Study of the interfacial mechanism of ZDDP tribofilm in humid environment and its effect on tribochemical wear; Part I: Experimental

Abstract

Wear performance of any tribological system can be influenced in a complex way by water contamination. Water can be the cause of steel corrosion which, in turn, can accelerate wear. It can decompose the additives in the oil and create a more corrosive environment which leads to the higher wear in the system. A key novelty of this study is to investigate the effect of relative humidity and the tribochemical changes on the tribological performance and tribofilm characteristics of boundary lubricated systems by means of designing a humidity control system integrated to the Mini Traction Machine (MTM) and Spacer Layer Interferometry Method (SLIM) for the first time. The system is capable of simulating rolling-sliding conditions continuously where lubricant can be contaminated with water. This paper is the first part of a two-part study and the theoretical aspects of the work is the subject of the second part of this investigation. It was observed that humidity hinders the tribofilm formation, especially at higher values of relative humidity and lower temperatures and it can significantly affect the wear process. The correlation between tribofilm thickness, water concentration, temperature and wear of the system was studied. The experimental results suggest that the higher the humidity, the higher the wear of the system and it is more noticeable at lower temperatures where the tribofilm in thinner. The surface chemistry of zinc polyphosphates was investigated as a function of humidity.

On the Crystallinity and Durability of ZDDP Tribofilm

“…Thus, Crobu et al [5] found that some depolymerization of zinc metaphosphate and polyphosphate occurred when this was rubbed against a glass sphere, though much less than when rubbed against steel. These changes were ascribed to the action of adventitious water, and recently Parsaeian et al [6] have shown that the deliberate addition of water does reduce the chain length of ZDDP tribofilms.…”

“…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.…”

In situ synchrotron XAS study of the decomposition kinetics of ZDDP triboreactive interfaces

“…can produce sulfate [10][11][12] or cleave the phosphate chains composing the tribolm. 11,13 Third, probing the contacting surfaces aer the test can conceal the formation and kinetics of any intermediate species during the initial stage of the decomposition reaction. 14 Fourth, during sliding at high temperature and under high contact pressure, triboemission from the substrate, 15 availability of cations, 16 heat and shear 17 make a continuous environment for chemical reactions to evolve, which cannot be captured completely using only ex situ technique.…”

Interactions of Ethanol with Friction Modifiers in Model Engine Lubricants

“…Advanced chemical analysis of the tribofilms formed in the presence of ethanol has suggested oxidation of sulfur species into sulfites and reduction in the length of the phosphate chains [14]. Water contamination has also been shown to reduce the thickness and change the structure of ZDDP tribofilms, apparently by reducing the length of the polyphosphate chains [15].…”