Evolution of microstructure during annealing of Mn_(1.5)Co_(1.5)O_4 spinel coatings deposited by atmospheric plasma spray

摘要

Plasma spraying is a promising cost-effective method to manufacture different functional layers, especially on the sophisticated surface. In this work, atmospheric plasma spray (APS) was used to manufacture Mn_(1.5)Co_(1.5)O_4 (MCO) coatings on T441 substrates. To further enhance the bond strength with the substrate and densify the protective coatings, reducing and subsequent oxidation were performed on MCO coatings. The redox behavior of spinel oxide during heat-treatment was investigated. Moreover, the evolution of vertical crack was in-situ observed by SEM. The results showed that dramatic changes had occurred on the microstructure of MCO coatings during heat-treatments. After reduced for 5 h in a 20% H_2/Ar environment at 800℃, the continuous nanoscale vertical cracks converted to be discontinuous and even extended in local area. However, after oxidation treatment for 10 and 20 h, all the vertical cracks had healed. Primary spinel layer that consisted of MnCo_2O_4, CO_3O_4 and Mn_3O_4 were obtained. With the increasing of heat-treating time up to 95 h and 250 h, cubic CO_3O_4 and tetragonal Mn_3O_4 were converted into CoMn_2O_4, dense spinel coatings were finally produced. The cross-sectional image indicates that the coating is still well-bonded with the substrate and free from cracks after oxidation in air for 250 h. The SEM image shows an oxide scale of less than 1 μm.