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C: Physical Processes in Nanomaterials and Nanostructures

Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis Revati Rani, Kalpataru Panda, Dr. Niranjan Kumar, Kamatchi Jothiramalingam Sankaran, Karuppiah Ganesan, and I-Nan Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00856 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on April 1, 2018

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The Journal of Physical Chemistry

Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis Revati Rania, Kalpataru Pandab, Niranjan Kumara*, Kamatchi Jothiramalingam Sankaranc,d, K. Ganesana, I-Nan Line

a

Materials Science Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603102,

Tamil Nadu, India b

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon

34141, Korea c

Institute for Materials Research (IMO), Hasselt University, 3590 Diepenbeek, Belgium

d

e

IMOMEC, IMEC vzw, 3590 Diepenbeek, Belgium

Department of Physics, Tamkang University, New-Taipei 251, Taiwan, Republic of China

ABSTRACT Tribological properties of diamond films are sensitive to the chemically reactive and inert triboatmospheric media and therefore, it is difficult to understand the underlying tribological mechanisms. In the present work, tribological properties of surface modified ultrananocrystalline diamond (UNCD) thin films were investigated in four distinct tribo-environmental conditions of ambient humid-atmosphere, nitrogen (N2), argon (Ar) and methane (CH4) gases. The in-situ depth-resolved X-ray photoelectron spectroscopy (XPS) showed the desorption of oxygen, oxyfunctional additives and sputtering of weakly bonded amorphous carbon species from UNCD

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film surface after the Ar+-ion sputtering process. After desorption of these chemical entities, friction and wear were decreased and run-in regime cycles became shorter in UNCD films. Friction in ambient humid-atmosphere was higher compared to other tribo-environmental conditions and it was explained by the oxidation mechanism of the sliding interfaces and formation of oxidized carbon transferfilm. However, low friction and wear in N2 atmosphere was associated to the adsorption of N2 species forming nitrogen-terminated carbon bonds at the sliding interfaces. This was directly investigated by XPS and energy dispersive X-ray spectroscopy (EDX) techniques. Furthermore, low friction in Ar atmosphere was explained by the physical adsorption of Ar gaseous species which tend to avoid the covalent carbon bond formation across the sliding interfaces. Moreover, ultralow friction in CH4 atmosphere was governed by the passivation of dangling carbon bonds by dissociative CH4 complexes which creates hydrogen-terminated repulsive sliding interfaces. More importantly, shorter run-in regime with low friction and wear in Ar+-ion sputtered UNCD films were explained by desorption of the oxygen and oxy-functional groups which are inherently present in the UNCD films.

1. INTRODUCTION Carbon based functionalized two dimensional (2D) nanomaterials1,2 and carbon/carbon nanocomposite thin films3,4 are useful for the tribological applications. Properties of diamondlike carbon (DLC) and diamond thin films are chemically and mechanically exceptional which are significantly useful for various mechanical and tribological applications.5–8 Tribological stability and tribo-performance of these films depend on several factors. For DLC, it is mainly associated to sp3/sp2 phase fraction and hydrogen concentration in the films.9,10 However, grain


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size and grain boundary volume fraction with distinct chemical properties are the main internal associates for tuning the tribological behavior of crystalline diamond films.11–13 Recently, nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) films with distinct morphology, microstructure and phase composition showed improved and sustainable tribological properties.13–16 This is specifically explained by the densely packed morphology, ultrasmall sp3 bonded grain size and sufficiently high volume fraction of short ranged sp2 crystalline phase occupying the grain boundaries of these films.12,17 The UNCD films showed superlow friction coefficient after the run-in regime and negligible wear.16 However, adverse wear from the ball counterbody was observed due to the high friction in run-in regime. In this condition, friction and wear are mainly described by the physical, chemical and mechanical nature of the sliding surfaces. The consideration of surface contamination and adsorption of atmospheric atoms/molecules at film surface are important for describing run-in regime.18–20 So far, the specific reason for high friction in this regime is unexplained and in most cases, it was explained by the surface roughness only.5,21,22 Wahl et al. found the decrease in the run-in regime towards low friction with the relative increasing amount of sp2 bonded graphitic carbon phase in NCD films. However, the description of such modification is still unknown.23 Moreover, Kim et al. investigated that the removal/wear of surface oxide layer is responsible for the run-in regime in DLC films exposed to various atmospheres.18 Wang et al. investigated variable run-in behavior which mainly depends on the atomic interactions at the contact interfaces. It was reported that the low friction was obtained due to the weak interactions between C and Al atoms in a-C and Al2O3 sliding interfaces.20 However, high friction depends on the strong interaction between C and O atoms leading to strong adhesion. From tribological point of view, DLC and diamond films are more sensitive to humidity11,14,24,25 and gaseous atmosphere26,27 which limits



