Effect of extreme load on plain ZDDP oil in the presence of FeF3 catalyst using design of experiment and fundamental study under two different rotational speeds

Abstract

Understanding the behavior of plain zinc dialkyldithiophosphate (ZDDP) oil in the presence of iron fluoride (FeF3) catalyst is of paramount practical significance. In this article, the tribological and chemical interactions of ZDDP/FeF3 underlying their improved wear performances were examined. Optimized 0.4 wt% FeF3 catalyst + 0.1% P (0.1% phosphorus concentration) plain ZDDP oil was investigated under the protocol of two different rotational speeds (100 rpm for the first 5,000 revolutions and 700 rpm until failure or 100,000 revolutions, whichever came first) and steady-state speed (700 rpm until failure or 100,000 revolutions). A Plint T53 SLIM modified ball-on-cylinder machine (Phoenix Tribology, Whitway, England) and different contact loads were used for all tests. A design of experiment (DOE) with a two-level factorial design was used to investigate the failure and wear responses with respect to catalyst interactions in plain ZDDP oils. An optimized load of 336 N (2.6 GPa Hertzian contact pressure) was reflected in the high-desirability DOE data when using different rotational speed protocols. The two different rotational speeds indicated better performance than the steady-state speed protocol, especially in the presence of FeF3 catalyst. The tribological properties of each compound under optimized loading conditions were evaluated using a ball-on-cylinder tribometer. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and auger electron spectroscopy (AES) were used to examine the wear tracks of FeF3/ZDDP mixtures and to characterize the chemical properties of tribofilms generated under extreme loading and different rotational speeds. The mechanism of tribofilm formation and breakdown was followed carefully by monitoring the friction coefficient over the duration of the test. It was found that the optimized loading sample under the two different rotational speeds performed better and the addition of catalyst increased the friction-reducing capacity of the tested samples. It was also shown that higher contact loads at 2.77 GPa Hertzian contact pressure resulted in premature breakdown of tribofilms, rendering their antiwear resistance under the steady-state rotational speed insignificant. However, the optimized loading sample under steady-state speed performed better than the extreme load (405 N) samples. Chemical analysis showed more phosphorus and more iron oxide (Fe3O4) nanoparticles in the tribofilm of the 0.4 wt% FeF3 + 0.1% P ZDDP sample conducted under 2.6 GPa Hertzian contact pressure and two different rotational speeds. Transmission electron microscopy analysis of the wear debris indicated that a larger fraction of the crystalline particles at higher contact loads of 405 N were Fe2O3. This was true even for different rotational speeds. Fe2O3 is significantly more abrasive and resulted in rapid breakdown of the tribofilms. Auger electron spectroscopy indicated thicker film in the optimized sample. This might be due to the higher phosphorus concentration and the greater reduction in oxygen inside the wear track.