Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface

A coarse-grain molecular dynamics study of the nanotribological properties of nanoparticle solutions

In this study, solutions of alkanethiol-capped nanoparticles in alkane are examined using molecular dynamics simulations for their nanotribological potential based on the hypothesis that fluid molecules of very different sizes may interrupt each other\u27s layering tendency to result in less layered or non-layered configurations and provide better lubrication for nanodevices. An effective nanoparticle-nanoparticle pair potential based on previous atomistic approach is used and the temperature and parallel pressure are controlled in place of chemical potential for defining thermodynamic state. When compressed, the confined nanoparticle-containing alkane films generate reduced oscillations in perpendicular forces and smoother expansion in lateral dimensions, indicating lesser extent of layering due to the presence of much bigger nanoparticles. The nanoparticles are found to be well dispersed by the alkane solvent throughout all separations, meaning no or little tendency to form clusters or aggregate towards the confining surfaces, which is important for the stability and quality of the nanoparticle solutions as nanotribological lubricant. When sheared by a sliding surface, the confined fluids tend to move in the same parallel direction so that their density profiles remain practically unchanged. The shear stress resulting from the sliding surface has been calculated and found to increase with faster sliding speed but not proportionally. More importantly, the presence of the nanoparticles in the lubricant films reduces the shear stress noticeably and thereby reducing the apparent viscosity and frictional force. This effect is particularly evident under relatively large sliding speed and large surface separations. Regarding mobility, the nanoparticles exhibit lower diffusivity in nanoconfinement than typical fluids and their diffusivity can be enhanced by shearing --Abstract, page iv

Atomistic Simulations of Defect Nucleation and Intralayer Fracture in Molybdenum Disulphide During Nanoindentation

Molybdenum disulphide (MoS2) is a layered, hexagonal crystal that has a very low coefficient of friction. Due to this low coefficient of friction, MoS2 has become a well-known solid lubricant and liquid lubricant additive. As such, nanoparticles of MoS2 have been proposed as an additive to traditional liquid lubricants to provide frictional properties that are sensitive to different temperature and pressure regimes. However, to properly design these MoS2 nanoparticles to be sensitive to different temperature and pressure regimes, it is necessary to understand the mechanical response of crystalline MoS2 under mechanical loading. Specifically, the fundamental mechanism associated with the nucleation and interaction of defects as well as intralayer fracture. This thesis addressed the mechanical response of crystalline MoS2 via contact deformation (nanoindentation) simulations, which is representative of the loading conditions experienced by these nanoparticles during synthesis and application. There are two main tasks to this thesis. First, a Mo-S interatomic potential (a combination of the reactive empirical bond-order (REBO) interatomic potential and the Lennard-Jones 12-6 interatomic potential) that has been parameterized specifically to investigate the tribological properties of MoS2 was implemented into the classical molecular simulation package, LAMMPS, and refined to provide improved predictions for the mechanical properties of MoS2 via molecular statics calculations. Second, using this newly implemented interatomic potential, molecular statics calculations were performed to investigate the mechanical response of MoS2 via nanoindentation with specific focus on the nucleation of defects. Nanoindentation force - displacement curves were compared to the Hertzian contact theory prediction. It was shown that MoS2 does not follow the Hertzian prediction due it anisotropic nature. In addition, it was shown that the initial sudden force drop event in the force - displacement curves corresponds to plastic deformation. It was hypothesized that the mechanism associated with plastic failure of MoS2 was the occurrence of broken bonds. However, it was proven that this initial plastic yield does not correspond to the occurrence of broken bonds in the MoS2 lattice; instead, a permanent slip occurred within or between the MoS2 layers

Adhesion and Deformation Mechanisms of Polydopamine and Polytetrafluoroethylene: A Multiscale Computational Study

