Structure formation on titanium during oxidation induced by cumulative pulsed Nd:YAG laser irradiation

Abstract

We investigated local oxidation and surface structure development of Ti targets under multi-pulse, high-repetition-rate Nd:YAG (λ =1.064 μm, τ∼300 ns, ν=30 kHz) laser irradiation in air at atmospheric pressure. The experiments were performed at laser intensity levels below the single-laser-pulse melting threshold of Ti. The morphology of the irradiated areas was studied by scanning electron microscopy and profilometry. The variation of the oxide compositions and the crystalline state with increasing laser pulse number was analysed by X-ray diffractometry. Besides the known phenomena related to laser irradiation and oxidation of metal surfaces (micro-crack or pore formation), we evidenced new morphological features such as droplet-like structures inside the surface micro-cracks and micro-columns, and with increasing laser pulse number the formation of a dome-shaped structure over the whole irradiated zone. The occurrence of melting under multi-pulse irradiation was associated with the rise in the surface temperature enhanced by the oxidation, and the progressive evolution of the surface structures was associated with the consecutive melting–solidification processes as well as with the different stages of oxidation.