has been introduced to the coating chamber. In this way, coatings of compounds that are not easily sputtered or do not evaporate stoichiometrically can be produced.

The microstructure of PVD coatings depends on the nature of the substrate and a number of process parameters including substrate temperature. Heating of the substrate is often used to obtain coatings that are free of metastable phases and residual stresses. A general description of the PVD methods and coating characteristics has been given by Bunshah (ref. 34).

As indicated in table 2, successful SiC coating of carbon fibers within yarns. by sputtering has been described recently in the literature (ref. 15). The coating process was performed continuously with the yarn leaving a supply spool, then moving between an anode plate and a cathode target plate. A take-up spool was used to rewind the coated yarn during the process. Two coating runs were made for each yarn. After the first run, the yarn was turned over so the side that originally faced the anode then faced the cathode, which was the source of the SiC vapor. Process parameters were varied, demonstrating that dense, adherent, amorphous SiC coatings of uniform thickness could be deposited in this manner. Coatings between 0.05 μm and 0.50 μm in thickness were produced.

Ion plating also was used to coat carbon fibers (ref. 27). In this work, aluminum coatings were made on individual fibers within yarns containing 6000 fibers. Batch processing was used in which 80-mm lengths of yarn were spread into a layer four or five fibers thick. Uniform and adherent aluminum coatings were reported. Recently, continuous carbon fibers have become available with aluminum, copper, and magnesium coatings deposited by ion plating.* Figure 3 shows a scanning electron micrograph of carbon fibers that have been ion plated with magnesium.

Figure 3. Carbon fibers coated with magnesium by ion plating. 1000 X. (Courtesy of Cordec Corporation.)

Electroplating

This process is used in many industries for depositing protective or decorative metal coatings. In electroplating, the material to be coated is immersed in a solution. containing ions of the coating metal. Electroplating is carried out by making the material to be coated the cathode in a direct-current circuit. Current is then passed between the cathode and an anode of the coating material through the electrolyte solution. This process creates metal ions from the anode and reduces the ions in order to deposit the metal on the cathode. Metals that are routinely electroplated include nickel, copper, tin, zinc, chromium, cadmium, gold, silver, platinum, and

brass alloys. The quality and adherence of the coatings are strong functions of the processed parameters, the chemical characteristics of the electrolyte, and the nature of the substrate.

Equipment that has been developed for the continuous electroplating of the individual fibers in carbon yarns is described in the literature (ref. 25). Nickel- and cobalt-electroplated carbon fibers were originally evaluated for use in metal-matrix composites, but this work was discontinued because of chemical incompatibility between the fibers and metals at high temperatures (ref. 24). However, nickeland copper-plated carbon fibers are sold commercially for resin matrix composite applications in which the high electrical conductivity of the fibers is beneficial (ref. 26). Thin electroplated nickel coatings do not detract significantly from the mechanical properties of the fibers, and dense coatings of uniform thickness can be made on individual fibers within the yarns, as shown in figure 4.

Liquid Precursor Methods

Liquid precursors to inorganic materials include a wide range of compounds in which organic groups are bound to metal atoms. For coating purposes, the most useful liquids of this type are those that can be gelled to form coherent and adherent

solid or semisolid polymeric films. Decomposition of the polymer by slow heating produces the desired inorganic coating. Precursors that do not contain oxygen as a major element are known as organometallics and yield nonoxides (refs. 35 to 38). Metal alkoxides are the principal source of oxides, but reducing conditions also can be used to produce nonoxides from these materials (refs. 39 and 40).

A second method is the use of an inorganic solution of the metal or a colloidal suspension of inorganic particles in a liquid. Massive precipitation of solids out of the solution or drying of the colloid allows the solid particles to link up and form a gel. Heating sinters the particles to yield a coherent solid. This method involves aqueous processing and usually yields oxide materials, although carbides and nitrides have been made by adding reducing agents and processing in reducing environments (ref. 40).

Individual carbon fibers within yarns were continuously coated with SiO2 using tetraethoxysilane (TEOS) as the liquid precursor (ref. 16). The process involved heating the fibers to remove the sizing, running the fibers through an ultrasonically agitated bath of diluted TEOS solution, gelling the TEOS by hydration in steam to stabilize the coatings, and, finally, heating the fibers in argon at 550°C to decompose the TEOS and form the SiO2 coatings. A sequence of two runs produced SiO2 coatings 0.07 μm to 0.15 μm thick. Carbon coatings were also made on ultrahigh-modulus carbon fibers by passing the fibers through a toluene solution of petroleum pitch, vaporizing the solvent, and then decomposing the pitch in a series of increasing-temperature furnaces. This process was used to improve the adherence of the SiO2 coatings.

Several organometallic compounds suitable for the development of nonoxide coatings such as SiC, B4C, Si3N4, AIN, and BN have been identified in the past few years; a wide range of metal alkoxides are commercially available for producing oxide coatings (refs. 35 to 40). A scanning electron micrograph of carbon fibers coated with Y203 by the alkoxide method at General Atomics is shown in figure 5.

Liquid Metal Transfer

This method has been used to deposit carbide coatings on carbon substrates by immersing the material to be coated in a bath of molten tin that contains one to several weight percents of a Group IV, V, or VI metal in solution (ref. 28). The metal in solution reacts with the carbon substrate at temperatures between 900°C and 1500°C to produce a carbide coating. Carbide coatings, 0.05 μm to 2.0 μm in thickness, have been made on individual carbon fibers within yarns using the liquid metal transfer technique (refs. 28 and 29). The coatings appear to be dense and uniform, with time and temperature determining the thickness. So far, only batch processing of short strands has been demonstrated. An apparent drawback to the process is that the tin alloy is retained between the fibers and must be removed by chemical dissolution.

Coated Fiber Properties

As stated earlier, the impetus for most of the carbon fiber coating development has been to provide a matrix or to protect the fibers and to enhance wetting by molten metals during the fabrication of metal-matrix composites. Carbon-fiberreinforced aluminum, magnesium, and copper have been the composites of primary interest. Fiber coatings of CVD titanium boride and alkoxide-derived SiO2 have proved successful for fabricating these materials (refs. 14 and 16).

The ability of nickel coatings made by electroplating and electroless deposition to protect carbon fibers from attack by molten aluminum has also been evaluated. However, the formation of embrittling and nonprotective Al3Ni compromised the composite properties (ref. 14). Although carbon fibers are unstable when in contact with nickel at temperatures over 600°C, because of the high solubility of carbon in nickel, carbon fibers coated with nickel by electroplating are being produced for low-temperature applications in which improved electrical and thermal conductivities are important (refs. 24 and 26). Copper coatings made by electroplating are significantly more effective than nickel coatings for providing high electrical and thermal conductivities; copper is chemically compatible with the fibers at high