Tribology of Si/SiO2 in Humid Air: Transition from Severe ChemicalWear to Wearless Behavior at NanoscaleLei Chen, Hongtu He,, Xiaodong Wang, Seong H. Kim,*, and Linmao Qian*,

Tribology Research Institute, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest JiaotongUniversity, Chengdu 610031, ChinaDepartment of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park,Pennsylvania 16802, United States

*S Supporting Information

ABSTRACT: Wear at sliding interfaces of silicon is a maincause for material loss in nanomanufacturing and device failurein microelectromechanical system (MEMS) applications.However, a comprehensive understanding of the nanoscalewear mechanisms of silicon in ambient conditions is stilllacking. Here, we report the chemical wear of single crystallinesilicon, a material used for micro/nanoscale devices, in humidair under the contact pressure lower than the material hardness.A transmission electron microscopy (TEM) analysis of the wear track conrmed that the wear of silicon in humid conditionsoriginates from surface reactions without signicant subsurface damages such as plastic deformation or fracture. When rubbedwith a SiO2 ball, the single crystalline silicon surface exhibited transitions from severe wear in intermediate humidity to nearlywearless states at two opposite extremes: (a) low humidity and high sliding speed conditions and (b) high humidity and lowspeed conditions. These transitions suggested that at the sliding interfaces of Si/SiO2 at least two dierent tribochemicalreactions play important roles. One would be the formation of a strong hydrogen bonding bridge between hydroxyl groups oftwo sliding interfaces and the other the removal of hydroxyl groups from the SiO2 surface. The experimental data indicated thatthe dominance of each reaction varies with the ambient humidity and sliding speed.

INTRODUCTIONTribological problems, such as high friction and severe wearthat lead to energy dissipation or materials failure, play a criticalrole in all length scales from earthquakes1,2 down to micro/nanoelectromechanical systems (M/NEMS).3,4 Although vari-ous contact and friction mechanisms have been proposed, ascientic understanding of wear mechanisms at the nanoscale isstill lacking. Normally, wear of sliding interfaces is thought tobe material removal by mechanical separation owing tomicrofracture, by chemical dissolution, or by melting at thecontact interfaces.58 The wear mechanism may change withvariations in surface properties or dynamic surface responsesduring a sliding process.9

Wear of materials is often described by the Archard lawwhich relates the wear volume to the applied load and thehardness of materials.57 In inert environments (such as dry orvacuum), deformation via phase transformation,8,10 viscousow,11 and dislocation formation12 were identied as the mainfactors for silicon wear. With the increase of normal load, themechanical damage on silicon could manifest as the protrusionof the surface (forming a hillock) at low contact pressure ormechanical wear (material removal) at high contact pressure.This transition takes place at a normal load that surpasses thehardness of silicon (13 GPa).13 The recent nanoscaleexperiment in high vacuum proposed a wear mechanism viaatom-by-atom removal for the materials of Si and diamond-like

carbon (DLC), in which wear rate depends on the stress-assisted bond dissociation of the substrate material.1417

The surface wear upon rubbing or sliding depends on notonly intrinsic properties of materials such as hardness and bondenergy but also many extrinsic factors of the slidinginterfaces.1119 Although the contact pressure was far lessthan the yield stress of silicon (7 GPa), the chemical reactionsinvolving water could induce serious damage of silicon contactinterfaces.18,19 The results obtained in MEMS applicationsindicated that this chemical wear would be more likely to occuron silicon surface after plasma cleaning.20 In order to minimizeor avoid the wear of material, depositions of hard coatings orhydrophobic organic layers on silicon or silica substrate havebeen attempted.2123 Recent experimental and theoreticalstudies demonstrated that water-induced chemical wear ofsilicon substrates can be prevented in an alcohol vaporenvironment.24 But, the success of the alcohol vapor lubricationapproach does not advance our understanding of the chemicalmechanism of the water-induced wear process itself.In this study, we present the eects of sliding speed (v) and

relative humidity (RH) on the wear of single crystalline siliconsurface rubbed with a SiO2 microsphere. Under humidconditions, the wear of silicon originates from surface reactions

Received: July 10, 2014Published: December 5, 2014

Article

pubs.acs.org/Langmuir

2014 American Chemical Society 149 dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156

without signicant subsurface damages, and the degree ofwater-induced wear of Si strongly depends on the sliding speedand RH. It was found that the wearless state of the Si/SiO2interface could be achieved at low RH/high v or high RH/low vconditions. Fundamental understanding of wear mechanism inhumid air would be useful to control or mitigate the nanoscalewear of the sliding interfaces of silicon parts in M/NEMSapplications.

