Lubricant Additives

88 Lubricant Additives: Chemistry and Applications

Rates of formation are, of course, temperature-dependent, but fi lms are produced up to at least 800°C.
Increases in temperature and TCP concentration caused an increase in deposit formation.
The use of TCP vapor to lubricate high-speed bearings made from M50 steel at 350°C was exam-
ined by Graham et al. in 1992 [96] with excellent results. In fact, the wear area was smoother than the
unused surface. Surprisingly, similar results were found when lubricating silicon nitride surface without
prior activation. Here, the results were clouded by the transfer of copper to the test specimens, and it was
thought that activation could have occurred by reaction of TCP with copper components of the vapor
delivery system, which was then deposited onto the ceramic surface. Analysis of the fi lm formed by TCP
on a ceramic surface was also investigated by Hanyaloglu and Graham [97]. In this case, the ceramic
was activated by a fi lm (∼20 atoms thick) of iron oxide. The presence of TCP at 0.5% in nitrogen or air
at 500°C gave a friction coeffi cient of 0.07 and produced a polymer containing mainly carbon, oxygen,
and a small amount of phosphorus with a molecular weight range of 6,000–60,000 g/mol.
A combination of vapor and mist lubrication has also been evaluated in the lubrication of gas
turbine bearings [98]. The data indicated that organophosphates worked well with ferrous metal due
to the rapid formation of a predominantly iron phosphate fi lm. This was followed by the development
of a pyrophosphate-based fi lm over the iron phosphate. As long as iron was present, the organophos-
phates worked well, but continued production of the phosphate/pyrophosphate fi lm reduced access
to iron and eventually led to surface failure. Morales and Handschuh [99] reported a solution to this
problem in which the phosphate contained a small quantity of ferric acetylacetonate. Evaluation of
this solution in comparison with the pure phosphate showed that the iron salt enabled a phosphate
fi lm to be successfully deposited onto an aluminum surface, which the pure phosphate is unable to
do. (Neutral phosphates are known not to wet the surface of aluminum.) Vapor/mist lubrication of a
gearbox using pure phosphate was compared with the performance of the phosphate containing the
iron salt; a signifi cant improvement in scuffi ng performance was noted. This was enhanced when
the mist was directed onto the gear teeth immediately before contact. Evaluation of the surface fi lm
on the gear teeth revealed no phosphorus when the pure phosphate was tested but showed the pres-
ence of “fair amounts” of both iron and phosphorus when using the soluble iron salt.
A recent study of the mechanism of fi lm formation by aryl phosphates [100] involved examin-
ing the reaction of phosphates with metal in the form of foil or powder and also with various metal
oxides in different oxidation states. The tests were carried out in both oxygen-rich and oxygen-
depleted environments and they revealed that the reactivity of both the commercial grade of TCP

Copper

Iron

Stainless steel

Nickel

Quartz

Tungsten

2100

1800

1500

1200

900

600

300

0

0 5 10 15 20 25 30

Time (min)

Deposit weight (mg/cm

2 )

FIGURE 3.11 Deposition on various substrates with 1.55% TCP in a nitrogen stream at 700°C. (From Klaus,
E.E., Jeng, G.S., Duda, J.L., Lubr. Eng., 45(11), 717–723, 1989. With permission.)