Beam Technologies for Integrated Processing (1992)

The application of beam technologies to produce new materials and components, other than microelectronics, is not a new development. This chapter reviews the function of beams in the development of coatings and surface modification and the formation of net shapes, composites, nanophases, and optical surfaces, including treatment of polymeric substrates. New exotic developments in beam processing of materials and some applications for beam technologies also are examined.

Metal, alloy, and ceramic coatings onto metal, alloy, ceramic, or polymer substrates of various sizes and shapes have been extensively developed and used in the past 30 years. They cover a wide spectrum of applications as summarized in Table 5-1, which lists specific materials and processing methods employed. The techniques used to prepare these materials are classified into two main groups, physical vapor deposition (PVD) and chemical vapor deposition (CVD). These techniques and their applications were discussed previously in Chapter 3.

Considerable interest has been aroused in the synthesis of diamond films because of their potential applications in microelectronics, optics, and tribology (Spear, 1989). Synthesis of diamond films by a variety of CVD, PACVD, and PVD techniques has been reported. Applications of these coatings have been limited by the inability to deposit films with smooth surface morphology and with desired optical and electrical properties at acceptable deposition rates, and at deposition temperatures low enough to be compatible with ultraviolet (UV) and infrared (IR) materials or with other process steps employed in microelectronics.

Some characteristic properties of individual carbon phases are compared in Table 5-2. The type of film deposited depends on the deposition technique. Table 5-3 lists some potential applications of diamond and diamond-like carbon (DLC) films along with the properties required for each application. Single-crystal diamond films have application for microelectronic and optoelectronic devices. Their high thermal conductivity makes them a candidate for heat sinks, and their electronic properties make them a candidate material for high-power microwave-and millimeter-wave devices and as heat sinks for various high-power devices. Moreover, their large bandgap (about 5.45 eV) can be used to make a variety of UV detectors, and their high resistivity makes possible low-noise UV detectors. Finally, because of their high hardness, there is great interest in tribological applications, such as cutting tools, except for the cutting of steel, where solution of the carbon into the steel can occur. In 1991, several companies have put on the market diamond-coated cutting tools for machining aluminum alloys and polymer-based composites.

Diamond films have been prepared by CVD, PACVD, and ion beam deposition techniques. CVD techniques used for the synthesis of diamond films are the chemical transport method, hot-filament thermal CVD techniques, electron-assisted CVD, and flame torch methods. It appears that independent and accurate control of atomic hydrogen in the process environment is key to the deposition of good-quality diamond films. PACVD techniques (with dc, rf, or microwave excitation) have been widely used for the synthesis of diamond and diamond-like carbon coatings. In addition, ion-beam-assisted and low-temperature ARE have been used to prepare diamond films. Recently, small-area diamond single-crystal films have been prepared on a copper substrate that have potential use for microelectronic applications (Narayan et al., 1991).

Diamond-like carbon produced by sputter deposition is principally used as a wear-resistant coating for magnetic disks. Some varieties of also are used for coating polymeric eyeglass lenses for abrasion resistance.

Cubic boron nitride (CBN), like diamond, is a metastable material at ambient temperatures and pressures. It possesses an attractive combination of properties similar to diamond—high thermal conductivity, high electrical resistivity, very high hardness, chemical inertness, transparency over a wide range of wavelengths, from UV through the visible into the IR, and has a large bandgap.

Synthesis of CBN films can be accomplished by various CVD, PACVD, and PACVD techniques, at deposition temperatures from ambient to 1000ºC, depending on the process used. Almost all the reactant species used in CBN synthesis are either toxic or explosive. To avoid this problem, a unique process has been developed using the reaction between nontoxic boric acid and ammonia in a PAPVD process to deposit CBN films at temperatures as low as 400ºC. Unlike diamond films, there is no difficulty in nucleating the material on a variety of substrates. Various applications in tribology, optoelectronics, and microelectronics have been studied. CBN is a very potent competitor to diamond films, and the state of development is considerably advanced. Unfortunately, it lacks the mystique associated with diamond.

Two beam techniques are predominant in surface modification of materials; ion implantation and laser treatment. Ion implantation is presently used in a limited but demanding number of high-technology, high-precision, and high-value-added, critical-use applications. The main advantages of ion implantation (Sioshansi, 1989) are flexibility in that any element can be introduced into any substance; no thermodynamic constraints exist, such as the requirement of an elevated temperature for diffusion or initiation of a chemical reaction; no refinishing or reheat-treating of the part is required, since the ion implantation can be performed at or near room temperature; no noticeable change occurs in the dimensional integrity of the part, no shape distortion, and no need for final finishing or polishing of the components; and no discrete interface is produced that can fail, since the ions penetrate into the original surface.

