Hugh O. Pierson

The chemical vapor deposition of carbon on carbon fibers

The relations between chemical vapor deposition (CVD) parameters and the resultant pyrolytic carbon microstructures have been examined for matrix deposition in fibrous carbon substrates. The parameters considered are temperature (1200–1450°C), pressure (20–630 Torr), C/H ratio (14–114), total flow rate (2–16) 1/min), and carbon felt density (0·12–0·23 g/cm3). Most of the data obtained are in agreement with a CVD model for carbon; where agreement is not obtained, it is surmised that the assumptions of the model may not be satisfied.

Boron Nitride Composites By Chemical Vapor Deposition

Composites of boron nitride (BN) have been made by the chemical vapor deposition (CVD) of a BN matrix on a BN felt fiber substrate. Reactant gases were boron trifluoride and ammonia. The composites have a relatively high density (1.70 g/cm3), a crystallite size LC = 150 A and an interlayer spacing d002 = 3.35 A. Measurements of elastic modulus and thermal conductivity and expansion showed some anisotropy as a result of the preferred fiber orientation of the substrate.

The effect of chemical-vapor-deposition conditions on the properties of carbon-carbon composites☆

Abstract Properties are given for as-deposited and heat-treated carbon-felt, carbon-matrix composites infiltrated at deposition temperatures of 1100 and 1400°C, and pressures of 20 and 630 Torr. A thermal stress figure of merit was calculated for each material, with the heat-treated composite infiltrated at 1400°C and 630 Torr yielding the highest value. As with most graphitizing carbon materials, heat-treatment resulted in a decrease of the flexural strengths and moduli. The strength-to-modulus ratios, however, increased, being highest for deposition conditions of 1400°C and 630 Torr. Heat-treatment also resulted in an increase in thermal conductivity and a decrease in thermal expansion. These changes were related to the degree of crystallinity and to the formation of cracks within the matrix.

Carbon-Felt, Carbon-Matrix Composites: Dependence of Thermal and Mechanical Properties on Fiber Precursor and Matrix Structure

Properties of carbon-felt, pyrolytic carbon-matrix composites have been measured as a function of fiber precursor [rayon and polyacrylonitrile (PAN)] and matrix microstructure (smooth laminar, rough laminar, and isotropic). The primary matrix effect is caused by the graphitic nature of the heat-treated rough laminar matrix which yields a high composite thermal conductivity. The increased modulus of the PAN-based fibers results in increased composite strength and modulus and a significantly reduced thermal expansion. A heat-treated, PAN-based carbon felt, rough laminar carbon matrix composite has a superior thermal shock figure-of-merit based on these results.

Carbides of Group IV: Titanium, Zirconium, and Hafnium Carbides

This chapter provides a review of the characteristics and properties of the interstitial carbides formed by the metals of Group IV: titanium, zirconium, and hafnium. The rationale for reviewing these compounds together is their similarity in atomic bonding, composition, and crystallography. These carbides also have similar properties and characteristics. Of the three, titanium carbide has been investigated more and is the most important from an application standpoint. It is produced industrially on a large scale in the form of powders, molded shapes, and thin films. Interstitial carbides are essentially non-stoichiometric compounds and the variations in the reported property values often found in the literature reflect this characteristic. The density of these carbides increases considerably with the increasing atomic number of the metal. The melting points of these carbides are higher in all cases than those of the other compounds and particularly those of the host metals. The thermal conductivity or k (i.e., the time rate of transfer of heat by conduction) of interstitial carbides is different from that of most other refractory materials as k increases with increasing temperature.

Carbon-Felt, Carbon-Matrix Composites: Dependence of Thermal and Mechanical Properties on Fiber Volume Percent

Carbon-felt, carbon-matrix composites were prepared by the chemical vapor deposition of carbon within carbonized viscose-rayon substrates. The thermal and mechanical properties of these carbon-carbon composites were investigated over a wide range (9-46 v/o) of fiber volume percent. In creases in fiber content resulted in: (1) decreases in both total porosity and pore size, (2) filament and matrix reorientation, and (3) reduction of the pyrolytic carbon matrix sheath thickness. Fiber content, and its inter related effects, are correlated with significant changes in flexural and ten sile strength and modulus, thermal conductivity and linear thermal expansion.

