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Recent progress on the tribology of doped diamond-like and carbon alloy coatings: a review
Surface
and CoatingsTechnology
100~101
(1998)
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Recent progress on the tribology of doped diamond-like carbon alloy coatings: a review
and
C. Donnet * l&ok
Centrale de Lyon. Laboratoire
de Trihologie
et Dynamique des Syst&nes. UMR 5513. BP 163, 69 131 lh~lly,
Crde.y, France
Abstract Diamond-like carbon (DLC) coatingshave beenwidely recognizedas beinga wear-resistantsolid lubricant with a low friction coefficient. Its tribological behavior strongly dependsboth on the tribotesting conditions and the nature of the coating, which in turn dependson the technique usedfor film deposition. Recently, there have beenseveralattempts to improve the tribological behavior of DLC coatings by the addition of elements.such as silicon, nitrogen, fluorine and various metals.The paper will presentan updated review of the tribological propertiesof doped DLC, in comparisonwith the conventional hydrogenatedand non-hydrogenatedcarbonaceousfilms. 0 1998ElsevierScienceS.A. Keywords:
Diamond-like carbon (DLC); Doped DLC; Carbon alloy coatings;Friction; Wear
1. Introduction Diamond-like carbon (DLC ) coatings have been the subject of intensive studies for the last 20 years. Recent reviews summarize the knowledges on film deposition, characterization, properties and tribological behavior [l-4]. The general term DLC describes hydrogenated and non-hydrogenated carbon materials prepared by a variety of methods (PVD and PACVD techniques) and presenting of wide range of structure, composition and properties, such as low friction, high wear resistance, chemical inertness, a relatively high optical gap and high electrical resistivity. DLC films present a noteworthy example of thin films whose tribological behavior strongly depends both on the nature of the coating (controlled by the deposition procedure) and the testing conditions, including mechanical (contact pressure), cinematic (speed), physical (temperature) and chemical (nature of the environment) parameters. In particular, for varied deposition methods, the hydrogen concentration ranges from less than a few at.% to about 50 at.%. Hydrogen is important for obtaining a wide optical gap and a high electrical resistivity, removing midgap defect states, stabilizing the random network and preventing its collapse into a graphitic phase. The * Tel: + 334 72 18 62 80; fax: 334 78 43 33 83; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier PII SO’57-897X(97)00611-7
Science
B.V. All rights
reserved.
nature and properties (electrical conductivity, surface energy, etc.) of the DLC may also be modified by controlling the incorporation of dopants, such as silicon, fluorine, nitrogen and various metals. For example, an increase of the water wetting angle, associated with a decrease of the surface energy, is observed with the incorporation of Si or F in the DLC, whereas the opposite behavior is observed with the incorporation of 0 or N [51. The effect of Si or F on the surface energy of the DLC films is attributed to the reduction of mainly of the polar part of the surface energy, due to the loss of sp’ C hybridization and dangling bonds. These carbon-based multicomponents films may also be combined to obtain property-controlled multilayer coatings [6], including: (1) interface layers, to improve the adhesion; (2) large number of repeated layers, with different intrinsic mechanical properties; and (3) diverse property layers, to combine several protective functionalities, such as corrosion protection, wear resistance,thermal isolation, electrical conductivity. diffusion barrier and adhesion to the substrate. The present paper reviews and summarizes the basic and latest understanding on the tribology of doped DLC films. A short review on the undoped DLC (a-C and a-C:H) coatings is provided for comparison at the beginning.
C. Donnef
2. Tribology
/ Surface
and Coatings
of conventional a-C and a-C:H coatings
Improvements in the tribological behavior and expansion of the use of DLC films for tribological protection in new applications have required an enhancement of the understanding of the friction and wear of these films and their dependence on the operating environment and film properties. Since review papers have recently been published on this subject [3,4], the tribology of DLC will be summarized here only in general terms. To be used as tribological coatings, DLC films must adhere well to the substrate material, the adhesive forces having to overcome the high internal stresses that otherwise would cause film delamination. Adhesion depends on the substrate material and can be affected by the deposition method. Good adhesion of DLC films to carbideand silicide-forming substrates has been found. The adhesion of DLC coatings to silicide forming metals can be improved by depositing an interfacial layer of amorphous silicon (2-4 nm thick) between the metal and the carbon film, thus forming an interfacial silicide layer promoted by the plasma even at a relatively low substrate temperature [ 71. Friction coefficients of adherent DLC films, typically ranging from 0.01 to more than 0.5, depending on the nature of the DLC film and the tribotesting conditions [3], are strongly governed by tribochemical effects [810]. The build-up of a transfer film, followed by easy shear within the interfacial material is the most frequently observed friction-controlling mechanism for DLC films. However, the shearing ability strongly depends on the nature of the surrounding gas present in the contact. From the pioneering experiment of Enke et al. [ll], DLC coatings are known to be extremely sensitive to the presence of oxidizing species (oxygen, water vapor) during friction, which give rise to a noticeable tribo-oxidation of the topcoats, generally increasing the friction and wear. Wear rates may reach extremely low values (down to 10e6 mm3 Nm-‘) but also very high values, which can inhibit the use of the coating in any applications. Liu et al. [ 121 have shown that the steady-state low friction of DLC films in ambient air is due to wear induced graphitization, i.e. formation of a low friction graphitized tribolayer. Both sliding velocity and applied load influence the graphitization process, due to temperature rise at contact asperities facilitating hydrogen release from the DLC structure. Lower friction coefficients ( lop2 range) have been frequently observed in dry and/or inert environments. However, the absence of a systematic control of oxygen and water vapor partial pressures in the reported experiments prevents precise identification of the partial pressure thresholds at which the friction changes. In the absence of oxidizing gas, as in an ultrahigh vacuum (UHV ), a large spread of friction values is observed in published results, from less than 0.01 up to 0.3 or more [3]. The ultralow
