Current issues and problems in the chemical vapor deposition (CVD) of diamond are those which relate to its characterization, its nucleation on foreign surfaces, the question of its formation in preference to the other phases of solid carbon (for example, graphite, chaoite, or lonsdaleite), why different morphologies and crystallographic orientations (textures) are seen in different experiments or with different parameters in the same experiment, and finally whether well-crystallized metastable phases can be obtained by CVD in other material systems or are only a peculiarity of carbon chemistry. Whether a given carbon coating is justly described as diamond has been such an issue, and coatings should clearly show evidence for diamond by x-ray diffraction and Raman spectroscopy before the claim of diamond is made. Experimental results have not been consistent in many cases, and much work remains to be done before an accurate assessment can be made of the technological impact of the development.

Polycrystalline boron nitride films have been deposited using microwave plasma-enhanced chemical vapor deposition. IR absorption spectra of films deposited using NaBH4 as the boron source in NH3 and H2 gases showed absorptions which are nearly the same as the characteristic vibrational modes seen in cubic and pyrolytic boron nitrides. Films deposited at 5 Torr also showed electron diffraction patterns for pyrolytic boron nitride, turbostratic boron nitride and cubic boron nitride. At higher gas pressures, only rings consistent with the formation of amorphous and cubic boron nitride were observed. Although the Raman spectra from a film deposited at 60 Torr showed broad peaks at ∼1080 and ∼1310 cm−1, the positions of the Raman lines for cubic boron nitride, no x-ray diffraction lines could be observed except that of the silicon substrate.

In view of (1×1):2H dihydride/(2×1):H monohydride reconstruction, structural stability of (100) surfaces of both cBN and diamond was comparatively investigated by semiempirical molecular orbital methods using isoelectronic clusters of B52N42H80−2n(10−), N52B42H80−2n(10+), and C94H80−2n, to model (100)B and (100)N of cBN, and diamond surface, respectively, where n=0, 1, 2, or 3. The n denotes the number of monohydride dimers formed. These clusters were nanometer-sized pyramidal crystallites bound by four of {111} faces and one (100). The (100)N of cBN was found unique because of the great stability as (1×1):2H dihydride phase, which retains the bulk structure truncated at the surface without reconstruction and is expected to be chemically inert. This passivation seems to be related to the difficulty in chemical vapor deposition of high quality cBN. The (100)B of cBN was predicted to stabilize as (2×1):H monohydride phase as much as hydrogenated (100) of diamond does.

Materials derived by precipitation or polymerization chemistry (e.g., “sol-gel” methods) are usually obtained in noncrystalline or otherwise metastable phases, and transformation to more thermodynamically stable phases generally occurs by a nucleation and growth process. In a fully reconstructive transformation, such as occurs in the alumina system, the activation energy for nucleation may be higher than that for simple short-range diffusion. Hence nucleation frequency can be a controlling factor in the development of microstructurc. The efficacy of seeding as a method of microstructural and phase control in solution-derived or so-called sol-gel materials has been clearly demonstrated for the alumina system. The epitaxial nature of this phenomenon is explored, using the polarizing microscope to follow the crystallographic orientation of the transformed material as the transformation proceeds, showing that this is epitaxial in nature, and that the nucleation frequency in unseeded material is relatively low (∼ 1010 cm−3). The microscope was then used to demonstrate the effect on nucleation frequency of seeding with materials selected to be isostructural, isotypic, and having little or no similarity to the corundum structure. Using these and other methods, the seeding phenomenon in alumina gels is shown to result from epitaxial growth of the stable corundum phase on isostructural or isotypic nuclei in the solid state. This approach is applied to formulate hypotheses for the mechanisms by which some of the previously reported effects of seeding, e.g., enhanced densification and microstructural refinement, can be understood and to formulate a set of generalizations for its potential application to other systems.

Considerable progress has been made toward the goal of diamond synthesis by chemical vapor deposition (CVD). This progress consists of improved methods for synthesis and understanding how diamond is formed. The field has rapidly expanded with industrial consortia, international joint ventures, specialized journals, novel methods for synthesis and processing, and the introduction of new products. Despite this expansion, many issues remain unresolved, generating considerable debate within the research community. Among these debates is the question of how diamond is formed. Both thermodynamics and kinetics are frequently debated at the many world‐wide meetings on this technology. The resolution of these issues awaits further progress and, with improved understanding, may have implications for the synthesis of other ceramic materials. The diamond research community includes not only ceramists and other materials scientists, but specialists in subjects that range from chemistry and chemical engineering to solid‐state physics and electrical engineering. Crystals are grown using methods that range from the use of high‐power plasmas and the combustion of acetylene in oxygen to the thermal decomposition of fluorocarbons and various hydrocarbons in the presence of fluorine. Although some evidence exists for diamond heteroepitaxy, the goal of large‐area heteroepitaxial diamond has proved elusive. Thick (> 100 μm), free‐standing, polycrystalline diamond layers are being grown, and their properties rival those of natural crystals. Methods have been developed for the cutting, polishing, and brazing of diamond, and products are being tested in the marketplace. Engineering of the diamond–substrate interface for acceptable adhesion and reliability has progressed, although much work remains to be done. The central issues for commercialization are less the question of whether diamond can be grown in sufficient amounts or with sufficiently attractive properties, but rather whether the fabrication methods can be made sufficiently cost‐effective for the markets envisioned.