Xtalic featured in The Economist: “Shine on me: A safer way to produce a bright surface”June 04, 2009
From The Economist print edition
A safer way to produce a bright surface
Chrome plating has long been used to show off everything from Harley-Davidson motorcycles to kitchen taps. This is not just because it can be buffed into a shine that you can see your face in. Chrome plating has been cheap to do, it ends up harder than steel and it is extremely durable because it resists corrosion. But working with the stuff can be an environmental nightmare. Now, though, researchers may have found a safer alternative.
Metal objects are coated with chrome in an electroplating process that involves running a current through a liquid bath of chemicals that contain chromium ions. These get deposited on the surface of the material in a thin layer. However, the chemicals are hazardous to health and the waste materials generated can poison groundwater if not disposed of carefully. When chrome plating came into widespread use in the 1940s these dangers were not well understood. Modern environmental and health regulations have significantly increased the costs of a chrome-plating factory. Industries that use a lot of chrome have tried to find alternatives, but have so far discovered nothing that looks as good or is as tough.
Christopher Schuh and his colleagues at the Massachusetts Institute of Technology set out to find an alternative by identifying the properties that make chrome so strong. Metals are composed of crystals that have their atoms lined up in neat rows. In most metals these rows can readily slip past one another, a characteristic that will lower the overall strength of the material. Dr Schuh’s team found that chrome benefits from a crystalline structure that is exceptionally small and this prevents its atomic rows from slipping.
For this reason they decided to work with nanocrystalline nickel, another material formed from minute atomic rows. They ran several trials plating objects with it. Although their nanocrystalline nickel was very tough at first, it lost its hardness when left at room temperature over a matter of months and so ended up as a poor substitute for chrome. To improve its properties they tried mixing it with a combination of metals.
To predict which metals would form the nanocrystalline structures that they were searching for, Dr Schuh and his colleagues used computer models that put atoms from different metals together. Because this was done at such a small scale, the way the atoms interact can produce new and different properties according to the rules of quantum physics. They found that when tungsten was added to the nickel it produced a suitable structure. The bonus was that the mixture was capable of being plated and in a way that was more environmentally friendly.
Initial tests showed that when materials were coated in the nickel-tungsten alloy, using a modified electroplating technique that keeps its crystals particularly small, the plating remained stable indefinitely at room temperature. Nor did it easily degrade when exposed to great heat. The nickel-tungsten alloy can be extremely bright and shiny, and even made to become harder than chrome. All in all, its characteristics and absence of environmental hazards make it the most appealing replacement yet found for chrome. The researchers intend to report as much to a forthcoming conference of the National Association for Surface Finishing in Louisville, Kentucky.
Proof, though, will come with use. Testing is already under way, with the nanocrystalline nickel-tungsten alloy being tried out as a plating on the bumpers of a fleet of trucks. It could soon find its way onto kitchen taps and other shiny fixtures. And if it can get to ride on a Harley, then its future will be assured.