The main application of ion implantation to date has been in the treatment of metals. However, in recent years there has been significant interest in the treatment of ceramic and polymer substrates by ion implantation. Table 5-4 shows the variety of materials treated by ion implantation and lists the properties influenced by the process. Many of these have been reviewed recently by Smidt (1990a,b).

There are numerous applications in which ion implantation has been tested in fields ranging from aerospace to biomaterials. Table 5-5 lists some successful ion implantation applications. There are many new applications where ion implantation is currently under investigation. These are mainly in areas of changing the optical properties of materials, influencing catalysis of surfaces, creating new magnetic alloys, improving adhesion properties of surfaces, and, finally, improving biocompatibility of materials. These applications are expected to have a significant impact on future trends in surface-engineered components.

Laser treatment, since it can be employed in atmosphere or nonvacuum ambients, offers greater flexibility for modifying large, irregular surfaces. Laser beams are being used to harden metal surfaces, and they provide corrosion resistance. In addition, selective etching using lasers is in fairly widespread use (e.g., as a marking tool). Laser welding also is finding application but on a limited basis. The ease of implementing lasers, compared to ion beams, makes this approach particularly suitable for integrated manufacturing.

The unique features of lasers have led to their use in a variety of shaping operations, including the removal (Copley, 1986) and deposition of materials (Deitz, 1990). They are routinely used for hole drilling and cutting sheets (Steen and Kamalu, 1983) and show promise for use in turning and milling operations (Copley, 1985). Lasers also can be used to build up shapes through processes such as solidification (Breinan et al., 1980), sintering (Deckard and Beaman, 1989), polymerization (Hull, 1986), and in CVD (Osgood et al., 1983). They have been used

routinely to build up shapes by welding (Mazumder, 1983) and soldering. The advantages in carrying out any of these processes with a laser in an integrated processing system are short interaction times, the atmosphere may be varied, a high degree of flexibility, and ease of on-line inspection and process control.

Important issues with respect to many material removal processes are the roughness and cleanliness of the machined surface, the material removal rate, and the strength and toughness of the laser-machined article. These issues, along with those related to process integration, are discussed in this section.

Hole drilling and sheet cutting with lasers have proven to be commercially viable processes. In fact, on a dollar basis, sheet metal cutting accounts for the largest fraction of CO₂ laser sales. In this application, laser cutting provides an economical alternative to a punch and die for cutting nonstandard shapes in small to moderate size batches. Creating a computerized numerical control program for laser cutting is a much less expensive step than fabricating a punch and die set for press work. Nevertheless, the use of laser cutting in combination with a punch and die to handle standard shapes has proven to be a cost-effective, flexible approach to sheet metal cutting. Both operations are now routinely carried out in series in continuous flow and transfer station systems. Material is removed by melting the substrate and blowing the melt away with an inert gas jet. In some cases a reactive gas jet is used (e.g., oxygen for cutting steel or titanium), where some of the metal burns, and the enthalpy of the oxidation reaction adds to the energy provided by the laser to melt additional material. Excellent edge quality is attained, so that the need for subsequent deburring operations is often eliminated. Because of the speed of the cutting, there is little time for heat transfer to the workpiece, so the heat-affected zone is normally very small.

Advantages of laser hole drilling include the capability of drilling fine holes with high aspect ratios, no machining forces are introduced, applicability to a wide range of materials, and adaptability to numerical control. Laser-drilled holes tend to be tapered, and resolidified material, vapor-deposited residues, and (in polymeric materials) charred regions are often observed. Nevertheless, laser hole drilling has been used successfully in a wide variety of applications, including drilling fine holes in diamond dies (e.g., with a ruby laser), cooling holes in nickel-base superalloy turbine blades, perforation of cigarette paper for use with low-tar and low-nicotine filters, and holes in baby bottle nipples. Truly remarkable results have been attained in machining polymeric and organic materials with excimer lasers. A high degree of spatial resolution and excellent surfaces can be attained, even to such accomplishments as holes drilled in a human hair (Znotins et al., 1987).

Laser-assisted machining, a form of hot machining, has been used in turning hard-to-machine metallic alloys. In this process the laser heats the material in front of the cutting tool, thereby improving its machinability. Under force-limited cutting conditions, for example, a factor of two increase in material removal rate, without increase in cutting force, has been reported for difficult-to-machine turbine materials, such as Inconel 718 and Ti-6Al-4V.

Lasers have also been used to shape hard ceramic materials by ablation. In Si₃N₄, for example, high material removal rates (1 cm³ in 200 sec) and smooth surfaces (3-µm arithmetic average surface roughness) have been reported for a moderate incident power of 560 W and a scan speed of 240 cm/sec (Wallace and Copley, 1989). Although laser machining causes some loss of strength, it has been shown that this loss can be completely recovered by an inexpensive etching

treatment (Tao et al., 1989). In some cases, reactive gases have been used in conjunction with laser ablation to help remove the reaction products, a process called laser-assisted chemical etching.

