Fluorine easily substitutes hydrogen in DLC films due to its monovalence ... films would be emphasized based on the surface passivation and repulsive forces induced by fluorine atoms ... carbon lms have attracted much attention due to their low ..... 2. The le model belongs to a low gas. ow rate ratio where electron-rich ...
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Recent advances in the mechanical and tribological properties of fluorine-containing DLC films Lifang Zhang,ab Fuguo Wang,a Li Qiang,a Kaixiong Gao,a Bin Zhang*a and Junyan Zhang*a Fluorine easily substitutes hydrogen in DLC films due to its monovalence and high electronegativity. The peculiarities of fluorine bestow low surface energy, low inner stress, good thermal stability, preeminent tribological properties and biocompatibility on fluorine-containing, diamond-like carbon (F-DLC) films. Although there are some reviews that introduce the important advances in DLC films, they are not particularly focused on the promising F-DLC films. In this review, we mainly concentrate on the mechanical and tribological properties of F-DLC films. The mechanical properties, including hardness, modulus, and inner stress, will be discussed thoroughly. More importantly, the eminent tribological properties of F-DLC
Received 7th November 2014 Accepted 23rd December 2014
films would be emphasized based on the surface passivation and repulsive forces induced by fluorine atoms from the surface chemical and micro-mechanical viewpoints. Finally, some existing challenges and promising breakthroughs about F-DLC films are also proposed. It is expected that these films would be produced on a
DOI: 10.1039/c4ra14078h
large scale and applied extensively in industrial applications such as micro-electro-mechanical systems, ultra-
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large scale integrated circuits, thin film transistor liquid crystal displays and biomedical devices.
1. Introduction In the past two decades, uorine-containing, diamond-like carbon lms have attracted much attention due to their low
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. E-mail: zhangjunyan@ licp.cas.cn; Fax: +86-931-4968295; Tel: +86-931-4968295
b
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Lifang Zhang received her Bachelor's degree from Hubei University in 2008. She joined Prof. Zhang's group as a Master's student at the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences in 2012. Her current scientic interests are devoted to the preparation of nanostructured carbon lms, exploring deposition mechanisms, and studying interfacial tribological behaviours.
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surface energy,1 antisticking,2 chemical inertness,3 anticorrosion,4,5 low friction,6 low dielectric constant7 and biocompatibility,8 which makes F-DLC lms a promising coating material for various applications. Ultra-thin amorphous uorine carbon lms (a-C:F) can be used as an anti-adhesion layer in the microelectro-mechanical system (MEMS) technology due to their low friction and low surface energy.9 Thin uorocarbon lms coated over a hard carbon lm are used to protect magnetic media and read/write heads due to their low friction and high hardness.10
Dr Junyan Zhang is a Professor at the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. He received his BS from Lanzhou University, China, in 1990, and MS and PhD from the Lanzhou Institute of Chemical Physics in 1997 and 1999, respectively. He did postdoctoral research at the University of California, Berkeley, University of Alabama, and Rice University (2000–2005). He was a guest scientist at Argonne National Laboratory (2007). He now serves as an editorial board member of Tribology Letters, Friction, Journal of Bio- &Tribo-Corrosion. His main interests are focused on graphene lms, the nanostructure carbon lms with super low friction.
