear occurs when seals chatter under conditions of dynamic
instability with one seal element moving normal to the seal interface.
Sometimes, very high vibration frequencies and acceleration forces might
develop. Rocking or precessing of the nose-piece relative to the wear plate
occurs and impact of the nose-piece edges is extremely destructive. This
type of phenomena occurs in undamped seals with low face pressures and
may be excited by friction or fluid behaviour, such as a phase change, as well
as by misalignment forces.
Fretting usually occurs on the secondary sealing surfaces as the primary
sealing interface moves axially to accommodate thermal growth, vibrations
and transient displacements including wear. Fretting of the piston ring
secondary seal in a gas seal can significantly increase the total seal leakage.
Some seal manufacturers report that 50 to 70 per cent of the leakage is past