Shell mold for casting niobium-silicide alloys, and related compositions and processes

A method of making a ceramic shell mold for casting niobium-silicide-based articles, comprising the steps of:

• (i) applying a facecoat-forming material to a wax pattern which has a configuration corresponding to a desired mold cavity, so as to form a facecoat, wherein the facecoat-forming material comprises at least one compound selected from the group consisting of yttria, zirconia, hafnia, rare earth compounds, and combinations thereof;
• (ii) applying at least one aqueous slurry over the facecoat in successive layers until a mold having a mold wall of a desired thickness is obtained, wherein the aqueous slurry comprises colloidal silica and at least one compound selected from the group consisting of yttrium silicates, zirconium silicates, hafnium silicates, rare earth silicates, and combinations thereof;
• and then
• (iii) heat-treating the mold to remove substantially all free silica.

A shell mold, comprising at least one mold cavity having a surface on which a facecoat is disposed;

• wherein the cavity is at least partially filled with molten niobium-silicide;
• and wherein:
• (I) the material forming the shell comprises at least one compound selected from the group consisting of yttrium silicates, zirconium silicates, hafnium silicates, rare earth silicates, and combinations thereof;
• and
• (II) the material forming the facecoat comprises at least one compound selected from the group consisting of yttrium silicates, zirconium silicates, hafnium silicates, rare earth silicates, and combinations thereof.