Effect of different plasma nitriding durations on the tribological characteristics of nickel-boron-nanodiamond electroless nanocomposite coatings

The aim of this research work was to investigate the effect of plasma nitriding duration on the tribological properties of nickel‑boron/nano-diamond (NiB/ND) coatings. In this regard, NiB/ND coatings were electrodeposited on AISI 4140 steel using the electroless plating technique. Subsequently, the coated samples were plasma nitrided in a gas mixture containing 25 % nitrogen and 75 % hydrogen at 400 °C for three different durations (60, 120, and 180 min). The coatings were then characterized using X-ray diffraction, scanning electron microscopy (SEM), surface roughness profilometer, and microhardness tests to evaluate their structural and mechanical properties. To assess the tribological behavior, a pin-on-disk test was performed with a load of 10 N applied over a sliding distance of 500 m. The results indicated that plasma nitriding led to the crystallization of the coatings, which can be attributed to the formation of Ni2B, Ni3B, and h-BN phases. Furthermore, the plasma nitriding process significantly increased the coating hardness from 700 to 1523 Vickers after 1 h of treatment. The wear resistance of plasma-nitrided coatings was improved, as evidenced by a considerable reduction in the specific wear rate compared to the untreated sample. However, for the samples plasma nitrided for >60 min, a reduction in coating hardness and wear resistance was observed, which is likely due to the changes in grain size during the plasma nitriding process. The optimal plasma nitriding duration for achieving desirable properties was found to be 60 min.