Structure stability and nucleation inhibition on the undercooled DD3 single crystal superalloy melt of the Si-Zr-B coating

Using the mixture of SiO₂ glassdust ZrO₂ powder as refractory material, SiO₂-ZrO₂ sol as binder and H₃BO₃ as softening agent, a substrate layer was prepared on the inner surface of the mold by adopting shell-molding technique, where the SiO₂-ZrO₂ sol was the product of the hydrolyzing-condensation reaction at 35°C for 180 min in the precursor solution with the mole ratio of 3H₂O, 4C₂H₅OH, 0.11 ZrOCl₂·8H₂O and 1 Si(OC₂H₅)₄, respectively. Then by adopting sol-gel processing, proper layers of SiO₂-ZrO₂-B₂O₃ thin film with the same composition as the substrate layer was compounded with the substrate layer step by step. After dipping-class heat treatment and glassing treatment at 800°C for 60 min, this film transformed into the SiO₂-ZrO₂-B₂O₃ glass coating. Furthermore, after holding the coating at a temperature of 1500°C for 30 min, the amount of crystalline is about 1%-3% (vol%). Finally, an undercooling experiment showed that high undercooling of DD3 single crystal superalloy up to 140 K was achieved in this coating mold. Based on the classical nucleation theory, the wetting angle corresponding to the largest undercooling achieved in the inhibitive coatings, θ^*, was calculated. It is found that they are far less than the measured one between coating and alloy melt. This phenomenon further proves that not only the SiO₂-ZrO₂-B₂O₃ coating has good high temperature structure stability, but also it is an ideal inhibitive nucleation coatings for DD3 single crystal superalloy melt.