Tribolayer Architecture and Subsurface Crystallographic Evolution in Nanocrystalline Ni Under Distinct Sliding Conditions

Tribological loading can substantially modify the near-surface structures of nanocrystalline metals, yet the microstructural evolution of worn tribolayers remains insufficiently understood. Here, an electrodeposited nanocrystalline Ni coating with an average grain size of ~10 nm was investigated under ball-on-disk sliding against AISI 52100 steel in laboratory air at sliding speeds of 0.03 and 0.3 m/s with identical normal load and sliding distance. The worn surfaces and subsurface regions were characterized using laser confocal microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), and four-dimensional scanning transmission electron microscopy (4D-STEM). The steady-state coefficient of friction was higher at 0.03 m/s (~0.05) than at 0.3 m/s (~0.02). Low-speed sliding produced greater material pile-up and surface topographic changes, accompanied by the formation of an ~1 μm-thick porous oxide-rich tribolayer containing transferred Fe and isolated Ni-rich regions. In contrast, high-speed sliding generated a thinner (~300 nm) oxide layer above a ~150 nm metallic lamellar subsurface zone. 4D-STEM orientation analysis of reliably indexed near-surface regions further revealed local crystallographic orientation features consistent with A1*/A2* and C components in the quasi-lamellar region at 0.03 m/s, whereas additional shear-related components, including C, B/B¯, and A1*/A2*, were identified in the highly deformed subsurface region under the 0.3 m/s condition. Combined TEM observations and 4D-STEM crystallographic analysis demonstrate that different sliding conditions produce distinct tribolayer architectures, subsurface deformation structures, and crystallographic evolution in nanocrystalline Ni/AISI 52100 steel contacts.