Templated epitaxial coatings on magnesium aluminate spinel using the sol-gel method

摘要

Polycrystalline magnesium aluminate (MgAl₂O₄) spinel has attractive properties for a range of applications including radomes, windows, and ballistic protection. A wet chemical approach using solutions of magnesium nitrate hexahydrate Mg(NO₃)₂ · 6H₂O in 2-methoxy-ethanol (2-MOE) and aluminum nitrate nonahydrate (Al(NO₃)₃ · 9H₂O) in ethylene glycol, was developed for spin coating on coarse-grain, polycrystalline spinel substrates. The coated substrates were subjected to isothermal heat treatments in the temperature range 1000–1400 °C, and subsequently examined using low voltage scanning electron microscopy, EBSD (electron back scattered diffraction), and transmission electron microscopy. The results showed that for annealing at 1200–1400 °C, the coatings were converted to crystalline MgAl₂O₄ which was epitaxial with the substrate grains. Heat treatment at lower temperatures, however, resulted in porous, fine-grained polycrystalline coatings. Thermal faceting of the grain surfaces was observed to occur. The observations suggest that faceting occurs preferentially on {100} and {110} planes. The morphology of the faceting was discussed in terms of the reported relative surface energy values for the low index planes in MgAl₂O₄ spinel. Finally, the influence of the coating process on spinel substrates which had been lightly abraded prior to spin coating was investigated.