Pressure Dependence of ZnDTP Tribochemical Film Formation: A Combinatorial Approach

Abstract

Combinatorial testing has been performed on zinc dialkyldithiophosphate (ZnDTP)-containing lubricants, to investigate the effects of contact pressure on the formation of tribochemical films. Contact pressures ranging from 25 to 500 MPa were applied in ball-on-disc tribotests with oscillating load. Both the ball and the disc were investigated by means of small-area and imaging X-ray photoelectron spectroscopy (XPS). The thickness and the composition of the reaction layer were estimated from the XPS data. The thickness of the reaction layer in the tribologically stressed areas of the ball and of the disc increased with both temperature and contact pressure. The reaction layer mainly consisted of short-chain poly(thio)phosphates, shorter chains being observed at higher contact pressures. At high pressures, the presence of a thick, high-toughness short-chain poly(thio)phosphate layer can explain the lower friction and dimensional wear coefficients observed. On the ball, similar anti-wear film formation mechanisms were observed as on the disc, zinc sulphide being deposited in the post-contact region.

Effect of Chain-Length and Countersurface on the Tribochemistry of Bulk Zinc Polyphosphate Glasses

“…The presence of shorter chain lengths inside the tribotracks compared to the non-contact regions was also previously observed [5,8]. In our previous study [13] it was shown that zinc metaphosphate is depolymerized against steel under sliding conditions.…”

“…This may explain why, despite more than 70 years of extensive research, the mechanisms of film formation and wear protection of the most common and successful class of anti-wear addives, Zinc dialkyldithiophosphates (ZnDTPs), are still incompletely understood [2,3]. ZnDTP is known to react at the steel interface, forming a tribofilm with thickness, chemical and physical properties that are dependent on conditions such as applied load, temperature, and additive concentration [4][5][6][7]. The composition of the tribofilm is heterogeneous, characterized by a pad-like structure and mainly constituted of amorphous zinc and iron polyphosphates [8][9][10].…”

“…Longer polyphosphates are formed at higher temperatures [4], while the application of higher pressures leads to the formation of thicker reaction films with shorter chain lengths. A shortening of chainlength has been correlated with rubbing time and applied load [5,8,13].…”

“…Nicholls [19] attempted to correlate anti-wear behavior with the degree of phosphate polymerization, via the enhanced mechanical properties at longer chain lengths. However, investigating the effect of pressure on ZnDTP tribofilm formation, Heuberger [5] came to the opposite conclusion: at higher contact pressures, thicker and tougher films of shorter polyphosphates were observed, which exhibited better wear resistance compared to those formed at lower contact pressures. Moreover, the ZnDTP tribofilm hardness was found to correlate with the mean applied pressure in the rubbing contact, showing the ability of the films to provide appropriate properties over a wide range of imposed conditions [20].…”

“…The characterization of different-chain-length zinc polyphosphates, ranging from orthophosphate to metaphosphate, was also reported, together with a strategy for chain-length identification by means of XPS and time-of-flight secondary-ion mass spectroscopy (ToF-SIMS) [22]. The intensity ratio between the bridging oxygen signal (BO) and the non-bridging (NBO) has been commonly used in order to estimate the polyphosphate chain-length from the XPS data [4,5,7,13,[22][23][24]. However, tribological samples generally exhibit a very heterogeneous composition and the presence of overlapping species can affect the measured BO/NBO value.…”