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Received Mar 20, 2017; Revised Jun 23, 2017; Accepted Jul 18, 2017
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1. Introduction
Laser-induced color marking of metals has been well known for 30 years [1] but to this day it has not been used widely in industry. High quality, difficulty to forge, wear resistance, and noncontact marking (no pressure, no deformation, and no pollution on the surface) are the main advantages of this method. The procedure utilizes a laser, which acts as a heat source allowing forming on a metal surface a thin transparent or semitransparent oxide film. If a beam of parallel monochromatic light falls, at an angle, onto a transparent or partly transparent layer, this beam is repeatedly reflected from the surfaces limited by the oxide layer (from the surface of both oxide and metal). By increasing the thickness of oxide on the surface and as a consequence by changing the interference effect (each wavelength has a different degree of attenuation or amplification), a color effect can be observed and controlled [2–5].
It is well known that titanium, because of its strong affinity with oxygen, when exposed to the atmosphere, becomes covered by a passive film with a thickness of a few nanometers. Numerous studies concerning various aspects of the laser-induced color marking of titanium as well as stainless steel have recently been published. In these papers, detailed characterizations of optical properties [3–9], changes in physicochemical properties [3, 5, 6, 8–11], oxide thicknesses [3–6, 8] and corrosion resistance [12–15] were conducted. It has also been found that a number of different process parameters such as laser power, scanning speed of the material, the size of the marked area, the temperature, and position of the sample exert a significant impact on the reproducibility of the resulting color [4]. In addition, the perception of the color depends on the angle of observation [5, 16, 17]. A single publication concerns...