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

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.