For many years, conventional electroless nickel systems used lead compounds to control the deposition reaction, and cadmium to add brightness and luster to the deposit. Recent requirements defined in the European Union's Restriction of Hazardous Substances (RoHS) and End of Life Vehicle (ELV) directives, and the U.S.'s National Sanitary Foundation Standard for Food Materials (NSF/ANSI 51-2005) have accelerated use of EN systems that do not contain lead or cadmium.
The most commonly used EN systems deposit an alloy of nickel and phosphorus, with other materials present in minute quantities. Many deposit properties are a direct function of the amount of phosphorus, which can vary from <1 wt% to as high as 13 wt%. The amount of phosphorus in the deposit is determined by the chemical formulation of the bath and operational parameters applied during plating.
Nickel-phosphorus (Ni-P) EN deposits, whether from RoHS- and ELV-compliant systems (hereafter referred to simply as compliant systems) or conventional systems, share certain characteristics—smoothness, high hardness, and a high degree of uniformity. These characteristics result in excellent corrosion and wear resistance and consistent deposit thickness regardless of part geometry.
Electroless nickel deposits offer excellent corrosion protection, provided there is total encapsulation of the substrate. They are especially well suited in applications where a component will be exposed to both corrosion and wear. Advantages have been seen with some compliant systems compared with conventional EN with the same phosphorus content. As with conventional electroless nickel systems, some compliant systems offer far better corrosion protection than others, and different systems may need to be evaluated to achieve optimal performance in a given application.
Wear—the loss of material from a surface by means of some mechanical action—can be addressed by EN deposits from compliant and conventional systems. As-deposited, EN deposits have a hardness ranging from 400–700 HV. Hardness can be readily increased to about 1,000 HV by heat treatment at 400°C for one hour. There is virtually no difference in hardness between compliant and conventional EN deposits containing the same amount of phosphorus. There are many different types of wear, which are briefly described below:
- Abrasive wear, which results from particles or protuberances rubbing the surface
- Adhesive wear, caused by contact bonding between two surfaces
- Surface fatigue, which is caused by fracture or degradation in the surface
- Erosion, caused by impinging particles, liquid or gas
- Fretting, which results from vibration between two contacting surfaces.
The wear properties of an EN deposit can be modified by incorporating other materials into the deposit. The most common materials used for this purpose are PTFE (polytetrafluoroethylene, or Teflon) and silicon carbide. If done properly, the incorporated particles are evenly dispersed in throughout the deposit. This is a big advantage over surface treatments, because more desirable particles are exposed as the surface wears away.
The wear properties of compliant EN deposits match those of conventional systems. The main factor impacting wear properties is the composition of the alloy deposited. Choosing the correct electroless nickel process for the specific wear type is critical and is primarily driven by the deposit alloy and is not significantly impacted by the stabilizer system utilized. Table 1 shows relative resistance of various EN alloys to different types of wear.
Table I Relative Wear Resistance of EN Alloys
EN Versus Electroplating
Unlike electroplating, electroless plating has an unmatched ability to coat parts completely with a deposit of uniform thickness, regardless of part size or geometry. This phenomenon makes EN suitable for the salvage of worn and mis-machined engineering components and ensures total coverage in components which would be difficult or impossible to plate using traditional electroplating processes.
Uniform deposit thickness over the entire part can be a big advantage for appearance, corrosion protection, and other attributes, especially on complex shapes where electroplated deposits can vary dramatically in thickness. Since both compliant and conventional EN deposits are both plated using a chemical reducing agent, both types of system offer the same deposit uniformity.
Electroless deposition also allows formation of conductive layers on plastics and other non-conductive substrate materials. Non-metallic surfaces lack catalytic properties and therefore require activating treatments that make their surfaces catalytic to electroplating. Solutions containing a catalytic metal such as palladium are generally used for surface activation. In plastic plating, activation allows for subsequent application of electroplated deposits onto a surface which would otherwise be non-platable. Lead- and cadmium-free EN systems have been used for decades to activate non-conductive substrates, notably plastics.
Other applications also take advantage of electroless nickel's ability to plate non-conductors. For example, synthetic fabric is plated to ground it and avoid static electricity, in electronics shielding applications. Both nickel and copper are considered highly effective electromagnetic shielding materials. On non-conductive materials, electroless plating can provide shielding either as a stand-alone coating or as a base layer for subsequent electroplating. No functional difference has been found between compliant and conventional EN processes in these applications.
