Preparation and Characterization of Nanostructured Gd₂Zr₂O₇ Feedstock for Plasma Spraying

It is an advanced surface modification technology to prepared environmental barrier coating (EBC) by plasma spraying, which can improve the corrosion resistance of SiC_f/SiC composite materials. However, the corrosion resistance of Calcium-Magnesiu-Alumino-Silicate (CMAS) for the EBC coating was not well solved at present. Rare earth zirconates are ideal materials for next generation thermal barrier coatings and environmental barrier coatings because of their low thermal conductivity, excellent sinterability and phase stability. This work aims to prepare high-performance nanostructured Gd₂Zr₂O₇ feedstocks with nanopowder regranulation technology to meet the needs of the next generation of thermal and environmental barrier coatings. The Gd₂O₃ and ZrO₂ with the average particle size of 50 nm were used to prepare the Gd₂Zr₂O₇ feedstocks. In order to make full use of nano effect, the nanoparticles must be reconstituted to the agglomerates by nanopowder regranulation technology. Nanopowder regranulation technology includes ball milling, spray drying, sintering and plasma treatment. The slurries which composed of 298 g Gd₂O₃, 202 g ZrO₂, 1 000 g deionized water and 2.5 g PVA were ball milled for 24 h. The prepared slurries were spray dried in a YC-018 spraying dryer to obtain the agglomerates and then sintered. The agglomerates were sintered in the heat treatment furnace at 1 200 ℃, 1 300 ℃, 1 400 ℃, 1 500 ℃ for 2 h, 4 h, and 6 h, respectively. In order to improve the density and mobility of the feedstocks, the agglomerates were plasma treated by Mecto 9MC spraying system. The plasma flame caused the agglomerates surface to melt rapidly and form droplets, which are then rinsed into deionized water to cool quickly and form spherical solid particles. Plasma treatment slurries were dried in a 120 ℃ dryer and screened through a 150-mesh sieve to obtain Gd₂Zr₂O₇ feedstocks. The microstructure, morphology, and chemical composition of the feedstocks were observed by a Quanta 200FEG scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The phases of the feedstocks were identified by X-ray diffraction analysis (XRD) with Cu Kα radiation. The results show that the nanostructure Gd₂Zr₂O₇ feedstocks can be prepared by nanopowder regranulation technology. The feedstocks have the dense structure and good sphericity. The particle size of feedstocks is 10~50 μm. In the process of nanopowder regranulation technology, the grain size increases with the increase of sintering temperature. The optimal sintering parameters for Gd₂Zr₂O₇ feedstocks are sintered at 1 300 ℃ for 2 hours. Compared with the other sintering parameters, the content of Gd₂Zr₂O₇ after sintering at 1 300 ℃ for 2 hours is highest and the grain size is smallest. Plasma treatment can improve the flowability of feedstocks effectively. After plasma treatment, the apparent density is 2.09 g/cm³, the tap density is 3.26 g/cm³, and the flowability is 22.3 s/(50 g). The hausner ratio is 1.19 which indicated the feedstocks is in a free flow state. The Gd₂Zr₂O₇ feedstocks show good flowability. Nanostructured Gd₂Zr₂O₇ powder feedstocks are successfully prepared by the nanopowder regranulation technology. The feedstocks have dense internal structure, smooth external surface and good spherical shape. It is suitable for plasma spraying and expected to become the next generation of TBC and EBC.