Tribological characterisation of MoSe2 and MoSe2/DLC-W coatings under oil-lubricated sliding conditions

Abstract

Reducing frictional losses and wear of the piston rings/cylinder liner tribo-pair in a heavy-duty vehicle’s internal combustion engine is a key enabler for minimising fuel consumption, boosting engine efficiency and durability, and minimising hazardous greenhouse gas emissions. One possible approach for reducing friction and improving wear resistance of the piston rings/cylinder liner tribo-pair is to use surface coatings with low coefficients of friction and excellent wear resistance. Molybdenum diselenide (MoSe2) coating can be a promising candidate for the tribo-pair due to its desirable properties such as low coefficient of friction, high load-bearing capacity, high thermal stability, low sensitivity to air humidity, and good wear resistance. For the actual assessment of the potential of MoSe2 coating for the piston rings/cylinder liner tribo-pair, an in-depth investigation and understanding of its tribological performance with oil lubrication are needed because this tribo-pair typically operates under oil-lubricated conditions. However, the entire attention of tribological research conducted on this coating has focused on its tribological performance under dry sliding conditions only, whereas its tribological performance under oil-lubricated sliding conditions has not gained overwhelming attention and remains unexplored. To evaluate its suitability for the piston rings/cylinder liner tribo-pair, it is imperative to have a thorough understanding of its tribological performance in conjunction with oil lubrication. Accordingly, the aim of the present work was to study and investigate the influence of oil lubrication on the tribological performance of MoSe2 coating when used under oil-lubricated sliding conditions.

Using magnetron sputtering, pure MoSe2 coating and layered coating comprising MoSe2 and tungsten-doped diamond-like carbon (MoSe2/DLC-W) were deposited on steel discs and characterised using a variety of experimental techniques. Their tribological properties were evaluated against steel balls using a reciprocating tribometer in polyalphaolefin 4 (PAO4 - the most common synthetic base oil used in industrial and automotive lubricants). Their tribological properties were also evaluated under dry sliding conditions for comparative purposes. Following the tribological tests, different experimental techniques, including profilometry, SEM, EDS, Raman spectroscopy, FIB, and TEM were used for analysing the worn surfaces of coatings and their sliding counterparts to develop a better understanding of the underlying friction and wear mechanisms involved.

When MoSe2 coating was assessed in PAO4-lubricated sliding, it demonstrated a high coefficient of friction (0.101). A crystalline MoSe2 tribolayer with basal planes aligned parallel to the sliding direction, which is known to be crucial for the low-friction mechanism of this type of coatings, was not formed in the coating wear track. Similarly, no beneficial MoSe2 transfer layer was formed on the sliding counterpart. The combined effect of these two factors accounted for this high coefficient of friction. This high friction was accompanied by a low wear rate (15.2 x 10-6 mm3/N.m) because the oil worked as a sealant and protected the coating from oxidation. In dry sliding, the coating exhibited a low coefficient of friction (0.054). This low friction was attributed to the combined effect of the formation of a lubricous MoSe2 transfer layer on the sliding counterpart and the presence of a crystalline MoSe2 tribolayer in the coating wear track with basal planes aligned parallel to the sliding direction. However, this low friction was accompanied by a high wear rate (20.8 x 10-6 mm3/N.m), caused by the oxidation of the coating during sliding and the formation of metal oxides that abraded the coating.

When MoSe2 coating was tribologically assessed in PAO4-lubricated sliding at different applied loads and temperatures, it exhibited coefficients of friction in the range 0.067-0.101 and in the range 0.075-0.101, respectively. As the applied load or temperature increased, the coefficient of friction and wear rate of the coating were observed to decrease. Increasing the applied load or temperature was found to enhance the degree of transfer layer formation of the coating on its sliding counterpart, resulting in low friction and wear.

When MoSe2/DLC-W coating was tribologically assessed in PAO4-lubricated sliding at different applied loads, it exhibited high coefficients of friction in the range 0.064-0.12. The oil acted as a barrier between sliding surfaces and significantly hindered the transfer of the coating to its sliding counterpart, making the formation of a beneficial transfer layer impossible. It also prevented the graphitisation of carbon during sliding. These two factors contributed to the observed high friction. Simultaneously, the absence of graphitised carbon and the suppression of coating oxidation contributed to a reduction in the consumption of the coating in PAO4-lubricated sliding, leading to improved wear resistance and low wear rates. In dry sliding, the coating exhibited low coefficients of friction, ranging from 0.025 to 0.085. This low friction was attributed to the graphitisation of carbon and the formation of a uniform and compact coating transfer layer on the sliding counterpart. However, this excellent friction performance was accompanied by poor wear resistance and higher wear rates than those obtained in PAO4-lubricated sliding, caused by the oxidation of the coating during sliding and the formation of soft graphitised carbon.