September 2004 Process and Mechanical Properties of in Situ SiC Nanowire-Reinfarced CVI SiC/SiC Composite 1723 Table L. Density, Interlayer, and Mechanical Properties of the Composites Carbon laver ssm) U (MPa '(MPw/(Mg/m'n K,(MPam) Kl (MPam A(Mg/n') NFC262±0.03 660±77 250±29 16.3±3.3 6.2±1.2 F-C 2.74±0.04 590±78 210±28 15.2±3.5 5.5±1.3 and Ai, are normalized ultimate flexural strength and fracture toughness, respectively, using composite densities. SiC/SiC composites. Many other SEM and TEM examinations features at their fracture surfaces, as shown typically in Figs. 5(a) confirmed successful growth of SiC nanowires directly in the and(b) for composite NF-C and Figs. 5(c)and (d) for composite composite preform with in situ deposition of thin and uniform carbon coating on all the nanowires, although the amount of the esults in sound fiber pullout fracture behaviors. Composite F-C nanowires was relatively small. exhibits slightly longer fiber pullouts than composite NF-C. likely because of its thicker carbon interlayer (Table I), which produces (2) Fabricated Composites Table I shows the density and the thickness of carbon fiber/ No clear debonding and pullout of the SiC nanowires from the hatrix interlayers of the two composites. The density is the matrix are observed at the fracture surfaces of composite NF-C. average value of the seven rectangular specimens of each com except occasional observations of fragments of nanorods posite from their masses and volumes. Composite NF-C has an average density of 2.62=0.03 Mg/m, which corresponds to a (4) Effect of SiC Nanowires porosity of -13%. A 60 nm carbon coating was deposited on the fibers as the fiber/matrix interlayer, as shown in Fig. 4(a) and the As shown in Table I. the two composites possess different nserted high-magnification SEM image. in which the fiber carbon interlayer and average density, which are important factors matrix. and the carbon coating around the fibers are evident. that affect mechanical performance. These differences make it However, no SiC nanowires are noted in the image. Similarly, at difficult to clearly understand the effects of the nanowires on he cross section of the composite where the matrix is densel mechanical properties by simply comparing the strength data of deposited. it is difticult to observe SiC nanowires in the matrix or the two composites. In previous studies . tensile and flexural around the fibers using other SEM examinations. Microscale trengths of a series of plain-woven Tyranno SA fiber-reinforced CVI-SiC/SiC composites showed close carbon layer thickness rods/structures are readily observed in areas with an insufficiently deposited matrix as shown typically in Fig. 4(b). Such microscale dependence up to-100 nm. When the carbon layer thickness ncreased from 50 to 100 nm. the ultimate tensile and flexural structures are not found in the companion composite, F-C. These strengths increased from 410+ 92 and 189+27 MPa to 606+ microscale structures are believed to have developed from the original nanowires during the CVI matrix densification process 28 and 285+ 20 MPa, respectively. The thickness of the carbon that followed. The absence of SiC nanowires in the dense matrix interlayer in composite NF-C is 60 nm, and it is 120 nm in in NF-C is likely because of the very thin carbon coating (--5 nm). composite F-C. Therefore, the real contributions of the SiC which is not sufficient to form a clearly visible interface between nanowires on the strength and toughness of composite NF-C might the nanowires and the matrix under current SEM examinations be partially covered by the effects of the thickness of the carbon Composite F-C has a thicker carbon interlayer, 120 nm, and high interlayer. In addition, statistical studies" have shown that the average density, 2.74 0.04 Mg/m flexural strength of SiC/SiC composites also increase with higher composite densities. However. composite NF-C shows higher (3) Flexural Strength and Fracture Toughness flexural strength and fracture toughness compared with composite F-C, despite its thinner carbon fiber/matrix interlayer and lower The obtained ultimate flexural strength(o )and fracture tough- density. The increased strength and toughness are believed to be ness(Ki)are summarized in Table L. The composite NF-C shows due to the contribution of the incorporated SiC nanowires. Here, average flexural strength and fracture toughness. 660+77 MP when the average strength and fracture toughness of composit and 16.3+3.3 MPam". respectively, which are slightly higher densities are simply normalized, composite NF-C shows% than those of composite F-C. The two composites have similar and -13% increases in flexural strength and fracture toughness Matrix Fiber Fiber Bilayer 5 m k Matrix Micro-rods 100m 5 u m Fig. 4. SEM images of the (a) cross section and (b) microrods/structures in composite NF-CSeptember 2004 Process iiinl Mechanical Properties of in Situ SiC Nanmvirc-Reinforced CVI SiC/SiC Composite Table I. Density. Interlayer. and Mechanical Propt'rties of the Composites Composite Dcnsii Carbon layer thickness Iniii) • r,, lMP;i) (MP;i-m' NF-C F-C 2.62 ± 0.03 2.74 ± 0.04 60 660 ± 77 120 590 ±7 8 250 ± 29 210 ± 28 16.3 ±3.3 15.2 ±3.5 iMPii-m"- 6.2 + 1.2 5.5 ± 1.3 ir;, and A*;, arc normalised ultimate flexural stroiiyih ami Iniilurc touj^hness, respociively. u.'