Molecular dynamics (MD) simulations are unique in their ability to elucidate atomic-scale phenomena because the positions, velocities, and forces of all atoms in the system are known as a function of time. Atomic-scale wear in nanoscale contacts is of particular importance for tip-based nanomanufacturing applications. As a result, wear resistant materials, such as diamond-like carbon (DLC), have been used to coat AFM tips to improve the lifespan and reliability of AFM probes. We have performed atomic force microscope (AFM) experiments and MD simulations aimed at examining adhesion and wear in diamond, ultrananocrystalline diamond (UNCD), and amorphous carbon (a-C:H) materials. We examine the normal and sliding contact of different shaped axisymmetric tips, composed of a-C-H and UNCD, with hydrocarbon-based substrates. The effects of tip shape, material, surface termination, impact point, and roughness on adhesion and wear were examined. Results from the MD simulations were compared, and lend insight into, adhesion data obtained in AFM experiments. The application of continuum mechanics models to data obtained using the AFM to extract work of adhesion values will also be discussed. Finally, we will describe the use of finite element analysis to create a unique tip geometry for MD simulations that maximizes contact area so that it approximately equals the contact size in complementary AFM tapping-mode experiments. The wear behavior of AFM tips coated with a common type of DLC, amorphous hydrogenated carbon (a-C:H), when it is scanned against an ultrananocrystalline diamond (UNCD) sample using amplitude modulation (AM) AFM and molecular dynamics (MD) simulations was compared. Both the AM-AFM and the MD results reveal gradual wear of a-C:H with no sign of fracture or plastic deformation. The atomistic wear events observed in the simulations are correlated with under-coordinated atomic species at the surface. For example, in the figure below the red carbon atom from the DLC tip (upper surface) began the simulation with a coordination number of 2, and the green carbon atom began the contact simulation with a coordination number of 1 on the UNCD (lower) surface. Other carbon atoms are represented as gray or blue spheres and hydrogen atoms on the DLC are represented as small white spheres. 

Elucidating atomic-scale wear processes in hydrocarbon-based materials via molecular dynamics and AFM

248th National Meeting (2014)

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