Polydopamine (PDA) has been shown to bond via covalent bonding, van der Waals forces, and hydrogen bonding and is known to adhere strongly to almost any material. The application of PDA between a substrate and a PTFE surface coating has resulted in low friction and a greatly reduced wear rate. Previous research probing the capabilities and limitations of PDA/PTFE films have studied the wear and mechanical properties of the film, but the overall adhesive and deformation mechanisms remain unclear. In this research, we investigate the tribological properties of PDA and PTFE molecules and composites from the atomic to the microscale using computational modeling. Molecular dynamics is used to investigate the mechanical properties of individual PTFE chains. The elastic moduli of varying lengths of PTFE molecules are tested in vacuum and in water and at varying temperatures, showing how the chain length and the surrounding environment affect the elastic strength of PTFE molecules. The deformation mechanisms of a nanoscale PTFE film are observed during an indentation and scratch test, and various scratch angles are used during the scratch tests to elucidate the deformation mechanisms of individual PTFE chains within a film. Based on molecular dynamics simulations, a coarse-grained model of PTFE is developed which allows modeling of PTFE particles and films up to the microscale. Micrometer sized PTFE particles are then modeled which show that frictional values of PTFE are dependent on the surface topography. Individual properties of PDA and PTFE molecules are investigated with density functional theory and molecular dynamics simulations. The adhesive properties of each molecule are tested as well as the deformation mechanisms. The primary source of adhesion between the PDA and PTFE molecules was observed to be van der Waals interactions, although, hydrogen bonding was also observed between PDA-PDA interactions. A PDA/PTFE thin film composite is studied, and an indentation and scratch test are performed to uncover deformation mechanisms. During scratch tests of the PDA/PTFE composite, a tenacious layer of PTFE is observed to adhere to the PDA substrate similar to experimental observations of PDA/PTFE composite films. Due to PTFE molecules penetrating the PDA substrate, and the unique deformation mechanisms of PDA oligomers, peeling is highly unlikely at the PDA/PTFE interface which increases wear resistance of the film. To continue the investigation of PTFE films and particles, the coarse-grained model was used to investigate PTFE films annealed for 0-, 4- and 8-minutes. PTFE films were created using images of experimental PTFE films taken by atomic force microscopy. The properties of PTFE films are investigated to understand how the chain length and film density affect the formation of the film, and the coefficient of friction. A machine learning algorithm is developed and used to evaluate whether the numerous models created can affectively predict the coefficient of friction prior to testing. Friction was seen to be dependent on the internal fiber orientation of the films and not just the surface topography

Tribological Characterization of Roles of Nanoparticles in Lubrication

This research investigates the tribological performance and rheological properties of nanoparticles as lubricant additives. Experimental approach combined with analysis were used to study the chemical and physical interactions between nanoparticles and lubricating system. Three areas of investigation were carried out as summarized in the following. Tribological performance and rheological properties of α-ZrP (Zr(HPO₄)₂•H₂O) and V₂O₅ nanoparticles were investigated as lubricant additives. α-ZrP showed 50% reduction in friction and 30% in wear compared to the conventional additives ZDDP. Spectroscopic characterization indicated that the tribofilm consists of iron oxide, zirconium oxide, and zirconium phosphates. Through Raman spectrum and EDS analysis, it was found that V₂O₅ involved tribochemical reaction during rubbing. Vanadium intermetallic alloy (V-Fe-Cr) was found to enhance the antiwear performance. This research revealed that nanoparticles could be effective additives to improve tribological performance. Tribofilms play vital roles in protecting lubricated surfaces in mechanical systems in motion. Strategically-selected-illuminative nanoparticles of NaYF₄ were added to a base oil in order to enable their tracking. Electrical conductivity was monitored during sliding that was found to be linked to the state of the interface and the tribofilm. This work discovered three stages to form a tribofilm: running in, reactive, and growth. Interestingly, the formation of a tribofilm was more dominated by frictional force than applied load. This is significant because we can now use alternative strategies to generate quality tribofilms. For the lubricating dynamics, the physical interaction between the nanoparticles and lubricating systems were investigated. Mechanisms of interfacial interaction between the shark skin and water have yet to be fully understood. In the present research, diamond particles worked as tracking particles in fluid. The shark-skinned surface with 90 degree orientation scale showed a more uniform distribution of diamond particles, which indicated to a lower gradient of velocity. Less momentum transfer between adjacent layers of fluid leads to a lower drag. Eventually, a viscosity map of shark-skinned surface with different scale orientation was created. It will facilitate the design of shark-skinned surface with better performance. The understanding generated in this study could be used as guideline for future study in surface design and texturing

Tribological Characterization of Roles of Nanoparticles in Lubrication

This research investigates the tribological performance and rheological properties of nanoparticles as lubricant additives. Experimental approach combined with analysis were used to study the chemical and physical interactions between nanoparticles and lubricating system. Three areas of investigation were carried out as summarized in the following. Tribological performance and rheological properties of α-ZrP (Zr(HPO₄)₂•H₂O) and V₂O₅ nanoparticles were investigated as lubricant additives. α-ZrP showed 50% reduction in friction and 30% in wear compared to the conventional additives ZDDP. Spectroscopic characterization indicated that the tribofilm consists of iron oxide, zirconium oxide, and zirconium phosphates. Through Raman spectrum and EDS analysis, it was found that V₂O₅ involved tribochemical reaction during rubbing. Vanadium intermetallic alloy (V-Fe-Cr) was found to enhance the antiwear performance. This research revealed that nanoparticles could be effective additives to improve tribological performance. Tribofilms play vital roles in protecting lubricated surfaces in mechanical systems in motion. Strategically-selected-illuminative nanoparticles of NaYF₄ were added to a base oil in order to enable their tracking. Electrical conductivity was monitored during sliding that was found to be linked to the state of the interface and the tribofilm. This work discovered three stages to form a tribofilm: running in, reactive, and growth. Interestingly, the formation of a tribofilm was more dominated by frictional force than applied load. This is significant because we can now use alternative strategies to generate quality tribofilms. For the lubricating dynamics, the physical interaction between the nanoparticles and lubricating systems were investigated. Mechanisms of interfacial interaction between the shark skin and water have yet to be fully understood. In the present research, diamond particles worked as tracking particles in fluid. The shark-skinned surface with 90 degree orientation scale showed a more uniform distribution of diamond particles, which indicated to a lower gradient of velocity. Less momentum transfer between adjacent layers of fluid leads to a lower drag. Eventually, a viscosity map of shark-skinned surface with different scale orientation was created. It will facilitate the design of shark-skinned surface with better performance. The understanding generated in this study could be used as guideline for future study in surface design and texturing

