Densification method of air-plasma-sprayed environmental barrier coatings achieved by pre-heat treatment

Abstract

High density of the top coat of environmental barrier coatings (EBC) is important for ensuring its performance in water vapor environment, which is also significant for prolonging service lifetime of SiCf/SiC aero-engine hot components. In this work, densification of the top coat of EBC was achieved by pre-heat treatment. In detail, the as-sprayed porous Yb2SiO5 coatings were heat-treated at the high temperature of 1250℃ to 1450℃, which improve the density significantly. Changes in microstructure and mechanical properties during the pre-heat treatment were studied by classifying defects into different types. The processes of defects-healing were observed, and the mechanism responsible for coating densification was revealed. Results show that the porosity of as-sprayed Yb2SiO5 coating is high due to the existence of three types of micro-defects, including two-dimensional (2D) inter-splat pores, 2D intra-splat cracks and three-dimensional (3D) spherical pores. During the pre-heat treatment process, 2D defects are healed to be reduced in a large amount in a short duration, while the 3D spherical pores seem unchanged. The mechanism of defects-healing in the heat treatment process is that grain growth inside the coating makes surface of the pores rough, which leads to multiple bridge-connection of 2D pores. As a result, the original continuous defects are divided into several sections and are further spheroidized. The method of pre-heat treatment for APS-EBC would make a fundamental contribution to its further engineering application.

Keywords

• environmental barrier coating
• air plasma spray
• pre-heat-treatment
• defects-healing
• densification