Laser-induced oxidation of titanium substrate: Analysis of the physicochemical structure of the surface and sub-surface layers

Applied Surface Science 325 (2015) 217–226

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Laser-induced oxidation of titanium substrate: Analysis of the physicochemical structure of the surface and sub-surface layers a,∗ b ´ ´ Arkadiusz J. Antonczak , Łukasz Skowronski , Marek Trzcinski b , Vasyl V. Kinzhybalo c,d , a a Łukasz K. Łazarek , Krzysztof M. Abramski a

Laser and Fiber Electronics Group, Faculty of Electrical Engineering, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland Institute of Mathematics and Physics, University of Technology and Life Sciences, Kaliskiego 7, 85-789 Bydgoszcz, Poland c Wroclaw Research Centre EIT+, Stabłowicka 147, 54-066 Wrocław, Poland d Institute of Low Temperature and Structure Research, Okólna 2, 50-422 Wrocław, Poland b

a r t i c l e

i n f o

Article history: Received 23 June 2014 Received in revised form 4 November 2014 Accepted 10 November 2014 Available online 22 November 2014 Keywords: Laser surface oxidation Laser color marking Titanium oxynitride Fiber laser

a b s t r a c t This paper presents the results of the analysis of the complex chemical structure of the layers made on titanium in the process of the heating of its surfaces in an atmospheric environment, by irradiating samples with a nanosecond-pulsed laser. The study was carried out for electroplated, high purity, polycrystalline titanium substrates using a Yb:glass fiber laser. All measurements were made for samples irradiated in a broad range of accumulated fluence, below the ablation threshold. It has been determined how the complex index of refraction of both the oxynitride layers and the substrate vary as a function of accumulated laser fluence. It was also shown that the top layer of the film produced on titanium, which is transparent, is not a pure TiO2 as had been supposed before. The XPS and XRD analyses confirmed the presence of nitrogen compounds and the existence of nonstoichiometric compounds. By sputtering of the sample’s surface using an Ar+ ion gun, the changes in the concentration of individual elements as a function of the layer’s cross-section were determined. Lastly, an analysis of the surface morphology has also been carried out, explaining why the layers crack and exfoliate from their substrate. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years we have been witnessing an intensive increase of interest in titanium oxides. They are cheap materials which are non-toxic, bio-compatible and stable, both chemically and mechanically [1]. Thanks to their unique physicochemical properties, titanium oxides have many applications which include: photoanodes in dye-sensitized solar cells (DSSC) [2,3]; photocatalysis for the selective synthesis of chemical compounds, the removal of environmental pollution, sterilization (viruses, bacteria and cancer cells), as well as the preparation of self-cleaning surfaces [4,5]; gas sensors [6,7]; biocompatible films covering medical implants [8,9] and simple anti-reflective layers [10]. Thanks to their high transparency in the visible light range, thin films of titanium dioxide (in the form of polymorphic rutile, anatase and brookite or in the amorphous phase) also allow one to obtain a broad range of colors resulting from the interference of white light at the border between different phases. This effect is particularly used

∗ Corresponding author. Tel.: +48 713204698. ´ E-mail address: [email protected] (A.J. Antonczak). http://dx.doi.org/10.1016/j.apsusc.2014.11.062 0169-4332/© 2014 Elsevier B.V. All rights reserved.

in decorative applications, advertising, electronics, automotive industry, art and jewelry making [11–13]. Various techniques can be used to obtain a titanium oxide coating: cathodic deposition [14], plasma deposition [15], laser deposition [16], magnetron sputtering [17], thermal oxidation [18], electrolytic oxidation (anodizing) [12,19] and laser oxidation [13,19,20]. The most prominent advantages of laser technology with respect to titanium substrates include a considerable shortening of the time required to manufacture coatings in the case when the surface area is small (of the order of a few cm2 ), significant cost reduction, and most importantly the possibility of having a selective, pointwise influence on the material, which in turn allows for repetitive color laser marking of titanium, among others [21]. Laser oxidation of metals is an example of gaseous corrosion occurring at high temperatures. In theory, the course of reaction and the reaction products composition may be foreseen, based on thermodynamic data and kinetic coefficients, however, in real conditions, this means the mixture of many gases and non-isothermal processes, the oxidation products composition may be very complex. Thus, the use of a laser for the oxidation of metals introduces a series of factors which may be difficult to account for, and the process itself should be considered to be a photophysical rather than

