Effect of Surface Silanol Groups on Friction and Wear between

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Effect of Surface Silanol Groups on Friction and Wear between Amorphous Silica Surfaces Ming Wang, FangLi Duan, and Xiaojing Mu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04291 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Effect of Surface Silanol Groups on Friction and Wear between Amorphous Silica Surfaces Ming Wang,† Fangli Duan,*,† Xiaojing Mu‡ †State

Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing 400030, China

‡College

of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China

ABSTRACT: Reactive molecular dynamics (ReaxFF) simulations are performed to explore the tribological behavior between fully hydroxylated amorphous silica (a-SiO2) surfaces as a function of surface silanol density. The results show that the interfacial friction and wear are greatly reduced by increasing surface silanol density, which originates from the suppression of the initial formation of interfacial 1 =;=1 bridge bonds. Two different tribochemical reactions resulting in the formation of interfacial 1 =;=1 bridge bonds are observed, i.e., one occurring between two silanol groups, which is insensitive to changes in silanol density, and the other occurring between a silanol group and a surface 1 =;=1 bond, which is strongly suppressed with the increase of silanol density. We decouple the contributions of these two 1 =;=1 bond formation mechanisms to the observed tribological behavior, and find that the latter formation mechanism plays a dominant role. Furthermore, the changes in the geometry and structure of fully hydroxylated a-SiO2 surface caused by the increased surface silanol groups also play an important role in the tribochemical reactions and the tribological performance of the a-SiO2/a-SiO2 system. This work provides a deeper insight into the effect of surface silanol groups on the tribological behaviors of silicon-based materials.

1. INTRODUCTION Silicon-based materials have been widely used for the manufacturing of microelectromechanical systems (MEMS),1,2 due to their importance in semiconductor devices and nanoscience technologies, such as accelerometer,3 microbearing systems,4 micromotor,5 biosensors,6,7 wafer bonding,8 etc. In MEMS, the surface damage of silicon-based components involved in physically moving and rubbing can occur,4-6 and thus the reliable operations of MEMS face significant challenges and limitations. * Corresponding author. Tel.: +86 138 8346 7096. E-mail address: [email protected] (Fangli Duan).

1

ACS Paragon Plus Environment

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For this reason, the tribology of silicon-based materials has been largely investigated in experiments. It was reported that the tribological behavior was strongly affected by lubricant,9-13 temperature,14 applied load,15,16 sliding velocity,17,18 relative humidity (RH),15,19,20 ambient gas type,9,21 etc. Besides the extrinsic factors, the intrinsic surface chemical properties of silicon-based materials were also found to be critical to the tribological behavior,22-26 which is a major concern in this study. The surface chemical groups (i.e., 1 =B, 1 =;B and 1 =;=1 ) of silicon-based materials have a direct effect on their tribological behavior. When H groups are adsorbed on silicon surface, the surface is hydrophobic, but when OH groups are adsorbed, the surface is hydrophilic.27 The nanowear of the two kinds of silicon surfaces against a silica microsphere was investigated using atomic force microscope at various RH and in water.19,20 Wang et al.19 found that with the increase of RH, the wear of hydrophilic silicon first increased to a maximum at about 50% RH and then decreased to no discernible surface damage in water, whereas Chen et al.20 found that the wear of hydrophobic silicon increased with the increase of RH and was maximum in water. This difference was explained by the fact that the adsorbed water layer thickness on silicon surface, depending on the surface hydrophilicity/hydrophobicity and RH, influences the formation of interfacial 1 =;=1 bonds.19 When the surface groups of silica are mainly 1 =;=1 bonds, the surface is hydrophobic, but when the coverage of 1 =;B groups is sufficiently high, the surface is hydrophilic.23,28 The hydrophilic silica surface was found to show a lower friction and wear compared to hydrophobic surface.23 In all these cases, tribochemical reactions between the sliding counter surfaces played an important role in the friction and wear. However, it is hard to fully clarify the atomistic mechanism of tribochemical processes in experiment. Reactive molecular dynamics (ReaxFF-MD) simulations have been widely used to reveal the tribochemical mechanisms in the tribological systems of silicon-based materials.29-35 For example, Li et al.29 studied the tribochemical wear of amorphous silica (a-SiO2) surface with only 1 =;=1 groups, and found that the main wear mechanism was the transfer of both individual atom and atomic cluster initiated by the preexisting 1 =;=1 bridge bonds between two a-SiO2 surfaces. When the surface groups on two sliding surfaces were mainly 1 =;B groups, the dehydration reaction between 1 =;B groups from the opposite surfaces can occur at sliding interfaces,30,31 resulting in the formation of interfacial 1 =;=1 bridge bonds. This was found to initiate the interfacial atomic mixing across the sliding interface of fully hydroxylated a-SiO2 and oxidized silicon,31 and to facilitate the hydrolysis reaction between the surface 1 =;=1 bonds of F %

