The enhancement effect and kinetics of rare earth assisted salt bath nitriding

Abstract

A rapid salt bath nitriding technology was primarily developed by adding rare earth lanthanum (RE La) in the salt bath media on the basis of normal salt bath nitriding technique (NT). Characterization of the modified surface layers was made by means of optical microscopy, X-ray diffraction, energy dispersive spectroscopy (EDS) and micro-hardness tester. The results showed that the nitriding efficiency could be significantly enhanced assisted by RE La, and the enhancement effect was RE La concentration dependent. The thickness of compound layer was increased from 13.3 μm up to 18.5 μm with 5% La addition at the temperature of 833 K for 2 h. Higher surface hardness, modestly higher sub-surface hardness and superior micro-hardness profile were obtained due to the solution of RE La atoms and consequent lattice expansion in the surface layer. Meanwhile, kinetics analysis showed that the diffusion coefficient was increased and activation energy was decreased with enhancement effect of RE La, the following kinetics formulas were obtained, dNT=0.1exp−21990/Tt and dRE=4.3×10−3exp−16319/Tt for NT and RE treatment, respectively. And the confidence level of the kinetics is examined to be perfectly agreed with the data. The main enhancement mechanism is that RE La can accelerate the decomposition rate of CNO− and thus increase the concentration of active nitrogen atoms in the salt bath medium, and especially increase the number of nitrogen atoms that meet the requirements for diffusing toward the substrate by reducing the energy barrier.