EXPERIMENTAL METHODSThe p-type Si(100) wafers with a thickness of 0.5 mm were purchasedfrom MEMC Electronic Materials, Inc. The root-mean-square (RMS)roughness of the silicon wafer measured by an atomic forcemicroscope (AFM, SPI3800N, Seiko, Japan) was about 0.05 nmover a 500 nm 500 nm area. The native oxide layer on silicon surfacewas not removed. Before nanowear test, the Si samples wereultrasonically cleaned in methanol, ethanol, and deionized water for10 min. The silicon wafer surface was relatively hydrophilic and had awater contact angle of 39.All the nanowear tests and in situ topography scanning were

performed with AFM within an environment chamber for relativehumidity (RH) control (Figure 1). A p-doped Si(100) wafer with a

native oxide layer was scratched with a silica ball attached to an AFMcantilever. AFM cantilevers with spherical SiO2 tips were purchasedfrom Novascan Technologies. The radii of the spherical SiO2 tips weremeasured with scanning electron microscopy and determined to be1.0 m (inset in Figure 1). The roughness of SiO2 microsphere wasmeasured as 0.4 nm over a 250 nm 250 nm area (see Figure S1 inSupporting Information), which was smaller than the elastic Hertziancontact deformation (1.3 nm) under our test conditions. Using areference probe with a force constant of 2.957 N/m, the normal springconstants of the cantilevers of SiO2 tips were calibrated as 10.513.8N/m.25 The friction forces were calibrated by using a silicon gratingwith a wedge angle of 5444 (TGF11, MikroMasch, Germany).26 Thenanowear tests were carried out at the sliding scan speed varying from0.008 to 50 m/s and RH from 0% to 70%. The applied normal load(Fn) was 3 N, the number of sliding cycles (N) was 100, and thesliding distance (D) was 200 nm, if not specially mentioned. Aftertests, the topography of wear area was imaged with a sharp siliconnitride tip (MLCT, Veeco) which had a nominal curvature radius of12 nm and a nominal spring constant of 0.1 N/m. All tests wererepeated multiple times to conrm the repeatability of the data (seeFigure S2 in Supporting Information). A selected set of wear scar onthe Si substrate was analyzed with TEM (Tecnai G2, FEI). The cross-section sample for TEM was prepared using a focused ion beamsystem (Nanolab Helios 400S, FEI, Holland). During the samplepreparation, an epoxy polymer, instead of Pt, was deposited on silicon

surface as the passivation layer to prevent the decrystallization ofsilicon due to high-energy impact of Pt during the deposition.

RESULTS AND DISCUSSIONTransition from Severe Chemical Wear to Wearless

State. Figure 2 shows the topographic images of wear scars ona silicon surface at three RH regimes (10%, 30%, and 65%).The corresponding cross-section proles of wear scars areshown in Figure S3 of the Supporting Information. At low RH(10%), the Si surface wore severely when sliding speed (v) wasless than 0.1 m/s. The wear of Si substrate was reduced as vincreased; at v > 2 m/s, there was no visible wear on the Sisurface (Figure 2a). At intermediate RH (30%), the wear ofsilicon resulted in grooves at all speed conditions (Figure 2b).When wear tests were operated at relatively high RH (65%),the silicon substrate wear was negligible at v < 0.1 m/s, andthere was sudden transition to severe wear at v > 0.1 m/s(Figure 2c). Small dierences in the length of wear scarsformed at various RH and sliding speeds might be due to thepile-up debris at the edge of the wear track (see Figures S4 andS5 in the Supporting Information).The entire wear rate data measured over wide ranges of RH

and sliding speed are shown in a 2D color plot in Figure 3. Theaverage wear rate at each condition is shown in Figures S6 andS7 of the Supporting Information. It is intriguing to note thatthere are wearless regions in the opposite corners of the v andRH parameter domains. In low RH conditions (from 0% to30%), the wearless behavior was observed when v was high(lower-right blue region in Figure 3). In contrast, the siliconsurface revealed negligible wear at low sliding speeds when RHwas above 55% (upper-left blue region in Figure 3). Other thanthese two regions, the wear rate of silicon decreasedmonotonically to a constant value as v increased or the wearrate increased to steady state as RH increased from 0% to 70%.For the normal load of 3 N and adhesion force (pull-o

force) of 1.35 N (see Figure S8 in the SupportingInformation) at 65% RH, the maximum contact pressure ofthe Si/SiO2 interfaces is calculated to be 1.3 GPa based on theDerjaguinMullerToporov (DMT) model.27 This contactpressure is much lower than the yield stress of Si (7 GPa) andSiO2 (8.4 GPa).

