posted on 2020-03-12, 18:40authored byEspen Sagvolden, Martin F. Sunding, Ole Swang
Phosphoric acid anodization
(PAA) is a candidate for replacement
of toxic chromates during the surface treatment of aluminum prior
to gluing in the aerospace industry. During PAA, a layer of AlPO4 forms on top of the alumina layer. We apply density functional
theory computations to investigate how the AlPO4 surface
reorganizes and how it bonds to water and adhesives. As our AlPO4 model, we use the α-berlinite (0001) surface. Taking
the structure of the α-quartz (0001) surface reported by Rignanese
et al. (Rignanese, G.-M.; De Vita, A.; Charlier, J.-C.; Gonze, X.;
Car, R., Phys. Rev. B 2000, 61,
13250–13255) as a starting point, we find that the α-berlinite
surface reconstructs. The lowest energy structure for α-berlinite
(0001) is found to have a buckled configuration, with three-coordinated
phosphorus protruding out of the surface and a neighboring aluminum
atom binding to five oxygens. Different structures for the hydrated
surface AlPO4·0.25H2O are presented, of
which the most stable involves hydroxylation of the aforementioned
buckle and of a new phosphorus buckle, accompanied by formation of
a P–Al dative bond. We report results for the adhesion of a
glue fragment derived from bisphenol A to the surface. The lowest
energy is found for a covalently bonded structure, mimicking the most
stable hydroxylated structure. The adhesion energy of the glue increases
strongly when it is covalently bonded to the surface rather than being
hydrogen bonded, providing superior adhesion to the material.