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a photothermal one [22]. The oxide layer growing on the substrate modulates the absorption of radiation with the time of the process [23]. The reaction induced by a laser is not isothermal and the temperature gradients triggered by it, both in the phase border plane and across the substrate, have considerable influence on the kinetics of the process. In the case of oxides which are semiconductors (including TiO2 ) the above mentioned gradient induces the Seebeck effect, which, depending on the substrate, either increases (copper) or decreases (cobalt) the dynamics of oxidation [22]. Abrupt temperature changes also cause lasting structural defects – dislocations and microcracks [21], which influence the diffusion of oxygen and change the reaction speed. The broad use of the coatings manufactured on titanium by the laser machining of its surface in an atmospheric environment requires a detailed characterization of the layer’s properties, which would account for its morphology, chemical composition and thickness as a function of laser process parameters. In the papers published to date, various authors have pointed out that these layers were only a conglomeration of titanium oxides (depending on the paper and the parameters of the process) Ti2 O, TiO, Ti2 O3 , TiO2 [11,24], Ti2 O, TiO, TiO2 [13], mainly TiO2 [19] or solely TiO [25]. Although it has been shown in paper [26] that the chemical composition of coatings may include compounds of nitrogen, it has also been emphasized that they are found only in the third, deepest sublayer, which is not optically transparent because it is located below the TiO layer. Thus, they should not affect the optical absorption of the layers. It is worth noting that the analyses performed to date have only included the stoichiometric compounds. As regards the structure of the manufactured coatings, a two-layer model (compact layer covered by a thin granulated layer) [24] and a three-layer model (TiO2/ TiO/Ti6 O+TiOx N1−x ) [26] have both been proposed. The overall thickness of layers, which depends on the laser fluence, was estimated at 0.1–10 ␮m [24]. In paper [26] it was specified that the fraction consisting of the TiO2 and TiO layers varied from 10 to 120 nm. The study presented in this paper is concerned with extended, detailed analysis of the complex chemical structure of the layers made on titanium in the process of the laser heating of its surfaces with a nanosecond laser in the atmospheric environment. In order to obtain a consistent picture of how morphology, chemical composition and layer thickness change with laser fluence in a broad range of values, limited only by the ablation threshold, research was carried out using a number of analytical techniques. Layers were characterized using, among others, the methods of ellipsometry, X-ray Photoelectron Spectroscopy (XPS) combined with an Ar+ ion sputtering of the sample’s surface using an ion gun, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). We also made use of an optical profilometer (confocal optical microscope).

2. Materials and methods 2.1. Substrates The study was carried out for electroplated, high purity (Ti > 99.9%), polycrystalline titanium substrates from MTI Corporation with a measured roughness of about Ra  12 nm. Depending on the type of measurements taken, two sample sizes were used. In the case of XRD measurements, the sample dimensions were 20 mm × 20 mm × 0.5 mm. The substrates were laser treated over their entire surface. All other analyses were performed using 10 mm × 10 mm × 0.5 mm samples, which were irradiated over an 8 mm × 8 mm surface. Before the experiment, the plates were washed with isopropyl alcohol in an ultrasonic cleaner. The samples were marked in atmospheric air.

Fig. 1. Block diagram of the system used for sample preparation, where: L – laser; F – F-theta lens; DS – distance sensor; T – temperature sensor; TEC – thermoelectric cooler (Peltier); S – tested sample; X, Y, Z – axes of the system.

2.2. Laser treatment Samples for the study were prepared using a MOPA configuration Yb: glass fiber laser (1.06 ␮m) with average output power up to 20 W, beam quality factor M2 ≤ 1.5, constant pulse duration 230 ns and pulse repetition rate PRR in the range of 20–80 kHz (IPG, YLP series). The laser system (TROTEC, SpeedMarker FL) was equipped with a galvanometer-based optical scanner allowing the beam to be deflected within the area of the irradiated substrate. The laser beam was focused on the target through a 160 mm focal length F-Theta lens (LINOS type 4401-305-000-21). The beam diameter at the focal point was approximately 40 ␮m. Substrates were placed on an adjustable Z-axis table. An external distance sensor allowed the setting of the sample in the focal plane with an accuracy of 10 ␮m. To stabilize the initial temperature of the substrate at 20 ± 0.1 ◦ C a thermoelectric cooler (TEC) and a temperature controller (ILX Lightwave type LDT-5525) were used. A block diagram of the system is shown in Fig. 1. The samples were irradiated in a unidirectional raster scanning mode with a hatching distance of 10 ␮m – Fig. 2. For all the investigated cases, the same maximum pulse repetition rate was used (PRR = 80 kHz). By changing the laser energy density, various colors of titanium surfaces can be achieved. These variations mainly result from the different thicknesses of the oxide layer formed on the surface. For the purpose of our experiment, six values of accumulated fluence FA were chosen in the range of 158–360 J/cm2 . The accumulated fluence (in one line of the process) was defined as: √ 2 2P FA max = (1) w0 V where P is the average laser power, w0 is the radius of the laser beam at the focal point, and V is the scanning speed of the sample.

