Phosphorescence a novel property of composite electroless nickel coatings

  • 13-Aug-2009 08:41 EDT
Composite EN Coatings.jpg
This cross-sectional photomicrograph of a composite electroless nickel coating shows a uniform dispersion of fine diamond.

Coatings can be advantageous, and in many applications even essential, for proper performance, protection, lifetime, and many other factors. Selecting the proper coating for each application, therefore, is vital. But choosing the right coating can be a challenge for two main reasons. First, there are many coatings available to the automotive engineering industry. Second, parts used in this industry come in a tremendous array of shapes, sizes, base metals, etc., and are used in an equally exceptional range of climates, requirements, and usage conditions.

One category of coatings is based on electroless nickel plating. Electroless nickel (EN) is a sophisticated yet reliable chemical process with many inherent features well suited to automotive applications, including hardness, corrosion resistance, and conformity to even the most complex geometries. In addition, it is possible to add super-fine particles into the EN to form composite EN coatings. These particles can provide hardness, wear-resistance, low-friction, release, heat-transfer, and/or even phosphorescent properties.

Electroless nickel has grown to be a mature segment of the metal finishing industry since its discovery in the 1940s. EN is generally an alloy of 88-99% nickel and the balance with phosphorous, boron, or a few other possible elements depending on the specific requirements of an application. It can be applied to numerous substrates including metals, alloys, and nonconductors, with enhanced uniformity of coating thickness to complex geometries. It is this last point that most commonly distinguishes electroless from electrolytic coatings, such as chrome plating.

Composite EN is exciting given the synergies possible between the EN and particles that can dramatically enhance existing characteristics and even add entirely new properties. This makes composite EN coatings especially advantageous for:

• Meeting ever more demanding usage conditions requiring less wear, lower friction, etc.

• Facilitating the use of new substrate materials such as titanium, aluminum, lower-cost steel alloys, ceramics, and plastics

• Allowing higher productivity of equipment with greater speeds, less wear, and less maintenance-related downtime

• Replacing environmentally problematic coatings such as electroplated chromium.

Composite EN coatings are regenerative, meaning that their properties are maintained even as portions of the coating are removed during use. This feature results from the uniform manner with which the particles are dispersed throughout the entire plated layer. Particles from a few nanometers up to about 50 µm (1.97 mil) in size can be incorporated into coatings from a few microns up to many mils in thickness. The particles can comprise about 10 to more than 40% by volume of the coating depending on the particle size and application.

Coatings designed for increased wear resistance have proven to date to be the most widely used composite EN coatings in the automotive sector. Particles of many hard materials can be used, such as diamond, silicon carbide, aluminum oxide, tungsten carbide, boron carbide, and others. But the unsurpassed hardness of diamond has made this material the most common composite. Despite the expensive-sounding name, composite EN with diamond is actually comparable to the cost of similar coatings.

Certain particles can be incorporated into EN to produce a coating with all the properties of EN (such as hardness and wear resistance) as well as a low coefficient of friction, dry lubrication, and repellency of water, oil, and/or other liquids.

Most commercial use of such composite lubricating coatings in the automotive industry has been with 20-25% by volume of sub-micron PTFE (polytetrafluoroethylene) particles in EN deposits. The properties of PTFE are widely recognized, and its enhancement of EN is clearly demonstrated in industry applications as well as standardized testing. The lowest coefficient of friction is achievable when both mating parts are coated with composite EN-PTFE.

In addition to EN-PTFE coatings, newer low-friction coatings have been developed and are being increasingly adopted in the auto industry. As beneficial as PTFE is, there are certain limitations that have been overcome by the incorporation of other materials. For example, particles of certain ceramics such as boron nitride provide lubricity, are significantly harder than PTFE, and can withstand temperatures above 850°C (1562°F). This tolerance for heat allows such coatings to be heat-treated after coating to achieve maximum hardness, which is a standard post treatment for most EN coatings.

Indication is a more recent category of composite EN coatings and a novel development in the field. These coatings have all the inherent features of EN, and appear normal under typical lighting, but when these phosphorescent coatings are viewed under UV light they emit a constant lighted glow. This is a feature that can be used in two ways.

First, the presence of a colored light emission from the coating can be valuable in authenticating parts from a distinct source. This is especially promising for the identification of genuine OEM parts that otherwise can be routinely counterfeited. Its value also extends to the identification of specific manufacturing lots where conventional methods of marking are not sufficient or durable.

Second, the light can serve as an indicator layer, warning when the coating has worn off and replacement, or recoating, is necessary. This feature permits the avoidance of wear into the part itself that may cause irreparable damage to a potentially costly part, or the production of inconsistent product from a worn manufacturing device. Such a layer can be employed in one of two options. Option one is to have a light-emitting indicator layer applied to a part prior to (or under) another functional coating to signal when the functional coating has worn through. In this case, the appearance of light signals wear to the functional layer and exposure of the indicator layer.

Option two is to use the light-emitting coating by itself, whereby the disappearance of the light following periodic inspections indicates wear. Fortunately, handheld, battery-operated UV lights are readily available and make inspection for the indicator layer at the operating site fast and convenient.

All varieties of composite EN coatings share some additional general features that make them further suited for automotive applications. For example, as with any conventional EN coating, these composite coatings can be chemically stripped, leaving the substrate ready for recoating. This can be a cost-effective alternative to disposing of overly worn parts and replacing them with entirely new parts.

For certain applications, customized composite coatings have been developed to satisfy unique requirements:

• Coatings with particles of two or more materials into the same layer to provide multiple properties

• Overcoating with a conventional EN layer for greater smoothness, cosmetics, or other priorities

• An underlayer of conventional (often high-phosphorous) EN applied to ensure maximum corrosion resistance.

Michael D. Feldstein, President of Surface Technology, Inc., wrote this article for SAE Magazines.

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