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“in situ” XPS studies of laser induced surface cleaning and nitridation of Ti
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Surface & Coatings Technology 202 (2008) 1486 – 1492 www.elsevier.com/locate/surfcoat
“in situ” XPS studies of laser induced surface cleaning and nitridation of Ti Ruth Lahoz a , Juan Pedro Espinós b , Germán F. de la Fuente a,⁎, Agustín R. González-Elipe b,1 a
Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, Laser Processing Laboratory, María de Luna, 3 Zaragoza 50018, Spain b Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Americo Vespucio 49, 41092 Sevilla, Spain Received 22 March 2007; accepted in revised form 29 June 2007 Available online 10 July 2007
Abstract A titanium foil has been subjected to laser irradiation “in situ” in a pre-chamber of an X-ray photoemission spectrometer under different atmospheres (vacuum, Ar, O2, air, N2 and H2). As a result of these treatments, a high amount of the carbon contamination layer was removed and other changes in composition were induced. Nitridation was achieved by laser irradiation under nitrogen. The most effective treatment protocol included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot has been also carried out. It is found that the outermost layers present a similar concentration of nitrogen. In addition, the measured nitrogen profile indicates that the amount of nitrogen within the laser spot region is relatively lower than within the immediately surrounding area. Almost no nitrogen remains in the spot area after sputtering for 30 min. A model is proposed to account for the observed titanium surface nitridation processes. © 2007 Elsevier B.V. All rights reserved. Keywords: Laser; Ti; Surface Cleaning; nitridation; XPS
1. Introduction Titanium nitride coatings obtained by laser techniques have been used for the production of high performance coatings with increased corrosion resistance [1], improved machining tools [2], biocompatible coatings [3] and increased wear resistance in tribology applications [4]. In addition, they have exhibited optical properties which make them attractive candidates for decorative coatings [5]. The latter have specifically been suggested for TiOx coatings obtained with a similar laser technique as the one presented in this paper [6]. A timely review paper on the subject has appeared recently in the literature [7]. The laser systems and conditions used to attain the above coatings vary considerably. For instance, laser nitridation may be achieved by using high power cw laser emission in order to melt Ti metal or alloy in a nitrogen gas atmosphere [1,8]. The
⁎ Corresponding author. Tel.: +34 976 762527. E-mail addresses: [email protected] (G.F. de la Fuente), [email protected] (A.R. González-Elipe). 1 Tel.: +34 95 448 9528. 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.06.061
product is a relatively thick TiN coating developed by reaction of the molten metal with N2 gas, which makes the original substrate suitable for tribology applications. In contrast, many papers report on work performed with pulsed lasers. In this case, powerful pulses with high irradiance values induce ablation of the substrate as well as surface reactions based on the combination of reactive species in the atmosphere with the intense generated plasma plumes and with the modified substrate's outermost surface. Most of the work published in the related literature is based on studies performed “ex-situ”, where the substrate is exposed to ambient conditions after the laser irradiation experiment. Contact with oxidising and reactive species in the atmosphere hinders the characterisation of the chemical changes that are intrinsic to this process, as the outermost surface may undergo a variety of unwanted reactions. In addition, these papers report on nitrided layers with thickness values ranging between several tens of nanometers to several hundred microns [1,2]. The present work presents an XPS study carried out “in situ” in the same spectrometer where “as received” titanium foils are subjected to pulsed-laser irradiation under different atmospheres. This procedure permits to separate the effects of the
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Fig. 1. Scheme of the laser ablation set-up used in the present work to study the “in-situ” interaction between 1064 nm pulsed radiation and Ti metal inside an XPS prechamber. The laser beam enters the chamber through a transparent oxide window and irradiates an area of approximately 4 mm in diameter.
laser treatment from those attributed to reactions with the air that occur when handling the samples under these conditions. Experiments have been performed in oxygen, air, argon, vacuum, hydrogen and nitrogen. The reason for using “as received” titanium foils for this study is related with the potential use of this laser technique in industrial decontamination and nitridation processes. The cleaning and nitridation behaviour of “as received” titanium have been characterized, and a model is proposed to account for the morphological and chemical changes observed. 2. Experimental A Ti foil (Goodfellow, 99.99% purity) was used as substrate for this study. It was introduced in the pre-chamber without undergoing any particular cleaning procedure, since one of the objectives of the present work is to determine the cleaning efficiency of the different laser treatments performed herein. A pulsed multimode Nd:YAG Q-switched laser (Model Brilliant, Quantel) emitting at 1064 nm with a repetition rate of 20 Hz and 5 ns pulse width was aligned on an angled support structure to irradiate the Ti metal samples onto their centre section, passing through an entry chamber borosilicate glass window. Each of the samples studied in-situ was irradiated with 60 pulses under vacuum conditions (1.33 × 10− 5 Pa) and under O2, synthetic air mixture, N2, Ar, and H2 atmospheres, all at a pressure of 1.33 × 102 Pa. A 50% N2 + 50% H2 mixture at a total pressure of 1.33 × 102 Pa was also used for some experiments. An energy per pulse of 18.5 mJ was determined with a Coherent Laser Energy Meter, fitted with an appropriate detector head to measure high energy pulses. The laser spot measured 4 mm in diameter, resulting in a laser fluence value of 0.147 J cm− 2 and an irradiance of 29.4 MW cm− 2. A schematic illustration of the experimental apparatus used is given in Fig. 1. The high irradiance values associated with the process provoke the appearance of intense shock waves due to the recoil pressure associated to the plasma plume present. Species at the surface are
ablated with a mechanism characteristic of ns regimes, where a thin transient melt appears. The melt layer is compressed at the spot and pushed away radially. It redeposits on a circle of several mm in diameter around the laser incidence spot [9]. The in-situ XPS experiments were performed in a VG ESCALAB 210 spectrometer supplied with a pre-chamber where gases (1.33 × 102 Pa under the present reaction conditions) can be dosed. Irradiation has been carried out in this pre-chamber through a glass window. After pumping, the samples were transferred from the preparation chamber to the analysis chamber, where the spectra were acquired. The latter were collected in the pass energy constant mode at a value of 50 eV. A Mg Kα source was used for the excitation of the spectra. The N1s peak at a value of 395.7 eV was used as the binding energy (BE) reference. This peak was preferred to the more common C1s peak for calibration purposes, because the latter was very much affected during the different laser irradiation treatments performed. This was particularly clear when comparing the original “as received” samples with those subjected to the various laser treatments. Table 1 Atomic percentages for the Ti specimen subjected to laser irradiation under the different atmospheres and conditions employed in this study Sample/gas a
Original Vacuum a Ar a H2 b O2 Air N2 b Vacuum + N2 Vacuum + (N2 + H2) Exposure to air
%Ti 1
0.9 231 30.9 30.9 27.7 23.4 28.2 34.3 28.4 9.1
%N
%O
%C
1.2 0.4 3.1 0.9 0.2 2.8 25.5 42.5 22.2 16.5
17.3 22.3 38.6 42.8 58.0 53.1 21.2 16.5 22.0 31.4
74.5 51.7 22.1 22.0 14.1 20.6 18.3 6.7 27.5 42.0
a Some silicon was detected for these samples (4.5% in the original sample and 2.6 and 0.8 after the treatment in vacuum and Ar, respectively. b Some calcium was detected in these samples (0.9% and 1.3% after treatment in H2 and N2, respectively).
