The tribological behavior of 316L stainless steel/CoCrMo alloy contacts under aqueous lubrication was investigated in this work. Three types of microgels including poly(N-isopropylacrylamide) (PNIPAM), polyacrylic acid (PAA), and poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-co-PAA) were prepared and used as lubricant additives in aqueous solutions. Tribological tests were conducted using a ball-on-disk reciprocating tribometer, over a range of temperatures, pH, and concentrations of the microgels. The PNIPAM-co-PAA microgels were proven to have excellent anti-friction and anti-wear properties, depending on the temperature and pH values of the solutions. The maximum friction coefficient coincided with the lower critical solution temperature of the PNIPAM-co-PAA microgels at 32 °C. At the same time, the friction coefficient increased with the increase in pH value of the lubricant solution with the PNIPAM-co-PAA microgels. The unique thermal- and pH-sensitive properties of the PNIPAM-co-PAA microgels provided a new strategy for controlling the friction and wear of steel/CoCrMo alloy contacts under aqueous lubrication.

In the present work, two types of novel nano additives, titanium sulfonate ligand/black phosphorus (TiL₄/BP) and titanium dioxide/black phosphorus (TiO₂/BP) nanocomposites, were prepared. The tribological behavior of the steel/steel friction pairs lubricated by polyalphaolefins type 6 (PAO6) containing the nanocomposites under boundary lubrication was studied. The worn surfaces were analyzed using modern surface techniques. The experimental results show that the rubbed surfaces became smooth and showed little wear with the addition of the nanocomposites. TiO₂/BP nanocomposites can significantly improve the lubricity of BP nanosheets under high contact stress. The synergistic roles of the load-bearing abilities and rolling effect of TiO₂ nanoparticles, the slip induced by the BP with its layered structure, and the establishment of a tribofilm on the sliding interface are the basis of the tribological mechanisms.