28 Since SiO2 (70 GPa) has lower elasticmodulus than Si (160 GPa), the indentation deformation willbe mostly at the SiO2 sphere at all test conditions of this study.Therefore, the mechanical process alone cannot explain the Siwear under ambient conditions. In fact, there was no materialremoval on Si surface when the Si wafer was scratched with theSiO2 sphere in dry conditions at the same mechanical loadconditions. Instead, small hillocks (protrusion) were generatedon the silicon surface (see Figure S9 in the SupportingInformation). This protrusion seems to be mainly due tosubsurface deformation.11 These results indicated that thechemical reactions induced by mechanical shear must beresponsible for serious wear of silicon in humid air.19 This isoften called tribochemical wear.To conrm the occurrence of tribochemical wear without

any mechanical damage to the substrate, the microstructure ofthe wear scar formed by sliding with a SiO2 ball in ambient airwas analyzed by the cross-sectional TEM. Since the amorphousoxide layer shows a featureless microstructure similar to that ofthe passivation layer, the amorphous oxide layer on the topsurface of silicon substrate is dicult to be identied in TEMimages. The groove depth shown in Figure 4a was 7.5 nm,which was consistent with the wear depth measured with AFM.

Figure 1. Nanowear test of the Si/SiO2 interfaces in humid air. TheSiO2 microsphere with a radius of 1 m moved horizontally on theSi(100) wafer over a distance (D) of 200 nm under applied load Fn = 3N. The sliding speed (v) was varied from 0.008 to 50 m/s, and thetotal number of reciprocating cycles (N) was 100. The relativehumidity (RH) was varied within 0% and 70%.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156150

The good agreement between TEM and AFM wear depthmeasurements implied that the amorphitization of Si beneaththe wear track was negligible. Also, it was noted that the lattice-resolved image in worn area (Figure 4b) shows the prefectcrystalline order of silicon even very close to the sliding surface.No dislocation or defects are observed in the subsurface ofsilicon lattice; thus, the subsurface plastic deformation can beruled out. The absence of the amorphous transformation orcrack formation in the silicon substrate beneath the weartrack29,30 supports that the main wear mechanism istribochemical, not mechanical, under the low contact pressure(1.3 GPa) and humid conditions.18,24

In previous studies, it is hypothesized that the chemical wearproceeds via the formation of SiOSi bonds bridging the twosolid surfaces.18,24,31 Upon the initial contact, silanol groups onthe native oxide surface of the Si wafer can form hydrogenbonds with the silanol groups at the SiO2 counter surface.Dehydration reactions of these groups during the sliding could

form SisubstrateOSisphere bonds, which bridge two solidsurfaces. The shear action may dissociate those interfacialbonds or the subsurface SiO bonds. The latter will lead towear of silicon substrate.18,24 During the sliding process, thetemperature rise due to frictional heat was negligible since thesliding speed was low (see Supporting Information for moredetails).32,33 Alternatively, it can be conceived that themechanical stress or shear could deform the Morse potentialof a specic chemical bond at the surface, lowering the energybarrier for bond dissociation.34 Then, the water-induceddissociation of the SiOSi network or SiSi network onsilicon substrate might take place readily, which leads totribochemical wear during mechanical shear in humid air.35,36

In any case, the data shown in Figures 2 and 3 clearly indicatethat the degree of these tribochemical wear processes appearsto strongly depend on the RH and sliding speed.In a previous study, the chemical species in the wear scars on

the silicon substrate were analyzed with time-of-ight secondary

Figure 2. AFM images of wear scars on silicon surface after sliding by a SiO2 microsphere at various speeds v and under dierent relative humidity.(a) RH = 10%, (b) RH = 30%, and (c) RH = 65%. Fn = 3 N, N = 100, and D = 200 nm. Note the height full scales for two right images in (a) andtwo left images in (c) are 5 nm, and all others are 50 nm.

Figure 3. Nanowear map of the single crystalline silicon surfacescratched with a SiO2 sphere (radius = 1 m) at an applied load of 3N for 100 reciprocating cycles. There are about 150 data points usedto show the RH and sliding speed dependence of wear behaviors ofsilicon surface. More details can be found in the SupportingInformation (Figures S6 and S7).

Figure 4. High-resolution TEM images of the wear scar on the siliconsubstrate. (a) TEM image showing a 7.5 nm deep wear scar formedon silicon surface after sliding by a SiO2 microsphere under theconditions of Fn = 3 N, RH = 60%, v = 24 m/s, and N = 100. Insetshows the AFM image of the wear scar. (b) Representative lattice-resolved image in worn area marked with a box (red dotted line) in(a). The EDX spectrum in inset reveals no oxygen is detected in thesubsurface of silicon.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156151

ion mass spectrometry (ToF-SIMS).37 The ToF-SIMSintensities of SiOH+ and SiH+ groups were found signicantlyhigher in wear debris of silicon compared to the original siliconsurface (see Figure S10 in the Supporting Information).37

These ToF-SIMS results also support the occurrence of thetribochemical reactions at the Si/SiO2 interface during thesliding process in humid air.Correlation between Wear Rate and Frictional Energy

Dissipation. Even though the main wear mechanism istribochemical rather than mechanical, the wear rate stillcorrelated with total frictional energy dissipated during theinterfacial slide.38,39 This also supported that the wear reactionwas activated by the mechanical shear at the sliding interface.Some fraction of the dissipated energy could be channeled intothe chemical reaction coordinate to surmount the activationbarrier of the SiOSi bond dissociation upon reaction withH2O.

