Tribochemistry of Aldehydes Sheared between (0001) Surfaces of α

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Tribochemistry of Aldehydes Sheared between (0001) Surfaces of α-Alumina from First-Principles Molecular Dynamics Sarah M. Haw and Nicholas J. Mosey* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6

bS Supporting Information ABSTRACT: First-principles molecular dynamics (FPMD) simulations are used to explore the tribological behavior of systems consisting of two Al2O3 (0001) surfaces separated by acetaldehyde molecules. The simulations were performed with normal pressures, P, that ranged from 0 to 20 GPa. The simulations show that sliding occurs with little or no significant changes in the structure of the aldehydes when P is low. Meanwhile, tribochemical reactions between aldehydes to yield oligomers, and between the oligomers and the surfaces, occur at higher P. The occurrence of these reactions leads to slip mechanisms that are dominated by the dissociation of chemical bonds. The different slip mechanisms affect the friction forces required to maintain motion of the surfaces. Slip mechanisms that do not involve bond rupture require low forces, with a friction coefficient of 0.034 to 0.044. The friction forces are much larger for the slip processes involving the rupture of bonds. Interestingly, the results indicate that the friction forces associated with slip mechanisms involving bond rupture are lower when the longer oligomers are involved in the slip process. Overall, this work sheds light on the atomic-level chemical processes that occur when lubricated surfaces move past one another, and may aid in the rational use of tribochemical reactions in functional lubrication.

I. INTRODUCTION Stress-induced changes in the atomic and electronic structures of materials due to compression and shear are important in many areas of science, engineering, and technology.1 Lubrication is one prominent area in which these stress-induced processes play a key role. Specifically, lubricant molecules undergo a wide range of reactions in response to the extreme stresses and temperatures that occur in sliding contacts.2,3 In many cases, these tribochemical reactions are deleterious, leading, for example, to the decomposition of the lubricant molecules, corrosion and wear of surfaces, or acidification of the lubricant.4
7 However, tribochemical reactions can also be useful. For example, it has been suggested that pressure-induced changes in the coordination of zinc atoms in films derived from antiwear additives used in motor oils contribute the protection of engine surfaces,8
10 while the transformation of biomolecules into protective films has been proposed to aid in lubricating artificial biological implants.11,12 Recently, efforts have been made to use tribochemical reactions in the context of lubrication.13
15 For example, studies have been performed in which simple alcohols were deposited on the surfaces of microelectromechanical (MEMS) devices, and transformed into lubricating polymers in response to the stresses experienced when the surfaces in these devices were moved past one another.16,17 In that case, the use of small molecules facilitated the introduction of lubricant precursors into the small spaces between contacts, which would be difficult with the larger, less mobile species typically used as lubricants, whereas the polymerization of the alcohols generated the lubricant within the r 2011 American Chemical Society

contact itself. This concept of functional lubrication, in which the lubricant adapts to the conditions in the contact by undergoing tribochemical reactions, may offer an improved ability to control friction and wear in sliding contacts and fine-tune lubricant performance. To realize this potential, it is necessary to develop a better fundamental understanding of the tribochemical processes that occur in sliding contacts and how these processes affect friction and wear. Identifying the chemical processes occurring within sliding contacts presents a significant experimental challenge. As such, chemical simulation can play a key role in this area. To model tribochemical reactions, it is necessary to perform simulations with potential energy functions that can account for changes in chemical bonding during reactions. Simulations using reactive force-fields have provided insights into tribochemical reactions,18,19 friction anisotropy, and wear.20 Quantum chemical (QC) methods are more general and transferrable than reactive FFs, but have high computational demands. QC-based simulations of compression-induced reactions involving a wide range of functional groups have been reported.21
25 However, such simulations have typically focused on bulk models and have Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: June 29, 2011 Revised: November 8, 2011 Published: November 14, 2011 2132

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The Journal of Physical Chemistry C not explicitly incorporated surfaces, which is necessary to understand how these reactions affect friction and wear. QC-based studies of tribochemical reactions using models that explicitly include moving surfaces have been reported, but are relatively rare due to computational expense.26
34 We have recently used first-principles molecular dynamics (FPMD) simulations to explore the chemical behavior of acetaldehyde molecules (CH3CHO, referred to as MeCHO throughout) under conditions of high compression in a bulk state21 and when placed between surfaces composed of alumina (Al2O3).26 The results of those simulations demonstrated that high stresses induce the formation of bonds between MeCHO molecules to yield polyethers. The role of compression was to reduce the distances between the reacting molecules to facilitate bond formation. Meanwhile, the surface promotes polymer formation by altering the electronic structure of adsorbed MeCHO molecules in a manner that renders the sp2 carbons more susceptible to nucleophilic attack, leading to the formation of bonds between these molecules. In addition, the surface acts as a medium with which protons can be exchanged to form electrically neutral oligomers. The formation of polyethers through tribochemical reactions between MeCHO molecules is intriguing in the context of functional lubrication because ethers are known to be good lubricants.35,36 In the present study, FPMD simulations are performed to explore how systems composed of two Al2O3 (0001) surfaces separated by MeCHO molecules behave under sliding conditions, and to connect the results of the observed behavior to friction and wear. The tribochemical behavior observed under the sliding conditions is analogous to that observed when these systems were compressed. The different tribochemical processes that occur lead to a variety of slip mechanisms, ranging from scenarios in which the surfaces moved smoothly with minimal changes in lubricant structure to those involving the dissociation of chemical bonds between the surface and tribochemical products present in the interface. Irreversible changes in surface structure, which would indicate wear, do not occur in any of the simulations. Meanwhile, the different slip mechanisms affect the friction forces required to maintain sliding. In particular, the friction forces are higher when slip requires the dissociation of bonds. Interestingly, it is found that the friction forces are reduced when longer oligomers are formed. Overall, this study provides atomic-level insight into tribochemical reactions and the influence of those reactions on friction and wear. The remainder of this paper is organized as follows. The models and methods used are described in section II. The results are presented and discussed in section III beginning with a description of the general slip processes that were observed, followed by discussions of the motions of the surfaces and tribochemical reactions in which the MeCHO molecules take part, and finishing with an examination of how these processes affect friction. The conclusions are reported in section IV.

