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National Advisory Committee for Aeronautics, Technical Notes - Friction of Surface Films Formed by Decomposition of Common Lubricants of Several Types

An experimental investigation was conducted to determine the
effect on friction of films formed on steel surfaces by decompo-
sition of common lubricants of several types. The films were
formed by heating, in air, surfaces to which a thin film of fluid
lubricant had been applied. The investigation was carried out
with a kinetic-friction apparatus employing an elastically restrained
spherical rider (l/4-in. diam) that radially traversed a spiral path
on a rotating steel-disk specimen. Experiments were conducted over
a range of sliding velocities between 75 and,8000 feet per minute
with loads from 269 to 1017 grams (initial Hertz surface stress of
126 ,000 to 194,000 lb/sq 111.). Surface studies were made using
standard metallurgical and physical-measurement equipment and
techniques.

The data obtained indicated that the prepared decomposition
films were, in general, beneficial to sliding surfaces with regard
to friction and wear. Measured values of friction between the
spherical riders and the steel disks covered with the decompo-
sition films were in the same general range as those obtained under
boundary lubrication conditions for the original fluid lubricants.
The mechanism by which these films decrease friction may be the
same as that of lubrication by thin metallic films. Discontinuities
and cracks indicated that the film materials do not serve as highly
viscous fluid lubricants.

Although film strength of neither a silicone-polymer fluid
nor its decomposition product was individually sufficient to satis-
factorily support the minimum load used, a combination of the two
materials permitted the load to be effectively supported. Very low
friction and acceptable film strength were obtained by using this
combination at surface stresses that approximate the maximum.
commonly used in the design of lubricated surfaces for aircraft
power plants. Such a combination should be especially satisfactory
‘in high-temperature lubrication because the decomposition film
would be regenerated.

A number of recently developed types of aircraft engine are
characterized by high operating temperatures. These temperatures
introduce critical cooling problems in lubrication systems because
the lubricants are required to contact hot surfaces ; the heat from
these surfaces causes thermal cracking of the lubricant and also-
accelerates the processes of oxidation, polymerization, or other
forms of decomposition (reference 1), resulting in lacquer deposits
on the surfaces.