Titanium carbonitride (TiCN) is an advanced, high-performance hard ceramic of great commercial importance that has been widely developed and employed. Nonetheless, it has only been in recent years that binderless titanium carbonitride bulk ceramics have been successfully fabricated using field-assisted sintering technology (FAST). However, the underlying structure-processing-property-performance relationships have yet to be fully evaluated, especially concerning indentation hardness of these materials across a broad range of loads and deformation length scales. In this work we aim to address these fundamental relationships and characterize the multiscale hardness phenomena in detail. It was found that the effects of soak temperature and time directly impacted the sintered microstructure and were reflected in the observed mechanical properties over various loads. Valuable insight into the load-dependence of hardness distributions, sensitivity/correlation with elasto-plastic parameters, and multiscale parameterization were developed using micro-/nanoindentation. Particularly, the load-dependent hardness sensitivity and resolvability demonstrate a fundamental tradeoff with respect to the manifested mechanical response influenced by the presence of underlying heterogeneities. These new insights relating the interplay of compositional/microstructural evolution with FAST processing parameters and multi -scale hardness are an important step in advancing next-generation hard ceramics.

Pt nanoparticles (NPs) are currently being investigated for use in fuel cells: however, Pt NP oxidation and Pt atom dissolution as a function of size morphology and temperature is not well understood or currently quantified.

The durability of platinum nanoparticles is investigated to determine the way in which particle size and oxygen coverage affect their reconstruction under gas-phase conditions and in oxidizing environments. Classical molecular dynamics simulations are performed using the third-generation charge-optimized many-body potential, and the findings are compared to experimental measurements. The diameters of the platinum nanoparticles range from 1.35 nm to 11.29 nm, and they are examined at temperatures of 300, 450, and 600 K. While these simulations indicate that the reconstruction of the oxidized nanoparticles becomes more pronounced as the temperature increases, some of the non-oxidized nanoparticles reconstruct with unexpectedly fast kinetic rates at 450 K and 600 K. As the adsorbed oxygen coverage increases, the simulations predict a decrease in nanoparticle stability and an increase in subsurface oxidization. These findings quantify the influence of oxygen and temperature on oxidized platinum nanoparticles' stability, which are essential to heterogeneous and homogeneous catalysis.

An investigation to optimize the application of the third-generation charge optimized many-body (COMB3) interatomic potential and associated input parameters was carried out through the study of solid-liquid interactions in classical molecular dynamics simulations. The rates of these molecular interactions are understood through the wetting rates of water nano-droplets on a bare copper (111) surface. Implementing the Langevin thermostat, the influence of simulation time step, the number of atoms in the system, the frequency at which charge equilibration is performed, and the temperature relaxation rate are all examined. The results indicate that time steps of 0.4 fs are possible when using longer relaxation times for the system temperature, which is almost double the typical time step used for reactive potentials. The use of the charge equilibration allows for a fewer atomic layers to be used in the Cu slab. In addition, charge equilibrium schemes do not need to be performed every time step to ensure accurate charge transfer. Interestingly, the rate of wetting for the nanodroplets is dominantly dependent on the temperature relaxation time, which is predicted to significantly change the viscosity of the water droplets. This work provides a pathway for optimizing simulations using the COMB3 reactive interatomic potential.