24

Since the dissipated energy is the integral of the friction forceFt over the total sliding distance, the friction force of the Si/SiO2 interface was recorded during the wear test. Figure 5 plots

the friction force (Ft) as a function of sliding cycles (N)measured at = 0.04, 0.8, and 20 m/s and RH = 10, 30, and65%. The friction force of the Si/SiO2 interface varied duringthe wear test, and its variance also depended on RH and . Ingeneral, Ft slightly increased initially (N < 10) and thendecreased to a steady value as the sliding continued (N > 10).The RH and dependences of Ft during the transient periodand at the nal steady state (see Figure S11 in SupportingInformation) indicated that the chemical and/or physicalconditions of the interfaces dynamically vary over time.40,41

The increase of initial friction with increasing RH could beattributed to the growth of a solidlike structured layer ofwater on the clean native oxide surface.42,43 The decrease ofinitial friction with increasing velocity could be attributed to thedecrease in the extent of water bridge bonds.43 With theincrease in the number of reciprocating cycles, the transition tolow friction may happen after the removal of native oxide layersof the silicon wafer by tribochemical wear.44

Figure 6 exhibits the correlation between wear rate of Si andfrictionally dissipated energy of the Si/SiO2 interface over 100sliding cycles under various RH and sliding speed conditions.With the increase of sliding speed, the total dissipated energy ofSi/SiO2 pair over 100 sliding cycles revealed a trend similar tothe wear rate of silicon substrate at RH = 65% (Figure 6a).Figure 6b shows the linear relationship between the averagewear rate measured at RH = 10%, 30%, and 65% and the totaldissipated energy through friction. A similar relationship wasreported for wear under fretting (oscillatory and reciprocatingsliding) conditions.4547 Density functional theory (DFT)calculations predicted that in the absence of any mechanicaldeformation or activation, the activation energy for the SiOSi dissociation upon reaction with H2O molecule impingingfrom the gas phase is about 113.8 kJ/mol when the siliconsurface is terminated by hydroxyl.24 The activation energyunder mechanical stress could be lower;34 but how much loweris not known. In any case, when the imposed energy issuciently high enough to overcome the activation barrier, thetribochemical reaction of Si/SiO2 pair will happen. The resultshown in Figure 6b indicated that the dissipated energy of Si/SiO2 pair is too low to cause the tribochemical damage ofsilicon when the wear test was performed at high speed/lowRH (20 m/s/10%) or at low speeds/high RH (0.04 m/s and0.08 m/s/65%) conditions.Interfacial Chemistry at Low RH Conditions. In low

humidity (RH below 30%), the average wear rate of Si substrate

Figure 5. Friction force vs number of reciprocating cycles (FtN)curves at representative sliding speed (v = 0.04, 0.8, and 20 m/s) andRH = (a) 10%, (b) 30%, and (c) 65%. The error of Ft estimated fromve dependent measurements is less than 20%. The asterisk marks thedierent value of stable friction force (N = 100) between theconditions of RH = 65%/v = 0.04 m/s (wearless case) and theconditions of RH = 65%/v > 0.2 m/s (wear case).

Figure 6. (a) Correlation between the average wear rate and the total dissipated energy during 100 cycles at RH = 65%. (b) Wear rate of the siliconsubstrate as a function of total dissipated energy during 100 sliding cycles at RH = 10%, 30%, and 65%.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156152

decreased linearly as a function of ln(v) or increased linearlywith 1/ln(1/RH) before transforming into the wearless state(see Figure S12 in the Supporting Information). This mightimply that the formation of capillary water bridge with stronghydrogen-bonded network between sliding interfaces isinvolved in the tribochemical reaction of the Si/SiO2 interfaceduring sliding process.18,34,37 The total free energy (E)needed for condensation of water vapor bridging two solidsurfaces could be expressed as48

=E k T Vln(1/RH)B

m (1)

where kB is Boltzmanns constant, T is the temperature, V is thevolume of water bridge, and m is the volume of a watermolecular. Assuming the condensation is an activation process,the time need to form such a water bridge can be approximatedwith the Arrhenius relationship4850

E k Texp[ /( )]0 B (2)Here, 0 is the critical time needed to condense one adsorbatelayer.51,52 Combining eq 1 and eq 2, the water bridge volume Vcan be related to the sliding speed v and RH as49,51,52

