Fortunately, there are a variety of other spectral techniques which can be used to establish sp 2 content in DLC films Electron Energy Loss Spectroscopy (EELS) and Nuclear Magnetic Resonance (NMR). As such, Raman analysis was not conducted here. Interlayers used to improve the DLC–metal bonding frequently form carbides and are not representative of the film, particularly for the top layer which is crucial to friction and wear performance. 7,8 DLC films are usually 1–3 μm thick and carbon bonding within the film is known to change with depth. These include an uncertainty in the beam penetration depth, and appropriate selection of excitation wavelength, which means that the technique may not assess the film accurately. 2,6 However, Raman spectroscopy is known to have limitations when examining DLC films. Most often, due to ease of use, the technique employed to detect changes in carbon hybridisation state within the film is Raman spectroscopy. Furthermore, the phenomena are complicated by accurate characterisation of carbon sp 2 : sp 3 ratios. Literature on tribologically induced changes of hybridisation of DLCs is somewhat lacking. 1–4 DLC coatings with high sp 3 fractions are frequently regarded as intrinsically low wearing, due to their high hardness. One prominent example of this is the change in sp 2 : sp 3 ratio that can occur upon high-temperature heating of DLC coatings. transform from carbon sp 3 to sp 2 bonds).
Like many carbonaceous materials, they are able to undergo changes in carbon hybridisation state, or “re-hybridise” ( i.e. Introduction DLC coatings are designed to be inherently low wearing materials. In comparison, the Si,O-doped DLC undergoes comparatively little change in carbon hybridisation state. The undoped a-C:H DLC coating shows an increase in non-planar sp 2 carbon content during wear testing which appears to be at the expense of sp 3-hybridised carbon. Both coatings show formation of protective tribofilms with tribochemically-relevant elements derived from the lubricant additives however the tribofilms on the two coatings exhibit key differences, with one containing pyrophosphate. The wear of the undoped a-C:H DLC coating is far lower than that of the Si,O-doped DLC. Physical–chemical characterisation allows the elucidation of both the physical and tribochemical mechanisms underpinning the respective wear behaviours.
Two diamond-like carbon coatings have been examined, both prior to, and post tribotesting.
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