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Vertical liquid alignment and enhanced anchoring strength makes a-C:F lms appropriate candidates for thin lm transistor liquid crystal displays (TFT-LCD).11 Combined with its bio- and hemo-compatibility, low friction F-DLC lms are also researched for use in biomedical devices.12,13 It is found that the properties of F-DLC lms are closely related to the peculiarities of the C and F elements as well as the structures of F-containing DLC lms. Table 1 gives the electronegativity of the elements constituting F-DLC lms and the energies of the bonds that exist in the lms. The electronegativity value of uorine is 3.98, which is far higher than that of carbon and hydrogen. During the process of the dissociation of hydrocarbons, highly reactive uorine free radicals are apt to react with hydrogen spices, giving rise to a decrease in the number and/or size of sp2 graphitic carbon clusters embedded in the carbon matrix.14 The loss of sp2 bonding means the elimination of the polarisation of p-electrons and unoccupied electrons, which are dominant for the polar components. The surface energy can be expressed by the summation of the polar and dispersive components. The polar component of the surface energy depends on the interaction of dipoles, whereas the dispersive component represents an attractive interaction between two nonpolar molecules. Therefore, the addition of uorine in the reactant gas can reduce the surface energy of conventional DLC lms by half.15 The element with the strongest electronegativity is uorine, and it bonds to carbon with the bond energy of 5.6 eV, which is higher than the C–H bond energy of 3.5 eV. These stronger C–F chemical bonds bestow DLC lms with chemical inertness. A uorine-rich polymer coated NiTi alloy could act as a barrier layer to mitigate the electron transportation and charge exchange on the surface of DLC lms because of its high corrosion potential, low corrosion current density and increased impedance.16 The corrosion resistance can be explained by the intrinsic chemical inertness of C–F bonds.9 Apart from the nature of C–F bonds, these outstanding properties have a close correlation with the chemical environment and structure of F-containing DLC lms. Surface energy is inversely proportional to the sp2 dominated structure and F content,17 which is also affected by –CF2 and/or –CF3 groups.18 However, the inner structure, F content and chemical environment are generally determined by the deposition conditions and mechanisms, which will produce signicant inuence on the mechanical and tribological properties of F-DLC lms. To date, a large number of preparation methods have been developed to deposit F-DLC lms such as the plasma enhanced chemical vapor deposition (PECVD),20–22 reactive magnetron sputtering,23,24 ion beam deposition,25 and plasma immersion ion implantation,26–28 and others.29 Freire Jr et al. deposited
The electronegativity of elements in F-DLC films and the energy of the bonds, which exist in F-DLC films19
Table 1
Electronegativity C 2.55
H 2.20
Bond energy (eV) F 3.98
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C–C 6.3
C–H 3.5
C–F 5.6
uorinated amorphous carbon lms (a-C:F:H) using radio frequency plasma enhance chemical vapor deposition (rf-PECVD) as the deposition method and a CH4/CF4 mixture as the plasma atmosphere.30 For a xed self-bias of 350 V, the F concentration increases gradually with CF4 partial pressure. They also found that, compared with amorphous carbon lms (a-C:H), the shi in the position of the G peak spans from 1538 to 1556 cm1 and the ratio between the D and G peaks (ID/IG) changed from 0.5 to 1.1 when the lm is deposited at the partial pressure of 80%. Raman information is associated with the size and amount of sp2-hybridized carbon domains.31,32 These results indicate that the amount of sp2 congurations changes and the ordered sp2 phase forms, leading to a transition from a diamond-like carbon structure to a polymer-like structure. However, the uorine content of a-C:H:F lms prepared using PECVD as the deposition method and the CH4–CF4 mixture as a gas source is limited to about 20 at.