Surface treatment on cobalt and titanium alloys using picosecond laser pulses in burst mode

Abstract

The authors report on the results of surface treatment experiments using a solid-state amplified laser source emitting laser pulses with a pulse duration of 10 ps. The laser source allows the generation of pulse trains (bursts) with an intra-burst pulse repetition rate of 80 MHz (pulse-to-pulse time interval about 12.5 ns) with up to eight pulses per burst. In this study a wavelength of 1064 nm was used to investigate both ablation of material and laser-induced surface modifications occuring in metallic implant alloys CoCrMo (cobalt-chromium-molybdenum) and TiAlV (titanium-aluminum-vanadium) in dependence of the number of pulses and fluences per pulse in the burst. By using the burst mode, a smoothing effect occurs in a certain parameter range, resulting in very low surface roughness of the generated microstructures. It is demonstrated that at fluences per pulse which are smaller than the material-specific ablation threshold, a self-organized pore formation takes place if a defined number of pulses per burst is used. Thus, the advantage of the MHz burst mode in terms of a possible surface modification is established.

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses

“…Depending on the pulse duration and the peak fluence, regimes where a melt film is formed leading to smooth surfaces can be identified [ 131 ]. Similar smoothing effects were already reported for steel 1.4301 (AISI 304) in [ 14 , 60 ] and for cobalt and titanium alloys in [ 78 , 132 ]. Neither for copper nor for brass could a smoothing effect be observed, and the surfaces only start to oxidize when they are machined with pulse bursts [ 14 , 60 ].…”

“…Using a higher number of pulses in the burst also helps to avoid the formation of nanoscopic cavities on the surface as shown in Figure 15 for cobalt alloy. The same also can be observed for titanium [ 132 ].…”

“…The same also can be observed for titanium [132]. surface roughness on cobalt and titanium alloys below = 100 nm [132]. Depending on the material and the used fluence of an individual pulse in the burst, different numbers of pulses in the burst (in general 6-8 pulses) need to be applied.…”

“…Using pulse bursts the smoothing effect can be maintained regardless of the structure depth. In another study, Metzner et al used bursts with up to 8 pulses and an intra-burst delay of 12.5 ns (corresponding to a repetition rate of 80 MHz) and a repetition rate of 100 kHz (inter-burst delay of 10 µs) to reduce the surface roughness on cobalt and titanium alloys below 100 nm [ 132 ]. Depending on the material and the used fluence of an individual pulse in the burst, different numbers of pulses in the burst (in general 6–8 pulses) need to be applied.…”

“… SEM images of the irradiated cobalt alloy surface as a function of the number of pulses in the burst at a fluence of 0.5 J/cm 2 per pulse. Reproduced under the terms of a Creative Commons Attribution 4.0 International License, ( (accessed on 31 May 2021)) [ 132 ]. Copyright 2020, the authors, published by Springer Nature.…”