II. METHODS AND MODELS FPMD simulations were performed using model systems consisting of two Al-terminated (0001) surfaces of alumina (Al2O3) separated by either 8 or 16 MeCHO molecules. The system with 8 MeCHO molecules, designated N8, is shown in Figure 1a, and that containing 16 MeCHO molecules, designated N16, is shown in Figure 2a. It is noted that these systems are relatively large for FPMD simulations, with the N8 and N16

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Figure 1. Structures of the N8 system before and after slip with P = (a) 0.0 GPa, (b) 7.5 GPa, and (c) 10.0 GPa. These structures were selected to represent the different slip mechanisms observed during the simulations. In all cases, the [0001] direction is aligned from bottom to top and the [2130] slip direction is aligned from left to right. Al, O, C, and H atoms are represented by large purple, medium red, medium silver, and small green spheres, respectively. All structures have been replicated twice along the directions defined by the a and b lattice vectors of the hexagonal simulation cell to better illustrate the structure. Atoms have been deleted in (b) and (c) to better illustrate the aldehydes involved in the slip processes.

models containing 176 and 232 atoms, respectively. The models were prepared by cleaving a 2  2  1 representation of the hexagonal unit cell of alumina (Al2O3) to yield two Al-terminated (0001) surfaces. This corresponds to the lowest energy surface for this system. The surfaces were then separated and the desired number of MeCHO molecules was introduced into the space between them. A 5 ps FPMD simulation was performed with the height of the system fixed to allow the atoms in the system to reorient. This was followed by an additional 2 ps of equilibration in which the thickness of the system was allowed to change to maintain a constant normal pressure, P, of 0 GPa. The systems were then compressed to obtain a series of structures with P between 0 and 20 GPa in 2.5 GPa intervals. The structures obtained at these different P were then equilibrated for an additional 2 ps while allowing the height of the simulation cell to fluctuate to maintain the desired normal load before shearing. Three-dimensional (3-D) periodic boundary conditions were employed in all simulations. The systems were sheared along the [2130] direction by altering the x and y components of c such that the top of the simulation cell moved along this direction at a rate of 1.0 Å/ps 2133

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Figure 2. Structures of the N16 system before and after slip with P = (a) 0.0 GPa, (b) 10.0 GPa, (c) 15.0 GPa, and 20.0 GPa. These structures were selected to represent the different slip mechanisms observed during the simulations. In all cases, the [0001] direction is aligned from bottom to top, and the [2130] slip direction is aligned from left to right. Al, O, C, and H atoms are represented by large purple, medium red, medium silver, and small green spheres, respectively. All structures have been replicated twice along the directions defined by the a and b lattice vectors of the hexagonal simulation cell to better illustrate the structure. Atoms have been deleted in b, c, and d to better illustrate the aldehydes involved in the slip processes.

according to Lees
Edwards boundary conditions.37 This direction was selected based on experiments38 indicating that it is the preferred basal slip direction in bulk Al2O3 and potential energy scans on the (0001) plane of bulk Al2O3. The shear simulations were performed at the values of P mentioned above, which were maintained by allowing the height of the simulation cell to change according to Parrinello
Rahman dynamics.39 In all cases, the systems were sheared for an initial shear distance of δ = 5.0 Å, where δ is defined as the distance the top of the simulation cell has moved along the slip direction relative to its original position. If no bonding between aldehydes was observed during this period, the simulation was stopped in the interest of minimizing computational effort. However, if bonding between aldehydes did occur, the simulation was continued until there was an evident peak in the associated stress
strain plot that could be associated with a slip event. Overall, this approach permitted the determination of representative slip mechanisms and associated shear strengths for these systems at all values of P considered. The FPMD simulations were performed using a version of the Quantum-Espresso software package that we modified to permit the application of constant strain rates.40 The electronic structure was evaluated with density functional theory41,42 using the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE).43 The valence states were represented as plane-waves expanded at the Γ-point to a kinetic energy cutoff of 30 Ry. The core states were represented by ultrasoft pseudopotentials44 using a kinetic energy cutoff of 180 Ry for the augmentation charges. The constant cutoff approach of Bernasconi et al.45 was used in all FPMD simulations. The dynamics were performed within the Car
Parrinello formalism46 using a time step of 5.0 au

(0.121 fs, 8268 time steps per ps of simulation) and an orbital mass of 400.0 au. The systems were equilibrated at 300 K using Nose-Hoover thermostats,47,48 and the temperature was not controlled during shearing. Stresses were obtained from the Hellmann
Feynman forces associated with the deformation of simulation cell, which are susceptible to errors in the form of Pulay forces. Tests indicated that these errors were minimal, with components of the stress tensor evaluated according to this procedure agreeing to within better than 0.1 GPa of values obtained with a kinetic energy cutoff of 80 Ry. Previous work in our group on similar systems have shown that this methodology is adequate for reproducing various properties related to the structures of alumina and aldehydes, as well as reactions involving these species.21,26
28

III. RESULTS AND DISCUSSION The purpose of the present study is to determine how tribochemical processes occurring in contacts comprising two Al2O3 (0001) surfaces separated by MeCHO molecules affect the sliding motion of the interface on the (0001) plane along the [2130] direction. The general details of the slip processes observed with different pressures, P, applied normal to the slip plane are described in section III.a. The motion of the surfaces under these conditions is explored in section III.b. A detailed examination of the tribochemical processes occurring in the interfaces is given in section III.c. The effects of the different slip processes on shear strengths and friction are discussed in section III.d. a. General Details of Slip Processes. Structures representing the different slip mechanisms observed during the simulations of 2134

