- Email: [email protected]
Growth and characterisation of boron–carbon–nitrogen coatings obtained by ion beam assisted evaporation
Vacuum 64 (2002) 199–204
Growth and characterisation of boron–carbon–nitrogen coatings obtained by ion beam assisted evaporation R. Gagoa,*, I. Jime! neza, I. Garc!ıab, J.M. Albellaa a
Instituto de Ciencia de Materiales de Madrid (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain Centro Nacional de Investigaciones Metalu!rgicas (CSIC), Gregorio del Amo 8, 28040 Madrid, Spain
b
Abstract Ion beam assisted deposition (IBAD) techniques have been employed to produce thin films composed of boron, carbon, and nitrogen atoms, including amorphous carbon (a-C), carbon nitride (CNx) and ternary compounds (BCxNy). The films were deposited by evaporating either graphite or boron carbide (B4C) targets, with simultaneous ion bombardment from a precursor N2 +CH4+Ar gas mixture. The composition and bonding structure of the films have been carefully studied, including the analysis with time of flight elastic recoil detection analysis (TOF-ERDA) and X-ray absorption near-edge spectroscopy (XANES). Mechanical characterisation of the films has been also performed, including measurements of hardness, elastic modulus, and friction coefficients. The optimal values encountered are hardness of B35 GPa, and friction coefficients of B0.05. Also, the thermal stability of the films has been examined by annealing under vacuum conditions. The applicability of the coatings is discussed in terms of all these parameters. r 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.05.Je; 81.15.Jj; 62.20.Qp; 68.60.Dv Keywords: Hard coatings; BCN; IBAD; Mechanical properties; XANES
1. Introduction At present, there are large efforts in many laboratories to synthesise coatings within the B– C–N system, capable of withstanding the requirements imposed by the technological applications (high hardness, good adherence, low friction coefficient and high wear resistance). We have studied the synthesis of BCN materials by ion beam assisted deposition technique (IBAD), and *Corresponding author. Tel.: +349-1-334-9000; fax: +3491-372-0623. E-mail address: [email protected] (R. Gago).
have related the composition and bonding structure of the coatings with their thermal stability and mechanical properties. In previous work, we have reported studies on Ar ion assistance during graphite evaporation for the deposition of amorphous carbon films [1,2] and nitrogen assistance for the growth of amorphous carbon nitride films [3,4]. Now, we have focused on the growth of ternary compounds with the addition of reactive ions during the deposition [5]. In the present case, we have evaporated boron carbide as the source of carbon and boron atoms. The simultaneous bombardment of the surface has been performed with ions from mixtures of Ar + N2 + CH4 gases
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 1 4 - 1
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R. Gago et al. / Vacuum 64 (2002) 199–204
at different ion energies and fluxes. Ar atoms are used to enlarge energy and momentum transfer in the ion–atom collisions to promote energetic sp3 hybridisation.
2. Experiment The films were grown in a high vacuum chamber with a base pressure of 2 10 7 mbar on p-type (1 0 0) oriented Si substrates. The substrates were successively cleaned with trichloroethylene, acetone and ethanol before entering the chamber. Further cleaning with an Ar+ beam of 10 mA and 300 V for 2 min was also performed prior to deposition. The deposition system is equipped with a 5 kW electron gun evaporator and a 3 cm Kauffman assisting ion gun. The carbon and boron atom beams were obtained by evaporating either graphite or boron carbide (B4C) lumps of approximately 3 mm size, stored in a 4 cm3 liner. The evaporation rate obtained was between 3 and ( /s at an electron acceleration voltage of 7 keV 4A and an electron current of 150 and 75 mA for graphite and B4C, respectively. The assistance was performed with a nitrogen and methane gas mixture at different current and ion voltages. The composition of the samples was determined by Time-Of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) [6]. The experiments were performed with the 5 MV tandem accelerator facility at University of Helsinki, using an incident beam of 48 MeV I9+ and the recoils were detected for a scattering angle of 401. This technique is very useful for the detection of light elements, since the simultaneous detection of kinetic energy and velocity allows the determination of the mass of the recoil. Thus, it is possible to separate the spectrum for each mass practically clean from unwanted background or interference from other signals. This feature is of great help in the evaluation of composition depth profiles. Additional information of the sample composition was obtained with X-ray absorption near edge spectroscopy (XANES). The XANES spectra also provide information about the bonding structure. The measurements were performed at the SACEMOR end-station (beam line SA72) of the
Laboratoire pour L’Utilisation du Rayonnement Electromagnetique (LURE) and the beamline 8 of the Stanford Synchrotron Radiation Laboratory (SSRL). The data were acquired in the total yield mode by monitoring the electron emission from the sample. The signal was normalised to the photocurrent obtained from a gold-covered grid, recorded simultaneously. The XANES spectrum is the result of electron transitions from a core level to the unoccupied states in one atom due to the absorption of X-ray photons. It provides a picture of the local density of unoccupied states of the material [7], and is very sensitive to the local environment and the bonding structure of the atoms. The hardness of the films was determined from the load–unload curves of nanoindentation experiments, as described in previous work [2,3]. The friction coefficient was measured by linear reciprocation, applying a force of 0.5 N with a corundum ball over 10,000 cycles. The measurements were performed with 50% of relative humidity and 221C of ambient temperature.