%. Thus, it is necessary to develop other deposition methods to obtain a higher uorine content. Reactive magnetron sputtering owing to the advantage of co-sputtering with uorocarbon and graphite can achieve a high ratio of uorine to carbon.10,33,34 However, the low availability of the graphitic target greatly restricts its practical application. In order to eliminate the inuence of hydrogen on lm properties, Ronning et al. deposited uorinated carbon lms by mass selected ion beam deposition, which can directly deposit energetic 12C+ and 19F+ ions at about 100 eV.35 The C+ : F+ charge ratio was varied from 1 : 0 (i.e., pure carbon) up to 3 : 7. As the F concentration increases, a three step progression of structure occurs as follows. Initially, the tetrahedrally bonded carbon atom (ta-C:F) network with a low uorine-doping concentration is characterized by a diamond-like structure. With a further increase in F concentration, uorinated amorphous carbon (a-C:F) lms occur with the graphitization of the three-dimensional amorphous network. Moreover, polymer-like uorocarbon structures, i.e., –CF2 chains and –CF3 endings, increase when the F concentration exceeds 20 at.%. The mechanical properties such as mass density and compressive stress decrease with increase in uorine concentration, whereas the water contact angle increases with increase in uorine concentration. Plasma immersion ion implantation (PIII) can provide the non-line-of-sight deposition of thin lms on largearea substrates and on complicated-shaped substrates at room temperature. Huang et al.36 deposited uorine-doped, diamond-like carbon lms with different uorine contents by using CF4 and a carbon cathode arc source. Both the increase in uorine content and the inner structural changes in F-DLC lms are considered to be associated with their deposition and growth mechanisms. Thus, it is crucial to study the deposition mechanisms and growth processes of F-DLC lms. As a kind of heterogeneous doped amorphous carbon lm, the initial intention of uorine incorporation into DLC lms is to solve the drawbacks of conventional DLC lms. The state-ofthe-art mechanical and tribological properties of conventional H-DLC lms are described as follows: (i) the friction coefficient spans from >0.7 to 0.4] seem to be so and have no wear resistance, moderate uorinated DLC lms [F/(F + C) < 0.2] can have a similar friction level and a lower surface energy compared to conventional a-C:H lms.94 Moreover, uorinated DLC lms not only have the same range of steady state friction as that of most typical DLC coatings, but also exhibit a comparable wear.95 It is still found that the repulsive force of F/F terminated surfaces is larger than that of H/H terminated surfaces under the same distance, whose repulsive force is greater than zero.93,96,97 Currently, it is widely believed that uorine, added into conventional DLC lms, plays two vital roles in the low friction behavior, which are passivation and repulsive forces.
4.1
Passivation
Strictly speaking, passivation is the elimination of dangling bonds and active carbon sites by the incorporation of termination atoms or groups, such as hydrogen, uorine, and hydroxyl. Covalent bonds can be formed in situations where sliding conditions are under ultra-high vacuum and/or high temperature, especially for hydrogen-free DLC lms. Hydrogenfree amorphous carbon (a-C) lms can easily form strong covalent bonds between the atoms in the counterpart ball and those at the sliding surface in dry inert atmosphere. These strong covalent bonds directly lead to the high friction coefficient, which can reach as high as 0.42.98 The passivation of dangling carbon bonds by hydrogen or hydroxyl is thought to hinder the interactions between the supercial carbon atoms and the counterface, which has been
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demonstrated by previous studies.99,100 It has been reported that the COF of non-hydrogenated DLC (NH-DLC) against 319 Al alloy tested in ambient air is 0.12, but evidently drops to 0.015 when tested in H2.101 Another study indicated that Al would be transferred to the clean diamond surface but not to the H or OH passivated diamond surfaces.38,102 The resulting Al(111)/C(111)–1 1 interface has a high adhesion force because of strong covalent Al–C bonds. It is proven that aluminum atoms form covalent bonds at surfaces consisting of carbon atoms with exposed dangling bond.103 Friction tests of NH-DLC coatings against Al pins in different testing environments have been performed to further verify the effect of passivation on tribological behavior. In terms of samples tested in both nitrogen and vacuum, it is observed from the SEM image of their wear track that a signicant