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National Advisory Committee for Aeronautics, Technical Notes - Effects of Chemically Active Additives on Boundary Lubrication of Steel by Silicones

A previous investigation showed that silicones, which in themselves
are poor lubricants, can be made to lubricate effectively by the addition
of a solvent, such as a diester, which is believed to increase the energy
of adhesion between the molecule and the surface.

In the present-report another method of improving the lubricating
quality of silicones was investigated, namely, that of providing chemi—
cally active additives. It has been hypothesized that silicones do not
maintain oxide or other reactive coatings on metal surfaces. Conventional
chemically active additives and more active compounds such as a peroxide
were investigated. It was found that conventional additives were not
effective, but that more active materials such as the peroxide did give
effective lubrication. However, all the chemically active-type additives
investigated were inferior to the solvent-type additions such as the
diesters previously studied.

During the past few years, the use of silicone fluids as lubricants
for specialized applications has increased steadily. They have the best
viscosity—temperature relation of any known class of fluids and a high—
temperature chemical stability which is at least as good as that of any
synthetic lubricant now being considered for use in aircraft turbine
engines (ref. 1). They therefore merit consideration as lubricants for
high-temperature applications. However, the silicones are extremely poor
boundary lubricants for ferrous surfaces.

An NACA research program has been directed toward finding means of
improving the boundary—lubrication characteristics of silicones for fer-
rous surfaces. Previously reported studies (ref. 2) have indicated that
blends of silicones with 50 to 40 percent by volume of various solvents
such as diesters will provide effective lubrication for steel surfaces. fl
This result may be associated with the effect of the solvents on the ge-
ometry of the silicone molecule. Silicone-diester blends appear to have
considerable promise as possible engine lubricants.