Filaments used in HF CVD

These are SEM photos of three types of metal filament used for hot filament CVD of diamond. They have been growing diamond in a 1%CH4/H2 atmosphere for about 4 hours.

W filament

W filament
Tungsten filament, showing the thin layer of WC formed at the edge. Re filament

Re filament

Re filament
Rhenium filament, showing cracks along the edge and a homogeneous cross section - no carbide.
Ta filament

Ta filament

Ta filament
Tantalum filament, showing cracks and plates along the edge - no obvious carbide.