Mechanical and tribological properties of CNx films deposited by reactive pulsed laser ablation

Diamond and Related Materials 11 (2002) 98–104

Mechanical and tribological properties of CNx films deposited by reactive pulsed laser ablation A. Zoccoa,*, A. Perronea, E. Broitmanb,c, Zs. Cziganyd, L. Hultmand, M. Anderled, N. Laidanic a

University of Lecce, Physics Department and Istituto Nazionale Fisica della Materia, 73100 Lecce, Italy ´ 850, (1063) Buenos Aires, Argentina Thin Film Laboratory, Engineering Faculty, University of Buenos Aires, Paseo Colon c ¨ ¨ Thin Film Physics Division, Department of Physics, Linkoping University, SE-581 83 Linkoping, Sweden d Divisione Fisica Chimica delle Superfici ed Interfacce, ITC-IRST, 38050 Povo, Trento, Italy

b

Received 6 September 2000; received in revised form 31 July 2001; accepted 3 August 2001

Abstract We report the tribological, mechanical, structural and compositional characteristics of CNx films deposited by excimer laser (XeCl, ls308 nm, tFWHMs30 ns) ablation of a graphite target in N2 atmosphere. The influence of growth conditions on structural, morphological, tribological and mechanical properties of the CNx films has been examined by X-ray Photoelectron Spectroscopy (XPS), Transmission and Scanning Electron Microscopy (TEM and SEM, respectively), nanoindentation measurements and ball-on-disk tests. All the as-deposited films have a microstructure consisting of nanometer-sized graphitic clusters in an amorphous matrix. The stresses of the films are tensile or compressive depending on the deposition conditions. Friction coefficients of the films, deduced by high speed steel balls, increase with laser fluence and nitrogen pressure from 0.12 to 0.14. Friction is thus, lower than what has been reported in literature for CNx films. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Tribology; Hardness; Carbon nitride; Laser ablation

1. Introduction New advanced materials are generally recognized as being of outermost importance for the development of new products. Particularly in the area of reducing friction and wear, new materials can be expected to play an important role in the future. For example, the larger requirements on magnetic recording devices with increased packing capacities and faster access times require increasingly higher demands for the thin wear protective films on both the recording media and on the read and write heads. Also for creating new types of bearings which operate in, e.g. non-lubricated conditions new low-friction and wear-resistant thin films are required. For these reasons, today hard and wear protective coatings are becoming of an increasing importance. The most common materials for these purposes are transition metal nitrides and carbides. In this last decade, carbon nitride compound as a hard material has become part of the rather dynamic and exciting field of materials * Corresponding author. Tel.: q39-0832-320502; fax: q39-0832320505. E-mail address: [email protected] (A. Zocco).

research. This development was initiated by Liu and Cohen w1x who suggested that a hypothetical compound of carbon and nitrogen, b-C3N4, could likely have a bulk modulus comparable to that of diamond. The dominating part of the research is today focused to synthesize the b-C3N4 phase using vapor phase film growth techniques w2,3x. All of the efforts made to realize b-C3N4 have resulted in a vast amount of different typesyphases of carbon nitrides CNx w4–7x. Considering the potential application of CNx films for hard-and wear-protective coatings, it is very surprising to find not so many existing studies on the tribological and mechanical properties of this compound. Much effort has been devoted to structural, compositional, optical properties of CNx films deposited by various synthesis methods like ion implantation in carbon films w8,9x, ion and vapour deposition method w10x, ECR plasma chemical vapour deposition w11,12x, sputtering w13–18x, ion-assisted arc deposition w13x combined ionbeam and laser ablation method w19,20x and reactive laser ablation w21–23x. In this study we present the results on the mechanical and tribological tests (nanoindentation, stress, friction)

0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 2 7 - 1

A. Zocco et al. / Diamond and Related Materials 11 (2002) 98–104

Fig. 1. Schematic diagram of the laser ablation system; MS: mass spectrometer, L: lens, T: target, S: substrate.

and on structural, compositional and morphological analysis (HRTEM, XPS, SEM) performed on the CNx films deposited by reactive laser ablation. 2. Experimental method

