Mechanical characterizations of diamond carbon films made by PACVD

Surface and Coatings Technology 151 – 152 (2002) 170–174

Mechanical characterizations of diamond carbon films made by PACVD a ´ P. Djemiaa,*, F. Tetard , F. Ganota, C. Metb, M.I. DeBarrosb, L. Vandenbulckeb a

LPMTM (CNRS UPR 9001), Universite´ Paris-Nord, 93430 Villetaneuse, France b ´ ´ LCSR (CNRS UPR 4211), Universite´ d’Orleans, 45071 Orleans, France

Abstract Mechanical and surface properties associated with their wettability were investigated for diamond films deposited on titanium alloy by a two-step microwave PACVD process at 600 8C. Polycrystalline and smooth fine grained diamond films were investigated. The elastic properties of the diamond layers were studied by Brillouin light scattering spectroscopy in order to determine their evolution as a function of the diamond purity. The effective elastic properties of these films with hexagonal effective symmetry were determined. The elastic constants decrease when diamond purity is lowered. The wettability tests can lead to the determination of superficial energy. A strong correlation is noticed between the wettability and superficial energy with the content of sp2. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Brillouin; Light scattering; Elastic waves; Elastic constants; Wettability; Diamond

1. Introduction The determination of the mechanical and surface properties of thin layers deposited on a substrate is important with regard to physical models and technical applications. Indeed, much is known about the physical and chemical properties of bulk materials but in most cases information is still lacking concerning the corresponding thin layers. Polycrystalline as well as smooth fine-grained diamond films with reduced roughness could be deposited by PACVD w1x on a titanium alloy substrate. Titanium alloys are used in applications requiring exceptional corrosion resistance, relatively low density and high specific strengths, but their tribological behavior is an important drawback for most mechanical applications w2x. Diamond coatings are proven to be very effective for improving their wear and frictional behavior w3x. However, the usefulness of the CVD polycrystalline diamond coatings is often restricted by the usually high deposition temperature which modifies the substrate structure and by their rough, faceted surface which causes high wear of counterpart materials during sliding friction w4,5x. Both problems can be overcome by depositing at moderate temperatures, approximately * Corresponding author. Tel.: q33-1-49403482; fax: q33-149403938. E-mail address: [email protected] (P. Djemia).

600 8C, smooth fine-grained diamond (SFGD) coatings with a very smooth surface finish w1,6,7x. To improve understanding of intrinsic thermo-mechanical and wettability properties, polycrystalline diamond and SFGD films with different diamond quality (amount of sp3hybridized carbons) was studied by Brillouin light scattering (BLS) and the drop sessile technique. 2. Experimental set-up 2.1. Contact angle measurements Advancing contact angles were measured directly with small drops of the test liquids: water, and glycerol on sample surface. The thin films were cleaned with acetone before measurement. The droplets were released in a controlled manner onto the polished surface of the samples from the tip of a microsyringe. For each sample, drops were performed with each test liquid on the polished surface and on the as-deposited surface of the diamond film; the drop volume was 5 ml. The angles of both sides of each droplet were measured and the mean values were used for calculations. The standard deviation due to experimental error was calculated as "18. The Lifshitz–van der Waals (LW) and acid–base (AB) surface tension components were calculated from the contact angle data according to the method of Fowkes w8x with the values of the surface tension components

0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 6 7 - 5

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Table 1 Diamond growth conditions and properties of coatings on Ti – 6Al – 4V substrates Sample number

A

B

C

D

E

Coating type Gaseous mixture Surface roughness (nm) Evaluated diamond quality (%) Residual stresses (GPa) Hardness (GPa) Film thickness (mm) r (kg my3)

