Organic Friction Modifi ers 207
The wear of the oil containing A and B is twice as high as those of the reference oil and that
oil with addition of 0.5% GMO. Consequently, the contact pressure p is twice as low as those of the
others and is what makes the ML/EHL transition shift to the left.
Thus it seems that organic FMs are predominantly active in the BL regime and that the shifts
observed in the mixed regime are likely to be caused by other phenomena that must not be ignored.
7.8.2 FRICTION AS A FUNCTION OF TEMPERATURE
Another aspect of FM performance is friction as a function of temperature. Temperature plays an
important role with regard to adsorption/desorption phenomena as for the formation of adsorbed
layers as well as regarding those of reacted layers.
The graph in Figure 7.12 shows the frictional behavior of some organic FMs as a function of
temperature, using the pin-on-ring tribometer as before with the same specimens and confi guration.
Again, CEC reference oil RL 179/2 was used, and the speed chosen (0.03 m/s) assured operation
well within the BL regime.
All the organic FMs studied show a signifi cant friction reduction over the temperature range
tested. GMO and oleylamide perform best, and the optimum adsorption seems to be obtained at
∼70°C. At higher temperatures, desorption may start to occur as well as some kind of competition
with other surface-active additives, leading to a higher coeffi cient of friction. Oleylamide, however,
continues to show high friction-reducing properties at elevated temperatures.
Figure 7.13 shows a comparison between organic FM GMO and metallic-type FM molybde-
num dithiocarbamate. Two sources of the latter were used at a concentration equivalent to 0.07%
molybdenum.
GMO and the molybdenum dithiocarbamates show a marked performance difference. Although
GMO is active over a wide temperature range, the molybdenum dithiocarbamates start to reduce
friction at temperatures of 120°C and above only. This has to be considered as an induction period
that can be explained by the necessary exchange of ligands between molybdenum dithiocarbamate
and zinc dialkyldithiophosphate (see Section 7.7). Once molybdenum dithiocarbamate has “lighted
off,” a fast drop of friction is noticed. At the end of the test cycle 140°C, the system has not stabi-
lized and the friction coeffi cient might decrease further.
FIGURE 7.12 Friction coeffi cient versus temperature—CEC RL 179/2 plus organic FMs.
40 50 60 70 80 90 100 110 120 130 1400.060.070.080.090.100.110.120.130.14Temperature (°C)Friction coefficientf (−)RL 179/2 + 0.5% GMO + 0.5% OFM A + 0.5% Oleylamide