Reactive laser plasma coating formation

Surface & Coatings Technology 200 (2005) 608 – 611 www.elsevier.com/locate/surfcoat

Reactive laser plasma coating formation Peter Schaaf T, Michael Kahle, Ettore Carpene Universita¨t Go¨ttingen, Zweites Physikalisches Institut, Friedrich-Hund-Platz 1, 37077 Go¨ttingen, Germany Available online 19 February 2005

Abstract Reactive laser plasma surface treatments have proven to have a large potential for technical applications. Here, we present a new promising fast, flexible and clean technique for a direct laser synthesis of carbide and nitride coatings by short pulsed laser irradiation in reactive atmospheres (e.g. methane, nitrogen). The corresponding material is treated by short but intense laser pulses involving a plasma formation just above the irradiated surface. Gas–Plasma–Surface reactions lead to a fast incorporation of the gas species into the material and subsequently to the desired coating formation if the treatment parameters are chosen properly. This reactive laser synthesis can also be used for the improvement of steel surfaces, achieving drastically improved hardness, corrosion and wear resistance. We report here on the irradiation of pure iron in methane atmosphere. At low number of laser pulses, this results mostly in the formation of hexagonal e-Fex C, which is more and more transformed into cementite (h-Fe3C) when increasing the number of pulses. D 2005 Elsevier B.V. All rights reserved. PACS: 52.50.Jm; 81.65.Lp; 61.80.Ba; 62.50.tp Keywords: Laser plasma nitriding; Laser plasma carburization; Iron nitrides; Iron carbides; Silicon carbide; Titanium nitride

1. Introduction Thin films and surfaces are playing a more and more important role in the application of smart materials. This covers the improvement of surface properties like hardness, corrosion resistance, wear resistance, friction, electrical, thermal, magnetic, and optoelectronic properties for specific applications [1,2]. Many methods evolved for the preparation of thin films and coatings. Laser surface treatments are now used for many years in order to improve the surface quality of tools and pieces, e.g. hardness, wear and corrosion resistance. Many methods, like laser hardening, re-melting, alloying, have been investigated and established for that purpose. Irradiation of surfaces with short laser pulses of high intensity in a reactive atmosphere can result in a direct coating formation if the treatment parameters are properly adjusted [3,4]. This Laser Plasma Synthesis, e.g. laser nitriding of aluminum and titanium or laser carburization of T Corresponding author. Tel.: +49 551 39 7672; fax: +49 551 39 4493. E-mail address: [email protected] (P. Schaaf). URL: http://www.uni-goettingen.de/~pschaaf. 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.01.028

iron, is an interesting process both for basic physics as well as industrial technology. The interaction between the molten substrate and the plasma produced by the laser in the ambient atmosphere is the basis for an effective treatment. Nevertheless, the physical mechanisms that lead to the formation of the new phases are not yet fully understood due to the complex phenomena involved [3]. The treatment performed in nitrogen atmosphere (laser nitriding) has been largely investigated due to the technological importance of nitrogen in metals and alloys [3,5–7]. For Ti even the formation of stoichiometric, adherent layers of TiN was reported [8]. The laser nitriding of aluminum and AlSi alloys was investigated by Barnikel and coworkers [9], while Sicard et al. [10] and Carpene et al. [11] reported the formation of AlN by excimer laser irradiation in nitrogen gas. The latter also produced a pure cementite layer by irradiating pure iron in pure methane [4]. Laser pulses ranging from femtosecond to nanosecond have been used to investigate their influence on the LaserPlasma Synthesis [12]. A number of laser types have been used for that (Excimer Laser, Nd:YAG, Ti:sapphire, Free Electron Laser) and a number of different nitride and carbide films have been

P. Schaaf et al. / Surface & Coatings Technology 200 (2005) 608–611 beam attenuator XeCl excimer laser λ=308 nm tp=55 ns

LASER BEAM

45° mirror lens/ homogenizer gas/ vacuum

quartz window chamber sample

X-Y stage

Fig. 1. Sketch of the laser irradiation experimental setup.

successfully produced (e.g. TiC, TiN, AlN, Fe3C, SiC) [3,4,11–16]. Here, the underlying mechanisms and some examples will be presented for iron irradiated in methane.

