Insigths into the tribochemistry of silicon-doped carbon based films by ab initio analysis of water/surface interactions

Diamond and diamond-like carbon (DLC) are used as coating materials for numerous applications, ranging from biomedicine to tribology. Recently, it has been shown that the hydrophilicity of the carbon films can be enhanced by silicon doping, which highly improves their biocompatibility and frictional performances. Despite the relevance of these properties for applications, a microscopic understanding on the effects of silicon is still lacking. Here we apply ab initio calculations to study the interaction of water molecules with Si-incorporated C(001) surfaces. We find that the presence of Si dopants considerably increases the energy gain for water chemisorption and decreases the energy barrier for water dissociation by more than 50%. We provide a physical rational for the phenomenon by analysing the electronic charge displacements occuring upon adsorption. We also show that once hydroxylated, the surface is able to bind further water molecules much strongly than the clean surface via hydrogen-bond networks. This two-step process is consistent with and can explain the enhanced hydrophilic character observed in carbon-based films doped by silicon

A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability.Comment: 22 pages, 4 figures, 1 tabl

Studies of mechano-chemical interactions in the tribological behavior of materials

Mechano-chemical interaction studies can contribute to the understanding of wear and friction of materials. Specific examples of experimental results relative to the subject are discussed. There are two parts: one describes the synergistic effect of corrosion and wear of iron sliding on sapphire in sulfuric acid, and the other describes the effect of surface films on the wear and friction of plasma-deposited diamondlike carbon (amorphous hydrogenated carbon) films in sliding contact with silicon nitride. The concentration of acid (pH) is an important factor in controlling the iron loss caused by wear-corrosion processes in sulfuric acid. The mechanical action can cause chemical reactions to proceed much faster than they would otherwise. The diamondlike carbon (DLC) films are shown to behave tribologically much like bulk diamond. In a dry nitrogen environment, a mechano-chemical reaction produces a substance which greatly decreases the coefficient of friction. In a moist air environment, mechano-chemical interactions drastically reduce the wear life of DLC films and water vapor greatly increases friction

Interfacial chemistry of a perfluoropolyether lubricant studied by XPS and TDS

The interfacial chemistry of Fomblin Z25, a commercial perfluoropolyether used as lubricant for space applications, with different metallic surfaces: 440C steel, gold and aluminum was studied. Thin layers of Fomblin Z25 were evaporated onto the oxide-free substrates and the interfacial chemistry studied using XPS and TDS. The reactions were induced by heating the substrate and by rubbing the substrate with a steel ball. Gold was found to be completely unreactive towards Fomblin at any temperature. Reaction at room temperature was observed only in the case of the aluminum substrate, the most reactive towards Fomblin Z25 of the substrates studied. It was necessary to heat the 440C steel substrate to 190 degree C to induce decomposition of the fluid. The degradation of the fluid was indicated by the formation of a debris layer at the interface. This debris layer, composed of inorganic and organic reaction products, when completely formed, passivated the surface from further attack to the Fromblin on top. The tribologically induced reactions on 440C steel formed a debris layer of similar chemical characteristics to the thermally induced layer. In all cases, the degradation reaction resulted in preferential consumption of the difluoroformyl carbon (-OCF2O-)

Tribochemistry of graphene on iron and its possible role in lubrication of steel

Recent tribological experiments revealed that graphene is able to lubricate macroscale steel-on-steel sliding contacts very effectively both in dry and humid conditions. This effect has been attributed to a mechanical action of graphene related to its load-carrying capacity. Here we provide further insight into the functionality of graphene as lubricant by analysing its tribochemical action. By means of first principles calculations we show that graphene binds strongly to native iron surfaces highly reducing their surface energy. Thanks to a passivating effect, the metal surfaces coated by graphene become almost inert and present very low adhesion and shear strength when mated in a sliding contact. We generalize the result by establishing a connection between the tribological and the electronic properties of interfaces, which is relevant to understand the fundamental nature of frictional forces.Comment: 19 pages, 6 figure