Fig. 2. Scheme of irradiation of the samples.

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Table 1 Process parameters corresponding to the selected colors and their measured color space coordinates. No.

Power

Speed [mm/s]

1 2 3 4 5 6

7 7 7 7 7 8

200 145 125 120 100 100

PRR [kHz]

80

Hatching [␮m]

Fluence

The color observed

10

157.6 216.1 252.1 262.6 315.1 360.1

Yellow Orange Red Purple Blue Light blue

This range was limited from below (with a certain margin) by the minimum value for which visual changes on the material’s surface were observed and from above by the material’s ablation threshold (∼420 J/cm2 ) [21]. A summary of process parameters for individual samples and the corresponding coordinates in the CIE Lab* color space are given in Table 1. The method of taking colorimetric measurements is described in one of the sections which follow (Section 2.3.1).

2

2

2

(L∗ ) + (a∗ ) + (b∗ ) ,

(2)

where Eab∗ (total color difference) is the Euclidean distance between two points in a three dimensional color space, and L*, a* and b* indicate how much a standard and a sample differ from one another. 2.3.2. EA: ellipsometry analysis Ellipsometric measurements were carried out using the VVase ellipsometer from J.A. Woollam Co., Inc. These measurements were performed for photon energy ranging from 0.58 to 6.5 eV (2200–193 nm) and for five values of incidence angle: 55–75◦ , with a 5◦ step. To obtain the optical constants of the substrate, the titanium plate was measured separately. The three-phase model (air/native oxide/substrate) was applied to determine the refractive index (n) and extinction coefficient () of the substrate and the thickness of the native oxide film. The complex refractive index of this film was taken from the Palik database of optical constants [28]. It should be noted, however, that such an approach is characterized by the high uncertainty of the estimation of the thickness of the native oxide layer. In this calculation, the thickness of 2.8 ± 0.75 nm was obtained (the MSE only varied by 0.0025 within this range). To describe optical properties of Ti substrate we adopted the Drude–Lorentz model of the dielectric function in form [29]: ε˜ (E) = ε∞ −

2 ωp2 E 2 + iE

+



Aj Ej2

j

Ej2 − E 2 − iEj

a*

b*

78.6 43.1 28.4 15.2 29.6 69.3

8.3 40 55.5 60.3 3.6 −17.7

62.5 60.7 2.9 −56.1 −73.3 −30.9

 1  2 2 ( im − ie ) + (im − ie ) N−P N

2.3.1. Spectrophotometric measurements For an objective evaluation of the color, an optical spectrometer Ocean Optics type USB 4000 was used. The light source was a GrafiLite lamp of the True Color type (color rendering index CRI = 82, color temperature TC = 5600 K). The Spectra Suite software used to control the spectrometer allows operation in several color spaces. For colorimetric identification, one of the most useful color spaces, the CIE Lab*, was used. In the CIE Lab* color space, the difference between the two measured colors can be expressed by the CIE color difference formula [27]: Eab∗ =

L*

absorption √ band, respectively. The n and  values were calculated as n + i = ε˜ . For the purpose of the modeling of the investigated layers a three-phase system was also assumed as above. The optical constants of oxide layers were parameterized by the Pole dispersion equation and the sum of Gauss and Tauc-Lorentz oscillators [30]. The measurement data were fitted to the model by minimizing the mean square error 2 using the Levenberg–Marquardt algorithm (with the WVASE32® software from J.A. Woollam Co., Inc.).