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friction of smooth DLC layers in inert environments is attributed by Gardos [ 131 to the high surface finish and the presence of ultrathin hydrocarbon-polymerlike topcoats with weak Van der Waals interactions between hydrogenated carbonaceous chains. The lo-fold, humidity and oxidation-caused, increase in friction from the lo-’ range to the lo- ’ range corresponds to an increase in bond strength from about 0.08 eV per bond, for Van der Waals bonding of hydrocarbons, to about 0.21 eV per bond, for hydrogen bonding at C=O sites by water molecules. Thus, most of the DLC films present a friction behavior evolution similar to MoS,, from UHV to ambient air, whereas diamond and graphite exhibit both higher friction in UHV (>0.5) but comparable friction values in ambient air, as mentioned by Buckley [ 141. The difficulty in correlating the properties of DLC with their tribological behavior, as determined by the preparation process, stems from the generally poor definition of all deposition conditions of a specific film, the difficulty of structural characterization of the amorphous materials, and the lack of any standardization of tribological characterization. Films with various hydrogen contents exhibit different physical properties, such as carbon hybridization, refractive index, hardness, stress, and tribological behaviors in controlled conditions. Recently, Donnet and Grill [ 151 have shown how to control the friction and wear of DLC films in high vacuum conditions, depending on the nature of the film. High-impact energy during the deposition (controlled directly by the bias and gas pressure in d.c. PACVD systems or by the power and the gas pressure in r.f. PACVD systems) induces high precursor dissociation and thus a more cross-linked carbon network, with a lower sp3 fraction, lower hydrogen content, and lower fraction of hydrogen bonded to carbon. Films deposited under such conditions are harder, with a higher stress and index of refraction, and a lower surface energy, and are characterized by a very high degree of friction (> 0.5) in UHV conditions. Ultralow friction and wear may be reached in UHV for DLC films if the hydrogen content is high enough (about 40 at.%), but with a carbon network sufficiently cross-linked and having a noticeable fraction of hydrogen (about 0.3) unbounded to carbon. With the deposition system used by the authors, an absolute bias increase from 500 to 800 V is sufficient to lower the hydrogen content (by about 6 at.%), and the fraction of hydrogen bounded to carbon in films deposited from acetylene, thus enhancing the friction in UHV from less than 0.01 to more than 0.5. This shows how the friction behavior of DLC films is sensitive to their structure and composition. As a result, much attention should be paid to the deposition process for controlling both friction and wear of DLC films. The main results related to the tribology of a-C and a-C:H films may be summarized as follows:
(1) relative high values of compressive stress (up to 3 GPa) result in poor adhesion to most of the substrates, requiring the deposition of adhesionpromoting interlayers, (2) transfer film build-up followed by easy-shear within the interfacial material is systematically observed, whatever the nature of the film and tribotesting conditions; ( 3 ) friction in ambient conditions (presence of oxygen and water vapor) leads to tribochemical oxidation of the DLC topcoats, confining the friction coefficients in the 10-r range; and (4) wear in ambient conditions as well as friction and wear in inert environments (including high vacuum conditions) are strongly affected by the deposition conditions. Therefore, the main objectives of the studies related to doped or alloyed carbon coatings are both to modify some of the surface-related properties of the films (i.e. surface energy) and to overcome tribological limitations, mainly by reducing the internal stress and inhibiting the triboreactivity of the coating regards oxygen and/or water vapor responsible for high friction and low wear resistance.
3. Tribology
of silicon-containing
DLC coatings
Oguri et al. [ 16,171 deposited a-C:H:Si coatings with a hydrogen content of about 40 at.% and a Si/C +Si ratio between 10 and 30 at.%, by a d.c. PACVD. Tribological tests (load lo-50 N, speed 0.2-3.0 m s- ‘) in ambient air (relative humidity RH=50-70%) gave wear rates lower than 1O-7 mm3 Nmt and friction coefficients ranging between 0.03 and 0.1. that is the lowest friction values observed with DLC films in ambient air conditions. The tribological behavior was attributed to the formation of SiO, wear particles and their interaction with the humid environment through tribochemical effects. Goranchev et al. [18] obtained a-C,:H:Si,-, films from r.f. reactive sputtering, with a carbon fraction .K ranging between 0.5 and 0.9. An increase in internal stress was associated with an increase in silicon content. Contrary to metal-containing films, the a-C,:H:Si, -x films were systematically amorphous and exhibited friction coefficients lower than 0.1. Hioki et al. [ 19,201 studied Si containing DLC films deposited from ion beam-assisted deposition (IBAD) with a composition C:Si:O =0.8:0.1:0.1 with the exception of 10 at.% of hydrogen. With relative humidities varying from 20 to 70% in air or nitrogen. friction coefficients ranged between 0.04 and 0.07. At higher humidities, friction increased up to 0.2. Ultralow friction values down to 0.02 or less were observed in dry nitrogen. A transfer film build-up was systematically observed on the steel counterpart. The same kinds of friction results
were already observed by Braun et al. [21] with films deposited by the same technique. The authors found also that for tool steel samples coated with an a-C:H:Si ( 100 nm thick) immersed in aqueous solutions of acetic acid or NaCl, the initiation of pitting corrosion compared to the bare substrate was delayed by a factor greater than 1000. Therefore, the coating was believed to have potentialities for corrosion protection. a-C:H:Si films with a silicon content ranging between 10 and 25 at.% were obtained from electron cyclotron resonance ECR-PCVD by Miyamoto and Miyake et al. [22-251. During microtribological tests (0.1 -mm diamond tip with a normal load of 10 mN), the films exhibited a high degree of wear resistance and low friction values. However, friction increased up to 1 when the load was decreased down to 0.5 mN. A low degree of friction was observed during macroscopic sliding tests against steel in ambient air conditions. Ultralow friction (0.007) was achieved when the film was rubbed with a steel ball in a high vacuum. Polarized microinfra-red spectroscopy revealed that high lubrication performance is attributed to hydrocarbons transferred from the rubbed film to the ball surface and oriented along the sliding direction [22]. Dorfman [26] obtained so-called diamond-like nanocomposites ( DLN ) films containing silicon from various deposition techniques, with a structure consisting of a mixture of diamond-like (a-C:H ) and quartzlike (a-Si) atomic-scale random networks. The DLN films combined low internal stress, high degree of hardness and elasticity together with low friction. In a review paper, Meneve et al. [27] investigated the tribological behavior of a-Sir-,:C,:H films deposited from r.f. PACVD, with 0.7
C. Donrwt
1 Swftice
and Coatings
moderate mechanical conditions, for the protection of low-stress aerospace or automotive components, precision ball bearings and gears, sliding bearings and magnetic recording media.
