Densification And Mechanical Properties Of C/SiC Fabricated By Machining Aided Chemical Vapor Infiltration

Chemical vapor infiltration（CVI）is a prominent process for fabricating carbon fiber/silicon carbide（C/SiC）composites.The"bottleneck"or"choking"effect in CVI,however,leads to nonuniform matrix infiltration/deposition（density gradient）between near-surface layers and far-surface middle layers in the fabricated composites.Although this nonuniform matrix infiltration/deposition can be inhibited and ignored when the preform/composite thickness is small,it can be exacerbated by increased preform/composite thickness,thus resulting in strength degradation.Therefore,it remains an important issue to mitigate the nonuniform matrix infiltration,particularly in the vapor fabrication of thick-section CMCs.To addressing the large density gradient,matrix infiltration in the inner/middle layers of the thick-section CVI CMCs should be increased.To increase this matrix infiltration,We have developped the concept of machining-aided CVI（MACVI）.In MACVI,diffusion-enhancing holes（DEHs）are used to increase the matrix infiltration in the middle layers.The MACVI is comprised of three stages:（1）pre-machining infiltration（PRMI）,（2）hole（DEHs）machining（HM）,and（3）post-machining infiltration（POMI）.The stage（1）is set to obtain partially dense CMCs.In stage（2）,DEHs are machined through the thickness of the partially dense CMCs to provide more diffusion channels for gaseous reactants in the subsequent matrix infiltration in stage（3）.Then the increased diffusion channels in stage（3）bring in increased matrix deposition in the inner/middle layers of thick-section CMCs.Based on this concept,systematic research has been conducted as follows.The densifications of thick-section MACVI 2D C/SiC composites were investigated.With the introdution of DEHs,the obtained densities and the density growth rates（DGRs）of MACVI 2D C/SiC composites were higher than those of the CVI counterparts（the ones without DEHs）.Through POMI,dense rings（DRs）appeared around DEHs and their widths increased with deposition cycles.The widths of DRs were not affected by the diameters of DEHs.Considering the decrease of the loading section caused by DEHs,therefore,DEHs with small diameters should be chosen in MACVI.When the DEHs were introduced to partially dense 2D C/SiC at the density of 1.6 g/cm³,the DEH-induced infiltration increase is contributed by the deposition increase in the large pores（diameters>52μm）,the inter-ply and the large inter-bundle pores,whereas the deposition in the small pores（diameters<52μm）,the intra-bundle and the small inter-bundle pores,can hardly be increased.These are the pores which have been sufficiently filled in PRMI.Therefore,the ration of the time of PRMI(t_prmi)to the time of POMI(t_pomi)is inferred as an imporantant parameter in MACVI.Reducing this ratio will increase the pore volume that can be affected in POMI.The densifications of thick-section MACVI 3DN C/SiC composites were investigated.Both the predicted final densities and the DGRs of the MACVI 3DN C/SiC increased compared with their CVI counterparts.The density gradient formed in the densification of the thick-section C/SiC composite was found to be inhibited with the introduction of DEHs,in a way dense rings（DRs）formed around DEHs.The DR width increased with the deposition time.But it was unaffected by the DEH size.Under POMI,the DR width formed in 3DN C/SiC was smalller than that in 2D C/SiC,which is attributed to the lower degree of order of the fibers in 3DN C/SiC.The short-cut web layers in the 3DN C/SiC composite indicates more complicated pore strusture and thus harder diffusion for gaseous reactants in vapor infiltration.The mechanical properties of thick-section MACVI 2D C/SiC composites have been investigated.Thick-section MACVI 2D C/SiC composites showed increased strength and toughness than those of the CVI counterpart.The fabricated 10-mm-thick MACVI 2D C/SiC exhibited a maximum flexrual strength of 351 Mpa and a tensile strength of 145.7±21 MPa,which indicates a 72.1%flexrual strength increase and a 31.8%tensile strength increase,comparesed to the CVI couterpart.The increased matrix deposition in MACVI 2D C/SiC has incresed the bonding between fiber layers and can efficiently inhibite the delamination that would occur in the failue of CVI counterpart.In the zone with increase matrix depositon,cracks tend to enter into a fiber bundle,rather than surpass it by entering to the inter-bundle porous matrix.When cracks enter into the fiber bundles,cracks deflect,resulting detachments of transverse fiber clusters and pullouts of longitudial fiber clusters.The mechanical properties of thick-section MACVI 3DN C/SiC composites with different hole-maching parameters（hole spcing,hole diameter）have been investigated.With MACVI,the flexrual and compressive strengthes of thick-section 3DN C/SiC can be increased.The fabricated 10-mm-thick MACVI 3DN C/SiC exhibited a maximum flexrual strength of 325±33 MPa and a maximum compressive strength of 415±40 MPa,which indicates a 13.6%flexrual strength increase and a 3.5%compressive strength increase,comparesed to the CVI couterpart.Tailoring hole diameter and hole spacing can tailor the mechanical preperties of MACVI 3DN C/SiC composites.By decreasing hole spacing,the flexrual and compressive strength of MACVI 3DN C/SiC composites can be increased.By decreasing the hole size,the compressive strength of MACVI 3DN C/SiC composites can be increased,which indicates small hole size should be chosen in MACVI.In this work,the maximum DR width of MACVI 3DN C/SiC obtained with maximum POMI cycles is 1.5 mm,which is lower than that of MACVI 2D C/SiC（2.2±0.2 mm）.