Fundamental Atomistic Insights into Tunable Tribological Performance of NbC/Nb Films through Thickness and Depth Effects

Abstract

Ceramic–metal nanolaminates (CMNLs) are promising scratch-resistant coatings, but knowledge gaps remain regarding the interactive effects of individual layer thickness and scratch depth. This study employed molecular dynamics simulations to investigate the tribological performance of NbC/Nb CMNLs, systematically varying ceramic and metal layer thicknesses (0.5–7.5 nm) and scratch depths (3, 5 nm). Models were loaded under displacement-controlled indentation followed by scratching. Mechanical outputs like material removal, friction coefficients, normal, and friction forces quantified scratch resistance. Material removal was even below that for NbC alone, demonstrating the multilayer benefit. Thinner layers showed complete penetration by the indenter, with material rolled in front rather than piled up. Thicker layers resisted penetration, enabling pile-up and lower friction coefficients due to higher normal forces. Excessive material removal decreased normal forces and raised friction coefficients. Peak coefficient occurred around 1.5–3 nm layer thicknesses where substantial top layer volumes were removed, minimizing ceramic under the indenter. Layer thickness corresponding to lowest material removal depended on scratch depth, with 3 nm and 7.5 nm layer thickness for 3 and 5 nm depths, respectively. Metallic layers reduced stiffness and drove material downward over piling up. Quantifying scratch resistance versus geometric parameters elucidates fundamental physics to facilitate superior CMNL coating fabrication.