amount of Al atoms transferred and adhered to the DLC coating surface. Energy Dispersive Spectroscopy (EDS) analysis also conrms that C and Cr removed from the DLC coatings are located at the interface, and their friction coefficients are 0.46 and 0.47, respectively. When the relative humidity in air increases from 0% to 85%, the width of the wear track becomes smaller and the contact surface is seriously oxidized, nally leading to the reduction of the friction coefficient from 0.16 to 0.085. Presumably, the adhesion interaction derived from dangling bonds between the surface carbon atoms and Al atoms in nitrogen and vacuum would be regarded as the most critical factor to cause the high uctuating COF and sever adhesion wear, which also remains the most plausible wear mechanism according to the qualitative analysis of the Gibbs free energy. In comparison with the tests in vacuum and nitrogen, the test in the ambient air provides a large amount of water vapor to passivate the dangling bonds on NH-DLC coatings, thus reducing the COF and wear due to the oxidation wear and abrasive wear.104 To elucidate the role of hydrogen in the tribological mechanism, the COF of partially hydrogenated DLC coatings with two different hydrogen contents have been investigated in an ultrahigh vacuum (UHV) and in different hydrogen gas pressures. In UHV, the COF of the lm with the lowest hydrogen content of 34 at.% ascends signicantly to a stable value of 0.6 aer a running-in period. However, provided that hydrogen with its pressure of 10 hPa (hectopascal) is introduced as the tribological atmosphere, the abovementioned lm exhibits a lower stable COF of 0.006 aer a short initial process. Such a phenomenon is also observed for the lm that contained 40 at.% hydrogen in UHV. This may be ascribed to the fact that the introduction of hydrogen increases the surface C–H bonds and reduces the p–p* interaction, forming more weak van der Waals interactions.105 To best of our knowledge, the type and extent of chemical interactions between the sliding-contact interfaces determine the friction and wear properties. It has been demonstrated that the di-hydrogen elimination of the s bonds and p–p* interactions is the main reason for the superlubricity of DLC lms.38 Both a carbonaceous tribolayer on the counterface and the passivation of the sliding surfaces by the chemisorptions of hydrogen co-induce the low friction of NH-DLC lms.103 The carbonaceous transfer layer with low shear strength is suggested to be a prerequisite attain a low friction coefficient of H-DLC, which is correlated with applied load, sliding velocity and cycle.106,107
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Similar to the hydrogen atoms discussed above, the passivation of surface dangling bonds by uorine atoms can be ascribed to the formation of uorocarbon groups, which terminate the surface active carbon. An F-DLC lm containing 18.6 at.% F was obtained using the plasma-assisted chemical vapour deposition technique. The following XPS analysis of C1s indicates that the content of –CF and –CF2 bonds approaches 21% and 4%, respectively.108 The XPS spectra of a-C:H:F lms prepared by RF magnetron sputtering can be deconvoluted into three peaks: –C–CF–, –CF, and –CF2. Herein, –C–CF– is the main bond structure at a low content of the uorine source, resulting in a three-dimensional network structure.74,109 It is also summarized that the incorporation of uorine into the lm not only substitutes H to terminate the surface carbon atoms, but also the total amount is kept roughly constant. Being the strongest electronegative element, uorine bonds to carbon with the bond energy of 5.6 eV, which is higher than that of C–H (3.5 eV). The stronger C–F chemical bond endows F-DLC lms with chemical inertness. Therefore, the substitution of hydrogen by uorine does not weaken the contribution of passivation to the tribological behaviors of F-DLC lms. Zhang et al.110,111 prepared uorinate-doped hydrogenated lms with a curved graphitic (CG-C:H:F) structure. The COF of CG-C:H:F lm against Al2O3 was sustained at a steady ultra-low value of 0.01 as seen in Fig. 7a(1), while the reference a-C:H lm is 0.03 as seen in Fig. 7a(2). The long-term, ultra-low friction for the CG-C:H:F lm is because uorine activates the formation of curved graphite interfacial layers dispersed in the amorphous carbon structure. Such curved graphite interfacial layers do not only diminish