99

(SEM). Plan-view HRTEM samples were obtained by floating-off, in de-ionized water, 30-nm-thick CNx films deposited in a separate run on freshly cleaved NaCl. X-ray photoelectron spectroscopy spectra were recorded in a Scienta Esca200 spectrometer, with monochromatized AlKa (1486.6 eV) radiation. The working pressure was less than 1=10y7 Pa. Detailed scans were recorded for the N 1s and C 1s regions. After a Shirleytype background subtraction, the raw spectra were fitted using a non-linear least squares fitting program adopting Gaussian–Lorentzian peak shape. The spectrometer was calibrated by assuming the binding energy (BE) of Ag 3d5y2 line at 368.16 eV with respect to the Fermi level. The nitrogen-to-carbon atomic ratio was determined by comparing the integrated areas under the peaks. The mechanical properties of the films have been obtained using a Nano IndenterTM II microprobe (Nano Instruments Inc.). Indentations were made by a trigonal (Berkovich) diamond tip in arrays with 30 mm between the indents, with maximum loads of 1, 3 and 5 mN. The resultant displacements were continuously recorded during both loading and unloading. The stress level in the films has been evaluated using a substrate curvature method. The curvature and the film thickness were determined by a stylus profilometer (Dektak 3030). The stress was then calculated using the Stoney equation:

2.1. Film growth ss

E ts2 w 1 1z x y | (1yv) 6t y R R0 ~

(1)

The deposition of film specimens was achieved using a 308-nm wavelength pulsed laser beam produced from a Lambda Physik LPX 315i excimer laser which was focused onto a graphite target rotating at a frequency of 3 Hz (Fig. 1). The laser beam was incident on the target surface with an angle of 458. The ablated material was collected on silicon (100) substrates at room temperature. The substrates were placed on a support parallel to the target at 4 cm from it. The depositions were performed inside a stainless steel vacuum chamber evacuated down 10y4 Pa by a pumping system consisting of a rotary and a turbomolecular pump. During the pumping the inner walls of the system were heated in order to facilitate the desorption of water vapor. The flux of high purity nitrogen (electronic grade 99.999%) was continuously blown inside the chamber and the pressure was measured by a penningypirani vacuometer. The laser fluence was set at 6 and 12 Jycm2 and the N2 pressure at 1 and 25 Pa.

where E is the Young’s modulus, R0 and R are the substrate curvature radii before and after deposition, tS and t are the substrate and film thickness, respectively, and n is the Poisson’s ratio w24x. We have used the values Es130 GPa and ns0.279 w25x for our Si (100) substrates with tss0.371 mm. Tribological properties of CNx samples were tested by a ball-on-disk equipment (VTT, Finland) using a bearing steel ball as a counter material. The ball had a diameter of 6 mm and the load was 2 N resulting in an initial contact pressure of;0.7 GPa, according to Hertz. The rotation frequency was 180 rpm, the track diameter was 5 mm and the number of revolutions was 3500. The test was performed in ambient atmosphere (228C and 35% relative humidity). The friction coefficient was continuously recorded during the test by measuring the tangential force of the ball.

2.2. Film characterization

3. Results and discussion

Structural and morphological characterizations were performed by high-resolution transmission electron microscopy (HRTEM) using a Philips CM 20 UT microscope operating at 200 kV (resolution ;0.19 nm) and a Philips XL-20 scanning electron microscope

3.1. Microstructure, composition and deposition rate Synthesized films have a good adherence to their substrates. The generation of a very good surface quality is, however, hampered by the droplets present on the

A. Zocco et al. / Diamond and Related Materials 11 (2002) 98–104

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Fig. 3. Note scale drawing of the traces produced by laser irradiation on the rotating target during the respective depositions.

Fig. 2. HRTEM plan view micrograph of CNx film deposited at N2 pressure Ps1 Pa and laser fluence Fs12 Jycm2.

surface. This feature is a characteristic drawback of pulsed laser deposition technique. However, SEM analyses show that the droplet density is limited to 104 y mm2 in all the deposited films. HRTEM studies have shown that all samples present a microstructure, consisting of nanometer-sized graphitic clusters in an amorphous matrix, as can be seen in Fig. 2. The nitrogen concentration of the films, as deduced from XPS analyses, is given in Table 1. It can be noticed that the change in the laser fluence or in the nitrogen pressure practically did not affect the nitrogen-to-carbon atomic ratio, wNx y wCx, in apparent contradiction with the earlier papers; indeed, the influence of number of laser pulsesysite was never taken into account. The composition of the plume strongly depends on the number of laser pulsesysite and it can be a critical parameter on the thin film properties.

