Study on the tribological properties of plasma nitrided bearing steel under lubrication with borate ester additive

•The tribological behavior was studied between borate ester and nitrided surface.•The borate ester was a phosphor and sulfur-free additive.•Bearing steel was nitrided and the untreated steel was applied to make a comparison.•BN has formed in the tribofilm on the worn nitrided surface.•This unique tribofilm provided better anti-wear and friction-reducing properties.

The nitrided layer on 52100 steel was prepared by plasma nitriding treatment. The tribological behaviors were examined under lubrication with borate ester containing nitrogen (B-N). The nitrided steel exhibited a lower friction coefficient and smaller wear scar diameter compared with the untreated steel. When 1.25 wt% B-N was used, the friction coefficient and wear scar diameter of nitrided steel were reduced by 34% and 45%, respectively. The main mechanism of this effect was attributed to the fact that the B-N additive can produce a tribofilm containing higher content of BN on the nitrided surface.