Si-HfO2 composite powders fabricated by freeze drying for bond layer of environmental barrier coatings

Abstract

Hafnia-doping is expected to improve the performance of silicon bond layer of EBC system for SiC-based CMC. However, obtaining a compositional uniform Si-HfO2 composite coating is still a challenge due to the big difference of their melting point, density and fluidity of powders. Fabrication of evenly distributed Si-HfO2 composite powders with good fluidity and activity is a promising strategy. Freeze drying, a cost-effective and green method, was applied to make Si-HfO2 composite powders for the first time. Highly active nano-sized Si and HfO2 powders were mixed uniformly into a slurry, which was frozen and dried into solid lamellar structure. After crushing down and sieving, composite powders with controllable particle size and good fluidity were achieved, which inherited the uniformity of the slurry. Thickness of the lamellar structure determines the size and shape of the final composite powders, which can be controlled by simply varying the freezing temperatures. Compact and smooth bond coatings with evenly distributed Si and HfO2 were successfully fabricated from these composite powders by plasma spraying. We hope this work could excite the whole society to focus more on the structural design of powders and coatings as well as material selection to further boost the performance of EBCs.