the supercial s dangling bonds, but also reduce the adhesion due to abundant saturated p-bonds, thus obtaining good tribological behaviors. Fig. 7b illustrates that the a-C:H:F lms have excellent tribological performance, because the C–F bond energy is higher than the C–H bond energy.111 The role of uorine bonded to carbon is both termination and reducing surface energy. The surface energy of F-DLC lms is close to that of tetrauoroethylene (PTFE), which is used for antisticking, and is far lower than that of a-C:H:Si and a-C:H.94,112 Notably, the gradually improved contact angle of water droplets on the surface of F-DLC lms suggests the formation of a great deal of hydrophobic –CFx groups on its surface, which further conrms its lower surface energy.2,18,19,73,113
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4.2
Repulsive force
Electrostatic forces, attractive or repulsive, are becoming an important factor at the interface of hydrogenated DLC lms and their counterparts. In particular, F with larger electronegativity, is introduced into hydrogenated DLC lms to lessen the surface energy.73 From the perspective of the Lewis acid-base theory, both terminated DLC lms with the same acid-base properties, either nucleophilicity or electrophilicity, produce repulsive forces. Correspondingly, these two lms with different acid-base properties, i.e. nucleophilicity and electrophilicity, result in attractive forces. Repulsive forces reduce the shear strength of the contact, leading to a weaker lateral friction force, thus lowering the friction coefficient. In contrast, attractive forces enhance the lateral friction force, leading to a higher friction coefficient. A special tribological characteristic of F-DLC is the formation of repulsive forces between two surfaces with uorine atoms. It has been reported that mutual action between two F-DLC surfaces have higher repulsive forces than that inicted by two H-DLC surfaces. Fig. 8a illustrates the electrostatic effects, which exist in two F-DLC and two H-DLC counterfaces. It is observed from le diagram plots in Fig. 8a that the two hydrogen terminated DLC counterfaces are mutually exclusive, whereas this repulsive force between the two uorine terminated DLC counterfaces leads to levitation without contact.93,97,114 Alpas et al.114 calculated the change in total energy of the system interface energy (DEtot) as a function of separation distance between Al and diamond:F surfaces. As these two surfaces become closer each other, the DEtot value presents two changing process. The rst process ranging from a local minimum value to a maximum value indicates the generation of attractive forces between the Al and the diamond:F, while the second process switching from the maximum value to a local minimum value indicates the development of one uorine atom, which is transferred to the Al surface. During the whole process, the global minimum value of the DEtot appears at the point where three uorine atoms are transferred to the diamond, indicating that the 3F transferred Al/diamond:F is the most stable structure. Moreover, electron charge density difference analysis is used to investigate the bond structure of Al and F atoms and explore the reconstruction of the interface in the process, which provides support for the formation of AlF3.115
Fig. 7 (a) Coefficient of friction of the CG-C:H:F film (1) and the a-C:H (2).110 (b) Friction coefficient of F-DLC films prepared by different modifications under different loads: (1) a-C:H film, (2) F-P-a-C:H film and (3) a-C:H:F film.111 Reproduced from ref. 110 and 111 with permission from Elsevier.
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Fig. 8 (a) The comparison of electrostatic repulsion between a hydrogen terminated DLC (left diagram) and fluorine terminated DLC (right diagram).93 (b) Schematic description of the evolution of COF with sliding time for F-DLC/Al. (1) The initial COF is relatively low for a thin oxide layer in the surface of F-DLC; (2) the breaking of the C–C, C–H and C–F bonds, after F (and C) transfer from the DLC to Al surfaces, the formation of new Al–F bonds at the Al surface, the progressive increase of the COF for the change of bond energy; (3) the formation of AlF3 at Al surface and some C linked to F atoms transferred to the Al surface. The final low steady-state COF is obtained from two F-terminated surfaces.114 Reproduced from ref. 93 and 114 with permission from Elsevier.
