80 Lubricant Additives: Chemistry and Applications
of opinion. These have probably arisen as a result of the different test conditions found in the
wide variety of test equipment developed for measuring wear. For example, different test specimen
geometries, surface fi nish, sliding speeds, and the use of additives with different levels of purity
have meant that the data have not been strictly comparable.
After a brief review of the early development of AW/EP additives, a number of papers exploring
the mechanism of action of different phosphorus-based additives are summarized in this section.
It is not inclusive, and the results of many other workers could have been mentioned. An additional
selection of papers on the topic is therefore given in Appendix B. Some papers evaluate several
classes of product (e.g., phosphates, phosphites, phosphonates, etc.); these may be located in sec-
tions other than that on neutral phosphates if information on these other structures is limited.
3.5.1 EARLY INVESTIGATIONS INTO ANTIWEAR AND EXTREME ADDITIVES
Some of the earliest experiments into the effects of different lubricants on friction were carried out
by Hardy in 1919 [70], who noted the superior performance of castor oil and oleic acid. He found
that good lubricating properties were closely related to the ability of substances to lower surface
energy. A series of papers from Hardy and Doubleday followed in 1922–1923 examining the activ-
ity of lubricants under boundary conditions.
In 1920, Wells and Southcombe [71] discovered that the addition of a small amount of a long-chain
fatty acid signifi cantly reduced the static coeffi cient of friction of mineral oil. Bragg postulated in
1925 [72] that long-chain molecules with a polar terminal group were attached to the surface by
adsorption of the polar group and that the long hydrocarbon chains were orientated perpendicular
to the surface. He also suggested that the formation of fi lms on both the moving surfaces assisted
lubrication by sliding over one another, with their long chains being “fl attened” as the distance
between the surfaces was reduced. However, in 1936, Clark and Sterrett [73] showed that the lubri-
cating fi lm could be up to 200 molecules in thickness but that only the fi rst layer would have the
strength to withstand the shearing stresses produced under sliding conditions. They also found
that certain ring structures (e.g., trichlorophenol) that were active as “fi lm strength” additives also
showed molecular orientation, in this case, parallel to the metal surface, and attributed the good
load-carrying performance to the ability of the layers to slide over one another. Orientation was not
the only factor involved, as compounds with a similar orientation could show a wide difference in
performance.
The mechanism and infl uence of additives on boundary lubrication were fi rst investigated and
reported by Beeck et al. [4]. They found that friction was reasonably constant with sliding velocity
up to a critical velocity, beyond which there was a signifi cant reduction. Additives were found to
reduce the friction at low speeds relative to the base oil alone and also had a signifi cant, but variable,
effect on the reduction at different critical velocities. Low critical velocities were found for com-
pounds that were strongly adsorbed and that showed orientation of the surface fi lm. It was recog-
nized that the adsorbed layer is thinner than the roughness of even the best machined surfaces and
that high temperatures (or loads) at points of contact would cause decomposition of the molecules
with the formation of a high-melting corrosion product and an increase in friction. If the surfaces
were highly polished, then sliding could take place without destruction of the surface fi lm. It was
concluded that most of the friction-reducing compounds, principally, the long-chain fatty acids,
were not able to produce a highly polished surface and therefore were not effective AW additives.
3.5.2 NEUTRAL ALKYL AND ARYL PHOSPHATES
3.5.2.1 Historical Background
One additive examined by Beeck et al. [5] that was able to reduce both friction and wear was TCP,
a product that was, at that time, beginning to fi nd widespread commercial use as an AW additive.
The authors proposed that TCP acted by a corrosive action, preferentially reacting with the high