Spatially resolved nanoscale chemical and mechanical characterization of ZDDP antiwear films on aluminum?silicon alloys under cylinder/bore wear conditions

Abstract

Understanding the lubrication of aluminum-silicon (Al-Si) alloys (>18% Si) under conditions similar to those in the cylinder/bore system is vital to determining their applicability to current engine designs. A novel investigation of the location of zinc-dialkyl-dithiophosphate (ZDDPs) antiwear (AW) film formation on an Al-Si alloy has been performed using X-ray absorption near edge structure (XANES) analysis, X-ray photoelectron emission spectroscopy (X-PEEM), and imaging nanoindentation techniques. A study of the initial stages of wear (10 min) to prolonged rubbing (60 min) was performed. The findings show that the film forms primarily on the raised silicon grains and is consistent with a zinc polyphosphate glass. The film has an elastic modulus of $70 GPa and a similar elastic response to a ZDDP AW film formed on steel under the same conditions. This provides the first direct observation and characterization of a ZDDP antiwear film on Al-Si alloys using spatially resolved chemical and mechanical techniques at the nanoscale.

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

“…The FIB-SIMS mapping clearly showed regions of preferential Zn and Mo rich layers dissociated on silicon and matrix respectively, although as shown from the depth profiles in Figure 8 this was not exclusively the case, with small amounts of Mo on Si and Zn in the matrix. 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].…”

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

“…The study of Al-Si alloys against steel lubricated by commercial engine oils under boundary lubrication conditions has been reported occasionally since the 1960s and 1970s [2,3], with research focused mainly on the tribological performance of Al-Si alloys lubricated by zinc dialkyl dithiophosphate (ZDDP) [4][5][6]. The challenge in understanding fully these interactions lie mainly in the research being performed using different Al-Si alloys and under different testing conditions.…”

“…Performance of this additive in maintaining low wear is due to its interaction with the ferrous material resulting in formation of a layered phosphate/sulphide tribofilm at the interface. Formation of the ZDDP tribofilm in Al-Si alloys has been recently studied [4][5][6] and, although the findings are still not conclusive, an insight on the interaction between this additive and the Al-Si alloy has started to develop. It is demonstrated that polyphosphates form on both Al and Si material [6] while the higher Si content promotes formation of the polyphosphates from the ZDDP additive [4].…”

The correlation between ZDDP tribofilm morphology and the microstructure of steel

“…Therefore, micro-scale variations in chemical composition may also be expected to a↵ect the tribofilm thickness on a similar scale. It has been shown that for Al-Si based alloys, which consist of Al-and Si-rich phases, under certain conditions tribofilm is preferentially formed on harder Si-rich phases in contrast to softer Al-rich phases [31]. 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.…”

“…It has been shown that for Al-Si based alloys, which consist of Al-and Si-rich phases, under certain conditions tribofilm is preferentially formed on harder Si-rich phases in contrast to softer Al-rich phases [31]. 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.…”