Single-pulse KrF laser ablation and nanopatterning in vacuum of β-titanium alloys used in biomedical applications

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We report the KrF laser (248 nm) nanosecond single-pulse irradiation in vacuum of a mirror-polished surface of a model β-titanium alloy. A series of single-pulse laser-annealing experiments is performed with increasing fluence adjusted to obtain conditions from submelting (a few mJ/cm2) up to intense laser ablation (30 J/cm2). The structural and morphological changes are followed by atomic force microscopy and scanning electron microscopy, as well as with other analytical techniques, and compared with the starting αβ equilibrium mixture of the untreated sample. The results are discussed and related to the theoretical estimation of fluences corresponding to the β-transus, melting, boiling and intense ablation (phase-explosion) thresholds as well as to the estimates of melt depth and lifetime evaluation as a function of fluence. By carrying out laser annealing in vacuum the surface oxidation is avoided while roughening and phase transformations are favoured within the laser-heated depth. The mirror-polished starting surface is composed of nanometric α-precipitates embedded in a β-matrix, whose surface is systematically 5–10 nm above the α-surface. Upon laser annealing with increasing fluence the α-nanocrystals transform progressively into β-phase, resulting in an increasing relief. At higher fluence the conditions for increasing roughness are studied. Submicrometre roughness is expected to improve cell adhesion to β-titanium alloys and hence osteointegration.