3. Results and discussion 3.1. Composition and bonding structure The composition depth profile for a BCN sample grown with N2+CH4 ion assistance is shown in Fig. 1 as an example. The films are free from oxygen in the bulk and only some signal appears at the film surface and film–substrate interface. As a result of the addition of CH4 there is appreciable hydrogen in the film, although the content is always below 20 at%. The relation between B, C and N covers a wide range of values depending on the deposition conditions. Finally, the films are homogeneous and, therefore, any influence of hetero-structures in the mechanical properties of the films is excluded. Fig. 2 shows the XANES spectra for a set of BCN films with different stoichiometry and bonding structure. The shape of the absorption edges is indicative of the bonding environment. Ternary BCN compounds can exhibit three different bonding structures: hexagonal (resembling gra-
R. Gago et al. / Vacuum 64 (2002) 199–204
phite), cubic (resembling diamond) and B4C-like, composed of B12 icosahedral units. In the bottom part of Fig. 2 the reference spectra are shown. Model curves for the B(1 s) are taken from hexagonal BN (h-BN) and cubic BN (c-BN) crystalline samples, as references for sp2 and sp3 hybridisations, respectively, and the spectrum
Fig. 1. Composition depth profile of a BCN film obtained from TOF-ERDA analysis.
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from crystalline B4C, representing the signature from B12 icosahedral units. The sharp peak at 192.0 eV in the h-BN curve is a p excitonic resonance [8]. The features starting at B194 eV in c-BN and B196 eV in h-BN correspond to s states. The sharp peak at 190.9 eV in the B4C spectrum is not well understood, and only appears in crystalline samples [9]. Amorphous B4C presents smooth features starting at B189 eV, with several less intense peaks in the 192–196 eV region [10]. Model spectra for the C(1 s) are taken from graphite and diamond, representing sp2 and sp3 references, and from crystalline B4C. The sharp peak at 287.0 eV only appears in crystalline samples, while amorphous B4C presents a smoother lineshape. Finally, the model spectra for the N(1 s) XANES are taken from h-BN and c-BN crystalline references, the former presenting a p* peak at 401 eV, that is absent in c-BN. With regards to the ternary compounds, Fig. 2 displays the spectra from three typical samples, each corresponding to a predominant bonding structure. The bottom curves represent a sample with a composition B0.7C0.2N0.1, and a dominant
Fig. 2. B(1 s), C(1 s) and N(1 s) XANES spectra from selected ternary BCN films, together with reference spectra from graphite, diamond, h-BN, c-BN and B4C.
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carbon, but now the incorporation of nitrogen is less effective, resulting in the production of BxClike compositions and structures (region 3). This is not surprising since BN and BxC materials are stable bulk compounds. There is a narrow window of deposition parameters yielding ternary compounds, which are not biased towards BN or BxC stoichiometries, or without segregated phases. 3.2. Mechanical properties and thermal stability Fig. 3. Composition of the BCN coatings for different growth conditions.
