Self-lubricating electroless nickel coatings

Published on 03/02/2022 by Giacomo Bordiga

SEM photo of Niplate 500 PTFE electroless nickel coating

The electroless nickel plating process makes it possible to create composite coatings with a matrix of nickel and functional particles. Through the process, it is possible to incorporate into the electroless nickel layer the particles introduced into the treatment bath.

This article describes the friction reduction and anti-adhesion properties of PTFE particles incorporated in the coating.

Historical background

In the early years of the industrial development of electroless nickel plating, a way was found to co-deposit self-lubricating particles such as PTFE (Polytetrafluoroethylene) by dispersing them in the electrolite for electroless nickel plating.

Other particles, which have the advantage of reducing the coefficient of friction, have also been experimentally incorporated into electroless nickel. These are mainly molybdenum disulphide (MoS2), graphite (C) and hexagonal boron nitride (hBN). These molecules have a flat hexagonal crystal structure, which means that the crystals can slide in parallel planes with very low friction. In order to exert their anti-friction power, several crystalline planes must slide over each other. The crystalline particles with lamellar structure all have a very low and comparable friction coefficient, but each has its own limit temperature for use. In this respect, the best performing product is hBN, which is also resistant to high temperatures.

The encapsulation of these hBN micro and nano powders in the electroless nickel layer is feasible, but due to plant problems caused by the tendency of the particles to agglomerate and coagulate, it is used in niche sectors and has never been widely developed in industry. In addition to the implantation problems, there is the need to encapsulate particles in high quantity so that, with sliding, there is an adequate amount of flakes on the surface to produce several sliding planes at the interface, which are effective in lowering friction.

State of the art

PTFE is the most widely used molecule in the field of friction reduction and is part of the family of fluorinated polymers that possess the common characteristics of low friction coefficient and non-stick properties. In particular, the PTFE polymer has a high continuous use temperature of 250°C and an initial degradation temperature of over 300°C. PTFE has the great advantage of being able to be used in an aqueous dispersion in electroless nickel baths with a deposited particle size of about 300nm (0.3 µm). Depending on the formulation of the electrolyte, PTFE particles are co-deposited in greater or lesser amounts, providing variations in the coefficient of friction proportional to the amount of PTFE available. PTFE, having a plastic structure, allows it to be “smeared” by the sliding of the counterpart on the surface, thus creating a very thin veil of solid lubricant at the interface of the two metals.

Surface view of a Nickel PTFE composite coating with 7% PTFE
Surface view of a Nickel PTFE composite coating with 7% PTFE
Surface view of a Nickel PTFE composite coating, Niplate 500 PTFE, with 30% PTFE
Surface view of a Nickel PTFE composite coating, Niplate 500 PTFE, with 30% PTFE

The dry sliding friction coefficient of electroless nickel on a metal counterpart is 0.4-0.5. If the electroless nickel layer contains a good amount of PTFE (at least 20-25% by volume), it lowers its coefficient of friction to about 0.1-0.12, which is very close to that of pure PTFE, which is 0.05. Niplate® 500 PTFE has a high percentage of PTFE (25-35%) and the coefficient of friction is 0.1 against steel and 0.08 for sliding on itself. The coating is suitable for sliding on surfaces with low surface roughness, low contact forces and where there is no particularly abrasive wear. In applications where abrasive wear occurs, electroless nickel coatings containing hard ceramic particles such as Niplate® 600 SiC work well.

Another important characteristic of PTFE is its anti-adhesion due to its chemical inertness, which does not bond with the materials it comes into contact with. While it is very useful as a release agent, the adhesion of the pure PTFE layer, applied as a coating, is poor. A significant example of this is non-stick cookware, where the coating unfortunately peels off very easily from the aluminium despite adequate sandblasting and preparation. The inclusion of PTFE in a very uniform and very adherent nickel matrix with the base metal avoids the problem of its poor adhesion and allows its properties to be used on internal cavities with high dimensional accuracy.

The high PTFE content of Niplate® 500 PTFE makes it preferable to other coatings of the same type but containing a lower content. Niplate 500®PTFE provides excellent release and is used, for example, on extruders for plastics to reduce the pressures required for extrusion. Usually, due to the friction produced by the high extrusion pressure, burns of the polymer to be extruded can be generated in the chamber, but these are eliminated by the sliding capacity of the deposit, which allows a lower extrusion pressure and consequently less friction. It is used on sealing plates, packaging films, gluing and labelling machine parts. Niplate 500®PTFE is very useful to avoid scale build-up on water solenoid valve nozzles and sludge and biofilm build-up on heat exchangers. A common use is on guides without lubrication, where it can withstand high dry running speeds.