Atomistic simulations of contact physics Alejandro Strachan Materials Engineering strachan@purdue.edu PRISM, Fall 2007

Atomistic materials simulations in PRISM Develop first principles-based constitutive relationships and provide atomic level insight for coarse grain models Identify and quantify the molecular level mechanisms that govern performance, reliability and failure of PRISM device using: Ab initio simulations Large-scale MD simulations PRISM, Fall 2007

PRISM Device simulation PRISM multi-physics integration Trapped charges in dielectric Predictions Electronic processes Validation Experiments: Microstructure evolution, device performance & reliability PRISM Device simulation MPM & FVM Elastic, plastic deformation, failure Micromechanics Defect nucleation & mobility in dielectric Fluid damping Fluid dynamics Dislocation and vacancy nucleation & mobility in metal Temperature & species Atomistics Thermal and mass transport Fluid-solid interactions Thermal & electrical conductivity Input Experiments: Surface roughness, composition, defect densities, grain size and texture PRISM, Fall 2007

Atomistic modeling of contact physics How: classical MD with ab initio-based potentials Size: 200 M to 1.5 B atoms Time scales: nanoseconds Predictions: Role of initial microstructure & surface roughness, moisture and impact velocity on: Force-separation relationships (history dependent) Mechanical response: Generation of defects in metal & roughness evolution Generation of defects in dielectric (dielectric charging) Interatomic potentials Thermal role of electrons in metals Current crowding and Joule Heating Electronic properties: Implicit description of electrons Chemistry: Surface chemical reactions Main Challenges PRISM, Fall 2007

Atomistic modeling of contact physics: II Smaller scale (0.5 – 2 M atom) and longer time (100 ns) simulations to uncover specific physics: Mobility of dislocations in metal, Interactions with other defects (e.g. GBs) Link to phase fields Surface chemical reactions Reactive MD using ReaxFF Defects in semiconductor Mobility and recombination Role of electric charging Fluid-solid interaction: Interaction of single gas molecule with surface (accommodation coefficients) for rarefied gas regime PRISM, Fall 2007

Obtaining surface separation-force relationships Contact closing and opening simulation 200 M to 1.5 billion atoms – nanoseconds (1 billion atom for 1 nanosecond ~ 1 day on a petascale computer) Characterize effect of: Impact velocities (4 values) Moisture (4 values) Applied force and stress (2 values) Surface roughness Peak to peak distance (2) and RMS (2) Presence of a grain boundary (4 runs) 16 runs 4 runs 4 runs 4 runs 28 runs PRISM, Fall 2007

Upscaling MD to: fluid dynamics Given a distribution of incident momenta characterize the distribution of reflected momenta: Accommodation coefficients: pi Fluid FVM models use accommodation coefficients from MD and predict incident distribution Role of temperature and surface moisture on accommodation coefficients PRISM, Fall 2007

Upscaling MD to: electronic processes Defect formation energies Equilibrium concentration Formation rates if temperature increases Impact generated defects Characterize their energy and mobility as a function of temperature Predict the distribution non-equilibrium defects Characterize energy level of defects SeqQuest PRISM, Fall 2007

Upscaling MD to: micromechanics Elastic constants Vacancy formation energy and mobility Bulk and grain boundaries Dislocation core energies Screw and edge Dislocation nucleation energies At grain boundaries, metal/oxide interface Nucleation under non-equilibrium conditions (impact) Dislocation mobility and cross slip Interaction of dislocations with defects Solute atoms and grain boundaries Upscaling MD to: thermals Thermal conductivity of each component Interfacial thermal resistivity Role of closing force, moisture and temperature PRISM, Fall 2007

MD simulations: challenges Accurate interatomic potentials Start with state-of-the-art Parameterize using ab initio calculations (ReaxFF, MEAM) Incorporate thermal and transport role of electrons Accurate description of thermal transport and Joule heating Extend new method for dynamics with implicit degrees of freedom - Strachan and Holian, Phys. Rev. Lett. (2005) PRISM, Fall 2007