their effective tribo-performance. Friction and wear of these films are high in mild humid conditions due to the restricted passivation mechanism of the sliding interfaces.11,14,24,28 Moreover, the nature of the run-in regime is distinct which depends on the gaseous atmosphere.18 The tribological properties of crystalline diamond films in various gaseous conditions are less reported compared to DLC films. Moreover, high friction and exceptionally high wear of diamond films during the run-in regime is very difficult to investigate which are scientifically relevant issues and yet not well understood. Chemical transformation of sp3 into a-C phase is common in this regime mainly due to the high frictional energy.9,29–31 Moreover, it is important to understand the chemical characteristics of the transferfilm which is mostly observed in the high friction regime.25 Furthermore, this may lead to high wear and large scale deformation of the sliding interfaces. Therefore, controlling the friction and wear in this regime is technically important for the sustainable applications of the diamond films. Furthermore, attaining sustainable tribological properties of diamond and DLC films at high temperature is quite challenging. The conversion of diamond sp3 into hydrogenated-graphitized phase in the sliding interfaces was investigated at tribo-temperature of 623 K in UNCD films.32 This was responsible for superlow friction coefficient ~0.002 and ultra high wear resistance of UNCD films. However, at high temperature (873 K), the tribological performance of UNCD films was degraded significantly due to the oxidation of sliding interfaces.33 Friction of diamond and DLC films is sensitive to the atmospheric conditions and changes in the relative humidity level. This is one of the most important issues in tribology area which needs to be addressed. However, comprehensive study of gas dependent behavior of NCD films is less reported. This study is important for gas applications as a lubrication medium for diamond surfaces during the tribological performances. In critical tribo-conditions, liquid lubrication is


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not applicable often because of the chemical purity requirements. The previous studies showed that solid lubricant applied as coatings in polymer composites are able to reduce the friction and wear in gaseous hydrogen tribo-condition.34 Asay et al. have investigated the vapor phase lubrication

of

silicon

oxide

which

completely

prevented

the

failure

of

the

microelectromechanical (MEMS) devices.35 Moreover, Alazizi et al. have reported the useful tribological applications of various materials including carbon films in gaseous and vapor phase lubrication medium.36 The high-resolution chemical characterization of the tribofilms is necessary for the investigation of various gas phase interactions. It was shown that the intrinsic hydrogen passivation mechanism is active to control the friction and wear in hydrogenated DLC and diamond films.9,37–39 However, in ambient humid-atmospheric conditions, water vapor passivates the sliding interfaces to control the friction and wear in hydrogen free carbon films.14,40 High frictional resistance and severe wear of DLC and diamond films were observed in inert gas and vacuum environmental conditions which are related to the active participation of the dangling σ bonds at the sliding contacts.32,40–43 However, ultralow friction value in hydrogenated DLC and hydrogenated amorphous carbon (a-C:H) films was observed in N2 and Ar atmospheres.26,44 In contrast, the friction value of these films was higher in O2 and ambient tribo-conditions. However, ultralow friction in UNCD films was observed in ambient and humid condition due to the effective termination of dangling bonds by decomposed water molecules.24,25,28,40 Adsorption of N2 gas molecules in the sliding interfaces of a-C:H and diamond films was responsible for the reduction in the frictional resistance by generating electrostatic repulsion of nitrogen terminated sliding interfaces.45 The experimental proof of such effect due to the adsorption of N2 gas is yet to be elucidated. Moreover, ultralow friction of DLC films in CH4 atmosphere was associated to the passivation of dangling carbon bonds by



hydrogen atoms/molecules which is chemically decomposed product of CH4 gas molecules at the sliding interfaces.26 Furthermore, comprehensive surface chemical analysis of tribo-interfaces is required to reveal the tribological mechanisms in different atmospheres. In this paper, chemically adsorbed oxygen and oxy-functional contaminations from UNCD film surface were removed by low energy Ar+ ion sputtering in ultra-high vacuum condition without changing the topography and morphology of the films. In-situ depth-resolved chemical analysis of sputtered film surfaces was obtained simultaneously by X-ray photoelectron spectroscopy (XPS). Further, bulk chemical properties of as-deposited and sputtered films were investigated by micro-Raman spectroscopy. A general relationship explaining the tribological properties of Ar+ ion modified UNCD films in gaseous atmospheres was established in ambient humid-atmosphere, nitrogen (N2), argon (Ar) and methane (CH4) environments. Moreover, friction and wear properties of these films in run-in regime were investigated in these gaseous environments. The surface chemical properties of UNCD films including chemical additives contamination and their interaction with gaseous molecules were considered for explaining the distinct tribological behaviors in different environments. Moreover, chemical properties of tribotransferfilm were investigated by energy dispersive X-ray spectroscopy (EDX) and highresolution XPS for revealing the friction and wear mechanisms in different tribo-atmospheric conditions.

2. EXPERIMENTAL SECTION 2.1. Film Deposition, Ar+ ion Sputtering of Film Surface and Characterizations. The UNCD films were deposited by microwave plasma enhanced chemical vapor deposition (MPECVD; 2.45 GHz 600 IPLAS-CYRANNUS) system using CH4(4%)/Ar plasma medium with microwave


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power of 1200 W, pressure 150 Torr and substrate temperature 550oC. The films were deposited on mirror polished silicon (100) substrates and detail deposition procedure is given in our earlier report.16 In the present work, low energy Ar+ ion beam (1 keV) coupled with XPS chamber in ultra-high vacuum (~10−10 mbar) condition was used for the surface modification of the UNCD films. For comprehensive chemical analysis of the film surface, depth-resolved XPS analysis was carried out simultaneously after each sputtering cycles of 5, 10, 15, 20, 25 and 30 minutes. For this purpose, the wide range and high-resolution XPS spectra of C1s and O1s were recorded by Sigma probe-Thermo VG Scientific equipped with hemispherical analyzer. The XPS was carried out using AlKα radiation (E = 1486.6 eV) with an energy resolution of 0.47 eV. The bulk chemical structure of as-deposited and Ar+-ion sputtered films were analyzed by micro-Raman spectrometer (Andor SR-500i-C-R, λ=532 nm). The surface topography and morphology of these films were analyzed by AFM (NT-MDT) and field emission scanning electron microscope (FESEM-Zeiss Supra 55), respectively. The microstructure of the UNCD films was examined using transmission electron microscopy (TEM; JEOL 2100). 2.2. Tribology Parameters and Test Conditions. The friction and wear behaviors of UNCD films were measured by a ball-on-disc tribometer (Anton Paar, Switzerland), operating in a linear-reciprocating mode. Tribology test parameters such as normal load, sliding speed and linear track length were kept constant as 0.5 N, 3 cm/sec and 4 mm, respectively, for all tribomeasurements. The tribology tests of films were performed in controlled tribo-conditions such as ambient humid-atmosphere, N2, Ar and CH4 mediums while sliding against the Al2O3 ball of 6 mm diameter with RMS roughness (Rq) ~45 nm. Residual relative humidity (RH) in the tribometer chamber was 5±2% while conducting tests in N2, Ar and CH4 gaseous media. However, RH value was 63±2% in ambient humid-atmosphere condition. Three UNCD films i.e.