4. Tribology
of fluorine-containing
DLC coatings
As mentioned in the introduction, fluorine and silicon both induce a significant reduction of the surface energy of the DLC films [5]. Indeed, since F and Si are unable to form double bonds, the authors claim that these elements force carbon into an sp3 bonding state. However, there is no reason that fluorine would prevent -C=C- bonds. Anyway, whereas the addition of silicon is able to reduce the polar part of the surface energy, it is unable to reduce also the dispersive component, as fluorine is able to do. As claimed by the author in Ref. [5], the main difference in the film formation is that with the addition of the Si-containing precursor [Si(CH,), in the study], a high amount of hydrogenated carbon is still present in the network structure, while with fluorine, many -CF, and -CF, groups are present with the deposition conditions used. This reduces the density of the network structure, resulting in a reduced dispersive component of the surface energy. Consequently, it is not surprising that the surface energy of a-C:F:H films was found to be intermediate (20 mN m-i), compared to PTFE (used for non-sticking purposes, with a surface energy of 18 mN rn- ‘), a-C:H:Si (31 mN m-‘) and a-C:H (43 mN m-r). Therefore, an advantage of the a-C:H:F films is to combine a low surface energy (comparable to the value of PTFE) with a hardness slightly lower but in the same range (between 5 and 20 GPa) than values related to undoped a-C:H and significantly higher than the hardness of PTFE (0.3 GPa). For some applications (i.e. magnetic recording media). the friction has to be further reduced by applying lubricants on the top surface of the protective DLC coating. It was shown that surface fluorination can reduce the friction and microwear of DLC [28]. However, fluorination in that case was applied subsequent to the deposition of DLC and therefore had a limited effect since its lubricating property was lost after a relatively short wear time, due to the localization on the topmost layer of the coating. Consequently, studies have used fluorinated hydrocarbons or their mixture with hydrocarbons or hydrogen to deposit fluorine-containing films by RF PACVD. Soft polymerlike fluorinated carbon films, with a small fraction of fluorine chemically bonded to the carbon matrix and the rest being trapped in the graphitic structure of the films, have been reported [29]. Other authors also reported deposition of fluorine-containing carbon films, some claiming that the films are abrasion-resistant but without reporting any abrasion results [30], and others
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obtaining soft and easily scratchable films [31]. Recently, Grill et al. [32] have reported on fluorinated DLC films prepared by d.c. PACVD from pure difluorobenzene (DFB) or hexafluorobenzene (HFB) with various dilutions. The films deposited from DFB contained lessthan 10 at.% fluorine and had a wear resistance similar to that of non-fluorinated DLC. The films deposited from HFB contained 45 at.% fluorine but were very soft and had no wear resistance at all. Argon dilution of the precursor reduced the fluorine concentration in the films, but had no effect on the wear resistance of the films. In contrast, films deposited from HFB diluted with hydrogen had lower fluorine concentrations and had a wear resistance that increased with increasing hydrogen dilution. At a sufficient dilution (71% hydrogen in the HFB + HZ), films containing 18 at.% fluorine and with a wear resistance similar to that of diamond-like carbon were obtained. Recently, Donnet et al. [33] have studied a:C:F:H films deposited by r.f. PACVD from hexafluorobenzene diluted with hydrogen. For a given precursor mixture, the wear rates of the a-C:F:H films can be controlled by the average impact energy of the ions on the growing film. Depth wear rates as low as 1 A per 1000 rotations, comparable to those observed for hard non-fluorinated DLC, have been obtained for films containing up to 20 at.% fluorine and 10 at.% hydrogen. The latter appears to be unbound to carbon. The highest wear resistance was achieved for films deposited at the highest deposition bias and lowest gas pressure. These films exhibited a more cross-linked structure with a lower fraction of unbound fluorine and higher compressive stresses,compared to lesswear-resistant films. The steady-state friction of the fluorinated films is in the same range as the steady-state friction of most of the typical DLC coatings. Contrary to wear, friction appears to be independent of the bias and gas pressure within the studied range, but depends very much on the contact pressure during friction. The combination of silicon and fluorine in a-C:F:Si:H films obtained by ECR-PCVD has been studied by Miyake et al. [24]. The microtribological behavior of the silicon-containing film was improved by fluorination. The surface energy is found to decreasedue to fluorination, in agreement with the results reported by Grischke et al. [ 51. The micro-wear is reduced on an atomic scale by fluorination. Moreover, the adhesion to the silicon substrate and strength of the carbon film is greatly improved by adding small quantities of silicon. Films containing silicon also have a significantly longer lubricating life. Miyake [34] has elucidated the complex frictional behavior of the investigated a-C:F:H films in ambient air: a running-in period (the first 100 sliding cycles) with coefficients of friction in the 0.21-0.35 range, a stable period (the next 300 cycles) in which the coefficient stabilized at 0.20, a transient period (the next 600 cycles) in which the value increased from 0.20 to
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0.65, and finally a failure period characterized by decomposition of the coating. Ex-situ I.R. measurements at the end of each friction regime provided a correlation of these frictional fluctuations with molecular transformations of the C:F structure. It was found that the halogenated carbon framework decomposed into amorphous carbon and activated carbon, which then reacted with fresh metal through bond cleavage between halogen and carbon. The carboncarbon bond cleavage also reduced the molecular size of carbonaceous chains. The low friction state occurred during the decomposition process, due to the presence of molecules containing fluorinated C =C moieties, such as polydifluorinatedacetylene (PDFA), that were oriented in the sliding direction (as seen by the polarized I.R. probe). In summary, like silicon, fluorine incorporation in the DLC structure affects the surface properties. The reduction in stress compared to conventional DLC is in the same range as with a-C:H:Si. However, the reduction in surface energy is higher with fluorine than with silicon. Highly fluorinated DLC [F/( F + C) > 0.41 appear to be soft with no wear resistance. Moderate fluorination [(F/( F + C ) < 0.21 can be controlled by the deposition conditions to obtain films with a comparable wear resistance and friction level than conventional a-C:H film, but with a lower degree of stress and surface energy.
5. Tribology
of nitrogen-containing
DLC coatings
From the prediction of Liu et al. [35], several groups have attempted to synthesize the b-C,N, crystalline phase, which would have a bulk modulus comparable to that of diamond. To achieve this goal, one way has been to incorporate nitrogen in the DLC structure by several methods summarized by Wan et al. [36]. Consequently, numerous published papers are dedicated to the analytical investigations of the composition, structure and properties of the a-C:N and a-C:H:N films. The main results indicate that the films are generally amorphous, with no unambiguous identification of the formation of the crystalline C,N, phase. However, it has been found that nitrogen incorporation in the DLC structure decreases the fraction of C sp3 hybridization. Indeed, the presence of the C=N and C=N bonds has often been detected by infra-red or electron energy loss spectroscopies. Consequently, the surface energy of a-C:H:N films should be significantly increased, and this has been confirmed by the results of Grischke et al. [5], showing a drastic increase from 43 mN rnF (for a typical a-C:H) to 59 mN rnk for the nitrogen-containing film due to the increase of the polar component. A N/(N + C ) atomic ratio up to 0.4 can be incorporated into the film structure, generally reducing the stress but preserving the hardness and wear resistance, even if the results do not agree systematically from one study to another [37-
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431. In 1993, Yeh et al. [37] obtained a protective coating for magnetic rigid disks by r.f. diode sputtering using a mixture of argon and nitrogen gases. In a continuous drag test with thin film head sliders, they showed that the CN, films have a better wear performance than a-C films obtained in similar conditions but without the nitrogen gas during deposition. Li et al. [38] synthesized nitrogen-containing DLC using d.c. unbalanced magnetron sputtering of graphite in a N,-containing plasma. Tribological tests indicated that the friction and wear behavior were both in the same range as for typical DLC films. Dekempeneer et al. [39] synthesized a-C:H:N films using r.f. PACVD and obtained a nitrogen content in the range of O-35 at.%, depending on the deposition conditions. The nitrogen incorporation led to softer, less stressed materials with a lower wear resistance than classical DLC films, and a steady-state friction remaining in the range of 0.2 in ambient air conditions. These results were explained on the basis of a structural model in which nitrogen atoms are arranged between the aromatic clusters, thereby modifying the sp3:sp2 ratio. More recently, Prioli et al. [40] deposited a-C:H:N films from PACVD and found a friction coefficient of 0.22 in air, against silicon nitride, for nitrogen concentration up to 11 at.%. A slight increase in the surface roughness of the top surface was observed due to the nitrogen incorporation increasing significantly the number and size of graphitic domains. Cutiongco et al. [41] presented promising results with CN, films (22 at.% of N) deposited on magnetic disks by sputtering in an argon/nitrogen plasma. The amorphous coatings exhibited hardness in the range of 22-28 GPa, better contact start-stop performances and three to four times better pin-on-disk contact durability compared with amorphous carbon overcoats. The IBAD technique has also been used by Khurshudow et al. [42] by argon-sputtering of a carbon target with concomitant bombardment of the growing film by nitrogen ions in the 0.5510-keV range. The intrinsic stresses could be varied from tensile to compressive, depending on the deposition conditions (current and energy of the ion beam). Some films exhibited friction coefficients as low as 0. IO-O. 12 in ambient air against a silicon nitride ball. However, no clear correlation was observed between the hardness, stress and wear resistance of the coatings. Koskinen et al. [43] deposited CN, films by the pulsed vacuum arc method, on silicon and metallic substrates. Friction and wear were measured with a pin-on-disk apparatus with various relative humidities (RH = 12 and 50%) and sliding speeds ( 1.4 and 14 cm s- ‘). After a run-in period with a friction decrease from 0.4 to 0.1-0.2, the steady-state friction ranges between 0.2 and 0.3. The wear rates remain near lo-’ mm3 Nm-i irrespective of nitrogen content. However, friction was found to increase with increasing nitrogen content, especially at lower values of the relative humidity. No
C. Donnet
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Coatings
friction-induced modification of the film structure was identified by micro Raman spectroscopy inside the wear tracks. In summary, less work has been performed on the tribological investigation of the nitrogen-containing DLC films, due to their recent discovery compared to conventional DLC and other doped DLC. If the nitrogen incorporation has an opposite effect on the surface energy than the silicon or fluorine incorporations, stress seems to be also systematically reduced, whereas the hardness evolution is more discussed. The first tribological investigations do not show any significant differences in terms of friction level and wear resistance, in comparison to undoped DLC. However, more work is encouraged to identify relationships between the nature/structure of the films, their properties and tribological behavior, in relation with the deposition process and conditions.
