Lubrication Chemistry Viewed from DFT-Based Concepts and Electronic Structural Principles
Abstract
:Introduction
DFT-Based Chemical Reactivity Indices and Electronic Structural Principles
Review of Historical Cases
Application of General Quantum Chemistry
- 1)
- Applying molecular dynamics to conduct simulation study of the non-equilibrium chemistry on the diamond and covalent interfaces, and to elucidate the relations between free radical chemistry and wear, including H-abstraction reactions of chemically adsorbed molecules, radical recombination, instantaneous surface adhesion and debris formation [26].
- 2)
- Employing computational chemistry algorithms to mimic the chemical degradation mechanisms of perfluoropolyether lubricants on the surfaces of metals, e.g. Al, and metallic oxides, and to design combinatorial systems of surface-lubricant-additive that meet requirements of sophisticated lubrication systems [27].
- 3)
- Based on the radical anionic model of multi-ring aromatics, semi-empirical molecular orbital theory was applied to calculate the effect of substituted groups on the electronic affinity of polybenzylthioether as a gas phase lubricant, for decreasing the temperature at which it decomposes to form frictional polymers under boundary lubrications [28].
- 4)
- Using DFT to determine the electronic structure of MoS2 related to the lubricating properties and the activation energy. A good agreement has been found between theoretical predictions and experimental results [26a].
Application of DFT-Based Concepts and Principles
Reference | Description |
Mori et al. (1987) | Describing adsorption activity of organics on fresh steel surfaces by hard and soft acid and base concepts and HSAB principle. |
Kajdas (1995) | Modeling tribochemical reactions based on low-energy electron emission from tribosurfaces and a generalized NIRAM-HSAB action mechanism. |
Fischer et al. (1995) | Characterizing solid tribosurfaces with Lewis acids and bases theory and frontier molecular orbital theory. |
Mansuy (1995) | Investigating effect of Lewis acid-base interactions between ZDTP and n-dodecylamine on the composition of ZDTP tribochemical films. |
Matin et al. (1995-2000) | Illustrating interactions in binary additive system, formation of ZDTP tribochemical films, synergism in MoDTC/ZDTP and MoDTC/calcium borates, and transfer of tribochemical film by Lewis acids and bases concept and HSAB principle, chemical hardness and MHP. |
Bhatia et al. (1999) | Elucidating tribochemical reactions of PFPE on magnetic head/disk interface by catalysis of Lewis acids. |
Li (2000) | Attempting Bond valence matching principle and Saville’s rule for understanding formation thermodynamics of inorganic and organic species yielded from ZDTP on rubbing steel surfaces. |
Zhang et al. (2001) | Employing electronegativity, electron affinity and ionization potential of functional antiwear additive elements (S, P) from ZDTP to account for their preferential residence on tribocoating surfaces of varied mechanical and chemical nature. |
ZDTP: Zinc Dithiophosphate; MoDTC: Molybdenum Dithiocarbamate
Application of Chemical Hardness and the HSAB Principle for Antiwear Films
- 1)
- Use of DFT approaches, DFT-based chemical reactivity descriptors and electronic structural principles in lubrication chemistry is almost exclusively confined to qualitative illustration of the formation chemistry of tribochemical films, antiwear (S, P) and extreme pressure (Cl, N) lubricious films in particular, with one documented citation of the HSAB principle for explaining the effect of “solubility” of ZDTP and MoDTC in bulk solution of lubricants on their friction reduction.
- 2)
- More qualitative elucidation has been made of tribochemical film formations with chemical hardness and by the HSAB principles, and very little use has been witnessed of other DFT-based chemical reactivity indicators and electronic structural principles in lubrication chemistry. Among the conceptual DFT applications in the field, more global parameters than their local counterparts have been explored.
- 3)
- Rather limited computational DFT cases have been available that are targeted on the components of tribochemical systems, i.e., almost no DFT characterization of tribochemical systems has been attempted.
Challenges in Lubrication Chemistry in View of DFT
Lubricant types | Critical challenges |
Passenger car engine oils | Fuel economy / Low or zero emissions |
Heavy-duty engine oils | Extended service interval |
Automotive gear oil | Durability |
Automatic transmission fluids | Shear stability |
Hydraulic fluids | High pressure |
Base oil | Pour point depressants/Biodegradability |
Challenges from Tribomaterials, Lubricants and Additives
- 1)
- Development of environmentally friendly and eco-compatible lubricants, and improvement of thermal and oxidative characteristics of renewable, biodegradable lubricant basestocks, and enhanced understanding of friction-reducing additive chemistry. For example, use of QM/QC methods can be attempted to design structurally novel wear inhibitors to replace Zn dithioalkylether phosphates, or to select alternative tolerant elements to substitute Zn and P in ZDTP, to develop ashless, sulfur and phosphorus-free engine oils, and to design new corrosion inhibitors to replace imidazolines.
- 2)
- Investigation of compatibility between lubricant basestocks and additives, and study of basestock and additive chemistry to cater for light-weight materials technology and durable, low-friction surface coatings, such as aluminum and magnesium alloys, titanium alloys and intermetallics, ceramics, and metal-matrix composites.
