Electroless nickel-phosphorus plating

Electroless nickel-phosphorus plating, also referred to as E-nickel, is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. It is the most common version of electroless nickel plating (EN plating) and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.

Unlike electroplating, processes in general do not require passing an electric current through the bath and the substrate; the reduction of the metal cations in solution to metallic is achieved by purely chemical means, through an autocatalytic reaction. This creates an even layer of metal regardless of the geometry of the surface – in contrast to electroplating which suffers from uneven current density due to the effect of substrate shape on the electric resistance of the bath and therefore on the current distribution within it. Moreover, it can be applied to non-conductive surfaces.

It has many industrial applications, from merely decorative to the prevention of corrosion and wear. It can be used to apply composite coatings, by suspending suitable powders in the bath.

Historical overview

The reduction of nickel salts to nickel metal by hypophosphite was accidentally discovered by Charles Adolphe Wurtz in 1844. In 1911, François Auguste Roux of L'Aluminium Français patented the process (using both hypophosphite and orthophosphite) for general metal plating.

However, Roux's invention does not seem to have received much commercial use. In 1946 the process was accidentally rediscovered by Abner Brenner and Grace E. Riddell of the National Bureau of Standards. They tried adding various reducing agents to an electroplating bath in order to prevent undesirable oxidation reactions at the anode. When they added sodium hypophosphite, they observed that the amount of nickel that was deposited at the cathode exceeded the theoretical limit of Faraday's law.

Brenner and Riddel presented their discovery at the 1946 Convention of the American Electroplaters' Society (AES); a year later, at the same conference they proposed the term "electroless" for the process and described optimized bath formulations, that resulted in a patent.

A declassified US Army technical report in 1963 credits the discovery to Wurtz and Roux more than to Brenner and Riddell.

During 1954–1959, a team led by Gregorie Gutzeit at General American Transportation Corporation greatly developed the process, determining the optimum parameters and concentrations of the bath, and introducing many important additives to speed up the deposition rate and prevent unwanted reactions, such as spontaneous deposition. They also studied the chemistry of the process.

In 1969, Harold Edward Bellis from DuPont filed a patent for a general class of processes using sodium borohydride, dimethylamine borane, or sodium hypophosphite, in the presence of thallium salts, thus producing a metal-thallium-boron or metal-thallium-phosphorus; where the metal could be either nickel or cobalt. The boron or phosphorus contents was claimed to be variable from 0.1 to 12%, and that of thallium from 0.5 to 6%. The coatings were claimed to be "an intimate dispersion of hard trinickel boride (Ni3B) or nickel phosphide (Ni3P) in a soft matrix of nickel and thallium".

Technology Summary

In electroless nickel plating, deposits are typically classified as functional coatings and have historically found uses in applications requiring either corrosion or wear protection, and in some cases, both. Due to the unique properties of the deposit and the uniformity of the resulting film, many other applications have emerged that capitalize on the multifunctional nature of electroless nickel. Most commercially used nickel films are deposited from solutions using sodium hypophosphite as the reducing agent. This results in films of nickel alloyed with phosphorus in the range of 1–12% by weight.

The mechanical properties of Ni+MP (PEV) deposits can be further improved not only by the co-deposition of inert particles such as Teflon, silicon carbide or boron nitride, but also by alloying with a third element, forming a ternary NiPX alloy, where X can be copper, tungsten, molybdenum or tin depending on the particular formulation. Also, by heat treatment at temperatures between 280 and 440oC, notable hardness increases can be achieved due to changes in the microstructure and precipitation of nickel phosphides. It is worth mentioning that before heat treatment, deposits with more than 10% P are amorphous, therefore, they are considered metallic glass, which has important implications.

Nickel boron (NIB) alloys also appear in the literature, although they are less commercially viable than NiP alloys. The films are generated using sodium borohydride or dimethylaminoborane as the reducing agent and can vary in boron content from 1 to 5 percent by weight. NiB films are typically used in the electronics industry where low resistivity coatings are required and also find use in industrial applications where extreme hardness and wear resistance are needed.

