The oxidation stability of boron nitride thin films on MgO and TiO2 substrates

The stability of BN thin film coatings (2-5 nm) on MgO and TiO2 substrates was investigated using transmission electron microscopy (TEM). The samples were heated in air for at least 16 hours at temperatures ranging from 773-1273 K. On MgO supports, the BN thin film coating was lost by 1073 K due to a solid state reaction with the substrate. This lead to the formation of Mg2B2O5. No such reaction occurred with the TiO2 substrate and the BN was stable even at 1273 K. However, the coating appeared to ball up and phase segregate into islands of near-graphitic BN and clumps of TiO2 (rutile). The oxidizing treatment appears to promote the transformation from turbostratic BN to graphitic BN.