Chrome and nickel electroplating have been used for decades for corrosion and wear protection and for sealing surfaces. Hexavalent chromium and nickel are also known to be hazardous toxins. Hexavalent chromium plating baths are the most widely used baths to deposit chromium on metal. Hexavalent chromium baths are composed of chromic acid, sulfuric acid, and water. The chromic acid is the source of the hexavalent chromium that reacts to form chromium deposits on the metal and is also emitted to the atmosphere. Nickel electroplating consists of cleaning and degreasing in alkaline water-based cleaners, electrolytic degreasing in alkaline water-based cleaners, acid dip pickling in acid-based solutions, and electroplating. Fumes generated in the electroplating process and subsequent finishing processes all contribute toward nickel exposure.
The cold spray (CS) deposition of protective coatings provides an environmentally friendly and field deployable alternative process for electroplating. This additive manufacturing technique utilizes a high-pressure gas stream to carry powder particles through a converging-diverging de Laval nozzle where they are accelerated to supersonic velocities before impacting on a solid substrate to form a coating or a part layer. The process is completely solid state and as such has no mechanism by which the powders will break down into more harmful versions of the elements.
In this SERDP project, Mr. Aaron Nardi and Dr. Vic Champagne from the U.S. Army Research Laboratory worked to advance the state-of-the-art for CS by developing a holistic approach to coating development and process optimization using advanced computational and analytical tools. This approach led to a better understanding of CS particle to substrate interaction and bonding mechanism and the compaction and consolidation of powders. It also resulted in the development of novel CS coating materials that can be used by the Department of Defense (DoD) and industry to replace chrome and nickel electroplating along with optimized process parameters and hardware.
Finite element modeling was used to predict how powder modifications affected the resulting CS deposit. These mathematical simulations pointed to the powder compositions most likely to yield the desired deposit. Over 160 different spray trials were performed on different powder formulations. From these experiments, solutions for nickel as well as chrome replacements have been identified. In addition, it has been noted from several wear test data points that some CS deposits show improvements over chrome in some wear situations.
To demonstrate CS hard surface repair, simulated helicopter engine turboshafts were cold sprayed with three of the selected powders for the evaluation of repair potential. Three CS deposits were evaluated on the simulated shafts. Qualitative results show each CS coating performed well in comparison to chrome, with one formulation providing performance superior to the chrome plating in wear tests.
Through their efforts on the project titled Cold Spray Coatings for Chromium and Nickle Plating Replacement, Mr. Nardi, Dr. Champagne, and their team have been awarded the 2020 SERDP Project of the Year for Weapons Systems and Platforms.
Project Team:
- Aaron Nardi - U.S. Army Research Laboratory - Weapons and Materials Directorate
- Victor K. Champagne, Ph.D. - U.S. Army Research Laboratory - Weapons and Materials Directorate
- Dr. Dennis Helfritch, Survice Engineering
- Dr. Matthew Siopis, U.S. Army Research Laboratory - Weapons and Materials Directorate