as-deposited and Ar+-ion sputtered for 15 and 30 minutes were selected for detail tribology measurements and for simplicity; these films will be further designated by: (a) as-deposited (b) FAr_15 and (c) FAr_30, respectively. Optical microscope was used for 2D wear analysis of these films and alumina balls. 2.3. Chemical Characterizations of Transferfilm. The transferfilm formed at the deformed Al2O3 ball scars in four different tribo-atmospheric conditions was analyzed by EDX and XPS (Sigma probe-Thermo VG Scientific). EDX elemental mapping image and X-ray spectra of elements were obtained at the large area of the deformed ball scar for quantitative chemical analysis of transferfilm formation. Moreover, comprehensive phase analysis of transferfilm was performed by XPS with spatial resolution of 50 µm which could completely focus into the deformed Al2O3 ball scar region.

3. RESULTS AND DISCUSSION 3.1. Microstructure, Topography and Bulk Chemical Structure of Films. The existence of ultranano diamond grains with a-C grain boundaries are highlighted in TEM image of Supplementary Information (Figure S1). The variation in roughness values of as-deposited and sputtered films was observed by AFM analysis (Figure S2). After the Ar+-ion sputtering, the FAr_15 and FAr_30 films showed sputtered depth of approximately 18±3 nm and 36±5 nm, respectively. Detail information on 2D and 3D topography, roughness and sputtered depth are provided in Figure S2. Bulk chemical properties of as-deposited, FAr_15 and FAr_30 films were analyzed by micro-Raman spectroscopy (Figure S3). The systematic reduction of peak intensity and blue shift of v1, v2 and v3 bands in TPA phase was observed in FAr_15 and FAr_30 films compared to as-deposited films. These two facts directly indicate the change of longer TPA


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chains in as-deposited films into shorter one after the Ar+-ion sputtering.46 Moreover, blue shift of G band in Ar+-ion sputtered films is fingerprint of in-plane smaller size of sp2 phase which was associated to increase in I(D)/I(G) ratio (Figure S3).47 3.2. Depth-resolved X-ray Photoelectron Spectroscopy Analysis of the Films. The XPS was used for comprehensive depth-resolved chemical analysis of the UNCD films. The Ar+-ion sputtered depth profile of these films against sputtering time is shown in Figure S2e. For each sputtering depth, the corresponding XPS spectra were recorded. Survey spectra clearly showed decrease in O1s/C1s peak intensity ratio with increasing sputtering depth (Figure 1a). For better clarity, the C and O atomic percentage (at%) is presented in Figure 1b. This fact described that more oxygen is physically/chemically adsorbed at the film surface which may be possibly associated to the (a) atmospheric contamination and (b) contamination appeared during the film deposition process. However, the presence of oxygen contamination was less at the more sputtered depth of the film surface which might be introduced during the film deposition process. Weak photoelectron emission lines of Si2p and Si2s were also noticed in minor impurities level which are related to silicon substrate.



Figure 1. (a) Wide range depth-resolved survey XPS spectra and (b) chemical composition of C and O in at.%; (a1), (a2), (a3), (a4), (a5), (a6) and (a7) correspond to film surface (a1) and sputtered film surface of depth 6, 12, 18, 24, 30 and 36 nm (a2-a7) corresponding to sputter time of 5, 10, 15, 20, 25, 30 minutes, respectively.

Depth-resolved high-resolution XPS analysis showed a broad band of C1s photoelectron emission which was deconvoluted into four individual components i.e. A, B, C and D following Shirley method48 (Figure 2a1-a7). Here, in each spectra, A and B components correspond to nonoxygenated carbon phase i.e. sp2 and sp3 hybridization, respectively.24,49 Other two components C and D are oxygenated carbon phase with carboxylic (C-O/C-OH) and carbonyl (C=O) functional groups, respectively.24,50 10 ACS Paragon Plus Environment

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Figure 2. Depth-resolved high-resolution XPS C1s spectra: (a1), (a2), (a3), (a4), (a5), (a6) and (a7) corresponding to the as-deposited film surface (a1) and sputtered film surface of depth 6, 12, 18, 24, 30 and 36 nm (a2-a7), respectively; (b) (A/B): sp2/sp3 and A/(B+C+D) ratio: sp2/(sp3+oxygen complexes) and (c) binding energy shift of components A and B at film surface (a1) and sputtered depth of film surface (a2-a7).

