Surface nitriding of titanium and aluminium by laser-induced plasma

Surface nitriding of titanium and aluminium by laser-induced plasma

Surface and Coatings Technology 97 (1997) 448–452 Surface nitriding of titanium and aluminium by laser-induced plasma A.L. Thomann a,*, E. Sicard a, ...

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Surface and Coatings Technology 97 (1997) 448–452

Surface nitriding of titanium and aluminium by laser-induced plasma A.L. Thomann a,*, E. Sicard a, C. Boulmer-Leborgne a, C. Vivien a, J. Hermann a, C. Andreazza-Vignolle b, P. Andreazza b, C. Meneau b ´ ´ ´ a GREMI, URA 831, Universite d’Orleans, B.P. 6759, 45067 Orleans, Cedex 2, France ´ ´ ´ b CRMD, UMR 0131, Universite d’Orleans, B.P. 6759, 45067 Orleans, Cedex 2, France

Abstract In this work we study a nitriding technique for titanium and aluminium by means of laser-induced plasma. The synthesised layers are composed of a nitrogen concentration gradient over several mm depth, and are expected to be useful for tribological applications with no adhesion problems. The method is tested on the synthesis of titanium nitride, and the role of the laser wavelength and pulse time duration is inspected by comparing layers obtained with two kinds of pulsed laser: a CO laser (l= 2 10.6 mm) and an XeCl excimer laser (l=308 nm). The laser-plasma treatment is then applied to the superficial nitriding of aluminium, which is known to be difficult to carry out. Comparison is made between the nitriding process efficiencies of these two metals. © 1997 Elsevier Science S.A. Keywords: Laser induced plasma; Nitride; Surface treatment; Hard coating

1. Introduction Material surfaces for tools or industrial pieces require special attention as they are often subjected to corrosive environments or mechanical wear. In many cases, it is interesting to confer specific superficial properties on the material for a given application (friction resistance, corrosion resistance, etc.). Diffusion coatings may be good candidates in such instances because of their chemical concentration gradients. There exist different laser treatments leading to the formation of diffusion layers of nitrides or carbides, for example. The element to be added is brought in gaseous or spray form and the laser (often a pulsed or continuous CO laser) is 2 used to melt the material surface [1]. In this work, we study nitrogen incorporation into metals from a nitrogen gas atmosphere by means of laser irradiation, which induces the creation of a plasma above the metal surface. This leads to the synthesis of a nitride layer at the metal surface [2]. The formation of nitrides is of great interest since these compounds exhibit better tribological properties than pure metals, such as hardness, wear resistance and chemical stability in corrosive environments [3]. In a previous work [4], the effect of the laser wavelength and pulse time duration (CO and XeCl lasers) was 2 * Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 19 9 - 0

studied; the main conclusions are summarised in Section 2. In Section 3 the transfer of this process to aluminium nitride synthesis is presented.

2. Nitriding method, titanium case 2.1. Description of the nitriding method In the method of interest, a laser beam is focused on a titanium target in a 760 torr nitrogen atmosphere (see Fig. 1). In order to study the effect of laser wavelength on the nitriding process, two kinds of pulsed laser are used: a CO laser emitting in the infra-red (IR) range 2 (10.6 mm, temporal pulse shape: 300 ns half-height width peak followed by a 70% energy-containing 2 ms tail ) and an XeCl excimer laser with a wavelength belonging to the ultraviolet ( UV ) domain (308 nm, trapezium temporal pulse shape of 28 ns half-height width). The laser action on the metal surface results in heating, melting, vaporisation and plasma formation. Since the laser/material interaction process depends on the laser, the required fluences (energy per surface unit) to obtain nitride synthesis are different. In the case of the CO 2 laser, the delivered fluence is 10–25 J cm−2, while the XeCl laser fluence is in the 1–4 J cm−2 range. Study of the nitriding process must be performed, on the one

A.L. Thomann et al. / Surface and Coatings Technology 97 (1997) 448–452

Fig. 1. Experimental set-up.

hand, by characterising the plasma and, on the other hand, by analysing the surface that has been in contact with the plasma. The synthesised layers are analysed by means of complementary techniques, giving information on the chemical composition, crystallographic structure and surface morphology. Depending on the laser used (CO or XeCl ), the characteristics of the laser/material 2 interaction, which is related to the wavelength and the pulse time duration, are different. As a result, the target temperature variation under laser heating differs, as well as the composition of the plasma created. The influence of both these aspects is discussed in this paper. The effective treated surface corresponds to the plasma spot (4–6 mm2), which is larger than the laser impact. In order to treat extended areas (1 cm2), many successive plasma spots are set side by side. Each irradiated area is submitted to a desired laser shot number in the range 100 to 4000.