V

c v vln( / )ln(1/RH)m 0

(3)

where c is a constant and v0 is the critical sliding speedcorresponding to a transition to the state where interfacialfriction becomes independent of sliding speed (see Figure S13in the Supporting Information).51 Equation 3 indicates that thevolume of water bridge between the substrate and the slidingcounter surface is proportional to ln(v0/v) and 1/ln(1/RH).Based on the Kelvin equation, the water bridge volume at the

equilibrium state can be calculated at each relative humid-ity.53,54 In this experiment, the minimum sliding speed was0.008 m/s. If the sliding process at this speed is assumed to bea quasi-equilibrium state, the water bridge volume V can beestimated for the lowest sliding speed at each RH; then V canbe estimated at all tested sliding speeds using eq 3. Figure 7aplots the average wear rate of the silicon substrate as afunction of water bridge volume V at RH below 30%. The dataclearly show that the average wear rate () is proportional tothe volume of water bridge (V). It is also intriguing to note thatthe linear relationship also holds at high humidity conditions(Figure 7b), but the slope is dierent, indicating the role ordynamics of water bridge is dierent at high humidity

conditions (see Interfacial Chemistry at High RH Conditionssection).Although further molecular details for tribochemical

reactions within the capillary bridge are beyond the scope ofthis experimental work, it is possible to postulate that thewearless behavior at high sliding speeds in the low RHcondition (

and then dropped to a low value within N < 10 cycles (seegreen line in Figure 5c and Figure S11c in the SupportingInformation). The substrate showed 0.5 nm thick wear in therst 20 cycles and then no more wear during the next 80 cycles(see Figure S15 in the Supporting Information). The initialwear thickness is close to typical native oxide thickness on thesilicon wafer.44 Once this oxide layer wears o, the chemicalbond formation across the interfaces would be less likely sincethe Si surface does not have OH groups. Although the siliconsurface in the sliding interfaces can be oxidized in the presenceof water and oxygen, the oxidation rate seems to depend on and RH. If the surface hydroxyl group formation is facilitated byvigorous interfacial shear, it might explain the less wear at lowsliding speeds.57 At the same shear rate, the transition to thenearly wearless behavior is not observed at lower RH (30%)(for example, Figure 2b), implying that the RH dependence ofthe adsorbed water layer structure may play a critical role.42,43,56

During the nearly wearless transition process at high RHconditions, not only the Si substrate but also the SiO2 spheresurface was also altered. This alteration was revealed in thefollowing control experiment. First, a new SiO2 tip was rubbedon a fresh silicon wafer surface with a displacement of 200 nmat RH = 65% and v = 0.04 m/s. After 100 sliding cycles, thedisplacement was increased to 400 nm, and the friction forcesFt inside and outside the original 200 nm track were compared.As shown in Figure 8a, once the Si/SiO2 interface was

preconditioned to the low-f riction and low-wear state, thefriction force of the preconditioned SiO2 sphere remained lowregardless of the substrate condition whether it is preconditioned(center 200 nm) or fresh (outside the initial 200 nm weartrack). A typical friction force measured on the fresh siliconsurface by a new SiO2 tip was 3.7 N at this test condition,but the friction force measured with the preconditioned SiO2 tipwas only 0.61 N on the fresh substrate, which was close to thevalue (0.45 N) measured on the preconditioned wear track(Figure 8b). Once the SiO2 sphere surface was preconditioned tothe low-friction and wearless state, then the friction forcemeasured by reciprocating cycles on the fresh substrate surfacewas always low regardless of sliding speed (see Figure S16 inthe Supporting Information). This state was stable andunaltered even after long exposure to the ambient humid air(see Figure S17 in the Supporting Information). The only wayto recover to the initial high friction state of the SiO2 sphere

was to keep sliding onto the fresh native oxide surface of thesilicon substrate (see Figure S17 in the SupportingInformation). These results indicated that the low-frictionbehavior is the consequence of the modication of the SiO2sphere surface.The low friction and negligible wear state induced by rubbing

could not be due to contaminations. If contaminations wereresponsible for this behavior, the same should have beenobserved at all RH tested. But, the transition to the nearlywearless state at low was observed only at high RH. The lowfriction and negligible wear at low and high RH could beexplained if silanol groups of the SiO2 sphere undergodehydration reactions with adjacent silanol groups during theslow shear against Si at high RH.58 This reaction would result inthe dehydroxylation of the SiO2 surface.

59 If the SiO2 sphereloses silanol groups, then the SisubstrateOSisphere bridges wouldnot be formed readily.18,31 Thus, tribochemical wear would besuppressed. The stability of this low friction and negligible wearstate also implies that the rehydration rate of the dehydroxy-lated silica surface is very slow.60

CONCLUSIONIn humid air, the wear of Si rubbed with SiO2 is not ubiquitous;it is highly dependent on the ambient humidity and the shearrate. The Si surface is mechanically robust in dry conditions; inhumid air, however, the water-induced chemical reactions makeit susceptible to wear. The TEM analysis conrms that there isno mechanically induced subsurface damage in the wear trackformed in humid conditions, supporting the hypothesis thatwear in humid conditions is purely tribochemical. Thetribochemical reactions appear to involve two pathways: (1)dehydration reaction between silanol groups at the substrateand those at the counter surface, which leads to wear, and (2)dehydration reactions between adjacent silanol groups on onesolid surface, which leads to a low-friction state. Thecompetition of these reaction channels determines whetherthe Si/SiO2 interfaces would wear severely or not. Thedominant reaction mechanism varies depending on relativehumidity (RH) and interfacial shear rate (v). It is especiallyintriguing to note that wearless behavior is observed at theopposite corners of the RH and v parameter domain: low RH/high and high RH/low v.