2. Experimental Pure iron (Armco) samples with 1.5 mm thickness were used. They were mechanically polished with SiC grinding paper followed by 1 Am diamond paste. A Siemens XP2020 XeCl Excimer laser with k=308 nm, 55 ns (FWHM) pulse duration, and 8 Hz repetition rate was used for the laser treatments. The laser fluence H=E/A was set to the desired value (0.5 to 5 J/cm2) with a spot size of A=5
5 mm2 employing a focusing beam homogenizer and a variable beam attenuator for changing the total pulse energy E [3]. The laser experimental setup is sketched in Fig. 1. For the irradiations, the samples were mounted in a chamber, which was first evacuated to less than 103 Pa and then filled with methane (99.5%) at a pressure of 1.5 bar prior to the treatment. Large sample areas were treated by meandering (i.e. scanning) the surfaces relative to the laser beam by a motorized and computer-controlled sample stage [3]. The shift of each laser spot was set according to the desired overlap and the number of laser shots. With this meandering treatment, the sample was shifted after each laser pulse in such a way that the displacement was a fraction of the spot size. If a is the size of the laser spot, the

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sample shift Dx=a/n in the x direction and Dy=a/m in the y direction lead to the meander treatment termed n
m, i.e. each area element of the sample surface is irradiated with nd m laser shots in toto. The carbon profiles have been obtained by Rutherford Backscattering Spectrometry (RBS) with a 900 keV He++ beam and a backscattering angle of 1658. The data analysis was performed with the RUMP code [17]. Phase analysis was done by X-ray diffraction in Bragg–Brentano geometry and under 1–158 grazing incidence (GIXRD) employing Cu-Ka radiation. Additionally, the samples have been analyzed by Conversion Electron and Conversion X-ray Mo¨ssbauer Spectroscopy (CEMS and CXMS). CEMS has an information depth of about 150 nm, whereas CXMS analyses a surface layer of about 10 Am. Finally, the surface roughness was measured by a Dektak profiler and the hardness depth profile was determined employing a nanoindenter Fischerscope HV100 using a Vickers diamond [3].

3. Results and discussion Due to the technological importance of carbon in iron, a carbon-containing atmosphere was chosen. The following will focus on the results of the meandering irradiation in methane (CH4) gas and the influence of experimental parameters, such as the spot overlap, on the mechanical and crystallographic properties will be illustrated. The preliminary investigation on the efficiency of the carbon incorporation has been performed with fixed laser fluence (4 J/cm2) and fixed methane gas pressure (1.5 bar), varying the spot overlaps of the meandering treatment. The CEM spectra of the samples treated with 8
8, 11
12 and 16
16 meander scans are shown in Fig. 2. The phase fractions resulting from these CEMS analyses are shown in Fig. 3a. The 8
8 scan produces a mixture of cFe(C) (austenite), e-Fex C and non-reacted e-Fe. The phase is fitted using a singlet [isomer shift dg0.06(1) mm/s] representing an iron site without carbon neighbors, and a doublet [dg0.02(1) mm/s, quadrupole splitting eg0.65(2) mm/s] due to an iron site with one carbon nearest neighbor.

Fig. 2. CEM spectra and corresponding fits of the iron samples irradiated in 1.5 bar CH4 with (a) 8
8, (b) 11
12 and (c) 16
16 spot overlap (from left to right). The thin lines show the subspectra used for the fit.

P. Schaaf et al. / Surface & Coatings Technology 200 (2005) 608–611

i

i

610

Fig. 3. Fe samples laser irradiated in 1.5 bar CH4: (a) phase fraction vs. the spot overlap as obtained from the CEMS analysis (left); (b) the corresponding GIXRD diffractograms measured at grazing angle of 58 (right). All main XRD peaks are labeled.

