792 Journal of the American Ceramic Society-Bertrand et al Vol 84. No 4 Table I. Interfacial Shear Stresses(T) Estimated Using Various Methods and Tow Youngs Modulus (M Batch Location HN/(C/SIC)1o Interior 105 78 HNT/(C/SIC)Io Interior Surface HNT/C Interior 150 trends have been observed on the Hi-Nicalon/SiC minicomposites 300 of the present study. The Hi-Nicalon/SiC minicomposites rein- forced with as-received fibers possessed the weaker fiber/matrix bonds. The interphase was deposited on a SiO,/PyC layer, which 250 is now well-known to lead to a weak fiber/coating interface. 3 Those Hi-Nicalon/SiC minicomposites reinforced with treated fibers appeared to exhibit features reflecting strong fiber/matrix 国200 bonds. The surface of the fibers was composed of an enriched carbon layer 50-100 nm thick, has been shown to give strong fiber/coating interfaces 150 Furthermore, the degree of organization of the carbon micro- structure also contributed to bonding. Droillard observed on two-dimensional Nicalon/SiC composites reinforced with as- received NL202 fibers that the PyC coating consisted of layers parallel to the fiber axis, whereas the PyC microstructure was rather disorganized in those composites reinforced with treated 50 Nicalon fibers. Similar observations were made on Hi-Nicalon minicomposites elsewhereas well as in the present work. In the minicomposites reinforced with treated fibers, the PyC sublayer exhibited a disordered microstructure (as shown in Fig. 2), an there was no discontinuity or interface between the fiber and the deposited PyC sublayer, which logically gave a strong fiber/ Its clearly indicate that longer in those minicomposites reinforced with treated fibers. This effect may be related to the presence of a stronger fiber/coating interface that limits debonding and crack-opening displacement. at 700C in air. Also given are lifetime data for Hi-Nicalon/BN/SIC The oxidized layers shown by Fig. 9 correspond exactly to the inicomposites18, 19 Arrows indicate those minicomposites that did not path of the deflected matrix cracks. In the minicomposites rein- forced with as-received fibers. the matrix cracks are deflected at the fiber/coating interface. Oxygen can diffuse instantaneously at the fiber surface and react with carbon in the sublayer on the fiber Increase these stresses in the minicomposites reinforced with and with the fiber itself. as-received fibers. This effect was supported by measurements of In those minicomposites reinforced with treated fibers, th the crack spacing distance that was smaller in the internal matrix matrix cracks are deflected first in the Pyc layers within th It could explain the large magnitude of T values determined on the coating, and finally in the PyC layer at the fiber surface. Oxygen ninicomposites reinforced with as-received fibers(90 MPa), reacts first with carbon in those sublayers within the multilayered although significant initial fiber/interphase debonding had been coating and ultimately with the carbon in the layer at the fiber surface. The oxygen propagation path is thus increased, whereas The T estimated for the minicomposites reinforced with treated the amount of oxygen is reduced, owing to the slight opening of fibers compared satisfactorily with those measured on twe the cracks. Thi yer ontributes to the dimensional woven Hi-Nicalon/(Py C/SiC)/SiC composites rein- forced with treated fibers for which t was -230 MPa. 8 treated fibers. Comparison of HNT/(C/SiC)o and HNT/C mini- The force-deformation curves predicted from the constituent composites indicates that this contribution is significant. However, properties and the flaw-strength statistical parameters(Table Il)fit it is worth emphasizing that this contribution is associated with the quite well the experimental ones(Fig. 7), which strongly supports presence of rather strong fiber/coating interfaces that determine the pertinence of the materials data and the model. The adjusted deviation of the matrix cracks, debonding, and crack opening. arameters agreed with those extracted using the other methods At high temperature, because of thermal expansion of the (Table IV). However, fitting was improved when tow Youngs composite constituents, the debond cracks and the matrix cracks moduli slightly larger than those measured using tensile tests on tend to close, or, at least, the crack-opening displacement tends to ingle filaments (Table IV) were introduced into the computations. decrease. Therefore, the effects associated with strong fiber This slight increase could be considered to reflect the tow coating interfaces that have been identified at room temperature stiffening associated with twisting. should be enhanced or. at least unaffected. As oxidation of the The composition of the superficial fiber layer has been shown to Py c sublayers proceeds, the influence of the above-mentioned determine the strength of the interphase/fiber bond in the Nicalon effects may decrease. The rate of this phenomenon is not known at (NL 202)SiC composites with Py C-based interphases. Simila this stage. However, comparison of either HN/C and HNT/C, orincrease these stresses in the minicomposites reinforced with as-received fibers. This effect was supported by measurements of the crack spacing distance that was smaller in the internal matrix. It could explain the large magnitude of t values determined on the