Designing artificial zinc phosphate tribofilms with tailored mechanical properties by altering the chain length

TL;DR

This study demonstrates how thermal treatment alters the chain length of zinc phosphate tribofilms, enabling the tailoring of their mechanical properties such as hardness and wear resistance.

Contribution

It introduces a method to modify zinc phosphate tribofilm properties by controlling annealing temperature and chain length, avoiding environmental sulfur concerns.

Findings

Increased annealing temperature reduces chain length and phosphorus content.

Shorter chain lengths correlate with higher hardness and wear resistance.

The coatings transition from metaphosphate to orthophosphate with thermal treatment.

Abstract

Zinc dialkyldithiophosphate (ZDDP), as the most prominent lubrication additive, forms tribofilms consisting primarily of zinc phosphate glasses containing sulfides. As sulfur is linked to environmental concerns, sulfur-free zinc phosphate coatings have been sputtered from a Zn3(PO4)2 target and investigated here. Based on the bridging to non-bridging oxygen ratio, determined by X-ray photoelectron spectroscopy (XPS), the as deposited coatings are classified as metaphosphates. As the annealing temperature is increased, the chain lengths are reduced, as witnessed by XPS data indicated by a loss of phosphorus and oxygen of the coating surface, likely due to hydrolysis with water from the atmosphere. Transmission electron microscopy energy-dispersive X-ray spectroscopy line scans show that the XPS-revealed composition change of the coating surface upon annealing occurs over the whole thickness of the coating. This alteration in composition and chain length reductions causes a rise in hardness, reduced Young's modulus, and wear resistance. Therefore, the properties of the artificial zinc phosphate tribofilms can be tailored via a thermally stimulated composition change, causing an alternation in chain length from meta- to orthophosphate and thereby enabling the design of coatings with desired mechanical properties.