Surface nitriding of metal should be conducted in a nitrogen atmosphere without oxygen. It is believed that treating in an atmosphere open condition results in an oxide formation. This study presents the experimental results against such conventional understanding: metal nitride layer without oxides is possible to be formed in an open atmosphere when the surface was exposed to a laser induced plasma generated by a focused pulsed laser irradiation. The laser induced plasma leads to the instantaneous melting of metal surface, and the surrounding gas is taken into the molten metal pool. The taken gas is then transported to a deeper melt region with the convection current, leading to the formation of modified surface layer. Herein, main component in a generic open atmosphere is nitrogen. Therefore, the resultant modified layer is possible to become a metal nitride layer.

Actually, irradiating a pulsed Nd:YAG laser beam on a pure Ti surface with a beam focusing led to the formation of a Ti-nitride layer comprising TiN and Ti₂N without a Ti oxide, even in an open atmosphere condition. Similarly, nitriding phenomenon in an open atmosphere was also observed on pure Nb and Zr surfaces, whereas formation of nitrides as well as the intake of nitrogen gas into surface were never observed on pure Cu surface. Penetration of nitrogen was also observed when irradiated the focused pulsed laser bean on a pure Fe surface, but its concentration was below 10 at.%, and iron-nitride layer was never formed despite small nitrogen precipitates were produced.

An open atmosphere nitriding phenomenon on metal surfaces, triggered by laser induced plasma, is an innovative finding that can be applied in automatic robot easily. However, applicable matarial is limited only several metals such as Ti, Zr, and Nb. Thus, further investigation elucidating the influence of metal properties to the resultant surface is indispensable as a future work.