Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 mu m and peeled away from the substrate due to the difference in thermal expansion between SiO2 and Al2O3 at temperatures above 1500 degrees C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 mu m to Ra 1.83 mu m, and no peeling was observed when heated to 1550 degrees C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by solgel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 degrees C and withdraw rate of 5 mm/min. While a uniform, 4-5 mu m thick HfO2 reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.