minicomposites reinforced with as-received fibers (;90 MPa), although significant initial fiber/interphase debonding had been observed.12 The t estimated for the minicomposites reinforced with treated fibers compared satisfactorily with those measured on two￾dimensional woven Hi-Nicalon/(PyC/SiC)n/SiC composites rein￾forced with treated fibers for which t was ;230 MPa.8 The force–deformation curves predicted from the constituent properties and the flaw–strength statistical parameters (Table II) fit quite well the experimental ones (Fig. 7), which strongly supports the pertinence of the materials data and the model. The adjusted t parameters agreed with those extracted using the other methods (Table IV). However, fitting was improved when tow Young’s moduli slightly larger than those measured using tensile tests on single filaments (Table IV) were introduced into the computations. This slight increase could be considered to reflect the tow stiffening associated with twisting. The composition of the superficial fiber layer has been shown to determine the strength of the interphase/fiber bond in the Nicalon (NL 202)/SiC composites with PyC-based interphases.1,3 Similar trends have been observed on the Hi-Nicalon/SiC minicomposites of the present study. The Hi-Nicalon/SiC minicomposites rein￾forced with as-received fibers possessed the weaker fiber/matrix bonds. The interphase was deposited on a SiO2/PyC layer, which is now well-known to lead to a weak fiber/coating interface.1,3 Those Hi-Nicalon/SiC minicomposites reinforced with treated fibers appeared to exhibit features reflecting strong fiber/matrix bonds. The surface of the fibers was composed of an enriched carbon layer ;50–100 nm thick, which has been shown to give strong fiber/coating interfaces.1,3 Furthermore, the degree of organization of the carbon micro￾structure also contributed to bonding. Droillard2 observed on two-dimensional Nicalon/SiC composites reinforced with as￾received NL202 fibers that the PyC coating consisted of layers parallel to the fiber axis, whereas the PyC microstructure was rather disorganized in those composites reinforced with treated Nicalon fibers. Similar observations were made on Hi-Nicalon minicomposites elsewhere12 as well as in the present work. In the minicomposites reinforced with treated fibers, the PyC sublayer exhibited a disordered microstructure (as shown in Fig. 2), and there was no discontinuity or interface between the fiber and the deposited PyC sublayer, which logically gave a strong fiber/ coating bond. The experimental results clearly indicate that the lifetime is longer in those minicomposites reinforced with treated fibers. This effect may be related to the presence of a stronger fiber/coating interface that limits debonding and crack-opening displacement. The oxidized layers shown by Fig. 9 correspond exactly to the path of the deflected matrix cracks. In the minicomposites rein￾forced with as-received fibers, the matrix cracks are deflected at the fiber/coating interface. Oxygen can diffuse instantaneously at the fiber surface and react with carbon in the sublayer on the fiber and with the fiber itself. In those minicomposites reinforced with treated fibers, the matrix cracks are deflected first in the PyC layers within the coating, and finally in the PyC layer at the fiber surface. Oxygen reacts first with carbon in those sublayers within the multilayered coating and ultimately with the carbon in the layer at the fiber surface. The oxygen propagation path is thus increased, whereas the amount of oxygen is reduced, owing to the slight opening of the cracks. Thus, the multilayered interphase contributes to the lifetime improvement in those minicomposites reinforced with treated fibers. Comparison of HNT/(C/SiC)10 and HNT/C mini￾composites indicates that this contribution is significant. However, it is worth emphasizing that this contribution is associated with the presence of rather strong fiber/coating interfaces that determine deviation of the matrix cracks, debonding, and crack opening. At high temperature, because of thermal expansion of the composite constituents, the debond cracks and the matrix cracks tend to close, or, at least, the crack-opening displacement tends to decrease. Therefore, the effects associated with strong fiber/ coating interfaces that have been identified at room temperature should be enhanced or, at least, unaffected. As oxidation of the PyC sublayers proceeds, the influence of the above-mentioned effects may decrease. The rate of this phenomenon is not known at this stage. However, comparison of either HN/C and HNT/C, or Table IV. Interfacial Shear Stresses (t) Estimated Using Various Methods and Tow Young’s Modulus Batch Location t (MPa) t# (MPa) Ef (GPa) Equation (1) Equation (3) Equation (4) Model of tensile behavior HN/(C/SiC)10 Interior 105 89 78 100 93 320 Surface 35 25 22 100 45 320 HNT/(C/SiC)10 Interior 216 117 103 350 197 350 Surface 113 61 54 350 144 350 HN/C Interior 120 81 71 100 93 320 Surface 50 34 29 100 53 320 HNT/C Interior 127 160 140 150 144 350 Surface 46 54 50 150 75 350 Fig. 8. Lifetime data for Hi-Nicalon SiC minicomposites in static fatigue at 700°C in air. Also given are lifetime data for Hi-Nicalon/BN/SiC minicomposites.18,19 Arrows indicate those minicomposites that did not fail. 792 Journal of the American Ceramic Society—Bertrand et al. Vol. 84, No. 4