Composite Electroless Nickel Coatings for the Gear Industry
1. Corrosion resistance,
2. Exceptional hardness,
3. Outstanding wear resistance,
4. Perfect uniformity of coating thicknesses on all geometries,
5. Lubricity,
6. Nonmagnetic properties,
7. Solderability,
8. Applicability to nearly all metals and alloys.
Since EN can be produced with a bright and reflective finish similar to chrome, it is also widely used in applications where cosmetics are critical.
Composite Electroless Nickel
The incorporation, or codeposition, of specific finely sized particles within EN coatings can greatly enhance their existing characteristics and, in some cases, add an entirely new feature to the coating.
In typical composite coatings, the fine particulate matter can range in size from 0.1µ to about 10µ and can be loaded up to about 40% by volume within the coating. The ratio of codeposited particles to the metal matrix in composite electroless plating can be adjusted to a fixed and constant ratio. Most commercial practices, however, focus on 18-25% by volume of the particle within the matrix. Because of the uniform manner in which the particulate matter is codeposited, these coatings are known as regenerative, maintaining their properties even when portions of the coating are removed by prolonged use.
Though it is possible to generate thicker coatings, deposits of 0.5-1.0 mil are adequate for most commercial applications. Composite EN coatings on gears are generally 0.8-1.0 mil in thickness.
While a wide variety of particulate matter can be codeposited, commercial composite electroless plating is limited to just a few types of particulate matter for three general purposes. For increased wear resistance and hardness, diamond or other hard particles are cornmonly codeposited within EN. Enhanced lubricity is achieved with the incorporation of polytetrafluoroethylene (PTFE) and certain inorganic particles that reduce the coefficient of friction. In addition, a new generation of composite coatings containing light-emitting particles has recently been developed. All three primary categories of composite coatings and their applicability to the gear industry will be discussed in the remainder of this article.
Wear Resistance
Composite EN coatings are most commonly used to improve the wear resistance of machinery parts. Wear to gears and many other machinery parts causes undesirable replacement costs, downtime, mechanical malfunction and inconsistent output or product.
Various test procedures have been employed to evaluate the degree of wear resistance achieved. The Taber wear test is the most common. It evaluates the resistance of surfaces to abrasive rubbing produced by the sliding rotation of two unlubricated, abrading wheels against a rotating sample. This test measures the worn volume.
The merits of composite EN coatings in comparison to conventional EN coatings as shown by Taber wear testing were illustrated by Parker, who measured the wear resistance of miscellaneous composites containing diamond, carbides, and aluminum oxide. The composites he tested, however, employed particulate matter of different sizes, and neither the concentration of particles within the matrix nor the surface roughness for the coating prior to testing was revealed. Accordingly, based on the data published, no definitive conclusion can be drawn about the performance of one particle versus another.
What is clear, however, is that all of the composites, regardless of the particulate matter incorporated, performed substantially better than the EN without any particulate matter.
Composite EN coatings with boron nitride and other types of inorganic particles have recently been investigated and commercially developed. These inorganic particles, compared to PTFE, have certain significant advantages that fulfill the demands of the gear industry and other industries, including:
1. Temperature resistance to above 600'C.
2. Exceptional hardnesses to about 1,000 Hv.
3. Greater abilities to take direct loads
4. Lower costs than composite PTFE coatings.
Light Emission
Light-emitting composite EN coatings are a recent and exciting development in the field. These coatings have all the inherent benefits of EN, but, when viewed under UV light, emit a distinct, brightly colored light. This novel property has two main uses. First, the presence
of light can be valuable in authenticating OEM machinery parts (for example, in the aircraft industry). Second, the light can serve as an "indicator" layer, warning when the coating has worn off and replacement or recoating is necessary before the part itself is worn and/or produces inconsistent product. This indicator layer can even be applied between the part and another coating to indicate when the first coating has worn through to the light-emitting layer.
Hand-held, battery operated UV lights are readily available and make of the indicator layer fast and convenient at the operating site. In the gear industry, this coating is particularly advantageous for applications employing very expensive gears; it avoids wear into the base metal and provides the opportunity to recoat the gear with a wear resistant coating. It is also beneficial in situations where operating with worn components must be avoided to insure consistant performance or product.