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Chromium Elimination in Medium Caliber Gun Barrels
Mark Miller | US Army Benet Laboratories
Current medium caliber gun barrels use chromium as a coating on their interior bore surfaces. This coating 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 current electrodeposition process is a hazardous substance. Hexavalent chromium, in the aqueous liquid and misting forms, is a known carcinogen and is extremely expensive to dispose of because of its toxic nature. Agencies that plate with chromium spend hundreds of thousands of dollars on environmental waste removal.
The objective of this project was to develop an environmentally benign (hexavalent chromium-free, low volatile organic compound [VOC], low hazardous air pollutant [HAP]) coating deposition process to replace the electrodeposition of hexavalent chromium without compromising wear life or performance of medium caliber guns (20-40 millimeter). The technology was demonstrated on full-length medium caliber gun barrels in an operational environment and achieved Technology Readiness Level (TRL) 7 in preparation for transition to production.
Explosive bonding is an innovative dry (i.e., non-aqueous) process for the deposition of tantalum (or possibly other high melting-temperature refractory metals), which is well suited for bore protection of a gun barrel. The process employs a cylindrical liner assembly comprised of explosive material surrounded by the coating material set at a specific distance from the gun barrel substrate. Detonation of the explosive forces the coating material onto the gun bore surface creating a superior bond. This project used various models to study and optimize the explosive bonding process. Numerous tests were performed on both flat and cylindrical coupons as well as actual gun barrels to determine how candidate coatings held up to the harsh conditions experienced inside a medium caliber weapon. Final analysis of the process included cost and environmental assessments to determine whether or not explosive bonding was a viable replacement for electrodeposition of hexavalent chromium.
In order to eliminate the hexavalent chrome used in the plating process, the development of explosively bonded refractory metal liners was carried out. After attempts by one vendor failed to produce a satisfactory product another vendor was selected who could explosively bond these materials to a short section of a gun tube. The quality and strength of these bonds were assessed in several ways. Visual observation of sectioned gun tubes showed good liner adhesion to the gun tube. Shear strength tests were conducted on several bonded liners. X-ray diffraction residual stress analysis revealed large residual stresses produced as a result of the explosive bonding process. Finally, hot hardness tests were conducted on three explosively-bonded liners. These tests led to the decision to select a tantalum-10% tungsten alloy (Ta-10W) as the liner material. The vendor was able to explosively bond a Ta-10W liner to a full-length M242 barrel. Efforts to machine the lands and grooves in the Ta-10W liner with conventional means met with failure, so a machining study was instituted to solve this problem. Part of the solution lay in a stress relief of the gun steel after explosive bonding. A satisfactory means of machining the liners was found, and several cannons were produced and successfully test fired.
An M242 cannon with an explosively-bonded Ta-10W liner was test fired in a side-by-side comparison with a conventional chromium-plated M242 cannon. A comparison of the wear and erosion of the Ta-10W lined barrel was made to the standard chrome-coated gun tube under the same firing conditions using fielded M791 and M793 ammunition. The comparison showed that the gun tube with the explosively-bonded Ta-10W liner had a useful life of more than three times that of the standard chromium-plated gun tube.
The main benefit of this research will be elimination of the Department of Defense’s dependence on electrodeposited hexavalent chromium to prolong the life of medium caliber gun barrels and reduction in the use of VOCs and HAPs during the explosive bonding process. In addition, there will be no loss in gun barrel performance by using Ta-10W lined barrels rather than conventional chromium coatings. The technology developed as part of this program offers the potential of eliminating chrome plating in all medium caliber cannons used by U.S. armed forces.