International Journal of Applied Ceramic Technolog-Sauder, Brusson and Lamon Vol. 7, No. 3, 2010 Pic The previous generations of SiC/SiC composites inal chemical bonding may be enhanced by mechanical ared via CVi and reinforced with nonstoichiomet- bonding as a result of thermally induced residual ic fibers(such as Nicalon or Hi-Nicalon fibers)possess stresses, which leads to gripping of the fiber by the ma- interesting mechanical properties like high trix. Radial gripping is enhanced when the interface is hness, high strength, and damage tolerance. How- rough. A thick fber coating(porous or pyrocarbon- ever,these fibers undergo shrinkage during irradiation based interphase) is used to reduce the clamping stresses by fast neutrons, which causes fiber/ matrix debonding induced by thermal expansion mismatch or surface and degradation of mechanical properties. By contrast, roughness. It has been reported that the SA3-1 the Hi-NicalonS and SA3 fibers exhibit microstructural forced SiC/SiC composites exhibit a brittle behavior stability because of a well-crystallized microstructure when the PyC interphase is very thin. .Then, it has and a lower impurity content, like CVD/CVI SiC. been shown experimentally that the failure strain in- Thus, it is necessary to investigate the mechanical be- creases with terphase thickness, whereas interfa- havior of CVI SiC/SiC composites reinforced by Hi- cial shear stress decreases. Various ranges of thickness NicalonS and SA3 fibers to check whether they can have been examined: <60 nm, 60-300 nm, and meet stringent material requirements for fuel cladding: >600 nm. hike mechanical properties and thermal conductivity sic gnis Morscher has ins estigfard fiber effets high operating (up to 1100C)and accidental polycrystalline fber. The Sylramic-reinforced minicom- (>1600C)temperatures, and posites had lower strain to failure and higher interfacial fast neutron and high fuence conditions. shear stress, compared with the Hi-Nicalon-reinforced Although both fibers exhibit comparable mechan- ones. This trend was attributed to the surface roughness ical properties, available data indicate that the tensile of the Sylramic fiber, which was reportedly quite resistance of composites is somewhat different: the significant: mean amplitude 69 nm against 4.3 nm for strain-to-failure of SA3 fiber-reinforced composites the Hi-Nicalon fiber. It is worth pointing out that (see for instance nanoinfiltration transient eutectic the mechanical behavior was not brittle, so that inter phase-sintered materials) is generally much smaller facial shear stress t in the range 60-160 MPa could be (<0. 3%)than that of the Hi-NicalonS-reinforced determined. The interfacial shear stress t increased ones(x 0.6-1%). This effect has not been satisfacto- with the applied stress, which was attributed to in- ly elucidated yet, although it was attributed to SA creased resistance to sliding with fber surface roughness in. Not much attention ha been paid to this surface roughness issue, and the cor- In order to analyze properly the role of surface relation between mechanical behavior and microstruc Important a ural features has not been well investigated. There are the reader of the elementary interface phenomena that some noticeable differences in the microstructure of are associated with matrix cracking under a uniaxial SA3 and Hi-NicalonS fibers. The average grain size of tensile load. When a crack initiates in the matrix SA3 fibers is reportedly 200 nm; the grain size increases pendicular to the fiber axis, its deflection results from from the core to the surface: grains as large as 400 nm coalescence with the interface crack that forms ahead of have been detected. By contrast, Hi-NicalonS contain the matrix crack tip, as a result of a polyaxial stress much smaller grains(R 20 nm)that are quite uni- state, 4. 5 and more particularly the stress component formly distributed. The presence of large grains results Orr perpendicular to the interface. Debonding is thus in a rough surface of SA3 fibers governed by the interface resistance to orr(opening The fiber/matrix interfacial domain is one of the strength). After deviation of the matrix crack, the load is decisive constituents of fiber-reinforced ceramic matrix transferred through the interface crack(composite be composites. Depending on the characteristics of this havior). The interface shear stress t reflects this load domain, the material either shows a brittle ceramic or a transfer. The higher the t, the shorter the interface crack damage-tolerant composite. Fiber failures can be pre- in the absence of roughness effects from the fiber sur- vented by crack deflection. For this purpose, the fber/ face. The sliding resistance may increase with the matrix interface must not be too strong. It can be tai applied stress because of roughness effects, leading lored via a pyrocarbon interphase. At the interface, ori to an increase in tThe previous generations of SiC/SiC composites prepared via CVI and reinforced with