Current and developmental armament systems require a protective layer in the gun tube to reduce the erosive effects of advanced propellant formulations. To date, all of the defense services use chrome plating as a wear enhancer for various weapons. Chrome plating protects the bore surface against the harsh environment of the hot propellant gases and the mechanical effects of the projectile, thereby increasing the life of the gun tube. However, the chromic acid used in the deposition process contains hexavalent chromium, a known carcinogen and toxic substance. This hazardous substance has a deleterious effect on the environment, and its disposal is extremely expensive.
This project developed a dry, environmentally benign replacement process for the existing aqueous electrodeposition chromium plating facility. The Cylindrical Magnetron Sputtering-Internally Magnetized (CMS-IM) process was chosen to replace the chrome plating process and extend the barrel life by introducing a superior wear and erosion mitigating material, namely tantalum (Ta).
CMS is a magnetically-enhanced sputtering system that is capable of providing high deposition rates over large areas while maintaining low substrate heating levels. Initially, test specimens were coated to determine principal process parameters such as inert gas sputtering pressure levels, substrate temperatures, inert gas flow rates, and substrate/target cleaning requirements. Every coated test specimen was characterized through a standardized battery of laboratory tests to examine the coating's adhesion, thickness, hardness, phase, morphology, density, thermal shock characteristics, interfacial properties, texture, and purity. After a number of reports were generated, a coating experts' review was conducted. The information and insights gained from the characterization of each coating specimen were used to adjust process parameters for subsequent coating runs. Post-firing characterization activities were conducted to determine why a coating did or did not perform well. The modeling efforts conducted in parallel by various leveraged projects also provided invaluable process-related information.
Throughout the project, many laboratory samples were coated and characterized to ascertain the optimum coating parameters. In addition, three 45mm full-length (i.e., 59 inches) gun barrels were coated and fired. These barrels were fired with high energy, non-ablated German L1M propellant with a flame temperature of 3,700 degrees Kelvin, far hotter than any propellant mixture used on current or proposed U.S. weapon systems. The firing results were compared to chrome plated 45mm barrels that were baselined with the same propellant mix. Results showed that the well adherent Ta coating (deposited via CMS-IM) provided less wear per shot than the chrome plated barrels. Though there was some gross delamination of Ta in some areas of the barrel, this issue has been successfully addressed in the aforementioned MANTECH follow-on program. The initial demonstration platform was intended to be 25mm barrels; however, it was determined that the CMS-IM process could not be applied to barrels with bore diameters less than 45mm. It has since been shown that the CMS variant known as Cyl indr ical Magnetron Sputtering-Externally Magnetized (CMS-EM) generates the magnetic field from outside the barrel and can accommodate repeatable and consistent plasma generation in smaller bore diameters. This project was completed in FY 2002.
This project maturated the applied research, developed the appropriate technology, and performed an advanced demonstration addressing specific technologies of interest to Army, Navy, and Air Force armament system sustainability requirements in the plating of medium caliber barrels. In addition, the project showed that the work could be spun off to large caliber gun barrels as demonstrated by the approval of an FY00-05 MANTECH program for this technology.