Laser nitriding of metals:Influences of the ambient pressure and the pulse duration

Abstract

Laser nitriding of metals such as iron, stainless steel, aluminum or titanium is an interesting phenomenon both in physics and industry. On a time scale of hundreds nanoseconds, high intensity pulsed laser irradiation ( I ~ 108 W/cm2 ) in ambient nitrogen atmosphere transforms the surface of metals to micron thick metal nitride layer, which greatly improve the metal surface mechanical properties. Since laser plume dynamics and incident pulsed laser are two most crucial factors determining the laser nitriding efficiency, the experiments focus on the influences of the ambient nitrogen pressure and the pulsed laser duration on laser nitriding. It is found that as nitrogen pressure increases from 0.05 bar to 2 bar, the nitriding efficiency increases rapidly, then remains nearly constant up to 10 bar. The optimal nitrogen pressure window lies between 2 and 3 bar. The nitrogen pressure series clearly demonstrated the transition from laser ablation to nitrogen d! iffusion dominated regime. The characteristic parameters of the nitrogen profile are extracted and qualitatively interpreted based on the laser supported combustion model. The investigations of the influence of the pulse duration on laser nitriding with nanosecond (ns) excimer and Nd-YAG laser, picosecond (ps) free electron laser and femtosecond (fs) Ti:sapphire lasers, have revealed that ns laser is superior compared to ps or fs laser. Nitrogen diffusion in molten metals is believed to be the primary mechanism of efficient laser nitriding. The competing process resulting in the depletion of nitrogen is a degassing process, which is confirmed by the annealing experiments and the comparison between iron and titanium nitrided by free electron laser. due to the enhanced laser-metal thermal coupling induced by the laser plasma, the nitriding efficiency is more or less independent of the wavelength of the incident laser. The thermal stability of laser produced metal nitrides and th! e iron nitride phase evolution are investigated in a series of annealing treatments conducted in vacuum and air. The results revealed that 973 K is the maximum temperature for laser produced iron nitrides, above 973 K, nitrogen escapes from the surface due to degassing. Titanium nitride is more stable than iron nitrides, which makes titanium an excellent candidate for effective laser nitriding.