High-Rate Sputter Deposition of Ultrathick Boron Carbide Coatings on Rolling Spherical Substrates

Amorphous boron carbide (B₄C) is a promising next-generation ablator material for inertial confinement fusion. However, the deposition of ultrathick B₄C coatings with submicron-scale density uniformity on spherical substrates, as required for ablator shell fabrication, remains challenging. Here, we use direct current magnetron sputtering to deposit B₄C onto 2-mm-diameter Si spheres rolling in a dish-shaped substrate holder. High deposition rates of μm/h are achieved. Ultrathick films have a columnar microstructure, with the column width determined by the density of nodular defects. Nodular defect nucleation is dominated by the pickup of particulates from the holder during substrate rolling. We demonstrate the fabrication of a hollow B₄C spherical shell with a wall thickness of 80 μm. Also demonstrated is the laser machining of holes in the B₄C coating, which is necessary for both the chemical removal of the Si template and the attachment of a fusion fuel fill tube.