B4C-like structure, as evidenced from the B(1 s) and C(1 s) lineshape. The 401 eV peak at the N(1 s) edge indicates that boron is forming p bonds within the B4C network. The middle curves represent a sample with composition B0.6C0.1N0.3 and a predominant hexagonal structure, as evidenced by the p peaks in the B(1 s) and N(1 s) spectra. The C(1 s) curve shows a manifold of p peaks corresponding to the p bonds of carbon with boron and nitrogen in a h-BCN network, compared with the graphite p peat at 285.4 eV. The top curves represent a sample with composition B0.55C0.10N0.35 and dominant tetrahedral bonding, typical of a cubic structure. The B(1 s) and N(1 s) positions of the main edge coincide with the reference c-BN, although there is a small presence of h-BN material. The position of the C(1 s) s edge also supports the tetrahedral coordination. A comparative study of the composition of the samples for different assistance processes is illustrated in Fig. 3. Three different composition regions can be distinguished. Region 1 corresponds to carbon evaporation with N2 assistance. In this case we obtain CNx films with a low nitrogen content (xo0.3). Alternatively, the evaporation of B4C with N2 bombardment gives rise to the incorporation of larger amounts of nitrogen in the structure in detriment of the carbon content. Under these conditions we are able to obtain ternary compounds, although with a low carbon content and a BN-like composition and structure (region 2). Finally, by using B4C evaporation with N2+CH4 bombardment the films become richer in
We have shown in the previous section the possibility of growing films with different film structure and composition by adequate selection of the processing parameters. However, we are mostly interested in the final properties of the coating in order to discern if they are appropriate for tribological applications, and if the films are capable of sustaining the high temperatures produced under operation. In this section we show the results of several mechanical tests performed in the samples, and the results of vacuum annealing experiments. Fig. 4 shows in the bottom panel the thermal stability of the different families of films, determined from vacuum annealing experiments. The films were annealed in vacuum in the XANES analysis chamber by direct flow of current through the Si substrate, and the temperature was monitored with an infrared pyrometer previously calibrated to the Si emissivity. The films are considered stable as long as the film composition and bonding structure are maintained. The central panel of Fig. 4 shows the ranges of the friction coefficient found for our samples by linear reciprocation. The top panel displays the hardness values measured by nanoindentation. As it is clear from Fig. 4, hardness is not the only mechanical property of interest. The performance of the coatings can be more affected by the adhesion to the substrate, the thermal stability or the friction coefficient, depending on the actual application of the coating. Films with different composition and structure within the B–C–N triangle of compositions present distinct mechanical properties. In this way, the optimal mechanical characteristics for a certain application can be chosen.
R. Gago et al. / Vacuum 64 (2002) 199–204
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independently the ion/atom ratio and the ion energy. Ternary BCN compounds can be grown by evaporation of B4C with simultaneous assistance of nitrogen ions. This yields carbon-poor ternary hexagonal compounds. The use of nitrogen and argon ions permits the growth of ternary tetrahedral phases also poor in carbon. The addition of methane to the gas mixture results in compositions richer in carbon, at the expense of hydrogen incorporation. Since the family of BCN compounds is varied in composition and structure, their mechanical characteristics also cover a wide range. In this way, coatings with a certain optimised property can be chosen, aiming towards a specific application. Hardness values up to 35 GPa, and friction coefficients as low as 0.06 have been obtained.
Acknowledgements
Fig. 4. Thermal stability, friction coefficent and hardness of different groups of materials within the BCN triangle of compositions.
The BxC coatings, based on the icosahedral structure, present the higher hardness although they are fragile and exhibit moderate adhesion to the substrate. On the contrary, the ternary films, although softer than BxC compounds, have an extremely good friction coefficient and a good adhesion. Finally, the a-C and a-CNx films do not have as good characteristics as the other film but can be economically competitive, since graphite targets are cheaper that boron carbide ones.
This work has been partially financed by the Spanish CICYT under Project MAT99-0830 and by the BRITE-EURAM contract BRPR-CT970487. We are indebted to L. J. Terminello for the synchrotron work at SSRL, a facility supported by the US Department of Energy, Office of Basic Energy Science. We are also indebted to P. Parent, and C. Laffon for their help with the synchrotron measurements at LURE, which were financed by the TMR Program of the European Union. We express our gratitude to T. Sajavarra, and E. Rauhala for the TOF-ERDA measurements performed at the University of Helsinky. The Department of Metallurgy and Materials Engineering of the Catholic University of Leuven (Belgium) is also acknowledged for the use of their tribological characterisation equipment.
