UC Merced

This dissertation investigates the chemical processes on surfaces, focusing on the growth of surface layers for applications in friction reduction using solid coatings and surface functionalization for investigations of mechanically driven chemical reactions.

First, we investigated how Ni doping affects the wear life of MoS2 by conducting experimental studies on sputtered undoped and 7% Ni-doped dry film lubricants. The results showed that Ni-doping improves wear life at low contact pressures, attributed to larger MoS2 debris in the wear track. Our findings suggest that wear life correlates more with the size of lubricious wear particles than with wear depth. A model was proposed to explain the differences between Ni-doped and undoped MoS2 wear processes, emphasizing the need for atomic-level insights into the sputtering process and resulting DFL microstructure.

To address this, we used a recently developed reactive molecular dynamics potential parameterized for Mo/S/Ni to simulated sputtering deposition and annealing simulations. Then, deposition of MoS2 with different Ni concentrations was simulated and the resulting films were analyzed for density, crystallinity, radial distribution function, and Ni-cluster formation. The highest density and crystallinity were observed just above the MoS2 substrate, and larger Ni clusters were observed at higher Ni concentrations. Shearing simulations revealed that films with 2% and 10% Ni had lower shear stress at steady state compared to those with 0% and 15% Ni, due to the formation of lubricious wear particles with a crystalline core and amorphous, Ni-containing surfaces. This non-monotonic dependence of tribological properties on Ni concentration provides insights for optimizing doped MoS2 dry film lubricants.

To study surface functionalization, we simulated the covalent attachment of aromatic hydrocarbons to amorphous silica. Surface densities of silanol-terminated phenyl, naphthyl, and anthracenyl molecules were lower than geometrically predicted, decreasing faster with more aromatic rings. Analysis of bonding configurations, tilt angles, and local conformational ordering showed that surface density is influenced by the size and symmetry of the hydrocarbons. Compression simulations further provided insights into the deformation of immobilized aromatic hydrocarbons during mechanochemical processes.

This research offers valuable insights into the effect of deposition or functionalization processes and parameters on the properties of functional surfaces. By integrating theory and experimentation, the results advance our understanding of surface chemistry, guiding the design and optimization of materials for tribological and mechanochemical applications.