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Mechanical properties of carbon nitride thin films prepared by ion beam assisted filtered cathodic vacuum arc deposition
Surface and Coatings Technology 112 (1999) 140–145
Mechanical properties of carbon nitride thin films prepared by ion beam assisted filtered cathodic vacuum arc deposition C. Spaeth *, M. Ku¨hn, T. Chudoba, F. Richter Technische Universita¨t Chemnitz-Zwickau, D-09107 Chemnitz, Germany
Abstract The mechanical properties of a tetrahedral amorphous carbon (ta-C ) film and three carbon nitride (CN ) films with nitrogen x concentrations of 2 and 20 at.% were examined by means of indentation tests, abrasive wear tests and scratch tests. The incorporation of 20 at.% nitrogen in carbon films leads, on the one hand, to a moderately reduced hardness and wear resistance but, on the other hand, to a considerably increased adherence, making this material a promising candidate for tribological applications. In a second part, a simple finite element (FEM ) analysis is applied to confirm qualitatively that the maximum thickness achievable of films with intrinsic compressive stress can be increased by an interfacial layer with a stress gradient. Both effects, i.e. the improved adherence of carbon nitride and the smoothing of the stress depth profile, are used to increase the maximum film thickness of ta-C film on a tungsten carbide insert from about 1.5 to 3.1 mm. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbon nitride; Thin films; Mechanical properties; Adherence
1. Introduction Since the proposal of Cohen [1] of the existence of the superhard carbon nitride, b-C N , much experimen3 4 tal research has been directed towards the synthesis of high nitrogen-containing carbon nitrides. Mostly, physical vapor deposition (PVD) methods or chemical vapor deposition (CVD) methods have been used and normally amorphous materials with 50 at.% nitrogen at maximum were produced. In the case of PVD methods, it was found that, for films with nitrogen concentrations of more than about 25–30 at.%, a significant fraction of nitrogen triply bonded leads to a rather soft paracyanogen-like material [2,3]. Thus, carbon nitrides with a lower nitrogen content seem to be more appropriate for mechanical applications. In Ref. [4], it is reported that sputtered carbon nitride films with 10 at.% nitrogen have an improved wear resistivity and a lower friction coefficient compared with sputtered carbon films. In Ref. [5], it was shown that the hardness of sputtered CN films x could be increased from 9 GPa for pure carbon films to
a maximum of 25 GPa for carbon nitride films with a nitrogen concentration of 18 at.%. In the case of ion beam and plasma beam deposition methods, where the films are grown from energetic carbon ions, the situation is different. The pure carbon films (ta-C ) have a rather high density of 3.0 g/cm3 [6 ] and a Young’s modulus of about 700 GPa [7] and no increase of the density and hardness could be obtained by the incorporation of nitrogen. In contrast, the density and hardness continuously decreased with increasing nitrogen content [8–10]. However, the wear resistivity of such carbon nitrides was found to be still an order of magnitude higher than those of conventional ceramics [11] and they exhibit much lower intrinsic stress than the carbon films, thereby facilitating the deposition of thick films. Very little is known about the mechanical properties of CVD carbon nitrides. In Ref. [12], a hardness of 20 GPa was measured and no dependence on the nitrogen content in a range of 0 to 37 at.% was observed.