Relative intensity counts and binding energy shift of XPS bands are two important parameters for comprehensive depth-resolved chemical analysis of the film surface. First of all, XPS results showed higher fraction of sp2/sp3 phase in the as-deposited film surface which significantly decreased at sputtering depth of 6 nm i.e. (a2) and then marginal and gradual decrease of this



phase continued with more sputtering depth of the film surface (Figure 2b). This describes that the sp2 phase with oxygen functional groups was dominating in the as-deposited film surface compared to the Ar+-ion sputtered film surface. This fact is also supportive to Raman analysis which described more sp2 phase in the as-deposited films (Figure S3). It was noticed that the presence of oxy-functional groups and sp2 phase in sputtered subsurface is bulk chemical entities of UNCD films. Moreover, it is confirmed that the Ar+-ion sputtering induced chemical conversion of sp2 into sp3 phase must be ruled out because significant change in sp2 phase was not continued in more sputtered depth of the film surface. Furthermore, binding energy of C1s photoelectrons from sp2 and sp3 hybridized states were found to be weaker with sputtered-depth of the film surface (Figure 2c). This must be related to the modification in electron wave function of sp2 and sp3 states which was governed by the removal of oxygen functional groups. This was the main reason to cause the change in chemical environment of sp2 and sp3 states with sputtered-depth of the film surface. High-resolution XPS analysis of O1s photoelectrons clearly showed removal of oxygen entities from the sputtered film surfaces (Figure 3a1-a7), which is in good support with the survey chemical analysis also (Figure 1).


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Figure 3. Depth-resolved high-resolution XPS of O1s spectra: (a1), (a2) (a3), (a4), (a5), (a6) and (a7) correspond to as-deposited film surface (a1) and sputtered film surface of depth 6, 12, 18, 24, 30 and 36 nm (a2-a7), respectively.

This confirmed the relieving of electronic-charge screening effect of sp2 and sp3 states by oxygen atoms which is responsible to weaken the binding energy of the C1s photoelectrons in more sputtered depth of the film surface.50 Moreover, significant low energy shift in sp2 and sp3 phases was observed at the more sputtering depth of the film surface which is related to the surface charging effect.50 Finally, it is important to mention that the as-deposited film surface is dominated by sp2 phase and chemically adsorbed oxygen and oxygen-functional groups which were removed by the Ar+-ion sputtering process. Therefore, sputtered film surface is chemically clean which contains more sp3 carbon phase and fewer amounts of oxygen and oxy-functional groups.



3.3. Tribology Properties of Films and Chemical Analysis of Sliding Contacts. Three films i.e. as-deposited, FAr_15 and FAr_30 were selected for tribological studies in four different environments. Close relationship between friction and wear behavior depending upon the gaseous tribo-environments were noticed. These are presented in Figure 4a-d and Figure 5a,b, respectively. Friction results versus gaseous atmospheres for as-deposited and FAr_15 and FAr_30 films are presented in Figure S4 for better clarity.

Figure 4. Friction coefficient versus sliding cycles (1k=1000 cycles) of: (a) as-deposited (b) FAr_15 and (c) FAr_30 films and (d) friction coefficient with standard deviation of (d1) as14 ACS Paragon Plus Environment

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deposited (d2) FAr_15 and (d3) FAr_15 films in different tribo-atmospheric condition: Amtambient humid-atmosphere, N2, Ar, and CH4 mediums.

Figure 5. (a) Optical image of wear track width of films and size of deformed scar formed at Al2O3 balls and (b) wear dimensions of sliding interfaces in bar graph: (a1 and b1) as-deposited (a2 and b2) FAr_15 and (a3 and b3) FAr_30 films in different tribo-atmospheric conditions: Amtambient humid-atmospheric, N2, Ar, and CH4 mediums.

Moreover, tribological properties were closely associated to the sputtering depth of the film surface due to the modification of oxygen/oxy-functional contamination and sp2/sp3 ratio (Figures 1-3). Commonly, friction coefficient and wear of the sliding interfaces were found to 15 ACS Paragon Plus Environment


decrease in sputtered films in all the tribo-environmental conditions as shown in Figure 4,5, respectively. However, roughness value is higher in sputtered films and therefore, relationship between friction and wear could not be explained by roughness parameter only. It is known that friction and wear generally increase with the increase in the roughness21,22 but in our case, this trend is not observed. Therefore, other factors will be taken into considerations which may influence the tribological properties. In all three films i.e. as-deposited, FAr_15 and FAr_30 films, the highest value of friction coefficient was noticed in ambient humid-atmosphere condition. However, this friction value was lowest with very high wear resistance in CH4 triboenvironmental condition. Moreover, moderate value of friction was measured in inert atmospheres such as Ar and N2 mediums. In order to understand the comprehensive tribological mechanisms in different tribo-mediums, the chemical analysis of alumina ball contact while sliding against FAr_30 films was carried out. Therefore, it is worth to discuss about the surface and bulk chemical analysis of deformed alumina ball scars before discussing the detail tribological properties and underlying mechanisms in the following section. 3.3.1. Bulk and Surface Chemical Analysis of the Deformed Ball Contacts. For understanding the governing tribological mechanisms, the comprehensive chemical analysis of the deformed ball contact was studied while sliding against FAr_30 films. The elemental analysis of deformed ball scars was performed by EDX mapping and the superimposed images of all the elements are shown in Figure 6 for various tribo-conditions. Moreover, detail mapping results are shown in the Supplementary Information (Figure S5-8). The EDX is considered as bulk sensitive technique which provides information about the bulk chemical composition of the sample. The EDX results showed that C atomic percentage (at.%) is high while Al and O at.% is less at deformed ball scar developed in ambient humid-atmospheric tribo-condition (Figure 6a).


