Microcolumn development on titanium by multipulse laser irradiation in nitrogen
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report the growth of titanium nitride microcolumns under multipulse Nd:yttrium aluminum garnet ( ⳱ 1.064 m, ∼ 300 ns, ⳱ 30 kHz) laser irradiation of titanium targets in nitrogen atmosphere. The laser intensity value was chosen below the single-pulse melting threshold of titanium. The evolution with the number of laser pulses of the target morphology, crystalline state, and chemical composition at the surface as well as in depth were investigated by scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, and wavelength dispersive x-ray spectroscopy. Under the action of the laser pulses, during progressive surface nitridation, an initial rippled morphology developed, which evolved with further irradiation to TiN microcolumns. In-depth investigations showed a granular zone beneath the surface consisting of rutile and anatase phase TiO2, followed by a compact needlelike layer of titanium until the interface with the unaffected target material.
I. INTRODUCTION
Titanium has interesting properties such as lightness, thermal stability, and biocompatibility, which make it suitable for industrial and medical applications.1 However, drawbacks such as poor wear resistance and low hardness limit the number of potential applications. To overcome these inconveniences, different surface techniques have been used, such as conventional furnace heating, plasma processing, or ion implantation. The change of the surface structure and/or chemical composition improves these properties over those of the bulk, corresponding to the requirements of the specific applications. As an alternative technique, processing of the titanium surface with laser radiation has attracted considerable interest during the last decade.2,3 Its main advantages over the conventional techniques are the accurate spatial control of the process and the short processing time. Among its possible applications stand surface compound layers formation, as nitridation or oxidation in controlled reactive atmosphere, surface structuring, smoothing, and hardening.
Laser scanning treatment of titanium surface in high-pressure nitrogen allowed us to grow high-quality titanium nitride thin layers with a smooth surface morphology.4,5 Moreover, laser multipulse irradiation of titanium at low laser intensities, below or in some cases just above the single-pulse melting threshold of Ti6 in air or vacuum, led to the development of a large variety of surface morphologies: crown-7 or dome-shaped8 microstructures or dendritic-shaped microrelief.9 On the other hand, columnar microstructures were recently reported to grow on titanium surface during multipulse laser irradiation in low (1 Pa) vacuum.10 Similar columnar structures were also reported to grow on silicon,11,12 or ceramic composites13 in oxygen-containing atmospheres. However, we could not find in the literature columnar titanium nitride microstructures, neither on the surface of irradiated TiN nor Ti in reactive nitrogen atmosphere. Nevertheless, TiN surface microcolumns could be interesting for field ele
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