Polycrystalline 8% COyH2 120 97

Fine-grained 37.5% CH4 yCO2 33 94

Fine-grained 41% CH4 yCO2 23 90

Fine-grained 44.5% CH4 yCO2 15 80

Fine-grained 50% CH4 yCO2 14 75

y5.5"0.4 106"1 5 3468"8

y5.3"0.4 – 2 3422"16

y4.5"0.4 86"2 5 3360"28

y3.6"0.4 81"2 5 3207"55

y3.2"0.4 74"2 5 3125"64

of test liquids from Van Oss w9x. The surface tension, gi is the sum of two components: AB gisgLW i qgi

(1)

the surface phonons probed in this experiment then typically lie at approximately 300 nm. The velocity (vS) is deduced from the measured frequency (V) by the relation:

LW i

where g is the Lisfshitz–van der Waals dispersive component and gAB is the Lewis acid–base or polar i component. The contact angle u for solid–liquid systems can be related to the surface thermodynamic properties of the solid (S) and liquid (L) via the Young–Dupre equation: AB AB Ž1qcosu.gLs2ŽygSLWgLW L qygS gL .

(2)

From Eq. (2), the surface free energy components of AB a solid (gLW S , gS ) could be determined. 2.2. Brillouin light scattering technique In BLS experiments, a beam of monochromatic light is used as a probe to reveal thermoactivated acoustic phonons. The experiments were performed at room temperature. The light source was an Arq laser tuned on a 514.5-nm single mode line. Incident 500-mW ppolarized light was focused on the surface of the sample. The scattered light was analyzed by means of a Sandercock-type 3q3 pass tandem Fabry–Perot interferometer characterized by a finesse of approximately 100 and a contrast ratio higher than 1010. For some spectra, an analyzer was inserted within the path of scattered light providing the s- or p-polarized part of the spectra. The typical duration for the acquisition of a Brillouin spectrum was 2–4 h. For a transparent film of several micron thickness, this technique permits a large amount of information to be obtained from a single spectrum. Indeed, two different geometries of interaction contribute to inelastic scattering of light from acoustic phonons in the film w10x. In the present work we used the backscattering geometry. In this condition the wave vector of the involved surface phonons propagating along the surface is determined by the relation: QSs2kIsinŽi.

(3)

where kI denotes the optical wave vector in air and where i is the angle of incidence. The wavelength of

v Ss

V QS

(4)

The wave vector of the involved bulk phonons is experimentally adjusted to the value: (5)

QBs2nkI

where n is the refractive index of the film corresponding to the propagation direction of light inside the film. The velocity (vB) is deduced from the measured frequency (V) by the relation: v Bs

V QB

3. Sample preparation characterizations

(6) and

structural

3.1. Deposition technique The Ti–6Al–4V substrates were prepared as previously reported w6,11x. The microwave plasma deposition apparatus was also already described w11x. All samples were coated at 600 8C using various deposition conditions to obtain different coating characteristics. Polycrystalline diamond and smooth-fine grained diamond (SFGD) coatings were deposited with a two-step process but SFGD coatings were obtained by varying the growth step w1x. The first step was always carried out at 600 8C with a 6–20% methane-rich mixture with hydrogen. The deposition conditions used during the second step are reported in Table 1. The polycrystalline coating was deposited from 8% CO–H2 and the SFGD coatings were deposited from different CO2 –CH4 mixtures. 3.2. Morphological and structural characterizations Diamond coatings were characterized by visible and UV Raman spectroscopy, atomic force microscopy

P. Djemia et al. / Surface and Coatings Technology 151 – 152 (2002) 170–174

172

Table 2 Contact angles u on different quality of polycrystalline and finegrained diamond thin films and deduced values of surface tension AB component gLS S , gS

