1. Atomistic simulations of contact physics Alejandro Strachan Materials Engineering
PRISM, Fall 2007

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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