Review of the tribology of diamond-like carbon

Diamond-like carbon (DE) films are characterized by very low friction coefficients, high wear resistance and high corrosion resistance. Depending upon the testing environment, the coefficient of friction can be as low as 0.01. As-deposited films are wear resistant in vacuum as well as in atmospheric ambient. However, the tribological properties of DLC are strongly affected by the deposition method. This paper reviews the friction and wear properties of DLC, and of similar materials derived from DLC, as a function of the preparation method and testing environment. Mechanisms proposed to explain the tribological properties are presented and discussed. Diamond-like carbon (DLC) is a term used to describe hard carbon films which are mostly metastable amorphous materials but can include a microcrystalline phase. DLC films have been prepared by a variety of methods and precursors, ~n~~udin~ r.f. or d.c. plasma-assisted chemical vapor deposition (CVD), sputtering, vacuum arc, and ion beam deposition, from a variety of carbonbearing solid or gaseous source materials [l]. DLC films are characterized by an extreme hardness, which is ‘measured to be in the range 2000-5000 kg mm-’ [2], a generally low friction coefficient and usually very high internal stresses 13-51. As a function of the deposition conditions, the films may contain varying amounts of hydrogen. The films deposited by plasmaassisted CVD (PACVD) usually incorporate up to 60% hydrogen f6], while those deposited by sputtering or a vacuum are may contain only small amounts of hydrogen or no hydrogen at all. The high hardness and chemical resistance of the DLC films makes them good candidates as wear-resistant protective coatings for metals, optical, or electronic components. The use of DLC is especially attractive in applications where it is required that the thickness of the protective film be less than 50 nm, as for example in the case of magnetic recording media. One such application is in hard magnetic disks, where the trend towards higher density data storage has led to the requirement of very low flying heights between a disk and a recording head 171. The protective coating must be resistant to wear and corrosion but also thin enough not to impede the achievement of high recording density. Therefore, special attention has been given to the tribological study of the head-disk interfaces in magnetic recording drives using thin film media, in order to develop reliable, high capacity disk files. During normal operation, the read/write head flies above the disk surface; however, when the disk starts or stops, the slider rubs on the surface of the disk. The friction and wear which can develop between the siider and disk during contact can result in the failure of the recording media. In addition, very little debris can be allowed to form during the start and stop cycles, because the debris can disrupt the flight of the head and lead to the failure of the disk surface. At the operating conditions of magnetic-recording media and micromechanics in general, microtribology becomes a key technology for interfaces [$I. Microtribology addresses the changes taking place at the atomic level in the very superficial layers in contact. Therefore, the characterization of microtribological properties has to be performed at ultralow loads. Microtribological testing tools, such as a contact profilometer [9] or point contact microscope flO] have been developed for this purpose by modi~ing a scanning tunnelling microscope. Measurements have been performed with this tool at loads as small as 1 pg [9]. In microtribology, the required surface material is not a self-sacrifice-type solid lubricant but has to be both wear resistant and lubricating. According to Miyake and Kaneko [8], to reduce atomic-scale wear, the structure of the tribological material has to satisfy the following requirements. ~43-1#8/93/$6.~