Friction of Diamond in the Presence of Water Vapor and Hydrogen

Article pubs.acs.org/JPCC

Friction of Diamond in the Presence of Water Vapor and Hydrogen Gas. Coupling Gas-Phase Lubrication and First-Principles Studies Maria-Isabel De Barros Bouchet,*,† Giovanna Zilibotti,‡ Christine Matta,*,† Maria Clelia Righi,‡ Lionel Vandenbulcke,§ Beatrice Vacher,† and Jean-Michel Martin† †

Ecole Centrale de Lyon, LTDS, 36 Avenue Guy de Collongue, 69134 Ecully, France CNR Istituto di Nanoscienze Centro S3, Dipartimento di Fisica, Universita’ di Modena e Reggio Emilia, 41100 Modena, Italy § Institut de Combustion, Aérothermique, Réactivité et Environnement, 45071 Orléans, France ‡

ABSTRACT: Nanocrystalline diamond (NCD) has attracted much attention in recent years because of improvements in growth methodologies that have provided increases in both film thickness and growth rate, while preserving the outstanding mechanical properties of diamond material. We provide here some evidence, based on combined experimental and firstprinciples analyses, that ultralow friction of nanocrystalline diamond in the presence of water vapor is associated with OH and H passivation of sliding surfaces, resulting from the dissociative adsorption of H2O molecules. The presence of these adsorbates (OH and H fragments) keeps the surfaces far apart preventing the formation of covalent bonds across the interface. H-passivated surfaces, resulting from the dissociative adsorption of H2 molecules, appears to be more efficient in further reducing friction than OH-terminated surfaces.

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INTRODUCTION Diamond coatings produced by various chemical vapor deposition (CVD) processes can display properties different from those of single-crystal diamond when deposited on metallurgical substrates. This is due to their microstructure, their chemical purity (i.e., their respective amounts of sp3- and sp2-hybridized carbons), and their surface roughness. The main concern with using high-purity polycrystalline diamond material as a tribological surface coating is the resulting high roughness due to competitive columnar growth between large grains of different crystallographic directions. However, an advantage is the low amount of sp2-hybridized carbon incorporated in amorphous and polyaromatic entities located especially inside grain boundaries of the coating and on its top surface. Very smooth nanocrystalline diamond coatings can be obtained by plasma-assisted chemical vapor deposition (PACVD) process, and greater sp2-carbon incorporation is accepted. Diamond's amazing ultralow friction performance and wear resistance in humid environments without any run-in period is commonly attributed either to formation of an ordered sp2-hybridized carbon layer1−5 or to passivation of dangling bonds formed during sliding.6,7 Recent spectroscopic studies carried out by Konicek et al.8 strongly support the hypothesis of dangling-bond passivation by H and OH species, excluding the sp2-hybridization hypothesis. In agreement with this work, we describe herein the elaboration and characteristics of such coatings for water and H2 lubrication experiments. Experimental and computational studies were combined to © 2012 American Chemical Society

highlight the role of each kind of termination, namely, H and OH terminations, in the frictional behavior.

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RESULTS 1. Characterization of Smooth Nanocrystalline Diamond Coatings. Nanosmooth fine-grained diamond coatings were deposited on titanium alloys or titanium-coated substrates by a Micro-Wave PACVD process at moderate temperature, equal to or lower than 600 °C, from CH4/CO2 species.9 These coatings are in fact duplex coatings with a pure diamond film on the top, a titanium carbide sublayer, and a diffusion layer forming a titanium solid solution.10 They can display high thicknesses despite isotropic biaxial residual stresses in the 2−5 GPa range depending on the diamond purity.11,12 They also exhibit a strong adherence to substrates as shown by various mechanical tests and very high induced stresses without peeling off13 thanks to the specific deposition process.9 These smooth fine-grained diamond (SFGD) coatings are described in terms of the ratio of sp2- to sp3-hybridized carbons. This important parameter influences the structure and intrinsic properties of the coatings (surface roughness, hardness, Young’s modulus, and residual stresses) and is correlated with the plasma-assisted CVD process through the formation of different concentrations of the gaseous precursors in the plasma.14,15 As shown in Table 1, fairly good SFGD coatings with high purity and low Received: November 24, 2011 Revised: February 17, 2012 Published: February 21, 2012 6966

dx.doi.org/10.1021/jp211322s | J. Phys. Chem. C 2012, 116, 6966−6972

The Journal of Physical Chemistry C

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Table 1. Diamond Growth Conditions (P = 1.33 kPa, T = 600 °C) and Coating Characteristicsa gaseous mixture

coating

surface roughness (nm)

diamond purity (%)

hardness (GPa)

biaxial modulus (GPa)