2. Experimental * Corresponding author. Tel: +49 371 5313553; Fax: +49 371 5313042; e-mail: [email protected]
The samples have been prepared by ion beam-assisted filtered cathodic vacuum arc deposition. A carbon
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plasma stream emerging from a vacuum arc discharge is passed through a magnetic coil, before deposition onto the substrate. The magnetic coil filters out neutral carbon atoms and macroparticles. Nitrogen is provided by admittance of a gas flow or by the bombardment of the growing film with nitrogen ions from a Kaufmantype ion source. The copper substrate holder is water cooled and it is possible to apply a d.c. or r.f. substrate bias voltage. The deposition system has been described elsewhere in more detail [13]. Four types of films were prepared: a tedrahedral amorphous carbon film and a carbon nitride film with 2 at.% nitrogen, both using an r.f. substrate bias voltage; a carbon nitride film with 2 at.% nitrogen grown onto a grounded substrate; and a carbon nitride film with 19 at.% nitrogen using the ion source. The deposition parameters are given in Table 1. In the cases where the films were grown onto a grounded substrate, a substrate bias voltage of −100 V was applied during the first 2 min of the deposition. Polished tungsten carbide inserts (surface finished with 3 m diamond paste) were used as substrates. They were pre-treated in an ultrasonic bath with alcohol, and sputter-cleaned by a 500 eV Ar+ ion beam prior to the deposition. The stoichiometry and density of the films was obtained from elastic recoil detection analysis ( ERDA) and profilometry of comparable films deposited onto silicon substrates [13]. Load-displacement curves were recorded by a Shimadzu DUH-202 microindenter using a Vickers-type indenter. The maximum load was 100 mN. The hardness and Young’s modulus were calculated from the unloading curves by a procedure described in Ref. [14]. The measurement was repeated 10 times for each sample. The intrinsic compressive stress was calculated from
the curvature of comparable samples deposited onto Si substrates using Stoney’s equation [15]: h2 E s s= s f 1−n 6Rh s f where a Young’s modulus of E =169 GPa and a Poisson s number of n =0.28 were assumed for the (001) silicon s substrates. h and h are the thickness of the substrate s f and the film, respectively. R is the measured radius of curvature of the coated substrate. The abrasive wear rate was measured using a calotte tester [16 ]. A 100Cr6 steel ball, rotating around its axis of symmetry with a speed of v=30 rpm, was pressed with a normal load of F =0.54 N onto the sample. A n diamond suspension with a grain size of 1 mm was used as an abrasive medium. The test was stopped after t= 3 min and the wear rate was calculated according to: r =1ph2(3R−h)/(F 2pRvt) n w 3
(1)
where h is the maximum depth and R=15 mm is the radius of the steel ball. The results were averaged over four tests. The scratch tests were performed with a load rate of 60 N/min and a speed of 10 mm/min. A Rockwell-type diamond stylus with 200 mm tip radius was used as an indenter. The critical load was determined from the length of the scratch track from the start to the position of the first optically observable film delamination and from the onset of the integrated acoustic emission (IAE ) signal. Here, the results from the IAE are omitted since both methods yield very similar critical loads. For each sample, four tests were done.
Table 1 Deposition parameters and E results for the carbon and CN films. F is the nitrogen flow, U the d.c. component resulting from the r.f. x N S substrate bias voltage, U the beam voltage of the Kaufman-type ion source, and j the nitrogen ion current density extracted from the beam N Kaufman-type ion source. The carbon ion current density was 1.5 mA cm−2 in all cases Sample
F (sccm) N
U (V ) S
U (V ) beam
j (mA cm−2) N
N content (at.%)
Density (g cm−3)
ta-C CN (I ) 0.02 CN (II ) 0.02 CN 0.23
– 3 3 3
−80 (r.f.) −80 (r.f.) 0 0
– – – −100
– – – 0.15
– 2 2 19
2.9 2.8 2.6 2.4
Table 2 The mechanical properties of the carbon and carbon nitride samples. F is the critical load determined by microscope investigation of the scratch track c Sample
Thickness (mm)
Hardness (GPa)
Young’s mod. (GPa)
Stress (GPa)
Wear rate (10−15m3 mN−1)
F (N ) c
ta-C CN (I ) 0.02 CN (II ) 0.02 CN 0.23 Substrate
1.2 1.6 1.1 1.9 –
56 57 45 37 23
530 602 499 405 520
11.5 8.9 3.3 3.1 –
0.47 0.91 0.97 2.69 –
12.6 16.9 26.4 50 –
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Fig. 1. The loading and unloading curves of all samples.
