Computational chemistry models leading to mediation of gun tube erosion

Understanding the interaction of propellant gas products with transition-metal surfaces, and in particular with iron surfaces, is of primary importance to understanding the erosion process in gun tubes. By modeling these interactions, and the ensuing reactions, we shall gain a better understanding of the physics and chemistry underlying gun tube erosion. Using spin-polarized density functional theory (DFT) with the generalized gradient approximation (GGA), calculations were performed on the interactions of a wide variety of small product gases with the (100) and (111) Miller surfaces of iron. For the (100) surface, we have located the adsorption sites and determined atomic configurations of HCO, HOC, H/sub 2/CO, OH, CHOH, H/sub 2/, H, N, and NO as well as the coadsorption of CO+H, NO+C, and CO+N. We have also inspected the dissociative chemisorption of H/sub 2/ on the iron (100) surface, and dissociation pathway of CO and HCO (the latter resulting in surface bound O+HC) using the nudged elastic band (NEB) method. For the iron (111) surface, we have predicted the adsorption sites of CO, NH/sub 3/, NO, N, C, and O, the coadsorption sites of C+O and N+CO, as well as mapping the reaction paths and energetic barriers for the migration and dissociation of CO on the iron (111) surface. The interaction of CO with a nitrited iron surface is also in the process of being studied for both the (100) and (111) iron surfaces. Ab initio direct molecular dynamics (DMD) has been used to model the collision of CO at various temperatures with a small iron cluster to compute, by a second means, the energy required for dissociation. DMD is also being used to model the dynamics of the CO/sub (gas/adsorbed)/+Fe/sub 14//spl rarr/O/sub (g)/+C/sub (ads)/-Fe/sub 14/ reaction. Monte Carlo MD simulations with embedded atom models (EAM) are being carried out on the interactions of H/sub 2/ with large iron surfaces to measure hydrogen diffusion rates into the iron, with particular interest in the relative rates of diffusion through a perfect surface versus through surface defects (e.g., grain boundaries). Through many different computational tools and approaches, and the inspection of a wide array of different systems, we are gaining a better understanding of the chemistry involved in gun tube erosion. Using the knowledge gained of the energetics of binding and surface dissociation, coupled with macroscopic (thermodynamic and kinetic) erosivity models being used or developed at ARL, we should be able to predict the erosivity of different propellant blends as a function of the propellant composition.