.ing composite dcnsitit 1723 SiC/SiC composites.*''' Many other SEM and TEM exaniin;iti(ins confirmed successful growth of SiC nanuwires directly in the composite preform with in situ dcpositit)n of ihin and Linilbrm carbon coating on all the nanowires. although the amount of the nanowires w;is relatively small. i2) Fabricated Composites Table I shows the density and the thickness of carbon tlber/ matrix interlayers of the iwo ctimposites. The density is the average value of the seven rectangular specimens of each com￾posite from their masses and volumes. Composite NF-C has an average density of 2.62 ± 0.03 Mg/tn"*. which corresponds lo a porosity of— 13%. A 60 nm carbon coating was deposited on the fibers as the tlber/inairix interlayer. as shown in Fig. 4(a) and the inserted high-magnillcation SEM image, in which the fiber, matrix, and ihe carbon coating around the fibers are evidcni. However, no SiC nanowires ari? noted in the image. Similarly, al ihe cross section of the composite where the matrix is densely deposited, it is difficult to observe SiC nanowires in ihe matrix or around the fibers using other SEM examinations. Microscale rods/struclures are readily observed in areas with an insufficiently deposited matrix, as shown typically in Fig. 4(b}. Such microscale structures are not found in the companion composite. F-C. These microscale structures are believed to have developed from ihe original nanowires during the CVI matrix densitlcatioii process that followed. The absence of SiC nanowires in the dense matrix in NF-C is likely because of the very thin carbon coating (—5 nm). which is not sufficient to form a clearly visible interface between the nanowires and the matrix under current SEM examinations. Composite F-C has a thicker carbon interlayer. 120nm, and higher average density, 2.74 ± 0.04 ' (3) Flexural Strength and Fracture Toughness The obtained uhimate tlexural strength (uj and fracture tough￾ness (A",,,) are summari/cd in Table I. The composite NF-C shows average tlexural strength and fracture toughness. 660 ± 77 MPa and 16.3 ± 3.3 MPa-m"-. rcspeciiveiy. which are slightly higher than those of composite F-C, The two composites have similar features al their fracture surfaces, as shown typically in Figs. 5(a) and |b) for composite NF-C and Figs. 5(c) and (d) for composite F-C. Inierfacial debonding occurs al the fiber surfaces, which results in sound fiber pulknit fracture behaviors. Composite F-C exhibits slightly longer fiber pullouts than composite NF-C. likely because of its thicker carbon interlayer (Table I), which produces lower interfacial shear strength/friclional stress in the materials.'' No clear debonding and pulloui oi' the SiC nanowires from the matrix are observed at the fracture surfaces of composite NF-C. excepi occasional observations of fragments of nant>rods. (4) Effect of SiC Nanowires As shown in Table 1. the iwo composites possess different carbon interlayer and average density, which are important factors that affect mechanical performance. These differences make it difficult to clearly understand the effects of the nanowires on mechanical properties by simply comparing the strength data of the two composites, in previous studies.'''' tensile and flexural sirengths of a series of plain-woven Tyranno-SA llber-reinforced CVI-SiC/SiC composites showed close carbon layer thickness dependence up to ~IOO nm. When the carbon layer thickness increased from 50 lo 100 nm. the ultimate tensile and tlexural strengths increased from 410 ± 92 and 189 ± 27 MPa to 606 ± 28 and 285 ± 20 MPa. respectively. The thickness of the carbon interlayer in composite NF-C is 60 nm. and it is 120 nm in composite F-C. Therefore, the real contributions of the SiC nanowires on the strength and toughness of composite NF-C might be partially covered by the etfects of the thickness of the carbon interlayer. In addition, statistical studies"'* have shown that Ihe Hexural strength of SiC/SiC composiics also increase with higher composite densities. However, composite NF-C shows higher tlexural strength and fracture toughness compared with composite F-C, despite its thinner carbon tlber/matrix interlayer and lower density. The increased strength and toughness arc believed to be due to the contribution of the incorporated SiC nanowires. Here. when the average strength and fracture toughness of composile densities are simply normali/.ed. composite NF-C shows —20^*^ and —13'^ increases in tlexural strength and fracture toughness Fig, 4. SHM image s of ihc fa) cross ii and ib) microrodsAirtictures in conipusiie NF-C