3. Results and discussion In Table 1, the composition and mass density of the samples are given. The small amount of nitrogen incorporated in sample CN (I ) does not change the mass 0.02 density significantly, compared with the ta-C sample. The density of sample CN (II ) is decreased. This is 0.02 very similar to the case of ta-C, where the critical influence of the substrate bias voltage on the film density is well known [6 ]. Nitrogen ion bombardment of the growing film further decreases the mass density in the case of the sample CN . 0.23 Table 2 gives the mechanical properties. The hardness and Young’s modulus of the samples correlate with their mass density. In all cases, the hardness of the coated substrate is higher than the hardness of the substrate. However, the Young’s modulus of 520 GPa of the uncoated substrate could not be further improved essentially by the coatings. In Fig. 1, the load-displacement
curves are demonstrated. The coated substrates show a lower indentation depth and a higher elastic recovery. The increasing hardness is accompanied by an increase of the compressive stress. Due to the compressive stress, the maximum film thickness is limited. For example, the maximum thickness of a ta-C samples is about 1.5 mm on a tungsten carbide inserts and only about 300 nm on a sputter-cleaned silicon substrate. In general, the results on the wear rates show similar trends as the results of the hardness measurements. However, the CN (I ) sample has twice the wear rate 0.02 of the ta-C sample at comparable hardness. The wear rate of the high nitrogen content sample, CN , is 0.23 further increased but is still lower than the typical wear rate of a-C:H films of about 3.4×10−15 m3 mN−1 [16 ]. The critical load in the scratch test depends on the Young’s modulus of the film, the film thickness, the compressive stress and the bonding of the film to the substrate. In general, thicker films with higher Young’s moduli have higher critical loads due to their ability to spread the load to a larger area of the substrate. However, due to the intrinsic compressive stress, the intrinsic shear stress at the interface increases with the film thickness and a small extrinsic shear stress will lead to the delamination of the film. For example, the ta-C film is thicker than the CN (II ) and has a higher 0.02 Young’s modulus but a lower critical load due to its higher compressive stress. The lower Young’s modulus of sample CN is compensated by its low intrinsic 0.23 stress and its larger thickness, which leads to the highest critical load of all samples. Fig. 2 shows a scanning electron microscopy (SEM ) image of the fracture surface of sample CN . Here, a 0.23 section of the scratch track produced by the Rockwelldiamond stylus is visible. The corresponding normal load was about 25 N, which is well below the critical
Fig. 2. SEM micrograph showing the fracture surface of sample CN , intersecting the scratch track. The corresponding normal load is 0.23 F #25 N. N
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Fig. 3. Deposition parameters and properties of the gradient layer system in dependence on the deposition time. The total deposition time was 1 h 45 min.
load. Nevertheless, the film and substrate are deformed, but the film still protects the substrate which is confirmed by the low penetration depth of the diamond stylus (1.0 mm at the maximum load of 50 N ). For comparison, at this load, complete film delamination occurs for the ta-C-coated sample and the penetration depth is 2.5 mm. The good adherence of the CN strongly suggests 0.23 the use of this material as an interfacial adhesion layer for ta-C films. Fig. 3 gives the deposition parameters and properties for such a layer system. The nitrogen concentrations and compressive stresses were measured on homogeneous films deposited on Si substrates with
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the same parameters. The actual dependence of the intrinsic stress on the substrate bias voltage was not measured. The material of the first layer is equal to the material of the CN sample. Then, the nitrogen 0.23 content is reduced by decreasing the nitrogen ion current density, j . For the reasons explained later, the CN N x layers are followed by a series of ta-C layers prepared at different substrate bias voltages to control the intrinsic stress of these layers [17]. Due to this interfacial layer system, it was possible to increase the maximum thickness of a ta-C film from about 1.5 to 3.1 mm. The hardness and Young’s modulus of this sample, ta-C(II ), are 72 and 741 GPa, respectively. They are even higher, as in the case of the ta-C sample without the interfacial layer system, because of the lower influence of the substrate on the results of the indentation test. To get more insight into the relationship between the intrinsic compressive stress and film adhesion, we have performed a simple finite element (FEM ) analysis. The aim of this analysis is to calculate the shear stress at the substrate film interface, which is crucial for the adhesion. Quadrilateral elements with a linear interpolation function were used. For simplicity, the strain, e , in y 22 direction was set to e =0 and a constant element size 22 was used. The normal and shear forces at the boundary were set to zero. Fig. 4 demonstrates the result of a film of 1 mm thickness on a 7.5 mm thick substrate. The film surface is located in the x–z-plane. Both the film and the substrate were assumed to have a bulk modulus of 500 GPa and a Poisson number of n=0.3. The stress in the film was s =s =9.6 GPa (e =e =0.01) in the 11 33 11 33 flat, unrelaxed starting-geometry. Most interesting is the non-linear curvature of the left and right boundary (x#±10 mm) since, in the theory of plates, a linear dependence is assumed [18]. Fig. 5 shows the shear
Fig. 4. The relaxed FEM mesh. For clarity, only every fourth row and column are drawn. The nodes of the left, right and top boundaries are drawn enlarged.
