Figure 3. Raman spectra of Diamond like Cartoon (DLC) film and PECVD diamond film.

attributed to sp3 bonding in the material.. Large single crystal graphite displays a single high frequency line at about 1575 cmor 1580 cm as a result of sp2 bonding. Polycrystalline graphite exhibits a line at 1355 cm'1. Subsidiary peaks occur near the main peaks due to the presence of defects, mixed phases and second order spectra.

The superior physical properties of diamond films, with particular reference to their mechanical and tribological properties make them desirable candidates for wear resistance applications. Some of the critical issues with respect to hard coatings for wear resistance include the control of the surface structure in terms of the grain size, the grain morphology and the surface smoothness. In addition the adhesion between diamond films and structural materials of interest is an important parameter that needs to be investigated.

It has been established that the microstructure of carbon films can be controlled over a wide range with the possibility of achieving grain sizes and morphologies ranging from large grain polycrystalline films with grain sizes in the several micron range to microcrystalline films with grain sizes below 100nM. At the extremes of this spectrum are amorphous carbon films on the one hand and single crystal diamond films on the other. With the current state of the technology amorphous films are largely composed of sp2 bonded carbon and these films are normally called diamond like carbon (DLC) films because they possess many of the properties of diamond. Diamond films fabricated by the plasma enhanced chemical vapor deposition techniques on the other hand are crystalline, characterized by true diamond bonding with ali of the desirable properties of natural diamond crystal, with some exceptions. The growth of single crystal diamond is a much sought after goal, primarily for the fabrication of electronic devices, and, perhaps for use in specialized applications where the mechanical properties of single crystals, unhampered by the presence of grain boundaries, would be advantageous. For instance, single crystal diamond film growth may be feasible on single crystal turbine blades made from nickel based superalloys since nickel exhibits the closest lattice match with diamond and thus commensurate epitaxy may be possible between diamond and nickel or nickel alloys. The growth of single crystal diamond for practical purposes must await the development of the technology of heteroepitaxial growth of diamond on lattice matched substrates such as copper and nickel.

In figure 4 are shown examples of the type of surface structures achieved in diamond films. It is possible to modify the grain size and morphology over a wide range and typical examples are shown in figure 5. The control of the grain size and the surface topography will be important for the particular end use. Two extreme examples of the structures that are suitable for cutting or machining applications and for hard coatings where smooth, non abrasive surfaces are required, are shown in figures 6 and 7.

Figure 6 shows the surface structure, as observed in a scanning electron microscope, of a silicon carbide tool bit before and after the deposition of a diamond film. The corresponding Raman spectrum of the diamond film indicates that the film is predominantly composed of carbon atoms directionally bonded with sp3 bonds characteristic of diamond bonding. The surface structure of the film is composed of sharp facets which should lead to an excellent, hard, cutting surface. Figure 7 is an example of of relatively smooth, hard coating which is less abrasive than the film shown in figure 6. In this case the film has been deposited on a sapphire surface with the intention of conferring additional abrasion resistance to the sapphire. The corresponding Raman spectrum of the diamond film indicates that the film is a two phase material with some degree of graphically (sp2) bonded material in the diamond matrix. The mechanical properties of such mixed phase materials have not been

Figure 4. Typical examples of the surface structure of diamond films. The left micrograph shows a rounded smooth grain structure and the right micrograph shows a sharply facetted structure.

Figure 5. An example of the range of grain sizes achievable in PECVD diamond films.

determined to date. It is generally observed that increasing surface smoothness of the films is achieved at the expense of the purity of the film with respect to diamond bonded material, i.e., the smoother films with rounded grains are frequently composed of mixed phases whereas the sharply facetted surface features are characteristic of single phase diamond bonded material.

One of the principle requirements of hard coatings is the nature of the bond between the overcoat and the substrate. The adhesion characteristics of diamond films have been investigated in some depth and the general conclusions are as follows: Materials that are strong carbide formers such as the refractory metals are observed to be strongly bonded to diamond films deposited on them whereas poor carbide formers ( e.g. Copper, Silver, Germanium ) are poorly bonded to diamond films. Table 6 shows the adhesion characteristics of typical materials to diamond films.

Table 6. Adhesion characteristic of established whether such adhesion promoters also lead to good tribological properties in tribological tests such as the pin against disc test.

The plasma enhanced CVD process for depositing diamond films is a non line-of-sight approach and as such the process can be adapted to diamond coat three dimensional objects such as complex tools and other components and mechanisms that can benefit from the superior mechanical properties of diamond films. It is expected that the development of appropriate plasma equipment for the deposition of films on various sized components of various shapes is well within the capability of the technology.

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