4.3 Femtosecond laser treatment
It is of fundamental importance for the method of laser-induced fixation (explained below in Section 4.4) that the substances involved differ in one essential property, the ablation threshold. In general, if a powder lies on top of a solid plate it can potentially be remove from the surface by the laser energy (laser cleaning) before it can be fixed. On the one hand, a laser energy density (fluence) is require to harmless to the powder because the crystalline structure must be preserve. On the other hand, the laser fluence should be high enough to cause a strong binding between the powder and the underlying material.
The pulses from a Ti:sapphire femtosecond laser (Femtolasers, Vienna, Austria) have a duration of about 30 fs.
A repetition rate of 1 kHz and an infrared centre wavelength of 790 nm. The focused laser beam had a beam diameter (1/e2) of about 200 μm. The beam has a Gaussian profile. Multipulse laser ablation thresholds of both substances were measured using the D2 method (Liu, 1982). The titanium alloy Ti6Al4V was found to have an ablation threshold of Fth = 0.15 J/cm2. Above this fluence a thin surface layer of the metal is disintegrate and remove, leaving behind a shallow ablation crater. Besides, the modified metal surface can form different characteristic nano- and microstructures, depending on the local fluence.
The structures produced on the metal consist of small parallel ridges on the nanometre scale formed at fluences just above the ablation threshold, and larger rods in the dimension of a few micrometres (Fig. 4.1). The cone-like spikes (Fig. 4.1(b)) in zones of a large fluence grow as a result of simultaneous ablation and accumulation next to each other. However the other unusual structure of nanoscale ripples (Fig. 4.1(c)) gradually fades out towards the edge of the ablation zone. There is no obvious sign of melting and clearly no lateral mass transport.