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In contrast, carbon at.% is significantly less with the increase in Al and O at.% in N2, Ar and CH4 tribo-conditions (Figure 6b-d). This is a direct evidence to claim that the carbon transferfilm formation on ball scar is more favorable in ambient humid-atmospheric tribo-condition.

Figure 6. In left panel: EDX elemental mapping of deformed alumina ball scar while sliding against FAr_30 films and in right panel: EDX spectra of whole area mapping of deformed ball scar developed in (a) ambient humid-atmospheric (b) N2 (c) Ar and (d) CH4 mediums.



The presence of small amount of N at the deformed alumina ball scar is evident in N2 triboatmospheric medium which confirm adsorption of nitrogen (Figure 6b). Detailed surface chemical analysis for phase composition of deformed alumina ball scars was carried out by XPS for all tribo-conditions and will be described below. The survey XPS spectra of deformed ball scars while sliding against FAr_30 films showed well-resolved photoelectron shift of carbon C1s peak in all different tribo-atmospheric conditions which confirmed the carbonaceous transferfilm formation (Figure 7a). Further, the chemical information of Al2O3 ball is indicated by the appearance of Al2p, Al2s and O1s shift of the photoelectron bands. Moreover, contribution of O1s may also include the atmospheric oxygen contamination at the deformed ball scar surface. The weak feature of Si2p and Si2s photoelectron shifts belong to substrate contamination. The bar graph in Figure 7b clearly shows quantitative chemical composition of C, O and Al in atomic percentage (at.%) at the deformed ball scars. The C/(O+Al) ratio is highest 1.70 in ambient humid-atmosphere condition which reduced to 1.47, 1.41 and 1.53 in N2, Ar and CH4 tribo-atmospheric conditions, respectively. This clearly confirmed that more carbonaceous transferfilm formation was favorable in ambient humid-atmosphere condition. These results perfectly match with EDX analysis of the ball scar as discussed above (Figure 6).


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Figure 7. (a) Survey XPS spectra of alumina ball scar sliding against FAr_30 films and (b) bar graph of elemental composition of C, O and Al in atomic percentage (at%) for: (a1) ambient humid-atmosphere (a2) N2 (a3) Ar, and (a4) CH4 mediums, respectively. Furthermore, the high-resolution XPS spectra were obtained for in-depth surface chemical analysis of the deformed ball scars in four different tribo-atmospheric conditions (Figure 8). The X-ray spot is marked by circle in optical micrographs and designated by a1, b1, c1 and d1 for ambient humid-atmospheric, N2, Ar and CH4 tribo-conditions, respectively. Well-resolved spectra of C1s photoelectron shifts are evident at these spots as shown in a2, b2, c2 and d2. In all tribo-atmospheric conditions, C1s band is deconvoluted into four individual bands, indicated as A, B, C and D components. First two components A and B are designated as sp2 and sp3 hybridized carbon phases, respectively. Other two bands C and D at higher binding energy side are C-O/C-OH and C=O functional groups, respectively. It is noticed that these functional groups are more active at the deformed ball scar32 compared to as-deposited and sputtered film surfaces



(Figure 2). This is mainly governed by the tribochemical reactions at the contact interface during the sliding process.51

Figure 8. Optical image of alumina ball scar sliding against FAr_30 films in: (a1) Amt-ambient humid-atmosphere (b1) N2 (c1) Ar and (d1) CH4 atmosphere, circle mark in optical image shows XPS spot during analysis, and corresponding high-resolution XPS spectra of ball scars: (a2 to d2) C1s spectra (a3 to d3) O1s spectra (b4) N1s spectra and (c4) Ar2p spectra (e1) (A/B): sp2/sp3 and A/(B+C+D): sp2/(sp3+C-O/C-OH+C=O) ratio and (e2) binding energy shift of sp2 (A) and sp3 (B) components. 20 ACS Paragon Plus Environment

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It is worth mentioning that the photoelectron shift of C and D bands may be contributed either from oxygen of the Al2O3 ball or atmospheric oxygen which is indistinguishable due to narrow difference in the energy shift which ultimately forms oxygen functional groups.52 It is confirmed by the C1s spectra that the sp2/sp3 ratio is tribo-atmospheric dependent. This ratio is lower i.e. 0.61 and 0.63 in ambient humid-atmospheric and Ar gas medium, respectively (Figure 8a2,c2). These values are almost similar to the FAr_30 films (Figure 2a7). However, sp2/sp3 ratio was significantly increased to 1.31 and 1.35 in N2 and CH4 tribo-atmospheric conditions, respectively (Figure 8b2,d2). This must be related to the tribochemical reactions at the sliding interfaces in the presence of N2 and CH4 gases. The full width at half maximum (FWHM) of sp2 and sp3 C1s bands are narrow i.e. 1.1 eV and 1.21 eV, respectively, in ambient humid-atmospheric conditions. However, FWHM of sp2 and sp3 phases is higher i.e. 1.3 eV and 1.41 eV in CH4 tribo-medium followed by 1.29 eV and 1.39 eV in Ar media, 1.26 eV and 1.31 eV in N2 atmosphere, respectively. This confirms the electronic interaction of sp2 and sp3 phases with the static residual charge of alumina surface which in turn is responsible for the higher binding energy shift and wider energy bands of these phases (Figure 8e2). The influence of static charge of alumina in case of ambient humid-atmosphere and Ar gas may diminish while forming the thicker carbon transferfilm. In N2 and CH4 atmospheres, the ambient atmospheric oxygen and other chemical species being absent cannot be taken into account to influence the energy width and higher binding energy shift of sp2 and sp3 phases. It is important to find the A/(B+C+D) ratio which is lower i.e. 0.43 and 0.42 in ambient humid-atmospheric and Ar gas medium, respectively. These values were increased to 0.93 in both N2 and CH4 tribo-atmospheric conditions. The summery of A/B (sp2/sp3) and A/(B+C+D) i.e. sp2/(sp3+C-O/C-OH+C=O) ratio are well presented in Figure 8e1. This confirmed more sp2 phase fraction in N2 and CH4 tribo-



