Influence of the ambient gas in laser structuring of the titanium surface

Surface structuring of titanium was performed by multipulse high repetition rate Nd:YAG (λ=1.064 μm, τ=∼300 ns, ν=30 kHz) laser irradiations in vacuum as well as reactive or inert ambient gases at an intensity value below the single-pulse melting threshold. After irradiations, the surface morphologies have been investigated by scanning electron microscopy (SEM). The surface structuring is strongly influenced by the ambient gas. In high-pressure nitrogen, rippled, and with the increase of the laser pulse number, columnar microrelief formation takes place. In vacuum or in high-pressure argon, the surface morphology is characterised by smooth polyhedral structures developing in the surface plane. In air, the initial surface morphology consisting of a network of microcracks evolves with further irradiation towards a porous microrelief. The physical phenomena involved in the specific surface structure formation are discussed.