Pull a silver coin out of your pocket. Look at the edge. You'll notice a thin copper line running through the middle. That's because the "silver" coin isn't silver at all. It's actually a metal composite comprised of a layer of copper sandwiched between two layers of nickel alloy, but vending machines and telephones can't tell the difference.
The U.S. Treasury saved a mint by replacing a precious metal with a metal composite. Now makers of pricey metal substrate catalytic converters are trying to use the same technology to compete with lower-cost ceramic substrates.
The key is making the typically brittle metal alloys used in metal converter substrates easier to fabricate.
Metal substrate converters offer automakers many advantages over ceramics, proponents argue, such as faster "light off" (they heat up and start catalyzing in seconds instead of minutes because they can be made with thinner walls and have higher thermal conductivity); better resistance to thermal shock (allowing them to be located closer to the engine exhaust manifold for faster light off); and they cause less horsepower-robbing back pressure. Despite these advantages, their costs -- 15% higher than ceramics -- have limited their use to mostly high-end European cars.
Texas Instruments Inc.'s Metallurgical Materials Div. in Attleboro, MA, says it has an answer to this manufacturing conundrum: cladding. U.S. coins now are formed by this centuries-old metalworking technique, which metallurgically bonds two or more distinct metal alloys in layers. It's also used for making ammunition, thermostats, cookware and some bright metal automotive trim.
The process takes several layers of different types of metal and squeezes them so tightly together in a rolling mill that they actually form a metallurgical bond. Heat-treating completes the synthesis, fully diffusing the separate metal chemistries so two or more metals actually can be formed into a new alloy.
The main benefit of cladding for catalytic converters is that the crucial substrate can be coiled and formed into the convoluted shapes required before it is transformed into the final alloy -- an extremely brittle material -- required for the catalyzation process.
TI says its new clad-metal material synthesis process solves the problem. The metal substrate first is formed into a thin foil consisting of stainless steel tightly sandwiched between two layers of aluminum. In this state, the foil easily is formed into the complex corrugations required to construct the interior of the converter. Once the assembly is made, it then is heat-treated and the steel and aluminum layers diffuse and meld to form the new alloy.
Voila! A tough, cost-effective metal substrate converter is made. It's a little like making a pie crust: You form the moist dough into the shape you want, and then you bake it. Try to shape it after it's been in the oven, and it'll just crumble.
A "wash" of precious metals then is added as in standard production operations.
TI officials emphasize this is no laboratory experiment. Pilot production on full-scale cladding mills now is producing material in volume for catalytic converter manufacturer field tests, and a clad-metal converter could appear on a production vehicle by mid-year, insiders say.
Will this put ceramics out of business? Probably not. Those folks have a technological trick or two up their sleeves as well. But metallics are expected to make significant gains in the next few years as Environmental Protection Agency emission requirements toughen. Meantime, remember: Composite nickels are okay, but don't take any wooden ones.