The evolution of the COF further demonstrates that the interaction of two F-DLC coatings alleviate adhesion. SEM and EDS analyses of the Al ball surface and wear track of Al/F-DLC show that plastic deformation occurs on the Al ball surface and Al fails to adhere to the F-DLC coating. The schematic in Fig. 8b depicts the repulsive effect based on the material transfer mechanisms, thus explaining the changes of the COF.114 Kubo et al.116 investigated the friction reduction mechanism of H, F-terminated DLC surfaces using molecular dynamics (MD) and tight-binding quantum chemistry (TBQC) calculations. MD can tackle atomic-scale chemical reaction dynamics along with the density functional theory, which can evaluate the reaction possibility, potential functions of the interactive atoms. TBQC-MD is an effective method that not only enables a long-range calculation to investigate the chemical reaction at the interface, but also can handle the friction properties at both the electronic-scale and atomic-scale. MD calculation results of the chemical reaction of H, F-terminated DLC models are concluded as follows. Coulombic energy with a positive value suggests that the larger repulsive forces work on the interface of H, F-terminated DLC models. Moreover, the Lennard-Jones (L-J) energies of the F-terminated DLC model is more stable than that of the H-terminated DLC model, which indicates the existence of weak van der Waals interactions at the interface of the H-terminated DLC model. Therefore, the friction coefficient of the F-terminated DLC model is lower than that of the Hterminated DLC model. Subsequently, the low friction mechanism of uorine-terminated, diamond-like carbon lms (F-terminated DLC) is investigated by TBQC-MD. The simulation results show that the friction coefficient of H-terminated DLC lms is 0.42, while it is 0.08 for F-terminated DLC lms at the high contact pressure of 7 GPa. The larger ion size and negative charge of uorine atoms account for the strong repulsive force of F-terminated DLC lms.117 4.3
Moisture sensitivity
It is generally recognized that the basic components of a tribological system are the tribo-pair and the surrounding
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environment. For conventional DLC lms, the concern is their tribological behavior, which is closely related to the humidity and usually leads to obvious discrepancies in the friction coefficients. This environmental sensitivity can be described as follows: a-C:H lms exhibit ultralow friction in vacuum/inert vapor and high friction in high humidity. In contrast, a-C lms exhibit high friction in vacuum/inert vapor and low friction in high humidity.38,48,92 Erdemir et al.38 depicted the friction sensitivity of hydrogenated and hydrogen-free DLC lms to the humidity. In dry nitrogen, the COF of hydrogenated and hydrogen-free DLC lms corresponds to 0.003 and 0.7; however, when the environment was switched to moist laboratory air, the COF values steeply converted to 0.06 and 0.25, respectively. The tribological behaviors of hydrogen-free DLC lms are explained on the basis of chemical and/or physical interaction mechanisms. Surface dangling bonds form covalent bonds between self-lubrication hydrogen-free DLC lms, thus producing a higher friction coefficient. Free s-bonds of surface carbon atoms are passivated by water molecules and the resulting friction coefficient drops suddenly. To comprehensively study the inuence of water vapor on the tribological behaviors of hydrogenated DLC lms, tribological tests were conducted by progressively increasing or decreasing the partial pressures of pure water vapor. At UHV, the friction coefficient of a-C:H lms initially remains near 0.01. However, it gradually increases up to 0.1 with the water vapor pressure rising to 2.3 103 Pa; moreover, when the water vapor pressure increases from 1.0 103 Pa, the friction coefficient remains at 0.1, while it suddenly decreases to 0.01 when the water vapor pressure reaches 104 Pa. It is speculated that more water vapor is physically absorbed onto the top surfaces, inhibiting the growth of the carbonaceous transfer lm, and thus the friction coefficient increases.118 Voevodin et al.119 observed the transfer layer mechanism of friction behaviors of hydrogen-free DLC lms. The reduction of friction with humidity of a-C/sapphire ball increasing aer 105 cycles may be attributed to the sp2 rich transfer lm, whereas the friction coefficient increases from 0.08 to 0.5 in vacuum at similar cycles. It is important to note that the sp2 phase with low shear strength plays the role of lubrication in the presence of water. In short, the friction coefficient of highly hydrogenated DLC lms is low in
