Manufacturing: Surface Finishing

Diamond Coatings CVD

To the scientist, the diamond is impressive because of its wide range of extreme properties. It is the hardest known material, has the lowest coefficient of thermal expansion, is chemically inert and wear resistant, offers low friction, has high thermal conductivity, is electrically insulating and optically transparent from the ultra-violet (UV) to the far infrared (IR). Given these many notable properties, it should come as no surprise to learn that diamond already finds use in many diverse applications including, of course, its use as a precious gem, but also as a heat sink, as an abrasive, and as inserts and/or wear-resistant coatings for cutting tools.

Obviously, given its many unique properties it is possible to envisage many other potential applications for diamond as an engineering material, but progress in implementing many such ideas has been hampered by the comparative scarcity of natural diamond. Hence the long running quest for routes to synthesise diamond in the laboratory. So called 'industrial diamond' has been synthesised commercially for over 30 years using >high-pressure high-temperature (HPHT) techniques, in which diamond is crystallised from metal solvated carbon at P~50-100 kbar and T~1800-2300 K.

Some of the outstanding properties of diamond

1. Extreme mechanical hardness (~90 GPa).
   Strongest known material, highest bulk modulus (1.2 x 1012 N/m2),
2. Lowest compressibility (8.3 x 10-13 m2/ N)
3. Highest known value of thermal conductivity at room temperature (2 x 103 W / m / K).
4. Thermal expansion coefficient at room temperature (0.8 x 10-6 K) is comparable with that of Invar.
5. Broad optical transparency from the deep UV to the far IR region of the electromagnetic spectrum.
6. Good electrical insulator (room temperature resistivity is ~1016 O cm).
7. Diamond can be doped to change its resistivity over the range 10-106 O cm, so becoming a semiconductor with a wide bad gap of 5.4 eV.
8. Very resistant to chemical corrosion.
9. Biologically compatible.
10. Exhibits low or 'negative' electron affinity.

World interest in diamond has been further increased by the much more recent discovery that it is possible to produce polycrystalline diamond films, or coatings, by a wide variety of chemical vapor deposition (CVD) techniques using, as process gases, nothing more exotic than a hydrocarbon gas (typically methane) in an excess of hydrogen. This CVD diamond can show mechanical, tribological, and even electronic properties comparable to those of natural diamond. There is currently much optimism that it will prove possible to scale CVD methods to the extent that they will provide an economically viable alternative to the traditional HPHT methods for producing diamond abrasives and heat sinks, whilst the possibility of coating large surface areas with a continuous film of diamond will open up whole new ranges of potential application for the CVD methods.