nonstoichiomet￾ric fibers (such as Nicalon or Hi-Nicalon fibers) possess several interesting mechanical properties like high toughness, high strength, and damage tolerance.4 How￾ever, these fibers undergo shrinkage during irradiation by fast neutrons, which causes fiber/matrix debonding and degradation of mechanical properties. By contrast, the Hi-NicalonS and SA3 fibers exhibit microstructural stability because of a well-crystallized microstructure and a lower impurity content, like CVD/CVI SiC.1 Thus, it is necessary to investigate the mechanical be￾havior of CVI SiC/SiC composites reinforced by Hi￾NicalonS and SA3 fibers to check whether they can meet stringent material requirements for fuel cladding: high mechanical properties and thermal conductivity under: high operating (up to 11001C) and accidental (416001C) temperatures, and fast neutron and high fluence conditions. Although both fibers exhibit comparable mechan￾ical properties, available data indicate that the tensile resistance of composites is somewhat different: the strain-to-failure of SA3 fiber-reinforced composites (see for instance nanoinfiltration transient eutectic phase-sintered materials) is generally much smaller (o0.3%) than that of the Hi-NicalonS-reinforced ones ( 0.6–1%).5 This effect has not been satisfacto￾rily elucidated yet, although it was attributed to SA3 fiber surface roughness in.5 Not much attention has been paid to this surface roughness issue, and the cor￾relation between mechanical behavior and microstruc￾tural features has not been well investigated. There are some noticeable differences in the microstructure of SA3 and Hi-NicalonS fibers. The average grain size of SA3 fibers is reportedly 200 nm; the grain size increases from the core to the surface6 : grains as large as 400 nm have been detected. By contrast, Hi-NicalonS contain much smaller grains ( 20 nm) that are quite uni￾formly distributed. The presence of large grains results in a rough surface of SA3 fibers. The fiber/matrix interfacial domain is one of the decisive constituents of fiber-reinforced ceramic matrix composites. Depending on the characteristics of this domain, the material either shows a brittle ceramic or a damage-tolerant composite. Fiber failures can be pre￾vented by crack deflection. For this purpose, the fiber/ matrix interface must not be too strong. It can be tai￾lored via a pyrocarbon interphase. At the interface, orig￾inal chemical bonding may be enhanced by mechanical bonding as a result of thermally induced residual stresses, which leads to gripping of the fiber by the ma￾trix. Radial gripping is enhanced when the interface is rough.7–9 A thick fiber coating (porous or pyrocarbon￾based interphase) is used to reduce the clamping stresses induced by thermal expansion mismatch or surface roughness.10 It has been reported that the SA3-rein￾forced SiC/SiC composites exhibit a brittle behavior when the PyC interphase is very thin.11,12 Then, it has been shown experimentally that the failure strain in￾creases with PyC interphase thickness, whereas interfa￾cial shear stress decreases.13 Various ranges of thickness have been examined: o60 nm, 60–300 nm, and 4600 nm. Greg Morscher has investigated fiber effects on SiC/BN/SiC minicomposites reinforced with a Sylramic polycrystalline fiber.14 The Sylramic-reinforced minicom￾posites had lower strain to failure and higher interfacial shear stress, compared with the Hi-Nicalon-reinforced ones. This trend was attributed to the surface roughness of the Sylramic fiber, which was reportedly quite significant: mean amplitude 69 nm against 4.3 nm for the Hi-Nicalon fiber. It is worth pointing out that the mechanical behavior was not brittle, so that inter￾facial shear stress t in the range 60–160 MPa could be determined. The interfacial shear stress t increased with the applied stress, which was attributed to in￾creased resistance to sliding with increasing sliding length. In order to analyze properly the role of surface roughness, it is important at this stage to remind the reader of the elementary interface phenomena that are associated with matrix cracking under a uniaxial tensile load. When a crack initiates in the matrix per￾pendicular to the fiber axis, its deflection results from coalescence with the interface crack that forms ahead of the matrix crack tip, as a result of a polyaxial stress state,14,15 and more particularly the stress component srr perpendicular to the interface. Debonding is thus governed by the interface resistance to srr (opening strength). After deviation of the matrix crack, the load is transferred through the interface crack (composite be￾havior). The interface shear stress t reflects this load transfer. The higher the t, the shorter the interface crack in the absence of roughness effects from the fiber sur￾face. The sliding resistance may increase with the applied stress because of roughness effects,16 leading to an increase in t. 292 International Journal of Applied Ceramic Technology—Sauder, Brusson and Lamon Vol. 7, No. 3, 2010