Friction reactions induced by selective hydrogenation of textured surface under lubricant conditions

The passivation of hydrogen atoms and the conformation of textured surfaces under oil-lubricated conditions are effective strategies to obtain amorphous carbon (a-C) films with extremely low friction. It is critical to understanding the influence mechanism of selective surface hydrogenation on the tribological behaviors of textured a-C film under oil-lubricated conditions. In particular, the interactions of hydrogen atoms and lubricants are confusing, which is enslaved to the in situ characterization technique. The reactive molecular dynamics (RMD) simulations were conducted to analyze the friction response of textured a-C films with selective hydrogenation surfaces under oil-lubricated conditions. The results indicate that the existence of hydrogen atoms on specific bump sites significantly decreases the friction coefficient (μ) of textured a-C film, which is highly dependent on the surface hydrogen content. The repulsion between hydrogen atoms and lubricant molecules prompts the formation of a dense lubricant film on the surface of the mating material. Interestingly, with the enhancement of the surface hydrogen content, the passivation of the friction interface and the repulsion between hydrogen atoms and lubricants play dominant roles in reducing the friction coefficient instead of hydrodynamic lubrication.


Introduction
Microelectromechanical system (MEMS) or nanoelectromechanical system (NEMS) is widely used in aerospace, biomedical, and control engineering fields [1,2].However, owing to the size effects, the presence of nanoscale friction leads to the generation of static friction, the increase of dissipation, and the reduction of reliability, which severely limit the further development and application of MEMS/NEMS [3,4].In addition, liquid lubricants, which are singly applied to MEMS/NEMS surfaces, such as perfluoropolyether, ionic liquids, and multiplyalkylated cyclopentane, are also susceptible to failure or dissociation under extreme operating conditions [5,6].
Amorphous carbon (a-C) film is mainly composed of sp 2 (graphite structure) and sp 3 (diamond structure) hybridization networks and is widely used as a protective film due to its excellent properties, such as high hardness, wear resistance, and low friction coefficient [7].In particular, ultra-thin a-C film is www.Springer.com/journal/40544| Friction considered as a promising protective film for MEMS/NEMS applications, which is ascribed to the atomic-level smooth surface [8,9].
To improve the tribo-induced self-consumption of ultra-thin a-C film and interfacial reconstruction of mating materials, a combined method, including surface texture treatment and liquid lubricants, was proposed and proved to be effective for reducing friction and wear [10,11].For example, a textured a-C film reported by He et al. [12] was prepared in situ by masking a metal mesh, which effectively improved the tribological properties.On the one hand, the micro-dimples on the textured surface played an essential role in reserving wear debris and lubricant oil.On the other hand, the graphitized textured layer and the liquid lubricant film formed the solid-liquid duplex lubrication system.Although the storage function of the textured surface reduced the friction and wear [13,14], it was highly dependent on the quality of the texture.In other words, the textured structure was prone to collapsing under high loads [15,16].Therefore, it is insufficient to meet the comprehensive requirements of a-C films for real-world applications.
To ensure the durable anti-friction properties of the textured layer, hydrogen atoms were introduced onto the a-C surface, which could saturate the carbon atoms, and thus reduce the chemical cross-linking of the mating interface [17,18].Song et al. [19] deposited hydrogenated amorphous carbon (a-C:H) films with textured surfaces and proposed that the textured surface captured hard abrasive particles during the friction process, allowing the a-C:H film to maintain low friction for a longer period of time.Besides, it was not reported about the synergistic effect of the hydrogen atom and hydrodynamic lubrication.Therefore, based on the reservoir function of the textured surface and the passivation of hydrogen atoms, both selective hydrogen passivation and surface texture are used to synergistically improve the friction behaviors of a-C film under oil-lubricated conditions [20,21].However, due to the limitation of the characterization scale, the interaction of hydrogen atoms and lubricants cannot be evaluated in situ by macroscopic experiments [21][22][23], which is a crucial point for demonstrating the influence of hydrogen atoms on the tribological properties of textured a-C film under oil lubrication.