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The Journal of Physical Chemistry C the N8 and N16 systems are shown in Figures 1 and 2, respectively. These structures are discussed in what follows to provide a general description of the slip mechanisms. More detailed descriptions of the motion of the surfaces and aldehydes are given below. Figure 1a shows the structure of the N8 system with P = 0.0 GPa before shearing and after the system was sheared to reach δ = 5.0 Å. A comparison of the structures demonstrates that the upper and lower surfaces behave as solid units, i.e., no significant shearing is evident within the Al2O3 slabs. Furthermore, little change is evident in the structure of the aldehydes, which remain adsorbed as a monolayer on each surface with the methyl groups pointing away from the surfaces. Overall, these structures are indicative of a smooth slip mechanism in which the surfaces forming the contact interact minimally with each other. The structures in Figure 1b correspond to the N8 system with P = 7.5 GPa just before and after a slip event that occurred at δ = 3.7 Å. The structure with δ = 3.0 Å shows that the aldehydes adsorbed to either side of the interface interact with each other more strongly than in Figure 1a, with the changes in structure required for slip to occur indicated by arrows. Specifically, the methyl group on an aldehyde bonded to the upper surface interacts with a methyl group of an aldehyde bonded to the lower surface. This interaction causes an aldehyde on the upper surface to reorient such that its methyl group adopts a location further back along the slip direction, while the aldehyde on the bottom surface reorients to become relatively parallel to the surface by moving its methyl group forward along the slip direction. In addition, an aldehyde on the lower surface oriented roughly parallel to the surface moves even closer to the surface, resulting in the formation of C
O bond between its sp2 carbon and an oxygen atom in the surface. The processes outlined in Figure 1a,b correspond to slip mechanisms that do not involve the formation of bonds between the aldehyde molecules, but involve varying degrees of reorganization of the aldehyde structure to permit continued sliding of the surfaces. Analogous slip mechanisms without the formation of bonds between aldehydes were also observed with P = 2.5 and 5.0 GPa, with the degree of reorientation of the aldehydes increasing with P. The structures in Figure 1c correspond to the N8 system with P = 10 GPa before and after a slip event that occurred when δ ≈ 6.0 Å. The slip mechanism involves the formation and dissociation of bonds between the aldehydes in the system. In this case, a C
O bond is formed between the oxygen atom of an aldehyde on the bottom surface and the sp2 carbon of an aldehyde on the upper surface. This process results in an aldehyde dimer that effectively connects the two surfaces during sliding. The continued motion of the top surface relative to the bottom surface occurs through the dissociation of the C
O bond between the two aldehydes, which is evident from the structure at δ = 7.0 Å. Other processes involving the formation and dissociation of bonds between aldehydes were observed in all simulations of the N8 system with P g 10 GPa; although, as discussed below, the specific bonds that formed and dissociated differed in the simulations. Figure 2a shows the structure of the N16 system with P = 0 GPa before shearing and after δ = 5.0 Å has been reached. At δ = 0.0 Å, the structure contains four layers of aldehydes, with a monolayer of these molecules bound to the lower Al2O3 surface and a half monolayer bonded to the upper Al2O3 surface. This structure persists after shearing to δ = 5.0 Å, with only minimal

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changes in the orientations of the aldehydes not bonded to the surface. In addition, no significant shearing of the Al2O3 slabs is evident, indicating that the slabs move as solid units. Overall, this behavior is consistent with a smooth sliding mechanism involving minimal interactions between the aldehydes. Similar processes involving the smooth motion of the surfaces without any bond formation or dissociation between aldehydes were observed in the simulations performed with P e 7.5 GPa. However, nonbonding interactions between aldehydes were more evident at higher P in this range, with the aldehydes undergoing significant reorientations at higher P. The structures in Figure 2b illustrate the slip process that occurred at δ ≈ 4.1 Å for the N16 system with P = 10 GPa. The structure at δ = 4.0 Å shows that an aldehyde trimer is bound to the upper surface through two Al
O bonds. As the upper surface moves along the slip direction, the trimer interacts with free aldehydes in the interface, and forces two of these aldehydes (circled in Figure 2b) downward. This process causes these aldehydes to react with other groups on the lower surface, ultimately forming two aldehyde trimers on that surface. This process is distinctly different from those observed at lower P because it involves the formation of bonds between aldehydes to yield larger groups that must move past one another. A similar process was observed during the simulation of N16 with P = 12.5 GPa. Figure 2c shows the structures involved in slip for the N16 system with P = 15 GPa. Prior to the slip event (δ = 5.75 Å), a polyether bridged the two Al2O3 surfaces. This ether was formed through pressure-induced reactions between aldehdyes within the interface as described below. Slip occurs through the dissociation of the Al
O bond between the polyether and the upper Al2O3 surface, as shown in the structure at δ = 6.5 Å. This process is analogous to that observed for the N8 system when P g 10 GPa; however, in this case, a more complex polyether bridges the interface, and bond rupture occurs between the ether and the surface, as opposed to the dissociation of bonds within the ether itself. Figure 2d outlines the slip process that occurred in the N16 system when P = 20.0 GPa. Before shearing, i.e., when δ = 0.0 Å, a polyether had formed through pressure-induced reactions during compression of the system to reach P = 20.0 GPa and is bound to the upper surface through an Al
O bond. As the system is sheared, this group interacts with aldehydes in the interface and a dimer bonded to the lower surface. This interaction causes these structures to reorient, with aldehydes in the interface becoming bonded to the surface. In addition, the polyether becomes detached from the upper surface through the dissociation of the Al
O bond, which allows this surface to continue sliding. This process does not involve the formation of new bonds between aldehydes in the interface, but rather involves nonbonding interactions between aldehydes and ethers in the interface that result in the dissociation of an Al
O bond. A similar process was observed for the N16 system when P = 17.5 GPa. b. Surface Motion. The influence of the different slip mechanisms on the movement of the Al2O3 slabs was examined by analyzing how the positions of the Al3+ ions immediately on either side of the interface move within the (0001) plane. Specifically, the average displacements, Δr(0001), of the four relevant Al3+ ions in the upper and lower surfaces were calculated relative to their corresponding positions in the (0001) plane when δ = 0.0 Å as a function of shear distance, δ. The values of the Δr(0001) versus δ are plotted in Figure 3 for the N8 and N16 2135

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Figure 3. Average displacements in the (0001) plane, Δr(0001), of the four Al3+ ions immediately on the upper (open symbols) or lower (closed symbols) side of the interface as a function of shear distance, δ, for the (a) N8 and (b) N16 systems. All values of Δr(0001) are referenced to the positions of the Al3+ ions when δ = 0.0 Å.

systems with P = 0, 5, 10, 15, and 20 GPa. If the slabs move in a perfectly smooth manner, the value of Δr(0001) for the lower surface should remain constant at 0.0 Å, and Δr(0001) = δ for the upper surface. Figure 3a provides the values of Δr(0001) as a function of δ for the N8 system. The values with P = 0 and 5 GPa lie close to Δr(0001) = 0.0 Å for the lower surface and Δr(0001) = δ for the upper surface. This is consistent with the slip mechanisms described above for the N8 system with P e 7.5 GPa, which involve the smooth movement of the surfaces with, in some cases, minor changes in the orientations of the aldehydes. Deviations from smooth sliding behavior are more evident at higher values of P. This is particularly true for δ > 5.0 Å, where the aldehyde dimers bridging the interface are present. Consider, for instance, the data obtained with P = 20 GPa, where deviations from smooth sliding behavior are largest when δ = 6.0 Å. At this point, Δr(0001) = 0.49 Å for the bottom surface and 5.35 Å for the upper surface. These deviations from smooth sliding are due to the presence of the bridging dimer, which prevents the independent motion of the two surfaces as solid slabs. It is noted, however, that these deviations from smooth sliding are relatively small, and have not caused the surfaces to adopt full stick
slip motion. In such a case, one would expect the system to exhibit behavior consistent with being sheared uniformly prior to undergoing sudden slip events. Assuming the positions of the Al3+ ions along the [0001] direction are preserved throughout the sliding process, under a uniform shearing process one would expect values of Δr(0001) = 2.1 Å and 4.9 Å for the upper and lower surfaces, respectively, at δ = 6.0 Å and P = 20 GPa. Clearly, the data are not close to such values, indicating that the tribochemical processes within the interface hinder only slightly the smooth motion of the surfaces. The values of Δr(0001) as a function of δ for the N16 system are given in Figure 3b. In all cases, the data for the upper surface remain close to the shear distance, δ, indicating that the upper surface slides smoothly. Deviations from smooth sliding are more significant for the lower surface, with values of Δr(0001) approaching 0.55 Å in some cases. Nonetheless, these are relatively small deviations, which, as in the case for the N8 system, suggest that the motion of the surfaces is relatively insensitive to the processes occurring between aldehydes, and between the aldehydes and surface, during sliding.

