Thermo-chemical erosion in gun barrels

When a gun fires the metal temperatures at the bore may reach 1100°C in a few milliseconds and decline to half this value by the time the projectile reaches the muzzle. Although the temperature fluctuation does not penetrate very far from the surface it does create a hard and brittle surface layer commonly referred to as the heat-affected zone. The diffusion of chemical species such as CO, CO2, H2, H2O and N2 into the hot surface forms a chemically-affected zone. Wear rate in guns is related to the thickness of this chemically-affected zone. From the equation of mass diffusion a simple equation of the Arrhenius type is derived relating the wear per round to the initial temperature, the maximum surface temperature, and the erosivity of the propellant. This equation is verified by trials on a vented vessel, which simulate gunfire, and also by data on the wear rates of numerous gun and propellant combinations. The erosivity of service propellants is correlated with their chemical composition and demonstrates that CO and H2 are very erosive whereas CO2, H2O and N2 are much less erosive.