Densification Behavior of a 3D Needled C/SiC Composite Fabricated by Machining‐Aided Chemical Vapor Infiltration

ABSTRACT Machining‐aided chemical vapor infiltration (MACVI) introduces machined through‐holes as additional diffusion channels, mitigating density gradients in thick‐section C/SiC composites. Using X‐ray imaging and image processing, we investigated the densification behavior of thick three‐dimensional needled (3DN) C/SiC composites during post‐machining infiltration (POMI) cycles. Results show that dense rings (DRs) form around the holes, enhancing central matrix deposition and reducing through‐thickness density gradient. DR width is independent of hole diameter. For a 10‐mm‐thick 3DN C/SiC composite (initial density 1.27 g/cm 3 ) with a through‐hole, DR width reaches ∼1.2 mm after two POMI cycles—29.4% smaller than that (∼1.7 mm) for 2D counterparts. Simulation comparison reveals this reduction originates from limited pore connectivity within the multiaxial short‐chopped fiber layers, imposing additional diffusion resistance beyond geometric control. These findings establish that hole spacing in MACVI hole‐array design is governed solely by DR width, which in 3DN C/SiC is significantly smaller than in 2D; consequently, denser hole arrays are required for 3DN C/SiC composites.