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 surface, wherein the porosity increases in a direction away from the surface, wherein forming the porous ceramic multilayer comprises applying multiple layers of a ceramic tape to the surface by at least:
• applying an inner tape layer to the surface and applying an outer tape layer to the inner tape layer, each of the inner and outer tape layers comprising a ceramic tape including ceramic particles in a polymeric binder, wherein a particle volume fraction and/or polymer content of the inner and outer tape layers are different;
• and
• laminating the inner and outer tape layers to the surface, 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 exhibiting an increasing gradient in porosity in the direction away from the surface and further having a reduced density compared to an inner portion of the surface coating, an outer portion of the surface coating thus being more readily machinable than the 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.