Chemical vapor deposition of boron carbides

Janson, U., Chemical Vapor Deposition of Boron Carbides, Materials and Manufacturing Processes, 6(3) 481-500 (1991)... 

Jansson U., Carlsson J.O. Area selective chemical vapor deposition of boron carbide achieved by molecular masking //J. Vac. Sci. Technol. A. 1988. V. 6. No 3. P. 1733-1735. 

As introduced above, the reaction of boron carbide with metal carbides can be used to fabricate metal borides or metal boride/boron carbide composites in a controlled way during densification if boron carbide or free boron is used in excess, or if carbon is bonded by another additive. Although the incompatibility of B4C and metal carbides is well known, many attempts have been undertaken to produce composites or coatings thereof but failed as soon as equilibrium conditions were approached. Physical or chemical vapor deposition of B4C on hard metal substrates, or WC coatings on boron carbides are typical problems (e.g., [252]). In both cases, interlayers of graphite form and hence result in an unsatisfactory adhesion of the deposited coating to the substrate. 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

Boron Trichloride. Boron trichloride is prepared commercially by the chlorination of boron carbide (equation 15). Direct chlorination of boric acid or a sodium borate in the presence of carbon is an alternative method. Most of the boron trichloride produced is converted to filaments of elemental boron by chemical vapor deposition (CVD) on tungsten wire in a hydrogen atmosphere. Numerous laboratory preparations of boron trichloride have been reported. One of the most convenient is the halogen exchange reaction of aluminum chloride with boron trifluoride or a metal fluoroborate. 

Chemical vapor infiltration (CVI) is widely used in advanced composites manufacturing to deposit carbon, silicon carbide, boron nitride and other refractory materials within porous fiber preforms. " Because vapor phase reactants are deposited on solid fiber surfaces, CVI is clearly a special case of chemical vapor deposition (CVD). The distinguishing feature of CVI is that reactant gases are intended to infiltrate a permeable medium, in part at least, prior to... 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

Single crystals a few mm long are obtained by chemical vapor deposition (cf. 5.3.2.2.3), or by reduction of B2O3 by graphite in an electrical arc . A 6-mm diameter sintered boron carbide rod can be zone melted under Ar... 

Boron fibers are used to reinforce different epoxy or light-metal matrices. To hinder interactions with metals, they should be protected by boron carbide deposits obtained by chemical vapor deposition. Boron carbide fibers can be prepared directly by reaction of boron obtained by the reduction of BCI3 on carbon fibers. ... 

Very fine boron carbide powders of spherical shape and 20-30 nm in size have been prepared by chemical vapor deposition according to (iii). In an Ar-H2-CH2-BCI3 atmosphere a radio frequency plasma produces stoichiometries between Bi5 gC and B3 9C [33, 166]. Also laser-induced pyrolysis of similar gas mixtures with or without acetylene has been employed for the preparation of nano-sized particles [167]. With similar success, composites of B4C and SiC have been produced by the pyrolysis of boron-containing polysilanes [168]. 

Boron, silicon carbide, diamond and other materials can be deposited by chemical vapor deposition on the surface of hot wires or hot fibers. If a minimal vapor deposit is applied, the process will modify only the surface of the fiber and produce a coating, while leaving its core functionality unchanged. If, however, a thick vapor deposit is applied, the process will create a new and very large diameter fiber that has the functionality of the sheath and a sacrificial core. 

Silicon carbide (SiC) monofilaments are usually made by chemical vapor deposition (CVD) by decomposing a silane such as methyltrichlorosilane (CHjSiCy in a hydrogen atmosphere onto a hot and fast-moving tungsten wire or pyrolitic carbon monofilament at a temperature of 1300°C. The equipment and process is the same as that used for making boron fibers (see Section 18.4.2). The chemical reaction occurring at the surface of the hot substrate is ... 

Ceramic fibers. The other fibers shown in Table 4.6 have varying uses, and several are still in development. Silicon carbide continuous fiber is produced in a chemical vapor deposition (CVD) process similar to that for boron, and it has many mechanical properties identical to those of boron. The other fibers show promise in metal matrix composites, as high-temperature polymeric ablative reinforcements, in ceramic-ceramic composites, and in microwave transparent structures (radomes or microwave printed wiring boards). 

---