4. Conclusions References The IBAD technique is a powerful method of growing BCN films with selected composition and bonding structure. This feasibility stems from the flexible election of the evaporated material and the reactive ions, including the possibility to control
[1] Gago R, Bo. hme O, Albella JM, Rom!an E. Diamond Rel Mat 1999;8:1944. [2] Gago R, Jim!enez I, Albella JM, Climent-Font A, C!aceres D, Vergara I, Terminello LJ, Banks JC, Doyle BL. J Appl Phys 2000;87:8174.
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R. Gago et al. / Vacuum 64 (2002) 199–204
[3] Jim!enez I, Gago R, Albella JM, C!aceres D, Vergara I. Phys Rev B 1999;62:4261. [4] Gago R, Jim!enez I, C!aceres D, Agullo! -Rueda F, Sajavara T, Albella JM, Climent-Font A, Vergara I, R.ais.anen J, Rau. hala E. Chem Matter 2001;13(1):129. [5] Gago R, Jim!enez I, Albella JM. Thin Solid Films 2000;373:277. [6] Withlow HJ, Possnert G, Petersson CS. Nucl Instrum Methods B 1987;27:448.
[7] Sto. hr J. NEXAFS spectroscopy. Berlin: Springer, 1990. [8] Jim!enez I, Jankowski AF, Terminello LJ, Sutherland DGJ, Carlisle JA, Doll GL, Tong WM, Shuh DK, Himpsel FJ. Phys Rev B 1997;55:12,025. [9] Jim!enez I, Sutherland DGJ, Carlisle JA, van Buuren A, Terminello LJ, Himpsel FJ. Phys Rev B 1998;57:13,167. [10] Jim!enez I, Terminello LJ, Himpsel FJ, Grush M, Callcot TA, Ederer DL. J Electron Spectros Relat Phenom 1999;101/103:611.
Growth and characterisation of boron–carbon–nitrogen coatings obtained by ion beam assisted evaporation R. Gagoa,*, I. Jime! neza, I. Garc!ıab, J.M. Albellaa a
Instituto de Ciencia de Materiales de Madrid (CSIC), Campus de Cantoblanco, 28049 Madrid, Spain Centro Nacional de Investigaciones Metalu!rgicas (CSIC), Gregorio del Amo 8, 28040 Madrid, Spain
b
Abstract Ion beam assisted deposition (IBAD) techniques have been employed to produce thin films composed of boron, carbon, and nitrogen atoms, including amorphous carbon (a-C), carbon nitride (CNx) and ternary compounds (BCxNy). The films were deposited by evaporating either graphite or boron carbide (B4C) targets, with simultaneous ion bombardment from a precursor N2 +CH4+Ar gas mixture. The composition and bonding structure of the films have been carefully studied, including the analysis with time of flight elastic recoil detection analysis (TOF-ERDA) and X-ray absorption near-edge spectroscopy (XANES). Mechanical characterisation of the films has been also performed, including measurements of hardness, elastic modulus, and friction coefficients. The optimal values encountered are hardness of B35 GPa, and friction coefficients of B0.05. Also, the thermal stability of the films has been examined by annealing under vacuum conditions. The applicability of the coatings is discussed in terms of all these parameters. r 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.05.Je; 81.15.Jj; 62.20.Qp; 68.60.Dv Keywords: Hard coatings; BCN; IBAD; Mechanical properties; XANES
1. Introduction At present, there are large efforts in many laboratories to synthesise coatings within the B– C–N system, capable of withstanding the requirements imposed by the technological applications (high hardness, good adherence, low friction coefficient and high wear resistance). We have studied the synthesis of BCN materials by ion beam assisted deposition technique (IBAD), and *Corresponding author. Tel.: +349-1-334-9000; fax: +3491-372-0623. E-mail address: [email protected] (R. Gago).
