Experimental and theoretical study on interaction between lanthanum and nitrogen during plasma rare earth nitriding

In present work, the interaction between lanthanum (La) and nitrogen (N) during plasma rare earth nitriding of M50NiL martensitic steel is analyzed. Phase compositions, elemental contents as well as microhardness profiles of surface layers are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and microhardness tester to observe the changes of the N contents in treated layers. The results of microhardness, XRD and EDS indicate that the addition of pure La can speed up the denitriding rate compared with the case without La addition. The XPS results reveal that the presence of the LaO and LaN bond reduces the peak intensity of the MeN bond, which indicates that the addition of La element can reduce the N contents in nitrided layers through the surface oxidation and the attraction of La atoms. The theoretical thermodynamic calculations are employed to further clarify the denitriding function of the surface oxidation and the attraction between La and N atoms.