u (8) Water u (8) Glycerol gAB (mJym2) S 2 gLS (mJym ) S

A

B

C

D

E

59"1 61"1 5"1 38"2

64"1 62"1 9"1 28"2

64"1 57"1 17"1 21"2

62"1 49"1 28"2 16"2

61"1 44"1 36"2 13"2

(AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Diamond purity in terms of sp3-carbon content of the polycrystalline and SFGD coatings was evaluated from visible micro-Raman spectra taking into account the significant influence of the diamond grain size on the intensity of the diamond peak w12,13x and the resonance Raman effect which enhances the sp2carbon contribution with respect to sp3-carbon w14,15x. The diamond purity of one sample was confirmed by electron energy loss spectroscopy (EELS). Furthermore, the residual stresses in the coatings were evaluated from the visible Raman spectra. The refractive indices needed for further analysis of BLS experiments have been measured by a prism coupling spectroscopy technique w10x. We measured lower refractive indices (ns 2.305"0.005) for the SFGD films compared to the polycrystalline diamond film A (ns2.435"0.001) for ls543.5 nm. A N011M fiber texture was found by XRD measurement for all samples w10x. We have calculated by a weight average a reasonable estimation of the mass densities of our films assuming that the mass density of sp3 diamond carbon is 3515 kg my3 and that the low density is entirely due to graphitic components (crystalline rs2250 kg my3 or disordered rs1170 kg my3). The characteristics of the films are provided in Table 1.

influence of roughness on the measured contact angles from the as-deposited surface which may prevent any correlation between the sp3 content and the contact angle. 4.2. Brillouin light scattering experiments Fig. 1 shows spectra from sample A taken at an angle of incidence is508 with different polarization of the analyzed scattered electric field: non-polarized, p-polarized and s-polarized. The two peaks in the p–p spectrum correspond to the Rayleigh surface wave (RW) and to the longitudinal mode (LM), travelling parallel to the film surface w10x. Measurements of the frequency position of these two Brillouin peaks enabled us to determine the phase velocity v of the corresponding acoustic modes, according to Eq. (4). It should be noted that the phase velocity of the LM is (C11 yr)1y2, so that C11 can be directly determined, using for the mass density the value in Table 1. As for the RW, its velocity is b(C44 y r)1y2, where b is only weakly dependent on C11, C13 and C33. The remaining peak in the p–s spectrum corresponds to a shear horizontal mode (SHM) travelling parallel to the film surface, and its phase velocity is simply (C66 yr)1y2. This enabled a direct evaluation of C66 that is usually very difficult because of the very

4. Results and discussion 4.1. Wettability tests Contact angles for the two probe liquids measured on the polished surface of samples are listed in Table 2. The sp2-hybridized carbon content in diamond film has a great influence on contact angle with water and glycerol. The increase of the contact angle for glycerol vs. the sp3 content is directly connected with the evolution of polar surface tension component of the films. As the diamond purity increases the polar surface tension decreases (see Table 2) due to the polarization potential of the involved p-electrons, as reported by Grischke et al. w16x. A linear correlation fits well the dependence between the sp3 content and the contact angle of glycerol. These are very interesting results concerning the biocompatibility applications of this new type of material. Finally, one should note the large

Fig. 1. Experimental spectra obtained for polycrystalline sample number A, for an angle of incidence equal to 508 and different polarizations of the analyzed scattered electric field: (a) non-polarized, (b) p-polarized and (c) s-polarized. We show also for comparison the calculated power spectrum for the related component (d) ux, (e) uy, (f) uz of the displacement field.

P. Djemia et al. / Surface and Coatings Technology 151 – 152 (2002) 170–174 B sinŽi. E

assiny1C D

Fig. 2. Experimental spectrum obtained (p – np geometry) at larger frequency for the SFGD sample number D, with an angle of incidence is608 corresponding to as228.

low scattering efficiency of shear horizontal modes. All these lines are reproduced in the relevant power spectrum of the displacement field calculated at the free surface (see Fig. 1). The peaks at larger frequency in the spectrum of Fig. 2 correspond to the bulk shear (BSW) and to the bulk longitudinal acoustic waves (BLW) observed for sample D (angle of incidence is 608). Different from the previous case, the interaction geometry is relative here to bulk phonons whose wave vector is expressed by Eq. (5) while the phase velocity is given by Eq. (6). Due to refraction at the airyfilm interface, the bulk waves associated with the BLW and BSW peaks have a wave vector at an angle:

n

F

173

(7)

