It has been proved that surface nanocrstallization pretreatment is beneficial to plasma nitriding of steel by enhancing nitrogen diffusion, while the tribological properties of the nitrided nanostructured steel under boundary lubrication are not clear. In this work, AISI 316 L stainless steel with and without ultrasonic cold forging technology (UCFT) pretreatment was plasma nitrided at 500 °C for 4 h. The effects of UCFT pretreatment on the microstructure and properties of the nitrided layer and the tribochemical interactions between the nitrided layer and friction modifier molybdenum dithiocarbamate (MoDTC) and antiwear additive zinc dialkyldithio-phosphate (ZDDP) were investigated using SRV tribometer, scanning electron microscopy (SEM), vickers hardness tester, optical microscope, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Surface analyses confirm the formation of a 20 μm thick nitrided layer on the UCFT-pretreated sample and it had higher hardness than that on the unpretreated sample. Furthermore, the nitrided UCFT-pretreated sample presented better synergetic effect with MoDTC and ZDDP on tribological behaviors than the nitrided unpretreated sample. This is attributed to the higher contents of Mo, S, Zn, P, and MoS2/MoO3 ratio in the tribofilms on the nitrided UCFT-pretreated sample.

Diamond-like carbon (DLC) coatings containing 0.7%, 5.8%, and 23.3% Ti were deposited via pulsed cathodic arc deposition and magnetron sputtering on AISI 316L stainless steel substrates. The varied Ti content was controlled by setting Ti target current at 3, 5, and 7A. The composition, microstructure, mechanical, and tribological properties of Ti-doped DLC (Ti-DLC) coatings were investigated using X-ray photoelectron spectroscopy, Raman spectroscopy, nanoindentation, and ball-on-disc tribometer. The results show that TiC formed when Ti content in the coating was higher than 5.8 at% and the I_D/I_G ratios increased gradually with the increasing Ti content. Ti-DLC with 0.7 at% Ti had the highest H/E and H³/E² ratios and exhibited optimal tribological properties under lubrication, especially when zinc dialkyldithio-phosphate (ZDDP) was contained in the oil. Furthermore, ZDDP tribofilms played an important role in wear reduction by protecting the rubbing surfaces against the adhesion and suppressing the tribo-induced graphitization of DLC coatings.