Nanoindentation Analysis of Evolved Bearing Steel under Rolling Contact Fatigue (RCF)

The bearing material operated under RCF is subjected to the triaxial stress state where work hardening followed by softening has been reported under the contact track. Such nonconformities (hardening/softening along with microstructural alterations) create complexities to model the cyclic hardening of bearing material under RCF. Current study presents a semi-empirical approach to evaluate the evolved subsurface response of bearing material with the help of a three-faced pyramidal Berkovich nanoindenter employing expanding cavity model for strain hardening materials. The expanding cavity model converts the localized measured hardness change to flow stresses which have been evolved during strain-hardening and microstructural phase changes of the bearing material. Moreover, to evaluate the representative stress-strain curve of the altered microstructure, a 5um spherical indenter was employed in a cyclic loading manner. The use of the spherical indenter with the integration of Field and Swain numerical model enables to extract the representative flow curve of the material at highly localized areas which cannot be possible even with miniature uniaxial tension/compression test

Mechanical and Frictional Behavior of Textured and non-Textured Surfaces

Tribology is the study of surfaces where two objects are sliding against another. Significant energy is lost due to friction between the sliding surfaces. Therefore, developing or designing surfaces to minimize friction is critical for the durability and reliability of the mechanical components. Several researchers have identified that surface texturing at the nanoscale (nanotexture) would reduce the friction between the contacting surfaces. The nanotextured surfaces have several applications in microelectromechanical systems and nanoelectromechanical systems. This dissertation employs molecular dynamics simulations to investigate the frictional and mechanical response of nanotextured aluminum (Al) and Al/amorphous silicon (a-Si) composite surfaces. This study determines the effective geometry (spherical or cylindrical) for texturing an Al surface that lowers the coefficient of friction of the nanotextured surface compared to a smooth surface. The results suggest that as the counter surface radius increases, the coefficient of friction decreases. For the lower counter surface radius, the coefficient of friction of the textured surface is higher than the smooth surface. But, after a specific increase in the radius of the counter surface, the coefficient of friction of the textured surface is lower than the smooth surface. The nanotextured surface consisting of Al has lower mechanical strength, which results in permanent failure even at low contact forces. Thus, a nanotextured hemispherical Al core surface is coated by an a-Si to protect the nanotextured surface from plastic deformation, and they are named as core-shell nanostructures (CSNs). The CSNs has previously shown remarkable deformation recovery to compression loading beyond the elastic limit. This study finds an optimum coating thickness that would protect the core from plastic deformation. i.e., the ratio of core radius to shell thickness should be between 0.5 and 2.0 to have deformation resistant CSNs. Additionally, this research investigates the core (single crystal and grain boundary) and substrate (crystalline and amorphous) material that affect the mechanical behavior of the CSNs subject to indentation. The results from this study conclude that CSNs with a single crystal core and crystalline substrate are more reliable for deformation-resistant behavior than those that contain grain boundary core and amorphous substrate. From our previous studies, it is clear that not all textured surfaces will have a lower coefficient of friction. The coefficient of friction also depends on the indenter or counter surface radius. Therefore, we investigate the relationship between surface texture (r, L) and counter surface (R) variables. The results from this study suggest that the counter surface radius should be greater than the difference between twice the pitch length and radius of the asperity (R \u3e (2L -r)) in order to have lower COF for the textured surface compared to a smooth surface. The relationship found between the textured surface and indenter surface variables is also confirmed for CSNs. Further, the relationship established in this study is also verified using experiments. This work provides the groundwork in designing the textured surfaces as well as deformation-resistant core-shell nanostructures that has both lower COF and deformation-resistant behavior. Additionally, this research finds the mechanisms behind the deformation-resistant behavior of the CSNs

On the mechanical interactions between TiO2 nanoparticles

In this work the mechanical interaction mechanisms between TiO2 nanoparticles at ambient conditions are investigated by using molecular dynamics (MD) and discrete element method (DEM) simulations in comparison to experimental findings. It is shown that the particle interaction forces are crucially determined by the nature of the surface adsorbed water layer of the particles. This dependence controls the characteristic trends of the interparticle forces at the nanoscale with humidity, surface roughness and hydrophilicity. From these insights, comprehensive equations are derived for the calculation of interparticle forces under ambient conditions at the nanoscale. The implementation of these equations into particle contact models enables the fast and accurate simulation of the mechanical behaviour of large nanoparticle assemblies and thus represents a link between the chemical properties of the particle surfaces and the macroscopic mechanical properties of entire nanoparticle films