atmospheric conditions and higher oxygen functional groups in ambient and Ar triboatmospheres. During sliding process, the formation of chemisorbed phases of N2 and CH4 gases at film surface protected against the oxygen functional groups formation. This is well evident in N1s photoelectron shift which showed three well-resolved bands of A, B and C components at energy shift of 397.2 eV, 398.5 and 399.5 eV, respectively. The N1s peak of component A is associated with C-N bonding which clearly demonstrate that nitrogen atoms were doped into the carbon phase of films which is possible only after the dissociation of N2 into isolated atoms.53 However, B and C components of N1s are related to the chemisorbed N2 into the sp2 and sp3 phases, respectively. This takes place due to the difference in the electronic charge dipole states of N2 and sp2/sp3 phase carbon atoms where the lone pair electron of N2 molecule interact with less electronegative carbon atoms. However, ambient humid-atmospheric medium containing moisture and water vapor that gets dissociated and chemically interact with film surface, ultimately forming more oxygen functional groups. The Ar gas being electrically neutral does not interact electronically with carbon atom of films which in turn get exposed to oxygen impurities at the sliding interfaces. Moreover, physical adsorption of Ar is possible at film surface due to dipole and induced electric dipole interaction between carbon and Ar atoms. However, the evidence of adsorbed Ar at the deformed ball scar was not noticed (Figure 8c4) due to absence of the chemisorptions. The peak shape of chemical shifts in O1s spectra is quite similar in all the four tribo-atmospheric mediums (Figure 8a3-d3). In these results, a broad spectra of O1s peak is deconvoluted into three sub-bands of components A, B and C as designated by C=O, C-OH and C-O chemical groups, respectively. The binding energies of these peaks are shifted to relatively lower sides in N2 and CH4 tribo-atmospheric conditions which indicate the effect of the electronic interaction. It is important to mention that two types of oxygen are


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contributing to these O1s spectra i.e. oxygen from Al2O3 ball and ambient-atmospheric oxygen which is indistinguishable due to wider photoelectron shift which ultimately forms oxygen functional complexes. 3.4. Tribological Mechanisms. EDX and XPS analyses of the deformed alumina ball scars showed more carbon transferfilm formation in ambient humid-atmospheric condition compared to other three tribo-conditions. More carbon transferfilm is an indication of the strong interaction between sliding interfaces in ambient atmospheric medium which may lead to high tangential stress (Figure 4). In contrast, in inert atmospheres such as N2 and Ar, the sliding interfaces becomes chemically inert due to the adsorption of these inert gases which reduce the possibility of transferfilm formation while protecting the sliding surfaces, leading to less tangential stress. Interestingly, sp2 phase rich transferfilm in N2 and CH4 media were caused by the adsorption and dissociation of these gaseous molecules at the sliding contacts. In CH4 tribo-condition, highest sp2 phase of transferfilm helps to reduce the tangential force significantly during the sliding process. Detail tribological mechanisms based on tribology results obtained in different triboatmospheres (Figure 4 & 5) will be discussed in the following sections. 3.4.1. Ambient Humid Tribo-atmospheric Condition. In ambient humid-atmosphere, water molecules dissociate and form COO, C-O and C-OH oxy-functional groups and various high binding energy radicals by breaking the sp2 and sp3 carbon networks of UNCD films.36,54 These are chemically energetic and active additives which oxidize the sliding interfaces, leading to the increase in tangential shear by forming strong chemical bonds. Furthermore, single atom of dissociated oxygen molecule reacts with the unsaturated carbon atom of film surface forming CO-C covalent bonds and other functional complexes, creating stronger adhesion at the sliding interfaces.37,55 However, the density functional calculations demonstrated more effective H2



passivation mechanism in diamond films compared to dissociative H2O passivation.56 Moreover, high friction of DLC in ambient-humid condition was experimentally investigated by Polaki et al.26 and Shi et al.44 However, super low value of friction was observed in CH4 atmosphere. In ambient condition, the evidence of more oxidized carbon and oxygen functional complexes was observed on ball sliding interface as evident from XPS (Figure 8a2). These are the valid reasons which explain the high friction of UNCD films in ambient humid-atmospheric condition. However, gradual decrease of friction value in FAr_15 and FAr_30 films was noticed compared to as-deposited films. The friction value was 0.08 in as-deposited films which reduced to 0.06 and 0.05 in FAr_15 and FAr_30 films, respectively (Figure 4). The standard deviation in friction value was also reduced to 26% and 22% of the as-deposited films in FAr_15 and FAr_30 films, respectively. This can happen due to the removal of adsorbed oxygen and oxy-functional groups from the film surface which undoubtedly reduced the oxidation and covalent linking possibility of functional groups across the shear-interfaces.55 The elimination of oxygen and oxy-functional groups in sputtered film surface was discussed in the above section in detail. 3.4.2. N2 Tribo-atmospheric Condition. It is considered that water molecules and ambient atmospheric chemical species are limited in N2 and Ar tribo-atmospheres. These chemically inert gases have ability to reduce the σ bonding across the sliding interfaces by forming adsorbed layer which is supportive for reducing the tangential-shear.45 Therefore, friction in ambient humidatmosphere was higher than that in N2 and Ar tribo-atmospheres. However, frictional differences in N2 and Ar atmospheres depend on difference in the chemical affinity of film surface towards these gaseous molecules. This affinity is mainly described by the physical and chemical adsorption of gaseous molecules at carbon film surface. In N2 tribo-atmosphere, one free electron pair of N2 molecule can interact with carbon of diamond film.45,57,58 This enables the adsorbed N2