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Fig. 5. The shear stress, s , versus the x and z coordinates. The film 13 with intrinsic compressive stress is located between z#0 mm and z#−1 mm. The element width and height is 1/8 mm. A Young’s modulus of 500 GPa and a Poisson number of n=0.3 were assumed for the film and the substrate. The initial strain was e =e =0.01. 11 33
stress in the film and the substrate. The maximum shear stress is located near the left and right boundaries. This is in accordance with the analytically-obtained result of Suhir [19]. The maximum shear stress reduces strongly in the substrate with increasing depth (decreasing z). Therefore, the deposition of a stress-free layer onto the substrate before the deposition of the stressed top layer will lead to a considerable reduction of the shear stress at the substrate surface. This is in contradiction to the results of Suhir, where a reduction in the shear stress can only be obtained if an additional layer with tensile stress is used. We believe that the reason for this discrepancy might be the restrictive assumption applied in the theory of plates. However, the result of the FEM calculation are supported by experimental results. In the case of cubic boron nitride (c-BN ), it was observed that a c-BN film deposited onto an about 100 nm thick hexagonal boron nitride (h-BN ) layer exhibits much better adherence than a c-BN layer deposited directly onto the silicon substrate [20,21]. By means of the stress-free layer, the maximum of the shear stress can be shifted but not reduced. Indeed, in the case of the boron nitride bilayer, the delamination then takes place at the c-BN/h-BN interface [21]. A reduction of the shear stress maximum can be obtained by a smoothing of the intrinsic stress depth profil. This is demonstrated in Fig. 6. As in the former example, the film at the top is 1 mm thick and has a compressive stress of 9.6 GPa. The two next layers are 0.5 mm in thickness and have compressive stresses of 6.4 and 3.2 GPa, respectively. The substrate is 7.5 mm in thick-
Fig. 6. The shear stress, s , versus the x and z coordinates of a stressed 13 layer deposited onto a gradient layer system. The gradient layer system is located between z#−1 mm and z#−2 mm.
ness. This smoother increase of the intrinsic compressive stress nearly bisects the maximum of the shear stress.
4. Conclusions We examined the mechanical properties of carbon nitride samples with nitrogen contents ranging from 0 to 20 at.%. On the one hand, the hardness, the Young’s modulus and the wear resistivity are reduced with increasing nitrogen content, but on the other hand, the adhesion of the films increases at higher nitrogen content. In a second part, the good adherence of carbon nitride and the possibility of controlling the intrinsic compressive stress of carbon films by applying different substrate bias voltages is used to form an interfacial layer system, which improves the adhesion of a highly stressed ta-C film. The effect of a smooth stress profile is explained by a simple FEM analysis.
Acknowledgements This work was partly supported by the Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie under grant number 03 N 5002 C. The authors are grateful for helpful discussions with N. Schwarzer. The scratch tests and abrasive wear tests were performed at Roth&Rau Oberfla¨chentechnik GmbH Wu¨stenbrand and by C. Siebert at the Fraunhoferinstitut fu¨r Schicht- und Oberfla¨chentechnik (Braunschweig), respectively.
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References [1] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. [2] Z.J. Zhang, S. Fan, C.M. Lieber, Appl. Phys. Lett. 66 (1995) 3582. [3] P. Hammer, M.A. Baker, C. Lenardi, W. Gissler, J. Vac. Sci. Technol. A 15 (1997) 107. [4] A. Khurshudov, K. Kato, D. Sawada, Trib. Lett. 2 (1996) 13. [5] E.C. Cutiongco, D. Li, Y.-W. Chung, C.S. Bhatia, J. Trib. 118 (1996) 543. [6 ] J. Koskinen, J. Appl. Phys. 63 (1988) 2094. [7] B. Schultrich, H.-J. Scheibe, G. Grandremy, D. Schneider, P. Siemroth, Thin Solid Films 253 (1994) 125. [8] C.A. Davis, Y. Yin, D.R. McKenzie, L.E. Hall, E. Kravtchinskaia, V. Keast, G.J. Amaratunga, V.S. Veerasamy, J. Non-Cryst. Solids 170 (1994) 46. [9] H.C. Hofsa¨ss, C. Ronning, U. Griesmeier, M. Gross, Mater. Res. Soc. Symp. Proc. 354 (1995) 93.