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vacuum, because hydrogen lubricates the friction surfaces in contact during the tribological process. However, the friction coefficient decreases with increasing humidity, which is ascribed to the tribochemical reactions because of the formation of high energy C]O bonds, dipole interaction and capillary forces, or to the inhibition of wear-induced graphitization mechanisms.120 Conversely, the friction coefficient of hydrogen-free DLC lms decreases with increasing humidity. This may be attributed to the passivation of dangling bonds by forming hydrogen and hydroxyl groups38,39 or to the water-lubricated sp2 rich transfer lm. In order to improve the humidity sensitivity of conventional DLC lms, the most common method is to introduce other elements into these lms such as Si, Ti and F.121 Among them, uorine is believed to be a considerable effective additive. Hauert and Gilmore122 investigated the tribological moisture sensitivity for the alloying elements of Ti and F, and compared these results with the as-reported ones for Si. As presented in Fig. 9a and b, the addition of Ti has little effect on the friction coefficient of DLC with respect to relative humidity. This also has been veried by the fact that the friction coefficient of DLC remains constant at 0.12 at the relative moisture of 65% and 85% over the full range of Ti contents (Fig. 9g and f). It is seen from Fig. 9c and d that a small content of Si can make Si-DLC become practically insensitive to ambient humidity. Its friction coefficient is stabilized at 0.075 over the relative humidity range of 5–85%. With regard to uorine, as shown in Fig. 9k, its contents of 2 at.% can stabilized the COF of F-DLC against steel
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at 0.15 from the detail of the evolution of m5%, m65%, m85%, while 2–4 at.% stabilize the COF at 0.1 for the Al/F-DLC lm counterpart, which is inferred from Fig. 9l. As far as wear behavior is concerned, F-DLC lms retains a low value, while Si-DLC lms present a linear increase in the wear rate; i.e., F-DLC is a promising candidate for friction coefficient tailoring and maintaining a wide range of relative humidity without compromising its wear resistance. Rubio-Roy et al.123 deposited F-DLC lms with different ratios of CHF3 : CH4 by PECVD. It can be observed in Fig. 10a that for any given uorine content, the friction shows a remarkable reduction over the humidity range of 20–60%. Importantly, when the humidity is beyond 60%, the friction is almost stabilized at approximately 0.2 despite the CHF3 contents. It should be noted that the F-DLC lms with 10% CHF3 display maximum friction for this given content of CHF3 at four selected relative humidities. In Fig. 10b, all friction curves initially show an increasing trend, and then begin to decrease when CHF3 is beyond 10%. It is well accepted that uorine modication can reduce the surface energy dramatically,124,125 and it is demonstrated by the water contact angle increase aer the incorporation of uorine into the DLC lms (Fig. 10d).17 The friction coefficient of a-C:F lms with a uorine content of 25 at.% against steel balls are tested in three different environments. It can be seen from Fig. 10c that friction coefficients in humid air and dry air have a similar behavior; hence, they are not affected by humidity.126
Fig. 9 Overview of the evolution of m5%, m65% and m85% as a function of dopant content for the dopants Ti (a and b), Si (c and d) and F (e and f). Results for the steel counterface are presented on the left (a, c, and e) and results for the alumina counterface on the right (b, d, and f). Detail evolution of m5%, m65% and m85% for low-friction behavior as a function of dopant content for the dopants Ti (g and h), Si (i and j) and F (k and l). Results for the steel counterface are presented on the left (g, i, and k) and results for the alumina counter-face on the right (h, j, and l).122 Reproduced from ref. 122 with permission from Elsevier.
