Thick Beryllium Coatings by Magnetron Sputtering
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Thick Beryllium Coatings by Magnetron Sputtering H. Xu,1 C. Alford,2 Eric Chason,3 A. Detor,2 T. Fuller,1 A. Hamza,2 J. Hayes,1 K.A. Moreno,1 A. Nikroo,1 T. van Buuren,2 M. Wang,2 J. Wu, and K.P. Youngblood1 1General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, USA 2Lawrence Livermore National Laboratory, Materials Science and Technology Division, 7000
East Avenue, Livermore, CA 94550, USA 3Brown University, Department of Engineering, Providence, RI, USA ABSTRACT Thick (>150 μm) beryllium coatings are studied as an ablator material of interest for fusion fuel capsules for the National Ignition Facility (NIF). As an added complication, the coatings are deposited on mm-scale spherical substrates, as opposed to flats. DC magnetron sputtering is used because of the relative controllability of the processing temperature and energy of the deposits. We used ultra small angle x-ray spectroscopy (USAXS) to characterize the void fraction and distribution along the spherical surface. We investigated the void structure using a combination focused ion beam (FIB) and scanning electron microscope (SEM), along with transmission electron microscopy (TEM). Our results show a few volume percent of voids and a typical void diameter of less than two hundred nanometers. Understanding how the stresses in the deposited material develop with thickness is important so that we can minimize film cracking and delamination. To that end, an in-situ multiple optical beam stress sensor (MOSS) was used to measure the stress behavior of thick Beryllium coatings on flat substrates as the material was being deposited. We will show how the film stress saturates with thickness and changes with pressure. INTRODUCTION Beryllium is one of the ablator materials for National Ignition Facility (NIF) due to its low x-ray opacity and higher density. In particular, a graded copper-doped Be capsule is desired for ignition experiments. The capsule requires thick Be coatings (up to 160 μm) which are made on spherical mandrels, typically CH in composition [1]. The coating has to be carried out at low temperatures to avoid any deformation of the mandrel in order to produce uniform wall thickness and smooth and spherical surface finish. The coating must be dense with low void fraction and be leak-free when filled with D2 fuel gas. DC Magnetron sputtering is used to produce the coating because of the energetics of the depositing species and relative low coating temperatures [2]. The coating process has been described previously in several articles [2–5]. Typically, Be coatings on a spherical surface show a consistent columnar microstructure with acceptable void content and size. However, the Be coating suffers from sufficient interconnected porosity leading
to rapid gas leakage. The measurements of D2 leak half-life at low temperature and ambient temperature indicated the leak is molecular flow through nanochannels [5]. Ion-assisted physical vapor deposition (IPVD) is the most commonly used method to produce dense films and to coat high aspect ratio trenches
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