E and water molecules.30 When silicon surfaces

2


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fully reacted with water, the surface groups of the oxidized-silicon surfaces were composed of 1 =B, 1 =;B and 1 =;=1

36

During scratching of the oxidized-silicon surface with a hydroxylated a-SiO2

tip, the formation of interfacial 1 =;=1 bonds through dehydration or dehydrogenation reaction was found to play a crucial role in the removal of Si atoms from the silicon substrate.32-34 Clearly, the tribochemical reactions involving the formation of interfacial 1 =;=1 bonds do play an important role in the tribological behaviors of silicon-based materials. However, the quantitative understanding on the tribochemical mechanisms is still lacking. In this paper, MD simulations with ReaxFF were used to investigate the tribological behavior of fully hydroxylated a-SiO2 surfaces with various silanol densities. ReaxFF-MD simulations showed that the presence of large amounts of surface silanol groups greatly reduced the friction and wear of the a-SiO2/a-SiO2 pair. We observed two different tribochemical reactions leading to the formation of interfacial 1 =;=1 bridge bonds, and found that they made different contributions to the observed tribological behavior. The chemisorption of silanol groups on fully hydroxylated a-SiO2 surfaces was found to alter the geometry and structure of the surfaces, which in turn could strongly influence the occurrence of the observed tribochemical reactions and the tribological performance of the studied system. The findings of this work would be useful to optimize the tribological design of silicon-based parts in M/NEMS applications.

2. SIMULATION METHODS The slab-on-slab sliding pair model used in this work, shown in Figure 1a, is constructed with two a-SiO2 substrates (periodic in x-z plane with dimensions of 53.8×17.5×53.8 Å3), in which both surfaces of the contact interface are fully hydroxylated and have a same silanol density. To generate the fully hydroxylated a-SiO2 surfaces with different silanol densities, different a-SiO2 substrates were cut out from bulk a-SiO2 produced from a melt-quench process of J %

E crystals, similar to

previous works.37,38 The surface hydroxylation of the obtained a-SiO2 substrates in the y direction was conducted by reacting with water molecules at 474 K in NVT ReaxFF-MD simulations.38 The temperature of 474 K in the hydroxylation simulations was chosen because a-SiO2 surface was hydroxylated to the maximum degree at this temperature in experiment.28 However, after the hydroxylation process, the resulted a-SiO2 surfaces were not fully hydroxylated. Then we found the undercoordinated oxygen and silicon atoms on the surfaces and added hydrogen atoms and hydroxyl groups to the corresponding atoms, respectively, obtaining the fully hydroxylated and relaxed a-SiO2 3


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respectively. Then these systems obtained from the loading process were well equilibrated under a NVT ensemble with a Nosé-Hoover thermostat at 300 K for 25 ps. For the friction processes, various normal pressures of 0.1, 0.5 and 1 GPa were applied on the rigid layer along the -y direction. The rigid layer was forced sliding with a constant velocity of 10 m/s along the z direction, whereas the fixed layer was completely frozen throughout the whole simulation. Thermostat layers, which are between the rigid layer and free surface layer, were coupled to a Langevin thermostat to control the system temperature at 300 K. Periodic boundary condition was employed in the x and z directions (both parallel to the interface). A Verlet algorithm was used to solve the equations of motion with a time step of 0.25 fs. This sliding friction lasted about 500 ps with configurations saved every 0.25 ps. We employed the ReaxFF reactive force field, which has been successfully applied to describe the interaction between a-SiO2 and water.38 All the MD simulations were performed with LAMMPS code.41