In addition to demonstrating that the surfaces move in a relatively smooth motion at all values of P, the data in Figure 3 illustrate that irreversible changes in surface structure did not occur in the simulations. Such changes would be associated with wear. The absence of such changes in structure indicates that the reactions between the surfaces and aldehydes, or surfaces and tribochemical products, do not damage the surfaces during sliding. c. Lubricant Motion and Tribochemical Reactions. The general details of the slip mechanisms described in part a showed that the aldehydes exhibited a wide range of chemical behavior when the N8 and N16 systems were subjected to sliding under the values of P considered in the calculations. The behavior observed in the simulations ranged from slip processes involving little, or no, reorientation of the aldehydes to tribochemical reactions forming oligomers along with the dissociation of chemical bonds during the slip event. A detailed examination of these structural changes is provided in what follows. A detailed description of the underlying changes in electronic structure is provided in the Supporting Information. The discussion in section III.a, as well as Figure 1a,b, demonstrated that the N8 system undergoes a slip process that involves neither the formation of bonds between MeCHO molecules nor the dissociation of chemical bonds within the interface. At P = 0.0 GPa, slip occurs through the movement of the upper surface past the lower surface with minimal changes in the structures of the MeCHO monolayers bonded to each surface. At P = 7.5 GPa, the surfaces still move smoothly, but changes in the orientations of the MeCHO molecules occur. The increased interaction between the MeCHO molecules in the interface at this pressure is evident from the data in Figure 4a, which plots the average, minimum, and maximum distances, dMe‑Me, between the methyl groups of MeCHO molecules that pass over each other during slip of N8 at P = 0.0 GPa and P = 7.5 GPa. These data show that the methyl groups come into closer proximity as P is increased, which is consistent with compression of the interface. Specifically, at P = 7.5 GPa the minimum values of dMe‑Me are between 2.75 and 3.25 Å throughout the entire range of δ considered in the simulations, while the average value oscillates just below 4.0 Å. Meanwhile, at P = 0.0 GPa, the methyl groups are much further apart, with the minimum distances ranging from 3.5 to 5 Å, and the average distances remaining above 4.3 Å. At both values of P, the maximum distance between the methyl groups 2136

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Figure 4. (a) Average, minimum, and maximum values of the distances between the methyl groups of the MeCHO molecules, dMe‑Me, that pass by each other during slip of the N8 system at P = 0.0 and 7.5 GPa as a function of shear distance, δ. (b) Average, minimum, and maximum values of the magnitude of the Al
O
CH
Me dihedral angles, ϕAl
O
CH
Me, of the MeCHO molecules adsorbed on the Al2O3 surfaces at P = 0.0 and 7.5 GPa as a function of shear distance, δ.

rises above 5 Å. At P = 0.0 GPa, this is due to the intrinsically large separations between these groups even without significant changes in the orientations of the MeCHO molecules. Meanwhile, at P = 7.5 GPa, these large separations are due to changes in the orientations of some MeCHO molecules as they reorient and form C
O bonds to the surface, as shown in Figure 1b. The changes in the orientations of the MeCHO molecules are evident from the data in Figure 4b, which plots the average, minimum, and maximum values of the magnitudes of the Al
O
CH
Me dihedral angles, ϕAl
O
CH
Me, for the eight MeCHO molecules in the N8 system at P = 0.0 and 7.5 GPa. The data obtained with P = 0.0 GPa show that the values of ϕAl
O
CH
Me are within a narrow range between 150° and 180°, which is consistent with the structures in Figure 1a, where the MeCHO molecules are largely aligned perpendicular to the Al2O3 surface with the methyl groups pointing toward the center of the interface. The values obtained at P = 7.5 GPa cover a much wider range. In this case, some dihedral angles remain near 180°, while others drop to as low as 0°. The latter correspond to scenarios in which rotation about the O
CH bond in the MeCHO molecule has occurred such that the methyl group points toward the surface. Such reorientations are due to interactions between the methyl groups during sliding. The structures in Figure 5a illustrate the tribochemical reactions that occurred during slip in N8 when P = 10 GPa. Corresponding changes in key interatomic distances are given in Figure 6a. When δ = 0.0 Å, the system exists in a state where one of the reacting MeCHO molecules is bonded to the upper surface through an Al
O bond, while the other MeCHO molecule is in the interface, but not bonded to either surface. The latter MeCHO molecule becomes bonded to the lower surface through the formation of the Al
O1 bond when δ = 1.0 Å and C2
O2 bond when δ = 1.9 Å. The orientation adopted by the lower MeCHO molecule exposes O1, allowing this atom to form a bond to C1 in the upper MeCHO molecule when δ = 4.0 Å. The data in Figure 6a show that the formation of this bond occurs rapidly once the C1
O1 distance drops to ∼2.5 Å. This is consistent with previous studies that have demonstrated the pressure-induced polymerization of MeCHO molecules through C
O bond formation occurs readily once the typical C
O distances reach ∼2.5 Å.21 From an electronic standpoint, processes of this type occur through nucleophilic attack involving a