8% CO/H2 37.5% CH4/CO2 41% CH4/CO2 44.5% CH4/CO2 50% CH4/CO2

polycrystalline SFGD SFGD SFGD SFGD

120 35 25 15 14

97 94 90 80 75

106 ± 1 − 86 ± 2 81 ± 2 74 ± 2

1250 930 810 780 620

residual stresses (GPa) −5.5 −5.3 −4.5 −3.6 −3.2

± ± ± ± ±

0.4 0.4 0.4 0.4 0.4

a

Surface roughness was determined by atomic force microscopy, diamond purity by Raman spectroscopy and electron energy-loss spectroscopy, hardness by nanoindentation, biaxial modulus by Brillouin light scattering, and residual stresses by Raman spectroscopy and calculated thermal stresses.

detected by surface analysis techniques at the top surface of most solids. TEM imaging also allows for an estimate of the roughness of the top surface that is in agreement with the tactile measurement. 2. Gas-Phase Lubrication in the Ultra-High-Vacuum (UHV) Tribometer. Gaseous lubricated friction tests were carried out using a specific device called an environmentally controlled analytical tribometer (ECAT) presented in Figure 2.

roughness can be deposited, especially when the CH4/CO2 ratio is low. Characteristics of a polycrystalline diamond coating are also reported in this table for comparison. The fine columnar structure of these diamond coatings (column widths of about 100 nm) and the presence of the titanium carbide sublayer are revealed by transmission electron microscopy (TEM) studies on transverse cross sections prepared by the focused-ion-beam (FIB) technique (Figure 1a). As previously mentioned, sp2-hybridized carbons are

Figure 1. (a) TEM image of an SFGD coating deposited with a 44.5% CH4/CO2 mixture. The coating is about 1350 nm thick, and a TiC sublayer of about 180 nm is present between the main coating and the Ti-6Al-4V alloy substrate. A Pt protective coating was deposited on the SFGD coating before cutting. (b) Energy-filtered image at 6 eV of the top of the coating. An sp2-carbon-rich layer with a thickness of about 5−15 nm depending on the area can be observed at the extreme surface (just below the protective resin and Pt layers deposited on the SFGD coating before nanomachining).

Figure 2. Schematic representation of the new environmentally controlled analytical tribometer (ECAT) setup. This representation shows the environmentally controlled tribometer (ECT) dedicated to friction experiments lubricated by the gas phase and the analysis chamber dedicated to in situ XPS surface analyses.

incorporated mainly at grain boundaries, and therefore, the lowest surface roughness corresponds to the lowest diamond purity. Special attention has been paid to the characterization of the extreme surface using energy-filtering transmission electron microscopy (EFTEM). An EFTEM image recorded at 6 eV energy loss, corresponding to the π/π* transition in graphitic carbon, is presented in Figure 1b. It clearly shows a higher sp2carbon incorporation in a thin nonhomogeneous bright layer, with a thickness of about 5−15 nm, at the top surface of the coating. This adventitious sp2-carbon incorporation occurs during the last step of the deposition process when the microwave plasma is just stopped. However, it allows for partial relaxation of the stresses at the top surface and better thermodynamic stability. At this stage, the SFGD coatings can be described as being composed of a fine diamond columnar structure with some sp2 carbons incorporated inside the grain boundaries and in a thin layer at the extreme surface. This thin layer enriched in sp2 carbons has also been observed in other hard amorphous carbon coatings16 and can be compared to the adventitious sp2-carbon contamination

The reciprocating hemispherical pin-on-flat tribometer was located inside an ultra-high-vacuum (UHV) chamber to control the gas pressure, and this setup [that is, the environmentally controlled tribometer (ECT)] was directly connected to a second UHV chamber dedicated to in situ X-ray photoelectron spectroscopy (XPS) and atomic emission spectroscopy (AES) surface analyses.17 Both Ti-6Al-4V titanium alloy counterparts were coated with SFGD material deposited with a 37.5% CH4/ CO2 mixture (see Table 1). Prior to friction tests, the coatings were just cleaned with heptane and alcohol ultrasonic baths and then introduced into the UHV chamber, where they were first heated at 150 °C to eliminate volatile surface hydrocarbon contamination species. The sp2-carbon-rich layer present at the extreme surface of the pristine diamond coatings (showed in Figure 1b) was not removed. The gaseous environment was achieved by introducing the selected gas, H2 or heavy water 6967



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Figure 3. (a) Friction coefficient versus number of passes for an SFGD/SFGD couple at room temperature under UHV (black) and under 1 mbar of 2 H2O vapor (continuous gray line). (b) Optical images of the corresponding wear tracks.