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Fig. 10 (a) Friction of a-C:H:F films with different contents of CHF3 as a function of humidity. (b) Friction of a-C:H:F films at different relative humidities in relation with contents of CHF3.123 (c) Friction coefficients of a-C:F films with a fluorine content of 25 at.% against stainless steel balls are tested in dry air (85% humidity) and deionized water environments.126 (d) Liquid droplet on the surface of DLC films before and after F doped: (1) DLC films; (2) F-DLC films, respectively.17 Reproduced with permission from: (a) and (b) ref. 123 Elsevier; (c) ref. 126 Elsevier; (d) ref. 17 Hindawi Publishing Corporation.
In conclusion, the friction decreases with increasing humidity, because surface dangling bonds produced mechanically can react with hydrogen and hydroxyl groups from water to terminate the dangling bonds, despite the fact that uorine bonds to carbon with a high bond energy. The incorporation of F reduces the surface energy so that the adhesion to the counterpart decreases. When the humidity increases up to some extent, physisorbed-water will mask the minor chemical difference of the lms.
5.
Conclusion and perspective
In this review article, we have summarized the recent progress in the development of F-DLC lms, especially with emphasis on their mechanical and tribological properties. Signicant advances have been made in developing F-DLC lms with tunable mechanical properties and excellent lubricating performances. A large number of DLC lms with different uorine contents have been designed and prepared to investigate the inuence of hetero-element doping on the performance of DLC lms. To date, these as-obtained F-DLC lms are being widely used in various elds including mechanic, electronic and medicine due to their low surface energy, hydrophobicity, low friction, low dielectric constant, chemical inertness, anticorrosion, biocompatibility, and antithrombogeneity, which is caused by the intrinsic properties of the uorine atom. Many previous reports have investigated the mechanical performances of F-DLC lms in depth, especially for hardness and stress. It is found that hardness decreases with increasing the
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uorine content in F-DLC lms,127 which strongly determines wear.128 For instance, a-C:H:F lms with F 18 at.% has a hardness of 16 GPa and the friction coefficient of 0.005 at UHV.129 Moreover, in one recent report, this hardness value even increases to 20 GPa when the F content is up to 18 at.%.130 Based on the surface passivation and repulsive forces induced by uorine atoms, the tribological properties of F-DLC lms have been explored in depth from the surface chemical and micromechanical viewpoint so as to better control their mechanical and tribological properties. However, it should be noted that the development of F-DLC lms are still facing many challenges and difficulties, and some of the problems still need to be resolved for further study. First and foremost, although a large number of synthetic methods have been developed to prepare F-DLC lms, there is still a lack of systematic process parameters to obtain DLC lms with a controllable uorine content by choosing uorocarbon gases with suitable F/C ratios and bias voltages. Importantly, F-DLC lms with a high F content show weaker mechanical properties. Therefore, it is important to obtain systematic process parameters to balance uorine content and mechanical properties. Notably, etching of uorine on silicon substrates prohibits the growth of lms in the deposition process; hence, it is necessary to build an interlayer. Moreover, the reduction of hardness is believed to be related with microstructure changes of F-DLC lms, which correspond to increasing sp2 congurations, but how this change reects on the carbon chemical environment is still an important issue to solve. Relationships between hardness and carbon chemical environment can be veried qualitatively. Last not but least, the tribological behaviors of F-DLC lms have been studied in depth by simulation and theoretical calculations. Experimental research on different sliding conditions and atmospheres need to be explored to meet working requirements. These will be benecial for the practical applications of F-DLC lms in our lives. As discussed in Section 4, the introduction of the F element to DLC induces a change in bonding and electronegativity inside, nally leading to excellent tribological performances. Our future focus will be primarily placed on the experimental research of hetero-element doping into DLC lms on a micro level, exploring the role of the chemical or adhesion interaction on the whole tribological performance of lms. The eventual aim is to design F-DLC lms with better tunable performance, and developing multifunctionalintegration F-DLC lms by doping with a variety of elements.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos 51275508 and 51205383), and the authors thank the colleagues who participated in the preparation and discussion of this paper.
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