Surface modification of SPK NL steel: relevance to tribological and electrochemical potency

SPK NL steel, as a hard and process-wise stable alloy, has myriad applications in sheet metal forming. However, to increase the quality of the products and decrease cost, applying a self-lubricant protective coating on the surface seems to be an efficacious strategy. The CrN and carbon-enriched CrN, i.e. (a-C: CrN), hybrid coatings are physically vapor-deposited on the SPK NL alloy surface. The C/N ratio on the morphology, tribological properties, mechanical characteristics, and anticorrosive features of the coated SPK NL substrates are studied. Results evidenced that high carbon inclusion increases the plastic hardness of the CrN coating up to about 35 GPa and significantly improves the plasticity index to ~0.2. The COF decreases from ~0.6 for CrN to ~0.1 for a-C:CrN. Coatings with a high sp2 fraction impart an excellent self-lubricant characteristic to SPK NL steel. Electrochemical impedance spectroscopy data approve that both coatings provide high corrosion resistance, and although CrN provides superior corrosion resistance nonetheless. As an outcome, (a-C: CrN) hybrid coatings with high carbon content, possessing high hardness, good tribological properties, and corrosion resistance seems to be a prospective candidate for surface protection of SPK NL steel.