Figure 4. (a) Friction coefficient versus number of passes for an SFGD/SFGD couple at room temperature under UHV (about 150 cycles) and after the introduction of 800 mbar of H2 vapor. (b) Optical image of the corresponding wear track at the end of the friction test.

vapor (2H2O), inside the UHV chamber through a precision leak valve until the chamber pressure reached the desired value. The pressure was adjusted to 1 hPa for 2H2O gas and to 800 hPa for H2 gas. Once the desired gas pressure was attained, the stationary hemispherical pin was loaded against the flat surface at ambient temperature or 80 °C with a normal force of 3 N. These conditions generated a contact pressure of about 300 MPa; the sliding speed was 0.001 m s−1. Under a gaseous environment, no viscous effect is possible as under hydrodynamic and elastohydrodynamic lubrication (EHL) regimes. The use of heavy water 2H2O (containing deuterium instead of hydrogen) is very convenient and useful for post-mortem secondary-ion mass spectrometry (SIMS) analyses after the friction tests. First, friction tests using nanosmooth fine-grained diamond surfaces at room temperature under ultrahigh vacuum (10 nPa partial pressure) were performed as a reference. After a few cycles of low friction due to the presence of surface contamination, the results showed a sudden increase to a high friction value of about 0.7 with stick−slip phenomena (see Figure 3a). The formation of a clearly visible black wear track on the specimens was observed by optical microscopy (Figure 3b). This high-friction regime is in agreement with previous results reported for diamond coatings in the literature.18 It is generally attributed to strong covalent interactions between the carbon dangling bonds formed by sliding leading to some kind of “welding” between the two diamond-coated counterfaces. In comparison, the introduction of 1 hPa of 2H2O vapor pressure dramatically decreased the friction coefficient below a value of 0.05 after only five cycles (Figure 3a). It is important to note that a very short induction period is necessary for the occurrence of tribochemical reactions in the case of SFGD

coatings, shorter than that obtained with other sp3-carbon-rich coatings such as amorphous diamond-like C coatings in the presence of OH-containing molecules.19,20 On the other hand, no visible wear was observed on specimens after ultralow friction, as shown in Figure 3b. A similar tribological behavior was obtained when the temperature was increased to 80 °C (results not shown here). To examine the first cycles of low friction and to provide evidence for the short induction period necessary to tribochemically activate the diamond-coated surfaces in the presence of a H-/OH-containing atmosphere, additional friction experiments were performed in the presence of 800 hPa of H2. In this case, to avoid any effects of surface contamination, the H2 gas was introduced after the high-friction regime related to the strong interactions between the C dangling bonds under UHV had been obtained (Figure 4a). Superlubricity (friction of about 0.01) was obtained directly after the introduction of H2 gas, showing the high speed of the reactivity of dangling bonds with H2. On the other hand, a wear track similar to that obtained under UHV was observed at the end of this test (Figure 4b), due to the first 150 cycles of friction carried out under UHV. 3. Surface Analysis by Time-of-Flight Secondary-Ion Mass Spectrometry (ToF-SIMS) after Friction Experiments. Gas-phase lubrication using isotopes is a powerful means of investigating friction reduction mechanisms thanks to advanced surface characterization technique such as ToF-SIMS. Just after the end of the test lubricated with 1 hPa of heavy water, ToF-SIMS spectra were obtained from inside and outside the wear track (Figure 3b) to clarify the molecular changes associated with the ultra-low-friction regime shown in Figure 3a. The results are reported in Table 2, and they show a 6968