have related the composition and bonding structure of the coatings with their thermal stability and mechanical properties. In previous work, we have reported studies on Ar ion assistance during graphite evaporation for the deposition of amorphous carbon films [1,2] and nitrogen assistance for the growth of amorphous carbon nitride films [3,4]. Now, we have focused on the growth of ternary compounds with the addition of reactive ions during the deposition [5]. In the present case, we have evaporated boron carbide as the source of carbon and boron atoms. The simultaneous bombardment of the surface has been performed with ions from mixtures of Ar + N2 + CH4 gases
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 3 1 4 - 1
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R. Gago et al. / Vacuum 64 (2002) 199–204
at different ion energies and fluxes. Ar atoms are used to enlarge energy and momentum transfer in the ion–atom collisions to promote energetic sp3 hybridisation.
2. Experiment The films were grown in a high vacuum chamber with a base pressure of 2 10 7 mbar on p-type (1 0 0) oriented Si substrates. The substrates were successively cleaned with trichloroethylene, acetone and ethanol before entering the chamber. Further cleaning with an Ar+ beam of 10 mA and 300 V for 2 min was also performed prior to deposition. The deposition system is equipped with a 5 kW electron gun evaporator and a 3 cm Kauffman assisting ion gun. The carbon and boron atom beams were obtained by evaporating either graphite or boron carbide (B4C) lumps of approximately 3 mm size, stored in a 4 cm3 liner. The evaporation rate obtained was between 3 and ( /s at an electron acceleration voltage of 7 keV 4A and an electron current of 150 and 75 mA for graphite and B4C, respectively. The assistance was performed with a nitrogen and methane gas mixture at different current and ion voltages. The composition of the samples was determined by Time-Of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) [6]. The experiments were performed with the 5 MV tandem accelerator facility at University of Helsinki, using an incident beam of 48 MeV I9+ and the recoils were detected for a scattering angle of 401. This technique is very useful for the detection of light elements, since the simultaneous detection of kinetic energy and velocity allows the determination of the mass of the recoil. Thus, it is possible to separate the spectrum for each mass practically clean from unwanted background or interference from other signals. This feature is of great help in the evaluation of composition depth profiles. Additional information of the sample composition was obtained with X-ray absorption near edge spectroscopy (XANES). The XANES spectra also provide information about the bonding structure. The measurements were performed at the SACEMOR end-station (beam line SA72) of the
Laboratoire pour L’Utilisation du Rayonnement Electromagnetique (LURE) and the beamline 8 of the Stanford Synchrotron Radiation Laboratory (SSRL). The data were acquired in the total yield mode by monitoring the electron emission from the sample. The signal was normalised to the photocurrent obtained from a gold-covered grid, recorded simultaneously. The XANES spectrum is the result of electron transitions from a core level to the unoccupied states in one atom due to the absorption of X-ray photons. It provides a picture of the local density of unoccupied states of the material [7], and is very sensitive to the local environment and the bonding structure of the atoms. The hardness of the films was determined from the load–unload curves of nanoindentation experiments, as described in previous work [2,3]. The friction coefficient was measured by linear reciprocation, applying a force of 0.5 N with a corundum ball over 10,000 cycles. The measurements were performed with 50% of relative humidity and 221C of ambient temperature.
3. Results and discussion 3.1. Composition and bonding structure The composition depth profile for a BCN sample grown with N2+CH4 ion assistance is shown in Fig. 1 as an example. The films are free from oxygen in the bulk and only some signal appears at the film surface and film–substrate interface. As a result of the addition of CH4 there is appreciable hydrogen in the film, although the content is always below 20 at%. The relation between B, C and N covers a wide range of values depending on the deposition conditions. Finally, the films are homogeneous and, therefore, any influence of hetero-structures in the mechanical properties of the films is excluded. Fig. 2 shows the XANES spectra for a set of BCN films with different stoichiometry and bonding structure. The shape of the absorption edges is indicative of the bonding environment. Ternary BCN compounds can exhibit three different bonding structures: hexagonal (resembling gra-
R. Gago et al. / Vacuum 64 (2002) 199–204
phite), cubic (resembling diamond) and B4C-like, composed of B12 icosahedral units. In the bottom part of Fig. 2 the reference spectra are shown. Model curves for the B(1 s) are taken from hexagonal BN (h-BN) and cubic BN (c-BN) crystalline samples, as references for sp2 and sp3 hybridisations, respectively, and the spectrum
Fig. 1. Composition depth profile of a BCN film obtained from TOF-ERDA analysis.
