Due to its high hardness, high thermal stability, high chemical resistance, and low-density, boron carbide stands out as a material of great interest for encapsulating carbonaceous materials. It has been reported that thin film deposition of boron carbide can improve the chemical stability of graphite films, even in plasma facing components for nuclear application. But deposition of boron carbide thin films by ALD is difficult to realize due to the limited number of precursor chemistries that exhibit self-limiting surface chemistry to deposit carbides by ALD. In this study, we explore the potential application of a recently developed chemical vapor deposition (CVD) process for boron carbide (B4C) thin films onto CNTs in a membrane structure. By using triethyl boron as a single source CVD precursor and Xe as a diffusion additive we demonstrate highly conformal deposition of B4C thin film onto CNTs with various diameters arranged randomly in a free-standing membrane structure. This method also illustrates the capacity of the CVD process to fine-tune the pore dimensions within the composite membrane. formation can be seen at the intersections or in the pores, suggesting that the CVD chemistry is highly conformal. The SEM micrographs further reveal a flat membrane surface without any wrinkles, exhibiting no bowing or curvature caused by the CVD process. The film thickness appears to be uniform, regardless of nanotube diameter (Figs. and). However, for ultranarrow tubes with a diameter of less than 2.5 nm, no deposition was observed. We ascribe this hindered nucleation of B4C films from TEB molecules to the inherent surface energy condition of ultra-narrow tubes, which is not favorable to the film formation. Figure shows the corresponding high magnification STEM image of a single B4C coated CNT, indicating that it is possible the CNTs remain intact after the CVD process. STEM imaging at the present experimental conditions promote mass contrast (~Z ). Hence, the lack of intensity in the core further suggests that it is possible the CNTs are intact. XPS was used to analyze the chemistry of the B4C deposited CNTs. The pristine membrane exhibit an intense narrow peak at 284.5 eV attributed to sp 2 hybridized C-C 39 bonds (Fig. ).