Ashless Phosphorus-Containing Lubricating Oil Additives 81
spot on the surface, where the surface temperatures are highest (from metal contact). It was thought
that in the reaction, the phosphate ester formed a lower melting phosphide (or possibly an iron/iron
phosphide eutectic) that fl owed over the surface and caused a smoothing or chemical polishing
effect. They also observed that there appeared to be an optimum level of addition of the TCP (1.5%),
a conclusion later confi rmed by other workers in the fi eld.
Beeck et al. claimed in these papers that their research had produced a better understanding of
the AW mechanism and enabled more precise distinctions to be drawn between the different types
of additives; more specifi cally, that
A wear prevention agent reduces pressure and temperature through better distribution of the load over
the apparent surface. If the resulting minimum pressure is still too high for the maintenance of a stable
fi lm, metal to metal contact will take place in spite of the high polish. Since in this case the surface of
actual contact is relatively very large, seizure and breakdown will follow very rapidly ...The intervention of the war years encouraged German researchers to prepare and evaluate a num-
ber of phosphorus compounds as EP/AW additives, principally phosphinic acid derivatives and
also acid phosphates [6,7], while other workers [74] continued to investigate the behavior of TCP.
The performance of the latter in white oil was examined, and it was suggested that the additive
reacted with steel to form a thin, solid, nonconducting fi lm that prevented seizure by shearing in
preference to metal-to-metal contacts. The improved behavior of blends of TCP with fatty acids
was explained as being due to the improved adsorption of the fatty acid onto the surface of the
chemically formed fi lm.
In 1950, an extensive evaluation of different neutral alkyl and aryl phosphates and phosphites,
in some cases containing chlorine and sulfur, was undertaken [75]. The results of this investigation
showed that the action of sulfur and chlorine on the surface is to form a sulfi de and a chloride fi lm,
respectively. In the presence of phosphorus, mixed fi lms of phosphide/sulfi de or phosphide/chloride
were formed. The presence of phosphide was established chemically by the liberation of phosphine
in the presence of hydrochloric acid.
Although the concept of phosphide fi lm formation was challenged at this time [76,77], it
remained as the generally held theory until the mid-1960s when several papers appeared with con-
tradictory data. Godfrey [78] pointed out that the experiments that had indicated the presence of
phosphide had all been static, high-temperature investigations, and none had identifi ed phosphide
on a sliding surface lubricated with TCP. He experimented with the lubrication of steel-on-steel sur-
faces by TCP followed by an examination of the metal surface. This revealed the presence of white
crystalline material, which was shown by electron diffraction measurements to be predominantly a
mixture of ferric phosphate, FePO 4 , and its dihydrate, FePO 4 .2H 2 O. Phosphides, if present, were in
extremely small quantities. Furthermore, a paste made from the dihydrate showed similar frictional
characteristics to TCP, whereas a paste from iron phosphide showed no signifi cant reduction in fric-
tion. Tests also suggested the importance of air to the performance of TCP as tests carried out under
nitrogen revealed substantially increased wear. Pure TCP was evaluated and, unlike commercial
material, showed no signifi cant friction-reducing properties.
The presence and role of impurities in the activity of commercial TCP was the subject of inves-
tigations using radioactive P^32 [79]. Results suggested that the phosphorus-containing polar impu-
rities—not the neutral TCP—were adsorbed onto the metal surface. The P^32 found in the wear
scar appeared to the chemically bound—not physically adsorbed, but the latter process seemed
to be the way that the phosphorus was initially made available on the surface. The authors indi-
cated that the impurities resembled acid phosphates (rather than phosphoric acid, which Godfrey
had assumed) and carried out wear tests comparing the neutral ester with both an acid phos-
phate (dilauryl acid phosphate) and hydrolyzed TCP. They found that lower concentrations of these
compounds generally gave equivalent performance to the neutral ester. Of interest was the obser-
vation that, although TCP showed no wear minimum in the reported tests (cf. the results given by