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Fig. 2. X-ray photoelectron spectra centred at the Ti 2p level for a Ti metal foil irradiated with a pulsed (5 ns width) Nd:YAG laser emitting at 1064 nm under different gases at a pressure of 1.33 × 102 Pa. Each condition included irradiation with 60 laser shots.
Calculation of the surface composition was performed by referencing the area of the different photoemission peaks to their sensitivity factors, provided by the spectrometer manufacturer. 3. Results and discussion 3.1. Laser irradiation under different atmospheres Table 1 summarises the percentages of the different elements detected on the surface of the titanium substrates after they have been subjected to irradiation under different atmospheres. The atomic percentages found in this Table clearly demonstrate that all of the laser treatments performed produce an increase in the intensity of all the signals, in detriment of the intensity of the carbon spectrum. This result suggests that the laser irradiation removes most of the contamination layer, observed to cover the original samples. These were used “as received” and are likely covered by a rather thick carbon contamination layer. This layer forms because the foil has been exposed to the atmosphere and,
Fig. 3. X-ray Photoelectron Spectra obtained as in Fig. 2, but in the region where the N1s peak is centered.
Fig. 4. X-ray photoelectron spectra obtained as in Fig. 2, in the region where the C1s peak is centered.
in addition, as a consequence of some remaining over-layer originating from the lamination oils typically used in metal deformation processes. A rough estimate of the thickness of this original contamination layer, calculated from the attenuation of the intensity of the Ti signal, yields a value of approximately 15 Å. This overlayer is efficiently removed by the laser processes applied here, according to the spectra shown in Figs. 2–5. Considering the percentages of carbon remaining on the surface of the samples after irradiation, it turns out that O2, N2 and Ar are the most effective gases in favouring this removal process. Further information derived from Table 1 suggests that nitrogen incorporates very effectively onto the titanium surface when the foil is irradiated under an N2 atmosphere. Some nitrogen also incorporates, however, when the irradiation is performed under air or Ar. Although this is expected in air, nitrogen incorporation under Ar can be attributed either to some contamination of the Ar employed for the experiment, and/or to the ablation of the small amounts of nitrogen-containing compounds present in the “as-received” samples, most likely as a part of the contamination layer covering the surface. Fig. 2 shows the Ti2p spectra recorded for each one of these experiments and reveals significant differences between their
Fig. 5. X-ray photoelectron spectra obtained as in Fig. 2, but in the region where the O1s peak is centered.
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Table 2 Type of atomic species/compound as deduced from the BEs of the peak/shoulders observed in the spectra (in parenthesis) Sample/gas
Ti
Original Vacuum Ar H2 O2 Air N2
TiO2 (457.8) Ti(0) (453.0) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) TiN (455.7) Ti(0) (453)
N
TiN (395.7)
TiN (395.7) TiN (395.7)
O
C
Adsorbed oxygen/water; C/O groups (531.7 TiO2 (529.8) TiO2 (529.8) TiO2 (529.8) TiO2 (530.1) TiO2 (530.1) TiO2 (530)
C\H/C\C (284.6) C\H/C\C (284.6) C/O groups (288.8) TiC (280.7) C\H/C\C (284.3) C/O groups (288.8) TiC (280.7) C\H/C\C (284.2) C/O groups (288.8) TiC (280.7) C\H/C\C (284.3) C/O groups (288.8) C\H/C\C (284.3) C/O groups (288.8) C\H/C\C (284.3) C/O groups (288.8) TiC (280.7)
The species written in bold correspond to the most abundant found in each case.
shapes. These can be rationalised by considering that the laser has induced different reactions according to the conditions imposed by the environment. For the original sample, although the intensity of the Ti 2p signal is very small, it has enough intensity as to prove that titanium is in the form of TiO2. In light of the existent XPS literature on titanium metal, oxides and nitrides [10–14], it is possible to assign the observed features to the formation of the compounds summarised in Table 2. This table also reports similar assignments for the species/compounds involving the other atoms present on the surface. The most significant information gathered from Table 2 and from Fig. 2 is that the treatments under vacuum, H2 and Ar lead to a partial reduction of the Ti species present at the contaminated surface to Ti metal. This process is most effective under vacuum. With hydrogen and Ar, Ti4+ species typical of TiO2 are formed together with Ti(0). Interestingly, no intermediate oxidation states of titanium (i.e., Ti3+, Ti2+ or Ti+, with BE values which are intermediate between those of Ti4+ and Ti0 [15] are formed under these conditions. The use of oxygen and air provokes an almost complete oxidation of titanium to TiO2, while the use of nitrogen for this reaction leads to nitridation of the Ti with the consequent formation of titanium nitride. Some remaining Ti(0) observed points to the fact that nitridation is not complete under these conditions. The information derived from the carbon and oxygen peaks and the percentages reported in Figs. 2–5 and Tables 1 and 2 is complementary to that given earlier concerning the titanium and nitrogen peaks. In fact, oxygen peaks exhibit maximum intensity for the Ti sample treated under oxygen and air. Under such conditions the carbon intensity is small, as expected for the activation of combustion processes. It is also interesting that with vacuum, Ar, H2 and N2 (i.e., non-oxidising conditions), some carbide species are detected, as deduced from the appearance of a C1s peak at 287.4 eV (2). These carbide species are likely to form via reaction of Ti metal with the carbon contaminants present at the sample's surface.
depicted in Fig. 2 reveal formation of Ti(0). To enhance the level of nitridation, a more complex procedure, consisting of a two-step treatment process, has been attempted. Fig. 6 shows the Ti2p photoemission peak of the Ti foil subjected to a first irradiation in vacuum, followed by a second irradiation in N2 or in N2 + H2 mixtures. With this latter mixture, H2 might contribute to remove oxygen and carbon, thus enhancing the nitridation efficiency. Actually, the opposite was found with respect to the results with N2. Compositions of the resultant species in atomic percent are reported, for these two treatments, in Table 1. Several features deserve a comment with respect to the results given in Table 1 and Fig. 6. First of all, the fact that the vacuum + N2 treatment is the most effective in removing oxygen and carbon, leading simultaneously to a maximum incorporation of nitrogen to the surface. Observing the Ti2p spectrum in Fig. 6 it is also apparent that most titanium is in the form of TiN, with less contributions from the other forms of titanium (e.g., Ti(0), according to the shape of the peak). In contrast, the vacuum + (N2 + H2) treatment is less effective for the cleaning and nitridation of the surface. This is evidenced for the relatively higher concentration of oxygen and carbon atoms and by the lower amount of nitrogen remaining on the surface. This latter effect correlates with the fact that an important amount of Ti(0) species remains on the surface after the laser treatment (cf. Table 1 and Fig. 6).
