Nucleation and growth of combustion flame deposited diamond coatings on silicon nitride

An investigation has been performed on the kinetics of diamond nucleation on silicon nitride (Si₃N₄) based materials during oxy-acetylene combustion flame chemical vapor deposition. The deposits were characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) techniques. Kinetic parameters of the nucleation process, such as nucleation rate (I), period of initial nuclei formation (t_ng), and maximum nucleation density (N_d) were experimentally determined. It was concluded that at low temperatures (T_s < 875°C), the observed nucleation density is due to epitaxial or pseudo-epitaxial growth on residual particles on the substrate surface. At higher temperatures (T_s > 875°C), heterogeneous nucleation of diamond on Si₃N₄ occurs with an apparent activation energy ($Eahet$) of $\sim$18 kcal/mol. From an Arrhenius plot of particle growth rate, the activation energy (E_a) for diamond growth was calculated to be $\sim$9 kcal/mol. These results suggest that the energy barrier associated with the heterogeneous nucleation process ($Eahet$ > E_a) may in fact be responsible for the observed low nucleation densities on Si₃N₄ substrates. Consequently, nucleation density on Si₃N₄ is limited by the concentration of available sites for nuclei formation. As surface diffusion to those sites was determined to be negligible, growth of stable nuclei occurs via direct impingement of gas phase species.; Based on these conclusions, a multistage deposition technique was developed to deposit continuous diamond coatings on untreated Si₃N₄ substrates. This two stage technique consisted of (i) an in situ flame pretreatment of the substrate to enhance nucleation through the formation of SiC and (ii) subsequent growth on the flame pretreated surface under optimized parameters. Using a previously developed compression test for brittle coating/substrate systems, the adhesion of multistage deposited coatings was compared to conventionally deposited coatings. Normalized values of adhesion for the multistage coatings were observed to be the same order of magnitude as the conventionally seeded diamond coatings. However, the failure mechanisms of the two coatings were observed to be significantly different. Multistage coating failure was characterized by partial ($\sim$typically 5%) delamination while conventionally seeded coatings were observed to completely disintegrate (100% delamination) at failure. Thus, it was projected that the fracture toughness of the multistage deposited coatings may be superior to that of conventionally seeded coatings.; Finally, a modified flame technique is introduced to remedy a number of deficiencies and increase the commercial viability of combustion flame synthesis. This technique consists of simple modifications to conventional apparatus which significantly enhances the deposition area and expands the number of candidate substrate materials and geometries that can be coated using combustion flame CVD.