Method to fabricate a machinable ceramic matrix composite

A method to form a ceramic matrix composite having a predetermined surface finish and/or dimensional tolerance, the method comprising:

• forming a porous ceramic multilayer on a surface of a fiber preform, the porous ceramic multilayer comprising a gradient in porosity in a direction normal to the surfacer, wherein forming the porous ceramic multilayer comprises:
• applying an inner slurry layer to the surface and applying an outer slurry layer to the inner slurry layer, each of the inner and outer slurry layers comprising ceramic particles in a carrier liquid, wherein a carrier liquid volatility, a particle volume fraction, and/or a polymer content of the inner and outer slurry layers are different;
• and
• removing the carrier liquid from the inner and outer slurry layers by passive and/or active drying, thereby forming the porous ceramic multilayer comprising the gradient in porosity;
• after forming the porous ceramic multilayer, infiltrating the fiber preform with a melt;
• cooling the melt to form a ceramic matrix composite with a surface coating thereon, an outer portion of the surface coating being more readily machinable than an inner portion of the surface coating;
• and
• machining the outer portion to form a ceramic matrix composite having a machined surface with a predetermined surface finish and/or dimensional tolerance.