Dense, crystalline, glass-ceramic coatings containing calcium metaphosphate and aluminum phosphate were prepared on aluminum oxide substrates by a three-step method. The processing involved glass (40 mol % CaO, 10 mol % Al2O3, 50 mol % P2O5) formation, deposition of a glass particle coating, and heat treatment to sinter the glass and crystallize the phosphates, Sintering and microstructure evolution were influenced by wet coating thickness, heat-treatment temperature, time, and heating rate. Heat treatment for 1 h at 725 °C using a 50 °C/min heating rate was found to give a dense, crack-free coating. The resultant coating microstructure has spherulitic morphology (0.3 μm size) with aluminum phosphate in the center of the spherulite. The hardness of the fully crystallized glass-ceramic coating was ∼5.2 GPa.
Bibliographical noteFunding Information:
The McKnight Foundation and the National Science Foundation (DMR-9357502) are gratefully acknowledged.