There has been considerable interest recently in the concept of ''desktop'' manufacturing, which permits production of a three-dimensional shape in two steps. The first step involves positioning a computer graphics rendering of the shape with respect to coordinate axis and sectioning the shape into thin layers. The second step converts the shape of each layer to a set of instructions that are communicated to a laser that scans a surface so as to reproduce the layer. In one version of this process, the layer is reproduced by selective polymerization by the laser of a layer of liquid resin. In another version the layer is reproduced by selective sintering by the laser of a layer of plastic or wax powder. So far, desktop manufacturing has been mostly applied to plastics and waxes, with considerable success in attaining smooth surfaces and good shape definition (Deitz, 1990). The technique has found application in prototyping and modeling with considerable interest in applying the technique to form waxes for prototype investment castings (ASM International, 1990). There is a possibility that the technique could be extended to structural materials through application of other processes, such as sintering, melting, and laser chemical vapor deposition (LCVD).

Lasers have been used with considerable success to produce autogeneous weldments and hard facing and for soldering. The laser's characteristic short interaction times have the beneficial effect of limiting undesirable reactions in the joining of dissimilar metals.

The increasingly stringent requirements on structural and electronic ceramics often cannot be met using traditional mechanically reduced powders. In addition, the increased prominence of covalently bonded ceramics (e.g., SiC and Si₃N₄) has increased the requirements for nonconventional, highly q powders. The improved powders should include the following characteristics (Bowen, 1980): small particle size, freedom from agglomeration, narrow size range, spherical shape, and highly controlled purity. Such powders can be produced either by liquid (e.g., sol-gel), solid (e.g., decomposition of salts such as carbonates), or vapor-phase techniques. The common theme in all these techniques is that the powders are built up or synthesized rather than broken down or comminuted. The vapor-phase techniques, in general, can be categorized as beam processes and will be considered briefly below.

Vapor-phase techniques for powder processing have been described in a number of recent papers (Kato, 1987; Rice, 1987; Marra and Haggerty, 1987). These techniques have a number of potential advantages for the production of closely controlled powders, such as high purity of the product because of the high-purity gaseous reactants, loosely agglomerated powders because of the highly diluted reaction, ultrafine powders with narrow size distributions, and versatility for producing a wide variety of powders, including metals as well as oxide and nonoxide ceramics. In laser processing the gas is heated directly by coupling the emitted photons with the absorption lines of the reactant gas. In an optimized process the efficiency is essentially equal to that for conversion of electrical energy to laser output. Silane (SiH₄) absorbs strongly at the 10.6-µm

wavelength of a CO₂ laser. Thus, silicon, silicon nitride, and silicon carbide are readily synthesized by the following reactions (Cannon et al., 1982):

The other reactant gases, such as methane and ammonia, where required, are heated indirectly by collisional processes.

Conventional CVD has also been used to produce a wide variety of powders. The key to producing fine powders is the use of a high degree of supersaturation so that homogeneous nucleation occurs. Particle size is determined from the relative nucleation and growth rates, which are controlled by the reaction parameters. Among the powders that have been produced by conventional CVD processes are Si₃N₄, SiC, TiN, ZrN, VN, TiC, Mo₂C, a-Fe₂O₃, TiO, a-A1₂O₃, ZrO₂, and Nb₃ Sn (Kato, 1987).

Other vapor-phase processes that have been used for powder processing include inductively coupled and microwave plasma systems, steam hydrolysis, chloride oxidation (Johnson, 1987), and high-pressure sputtering (Suh et al., 1991).

Metal-matrix and ceramic-matrix composites, and the ceramic whiskers and fibers on which they are based, are advanced technologies whose applications are just beginning. A variety of innovative processes have been developed for them, many based on beam processing techniques. Vapor deposition techniques, discussed below, have been used to form both the reinforcing whiskers and filaments for metal and ceramic composites, and chemical vapor infiltration of fiber preforms is used for matrix formation in ceramic composites. Recent applications of beam technologies to composites are summarized in Table 5-6.

Chemical vapor processing is widely used to produce ceramic whiskers, platelets, and continuous fibers for reinforcement of composites. The chemistry and properties of the fiber-matrix interface are important in developing ceramic-matrix composites, and each interface must be tailored to give a bond strength that is suitable to provide an appropriate toughening mechanism (e.g., to control fiber pullout). CVD is probably the most common method currently used for coating fibers to thicknesses ranging from nanometers to several micrometers to provide an appropriate interface between fibers and matrix (Cranmer, 1989; Kerans et al., 1989).

