Influence of the manufacturing process on the crystallization behavior of a CZS glass system

A glass belonging to the CaO–ZrO2–SiO2 (CZS) system has been plasma-sprayed on ceramic tiles. The crystallization behavior of plasma-sprayed coatings has been compared to the devitrification processes of the bulk glass and sintered glass powders of the same composition. DTA experiments reveal a crystallization peak at about 1323 K, with a quite high activation energy, indicating the bulk crystallization process is difficult, because of the high viscosity of the glass. Bulk samples require higher treatment temperatures (≈1423 K) to develop significant amounts of crystalline phases. Furthermore, being the crystallization mostly superficial, high residual stresses develop between the crystalline surface and the glassy core. Sintered samples are fully crystalline, because crystallization starts from the particle surface, the specimens show a considerable shrinkage. After an 1123 K 30′ + 1323 K 15′ thermal treatment, plasma-sprayed coatings, possessing numerous defects and a slight degree of devitrification induced by thermal cycling during plasma deposition, become very well sintered and completely crystalline, because the as-sprayed microstructural features promote heterogeneous nucleation. In this case, the shrinkage is quite negligible, since partial sintering occurs already during the deposition process. In the fully crystalline coatings, excellent mechanical properties (microhardness, fracture toughness and abrasion resistance) are achieved, overcoming those of traditional industrial glazes.