Chemical, mechanical and tribological characterization of ultra-thin and hard amorphous carbon coatings as thin as 3.5 nm: recent developments

Abstract

Diamond material and its smooth coatings are used for very low wear and relatively low friction. Major limitations of the true diamond coatings are that they need to be deposited at high temperatures, can only be deposited on selected substrates, and require surface finishing. Hard amorphous carbon (a-C), commonly known as diamondlike carbon (DLC), coatings exhibit mechanical, thermal and optical properties close to that of diamond. These can be deposited with a large range of thicknesses by using a variety of deposition processes, on variety of substrates at or near room temperature. The coatings reproduce substrate topography avoiding the need of post finishing. Friction and wear properties of some DLC coatings can be very attractive for tribological applications. The largest industrial application of these coatings is in magnetic storage devices. Recent developments in the chemical, mechanical and tribological characterization of the ultra-thin coatings are reviewed in this paper. The prevailing atomic arrangement in the DLC coatings is amorphous or quasi-amorphous with small diamond (sp³), graphite (sp²) and other unidentifiable micro- or nanocrystallites. The mechanical and tribological properties of the DLC coatings are dependent upon the deposition technique. Thin coatings deposited by filtered cathodic arc, ion beam and ECR-CVD hold a promise for tribological applications. Coatings as thin as 5 nm in thickness provide wear protection.

Introduction

Carbon is an unusual material in that it exhibits both metallic and nonmetallic characteristics. Carbon exists in both crystalline and amorphous forms [1], [2]. Crystalline carbon includes graphite, diamond and a family of fullerenes, Fig. 1. The graphite and diamond are infinite periodic network solids with a planar structure, whereas the fullerenes are a molecular form of pure carbon with a finite network with a nonplanar structure. Graphite has a hexagonal, layered structure with weak interlayer bonding forces, and exhibit excellent lubrication properties. The graphite crystal may be visualized as infinite parallel layers of hexagons stacked 0.34 nm apart with a 0.1415 nm interatomic distance between the carbon atoms in the basal plane. The atoms lying in the basal planes are trigonally coordinated and closely packed with strong σ (covalent) bonds to its three carbon neighbors using the hybrid sp² orbitals. The fourth electron lies in a p_z orbital lying normal to the σ bonding plane and forms a weak π bond by overlapping side to side with a p_z orbital of an adjacent atom to which carbon is attached by a σ bond. The layers (basal planes) themselves are relatively far apart and the forces that bond them are weak van der Waals forces. These layers can align themselves parallel to the direction of the relative motion and slide over one another with relative ease, thus providing low friction. Strong interatomic bonding and packing in each layer is thought to help reduce wear. Operating environment has a significant influence on lubrication, i.e. low friction and low wear, properties of graphite. It lubricates better in a humid environment than a dry one which results from adsorption of water vapor and other gases from the environment which further weakens the interlayer bonding forces, resulting in easy shear and transfer of the crystallite platelets to the mating surface. Thus transfer plays an important role in controlling friction and wear. Graphite oxidizes at high operating temperatures and can be used up to about 430°C.

One of the fullerene molecule is C₆₀, commonly known as Buckyball. Since the C₆₀ molecules are very stable and do not require additional atoms to satisfy chemical bonding requirements, they are expected to have low adhesion to the mating surface and low surface energy. Since C₆₀ molecules with a perfect spherical symmetry are weakly bonded to other molecules, C₆₀ clusters get detached readily, similar to other layered-lattice structures, and either get transferred to the mating surface by mechanical compaction or are present as loose wear particle which may roll like tiny ball bearings in a sliding contact, resulting in low friction and wear. The wear particles are expected to be harder than as-deposited C₆₀ molecules because of their phase transformation at high asperity contact pressures present in a sliding interface. The low surface energy, spherical shape of C₆₀ molecules, weak intermolecular bonding, and high load bearing capacity offer potential for various mechanical and tribological applications. The sublimed C₆₀ coatings and fullerene particles as an additive to mineral oils and greases, have been reported to be good solid lubricants comparable with graphite and MoS₂ [3], [4], [5].

Diamond crystallizes in the modified face-centered cubic (fcc) structure with an interatomic distance of 0.154 nm. The diamond cubic lattice consists of two interpenetrating fcc lattices displaced by one-quarter of the cube diagonal. Each carbon atom is tetrahedrally coordinated, making strong σ (covalent) bonds to its four carbon neighbors using the hybrid sp³ atomic orbitals which accounts for its highest hardness (80 to 104 GPa) and thermal conductivity (900–2100 W m⁻¹ K⁻¹, on the order of five times that of copper) of any know solid, and a high electrical resistivity, optical transmission and a large optical bandgap. It is relatively chemically inert, and it exhibits poor adhesion with other solids with consequent low friction and wear. Its high thermal conductivity allows dissipation of frictional heat during sliding and protects the interface, and the dangling carbon bonds on the surface react with the environment to form hydrocarbons which act as good lubrication films. These are some of the reasons for low friction and wear of the diamond. Diamond and its coatings find many industrial applications: tribological applications (low friction and wear), optical applications (exceptional optical transmission, high abrasion resistance), and thermal management or heat sink applications (high thermal conductivity). The diamond can be used to high temperatures and it starts to graphitize at about 1000°C in ambient air and at about 1400°C in vacuum. Diamond is an attractive material for cutting tools, as an abrasive for grinding wheels and lapping compounds, and other extreme wear applications.

