Tribological Behavior of Plasma Nitrided 1Cr18Ni9Ti Austenitic Stainless Steel Under the Effect of Lubricant Additives

To expand the engineering application of stainless steel as tribological material, it is important to study the tribological interaction of the nitrided layer with lubricating additives. The friction and wear properties of plasma nitrided 1Cr18Ni9Ti stainless steel were investigated under lubricated conditions on an Optimol Schwinyung Reibung Versch oscillating friction and wear tester. The lubrication oil was 1,1,1-trihydroxymethylpropyl trioctoate containing zinc dibutyl dithiophosphate, bismuth dibutyl dithiophosphate, and bismuth $N$⁠, $N$-dibutyldithiocarbamate as the antiwear and extreme pressure additives. The variations in the nitrided stainless steel and the unnitrided one under the tribological action of the additives were contrasted and the tribological chemical interaction between the nitrided layer and the additives was revealed. The results showed that the selected additives had good synergetic effect with the nitrided layer on tribological performance and the bismuth containing additive had better friction-reducing and antiwear abilities than the zinc containing additive. Meanwhile, under the effect of these additives, the $N/(Fe+Cr)$ ratio rose and the Fe/Cr ratio decreased in the nitrided layer, while the Fe/Cr ratio in the unnitrided stainless steel varied little. Three main elements, N, Cr, and Fe, in nitrided layer had different actions with the additives and contributed to tribological performance by different methods.