Essentially, pyrolytic deposition (literally, deposition by thermal decomposition) is a form of so-called gas or vapor plating. Gas or vapor plating can be accomplished by (1) hydrogen reduction, (2) displacement, or (3) thermal decomposition. Pyrolytic deposition is accomplished by the last mechanism.
The process involves passing the vapors of a compound over a surface maintained at a temperature above the decomposition temperature of the compound, in a vacuum furnace. The surface provides a source for nucleation of the desired material, which is built up to the desired section thickness.
Elemental materials are deposited from single-compound vapors, e.g., carbon, from a hydrocarbon; metal, from its halide; compound materials, from mixtures of compounds, e.g., boron nitride from boron halide and ammonia.
In producing coatings, the substrate serves as the surface on which the coating is deposited.
In producing self-supporting parts, the substrate serves as a mandrel or mold from which the pyrolytic material is removed after deposition.
The pyrolytic deposition process is used to produce pyrolytic graphite, as well as coatings or self-supporting structures of an extremely broad range of materials. Theoretically, the only limitation on the type of material that can be produced is that (1) it must be available in the form of a compound whose vaporization temperature is below its decomposition temperature, and (2) the desired material must separate cleanly from the vapor of the compound.
General Properties
The major benefits of pyrolytic materials are the following:
1. Highly directional properties are obtained in some materials by the substantial degree of orientation of crystals or grains. (Note: Highly directional properties are only obtained in materials such as pyro-lytic graphite and boron nitride, which possess the unique and aniso-tropic graphite crystal structure.)
2. High densities, equivalent to theoretical densities, are obtainable.
3. High purity of material and close control of ingredients in "alloys" are obtainable by control of the reactant gases.
All initial work has been aimed at producing high-temperature materials, primarily for aerospace use. The unique properties of these materials make them attractive for a number of commercial applications.
Materials and Forms
To date, pyrolytic graphite has been the largest-volume material produced, but newer materials include the following:
1. Graphite-boron compounds: pyro-lytic graphite to which less than 2% boron has been added for increased strength and oxidation resistance as well as lower electrical resistivity.
2. Other graphite compounds: pyrolytic graphite to which varying percentages of columbium, molybdenum, or tungsten have been added.
3. Boron nitride: pyrolytic boron nitride containing 50 atm% of boron and nitrogen.
4. Carbides: pyrolytic carbides of tantalum, columbium, hafnium, and zirconium.
5. Tungsten: pyrolytic tungsten has been produced in the form of coatings and parts such as crucibles and tubes.
Coatings can be deposited on complex surfaces, providing gas impermeability in extremely thin sections. On the other hand, only those surfaces of the shape that can be exposed to the flow of gases will be coated. Also, differences in coefficients of thermal expansion between substrate and coating materials must be carefully considered.
Self-supporting shapes are limited by the fact that the material must be produced by deposition on a mandrel that must be removed after the part is formed.
Directionality Depends on Crystal structure
The directional properties of the produced part or coating depend on the inherent crystal structure of the material. In general, materials are either highly anisotropic or nearly isotropic.
Pyrolytic deposition of material with hexagonal graphite crystal structures (i.e., pyrolytic graphite and boron nitride) results in preferred crystal orientation producing a high degree of directionality of properties. The hexagonal or layer plane alignment of the grains is essentially parallel to the substrate surface. Directionality results from strong atomic bonds within layer planes and weak bonds between layer planes, and also from the mode of heat transfer through the material, which is predominantly by lattice vibration.
Pyrolytic deposition of materials with face-centered or body-centered cubic structures (i.e., carbides or tungsten) results in relatively iso-tropic properties. Although such materials can have a high degree of crystal orientation, orientation does not necessarily produce directional properties.
Properties of Anisotropic Materials
Pyrolytic graphite and its compounds and boron nitride all have substantial directionality of properties. The ratio of the number of crystallites with layer planes parallel to the deposition surface (i.e., a axis) to the number normal to the surface (i.e., c axis) can be varied by process control. For example, in graphite, ratios may range from 100 to 1000 to 1, compared with ratios of 3 or 4 for some commercial graphites. Orientation obtained in boron nitride has been as high as 1900 to 1.
Conductivity
One of the most useful properties of pyrolytic graphite is its insulating ability. In the direction normal to the deposition surface, pyrolytic graphite is a better insulator than the most refractory ceramic materials. In addition, the thermal conductivity parallel to the deposition surface is comparable to the more conductive metals, tungsten and copper. This high conductivity evens out hot spots over the total surface.
The high destruction temperature of pyro-lytic graphite combined with low conductivity normal to the surface allows the surface temperature to become very high. This cuts down heat absorbed by the component by reradiating heat back to the atmosphere, thus acting as a "hyperinsulator."
Tensile Strength Improved
Tensile strengths in the a axis are orders of magnitude higher than in the c axis. For example, at room temperature, pyrolytic graphite has an a-axis average tensile strength of about 95.2 MPa, compared with about 3.5 MPa in the c direction. The graphite-boron compound has room temperature tensile strengths of 112.2 and 4.9 MPa in the a and c axes; boron nitride has a- and c-axis tensile strengths of 84 and 4.5 MPa, respectively.
Pyrolytic graphite (like conventional graphite) offers the singular advantage of increasing in strength with increasing temperature. Preliminary data indicate that the c-axis strength decreases with temperature.
Addition of alloying elements has been found to improve c-axis strength. Additions of low concentration of less than 1% of tungsten and molybdenum have increased c-axis tensile strength by 50 to 90%.
Oxidation Resistance improved
Pyrolytic graphite has somewhat greater oxidation resistance than normal graphite due largely to its imperviousness. Addition of boron improves oxidation resistance of pyrolytic graphite by a factor of 1. Oxidation resistance of boron nitride is superior to other pyrolytic materials produced to date specifically at temperatures below 2000°C.
Other Properties
Owing to the atom-by-atom deposition process, pyrolytic materials are all near theoretical density, are impervious, and have extremely high purity levels. The materials do exhibit substantial directionality of thermal expansion, which must be carefully considered in designing components.
Properties of Isotropic Materials
Performance data on the isotropic pyrolytic materials, tungsten, and the carbides of tantalum, hafnium, and columbium are much more limited than those for the anisotropic pyrolytic materials.
As mentioned before, although such materials are considered to be isotropic (in comparison with the anisotropic materials), crystal growth does tend to provide a preferred orientation in a plane normal to the deposition surface. Thus, generally speaking, the strength of such materials is greater in the plane normal to the surface than in the plane parallel to the surface.
Carbides
The pyrolytic deposition process can be controlled to produce carbides of varying metal-to-carbon ratio, resulting in carbides of differing microhardness. Following are Knoop hardness number (K100) for three carbides, hardness increasing with increasing carbon-to-metal ratio:
|
TaC |
1400 to 3500 |
|
HfC |
2000 to 3600 |
|
NbC |
1700 to 4000 |
The very hard grades of carbide are extremely brittle and difficult to handle. Their strength is low but they promise to be useful as thin, well-bonded coatings.
As hardness decreases, the ductility and strength of the carbides increase. Bend strengths, except for the high-hardness grade, were found to fall in the range of 68 to 272 MPa; a value as high as 1030 MPa was observed for tantalum carbide low in carbon.
Tungsten
Pyrolytic tungsten, which can now be produced in thicknesses over 6.4 mm as coatings on components to 457 mm in diameter, is being evaluated for missile application. The increased strength and lower impurity level of pyrolytic tungsten it believed to be a major factor permitting production of sound coatings on large rocket nozzles.