The relative area of the singlet A s and the doublet A d can be related to the carbon occupancy of the interstitial octahedral sites, and the average carbon content y in FeCy can be estimated according to the relation [3]: 
1 As 1 y¼ ; ð1Þ 6 As þ Ad obtaining yg0.12. This value is higher than the maximum solubility of carbon in c-Fe under equilibrium conditions, but the laser irradiation produces extreme pressures and temperature gradients that can lead to farfrom-equilibrium solid solutions. The e-Fex C is represented by two sextets with hyperfine fields of B 1~28 T and B 2~21 T. They are attributed to the hexagonal arrangement of iron atoms with one (e-Fe6C) and two (e-Fe3C) neighboring interstitial carbon atoms, respectively. The relative fraction of the two components allows to determine the average stoichiometry of the phase, obtaining x=4.33. With the 11
12 meander scan, no austenite is observable with CEMS, although a mixture of c and e phases is visible with GIXRD (grazing incidence of 58), as shown in Fig. 3b. Since the information depth of GIXRD with the incidence angle of 58 is about 0.8 Am, while CEMS is sensitive to the first 150 nm from the surface, we can conclude that the austenite is not formed at the surface of the sample, but deeper inside, most probably close to the maximum melting depth, where the carbon content is lower. Opposite to the austenite, some h-Fe3C appears in the CEM spectrum, but not in the X-ray diffractogram, indicating that its formation is limited to the surface (first 150 nm) of the specimen. Notice that in the sample meandered with the 8
8 scan, only the austenite is visible with XRD, while both the c and the e phases are observed in the CEM spectra, suggesting that the latter is mainly confined on the sample surface. The treatment performed with the 16
16 spot overlap reveals the formation of a cementite (h-Fe3C) layer and no other carbide is observed by means of CEMS and GIXRD.

The mechanical properties of the irradiated samples have been investigated by nanoindentation, and the hardness curves are reported in Fig. 4. Jo¨nsson and Hogmark [18] developed a model to estimate the composite hardness H(d) of a coating on a softer substrate as a function of the indentation depth. According to their analysis the hardness is obtained by: " 2# t t 2 : ð2Þ H ðd Þ ¼ Hs þ ðHc  Hs Þ 2k  k d d

ðÞ

H c and H s are respectively the hardness of the coating and the substrate, d is the indentation depth, t is the coating thickness and k (with 0.073bkb0.14) is a parameter describing the plastic deformation of the film as the diamond tip indents the sample. For simplicity we assumed k=0.1. The model works properly if the ratio d/tN0.1; since the thickness t of the carburized layer should not exceed the melting depth (g1 Am), the model can be applied to indentation depths dN100 nm. The continuous lines in Fig. 4 are the fits of the hardness profiles according to Eq. (2), using H s, H c and t as fitting parameters. The results are reported in Table 1.

Fig. 4. Microhardness curves of the Fe samples meandered in 1.5 bar CH4 (symbols). The continuous lines are fits according to Eq. (2).

P. Schaaf et al. / Surface & Coatings Technology 200 (2005) 608–611 Table 1 Results of the hardness profiles fits Meandering

H s [MPa]

H c [MPa]

t [Am]

8
8 11
12 16
16

1761 (7) 1701 (10) 1588 (3)

4888 (85) 5668 (70) 7927 (20)

0.95 (3) 1.04 (3) 0.91 (1)

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influence of the number of laser pulses on the structural and mechanical properties of the materials. With increasing number of laser shots the produced phases change form austenite to hexagonal carbide to cementite.