Silicon carbide whiskers are grown today on a commercial basis from the pyrolysis of rice hulls, which are composed mostly of cellulose with hydrated amorphous silica (Lee and Cutler, 1975). Heating rice hulls in a coking furnace at 1200º to 1800ºC causes a reaction between silicon suboxide and carbon via the gas phase to form silicon carbide whiskers (Nutt, 1988). These whiskers currently are finding commercial application as reinforcements in ceramic cutting tools and are being intensively evaluated in structural composites. Vapor-grown particulates, as well as whiskers, have been used as reinforcements in ceramic cutting tools (Rothman et al., 1986; Lee and Borom, 1988).

In addition to SiC, whiskers of TiC, ZrN, TiN, and ZrC have been formed by CVD. In each case, the source of carbon was methane, with metal tetrachlorides for the metal ion and nitrogen for the nitride ion. Nickel, palladium, and platinum, among others, were effective for catalytic growth of these whiskers on a mullite procelain (Kato et al., 1977; Kato and Tamari, 1980; Wokulski and Wokulska, 1983). The growth temperature, typically 1100º to 1300ºC, produced whiskers with diameters ranging from 10 to 300 µm and lengths of a few millimeters. The TiC whiskers appear to grow by a vapor-liquid-solid (VLS) mechanism on Ni at the initial stage and by a vapor-solid (VS) mechanism on mullite after the initial stage (Tamari and Kato, 1979). Whisker technology reviewed by Levitt (1970) provides rather extensive coverage of CVD and other growth methods.

Catalyzed growth of both carbon and silicon carbide whiskers has been reported. Japanese (Koyama and Endo, 1972, 1983), French (Oberlin et al., 1976), and American (Tibbetts, 1989) investigators have reported the growth of carbon whiskers from metal catalyst particles (usually Fe), of a few nanometers in diameter, exposed to a mixture of hydrocarbon gases and hydrogen at a temperature near 1000ºC. The growth rate may be as high as several millimeters per minute. The filament length is several centimeters, while the whisker diameter remains about the same as the catalyst particle (Tibbetts, 1989). Baker and coworkers have been actively studying the catalyzed growth of carbon filaments using controlled-atmosphere electron microscopy, in which a gas reaction cell is incorporated within an electron microscope (Baker and Harris, 1978). This permits continuous observation of the gas-solid reactions as they occur. A comprehensive review of the literature on the formation of filamentous carbon was published by Baker and Harris (1978). The whiskers can be made thicker by in situ CVD to diameters of 7 to 10 µm and can be grown to several centimeters in length. Strengths of 2.9 GPa (420 Kpsi) and modulus values of 240 GPa (34 Mpsi) have been measured. The whiskers can be given a very high degree of preferred orientation by high-temperature heat treatment to produce a morphology consisting of graphitic "C" planes wrapped around the whisker axis. This configuration gives many unique properties, including exceptionally high thermal conductivity along the whisker axis. A detailed theoretical analysis of the growth process has been presented by Tibbetts (1989).

In a somewhat analogous development, workers at the Los Alamos National Laboratory (Milewski et al., 1985) have reported the VLS process for producing ß-SiC whiskers. Growth of the whisker occurs by precipitation from the supersaturated catalytic liquid at the solid whisker/liquid catalyst interface. A basic requirement is that the molten catalyst take into solution the atomic components of the whiskers to be grown. For silicon carbide growth, transition metals and iron alloys satisfy this requirement (Milewski et al., 1985). The process, operated at 1400ºC using 30-µm stainless steel catalyst and methane, hydrogen, and silicon monoxide precursors, produces whiskers of a rounded triangular cross-sectional shape (4 to 6 µm in diameter) with lengths ranging from 10 to 100 mm. Tensile strength values average 8.4 GPa (1,220 Kpsi), with average fracture strain of 1.74 percent and an average elastic modulus of 578 GPa (83 Mpsi). These mechanical properties are superior to those of the best polymer-derived ceramic fiber yarns (Petrovic et al., 1985).

Continuous monofilaments of boron and silicon carbide are being produced on a fine wire substrate by CVD. Boron fibers (Table 5-7) are formed by CVD onto an electrically heated fine tungsten filament. A gas mixture of boron trichloride and hydrogen is used to deposit the boron. The tungsten wire core is about 3 µm in diameter, with the final boron filament being 100 to 200 µm in diameter. The properties of this fiber are quite good, having an elastic modulus of 58 Mpsi, a tensile strength >500 Kpsi, and a density <2.6 g/cm³. To avoid reactions between the boron filament and metals during high-temperature fabrication of the composites, the surface of the fiber is coated by CVD with a thick diffusion barrier of B₄C. The B₄C coating also increases the tensile strength by about 10 percent and improves composite properties.

Silicon carbide filaments are produced by the decomposition of methyl trichlorosilanes on a carbon core (Suplinskas and Henze, 1982; Marzik, 1984). The CVD-produced SiC fiber has properties superior to polycarbosilane-derived fibers (Yajima, 1980), although its fiber diameter (>100 µm) is too large for weaving or for some fabrication processes. The fibers are available with a final coating of carbon that improves chemical compatibility in certain matrices and also improves fiber strength and handleability.