3.2. Laser induced nitridation processes One of the goals of the present work is to establish effective reaction conditions to induce a complete nitridation of “as received” titanium foils by laser ablation. With this point in mind, the reaction has been studied just by irradiating the Ti specimen in N2. Although the nitridation is rather effective, the results
Fig. 6. Ti 2p photoelectron spectra of the Ti foil subjected to a two-step treatment process consisting of in-situ laser irradiation in vacuum followed by laser irradiation under 1.33 × 102 Pa of different gases.
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Fig. 7. Re-oxidation of a sample previously nitrided at 1064 nm under (vacuum + N2) conditions, following exposure to ambient atmosphere for one month. Spectrum centred around the BE corresponding to Ti2p (left) and N1s (right).
3.3. Long term stability of the nitride surface layers Most studies by XPS related to the laser nitridation of titanium deal with samples that are exposed to air after their treatment (i.e. “ex situ” studies). It is likely that this exposure may affect the surface composition by reaction with O2 and other species present in air. In order to study the main effects caused by these species on the nitride layer formed by in-situ laser nitridation, XPS spectra were recorded on samples whose spectra are shown in Figs. 2–5, upon exposure to the atmosphere. This resulted in a progressive oxidation of the surface, accompanied by an increased accumulation of contaminating carbon. Table 1 and Fig. 7 report, respectively, the surface composition and the spectra of the surface atoms for a nitrided sample that was kept in ambient air during one month. The spectra show the incorporation of oxygen, the loss of nitrogen and the accumulation of carbon on the surface layers examined by XPS. In addition, The Ti 2p spectra reveals the formation of TiO2 and some Ti(0), while some defective TiN must still remain, as deduced from the shape of the spectrum (continuous shape in the zone with a BE at ca. 455 eV, the presence of some components at this BE has been also proven by a fitting analysis not shown in the figure). The permanence of some nitride after this prolonged exposure to ambient air is also justified by the N1s spectrum, where a peak at 395.7 eV is
clearly visible. In addition, the shoulder at around 398.5 eV, observed in the same spectrum, can be attributed to the formation of a titanium oxinitride phase [16]. 3.4. Lateral and depth distribution of nitrogen in the laser treated samples The performed laser experiments consist on irradiating a reduced surface area of the treated metal, directly with the laser beam output. Since there are no focusing optics involved in this process, the diameter of the laser spot is ∼4 mm, in agreement with the ideal beam diameter output specified for this laser. An interesting issue is if the laser-induced surface reactions concentrate in the area of the spot or whether they extend to the surrounding zones. In other words, it is of interest to determine the reaction area as compared to the initial 4 mm diameter laser incidence zone. In order to study this surface spatial reactivity phenomenon, a series of experiments have been carried out using SEM and XPS with the samples that have been subjected to the vacuum + N2, two-step treatment. Fig. 8 shows SEM micrographs obtained from within the laser spot area. Both images shown exhibit the characteristic ablation microstructure expected from a high-energy multimode laser beam. The observed typical waves within the melted surface are indicative of high-energy maxima focalised into several points throughout the spot. This situation
Fig. 8. SEM micrographs obtained on the center (left) and edges (right) of the spot generated by irradiating a Ti metal foil “in-situ” under 1.33 × 102 Pa N2 with a 1064nm 5ns pulsed laser beam, inside a vacuum pre-chamber of an XPS apparatus. The morphology observed corresponds to typical ablation profiles previously described for the ns regime [15,16]. The scale bar corresponds to 20 μm.
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found for the surface composition distribution. This point was addressed by performing a depth profile analysis via Ar+ bombardment during increasing periods of time. Spectra were recorded in the centre of the spot and at a point 8 mm outwards, well outside the spot area. From these spectra the atomic percentages have been calculated and the N/Ti ratio represented in Fig. 10. An estimated error bar of 0.2 can be associated to the calculated values. This figure clearly shows that, although the nitrogen content in the surface is equivalent for the two sample zones observed, it does not follow the same trend in depth. In fact, this ratio decreases initially fast at the outer area, but it levels off after 5 min of thinning, keeping constant thereafter. In contrast, below the spot area it decreases more progressively during the first 20 min of thinning, levelling off thereafter at a much lower value than below the surrounding area. It must be stressed that practically no N remains in the centre of the spot after prolonged bombardment with Ar+ ions. 3.5. Reaction model
Fig. 9. Multimode intensity distribution expected for the Nd:YAG laser emission used in the present experiments (a). The overall summation of each of the maxima produces the Gaussian type effective energy distribution illustrated (b).
appears to be the trademark of a multimode beam laser ablation process, in contrast to the characteristic ‘crater’ cone shape morphology observed for TEM00 mode laser ablation [17,18]. Melting waves are wider in the centre of the spot (Fig. 8 — left) than in the border zone (Fig. 8 — right). This is due to the average summation of energy maxima that results in a nearly Gaussian global distribution of the laser irradiance, in spite of localised statistic distribution of maxima and minima in the multimode energy distribution. This effect implies higher energy contribution in the centre of the spot that leads to an increase in wave sizes of melted material. Fig. 9 shows an illustrative scheme of this energy distribution. Image (a) shows the characteristic energy profile of a multimode beam and image (b) shows the average sum of the profiles to achieve a pseudo Gaussian profile, which explains the special morphology obtained with this laser ablation process. XPS analysis has been carried out by focussing the electron analyzer onto the centre of the spot and by recording spectra at different lateral positions, relative to this centre and up to 10 mm outside it. It is interesting that quite similar spectra are recorded at any point, indicating that the surface composition is similar both, in the incidence spot and in the surrounding areas (characterized by the surface microstructure depicted in Fig. 8). Since XPS is a surface-sensitive technique, it seemed relevant to check whether the nitrogen content in the centre and in the surrounding areas follows a similar depth profile as
The above results clearly demonstrate that pulsed laser irradiation is very effective for cleaning the surface of titanium metal even if it is heavily contaminated from its exposure to ambient air and to mechanical deformation additives. These results show, in addition, that the surface composition of the Ti metal can be modified during the cleaning treatment if the surrounding atmosphere is well controlled. In fact, this work demonstrates that oxidation and nitridation processes can be effectively induced with “as received” titanium foils by following an appropriate reaction protocol. In particular, they show that nitridation of “as received” Ti foils may be carried out conveniently and effectively performing an initial laser irradiation in vacuum, followed by a second one under nitrogen. The results show, in addition, that irradiation in vacuum leads to the removal of a substantial part of the carbon contamination layer, while the irradiation in nitrogen further contributes to the removal of the remaining contamination (i.e., O, C, see Table 1) and produces a very effective incorporation of nitrogen into the substrate, in the form of titanium nitride.