201
from crystalline B4C, representing the signature from B12 icosahedral units. The sharp peak at 192.0 eV in the h-BN curve is a p excitonic resonance [8]. The features starting at B194 eV in c-BN and B196 eV in h-BN correspond to s states. The sharp peak at 190.9 eV in the B4C spectrum is not well understood, and only appears in crystalline samples [9]. Amorphous B4C presents smooth features starting at B189 eV, with several less intense peaks in the 192–196 eV region [10]. Model spectra for the C(1 s) are taken from graphite and diamond, representing sp2 and sp3 references, and from crystalline B4C. The sharp peak at 287.0 eV only appears in crystalline samples, while amorphous B4C presents a smoother lineshape. Finally, the model spectra for the N(1 s) XANES are taken from h-BN and c-BN crystalline references, the former presenting a p* peak at 401 eV, that is absent in c-BN. With regards to the ternary compounds, Fig. 2 displays the spectra from three typical samples, each corresponding to a predominant bonding structure. The bottom curves represent a sample with a composition B0.7C0.2N0.1, and a dominant
Fig. 2. B(1 s), C(1 s) and N(1 s) XANES spectra from selected ternary BCN films, together with reference spectra from graphite, diamond, h-BN, c-BN and B4C.
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R. Gago et al. / Vacuum 64 (2002) 199–204
carbon, but now the incorporation of nitrogen is less effective, resulting in the production of BxClike compositions and structures (region 3). This is not surprising since BN and BxC materials are stable bulk compounds. There is a narrow window of deposition parameters yielding ternary compounds, which are not biased towards BN or BxC stoichiometries, or without segregated phases. 3.2. Mechanical properties and thermal stability Fig. 3. Composition of the BCN coatings for different growth conditions.
B4C-like structure, as evidenced from the B(1 s) and C(1 s) lineshape. The 401 eV peak at the N(1 s) edge indicates that boron is forming p bonds within the B4C network. The middle curves represent a sample with composition B0.6C0.1N0.3 and a predominant hexagonal structure, as evidenced by the p peaks in the B(1 s) and N(1 s) spectra. The C(1 s) curve shows a manifold of p peaks corresponding to the p bonds of carbon with boron and nitrogen in a h-BCN network, compared with the graphite p peat at 285.4 eV. The top curves represent a sample with composition B0.55C0.10N0.35 and dominant tetrahedral bonding, typical of a cubic structure. The B(1 s) and N(1 s) positions of the main edge coincide with the reference c-BN, although there is a small presence of h-BN material. The position of the C(1 s) s edge also supports the tetrahedral coordination. A comparative study of the composition of the samples for different assistance processes is illustrated in Fig. 3. Three different composition regions can be distinguished. Region 1 corresponds to carbon evaporation with N2 assistance. In this case we obtain CNx films with a low nitrogen content (xo0.3). Alternatively, the evaporation of B4C with N2 bombardment gives rise to the incorporation of larger amounts of nitrogen in the structure in detriment of the carbon content. Under these conditions we are able to obtain ternary compounds, although with a low carbon content and a BN-like composition and structure (region 2). Finally, by using B4C evaporation with N2+CH4 bombardment the films become richer in
We have shown in the previous section the possibility of growing films with different film structure and composition by adequate selection of the processing parameters. However, we are mostly interested in the final properties of the coating in order to discern if they are appropriate for tribological applications, and if the films are capable of sustaining the high temperatures produced under operation. In this section we show the results of several mechanical tests performed in the samples, and the results of vacuum annealing experiments. Fig. 4 shows in the bottom panel the thermal stability of the different families of films, determined from vacuum annealing experiments. The films were annealed in vacuum in the XANES analysis chamber by direct flow of current through the Si substrate, and the temperature was monitored with an infrared pyrometer previously calibrated to the Si emissivity. The films are considered stable as long as the film composition and bonding structure are maintained. The central panel of Fig. 4 shows the ranges of the friction coefficient found for our samples by linear reciprocation. The top panel displays the hardness values measured by nanoindentation. As it is clear from Fig. 4, hardness is not the only mechanical property of interest. The performance of the coatings can be more affected by the adhesion to the substrate, the thermal stability or the friction coefficient, depending on the actual application of the coating. Films with different composition and structure within the B–C–N triangle of compositions present distinct mechanical properties. In this way, the optimal mechanical characteristics for a certain application can be chosen.