In this study, the aim is to explore the coupling impact of the textured surface and hydrogen passivation under oil-lubricated conditions.Therefore, hydrogen atoms were selectively injected onto the bumps of the textured a-C surface [23].The friction behaviors were investigated using the reactive molecular dynamics (RMD) simulations.The evolvement of the interfacial structure, the interaction of hydrogen atoms with lubricant, and the flow behaviors of the lubricant were investigated to demonstrate the lubrication mechanisms, which provided theoretical guidance for employing a-C films in the fabrication of MEMS and NEMS devices.

Methods
The large-scale atomic/molecular massive parallel simulator was used for all RMD calculations [23,24].A "sandwich" friction model, as displayed in Fig. 1, includes a textured a-C film with selective hydrogenation surfaces (the lower layer), lubricants (poly alpha olefin of C 24 H 50 molecules, the middle layer), and an untextured a-C film (the upper layer).The hydrogen content was controlled to reveal the effect of hydrogenation degree on the tribological performance of textured a-C films.The atom-byatom deposition method was conducted to construct the initial H-free a-C films and textured a-C films [25][26][27].The model size, the atom numbers, and the percentages of sp 2 and sp 3 hybridization were consistent with those reported in Ref. [23].The high-pressure gases containing varying amounts of hydrogen molecules were injected onto the initial a-C surface.Then, a 600 K of annealing step was applied to produce the hydrogenation surfaces with varying hydrogen contents.After that, they were further moulded to the textured surfaces with a rectangular shape, whose textured values of the width along the x-direction, depth along the z-direction, and the length along the y-direction were 30.00,10.00, and 40.36 Å, respectively [23,25].The selectively hydrogenated model is represented by the lower film (Fig. 1), and it is observed that the hydrogen atoms are left at the bump locations of the textured a-C surface.Based on the different surface hydrogen contents, the above a-C films were labeled as a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1%.In addition, the texture scale utilized in this study was enough to illustrate the friction response caused by hydrogen atoms under oil lubrication [23,28].
The whole system (Fig. 1) includes a fixed layer, a thermostatic layer, and a free layer.The fixed layer is used to simulate the infinite system.The thermostatic layer is maintained at 300 K using the Berendsen thermostat's microcanonical ensemble (NVE) set [29].For the NVE, the system does not exchange energy with the outside world, and the number and volume of particles remain constant.In addition, the ensemble evolves along a constant energy orbit in the phase space.The free layer consists of the C 24 H 50 lubricant and the remaining upper and lower a-C structures.Periodic boundary conditions were enforced in both the x and y axes with a time step of 0.25 fs.The interactions of C-C, C-H, and H-H in the complete system were properly described using the reactive force field (ReaxFF) potential developed by Tavazza et al. [30], and the correctness and reliability have been confirmed in Refs.[23,26,31].
Before friction simulation, the model first relaxed 2.5 ps at 300 K to obtain the optimal structure.Then a contact pressure of 5 GPa was reached within 25 ps by applying a load to the upper a-C layer.Finally, the system performed a sliding simulation of 1,250 ps, and the upper fixed layer moved along the x-direction at a constant sliding speed of 100 m/s.Notably, Refs.[23,[32][33][34] proved that the contact pressure of 5 GPa was suitable for studying the friction behaviors of a-C films at the nanometer scale.Due to the limitation of simulation time and computer resources, a higher speed than the experimental value was used to simulate a longer sliding distance to sample enough spatial phases [16].

Results and discussion
To investigate the relation of the hydrogenation degree and the friction behaviors of textured a-C films [25], Fig. 2(a) gives the friction forces (f) and normal loads (w) of textured a-C film with different hydrogen contents during the sliding process.It is Fig. 2 (a) f and w curves and (b) friction coefficients of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems.
www.Springer.com/journal/40544| Friction found that the f curves of each system reach a steady state quickly, and they do not exhibit remarkable running-in periods, which is ascribed to the low contact pressure (5 GPa) and the shielding effect of the lubricant oil on the chemical bonding of the mating a-C surfaces [31].Therefore, the f curve is not significantly affected by the addition of hydrogen atoms.