2.3. Models and measurement techniques



CIE coordinates [J/cm2 ]

,

(3)

where ε∞ – is the dielectric constant, ωp and  – are the plasma frequency and the free-carrier damping, respectively. In turn, Aj , Ej and  j – represent the amplitude, energy and broadening of the jth

2 =

(4)

i

where N – total number of independent measurement data, P – number of fitted parameters of the model, im , im , ie , ie – measurement data (m) and angles estimated in the fitting process (e), respectively. The 2 parameter, which defines the goodness of fit and thus the quality of model, typically did not exceed 2 = 2.3, except for the fit for the blue color, for which 2 = 3.4 [30]. 2.3.3. XPS: X-ray photoelectron spectroscopy The XPS measurements were performed in a UHV chamber at <2 × 10−10 mbar using a roentgen lamp as the source of monochromatic radiation: Al K␣ (ω = 1486.6 eV). The source was placed at 55◦ to the normal of investigated samples. The analysis of photoelectrons was carried out using a hemispherical spectrometer (VG Scienta R3000) positioned perpendicularly to the analyzed surface (acquisition time: 0.5 s). The composition of subsurface layers was obtained by Ar+ sputtering of the sample’s surface using a differentially pumped ion gun (energy: 4 keV, incidence angle: 69◦ ). In order to ensure homogeneous etching, an ion beam was scanning the material over an area of 1.5 mm × 1.5 mm. The photoemission spectra were analyzed for 700 min, with a variable step (every 2–10 min). These spectra were calibrated based on the position of the carbon C 1s peak (285 eV), the presence of which was recorded only at the sample’s surface (<27 at.%). The atomic concentration of individual components was estimated by fitting to the Gauss–Lorentz line (using the CasaXPS® software from Casa Software Ltd.). The quantitative analysis was conducted by measuring the peak areas of specific elemental core lines and by applying appropriate atomic relative sensitivity factors. For the background correction, Shirley’s method was applied. 2.3.4. XRD: X-ray diffraction The XRD analysis was carried out using the Empyrean X-ray diffractometer from Panalytical. Research was performed in the Bragg–Brentano configuration (–). Measurements were taken for angles from 5◦ to 125◦ with a  = 0.013◦ step and the averaging time of 5.9 s/step. ˇ-Filtered (Ni filter) radiation of a Cu K␣ roent˚ U = 40 kV, Ie = 30 mA) and a fast detector gen lamp ( = 1.54184 A; (PiXel3D ) were used. The obtained diffractograms were analyzed using the Rietveld method [31] (the model accounts for the position and intensity of reflections) for which the Highscore Plus 3.0e software from Panalytical was used.

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Fig. 3. (a) Photos of individual samples, (b–g) microscope images of oxide layers on the surface of titanium for various values of accumulated fluence (in the same order as in point (a)).

2.3.5. Measurements of the surface morphology The microscope photos and the measurements of roughness were carried out with a confocal optical microscope (Olympus Lext OLS 4000). The roughness parameters were determined according to ISO 4287 for a field of observation of 129 ␮m × 129 ␮m by taking the average of 10 measurement lines orthogonal to the trajectory of the laser beam. Samples were also visualized using the Zeiss EVO MA 25 scanning electron microscope. Measurements were taken at <2 × 10−5 mbar and with the accelerating voltage of 20 kV. 3. Results and discussion 3.1. Surface morphology Photos of samples with visible colored areas of irradiation and microscope images showing the morphology of the surface of each colored area are given in Fig. 3. The inhomogeneity of obtained colors visible under the microscope is a direct consequence of the raster method of the exposure of a sample with multiple

repetitions. The non-isothermal character of the process and the diversity of conditions in which the material was irradiated (in each line the beam was partly scanning the substrate area and partly the oxide which had been formed previously) prevented the formation of an homogeneous layer. As can be seen, the oxide layers formed in the laser process were cracking while cooling down, creating irregular agglomerates averaging 5–20 ␮m in size. This is the result of the high speed cooling of the oxide layer being formed and of the residual stress resulting from the raster method of scanning the sample [32]. With the increase of laser fluence (temperature of the sample) the size of agglomerates was increasing and the number of cracks was diminishing – Fig. 4a. This was for two reasons: first, the smaller microcracks were melting [33] and, second, the oxide layer’s chemical composition was changing. The cracks we observed were related to the formation of the phase alpha which will be discussed further on. Moreover, with the increase of laser fluence the roughness of investigated samples was decreasing. The values of: Ra – the average arithmetic profile deviation from the average line, measured along the measuring