Fig. 10. N to Ti ratio as a function of the Ar+ sputtering time in a Ti foil irradiated in vacuum and then under 1.33 × 102 Pa N2 at the centre of the laser spot and in the periphery (8 mm away).
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First layers of titanium nitride are not stable after long exposure of the samples to ambient air. It is thus found that the outermost layers become partially oxidised. These are typically found to contain titanium oxide and oxynitride, together with some Ti(0). This surface composition suggests that the surface layers of titanium nitride are partially decomposed by oxidation and/or hydrolysis in the ambient atmosphere. This effect must also occur at the outermost surface layers of the thicker TiN coatings prepared by other, more energy intensive laser treatments, as well as by alternative techniques. For example, the presence of TiN, TiOxNy and TiO2 has been reported recently at the surface of TiN coatings obtained by dc magnetron sputtering [19]. These authors established that TiN was predominantly obtained at high N2 concentration and film thickness, consistent with the results found in this work. Concerning the spatial distribution of the nitrides produced during laser irradiation, it may be emphasised that the N/Ti ratio is found similar in values, or even higher at considerable distances from the centre of the incident beam spot. It is also particularly interesting that, according to the depth profiles in Fig. 10, N is only located in the outmost surface layers of the centre of the spot. This phenomenon is not surprising and may be discussed in terms of the reactivity and physical phenomena related to the presence of an intense plasma. We can understand the previous result by assuming the following reaction model. The break down of gaseous species at the irradiance level reached, combined with the intense shock waves associated to the recoil pressure of the plasma – which should be most intense at the spot – may well explain the surface and in-depth N/Ti compositional profiles shown in Fig. 10. On the one hand, species break-down causes formation of the Ti nitride above the substrate. Part of these species is compressed by the shock waves onto the spot area, and another part is dispersed radially outwards into the surrounding area. They are thus found in both zones with a similar surface distribution. Their in-depth distribution, however, is modified by the expected removal of transient melt from the centre of the spot, as illustrated in the scheme of the reaction model presented in Fig. 1, where a typical ablation process is represented within an illustration of the ablation procedure employed in this work. In this particular case, a very limited melt layer may be formed as expected for nanosecond regime ablation [17,18]. The intense shock waves should follow a similar profile as that given in Fig. 9. They would thus push the melt radially outwards, away from the spot area, thus contributing to the observed thicker nitrided layers localised away from the laser spot, as well as to the local microstructure and morphology observed in Fig. 8. 4. Conclusion Surface cleaning of Ti metal has been demonstrated for the first time in an in-situ configuration, inside the pre-chamber of an XPS apparatus, by introducing a Nd:YAG laser emitting at 1064 nm in pulsed mode. In addition, laser-induced reactivity of the Ti surface has been studied under Ar, O2, synthetic air, N2 and H2. The results obtained indicate that, on the one hand, a high amount of the initial carbon contamination layer was removed. On the other hand, Ti nitridation was achieved by
laser irradiation under nitrogen. The most effective sequence included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot indicates that the surface concentration of nitride is similar within a wide area away from the spot and that the depth profile is different at the spot and several mm away, where more nitride is present. Within the deep layers located below the centre of the laser spot, only small amounts of nitrogen are found. A model is consequently proposed to account for the observed titanium surface cleaning and nitridation processes. Acknowledgements The authors acknowledge the Spanish Ministry of Education and Science for financial support through Project no. MAT 2004-01558 and INGESNET (Surface Engineering Network), as well as the Government of Aragón (DGA) for partial support of the Laser Materials Processing Group through its Consolidated Group Programme. References [1] I. García, J.J. de Damborenea, Corros. Sci. 40 (1998) 1411; M.S.F. Lima, F. Folio, S. Mischler, Surf. Coat. Technol. 199 (2005) 83. [2] J.M. Lackner, W. Waldhauser, R. Berghauser, R. Ebner, B. Major, T. Schöberl, Thin Solid Films 453–454 (2004) 195; C.Y.H. Lim, S.C. Lim, K.S. Lee, Tribol. Int. 32 (1999) 393; R.R. Manory, A.J. Perry, D. Rafaja, R. Nowak, Surf. Coat. Technol. 114 (1999) 137. [3] H.C. Man, Z.D. Cui, X.J. Yang, Appl. Surf. Sci. 199 (2002) 293. [4] B.S. Yilbas, J. Nickel, et al., Wear 212 (1997) 140; P. Jiang, et al., Surf. Coat. Technol. 130 (2000) 24; C. Langlade, A.B. Vannes, J.M. Krafft, J.R. Martin, Surf. Coat. Technol. 100–101 (1998) 383; E. Bemporad, M. Sebastiani, C. Pecchio, Surf. Coat. Technol. 201 (2006) 2155. [5] E. György, A. Pérez del Pino, P. Serra, J.L. Morenza, Appl. Surf. Sci. 186 (2002) 130. [6] A. Pérez del Pino, P. Serra, J.L. Morenza, Thin Solid Films 415 (2002) 201. [7] J.M. Lackner, Vacuum 78 (2005) 73. [8] A.I.P. Nwobu, R.D. Rawlings, D.R.F. West, Acta Mater. 47 (1999) 631. [9] B. Gakovic, I. Pongrac, S. Petrovic, D. Minic, M. Trtica, Mat. Sci. Forum 518 (2006) 161; R.K. Thareja, A.K. Sharma, Laser Part. Beams 24 (2006) 311. [10] Milosev, H.-H. Strehblow, B. Navinsek, M. Metikos-Hukovic, Surf. Interface Anal. 23 (1995) 529. [11] L.I. Johansson, Surf. Sci. Rep. 21 (1995) 177. [12] J. Zhao, E. Gene Garza, K. Lam, C.M. Jones, Appl. Surf. Sci. 158 (2000) 246. [13] I.leR. Strydom, S. Hofmann, J. Electr. Spectrosc. Relat. Phenom. 56 (1991) 85. [14] I. Bertóti, Surf. Coat. Technol. 151 (2002) 194. [15] D. Leinen, A. Fernández, J.P. Espinós, A.R. González-Elipe, Appl. Phys., A 63 (1996) 237. [16] I. Bertóti, R. Kelly, M. Mohai, A. Tóth, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 80/81 (1993) 1219. [17] D. Bäuerle, Laser Processing and Chemistry, Springer-Verlag, 1996. [18] H.-G. Rubahn, Laser Applications in Surface Science and Technology, J. Wiley & Sons, 1999. [19] Y. Jeyachandran, S.K. Narayandass, D. Mangalaraj, S. Areva, J.A. Mieluarski, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 445 (2007) 223.