- 3)
- Mechanism study of tribochemical compatibility among advanced coatings, low-viscosity base oils and friction modifiers (Mo series and organic esters) in the presence of simulated combustion products and biofuels (methanol, ethanol and other biomass-derived fuels). Hierachical modelings starting from QC simulation of the most elementary triboelements, tribomaterials, lubricant basestocks and tribological additives, will pave viable routes leading to fundamentally chemical solutions to the issue.
- 4)
- Development of virtual laboratory test methods to screen high performance lubricants at competitive costs.
Challenges from Boundary Lubricants and Tribochemistry
Challenges from Nano-Lubrication
- 1)
- Fluid lubrication (Hydrodynamic and Elastohydrodynamic Lubrications): determining what atomic scale behavior influences liquid lubrication, and how changes in phase or chemical composition influence wear, and designing of contact geometry and controlling of operating parameters to produce sufficient fluid pressure to prevent the surfaces from direct contacts. Under fluid lubrication, it is the mechanical forces occurring in the bulk lubricants that dominate the tribosystems. And so mechanical force responsiveness of lubricant molecules is of primary significance.
- 2)
- Boundary lubrication: using chemistry to generate sacrificial surface chemical films to protect the surfaces from shear stresses from rubbing and abrasion. Under boundary lubrication, it is the chemical forces between the lubricant (basestocks and additives) molecules and the tribomate materials that prevail in the tribosystems. One closely related subject is designing molecular or formulation chemistry to self-recondition or regenerate worm tribosurfaces in the course of lubrication.
- 3)
- Nano-lubrication: using sophisticated molecular engineering techniques currently available to construct monolayer films to control adhesion, stiction, friction, and wear of surfaces coming into contact at the micro/nano-scale. These films should be of, among others (non-volatile, oxidation and thermal decomposition resistant), good adhesion and cohesion, and self-repairing or self-regeneration. Under nano-lubrication, it is the physical forces and tribochemical interactions between the specific lubricant molecules (e.g., PFPE, X-1P) and/or the surfaces (e.g., single and polycrystalline silicon, silicon nitride, silicon carbide, Nickel, diamond-like carbon, and hydrogenated carbon films) that govern the tribosystems.
Exploration of DFT-Based Strategies in Lubrication Chemistry
Solution Chemistry
Category | Surface protecting | Performance enhancing | Lubricant protective |
Function | prevent wear, corrosion, or rust by forming adsorptive or reacted protective films. | make the oil perform in a desired manner for specific applications. | protect the lubricant itself. |
Chemistry | tribo-additives (oiliness, friction reduction, antiwear, extreme pressure); rust &corrosive inhibitors; detergents & dispersants. | viscosity index improvers; pour point depressants; demulsifiers. | oxidation inhibitors; antifoaming agents. |
- 1)
- Application of DFT-derived concepts and related electronic structural principles to account for addition, synergism and antagonism in additive-additive interaction chemistry;
- 2)
- Application of the same concepts and principles to account for compatibility of additives with lubricant solutions, the synthetic basestocks in particular.
- 1)
- Determination of critical DFT-based reactivity descriptors for additive molecules and general basestock molecules;
- 2)
- Computation of such indices on the global and local scales. In the latter, the distributions of the DFT-based reactive sites should be mapped for individual additive molecules.
Contact Chemistry
Tribochemistry
- 1)
- There exists an energy gap in the bonding energy spectrum in solids and molecules. Either solids or molecules are chemical systems with fixed ‘hardness’ that is determined by the energy gap in the energy spectrum of bonding electrons. There are different nomenclatures for such a gap depending on the chemical systems: the work function in metals, the forbidden band width (Eg) in non-metals (semiconductors and insulators), and the chemical hardness in molecules.
Table 4. Traditional and modern mechanochemistry. About ‘mechano’ About ‘chemo’ Comparisons Traditional -hydrostatic stress state
-plastic deformation
-dynamic loading & relaxation
-fine grinding & comminution
-ultrasonic irritation
-explosive shock wave-dissolution
-dissociation
-extractive process
-polymer stress reactions
-electrochemistry
-solid state reactions
-mechanical alloying
-tribochemistry & wear
-stress corrosion
-synthesis of organic & inorganic compoundsMacrosystems (solid-solid and solid-molecule interaction mechanochemistry) - ▪
- Mechanochemical effects — mechanical stresses on the course of chemical reactions and physico-chemical transformations of solids
Modern -chemical bonding forces
between atoms and/or molecules
-molecular interaction energies (electrostatic, exchange, induc-tion, dispersion, and other terms)-the processes of directing chemical reactions by mechanical forces on the reactants, i.e., position specific reactions (Molecular nanotechnology or machine phase chemistry)
-how cells sense, respond, and adapt to physical forces (Biophysics)Micro/nanosystems (atomic or molecular interaction mechanochemistry) - ▪
- Chemomechanical effects — how chemical bondings or forces of chemical nature drive mechanical movements, or change mechanical states of interactive bodies in miromachines
- 2)
- The energy gap will diminish or even close when solids or molecules are sheared. Under shearing strains, there occurs symmetry breakdown in solids and molecules resulting in the loss of the structural stability of bonding electrons. Under such circumstances, there happens the decrease of the electronic work functions of metals, narrowing of the forbidden gap between the valence and conduction bands of non-metals, and reduction of the energy gap between the bonding and antibonding orbitals of molecules [48]. All these will lead to easier transferring of the localized electrons into the conduction bands of solids or antibonding orbitals of molecules resulting in free or non-localized electrons, which greatly decreases the chemical stability of solids and molecules, and hence facilitate their mechanochemical reactions.