3. RESULTS AND DISCUSSION 3.1. Effect of Surface Silanol Density on the Tribology Behavior 2

1.17 OH/nm 2 2.38 OH/nm 2 3.05 OH/nm 2 5.08 OH/nm

250 200

(b)

0.5 GPa

No. of Si–O–Si Bridge Bonds

(a) Friction Force /nN

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 100 50 0 0

100

200

300

400

2

1.17 OH/nm 2 2.38 OH/nm 2 3.05 OH/nm 2 5.08 OH/nm

100 80 60 40 20 0 0

500

0.5 GPa

Time /ps

100

200

300

400

500

Time /ps

Figure 2. Variation of (a) friction force and (b) the number of interfacial 1 =;=1 bridge bonds at the sliding interfaces of fully hydroxylated a-SiO2 with sliding time for various surface silanol densities.

The friction and wear behaviors between fully hydroxylated a-SiO2 surfaces were investigated as a function of silanol density. Figure 2a shows the variation of friction force during the sliding under a normal pressure of 0.5 GPa. The configurations of the corresponding interfacial atomic mixing and the total amount of transferred atoms across the interface are shown in Figures S2 and S3, respectively. When silanol density is less than or equal to 2.38 OH/nm2, the friction force increases drastically with the increase of sliding time, and severe wear occurs at the tribological interfaces after sliding. For 5


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silanol density of 3.05 OH/nm2, the friction force increases slightly during the initial 300 ps and then remains stable as the sliding continues. Meantime, the occurrence of wear becomes slowly and the wear of the contact interface is slight. However, when silanol density is equal to or more than 5.08 OH/nm2, the friction force remains stable during the entire sliding process, and the interfacial wear is negligible after sliding for 500 ps. It is clear that the friction and wear are greatly reduced as silanol density is increased. The same dependence of tribological behavior on silanol density is also observed under normal pressures of 0.1 and 1 GPa (Figures S3 and S4). This suggests that the presence of large amounts of surface silanol groups significantly suppresses the friction and wear between fully hydroxylated a-SiO2 surfaces. This observation is consistent with the result of experimental study, which showed that hydrophobic a-SiO2 surfaces had a higher friction and a worse wear resistance compared to hydrophilic surfaces.23 The friction force between silicon-based materials was found to have a strong correlation with the number of 1 =;=1 bridge bonds formed across the contact interface.29,34,35 Here, we identified any one interfacial 1 =;=1 bridge bond formed between fully hydroxylated a-SiO2 surfaces by that two silicon atoms located on the opposite surfaces were connected by an oxygen atom, and the distance between the silicon and oxygen atoms was less than a cutoff distance of 1.8 Å.29,30 Figure 2b shows the variation of the number of interfacial bridge bonds (NSiOSi) with sliding time for different surface silanol densities. When silanol density is less than or equal to 2.38 OH/nm2, NSiOSi increases significantly with the sliding time. For silanol density of 3.05 OH/nm2, NSiOSi first increases slightly and then becomes stable. However, when silanol density is equal to or more than 5.08 OH/nm2, NSiOSi is very small (only 1 or 2 bonds). Clearly, the formation of interfacial bridge bonds is strongly suppressed by increasing the surface silanol density. Moreover, the variations of both the friction force (Figure S4) and NSiOSi (Figure S5) with sliding time exhibit a strong correlation under different silanol density conditions. This indicates that the increased surface silanol groups strongly suppress the formation of interfacial bridge bonds, and thus greatly reduce the friction force between fully hydroxylated a-SiO2 surfaces.