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form (bcc) ˇ-Ti [34]. At temperatures above 600 ◦ C, both these forms dissolve in itself large quantities of oxygen and nitrogen, which locate themselves in the interstitial positions of the lattice. The ˛-Ti phase is capable of dissolving about 33 at.% (14 wt.%) of oxygen or 23 at.% (8 wt.%) of nitrogen. During the process of hightemperature oxidation of titanium under the main oxide layer, when the concentration of oxygen exceeds 15 at.%, a thick (up to several hundred ␮m for long exposure times) oxygen-rich metallic layer is formed – the Ti(O) interstitial solid solution, which in the literature is referred to as the “alpha-case” [35]. The presence of oxygen hardens the layer and is responsible for its embrittlement, which considerably reduces the durability of products made of titanium [36]. Since the molar volume of the alpha phase is greater than that of titanium, shearing stress occurs on the border between these two. When the layer reaches an appropriate thickness, the generated relaxing stress causes exfoliation either on the phase border or within the layer. Fig. 5 shows the probable exfoliation of such a layer. This effect was observed in all the analyzed cases of laser oxidation of titanium (FA = 158–360 J/cm2 ). However, for the investigated samples, no correlation was found between the thickness of the exfoliating phase and the fluence of laser radiation. This thickness varied from 0.9 to 2.9 ␮m, both for an individual sample and for the entire measurement.

3.3. The optical properties of the layers

Fig. 4. Surface morphology: (a) the average value of the surface area of the oxide scales S and the number NS of the oxide scales that were fully visible in the field of 40 ␮m × 50 ␮m, (b) a comparison of selected roughness parameters (Ra , Rp , Rv ) as a function of accumulated fluence of laser radiation; values averaged over 10 measuring lines.

section, Rp – the highest point and Rv – the lowest point, decreased with the accumulated fluence – Fig. 4b, and converged to the values characteristic for the substrate (Ra = 12 nm, Rp = 58 nm, Rv = 24 nm, respectively). 3.2. Problem “alpha-case” Titanium has two allotropic forms: the hexagonal-close-packed form, (hcp) ˛-Ti (stable up to 882.5 ◦ C) and the body-centered cubic

The optical properties of laser induced oxynitride films on titanium was investigated in detail in our previous paper [30]. A comparison of the reflectance characteristics, modeled theoretically under the assumption that layers formed on the surface of titanium are made solely of titanium dioxide and having a thickness determined by the colorimetric method (by minimizing Eab∗ between the synthesized color and the actual sample) [21] with the characteristics determined based on ellipsometric measurements yields significant differences in the value of the reflection coefficient in the spectral range of ∼300–580 nm [30]. The observed discrepancy increases with the laser fluence and its maximum shifts from UV to the visible spectrum [30]. This may suggest that an outer, transparent, layer may exist, consisting of titanium oxides doped with nitrogen TiOx Ny [37] or reduced, nonstoichiometric compounds, because pure stoichiometric titanium dioxide does not absorb beyond 385 nm (energy bandgap ∼3.2 eV). The differences in the reflectance characteristics are directly correlated with the extinction coefficient  (which changes with fluence) determined

Fig. 5. A probable exfoliation of the alpha-case due to the shearing stress caused by the difference in the molar volume between the phase and the substrate – measurements at two different points within the blue color sample (FA = 315 J/cm2 ).

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ablation threshold) and by an N-fold repetition of the process with a low fluence value – Fig. 7. It should be noted that in the case of the N-fold repetition of the process due to other conditions of heat accumulation, the resulting layer thickness may be smaller. 3.4. The chemical structure of the layers

Fig. 6. The index of refraction values n (continuous lines) and of the coefficient of extinction values  (broken lines) determined ellipsometrically for individual colors and the substrate compared with the values of n and  which are available in the literature for Ti [28] and TiO2 [38].