6. Tribology
of metal-containing
DLC coatings
Alloying DLC films has been obtained with different metals (Ti, Nb, Ta, Cr, MO, W, Ru, Fe, Co, Ni, Al, Cu, Au, Ag) to form a-C:H:M coatings [43-491. The structure of these films has not been systematically investigated, but in many cases, metals are in the form of small nanocrystallites of pure metal or metal carbide (depending on the nature and concentration of the metal) dispersed throughout the carbon network. This type of composite film, mainly obtained by reactive sputtering, differs from films constituted by multilayers, such as the superposition of DLC and metal carbide thin layers, even if the global composition [M/(M + C)] may be in the same range for both types of coatings. Generally, the metal incorporation reduces the compressive stress (< 1 GPa) compared to conventional undoped DLC. Most of the tribological tests have been performed in ambient air conditions and exhibited steady-state friction in the range of 0.10-0.20, with a slight dependence on the humidity, load and metal concentration. For Ta and W incorporation in DLC obtained by reactive sputtering, frictions below 0.1 have been observed with lower wear rates than TIN and TiAlN coatings in the same testing conditions [45]. Interesting optimum wear rates may be obtained for each type of metal dopant, with a concentration that seems to depend on its nature. The tribological behaviors of the metalcontaining DLC films have been explained by a combination of ceramic-like properties (high hardness, H) and polymer-like properties (high elasticity, E, and low surface energy, S), thus leading to high H/E values and low S/H values. In summary, the number of different material compositions and structures appears to be enormous when one starts to coat metal-containing DLC coatings. One
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should bear in mind that the optimization of the material combination and deposition parameters is a challenging subject for each element or combination of elements. When it is achieved, the metal-containing DLC films may exhibit promising tribological properties in terms of steady-state friction level and wear rates for many applications.
7. Conclusion The literature survey presented above shows how the incorporation of dopants in the DLC structure may be used to achieve precise combinations of design surfacerelated properties with a low friction and high wear resistance. However, the need for correlating between doped DLC film processing, their properties and tribological behavior becomes more pressing, taking into account the diversity of coatings that can be produced by the deposition technology. The ultimate goal is to identify a limited seriesof significant coating properties that are easily measurable, which may be correlated with the tribological behavior quantified through standardized and reproducible procedures. This will afford, finally, a control and monitoring of the tribological behavior in given experimental conditions, directly from the control of the deposition process. Such a direct link between coating designers and end-users offers a great opportunity for a significant increase in mass market applications of tribological coatings in the next future.
Acknowledgement The author gratefully acknowledges Dr A. Grill for valuable comments on the manuscript.
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[ 141 [l5] [ 161 [ 171 [I81 [ 191 [20] [21] [22] [23] [24] [25] [36] [27] [28] [29]
Pauleau. P.B. Barna (Ed%), Protective Coatings and Thin Films, NATO-AS] Series Vol. 21. Kluwer, Dordrecht, 1997, p. 229. K. Enke, H. Dimigen. H. Hubsch, Appl. Phys. Lett. 36 ( 1980) 291. Y. Liu, A. Erdemir. E.I. Meletis. Surf. Coat. Technol. 8687 ( 1996) 564. M.N. Gardos, in: K.E. Spear. J.P. Dismuke (Eds.). Synthetic Diamond: Emerging CVD Science and Technology, Wiley, New York, 1994, p. 419. D.H. Buckley, in: Surface Etfects in Adhesion, Friction, Wear and Lubrication, Elsevier, New York, 198 1, p. 574. C. Donnet, A. Grill, Surf. Coat. Technol. 94-95 ( 1997) 456. K. Oguri. T. Arai, Surf. Coat. Technol. 47 (1991) 710. K. Oguri, T. Arai. Thin Solid Films 208 ( 1992) 158. B. Goranchev. K. Reichelt. J. Chevallier, P. Hornshoj, H. Dimigen, H. Htibsch, Thin Solid Films 139 (1986) 275. T. Hioki. Y. Itoh, A. Itoh. S. Hibi, J. Kawamoto. Surf. Coat. Technol. 46 ( 1991) 233. Y. Itoh, S. Hibi, T. Hioki. J. Kawamoto. J. Mater. Res. 6 (1991) 871. M. Braun. Nucl. Instrum. Meth. B 39 (1989) 556. 1. Sugimoto, S. Miyake, Appl. Phys. Lett. 56 (19) (1990) 1868. T. Miyamoto, R. Kaneko. S. Miyake. J. Vat. Sci. Technol. B 9 (1991) 1336. S. Miyake, R. Kaneko, Y. Kikuya, 1. Sugimoto, Transactions of the ASME, J. Tribol. 113 (1991) 384. S. Miyake. R. Kaneko. Thin Solid Films 212 (1992) 256. V.F. Dorfman, Thin Solid Films 212 (1992) 267. J. Meneve, E. Dekempeneer, J. Smeets, Diamond Films Technol. 4(1)(1994)23. K. Trojan, M. Grischke, H. Dimigen. Phys. Status Solidi A 145 (1994) 57. R. D’Agostino, R. Lamendola. P. Favia, A. Giquel, J. Vat. Sci. Technol. A 12 (1994) 308.
