Reaction mechanism of plasma spraying TiO2-Al system and formation mechanism of the coating

Abstract

The reaction mechanism of plasma spraying TiO2-Al composite powder and the formation mechanism of the in-situ microstructure were revealed through thermal analysis, quenching experiments, simulation calculation and microstructure characterization. The reaction process of the TiO2-Al system was accomplished in a two-step reaction. Reaction 1 was: Al+TiO2→Al2O3 +TinO2n-1/AlxTiy；Reaction 2 was: AlxTiy +TinO2n-1→Al2O3 +TiwO. The starting reaction temperature of the TiO2-Al composite powder with a molar ratio of 3:4 was about 800 °C, and the reaction can be fully carried out at 1200 °C. The reaction types of Al and TiO2 under plasma spraying conditions were solid-liquid and liquid-liquid reactions. The molecular dynamics trajectory shows that the reaction between Al and TiO2 was carried out by Al continuously capturing O in TiO2, but not all O in TiO2 was captured by Al. During the cooling solidification stage, the main phase-separation mechanisms of the TiO2-Al droplet were nucleation-growth mechanism and spinodal decomposition mechanism. In the metastable alumina-titanium oxide coating, the spinodal decomposition mechanism corresponds to self-locking microstructure, while the nucleation and growth mechanism corresponds to fine crystal and intracrystalline/intergranular microstructure.