Surface & Coatings Technology 202 (2008) 1486 – 1492 www.elsevier.com/locate/surfcoat
“in situ” XPS studies of laser induced surface cleaning and nitridation of Ti Ruth Lahoz a , Juan Pedro Espinós b , Germán F. de la Fuente a,⁎, Agustín R. González-Elipe b,1 a
Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, Laser Processing Laboratory, María de Luna, 3 Zaragoza 50018, Spain b Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Americo Vespucio 49, 41092 Sevilla, Spain Received 22 March 2007; accepted in revised form 29 June 2007 Available online 10 July 2007
Abstract A titanium foil has been subjected to laser irradiation “in situ” in a pre-chamber of an X-ray photoemission spectrometer under different atmospheres (vacuum, Ar, O2, air, N2 and H2). As a result of these treatments, a high amount of the carbon contamination layer was removed and other changes in composition were induced. Nitridation was achieved by laser irradiation under nitrogen. The most effective treatment protocol included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot has been also carried out. It is found that the outermost layers present a similar concentration of nitrogen. In addition, the measured nitrogen profile indicates that the amount of nitrogen within the laser spot region is relatively lower than within the immediately surrounding area. Almost no nitrogen remains in the spot area after sputtering for 30 min. A model is proposed to account for the observed titanium surface nitridation processes. © 2007 Elsevier B.V. All rights reserved. Keywords: Laser; Ti; Surface Cleaning; nitridation; XPS
1. Introduction Titanium nitride coatings obtained by laser techniques have been used for the production of high performance coatings with increased corrosion resistance [1], improved machining tools [2], biocompatible coatings [3] and increased wear resistance in tribology applications [4]. In addition, they have exhibited optical properties which make them attractive candidates for decorative coatings [5]. The latter have specifically been suggested for TiOx coatings obtained with a similar laser technique as the one presented in this paper [6]. A timely review paper on the subject has appeared recently in the literature [7]. The laser systems and conditions used to attain the above coatings vary considerably. For instance, laser nitridation may be achieved by using high power cw laser emission in order to melt Ti metal or alloy in a nitrogen gas atmosphere [1,8]. The
⁎ Corresponding author. Tel.: +34 976 762527. E-mail addresses: [email protected] (G.F. de la Fuente), [email protected] (A.R. González-Elipe). 1 Tel.: +34 95 448 9528. 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.06.061
product is a relatively thick TiN coating developed by reaction of the molten metal with N2 gas, which makes the original substrate suitable for tribology applications. In contrast, many papers report on work performed with pulsed lasers. In this case, powerful pulses with high irradiance values induce ablation of the substrate as well as surface reactions based on the combination of reactive species in the atmosphere with the intense generated plasma plumes and with the modified substrate's outermost surface. Most of the work published in the related literature is based on studies performed “ex-situ”, where the substrate is exposed to ambient conditions after the laser irradiation experiment. Contact with oxidising and reactive species in the atmosphere hinders the characterisation of the chemical changes that are intrinsic to this process, as the outermost surface may undergo a variety of unwanted reactions. In addition, these papers report on nitrided layers with thickness values ranging between several tens of nanometers to several hundred microns [1,2]. The present work presents an XPS study carried out “in situ” in the same spectrometer where “as received” titanium foils are subjected to pulsed-laser irradiation under different atmospheres. This procedure permits to separate the effects of the
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Fig. 1. Scheme of the laser ablation set-up used in the present work to study the “in-situ” interaction between 1064 nm pulsed radiation and Ti metal inside an XPS prechamber. The laser beam enters the chamber through a transparent oxide window and irradiates an area of approximately 4 mm in diameter.
laser treatment from those attributed to reactions with the air that occur when handling the samples under these conditions. Experiments have been performed in oxygen, air, argon, vacuum, hydrogen and nitrogen. The reason for using “as received” titanium foils for this study is related with the potential use of this laser technique in industrial decontamination and nitridation processes. The cleaning and nitridation behaviour of “as received” titanium have been characterized, and a model is proposed to account for the morphological and chemical changes observed. 2. Experimental A Ti foil (Goodfellow, 99.99% purity) was used as substrate for this study. It was introduced in the pre-chamber without undergoing any particular cleaning procedure, since one of the objectives of the present work is to determine the cleaning efficiency of the different laser treatments performed herein. A pulsed multimode Nd:YAG Q-switched laser (Model Brilliant, Quantel) emitting at 1064 nm with a repetition rate of 20 Hz and 5 ns pulse width was aligned on an angled support structure to irradiate the Ti metal samples onto their centre section, passing through an entry chamber borosilicate glass window. Each of the samples studied in-situ was irradiated with 60 pulses under vacuum conditions (1.33 × 10− 5 Pa) and under O2, synthetic air mixture, N2, Ar, and H2 atmospheres, all at a pressure of 1.33 × 102 Pa. A 50% N2 + 50% H2 mixture at a total pressure of 1.33 × 102 Pa was also used for some experiments. An energy per pulse of 18.5 mJ was determined with a Coherent Laser Energy Meter, fitted with an appropriate detector head to measure high energy pulses. The laser spot measured 4 mm in diameter, resulting in a laser fluence value of 0.147 J cm− 2 and an irradiance of 29.4 MW cm− 2. A schematic illustration of the experimental apparatus used is given in Fig. 1. The high irradiance values associated with the process provoke the appearance of intense shock waves due to the recoil pressure associated to the plasma plume present. Species at the surface are
ablated with a mechanism characteristic of ns regimes, where a thin transient melt appears. The melt layer is compressed at the spot and pushed away radially. It redeposits on a circle of several mm in diameter around the laser incidence spot [9]. The in-situ XPS experiments were performed in a VG ESCALAB 210 spectrometer supplied with a pre-chamber where gases (1.33 × 102 Pa under the present reaction conditions) can be dosed. Irradiation has been carried out in this pre-chamber through a glass window. After pumping, the samples were transferred from the preparation chamber to the analysis chamber, where the spectra were acquired. The latter were collected in the pass energy constant mode at a value of 50 eV. A Mg Kα source was used for the excitation of the spectra. The N1s peak at a value of 395.7 eV was used as the binding energy (BE) reference. This peak was preferred to the more common C1s peak for calibration purposes, because the latter was very much affected during the different laser irradiation treatments performed. This was particularly clear when comparing the original “as received” samples with those subjected to the various laser treatments. Table 1 Atomic percentages for the Ti specimen subjected to laser irradiation under the different atmospheres and conditions employed in this study Sample/gas a
Original Vacuum a Ar a H2 b O2 Air N2 b Vacuum + N2 Vacuum + (N2 + H2) Exposure to air
%Ti 1
0.9 231 30.9 30.9 27.7 23.4 28.2 34.3 28.4 9.1
%N
%O
%C
1.2 0.4 3.1 0.9 0.2 2.8 25.5 42.5 22.2 16.5
17.3 22.3 38.6 42.8 58.0 53.1 21.2 16.5 22.0 31.4
74.5 51.7 22.1 22.0 14.1 20.6 18.3 6.7 27.5 42.0
a Some silicon was detected for these samples (4.5% in the original sample and 2.6 and 0.8 after the treatment in vacuum and Ar, respectively. b Some calcium was detected in these samples (0.9% and 1.3% after treatment in H2 and N2, respectively).
