Protective tubes ( Ø 90x1500 mm) were manufactured by spraying with a water stabilized plasma gun. Mixtures of zircon (ZrSiO4) and alumina (Al2O3) were used as feedstock powders. Products made of these powders exhibit very good properties during thermal cycling. Previous results of the phase composition and phase changes were obtained from as-sprayed and annealed samples using X-ray diffractometry (XRD) and scanning electron microscopy. During plasma spraying zircon decomposed into ZrO2 and SiO2, which on impact and after rapid quenching formed a very fine eutectic mixture of tetragonal or monoclinic ZrO2 and amorphous SiO2. During this process alumina, in feedstock as α-phase (corundum), formed the spinel γ-phase. On annealing the y-phase transformed into the a-phase, whereas amorphous SiO2 crystallized and reacted with tetragonal ZrO2 to form ZrSiO4. Mullite (3Al2O3.2SiO2) was found at the highest annealing temperature of 1500°C when alumina reacted with SiO2. High temperature XRD was used for direct observation of phase changes during heating and cooling between room temperature and 1500°C in powdered as-sprayed deposits. This method confirmed the phase changes observed at room temperature in annealed samples, in particular the partial transformation of tetragonal to monoclinic ZrO2.

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