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the carbon atoms of each dimer, and the double CC bond transforms into a single C−C bond. We considered fully passivated surfaces, resulting from the dissociative adsorption of one H2O or H2 molecule per (2 × 1) cell. The effects of surface passivation in reducing the adhesion and friction of diamond have been theoretically demonstrated by both classical molecular dynamics24−26 and ab initio calculations.27−32 In this work, we quantitatively compare the works of adhesion and ideal shear strengths calculated for the three interfaces obtained by self-mating clean, H2O- and Hterminated diamond surfaces. The interfacial work of adhesion, γ, is defined as the work per unit area required to separate two surfaces from contact to infinity,33 and the interfacial shear strength, τ, which has the dimensions of a frictional force per unit area, describes the intrinsic resistance to sliding of an interface.34 These quantities, usually derived by fitting friction force measurements with contact mechanics models, along with the friction coefficient quantify the tribological performances of interfaces.35 We recently proposed an approach to obtain γ and τ by means of first-principles calculations.32 The calculated values should be regarded as limiting values because the simulated surfaces are commensurate and infinitely flat. Nevertheless, comparison of the different interfaces can provide useful information to understand the effects of the surface termination observed experimentally. Methods. We performed density functional theory (DFT) calculations.36 The Perdew−Burke−Ernzerhof (PBE)37 generalized gradient approximation was used for the exchangecorrelation functional on the basis of test calculations of the structural and electronic properties of bulk diamond.27 The ionic species were described by ultrasoft pseudopotentials, and the electronic wave functions were expanded in a plane-wave

Table 2. Results of ToF-SIMS Analysis of a Diamond Surface Lubricated with Heavy Water, in Terms of Relative Intensities of the 2H and O2H Ion Fragments of the 2H2O Molecule versus the Sum of the Cluster-Ion Cn Intensitiesa area tribofilm outside tribofilm a

2

H

O2H

42.85 5.27

1.75 0.61

Characteristic of diamond coating.

significant increase in 2H and O2H ionic species (a factor of at least 3) inside the tribofilm in comparison with outside it. These results clearly confirm the passivation mechanism of the extreme carbon surface by hydrogen and hydroxyl groups during the friction process in the presence of OH-containing environments, contributing to the ultralow friction. 4. Computational Part. To analyze the microscopic origin of the friction coefficient decrease observed when passing from UHV conditions to H2O or H2 vapor, we studied the interaction between diamond surfaces with terminations representative of the three considered experimental conditions. We took into account the (001) surface of diamond that presents a (2 × 1) dimer reconstruction where carbon atoms form double CC bonds. Even if the simulated surface is crystalline and thus differs from the experimental one, the presence of a stable surface layer of carbon atoms forming double bonds resembles the experimental situation of a thin surface layer of sp2 carbon. The dissociative adsorptions of H2O and H2 molecules on the C(001)-(2 × 1) surface are exothermic reactions.21−23 The (2 × 1) reconstruction is preserved upon adsorption: The molecular fragments (H and OH in the case of H2O, H atoms in the case of H2) attach to

Figure 5. (a) Ball-and-stick representations of the three diamond interfaces considered: clean interface, H2O-terminated interface, and H-terminated interface. (b) Corresponding PESs, where the grayscale indicates the variation of the surface interaction energy Δv per (1 × 1) contact with respect to its minimum value. Note that different scales are used. (c) Potential profile (solid line) and lateral force (dashed line) per (1 × 1) contact plotted as functions of the surface displacement along the MEPs indicated by arrows on the PESs. 6969



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Table 3. Interaction Energy Calculated for the Surface Relative Position Corresponding to the PES Absolute Minimum, vmin = v[(x,y)min, zeq]; PES Corrugation Δv; and Maximum Restoring Forces along the Minimum-Energy Paths per (1 × 1) Cell; Interfacial Quantities, Work of Adhesion γ and Shear Strength τ (1 × 1) contact interface

vmin (meV)

Δv (meV)

clean H2O-terminated H-terminated

−3 × 103 −6 −4

3 × 103 4 2

interfacial f xs ,

f ys

(pN)

2 × 103, 6 × 103 2, 3 1, 2

γ (mJ/m )

τx, τy (MPa)

