Tribological performance of an Al—Si alloy lubricated in the boundary regime with zinc dialkyldithiophosphate and molybdenum dithiocarbamate additives

Abstract

The tribolochemical performance of a cast iron/Al-Si alloy system lubricated in the boundary regime using lubricants containing Zinc Dialkyldithiophosphate (ZDDP) and Molybdenum Dialkyldithiocarbamate (MoDTC) additives has been investigated. Tests were conducted on a Plint TE 77 reciprocating tribometer in contact conditions comparable to the conditions in a piston ring/cylinder liner system of the internal combustion engine. Tests were performed for 4 different time durations, from 1 minute to 3 hours, to investigate the tribofilm evolution formed on the Al-Si alloy surfaces. Surface sensitive analytical techniques such as atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS) have been used to determine the nature of the surface topography as well as the chemical nature of the tribofilms formed on the Al-Si material. The additive effect on the tribofilm formation as well as tribological performance is discussed. It is shown that the MoDTC additive, blended into a ZDDP-containing lubricant, improves the effectiveness of friction and wear reduction by providing an interface in the tribofilm that can be easily sheared, thus protecting the ZDDP-derived phosphate film from high shear stress. Tribofilms formed on all the Al-Si test surfaces after short times and the lubricant also affected the physical nature of the Al-Si microstructure. The tribofilm evolution processes for different lubricants is discussed in relation to recent literature and hypotheses of tribofilm evolution on Al-Si material are proposed.2

The influence of start–stop transient velocity on the friction and wear behaviour of a hyper-eutectic Al–Si automotive alloy

“…Dynamic friction coefficients of all tests were in the range 0.08-0.1 and therefore in agreement with published literature on lubricated sliding of hyper-eutectic Al-Si against ferrous counter-surfaces [28][29][30][31].…”

“…This was further evidence of boundary additive function reducing the static re-start friction coefficient to 0.15. When re-starting, contact potential readings exhibited a peak associated with the static friction peak, again confirming tribo-chemical reaction associated with frictional energy input from local flash temperatures and also consistent with other studies on anti-wear pad formation [19,31,33,34].…”

“…This was broadly consistent with work by Nicholls et al [34,36,37] who used a number of X-Ray techniques (XANES, X-PEEM) to determine the mechanism of preferential long chain poly-phosphate anti-wear pad formation from ZDDP additives at contacting Si interfaces. The presence of iron was also noted on the contact surface of the Si particles, Figure 5 d), and would be consistent with the theory that a ferrous oxide surface is necessary to allow desorption of Zn based additives prior to formation of a glassy phosphate anti-wear pad [19,31,34,36,38]. The presence of Al also on the surface of the Si particles, Placing the current work in a wider context, it was clear that whilst friction behaviour was not significantly inhibited, start-stop interrupted velocities had a detrimental effect on the thickness of antiwear film formation on Al-Si cylinder liner materials.…”

The correlation between ZDDP tribofilm morphology and the microstructure of steel

“…The tribofilm is then preferentially formed on protruding Si-rich precipitates, which remain in contact with the counterface. Other tribological behaviours were observed by Xia et al [32] when oils containing only ZDDP or ZDDP and high concentration of MoDTC were used. Although the tribofilm formation on Al-Si based alloys has been shown to be associated with microstructure of the material and it is well known that the microstructure of typical hard steels used in tribological applications, is also highly inhomogeneous, investigating and quantifying the microstructuretribofilm relationship of such materials has never been tried.…”

“…In a study of ZDDPderived tribofilm on an Al-Si alloy performed with the use of various chemical micro-analysis techniques, Nicholls et al [31] attributed this e↵ect to the increased contact pressure, to which Si-rich precipitates are exposed, as these were shown to protrude by about 150 nm from the Al-rich matrix. Xia et al [32] have noted that when oil containing ZDDP and a low concentration of molybdenum dialkyldithiocarbamate (MoDTC) is used to lubricate Al-Si alloy -cast iron tribocouples, it results in more pronounced wear of the Al-matrix with respect to hard Si-rich phase. The tribofilm is then preferentially formed on protruding Si-rich precipitates, which remain in contact with the counterface.…”