Electroless nickel plating, like electroless nickel plating, is only as strong as the weakest link in the process. More clearly, successful electroless nickel plating requires strict adherence to bath operating guidelines and optimal surface preparation. A well-formulated electroless nickel plating process cannot overcome inadequate surface preparation. Similarly, improperly applying electroless nickel plating will also result in a poor coating even if the substrate is perfectly prepared.

The following links are essential:

Proper surface preparation

Choice of electroless nickel

Compliance with the Operating Guidelines.

Failure to recognize the interconnectedness of these three basic elements ultimately leads to failure at some point in the manufacturing process.

Difference to electroplated nickel

One of the differences to electroplated nickel is that no external electrical current, such as from a rectifier, is used for deposition; instead, the electrons required for the deposition (reduction) of the nickel ions are generated in the bath itself through a redox reaction. This results in contour-accurate coatings with dimensions ranging from 8 µm to 80 µm, with a tolerance of ± 2 µm to ± 3 µm. However, stresses in the layer must be expected above 50 µm.

Non-conductive base bodies

Due to the electroless deposition process, it is possible to coat even electrically non-conductive materials, e.g. plastics such as polyamide. ABS (acrylonitrile butadiene styrene) is the easiest to coat with good adhesion: after etching the ABS with a chromic sulfuric acid pickling solution, the nickel is deposited after nucleation with a precious metal (palladium) even in the fine holes that the chromic acid forms by dissolving butadiene. The coating interlocks with the plastic. Nucleation is necessary for non-conductive base materials, since the electrolyte begins to deposit (almost) only on bare metal surfaces. Otherwise the electrolyte would decompose itself.

Layer properties

Nickel - phosphorus coatings are predominantly used in technology. Several properties of the coating can be controlled by the phosphorus content. A distinction is made between high (10–14%), medium (9–12%), and low (3–7%) phosphorus content.

Effective corrosion protection for coated steel requires a high phosphorus content and the deposition of a pore-free layer, which is influenced by the base material and its surface structure. Polished surfaces require different treatment than ground or turned and milled surfaces. The surface of the material also influences the adhesion strength of the coating. The thickness of a corrosion protection layer is usually at least 30–50 µm, depending on the base material and its processing.

The deposition hardness increases with decreasing phosphorus content and can be increased to values of 800 to 1100 HV 0.1 by heat treating the coating at a maximum of 400 °C and a one-hour holding time. The coating thicknesses range from 10 µm to 50 µm, depending on the application.

The adhesion strength of the coating depends primarily on the base material and the material's pretreatment. The adhesion strength can also be improved through heat treatment, which involves using lower temperatures and slightly longer holding times.

The appearance of the coating depends on the pre-processing of the base material on which the coating is deposited: blasted surfaces remain matte, polished surfaces remain shiny. Unlike electroplated coatings, the appearance of the coating can only be adjusted to a limited extent by additives in the electrolyte (e.g., brighteners). With selected components, certain coating properties such as grain boundary density can be varied and, to a certain extent, the appearance can be influenced.

Due to the high cost of this coating, layers thicker than 50 µm are rarely deposited. The deposition of 10 µm of electroless nickel takes approximately 1 h. 

Procedure

Surface cleaning

Before plating, the surface of the material must be thoroughly cleaned. Unwanted solids left on the surface cause poor plating. Cleaning is usually achieved by a series of chemical baths, including non-polar solvents to remove oils and greases, as well as acids and alkalis to remove oxides, insoluble organics, and other surface contaminants. After applying each bath, the surface must be thoroughly rinsed with water to remove any residue of the cleaning chemicals.

Internal stresses in the substrate created by machining or welding can affect the plating.

Plating bath

The main ingredients of an electroless nickel plating bath are source of nickel cations Ni2+, usually nickel sulfate and a suitable reducing agent, such as hypophosphite H2PO−2 or borohydride BH−4. With hypophosphite, the main reaction that produces the nickel plating yields orthophosphite H2PO−3, elemental phosphorus, protons H+ and molecular hydrogen H2:

2Ni2+ + 8H2PO−2 + 2H2O → 2Ni0 (s) + 6H2PO−3 + 2H+ + 2P (s) + 3H2 (g)

This reaction is catalyzed by some metals including cobalt, palladium, rhodium, and nickel itself. Because of the latter, the reaction is auto-catalytic, and proceeds spontaneously once an initial layer of nickel has formed on the surface.

