Hard chrome (Cr) coatings (0.001 to 0.015 inches thick) are used extensively for imparting wear and erosion resistance to components in both industrial and military applications because of their intrinsic high hardness (600-1,000 VHN) and low friction coefficient (less than 0.2). The most common means of depositing such hard Cr deposits has been through the use of chromic acid baths. Health risks associated with the use of hexavalent Cr baths have been recognized since the early 1930s. In addition to the health risks, there are several other process and performance drawbacks to hard Cr coatings. Electrodeposited nanocrystalline metal and alloy coatings, in addition to being fully compatible with current hard Cr plating infrastructure, have displayed properties that render them a superior alternative to hard Cr coating technology. Of particular importance is that these properties are attained using more environmentally benign chemistries.
The objective of this project was to develop and optimize an advanced nanoscale coating technology based on the modification of environmentally benign conventional electroplating techniques that yield coatings that meet or exceed the overall performance and life-cycle cost of existing hard Cr electroplating.
This project’s technical approach consisted of three phases—technology viability, coating optimization, and extension to complex shapes. Phase I involved identifying and conducting a preliminary experimental assessment of suitable nanoscale electrodeposition systems that satisfy the environmental objective and provide cost and performance requirements. Phase II involved developing and optimizing the most promising system and incorporated additional performance evaluation, including wear, fatigue, and corrosion testing. Phase III focused on the optimization of nanoscale non-line of sight (NLOS) techniques that represent key applications for the Department of Defense (DoD).
In Phase I, three different alloy coatings were synthesized on a laboratory scale. Binary alloys, Co-P and Co-Mo, were produced along with the ternary alloy, Co-Fe-P. These alloys were evaluated for grain size, hardness, coating thickness, and thermal stability. Phase I also included an effort to establish baseline emission, chemical waste production, and process cost data. In Phase II, a nanocrystalline Co-P alloy and associated process was established, studied, and optimized. Performance tests were conducted on uncoated and coated samples (as well as reference materials) to establish properties and gauge performance. As part of Phase III, the nanocrystalline Co-P alloy coating was applied to internal (NLOS) diameter surfaces. Anode selection and plating parameters were selected and tested on mock-ups designed to simulate the typical internal diameter surfaces encountered in DoD applications. As a final demonstration, the coating process was successfully applied to an ID surface of the shock strut of a landing gear component.To further determine the viability of this technology as an alternative to hard chrome coatings, demonstration/validation efforts are ongoing under ESTCP project WP-200411.
If successfully validated, the results of this project will make possible the complete elimination of hexavalent Cr at rework, maintenance, and manufacturing facilities within DoD. The DoD currently spends more than $10 million per year in hazardous material disposal costs associated with hard Cr electroplating. The approach, which is based on electroplating, allows for the retention of numerous benefits associated with hard Cr plating technology (i.e., NLOS applications, excellent coating adhesion, dimensional consistency, and superior surface finish). In addition, this technology will be able to utilize existing hard Cr plating infrastructure within the defense sector, significantly reducing time and cost to practical implementation. Moreover, this technology is expected to provide significant performance and life-cycle cost advantages over current hard Cr plating technology. (Project Completed – 2003)