Critical thickness of polymer-derived ceramic coatings with particulate fillers

In this study, we demonstrate a novel environmental barrier coating processed from polymer-derived ceramics (PDCs) with homogeneously distributed sub-micrometer Y₂O₃ as the filler. Under suitable conditions, dense and crack-free coatings can be achieved for all the designed compositions with the volumetric content of Y₂O₃ varied from 45 to 93 vol%. To process the PDC SiC–Y₂O₃ composite coatings, Y₂O₃ particles and SiC liquid precursor were uniformly dispersed in hexane and then dip-coated on SiC substrates. After cross-linking at 250°C and heat-treated at 900°C in argon, dense and crack-free PDC SiC–Y₂O₃ composite coatings were formed. The effect of coating thickness and heat-treatment temperature on the formation of cracks due to constrained pyrolysis was studied. The critical thickness for realizing crack-free coatings of three compositions (i.e., 93, 77, and 45 vol% Y₂O₃) was studied for heat treatment from 1000 to 1300°C using atomic force microscope and scanning electron microscopy. As heat-treatment temperature increases, the critical coating thickness decreases for the same coating compositions due to enhanced shrinkage at higher temperature. With higher Y₂O₃ content, the critical thickness of the coating increased. The inert Y₂O₃ particles reduce the amount of polymer leading to reduction in the overall constrained shrinkage of the coating during heat treatment.