7 × 103 15 10

29 × 103, 88 × 103 31, 47 11, 39

2

the self-mating surfaces). The high repulsion at short range is dictated by the presence of a layer of fully saturated polar bonds,27 whereas the small attraction at long range is due to nonbonding interactions such as electrostatic, polarization, and van der Waals (vdW) interactions. The interaction energy obtained for the most favorable relative lateral position of the two surfaces in contact, vmin, is reported in Table 3. The negative sign indicates adhesion in all of the cases considered. vmin is a few electronvolts per (1 × 1) contact in the case of clean surfaces and decreases by 3 orders of magnitude if the surfaces are fully passivated. A slightly higher adhesion, and a consequent shorter equilibrium distance, is obtained for the interface passivated by water fragments where hydrogen bonding is present across the interface. It is well-known that DFT calculations in which the exchange-correlation energy is a functional of the local electronic density and its gradient are not able to accurately describe the nonlocal part of vdW interactions. The adhesion energies reported in Table 3 should then be considered as affected by an error, the magnitude of which depends on the chosen approximation for the exchange correlation functional. Usually, the PBE approximation produces an underestimation of physisorption energies.21 The work of adhesion calculated for the fully hydrogenated (001) interface by means of the adaptive intermolecular reactive empirical bond-order (AIREBO) potential is 1 order of magnitude higher26 and is in agreement with work of adhesion measured for an amorphous carbon tip on the H-C(001) surface.39 However, the measured value for a diamond tip on a fully hydrogenated ultrananocrystalline diamond surface, γ = 10 mJ/m2, is in very good, maybe fortuitous, agreement with our result.40 Here, we focus on the trends highlighted by a comparison of the results reported in Table 3, which can be considered reliable. The decrease of the adhesion energy caused by the presence of adsorbates at the interface is accompanied by a corresponding decrease of the PES corrugation. By comparing the values reported in Table 3, one can observe that Δv decreases by about 3 orders of magnitude on passing from the clean interface to the passivated ones. The modifications in the PESs caused by surface passivation are clearly visible in Figure 5b. The grayscale indicates the variation of the surface interaction energy as a function of the relative lateral position. The absolute minimum (black) is taken as the reference. Note that two different energy scales are used for the clean and fully passivated interfaces. The deep minima present in the PES of clean diamond surfaces are located at the grid points corresponding to the relative positions where covalent carbon bonds are established between the two surfaces in contact. The differences in the PESs for the two fully passivated interfaces are mostly dictated by the different morphologies of the surfaces: The presence of the OH groups makes the H2Oterminated interface less smooth than the H-terminated one. This is reflected in the PES shape, which presents a slightly

basis with a 30-Ry cutoff. The system consisting of two interacting surfaces, henceforth referred to as an “interface”, was simulated by periodic supercells with a (2 × 1) in-plane size, containing two diamond slabs in contact and a vacuum region of 30 Å to separate the periodic replicas along the z direction. Test calculations revealed that a slab thickness of nine carbon layers was sufficient to simulate the (001) surface. The k points used in the calculations were generated by the Monkhorst−Pack (MP) algorithm.38 A (5 × 10 × 1) MP grid was used to sample the Brillouin zone. The interaction energy of the two mating surfaces was tot tot tot − E1+2 , where E12 is the energy of two calculated as v = E12 tot is the interacting slabs at their equilibrium distance and E1+2 energy of two noninteracting slabs, that is, two slabs separated by a vacuum region 15 Å thick on both sides. We evaluated v for different relative lateral positions of the two surfaces; in particular, we considered a homogeneous grid of eight points per (2 × 1) cell. A structural relaxation of the system was performed at each lateral position by keeping the three bottom layers of the lower slab fixed and optimizing all of the degrees of freedom except for the (x, y) coordinates of the three topmost layers of the upper slab. In this way, during the relaxation, the distance between the two surfaces (initially fixed to 1.5 Å) could reach its equilibrium value, zeq, at each fixed lateral position. Through a bicubic interpolation of the calculated energies, we obtained the potential energy surface (PES) experienced by a surface cell when translating along the substrate, v(x, y, zeq). We calculated the work of adhesion from the PES absolute minimum, γ = −vmin/A, where A is the area of the surface unit cell, and from the PES absolute maximum, we obtained the potential corrugation Δv = vmax − vmin. When pulled along a given direction, the top surface follows the minimum-energy path (MEP). The MEP is the path that has the greatest statistical weight; it connects the PES minima passing thorough the PES saddle points (avoiding the PES maxima). The lateral force f experienced by the surface during this displacement can be obtained as the derivative of the potential profile v along the MEP. The most negative value of the force, fs, is the maximum resistance to sliding along the considered direction. We considered this quantity to define the ideal interfacial shear strength: τ = fs/A. Computational Results. The optimized configurations of the three considered interfaces are shown in Figure 5a. One can observe that the interaction between two bare diamond surfaces (left panel) resulted in a cold welding of the surfaces, even if a stable reconstruction was present on both surfaces in contact. On the contrary, the presence of adsorbates such as dissociated water molecules and hydrogen atoms keeps the surfaces far apart. During the process of structural relaxation, we observed the passivated surfaces to repulse each other and increase their distance until it reached the values zeq = 2.5 Å for the H2Oterminated interface and zeq = 3.0 Å for the H-terminated one (zeq indicates the distance between the outermost adsorbates of 6970



