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National Advisory Committee for Aeronautics, Technical Notes - Effect of Chemical Reactivity of Lubricant Additives on Friction and Surface Welding at High Sliding Velocities

The effect of chemical reactivity of lubricant additives on
friction at high sliding velocities was investigated. The investi-
gation was conducted with a kinetic-friction apparatus consisting
essentially of an elastically restrained spherical rider specimen
sliding on a rotating steel disk lubricated with cetane (normal
hexadecane) containing lubricant additives of different chemical
reactivities. The lubricant additives investigated were benzyi
chloride 063503201, p-dichlorobenzene 0634012, free sulfur S,
benzyl disulfide (06350E2)282, and phenyl monosulfide (CBES)ZS'

The experiments were conducted over a range of sliding velocities
from 75 to 7000 feet per minute and with loads from 269 to 1017
grams (initial Hertz surface stresses, 125,000 to 194,000 lb/sq in.).
Supplemental surface studies were made using standard physical,
chemical, and metallurgical equipment and techniques.

The data indicated that higher critical sliding velocities
were obtained with additives the active atoms of which had the
greatest chemical reactivity; that is, greater reactivity allowed
operation at higher sliding velocities before lubrication failure
occurred. The effect on critical sliding velocity of concentra-
tions of active'atoms between 1 and 10 percent was small indicating
that, within the limits of the experiments, the factor of activity
of the individual active atoms was of more importance than the
number of such atoms available for reaction. For all additives,
at velocities greater than the critical, the friction coefficient
increased and surface welding occurred. Increase in load, for one
concentration of sulfur atoms, decreased the critical sliding
velocity. For both free sulfur and phenyl-monosulfide additives,
increase in initial Hertz surface stress decreased critical sliding
velocity as a linear function. Such a change is to be expected because
of the increased tendency toward surface welding at higher surface-
contact stresses.

Rate of reaction between extreme-pressure lubricant additives
and sliding surfaces was a factor of appreciable importance in
establishing quality of lubrication obtained by use of additives
at high sliding velocities.