Synergistic investigation on Fe[sbnd]N phases and gradient microstructure supported anti-indentation and anti-scratch performance of low-temperature plasma ion nitrided Ti[sbnd]Nb stabilized IF steel

Abstract

Plasma ion nitriding (PIN) is a remarkable surface modification process that utilizes a thermochemical environment to treat the substrate by diffusion-induced phenomena, allowing it to modify complex-shaped objects, especially steel components. However, the foremost shortcoming of PIN is the higher processing cost, which can be minimized by reducing the nitriding temperature. This leads to a prominent variation in the sample's microstructure, often resulting in a gradient microstructure in the depth direction, which can be beneficial for specific industrial applications like scratch resistance, abrasion resistance, etc. The current investigation performs an extensive study to extrapolate the gradient microstructure-induced indentation and scratch resistance of low-temperature PINed (400 °C to 500 °C) interstitial-free steel through microscopy, diffractometry, spectroscopy, microhardness test, indentation test, and scratch test (constant and progressive loading). The transmission electron microscopy and scanning electron microscopy findings suggested a clear trace of gradient microstructure containing various Iron nitride phases (?-Fe<inf>2-3</inf>N, ?'-Fe<inf>4</inf>N, and ??-Fe<inf>16</inf>N<inf>2</inf>), size and distribution of which affect the scratch resistance. The sample treated at 450 °C shows the best result, with an overall improvement in scratch hardness of 3.2 times the base value. The coefficient of friction, track depth, traction force variation, etc., are also studied and correlated with spectroscopy and microscopy findings. © 2025 Elsevier B.V.