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National Advisory Committee for Aeronautics, Technical Notes - Formation of Sulfide Films on Steel and Effect of Such Films on Static Friction

Experimental studies were'conducted to evaluate the formation,
under transient temperature conditions, of sulfide films on heated
steel specimens immersed in solutions of free sulfur in cetane and to
establish the necessary sulfide-film.thickness for effective lubrication
under static-friction conditions.

With the particular steel specimens used in these experiments, the
minimum temperatures at which iron sulfide is formed on the heated
specimens in cetane solutions containing 0.1 to 1.0 percent free sulfur
by weight were from 7000 to 575° F, respectively. By establishing the
cooling rates of hot specimens inserted in cool solutions, amounts of
reaction product formed at various temperatures with varied concentra-
tions, and time—temperature relations in all experiments, it was possi-
ble to calculate the rate of formation of the reactiOn film. The calcu-
lated rate of film formation on steel in a solution of 0.5 percent free
sulfur in cetane was approximately 1.8X105 Angstrom units per second in
the temperature range from 11000 to 10000 F.

Static—friction data were obtained with clean dry steel (coeffi—
cient of static friction, 0.79) and for steel coated with sulfide films
of various thickness. Dry film thicknesses of 5000 Angstrom units or
greater were found necessary to prevent surface welding completely and
to produce relativexy low friction (coefficient of static friction, O. 52
to O. 39). Welding was appreciably reduced, however, with film thick-
nesses as low as 5400 Angstrom units,.although friction was reduced only
slightly. This result indicated that thin sulfide films were somewhat
effective even though complete prevention of welding was not obtained.

Experimental studies by numerous investigators (references 1 to
5) have established the concept that the effectiveness of so-called
extreme-pressure lubricant additives is dependent on chemical reaction
between the additives and the lubricated surfaces to produce a solid
surface film having desirable lubricating properties. High surface
temperatures are necessary for the proper functioning of extreme—pressure
lubricants. Bowden and.Ridler (reference 6) have demonstrated that
surface temperatures occurring on effectively lubricated sliders may be
above 6000 C (11120 F) and that, in many cases, localized surface-
temperature flashes approach the melting point of one of the slider
materials. With continued motion of sliders, a series of localized
temperature flashes occurs with the total amount of heat energy (but
not necessarily the maximum temperature of the surface) being dependent
on the loading. As suggested in reference 5, "extreme temperature"
lubricants might be a more accurate name because they are primarily
temperature active.