HYDROGEN RARE-GAS OPTIMIZATION BY SIMPLE BOUNDARY-LAYER THEORY FOR DIAMOND FILM DEPOSITION IN ARC-HEATED GAS-FLOWS

This analysis optimizes the amount of monatomic hydrogen produced by a n are-heated plasma torch against the thickness of the boundary layer above a substrate target in stagnation how. Large amounts of monatomic hydrogen are believed to be ideal for diamond growth, and thinner bou ndary layers will permit more of the free-stream H to penetrate to the growing surface. A plateaulike optimum is found in the range 50%-80% hydrogen in the feedstock; this hat plateau indicates that process con trol can be made insensitive to composition. The optimum bulk temperat ure for a constant property boundary layer is found to be 4000 K, with the curve approaching this temperature being steepest around 3500 K. Since 3500 K is the typical bulk temperature achieved by arc-heaters, process control is particularly sensitive to the amount of power intro duced into the gas stream. Finally, additional means of thinning the b oundary layer further are discussed.