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[10] F.-R. Weber, H. Oechsner, Surf. Coat. Technol. 74/75 (1995) 704. [11] J. Koskinen, J.-P. Hirvonen, J. Levoska, P. Torri, Diam. Rel. Mater. 5 (1996) 669. [12] F.L. Freire, D.F. Franceschini, C.A. Achete, Nucl. Instr. Meth. B 85 (1994) 268. [13] C. Spaeth, M. Ku¨hn, F. Richter, Diam. Rel. Mater. 6 (1997) 626. [14] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [15] G.G. Stoney, Roy. Soc. Proc. A 82 (1909) 172. [16 ] C.P. Klages, R. Memming, Mater. Sci. Forum 52/53 (1989) 609. [17] C.A. Davis, Thin Solid Films 226 (1993) 30. [18] L.D. Landau, E.M. Lifschitz, Theoretical Physics VII, Pergamon Press, Oxford, 1995. [19] E. Suhir, J. Appl. Mech. 55 (1988) 143. [20] A. Schuetze, K. Bewilogua, H. Lu¨thje, S. Kouptsidis, M. Gaertner, Surf. Coat. Technol. 97 (1997) 33. [21] J. Hahn, private communication.
Mechanical properties of carbon nitride thin films prepared by ion beam assisted filtered cathodic vacuum arc deposition C. Spaeth *, M. Ku¨hn, T. Chudoba, F. Richter Technische Universita¨t Chemnitz-Zwickau, D-09107 Chemnitz, Germany
Abstract The mechanical properties of a tetrahedral amorphous carbon (ta-C ) film and three carbon nitride (CN ) films with nitrogen x concentrations of 2 and 20 at.% were examined by means of indentation tests, abrasive wear tests and scratch tests. The incorporation of 20 at.% nitrogen in carbon films leads, on the one hand, to a moderately reduced hardness and wear resistance but, on the other hand, to a considerably increased adherence, making this material a promising candidate for tribological applications. In a second part, a simple finite element (FEM ) analysis is applied to confirm qualitatively that the maximum thickness achievable of films with intrinsic compressive stress can be increased by an interfacial layer with a stress gradient. Both effects, i.e. the improved adherence of carbon nitride and the smoothing of the stress depth profile, are used to increase the maximum film thickness of ta-C film on a tungsten carbide insert from about 1.5 to 3.1 mm. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbon nitride; Thin films; Mechanical properties; Adherence
1. Introduction Since the proposal of Cohen [1] of the existence of the superhard carbon nitride, b-C N , much experimen3 4 tal research has been directed towards the synthesis of high nitrogen-containing carbon nitrides. Mostly, physical vapor deposition (PVD) methods or chemical vapor deposition (CVD) methods have been used and normally amorphous materials with 50 at.% nitrogen at maximum were produced. In the case of PVD methods, it was found that, for films with nitrogen concentrations of more than about 25–30 at.%, a significant fraction of nitrogen triply bonded leads to a rather soft paracyanogen-like material [2,3]. Thus, carbon nitrides with a lower nitrogen content seem to be more appropriate for mechanical applications. In Ref. [4], it is reported that sputtered carbon nitride films with 10 at.% nitrogen have an improved wear resistivity and a lower friction coefficient compared with sputtered carbon films. In Ref. [5], it was shown that the hardness of sputtered CN films x could be increased from 9 GPa for pure carbon films to
a maximum of 25 GPa for carbon nitride films with a nitrogen concentration of 18 at.%. In the case of ion beam and plasma beam deposition methods, where the films are grown from energetic carbon ions, the situation is different. The pure carbon films (ta-C ) have a rather high density of 3.0 g/cm3 [6 ] and a Young’s modulus of about 700 GPa [7] and no increase of the density and hardness could be obtained by the incorporation of nitrogen. In contrast, the density and hardness continuously decreased with increasing nitrogen content [8–10]. However, the wear resistivity of such carbon nitrides was found to be still an order of magnitude higher than those of conventional ceramics [11] and they exhibit much lower intrinsic stress than the carbon films, thereby facilitating the deposition of thick films. Very little is known about the mechanical properties of CVD carbon nitrides. In Ref. [12], a hardness of 20 GPa was measured and no dependence on the nitrogen content in a range of 0 to 37 at.% was observed.