ASSOCIATED CONTENT*S Supporting InformationCharacterization of SiO2 microspheric tip used in the test;conrmation of reproducibility; cross-section proles of wearscars at selected RH and sliding speed; eect of pile-up debrison wear scar length; wear rate at selected relative humidity andsliding speed; humidity eect on the adhesion force of Si/SiO2pair; wearless behavior of Si/SiO2 pair in dry air condition;estimation of frictional temperature rise; measurement ofchemical contents on the wear scars formed in microwear tests;comparison of the initial (N = 1) and steady (N > 20) frictionat various RH and v; wear rate at various sliding speeds and RH(RH < 30%); critical sliding speed v0 at various relativehumidity (RH < 30%); negligible wear of the Si substrate athigh RH and low v conditions; stability of the low-friction andlow-wear state of the SiO2 sphere surface formed at high RHand low v conditions. This material is available free of chargevia the Internet at http://pubs.acs.org.

Figure 8. (a) Friction loop (Ft vs D) curve measured afterpreconditioning the Si/SiO2 interfaces to the low-friction and low-wear state by rubbing at v = 0.04 m/s and under RH = 65%. After N= 100, the sliding displacement distance was increased from 200 to400 nm. (b) Comparison of the friction force Ft measured for freshsurface by new SiO2 tip, for inside and outside the preconditionedwear track by modied tip. The error of Ft estimated from threedependent measurements is less than 20%.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156154

AUTHOR INFORMATIONCorresponding Authors*E-mail shkim@engr.psu.edu (S.H.K.).*E-mail linmao@swjtu.edu.cn (L.Q.).NotesThe authors declare no competing nancial interest.

ACKNOWLEDGMENTSThe authors are grateful for the nancial support from theNatural Science Foundation of China (91323302, 51175441,and 51375409). S.H.K. acknowledges the support from theNational Science Foundation (Grant DMR-1207328).

REFERENCES(1) Snyder, G. J.; Lim, J. R.; Huang, C.-K.; Fleurial, J.-P.Thermoelectric Microdevice Fabricated by a MEMS-like Electro-chemical Process. Nat. Mater. 2003, 2 (8), 52831.(2) Bartsch, S. T.; Lovera, A.; Grogg, D.; Ionescu, A. M.Nanomechanical Silicon Resonators with Intrinsic Tunable Gain andSub-nW Power Consumption. ACS Nano 2012, 6 (1), 256264.(3) Alsem, D. H.; Stach, E. A.; Dugger, M. T.; Enachescu, M.;Ritchie, R. O. An Electron Microscopy Study of Wear in Poly SiliconMicroelectromechanical Systems in Ambient Air. Thin Solid Films2007, 515, 32593266.(4) Kim, S. H.; Asay, D. B.; Dugger, M. T. Nanotribology andMEMS. Nano Today 2007, 2, 2229.(5) Archard, J. F. Contact and Rubbing of Flat Surfaces. J. Appl. Phys.1953, 24, 981988.(6) Kato, K. Classication of Wear Mechanisms/Models In Wear-Materials, Mechanisms and Practice; Stachowiak, G. W., Ed.; Wiley:New York, 2006; pp 918.(7) Chung, K. H.; Lee, Y. H.; Kim, D. E. Characteristics of Fractureduring the Approach Process and Wear Mechanism of a Silicon AFMTip. Ultramicroscopy 2005, 102, 161171.(8) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nanotribology:Friction, Wear and Lubrication at the Atomic Scale. Nature 1995, 374,607616.(9) Kato, K.; Adachi, K. Wear Mechanisms. In Modern TribologyHandbook; Bhushan, B., Ed.; CRC Press: Boca Raton, FL, 2001; pp273274.(10) Kovalchenko, A.; Gogotsi, Y.; Domnich, V.; Erdemir, A. PhaseTransformations in Silicon under Dry and Lubricated Sliding. Tribol.Trans. 2002, 45, 372380.(11) Zarudi, I.; Cheong, W. C. D.; Zou, J.; Zhang, L. C. AtomisticStructure of Monocrystalline Silicon in Surface Nano-modification.Nanotechnology 2004, 15, 104107.(12) Ribeiro, R.; Shan, Z.; Minor, A. M.; Liang, H. In SituObservation of Nano-abrasive Wear. Wear 2007, 263, 15561559.(13) Yu, B. J.; Dong, H. S.; Qian, L. M.; Chen, Y. F.; Yu, J. X.; Zhou,Z. R. Friction-induced Nanofabrication on Monocrystalline Silicon.Nanotechnology 2009, 20, 465303.(14) Gotsmann, B.; Lantz, M. A. Atomistic Wear in a Single AsperitySliding Contact. Phys. Rev. Lett. 2008, 101, 1125501.(15) Lantz, M. A.; Wiesmann, D.; Gotsmann, B. DynamicSuperlubricity and the Elimination of Wear on the Nanoscale. Nat.Nanotechnol. 2009, 4, 586591.(16) Jacobs, T. D. B.; Carpick, R. W. Nanoscale Wear as a Stress-Assisted Chemical Reaction. Nat. Nanotechnol. 2013, 8, 108112.(17) Liu, J. J.; Notbohm, J. K.; Carpick, R. W.; Turner, K. T. Methodfor Characterizing Nanoscale Wear of Atomic Force Microscope Tips.ACS Nano 2010, 4 (7), 37633772.(18) Yu, J. X.; Kim, S. H.; Yu, B. j.; Qian, L. M.; Zhou, Z. R. Role ofTribochemistry in Nanowear of Single-Crystalline Silicon. ACS Appl.Mater. Interfaces 2012, 4 (3), 15851593.(19) Varenberg, M.; Etsion, I.; Halperin, G. Nanoscale Fretting WearStudy by Scanning Probe Microscopy. Tribol. Lett. 2005, 18 (4), 493497.

