The efforts of U.S. Army engineers to implement an improved coating for howitzer breech spindles will provide several benefits, including easing the logistical burden on soldiers, reducing hazardous waste, and saving millions of dollars.

Rust, wear and corrosion problems currently force the Army to condemn breech spindles before their full service lives. A team at the U.S. Army Research, Development and Engineering Command (RDECOM) is leading a project to identify, validate, test and evaluate a solution. Project officer Maira Senick and technical lead Dr. Christopher Mulligan, both with RDECOM's Armament Research, Development and Engineering Center, in partnership with Product Manager Towed Artillery Systems, are aiming for a production-ready coating within six months to a year.

An M776 howitzer corroded chrome-plated standard obturator spindle sits next to newly plated production at the U.S. Army Aberdeen Test Center, Aberdeen Proving Ground, MD. (Photo: Conrad Johnson, RDECOM)

"We're improving the performance of the howitzer and reducing the logistical burden on the soldier," said Mulligan, a materials engineer. "A lot of times when you're trying to improve performance or eliminate hazardous materials, you end up with a more expensive process. Here, we're saving a significant amount of money over the life cycle of the weapon with minimal to no increase in production cost." Picatinny Arsenal, N.J., officials estimate that a minimum of $5 million will be saved over the life of the M777A2 howitzer, but that amount would increase if the improved coating were to be used with other artillery as well.

Mulligan explained that the spindle is the howitzer component that seals the chamber and holds the pressure to prevent gas from leaking from the breech. Chromium has been used to coat spindles for decades, but the Army has found this method often leads to a shortened service life when subjected to the rigors of soldiers using the weapons in training or combat.

"Any time you have corrosion, wear or chipping, it could result in loss of the seal and affect chamber pressure and accuracy. It needs to function properly. Wear and corrosion cause malfunctions," Mulligan said.

To find and evaluate possible replacement technologies, the team developed a list of 10 primary metrics necessary for a new coating and application process. These included resistance to corrosion, mechanical wear and high temperatures. The group evaluated 12 material formulations in small samples and then down-selected to three based on performance and cost. The candidates currently being tested are High Power Impulse Magnetron Sputtering from Sheffield Hallam University in the United Kingdom, Accelerated Plasma Arc from Phygen Coatings, and Electroless Nickel Plating. Mulligan said they are all vastly outperforming the chrome plating in terms of corrosion and wear.

To ensure the coatings can withstand the rigors of soldier use, the ARDEC team then turned to the Aberdeen Test Center at Aberdeen Proving Ground for live-fire testing on a howitzer range. After the first round of firing, the spindle undergoes 30 days of weathering in a caustic and acidic propellant byproduct, known as swab water, to simulate potential conditions in combat, followed by another round of firing and then a final weathering cycle.

"[Soldiers] use swab water to clean after firing, and sometimes proper maintenance is not done," Mulligan said. "We need to make sure it can withstand the firing environment and still maintain its corrosion resistance. After these are all fired and go through the second 30-day weathering cycle, we're going to put them back through accelerated corrosion testing for a five-day cycle in an environmental chamber, including salt fog as the final step."

Mulligan explained that following one aggressive cycle in the chamber, the chromium coating exhibits severe corrosion. Senick said the team is constantly searching for new coating technologies and has identified a fourth option from Canada. Funding has recently been secured, and testing will begin within a few months.

"Even though we have engaged on a path forward with these promising alternatives, we continue to monitor trends and advances on the corrosion- mitigation coating field," Senick said. "We have identified a newly developed promising chemical vapor deposition type coating known as Carbonyl."

After all testing is complete, a final decision on the best process is expected within 90 days.

As the improved coatings extend the life of spindles, the logistical demands on soldiers will decrease as fewer spare parts will need to be transported and stored. The Army would then buy fewer spindles as the requirement for replacements is reduced.

Another important aspect of the project is the environmental benefit, Mulligan said. While the current spindle chrome electroplating process is federally permitted and under close engineering controls, it generates a waste stream of hexavalent chrome, which is highly carcinogenic. The goal is to establish a dry process that allows for a completely clean environmental method. The project aims to comply with a Department of Defense directive issued in April 2009, which mandates the minimization of hexavalent chromium in defense-related industrial base manufacturing, Senick said.

Source