Method to fabricate a machinable ceramic matrix composite
View Patent ↗A method to form a machinable ceramic matrix composite comprises forming a porous ceramic multilayer on a surface of a fiber preform. In one example, the porous ceramic multilayer comprises a gradient in porosity in a direction normal to the surface. In another example, the porous ceramic multilayer includes low-wettability particles having a high contact angle with molten silicon, where an amount of the low-wettability particles in the porous ceramic multilayer varies in a direction normal to the surface. After forming the multilayer, the fiber preform is infiltrated with a melt, and the melt is cooled to form a ceramic matrix composite with a surface coating thereon. An outer portion of the surface coating is machined to form a ceramic matrix composite having a machined surface with a predetermined surface finish and/or dimensional tolerance.
1. 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.
2. The method of claim 1 , wherein the outer tape layer has a polymer content higher than that of the inner tape layer.
3. The method of claim 1 , wherein the ceramic matrix composite comprises silicon carbide fibers in a matrix comprising silicon carbide, and wherein the surface coating comprises silicon carbide.
4. The method of claim 1 , wherein the outer tape layer contains the ceramic particles at a volume fraction lower than that of the inner tape layer.
5. The method of claim 1 , wherein a plurality of outer tape layers are applied to the inner tape layer, each successive outer tape layer comprising a reduced volume of ceramic particles and/or an increased polymer content compared to an underlying tape layer.
6. A gas turbine engine component comprising a ceramic matrix composite having a predetermined surface finish and/or a dimensional tolerance formed by the method of claim 1 .