lone pair of electrons on O1 attacking C1. This process is facilitated by the formation of the Al
O bond between the MeCHO molecule and the upper surface, which draws electron density away from C1.26 The formation of the C1
O1 bond results in an ether that bridges the interface. Continued sliding occurs through the dissociation of the C1
O1 bond at δ = 7.0 Å. A similar process involving the formation of a bridging ether followed by C
O bond rupture within the ether to permit sliding was observed when P = 17.5 GPa. Figure 5b outlines the tribochemical reaction that occurred in the N8 system when P = 15 GPa, and associated changes in interatomic distances are given in Figure 6b. At the start of the simulation, the reacting species correspond to an MeCHO molecule (containing C3) bonded to an Al ion in the lower surface and an OCHdCH2 moiety (containing O1, C1, and C2) bonded to the upper surface through the Al1
O1 bond. The C1
C2 bond is a double bond at the start of the simulation due to the transfer of a hydrogen atom from C2 to an oxygen atom in the surface as the system was compressed to P = 15 GPa before shear. The formation of a C
C bond between C2 and C3 occurs when the system is sheared to δ = 1.0 Å. Compression-induced reactions leading to the formation of C
C bonds between sp2 carbons have been observed and described previously.25,26 Here, the formation of the C2
C3 bond results in the formation of a species corresponding to a deprotonated diol that bridges the interface. The C2
C3 bond persists as the system is sheared, and the slip process involves the dissociation of the Al
O1 bond when δ = 7.0 Å. This bond rupture process is followed at δ = 8.1 Å by the formation of a bond between O1 and another Al ion (Al2) further back along the slip direction, along with the formation of a C
O bond between C3 and an oxygen atom (O2) in an ether present in the system. A similar process involving the formation of a C
C bond followed by the dissociation of an Al
O bond was observed in the simulations with P = 12.5 GPa. The tribochemical reactions that occurred in the N8 system with P = 20 GPa are shown in Figure 5c and the corresponding changes in relevant interatomic distances are given in Figure 6c. At the start of the simulation (δ = 0.0 Å) the species involved in the reaction correspond to an MeCHO molecule (containing C1 and O2) bonded to the upper surface through the Al1
O2 bond and the C1
O1 bond, and another MeCHO molecule 2137

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Figure 5. Structures illustrating the tribochemical reactions involved in the slip mechanisms for the N8 system at P = (a) 10.0 GPa, (b) 15.0 GPa, and (c) 20.0 GPa. Al, O, C, and H atoms are represented by large purple, medium red, medium silver, and small green spheres, respectively. Labels on the atoms correspond to those mentioned in the main text and are used to define the interatomic distances in Figure 6.

(containing C2) bonded to the lower surface at two sites through an Al
O bond and the C2
O3 bond. Shearing to δ = 1.2 Å results in the dissociation of the C2
O3 bond, which is followed by the formation of the C2
O2 bond when δ = 2.0 Å. The latter process yields an ether that bridges the two sides of the interface. The Al1
O2 bond ruptures at δ = 2.8 Å; however, the bridge between the sides of the interface persists until the C1
O1 bond dissociates at δ = 6.0 Å to allow continued motion of the upper surface to occur. The rupture of the C1
O1 bond is followed by the formation of a bond between O1 and an Al ion (Al2) further back along the slip direction at δ = 6.6 Å. The discussion in section III.a indicated that slip in the N16 system occurs without reactions between the MeCHO molecules in the interface for P e 7.5 GPa; although interactions between MeCHO molecules leading to changes in the lubricant structures became apparent with increasing pressure. This behavior is illustrated through the data in Figure 7, which plots the minimum and maximum values of the centers of mass of the MeCHO molecules in each of the four layers of these molecules that are evident in the interface (see Figure 2a). The data in Figure 7a show that the layers are well separated when P = 0.0 GPa. For example, the difference in the minimum and maximum COM[0001] values for any two adjacent layers is 1.7 Å between the minimum value for layer 2 and the maximum value for layer 3 at

δ ≈ 3.0 Å, and remains above 3.0 Å for the majority of the simulation. The interlayer separations are clearly reduced when P = 7.5 GPa, with the minimum interlayer distances reaching below 1.0 Å in many cases. Despite the reduction in interlayer distance, there is no evidence of mixing of layers, which would involve the minimum and maximum values of COM[0001] for two adjacent layers crossing. As mentioned in section III.a, tribochemical reactions occur within the N16 system when P g 10.0 GPa. The reactions that occurred at P = 20.0 GPa, correspond to only those events described in the text above associated with Figure 2d. That is, the relevant tribochemical products were present in the system after it was compressed to P = 20.0 GPa prior to shearing. Similar processes were observed when P = 17.5 GPa. As such, these processes are not discussed further here. Instead, the events that occurred at P = 10.0 and 15.0 GPa will be discussed in detail. The changes in chemical structure that occurred during these reactions are outlined in Figure 8 and the associated changes in relevant interatomic distances are provided in Figure 9. The data in Figure 8a detail the tribochemical reactions that occurred during sliding of the N16 system with P = 10.0 GPa, and the associated changes in distances are provided in Figure 9a. When δ = 0.0 Å, the relevant portion of the system exists as a set of seven disconnected MeCHO molecules. Four of these 2138

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Figure 6. Changes in key interatomic distances associated with the tribochemical reactions involved in the slip mechanisms for the N8 system at P = (a) 10.0 GPa, (b) 15.0 GPa, and (c) 20.0 GPa as a function of shear distance, δ. Labels for distances correspond to those assigned in Figure 5 for processes occurring at the same value of P.

molecules are bonded to the surfaces through Al
O bonds, and the remaining three are present within the interface. At this stage, the C3
C4 bond corresponds to a double bond because a hydrogen atom was transferred from C4 to an oxygen in the surface at an earlier stage of compression. Shearing to δ = 2.5 Å results in the formation of bonds between C2 and C4, and C1 and O1. This process yields an ether derived from three MeCHO molecules that is bound to the upper surface through two Al
O bonds. Further sliding to δ = 4.2 Å induces reactions between MeCHO molecules near the lower surface, leading to