The thickness of the deposited films, shown in Table 1, was not always related to the total number of laser pulses because the ablation rate (and thus, the deposition rate) decreases when the laser pulses hit the same spot on the target surface. For the growth of the sample 噛1 we used a trace with a smaller diameter than one used for the growth of the film 噛2 (see Fig. 3), resulting in an higher number of laser pulses per spot and consequently in a lower ablation rate w26x. 3.2. Chemical structure Some information about the film chemical structure can be derived from the N 1s and C 1s spectra. Fig. 4 shows the N 1s spectrum of film 噛1. We obtained the best fit of the experimental spectrum by using five individual peaks N1–N5. The same peak line shape was observed for the spectra recorded for films 噛2 and 噛3. The results of the N1s curve fitting (energy position) are given in Table 2. Regardless of peaks N4 and N5 that are due to different oxidized states of nitrogen w27x, the fit clearly indicates that nitrogen is mainly involved in three different chemical environments corresponding to peaks N1–N3. The latter are due to carbon–nitrogen bonds, though they are not easily assignable to definite chemical states among N-sp3C single bonded, N-sp2C double bonded or N-spC triple bonded, since various

Table 1 Thickness, stress value and wNxywCx atomic ratio of CNx films deposited under different conditions Fluence (Jycm2)

PN2 (Pa)

噛1

12

1

噛2

12

25

噛3

6

25

Total number of pulses (pulsesysite)

Thickness (mm)

Stress, s (GPa)

wNx y wCx ratio

30000 (500) 40000 (300) 40000 (450)

0.33"0.03

q (0.135"0.015)

0.075

1.02"0.03

y (1.445"0.045)

0.072

0.41"0.03

q (0.470"0.050)

0.079

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Fig. 4. N1s spectrum of the film 噛1 with the curve fitting. Table 2 Results of the N 1s XPS curve fitting for the 3 different CNx samples Sample

噛1

Gaussian peaks N1 N2 N3 N4 N5

398.7 399.8 400.3 401.4 402.5

噛2 eV eV eV eV eV

398.4 399.5 400.3 401.1 402.0

噛3 eV eV eV eV eV

398.8 399.8 400.2 401.9 401.7

eV eV eV eV eV

and contrasting assignments are given in the current literature dealing with nitrogenated carbon. The peak at ;398.6 eV can be attributed to N-sp3C bond w28,29x while the peak at ;399.7 and at ;400.3 eV are due to N-sp2C with different chemical environments w30x. What it appears from this analysis is that the change of the

deposition parameters, whatever fluence or pressure, did not affect greatly the distribution of nitrogen in its different chemical states. The C1s spectra, in turn, did not give more information about the nitrogen chemical bonds; nevertheless, some insight about the carbon phase structure could be derived from the plasmon loss features associated with the spectra, as will be discussed. Fig. 5 shows the spectra with their curve fitting, recorded on films 噛2 (a) and 噛1 (b) (the spectrum from film 噛3 is similar to the one of film 噛1 and it is not shown). In Table 3 the results of C 1s curve fitting are reported. It can be observed that the major contribution to the C1s spectrum from both films is due to peak C1, at a binding energy of approximately 284.5 eV, which is commonly attributed to CsC double bond w31x either in a graphitic or

Fig. 5. C1s spectra of the film 噛2 (a) and 噛1 (b) with their curve fitting.

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Table 3 Results of the C 1s XPS curve fitting for the 3 different CNx samples Sample

