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

Abstract

Zinc polyphosphate glasses are considered to be chiefly responsible for the anti-wear efficiency of ZnDTP tribofilms. In this work, the tribochemical properties of amorphous bulk zinc polyphosphates of different chain lengths (ranging from zinc metaphosphate to zinc pyrophosphate) have been investigated. Tribological tests on bulk polyphosphate discs have been carried out using steel and quartz balls as counter-surfaces in a poly-a-olefin (PAO) bath at room temperature. The composition in the wear track and on the contact region of the balls has been monitored by small-area and imaging X-ray photoelectron spectroscopy (i-XPS). The XPS analysis revealed that the composition of short-chain-length polyphosphates remained unchanged following tribological stress. Long-chain-length polyphosphates are depolymerized in the wear track as a consequence of a tribochemical reaction. By comparing the results obtained using quartz and steel balls, it could be observed that while the reaction of iron oxide with the polyphosphates certainly plays a role in the depolymerization of the samples under sliding conditions, pressure and shear stress alone and also in the presence of water or oil-oxidized species are able to depolymerize the glass when an inert material is used as counterpart; the composition of the wear track, in this case, is dependent on the applied load. All samples were able to form an adhesive, glassy transfer film on both steel and quartz balls, but the short-chain-length polyphosphates showed a lower friction coefficient and wear coefficient. The results suggest a third-body mechanism with the polyphosphates acting as a solid lubricant. Differences in tribological behavior of the different-chain-length polyphosphates are attributable to their mechanical and rheological properties.

Tribochemical study of micropitting in tribocorrosive lubricated contacts: The influence of water and relative humidity

“…This can be the case especially in the event of high contact pressure and shear stress [60,65]. As can be seen in Figure 8, the absorbed water to some extent is present in the BO + ZDDP lubricants (tests in laboratory condition) throughout the tests which can have a catalytic influence on short-chain (poly)phosphate formation.…”

“…The Zn 3s -P 2p3/2 binding energy difference tends to increase with the decrease in the (poly)phosphate chain length [65]. In addition, with an increase in the zinc phosphate chain length a decrease in the modified Auger parameter ( ') binding energy was observed [45,66].…”

The influence of anti‐wear additive ZDDP on doped and undoped diamond‐like carbon coatings

“…In the cast iron counter body, the phosphate occurs as short‐chain pyrophosphate (100%) after both 6 and 20 h testing. It is seen in the literature that short‐chained pyrophosphate derived from the ZDDP anti‐wear additive is more effective in protecting the surfaces than the long‐chain metaphosphates . This suggests that the film transferred to the DLC surface changes from long‐chained metaphosphates to short‐chained pyrophosphates at a slower rate, or that it is transferred gradually from the cast iron pin to the DLC.…”

Effect of loading on the friction and interface microstructure of lubricated steel tribopairs

“…This paper uses a ball-on-disc high temperature tribometer in which the disc surface is pre-coated with sodium metaphosphate before heating, this lubricant was found to have an excellent wettability with a steel surface at high temperature [6]. This layer of phosphate film can form a viscous tribofilm in the contacting zone and help to reduce the friction and wear [17,18]. This paper comprehensively considers the effect of applied load on the tribological behaviour of sodium metaphosphate.…”