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Composite coatings incorporating particles of two or more materials—in this case, Teflon and diamond—can provide both significant wear resistance and a low coefficient of friction.
Cross section through composite EN-diamond coating at magnification of 1000× shows uniform dispersion of diamond particles throughout coating thickness.
In recent months, finishing industry publications and conferences have been filled with information on the drastic reduction in the hexavalent chrome Permissible Exposure Limit (PEL). As is now well known, The U.S. Occupational Health and Safety Administration (OSHA) has slashed the PEL from 52 down to 5 μg/m3.
To date, the focus (of these issues) has been on the regulatory battle and the technical and financial challenges a drastically lower PEL would mean for shops using hex chrome. While these important considerations will continue, the purpose of this article is to look beyond these regulations to a technology with potential performance and environmental advantages over chrome.
Much of the difficulty in recent years in looking for a “chrome replacement” is the goal itself. There really is no exact chrome replacement. The periodic table of the elements is divided into little squares for a reason; each element is unique in its properties. No one material can replace chrome—or any other material, for that matter. Chrome is widely used for a multitude of applications based on its corrosion resistance, hardness, wear resistance, shiny appearance, and other properties. An across-the-board replacement is impossible, and settling for only equal performance of other materials to “replace” chrome for specific uses sets the bar too low and ignores the progress our industry has made over decades to develop new technologies.
The search therefore should be for other materials that can actually surpass chrome in each of its conventional uses. This article focuses specifically on wear resistance, and how composite electroless nickel coatings can actually surpass chrome plating in that regard.
Electroless nickel (EN) has long since been a key segment of the metal finishing industry. Its properties are widely recognized and used in an array of industrial applications. The inclusion of particulate matter within EN deposits can provide powerful enhancement of the coating’s inherent characteristics, and, in many instances, adds entirely new properties to the nickel layer. Composite EN coatings with lubricating particles such as PTFE (Teflon) are widely recognized and used around the world. So too are coatings incorporating wear-resistant particles to surpass chrome plating for hardness, wear resistance, uniformity, thermal transfer, environmental considerations, and economy.
All varieties of composite EN coatings can be applied to metals, alloys, and nonconductors with outstanding uniformity of coating thickness to complex geometries. This uniformity and the ability to plate in non-line of sight areas are important distinctions between electroless and electrolytic plating methods such as electroplated chrome. When chrome plating parts with complex geometries such as extrusion screws, molds or gears, the chrome must often be applied with uneven thickness from one area of the part to another due to inconsistencies in the electric current. While the thinnest areas of the coating may meet the specification, the remainder of the plating is too thick and may need to be re-worked to achieve the specified thickness. After plating, EN and composite EN coatings do not need to be ground, re-machined, or otherwise re-worked to get the part into proper tolerance. This benefit translates into cost, time, and material savings; as well as environmental benefits.
Composite EN coatings are applied using specialized plating baths that overcome the natural incompatibility between a plating bath and an extraordinary loading of insoluble particles. Development of patented particulate matter stabilizers (PMSs) have made composite EN plating reliable and commercially viable by treating the particles so they will disperse without agglomerating, thereby remaining viable in the bath for incorporation into the coating. Considering the surface area loading of particles in a typical composite EN bath can be approximately 800–1,500 times the preferred loading of a conventional EN bath, this technology is significant. Also enabling optimal and consistent composite EN plating are specially designed tanks and process equipment.
Proper plating chemistry and apparatus produce composite EN coatings that 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.
Depending on the particle sizes and certain plating conditions, coatings can be produced with a particle density of up to 40% by volume. Lesser densities may not provide the maximum benefit available from the particulate matter, and significantly higher densities risk premature wear of the coating since there may not be enough of the metal “glue” to prevent particles from being removed. This observation indicates that the typical wear mechanism of composite EN coatings is not wear to the particles themselves but rather wear to the surrounding metal matrix. This eventually allows the particles to be removed. Based on this understanding of the tribology involved, experienced selection of specific particle sizes for individual applications can be helpful.
As with conventional EN, composite EN coatings can be heat treated after plating to enhance their hardness and their adhesion to the substrate. Most composite EN coatings can operate at continuous temperatures of 750°F. They have a shear strength of 20,000–45,000 psi on aluminum substrates and 30,000–60,000 psi on steel substrates.
Composite EN coatings designed for increased wear resistance have been developed with particles including diamond, silicon carbide, aluminum oxide, tungsten carbide, boron carbide and chromium carbide. These materials differ not only in hardness and wear resistance, but also in their shape and other properties. Any of these factors can affect surface and performance characteristics.