Historically, many efforts have been made to make ternary EN alloys because it was believed that these would give considerable advantages over conventional Ni-P deposits in certain applications. In practice, these alloys either rarely gave advantages over existing products or the advantages were insufficient to compensate for increased difficulty in process operation.
But new ternary alloys are still being developed, and the latest to be promoted is a nickel-phosphorus-tin deposit that has found some commercial application. The process is also compliant and is accepted in food contact applications.
The first application for the Ni-P-Sn deposit was coating food handling equipment to prevent wear and corrosion. The machines need to be cleaned regularly to ensure there is no contamination, and the preferred cleaning method relies on use of very strong oxidizers. Historically, EN has been used in this application, but even high-phosphorus EN deposits cannot resist the cleaning chemistry over an extended period of time. The ternary EN can withstand nitric acid attack for up to 4 hr, and there have been circumstances where up to 6 hr of resistance has been achieved.
Another advantage of the ternary alloy is its ability to maintain properties at high temperature. The deposit resists hardening or crystallization at 300°C and maintains its corrosion resistance.
This is extremely important when using the alloy as the initial layer in a duplex EN coating, such as in a new application combining the ternary alloy with a very low-phosphorus (1% P) compliant EN top layer. The resulting deposit can be heat treated at 300°C for 1 hr to give very high hardness while maintaining the excellent corrosion resistance of the ternary undercoat. Testing of this duplex system has shown more than 2000 hr neutral salt-spray (NSS) resistance and excellent substrate protection in copper-accelerated salt-spray (CASS) testing combined with a hardness in excess of 900 HV.
This combination of deposits has replaced hard chrome in some applications. Hard chrome deposits on cylinders, for example, often use an undercoat of bronze. The cylinders required machining after bronze and chrome plating, and the bronze plating chemistry used cyanide. Using the low-phosphorus/ternary duplex EN deposit eliminated cyanide and secondary machining, and gave improved corrosion resistance.
Duplex ternary/low-phosphorus EN is not a viable replacement for all hard chrome applications. It has a higher coefficient of friction, wear resistance is normally not as good and chemical cost is increased. However, the duplex process is an alternative to existing options in applications requiring both abrasion and corrosion resistance.
Another application for this ternary alloy is improving properties of EN deposits that are to be blackened. Methods to commercially blacken EN have been known and utilized for many years, but because the black color is formed by oxidation of the EN surface, blackened deposits were unable to offer any real corrosion resistance to a substrate. And, once the substrate started to corrode, the black color would also be lost.
Using the ternary EN, which resists the oxidizing solution, results in a black finish with improved corrosion resistance. This has opened opportunities for black EN in areas as diverse as firearms, absorbent solar panels and decorative internal fittings for houses and offices. In firearms, blackened EN is very consistent in color, does not require oiling and does not use hexavalent chrome. It is also a fairly hard deposit. For furniture and fittings, the conductive nature of the deposit means it is possible to apply electrophoretic lacquer to parts to give an even greater luster, improved abrasion resistance and improved corrosion protection.
Other new applications for EN are based on developments in nickel-boron (Ni-B) EN formulations and Ni-P formulations using non-metallic stabilizers.
A RoHS-compliant electroless Ni-B bath has found new application in manufacturing of specialized liquid measurement and metering systems. In service, these devices use ceramic discs plated with a compliant N-B deposit on either side of a flowing solution. Nickel-boron was used rather than a conventional Ni-P process because RoHS compliance was required and boron-reduced systems have higher reduction energy than hypophosphite systems used to create Ni-P deposits. This higher reduction energy allowed deposition of the nickel onto non-conductive ceramic surfaces, which was not achievable with conventional Ni-P chemistry.
Finally, most early compliant EN systems using metallic stabilizers had issues activating onto copper and copper alloy substrates. One method to overcome this is to use organic stabilizers. An application where this was clearly seen was in Europe, where bronze automotive parts had been successfully plated in a conventional bright EN for many years. However, the existing bath had to be changed to an ELV-compliant chemistry, which had relatively poor initiation and coverage of the bronze substrate.
The answer was use of organically stabilized EN, which showed none of the initiation issues seen with the metallically stabilized chemistry and in fact offered several advantages in terms of speed and stability. The manufacturer and OEM were so pleased with the results that the new process was rushed through the approval process in a month.