Metal-matrix composites are of strong interest today for advanced applications where lightweight, high specific stiffness, and good high-temperature strength are required. An

example of a successful application of metal-matrix composites is the boron-aluminum composite tubing used as cargo bay stiffeners on the space shuttles.

CVD-derived filaments are of particular interest for high-performance metal-matrix composites, both for their superior properties and because the monofilament form lends itself well to metal-matrix fabrication processes. In addition to the cargo bay stiffeners mentioned above, extensive work is under way on titanium alloys reinforced with CVD-derived SiC fibers (Lancin et al., 1988).

Beam technologies have been employed to produce metal-matrix composites, although squeeze casting or spray techniques (Zanchuk, 1988) are used more commonly. Titanium alloy (Ti-6Al-4V) foil was made some years ago on a pilot plant scale using electron beam evaporation (Hughes, 1974). Even earlier, Bunshah and Juntz (1965) showed that electron beam distillation is an attractive technique for depositing sheets of reactive metals such as titanium.

Ceramic whiskers can be processed with ceramic powders to give a ceramic-matrix composite having improved fracture toughness (Milewski, 1986). Ceramic cutting tools, formed with SiC whiskers and TiC particulates in an alumina matrix, have met with considerable success (Rothman et al., 1986; Lee and Borom, 1988) and are now available commercially. The matrix is based on SiC whiskers in an oxide matrix (Rhodes, 1985; Tiegs and Becher, 1985). The SiC whisker-reinforced alumina cutting tool produces a twofold increase in toughness relative to monolithic alumina. The aerospace industry considers it to be the state-of-the-art cutting tool material for rough machining of Ni-based superalloys (Baldoni and Buljan, 1988). However, it is chemically unsuitable for machining ferrous alloys (Billman et al., 1988) because of dissolution of the carbon of the matrix into the iron.

Whisker-reinforced ceramic composites also are under development for a variety of structural applications, including automotive turbocharger rotors (Akimune et al., 1989). Continuous ceramic-fiber/ceramic-matrix composites are being produced by a variety of techniques (Prewo, 1989). Prominent among these is chemical vapor infiltration (CVI). The CVI process was first employed for processing carbon-carbon composites (Buckley, 1988) and is the dominant process for producing carbon-carbon composite aircraft brake linings.

CVI has also been used to form a Nicalon-fiber/SiC-matrix and carbon-fiber/SiC-matrix composites. Ceramic-ceramic composites made of alumina fibers embedded in an alumina matrix were obtained from fibrous alumina preforms using a CVI technique based on gaseous alumina precursors (AlCl₃-H ₂-C1₂). This process is similar to that used for the SiC matrix mentioned earlier. CVI has also been used for forming B₄C, TiC, and BN matrices (Colmet et al, 1986). Silicon carbide fiber-reinforced silicon composites have been formed by CVD of silicon from a mixture of SiCl₄ and H₂ (Hwan et al., 1988). Silicon nitride matrices with both carbon and mullite fibers also have been reported (Gulden et al., 1990a,b). Improvements in fracture toughness have stimulated theoretical analyses of CVI in ceramic-to-ceramic composites, giving models for silicon carbide, alumina, and titanium carbide-matrix deposition within a ceramic-fiber bundle (Tai and Chou, 1988).

CVI has many advantages that have gained it a prominent position as a process for consolidating large, complex, thin-section ceramic-and carbon-matrix composites. Among the key advantages are the following:

• It is a near net-shape process.

• It involves no high-pressure sintering compaction process, so mechanical fiber damage is minimized.

• It is a very pure matrix with no sintering additives that would compromise high-temperature performance.

• It employs processing temperatures that are relatively modest, so thermal damage to the fibers is minimized.

Current commercial CVI processes used for both carbon and ceramic matrices are performed in isothermal reaction chambers (Theis, 1972; Newkirk, 1981; Naslain et al., 1983). In this consolidation process the reactant gases diffuse into the preform and deposit the material on the fiber surfaces to produce a relatively dense composite. The decomposition products diffuse out of the preform and are exhausted from the reaction chamber. The infiltration process is most effective at relatively low temperatures and pressures, where the deposition rates are low. Under these conditions the process is limited by surface reaction rates rather than by mass transport, and deposition occurs uniformly throughout the porous preform. In practice, economics drive the process to higher deposition rates, where localized sealoff can prematurely terminate the infiltration process.

"Directed" CVI processes, involving thermal-gradient, forced flow, or combined thermal-gradient and forced flow systems, have been developed to obviate the limitations of the isothermal approach (Caputo and Lackey, 1984). These approaches minimize the kinetic limitations of the CVI process and reduce the potential for preferential deposition at the external surface of the preform. A new and innovative class of composite materials has been termed microlaminate condensates. Microlaminate condensates are thick coatings or self-supported shapes produced by alternate deposition of the two species from evaporation or sputtering sources. The boundaries between the two laminate layers are noncoherent. The thickness of each laminate varies from 0.1 to 300 µm, and the entire thickness of the deposition can be as high as 1 mm.

