Optimizing diamond growth for an atmospheric oxyacetylene torch

Diamond growth conditions for an atmospheric combustion flame have been optimized using statistical experimental design. Films are grown on a molybdenum bolt for 40 min at a distance of 1 mm from the flame cone. The diamond films have been characterized using Raman spectroscopy, x-ray diffraction, and scanning electron microscope. The input process variables are varied over a range of conditions: total gas flow rate Q = 2–4 standard liter/min, substrate surface temperature T_s = 800–1000 °C, and flow ratio of O₂/C₂H₂ = R = 0.93–0.99. The experimental response outputs are growth rate, full width half maximum (FWHM) of the diamond Raman peak, Raman diamond fraction (β) in the film, ratio of luminescence to diamond peak height (LR), and the relative intensity of the {220}, {311}, {400}, and {331} orientations. The film quality indices FWHM, β, and LR improve by increasing the gas ratio (R), by increasing substrate surface temperature (T_s), and lowering the growth rate by decreasing total gas flow rate. Diamond film shows a small amount texturing in {220} and {400} orientation at low R and T_s. At high R and low T_s crystals are oriented with the {111} direction normal to the substrate surface. Jet and boundary layer theory have been applied to understand the growth rate, the thickness profile, and the morphological instability of the diamond films. Surface Damkühler calculation shows that the deposition process is marginally controlled by mass transfer. Growth rate of an open flame is higher than for an enclosed flame, while the Raman quality measurements of the enclosed flame are more uniform than open flame over the range of the comparison.