[30] R.E. Sah. B. Dischler. P. Koidl, A. Bubenzer, Appl. Phys. Lett. 46 (1985) 739. [3l] J. Seth. S.V. Babu, Thin Solid Films 230 (1993) 90. [32] A. Grill. V. Patel, Diamond Films Technol. 6 ( 1996) 13. [33] C. Donnet, J. Fontaine, A. Grill, V. Pate], C. Jahnes, M. Belin, Surf. Coat. Technol. 94495 (1997) 531. [34] S. Miyake, in: K. Miyoshi. Y.W. Chung (Eds.), Surface Diagnostic in Tribology. Series in Modern Tribology. Vol. I, World Scientific, Singapore, 1993, p. 183. [35] A.Y. Liu. M.L. Cohen. Science 245 (1989) 841. [36] L. Wan, R.F. Egerton. Thin SoIid Films 279 ( 1996) 34. [37] T.A. Yeh. C.L. Lin, J.M. Sivertsen. J.H. Judy, J. Magnetism Magnetic Mater. 120 (1993) 314. [38] D. Li, V.P. Dravid, Y.W. Chung. M.Y. Chen, M.S. Wong, W.D. Sproul, Diamond Films Technol. 4 (2) ( 1994) 99. [39] E.H.A. Dekempeneer. J. Meneve. J. Smeets, S. Kuypers, L. Eersels, R. Jacobs, Surf. Coat. Technol. 6869 ( 1994) 621. [40] R. Prioli, S.I. Zanette, A.O. Caride, D.F. Franceschini. F.L. Freire Jr, J. Vat. Sci. Technol. A 14 (1996) 2351. [4l] E.C. Cutiongco, D. Li. Y.W. Chung, C.S. Bhatia, Trans. ASME I18 (1996) 543. [42] A. Khurshudov, K. Kato. D. Sawada, Tribal. Lett. 2 (1996) 13. [43] H. Dimigen, H. Htibsch. R. Memming, Appl. Phys. Lett. 36 (1987) 1056. [44] C.P. Klages, R. Memming, Mater. Sci. Forum 5253 ( 1989) 609. [45] H. Dimigen. C.P. Klages. Surf. Coat. Technol. 49 ( 1991) 543. [46] M. Wang, K. Schmidt, K. Reichelt. H. Dimigen. H. Htibsch, J. Mater. Res. 7 (1992) 667. [47] D. Klaffke, R. Wasche, H. Czichos, Wear 153 (1992) 149. [48] M. Grischke, K. Bewilogua. H. Dimigen. Mater. Manufact. Processes 8 ( 1993) 407. [49] H. Sjostrom, L. Hultman, J.E. Sundgren, L.R. Wallenberg, Thin Solid Films 232 ( 1993) 169.
and CoatingsTechnology
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Recent progress on the tribology of doped diamond-like carbon alloy coatings: a review
and
C. Donnet * l&ok
Centrale de Lyon. Laboratoire
de Trihologie
et Dynamique des Syst&nes. UMR 5513. BP 163, 69 131 lh~lly,
Crde.y, France
Abstract Diamond-like carbon (DLC) coatingshave beenwidely recognizedas beinga wear-resistantsolid lubricant with a low friction coefficient. Its tribological behavior strongly dependsboth on the tribotesting conditions and the nature of the coating, which in turn dependson the technique usedfor film deposition. Recently, there have beenseveralattempts to improve the tribological behavior of DLC coatings by the addition of elements.such as silicon, nitrogen, fluorine and various metals.The paper will presentan updated review of the tribological propertiesof doped DLC, in comparisonwith the conventional hydrogenatedand non-hydrogenatedcarbonaceousfilms. 0 1998ElsevierScienceS.A. Keywords:
Diamond-like carbon (DLC); Doped DLC; Carbon alloy coatings;Friction; Wear
1. Introduction Diamond-like carbon (DLC ) coatings have been the subject of intensive studies for the last 20 years. Recent reviews summarize the knowledges on film deposition, characterization, properties and tribological behavior [l-4]. The general term DLC describes hydrogenated and non-hydrogenated carbon materials prepared by a variety of methods (PVD and PACVD techniques) and presenting of wide range of structure, composition and properties, such as low friction, high wear resistance, chemical inertness, a relatively high optical gap and high electrical resistivity. DLC films present a noteworthy example of thin films whose tribological behavior strongly depends both on the nature of the coating (controlled by the deposition procedure) and the testing conditions, including mechanical (contact pressure), cinematic (speed), physical (temperature) and chemical (nature of the environment) parameters. In particular, for varied deposition methods, the hydrogen concentration ranges from less than a few at.% to about 50 at.%. Hydrogen is important for obtaining a wide optical gap and a high electrical resistivity, removing midgap defect states, stabilizing the random network and preventing its collapse into a graphitic phase. The * Tel: + 334 72 18 62 80; fax: 334 78 43 33 83; e-mail: [email protected] 0257~8972/98/$19.00 0 1998 Elsevier PII SO’57-897X(97)00611-7
Science
B.V. All rights
reserved.
nature and properties (electrical conductivity, surface energy, etc.) of the DLC may also be modified by controlling the incorporation of dopants, such as silicon, fluorine, nitrogen and various metals. For example, an increase of the water wetting angle, associated with a decrease of the surface energy, is observed with the incorporation of Si or F in the DLC, whereas the opposite behavior is observed with the incorporation of 0 or N [51. The effect of Si or F on the surface energy of the DLC films is attributed to the reduction of mainly of the polar part of the surface energy, due to the loss of sp’ C hybridization and dangling bonds. These carbon-based multicomponents films may also be combined to obtain property-controlled multilayer coatings [6], including: (1) interface layers, to improve the adhesion; (2) large number of repeated layers, with different intrinsic mechanical properties; and (3) diverse property layers, to combine several protective functionalities, such as corrosion protection, wear resistance,thermal isolation, electrical conductivity. diffusion barrier and adhesion to the substrate. The present paper reviews and summarizes the basic and latest understanding on the tribology of doped DLC films. A short review on the undoped DLC (a-C and a-C:H) coatings is provided for comparison at the beginning.