for individual samples by means of ellipsometric spectroscopy – Fig. 6. As has been shown previously, the thicknesses of the optically active sublayer increased in an ideally linear manner (coefficient of determination R2 = 0.994) with the increase of accumulated laser fluence, according to the following empirical relationship: d [nm]  2.9 + 0.1FA [J/cm2 ] [30]. The linear growth dynamics of the oxide layer may be due to two reasons. As has been determined from the XRD measurements of investigated structures (discussed further on), the components of the laser-induced oxide layers (including the ˛-case) include, among others, the titanium compounds having a low degree of oxidation, for which the PillingBedworth coefficients are close to one (1.04 for TiO; 1.12 for Ti2 O and 1.22 for TiN), which does not guarantee the formation of a passivation layer (lack of compactness). This, in turn, combined with the microcracks (molecular diffusion of oxygen) provides favorable conditions for the linearity of the process [39]. Since the process is linearly scalable, the layer of required thickness may be formed both by a single use of an appropriate value of fluence (below the

In order to fully explain the morphology of oxide layers formed on the surface of titanium, the layers’ chemical composition was studied, which also included its dependence on their thickness. The chemical composition of samples was determined using X-ray Photoelectron Spectroscopy and X-Ray Diffraction. The photoemission spectra for the Ti 2p lines for three selected cases as a function of the ion sputtering time are shown in Fig. 8. Due to the complex chemical structure of the oxide layers involved, the widely stretched XPS spectra with no visible maxima and the mutual overlap between the emission lines of factors which may potentially occur in the investigated layers (e.g. Ti2 O3 and TiOx Ny or TiO and TiN) – Fig. 8c, the chemical quantitative analysis of specific oxides, nitrides or nitroxides was practically impossible or at least would not have been reliable. However, the measurements allowed us to establish some important facts. By using the results from [40,41] we determined the characteristic positions of the Ti 2p3/2 lines for selected factors (Fig. 8c). In all the cases the presence of TiO2 on the surface of samples was observed (Ti+4 2p doublet: binding energy 2p3/2 458.6 eV; spin–orbit splitting E = 5.6 eV, characteristic position of the O 1s oxygen peak: 529.8 eV) – Figs. 8a and 9a and b. These values correlate well with those presented in other works [42] – both for titanium (Ti 2p3/2 458.59 eV; E = 5.72 eV and for oxygen O 1s 529.87 eV (anatase). During ion sputtering the doublet was widened, passing into band emission. It should be emphasized that the changes were smooth and gradual. Also the binding energy was decreasing, which was due to the change in the degree of oxidation of titanium Ti+4 → Ti0 and, consequently, the decrease in the relative concentration of oxygen (or/and nitrogen) – Fig. 9c. After the sputtering time (which depended on the sample’s color), the

Fig. 7. Photos showing the summing of the thickness (color) of the oxide layer: (a–d) on the example of the layer of the same thickness (color), (e) of three different thicknesses (within the intersections of the circles, titanium was irradiated either two or three times), (f) a sample with a matrix irradiated N times with a constant fluence value FA = 45 J/cm2 ; the number of irradiations corresponds with that of the field (numbering according to the reading order). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8. The change in the XPS photoemission spectra of the Ti 2p lines versus the ion sputtering time (Ar+ , 4 keV): (a) yellow color, (b) red color, (c) light blue color sample; location of the reference peaks was determined based on [40,41].

Fig. 9. The change in the ion sputtering time (Ar+ , 4 keV) of the: (a) photoemission spectra of the O 1s lines, (b) N 1s photoemission spectra; in both cases for the yellow color samples, (c) atomic concentration of titanium, oxygen and nitrogen for two arbitrary samples (yellow – Y and red – R).

spectrum characteristic for the Ti substrate (Ti0 2p doublet: binding energy 2p3/2 E = 453.2 eV; spin-orbit splitting E = 6.2 eV) was emerging, which took several dozen minutes on average. The reference values are as follows: 2p3/2 E = 453.7 eV and E = 6.1 eV [42]. In addition, the location of the O 1s oxygen peak, after the ion sputtering of the oxide layer, returned to the position of 530.8 eV, which is characteristic for electrophilic oxygen – Fig. 9a. With the increase of fluence it has also been observed that the thickness of the transitory layer increases (Fig. 8) – the specific shape of the emission spectra appeared after a longer sputtering time, which was in line with our expectations. Due to the surface inhomogeneity (resulting from the method used for the manufacturing of films), the microcracks in the oxide layer and the chemical compositional variation with depth (etching speed), the graduation of the ion sputtering time (composition of the oxide layer) with the depth of sputtering was impracticable. Moreover, a small quantity of nitrogen was also found in the samples, typically <8.6–11.2 at.%, which, however, did not occur substantially on the surface (concentration at surface <0.5 at.%) – Fig. 9b and c. The presence of nitrogen (depending on the color of the sample) was recorded after several minutes from sputtering (≤4 min for yellow – concentration 2.03 at.%). A wide and flat maximum was observed for several hundred minutes of sputtering (∼80–220 min for the yellow color) and shifts to higher times with increasing laser fluence. The rate of buildup of a nitrogen concentration was inversely proportional to the thickness of the produced coatings. The low concentration of nitrogen at the surface can be explained in two ways. Firstly, the enthalpy of the TiO2 reaction (−944 kJ/mol) is significantly smaller than that of the TiN reaction (−338 kJ/mol), thus as long as the process is mainly controlled thermodynamically (surface, no influence of kinetics) the formation of oxides prevails [43]. Secondly, the nitrides present on the surface oxidize. Although TiN is thermodynamically stable, it oxidizes in oxygen or an air environment and at an increased temperature (>350 ◦ C) according to the reaction [44]: TiN + O2 → TiO2 + 0.5 · N2

