A new insight into the interfacial mechanisms of the tribofilm formed by zinc dialkyl dithiophosphate

Abstract

Understanding the true interfacial mechanisms involved in the growth of tribofilms generated by Zinc Dialkyl Dithiophosphate (ZDDP) is important because it is the most widely used antiwear additive and there is legislative pressure to find efficient environmentally-friendly replacements. The main focus of this study is to investigate the durability of the ZDDP tribofilm and correlate it to the chemical and physical properties of the glassy polyphosphates.A novel experimental method has been developed to study the effect of lubricant temperature and contact load on tribofilm growth and durability. Results show that physical parameters such as temperature and pressure significantly influence the tribofilm durability. XPS analyses were carried out before suspending the test and after changing the oil to assess the difference in chemical structure of the tribofilm before and after stopping the test. The chemical analyses suggest that there are different chemical properties across the thickness of the tribofilm and these determine the durability characteristics.

Study of the Interfacial Mechanism of ZDDP Tribofilm in Humid Environment and its Effect on Tribochemical Wear; Part II: Numerical

“…Parsaeian et al [12,16] found that water can affect the performance of ZDDP by altering the polyphosphates chain length. This effect manifests itself in the formation of shorter polyphosphate chain at higher levels of relative humidity [16,23]. The results are in a good agreement with the study done by Nedelcu et al [24].…”

On the Crystallinity and Durability of ZDDP Tribofilm

“…One way to assess the durability of a ZDDP tribofilm is to monitor changes in ZDDP tribofilm thickness after the replacement of a ZDDP-containing oil by a ZDDP-free oil [1][2][3]. Fujita et al [1] measured the growth of a thick ZDDP tribofilm in rolling-sliding contact using optical interferometry and then the effect on this pre-formed tribofilm of replacing the ZDDP solution by base oil.…”

“…This picture has, however, recently become more complicated since Parsaeian et al [3] have shown that the durability of a ZDDP tribofilm appears to depend on how long it has been rubbed. Like Fujita et al, they found that if ZDDP solution was replaced by base oil after prolonged rubbing, the tribofilm was not significantly removed during further rubbing and was thus durable.…”

“…The chain length of phosphates in ZDDP tribofilms can be estimated from the ratio of bonding oxygen, BO (P-O-P), to non-bonding oxygen, NBO (P=O and P-O-M), intensity, the BO/NBO ratio [4][5][6][7]. It should be noted that many studies have shown that the chain length of zinc polyphosphate reduces with rubbing time [3,6,[8][9][10] though a few have proposed precisely the opposite [11,12]. Parsaeian et al's work [3] thus suggests that the strength/durability of ZDDP tribofilm may depend on its phosphate chain length and thus change with rubbing time.…”

Ethoxylated Amine Friction Modifiers and ZDDP

“…One is that the ZDDP tribofilm formation stage 1 was quite long in this work, at 3-h rubbing. A number of studies have found that ZDDP tribofilms gradually transform from polyphosphate to short-chain phosphate during extended rubbing [42,43] and it is thus quite likely that most of the tribofilm formed in the current study was, in fact depolymerised orthophosphate. Concerning the immediate surface where polyphosphate might be expected to be most likely to be present, this may have been removed by cleaning.…”