Gas-Phase Lubrication of ta-C by Glycerol and Hydrogen Peroxide. Experimental and Computer Modeling

Tetrahedrally coordinated hydrogen-free amorphous diamond-like carbon coating (denoted as ta-C) presents ultralow friction under boundary lubrication conditions at 80 °C in presence of OH-containing molecules. To understand the mechanism of ultralow friction, we performed gas-phase lubrication experiments followed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses and this using two simple molecules: deuterated glycerol and hydrogen peroxide. The experiments were complemented by computer simulations using the ReaxFF reactive force field. These simulations suggest a ta-C surface rich in sp^2 carbon with some reactive sp^1 carbon atoms, in agreement with previous energy filtered transmission electron microscopy (EFTEM) results. Sliding simulations show that the carbon surface atoms react with glycerol and hydrogen peroxide to form OH-termination. Moreover, the hydroxylation is then followed by the chemical dissociation of some of the glycerol molecules leading to the formation of water. This is in agreement with the secondary ion mass spectrometry (SIMS) analyses and mass spectrometer results obtained with gas-phase lubrication experiments with the same molecules. Both experimental and computer simulations strongly suggest that the hydroxylation of the carbon surface is at the origin of ultralow friction together with the formation of water-rich film in the sliding interface

Polyimide/mesoporous silica nanocomposites: Characterization of mechanical and thermal properties and tribochemistry in dry sliding condition

Mesoporous silica (MPS) with tunable mesopore channels can be used to reinforce polymers and has great potential in tribological applications, which is rarely investigated by research community. In this study, comprehensive properties of polyimide (PI)/MPS nanocomposites were investigated. The results demonstrated a slightly decreased tensile strength but increased modulus, microhardness and thermal stability of the PI/MPS nanocomposites. In particular, tribological properties of PI/MPS nanocomposites as functions of MPS content, applied load and sliding speed were systematically investigated. The incorporation of 1.5 wt.% MPS increased the anti-wear resistance of PI by more than 14-fold while the coefficient of friction decreased by 27%. This was highly associated with the formation of high-quality transfer film on the bearing steel counterpart surface.Relevant tribochemistry was thoroughly revealed by X-ray photoelectron spectroscopy (XPS) analysis on the transfer film and Raman analysis on the worn surfaces. This study confirmed the high efficiency of using MPS to reinforce PI polymer for tribological applications and elaborated tribochemistry to further understand the tribochemical process in polymer-metal rubbing systems

Development of Single Cam Rig for Accurate Simulation of Valve Train Tribochemistry

The study of cam-tappet tribochemistry is on the rise due to the need for a better understanding of how nanoscopic tribofilms reduce friction (improved engine efficiency) and wear (durability) in internal combustion engine. Environmental legislation on exhaust gas emissions have further stimulated research on the use of less phosphorus and sulphur containing additives because phosphorus clogs the catalytic converters in an engine exhaust system. Current tests evaluate the resultant surface films formed on the contact by the additive package which has made understanding of the test conditions crucial due to the increased complexity of tribochemistry. Diamond Like Carbon (DLC) surface coatings are also receiving significant attention even though their interaction with conventional lubricants additives is still unclear. A vast majority of published studies look at these systems under steady-state conditions whereas, dynamic conditions are predominant. In this work, a newly modified ‘SLICE’ single cam tribometer, which has incorporated a programmable dynamic speed and lubricant supply system, was designed and employed for the study of cam-follower tribochemistry. Information such as frictional torque and lubricant film thickness were obtained using a torque transducer and the Dowson mathematical model respectively. XPS, RAMAN/FTIR, SEM/EDX surface analytical techniques are used to study the tribofilms. Properties of the tribofilms are evaluated with the aid of AFM, Nano-indenter and Surface Profilometry. Comparison of data with laboratory/conventional tribometers showed that the films had similar characteristics in the boundary lubrication regime and the friction data in a single cam rig closely mimics those in reciprocating pin-on-plate tribometers. The tribofilm was mapped in a unique spot wise manner on the cam and had similar trends with those of the plate in a PoP tribometer. A unique observation in this study was the effect of coating on cam wear. These values closely support those in the PoP reciprocating tribometer. This illustrates that reciprocating laboratory tribometers and bench test data can be used to establish how components in real engines may behave. The rig is capable of ranking candidate materials, surface coatings and fully formulated lubricants for valve train applications

