When a medium caliber (i.e., 20-60 millimeter) gun is fired, the interior surfaces of the barrel are exposed to the erosive effects of hot propellant gases and the mechanical effects of the projectile. To protect barrels, the Department of Defense (DoD) uses chromium coatings that are electrodeposited from aqueous solutions of hexavalent chromium (Cr6+), a known carcinogen and toxic substance that is strictly regulated. A potential replacement for chromium is tantalum (Ta), which provides excellent protection to gun barrels and is environmentally friendly since it is applied through a dry process. Internally magnetized cylindrical magnetron sputtering (IM-CMS) has been demonstrated to deposit Ta coatings to large caliber gun bores (greater than or equal to 60 millimeter); however, this process performs poorly on cylinders with diameters less than 45 millimeter because of the critical ionization distance required.
This project investigated the use of chemical vapor deposition (CVD) to produce high-quality Ta coatings for wear and erosion protection of medium caliber gun barrels.
Chemical vapor deposition is used extensively in a wide range of coating applications. This project demonstrated the viability of using CVD to apply Ta coatings to the interior surfaces of medium caliber gun barrels. In this collaborative effort, the New Jersey Institute of Technology (NJIT) focused on the synthesis and characterization of the Ta coatings. NJIT evaluated the CVD process and determined how changing the processing parameters affects throughput and the chemical purity of the coating. Additionally, NJIT studied how surface preparation treatments affect the quality and performance of the coating.
This project demonstrated that CVD Ta coatings could be successfully synthesized on steel substrates. The chemical, structural, and morphological properties of the Ta coatings were established. The results indicated that the coatings were essentially pure and dense independent of deposition parameters. The results also indicated that these coatings exhibit a near perfect step coverage and a crystallographic structure that is dependent on pre-treatment procedures. A major shortcoming of in-situ sequential deposition of CVD Ta nitride and CVD Ta was the low growth rate achieved as a result of the relatively lower temperatures imposed by the experimental constraints of the plasma-enhanced CVD deposition apparatus. That constraint was lifted by making use of ultraviolet-assisted CVD, which proved to produce alpha phase Ta coatings with significantly higher growth rates at temperatures higher than those used in plasma-enhanced CVD but still within the tempering temperature range of medium caliber gun barrel steel.
This project demonstrated that it was feasible to deposit Ta coatings on gun barrels using CVD but additional work will be required to increase deposition rates and validate performance under firing conditions. (Project Completed - 2008)