R. Gago et al. / Vacuum 64 (2002) 199–204
203
independently the ion/atom ratio and the ion energy. Ternary BCN compounds can be grown by evaporation of B4C with simultaneous assistance of nitrogen ions. This yields carbon-poor ternary hexagonal compounds. The use of nitrogen and argon ions permits the growth of ternary tetrahedral phases also poor in carbon. The addition of methane to the gas mixture results in compositions richer in carbon, at the expense of hydrogen incorporation. Since the family of BCN compounds is varied in composition and structure, their mechanical characteristics also cover a wide range. In this way, coatings with a certain optimised property can be chosen, aiming towards a specific application. Hardness values up to 35 GPa, and friction coefficients as low as 0.06 have been obtained.
Acknowledgements
Fig. 4. Thermal stability, friction coefficent and hardness of different groups of materials within the BCN triangle of compositions.
The BxC coatings, based on the icosahedral structure, present the higher hardness although they are fragile and exhibit moderate adhesion to the substrate. On the contrary, the ternary films, although softer than BxC compounds, have an extremely good friction coefficient and a good adhesion. Finally, the a-C and a-CNx films do not have as good characteristics as the other film but can be economically competitive, since graphite targets are cheaper that boron carbide ones.
This work has been partially financed by the Spanish CICYT under Project MAT99-0830 and by the BRITE-EURAM contract BRPR-CT970487. We are indebted to L. J. Terminello for the synchrotron work at SSRL, a facility supported by the US Department of Energy, Office of Basic Energy Science. We are also indebted to P. Parent, and C. Laffon for their help with the synchrotron measurements at LURE, which were financed by the TMR Program of the European Union. We express our gratitude to T. Sajavarra, and E. Rauhala for the TOF-ERDA measurements performed at the University of Helsinky. The Department of Metallurgy and Materials Engineering of the Catholic University of Leuven (Belgium) is also acknowledged for the use of their tribological characterisation equipment.
4. Conclusions References The IBAD technique is a powerful method of growing BCN films with selected composition and bonding structure. This feasibility stems from the flexible election of the evaporated material and the reactive ions, including the possibility to control
[1] Gago R, Bo. hme O, Albella JM, Rom!an E. Diamond Rel Mat 1999;8:1944. [2] Gago R, Jim!enez I, Albella JM, Climent-Font A, C!aceres D, Vergara I, Terminello LJ, Banks JC, Doyle BL. J Appl Phys 2000;87:8174.
204
R. Gago et al. / Vacuum 64 (2002) 199–204
[3] Jim!enez I, Gago R, Albella JM, C!aceres D, Vergara I. Phys Rev B 1999;62:4261. [4] Gago R, Jim!enez I, C!aceres D, Agullo! -Rueda F, Sajavara T, Albella JM, Climent-Font A, Vergara I, R.ais.anen J, Rau. hala E. Chem Matter 2001;13(1):129. [5] Gago R, Jim!enez I, Albella JM. Thin Solid Films 2000;373:277. [6] Withlow HJ, Possnert G, Petersson CS. Nucl Instrum Methods B 1987;27:448.
[7] Sto. hr J. NEXAFS spectroscopy. Berlin: Springer, 1990. [8] Jim!enez I, Jankowski AF, Terminello LJ, Sutherland DGJ, Carlisle JA, Doll GL, Tong WM, Shuh DK, Himpsel FJ. Phys Rev B 1997;55:12,025. [9] Jim!enez I, Sutherland DGJ, Carlisle JA, van Buuren A, Terminello LJ, Himpsel FJ. Phys Rev B 1998;57:13,167. [10] Jim!enez I, Terminello LJ, Himpsel FJ, Grush M, Callcot TA, Ederer DL. J Electron Spectros Relat Phenom 1999;101/103:611.