Researchers at Purdue University have made a surprising discovery that could open up numerous applications for metal "nanocrystals," or tiny crystals that are often harder, stronger and more wear resistant than the same materials in bulk form.
The research engineers have discovered that the coveted nanocrystals are contained in common scrap, the chips that are normally collected and melted down for reuse.
"Imagine, you have all of these bins full of chips, and they get melted down as scrap," said Srinivasan Chandrasekar, a professor of industrial engineering. "But, in some sense, the scrap could be more valuable pound-for-pound than the material out of which the part is made."
Nanocrystals might be used to make super-strong and long-lasting metal parts. The crystals also might be added to plastics and other metals to make new types of composite structures for everything from cars to electronics.
However, nanocrystals have been far too expensive and difficult to produce to be of any practical industrial or commercial use. The cost of making nanocrystals is at least $100 per pound, while nanocrystals of certain metals critical to industry cannot be made at all with present laboratory techniques, said Chandrasekar and Dale Compton, also a professor of industrial engineering at Purdue.
"Our contribution has been in developing a process that we think can be used to make these materials in large quantities at very low cost," Chandrasekar said. "The cost is expected to be no more than $1 per pound, plus the initial cost of the bulk material."
Findings will appear in the October issue of the Journal of Materials Research, published by the Materials Research Society. The paper was written by Travis L. Brown and Srinivasan Swaminathan, graduate students in Purdue's School of Industrial Engineering, Chandrasekar and Compton, Alexander King, head of the School of Materials Engineering, and Kevin Trumble, a professor in the School of Materials Engineering.
One process now used to make nanocrystals in research labs involves heating a metal until it vaporizes and then collecting nanocrystals as the vaporized metal condenses onto a cold surface.
"The process is cumbersome, and if you want to make a pound of the material, or a few hundred pounds, it's time-consuming," Chandrasekar said. "There are other techniques, but all of them have serious limitations."
Chandrasekar and Compton have discovered that the chips left over from machining are either entirely or primarily made of nanocrystals. The chips, which are shaved away from metals as they are machined, ordinarily are collected as scrap, melted down and reused. But melting down the chips turns nanocrystals back into ordinary bulk metals, removing their super strength, wear resistance and other unusual properties.
These chips, then, might be saved and processed for use in a wide range of products. Metal nanocrystals might be incorporated into car bumpers, making the parts stronger, or into aluminum, making it more wear resistant. Metal nanocrystals might be used to produce bearings that last longer than their conventional counterparts, new types of sensors and components for computers and electronic hardware.
Nanocrystals of various metals have been shown to be 100 percent, 200 percent and even as much as 300 percent harder than the same materials in bulk form. Because wear resistance often is dictated by the hardness of a metal, parts made from nanocrystals might last significantly longer than conventional parts.
"One of the really big advantages of this is that you can do it with almost any material," Compton said. "You can make nanocrystals of steels, tungsten, titanium alloys, nickel alloys."
The engineers have measured increased hardness in nanocrystals of copper, tool steel, stainless steel, two other types of steel alloys and iron.
"We have a lot of data demonstrating that these materials are nanocrystalline and that they have enhanced mechanical properties," Chandrasekar said.
Currently, though, it is either prohibitively expensive or impossible to make nanocrystals of many alloys, including steel alloys critical to industry and commercial products.
The Purdue researchers were led to their discovery by findings in scientific literature.
"There is some work in the literature that says if you introduce very large strains into a material it will be converted into nanocrystalline," Compton said. "In our research, we knew that there was strain being introduced at the point of the cutting tool."
The very strains caused by a cutting tool also produces nanocrystals about 100 nanometers in diameter, he said.
Nano is a prefix meaning one-billionth, so a nanometer is one-billionth of a meter, which is roughly 10 atoms wide.
Nanocrystals are not currently used to make products. However, experimental uses for nanocrystals include research aimed at developing high-performance bearings, such as those used for helicopter rotors; creating new types of high-strength, lightweight composite materials; making superior fuel-injection components for diesel engines; and producing new types of chemical catalysts.
Further research will be needed to determine whether the nanocrystals contained in scrap chips retain their desired properties after standard processing steps. Those steps include milling the chips to make powders and then compressing and heating the powders to make metal parts. Nanocrystals currently produced in laboratories have been subjected to such processes, and they have retained their nanocrystalline properties, the engineers said.
Purdue has filed a patent application. The work has been funded by private donations and the Trask Pre-Seed Venture Fund, originally established in 1974 through an estate gift from Vern and Ramoth Trask, both Purdue alumni.
Journal of Materials Research
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