Various material systems have been prepared and studied, such as Fe-Cu, Cu-Ni, Cu-Ca, TiC-Ni, and TiC-TiB. The strength of the composite increases with decreasing laminate thickness, particularly below 5 µm. The strength and hardness properties obey the Hall-Petch relationship (Mocvhan and Bunshah, 1982). Thermal conductivity across the thickness of the laminate is markedly reduced. Thus, a metal-ceramic laminate can have the low thermal conductivity of a ceramic without the brittleness. This suggests potential use of thermal barrier coatings (Radhakrishna et al., 1988).

Nanophase materials (at least one dimension in the nanometer size range) possess properties that in many cases are considerably different from the bulk material because of their very high surface area-to-volume ratio (A/V). The large A/V values can be achieved with films (one dimension), fibers (two dimensions), and fine particles (three dimensions). Nanophase matter can be incorporated into composites (particulate, fiber, or laminate) or in applications that use the properties of the fine particles themselves. Examples include those concerned with ultrafine particles of TiO₂ in paints and pigments to the more high-technology applications such as catalysts, ultrahigh-modulus composites, sintered powder products, magnetic recording media,

optical applications, and biomaterials applications. Some typical application areas are listed in Table 5-8. More recently, ultrafine particles of organic molecules have been prepared by gas-phase condensation techniques for potential use in medicine. Some radical changes in properties have been observed (e.g., organic materials that are hydrophobic in bulk form become hydrophyllic when finely divided).

The preparation methods for ultrafine particles can be divided into two major classifications: (1) chemical methods (e.g., redox reactions, sol-gel techniques, polyol processes, flame pyrolysis, CVD, PACVD, and arc or plasma torches) and (2) physical methods (e.g., gas-phase condensation of vapor species in which the depositing species are produced by evaporation using resistance heating, induction heating, arc or electron beam sources, and sputtering). Metal as well as compound particles (oxides, nitrides, etc.) can be prepared; the latter group (compounds) is often produced by reaction of the metal particle with the corresponding reactive gas (Hayashi, 1987; Kato, 1987; Hayashi et al., 1988; Eastman and Siegel, 1989).

Japanese scientists have placed very high priority on basic studies of ultrafine particles in the size range 1 to 100 nm. Their basic work dates back to the early 1970s. In 1987 a 5-year project, operated by the Research and Development Corporation of Japan (JRDC), was concluded at a cost of 1.5 billion yen. The results are published in research articles referenced below as well as in a book in Japanese published in 1988. Recent issues of the Materials Research Society Bulletin have been dedicated to fine particles (1989-1990). In addition, the National Materials Advisory Board published a comprehensive, broad-based committee report on nanosized particles (NMAB-454, 1989). Only nanosized materials formed by beam technologies are the concern of the present study.

In contrast to vapor-grown nanosized carbon fibers, iron whiskers of 5 to 30 nm were produced by CVD (Schladitz, 1968; Lashmore et al., 1977). These a-iron crystallites (Fe-1.8C) showed an exceptional strength of 8 GPa, indicating a potentially large increase in strength (as with the nanolaminates to be mentioned later) with a decrease in crystal size. Furthermore, a joint effort involving Japan's National Defense Academy (Yokosuka, Japan) and Tohuku University (Sendai, Japan) has shown that dispersions of nanoscale silicon carbide or silicon nitride particles greatly strengthen and toughen alumina, magnesia, and mullite-based ceramics (Tennery, 1989).

Synthesis of fine ceramic powders from chloromethylsilanes using pulsed excimer radiation is presently being worked on in Australia. In this technique the silane is placed in a glass container and heated to approximately 10ºC below its boiling point to produce a sufficiently high vapor pressure in the reaction chamber (13.3 KPa). The vapors are photodecomposed by an excimer laser (Ar-F gas mix) operating at 193 nm. The laser energy is absorbed by all of the silane and the decomposition fragments used in the study. The resultant particle sizes of amorphous silicon or ß-SiC varied from less than 10 nm to a maximum of 1.5 µm. Earlier work using continuous wave CO₂ infrared laser-induced photodecomposition of silanes produced ultrafine silicon powders for the production of reaction-bonded Si₃N₄ monoliths with superior strength properties (Cannon et al., 1982; Haggerty et al., 1986).

