In aerospace and defense, the use of high strength steel is common for structural components like landing gear. While these materials offer good strength, they have poor wear and corrosion surface properties, requiring protective coatings for wear and corrosion resistance to be added to the steel. Engineering hard chrome plating (EHC - also known as engineered hard chrome, functional hard chrome, electrolytic hard chrome and industrial hard chrome plating) is widely used to protect the surface of the steel. Due to increasingly strict regulations on toxic hexavalent chromium emissions, a trend that will undoubtedly continue, production costs are on the rise and there is a huge push to find environmentally benign alternatives.
In the 1990’s, work started on alternatives to hard chrome utilizing thermal spray processing. The most popular solution being high velocity oxygen fuel (HVOF) applied WC-Co-Cr, a ceramic-metal composite (or “cermet”) of tungsten carbide in a cobalt-chrome matrix. In the early 2000’s, Integran began work on nanocrystalline cobalt plating, known as Nanovate CoP, with funding from many of the same agencies that supported development of the HVOF materials and processes. These two materials are considered the leading alternatives to hard chrome. While the HVOF materials have a number of interesting attributes, there are a number of reasons to prefer Nanovate CoP plating.
1. Spalling Resistance
While the WC-Co-Cr HVOF coatings can be very hard (>1000VHN), lending to good wear resistance, cermets can tolerate very limited strain at the material elastic limit. While designers can compensate using models and FEA analysis to control strain distributions, reality does not always match predictions and spalling in high load, high strain, and high fatigue applications is a possibility. Often these issues can arise many years after a part is put into service, particularly when side loads or geometry transitions in the design (filets, weld lines and other features that are difficult to accurately) are present.
In contrast, Nanovate CoP is a tough material with a unique combo of a high elastic limit (above 1%), high strength, good adhesion AND good ductility, which lends to impressive spalling and crack resistance in high strain applications.
These material properties, coupled with a lower coefficient of friction, also contribute to corrosion and sliding wear resistance that exceeds that of hard chrome.
2. Setup Cost/Infrastructure
The HVOF process requires costly and fundamentally different equipment (such as spray guns, dust handling equipment, sound proof enclosures, robotic arms etc.) as well as considerably different skilled employees from electroplating of hard chrome. The surface finish requirements and high hardness of the cermet material also means more elaborate polishing and grinding equipment.
Nanovate CoP is a drop in replacementfor hard chrome, both in terms of plating as well as grinding and polishing infrastructure.
3. Operational Costs
HVOF coatings for sliding wear applications are widely recognized to have significant post processing costs two reasons. Firstly, the HVOF coating is much harderthan hard chrome, and therefore it takes longer to post-process the material to the same surface finish. Secondly, in order to maintain similar seal life, the surface needs to be super-finished (meaning Ra<4 µin) so it does not “chew” the seals prematurely, further extending the time required to correctly post process the parts. For reference, hard chrome is often specified as Ra of 4-12 µin.
Nanocrystalline cobalt plating is specified to the same surface roughness and has similar hardness to hard chrome, which means the polishing times are identical.
Aside from surface finishing, the Nanovate CoP process can also achieve cost reductions over even hard chrome plating, as it is also very fast (up to 200microns deposition rate per hour).
4. Line of Sight (LOS)/ Non-Line of Sight (NLOS) Applications
Because HVOF is applied through spraying, it is fundamentally a line of sight (LOS) process. In landing gear applications, chrome-free NLOS surfaces are typically coated with electroless nickel, meaning that two finishing processes are required.
Nanovate CoP is amenable to both LOS and NLOS surfaces, just as with hard chrome plating.
5. Supply Chain Availability
Integran’s Nanovate CoP can be licensed into any electroplating shop in your supply chain, and options are generally plentiful.