G

from the surface normal. For as08 the BLW phase velocity is simply (C33 yr)1y2, while the BSW one is (C44 yr)1y2. For increasing angle a, the phase velocity depends also on C11, C13 and C44. Measurements of the bulk wave velocities were performed from orthogonal incidence to a larger angle (is708 corresponding to as 238). A best-fit procedure of the BLW, BSW and RW experimental data to the calculated phase velocities of the corresponding modes, enabled us to determine C33, C13 and C44. The experimental values of the phase velocity of the acoustic modes revealed in the different samples are summarized in Table 3, while the elastic constants are shown in Table 4. For comparison purposes, the calculated values of the elastic constants of polycrystalline diamond with N011M fiber texture following a Voigt and Reuss averaging procedure, using the known bulk constants, are also reported. An excellent agreement is observed for elastic constants obtained in the polycrystalline diamond layer A with approximately 97% diamond purity. Whereas the polycrystalline sample A shows an almost isotropic behavior, the SFGD films have a pronounced anisotropic elastic symmetry. 5. Summary and conclusion We have determined the mechanical properties of polycrystalline and smooth fine-grained diamond films grown by PACVD on Ti–6Al–4V substrate, using

Table 3 Experimental values of the phase velocity of the different acoustic modes detected in Brillouin spectra Sample

vRW (kmys)

vLM (kmys)

vSHM (kmys)

vBSW (kmys) (as08)

vBLW (kmys) (as08)

A B C D E

10.8"0.3 8.78"0.14 8.66"0.26 8.30"0.3 7.46"0.1

18.32"0.3 15.15"0.29 13.40"0.23 14.00"0.3 13.53"0.43

12.35"0.3 9.77"0.13 7.70"0.25 9.73"0.3 9.53"0.6

12.38"0.06 9.85"0.04 9.78"0.05 9.41"0.04 8.29"0.04

18.16"0.07 16.23"0.08 16.94"0.08 16.57"0.06 15.35"0.07

Table 4 Experimental values of the five independent effective elastic constants of the diamond films analyzed in the present work Sample

C11 (GPa)

C33 (GPa)

C44 (GPa)

C66 (GPa)

C13 (GPa)

A B C D E N011M textured polycrystalline diamond

1165"40 785"30 605"30 630"40 570"40 1163 w1157x

1145"15 900"10 965"10 880"10 735"10 1174 w1170x

530"25 330"10 320"20 285"20 215"5 530 w525x

530"20 325"10 200"10 305"15 285"35 541 w537x

N.D. N.D. N.D. N.D. N.D. 68 w70x

The value of C13 has not been measured with accuracy but is below 150 GPa. The Voigt and Reuss estimated values of the elastic constants for a bulk N011M textured polycrystalline diamond are also reported for comparison in the last line winside the square bracketsx. N.D. indicates no determination.

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P. Djemia et al. / Surface and Coatings Technology 151 – 152 (2002) 170–174

Brillouin light scattering, and their wettability properties using the sessile drop technique. We measured the five independent effective elastic constants of diamond films in the micron range of thickness, which is typical of many applications. The values of the elastic constants of the polycrystalline film compare fairly well with those calculated using the Voigt or Reuss average procedure for a N011M oriented cubic polycrystalline material, using the known bulk values of elastic constants of diamond. A clear correlation was found between the mechanical and wettability properties and the sp3-carbon content which depends on the deposition conditions. The results demonstrate that FGD coatings with very smooth surface finish and fairly good quality can be deposited on titanium alloys at a temperature of 600 8C. References w1x M.I. De Barros, L. Vandenbulcke, Diamond Relat. Mater. 9 (2000) 1862. w2x K.G. Budinski, Wear of Materials, ASME, 1991, p. 289.

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