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2.2.2. Plasma creation and characterisation According to the different laser wavelengths and to the laser/metal interaction features [5,6 ], the ionisation processes leading to the creation of a plasma over the surface are different depending on the laser [7]. Spaceand time-resolved spectroscopy measurements have revealed great differences in plasma composition and time scale (related to the time duration of the laser pulse). This is illustrated in Fig. 2 by spectral line kinetics of the excited species present in each plasma. It should be pointed out that in our experimental conditions no spectral line self-absorption has been observed. In the case of CO irradiation, an absorbing nitrogen 2 breakdown plasma is created, shielding the metal surface from the laser beam action and especially evaporation ( Fig. 2a). On the contrary, with the XeCl laser, the plasma is created in the metallic vapour ( Fig. 2b). However, some nitrogen excited species have been detected in this erosion plasma too [8]. An example of electron density (N ) evolution with time is given in e Fig. 3. N is obtained from Stark broadening of N or e Ti spectral lines, depending on the laser-induced plasma. The Stark broadening effect is known to prevail in highly ionised plasmas [9] and is tabulated for both species of interest [8,10]. An ionisation phase occurring

(a)

2.2. Laser/materialinteractionandplasmacharacterisation 2.2.1. Laser/material interaction features Beside other parameters, the reflectivity R and the absorption length d can be used to characterise a laser/metal interaction [5]. Both depend on the laser wavelength and on the metal characteristics. In the case of a titanium target at 10.6 mm and 308 nm, R=0.96, d =41 nm and R=0.39, d =15 nm, respectively. In the a a IR case the majority of the laser intensity is reflected and the interaction involves a large depth. Temperature calculations have shown that the titanium is deeply heated and for a long time under CO irradiation [4]. 2 Conversely, in the UV range, better laser energy absorption occurs over a smaller thickness, resulting in an intense superficial evaporation process.

(b)

Fig. 2. (a) Typical spectral line kinetics of the excited species present in the CO laser-induced breakdown plasma. (b) Typical spectral line 2 kinetics of the excited species present in the XeCl laser-induced erosion plasma.

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Fig. 3. Temporal evolution of the electron density (N ) in the case of e CO laser-induced plasma. N is determined from the spectral line 2 e Stark broadening of an ion which is present early in the plasma and of a neutral existing at the end of the plasma.

during the laser pulse cannot be evidenced; this curve only shows N variation during the recombination phase. e The maximum values are 1017 to 1018 cm−3. The mean electron temperature, kT , can also be evaluated to be e about 3 eV in this phase [4]. It is interesting to note that both N and kT have been found of the same order e e of magnitude whatever the laser used. This shows that the main difference observed between both laser interactions is the plasma’s chemical composition. 2.2.3. Coating analysis Surface chemical composition has been studied by X-ray photoelectron spectroscopy ( XPS), which reveals the effective synthesis of TiN and exhibits the presence of oxidised species ( TiO , TiO N ). These oxides are 2 x y usually formed over TiN surfaces exposed to air [11]. The in-depth (over several mm) chemical dosage has been examined by Rutherford backscattering spectroscopy (RBS ), revealing a nitrogen concentration gradient and, in the case of CO laser-synthesised samples, 2 in-depth oxygen contamination, as well as large thickness (5 mm versus 2 mm when XeCl laser is used ). In addition, in-depth analysis of crystalline phases has been performed with grazing-incidence X-ray diffraction (GIXRD). In both cases a-Ti, a-Ti(N ) solid solution and d-TiN phases have been detected; with the CO 2 laser, however, the nitrided phases are mixed over a large depth whereas, in the case of XeCl laser, d-TiN is mainly present close to the surface, followed by a-Ti(N ) and then a-Ti. Surface states have been also characterised by roughness measurements, which show the high rugosity present on the CO laser-synthesised samples 2 (2 mm). The efficient nitriding process and the in-depth oxygen contamination, observed in the case of CO laser-plasma 2

treatment, are explained by the long and deep laser heating of the Ti target (promoting TiN synthesis and oxygen diffusion), and by the restricted evaporation process as a result of the plasma shielding effect. In the case of the XeCl laser-induced erosion plasma, target evaporation competes with the nitrogen incorporation process, limiting the TiN layer thickness, and also acts like an in situ surface cleaning process (oxygen removal ). The large surface roughness observed when the CO 2 laser is used is due to the formation of a detonation wave during the plasma propagation phase, the violent recoil action of which causes on the surface much damage. The layer analysis results provide evidence for the important part played by the nitrogen diffusion process. The nitride layers are obtained by progressive incorporation and diffusion of nitrogen into titanium [4]. This mechanism only takes place in the near-surface region for XeCl laser-irradiated samples, and involves a more extended depth in the case of CO laser-plasma treat2 ment. Moreover, it has been shown that the plasma is necessary for nitrogen incorporation into titanium and TiN synthesis. Its creation above the surface allows a better coupling between primary energy provided by the laser beam and the target (N dissociation, extreme UV 2 radiation) [5]. Comparison between both kinds of laser shows that the nitriding of a titanium target may be efficiently performed in both cases. However, the corresponding layers exhibit some differences (depth, oxygen contamination, roughness), correlated to the features of the initial laser/material interaction and to the kind of plasma created above the target surface.

