Ion Beam Methods Applied to Interior Ballistic Studies

High temperature, pressure and velocity gases produced during the interior ballistic cycle of a gun firing are responsible for considerable damage to the steel surfaces of a gun bore. This damage is studied by exposing steel samples to the erosive flows of burning propellant gases in a modified 37mm gun chamber where pressures of 200 MPa and flame temperatures of 3000°K are typical. Ion beam methods are used to characterize the composition of the steel surfaces by combined nuclear reaction (NR) and elastic backscattering (EBS) analysis and thin layer activation (TLA) is used to measure surface wear rates. Combined fits to the EBS and NR distributions yield concentrations and depth profiles of carbon, nitrogen and oxygen as well as iron and other heavier elements. Hydrogen concentrations have also been measured on some of the samples. The results of these experiments show the presence of two different erosion mechanisms. In one, the surface is softened by thermo-chemical processes prior to removal by the shear forces of the gas flow while in the other surface layer melting occurs prior to removal. TLA using the 56Fe(p,n)56Co reaction has been used to measure wear from a 20 mm barrel and is being instrumented for larger barrels. EBS is being used to characterize the interfaces between steel substrates and coatings designed to reduce erosion.