AmericanChemicalSociety.com

Oil drilling bits and rock drills post severe requirements for material properties, which include high strength and hardness and low friction and wear. Ultra-nanocrystalline diamond (UNCD) is an excellent candidate for drill coatings because its tribological, chemical, and mechanical properties are very similar to that of single crystal diamond, which is the strongest material known. Recently UNCD has been successfully deposited on micro end mills, which resulted in a considerable reduction in cutting forces and burr formation. In this project we employ large scale molecular dynamics (MD) simulations to unravel fundamental mechanisms underlying friction and wear of UNCD. By simulation atomic force microscope (AFM) experiments and nanoindentation, relationships will be determined between a structure of UNCD and friction/wear at a single asperity level.

In the first year of the project we have performed large MD simulations to study friction in nanoscale contacts between diamond like carbon tips and diamond surfaces. We have discovered friction laws that apply at such small length scales, i.e., we have shown how the friction force depends on contact area and on the applied load. The modeling system consisted of a tip made of H-terminated diamond-like carbon and a H-terminated diamond sample. Simulations were carried out using a reactive bond order interatomic potential integrated with dispersive interactions. A quantitative agreement in contact pressures and shear strengths is achieved between our simulations and previously reported experimental studies. We show that the roughness theories capture the correct physics of deformation at the nanoscale. Our study provides a consistent explanation of the widely observed transition from a linear to sub-linear dependence of friction force on applied load and we demonstrate that both regimes of friction are governed by the same physical phenomenon. Specifically, we show that friction is controlled by the number of atoms that interact chemically across the contact interface. This discovery creates an opportunity to build a unified theory of friction all length scales, and more specifically to modify roughness theories to describe friction at multiple length scales simultaneously.

In order to be able to make quantitative predictions of friction, one needs to identify elemental instabilities that occur during sliding and these instabilities are strongly dependent on a specific chemistry of the interfaces. We have employed MD simulations of friction of diamond passivated with different isotopes of H atoms and we have proposed a physical explanation for the experimentally observed isotope effect on solid friction, i.e., we explained why increasing mass of adsorbate atoms leads to reduction in friction. Such reduction in friction (isotope effect) has been found in SFM studies on diamond and silicon, however this effect had not been explained convincingly. Using theoretical analysis and MD simulations we have shown that the isotope effect can be explained quantitatively by small differences in surface coverage, which are due to isotope-dependent bond stabilities. The isotope effect on solid friction has important technological consequences, because it creates a path for design of surfaces with ultralow friction. Similar isotope effect in lubrication is now utilized in commercial deuterium-based oils.

The next step in our studies is to perform nanoindentation simulations of UNCD samples. We will first investigate how mechanical properties (hardness in this case) change with the H content in grain boundaries of UNCD. Since H content can be controlled during deposition of UNCD, this understanding will allow to optimize mechanical properties of UNCD films. This study will also provide a fundamental understanding of deformation mechanisms in this technologically important material.