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

1. 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. Non- adhesive contact Transition from linear to non-linear friction due to increased adhesion Adhesive contact

2. Education in Computational Materials ScienceIzabela 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 Undergraduate student Paul Kamenski • 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

3. Education in Computational Materials ScienceIzabela Szlufarska (University of Wisconsin) DMR 0512228 Ferrite (α) BCC a = 2.870 Å Austenite (γ) FCC a = 3.515 Å 910 °C 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