Oxidation protective coating of Y2O3-Na2SixO2x+1 composite on graphite crucible for high temperature applications

Oxidation and corrosion protective coatings on graphite are essential for its high temperature applications. In the present work, oxidation protective coating on graphite was achieved with a combination of Y2O3 and SiO2 suspension with NaOH in methanol. Study showed that the optimized composition of the desired coating was Y2O3-15 wt% SiO2 (i.e., 2 g mixture of Y2O3 and SiO2 powder containing 0.3 g of SiO2 which is 15% of the total powder content if expressed in weight) with 5 M NaOH in 10 mL methanol. This protective coating on graphite was stable up to 1473 K for 30 min in Ar ambient before visible cracks appeared. The compositions of SiO2: NaOH were varied (5 mmol:5 M, 10 mmol:5 M and 5 mmol:10 M) to examine the roles of these two components in the overall adhesiveness of the coating. SiO2 and NaOH acted as inorganic binders and formed in-situ intermediate layers during sintering which held Y2O3 layer with the graphite surface. The coated graphite blocks and crucibles were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), energy dispersive X-ray spectroscopy (EDS), differential thermal analysis and thermogravimetric analysis (DTA/TGA) and Fourier transform infrared spectroscopy (FTIR). The oxidation protective nature of the coating was tested by heating the coated crucible at 1273 K for 1-15 h in air atmosphere. The effective adhesion of oxidative coating on graphite was explained.