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Thin-film diamond at the cutting edge.

What could be better than polycrystalline diamond (PCD) for machining of high-silicon aluminum alloys and other highly abrasive nonferrous materials? How about a tool that combines the hardness of pure diamond with the toughness, multiple cutting edges, and chipbreaker geometries of carbide inserts?

"What we're talking about is really almost a new class of tool," says Laurie C Conner, vice president of marketing for Crystallume Inc, a Menlo Park, CA, company that specializes in chemical vapor deposited (CVD) diamond coatings. "It's not carbide, and it's not PCD. It kind of combines the best attributes of both."

What we're talking about is carbide inserts coated with thin (under 100 |micrometer~) diamond films. After years of trying, researchers at Crystallume and elsewhere appear to have finally solved the coating adhesion problems that have prevented widespread use of diamond-coated carbide (DCC) tooling. Now, the race to commercialize DCC technology is on, and the real winners may be cutting tool users.

Making it stick

The properties of diamond are well known. For a tool material, probably the most important are extreme hardness (9000 kg/m|m.sup.2~, versus 4500 kg/m|m.sup.2~ for cubic boron nitride) and a very low coefficient of friction (0.05, equal to Teflon).

Regardless of the substrate material, adhesion of diamond coatings has been a barrier to widespread application. In the case of carbide substrates, though, adhesion problems are magnified by a specific culprit that until recently was thought by many to be an insurmountable obstacle: the cobalt binder phase found in carbide tools essentially "poisons" the diamond nucleation and growth process, resulting in formation of graphitic carbon.

"Cobalt is a killer when it comes to achieving diamond adhesion on carbide," says Ed Oles, manager of superhard materials development at Kennametal Inc, Latrobe, PA. "If cobalt is at the surface of the tool, you form graphite at the cobalt regions and diamond on the carbide regions of the surface. The presence of graphite gives you an inherently weak interfacial layer and consequently poor coating adhesion."

Tool suppliers have tried several ways around the cobalt problem. One method is limiting cobalt content, and Japanese insert manufacturers have developed and marketed diamond-coated carbide tools based on this idea. Another alternative, as in the case of Norton Co's CVD diamond coated silicon nitride tools, is eliminating cobalt altogether by switching to a ceramic substrate. Either way, tool life in some applications tends to suffer as a result of substrate brittleness, says Rob Hay, manager of product development, Norton Diamond Film, Northborough, MA.

"Diamond-coated silicon nitride right now is used in machining of plastics and polymer composites, and DCC wouldn't work any better in those applications," he says. "But for aluminum alloys, the carbide will keep machining after you wear through the coating. The silicon nitride, when the coating's worn through, will catastrophically fail in contact with aluminum." Norton is pursuing diamond-coated carbide as an extension of its existing diamond tooling technology, which includes both diamond-coated silicon nitride and thick-film diamond, he adds.

A consortium coordinated by the National Center for Manufacturing Sciences (NCMS) is having success with tougher, higher-cobalt substrate materials. The group includes Crystallume and General Motors, as well as insert suppliers Valenite Inc and Teledyne Cutting Tools.

"We wanted to put diamond on a standard 6% cobalt (C-2) grade," says NCMS project manager Jerry Reimann. "Lower- cobalt materials are brittle and tend to chip easily. At 6% you get the toughness you want, and you can make sure the failure mechanism is wearing of the diamond, not coating delamination."

With a patent on the technique pending, Ms Conner refuses to say exactly how Crystallume is able to achieve good coating adhesion on carbide. "What counts is, it sticks like hell," she says.

The secrecy--and the sentiment--are echoed by Kennametal's Mr Oles. "We accomplished our technical objective: to get diamond coatings to stick on a carbide substrate. We think we have a patentable adhesion technology." He will say only that Kennametal uses a proprietary CVD process for diamond deposition and has developed a carbide grade specifically as a substrate for diamond coating.

Testing, testing

The NCMS consortium has been collecting and evaluating test data on the life of its DCC inserts produced by Crystallume for some time. The testing effort has been spearheaded by General Motors, with much of the initial work performed at GM's Technical Center in Warren, MI.

The benchmark for the GM laboratory tests was PCD, but the tests have also included comparisons to other diamond-coated tools with both ceramic and carbide substrates. "We've done a lot of machining evaluation to establish the viability of all the diamond-coated tools we've been able to get our hands on, including ceramic and carbide substrates," says CH Shen, supervisor of machining technology at the GM Tech Center.

Testing so far has involved mainly turning and interrupted turning to evaluate resistance to thermal shock, although some milling tests have been performed. Results indicated that coating adhesion is not a problem for the Crystallume tools, and those inserts wear at rates comparable to PCD until the coating is worn through. The next phase, beta site testing in GM production facilities, is well under way.

The beta test program also is open to NCMS members, says Mr Reimann. "We will supply DCC inserts, but the companies have to agree to give us access to their test data, so they sign on for what are essentially controlled field tests. Members can send in inserts to be coated or send a drawing of the geometry they want to try."

Early this year, Kennametal embarked on a field testing program of its own. Baseline for the tests was whatever the customer happened to be using at the time. "For example, in wheel turning applications, especially roughing, the benchmark is uncoated carbide. For a lot of other applications, customers are already using PCD, so PCD is the baseline," says Mr Oles. "To be honest, though, the benchmark has to be PCD," he adds.

