Tribological Considerations of Metal Coatings
Superior Wear...Excellent Lubricity...lower coefficient of friction
Understanding the friction properties of various platings and coatings is helpful in choosing an appropriate finish for a given application. Unfortunately, data on the respective coefficients of friction for plated or coated surfaces is dependent on a number of variables, including but not limited to: substrate, thickness, pre-treatments, post-treatments, and environment. The purpose of this document is to briefly address some of the relevant variables and thus qualify an included table of data of coefficients of friction.
Static(1) Coefficient of friction (u) is defined as the ratio of the tangential force (F) required to produce sliding motion between divided by the normal force (N) between two surfaces.
For a horizontal surface, N is simply the mass of the solid multiplied by gravity, so for a constant mass, the coefficient of friction is directly proportional the force required to initiate motion. In other words, the lower the coefficient of friction, the more "slippery" the surface or the greater its lubricity.
Examination of the above definition highlights one of the foremost considerations of any comparison of friction characteristics of any surface: a number of factors must be held constant to generate any meaningful data. For example, if coefficients of friction are being derived from experiments initiating motion in a block on a horizontal surface, the mass of the block must be identical for each instance of the experiment.
What are less obvious are environmental factors, such as humidity, temperature, and air quality, yet these can all influence the outcome of any effort to experimentally determine coefficients of friction. Furthermore, they will affect various materials in different ways. Plated silver, for example, is more affected by humidity than other plated metals because of its tendency to tarnish, yet it lubricates at elevated temperatures where other plated metals fail entirely. Thus environmental conditions also need to be held constant if data is to be meaningful. "...only coefficients of friction obtained under the same conditions can be compared. [Even then] the values should not be considered as absolute."(2)
This being the case, it calls in to question the usefulness of coefficient of friction data in isolation. After all, real-world applications usually involve ranges, not points, of some or all of the aforementioned factors. Loads, humidity, temperature and air quality all vary interdependently, and so demand consideration of all the finishes engineering characteristics before choice can be made. For example, where high humidity is a factor, corrosion protection will be as important a consideration as lubricity. In the case of the previously mentioned silver, it may be the only option because of high temperatures. Where temperature extremes will be encountered, such as aerospace applications, its coefficient of friction relative to other coatings will not influence its choice.
Another factor to discuss that is especially familiar to platers and coaters is the formation of oxide films. The above mentioned tarnishing of silver is an example of this process. Almost any metal, upon exposure to air, will spontaneously form an oxide layer on its surface. Even if this layer is only a few atoms thick, it will significantly affect the coefficient of friction between materials. "Practically all published work agrees that extremely small amounts of oxygen or other contaminants can greatly reduce the adhesion between metals."(3)
To these qualifications an almost infinite number could be added, but no less an authority than Nobel Prize winner Richard Feynman states, "The tables that list purported values of the friction coefficient of 'steel on steel' or 'copper on copper' and the like are all false because they ignore a couple of important factors." He lists these factors as:
- The area of real contact between the sliding surfaces.
- The type of strength of bond that is formed at the interface where contact occurs.
- The way in which the material in and around the contacting regions is sheared and ruptured during sliding.4
Having hopefully presented sufficient qualification, we present the following table of coefficients of friction for plated and coated steel, static, on bare steel. These numbers are drawn from a variety of sources and, given the difficulty of extracting data even from tightly controlled experiments, serve as little more than a rough guide of where to start. Any application should be thoroughly tested before choosing a plating or coating.
- Material Plated/Coated on Steel
- Static Coefficient of Friction against Steel
- Hard Chrome
- Electroless Nickel
Beyond the considerations above, other factors influence the performance of each coating listed. The quality of the plate, for instance, is important, since any surface roughness would affect the coefficient of friction. Pretreatment is important as it can change the surface qualities of the substrate and thus the characteristics of the plated surface. Post treatments, such as chromate conversion coatings on zinc, cadmium or silver, can also change surface characteristics, and really necessitate a new data set. Post plate bakes (or curing in the case of Teflon) also modify the surface characteristics. While plated finishes are often baked to relieve hydrogen, electroless nickel can be baked to reorient the atomic crystal structure and thus change its friction properties.
Yet another factor to be considered is part geometry. In calculating coefficients of friction, geometry may be less important, since for lower masses the resultant number is independent of surface area. But in real world applications, geometry can be very restrictive. Hard chrome, as the table shows, has a low coefficient of friction, and, "The coefficient of friction of hard chromium against hard metals are generally the lowest of any electrochemically deposited coatings."(5) But hard chrome plating has very poor "throw," which is a plater's way of saying it requires extensive tooling to plate complicated geometries. This equates to high costs. Electroless nickel, on the other hand, while having a higher coefficient of friction, requires no current and can thus plate almost any geometry, even blind holes.
That brings us to a final consideration, cost, which is often left out of engineering tables but absolutely necessary to consider in real-world applications. The friction properties of a finish must be weighed against its cost. It is worthwhile to involve an experienced plating firm early in the engineering process even if all the tribological characteristics of the various finishes under consideration are well understood, as only the plater is going to be able to give a real sense of what the final process will cost.
For more information about various plated finishes, fluoropolymer coatings and their tribological properties, contact Chem Processing, Inc.
- (1) This document only deals with static coefficients. Sliding coefficients of friction are related, but add on another layer of variables in the form of kinetic interactions.
- (2) Rolf Weil and Keith Sheppard, "Electroplated Coatings," ASM Handbook Vol. 18 (1992), pg. 835
- (3) David Tabor, "Status and Direction of Tribology as a Science in the 80's: Understanding and Prediction," New Directions in Lubrication, Materials, Wear and Surface Interactions: Tribology in the 80's (Noyes, New Jersey 1985), p. 3
- (4) Horst Czichos, "Importance of Properties of Solids to Friction and Wear Behavior," New Directions in Lubrication, Materials, Wear and Surface Interactions: Tribology in the 80's (Noyes, New Jersey 1985), p. 72
- (5) Rolf Weil and Keith Sheppard, "Electroplated Coatings," ASM Handbook Vol. 18 (1992), pg. 835
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