Effect of lanthanum rare earth addition on low temperature plasma nitriding

Abstract

The effect of lanthanum additions on the plasma nitriding of nanocrystalline steel 3J33 is reported. Nitriding was carried out in 20N₂ + 80H₂ at a pressure of 400 Pa at 350 and 410°C. Glow discharge spectrometry shows that although its atomic radius is larger than that of Fe, La can diffuse into the surface layer to a significant depth; the La addition increases the compound layer thickness. Based on the microhardness profile, the diffusion layer thickness also increases due to La addition. XRD analyses indicate that the presence of La did not change the phase structure of the surface, i.e. a compound layer of γ′-Fe₄N type. The incorporation of La not only enhances the microhardness but also improves the toughness of the nitrided layer. The atomic and electron structures of phases γ′-Fe₄N and γ′-(Fe,La)₄N were calculated using the plane wave pseudopotential method. The (Fe,La)₄N phase is more stable than Fe₄N and the shear/bulk modulus ratio G/K of (Fe,RE)₄N is much smaller. This finding provides a possible mechanism for the increased microhardness and improved toughness of nitrided layers containing La.