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Atomistic simulations of contact physics

Alejandro StrachanMaterials Engineering

strachan@purdue.edu

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Develop first principles-based constitutive relationships and provide atomic level insight for coarse grain models

Atomistic materials simulations in PRISM

Identify and quantify the molecular level mechanisms that govern performance, reliability and failure of PRISM device using:

•Ab initio simulations•Large-scale MD simulations

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Input Experiments:Surface roughness, composition, defect densities, grain size and texture

Atomistics

PRISM Device simulation

MPM & FVM

Validation Experiments:Microstructure evolution,

device performance & reliability

Predi

ctio

ns

•Defect nucleation & mobility in dielectric

•Dislocation and vacancy nucleation & mobility in metal

•Fluid-solid interactions

•Thermal & electrical conductivity

Electronic processes

Micromechanics

Fluid dynamics

Thermal and mass transport

•Trapped charges in dielectric

•Elastic, plastic deformation, failure

•Fluid damping

• Temperature & species

PRISM multi-physics integration

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Atomistic modeling of contact physicsHow: classical MD with ab initio-based potentialsSize: 200 M to 1.5 B atomsTime scales: nanoseconds

Mechanical response:Force-separation relationships (history dependent)Generation of defects in metal & roughness evolutionGeneration of defects in dielectric (dielectric charging)

Electronic properties:Thermal role of electrons in metalsCurrent crowding and Joule Heating

Chemistry: Surface chemical reactions

Predictions:Role of initial microstructure & surface roughness, moisture and impact velocity on:

Main Challenges

Interatomic potentials

Implicit description of electrons

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Atomistic modeling of contact physics: II

Mobility of dislocations in metal, •Interactions with other defects (e.g. GBs)•Link to phase fields

Defects in semiconductor•Mobility and recombination•Role of electric charging

Surface chemical reactions•Reactive MD using ReaxFF

Fluid-solid interaction: •Interaction of single gas molecule with surface (accommodation coefficients) for rarefied gas regime

Smaller scale (0.5 – 2 M atom) and longer time (100 ns) simulations to uncover specific physics:

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Obtaining surface separation-force relationships

Contact closing and opening simulation200 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 runs4 runs

28 runs

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Upscaling MD to: fluid dynamics

Given a distribution of incident momenta characterize the distribution of reflected momenta:

pi

Accommodation coefficients:

iw

irE EE

EE

iw

irp pp

pp

Fluid FVM models use accommodation coefficients from MD and predict incident distribution

Role of temperature and surface moisture on accommodation coefficients

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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

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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

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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)