The plating bath also often includes:

complexing agents, such as carboxylic acids or amines to increase phosphate solubility and to prevent the white-out phenomena by slowing the reaction.

stabilizers, such as lead salts, sulfur compounds, or various organic compounds, to slow the reduction by co-depositing with the nickel.

buffers, to maintain the acidity of the bath. Many complexing agents act as buffers.

brighteners, such as cadmium salts or certain organic compounds, to improve the surface finish. They are mostly co-deposited with nickel (like the stabilizers).

surfactants, to keep the deposited layer hydrophilic in order to reduce pitting and staining.

accelerators, such as certain sulfur compounds, to counteract the reduction of plating rate caused by complexing agents. They are usually co-deposited and may cause discoloration.

Surface activation

Because of the autocatalytic character of the reaction, the surface to be plated must be activated by making it hydrophilic, then ensuring that it consists of a metal with catalytic activity. If the substrate is not made of one of those metals, then a thin layer of one of them must be deposited first, by some other process.

If the substrate is a metal that is more electropositive than nickel, such as iron and aluminum, an initial nickel film will be created spontaneously by a redox reaction with the bath, such as:

Fe0 (s) + Ni2+ (aq) → Ni0 (s) + Fe2+ (aq)

2Al0 (s) + 3Ni2+ (aq) → 3Ni0 (s) + 2Al3+ (aq)

For metals that are less electropositive than nickel, such as copper, the initial nickel layer can be created by immersing a piece of a more electropositive metal, such as zinc, electrically connected to the substrate, thus creating a shorted Galvanic cell.

On substrates that are not metallic but are electrically conductive, such as graphite, the initial layer can be created by briefly running an electric current through it and the bath, as in electroplating. If the substrate is not conductive, such as ABS and other plastics, one can use an activating bath containing a noble metal salt, like palladium chloride or silver nitrate, and a suitable reducing agent. 

Activation is done with a weak acid etch, nickel strike, or a proprietary solution, if the substrate is non-metallic.

After-plating treatment

After plating, an anti-oxidation or anti-tarnish chemical coating, such as phosphate or chromate, is applied, followed by rinsing with water and dried to prevent staining. Baking may be necessary to improve the hardness and adhesion of the plating, anneal any internal stresses, and expel trapped hydrogen that may make it brittle.

Variants

The processes for electroless nickel-phosphorus plating can be modified by substituting cobalt for nickel, wholly or partially, with relatively little changes. Other nickel-phosphorus alloys can be created with suitable baths, such as nickel-zinc-phosphorus.

Composites by codeposition

Electroless nickel-phosphorus plating can produce composite materials consisting of minute solid particles embedded in the nickel-phosphorus coat. The general procedure is to suspend the particles in the plating bath, so that the growing metal layer will surround and cover them. This procedure was initially developed by Odekerken in 1966 for electrodeposited nickel-chromium coatings. In that study, in an intermediate layer, finely powdered particles, like aluminum oxide and polyvinyl chloride (PVC) resin, were distributed within a metallic matrix. By changing the baths, the procedure can create coatings with multiple layers of different composition.

The first commercial application of their work was electroless nickel-silicon carbide coatings on the Wankel internal combustion engine. Another commercial composite in 1981 incorporated polytetrafluoroethylene (nickel-phosphorus PTFE). However, the co-deposition of diamond and PTFE particles was more difficult than that of aluminum oxide or silicon carbide. The feasibility to incorporate the second phase of fine particles, the size of a nanometer to micrometer, within a metal-alloy matrix has initiated a new generation of composite coatings.

Characteristics

Advantages and disadvantages

Compared to the electrolytic process, a major advantage of electroless nickel plating is that it creates an even coating of a desired thickness and volume, even in parts with complex shape, recesses, and blind holes. Because of this property, it may often be the only option.

Another major advantage of EN plating is that it does not require electrical power, electrical apparatuses, or sophisticated jigs and racks. 

If properly formulated, EN plating may also provide a less porous coating, harder and more resistant to corrosion and hydrogen absorption.

