86 Lubricant Additives: Chemistry and Applications
The concept of corrosive wear and of phosphates as chemical polishing agents as expressed by
Beek et al. [5] was examined by Furey in 1963 [88]. In his work, surfaces of different roughness were
prepared and friction measurements were made when in contact with a solvent refi ned oil under dif-
ferent applied loads. In tests on an additive-free oil (unfortunately, no information was available on
the sulfur or aromatic content), it was found that friction, in addition to being load-dependent, was
low for highly polished surfaces and rose with increasing roughness up to a roughness of ∼ 10 μin. At
about this roughness, the percentage metal contact was also found to be at its maximum but decreased
thereafter. The explanation given for this was that with increasing roughness, the distances between
the peaks and troughs increase but the peaks become fl atter. The fl atter the peak, the better the load-
carrying capacity, whereas the deeper troughs allow for a greater reserve of oil available locally for
lubrication and cooling. When several AW/EP additives were evaluated in the oil, it was found that,
although there was a reduction in surface roughness, it was less than that found by the oil alone. Fur-
thermore, at low loads, TCP was able to reduce metal contact signifi cantly but had no effect on surface
roughness. At moderate to high loads, although the metal contact was reduced, the surface roughness
was increased. The author concluded that TCP was not acting through a polishing action.
In 1981, Gauthier et al. [89] looked again at the wear process and fi lm formation. They catego-
rized the process into three wear phases: an initial, very rapid phase followed by a medium wear
rate, and fi nally a slow wear phase. In the rapid wear phase, a brown fi lm was formed that, on analy-
sis, was found to be a mixture of ferrous oxide and phosphate. A blue fi lm, which is formed as the
wear rate slows (and the surface becomes smoother), contained no iron and was described as a poly-
meric acid phosphate. (No mention was made of the “white crystalline fi lm” Godfrey reported.)
When both fi lms were removed and the roughness of the underlying surface was measured,
it was found that the surface below the brown fi lm was very smooth. The surface under the blue
fi lm was much rougher and ∼1000 Å thicker. The authors suggested that the smooth surface was
the result of polishing arising from corrosive wear. They concluded that in the fi rst phase of wear,
a corrosive wear process is involved because of the presence of ferrous phosphate on the surface.
When a “critical value” for the surface coverage by the phosphate was achieved, the organic phos-
phoric acids produced by the decomposition of TCP polymerized to form a polyphosphate. As a
result, in the last two wear phases, “the wear of metal is almost completely replaced by the wear of
the additive.” In this way, the disparate observations of TCP behavior (polishing versus increased
surface roughness) could be related and combined.
The presence of polyphosphate was also noted by Placek and Shankwalkar [90] when investigating
the fi lms produced on bearing surfaces by pretreatment with phosphate esters. Tests were carried out on
100% phosphates and also on their 10% solutions in mineral oil, the latter condition because the combi-
nation had been repor ted to provide better wear protection than the individual components alone, appar-
ently by the formation of a “friction polymer” [91,92]. Phosphates chosen for the work included both
aryl and alkyl types. Analyses of the fi lms formed by immersion in the phosphates at 250°C revealed
the presence of a high level of carbon together with iron phosphate/polyphosphate and a small amount
of phosphide. At 300°C, the hydrocarbon had all but disappeared and no phosphide was detected. The
fi lms formed by the mineral oil solutions were mainly hydrocarbon-based, but the fi lm formed by the
alkyl phosphate was unique in that it contained needlelike fi bers. The effect of pretreatment on wear
found under four-ball test conditions is indicated in Table 3.8. The bearings treated with the mineral oil
solutions displayed at least as good wear reduction as those treated with the 100% phosphate.
3.5.2.2 Recent Technical Developments
In 1996, Yansheng et al. [93] reported on the effect of TCP on the wear performance of sulfurized,
oxy-nitrided, and nitrided surfaces. A synergistic effect on nitrided and oxy-nitrided surfaces was
found, resulting in signifi cant increases in load-bearing capacity while reducing friction and wear,
but no improvement was seen on sulfurized surfaces.
A recent application in this brief survey relates to the use of aryl phosphates as vapor-phase
lubricants. Although not strictly an additive application, this development has been the focus of