The natural diamond particularly in large sizes is very expensive and its coatings, a low cost alternative, are attractive. The true diamond coatings are deposited by chemical vapor deposition (CVD) processes at high substrate temperatures (on the order of 800°C). They adhere best on a silicon substrate and require an interlayer for other substrates. A major roadblock to the widespread use of true diamond films in tribological, optical and thermal management applications, is the surface roughness. Growth of the diamond phase on a nondiamond substrate is initiated by nucleation either at randomly seeded sites or at thermally favored sites due to statistical thermal fluctuation at the substrate surface. Based on growth temperature and pressure conditions, favored crystal orientations dominate the competitive growth process. As a result, the grown films are polycrystalline in nature with relatively large grain size (>1 μm), and terminate in very rough surfaces with rms roughnesses ranging from few tenth of a micron to tens of microns. Techniques for polishing these films have been developed. It has been reported that the laser-polished films exhibit friction and wear properties almost comparable with that of bulk polished diamond [6], [7].

Amorphous carbon has no long-range order and the short-range order of carbon atoms can have one or more of three bonding configurations — sp³ (diamond), sp² (graphite) or sp¹ (with two electrons forming strong σ bonds and the remaining two electrons left in orthogonal p_y and p_z orbitals to form weak π bonds). Short-range order controls the properties of amorphous materials and coatings. Hard amorphous carbon (a-C) coatings, commonly known as diamondlike carbon or DLC (implying high hardness) coatings, are a class of coatings which are mostly metastable amorphous materials but include a micro- or nanocrystalline phase. The coatings are a random network of covalently bonded carbon in hybridized tetragonal (sp³) and trigonal (sp²) local coordination, with some of the bonds terminated by hydrogen. These coatings have been successfully deposited by a variety of vacuum deposition techniques on variety of substrates at or near room temperature. These coatings generally reproduce substrate topography and do not require any post-finishing. However, these coatings mostly adhere best on silicon substrates. Best adhesion is obtained on substrates that form carbides, e.g. Si, Fe and Ti. Based on depth profile analyses using Auger and XPS of DLC coatings deposited on silicon substrates, it has been reported that a substantial amount of silicon carbide (on the order of 5–10 nm in thickness) is present at the carbon–silicon interface for the coatings with good adhesion and high hardness (e.g. Ref. [8]). For good adhesion of DLC coatings to other substrates, in most cases, an interlayer of silicon is required except for cathodic-arc-deposited coatings.

There is significant interest in DLC coatings because of their unique combination of desirable properties. These properties include high hardness and wear resistance, chemical inertness to both acids and alkalis, lack of magnetic response, and an optical band gap ranging from zero to a few eV, depending upon the deposition conditions. These are being developed for a wide range of applications including tribological, optical, electronic, and biomedical applications [1], [9], [10]. The high hardness, good friction and wear properties, versatility in deposition and substrates and no requirements of post-finishing make them very attractive for tribological applications. Two primary examples include overcoats for magnetic media and heads for magnetic storage devices [11], [12], [13], [14], [15] and the emerging field of microelectromechanical systems [16], [17], [18]. The largest industrial application of the family of amorphous carbon coatings, typically deposited by DC magnetron sputtering, plasma-enhanced chemical vapor deposition, or ion beam deposition techniques, is in magnetic storage devices. These are employed to protect against wear and corrosion, magnetic coatings on thin-film rigid disks and metal evaporated tapes (Fig. 2a) and the thin-film head structure of a read/write disk head (Fig. 2b). To maintain low physical spacing between the magnetic element of a read/write head and the magnetic layer of a medium, thicknesses ranging from 3.5 to 15 nm are employed. Mechanical properties affect friction wear and these need to be optimized. In 1998, Gillette introduced Mach 3 razor blades with ultra-thin DLC coatings which have the potential of being a very large industrial application. DLC coatings are also used in other commercial applications such as glass windows of supermarket laser barcode scanners and sunglasses.

In this invited article, a state-of-the-art review of recent developments in the chemical, mechanical and tribological characterization of ultra-thin amorphous carbon coatings is presented. An overview of most commonly used deposition techniques is presented next followed by typical chemical and mechanical characterization data and typical tribological data both from coupon level testing and functional testing.