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Fig. 2. X-ray photoelectron spectra centred at the Ti 2p level for a Ti metal foil irradiated with a pulsed (5 ns width) Nd:YAG laser emitting at 1064 nm under different gases at a pressure of 1.33 × 102 Pa. Each condition included irradiation with 60 laser shots.
Calculation of the surface composition was performed by referencing the area of the different photoemission peaks to their sensitivity factors, provided by the spectrometer manufacturer. 3. Results and discussion 3.1. Laser irradiation under different atmospheres Table 1 summarises the percentages of the different elements detected on the surface of the titanium substrates after they have been subjected to irradiation under different atmospheres. The atomic percentages found in this Table clearly demonstrate that all of the laser treatments performed produce an increase in the intensity of all the signals, in detriment of the intensity of the carbon spectrum. This result suggests that the laser irradiation removes most of the contamination layer, observed to cover the original samples. These were used “as received” and are likely covered by a rather thick carbon contamination layer. This layer forms because the foil has been exposed to the atmosphere and,
Fig. 3. X-ray Photoelectron Spectra obtained as in Fig. 2, but in the region where the N1s peak is centered.
Fig. 4. X-ray photoelectron spectra obtained as in Fig. 2, in the region where the C1s peak is centered.
in addition, as a consequence of some remaining over-layer originating from the lamination oils typically used in metal deformation processes. A rough estimate of the thickness of this original contamination layer, calculated from the attenuation of the intensity of the Ti signal, yields a value of approximately 15 Å. This overlayer is efficiently removed by the laser processes applied here, according to the spectra shown in Figs. 2–5. Considering the percentages of carbon remaining on the surface of the samples after irradiation, it turns out that O2, N2 and Ar are the most effective gases in favouring this removal process. Further information derived from Table 1 suggests that nitrogen incorporates very effectively onto the titanium surface when the foil is irradiated under an N2 atmosphere. Some nitrogen also incorporates, however, when the irradiation is performed under air or Ar. Although this is expected in air, nitrogen incorporation under Ar can be attributed either to some contamination of the Ar employed for the experiment, and/or to the ablation of the small amounts of nitrogen-containing compounds present in the “as-received” samples, most likely as a part of the contamination layer covering the surface. Fig. 2 shows the Ti2p spectra recorded for each one of these experiments and reveals significant differences between their
Fig. 5. X-ray photoelectron spectra obtained as in Fig. 2, but in the region where the O1s peak is centered.
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Table 2 Type of atomic species/compound as deduced from the BEs of the peak/shoulders observed in the spectra (in parenthesis) Sample/gas
Ti
Original Vacuum Ar H2 O2 Air N2
TiO2 (457.8) Ti(0) (453.0) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) Ti(0) (453) TiO2 (458.5) TiN (455.7) Ti(0) (453)
N
TiN (395.7)
TiN (395.7) TiN (395.7)
O
C
Adsorbed oxygen/water; C/O groups (531.7 TiO2 (529.8) TiO2 (529.8) TiO2 (529.8) TiO2 (530.1) TiO2 (530.1) TiO2 (530)
C\H/C\C (284.6) C\H/C\C (284.6) C/O groups (288.8) TiC (280.7) C\H/C\C (284.3) C/O groups (288.8) TiC (280.7) C\H/C\C (284.2) C/O groups (288.8) TiC (280.7) C\H/C\C (284.3) C/O groups (288.8) C\H/C\C (284.3) C/O groups (288.8) C\H/C\C (284.3) C/O groups (288.8) TiC (280.7)
The species written in bold correspond to the most abundant found in each case.
shapes. These can be rationalised by considering that the laser has induced different reactions according to the conditions imposed by the environment. For the original sample, although the intensity of the Ti 2p signal is very small, it has enough intensity as to prove that titanium is in the form of TiO2. In light of the existent XPS literature on titanium metal, oxides and nitrides [10–14], it is possible to assign the observed features to the formation of the compounds summarised in Table 2. This table also reports similar assignments for the species/compounds involving the other atoms present on the surface. The most significant information gathered from Table 2 and from Fig. 2 is that the treatments under vacuum, H2 and Ar lead to a partial reduction of the Ti species present at the contaminated surface to Ti metal. This process is most effective under vacuum. With hydrogen and Ar, Ti4+ species typical of TiO2 are formed together with Ti(0). Interestingly, no intermediate oxidation states of titanium (i.e., Ti3+, Ti2+ or Ti+, with BE values which are intermediate between those of Ti4+ and Ti0 [15] are formed under these conditions. The use of oxygen and air provokes an almost complete oxidation of titanium to TiO2, while the use of nitrogen for this reaction leads to nitridation of the Ti with the consequent formation of titanium nitride. Some remaining Ti(0) observed points to the fact that nitridation is not complete under these conditions. The information derived from the carbon and oxygen peaks and the percentages reported in Figs. 2–5 and Tables 1 and 2 is complementary to that given earlier concerning the titanium and nitrogen peaks. In fact, oxygen peaks exhibit maximum intensity for the Ti sample treated under oxygen and air. Under such conditions the carbon intensity is small, as expected for the activation of combustion processes. It is also interesting that with vacuum, Ar, H2 and N2 (i.e., non-oxidising conditions), some carbide species are detected, as deduced from the appearance of a C1s peak at 287.4 eV (2). These carbide species are likely to form via reaction of Ti metal with the carbon contaminants present at the sample's surface.
depicted in Fig. 2 reveal formation of Ti(0). To enhance the level of nitridation, a more complex procedure, consisting of a two-step treatment process, has been attempted. Fig. 6 shows the Ti2p photoemission peak of the Ti foil subjected to a first irradiation in vacuum, followed by a second irradiation in N2 or in N2 + H2 mixtures. With this latter mixture, H2 might contribute to remove oxygen and carbon, thus enhancing the nitridation efficiency. Actually, the opposite was found with respect to the results with N2. Compositions of the resultant species in atomic percent are reported, for these two treatments, in Table 1. Several features deserve a comment with respect to the results given in Table 1 and Fig. 6. First of all, the fact that the vacuum + N2 treatment is the most effective in removing oxygen and carbon, leading simultaneously to a maximum incorporation of nitrogen to the surface. Observing the Ti2p spectrum in Fig. 6 it is also apparent that most titanium is in the form of TiN, with less contributions from the other forms of titanium (e.g., Ti(0), according to the shape of the peak). In contrast, the vacuum + (N2 + H2) treatment is less effective for the cleaning and nitridation of the surface. This is evidenced for the relatively higher concentration of oxygen and carbon atoms and by the lower amount of nitrogen remaining on the surface. This latter effect correlates with the fact that an important amount of Ti(0) species remains on the surface after the laser treatment (cf. Table 1 and Fig. 6).
