TOUGHER THAN TOUGH

A diamond-containing composite will give a new spin to drill bits and other tools.
 

A new material so sharp and tough it can cut through cast iron and granite without wearing out could make coal mining safer, cheaper, and more productive.

Developed by SIUC materials scientists Dale Wittmer and Peter Filip, the composite consists of a mixture of nickel, aluminum, metal carbide, and industrial diamond powders processed at temperatures over 1,400 degrees Celsius. Engineers at the Robert Bosch Tool Co., a manufacturing plant in Louisville, Ky., found this composite to be 800 times more wear-resistant than the company's toughest carbide now used commercially in making mining tools, drill bits, ceramic tile routers, and other such items.

"It's a material unlike any that have been used in the manufacture of cutting tools," says Wittmer, a professor of mechanical engineering.

"It could be used in mining coal, machining metal, drilling for oil, cutting rock, masonry, or ceramic tile. It has numerous applications, but because we are funded by the Illinois Clean Coal Institute, we're focusing on mining. With bits made from our material, maybe coal companies could mine seams they'd shied away from before because it would have been too hard on their equipment."

Most drill bits used in mining coal consist of tungsten carbide and cobalt. They can wear out in as little as 20 minutes, depending on the type of mining being done, the coal's make-up, and the presence of rock. When worn, these bits can fall out of the drill.

"If a tool lasts longer, a miner can continue mining without interruptions, which saves money, but there's also a safety issue," Wittmer says. "If the drill is going at 6,000 rpm and the tip comes flying off at that velocity, it could create a lot of damage."

Traditional bits also create more dust than a sharper diamond tip would make. "It gets into everything--lungs, equipment--which relates to both safety and productivity," Wittmer says.

Because industrial diamonds, like their engagement ring counterparts, rank among the world's hardest substances, manufacturers have for years mixed them with soft metals, epoxies, and organic resins to make polishing and grinding wheels. They have not, however, been able to find a way to incorporate that hardness in super-tough cutting tools.

They can't use the same materials used for grinding wheels, because with heavy pressure or high temperatures--factors involved in both making and using a cutting tool--the diamonds readily pop out, and the materials themselves lose their shape. In addition, at high temperatures, the diamonds in the materials either vaporize or turn into graphite, another form of carbon.

"When they change their crystal structure, they're no longer hard--and once they turn to graphite, that's irreversible," Wittmer says.

Wittmer and Filip, who directs SIUC's Center for Advanced Friction Studies, got into the game about two years ago. That's when Benton, Ill., resident Ken Burkett showed up at the University with a small jar containing roughly a teaspoon of industrial diamonds and the conviction that the two scientists could somehow make a coal-mining tool that contained diamonds.

After Burkett left, Wittmer and Filip tossed some ideas around, but it wasn't until the next morning, when Wittmer was in the shower, that inspiration hit.

Wittmer had a lot of previous experience working with nickel aluminide, a metal compound formed by the reaction of nickel and aluminum. He knew that when the compound heats, it expands a great deal, whereas diamonds do not. He thought the heated compound might expand enough to wrap around the diamonds--then, as it cooled, might shrink enough to clamp them into place so they wouldn't pop out. This "shrink-wrap" might also protect the diamonds from the heat's effects.

"The first composites we did were made by 'shake and bake,'" he says. "I put [the ingredients] in a plastic zippered bag, shook them up to mix them, and then pressed them into four pellets in a steel die we have. Then I put them in ceramic boxes and sent them through my furnace (a unique, continuous-belt furnace capable of reaching temperatures as high as 2,400 degrees Celsius).

"I was as amazed as anybody that the diamonds didn't disappear or convert to graphite--typically a scientist doesn't expect something to work the first time it's tried. But we dared to be bold, to try something we thought might not work. That's American ingenuity!"

At this point, the Illinois Clean Coal Institute got into the picture. Funding from the Illinois Department of Commerce and Economic Opportunity via the institute paid for the production of 53 different composite formulations made with differing proportions of elements and diamond sizes, allowing Wittmer and Filip to search for the combination that would produce the densest--and therefore hardest--material. It also paid for wear-testing of the best formulations at SIUC.

The two best composites easily cut through cast iron and granite with hardly a sign of wear. In fact, when testers cranked up the power in the granite test, the granite exploded, while the composites, though red-hot, remained intact. Mounted face-down under 50 pounds of pressure for 30 hours on a diamond polishing wheel running at 400 rpm, the new composites wore out the disk.

"That's how we found out how tough our material really was," Wittmer says. "These wheels typically last for months in a machine shop but only lasted for a few hours in contact with our diamond composite."

In Louisville, where Bosch company officials agreed to run wear tests of their own, engineers used a diamond-bladed saw to try to cut through both the diamond composites and the firm's toughest grade of tungsten carbide. It took a little over a minute to slice through the tungsten carbide, but even after 20 minutes, they failed to cut through the diamond composites--though they wore out the saw blades trying.

"Tungsten carbide tools have been the standard for over 50 years in the mining industry because nothing else has been able to improve on the abrasion resistance and performance, but those engineers said our materials may be the next step in the evolution of better and longer-lasting materials," Wittmer notes.

SIUC has filed a provisional patent application on the new composite materials. Meanwhile, Wittmer and Filip are still tinkering with formulations of the original ingredients--they now have 84--searching for the best possible mix. They also are working on bonding other metals to the diamonds and looking at the effects of various temperatures on density. "Our goal is to make a fully dense diamond composite material," Wittmer says.

By this time next year, they hope to have made and tested some sample tools in an actual coal mine.

"We have the hardness and the wear resistance," says Wittmer, "but the main issue will be proving that these tools are tough enough not to fracture under the kinds of stresses present in such an aggressive environment as a coal mine."

--by K. C. Jaehnig, Media & Communication Resources

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