Argonne National Laboratory is generating equiaxed nanophase materials by the gas condensation process reported by Kashu et al. (1974). The gas condensation method consists of evaporating materials inside a vacuum chamber at I x 10^-7 Torr and then backfilling with low-pressure gas, typically a few hundred pascals of an inert gas such as He. If compounds are to be formed in the condensation process, reactive gases are used. Evaporated atoms collide with gas atoms inside the chamber and form small gas-borne particles rather than a continuous film that would form in vacuo. The condensed fine powders collect on a hollow tube (cold finger) filled with liquid nitrogen from outside the chamber. This synthesis method can produce pure

metals as well as oxides by using oxygen as the gas (e.g., TiO₂, A1₂O₃, and MgO). To produce A1₂O₃, gas-condensed Al-metal particles need to be heated to 1000ºC in air to transform the metal particles to a-Al₂O₃; this results in a very small increase in particle size (about 18 nm), since the low-temperature oxidation is incomplete and produces only an oxide film on the metal particles.

In contrast to molecular beam epitaxy, which is not a volume production method, a semicommercial-scale electron beam vapor deposition process for forming multilayer structures has been constructed. This technique produces alternate layers of aluminum (20 to 1600 nm thick) and a second metal (0.1 to 20 nm thick of Cr, Fe, Mg, Mn, Ni, or Ti) (Bickerdike et al., 1984-1985). This material can be deposited at rates of up to 5-cm thickness in an 8-hour period—a yield of layered nanocrystalline material adequate for mechanical property evaluation. Alternatively, annealing of suitable combinations of such multilayer deposits produces amorphous phases at low temperatures, which can be crystallized to give extremely fine grained equiaxed crystallites (Johnson, 1986). Similarly, using electron beam vapor deposition, multilayered nanostructure laminates of Al-Cu and Al-Ag were formed on a substrate. The Al-Cu laminates showed a tensile yield stress of about 650 MPa when the layer thickness was below 70 rim; above that thickness the yield stress rapidly dropped off (e.g., about 250 MPa at 200 nm thickness). These laminates were formed at impractical deposition rates (4 µm/h) for structural fabrication, relative to those formed by Bickerdike and associates (1984-1985) mentioned above (Lechoczsky, 1978).

Structures synthesized for the modification of optical properties generally rely on interference effects produced by the wave nature of light. Thus, the structure dimensions (number of wavelengths) and refractive index (wavelength in the medium comprising the structure) must be precisely controlled.

Films on surfaces, generally multilayers of insulators or semiconductors, are used to make such items as band-pass filters, dichroic filters, and reflecting or antireflecting surfaces (MacLeod, 1986). These require one-dimensional uniform control of thickness. This control is provided by monitoring, during deposition, optical transmission and reflection of the film with an optical beam. Theoretical analysis of the change in property with thickness is required for interpretation of the monitor, and extensive studies have been undertaken to determine the best method to analyze the light beam for optimal control. This technology is presently highly advanced, achieving thickness control on each of the 20 to 30 layers in the total fiber of between 2 and 10 percent, depending on the application. Physical vapor deposition, often electron beam evaporation, is used. Laser mirror coatings are an important application of such films. Careful use of conventional technology suffices for most optoelectronic devices, and special deposition to avoid absorption is necessary only in large high-power devices. Low absorption requirements also exist for Rugate filters used in laser filters, where high incident power necessitates low absorption and many carefully controlled layers to achieve significant power rejection.

Optically transmitting and electrically conducting films are used in some solar cells, matrix displays, and light shutters. In such applications the films are used for their bulk properties (and only incidentally provide interference effects), although they are often patterned. Optical absorption by the free carriers is a problem that has been solved by the use of large bandgap indium-tin oxide. This material can be applied in many ways, but sputtering is preferred because of cost, and it has become a common commercial process.

Architectural and automotive films are deposited on windows to control energy transfer within some specified aesthetic or visibility constraint (Granquist, 1989). For maximum reduction of air conditioning loads, high reflectance is needed over both the visible and infrared spectra. For colder climates, however, maximum energy conservation requires high transmission of the solar energy in the visible and near infrared, but high reflectance (low emissivity) in the intermediate infrared to return radiation back into the building. A few examples of such practical optical solutions are given in Table 5-9. Some of these films are routinely manufactured, with annual outputs of over 2 million square meters per system, for use in such volume applications as automobiles and buildings.

Dielectric optical waveguides on substrates (Miller and Kaminow, 1984) are used today chiefly in discrete couplers, which may be directional, star, wave-division multiplexing, or polarization retaining. As optoelectronics evolves, applications may be expected in hybrid circuits. Even with fully integrated circuitry available, some applications for discrete and hybrid components are expected to exist. Planar devices, most amenable to beam processing, compete with discrete couplers made by commercially available bulk methods. Most planar devices are used for research and are made by diffusion, plasma-enhanced CVD, or silica soot (fume) deposition. Microlenses (Iga et al., 1984) also may become part of hybrid circuits; they have been made by diffusion or, as Fresnel lenses, by electron beam machining. Presently, commercial lenses are made by bulk processing.

