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National Advisory Committee for Aeronautics, Technical Notes - Friction at High Sliding Velocities of Oxide Films on Steel Surfaces Boundary Lubricated with Stearic-Acid Solutions

Experiments were conducted to establish the capabilities of a
fatty acid (stearic) as an additive for lubrication of steel sur-
faces at high sliding velocities (75 to 7000 ft/min) with severe
loading (169 to 1543 grams, initial Hertz surface stress, 108,000 to
194,000 lb/sq in.) and to determine the efficacy of prepared oxide
films in this type of lubrication.

It was found that lubrication with stearic acid as an additive
in cetane was effective at sliding velocities up to 3000 feet'per
minute both for clean steel surfaces and for surfaces coated with
ferric oxide F6203 (1000 A thick). A prepared film of ferroso-
ferric oxide F6304 (1000 A thick) prevented lubrication failure
with 0.5-percent stearic acid in cetane at sliding velocities higher
than 7000 feet per minute (269 gram load) and with loads to 769 grams
at a sliding velocity of 6000 feet per minute. Lubrication failures
were probably caused by melting of the metallic soap formed by inter-
action of the stearic acid with the steel surfaces. The experiments
indicate that the type of surface oxide and the thickness of the
oxide film are important in determining the effectiveness of stearic
acid as an additive in lubrication at high sliding velocities.

The use of low-viscosity oils in aircraft engines to allow
satisfactory operation at low temperatures has resulted in marginal
boundary lubrication at operating temperatures (reference 1). The
use of lubricant additives for providing increased load capacity
of sliding surfaces is being considered.. References 2 and 3 point
out that with a temperature-active (extreme-pressure) type of addi-
tive lubrication.may be ineffective at high sliding velocities
because an effective chemical reaction between the surfaces and the
additives to provide a lubricating film.may not have time to occur.

Studies made at relatively low temperatures (references 4 to 9)
have established the point that, with a fatty-acid type of additive,
high contact temperatures or pressures are unnecessary for formation
of an effective lubricating film. Strongly adsorbed, oriented mole-
cules of the fatty acids are deposited on the metallic surfaces with
which they react to form.metallic soap. Because a preformed lubri-
cating film is available, a fatty-acid type of additive would offer
greater possibilities for lubrication at high sliding velocities
than would the temperature—active (extreme-pressure) type of addi—
tive, which must form its lubricating film under contact conditions.
Fatty acids are, however, generally considered to provide less load-
carrying capacity than is obtainable from temperature-active mate-
rials. Furthermore, fatty acids are not considered as effective
at high temperatures (reference 10).