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molecules to exist in a relatively more electronegative state and it would act as electron donors. The electrostatic repulsion at frictional sliding interface could be developed if twofold of N2 terminated carbon film surfaces are brought into contact. Thus, electrostatic repulsion of N2 terminated carbon of UNCD films is more effective in sputtered films when oxygen and oxyfunctional groups are limited. The evidence of adsorbed N2 and C-N phase at the deformed ball scar was shown by XPS spectra (Figure 8b4) which confirmed the electrostatically repulsive mechanism for reducing the friction in N2 tribo-condition. So far, evidence of such adsorbed phases was not reported in tribology contact condition. The mean value of friction coefficient in FAr_15 and FAr_30 films is 0.05 and 0.04, respectively, compared to 0.07 in as-deposited films (Figure 4). Moreover, the standard deviation in friction value was reduced to 45% and 57% of the as-deposited films in FAr_15 and FAr_30 films, respectively. In run-in regime, the friction value and standards deviation is higher especially in as-deposited films in N2 tribo-atmosphere due to the presence of oxygen and oxy-functional groups at the film surface. In this case, oxygen functional additives interact with N2, destroying the electrostatic repulsion across the sliding interfaces thus giving rise to frictional resistance due to oxidation.55 3.4.3. Ar Tribo-atmospheric Condition. The physical and chemical adsorption of Ar gas towards the C and H of diamond film surface is scarcely realized. This is a monatomic gas molecule with stable electronic configuration, which does not interact with C and H atoms of the diamond films. In this condition, dangling covalent σ bonds of carbon in the sliding interfaces do not chemically interact with the Ar and therefore, the dangling bonds are active. This could be the main reason for high friction of carbon films in Ar compared to N2 tribo-atmospheric condition.58 However, friction is less in Ar tribo-atmosphere compared to ambient humidcondition. Moreover, friction value in Ar atmosphere is less than N2 in as-deposited and FAr_15



films. This could be related to the physical adsorption of Ar which tends to destroy the covalent interaction between the dangling carbon bonds at the sliding interfaces. This could be possible at moderate pressure of Ar during the tribology test. The physical adsorption of Ar at moderate pressure keeps the sliding surfaces apart, limiting the covalent interaction across the shearinterfaces. It is interesting to note that the friction value in this condition does not change significantly in as-deposited and Ar+ ion sputtered films. This directly indicates the absence of chemical interaction of Ar with diamond films containing C, H and O atoms/molecules. Therefore, in XPS spectra, the signature of Ar is absent (Figure 8c4) but it was definitely physically adsorbed during the tribo-interaction of the surfaces. The mean value of friction coefficient slightly increases to 0.036 and 0.04 in FAr_15 and FAr_30 films, respectively, compared to 0.035 in as-deposited films (Figure 4). The standard deviation in friction value was also found to increase by 15% and 55% of the as-deposited films in FAr_15 and FAr_30 films, respectively. This result is contrasting to the rest of the tribo-atmospheric conditions and can be explained by the physical adsorption of Ar at the sliding interfaces. Moreover, physical adsorption of Ar is relatively stable when diamond film surface contains oxygen impurities in asdeposited films and, therefore, it effectively destroys the covalent interaction across the sliding interfaces. 3.4.4. CH4 Tribo-atmospheric Condition. Friction coefficient was significantly reduced in CH4 tribo-atmospheric condition. The mean value of friction is 0.033 in as-deposited films which reduced to 0.029 in FAr_15 and FAr_30 films (Figure 4). Furthermore, standard deviation in friction value in all three films is almost similar which describes not much influence of surface contamination and oxygen/oxy-functional groups. Here, residual σ covalent dangling bonds of film surface and dangling bonds created by deformation during interfacial sliding process are


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instantly saturated by hydrogen atoms/molecules dissociated form CH4 molecules.59 In this condition, covalent bonding of carbon atoms across the sliding interfaces is largely avoided and tangential-shear takes place between the hydrogenated carbon films which generates weak repulsive Van der Waals force.59 It could be explained more elaborately by considering one fold of hydrogenated carbon surface (either sp2-hydrogenated and/or sp3-hydrogenated) interacts with other fold of surface having same hydrogenated carbon configuration. In sliding interfaces, interhydrogen electronic cloud from the two-hydrogenated UNCD surfaces interact which generates electrostatic repulsion. This is well known mechanism by which low friction is explained in CH4 tribo-condition.26,59 Moreover, such mechanism is more effective when oxygen and oxyfunctional additives are absent or limited at the film surface. In contrast, oxygen and oxyfunctional additives may destroy the effective electrostatic repulsion of the sliding interfaces. However, no influence of oxygen and oxy-functional additives in as-deposited films was observed on friction results which points towards excess adsorption of CH4 molecules and its dissociative complexes. 3.4.5. Wear and Tribological Properties in Run-in Regime. Generally, the wear resistance of the sliding surfaces was improved with decrease in friction coefficient and shorter run-in regime in sputtered films. It is interesting to note that the magnitude and trend of wear is distinct and depends on the gas atmospheres and oxygen/oxy-functional complexes at the film surface (Figure 6). However, wear is not always complementary to friction value and it is rather more complex process. The frictional resistance of the sliding interfaces was decreased and wear resistance was improved in ambient humid-atmosphere and N2 tribo-condition. However, frictional resistance increased and wear resistance is improved in sputtered films in Ar triboatmosphere which is contradictory to the traditional behavior. It can be seen that the friction