Nominal particle size can range from nanometers up to about 50 μm. The smaller the particles, the smoother the coating. Those with nanometer sized particles, for example, look much like conventional matte EN coatings. Particles of about 20–50 microns yield a noticeably textured surface useful for frictional and non-reflective properties. The most common composite EN coatings for wear resistance rely on particles that are between one and eight microns in size. These coatings are smooth to the touch.
Such coatings generally have particle densities of 25–40% by volume and can provide coating thicknesses of 25–50 μm (0.001–0.002 inch), although in recent years thicknesses up to and beyond 500 μm (0.01 inch) have become popular in high-wear applications.
Hybrid combination composite coatings incorporate particles of two or more materials into the same plated layer, and can satisfy some application requirements. When both significant wear resistance and a low coefficient of friction are necessary, for example, wear-resistant particles can be combined with lubricating particles in the EN bath to produce a coating with both characteristics. Light-emitting particles can also be combined with particles of the other categories to create a more wear resistant or lubricious coating that also emits light to identify the origin of the part or to indicate when the layer is worn from use. Thicknesses, materials, particle sizes, and densities for these combination composites all depend on the specific application.
Overcoating is often used for composite wear-resistant coatings. Composites containing particles (as discussed above) are smooth to the touch and sufficient as-is for most applications. There may be, however, some particles on the surface of the coating that are only partially entrapped in the coating. When the coating is intended to contact delicate materials such as textile and paper products, these protruding particles may be deleterious or require a break-in period of use to smooth the surface.
Instead of employing mechanical means to smooth the surface or operating a coated part for a less productive “break-in” period, an overcoat can be applied. For a composite EN coating, a conventional EN overcoat layer about 5 μm thick is sufficient to cover the composite surface and provide a new surface that will be smoother and more easily leveled by use. A bright EN overcoat may also be desirable for applications previously chrome plated.
Proper selection of particle material—including size, shape, density and material—for specific applications depends on many factors including the specific wear mechanism involved. Standardized wear testing methods are instructive, but cannot substitute for controlled testing of various composites under the actual intended use conditions.
Various test methods have been used to evaluate wear resistance of different materials and coatings. In a recent study, coating thickness, nickel-phosphorous alloy, particle density, particle size range and post-plating heat treatment were all tightly controlled on two-sq-inch steel panels, leaving particle material as the only variable.
|Table I: Abrasive Slurry Wear Resistance of Composite EN Coatings|
||Wear Constant||Wear vs. Steel||Wear vs. EN|
Seven such panels were tested, with one bare steel panel and one conventional EN panel as controls. Composite-plated panels used aluminum oxide, boron carbide, diamond, silicon carbide and tungsten carbide particles. All panels were wear tested using a 5-μm alumina slurry contacting the panel surface under a constant load. Wear data was converted to a new constant equating the volume of material lost (cubic microns) per unit force (Newton) for unit length (mm). Results are shown in the table.
These and other test results, as well as numerous industrial applications, demonstrate the advantages of composite EN over chrome and even over conventional EN, which has from time to time been the subject of health and environmental questions regarding nickel metal.
Composite EN coatings use no chrome. The environmental and worker safety issues inherent with plating and using chrome are therefore entirely eliminated.
They also use less nickel than conventional EN coatings. Composite EN coatings are produced with up to 40% by volume of co-deposited particles. This means that at least 40% less nickel is required to produce composite coatings of equal thickness to a conventional coating without such particles. Given the greater wear resistance of composite EN coatings versus conventional coatings, the deposit thickness of composite coatings can be significantly less than conventional EN coatings, further decreasing nickel usage. And composite EN coatings last longer, meaning parts will need to be recoated or replaced less frequently. The less nickel a plating shop uses, the longer baths will last. This means less baths required, less waste and less waste treatment.
Concern about the release of chrome, nickel and other metals into the environment does not stop at the plating shop’s door. As coatings wear, their constituents are released. Depending on the application, they can be released into work areas, food preparation equipment, sensitive assemblies and the environment as a whole. The same advantages outlined above accrue here, and, because composite EN coatings can be chemically stripped, used parts can be stripped and recoated, thereby reclaiming the nickel metal in solution form for recycling.
Composite EN technology has recently been developed to avoid the use of lead and cadmium and ensure compliance with ELV and similar directives. Considering the stability ramifications of the intense surface area loading of particles into an EN bath, eliminating lead is a significant accomplishment with patents pending.
Composite electroless plating technologies have also been developed recently with base metals other than nickel. The hardness and wear test data of the nickel-free versions are similar to those of composite EN coatings, but the chemistries are more expensive since they are not commercially used. But the development of such alternatives ensures the survival and growth of composite electroless plating technologies regardless of possible future regulatory issues.
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