C. Donnef
2. Tribology
/ Surface
and Coatings
of conventional a-C and a-C:H coatings
Improvements in the tribological behavior and expansion of the use of DLC films for tribological protection in new applications have required an enhancement of the understanding of the friction and wear of these films and their dependence on the operating environment and film properties. Since review papers have recently been published on this subject [3,4], the tribology of DLC will be summarized here only in general terms. To be used as tribological coatings, DLC films must adhere well to the substrate material, the adhesive forces having to overcome the high internal stresses that otherwise would cause film delamination. Adhesion depends on the substrate material and can be affected by the deposition method. Good adhesion of DLC films to carbideand silicide-forming substrates has been found. The adhesion of DLC coatings to silicide forming metals can be improved by depositing an interfacial layer of amorphous silicon (2-4 nm thick) between the metal and the carbon film, thus forming an interfacial silicide layer promoted by the plasma even at a relatively low substrate temperature [ 71. Friction coefficients of adherent DLC films, typically ranging from 0.01 to more than 0.5, depending on the nature of the DLC film and the tribotesting conditions [3], are strongly governed by tribochemical effects [810]. The build-up of a transfer film, followed by easy shear within the interfacial material is the most frequently observed friction-controlling mechanism for DLC films. However, the shearing ability strongly depends on the nature of the surrounding gas present in the contact. From the pioneering experiment of Enke et al. [ll], DLC coatings are known to be extremely sensitive to the presence of oxidizing species (oxygen, water vapor) during friction, which give rise to a noticeable tribo-oxidation of the topcoats, generally increasing the friction and wear. Wear rates may reach extremely low values (down to 10e6 mm3 Nm-‘) but also very high values, which can inhibit the use of the coating in any applications. Liu et al. [ 121 have shown that the steady-state low friction of DLC films in ambient air is due to wear induced graphitization, i.e. formation of a low friction graphitized tribolayer. Both sliding velocity and applied load influence the graphitization process, due to temperature rise at contact asperities facilitating hydrogen release from the DLC structure. Lower friction coefficients ( lop2 range) have been frequently observed in dry and/or inert environments. However, the absence of a systematic control of oxygen and water vapor partial pressures in the reported experiments prevents precise identification of the partial pressure thresholds at which the friction changes. In the absence of oxidizing gas, as in an ultrahigh vacuum (UHV ), a large spread of friction values is observed in published results, from less than 0.01 up to 0.3 or more [3]. The ultralow
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friction of smooth DLC layers in inert environments is attributed by Gardos [ 131 to the high surface finish and the presence of ultrathin hydrocarbon-polymerlike topcoats with weak Van der Waals interactions between hydrogenated carbonaceous chains. The lo-fold, humidity and oxidation-caused, increase in friction from the lo-’ range to the lo- ’ range corresponds to an increase in bond strength from about 0.08 eV per bond, for Van der Waals bonding of hydrocarbons, to about 0.21 eV per bond, for hydrogen bonding at C=O sites by water molecules. Thus, most of the DLC films present a friction behavior evolution similar to MoS,, from UHV to ambient air, whereas diamond and graphite exhibit both higher friction in UHV (>0.5) but comparable friction values in ambient air, as mentioned by Buckley [ 141. The difficulty in correlating the properties of DLC with their tribological behavior, as determined by the preparation process, stems from the generally poor definition of all deposition conditions of a specific film, the difficulty of structural characterization of the amorphous materials, and the lack of any standardization of tribological characterization. Films with various hydrogen contents exhibit different physical properties, such as carbon hybridization, refractive index, hardness, stress, and tribological behaviors in controlled conditions. Recently, Donnet and Grill [ 151 have shown how to control the friction and wear of DLC films in high vacuum conditions, depending on the nature of the film. High-impact energy during the deposition (controlled directly by the bias and gas pressure in d.c. PACVD systems or by the power and the gas pressure in r.f. PACVD systems) induces high precursor dissociation and thus a more cross-linked carbon network, with a lower sp3 fraction, lower hydrogen content, and lower fraction of hydrogen bonded to carbon. Films deposited under such conditions are harder, with a higher stress and index of refraction, and a lower surface energy, and are characterized by a very high degree of friction (> 0.5) in UHV conditions. Ultralow friction and wear may be reached in UHV for DLC films if the hydrogen content is high enough (about 40 at.%), but with a carbon network sufficiently cross-linked and having a noticeable fraction of hydrogen (about 0.3) unbounded to carbon. With the deposition system used by the authors, an absolute bias increase from 500 to 800 V is sufficient to lower the hydrogen content (by about 6 at.%), and the fraction of hydrogen bounded to carbon in films deposited from acetylene, thus enhancing the friction in UHV from less than 0.01 to more than 0.5. This shows how the friction behavior of DLC films is sensitive to their structure and composition. As a result, much attention should be paid to the deposition process for controlling both friction and wear of DLC films. The main results related to the tribology of a-C and a-C:H films may be summarized as follows:
(1) relative high values of compressive stress (up to 3 GPa) result in poor adhesion to most of the substrates, requiring the deposition of adhesionpromoting interlayers, (2) transfer film build-up followed by easy-shear within the interfacial material is systematically observed, whatever the nature of the film and tribotesting conditions; ( 3 ) friction in ambient conditions (presence of oxygen and water vapor) leads to tribochemical oxidation of the DLC topcoats, confining the friction coefficients in the 10-r range; and (4) wear in ambient conditions as well as friction and wear in inert environments (including high vacuum conditions) are strongly affected by the deposition conditions. Therefore, the main objectives of the studies related to doped or alloyed carbon coatings are both to modify some of the surface-related properties of the films (i.e. surface energy) and to overcome tribological limitations, mainly by reducing the internal stress and inhibiting the triboreactivity of the coating regards oxygen and/or water vapor responsible for high friction and low wear resistance.
3. Tribology
of silicon-containing
DLC coatings
Oguri et al. [ 16,171 deposited a-C:H:Si coatings with a hydrogen content of about 40 at.% and a Si/C +Si ratio between 10 and 30 at.%, by a d.c. PACVD. Tribological tests (load lo-50 N, speed 0.2-3.0 m s- ‘) in ambient air (relative humidity RH=50-70%) gave wear rates lower than 1O-7 mm3 Nmt and friction coefficients ranging between 0.03 and 0.1. that is the lowest friction values observed with DLC films in ambient air conditions. The tribological behavior was attributed to the formation of SiO, wear particles and their interaction with the humid environment through tribochemical effects. Goranchev et al. [18] obtained a-C,:H:Si,-, films from r.f. reactive sputtering, with a carbon fraction .K ranging between 0.5 and 0.9. An increase in internal stress was associated with an increase in silicon content. Contrary to metal-containing films, the a-C,:H:Si, -x films were systematically amorphous and exhibited friction coefficients lower than 0.1. Hioki et al. [ 19,201 studied Si containing DLC films deposited from ion beam-assisted deposition (IBAD) with a composition C:Si:O =0.8:0.1:0.1 with the exception of 10 at.% of hydrogen. With relative humidities varying from 20 to 70% in air or nitrogen. friction coefficients ranged between 0.04 and 0.07. At higher humidities, friction increased up to 0.2. Ultralow friction values down to 0.02 or less were observed in dry nitrogen. A transfer film build-up was systematically observed on the steel counterpart. The same kinds of friction results
were already observed by Braun et al. [21] with films deposited by the same technique. The authors found also that for tool steel samples coated with an a-C:H:Si ( 100 nm thick) immersed in aqueous solutions of acetic acid or NaCl, the initiation of pitting corrosion compared to the bare substrate was delayed by a factor greater than 1000. Therefore, the coating was believed to have potentialities for corrosion protection. a-C:H:Si films with a silicon content ranging between 10 and 25 at.% were obtained from electron cyclotron resonance ECR-PCVD by Miyamoto and Miyake et al. [22-251. During microtribological tests (0.1 -mm diamond tip with a normal load of 10 mN), the films exhibited a high degree of wear resistance and low friction values. However, friction increased up to 1 when the load was decreased down to 0.5 mN. A low degree of friction was observed during macroscopic sliding tests against steel in ambient air conditions. Ultralow friction (0.007) was achieved when the film was rubbed with a steel ball in a high vacuum. Polarized microinfra-red spectroscopy revealed that high lubrication performance is attributed to hydrocarbons transferred from the rubbed film to the ball surface and oriented along the sliding direction [22]. Dorfman [26] obtained so-called diamond-like nanocomposites ( DLN ) films containing silicon from various deposition techniques, with a structure consisting of a mixture of diamond-like (a-C:H ) and quartzlike (a-Si) atomic-scale random networks. The DLN films combined low internal stress, high degree of hardness and elasticity together with low friction. In a review paper, Meneve et al. [27] investigated the tribological behavior of a-Sir-,:C,:H films deposited from r.f. PACVD, with 0.7
C. Donrwt
1 Swftice
and Coatings
moderate mechanical conditions, for the protection of low-stress aerospace or automotive components, precision ball bearings and gears, sliding bearings and magnetic recording media.