(4)

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Similarly, this process is favored thermodynamically – the reaction’s free energy is 139 kJ/mol [44]. The location of the N 1s nitrogen peak oscillated with sputtering within the range of 396.4–396.8 eV. The range of 396.3–396.77 eV points at the titanium nitroxides TiOx Ny [45], and the second value (396.8 eV) is characteristic of the titanium nitride TiN (reference position 396.8 ± 0.2 eV – the center of an asymmetric peak) [40,44]. Many authors claim that on the oxidation border TiO2 /TiN a transitory phase should form TiOx Ny [44,46] and it does seem to be the case here also. The presence of gases (mainly oxygen) in the samples under analysis, even after several hundred minutes of sputtering, confirms the existence of the interstitial solid solution – Fig. 9c. The XPS measurements were validated by those based on the XRD. The XRD diffractograms for angles from 20◦ to 80◦ are shown in Fig. 10. The determination of theoretical diffraction patterns for identified phases based on the crystallographic information from the ICSD or ICDD databases [47,48] and fitting them, using the least squares method, to the experimental diffraction patterns allows one to obtain a broad set of crystallographic data, including quantitative information on the relative content of separate crystalline phases (amorphous factors are not taken into account). When applied to the multi-phase systems, in which some of the factors (in pairs) have almost identical crystallographic properties and, consequently, almost identical diffraction patterns, the method does not yield unequivocal results: the assessment of which phase and in what share it appears in the structure under analysis is ambiguous – which was also the case in our study – Table 2 and Fig. 10 – lower signatures (main reflections). Taking the above into account, we determined the two most likely sets of factors which might potentially contribute to the composition of the analyzed layers, in which we were interchangeably pinpointing Ti3 O and TiO0.892 and TiN0.26 and TiN0.22 O0.78 , among others. The proposed phases of titanium oxides and nitrides (as well as mixed oxides-nitrides) were selected from hundreds of ICDD and ICSD database entries by matching their diffraction patterns to experimentally obtained diffractograms for colored titanium plates. The relative content of the phases determined by means of this method for both these cases and the corresponding ICSD (or ICDD) numbers are given in Table 3. The most likely scenario (taking into account the XPS analysis) is that both of the determined phases occur; however, it is impossible to distinguish between them from XRD data and to determine the ratio in which they do occur or with which probabilities. It should be underlined that the XRD analysis has only proved the presence of the main compounds. The titanium dioxide (TiO2 ) marked for all the samples in the XPS analysis was visible, in the case of the XRD analysis, for only two samples irradiated with a fluence above 315 J/cm2 . This is due to the low thickness of the analyzed layers and also because of the low degree of their crystallinity. Thin, nonhomogeneous, strongly defected layers do not have any long range order, such as the one created, among others, in the discussed non-isothermal laser process [22], and may be hard to detect by means of the XRD technique. Therefore, the results obtained on this subject should be treated with caution. In diffractometry measurements an important role is played by the substrate on which the sample is placed. In our study the analyzed layers were deposited on titanium, which implied a strong, dominating share of the metallic phase in the individual diffractograms. It should

Fig. 10. The XRD diffractograms of the substrate (in gray), the samples corresponding to the individual colors being analyzed and the diffraction patterns of the determined factors with the main reflections (intensity over 5%) marked on them, based on the ICDD [47] and ICSD [48] databases.

