The bore of gun barrels are typically coated with electroplated chromium to provide both corrosion protection and long service life. Agencies that plate with chromium spend hundreds of thousands of dollars on environmental waste removal to dispose of the waste, incurring additional costs to minimize employee exposure to chromic acid. Hexavalent chromium, the by-product of this plating, has been identified as a prohibited substance in the Hazardous Material Management Plan for numerous weapon systems. For the past 80 years, there has been no viable alternative due to the extreme nature of the interior ballistic environment and utilization of aggressive munitions that expose the gun barrel to elevated flame temperatures and erosive gases.
The objective of this project is to demonstrate the suitability of refractory metal alloy coatings as a replacement for chrome plating in gun barrels. As part of this project we will test fire a 5 inch gun barrel with the new refractory metal alloy coating and compare its performance against that of a traditional chrome plated gun barrel.
The refractory alloy coating will be applied using the Electromagnetically-enhanced Physical Vapor Deposition (EPVD) process. This is a process where the material is removed from a target made from the refractory alloy and deposited on the bore of the gun barrel in a low pressure inert environment. While the concept is based on a conventional technology that is typically used for thin films (10 to 15 micrometers), the EPVD process was developed to generate thick layers (125 to 200 microns) that are required for the gun barrel application. The process is easily adapted to accommodate small, medium, and large caliber gun barrels, applying engineered multilayer coatings. This process is robust and able to apply coatings of nearly any metallic or ceramic material. Coatings employed for gun barrel applications have been predominantly designed with tantalum, tungsten, and other refractory metals such as zirconium and niobium. The Mk 45 5-inch barrels will be test-fired and evaluated for coating adhesion and absence of stress cracking. GAU-12 25mm barrels will be also coated and evaluated during this effort and then test-fired as part of a separate effort.
The primary benefit of this effort is the removal of chromium from Department of Defense and contractor operated manufacturing plants that make the various gun barrels for the Armed Forces. The producers of these barrels estimate their environmental compliance costs to be about 8% of the total barrel cost, which represents close to $300 million just in environmental costs. Without this technology insertion, electroplated chromium will likely be in use for the next 50 - 75 years, potentially generating an environmental impact of close to $2 billion. In addition to environmental concerns, the increase in service life from the application of this coating will mean a significant reduction in the quantity of spare and replacement barrels that will be required over the life of these gun systems. (Anticipated Project Completion - 2015)