Diamond is the hardest of all natural materials. The hardness of any given material is usually determined by pressing a carefully shaped indenter onto a surface under a load, resulting in a permanent plastic deformation. The indenter must be at least as hard as the substance being indented; for measuring the hardness of diamond, only a diamond indenter is useful. Even then, the indenter itself becomes misshapen after several uses and must be replaced. The applied force on the indenter is divided by area of the impression left on the surface, and so hardness is expressed in units of pressure.
The Mohs hardness scale assigns the hardness of materials based on a scale of 1-10, with each number represented by a known/defined material. On the Mohs scale, 10 represents diamond, but this extreme hardness is not adequately represented due to the non-linearity of the scale. The Mohs scale gives the impression that #9, corundum (also called sapphire), is nearly as hard as diamond, when in reality, there is a 4x increase in hardness between corundum and diamond.
The carbon-nitrogen bond is thought to be stronger than the hybridized carbon-carbon bond of diamond [1]. If so, a material that could possibly be harder than diamond is C3N4 [2]. The in-plane doubled bonded C=C bonds of graphite are also stronger that the C-C bonds of diamond. The atomic density of diamond is an unequalled 1.76×1023 atoms/cm3.
Diamond has stimulated much interest in the field of tribology, the study and application of friction [3-5]. CVD grown diamond-like carbon (DLC) films have attracted an overwhelming interest from both industry and the research community. These films offer a wide range of exceptional physical, mechanical, biomedical and tribological properties that make them scientifically very interesting and commercially essential for numerous industrial applications.
Mechanically, certain DLC films are extremely hard (as hard as 90 GPa) and resilient, while tribologically they provide some of the lowest known friction and wear coefficients. Because of their excellent chemical inertness, these films are resistant to corrosive and oxidative attacks. The combination of such a wide range of outstanding properties in one material is rather uncommon, so DLC can be very useful in meeting the multifunctional application needs of advanced mechanical systems. In fact, these films are now used in numerous industrial applications, including razor blades, magnetic hard discs, critical engine parts, mechanical face seals, scratch-resistant glasses, invasive or implantable medical devices, optical windows, and microelectromechanical systems (MEMs) [6].
Recent systematic studies of DLC films have confirmed that the presence or absence of certain elemental species, such as hydrogen [7], nitrogen, sulfur, silicon, tungsten, titanium and fluorine [8], in their microstructure can also play significant roles in their properties.
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6. Krauss, A.R., Auciello, O., Gruen, D.M., et al., Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices. Diamond and Related Materials, 2001. 10(11): p. 1952-1961.
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8. Touhara, H. and Okino, F., Property control of carbon materials by fluorination. Carbon, 2000. 38(2): p. 241-267.