On the kinetics of nitride and diffusion layer growth in niobium plasma nitriding

Abstract

This work reports results of a study on niobium plasma nitriding kinetics, conducted at relatively low temperatures, regarding the N-Nb system (1523 K) eutectoid temperature, and short times (on the 1253–1453 K and 1–4 h ranges). Results confirm that the niobium plasma nitriding is diffusion-controlled. The obtained nitride (compound) layer is multiphase, mainly constituted of ε-NbN, γ-Nb4N3 and β-Nb2N nitrides. The estimated compound and diffusion layer growth activation energy was 97.11 and 120.11 kJ mol−1, respectively. Considering the hardness distribution all over the nitrided layer, the transition between the compound and diffusion layers is very sharp. As example, it can vary from 18.5 GPa (⁓1850 HV) at the surface compound layer to decreasing values from ⁓480 to 100 HV, along the diffusion layer, depending on the studied condition. All results obtained from the Sa and Sz roughness characterization indicated increasing measured values for longer treatment times and higher temperatures, for atom clusters islands-like surface morphology. Finally, apparently crack-free compound layers were yielded, highlighting the importance of the use of relatively low treatment temperatures, below the N-Nb system (1523 K) eutectoid temperature, and relatively short treatment times for the plasma nitriding treatment studied here.