Boron carbon nitride films (BCN) were grown by B$_4$C evaporation under concurrent N$_2$ ion beam assistance. The films were characterized by x-ray absorption near-edge spectroscopy, infrared and Raman spectroscopies, and high-resolution transmission electron microscopy. The bonding structure and film composition correlate with the momentum transfer per incoming atom during deposition. As the momentum transfer is increased, the film structure evolves from an amorphous boron carbide network towards a hexagonal ternary compound ($h$–BCN) with standing basal planes. The growth of $h$–BCN takes place for momentum transfer in the window between 80 and 250 (eV $\times$ amu)$^{1/2}$ . The characteristic vibrational features of the $h$–BCN compounds have also been studied. Finally, the solubility limit of carbon in the hexagonal BN structure, under the working conditions of this article, is found to be $≈$15 at. %.