3.2. Initial Formation of Interfacial Si3O3Si Bridge Bonds It was reported that the formation of interfacial 1 =;=1 bridge bonds had a direct influence on the wear evolution and degree of silicon-based materials during sliding.19,30,34 When an interfacial bridge bond is formed between fully hydroxylated a-SiO2 surfaces, then with the help of interfacial 6


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shearing, it can facilitate the breaking of surface 1 =; bonds,19,33 or be strained to be broken.29,35 In such breaking events, initial wear indeed takes place, generating wear atoms (i.e., atoms that have deviated from their original surface or have been transferred to the counter surface),30,34 or exposing 1 = and 1 =;= dangling bonds at the interface.15,29 With the further sliding, both the wear atoms and dangling atoms would have a strong tendency to participate in the formation of new interfacial bridge bonds because they are highly reactive.15,29,30 During the frictional sliding, the continually formed new interfacial bridge bonds can aggravate the initial wear or facilitate the occurrence of new initial wear,29,32 finally leading to severe wear (e.g., a large number of transferred and mixed atoms at the sliding interface).31,34 In the tribological systems of this study, because fully hydroxylated a-SiO2 surfaces have no dangling atoms, the initial formation of interfacial 1 =;=1 bridge bonds through tribochemical reactions is the origin of the occurrence of interfacial wear. For this reason, we focus on the initial formation pathways of interfacial 1 =;=1 bridge bonds.

Figure 3. Two kinds of initial formation mechanisms of interfacial 1 =;=1 bridge bonds occurring between fully hydroxylated a-SiO2 surfaces: (a) the tribochemical reaction between a silanol group and a surface 1 =;=1 bond on the counter surface, and (b) the tribochemical reaction between two silanol groups from the opposite surfaces. The sliding time is shown in each frame. Black arrow points to the rupture location of bond, and blue dotted circle marks a three-coordinated oxygen transition configuration.

In our simulations, the initial formation of interfacial 1 =;=1 bridge bonds means that they are formed through tribochemical reactions between the chemical groups that come from the opposite surfaces and remain locally intact (i.e., no breaking of 1 =; and ;=B bonds) before reacting with 7


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each other. This kind of interfacial 1 =;=1 bridge bonds are called initial interfacial 1 =;=1 bridge bonds. We monitored the formation processes of these interfacial bridge bonds as a function of sliding time, and observed two kinds of formation mechanisms (Figure 3). One is the reaction between a silanol group and a counterface 1 =;=1 bond (Figure 3a), and the other is the reaction between two silanol groups from the opposite surfaces (Figure 3b). As shown in Figure 3, a silanol group (SiB=;b=B1) on one surface is moved close to the reactive groups on the counterface during the sliding. When they get close enough, the H1 atom attaches to an oxygen atom (Oa) of a silanol group (SiA=;a=B2) or a siloxane bond (SiA=;a=1 C), creating a threecoordinated oxygen transition configuration (marked by blue dotted circle). With the further sliding of upper substrate, the Ob=B1 and SiA=;a bonds are almost dissociated simultaneously (indicated by black arrows), and then the undercoordinated Ob and SiA atoms are bonded to form the interfacial SiA=;b=1 B bridge bond. During this process, the attachment of H atoms from silanol groups to the O atoms of 1 =; bonds is found to play a crucial role in the rupture of the 1 =; bonds. This is very similar to the breaking behavior of 1 =; bonds in previous tribo-simulations,34,35 which revealed that hydrogen had a weakening effect on 1 =; bonds. The formation of interfacial 1 =;=1 bridge bonds is generally thought to be due to the dehydration reaction between silanol groups at tribological interfaces,16,42-44 and this formation pathway has also been observed in recent MD simulations.30-32 However, in some other recent MD studies, although the atomic details of bond formation between specific atoms were revealed, it is unclear what specific reactions are able to form interfacial bridge bonds.33-35 In this work, we distinguish two reactions inducing the formation of interfacial 1 =;=1 bridge bonds, and as will be discussed later, the two formation mechanisms are found to have different contributions to the initial wear of fully hydroxylated a-SiO2 surface. Figure 4 shows the number of initial interfacial 1 =;=1 bridge bonds formed by two formation mechanisms (Figure 3). Each data point was obtained only during the sliding process in which severe wear has not occurred at the a-SiO2/a-SiO2 interface, and the corresponding sliding time is listed in Table S1 (see Supporting Information for more calculation details). As silanol density increases, the total number of initial interfacial bridge bonds (NTotal) decreases (Figure 4a), and so does the number of initial interfacial bridge bonds formed between a silanol group and a counterface 1 =;=1 bond (NSiOH+SiOSi) (Figure 4b). However, the number of initial interfacial bridge bonds formed between two silanol groups (NSiOH+SiOH) is quite small with no clear trend with silanol density (Figure 4c). Each value of NSiOH+SiOSi is very close to the corresponding value of NTotal in all cases, and both are much 8


