Friction Laws for Dry Nanoscale ContactsIzabela Szlufarska (University of Wisconsin - Madison) DMR 0512228

How does friction force depend on applied load and contact area?

• Macroscopic contacts: (Amontons’ law 1699),

• Nanoscale contacts: Laws Unknown

Approaches:

• AFM experiments for contacts nm-µm in size. Interpreted through continuum mechanics models:

• Continuum mechanics breaks down in nanoscale contacts.

This study:

• PI performed molecular dynamics simulations of AFM experiments at realistic length scales. Highly accurate REBO potentials are used to simulate mechanical deformation and chemical reactions of diamond simultaneously.

• Models give very good agreement with experiments on H-terminated diamond interfaces. Friction coefficient ~0.05 (exp: ~0.02), shear strength ~1,000 MPA (exp: 200 - 1,000 MPa).

Discovery:

AFM tip is not smooth. It consists of multiple atomic size asperities (total area Areal). Friction force is always proportional to this area:

Atomic roughness and interfacial interactions govern friction behavior

Non-adhesive nanoscale contacts follow macroscopic laws of friction:

Adhesive contacts are well described by continuum models:

Atomic multi-asperity model is proposed to describe simulation results.

Transition from linear to non-linear friction due to increased adhesion

Non- adhesive contact

Adhesive contact

Ff L

Ff A

Ff A

Ff L2 / 3

Ff Areal

Ff L

Ff L2 / 3

Education in Computational Materials Science

Izabela Szlufarska (University of Wisconsin) DMR 0512228

Undergraduate mentoring: Paul Kamenski (Materials Science & Engineering)

• Supported by PI’s lab for 2 years

• Wrote codes to support molecular dynamics simulations of nanocrystalline materials

• Co-op at the Oak Ridge National Laboratory through PI’s collaborations

• In the Spring ‘08 won NSF Graduate Research Fellowship (GRFP)

• In the Fall ‘08 begins graduate studies in materials science at Oxford University

Interdisciplinary course: Molecular Dynamics and Monte Carlo Simulations in Materials Science

• Taken by students across different colleges and departments (materials science & engineering, mechanical engineering, chemical engineering, chemistry, nuclear engineering, engineering mechanics, geophysics). Most students come from experimental groups

• Students work on interdisciplinary teams and on individual projects

• Final project examples:

• “Reverse Monte Carlo for Amorphous Si”

• “Radiation damage in nanoparticles”

• “Investigation of solid-water interface using LAMMPS”

• “Polymer bulk erosion: Monte Carlo simulations”

• “Modeling of phonon density of states for a Si/Ge heterostructures”Graduate student Sarah Khalil presents her final class project

Undergraduate student Paul Kamenski

Ferrite (α)

BCC

a = 2.870 Å

Austenite (γ)

FCC

a = 3.515 Å

910 °C

Education in Computational Materials Science

Izabela Szlufarska (University of Wisconsin) DMR 0512228

Example of student’s work from the course: MD and MC Simulations in Materials Science

Andy Nelson (graduate student in experimental group in Nuclear Engineering):

“Modeling of Ferrite-Austenite Transition”

ASM Materials Handbook Vol. 9