Nanochemical hardening and wear resistance of multi-component Cr-Ni-Co-N thin films synthesized via reactive rf plasma sputtering

Abstract

Modifying AISI 4140 alloy steel via physical vapor deposition (PVD) is essential to mitigate surface degradation under demanding tribological conditions. This study investigates the phase evolution, surface topography, and nanomechanical performance of chromium-based nitride coatings co-sputtered with nickel and cobalt on AISI 4140 substrates using reactive radio-frequency (RF) sputtering at 13.56 MHz. By systematically varying the reactive nitrogen flow ratio (10%, 15%, and 20% N₂) and target surface metallic configurations (Cr, Cr-Ni, Cr-Co, and Cr-Ni-Co), the interplay between deposition dynamics and film properties was established. Cross-sectional thickness analysis revealed a reduction in film thickness from 2.9 mm to 1.3 mm with increasing N₂ content, governed by target poisoning and gas-phase scattering mechanics. X-ray diffraction (XRD) confirmed the formation of cubic (Cr,M)N structures with nano-crystallite sizes ranging from 19.44 nm to 53.50 nm. Surface topography via atomic force microscopy (AFM) demonstrated an increase in average surface roughness (S_a = 8.42-12.30 nm) as a function of N₂ incorporation. Nanoindentation characterization demonstrated a continuous increase in mechanical response with nitrogen flow: maximum hardness (H = 25.5 GPa) and reduced elastic modulus (E= 285 GPa) were obtained for Cr-Ni coatings at 20% N₂. The calculated elastic strain index (H/E = 0.089) and plastic deformation resistance (H³/E² = 0.204 GPa) indicate high load-bearing capacity and crack-initiation resistance, demonstrating that metallic co-sputtering combined with optimized nitrogen stoichiometry significantly elevates the surface performance of AISI 4140 steel.