Chemical Vapor Deposition of Diamond
Applications to Tool Inserts

Introduction
Chemical vapor deposition (CVD) of diamond films has evolved quite dramatically in recent years, and several applications for these films have now been commercialized. The two most promising applications in the near term are thick diamond films for thermal management and wear applications, and thin film coatings for cutting tool inserts.

CVD diamond-coated tool inserts offer several advantages over polycrystalline diamond (PCD) inserts, making CVD diamond tools a potential threat to today's large PCD market. These advantages include:

• lower cost
• multiple cutting edges on a single tool
• tools with advanced chip breaker designs can be coated
• elimination of the need to polish the diamond edge since, with the proper control of the surface morphology, tools can be used as grown.

SEM photograph of one corner of a diamond-coated SiNx insert.

An example of a diamond-coated SiNx tool insert is shown in the SEM photographs. Nicely faceted diamond material is clearly visible.

Technical Issues Diamond is highly soluble in iron at high temperatures and, thus, is unsuitable for machining ferrous metals. Therefore, the focus application for coated inserts will be to machine non-ferrous materials and ceramics. A key application area for diamond is machining of high silicon content aluminum -- a very hard material which will be widely used for engine blocks required by lighter, more fuel efficient automobiles. Another key application is for woodworking tools.

Closer view of tool insert shown above.

Adhesion of a deposited diamond film to the tool has presented the greatest technological hurdle to commercial introduction of diamond-coated tool inserts. Film delamination has been a common occurrence during initial development stages. Because of its random nature, failure by delamination is unacceptable for cutting tools. Thus, an important early goal was to develop adhesion methods which, instead, permitted the diamond films to wear out.

Tungsten carbide is the most widely used tool insert material today. It typically contains 6% cobalt to act as a binder during the sintering process. Cobalt also provides added strength to the tool and, similar to iron, has high carbon solubility. Therefore, it is difficult to deposit high quality diamond on an untreated tungsten carbide tool, and delamination of the diamond film represents the most common mode of failure. Because of these difficulties, the initial work on diamond coatings for tools focused on SiN_x inserts for which adhesion is easier to accomplish.

The presence of cobalt at the surface has created the most significant technological challenge to achieving proper adhesion between tungsten carbide tools and the diamond films deposited on them. Several researchers have reported techniques which address this issue but, in all cases, have revealed little detail. Following are some of the techniques which may successfully lead to adequate tool performance: addition of interlayers between the tool and diamond acid dilution of cobalt near the surface other treatments to eliminate cobalt from the surface and prevent its diffusion back to the surface. re-sintering to enlarge crystal size near the surface

Based on the input of several tool insert manufacturers, the estimated world market for diamond-coated inserts is on the order to $8 million per year. CVD diamond-coated inserts are presently targeted for applications in machining high silicon content aluminum. The projected growth of the aluminum content in automobiles indicates that the growth of the diamond-coated tool insert market will be significant. The greatest potential depends on CVD diamond inserts supplanting the present PCD market. When the performance and price of CVD tools compare favorably with those of PCD tools, CVD tools may displace PCD in some markets. Diamond-coated insert machining aluminum for an automotive application (courtesy of Allison Engine Company).

The AX6560 is designed as a first generation tool coated for production. The AX6560 offers fast deposition rates based on ASTeX's high power density concept

The AX6560 can deposit diamond with better than 15% uniformity over an area up to 3 to 4 in. diameter. Sample depositions have been performed in ASTeX reactors for several potential customers, using both tungsten carbide and silicon nitride tools. In all cases, the customers provided their proprietary surface treatment for the tools prior to deposition but shared the performance results of the coated cutting tools.

Cutting tests performed on SiNx tools diamond-coated in ASTeX equipment are promising. In tests performed on silicon aluminum alloy AC8A-T6, machining at 600 m/minute, 0.10 mm/revolution, and to a depth of 0.25 mm, the tool was still in service after 50 km. The figure below shows a plot of surface finish as a function of time comparing a diamond-coated tungsten carbide insert with a PCD insert. The cutting conditions were similar to those cited above. The surface finish (obtained by proper control of the diamond film morphology) is similar to the finish provided by the PCD insert.

Second Generation Reactors
The ASTeX AX6560 reactors in a factory will enable tool insert companies to enter the CVD diamond-coated tool market. With a batch size on the order of 20 to 30 inserts, the AX6560 is adequate for pilot plant production and development of consistent surface treatments. It offers a conservative approach to market entry by companies which have not yet fully developed a reliable surface treatment method. The economics for this reactor are good, but the costs per inset are higher than the $5/insert figure quoted earlier.

The AX6600 reactor, with a batch size of 75 to 200 tools, provides full-scale production capability. This should be the reactor of choice for tool companies with the proper adhesion technology who are ready to enter the tool market in an aggressive way. Cutting tools in the process of being coated with CVD diamond in an ASTeX reactor.

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