Devitrification behaviour of plasma-sprayed glass coatings

Glass and glass-ceramic coatings on ceramic tiles have been manufactured by plasma-spraying high-performance CAS (in wt%—SiO2, 60%; Al2O3, 15%; CaO, 23%; others, traces) and CZS (in wt%—SiO2, 50%; CaO, 31%; ZrO2, 16.5%; Al2O3, 2%; others, traces) glass frits. The CZS system has a surface crystallization at about 1050 °C. Such behaviour would not easily allow to obtain a fully crystalline bulk glass-ceramic, but the defectiveness of the plasma-sprayed coating supplies many nucleation sites. Thus, it becomes completely crystalline and well sintered after a 850 °C for 30 min + 1050 °C for 15 min treatment. The CAS frit, designed not to produce significant crystallization, is well sintered after a 850 °C for 30 min + 950 °C for 30 min thermal treatment, but remains too brittle due to its glassy nature. A 1050 °C treatment allows a few pseudowollastonite crystals to form in a glassy matrix; their formation also hinders sintering. Thus, mechanical properties are inferior to heat-treated plasma-sprayed CZS.