Interstitial Nitrides: Properties and General Characteristics

This chapter reveals the properties and general characteristics of the interstitial nitrides formed by the metals of Group IV (titanium, zirconium, and hafnium) and Group V (vanadium, niobium, and tantalum). These six nitrides are the only refractory transition-metal nitrides. They have similar properties and characteristics and, of the six, titanium nitride has the greatest importance from an application standpoint. These nitrides are produced mostly in the form of coatings or powders. Like the interstitial carbides, interstitial nitrides are essentially non-stoichiometric compounds. The density of these nitrites increases considerably with the increasing atomic number of the metal. The melting point of the nitrides is lower in every case than that of the corresponding carbides but, with the exception of niobium nitride (NbN), higher than the parent metals. The interstitial nitrides are relatively good electrical conductors, although with a resistivity slightly higher than that of the corresponding carbides and the parent metals, but still reflecting the essentially metallic character of these compounds.

Carbides of Group VI: Chromium, Molybdenum, and Tungsten Carbides

This chapter provides a review of the characteristics and properties of the interstitial carbides formed by the metals of Group VI: chromium, molybdenum, and tungsten. These three carbide systems have similar atomic bonding, composition, and crystallography. Their properties and characteristics are also similar. The carbides of Group VI are important industrial materials, particularly tungsten carbide and chromium carbide. The density of these carbides increases considerably with the increasing atomic number of the metal. The carbides of Group VI have melting points that are lower than those of their respective host metals but are relatively close to those of the borides. Like the other interstitial carbides, the carbides of Group VI are good thermal conductors, thus, reflecting their metallic character. This is especially true of tungsten carbide (WC) that has the highest thermal conductivity of any of the transition-metal carbides and can be considered as an excellent thermal conductor. The Group VI carbides are chemically stable and have a chemical resistance similar to that of the Group IV carbides.

Processing of Refractory Carbides and Nitrides (Coatings)

This chapter reviews the coating processes of refractory carbides and nitrides. Coatings of refractory carbides and nitrides have great industrial importance with a wide range of applications in semiconductors and other electronic components, in cutting tools, gas-turbine vanes and blades, precision bearings, punch sets, extruders, prostheses, and many other products. The surface of a material may be exposed to wear, corrosion, radiation, electrical or magnetic fields, and other phenomena and hence, it must have the ability to withstand these environments. This can be accomplished by coating the base material to obtain a composite in which the surface properties may be considerably different from those of the substrate. Chemical vapor deposition (CVD) and physical vapor deposition (PVD) belong to the class of vapor-transfer processes, which are atomistic in nature—that is, the deposition species are atoms or molecules or a combination of these. The coatings are also commonly known as thin-films when their thickness is less than 10 μm. CVD is a versatile process that is well adapted to the production of all the refractory carbides and nitrides, not only as coatings but also as powders, bulk/monolithic components, and fibers. It may be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapor phase.

Covalent Nitrides: Properties and General Characteristics

This chapter reveals the properties of covalent nitrides—boron nitride, aluminum nitrides, and silicon nitride—and provides a summary of the fabrication processes and applications of these compounds. The refractory covalent nitrides have remarkable properties and are industrial materials of major importance, produced on a large scale in the form of powders, monolithic shapes, and coatings. The three covalent nitrides are low-density materials with melting points, which are higher than those of their parent elements—boron, aluminum, and silicon. Of the three, boron nitride has the highest melting point and is more refractory than boron carbide. On the other hand, silicon nitride is not as refractory as silicon carbide. The thermal conductivity of the covalent nitrides decreases with increasing temperature. Thermal expansion of the covalent nitrides is low and, like that of the covalent carbides, increases with increasing temperature. This increase is not entirely linear and is slightly more rapid at high temperature. The covalent nitrides are excellent electrical insulators. Their electrons are strongly and covalently bonded to the nucleus and are not available for metallic bonding.