What to Consider When Applying a Classic Black Finish

Q. What are the best options for a durable black finish? 

A. The classic black coatings used by general metal finishing (GMF) shops include nickel alloys, chrome-based overplates and chrome conversion coatings. The chrome-based conversion coatings are called chromates and have a long history. Formulations and performance differ, but they all share the same downsides.

First, shops are reluctant to use chrome-based chemistries because of environmental concerns. Also, the nickel alloy overplates and chromate conversion coatings are thin, often fragile and inherently unable to provide the corrosion protection OEMs require. Many try to avoid plating chrome. Mercury aside, hex chrome is the worst among heavy metal contaminants, and adapting to what’s required for its application is not an appealing proposition. 

Recent developments in nickel overplates and co-deposited blackening agents show promise in delivering both a dark black surface and high hardness values. The process has also been adapted to use an electroless nickel rather than an electroplated nickel underlayer and overplate. This allows the cost-effective plating of complex part geometries and designs with a uniform nickel thickness.

GMF platers will be reassured that the process uses an industry workhorse, an electroless nickel underplate and overplate, a chemistry they know well that is already operating in their facilities.

This alternative process uses a standard EN underplate followed by a thin mid-phos electroless NiP deposit layer. This thin, 5 micron electroless NiP overplate is a lead-free ELV, WEE and RoHS compliant layer serving as the seed layer and a topography matrix for the blackening deposit. 

The last step is immersing the parts in an alkaline blackening bath for approximately 15-20 min. at 50°C. The treatment time is adjusted based on the complexity of the part geometry and the solution dynamics of the tank. 

The final result is an ultra-uniform, high surface area matrix that does not necessarily take on the typical appearance of electroless nickel. Rather, it has a distinctive topography that is the key to creating an extremely dark black surface. The deposit shows that the matrix film has some depth versus simply being superficial, and this undoubtedly contributes to the lower reflectivity of the deposit and its unusually dark and highly uniform color.

Notably, the final deposit can be heat treated to improve durability and wear resistance. There is, predictably, a minor loss in the darkness level of the deposit as a result of the baking process. However, compared to conventional processes, even after baking, the darkness level improves by 60-70 percent. L values (SCI) for the standard process are between 30 and 35; this new process ranges from 8-12 L value (SCI). Lower reflectance, critical for dark appearance, is also achieved with values as low as 1 percent. Conventional processes are in the 8- to 10- percent reflectance range. 

After heat treatment, depending on the time and temperature applied, a Vickers hardness number of 600-800 can be achieved. 250°C for 60 minutes is a typical baking cycle.

Improved L values and low reflectance values appear to be the result of two symbiotic factors: the deposited matrix that has been developed using a specific mid-phos electroless NiP overplate, and a blackening agent electrolyte that uniformly etches and deposits a Ni2O3 enriched surface. 

The deposit is atypical of etched electroless nickel surfaces in its micro-uniformity and high surface area. This leads to a darker and more uniform black surface with lower reflectivity. The process also keeps the underlying higher phos nickel underplates “intact.” This produces higher corrosion resistance in the final protective layer. 

What is the key distinguisher of this new technology is the film’s morphology. Chemists describe these deposits as an entirely new film “shape.” What they have developed is a nickel phos surface that has a strong affinity for a secondary blackening agent. The subsequent Ni₂O₃ growth layer is comprised of shortened, high-density topography. The combination of receptive topography and unique blackening agent is what creates the final result.

What improves the durability of this deposit versus conventional deposits? The major contributor is the deposit nickel matrix that is uniform, dense and highly Ni₂O₃ enriched. The mechanism that has been studied shows that during the heat treatment process there is further enrichment and oxidation of the Ni₂O₃ layer to NiO, and this oxidation state of nickel oxide is less fragile. 

An auger scan of the surface before and after heat treatment shows significant reduction in percentage of oxygen at the surface, denoting a conversion from Ni₂O₃ to NiO with a corresponding increase in surface durability.   

 

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Originally published in the April 2017 issue.

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