- 3)
- The potential barrier of mechanochemical reactions is the function of shearing strains. Kinetically, mechanochemical reactions can be well described by Zener tunneling effect, i.e., the transition of electrons from HOMO to LUMO. Rate of the Zener tunneling process, r, in given chemical bonding states is expressed as [48]:
- 1)
- Dynamic responses of chemical reactivity indices of all the involved tribo-elements under shearing;
- 2)
- Formation kinetics of lubricious films in terms of mechanochemistry;
- 3)
- Application of the frontier orbital theory and the energy gap concepts to tribomaterials and triboadditives under mechano-activation.
- 1)
- Mapping of tribosurfaces in terms of DFT-based chemical reactivity descriptors;
- 2)
- DFT-based chemical reactivity descriptors for additive molecules and general basestock molecules, and computations of such indices both on the global and local scales;
- 3)
- Computation and measurement of narrowing of Eg of tribomaterials and of triboadditives.
Summary and Outlook
- 1)
- Lubrication chemistry is generally explored from solution chemistry, contact chemistry and tribochemistry. Each of the three fields needs involvement of DFT-based chemical reactivity indices and related electronic structural principles from different angles and to varied extents.
- a)
- For the solution chemistry, as all chemistry is occurring between lubricant basestocks and additives on the molecular scale, all of the current DFT-based molecule-targeted chemical reactivity indices and related electronic structural principles, either global or local and either qualitative or quantitative, can be adopted with highly expectations. They are anticipated to be of particular significance in understanding and prognosticating varied complicated additive-additive interaction modes.
- b)
- Contact chemistry concerns with heterogeneous catalytic chemistry involving adsorption and reaction of reactive atoms and functional groups, molecules and free radicals of lubricant basestocks and additives on either tribo- or non-tribo surfaces of metals/alloys and ceramics. Success of DFT application to contact chemistry, given the fine DFT characterization of lubricant molecules, depends on its provision of chemical reactivity parameters of solid tribomaterials, particularly tribocoating surfaces of different physical, chemical, mechanical and metallurgical properties.
- c)
- The core of tribochemistry is mechanochemical interactions of lubricant basestocks, tribological additives and tribomate surfaces under time-dependent, excited states, and external shearing stress fields. It is the most complicated branch of DFT applications to lubrication chemistry, and devotion to it will surely breed smarter tribochemical systems of high efficiency and low emissions. Non-equilibrium mechanochemistry and advanced computational DFT will have to be integrated for dynamic illustration and molecular chemistry understanding of tribochemical events and underlying mechanisms.
- 2)
- DFT is being developed in theoretical, conceptual and computational aspects, and the latter two will be of more direct and imminent potentiality in understanding lubrication chemistry issues, the tribochemistry in particular. Concretely it can be stated as follows.
- a)
- On the theoretical DFT level, it is necessary to develop formalism of density functionals of tribo-elements in lubrication chemistry systems: model compounds of mineral basestocks or specific compounds of synthetic basestocks, critical additive species, and conventional solid tribomaterials and advanced tribological coatings whose characters can be more proactively designed.
- b)
- On the conceptual DFT level, more chemical reactivity indicators of local and quantitative nature should be sought for chemically mapping the tribo-elements in lubrication chemistry systems besides empirical or semi-quantitative global hardness or softness. And on the other hand, not only HSAB principle but also EEP and MHP based on the DFT-derived chemical reactivity indicators should be exploited for illustrating the interactions among the tribo-elements in lubrication chemistry systems.
- c)
- On the computational DFT level, algorithms are greatly needed for finer quantitative characterization of chemical and mechanochemical reactivity of the tribo-elements in lubrication chemistry systems.
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Shenghua, L.; He, Y.; Yuansheng, J., 1. Lubrication Chemistry Viewed from DFT-Based Concepts and Electronic Structural Principles. Int. J. Mol. Sci. 2004, 5, 13-34. https://doi.org/10.3390/i5010013
Shenghua L, He Y, Yuansheng J 1. Lubrication Chemistry Viewed from DFT-Based Concepts and Electronic Structural Principles. International Journal of Molecular Sciences. 2004; 5(1):13-34. https://doi.org/10.3390/i5010013
Chicago/Turabian StyleShenghua, Li, Yang He, and Jin Yuansheng, 1. 2004. "Lubrication Chemistry Viewed from DFT-Based Concepts and Electronic Structural Principles" International Journal of Molecular Sciences 5, no. 1: 13-34. https://doi.org/10.3390/i5010013