Electroless nickel plating also can produce coatings that are free of built-in mechanical stress, or even have compressive stress.

A disadvantage is the higher cost of the chemicals, which are consumed in proportion to the mass of nickel deposited; whereas in electroplating the nickel ions are replenished by the metallic nickel anode. Automatic mechanisms may be needed to replenish those reagents during plating.

The specific characteristics vary depending on the type of EN plating and nickel alloy used, which are chosen to suit the application.

Types

The metallurgical properties of the alloy depend on the percentage of phosphorus.

Low-phosphorus coatings have up to 4% P contents. Their hardness reaches up to 60 on the Rockwell C scale.

Medium-phosphorus coatings, the most common type, are defined as those with 4 to 10% P, although the range depends on the application: up to 4–7% for decorative applications, 6–9% for industrial applications, and 4–10% for electronics.

High-phosphorus coatings have 10–14% P. They are preferred for parts that will be exposed to highly corrosive acidic environments such as oil drilling and coal mining. Their hardness may score up to 600 on Vickers test. Note that the Vickers hardness is not easily comparable to the Rockwell scale.

Surface finish

Electroless nickel plating can have a matte, semi-bright, or bright finish.

Structure

Electroless nickel-phosphorus coatings with less than 7% phosphorus are solid solutions with a microcrystalline structure, with each grain 2–6 nm across. Coatings with more than 10% phosphorus are amorphous. Between these two limits, the coating is a mixture of amorphous and microcrystalline materials.

Physical properties

The melting point of the nickel-phosphorus alloy deposited by the EN process is significantly lower than that of pure nickel (1445 °C), and decreases as the phosphorus content increases, down to 890 °C at about 14% P.

The magnetic properties of the coatings decrease with increasing phosphorus contents. Coatings with more than 11.2% P are non-magnetic.

Solderability of low-phosphorus coatings is good, but decreases with increasing P contents.

Porosity decreases as the phosphorus contents increases, while hardness, wear resistance, and resistance to corrosion increase.

Surface coating systems

To reliably prevent co-coating of system components such as tanks, pumps, etc., these components are often made of stainless steel and artificially passivated, then subjected to a low positive voltage (tank protection) so that any nickel deposited is immediately dissolved. A passivated stainless steel cathode usually serves as the counterelectrode, on which nickel is then deposited as in a purely electroplating process. However, if the electrolyte is incorrectly prepared, if foreign substances are introduced, or if the system's passivation layer is destroyed, for example due to mechanical damage, the electrolyte can spontaneously decompose, resulting in heavy metal deposition.

The equipment and baths used for electroless nickel plating are more complex than those used for electroplating processes. Because the nickel ions are contained in the bath mix, the electrolyte "bleeds." The stabilizers, temperature, and pH value in the bath must be kept constant within certain tolerances; this is achieved using metering pumps and regular monitoring of the values. After approximately 1–2 weeks, the nickel in the bath is consumed, and a new bath must be prepared. The bath age is expressed in so-called MTOs (metal turn overs). The costly bath management and the slow deposition rate lead to significantly higher costs than with electroplating. 

Deployment requirements and shift selection

Special features of chemical nickel coatings:

Higher dimensional accuracy than electroplated nickel coatings (no “dogbone” effect due to increased deposition on outer edges)

High corrosion resistance, including against alkaline media

High hardness and resistance to abrasive wear

Medium-phosphorus coatings are used in approximately 60% of all applications, high-phosphorus coatings in 30–35% of cases and low-phosphorus coatings in 5–10%.

If the electroless nickel coating is to achieve the highest possible hardness without the heat treatment required for precipitation hardening in the range of 350 °C, as is the case with many aluminum alloys or hardened steels, low-phosphorus electroless nickel coatings are generally used. If the substrate material allows for appropriate heat treatment, medium- or high-phosphorus coatings can be used. The latter are often used as corrosion protection coatings due to their amorphous structure in the deposited state. In this case, no heat treatment should be performed to maintain the amorphous state.

