High-rate sputter deposited tantalum coating on steel for wear and erosion mitigation

A proto-type, rifled steel liner of 20-mm inside diameter was coated with ∼50–125 μm tantalum at ∼22.6 μm/h in krypton gas using a triode sputter system. It was test fired with 1500 rounds for the mitigation of high-temperature and pressure wear and erosion. Growth surface analysis showed: (1) predominately α-phase and minimal β-phase surface tantalum, (2) near random, slight preferred (211) crystalline orientation, (3) a thin layer of surface zinc oxide and copper firing debris, (4) compressive surface residual stress peaked near the center of the tube. Photomicrograph metallurgical examination showed: (1) general good adhesion in major parts of the bulk coatings, (2) a thin layer of β phase nucleation, and α phase or mixed α and β phase grain growth, (3) excellent behavior in soft and ductile α tantalum areas, but cracking and disbonding occurred in β tantalum areas, (4) coating lift-off and coating loss occurred most frequently in land-groove transition and land areas, less in groove areas, (5) near the breech end where no coating was sputtered, the steel substrate suffered extensive cracking and environmentally-assisted heat damages.