Plasma-sprayed graded ceramic coatings on refractory materials for improved chemical resistance

Plasma-sprayed ceramic coatings were manufactured on sintered alumina-mullite refractory bricks to improve their chemical resistance to molten glass. Mullite and alumina powders were employed. Graded layered coatings were designed and produced, to reduce the thermal expansion mismatch with the substrate: in all cases, the upper layer consisted in pure alumina (very resistant to chemical attack); alumina-mullite intermediate layers were added to match the low thermal expansion of the porous substrates. Plasma-sprayed coatings definitely improved both the abrasion resistance and the chemical resistance to long-time (8 h at 1400 degrees C) contact with molten glass, since the coating preserved its original microstructure. However, in thermal shock tests, some transverse cracks appeared; thus, thermal cycling tests in presence of molten glass indicated that, after a few cycles, the glass can penetrate down to these cracks. The tested samples were studied by means of scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD).