Characterization and Comparison of Magnetron Sputtered and Electroplated Gun Bore Coatings

The demands to increase range, rate of fire, and muzzle velocity have resulted in increased wear and erosion problems in gun tubes. To increase the service life of gun tubes, a number of bore-coating systems are being considered for replacement of the current electroplated high-contractile chromium coating. Two such coating systems are cylindrical magnetron sputtered (CMS) Cr coatings and CMS $Ta∕Cr$ bilayer coatings. Cylindrical magnetron sputtering is a high-rate vacuum deposition process that has been applied to $120mm$ tubes. Characterization studies of the electroplated and CMS coatings were completed to determine the applicability of these coating/substrate systems for gun bore protection. Each coating system is subjected to a series of tests, including adhesion, microhardness, compositional analysis, and vented erosion-simulation testing (VES). VES testing is completed via a laboratory combustion chamber that reproduces the transient thermal and chemical environments of tank cannon firing on small chord sections of $120mm$ coated gun tubes. In addition to the aforementioned characterization tests, metallography, scanning electron microscopy, and energy dispersive spectroscopy are conducted on each specimen before and after VES testing to evaluate the thermal stability of the coating and the severity of the thermal damage imposed. The mechanisms of damage are investigated, including void formation and micropit growth, oxidation and erosion, and thermomechanical cracking. In addition, methods to further increase resistance to thermal damage are discussed to increase the service life of future gun tube systems.