In this project, hexavalent-chromium-free cold spray (CS) coatings were evaluated as an alternative to hard chrome and nickel plating. Coating materials and CS processing conditions were initially developed in SERDP program WP-2607. The purpose of this ESTCP project was to perform more extensive characterization of the CS coatings, demonstrate the application of CS coatings to real Department of Defense (DoD)-applications, and transition the CS processing capability to DoD repair and overhaul facilities.

CS is a solid-state powder consolidation process in which powder particles are accelerated to high velocity in a heated gas stream and deposited onto a metallic surface. Bonding of material is achieved primarily by kinetic energy as opposed to thermal energy. Therefore, CS causes negligible thermal damage to the underlying substrate. CS can reliably produce thick, dense  coatings, offering capability not possible with chrome plating. Since the process requires relatively low heating, the CS application tool can be shrunk to fit inside long tubes so that inner diameter coatings can be applied. CS typically uses fully metallic feedstock powders since the process requires the particles to plastically deform to achieve deposition. However, harder ceramic materials can be deposited by combining with metallic materials. One of the key results of the SERDP project that preceded this project was engineering tailored feedstock powders with as much as 63% ceramic material by volume that could be successfully deposited using CS to produce robust wear-resistance coatings.

The testing performed in this project showed that CS coatings can be used as chrome and nickel alternatives for some applications. The CS coatings are generally softer than hard chrome plating, but some coatings exhibited superior wear resistance to chrome in some loading conditions. These results underscore the fact that hardness is not the only material property which affects wear resistance. The testing also showed that the optimal CS coating will be application dependent. For example, some coatings are less corrosion resistant than others, some coatings are difficult to surface finish, and there is a wide range of material cost per unit volume. The final selection of coating material will be dependent on how these factors are weighted. The final report should provide the information required to select a CS coating for a given application.

The technology demonstration resulted in a component that was successfully fielded on a platform and other components in which fielding is being actively pursued. Over the course of the project, CS systems became operational at two DoD overhaul facilities and in process for a third. The primary challenge encountered in standing up CS capability at DoD facilities was obtaining permission to use CS equipment by Environmental Health and Safety (EH&S) approval authorities. This is a challenge because CS is a relatively new process that is not well understood by EH&S approval authorities and the process for obtaining approval often varies at each DoD facility. CS vendors can produce dust and fire hazard analyses of their systems to help justify EH&S approvals but may be resistant to incurring additional cost if it is not included in their contract. DoD facilities are advised to include support of EH&S approval justification when establishing new contracts with CS vendors. Staffing is also an issue with CS implementation since CS is still a relatively new technology and it is difficult to find laborers with prior CS experience. CS training courses are available, but do not provide the full depth of experience required to successfully utilize CS. The best training for CS is shadowing a skilled CS operator for an extended period of time. Best practice is to maintain a staff with at least two experienced operators at any time so that if one leaves the other is still there to train the replacement.

Overall, this project demonstrated that CS coatings can be a viable alternative to chrome and nickel plating, but there is no “one-size-fits-all” solution. The optimal CS coating should be determined based on a weighting of various application-dependent factors. Implementation of CS capabilities at DoD facilities was shown to be possible, but issues were encountered that caused delays. DoD facilities that wish to implement CS in the future can leverage lessons learned from this project to avoid issues and accelerate implementation.