Diamond coating

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

Fig. 14. A scanning electron micrograph of a diamond coating on a siUcon nitride cutting tool (b) at higher magnification, the octahedral growth of...

The activated CVD diamond techniques can be mote attractive in cases where the huge capital investment (several hundred million dollars) requited for the HP—HT technology is not available or where the high level of technical knowledge requited for HP—HT synthesis is not available. In addition, most wear-resistant apphcations requite diamond coatings only of the order of a few micrometers thick. Such coatings can be deposited ditecdy on the finished product without the need for further finishing if CVD techniques are employed. 

Another approach is to coat the cutting tool material with a carbide former, such as titanium or siUcon or their respective carbides by CVD and deposit diamond on top of it. The carbide layer may serve as an iaterface between diamond and the cemented carbide, thus promoting good bonding. Yet another method to obtain adherent diamond coatings is laser-iaduced microwave CVD. By ablating the surface of the substrate with a laser (typically, ArF excimer laser) and coating this surface with diamond by microwave CVD, it is possible to improve the adhesion between the tool and the substrate. Partial success has been achieved ia this direction by many of these techniques. 

Diamond Coating, A World of Opportunity, Technical Brochure, Genasystems Inc., Worthington, OH (1991)... 

Oles, E. J., et al., The New Diamond-Coated Carbide Cutting Tools, Diamond Conference, Barcelona, Spain (Sept., 1995)... 

AI2O3 ball. Steel ball Diamond coated Si3N4 ball Si3N4 ball... 

Figure 5.7. A diamond-coated tungsten wire that is about the same diameter as a human hair.

Diamond coatings as bulk diamonds may have important applications due to the unique properties of this substance (high hardness, low thermal expansion, high-thermal conductivity). 

After solidification, the ingots are shaped using diamond-coated band saws or wires in an abrasive medium in order to have square multicrystalline or pseudosquare rounded angle monocrystalline blocks. This process removes the outer parts of the ingots, generally out of specification for dimensions and with some contamination from the containers or the furnaces. 

Plasma-sprayed, flame-plated, or electrolytically deposited coatings of powders of Al O Ci O TiN, WC, and Ti02 can be applied as wear-resistant ceramics on metal substrates with or without Co, Ni, or Cr incorporated to improve mechanical properties. Silver, barium fluoride—calcium fluoride, and other modifying materials have also been found useful in ceramic coatings for improved friction and wear properties (35). Diamond coatings are also being developed (36). 

Boron-doped diamond coating on Pt electrode. [From J. Cvacka et at. Anal. Chem. 2003, 75,2678. Courtesy G. M. Swain, Michigan Slate University.]... 

Further improvements in deposition technology are still needed—only substrates stable at a temperature of 1000°C can be coated at present—but the time may not be too far off before diamond coatings on a variety of consumer products become commonplace. 

Miyamoto, Y., Kashiwagi, T., Hirota, K., Yamaguchi, O., Moriguchi, H. Tsuduki, K., Ikegaya, A., (2003), Fabrication of new cemented carbide containing diamond coated with nanometer-sized SiC particles , J. Am. Ceram. Soc., 86, 73-76. 

Diamond coatings - and in particular semiconducting variations like boron-doped diamond - are currently available on different support materials from various suppliers. The most prominent supports utilize silicon, titanium, or niobium. 

When a single crystal diamond (synthetic or natural) is the substrate, epitaxial growth occurs the growing diamond replicates the substrate crystal lattice and turns to single crystal film. The film thickness usually comes to a few microns however, films of 1 mm in thickness were reported. The diamond-coated area would achieve 10 cm in diameter by order for industrial applications, much larger areas (e.g. 40 by 60 cm) are covered. Samples destined for the electrochemical measurements used to have dimensions ca. 1 by 1 cm. 

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