Boron Nitride Coatings on Oxide Substrates: Role of Surface Modifications

Physical mixtures of a BN polymeric precursor with oxides such as MgO, Al2O3, and TiO2 have been found to yield crystalline, tightly adherent coatings of hexagonal BN on the oxide surface, after appropriate heat treatments. In every instance, the BN (0001) basal planes are parallel to the oxide surface. Generally, the coating conforms very well to the contours of the oxide surface. However, an exception was the silica surface where no coatings were observed. It appears that the surface silanols condense to form siloxanes at high temperatures, yielding a surface that is unreactive and unable to bind to the BN. Modification of the silica surface by incorporation of a monolayer of titania dramatically alters the ability of the silica to stabilize BN coatings. It was found that stable BN coatings could be obtained when the silica surfaces were thus modified.