Tribocatalytic NiCrMo Superlattice Nanolayer Coatings for Wear Protection of Steel under Boundary Lubrication

Steel components operating under boundary lubrication often fail by wear, even when their bulk strength is adequate. Recent work has shown that high-chromium steels can exhibit unusually low friction and wear in hydrocarbon lubricants, not simply because of hardness, but because chromium promotes the formation of protective carbonaceous tribofilms during sliding. This observation suggests that tribochemical activity at the surface, rather than bulk alloy substitution, may provide a practical pathway to improved performance. Here, we report the design and evaluation of a nanocrystalline NiCrMo multilayer coating that combines chromium-driven tribocatalysis with mechanical robustness. Nichrome (Ni80Cr20) was selected to provide a chromium-rich, face-centered cubic matrix. To increase hardness and stabilize the nanostructure, molybdenum was introduced through both solid-solution alloying and an alternating nanolayer architecture consisting of NiCr(Mo) layers (~ 8 nm) separated by ultrathin Mo layers (~ 1 nm). This design suppresses columnar growth through repeated re-nucleation and limits grain size to less than 20 nm, yielding hardness values of 8.5–12.0 GPa. A graded interlayer was incorporated to improve adhesion to hardened 52100 steel substrates. Under boundary-lubricated reciprocating sliding (2 N, 2 Hz), the coating maintained a stable friction coefficient of 0.13–0.15 and reduced wear track width by approximately a factor of two relative to uncoated steel, corresponding to an estimated order-of-magnitude reduction in counterface wear. Raman analysis confirmed the formation of carbonaceous tribofilms. These results indicate that a mechanically stable, chromium-rich nanostructured coating can promote in situ formation of protective tribofilms and improve wear resistance without altering the bulk steel composition.