Hard Chrome Plating
Hard chrome plating is a process that is commonly applied in manufacturing industries, especially in machinery. For components where wear resistance and hardness are important, surfaces are often coated with a hard chrome layer (e.g. landing gear sliding tubes, printing cylinders, engine components, shock absorbers).
New regulations, such as European Community's REACH, aim to improve the protection of human health and the environment, and have intensified the search for alternatives to hard chrome, such as cobalt-phosphor.
Hard chrome plating baths have a very limited plating efficiency resulting in long process times. Furthermore, their poor throwing power and the need to reach minimum thickness specification in recessed areas results in significant overplating elsewhere at high current density areas. Consequently, edge build-ups need to be ground leading to further expense related to the use of diamond tools and labor. An additional challenge for replacement alloy coatings is to keep the content of the alloying metal under strict control, which is significantly affected by the local current density and flow conditions.
More about hard chrome plating
Still today hard chrome plating is one of the most common plating processes. Due to the excellent mechanical properties of hard chrome layers, such as hardness and wear resistance, this coating is applied for a wide range of sliding or moving components.
Because of the toxicity of hexavalent chrome ions, alternative coatings have been pursued for years. HVFO thermal spray techniques heat-up and spray the coating metal alloy onto the substrate surface. This technique often uses alloys that also incorporate chrome. HVOF is mainly a line of sight process, hence difficult to reach surfaces (like internal diameters) cannot be coated with this technique. Other metal coatings, like cobalt or nickel based coatings, have also been presented as a Cr(VI) alternative, but hard chrome plating is still by far the most widely used process and coating.
Hard chrome plating solutions have a very limited cathodic plating efficiency (ranging from 10 to 30 % in industrial baths), due to the inherent hydrogen gas evolution and other side-reactions such as partial Cr(VI) reduction to Cr(III). Gas evolution in chrome plating promotes local agitation and for that reason no mass transfer limiting current density is observed in chrome baths, however extensive gas evolution can accumulate and inhibit chrome deposition at some locations.
Despite the high current density values being used, the low plating efficiency often results in long plating times. The standard practice to tackle under-specification areas is to plate longer and thus to overplate at locations that receive moderate and high current density areas. Hard chrome plating is often followed by grinding operations where the overplated deposit is grinded down to the target layer thickness. Overplating by default not only represents a waste of metal and plating capacity, but the grinding operations represent a huge manpower and hardware cost, especially since the grinding tools are based on a nickel matrix with diamond dispersion.
Rather than through overplating the biggest process improvement can be achieved by engineering the plating process towards a more uniform current density distribution. Due to the afore mentioned problems that can occur with extensive gas evolution, the process improvement wil often comprise a combination of a more advanced plating tooling configuration with a dedicated forced flow system.
Elsyca's Advanced Engineering Services (AES) offer different solutions for hard chrome plating.
The electrolyte characterization services collect all relevant electrochemical properties from a given bath sample as supplied by the customer. Laboratory experiments are executed to measure critical plating bath characteristics, such as conductivity, anodic / cathodic polarization and anodic/cathodic efficiency. The data collected from the plating bath characterization are used as input for the PlatingMaster v5.0 simulations, ensuring an improved accuracy of the simulation results. On its own, the electrolyte characterization already provides valuable insight in the bath characteristics, such as the allowable operating window (current density, temperature, etc.) for achieving a specified deposit quality.
Rack plateability studies enable a costumer to validate a rack/tooling design without actually having to construct it. Upon receiving the component geometry, infrastructure details and the tooling/rack design to be evaluated from the customer, Elsyca will deliver the computer simulated current density and deposit distributions that can be expected from this rack / tooling configuration when it would be implemented in the existing plating infrastructure of the customer. This solution enables plating shops to validate their rack or tooling design before proceeding to the manufacturing. If the achieved performance is not satisfactory, the costumer can update the design and re-validate it. Via iterative simulations platers can develop their own rack or tooling designs without having to physically construct and update the rack / tooling along its development stages.
Computer Aided Engineering (CAE) tooling design projects include the iterative development of rack and/or tooling configurations by Elsyca. Starting from a simulation without any tooling, the component or component position in the plating tank is optimized in a sequence of simulations. Problem zones are tackled individually by deploying localized tooling elements, such as auxiliary anodes, current robbers or insulating shields. After a process of virtual trial-and-error by means of Elsyca PlatingMaster simulations a final configuration with optimal performance is retained. The fixture which will accommodate all tooling elements is designed and the final CAD assembly and Bill of Materials (BoM) are delivered to the costumer, together with a report that describes the expected tooling performance for a given set op process parameters. Optionally 2D technical drawings can also be supplied.
Click here for an example of a CAE study (computer aided engineering) that evaluates the impact of a dedicated conforming anode system for hard chrome plating of a mandrel of piston rings.