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larger than the corresponding value of NSiOH+SiOH for lower silanol densities, suggesting that the reaction between a silanol group and a surface 1 =;=1 bond plays a dominant role in forming initial interfacial bridge bonds. It also indicates that large amounts of surface silanol groups strongly suppress the formation of initial interfacial 1 =;=1 bridge bonds, achieved mainly by preventing the tribochemical reactions between silanol groups and surface 1 =;=1 bonds. Furthermore, we observe that the variation of NSiOH+SiOSi with silanol density is consistent with the variation of interfacial wear degrees under different normal pressures (Figures 4b and S3). It is suggested that the initial interfacial 1 =;=1 bridge bonds formed between silanol groups and surface 1 =;=1 bonds play a leading role in the wear evolution of the a-SiO2/a-SiO2 interface.

No. of Initial Interfacial Si–O–Si Bridge Bonds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1 GPa

1 GPa

0.5 GPa

(a) Formed by Two Initial Formation Mechanisms

16 12 8 4 0 16

(b) Formed between Si–OH and Si–O–Si

12 8 4 0 16

(c) Formed between Si–OH and Si–OH

12 8 4 0 1

2

3

4

5

Density of Silanol Group /nm-2 Figure 4. The number of initial interfacial 1 =;=1 bridge bonds (a) formed by two initial formation mechanisms (Figure 3), (b) formed between a silanol group and a surface 1 =;=1 bond, and (c) formed between two silanol groups, as a function of surface silanol density. Each data point is obtained only during the sliding process before severe wear occurring between fully hydroxylated a-SiO2 surfaces, and error bars represent one standard deviation.

To confirm the origin of the difference between NSiOH+SiOH and NSiOH+SiOSi (Figure 4b,c), the energy barriers for 1 =;=1 bond formation reactions (Figure 3) were estimated via ReaxFF-MD simulations and density functional theory (DFT) calculations (see Supporting Information for more details). To simplify the calculations of transition state search, we used two simplified reaction models, i.e., the reaction between a SiO4H4 cluster and a Si7O19H14 cluster (Figure 5a), and the 9


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reaction between two SiO4H4 clusters (Figure 5b). Some atoms of one of the reactant clusters in each reaction were fixed (marked by gray color) to mimic the real contact situation. From Figures 5 and S9, it can be seen that both reaction processes all have one transition state. The energy barrier for the reaction between two silanol groups is ~283 kJ/mol for ReaxFF and ~249 kJ/mol for DFT, falling within the range of experimental values of

kJ/mol.45,46 Whereas the energy barrier for the

=

reaction between a silanol group and a surface 1 =;=1 bond is ~110 kJ/mol for ReaxFF and ~164 kJ/mol for DFT. Both ReaxFF and DFT show that the energy barrier of the reaction between two silanol groups is much higher than that of the reaction between a silanol group and a surface 1 =;=1 bond. This means that the reaction between a silanol group and a surface 1 =;=1 is much easier to occur compared to the reaction between two silanol groups. In our simulations, since thermal energy at 300 K (~2.5 kJ/mol) is much lower than the calculated energy barriers, the 1 =;=1 bond formation reactions cannot occur at 300 K. Thus, we explored the effect of mechanical interaction (compression and shear) on the 1 =;=1 bond formation reactions. It was shown that the mechanical interaction played a key role in driving these two reactions to happen during the sliding process (see Supporting Information for more information). (a)

(b)

D (Å)