2. Experimental * Corresponding author. Tel: +49 371 5313553; Fax: +49 371 5313042; e-mail: [email protected]
The samples have been prepared by ion beam-assisted filtered cathodic vacuum arc deposition. A carbon
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plasma stream emerging from a vacuum arc discharge is passed through a magnetic coil, before deposition onto the substrate. The magnetic coil filters out neutral carbon atoms and macroparticles. Nitrogen is provided by admittance of a gas flow or by the bombardment of the growing film with nitrogen ions from a Kaufmantype ion source. The copper substrate holder is water cooled and it is possible to apply a d.c. or r.f. substrate bias voltage. The deposition system has been described elsewhere in more detail [13]. Four types of films were prepared: a tedrahedral amorphous carbon film and a carbon nitride film with 2 at.% nitrogen, both using an r.f. substrate bias voltage; a carbon nitride film with 2 at.% nitrogen grown onto a grounded substrate; and a carbon nitride film with 19 at.% nitrogen using the ion source. The deposition parameters are given in Table 1. In the cases where the films were grown onto a grounded substrate, a substrate bias voltage of −100 V was applied during the first 2 min of the deposition. Polished tungsten carbide inserts (surface finished with 3 m diamond paste) were used as substrates. They were pre-treated in an ultrasonic bath with alcohol, and sputter-cleaned by a 500 eV Ar+ ion beam prior to the deposition. The stoichiometry and density of the films was obtained from elastic recoil detection analysis ( ERDA) and profilometry of comparable films deposited onto silicon substrates [13]. Load-displacement curves were recorded by a Shimadzu DUH-202 microindenter using a Vickers-type indenter. The maximum load was 100 mN. The hardness and Young’s modulus were calculated from the unloading curves by a procedure described in Ref. [14]. The measurement was repeated 10 times for each sample. The intrinsic compressive stress was calculated from
the curvature of comparable samples deposited onto Si substrates using Stoney’s equation [15]: h2 E s s= s f 1−n 6Rh s f where a Young’s modulus of E =169 GPa and a Poisson s number of n =0.28 were assumed for the (001) silicon s substrates. h and h are the thickness of the substrate s f and the film, respectively. R is the measured radius of curvature of the coated substrate. The abrasive wear rate was measured using a calotte tester [16 ]. A 100Cr6 steel ball, rotating around its axis of symmetry with a speed of v=30 rpm, was pressed with a normal load of F =0.54 N onto the sample. A n diamond suspension with a grain size of 1 mm was used as an abrasive medium. The test was stopped after t= 3 min and the wear rate was calculated according to: r =1ph2(3R−h)/(F 2pRvt) n w 3
(1)
where h is the maximum depth and R=15 mm is the radius of the steel ball. The results were averaged over four tests. The scratch tests were performed with a load rate of 60 N/min and a speed of 10 mm/min. A Rockwell-type diamond stylus with 200 mm tip radius was used as an indenter. The critical load was determined from the length of the scratch track from the start to the position of the first optically observable film delamination and from the onset of the integrated acoustic emission (IAE ) signal. Here, the results from the IAE are omitted since both methods yield very similar critical loads. For each sample, four tests were done.
Table 1 Deposition parameters and E results for the carbon and CN films. F is the nitrogen flow, U the d.c. component resulting from the r.f. x N S substrate bias voltage, U the beam voltage of the Kaufman-type ion source, and j the nitrogen ion current density extracted from the beam N Kaufman-type ion source. The carbon ion current density was 1.5 mA cm−2 in all cases Sample
F (sccm) N
U (V ) S
U (V ) beam
j (mA cm−2) N
N content (at.%)
Density (g cm−3)
ta-C CN (I ) 0.02 CN (II ) 0.02 CN 0.23
– 3 3 3
−80 (r.f.) −80 (r.f.) 0 0
– – – −100
– – – 0.15
– 2 2 19
2.9 2.8 2.6 2.4
Table 2 The mechanical properties of the carbon and carbon nitride samples. F is the critical load determined by microscope investigation of the scratch track c Sample
Thickness (mm)
Hardness (GPa)
Young’s mod. (GPa)
Stress (GPa)
Wear rate (10−15m3 mN−1)
F (N ) c
ta-C CN (I ) 0.02 CN (II ) 0.02 CN 0.23 Substrate
1.2 1.6 1.1 1.9 –
56 57 45 37 23
530 602 499 405 520
11.5 8.9 3.3 3.1 –
0.47 0.91 0.97 2.69 –
12.6 16.9 26.4 50 –
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Fig. 1. The loading and unloading curves of all samples.
