Preliminary study of CVD diamond film to improve the performance of heat flux sensors for hypersonic experiments

The present work investigated the use of conventional hot filament chemical vapor deposition technique (HFCVD) diamond film as a coating on Platinum Thin Film Heat Transfer Gauges to improve their operation performance in the range of Mach 4 to Mach 15 airflow, in order to acquire accurate heat transfer data in hypersonic experiments. The gauge under development has a substrate made of sintered silicon nitride (Si3N4). It has cylindrical shape, with 5 mm diameter and 15 mm length. On it's top surface there is a platinum thin film, which is the heat flux-sensing element. Diamond films were deposited at 870 K and 6.5 kPa by HFCVD using a gas mixture of methane 1.0% or 1.5% vol. in hydrogen. For improving the adherence, films were grown onto Pt/Si3N4. A SiO2 interlayer was also used to minimize the dissolution of carbon into the platdinum substrate during film growth. Scanning Electron Microcopy (SEM) has evidenced a faceted polycrystalline grains with orientation (111) and (100) covering the entire surface. Raman spectroscopy has demonstrated a film with high quality and low stress, by considering the small peak shift presents in the spectrum compared to natural diamond.