Heat treatment of high-phosphorus coatings is carried out when, in addition to the high hardness that can be achieved, the higher elongation at break of the high-phosphorus coatings is relevant. If both high hardness and good corrosion resistance are required, medium-phosphorus coatings are advantageous. Due to the almost twice the deposition rate, these are generally also more cost-effective than high-phosphorus coatings for the same layer thickness. To achieve the highest possible corrosion protection, the coatings must not have any pores that extend through to the base material, which is the case with layer thicknesses of 20–25 µm and above. Pores can form because elemental hydrogen is always produced during deposition, which can create bubbles and form the origin of a pore if suitable measures such as the use of surfactants or sufficient convection are not taken.

The high dimensional stability, combined with the very high corrosion resistance and the very high hardness, is unique for electroless nickel. As with all coatings, the protection achievable in practice depends on the specific load spectrum and the applied layer thickness. The corrosion protection provided by electroless nickel coatings on ferrous substrates is generally anodic in nature, such as tin /nickel coatings. Alternatives from the field of cathodic corrosion protection include zinc and zinc alloy coatings. With regard to point 3, only electroplated hard chrome coatings exhibit a higher layer hardness. In summary, however, it can be stated that an electroless nickel coating, like basically any other coating system, is unique in its property spectrum, so that depending on the load spectrum, requirements for dimensional stability and layer thickness, as well as the coating costs involved, electroless nickel can meet the requirements in a unique way.

Applications

Electroless nickel-phosphorus is used when wear resistance, hardness and corrosion protection are required. Applications include oilfield valves, rotors, drive shafts, paper handling equipment, fuel rails, optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment.

Due to the high hardness of the coating, it can be used to salvage worn parts. Coatings of 25 to 100 micrometers can be applied and machined back to the final dimensions. Its uniform deposition profile means it can be applied to complex components not readily suited to other hard-wearing coatings like hard chromium.

It is also used extensively in the manufacture of hard disk drives, as a way of providing an atomically smooth coating to the aluminium disks. The magnetic layers are then deposited on top of this film, usually by sputtering and finishing with protective carbon and lubrication layers.

Its use in the automotive industry for wear resistance has increased significantly. However, it is important to recognize that only End of Life Vehicles Directive or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.

Ni/P coatings on light metal components

Light metals such as aluminum and magnesium alloys can be used as materials under atmospheric conditions because a dense oxide layer shields the reactive base material from aggressive environmental influences. However, for many applications, the chemical and mechanical stress profile of a component exceeds the resistance of these oxide layers. For such applications, surface finishing is essential. Since complex geometric shapes are common for light metal components, electroless metal deposition, particularly electroless nickel plating, is preferred. Optimized pretreatment sequences for the base material are responsible for suppressing the formation of partial oxide layers in the initial phase of the coating. Oxide islands limit the adhesion between the base metal and the coating and can thus lead to component failure. Once this key process step is controlled, the entire range of properties of Ni/P alloy layers is available for component optimization.

Applications of Ni/P coatings

Chemical nickel coatings are often used in the following areas due to their excellent dimensional stability, very high hardness and excellent corrosion protection:

Automotive and aerospace industries, mechanical engineering: Components for injection systems, air conditioning compressors, coolant pumps, airbag systems, synchronizer rings, servo valves, chassis components, bearing journals, flanges, compressor wheels, textile machine components, etc.

Chemical industry, petrochemicals: heat exchangers, turbine blades, spray heads, pressure vessels, reactors, etc.

Electronics: PC drive components, connectors, base layer for solder joints, electromagnetic shielding of sensitive components, etc.

Human medicine: base layers for metal-ceramic solder joints (joint prostheses) etc.

Plastic metallization: decorative and functional layers

Optics: Coating of metallic base bodies with high-phosphorus-containing layers for the production of mirror optics by polishing, diamond turning or diamond milling 

Printed circuit boards

Electroless nickel plating, covered by a thin layer of gold, is used in the manufacture of printed circuit boards (PCBs), to avoid oxidation and improving the solderability of copper contacts and plated through holes and vias. The gold is typically applied by quick immersion in a solution containing gold salts. This process is known in the industry as electroless nickel immersion gold (ENIG). A variant of this process adds a thin layer of electroless palladium over the nickel, a process known by the acronym ENEPIG.

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