Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 87

most recent analytical studies into the mode of action of these additives; therefore, the conclusions
represent the current thinking. Aryl phosphates were chosen for this application because of their
oxidation stability and good boundary lubrication performance at high temperatures. The initial
studies took place with TCP [94] and involved examination of the fi lms formed on tool steel balls
and on iron, stainless steel, copper, nickel, tungsten, and quartz wire specimens. (TCP vapor had
previously been shown to form tenacious fi lms on graphite, tungsten, and aluminum at temperatures
above its thermal decomposition point [95].) Wear tests on tool steel with vapor at 370°C showed
low levels of wear even at 0.1 mol% concentration (Figure 3.10). An optimum concentration was
reached at ∼0.5 mol%. Reaction with the metals indicated above is displayed in Figure 3.11, which
shows that deposition on iron and copper is relatively fast but slow for quartz, nickel, and tungsten.

TABLE 3.8

Friction and Wear Reduction from Bearing Surface Pretreatment by Phosphate

Esters

Bearing Preparation

Average Scar

Diameter (mm)

Improvement

(%)

Maximum

Torque (gf m)

Improvement

(%)

Untreated reference 1.00 – 46.1 –

TCP 0.72 28 18.4 60

IPPP 0.75 25 18.4 60

TOF 0.81 19 18.4 60

10% TCP in mineral oil 0.72 28 18.4 60

10% IPPP in mineral oil 0.72 28 15.0 68

10% TOF in mineral oil 0.64 36 19.6 58

Note: ASTM D 4172–88. Four-ball wear test conditions: test time, 60 min; test temperature, 75°C; test load,

40 kgf; test speed, 600 rpm. IPPP = isopropylphenyl phosphate; TCP = tricresyl phosphate; TOF = tris (2-

ethylhexyl) phosphate. All wear tests performed in 100 solvent neutral paraffi nic mineral oil.

Source: Placek, D.G., Shankwalkar, S.G., WEAR, 173(1-2), 1994. Permission from Elsevier.

Hertz line

0

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

Wear scar (mm)

TCP (mol %)

FIGURE 3.10 Four-ball wear values at 370°C with vapor lubrication as a function of tricresyl phosphate
(TCP) vapor concentration. (From Klaus, E.E., Jeng, G.S., Duda, J.L., Lubr. Eng. 45(11), 717–723, 1989. With
permission.)