噛1

Gaussian peaks C1 C2 C3 C4 C5 C6

284.7 285.7 286.4 287.6 288.7 290.3

噛2 eV eV eV eV eV eV

284.5 285.3 286.3 287.7 288.8

噛3 eV eV eV eV eV

284.6 285.5 286.4 287.6 288.7 290.2

eV eV eV eV eV eV

in an amorphous phase. In contrast with film 噛2, for which a fit with five individual peaks, C1–C5, nicely describes the spectrum, the XPS profile of film 噛1 presents a high binding energy tail, which represents a sixth contributor, C6. The C2 peak at ;285.7 is attributed to CsN double bond, the peak C3 at ;286.6 is due to C–N single bond and C4 and C5 peaks are due to different oxidized states of carbon w32x. The C6 contributors, at 290.3 eV, present in the C1s spectrum of film 噛1 (and 噛3) and not in that of film 噛2, corresponds to a plasmon loss peak. It can be due to p-electrons oscillations or to p-p* transitions or both w33,34x and is known to occur at 6–7 eV from the principal peak in systems containing aromatic groups. The presence of this contribution in the case of carbon films is a fingerprint of sp2-hybridized carbon sites, arranged in graphitic domains. Clearly, the carbon phase structure of film 噛2 appears to deviate from this kind of structure, although it is not clear in what this deviation is consisting: either a decrease in the trigonally sp2bonded carbon or a stronger p-electron localization. 3.3. Mechanical properties The stress values of the as-deposited films (which were found to be tensile or compressive depending on the deposition conditions) are shown in Table 1. When the films were deposited at the lower fluence or lower pressure, they had tensile stress but when the films were deposited at high fluence and pressure the films resulted to have a high level of compressive stresses. Fig. 6 compares the load–displacement curves from nanoindentation measurements for the films labeled as 噛1, 噛2 and 噛3 and a fused silica reference sample (hardness H;10 GPa). The larger penetration depth of the indenter at maximum load (hmax) and the smaller elastic recovery (%R) of the samples 噛1 and 噛3 indicate both lower hardness and elasticity as compared to the SiO2 reference. Film 噛2 exhibited a hardness of 14 GPa and an elastic recovery comparable to SiO2. We can observe that in the case of the film 噛3, there is a fracture of the film during loading. Table 4 shows the mechanical properties of the analyzed films in terms of hmax and %R. The nanoindentation response of sample 噛2 is similar to reported results of CNx films deposited

Fig. 6. Nanoindentation load – displacement curves for the films deposited under different conditions. The SiO2 data are given as a reference.

by magnetron-sputtering in 100% N2 at room temperature, and tested under the same conditions w14x. In all the samples it was found that the friction coefficient as a function of the sliding cycles is characterized by two stages (Fig. 7). In the initial stage, the increase of friction is controlled by film roughness, droplets from the laser deposition method and the buildup of a transfer layer (tribolayer). In the second stage, the reduction of friction coefficient is controlled by the nature of the tribolayer. This friction regime has been reported previously for CNx films w35x. Fig. 8 shows the steady state friction coefficient of CNx films deposited on Si substrate. For comparison, the friction coefficient of the silicon is also indicated. According to a recent compilation of friction coefficients of DLC films w36x, the steady-state friction values of aC and a-C:H films are ranging from 0.01 to more than 0.5. In the case of CNx films, a large spread of friction values has also been reported, from 0.08 w37,38x up to 0.45 or more w39–41x. The difficulty to correlate the published properties of DLC and CNx films with their tribological behaviour stems from the anticipated difference in microstructure (generally the structural characterization of the materials has been lacking) and the lack of standardization of tribological tests w35x. In our experiments, the film deposited at higher fluence and pressure has the higher friction coefficient; however, Table 4 Nanoindentation parameters for the CNx films. The SiO2 data are given as a reference. Sample

hmax (nm)

%R

H (GPa)

噛1 噛2 噛3 SiO2

166 107 196 149

38 47 35 52

8.9 14.0 7.6 10.1

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in the level of stresses from tensile to compressive, seems to be beneficial for the mechanical properties of the film: they show an increase in the elastic recovery (%R) and hardness (up to 14 GPa). These films also have friction coefficient values among the lowest ones reported in literature for CNx films tested under high contact pressures. Acknowledgements

Fig. 7. Friction coefficient (m) vs. number of sliding cycles for the CNx film deposited on Si with Fs12 Jycm2 and PN2s25 Pa.

this value is still among the lower values reported for CNx films tested under high contact pressures w35x. SEM image shows that the surface of the wear track has no apparent damages. The sliding wear behaviour of CNx is consistent with the microfracturing of the tips of contacting asperities leading to film smoothing w35x. 4. Conclusions CNx thin films have been synthesized by reactive pulsed laser ablation technique on Si(100) substrate at room temperature using different laser fluences and different nitrogen pressures and different laser pulsesy site. In this experiment, the change in the laser fluence or in the nitrogen pressure does not affect significantly the nitrogen-to-carbon atomic ratio wNx y wCx. It is probably due to the influence of pulsesysite on the plume composition. However, at the higher fluence (12 Jycm2) and pressure (25 Pa), a decrease in the trigonally sp2bonded carbon (or a stronger p-electron localization) is observed. Such structural change, followed by a change

Fig. 8. Steady state friction coefficient (m) measured by ball bearing steel test for CNx film deposited on Si under different conditions. The friction coefficient of the Si substrate is also indicated.

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