The effect of N2 concentration in pre-plasma nitriding on the tribological behavior of CrN/TiN nano multilayer coatings: A study on duplex-treatment
Ali Fallah-Lalehzari, F. Mahboubi, Hassan Elmkhah
Abstract
: To extend the service life of industrial components under demanding operating conditions, duplex treatments employing physical vapor deposition (PVD) coating and plasma nitriding are crucial surface engineering approaches, and the CrN/TiN nano multilayered coating is selected because of its optimal balance between hardness, toughness, and adhesion, making it a suitable choice where durability and wear resistance are critical. This study examines how the tribological behavior of CrN/TiN multilayer coatings on an AISI 4140 cold-worked steel substrate is affected by plasma nitriding as a pre-treatment. Using a gas mixture of (N 2 + H 2 ) at four distinct nitrogen concentrations (10%, 25%, 50%, and 75%), plasma nitriding was performed for 8 hours at 520°C. Using cathodic arc evaporation, a CrN/TiN multilayer coating was deposited from the vapor phase. The characterization of duplex-treated materials was evaluated using optical microscopy, field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). Testing of adhesion using the Rockwell C indenter was applied based on the VDI 3198 standard. Mechanical properties were assessed by evaluating roughness and microhardness. To assess the tribological behavior of the samples, pin-on-disk wear tests were performed. The results showed that the CrN/TiN coating achieved a hardness of 18 GPa from 4.8 GPa and an elastic modulus of 340 GPa, indicating a substantial improvement in surface mechanical performance compared to the untreated substrate. The optimal tribological performance was exhibited by the duplex-treated sample with 25% nitrogen, which experienced the minimum pin penetration during the wear test into the substrate. We attributed this hardness improvement to the duplex-treated sample's increased wear resistance at the 5-micron white layer thickness related to (25% N 2 + 75% H 2 ). Also, the specific wear rate dropped by 91%, from 18×10 -5 mm 3 /N·m for the PVD sample to 1.63×10 -5 mm 3 /N·m for the sample that was duplex treated with 25% nitrogen in pre-plasma nitriding. Profilometry results indicated that the wear track width and depth decreased from 781 μm and 5 μm for the PVD sample to 385 μm and 1 μm for the duplex-treated sample with 25% nitrogen.