BCxNy thin films deposited at 250°C by pulsed reactive magnetron sputtering of a B4C target in an Ar/N2 plasma were studied by elastic recoil detection analysis, Fourier transform infrared, Raman, and photoelectron spectroscopy, electron microscopy, and nanoindentation. In the concentration range of 6% to 100% N2 in the sputter plasma the segregation into nanocrystalline hexagonal boron nitride and amorphous sp2 carbon is the dominant process during the film growth. The stoichiometric ratio and structural details of the major phases depend on the N2 concentration in the plasma and have significant influence on the Young′s modulus and the elastic recovery of the BCxNy thin films.