3.2. Laser induced nitridation processes One of the goals of the present work is to establish effective reaction conditions to induce a complete nitridation of “as received” titanium foils by laser ablation. With this point in mind, the reaction has been studied just by irradiating the Ti specimen in N2. Although the nitridation is rather effective, the results
Fig. 6. Ti 2p photoelectron spectra of the Ti foil subjected to a two-step treatment process consisting of in-situ laser irradiation in vacuum followed by laser irradiation under 1.33 × 102 Pa of different gases.
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Fig. 7. Re-oxidation of a sample previously nitrided at 1064 nm under (vacuum + N2) conditions, following exposure to ambient atmosphere for one month. Spectrum centred around the BE corresponding to Ti2p (left) and N1s (right).
3.3. Long term stability of the nitride surface layers Most studies by XPS related to the laser nitridation of titanium deal with samples that are exposed to air after their treatment (i.e. “ex situ” studies). It is likely that this exposure may affect the surface composition by reaction with O2 and other species present in air. In order to study the main effects caused by these species on the nitride layer formed by in-situ laser nitridation, XPS spectra were recorded on samples whose spectra are shown in Figs. 2–5, upon exposure to the atmosphere. This resulted in a progressive oxidation of the surface, accompanied by an increased accumulation of contaminating carbon. Table 1 and Fig. 7 report, respectively, the surface composition and the spectra of the surface atoms for a nitrided sample that was kept in ambient air during one month. The spectra show the incorporation of oxygen, the loss of nitrogen and the accumulation of carbon on the surface layers examined by XPS. In addition, The Ti 2p spectra reveals the formation of TiO2 and some Ti(0), while some defective TiN must still remain, as deduced from the shape of the spectrum (continuous shape in the zone with a BE at ca. 455 eV, the presence of some components at this BE has been also proven by a fitting analysis not shown in the figure). The permanence of some nitride after this prolonged exposure to ambient air is also justified by the N1s spectrum, where a peak at 395.7 eV is
clearly visible. In addition, the shoulder at around 398.5 eV, observed in the same spectrum, can be attributed to the formation of a titanium oxinitride phase [16]. 3.4. Lateral and depth distribution of nitrogen in the laser treated samples The performed laser experiments consist on irradiating a reduced surface area of the treated metal, directly with the laser beam output. Since there are no focusing optics involved in this process, the diameter of the laser spot is ∼4 mm, in agreement with the ideal beam diameter output specified for this laser. An interesting issue is if the laser-induced surface reactions concentrate in the area of the spot or whether they extend to the surrounding zones. In other words, it is of interest to determine the reaction area as compared to the initial 4 mm diameter laser incidence zone. In order to study this surface spatial reactivity phenomenon, a series of experiments have been carried out using SEM and XPS with the samples that have been subjected to the vacuum + N2, two-step treatment. Fig. 8 shows SEM micrographs obtained from within the laser spot area. Both images shown exhibit the characteristic ablation microstructure expected from a high-energy multimode laser beam. The observed typical waves within the melted surface are indicative of high-energy maxima focalised into several points throughout the spot. This situation
Fig. 8. SEM micrographs obtained on the center (left) and edges (right) of the spot generated by irradiating a Ti metal foil “in-situ” under 1.33 × 102 Pa N2 with a 1064nm 5ns pulsed laser beam, inside a vacuum pre-chamber of an XPS apparatus. The morphology observed corresponds to typical ablation profiles previously described for the ns regime [15,16]. The scale bar corresponds to 20 μm.
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found for the surface composition distribution. This point was addressed by performing a depth profile analysis via Ar+ bombardment during increasing periods of time. Spectra were recorded in the centre of the spot and at a point 8 mm outwards, well outside the spot area. From these spectra the atomic percentages have been calculated and the N/Ti ratio represented in Fig. 10. An estimated error bar of 0.2 can be associated to the calculated values. This figure clearly shows that, although the nitrogen content in the surface is equivalent for the two sample zones observed, it does not follow the same trend in depth. In fact, this ratio decreases initially fast at the outer area, but it levels off after 5 min of thinning, keeping constant thereafter. In contrast, below the spot area it decreases more progressively during the first 20 min of thinning, levelling off thereafter at a much lower value than below the surrounding area. It must be stressed that practically no N remains in the centre of the spot after prolonged bombardment with Ar+ ions. 3.5. Reaction model
Fig. 9. Multimode intensity distribution expected for the Nd:YAG laser emission used in the present experiments (a). The overall summation of each of the maxima produces the Gaussian type effective energy distribution illustrated (b).
appears to be the trademark of a multimode beam laser ablation process, in contrast to the characteristic ‘crater’ cone shape morphology observed for TEM00 mode laser ablation [17,18]. Melting waves are wider in the centre of the spot (Fig. 8 — left) than in the border zone (Fig. 8 — right). This is due to the average summation of energy maxima that results in a nearly Gaussian global distribution of the laser irradiance, in spite of localised statistic distribution of maxima and minima in the multimode energy distribution. This effect implies higher energy contribution in the centre of the spot that leads to an increase in wave sizes of melted material. Fig. 9 shows an illustrative scheme of this energy distribution. Image (a) shows the characteristic energy profile of a multimode beam and image (b) shows the average sum of the profiles to achieve a pseudo Gaussian profile, which explains the special morphology obtained with this laser ablation process. XPS analysis has been carried out by focussing the electron analyzer onto the centre of the spot and by recording spectra at different lateral positions, relative to this centre and up to 10 mm outside it. It is interesting that quite similar spectra are recorded at any point, indicating that the surface composition is similar both, in the incidence spot and in the surrounding areas (characterized by the surface microstructure depicted in Fig. 8). Since XPS is a surface-sensitive technique, it seemed relevant to check whether the nitrogen content in the centre and in the surrounding areas follows a similar depth profile as
The above results clearly demonstrate that pulsed laser irradiation is very effective for cleaning the surface of titanium metal even if it is heavily contaminated from its exposure to ambient air and to mechanical deformation additives. These results show, in addition, that the surface composition of the Ti metal can be modified during the cleaning treatment if the surrounding atmosphere is well controlled. In fact, this work demonstrates that oxidation and nitridation processes can be effectively induced with “as received” titanium foils by following an appropriate reaction protocol. In particular, they show that nitridation of “as received” Ti foils may be carried out conveniently and effectively performing an initial laser irradiation in vacuum, followed by a second one under nitrogen. The results show, in addition, that irradiation in vacuum leads to the removal of a substantial part of the carbon contamination layer, while the irradiation in nitrogen further contributes to the removal of the remaining contamination (i.e., O, C, see Table 1) and produces a very effective incorporation of nitrogen into the substrate, in the form of titanium nitride.