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higher corrugation and anisotropy in the case of the H2Oterminated interface. The differences observed in the PES corrugation appear also in the frictional forces. In Figure 5c, we report the potential profiles experienced by the slider when dragged along the [110] and [1̅10] directions. In the case of the water-passivated interface, the MEP along the [1̅10] direction is a zigzag path (central panel of Figure 5b), whereas all of the other MEPs are straight paths. The lateral forces acting on the slider unit cell when moving along the MEPs are represented by dashed lines. The maximum restoring forces are a few piconewtons per (1 × 1) contact in the case of the passivated interfaces and a few nanonewtons in the case of the bare interface. As can be seen in Table 3, these values correspond to shear strengths of a few tens of megapascals in the case of passivated interfaces and tens of gigapascals in the case of the clean interface. The latter interface shows a marked friction anisotropy, as the shear strength along the [1̅10] direction is much higher than that along the [110] direction and approaches the ideal shear strength calculated for bulk diamond (τbulk = 93 GPa).41

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reducing friction than termination resulting from the dissociative adsorption of H2O molecules. There is a good agreement between the experimental results and ab initio calculations. These results confirm the major role of dangling-bond passivation by H and OH species to generate superlow and ultralow friction, respectively. Nevertheless, the hypothesis of sp2 rehybridization of sliding carbon surfaces during the first friction cannot be excluded because the simulated surfaces are considered as commensurate and infinitely flat. These considerations are not realistic, and some structural modifications of the surface of the coatings can occur during the first cycles of friction, leading to the flattening and accommodation of both sliding surfaces. Thus, EFTEM coupled with FIB should allow for the monitoring of the structural changes of the coating during friction tests from its extreme surface to the interface with the substrate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.-I.D.B.B.), [email protected] (C.M.).

CONCLUSIONS

Notes

We investigated the origin of the ultra-low- and super-lowfriction behaviors of smooth nanocrystalline diamond coatings obtained in the presence of OH- and H-containing environments, respectively. Toward that end, we first fully characterized the extreme surface of pristine nanocrystalline diamond coatings by FIB/EFTEM to determine the real nature of the top carbon layers. Then, we combined experimental simulations using gas-phase lubrication, involving reactions at the macroscopic scale, with first-principles calculations to investigate the corresponding lubrication conditions. The results can be summarized as follows: • Nanosmooth fine-grained diamond coatings elaborated by the PACVD process display a thin nonhomogeneous layer at the top surface, with a thiskness of about 5−15 nm, rich in sp2-hybridized carbon. For the theoretical analysis the C(001) surface has been considered. This surfaces presents a (2 × 1) dimer-reconstruction consisting of a stable surface layer of double CC bonds. Thus, the simulated surface resembles the experimental one described by EFTEM results. • SFGD provide high friction under UHV after removal of the initial surface contamination. Dangling bonds produced by the unsaturated C atoms at the interface can react by forming and breaking covalent bonds between the carbon atoms of two slabs, producing high shear forces. The quasistatic quantum mechanical simulation of two self-mating surfaces revealed, in fact, the welding of clean diamond surfaces, and the calculated ideal shear strength was found to approach that of bulk diamond. • Nanosmooth fine-grained diamond surfaces provide ultralow friction when lubricated by water vapor and superlubricity in the presence of hydrogen. The ultralow friction in the presence of water vapor is attributed to the formation of H-/OH-terminated carbon surfaces. This was confirmed by the dramatic decrease observed for the calculated work of adhesion γ and ideal shear strength τ for interfaces passivated by dissociated water molecules. • As indicated both by experiments and calculations, H termination seems to be more efficient in further

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS M.C.R is grateful to S. Corni for useful discussions. The calculations were performed using the supercomputing facilities at CINECA, Bologna, Italy.

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REFERENCES

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Friction of Diamond in the Presence of Water Vapor and Hydrogen

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