Influence of the ambient gas in laser structuring of the titanium surface E. Gyo ¨rgy 1 , A. Pe ´rez del Pino, P. Serra, J.L. Morenza * Departamento de Fı ´sica Aplicada i O ` ptica, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona, Spain Received 5 May 2003; accepted in revised form 10 March 2004 Available online 6 May 2004 Abstract Surface structuring of titanium was performed by multipulse high repetition rate Nd:YAG (k = 1.064 Am, s = f 300 ns, m = 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. D 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 68.37.Hk Keywords: Pulsed laser interaction; Titanium; Surface structures; Microcolumns 1. Introduction Laser radiation induces on the materials surface mor- phologies such as ridges, resonant or large-scale periodic structures [1,2], cones [3–5], or columns [6–13]. These structures have been observed on various target materials, ambient atmospheres, and laser beam characteristics. In previous works, we studied the surface microstructuring of titanium by accumulation of high repetition rate Nd:YAG (k = 1.064 Am) laser pulses in vacuum [14], air [15,16], or nitrogen [17] atmospheres. Specific pro- cessing parameters promoted the formation of polyhedral surface structures when the irradiations were performed in vacuum, and dome-shaped, or columnar microreliefs in reactive atmospheres. The present work’s purpose is the investigation of the influence of the ambient gas nature as well as its pressure on the titanium target surface morphology evolution during multipulse laser irradiation, keeping constant all other experimental conditions (laser beam parameters and irradi- ation geometry). Our aim is also to study the role of the laser-induced chemical reactions and compounds formation on the target surface. Thus, parallel experiments were performed in vacuum, and both reactive (air and nitrogen) and inert (argon) gas ambience, at similar pressure values. This way, we intend to clearly delimit two effects: one caused purely by the presence of a high-pressure ambient gas, and the other by the change of surface chemical composition when the irradiation is conducted in a reactive gas. Moreover, for a fixed number of laser pulses applied onto the same target location, we followed the development of the surface morphology by increasing the ambient reac- tive gas pressure. Irradiations were performed also in successive trains of pulses, in both reactive and inert gases. These studies will provide a better understanding of the physical processes which lead to surface structuring and, in particular, to the formation and further development of the specific columnar morphology in titanium. 2. Experimental The irradiations were conducted in a stainless steel vac- uum chamber, with a Baasel LBI 6000 Nd:YAG (k = 1.064 Am) laser system generating pulses of 300 ns, at a repetition rate of 30 kHz. The laser beam had a circular cross-section 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.015 * Corresponding author. Tel./fax: +34-93-4021138. E-mail addresses: eniko@ifin.nipne.ro (E. Gyo ¨rgy), jmorenza@ub.edu (J.L. Morenza). 1 On leave from Institute of Atomic Physics, PO Box MG 36, Bucharest V, 76900, Romania. Tel./fax: +40-14231791. www.elsevier.com/locate/surfcoat Surface & Coatings Technology 187 (2004) 245– 249