4. Tribology
of fluorine-containing
DLC coatings
As mentioned in the introduction, fluorine and silicon both induce a significant reduction of the surface energy of the DLC films [5]. Indeed, since F and Si are unable to form double bonds, the authors claim that these elements force carbon into an sp3 bonding state. However, there is no reason that fluorine would prevent -C=C- bonds. Anyway, whereas the addition of silicon is able to reduce the polar part of the surface energy, it is unable to reduce also the dispersive component, as fluorine is able to do. As claimed by the author in Ref. [5], the main difference in the film formation is that with the addition of the Si-containing precursor [Si(CH,), in the study], a high amount of hydrogenated carbon is still present in the network structure, while with fluorine, many -CF, and -CF, groups are present with the deposition conditions used. This reduces the density of the network structure, resulting in a reduced dispersive component of the surface energy. Consequently, it is not surprising that the surface energy of a-C:F:H films was found to be intermediate (20 mN m-i), compared to PTFE (used for non-sticking purposes, with a surface energy of 18 mN rn- ‘), a-C:H:Si (31 mN m-‘) and a-C:H (43 mN m-r). Therefore, an advantage of the a-C:H:F films is to combine a low surface energy (comparable to the value of PTFE) with a hardness slightly lower but in the same range (between 5 and 20 GPa) than values related to undoped a-C:H and significantly higher than the hardness of PTFE (0.3 GPa). For some applications (i.e. magnetic recording media). the friction has to be further reduced by applying lubricants on the top surface of the protective DLC coating. It was shown that surface fluorination can reduce the friction and microwear of DLC [28]. However, fluorination in that case was applied subsequent to the deposition of DLC and therefore had a limited effect since its lubricating property was lost after a relatively short wear time, due to the localization on the topmost layer of the coating. Consequently, studies have used fluorinated hydrocarbons or their mixture with hydrocarbons or hydrogen to deposit fluorine-containing films by RF PACVD. Soft polymerlike fluorinated carbon films, with a small fraction of fluorine chemically bonded to the carbon matrix and the rest being trapped in the graphitic structure of the films, have been reported [29]. Other authors also reported deposition of fluorine-containing carbon films, some claiming that the films are abrasion-resistant but without reporting any abrasion results [30], and others
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obtaining soft and easily scratchable films [31]. Recently, Grill et al. [32] have reported on fluorinated DLC films prepared by d.c. PACVD from pure difluorobenzene (DFB) or hexafluorobenzene (HFB) with various dilutions. The films deposited from DFB contained lessthan 10 at.% fluorine and had a wear resistance similar to that of non-fluorinated DLC. The films deposited from HFB contained 45 at.% fluorine but were very soft and had no wear resistance at all. Argon dilution of the precursor reduced the fluorine concentration in the films, but had no effect on the wear resistance of the films. In contrast, films deposited from HFB diluted with hydrogen had lower fluorine concentrations and had a wear resistance that increased with increasing hydrogen dilution. At a sufficient dilution (71% hydrogen in the HFB + HZ), films containing 18 at.% fluorine and with a wear resistance similar to that of diamond-like carbon were obtained. Recently, Donnet et al. [33] have studied a:C:F:H films deposited by r.f. PACVD from hexafluorobenzene diluted with hydrogen. For a given precursor mixture, the wear rates of the a-C:F:H films can be controlled by the average impact energy of the ions on the growing film. Depth wear rates as low as 1 A per 1000 rotations, comparable to those observed for hard non-fluorinated DLC, have been obtained for films containing up to 20 at.% fluorine and 10 at.% hydrogen. The latter appears to be unbound to carbon. The highest wear resistance was achieved for films deposited at the highest deposition bias and lowest gas pressure. These films exhibited a more cross-linked structure with a lower fraction of unbound fluorine and higher compressive stresses,compared to lesswear-resistant films. The steady-state friction of the fluorinated films is in the same range as the steady-state friction of most of the typical DLC coatings. Contrary to wear, friction appears to be independent of the bias and gas pressure within the studied range, but depends very much on the contact pressure during friction. The combination of silicon and fluorine in a-C:F:Si:H films obtained by ECR-PCVD has been studied by Miyake et al. [24]. The microtribological behavior of the silicon-containing film was improved by fluorination. The surface energy is found to decreasedue to fluorination, in agreement with the results reported by Grischke et al. [ 51. The micro-wear is reduced on an atomic scale by fluorination. Moreover, the adhesion to the silicon substrate and strength of the carbon film is greatly improved by adding small quantities of silicon. Films containing silicon also have a significantly longer lubricating life. Miyake [34] has elucidated the complex frictional behavior of the investigated a-C:F:H films in ambient air: a running-in period (the first 100 sliding cycles) with coefficients of friction in the 0.21-0.35 range, a stable period (the next 300 cycles) in which the coefficient stabilized at 0.20, a transient period (the next 600 cycles) in which the value increased from 0.20 to
184
C. Donnet
1 Surfice and Contings
0.65, and finally a failure period characterized by decomposition of the coating. Ex-situ I.R. measurements at the end of each friction regime provided a correlation of these frictional fluctuations with molecular transformations of the C:F structure. It was found that the halogenated carbon framework decomposed into amorphous carbon and activated carbon, which then reacted with fresh metal through bond cleavage between halogen and carbon. The carboncarbon bond cleavage also reduced the molecular size of carbonaceous chains. The low friction state occurred during the decomposition process, due to the presence of molecules containing fluorinated C =C moieties, such as polydifluorinatedacetylene (PDFA), that were oriented in the sliding direction (as seen by the polarized I.R. probe). In summary, like silicon, fluorine incorporation in the DLC structure affects the surface properties. The reduction in stress compared to conventional DLC is in the same range as with a-C:H:Si. However, the reduction in surface energy is higher with fluorine than with silicon. Highly fluorinated DLC [F/( F + C) > 0.41 appear to be soft with no wear resistance. Moderate fluorination [(F/( F + C ) < 0.21 can be controlled by the deposition conditions to obtain films with a comparable wear resistance and friction level than conventional a-C:H film, but with a lower degree of stress and surface energy.
