Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

Abstract

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.