In order to quantitatively investigate the relation between the hydrogen content and the frictionreduction effect of textured a-C systems, the friction coefficient (μ), as shown in Fig. 2(b), is calculated by Eq. ( 1): where w and f represent the normal load and the friction force, respectively, and their values are calculated from the last 200 ps of the f curves (Fig. 2(a)), which are summarized in Fig. S1 in the Electronic Supplementary Material (ESM).It is seen in Fig. 2(b) that the friction coefficient exhibits a decreasing trend with the enhancement of the hydrogen content, and the friction coefficient of the a-C@32.1% system shows a reduction of about 63% than that of the a-C@0% system, suggesting that hydrogen atoms play a crucial role in anti-friction.Reference [35] indicated that textured surfaces improved the tribological properties of a-C film by increasing the mobility of the lubricants at the interface.This implies that textured surface and hydrogenation play a synergistic role in anti-friction.
The interfacial morphologies of a-C@0% and a-C@24.2%systems are displayed in Fig. 3. Notably, the lubricant oil between the mating a-C surfaces hinders the formation of C-C covalent bonds, which holds the friction system in a low-energy state.Moreover, numerous lubricant molecules are adhered to the upper surface, and thus form a lubricant film, while the others are intercepted by the textured edge and stored in the textured structure, which is consistent with the experimental phenomenon [36][37][38].However, compared with that of the a-C@0% system, the number of lubricant molecules captured by the a-C@24.2%system at the end of the sliding is significantly less, and a more visible gap at the interface is also observed.Reference [35] revealed that such gaps implied a synergistic effect between hydrodynamic lubrication and textured surface on anti-friction.In addition, no significant flow of hydrogen atoms is observed in the lower bump surfaces, which is owing to the shielding effect of the lubricant molecules.
In general, a tribological behavior is highly dependent on the interfacial structure of a-C film [24,39].The interaction between the lubricant and the textured a-C films with different hydrogen contents, Fig. 3 Evolution of interfacial morphologies for a-C@0% and a-C@24.2%systems during sliding processes.4, the interfacal widths of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems during sliding are determined, suggesting that the structural changes caused by friction appear at the friction interface instead of intrinsic a-C film.Notably, the width of the friction interface shows a significant increasing trend with the increase of the hydrogen content, which is related to the improvement of the load-bearing capacity.It is caused by the repulsion between the hydrogen atoms from the lubricant and the lower textured surface.It will be analyzed in detail below.Before quantifying the interfacial structure, the coordination number along the z-direction is shown in Fig. S2 in the ESM to analyze the bonding state between textured a-C films and the lubricant.For the above friction system, the coordination number values from the contribution of lubricant to H and lubricant to a-C film are zero, showing that intermolecular interactions instead of chemical bonds are the dominant style of the interaction between textured a-C film and lubricant [40].Moreover, the coordination number of hydrogen atoms contributed by a-C film remains constant, suggesting that the C-H bonding state between hydrogen atoms and a-C is always maintained.Fig. 4 Density distributions of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems along film depth direction.
The evolution of hybridized structures can provide important information for revealing the tribological properties.Figure S3 in the ESM exhibits the variations in the percentages of carbon-hybridized structure.It is found that the percentages of sp-C and 1-coordinated C-hybridized bonds show a trend of decreasing first, and then stabilizing, while the percentages of sp 2 -C and sp 3 -C increase obviously compared with those of the initial state (0 ps), which is related to the interface passivation.It includes the self-passivation of the a-C film caused by friction and the passivation caused by hydrogen atoms from the lubricant and the textured a-C surface.The sp 3 -C percentages of a-C@24.2%and a-C@32.1% systems remain stable, which is attributed to the high content of hydrogen atoms occupying the active sites of the textured a-C surface.In addition, high hydrogen contents can not only support the applied w, but also prevent the direct interaction of lubricant molecules with the intrinsic a-C matrix.