(20) Ku, I. S. Y.; Reddyhoff, T.; Holmesb, A. S.; Spikes, H. A. Wearof Silicon Surfaces in MEMS. Wear 2011, 271, 10501058.(21) Erdemir, A.; Donnet, C. Tribology of Diamond-like CarbonFilms: Recent Progress and Future Prospects. J. Phys. D: Appl. Phys.2006, 39 (18), 311327.(22) Chen, L.; Yang, M. C.; Song, C. F.; Yu, B. J.; Qian, L. M. Is 2 nmDLC Coating Enough to Resist the Nanowear of Silicon. Wear 2013,302 (12), 909917.(23) Carpick, R. W.; Salmeron, M. Scratching the Surface:Fundamental Investigations of Tribology with Atomic ForceMicroscopy. Chem. Rev. 1997, 97, 11631194.(24) Barnette, A. L.; Asay, D. B.; Kim, D.; Guyer, B. D.; Lim, H.;Janik, M. J.; Kim, S. H. Experimental and Density Functional TheoryStudy of the Tribochemical Wear Behavior of SiO2 in Humid andAlcohol Vapor Environments. Langmuir 2011, 27, 1270212708.(25) Torii, A.; Sasaki, M.; Hane, K.; Okuma, S. A Method forDetermining the Spring Constant of Cantilevers for Atomic ForceMicroscopy. Meas. Sci. Technol. 1996, 7, 179184.(26) Varenberg, M.; Etsion, I.; Halperin, G. An Improved WedgeCalibration Method for Lateral Force in Atomic Force Microscopy.Rev. Sci. Instrum. 2003, 74, 33623367.(27) Schwarz, U. D. A Generalized Analytical Model for the ElasticDeformation of an Adhesive Contact between a Sphere and a FlatSurface. J. Colloid Interface Sci. 2003, 261, 99106.(28) Petersen, K. E. Silicon as a Mechanical Material. Proc. IEEE1982, 70 (5), 420457.(29) Zarudi, I.; Cheong, W. C. D.; Zou, J.; Zhang, L. C. AtomisticStructure of Monocrystalline Silicon in Surface Nano-modification.Nanotechnology 2004, 15, 104107.(30) Ribeiro, R.; Shan, Z.; Minor, A. M.; Liang, H. In SituObservation of Nano-abrasive Wear. Wear 2007, 263, 15561559.(31) Liu, Y.; Szlufarska, I. Chemical Origins of Frictional Aging. Phys.Rev. Lett. 2012, 109, 186102.(32) Lim, S. C.; Ashby, M. F. Overview No. 55, Wear MechanismMaps. Acta Metall. 1987, 35 (1), 124.(33) Liu, Y.; Erdemir, A.; Meletis, E. I. A Study of the WearMechanism of Diamond-like Carbon Films. Surf. Coat. Technol. 1996,82, 4856.(34) Beyer, M. K. The Mechanical Strength of a Covalent BondCalculated by Density Functional Theory. J. Chem. Phys. 2000, 112(17), 73077312.(35) Mizuhara, K.; Hsu, S. M. Tribochemical Reaction of Oxygen andWater on Silicon Surfaces. Tribol. Ser. 1992, 21, 323328.(36) Katsuki, F. Single Asperity Tribochemical Wear of Silicon byAtomic Force Microscopy. J. Mater. Res. 2009, 24, 173178.(37) Yu, J. X.; Chen, L.; Qian, L. M.; Song, D. L.; Cai, Y.Investigation of Humidity-dependent Nanotribology Behaviors ofSi(100)/SiO2 Pair Moving from Stick to Slip. Appl. Surf. Sci. 2013,265, 192200.(38) Mohrbacher, H.; Blanpain, B.; Celis, J. P.; Roos, J. R. LowAmplitude Oscillating Sliding Wear on Chemical Vapour DepositedDiamond Coatings. Diamond Relat. Mater. 1993, 2, 879884.(39) Yu, J. X.; Qian, L. M.; Yu, B. J.; Zhou, Z. R. NanofrettingBehavior of Monocrystalline Silicon (100) Against SiO2 Microspherein Vacuum. Tribol. Lett. 2009, 34, 3140.(40) Xiao, X. D.; Qian, L. M. Investigation of Humidity-dependentCapillary Force. Langmuir 2000, 16, 81538158.(41) Nevshupa, R. A.; Scherge, M.; Ahmed, S. I.-U. TransitionalMicrofriction Behavior of Silicon Induced by Spontaneous WaterAdsorption. Surf. Sci. 2002, 517, 1728.(42) Asay, D. B.; Kim, S. H. Evolution of the Adsorbed Water LayerStructure on Silicon Oxide at Room Temperature. J. Phys. Chem. B2005, 109, 1676016763.(43) Asay, D. B.; Kim, S. H. Effects of Adsorbed Water LayerStructure on Adhesion Force of Silicon Oxide Nanoasperity Contact inHumid Ambient. J. Chem. Phys. 2006, 124, 174712.(44) Chen, L.; Kim, S. H.; Wang, X. D.; Qian, L. M. Running-inProcess of Si-SiOx/SiO2 Pair at Nanoscale Sharp Drop of Frictionand Wear Rate during Initial Cycles. Friction 2013, 1, 8191.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156155