3. Results and discussion In Table 1, the composition and mass density of the samples are given. The small amount of nitrogen incorporated in sample CN (I ) does not change the mass 0.02 density significantly, compared with the ta-C sample. The density of sample CN (II ) is decreased. This is 0.02 very similar to the case of ta-C, where the critical influence of the substrate bias voltage on the film density is well known [6 ]. Nitrogen ion bombardment of the growing film further decreases the mass density in the case of the sample CN . 0.23 Table 2 gives the mechanical properties. The hardness and Young’s modulus of the samples correlate with their mass density. In all cases, the hardness of the coated substrate is higher than the hardness of the substrate. However, the Young’s modulus of 520 GPa of the uncoated substrate could not be further improved essentially by the coatings. In Fig. 1, the load-displacement
curves are demonstrated. The coated substrates show a lower indentation depth and a higher elastic recovery. The increasing hardness is accompanied by an increase of the compressive stress. Due to the compressive stress, the maximum film thickness is limited. For example, the maximum thickness of a ta-C samples is about 1.5 mm on a tungsten carbide inserts and only about 300 nm on a sputter-cleaned silicon substrate. In general, the results on the wear rates show similar trends as the results of the hardness measurements. However, the CN (I ) sample has twice the wear rate 0.02 of the ta-C sample at comparable hardness. The wear rate of the high nitrogen content sample, CN , is 0.23 further increased but is still lower than the typical wear rate of a-C:H films of about 3.4×10−15 m3 mN−1 [16 ]. The critical load in the scratch test depends on the Young’s modulus of the film, the film thickness, the compressive stress and the bonding of the film to the substrate. In general, thicker films with higher Young’s moduli have higher critical loads due to their ability to spread the load to a larger area of the substrate. However, due to the intrinsic compressive stress, the intrinsic shear stress at the interface increases with the film thickness and a small extrinsic shear stress will lead to the delamination of the film. For example, the ta-C film is thicker than the CN (II ) and has a higher 0.02 Young’s modulus but a lower critical load due to its higher compressive stress. The lower Young’s modulus of sample CN is compensated by its low intrinsic 0.23 stress and its larger thickness, which leads to the highest critical load of all samples. Fig. 2 shows a scanning electron microscopy (SEM ) image of the fracture surface of sample CN . Here, a 0.23 section of the scratch track produced by the Rockwelldiamond stylus is visible. The corresponding normal load was about 25 N, which is well below the critical
Fig. 2. SEM micrograph showing the fracture surface of sample CN , intersecting the scratch track. The corresponding normal load is 0.23 F #25 N. N
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Fig. 3. Deposition parameters and properties of the gradient layer system in dependence on the deposition time. The total deposition time was 1 h 45 min.
load. Nevertheless, the film and substrate are deformed, but the film still protects the substrate which is confirmed by the low penetration depth of the diamond stylus (1.0 mm at the maximum load of 50 N ). For comparison, at this load, complete film delamination occurs for the ta-C-coated sample and the penetration depth is 2.5 mm. The good adherence of the CN strongly suggests 0.23 the use of this material as an interfacial adhesion layer for ta-C films. Fig. 3 gives the deposition parameters and properties for such a layer system. The nitrogen concentrations and compressive stresses were measured on homogeneous films deposited on Si substrates with
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the same parameters. The actual dependence of the intrinsic stress on the substrate bias voltage was not measured. The material of the first layer is equal to the material of the CN sample. Then, the nitrogen 0.23 content is reduced by decreasing the nitrogen ion current density, j . For the reasons explained later, the CN N x layers are followed by a series of ta-C layers prepared at different substrate bias voltages to control the intrinsic stress of these layers [17]. Due to this interfacial layer system, it was possible to increase the maximum thickness of a ta-C film from about 1.5 to 3.1 mm. The hardness and Young’s modulus of this sample, ta-C(II ), are 72 and 741 GPa, respectively. They are even higher, as in the case of the ta-C sample without the interfacial layer system, because of the lower influence of the substrate on the results of the indentation test. To get more insight into the relationship between the intrinsic compressive stress and film adhesion, we have performed a simple finite element (FEM ) analysis. The aim of this analysis is to calculate the shear stress at the substrate film interface, which is crucial for the adhesion. Quadrilateral elements with a linear interpolation function were used. For simplicity, the strain, e , in y 22 direction was set to e =0 and a constant element size 22 was used. The normal and shear forces at the boundary were set to zero. Fig. 4 demonstrates the result of a film of 1 mm thickness on a 7.5 mm thick substrate. The film surface is located in the x–z-plane. Both the film and the substrate were assumed to have a bulk modulus of 500 GPa and a Poisson number of n=0.3. The stress in the film was s =s =9.6 GPa (e =e =0.01) in the 11 33 11 33 flat, unrelaxed starting-geometry. Most interesting is the non-linear curvature of the left and right boundary (x#±10 mm) since, in the theory of plates, a linear dependence is assumed [18]. Fig. 5 shows the shear
Fig. 4. The relaxed FEM mesh. For clarity, only every fourth row and column are drawn. The nodes of the left, right and top boundaries are drawn enlarged.