5. Tribology
of nitrogen-containing
DLC coatings
From the prediction of Liu et al. [35], several groups have attempted to synthesize the b-C,N, crystalline phase, which would have a bulk modulus comparable to that of diamond. To achieve this goal, one way has been to incorporate nitrogen in the DLC structure by several methods summarized by Wan et al. [36]. Consequently, numerous published papers are dedicated to the analytical investigations of the composition, structure and properties of the a-C:N and a-C:H:N films. The main results indicate that the films are generally amorphous, with no unambiguous identification of the formation of the crystalline C,N, phase. However, it has been found that nitrogen incorporation in the DLC structure decreases the fraction of C sp3 hybridization. Indeed, the presence of the C=N and C=N bonds has often been detected by infra-red or electron energy loss spectroscopies. Consequently, the surface energy of a-C:H:N films should be significantly increased, and this has been confirmed by the results of Grischke et al. [5], showing a drastic increase from 43 mN rnF (for a typical a-C:H) to 59 mN rnk for the nitrogen-containing film due to the increase of the polar component. A N/(N + C ) atomic ratio up to 0.4 can be incorporated into the film structure, generally reducing the stress but preserving the hardness and wear resistance, even if the results do not agree systematically from one study to another [37-
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431. In 1993, Yeh et al. [37] obtained a protective coating for magnetic rigid disks by r.f. diode sputtering using a mixture of argon and nitrogen gases. In a continuous drag test with thin film head sliders, they showed that the CN, films have a better wear performance than a-C films obtained in similar conditions but without the nitrogen gas during deposition. Li et al. [38] synthesized nitrogen-containing DLC using d.c. unbalanced magnetron sputtering of graphite in a N,-containing plasma. Tribological tests indicated that the friction and wear behavior were both in the same range as for typical DLC films. Dekempeneer et al. [39] synthesized a-C:H:N films using r.f. PACVD and obtained a nitrogen content in the range of O-35 at.%, depending on the deposition conditions. The nitrogen incorporation led to softer, less stressed materials with a lower wear resistance than classical DLC films, and a steady-state friction remaining in the range of 0.2 in ambient air conditions. These results were explained on the basis of a structural model in which nitrogen atoms are arranged between the aromatic clusters, thereby modifying the sp3:sp2 ratio. More recently, Prioli et al. [40] deposited a-C:H:N films from PACVD and found a friction coefficient of 0.22 in air, against silicon nitride, for nitrogen concentration up to 11 at.%. A slight increase in the surface roughness of the top surface was observed due to the nitrogen incorporation increasing significantly the number and size of graphitic domains. Cutiongco et al. [41] presented promising results with CN, films (22 at.% of N) deposited on magnetic disks by sputtering in an argon/nitrogen plasma. The amorphous coatings exhibited hardness in the range of 22-28 GPa, better contact start-stop performances and three to four times better pin-on-disk contact durability compared with amorphous carbon overcoats. The IBAD technique has also been used by Khurshudow et al. [42] by argon-sputtering of a carbon target with concomitant bombardment of the growing film by nitrogen ions in the 0.5510-keV range. The intrinsic stresses could be varied from tensile to compressive, depending on the deposition conditions (current and energy of the ion beam). Some films exhibited friction coefficients as low as 0. IO-O. 12 in ambient air against a silicon nitride ball. However, no clear correlation was observed between the hardness, stress and wear resistance of the coatings. Koskinen et al. [43] deposited CN, films by the pulsed vacuum arc method, on silicon and metallic substrates. Friction and wear were measured with a pin-on-disk apparatus with various relative humidities (RH = 12 and 50%) and sliding speeds ( 1.4 and 14 cm s- ‘). After a run-in period with a friction decrease from 0.4 to 0.1-0.2, the steady-state friction ranges between 0.2 and 0.3. The wear rates remain near lo-’ mm3 Nm-i irrespective of nitrogen content. However, friction was found to increase with increasing nitrogen content, especially at lower values of the relative humidity. No
C. Donnet
J Surjbce
and
Coatings
friction-induced modification of the film structure was identified by micro Raman spectroscopy inside the wear tracks. In summary, less work has been performed on the tribological investigation of the nitrogen-containing DLC films, due to their recent discovery compared to conventional DLC and other doped DLC. If the nitrogen incorporation has an opposite effect on the surface energy than the silicon or fluorine incorporations, stress seems to be also systematically reduced, whereas the hardness evolution is more discussed. The first tribological investigations do not show any significant differences in terms of friction level and wear resistance, in comparison to undoped DLC. However, more work is encouraged to identify relationships between the nature/structure of the films, their properties and tribological behavior, in relation with the deposition process and conditions.
6. Tribology
of metal-containing
DLC coatings
Alloying DLC films has been obtained with different metals (Ti, Nb, Ta, Cr, MO, W, Ru, Fe, Co, Ni, Al, Cu, Au, Ag) to form a-C:H:M coatings [43-491. The structure of these films has not been systematically investigated, but in many cases, metals are in the form of small nanocrystallites of pure metal or metal carbide (depending on the nature and concentration of the metal) dispersed throughout the carbon network. This type of composite film, mainly obtained by reactive sputtering, differs from films constituted by multilayers, such as the superposition of DLC and metal carbide thin layers, even if the global composition [M/(M + C)] may be in the same range for both types of coatings. Generally, the metal incorporation reduces the compressive stress (< 1 GPa) compared to conventional undoped DLC. Most of the tribological tests have been performed in ambient air conditions and exhibited steady-state friction in the range of 0.10-0.20, with a slight dependence on the humidity, load and metal concentration. For Ta and W incorporation in DLC obtained by reactive sputtering, frictions below 0.1 have been observed with lower wear rates than TIN and TiAlN coatings in the same testing conditions [45]. Interesting optimum wear rates may be obtained for each type of metal dopant, with a concentration that seems to depend on its nature. The tribological behaviors of the metalcontaining DLC films have been explained by a combination of ceramic-like properties (high hardness, H) and polymer-like properties (high elasticity, E, and low surface energy, S), thus leading to high H/E values and low S/H values. In summary, the number of different material compositions and structures appears to be enormous when one starts to coat metal-containing DLC coatings. One
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should bear in mind that the optimization of the material combination and deposition parameters is a challenging subject for each element or combination of elements. When it is achieved, the metal-containing DLC films may exhibit promising tribological properties in terms of steady-state friction level and wear rates for many applications.
7. Conclusion The literature survey presented above shows how the incorporation of dopants in the DLC structure may be used to achieve precise combinations of design surfacerelated properties with a low friction and high wear resistance. However, the need for correlating between doped DLC film processing, their properties and tribological behavior becomes more pressing, taking into account the diversity of coatings that can be produced by the deposition technology. The ultimate goal is to identify a limited seriesof significant coating properties that are easily measurable, which may be correlated with the tribological behavior quantified through standardized and reproducible procedures. This will afford, finally, a control and monitoring of the tribological behavior in given experimental conditions, directly from the control of the deposition process. Such a direct link between coating designers and end-users offers a great opportunity for a significant increase in mass market applications of tribological coatings in the next future.
Acknowledgement The author gratefully acknowledges Dr A. Grill for valuable comments on the manuscript.
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