C

A a 1

b B

A3a

1.96

a31

1.09

13b b3A

D (Å) A3a

249 kJ/mol

1.03

1.58

13b

1.61

2.66

b3A

2.33

164 kJ/mol C

A A

C

A

1 1

b

a

B

b

1 b 2

1 b

B

2

a

A

a

a

1.92

a31

B

A b

B

2

a

1

B

0 kJ/mol

0 kJ/mol -23 kJ/mol Initial

TS

-43 kJ/mol Initial

Final

TS

Final

Figure 5. The model used to estimate the energy barriers of tribochemical reactions (a) between two silanol groups and (b) between a silanol group and a surface 1 =;=1 bond to form initial interfacial 1 =;=1 bridge bonds, showing the configurations of the initial, transition and final states of the reactions. Inset table shows the distances between the specific atoms of transition states. Black arrow points to the rupture location of a bond, and green arrow means that two atoms will form a covalent bond. The atoms that are fixed are colored in gray and other color codes in the figures are the same as Figure 1a.

3.3. Interfacial Separation Increased by Surface Silanol Groups To understand the observed suppression effect of surface silanol groups (Figures 2 and 4), we investigated the surface geometry and structure of fully hydroxylated a-SiO2. We first explored the 10


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surface geometrical morphology (Figure S11) of fully hydroxylated a-SiO2 surfaces. The root-meansquare (RMS) roughness of these a-SiO2 surfaces were calculated. Figure 6a shows the variation of the RMS roughness versus silanol density, in which each data point was obtained from the coordinates of the topmost Si atoms of a-SiO2 substrate (i.e., about 230 data points). The surface roughness shows a strong dependence on silanol density. When increasing silanol density from 1.17 to 5.08 OH/nm2, the surface roughness increases from 0.78 to 1.07 Å. The roughness values match well with the data &Q

=

Å) in recent MD studies,47,48 and are very close to experimental data &Q = @ Å).49 Figure

6b shows the interfacial separation between fully hydroxylated a-SiO2 surfaces, defined as the average distance between the topmost and bottommost Si layers at the contact interface, as a function of silanol density. The interfacial separation increases upon increasing silanol density under various normal pressures. Such a trend could be explained by the fact that higher surface roughness can lead to a larger interfacial separation.50,51 In this case, as silanol density increases, the increased interfacial separation would have a lower probability for the reactive groups from the opposite surfaces to react with each other to form interfacial 1 =;=1 bridge bonds,19 and thus reduce the interfacial friction and wear. (a)

1.08

(b) Interfacial Separation /Å

1.04

RMS Roughness /Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.00 0.96 0.92 0.88 0.84 0.80 0.76

1

2

3

4

6.8

0.1 GPa 0.5 GPa 1 GPa

6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0

5

1

2

3

4

5 -2

-2

Density of Silanol Group /nm


Figure 6. (a) Root-mean-square (RMS) roughness of fully hydroxylated a-SiO2 surfaces as a function of silanol density. (b) Interfacial separation between two fully hydroxylated a-SiO2 surfaces as a function of silanol density under different normal pressures. Each data point represents the average of four mean interfacial separations calculated from four different contact interfaces, and error bars represent one standard deviation.

To confirm the origin of the variation of surface roughness with silanol density (Figure 6a), a control test was performed on the 1.17 OH/nm2 a-SiO2 surface. Some new silanol groups were generated on the surface “by hand”, which was achieved by removing the O atoms of some surface 1 =;=1 bonds and then adding hydroxyl groups to the dangling Si atoms. Then we obtained other more-hydroxylated surfaces with silanol density of 2.38, 3.05 and 4.18 OH/nm2. It was found that as silanol density on the 1.17 OH/nm2 surface was increased, the RMS roughness of the surface was 11