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the formation of the C5
O3 and C6
O4 bonds. This process occurs as a result of the ether on the upper surface pushing the MeCHO molecule containing O3 and C6 downward as it moves along the slip direction. Essentially, the coordination of this ether to the upper surface through two sites reduces its ability to undergo changes in structure and orientation during sliding, and instead it interacts strongly with other species in the system. The formation of the C5
O3 and C6
O4 bonds yields a second ether derived from three MeCHO molecules, which is bonded to the lower surface through two Al
O bonds. Sliding to δ = 4.8 Å increases the size of the ether bonded to the upper surface through the formation of the C3
O2 bond, which occurs along with the transfer of a proton to the surface (circled in the last frame of Figure 8a) to yield an electrically neutral species. Similar processes involving the formation of ethers bonded to the upper and lower surfaces without the dissociation of bonds during the slip process were also observed during the simulations of N16 performed with P = 12.5 GPa. The structures in Figure 8b illustrate the tribochemical reactions that occurred in the N16 system when P = 15 GPa, and the associated changes in interatomic distances are shown in Figure 9b. At the start of the simulation, δ = 0.0 Å, the species involved in the reaction comprise an O
CH=CH2 moiety containing O1 and C1 bonded to the upper surface, an MeCHO dimer bonded to an Al atom in the lower surface, and three MeCHO molecules moving freely in the interface. A bond was formed between C1 and C2 upon reaching δ = 3.6 Å, which yielded an oligomer that is bonded to Al ions on both sides of the interface. Shearing to δ = 3.9 Å results in the formation of a trimer between the three MeCHO molecules initially moving freely in the interface. This process also involves the transfer of a hydrogen atom (circled in the second and third frames of the figure) from C1 to the terminal oxygen atom of the trimer. This process involves the movement of this hydrogen atom across the wall of the periodically repeated simulation cell. The bridging oligomer and trimer connect through the formation of the C1
C3 bond when δ = 4.6 Å. This has the effect of weakening the Al
O1 bond, as evidenced through an increase in this bond distance at δ = 4.6 Å in Figure 9b. Further shearing leads to the dissociation of this weakened bond when δ = 6.5 Å to yield the structure shown as the slip product in Figure 2c. The reactions described above are analogous to those observed during the compression of systems containing MeCHO molecules.21,26 Those studies showed that compressing the aldehydes forms dimers and higher-order oligomers through processes involving the formation of C
O and C
C bonds in conjunction with proton transfer. The application of P was found to promote these reactions by reducing the distances between the reacting atoms to the point at which the barriers to the bond formation and proton transfer processes become small. Adsorption of the MeCHO molecules on the surfaces draws electron density away from the sp2 carbons, rendering those atoms more susceptible to nucleophilic attack that leads to the formation C
O and C
C bonds. Additionally, the surfaces provide a medium with which protons can be exchanged to form neutral oligomers. The similarities between the reactions described above and those observed in the previous studies suggest that P and the presence of surfaces are playing the same role here. The earlier studies in which the MeCHO molecules were subjected to compression showed that the oligomers generally persisted once formed. Meanwhile, the interface structure changed during the present simulations, with oligomers detaching from the surface 2139

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Figure 7. Minimum and maximum positions of the centers of mass along the [0001] direction, COM[0001], of each of the four MeCHO molecules in each layer of the N16 system as a function of shear distances, δ, at P = (a) 0.0 GPa and (b) 7.5 GPa. A layer refers to the group of four MeCHO molecules residing at approximately the same position along the [0001] direction, and the layers are numbered from the top to bottom of the interface.

Figure 8. Structures illustrating the tribochemical reactions involved in the slip mechanisms for the N16 system at P = (a) 10.0 GPa, and (b) 15.0 GPa. Al, O, C, and H atoms are represented by large purple, medium red, medium silver, and small green spheres, respectively. Labels on the atoms correspond to those mentioned in the main text and are used to define the interatomic distances in Figure 9.

and rebonding at different sites, or decomposing altogether before reacting with other species. This suggests that shear plays a role in terms of changing the interface structure of the interface over time. Specifically, shearing stretches the oligomers bridging the interface, resulting in detachment or decomposition, and brings species in the interface closer to one another, which also leads to decomposition and changes in bonding. In addition, the movement of the surfaces introduces new sites for reattachment of the oligomers and the movement of the species in the interface facilitates changes in structure by bringing different species in sufficiently close proximity to react.

It should be noted that the simulated models are an approximation to real systems, where surfaces have curvature, defects, and other adsorbed species. These details will likely affect the occurrence of these reactions. From the standpoint of P, one would rarely expect real devices to contain large regions with atomically flat surfaces that could provide a uniform pressure distribution. Instead, real surfaces generally have large variations in height, and one would only expect the values of P required to induce these reactions to occur at the small number of points where the surfaces actually come into intimate contact. At these points, the presence of defects and other adsorbed species will 2140

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Figure 9. Changes in key interatomic distances associated with the tribochemical reactions involved in the slip mechanisms for the N16 system at P = (a) 10.0 GPa and (b) 15.0 GPa as a function of shear distance, δ. Labels for distances correspond to those assigned in Figure 8 for processes occurring at the same value of P.

affect the reactions. These details will alter the ability of the MeCHO molecules and oligomers to bind to the surfaces, e.g., by altering the strength of adhesion or through competition for available sites, and change the effect of adsorption upon the electronic structure of the MeCHO molecules. d. Effects on Shear Strengths and Friction. The results presented above indicate that sliding of the contacts occurs in conjunction with a wide variety of chemical processes that can be broadly categorized according to the value of P at which the simulations were performed. When P e 7.5 GPa, both the N8 and N16 systems undergo slip processes that do not involve any reactions between aldehydes, although some aldehydes do react with the surfaces at the higher end of this pressure range. Processes analogous to that described at P = 7.5 GPa for the N8 system were also observed at higher values of P for that system when δ was low. The N8 system also underwent chemical reactions involving the formation of bridging dimers in all cases where P g 10.0 GPa. In these cases, slip was associated with the dissociation of chemical bonds, either within the bridging dimer or between the dimer and the surface. The specific natures of these processes are outlined in Table 1. At P = 10.0 and 12.5 GPa, reactions occur between aldehydes in the N16 system to form oligomers that are bonded to the surfaces at multiple positions. The slip process in these cases involves these bound oligomers moving past each other without the dissociation of chemical bonds within the oligomer, or between the oligomers and the surface. Similar processes involving the motion of the surfaces without the dissociation of chemical bonds were also observed in the simulations of the N16 system at higher P when δ was low, except at P = 20.0 GPa, where movement of oligomers that were present at the start of the simulation past each other led to bond rupture. As described above, the N16 system also exhibited slip processes dominated by the dissociation of chemical bonds when P g 15.0 GPa. Key details of these processes are summarized in Table 1. In this section, the effects of these different processes on friction and wear are explored. The different processes that occurred during sliding affected the shear stresses that must be applied to allow the surfaces to remain in relative motion. This is evident from the data in Figures 10a and 10b, which plot the shear stress applied along the slip direction, τ, as a function of shear distance, δ, for the N8 and N16 systems associated with the slip processes outlined in

