Nanosecond, picosecond and femtosecond laser surface treatment of magnesium alloy: role of pulse length

Despite having a whole range of applications as in weight-saving structures, biodegradable implants and rechargeable batteries, the usage of magnesium is still hindered by its high tendency to corrosion. Pulsed laser treatments are an interesting approach to modify the surface of magnesium aiming the improvement of the corrosion properties as well as the generation a controlled roughness, which is useful for improving coatings adhesion or tailoring the interactions with living cells at the surface of an implant. In this work, a novel study on the role of the pulse length on topography and corrosion properties of AZ31 magnesium alloy laser-treated surfaces is presented. Three different laser sources with pulse lengths of 20 ns, 800 ps and 266 fs were employed with a similar experimental setup. Surface topography analysis revealed a reduction of the amount of melt generated and an increase in the aspect ratio (depth/width) of the ablated features as the pulse length is reduced. All three treatments significantly improved the corrosion performance of the untreated base material. Thicker and more homogenously distributed recast material for the longer pulses of the nanosecond treatment resulted in a longer-lasting protection (11-12 fold reduction in mass loss rate vs base material) compared to the pico- and femtosecond treatments (4-5 fold reduction).