By Ruben Prado, Diana Facchini, Neil Mahalanobis, Francisco Gonzalez and Gino Palumbo
Hexavalent chrome plating has been used for decades in such applications as aircraft landing gear, hydraulic actuators, gas turbine engines, helicopter dynamic components and propeller hubs, but there have been safety and environmental concerns mainly because of the carcinogenic nature of vapor emissions produced during the process.
Electrodeposited nanocrystalline cobalt-phosphorus (nCoP) has emerged as a viable environmental alternative to hard chromium coatings for both line-of-sight (LOS) and non-line-of-sight (NLOS) applications. Such coatings were developed under the Strategic Environmental Research and Development Program (PP-1152) and were put through demonstration-validation testing in a project under the Environmental Security Technology Certification Program (WP-0411).
The coatings’ material properties revealed that nCoP has high hardness, enhanced ductility, lower wear rates, superior corrosion resistance and no issues with hydrogen embrittlement after baking. They exhibit:
- Higher process efficiency with reduced energy consumption
- Higher deposition rates
- Large thicknesses deposited
- No pitting, microcracks or pores
- Lower environmental impact.
Significant Improvements Over Hard Chrome
The research projects showed that nCoP exhibits properties equivalent to (and in many ways better than) electroplated engineering hard chromium (EHC), as summarized in Table 1. Like EHC, nCoP is an aqueous bath process produced by electrodeposition. It represents a drop-in alternative technology that is fully compatible with the current hard chrome electroplating infrastructure, and renders nCoP well-suited for application to both LOS and NLOS surfaces.
The nCoP process differs, however, in that it uses pulse plating technology for controlling and building fully dense, nano-grain size deposits leading to improved material properties as compared with standard polycrystalline electro-deposited coatings. As the coating is built up, it remains fully dense and nanocrystalline in structure. The process uses no constituents on the EPA lists of hazardous materials, nor does it generate hazardous emissions or by-products. Significant reductions in energy consumption and increases in throughput can be achieved with the nCoP process as a result of higher overall plating efficiency (approximately 90 percent for nCoP compared to less than 35 percent for EHC).
In addition, nCoP has a high deposition rate ranging from 0.002 to 0.008 inch (50 to 200μm) per hour depending on current density, in contrast to EHC which typically plates at 0.0005-0.001 inch per hour.
Surface Morphology and Coating Integrity: Figure 2 (a) is an optical micrograph (500x magnification) of an nCoP coating showing the surface morphology typically observed in nanocrystalline materials. Visually, nCoP coatings are uniformly smooth and shiny, similar to EHC. Microscopically, nCoP has a fully dense structure, free from pits, pores and microcracks as shown in Figure 2(b).
Nanocrystalline Microstructure: X-Ray diffraction (XRD) analysis was performed on polycrystalline, nanocrystalline and amorphous nCoP coatings as a means of determining the crystal structure and texture, and for estimating the average grain size of the material. Figure 3 shows a typical XRD pattern of a nanocrystalline cobalt phosphorous electrodeposit produced under the SERDP project work.
Hardness and Composition: As a result of Hall-Petch strengthening, nanocrystalline alloys like nCoP display significant increases in hardness and strength relative to their coarser grained counterparts due to their ultrafine grain size. Hardness of samples was determined using ASTM E384. Microindentation tests were completed using Clark Microhardness Tester CM-700AT with an applied load of 100 g and sample thickness of 0.004 inch.
Corrosion Resistance: In salt spray testing, nCoP has exhibited superior corrosion resistance to EHC. Salt spray corrosion testing was conducted according to the requirements of ASTM B117. Figure 6 shows the ASTM B537 Protection Rating as a function of exposure time for nCoP and EHC. nCoP performed very well, decreasing to only a protection/appearance rating of 8 after 1,000 hrs exposure time, compared with a rating of less than 2 for EHC after the same exposure time. Note that the nCoP coating was 50 percent thinner than the EHC coating.
Wear Resistance and Lubricity: As shown in Table 3, pin-on-disc sliding wear testing indicates that nCoP exhibits less wear loss than EHC. In addition, the wear loss of the mating material is significantly less severe. nCoP has a lower coefficient of friction than EHC, resulting in enhanced lubricity.
Hydrogen Embrittlement: The high plating efficiency of the nCoP process leads to significantly less hydrogen generation at the cathode compared to EHC processes, thus minimizing the likelihood of hydrogen uptake and subsequent embrittlement of susceptible materials (i.e., high-strength steels). Tests conducted in accordance with ASTM F519 indicate that the standard hydrogen embrittlement relief baking procedures for EHC can be applied to the nCoP to fully eliminate the risk of embrittlement.
Scale up and demonstration/validation of the nCoP technology was performed at the Fleet Readiness Center in Jacksonville, Fla., as part of the Environmental Security Technology Certification Program (WP-0411) project.
The plating process works similarly to that of many electrodeposition processes in that parts still go through a cleaning and an activation process to ensure optimal conditions for plating. The difference is in the plating step itself: all present-day depot (and most commercial) electroplating uses direct current between the cathode and anode to build the coating. The nCoP technology uses pulse plating to control the nucleation and growth of the coating material and create a nanocrystalline grain structure. Pulse control allows the optimal ratio of grain nucleation and growth, which determines the final grain size of the material. The pulse plating process is achieved by the use of a high capacity pulse plating power supply designed and built to deliver 1,500-amp peak and 500-amp average current using a particular set of pulse conditions.
A large portion of the validation and producibility testing involved electrodeposition of various material substrates and components with different sizes and complex geometries to better understand the capabilities and limitations of the technology. Much of this focus included looking at the best configuration for anodes, the use of thieves, masking/demasking, activation and plating of different alloys, non-destructive testing, coating thickness uniformity and appearance, grinding, and plating bath stability just to mention a few. Initial coupon trials also were conducted on flat plates and internal diameter surfaces.
Component Plating Trials
Component plating trials also were conducted to demonstrate ID plating of areas where HVOF deposits are difficult to apply. Plating of ID journals was demonstrated by using cobalt plated titanium anode rods as well as titanium basket anodes filled with cobalt pieces. Additional proposed classes of demo components include a P-3 MLG actuating cylinder ID section and crash crane hydraulic cylinder.
Ruben A. Prado, CEF, is with the Naval Air Systems Command, Materials Engineering Laboratory in Jacksonville, Fla. Diana Facchini, Neil Mahalanobis, Francisco Gonzalez and Gino Palumbo are with Integran Technologies Inc. in Toronto, Canada.