Figures 1 and 2. As discussed above, in cases where no bonding between aldehydes occurred, the data were collected up to δ = 5.0 Å. In cases where bonds were formed between aldehdyes, the data were collected up to a value of δ exceeding the point at which bonds ruptured during the slip event. The data in Figure 10a with P = 0.0 GPa are consistent with the description above in which the motion of the upper surface with respect to lower one occurred in a smooth manner with little interaction between the aldehyde monolayers on each of these surfaces. Essentially, the values of τ oscillate between (1.0 GPa. The interactions between the aldehydes bonded to the two surfaces are more apparent from the data in Figure 10a at P = 7.5 GPa, where the values of τ lie predominantly above 0.0 GPa. The maximum shear stress is reached at this value of P when δ ≈ 3.7 Å, and is associated with the slip event outlined in Figure 1b. The data obtained with P = 10.0 GPa are similar to those plotted at P = 7.5 GPa for δ < 5.0 Å, with a peak in τ due to a slip event occurring without bond rupture at δ ≈ 3.8 Å. This event was followed by the formation of an aldehyde dimer that bridges the interface, as described above, and continued shearing of the system leads to a large increase in τ. The dissociation of the dimer at δ ≈ 7.0 Å leads to peak in the τ at that point. The data in Figure 10b with P = 0.0 GPa are consistent with a smooth sliding mechanism in which minimal interactions exist between the components of the interface to impede sliding. This is clear from the values of τ, which oscillate between (0.5 GPa throughout the simulation. The data obtained with P = 10.0 GPa exhibit a peak in τ when δ ≈ 1.5 Å, which is due to slip without the dissociation of bonds between species in the interface. This is followed by an increase in τ, with large oscillations, before reaching a second peak at δ ≈ 4.0 Å. The latter peak is associated with the slip process outlined in Figure 2b. The data obtained with P = 15.0 GPa also exhibit a peak in τ when δ ≈ 1.5 Å due to a slip process that does not involve bond dissociation. This is followed by the formation of an oligomer that bridges both sides of the interface, which was described above. Continued shearing in the presence of this bridging oligomer leads to a steady increase in τ until the dissociation of an Al
O bond occurs when δ ≈ 6.5 Å. The data obtained at P = 20.0 GPa exhibit a steady increase in τ immediately upon shearing from δ ≈ 0.0 Å. This is due to interactions between oligomers formed during the compression of the system prior to shearing, which led to a slip 2141

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Table 1. Key Details of Tribochemical Reactions Associated with Slip Events for the N8 System with P g 10.0 GPa and the N16 System with P g 15.0 GPaa system

P [GPa]

bridging

N8

10.0

yes

C
O

N8

12.5

yes

Al
O

N8

15.0

yes

Al
O

N8

17.5

yes

C
O

N8 N16

20.0 15.0

yes yes

C
O Al
O

N16

17.5

no

C
O

N16

20.0

no

Al
O

dissociating bond

a

The data indicate whether the oligomers formed were bonded to both sides of the interface (bridging) and characterize the type of bond that dissociated during the slip event.

process involving bond dissociation at low values of δ. In this case, τ increases until reaching a maximum value when the Al
O bond dissociates at δ ≈ 2.5 Å. The maximum shear strengths, τc, associated with the slip events are directly related to friction forces, Ff = Aτc, where A is the cross-sectional area of the surface. The values of τc obtained from the simulations performed for both the N8 and N16 systems at all values of P are plotted in Figure 10c for slip events that occurred with and without the dissociation of the chemical bonds. Note that due to computational expense these values are each obtained through a single MD simulation performed for each system at each value of P, and thus one should not place too much emphasis on the specific values of τc in Figure 10c. Nonetheless, these data can be grouped in a way that provides general insights into how friction is affected by the observed tribochemical processes, the size of the system, and the applied value of P. The values of τc for slip events without bond rupture in the N8 system exhibit a trend of increasing approximately linearly from τc = 0.9 to 3.2 GPa between P = 0.0 and 5.0 GPa, and then level off to remain between τc = 2.5 and 3.1 GPa between P = 7.5 and 20.0 GPa. Similar behavior is observed for the N16 system, where the slip events that occur without bond rupture have values of τc that increase from 0.5 to 1.7 GPa over P = 0.0 to 5.0 GPa, and then remain between 1.9 and 2.5 at higher P. The values of τc are significantly affected by the occurrence of tribochemical reactions and bond rupture during slip. The values of τc for the N8 system when P g 10.0 GPa, i.e., all instances in which ethers bridging the interface were formed, are much higher than those associated with the slip events that occurred without bond rupture. Specifically, the values of τc associated with bond rupture events in N8 range between 6.4 and 9.6 GPa. By way of comparison, the shear strength of crystalline Al2O3 along [2130](0001) has been calculated to be 20.7 GPa when P = 0.0 GPa.27 Clearly, the large values of τc associated with these events arise from the increased force which must be applied to induce bond dissociation. The values of τc for the bond rupture events in N16 range from 4.2 to 5.1 GPa. These values are larger than those for the slip events in this system that occurred at the same values of P without bond rupture, but are lower than those associated with bond rupture events in the N8 system. Overall, these results indicate that the occurrence of tribochemical reactions involving bond rupture lead to higher friction; although the specific effects on τc may be affected by the structure and size of the interface.

Figure 10. Shear stress, τ, along the [2130] direction on the (0001) plane as a function of shear distance, δ, for the (a) N8 and (b) N16 systems corresponding to the slip mechanisms described in Figures 1 and 2. (c) Summary of shear strengths, τc, associated with slip events occurring with and without the dissociation of chemical bonds for the N8 and N16 systems at all values of P considered in the simulations.

The value of τc at a given P can be expressed in terms of the intrinsic dependence of the shear strength on applied load (Amontons’s law)49 and the number and strengths of the adhesive interactions in the interface:27 τc ¼ μ P þ

∑i Ni τi

ð1Þ

where μ is the friction coefficient, i is an index denoting the different types of interactions in the system, Ni is the number of interactions of type i, and τi is an intrinsic shear stress associated 2142

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The Journal of Physical Chemistry C with that type of interaction. The relatively steep increase in τc when P e 5.0 GPa for both the N8 and N16 systems reflects the fact that the number and strengths of adhesive interactions in the interface increase steadily upon raising P from 0.0 GPa. The leveling-off of τc at higher P likely results from the fact that the adhesive interactions between aldehydes, and thus their contributions to τc, i.e., the sum in eq 1, are roughly constant for P > 5.0 GPa, leading to a smaller slope in the relationship between τc and P. Assuming the summation in eq 1 is constant for P > 5.0 GPa and performing a linear least-squares fit of the τc versus P data for slip events occurring without bond rupture at these pressures yields μ = 0.034 ( 0.016 and 0.044 ( 0.032 for the N8 and N16 systems, respectively. Such low values of μ indicate that the aldehydes and tribochemical products may be effective lowfriction materials under high loads, provided that bond rupture events do not occur. In fact, polyethers such as the products of the tribochemical reactions between aldehydes are used as lubricants.35,36 The large values of τc associated with slip processes that involve the rupture of bonds arise from the fact that the dissociation of bonds requires the application of large forces, which will largely dictate the value of τc. In other words, the value of τi associated with the bond that ruptures will likely be the largest contributor to eq 1. Of course, the specific values of τi will depend on the details of the bond that ruptures and the structure of the interface. Furthermore, the contribution of other, weaker interactions will vary among the simulations. Together, these factors lead to differences in the values of τc even when the types of bonds that dissociate during the slip process are the same. For example, the slip processes involving rupture in the N8 system with P = 10.0, 17.5, and 20.0 GPa all involve the dissociation of C
O bonds; however, the associated values of τc are 6.4, 8.6, and 7.3 GPa, respectively. The lower value of τc with P = 10.0 GPa can be understood in terms of the nature of the C
O bond that dissociates during slip. At this pressure, the oxygen atom involved in the scissile C
O bond is tricoordinate (see Figure 5a) and hence presumably weaker than the C
O bonds that rupture at the higher pressures where the oxygen atom is dicoordinate. More detailed analyses of the interactions contributing to each value of τc can shed light on the origin of the variation in these values; however, such efforts are of little value considering the data correspond to very specific processes obtained through single simulations of a given system under a particular set of conditions. In addition to demonstrating that slip processes involving bond rupture have higher values of τc than those that do not involve bond rupture, the data in Figure 10c show that the values of τc for slip events involving bond rupture in the N16 system are consistently lower than those for the N8 system. This suggests that τc is related not only to P and the occurrence of bond dissociation, but also the number of aldehydes present in the contact. This is particularly interesting because actual devices are unlikely to be lubricated solely with two monolayers of aldehydes as in the N8 system, but will rather contain a large number of these molecules. The size-dependence of shear strengths in ideal crystals is known, with τc decreasing as the thickness of the sheared material is increased normal to the slip plane, with50 pffiffiffiffi τN µ τ1 = N ð2Þ where τN is the shear strength of a crystal containing N units normal to the slip direction, and τ1 is the shear strength of an