Active dielectric devices in electrooptics include such components as switches, isolators, and spectrum analyzers. Almost all such switches are presently made by diffusing titanium into lithium niobate to make the device structure (Thylen, 1988), but ion implantation has also been used. The titanium provides the refractive index change to create the waveguides within the electrooptic material. These switches have the largest bandwidth from low frequencies of any switch and thus potentially have a broad range of applications. They are presently employed in commercially available equipment for signal processing. Faraday isolators are presently discrete devices, and attempts to make them in a form suitable for integrated optics are still beset with problems.

The volume of polymeric substances used in industry is constantly growing, and plastics are used as substitutes for metals in a variety of applications. Beam technologies are responsible for formation of a variety of specialty plastic materials. These technologies are used either for treatment of bulk or surfaces of polymers. The main objectives for treatment of polymers are outlined in Table 5-10.

The unique ability to tailor the characteristics and properties of materials produced by CVD has led to the use of these materials in a number of advanced energy applications where conventional materials were inadequate. In this sense, CVD has been an enabling technology.

The coated-particle nuclear fuel concept is based on CVD of carbon and ceramic coatings on a fuel ''kernel'' in a fluidized bed (Gulden, 1986). One of the most promising concepts for nuclear power in space is in-core thermionics. The heart of this concept is the tungsten emitter produced by CVD. The emitter has a duplex structure with the inner coating, deposited from tungsten hexafluoride, providing strength and creep resistance, and the outer layer, deposited from the chloride, has a (110) texture to provide the maximum electron work function (Yang and Hudson, 1967).

The first wall of magnetic confinement fusion devices will require structures with low neutron activation, high melting temperature, and low physical and chemical sputtering rates. "Armor tiles" coated by CVD processes with carbon and refractory ceramic materials have been developed for this application. A codeposited coating of pyrolytic carbon and silicon carbide has proven particularly effective because of its unusually high resistance to chemical sputtering (Hopkins et al., 1984). In situ boron carbide coatings also have been used (Veprek et al., 1989).

Ion implantation has been shown to be very effective in reducing the wear of titanium-based total joint replacements in the orthopedic field (Sioshansi, 1987). The superior wear resistance results from both increased hardness of the titanium alloy (Ti-6Al-4V) and the lower coefficient of friction from homogenization of the two-phase alloy and formation of nitride, oxide, and carbide precipitates on the surface of titanium components.

The new-generation orthopedic implants are manufactured from Ti-6Al-4V alloy for its ideal biocompatibility. The Ti-6Al-4V alloy has a superior corrosion and fatigue resistance compared

to the traditional cobalt-chromium-molybdenum alloy and has a lower modulus of elasticity that provides a better match for the bone. In total joint replacement the titanium component articulates against an ultrahigh molecular weight polyethylene (UHMWPE) surface. Improvements in the wear resistance of Ti-6Al-4V are of significant interest to the orthopedic community.

Ion implantation of species, such as nitrogen and carbon, into the Ti-6Al-4V component has been shown to increase the microhardness of the alloy (Sioshansi, 1987). A threefold increase in microhardness (at loads of I to 2 g) can easily be achieved in these alloys. Recent work (Sauer, 1986) at MIT has shown that nitrogen implantation into Ti-6Al-4V changes the two-phase microstructure (the appearance of alpha and beta microplates) of the Ti-6Al-4V plates and renders the material impervious to standard etchant solutions. Earlier work showed that a reduction in the coefficient of friction of the Ti-6Al-4V alloy from 0.48 to 0.15 results from nitrogen-ion implantation at 100 keV to a total dose of 4 × 10¹⁷ ions/cm² (Oliver et al., 1984).

The increased hardness and lower coefficient of friction in the titanium alloy are believed to be the reason for a lower wear rate of the alloy and a much reduced wear rate of the articulating UHMWPE surface; a 1000-fold reduction has been observed in the corrosive wear of the titanium-polyethylene couple. The significant reduction of the wear of the titanium-UHMWPE system has convinced orthopedic manufacturers to specify this process in treating their products. (IONGUARD is the registered trademark of Spire Corporation, and the 1000 Series is used for processing titanium-based orthopedic implants.) This application has already reached market maturity, and large quantities of titanium-based orthopedic knees and hips and smaller quantities of wrists, shoulders, fingers, and toes are routinely prepared using the ion implantation process.

Pyrolytic carbon has been found to be exceptionally biocompatible (Bokros et al., 1972; Humbold et al., 1981). Addition of a few percent SiC in a CVD codeposition process provides the strength and wear resistance required for artificial heart valves (Kaae and Gulden, 1971; Shim and Schoen, 1974). As a result of these unique characteristics, heart valve components coated with carbon and SiC, codeposited by CVD, have come to dominate the artificial heart valve market in the past decade. These materials are also being evaluated for other applications, such as subcutaneous leads, joint replacements, and dental implants.

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