Acknowledgements The substrate hardness H s is ~1700 MPa for all samples and the thickness t of the layer is ~0.95 Am, in good agreement with the calculated melting depth. The hardness H c of the carburized film increases with the spot overlap and therefore with the amount of cementite (the hardest carbide [19]). Despite the fact that the model was developed for homogeneous coatings on softer substrates, the fits are quite satisfactory and the average hardness of the carburized layers is consistent with the phase analysis. Since the mechanical properties and the crystallographic phases obtained with the 8
8 meander scan in CH4 closely resemble what has been observed with nitrogen gas under identical experimental conditions (same spot overlap, same pressure and laser fluence) [20], the further investigation has been focused on the higher spot overlap. With the 11
12 scan, a complex overlap of magnetic phases has been observed with Mo¨ssbauer spectroscopy. In order to achieve a better understanding of the carburization process we performed the same meandering treatment at various methane pressures ranging from 0.1 bar to 10 bar. The results have to be reported elsewhere [21]. As already mentioned, with the 16
16 meander scan at an intermediate methane pressure (1.5 bar), the only ironcarbide formed in the irradiated sample is h-Fe3C. This has been reported in [4].

4. Summary The reactive laser processing of aluminum, iron, stainless steel, or silicon substrates in reactive nitrogen or methane atmospheres is an efficient method to incorporate nitrogen/ carbon into the irradiated material, and it represents a valuable alternative to other nitriding and carburizing techniques. It is shown that the irradiation of iron in methane produces various carbon containing phases. The detailed analysis of the irradiated samples revealed the

This work is supported by the Deutsche Forschungsgemeinschaft under grant DFG Scha 632.

References [1] D. B7uerle, Laser processing and chemistry, Springer Verlag, Berlin, 2000. [2] R.W. Cahn, P. Haasen, Physical metallurgy, North-Holland Physics Publishing, Amsterdam, 1983. [3] P. Schaaf, Prog. Mater. Sci. 47 (1) (2002) 1. [4] E. Carpene, P. Schaaf, Appl. Phys. Lett. 80 (2002) 891. [5] E. Carpene, F. Landry, P. Schaaf, Appl. Phys. Lett. 77 (2000) 2412. [6] P. Schaaf, F. Landry, K.-P. Lieb, Appl. Phys. Lett. 74 (1) (1999) 153. [7] F. Landry, K.-P. Lieb, P. Schaaf, J. Appl. Phys. 86 (1999) 168. [8] E. D’Anna, M.L. De Giorgi, G. Leggieri, A. Luches, M. Martino, A. Perrone, I.N. Mihailescu, P. Mengucci, A.V. Drigo, Thin Solid Films 213 (1992) 197. [9] J. Barnikel, T. Seefeld, K. Schutte, H.W. Bergmann, H7rterei-Tech. Mitt. 52 (1997) 94. [10] E. Sicard, C. Boulmer-Leborgne, T. Sauvage, Appl. Surf. Sci. 129 (1998) 726. [11] E. Carpene, P. Schaaf, Phys. Rev., B 65 (2002) 224111. [12] P. Schaaf, M. Han, K.-P. Lieb, E. Carpene, Appl. Phys. Lett. 80 (2002) 1091. [13] E. Carpene, A.-M. Flank, A. Traverse, P. Schaaf, J. Phys. D: Appl. Phys. 35 (2002) 1428. [14] E. Carpene, P. Schaaf, M. Han, K.-P. Lieb, M. Shinn, Appl. Surf. Sci. 186 (2002) 195. [15] M. Schwickert, E. Carpene, M. Uhrmacher, K.-P. Lieb, P. Schaaf, Appl. Phys., A 77 (2003) 793. [16] M. Schwickert, E. Carpene, K.-P. Lieb, M. Uhrmacher, P. Schaaf, Appl. Phys. Lett. 84 (2004) 5231. [17] L.R. Doolittle, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 15 (1986) 227. [18] B. Jo¨nsson, S. Hogmark, Thin Solid Films 114 (1984) 257. [19] M. Ron, in: R.L. Cohen (Ed.), Applications of Mo¨ssbauer spectroscopy, vol. II, Academic Press, New York, 1980, p. 329. [20] M. Han, E. Carpene, F. Landry, K.-P. Lieb, P. Schaaf, J. Appl. Phys. 89 (2001) 4619. [21] M. Kahle, E. Carpene, P. Schaaf, Phys. Rev., B, in preparation.