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increased (Figure S12). The results shown in Figures S11 and S12 indicate that the silanol group chemisorption on fully hydroxylated a-SiO2 surface can alter the surface morphology and roughness. This is very similar to Araki’s study,52 it was reported that when a-SiO2 surface was treated by H2O2, the strong oxidation led to the increase of surface roughness and altered the surface structure relative to an initial surface. We next investigated the orientations of surface silanol groups, characterized by the angles between the 1 =; (and ;=B) bonds of silanol groups and the surface normal. Figure 7a shows the average values of the silanol orientation angles as a function of silanol density. For the 1.17 OH/nm2 case, the average orientation angle for the 1 =; and ;=B bond is 26.5° and 63.4°, respectively. When silanol density increases to 5.08 OH/nm2, the average orientation angle for the 1 =; and ;=B bond respectively increases to 51.9° and 81.1°. The corresponding configurations are displayed in Figure 7b. It can be seen that both the silanol groups orient with an upward slant with respect to the substrate, but the silanol group is more tilted away from the surface in the case of 1.17 OH/nm2, compared to the 5.08 OH/nm2 case. The variation of the silanol orientation angles with silanol density (Figure 7a) can be explained by the changes in both the types of silanol groups and the number of hydrogen bonds formed among the silanol groups themselves (detailed explanation in the Supporting Information). Furthermore, it should be noted that the silanol orientation angles present negative correlation with the number of interfacial 1 =;=1 bridge bonds (Figures 4b and S5) and the interfacial friction and wear (Figures S3 and S4). (a)

Ave of Orientation Angle /°

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

(b)

O–H Bond Si–O Bond

72 64 56 48 40 32 24

1

2

3

4

5 -2


Figure 7. (a) Average orientation angles of silanol groups on fully hydroxylated a-SiO2 surfaces as a function of silanol density. The orientation angles of silanol group are defined as two angles between both its 1 =; and ;=B bonds and surface normal. (b) Schematic image of the average orientation angles of silanol groups. The color codes in the figures are the same as Figure 1b.

According to the results of this work, it is clear that the surface reactivity of fully hydroxylated a-SiO2 surfaces directly determines the chemical reactions observed in this study, and the mechanical 12


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interaction (compression and shear) can drive these reactions to occur at the a-SiO2/a-SiO2 interface. Moreover, the interfacial separation that is small enough to make the reactive groups from the opposite surfaces come close enough is a necessary condition for the occurrence of the reactions.19,29 It should be noted that the interfacial separation is determined by the surface geometry and structure of fully hydroxylated a-SiO2 under a given normal pressure.53,54 In our cases, increasing silanol density increases the surface roughness of fully hydroxylated a-SiO2 and alters the orientations of silanol groups themselves. As a result, the direct contact between silanol groups and counterface 1 =;=1 bonds is prevented, suppressing the initial formation of interfacial 1 =;=1 bridge bonds, and thereby lowering the interfacial friction and wear.

4. CONCLUSION This work has investigated the friction and wear at the sliding interface of fully hydroxylated aSiO2 as a function of surface silanol density using ReaxFF-MD simulations. The increase of silanol density has been shown to strongly suppress the initial formation of 1 =;=1 bridge bonds across the interface thereby greatly reducing the interfacial friction and wear. We observe two kinds of interfacial 1 =;=1 bond formation pathways. One is the reaction between two silanol groups from the opposite surfaces, the occurrence of which is less correlated to silanol density, and the other is the reaction between a silanol group and a counterface 1 =;=1 bond, the occurrence of which is strongly suppressed with the increase of silanol density. The latter formation mechanism is found to play a dominant role in dominating the evolution of interfacial friction and wear. ReaxFF-MD simulations and DFT calculations are used to estimate the energy barriers for the observed chemical reactions. We find that the energy barrier for the reaction between two silanol groups is much higher than that between a silanol group and a surface 1 =;=1 bond. Furthermore, the changes in the geometry and structure of fully hydroxylated a-SiO2 surface induced by increasing silanol groups are found to have a direct influence on the formation of interfacial bridge bonds and the tribological performance of the a-SiO2/a-SiO2 system. Our work reveals the important role of silanol groups in altering the surface geometry and structure of fully hydroxylated a-SiO2, and in suppressing the friction and wear between fully hydroxylated a-SiO2 surfaces.

AUTHOR INFORMATION Corresponding Author 13


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*E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The project is supported by the National Natural Science Foundation of China (Grant No. 51775066).

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Effect of Surface Silanol Groups on Friction and Wear between

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