Fig. 10. N to Ti ratio as a function of the Ar+ sputtering time in a Ti foil irradiated in vacuum and then under 1.33 × 102 Pa N2 at the centre of the laser spot and in the periphery (8 mm away).
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First layers of titanium nitride are not stable after long exposure of the samples to ambient air. It is thus found that the outermost layers become partially oxidised. These are typically found to contain titanium oxide and oxynitride, together with some Ti(0). This surface composition suggests that the surface layers of titanium nitride are partially decomposed by oxidation and/or hydrolysis in the ambient atmosphere. This effect must also occur at the outermost surface layers of the thicker TiN coatings prepared by other, more energy intensive laser treatments, as well as by alternative techniques. For example, the presence of TiN, TiOxNy and TiO2 has been reported recently at the surface of TiN coatings obtained by dc magnetron sputtering [19]. These authors established that TiN was predominantly obtained at high N2 concentration and film thickness, consistent with the results found in this work. Concerning the spatial distribution of the nitrides produced during laser irradiation, it may be emphasised that the N/Ti ratio is found similar in values, or even higher at considerable distances from the centre of the incident beam spot. It is also particularly interesting that, according to the depth profiles in Fig. 10, N is only located in the outmost surface layers of the centre of the spot. This phenomenon is not surprising and may be discussed in terms of the reactivity and physical phenomena related to the presence of an intense plasma. We can understand the previous result by assuming the following reaction model. The break down of gaseous species at the irradiance level reached, combined with the intense shock waves associated to the recoil pressure of the plasma – which should be most intense at the spot – may well explain the surface and in-depth N/Ti compositional profiles shown in Fig. 10. On the one hand, species break-down causes formation of the Ti nitride above the substrate. Part of these species is compressed by the shock waves onto the spot area, and another part is dispersed radially outwards into the surrounding area. They are thus found in both zones with a similar surface distribution. Their in-depth distribution, however, is modified by the expected removal of transient melt from the centre of the spot, as illustrated in the scheme of the reaction model presented in Fig. 1, where a typical ablation process is represented within an illustration of the ablation procedure employed in this work. In this particular case, a very limited melt layer may be formed as expected for nanosecond regime ablation [17,18]. The intense shock waves should follow a similar profile as that given in Fig. 9. They would thus push the melt radially outwards, away from the spot area, thus contributing to the observed thicker nitrided layers localised away from the laser spot, as well as to the local microstructure and morphology observed in Fig. 8. 4. Conclusion Surface cleaning of Ti metal has been demonstrated for the first time in an in-situ configuration, inside the pre-chamber of an XPS apparatus, by introducing a Nd:YAG laser emitting at 1064 nm in pulsed mode. In addition, laser-induced reactivity of the Ti surface has been studied under Ar, O2, synthetic air, N2 and H2. The results obtained indicate that, on the one hand, a high amount of the initial carbon contamination layer was removed. On the other hand, Ti nitridation was achieved by
laser irradiation under nitrogen. The most effective sequence included an initial cleaning procedure induced by irradiation in vacuum, followed by a second irradiation process performed under nitrogen. Partial nitridation is also observed when irradiating under synthetic air. Lateral and depth analysis of the nitrogen concentration around the laser spot indicates that the surface concentration of nitride is similar within a wide area away from the spot and that the depth profile is different at the spot and several mm away, where more nitride is present. Within the deep layers located below the centre of the laser spot, only small amounts of nitrogen are found. A model is consequently proposed to account for the observed titanium surface cleaning and nitridation processes. Acknowledgements The authors acknowledge the Spanish Ministry of Education and Science for financial support through Project no. MAT 2004-01558 and INGESNET (Surface Engineering Network), as well as the Government of Aragón (DGA) for partial support of the Laser Materials Processing Group through its Consolidated Group Programme. References [1] I. García, J.J. de Damborenea, Corros. Sci. 40 (1998) 1411; M.S.F. Lima, F. Folio, S. Mischler, Surf. Coat. Technol. 199 (2005) 83. [2] J.M. Lackner, W. Waldhauser, R. Berghauser, R. Ebner, B. Major, T. Schöberl, Thin Solid Films 453–454 (2004) 195; C.Y.H. Lim, S.C. Lim, K.S. Lee, Tribol. Int. 32 (1999) 393; R.R. Manory, A.J. Perry, D. Rafaja, R. Nowak, Surf. Coat. Technol. 114 (1999) 137. [3] H.C. Man, Z.D. Cui, X.J. Yang, Appl. Surf. Sci. 199 (2002) 293. [4] B.S. Yilbas, J. Nickel, et al., Wear 212 (1997) 140; P. Jiang, et al., Surf. Coat. Technol. 130 (2000) 24; C. Langlade, A.B. Vannes, J.M. Krafft, J.R. Martin, Surf. Coat. Technol. 100–101 (1998) 383; E. Bemporad, M. Sebastiani, C. Pecchio, Surf. Coat. Technol. 201 (2006) 2155. [5] E. György, A. Pérez del Pino, P. Serra, J.L. Morenza, Appl. Surf. Sci. 186 (2002) 130. [6] A. Pérez del Pino, P. Serra, J.L. Morenza, Thin Solid Films 415 (2002) 201. [7] J.M. Lackner, Vacuum 78 (2005) 73. [8] A.I.P. Nwobu, R.D. Rawlings, D.R.F. West, Acta Mater. 47 (1999) 631. [9] B. Gakovic, I. Pongrac, S. Petrovic, D. Minic, M. Trtica, Mat. Sci. Forum 518 (2006) 161; R.K. Thareja, A.K. Sharma, Laser Part. Beams 24 (2006) 311. [10] Milosev, H.-H. Strehblow, B. Navinsek, M. Metikos-Hukovic, Surf. Interface Anal. 23 (1995) 529. [11] L.I. Johansson, Surf. Sci. Rep. 21 (1995) 177. [12] J. Zhao, E. Gene Garza, K. Lam, C.M. Jones, Appl. Surf. Sci. 158 (2000) 246. [13] I.leR. Strydom, S. Hofmann, J. Electr. Spectrosc. Relat. Phenom. 56 (1991) 85. [14] I. Bertóti, Surf. Coat. Technol. 151 (2002) 194. [15] D. Leinen, A. Fernández, J.P. Espinós, A.R. González-Elipe, Appl. Phys., A 63 (1996) 237. [16] I. Bertóti, R. Kelly, M. Mohai, A. Tóth, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 80/81 (1993) 1219. [17] D. Bäuerle, Laser Processing and Chemistry, Springer-Verlag, 1996. [18] H.-G. Rubahn, Laser Applications in Surface Science and Technology, J. Wiley & Sons, 1999. [19] Y. Jeyachandran, S.K. Narayandass, D. Mangalaraj, S. Areva, J.A. Mieluarski, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 445 (2007) 223.