be stressed that the relative shares of individual factors are not the percentage shares of the surface layer’s composition (in the sense of weight) but rather a measure of intensity of the signal originating from each individual phase. Similar to the statistical approach [49], our analysis confirms yet again the possibility of there being nitrides and nitroxides in the laser-made oxide layers on the surface of titanium in an atmospheric environment, which, in turn, proves that the colors obtained in this way result from both the interference and the absorption effects. We are not able to specify in which layers these compounds were located. The XRD

Table 2 Summary of crystallographic data and information on the peaks for TiO0.892 and TiN0.22 O0.78 [47,48]. Compound

System/space/group/ number

Lattice parameters a; b; c [Å]

Cell volume [Å3 ]

2 [◦ ] positions and (intensities [%]) of the main (intensity over 5%) reflections

TiO0.892 TiN0.22 O0.78

Cubic/Fm-3m/255

3 × 4.189 3 × 4.200

73.51 74.09

37.145 (61.4); 43.157 (100); 62.679 (52.6); 75.162 (20.4) 37.043 (57.3); 43.038 (100); 62.498 (45.1); 74.928 (14.5)

A.J. Anto´ nczak et al. / Applied Surface Science 325 (2015) 217–226 Table 3 The relative contribution percentage RPC compounds (phases) determined by Rietveld.

225

consisting of titanium oxides (mainly TiO2 ) doped with nitrogen (in the form of TiOx Ny ) as well as reduced, nonstoichiometric titanium oxides. Oxynitride coatings, whose absorption is partially extended to the visible range and which are produced in such a simple way, may be particularly attractive in photocatalytic applications based on the TiO2 compounds. The XPS and XRD analyses have confirmed the possible presence of nitrogen compounds and the existence of nonstoichiometric factors (TiN0.26 , TiN0.22 O0.78 , TiO0.892 ). In this way, it was also shown that the top layer of the film produced on titanium is not a pure TiO2 , as had been supposed before. By the sputtering of the sample’s surface using an Ar+ ion gun the changes in the concentration of individual elements as a function of the layer’s cross-section were also determined. These measurements indicate that the chemical composition of coatings changes gradually (in a gradient manner) and not discontinuously (in a stepwise manner). An analysis of surface morphology has revealed that the roughness of layers produced in this way diminishes with increasing fluence (for an energy density below the ablation threshold) and converges to the values characteristic for the substrates used in the experiment. Acknowledgments This research was partly supported by the Wroclaw Research Center EIT+ in the framework of the project “The use of nanotechnology in modern materials” – NanoMat (POIG.01.01.0202-002/08) financed by the European Regional Development Fund (Operational Program Innovative Economy Measure 1.1.2). Financial support for part of the instrumentation was obtained from (Stage 2 of the Regional Center of Innovativeness) and The Polish Ministry of Science and Higher Education. The authors would also like to thank M.Sc. Patrycja Szymczyk, for carrying out the SEM (Fig. 5) measurements.

experiment in Bragg–Brentano geometry does not provide information on the depth of layers relative to the surface. Taking into account our previous studies, there is a high probability that nitrogen compounds are located in the outer optically transparent layer, which affects the absorption characteristics of these coatings [30]. 4. Conclusions In this paper we have characterized selected optical properties, surface morphology evolution and chemical composition changes of the complex chemical structure of the layers made on titanium in the process of the heating of its surfaces in the atmospheric environment, by irradiating samples with a nanosecond pulsed (230 ns) fiber (Yb:glass, 1062 nm) laser. The study was carried out over a wide range of values of accumulated laser fluence (158–360 J/cm2 ), limited from below by the minimum value for which clear visual changes on the material’s surface were observed and from above by the material’s ablation threshold. Layers were characterized using ellipsometry, X-ray Photoelectron Spectroscopy, which was combined with Ar+ ion sputtering of the sample’s surface using an ion gun, X-Ray Diffraction, Scanning Electron Microscopy and optical profilometry. Changes in optical properties of the layers were presented in the form of complex refractive index characteristics of the material (n and ) as a function of the wavelength for each individual value of layer’s thickness (color). A comparison of the extinction coefficient characteristics of the laser induced layers with the characteristic for the pure TiO2 yielded significant differences in value in the spectral range of ∼300–580 nm. The observed discrepancy increases with the laser fluence and its maximum shifts from a UV to visible spectrum. This suggests that an outer, optically active layer may exist,

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