Diamond derby

Now that the coatings are sticking, the race to market DCC tools is heating up. The main challenge facing the contestants is expanding their developmental processes to commercial levels.

That means both more and bigger CVD reactors, and the scale-up process is almost never without problems.

"One of the big hurdles that remains to be overcome is scale-up of the Crystallume technology, but I'd anticipate that within a year or so we'd have production machinery to at least make hundreds of inserts in a run," says Jim Oakes, manager of research and development, Teledyne Cutting Tools, Lavergne, TN.

Initial applications for DCC inserts will be in turning, with milling grades coming later. Kennametal, for example, expects to roll out DCC inserts for wheel-turning applications first, then follow up with a more general product introduction in the next year or so, says Mr Oles.

Another unknown facing DCC suppliers is the trade-off between price and coating thickness. "We're probably going to have two diamond-coated grades, one with a thick coating (still less than 100 |micrometer~ thick) and one with a thinner (10-20 |micrometer~) coating," says Kennametal's Mr Oles. "The thick one would be for very abrasive, very aggressive workpiece materials, and the thinner one for less demanding applications."

Both the NCMS consortium and Kennametal expect prices to initially come in about the same as PCD, with prices eventually settling between "conventional" (example: TiN) coated carbide inserts and PCD. "It's feasible that the long-term price of these tools could be in the $20 to $30 range, which, compared to PCD in the $100 range, is extremely competitive. In the near term, though, I think prices will be in the $60 to $100 range," says Mr Reimann.

"Of course, DCC gives the customer multiple cutting edges, which they don't now have, and the ability to form the coating on complex chipbreaker geometries," says Mr Oles.

Even at prices comparable to PCD, Mr Reimann believes the US market will accept DCC tools more readily than, for example, cermets. "A390 aluminum (a casting alloy containing 16 to 18% silicon) is one tough material to machine. People machining highly abrasive materials like that will understand the benefits, because right now they're trying to machine without it."

A CVD diamond primer

Creating diamond from methane and hydrogen sounds like a bit of alchemy, and even some of the people most closely involved in making DCC a reality marvel that the process works at all.

"If you had told me five years ago that you could make diamond from methane gas and a microwave, I wouldn't have believed it. Beyond the problems specific to putting diamond coatings on carbide substrates, that's still something I find amazing," says Jerry Reimann of NCMS.

So how does the CVD diamond process work? There are three basic deposition techniques currently in use. All involve dissociation and ionization of the hydrogen and methane precursor gases, which are then passed over and deposited onto the heated substrate. The main difference is in how the hydrogen and methane precursors are dissociated and ionized.

The heated filament method, as the name implies, uses a tungsten or tantalum filament similar to that in a lightbulb to heat the precursor gases to about 2000 C. Substrate temperature ranges from 600 C to 1100 C. Using hydrogen and methane precursors, deposition rates of 1 to 10 |micrometer~ per hour are possible. Disadvantages of the process include limited filament life and some contamination of the resulting film by the filament metal.

DC plasma deposition, the technique being investigated by the Westinghouse/SGS team, uses a DC arc to dissociate the precursor gases, and can provide higher gas volumes and velocities than other processes. The high-rate, large-area DC plasma process being developed at Westinghouse has projected deposition rates of up to 100 carats per hour.

Microwave plasma CVD, the method of choice for Crystallume, is probably the most commonly used diamond deposition method. The process uses microwaves to excite the precursor gases, resulting in deposition rates of several microns per hour. What the microwave process might lack in speed, however, it makes up for in purity: coatings deposited using this method are of very high purity, closer to pure diamond than the other techniques described.

Round tools, too

Development of DCC tools is not limited strictly to inserts, although work on rotating tools is still in the preliminary phases. Backed by the National Institute of Standards and Technology (NIST), Westinghouse Electric Corp, Pittsburgh, PA, and SGS Tool Co, Munroe Falls, OH, have launched a three-year effort aimed at reducing the cost of diamond coatings from the current $30 to less than $5 per carat.

Westinghouse has done diamond coating work since the mid-1980s, mostly in market segments such as electronics. The collaboration with a supplier of carbide cutting tools is an attempt to put Westinghouse DC plasma deposition technology to work on a practical problem, says diamond program manager Art Long.

"We felt tooling was probably the first marketplace that would readily accept diamond. People who use tooling know very well the kinds of things they need, and they can apply it in places where you can see results quickly," he says.

Westinghouse has already built several industrial plasma systems of the type it intends to use for the diamond film project, says Mr Long. "Our thought is to use these arc heaters to produce diamond coatings," he says. "We have a 15-kW system that has been a workhorse. The next steps from there are to proceed to a megawatt reactors with product diameters of 2 to 3 ft."

SGS vice president of technical services Jeff Burton says depositing diamond on a round tool may present more of a challenge than coating inserts, which have relatively flat surfaces compared to the drills, end mills, and other rotating tools manufactured by SGS. "It's a little different than what other people have done. We're not talking about inserts for turning or milling, we're talking about solid carbide rotary cutting tools," he says.
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Title Annotation:includes related articles
Author:Destefani, James D.
Publication:Tooling & Production
Article Type:Cover Story
Date:Jul 1, 1993
Words:1999
Previous Article:Presetting off-line offers CNC savings.
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