Fig. 7 EPMA Analysis of pin specimen after wear test
(c) and (d) show the distribution of sulfur and phosphor to examine the effect of the extreme-pressure additive, etc., contained in the lubricating oil. In comparison of Fig. 7(b) with Fig. 6(b) above, it is understood that the white metallic luster area in the observation by the optical microscope is molybdenum. The results of (c) and (d) indicate that sulfur and phosphor are present in the crack (part A) and the crescent-shaped recess (part B). These are derived from the lubricating oil composition, and it is believed that the degraded lubricating oil is left in the recessed part difficult to discharge by the wear.
On the other hand, little sulfur and phosphor is left on the worn surface and no formation of the extreme- pressure film was detected. It is well known that the extreme-pressure sulfur or phosphor additive to be added to the lubricating oil forms a reactive film on the surface when the temperature reaches the prescribed value by the friction and is useful in preventing the direct contact of metals with each other. The temperature is reportedly around 423K in the case of the extreme-pressure phosphor additive1). In the present test condition, the temperature of the lubricating oil was 347K, the sliding speed was 2.1 m/s, and the load was 4500N, and the initial bearing pressure when the test was started was about 1.47 Gpa, and it is presumed that higher temperature over 423K was generated. The reason that the reactive product with the extreme-pressure additive could not be detected is considered to be attributable to the fact that the reactivity of the material with the extreme-pressure additive is low.
As for the working mechanism of the extreme-pressure additive, the following findings are obtained2). In the case of the steel material, the sulfur additive composition forms iron sulfide on its surface, and the phosphor additive composition forms iron phosphate on its surface, and the formed substance plays a role as the boundary lubrication film which is the reactive film weak in strength in the vicinity of the contact interface.
In the experience of one of the authors, the phosphor extreme-pressure film was detected3) on the worn surface though the frictional condition was not so severe, for example, the maximum Hertzian contact pressure was 1.44 Gpa, and the sliding speed was 1.7 mm/s in the reciprocating friction in the spherical/plane contact of bearing steels at the normal temperature, and temperature of the lubricating oil was not set to above 423K3).
