Thermal stability of laser-produced iron nitrides

Laser nitriding is a very efficient method to improve the mechanical proper ties, surface hardness, corrosion, and wear resistance of iron and steel, w ith the advantages of a high nitrogen concentration, fast treatment, and ac curate position control, and without any undesired heating effect on the su bstrate. However, the stability of laser-produced iron nitrides is still un der investigation. This article reports investigations of the thermal stabi lity of these iron nitrides upon annealing treatments, which were conducted both in vacuum and air. The phase and elemental composition of the nitride layers were deduced from conversion electron Mossbauer spectroscopy, reson ant nuclear reaction analysis, and grazing incidence x-ray diffraction. The surface hardness was measured by the nanoindentation method. In laser-nitr ided iron, two critical temperatures are found: at 523 K the predominant ir on-nitride phase changes from the gamma/epsilon to the gamma (') phase. Whe n the temperature exceeds 773 K, all of the nitrogen has escaped from the s urface layer. For annealing in air the nitrogen escapes completely already at 673 K, where a thick oxide layer has formed. Stainless steel proved to b e more stable than iron, and even up to 973 K no new phases or oxides were produced, here, also, only at 973 K the nitrogen content decreased signific antly. Therefore, laser-nitrided stainless steel is well suited for applica tions. (C) 2001 American Institute of Physics.