Figure 5 further gives the interfacial hybridized structures of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems after sliding processes, in which the contribution from lubricant oil is excluded.First, as the hydrogen content of the textured a-C surface increases, the sp 3 -C fraction of the total friction interface increases gradually, following the reduction of sp 2 -C, sp-C, and 1-coordinated fractions.It suggests the positive role of hydrogen addition in passivating the friction interface and reducing the friction coefficient, which is consistent with that reported in Refs.[7,17,18,20].The passivation of the dangling bonds results in weak van der Waals interaction at the friction interface dominated by C-H.Combined with the existence of electrostatic repulsion between hydrogen atoms, these contribute to an ultra-low friction coefficient.
Moreover, except for the lubricant oil, the friction interface is divided into upper a-C and lower textured a-C layers (Fig. 4), and the contribution of the hybridized structure of each layer to that of the total friction interface is further evaluated (Fig. 5).It shows that the contribution of the lower textured a-C layer is dominant for the transition of the interfacial hybridized structure.The increase in the hydrogen content results in the enhancement of the repulsion between hydrogen atoms from the textured a-C surface www.Springer.com/journal/40544| Friction and lubricant oil, and thus causes the decrease in both sp-C and 1-coordinated C fractions at the upper naked a-C surface, which improves the shearing of the friction interface.Additionally, the increase of the hydrogen content on the textured bump surface causes more oil molecules distributed at the upper naked a-C surface, which promotes the passivation of the upper a-C layer.
The repulsion effect between the hydrogen atoms from the lubricant and the textured a-C film is further analyzed.Figure 6 shows the stress distributions of hydrogen atoms from both C 24 H 50 oil and the bump surface of textured a-C film.These hydrogen atoms exhibit tensile stress, corresponding to the strong repulsive force.As shown in Fig. 6(a), the stress distribution in the lubricant oil of the a-C@0% system is relatively uniform.In contrast, the introduction of hydrogen atoms results in the stress being primarily concentrated on the upper surface, which corresponds to the flow behavior of the lubricant oil.This is in consistence with the results in Fig. 3.The stress distributions of the hydrogen atoms from the selective bump sites of the textured a-C surface are given in Fig. 6(b), which is enhanced with the increasing hydrogen content.This promotes the C 24 H 50 lubricant molecules close to the upper naked a-C surface, and will be discussed later.In addition, Fig. S4 in the ESM plots the mean square displacement (MSD) curves of the hydrogen atoms from the textured a-C surface for each case, which remains stable (< 1 Å 2 ).It indicates that these hydrogen atoms are fixed on the textured a-C surface, which is also proved by Fig. 3 and Fig. S2 in the ESM.
Besides, the MSD of C 24 H 50 base oil is estimated by Eq. ( 2) to evaluate the influence of lubricant fluidity on the friction performance [25,31]: where N, r i (t), and r i (0) are the number of i atoms in the system and the positions of the ith atom at time t and 0, respectively; D and t represent the diffusion coefficient and sliding time, respectively.The MSD curves of the lubricant oil for a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems were obtained, as displayed in Fig. 7, exhibiting an increasing trend in the sliding process.Importantly, the existence of hydrogen atoms on textured a-C surface restricts the mobility of lubricant oil substantially.Using the Fig. 5 Interfacial hybridized structures of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems after sliding processes, in which only the contributions from the upper a-C layer and lower textured a-C layer are considered.