Modelling interfacial tribochemistry in the mixed lubrication regime

The need to reduce the cost of components is driving more and more machine elements to operate under mixed lubrication conditions. With higher operating pressures, the lubricant film is becoming thinner and eventually reaches nanometre scales, comparable to the surface roughness. Thus, understanding the mixed lubrication phenomenon is becoming increasingly important. However, the mixed lubrication phenomenon is difficult to capture experimentally and the lubricant additive ZDDP (Zinc Dialkyl Dithio Phosphate) shows its full antiwear character in the mixed lubrication conditions. This research stems from the need for models that can simulate contact mechanics, lubrication and tribochemistry in a single framework. This is the key to understanding and optimizing the lubrication systems to meet future needs. To this end, a numerically efficient procedure based upon the tridiagonal solution of the Reynolds equation is developed and is implemented, in Fortran to solve the equations line by line to incorporate more information from the current iteration step. The asperity contacts are handled by the unified solution algorithm. A new strategy to simulate plastic deformation in a lubricated contact is developed. Under practical loading conditions, the pressures inside the contact can reach values far above the material yielding limit. Thus, an efficient numerical scheme is devised to include the elastic perfectly plastic behaviour in the EHL solution procedure to simulate realistic contact conditions with minimal increase in computational cost. The Boussinesq deformation integrals result in a convolution of pressure and the deformation which is solved using Fast Fourier Transforms (FFTs) by modifying the solution domain to create a cyclic convolution. Code is developed to allow exploration of the highly optimized C-based library (www.fftw.org). The use of FFTs speeds up the solution process many times and makes the use of denser grids and larger time scales accessible. A mesh size of 129 x 129 is found to give reasonable results. The simulation results from the current study agree very well with the previously published results. The evolution of contact area ratio and the central film thickness exhibit a Stribeck type behaviour and the transition speeds at which the contact transits from EHL to mixed and from mixed to complete boundary lubrication can be precisely identified. Existing tribofilm growth models developed for boundary lubrication are reviewed and a model based on the interface thermodynamics is adapted and integrated with the mixed lubrication model to simulate tribochemistry. The problems with existing EHL concepts such as lambda ratio and central film thickness are identified and new definitions are proposed to allow understanding of the mixed lubrication mechanics. The mutual interaction between the tribofilm growth and lubricant film formation is studied. Finally the wear of the tribological system is studied and the wear track profiles are predicted. The new model is then applied to study a ball-on-disc system to explore wear, tribochemistry and roughness evolution. The ZDDP tribofilm growth is studied and the it is found that the final ZDDP tribofilm thickness is very weakly affected by increasing SRR but the rate of formation and removal are strongly affected by the SRR value. The tribofilm growth results are validated against published numerical and experimental results. It is found that the antiwear action of the ZDDP tribofilm is not only due to its chemical action but the ZDDP tribofilm helps to entrain more lubricant and improves contact performance. The presence of tribofilm roughens the contact and the contact area and load ratio both increase due to tribofilm growth. It was also found that the use of conventional EHL parameters to analyse the behaviour of tribosystem is misleading. The flattening of the roughness inside the contact and the proper identification of the central film thickness are crucial to the interpretation of the mixed lubrication results. The roughness of the ball generally decreases due to wear but the presence of tribofilm limits this reduction. Contrary to this, the surface roughness of the ball generally increases due to wear but the presence of tribofilm results in increased roughness of the ball