value in CH4 atmosphere is almost similar in all the three films i.e. as-deposited and sputtered films, but wear is significantly reduced in FAr_30 films compared to other two films (Figure 5). This can be explained by the excess adsorption of CH4 gas in the absence of oxygen and oxyfunctional groups at the sputtered film surface that protects against wear. Controlling the friction and wear in run-in regime is technically important for sustainable application of tribomechanical devices. Generally, adverse wear takes place in this regime due to high friction caused by the surface contamination, surface chemical additives and high surface roughness.18-22 However, controlling the frictional energy is challenging in this regime and this problem is less reported especially for crystalline diamond films. In the present work, low energy Ar+-ion was used to modify the adsorbed surface contamination without destroying the sp3 and sp2 carbon network in subsurface/bulk of UNCD films. A longer run-in regime cycles with high friction value was observed in as-deposited UNCD films (Figure 4a). However, this regime was found to be shorter in sputtered films (FAr_15 and FAr_30) (Figure 4b,c). It can be clearly seen that not only friction value but also a decrease in the magnitude of standard deviation occurred in the sputtered films, mainly due to the shorter run-in regime (Figure 4d). High friction in this regime was mainly associated to the enhancement of wear at the sliding surfaces. In addition, significantly high wear resistance of the sliding surfaces was shown with decrease in run-in regime cycles in sputtered films. Therefore, it must be noted that frictional energy is directly related to wear.60 It is confirmed from the above experiments that the surface chemical properties are accountable for such distinct tribo-behavior. Thus, the improvement in tribological properties of sputtered films was associated to the elimination of reactive oxygen and oxy-functional additives form the film surface which reduced the run-in regime cycles. This fact is comprehensively studied in the above section by depth-resolved XPS analysis. The run-in regime


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cycles were still similar for the sputtered films in ambient tribo-condition due to the unavoidable atmospheric oxygen contamination during the sliding process. 4. CONCLUSIONS UNCD films were sputtered by the low energy Ar+-ion in ultrahigh vacuum condition to remove the oxygen/oxy-functional and excess a-C/graphitic phase contaminations from the film surface. Chemical quantification of these contaminations was carried out by depth-resolved XPS analysis in ultra-high vacuum condition. Friction and wear properties of these films were found to be largely modified and improved after the elimination of sputtered contaminated chemical entities. These tribo-properties were also dependent on the tribo-test environments. Friction was higher in ambient humid-atmospheric condition which is mainly associated to the oxidation of sliding interfaces in the presence of oxygen functional groups. In this condition, extensive carbon transferfilm formation is also responsible to increase the frictional value. However, low friction in N2 tribo-atmosphere is evident of adsorption and dissociation of N2 gas molecules forming repulsive electrostatic interaction across the sliding interfaces. Moreover, low friction in chemically inert Ar-atmosphere is described by the adsorption of non-reactive Ar gaseous species which tend to avoid the covalent carbon bond formation across the sliding interfaces. Furthermore, friction value in chemically reactive CH4 tribo-atmosphere is lowest due to hydrogen-terminated carbon bonds formation at the sliding interfaces. Friction of sputtered films showed lower value in run-in regime in all the tribo-atmospheric conditions. This is explained by the elimination of oxygen/oxy-functional additives and a-C contamination from the film surface. Moreover, wear resistance was significantly improved in Ar+ ion sputtered films in all the triboatmospheric conditions. Further, the highest wear resistance is observed in CH4 triboatmospheric condition which is directly related to the lowest value of friction coefficient.



However, lowest wear resistance is noticed in Ar and ambient humid-atmospheric conditions. This is related to the weak physical adsorption of Ar gas atoms in Ar tribo-atmospheric condition and oxidational wear in ambient tribo-condition, respectively.

ASSOCIATED CONTENT Supporting Information. Microstructure of UNCD films, 2D and 3D AFM topography, surface roughness of as-deposited and sputtered film surface, Ar+-ion induced sputtered depth of UNCD film surface, Friction coefficient of as-deposited, FAr_15 and FAr_30 films in ambient humidatmosphere, N2, Ar and CH4 tribo-mediums, EDX elemental mapping of deformed alumina ball scar developed in ambient-humid atmosphere, N2, Ar and CH4 mediums. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *Corresponding author Email: [email protected], [email protected] Tel.: +91 44 27480500 (ext. 22537) Fax: +914427480081

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS Kamatchi Jothiramalingam Sankaran is a Postdoctoral Fellow of the Research FoundationFlanders (FWO). Kalpataru Panda acknowledges Institute for Basic Science (IBS), S. Korea. We thank the Department of Atomic Energy, India for support. 30 ACS Paragon Plus Environment

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