Effect of N2 flow rate on the microstructure, mechanical properties, and dry-sliding behavior of Cr/CrN/LaB6-N multilayer coatings on AISI 4130 steel

Abstract

Lanthanum hexaboride (LaB6) is a refractory ceramic widely used in electron-emission applications, whereas its potential as a nitrogen-modified protective coating for engineering steels remains insufficiently explored. This study presents, for the first time, a systematic nitrogen flow rate series of Cr/CrN/LaB6-N coatings on AISI 4130 steel by hybrid DC/pulsed-DC closed-field unbalanced magnetron sputtering (0–8 sccm N2; R1–R4). With increasing N2, scanning electron microscopy/energy-dispersive x-ray spectroscopy showed a columnar-to-dense transition and rising nitrogen content (0.2–40.4 at. %), while x-ray diffraction revealed progressive amorphization of the LaB6 phase and a new BN-related reflection at 8 sccm. x-ray photoelectron spectroscopy confirmed a shift toward more ionic La–N/Cr-N and B–N/B–O bonding. Nanoindentation showed a non-monotonic hardness trend, peaking at 28 GPa (R3, 4 sccm) before declining to 19.8 GPa (R4), while scratch adhesion peaked at R2 (Lc = 50 N). Dry-sliding friction increased monotonically from 0.31 (R1) to 0.39 (R4). These results identify N2 flow rate as an effective tailoring parameter, with 2–4 sccm offering the best combination of hardness and adhesion for wear-resistant coating applications.