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individual unit. As discussed in reference 50, slip requires a crystalline system to overcome an energy barrier that is independent of its thickness. The energy needed to overcome the barrier is acquired by shearing the system along the slip direction, which increases the potential energy by stretching the bonds in the system. The total increase in potential energy due to bond stretching reaches the barrier at smaller shear strains for thicker systems containing more bonds than it does for thinner systems. Since smaller deformations require lower shear stresses, systems with a higher value of N will have lower shear strengths. While the N8 and N16 systems do not correspond to ideal crystals, the same general concepts likely apply. Bond rupture during sliding has an associated energy barrier that is independent of the size of the system, yet the shear stress applied to rupture that bond will depend on the number of units in the associated oligomer that are deformed in response to that applied stress. In the case of the N8 system all bond rupture events involved dimers derived from aldehydes, yielding N = 2. Meanwhile, the oligomers involved in the bond rupture processes in the N16 system at P = 15.0, 17.5, and 20.0 GPa are derived from 3, 3, and 5 MeCHO units, respectively, if branches off the oligomers are neglected. In the context of eq 2, the increased values of N for the N16 system are consistent with the lower values of τc for bond rupture processes in this system. As such, it is likely that the shear strengths will be even lower in larger systems, where longer oligomers can be formed through tribochemical reactions.

IV. CONCLUSIONS The present study used FPMD simulations to identify and study the processes that occur when systems composed of two Al2O3 (0001) surfaces separated by either 8 (N8 system) or 16 (N16 system) MeCHO molecules are sheared along the [2130](0001) direction under different normal pressures, P. The details of the tribochemical reactions that occurred during the slip processes were discussed, and these processes were connected to friction and sliding. The simulations led to the identification of a variety of slip mechanisms. At low values of P, sliding within both systems occurred without the formation of bonds between aldehydes, and without the rupture of any bonds in the system. Indeed, at very low pressures, little change in the structure was apparent at all during the slip processes. At higher pressures, tribochemical reactions occurred, which led to the formation of MeCHO dimers, trimers and even higher order oligomers. In the case of the N8 system, the reactions led to the formation of dimers that bridged the interface in all cases where P > 7.5 GPa. The associated slip mechanisms involve the dissociation of a bond either within the dimer itself or between the dimer and one of the surfaces. In the case of the N16 system, tribochemical reactions also occurred in all cases where P > 7.5 GPa. At P = 10.0 and 12.5 GPa, these reactions led to oligomers that were bonded to only one surface at multiple points. At higher values of P, oligomers were also formed in the N16 system and extend into the interface, including one case of bridging the surfaces, and the associated slip mechanisms involve bond rupture. These reactions are consistent with those reported previously when the N8 and N16 system were subjected to uniaxial compression along [0001].26 This suggests that shearing the system does not induce any new types of reactions, but rather these processes are dictated by the degree of compression, which brings the molecules sufficiently close to 2143

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The Journal of Physical Chemistry C induce reactions, as well as interactions with the surface, which alter the electronic structures of the aldehydes to facilitate bond formation processes and provides a medium with which protons can be exchanged to form electrically neutral species. Shear does play a role, however, in altering the interface structure by inducing the detachment of oligomers from the surfaces or the decomposition of the oligomers themselves. The resulting species then react with other species in the interface, e.g. the surfaces, MeCHO molecules, or other oligomers, resulting in a dynamic structure as reacting species are brought into close proximity through shear. In terms of tribology, the results demonstrate that the surfaces move smoothly as relatively solid slabs at all P. The occurrence of tribochemical reactions, particularly those which lead to bond rupture during slip, have small effects on the motion of the surface, but do not bring the movement of the surfaces into the stick
slip regime. In addition, irreversible changes in the structures of the surface, which would be indicative of wear, were not observed in the simulations. The friction forces, which are proportional to the values of τc, depend significantly on the occurrence of tribochemical reactions and the nature of the slip mechanism. For both systems, increasing P from 0.0 to 5.0 GPa leads to a steady increase in τc as the strength and number of adhesive interactions increases. At higher P, τc is relatively insensitive to P when the systems undergo slip mechanisms that do not involve bond rupture. This is likely due to the fact that the number and strengths of the adhesive interactions within the interface are relatively constant. Slip processes involving the rupture of bonds lead to much higher values of τc, which can be attributed to the need to apply large stresses to induce bond dissociation. The values of τc associated with bond rupture processes are consistently higher for the N8 system than they are for the N16 system. These differences are due to the presence of longer oligomers in the latter system, which require lower shear strains, and hence magnitudes of applied stress, to acquire the potential energy needed to dissociate bonds. The possibility that the formation of longer oligomers may further reduce the friction forces is intriguing in the context of functional lubrication because real contacts are likely to contain greater numbers of monomers than the systems considered here, which may permit the formation of larger tribochemical products. The exploration of this possibility is a focus of ongoing research in our group.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed description of the changes in electronic structure that occurred during the tribochemical reactions. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Ontario Ministry of Research and Innovation, and Queen’s University is gratefully acknowledged. Computing resources were provided by the

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Shared Hierarchical Academic Research Computing Network (SHARCNET), the Resaue Quebecois de Calcul de Haute Performance (RQCHP), and the Scinet High Performance Computing Consortium.

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