wwceramics. org/ACT Infuence of Interface Characteristics on the Mechanical Properties matrix 100 0 0 100 Radius(um) Radius (um) fiber -Radial stress -Hoop stress 500 Radial stress 400 Axial stress Hoop stress Axial stress 700 Fig. 10. Thermally induced residual stresses in the interface area for(a) Hi-Nicalon S/Py C/SiC and(b) SA3/pyC/SiC combinations eryc= 150nm, Vm= VE On the Hi-Nicalon S/SiC minicomposites with The stress on fiber required to cause sliding is re- thinner PyC fiber coating, t was found to be higher. On lated to the clamping stress by the following equation the SA3/SiC minicomposites, brittle-like or damage-tolet t behavior was observed when the Pyc coating thickness (τo+μoR) (epyc) was larger than available Rmax: epyc:=150 nm(M2) Brittle-like behavior was only observed when epyc< Rmax (M4, epC=30 nm). These trends can be logically attrib- where t. is the interface shear stress in the absence of uted to the clamping stress generated by surface roughness clamping stress, H is the friction coefficient, and La is the in the debonded interfaces. First. the contribution of debond length clamping stress to t is described by iding occurs when of or(where of is the stress at fiber failure): which corresponds to the above- t=to+HoR (6) mentioned noncatastrophic influence of roughness. It where to is the interface shear stress in the absence of corresponds to the above-mentioned catastrophic influ clamping stress and u is the friction coefficient. ence of ro Thus, according to Eqs.(4)-(6), PyC coating thick Rough estimates of of were determined from the ness decreases cause A and oR increases, and, conse- following characteristics, which can be considered to be t increases(Fig. 8a). This trend has b realistic:R=4, to=10 MPa, H=0.4, and A= 2RRMS observed on SA3/SiC and Hi-Nicalon S/SiC minicom df=496 Mpa for Hi-NicalonS and of =3056 MPa sites(this work and Sauder et al. for SA3 were obtained. It appears that sliding requires a After matrix crack deviation, the following post much higher stress on the SA3 fiber; this stress can debonding behavior can take place exceed the fiber strength Table ID). Thus, the nonskid- (1) either fiber/matrix sliding, with a constant t ing condition of >of can be fulfilled on the SA3 when the roughness effects are limited, or increasing t fiber. It is clearly quite difficult to be reached with the when they are effective(noncatastrophic infuence of Hi-NicalonS fiber High OR causes interface crack closure, which pre- (2)or no-sliding because of high sliding resistance vents relaxation of the stress concentration at the matrix induced by the clamping stress OR(catastrophic influ- crack tip(Fig. 11b). Thus, very high tensile stresses op nce of roughness) erate on fibers: Ko(K,= stress concentration factor,On the Hi-NicalonS/SiC minicomposites with a thinner PyC fiber coating, t was found to be higher. On the SA3/SiC minicomposites, brittle-like or damage-toler￾ant behavior was observed when the PyC coating thickness (ePyC) was larger than available Rmax: ePyC 5 150 nm (M2). Brittle-like behavior was only observed when ePyCoRmax (M4, ePyC 5 30 nm). These trends can be logically attrib￾uted to the clamping stress generated by surface roughness in the debonded interfaces. First, the contribution of clamping stress to t is described by: t ¼ to þ msR ð6Þ where to is the interface shear stress in the absence of clamping stress and m is the friction coefficient. Thus, according to Eqs. (4)–(6), PyC coating thick￾ness decreases cause A and sR increases, and, conse￾quently, t increases (Fig. 8a). This trend has been observed on SA3/SiC and Hi-NicalonS/SiC minicom￾posites (this work and Sauder et al. 5 ). After matrix crack deviation, the following post debonding behavior can take place: (1) either fiber/matrix sliding, with a constant t when the roughness effects are limited, or increasing t when they are effective16 (noncatastrophic influence of roughness), (2) or no-sliding because of high sliding resistance induced by the clamping stress sR (catastrophic influ￾ence of roughness). The stress on fiber required to cause sliding is re￾lated to the clamping stress by the following equation: ss f ¼ 2Ld Rf ðÞ ð to þ msR 7Þ where to is the interface shear stress in the absence of clamping stress, m is the friction coefficient, and Ld is the debond length. Sliding occurs when ss f < sR f (where sR f is the stress at fiber failure): which corresponds to the above￾mentioned noncatastrophic influence of roughness. It does not occur if the fiber fails first: sR f < ss f, which corresponds to the above-mentioned catastrophic influ￾ence of roughness. Rough estimates of ss f were determined from the following characteristics, which can be considered to be realistic: Ld R ¼ 4, to 5 10 MPa, m 5 0.4, and A 5 2RRMS. ss f ¼ 496 Mpa for Hi-NicalonS and ss f ¼ 3056 MPa for SA3 were obtained. It appears that sliding requires a much higher stress on the SA3 fiber; this stress can exceed the fiber strength (Table II). Thus, the nonslid￾ing condition ss f > sR f can be fulfilled on the SA3 fiber. It is clearly quite difficult to be reached with the Hi-NicalonS fiber. High sR causes interface crack closure, which pre￾vents relaxation of the stress concentration at the matrix crack tip (Fig. 11b). Thus, very high tensile stresses op￾erate on fibers: Kts (Kt 5 stress concentration factor, Fig. 10. Thermally induced residual stresses in the interface area for (a) Hi-NicalonS/PyC/SiC and (b) SA3/PyC/SiC combinations (ePyC 5 150 nm, Vm 5 Vf). www.ceramics.org/ACT Influence of Interface Characteristics on the Mechanical Properties 301