High-Rate Carbide Growth by Evaporation

Project Overview

Boron carbide (B4C) is an attractive inertial confined fusion (ICF) ablator and a unique ultra-hard material with numerous current and potential applications. What makes B4C attractive as an ablator is its relatively high density and the ability to form a stable glassy phase combined with excellent chemical and mechanical stability. Despite these advantages, the fabrication of B4C ablators has remained a challenge. Magnetron sputtering, our current frontrunner approach for B4C deposition, exhibits low deposition rates (< 2 μm/h) and undesirable nodular defects within the film structure that are believed to generate from the dusty plasma around the sputtering source. The goals of this project were to (1) establish alternative plasma-free, electron beam evaporation (EBE) capabilities for the deposition of thin films that are needed in mission-critical programs and; (2) demonstrate the feasibility to deposit B4C films at high rates by EBE in a bottom-up geometry and characterize their stoichiometry. In this feasibility study, we initially developed the capability to convert a general-use vacuum chamber into an electron beam evaporator as needed, or on demand. Then, we successfully used this setup to deposit films made of boron carbide and various other materials. By adjusting the deposition parameters and the evaporation source material, we were able to demonstrate the feasibility of reaching deposition rates as high as 8 μm/h, which is about 4 times the maximum rates obtained through sputtering methods. We predict that these values can still be further improved by implementing enhanced thermal control measures for the target source and optimizing electron beam parameters (ie. current, sweeping and frequency) since ceramic materials with similar densities to B4C can exhibit deposition rates of about twice this value. We anticipate that the skills and capabilities developed throughout this project will prove relevant not only to the HED and ICF campaigns but also to numerous existing and prospective applications focused on the development of ultra-hard carbide coatings. The fresh set of tools here developed will also facilitate the preparation and research on future new target materials of interest beyond B4C.

Mission Impact

The new EBE capabilities developed within target fab under this FS study will enable the development of new materials required for future HED and ICF experiments that are beyond our current technical capabilities. Furthermore, it equips us with new tools to manufacture current materials of interest faster and more efficiently. The successful production of boron carbide films using EBE technology not only enhances our Core Competency in Advanced Materials and Manufacturing, particularly in the realms of ultrahard and chemically inert materials/coatings but also carries technological implications that extend beyond the scope of the HED and ICF programs. It is worth noting that the results and developments achieved during this FS study will serve as a cornerstone for an upcoming LDRD project, recently funded by NIF & PS for FY24, that will explore the feasibility of mitigating nodular defects in boron carbide films deposited by EBE.