Combustion assisted (CA) CVD is one of a range of CVD methods that have been used successfully to grow diamond films on a range of substrates. Of critical interest is the role of interfacial carbides in the nucleation and adhesion of diamond at the substrate/film interface. Deposition experiments with substrates including titanium and its alloys, suggest the nature of the substrate is important in aiding carbide and subsequently diamond formation. The formation of a stable carbide clearly assists in the nucleation phase and in enhancing bonding at the interface, but it is far from clear that it is an essential precursor to the formation of diamond. Rather it appears to stabilize the substrate surface while diamond growth is established. On the other hand, refractory oxide films accelerate deposition by aiding in the stabilization of the substrate surface under the combustion flame, but are ultimately deleterious to film bonding because of the weakened bonding and increased lattice mismatch with the overlying diamond.
The role of carbide and oxy-carbide films in the adhesion of diamond films on commercial pure Ti metal, and a Ti, 6%Al, 4%V substrates was examined. It was found that the carbon to oxygen ratio in an oxy-carbide structure formed at the interface broadly correlates with the adhesive strength of the diamond/substrate bond. Deposition experiments with substrates including titanium and its alloys, suggest the nature of the substrate is important in aiding carbide and subsequently, diamond formation. XPS depth profiling for Ti-6Al-4V indicates that the mode of TiC formation on pure Ti and Ti-6Al-4V is significantly different because of the segregation of aluminium. This contributes to a change of TiC stoichiometry and morphology. SIMS depth profiles also show that the interfacial surface on Ti-6Al-4V contains several percent of aluminium and nitrogen, both at the outermost surface and in the interior of the interfacial layer. The aluminium is enriched at the surface as compared with the bulk concentration.
A major target in diamond deposition is the growth of heteroepitaxial diamond films that are free from impurities, grain boundaries and defects for use in electronic applications. Nickel has been identified as a material with a close lattice match to diamond that minimises defect formation in diamond films but as a substrate, nickels' higher thermal expansion coefficient with respect to diamond, and the interfacial formation of amorphous and graphitic carbon before nucleation of diamond, leads to a weakened diamond/metal bond. Haynes 214 alloy is a nickel-based alloy used in high-temperature environments. Regions within this alloy contain amounts of aluminium known to affect the ability of nickel to stabilise amorphous carbon and graphite. Areas rich in aluminium discourage the formation of sp 2 carbon, creating a barrier to carbon diffusion and allowing diamond nucleation to occur on the metal surface. Diamond deposition on Haynes 214 alloy using combustion assisted chemical vapour deposition (CACVD) rapidly produced a well structured, polycrystalline diamond film that typically delaminated with sample cooling. The film exhibited areas that were transparent corresponding to areas on the substrate rich in aluminium and dark areas matching nickel rich areas of the substrate surface catalysing the formation of amorphous and graphitic carbon.
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