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Fig. 5. The shear stress, s , versus the x and z coordinates. The film 13 with intrinsic compressive stress is located between z#0 mm and z#−1 mm. The element width and height is 1/8 mm. A Young’s modulus of 500 GPa and a Poisson number of n=0.3 were assumed for the film and the substrate. The initial strain was e =e =0.01. 11 33
stress in the film and the substrate. The maximum shear stress is located near the left and right boundaries. This is in accordance with the analytically-obtained result of Suhir [19]. The maximum shear stress reduces strongly in the substrate with increasing depth (decreasing z). Therefore, the deposition of a stress-free layer onto the substrate before the deposition of the stressed top layer will lead to a considerable reduction of the shear stress at the substrate surface. This is in contradiction to the results of Suhir, where a reduction in the shear stress can only be obtained if an additional layer with tensile stress is used. We believe that the reason for this discrepancy might be the restrictive assumption applied in the theory of plates. However, the result of the FEM calculation are supported by experimental results. In the case of cubic boron nitride (c-BN ), it was observed that a c-BN film deposited onto an about 100 nm thick hexagonal boron nitride (h-BN ) layer exhibits much better adherence than a c-BN layer deposited directly onto the silicon substrate [20,21]. By means of the stress-free layer, the maximum of the shear stress can be shifted but not reduced. Indeed, in the case of the boron nitride bilayer, the delamination then takes place at the c-BN/h-BN interface [21]. A reduction of the shear stress maximum can be obtained by a smoothing of the intrinsic stress depth profil. This is demonstrated in Fig. 6. As in the former example, the film at the top is 1 mm thick and has a compressive stress of 9.6 GPa. The two next layers are 0.5 mm in thickness and have compressive stresses of 6.4 and 3.2 GPa, respectively. The substrate is 7.5 mm in thick-
Fig. 6. The shear stress, s , versus the x and z coordinates of a stressed 13 layer deposited onto a gradient layer system. The gradient layer system is located between z#−1 mm and z#−2 mm.
ness. This smoother increase of the intrinsic compressive stress nearly bisects the maximum of the shear stress.
4. Conclusions We examined the mechanical properties of carbon nitride samples with nitrogen contents ranging from 0 to 20 at.%. On the one hand, the hardness, the Young’s modulus and the wear resistivity are reduced with increasing nitrogen content, but on the other hand, the adhesion of the films increases at higher nitrogen content. In a second part, the good adherence of carbon nitride and the possibility of controlling the intrinsic compressive stress of carbon films by applying different substrate bias voltages is used to form an interfacial layer system, which improves the adhesion of a highly stressed ta-C film. The effect of a smooth stress profile is explained by a simple FEM analysis.
Acknowledgements This work was partly supported by the Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie under grant number 03 N 5002 C. The authors are grateful for helpful discussions with N. Schwarzer. The scratch tests and abrasive wear tests were performed at Roth&Rau Oberfla¨chentechnik GmbH Wu¨stenbrand and by C. Siebert at the Fraunhoferinstitut fu¨r Schicht- und Oberfla¨chentechnik (Braunschweig), respectively.
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