(45) Aghdam, A. B.; Khonsari, M. M. Prediction of Wear inReciprocating Dry Sliding via Dissipated Energy and TemperatureRise. Tribol. Lett. 2013, 50, 365378.(46) Fouvry, S.; Paulin, C.; Liskiewicz, T. Application of an EnergyWear Approach to Quantify Fretting Contact Durability: Introductionof a Wear Energy Capacity Concept. Tribol. Int. 2007, 40, 14281440.(47) Bryant, M. D.; Khonsari, M. M.; Ling, F. F. On theThermodynamics of Degradation. Proc. R. Soc. A 2008, 464, 20012014.(48) Bocquet, L.; Charlaix, E.; Ciliberto, S.; Crassous, J. Moisture-induced Ageing in Granular Media and the Kinetics of CapillaryCondensation. Nature 1998, 393, 2431.(49) Riedo, E.; Levy, F.; Brune, H. Kinetics of CapillaryCondensation in Nanoscopic Sliding Friction. Phys. Rev. Lett. 2002,88 (18), 185505(4).(50) Tambe, N. S.; Bhushan, B. Friction Model for the VelocityDependence of Nanoscale Friction. Nanotechnology 2005, 16, 23092324.(51) Szoszkiewicz, R.; Riedo, E. Nucleation Time of NanoscaleWater Bridges. Phys. Rev. Lett. 2005, 95, 135502.(52) Noel, O.; Mazeran, P.-E.; Nasrallah, H. Sliding VelocityDependence of Adhesion in a Nanometer-Sized Contact. Phys. Rev.Lett. 2012, 108, 015503.(53) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.;Academic Press Inc.: San Diego, 1992.(54) Orr, F. M.; Scriven, L. E.; Rivas, A. P. J. Comments on theCapillary Binding Force of a Liquid Bridge. Fluid Mech. 1975, 67,723742.(55) Luzar, A.; Chandler, D. Hydrogen Bond Kinetics in LiquidWater. Nature 1996, 397, 5557.(56) Vogler, E. A. Structure and Reactivity of Water at BiomaterialSurfaces. Adv. Colloid Interface Sci. 1998, 74, 69117.(57) Gee, M. G.; Butterfield, D. The Combined Effect of Speed andHumidity on the Wear and Friction of Silicon Nitride. Wear 1993,162-164, 234245.(58) Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica.Colloids Surf., A 2000, 173, 138.(59) Feng, A.; McCoy, B. J.; Munir, Z. A.; Cagliostro, D. E. WaterAdsorption and Desorption Kinetics on Silica Insulation. J. ColloidInterface Sci. 1996, 180, 276284.(60) Snyder, L. R.; Ward, J. W. The Surface Structure of PorousSilicas. J. Phys. Chem. 1966, 70, 39413952.

Langmuir Article

dx.doi.org/10.1021/la504333j | Langmuir 2015, 31, 149156156