We used X-ray reflectivity and X-ray glancing-angle diffraction in order to study the crystallisation process in ion-plated thin boron nitride films. Both experiments show adistinct threshold plating energy of E*ion=100eV. Above E*ion the average density (deduced from X-ray reflectivity) shows astrong increase, indicating the sudden appearance of the cubic boron nitride phase consistent with the sp3 concentration deduced from IR absorption spectroscopy. The in-plane X-ray diffraction shows that this cubic phase consists of small nanocrystals of 70 linear size.