Applications

In order to place IBAD processing in the proper context with respect to potential applications, examples of current and projected IBAD uses are described below. Applications are discussed in the areas of optical films, oxidation- and corrosion-protection coatings, and tribological coatings. Applications are quite extensive in the optical thin-film industry, where the primary advantage offered by IBAD is film densification. Refractive index stability and freedom from environmental degradation are thus direct benefits of using the IBAD process in these applications. Research in the area of wear- and corrosion-resistant coatings for metals and ceramics is in an early development stage.

Films deposited using dry, benign, energetic deposition techniques are also attractive for replacing wet electrochemical processes, such as chromium and cadmium plating, that can pollute the environment. As a result of research already conducted, corrosion and wear problems involving planar and cylindrical geometries of parts less than 1 m (40 in.) in size can already be addressed technically using these energetic-deposition techniques. The issue that needs further attention, then, is how each technique and the economics thereof relates to the scale-up of specific applications and associated geometries.

Optical Films. As already stated, the IBAD process was promoted earliest by those workers interested in optical thin films. This involved, in turn, two kinds of applications: those in which densification is the primary concern and those in which graded refractive index profiles are required.

For applications involving densification, low-energy argon or oxygen ions are typically used to bombard the optical thin films during deposition (Ref 16). As a result, the density is increased, sometimes up to that of the bulk material. The primary attraction is not necessarily that the refractive index is increased to near-bulk values, but that the index value is stable under humidity and temperature variations, because there are no voids or pores in the film that adsorb water vapor. This simplifies optical-coating design and promotes better control and reproducibility in the fabrication process. Another benefit of using low-energy ions is that adhesion to the substrate is improved, which also helps to increase production yields.

The IBAD process is just starting to be used to fabricate graded index coatings for antireflection coatings, reflection filters, and mirrors (Ref 115). Some of these devices can be tens of microns thick, which means that stress control becomes very important. In general, low energies are desired for the deposition of optical films to reduce absorption caused by radiation damage.

Ion-Beam Deposition. Several groups are depositing diamond-like carbon (DLC) by either direct ion-beam deposition or sputter deposition of carbon in the presence of ion bombardment (Ref 90). Using a Kaufman-type ion source, methane is introduced into the plasma and, at energies between 100 and 1000 eV, DLC is deposited at rates between 0.1 and 1

nm/s (1 and 10 A/s). Applications include hard, protective coatings for optics and windscreens on vehicles. Although

DLC absorbs strongly in the visible range, coatings that are between 20 and 200 nm (200 and 2000 A) in thickness remain transparent enough to be used as protective transmission coatings. As of 1994, commercial sources are available to produce these coatings over reasonably large areas (200 mm, or 8 in., in diameter). The advantages of these coatings include low porosity, high scratch hardness, and high adhesion to most substrates.

Aqueous Corrosion. The first industrial application of IBAD for purposes of wear or corrosion resistance appears to be the coating of electric razor screens with titanium nitride (Ref 7). In this case, the choice of IBAD processing was dictated by both the superior adhesion of the films and a decreased number of pin holes, combined with low-temperature deposition on 316 stainless steel substrates. Because IBAD films are dense and have few pin holes, they are attractive for corrosion-protection applications.

Only a limited number of studies on the corrosion behavior of IBAD coatings have been made, but the early results look promising. Platinum, titanium carbide, titanium nitride, boron, diamond-like carbon, chromium nitride, boron nitride, chromium oxide, silicon nitride, and silicon coated on metals using the IBAD process provide excellent corrosion resistance (Ref 7, 8, 9, 43, 45, 116).

One company (Ref 13) has set up a processing line that coats steel sheet continuously with aluminum, titanium nitride, or aluminum oxide for decorative panel applications. Figure 3(b) depicts the apparatus. Part of the cost of the process is recovered by the use of inexpensive steel substrate and coating materials. The resulting films are adherent and more ductile than bulk materials, because of the microcrystalline or amorphous structures. Therefore, they can yield with the metal sheet so that some working of the metal should be possible after the coating is applied.

High-Temperature Oxidation. Work on achieving oxidation protection with IBAD coatings is also very new. Only a few results are in the literature describing the use of chromium nitride (Ref 100), titanium nitride (Ref 100), and silicon nitride (Ref 76) to protect titanium alloys. This seems to be a promising area for further research.

Ion-Induced Chemical Vapor Deposition (CVD). Several researchers have used reactive IBAD (mode 3 of Table 1) to produce unique hydrocarbon or ceramic films (Ref 117). In this process, a gas is introduced into the chamber, the substrate is cooled to induce condensation of the gas, and an ion beam strikes the surface. During the process, hydrocarbon bonds are broken, volatile species are released, and a coating is produced. For silicone oil vapor, the films can range from very low friction solid lubricants to very hard, corrosion-resistant silicon oxycarbide (SiOxCy) coatings, depending on the arrival ratio of ions to vapor-condensed atoms. This process is similar to CVD, in which the high temperature of the substrate provides the energy to initiate chemical reactions that are responsible for film formation. In the ion-beam case, the same or similar reactions can be beam-induced at room temperature, opening the possibility of depositing CVD-like films on polymers and other temperature-sensitive substrates.

Friction and Wear. The IBAD process is being used to deposit solid-lubricant coatings, such as molybdenum disulfide. Advantages are that the coatings adhere to the substrate and have a longer lifetime as a result of densification (better coherence) (Ref 87, 118).

Hard coatings, such as titanium nitride, are by far the most extensively studied (Ref 1). The hardness of these coatings can be varied over a large range by microstructure control (Ref 55). The ductility of these films, even for the highest hardness, is much larger than that for bulk material or CVD titanium nitride films. Boron nitride is also a coating that is readily deposited by IBAD and has good wear characteristics (Ref 119). The IBAD technique is the only one, as of 1994, that is capable of depositing cubic boron nitride that is theoretically as hard as diamond (Ref 72). Currently, molybdenum disulfide, titanium nitride, and ion-stimulated CVD of silicone are the most developed materials for tribological applications.

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