Friction coefficient MoDTC/ZDDP tribofilms

FIGURE 9.6 Nanoscratch data of the MoDTC/ZDDP tribofilm at a constant normal force of 1000 iN. (a) Normal force vs. time and scratch displacement vs. time, (b) lateral force vs. time and normal displacement vs. time, and (c) friction coefficient vs. time and in situ AFM image of the residual scratch groove. 

FIGURE 9.20 Model of the functionally graded structure of the MoDTC/ZDDP tribofilm. A low-friction inner-skin layer formed hy MoSj nanostrips has low yield strength and acts as a lamellar solid lubricant in lowering the friction coefficient of the MoDTC/ZDDP tribofilm. High concentrations of Ca, O, P, and S oil additive elements in the middle layer decreased and disappeared from the base layer, while the Fe concentration increased from an ultralow level in the surface layer to the same level as the steel disk at the tribofihn-disk interface. 

To understand the wear mechanism in valve train wear tests, samples of the worn tappet surface were analyzed for surface elements by electron probe microanalysis (EPMA) and X-ray photo electron spectroscopy (XPS). Results of EPMA analysis of the worn surface in terms of concentration of phosphorus and sulfur atoms for oil with primary ZnDDP without MoDTC, showed an increase of zinc and sulfur intensity after 100 hrs of test time, in spite of decreasing phosphorus intensity. Examination of the worn surface by XPS with primary and secondary ZDDP with addition of MoDTC showed the presence of MoS2 in the tribofilm. Using mixtures of ZDDP and MoDTC, the friction coefficient is reduced, and wear is comparable to that of using ZDDP alone (Kasrai et ah, 1997). 

Wear scar and coefficient of friction. Study of friction coefficient and wear scar were performed for tribofilms generated from a range of MoDTC and ZDDP concentrations, see Table 5.14 (Kasrai et ah, 1998). The wear scar for base oil is 225 /urn wide. The wear scar for neat ZDDP shows a 75 ptm value, over the whole range of ZDDP concentrations (100-2000 ppm). 

As shown in fig. 9.8, both the MoDTC/ZDDP and ZDDP tribofilms displayed the same friction coefficient distributions relative to the scratch depth [32], The friction coefficient retained a high constant value of 0.35 when the scratch depth was greater than 60 nm. However, when the scratch depth was around 10 nm near the film surface. 

These results indicated that the MoDTC/ZDDP and ZDDP tribofilms possessed the same mechanical and frictional property distributions relative to the film depth except near the triboflhn surface. Their hardness, modulus, and friction coefficient became smaller with decreasing film depth. Most importantly, there was no evidence that the friction reduction due to the MoDTC additive could be attributed to these properties at a depth greater than 10 nm. 

Table 5.14. The wear scar and coefficient of friction for tribofilms generated from the combination of ZDDP + MoDTC...

---