Laser-induced growth of titanium nitride microcolumns on biased titanium targets
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tanium targets with a bias voltage ranging from −500 to +500 V were submitted to multipulse high repetition rate Nd:yttrium aluminum garnet (YAG; ⳱ 1.064 m, ∼ 300 ns, ⳱ 30 kHz) laser irradiations in nitrogen at intensity values below the single-pulse melting threshold. The morphology of the TiN structures formed under the cumulative action of the laser pulses on the surface of the unbiased and biased targets was investigated by profilometry and scanning electron microscopy. Under these irradiation conditions, a specific columnar surface microrelief developed. The height of the microcolumns reached about 10–15 m, and their diameter about 1–2 m. The development of TiN microcolumns was enhanced by the applied bias voltage. The enhancement in the negative biased samples was stronger than that in the positive biased ones.
I. INTRODUCTION
Laser processing of solid surfaces has been extensively investigated during the last decade because of its large variety of applications in recrystallization, doping, smoothing, hardening, or compound formation.1 Moreover, specific irradiation conditions permit the creation of different surface morphologies as laser-induced periodic structures, microcones, or microcolumns. We have recently reported for the first time the growth of titanium nitride microcolumns during the cumulative action of high repetition rate laser pulses.2,3 In our experiments, we used a pulsed Nd:yttrium aluminum garnet (YAG; ⳱1.064 m) laser for the irradiation of titanium surfaces in high-pressure nitrogen atmosphere. Surface morphologies, in some aspects similar to those obtained by us, have been recently reported to develop during laser irradiation of various material surfaces. More precisely, microcolumns have been grown on titanium in low vacuum (1 Pa),4 on tungsten in low-pressure He gas,5 as well as on silicon6,7 and ceramic composites8 in oxygen-containing atmospheres. Titanium nitride microcolumns grown on titanium can be useful for certain applications. Indeed, in medical implants it is generally accepted that rough surfaces ensure a better osteointegration than smooth ones do.9 In
a)
On leave from Institute of Atomic Physics, P.O.Box. MG. 36, Bucharest V, 76900, Romania b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0021 62
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 1, Jan 2005 Downloaded: 13 Mar 2015
addition, due to its biocompatibility and bioinertness, titanium nitride coatings can act as efficient diffusion barriers against the metal. On the other hand, recent investigations have also been focused on the field electron-emission properties of the titanium nitride microcolumns.10 Our purpose in the present work was the investigation of the effect of negative or positive bias voltages applied to the target on the columns growth process. More precisely, we wanted to see the effect of the external electric field generated by the target bias on the species in the ablation plasma and especially on their preferential redeposition o
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