3. Aluminium nitride synthesis After this study on titanium, the nitriding process was transferred to aluminium metal, which is of interest for industrial applications. In this case, the reflectivity coefficient at 10.6 mm is very high (R=0.99), so AlN synthesis with the CO laser could not be carried out. 2 With the XeCl laser, owing to the particular laser interaction with the aluminium target [high reflectivity coefficient (0.92), low evaporation temperature, large thermal diffusivity], the laser fluence required to perform the nitriding treatment must be larger (4 to 10 J cm−2) than in the case of titanium. Consequently, heterogeneity of the spatial energy distribution in the plasma spot has been observed. To avoid this problem, several samples have been synthesised with overlapping of the plasma spots. Results from emission spectroscopy are given in Fig. 4. It is shown that, as in the XeCl/titanium case, only metal spectral lines are detected, with no selfabsorption. Thus, in spite of the high reflectivity of

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Fig. 4. Typical line kinetics of the main excited species present in the erosion plasma induced by XeCl laser over an Al target.

aluminium, the evaporation process is very efficient and an erosion plasma is also created in front of the aluminium surface. As for titanium, oxidised species (Al O and 2 3 AlN O ) have been detected by XPS at the surface. x y From electron diffraction, AlN wurtzite, Al cubic and oxynitride phases have been evidenced at 5 J cm−2 laser fluence ( Fig. 5a). The presence of pure metal at the near-surface region was not observed with a titanium target, and may indicate an incomplete nitriding process of the aluminium atoms. The in-depth phase distribution, illustrated by the GIXRD spectra in Fig. 6, is very surprising. The average AlN concentration is low close to the surface (100 nm) and then increases towards 1–2 mm thickness, while the aluminium concentration decreases. At depths >2 mm this trend is inverted, with the AlN phase concentration being largest at a thickness of #1–2 mm under the surface. This may be explained by decomposition of an initially synthesised AlN layer under further laser irradiation. Indeed, it has been shown that an AlN layer submitted to UV laser irradiation is

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easily decomposed with the formation of aluminium metal islands at the surface [12]. Fig. 5b presents the electron diffraction pattern of a sample submitted to 1000 laser shots but with 80% overlapping of the plasma spots at 10 J cm−2. The pure aluminium phase can no longer be observed in this figure, and the presence of an amorphous AlN phase is indicated. This may be due to an intense evaporation phenomenon occurring for fluences above 6 J cm−2, which leads to evaporation of AlN and its redeposition on the surface in an amorphous form [13]. The influence of the overlapping has been studied by scanning electron microscopy (SEM ) and roughness measurements. For the same total number of laser shots, samples synthesised with 50% or 80% overlapping, compared with nonoverlapped ones, exhibit better surface homogeneity but high roughness, possibly due to the redeposition process. This first study on aluminium nitriding by a laserplasma treatment method shows that superficial AlN synthesis can be carried out under UV laser irradiation. However, in comparison with titanium nitriding, the process is more difficult to control because of the apparent fragility of the AlN compound. This ability of AlN to be easily decomposed or evaporated by the XeCl laser has to be related to its very low reflectivity value at 308 nm [14]. Whereas TiN better reflects the 308 nm line than titanium, AlN absorbs a larger part of the laser beam energy than aluminium, which strongly modifies the laser/surface interaction. This trend may explain the difference between the nitriding process efficiencies for these two metals.

4. Conclusions The result presented in this paper provide evidence for the successful nitriding of titanium and aluminium targets by the laser-induced plasma method. In a first

Fig. 5. Selected-area diffraction patterns of samples submitted to 1000 XeCl laser shots: (a) 5 J cm−2 without overlapping, (b) 10 J cm−2 with 80% overlapping. AlN wurtzite, Al cubic and one of the AlN O phases are noted. x y

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Fig. 6. Typical evolution of main phases at increasing incidence angle, i.e., analysis depth, of an XeCl laser-treated Al target. AlN concentration is low for (A) 10 nm analysis depth and (B) 100 nm, then increases for (C ) 700 nm and (D) 1500 nm before decreasing for (E ) 3000 nm.

study on titanium, the diffusion of nitrogen into the metallic lattice was found to be the main process in this nitriding method. The creation of a plasma has been found to be necessary for the nitriding process. The synthesised layers exhibit differences that depend on the pulsed laser used (CO or XeCl laser). 2 In the case of aluminium substrates, the nitriding process appears to be more difficult to perform because of the fragility of AlN under UV irradiation. Nevertheless, with precise control of the treatment parameters ( laser fluence, shot number, overlapping) it should be possible to synthesise AlN layers with the special properties needed for a particular application. Owing to the laser technology, the nitriding treatment can be carried out on a restricted area with no piece size enhancement, making this method attractive for industry.

Acknowledgement We wish to thank G. Blondiaux for RBS measurements and H. Szwarckopf-Estrade for XPS analyses. This work was supported by PSA industry and the Region centre.

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