Structural stability and non-catalytic nucleation inhibition effect of Si-Zr-B mould coating on superalloy melt

The investment moulding technique was first adopted to prepare a SiO2 - ZrO 2 - B2O3 (Si - Zr - B) substrate layer on the inner surface of the mould, b y employing SiO2 glass dust and ZrO2 powder, SiO2 - ZrO2 sol, and analytica l grade H3BO3 as refractory material, binder, and softening agent, respecti vely. Then using sol - gel processing, seven layers of Si - Zr - B film of the same formula as the aforementioned Si - Zr - B substrate layer were com pounded with the substrate layer step by step. After glassing treatment at 850 degreesC for 60 min, this film transformed into a glass lined coating. It was shown from X-ray diffraction analysis that, after holding it at a te mperature of 1500 degreesC for 30 min, the amount of crystallinity in the S i - Zr - B coating was about 1 - 3% (vol.%). Finally, the undercooling expe riment showed that a large undercooling (up to 140 K) was achieved in a DD3 (Ni - Cr - Mo - Al - Ti - Co - W) single crystal superalloy melt in this c oated mould. So it is concluded that a Si - Zr - B coating has got a good s tructural stability at high temperature and provides ideal non-catalytic nu cleation inhibition for an undercooled superalloy.