Friction 12(1): 174-184 (2024) | https://mc03.manuscriptcentral.com/frictionvalues from the gray backgrounds in part of the MSD curves (Fig. 7), the D of the lubricant oil are quantified for a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems to characterize the degree of lubricant oil diffusion, as depicted in the inset of Fig. 7.The inhibitory effect of hydrogen atoms from textured a-C film on lubricant molecules is not linearly dependent on the degree of selective hydrogenation.This is related to the aggregation of oil molecules, which is caused by the highly repulsive squeezing of hydrogen atoms from the lower bump surfaces, as confirmed by Fig. 6.Nevertheless, the above study still suggests that the friction coefficient is related to the flow behavior of the lubricant [41].In particular, Refs.[42,43] reported that the textured layer provided secondary lubrication as an oil reservoir to reduce friction.The coupling effect of the introduction of hydrogen atoms and the textured surface needs further investigation.Furthermore, Fig. 8 provides the distributions of oil atoms during the sliding process.Most of the oil molecules are distributed on the upper naked a-C surface, remarkably decreasing the total energy of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems and the energy barrier of oil diffusion, which Fig. 7 MSD curves of C 24 H 50 lubricant molecules with t for a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems.
is attributed to the existence of numerous dangling bonds and small roughness.Importantly, the presence of hydrogen atoms causes an extreme distribution of the lubricant, and there is a very small number of lubricant molecules free at the middle interface.Moreover, being consistent with what is observed in Fig. 3, the captured oil atoms are relatively small for the hydrogenated systems, which is related to the H-induced low mobility.Compared to that of the Fig. 6 Stress distributions of hydrogen atoms from both (a) C 24 H 50 lubricant molecules and (b) textured bump sites of a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems.
www.Springer.com/journal/40544| Friction a-C@0% system, more lubricant molecules are adhered to form a dense lubricant film on the upper surface owing to the strong repulsive effect of the hydrogen atoms of the textured surface, but it also prohibits the diffusion of lubricant.
In addition, Fig. 9 exhibits the displacements of lubricant molecules obtained from 1,100 to 1,250 ps.These lubricant oil molecules adhered on the upper surface produce high mobility, while those trapped by the textured layer exhibit low mobility.Notably, the displacement distribution cloud is presented in the inset of Fig. 9.The denser lubricant oil film is clearly observed in the hydrogenated systems.Moreover, with the enhancement of the hydrogen content, the hydrogenated bump surface of the system is hardly attached by oil atoms, while more oil molecules are distributed at the upper naked a-C surface and friction interface.This supports the separation and repulsion observed in Fig. 8 and also causes the real friction interface transformed from oil/oil to oil/hydrogenated   8 Distributions of lubricant oil atoms for a-C@0%, a-C@10.5%,a-C@24.2%,and a-C@32.1% systems.

Conclusions
In this study, the RMD simulation was conducted to explore the tribochemical reactions and corresponding mechanism of hydrogenated textured a-C film under oil-lubricated conditions.The results revealed that the selective hydrogenation of the textured surface highly affected the friction properties of a-C film.For the a-C@0% system, the shielding effect of the lubricant made the tribological behavior dominated by hydrodynamic lubrication.With the selective introduction of hydrogen atoms onto textured bump sites, the friction coefficient was reduced by 63% maximally compared to that of the a-C@0% system.A large number of added hydrogen atoms occupied the active sites on the textured bump surface, promoting the interface passivation and also causing the lubricant molecules to interact primarily with the hydrogen atoms.The strong repulsion of hydrogen atoms from the lubricant and hydrogenated bump surface allowed the formation of a dense lubricant film on the upper a-C layer, indicating that the real friction interface changed from oil/oil to oil/hydrogenated bump.The underlying friction mechanism also changed from hydrodynamic lubrication to both H-induced passivation of friction interface and repulsion between hydrogenated bump surface and lubricant oil, accounting for the decrease of the friction coefficient.This study can guide the design of ultra-thin a-C film with high performance for MEMS/NEMS applications.

Fig. 1
Fig. 1 Friction model consisting of intrinsic a-C as upper mating film, textured a-C with selective surface hydrogenation as lower film, and C 24 H 50 molecules as middle lubricant.

Friction 12 ( 1 )
: 174-184 (2024) | https://mc03.manuscriptcentral.com/friction the evolvement of the hybridized structure, and the flow state of the lubricant were analyzed to demonstrate the friction mechanisms caused by different hydrogen contents.From the density distributions, as shown in Fig.