Density functional theory has become an indispensable tool for investigating nanoscale friction and wear mechanisms at the atomic level. By solving the quantum mechanical many-body problem, DFT provides insights into interfacial interactions, adhesion energies, and electronic structure changes that govern tribological behavior in nanoscale contacts. Unlike empirical or continuum models, DFT captures the explicit electronic contributions to friction without relying on macroscopic assumptions, making it particularly suited for studying single-asperity contacts and molecular lubricants.

At the heart of DFT-based tribology studies lies the calculation of adhesion energy between contacting surfaces. The adhesion energy, defined as the work required to separate two surfaces from equilibrium contact to infinite separation, directly influences static friction forces. For graphene-graphene interfaces, DFT calculations reveal adhesion energies ranging from 0.2 to 0.5 J/m² depending on stacking configuration and interlayer distance. The AB-stacked configuration typically shows higher adhesion than AA-stacking due to favorable orbital overlap. In diamond-like carbon films, adhesion energies vary significantly with hydrogen termination, with hydrogenated surfaces showing up to 50% reduction in adhesion compared to bare carbon interfaces. These quantitative predictions enable rational design of low-adhesion coatings.

Sliding potential energy surfaces provide the fundamental basis for understanding frictional energy dissipation. DFT constructs these surfaces by calculating the total system energy at various lateral positions of the upper surface relative to the lower surface. For a graphene flake sliding on copper, DFT reveals a periodic energy corrugation with amplitude 5-10 meV per atom, directly corresponding to the electronic friction contribution. The energy barrier periodicity matches the substrate lattice constant, demonstrating how atomic structure dictates frictional anisotropy. In DLC films, the potential energy surface becomes more complex due to mixed sp²/sp³ bonding, with local energy minima corresponding to favorable carbon ring alignments.

Interfacial charge redistribution during nanoscale contact significantly affects tribological behavior. DFT calculations show that when two surfaces approach within van der Waals contact, charge density accumulates in the interfacial region while depleting in adjacent atomic layers. This redistribution creates electrostatic interactions that contribute to adhesion. For graphene on nickel, charge transfer of approximately 0.05 electrons per carbon atom occurs, creating a dipole layer that increases adhesion energy by 20% compared to non-charge-transfer cases. In lubricant monolayers such as perfluoropolyether on iron oxide, DFT reveals that terminal oxygen atoms withdraw electron density from surface iron atoms, creating strong electrostatic bonds that resist shear.

DFT models single-asperity contacts by simulating tip-surface interactions with atomic precision. A common approach uses a pyramidal or spherical cluster of atoms to represent the asperity tip sliding over a periodic surface. For a silicon tip on diamond, DFT shows that the onset of wear occurs when interfacial shear stresses exceed 15 GPa, corresponding to the breaking of C-Si bonds and subsequent formation of new Si-C cross-interface bonds. The theory also predicts that oxygen-terminated diamond surfaces reduce tip adhesion by 40% compared to hydrogen-terminated surfaces due to Pauli repulsion between oxygen lone pairs and silicon orbitals.

Lubricant monolayer studies benefit from DFT's ability to model molecular-scale interactions. For zinc dialkyldithiophosphate on iron oxide, DFT calculations demonstrate that the molecule chemisorbs through sulfur-iron bonds with binding energies around 2 eV, explaining its effectiveness as an antiwear additive. The theory also predicts shear-induced decomposition pathways where the molecule dissociates at loads exceeding 1 nN, releasing phosphate fragments that form protective tribofilms. In the case of ionic liquid lubricants, DFT reveals that alkyl chain length and anion geometry control the formation of ordered interfacial layers that minimize energy dissipation during sliding.

Despite its strengths, static DFT has inherent limitations for tribological studies. The method assumes zero temperature and quasi-static sliding conditions, neglecting thermal fluctuations and dynamic effects that dominate real friction processes. The typical time scale of DFT simulations (picoseconds) is many orders of magnitude shorter than experimental sliding speeds. Additionally, standard DFT cannot capture non-equilibrium phenomena like phonon excitation or electronic friction mechanisms that contribute to energy dissipation. These limitations necessitate coupling DFT with molecular dynamics or non-equilibrium Green's function methods for complete tribological understanding.

Graphene interfaces serve as an important test case for DFT tribology predictions. Calculations show that bilayer graphene exhibits superlubricity when the layers are rotationally misaligned by more than 5 degrees, with friction coefficients below 0.001. This arises from the cancellation of lateral forces due to incommensurate lattice matching. However, defects or edge contacts can break superlubricity by creating localized high-adhesion regions. DFT predicts that single vacancies increase local friction by a factor of three due to enhanced electronic states near the Fermi level that strengthen interfacial bonding.

Diamond-like carbon films present complex tribological behavior that DFT helps unravel. The theory shows that hydrogen content controls friction by passivating dangling bonds that would otherwise form strong interfacial connections. At 30% hydrogen content, the coefficient of friction drops below 0.1 due to the formation of a passivated sliding interface. DFT also explains the role of sp² clusters in DLC films, which act as internal slip planes during shear while maintaining overall structural integrity. The calculations reveal that optimal sp²/sp³ ratios around 30% provide both low friction and high wear resistance.

Recent advances in DFT methodology have improved nanoscale tribology predictions. Van der Waals corrections now accurately capture long-range dispersion forces crucial for layered materials and lubricant films. Hybrid functionals provide better description of electronic excitations during sliding contacts. High-throughput DFT screening enables rapid evaluation of material combinations for low-friction applications. These developments continue to expand DFT's role in understanding and designing nanoscale tribological systems, bridging the gap between quantum mechanics and macroscopic friction behavior.