ournal J An. Ceran. Soc., 81 [7] 1881-87(1998) Interface Properties in High-Strength Nicalon/C/SiC Composites, As Determined by Rough Surface Analysis of Fiber Push-Out Tests Ronald J Kerans and Triplicane A Parthasarathy Wright Laboratory Materials Directorate, Wright Patterson AFB, Ohio 45433-7817 Francis Rebillat and Jacques Lamon Laboratoire des Composites Thermostructuraux, Unite Mixte de Recherche, Centre National de la recherche Scientifique-Societe Europeene de Propulsion-Universite de Bordeaux I, 33600 Pessac, France fiber Nicalon/C/SiC composites indicate behavior that is vapor-deposited silicon carbide(CVI SiC). In one case, the distinctly different from other composites. These compos- fibers were subjected to a proprietary treatment that is reported ites have been analyzed using a model that explicitly i to clean the fiber surface and remove any native oxide layer; cludes two sets of debond-crack-surface topographies that this treatment also resulted in a somewhat different structure correspond to two stages of crack propagation. The model for the deposited carbon. , The composites made with treated successfully duplicates the observed unusual push-out fibers demonstrated 30% higher strength at the same -1% load-deflection behavior by adjusting interfacial toughness strain-to-failure value, much-finer matrix-crack spacing, and and topographical parameters to fit the experimental data. different stress-strain behavior Although the topographical parameters of the debonding Interface-property measurements on the treated-fiber mate cracks are largely unconfirmed, the fitted values are con- rial have been difficult to rationalize by conventional means sistent with observations and expectations. This analysis Features of the unloading-reloading cycles, as well as predic has determined interfacial fracture energies that are un- tions of the tensile stress-strain behavior of microcomposites usually high for composites with acceptable mechanical be- and havior but are consistent with measurements on bulk py the friction stress), imply T values that can reach 300 MPa 2,6, 0 rocarbons. The analysis provides a unifying rationalization Transmission electron microscopy (TEM)analysis of tensile of the apparent inconsistencies in results reported for such specimens has indicated that the debonding fracture consists of materials fine, diffuse multiple cracking of the carbon layer itself, as pposed to the single crack at or very near the coating/fil interface that is common in the untreated -fiber material,, . Introduction Push-out tests have shown curves that differ from those ex- EX XPERIENCE with successful composites, and subsequen pected from commonly used models and observed for conven- theoretical treatments has led to the prevalent assumption tional composites. A combination of nanoindentor push-in tests that good composite behavior requires that the debond crack and conventional push-out tests(Fig. 1)revealed that an initial ghness and interfacial sliding friction should both be quite debonding crack initiates and grows very slightly as the load low(see, for example, Cao et al. ) Reports of substantially increases to what, on first inspection, seems to be the initial roved properties of Nicalon TM/C/SiC composites due to the deviation from linearity(to area"b"in Fig. 1), then jumps behavior of the material is similar to other tough composites, in 4-6 The fracture lation of two different cracking modes, with a transition from that the interfacial region is sufficiently weak that matrix one to the other in the unstable b-to-c region in Fig. 1. As with most push-out data, there is a wide variation in behavior. Many cracks deflect into debonding cracks. However, analysis indi- curves indicate greater displacement between areas a and b cates interface property values that are significantly higher than han that shown in Fig. 1; however for most curves, that dis- usually assumed to be permissible Related questions regarding placement remains quite small. Similarly, most curves demon- laminates. for a concise discussion and review see Lee et al. 7 strate upward curvature in the c-to-d region in Fig. 1, many of (for a new approach to the detailed analysis of the problem in seems reasonably representative, and the very small a-to-b dis- fibrous composites, see Pagano) acement provides an ially rigorous test of any model The original and the improved systems both consisted of Conventional analyses do not allow for curves with such fea- pyrolytic carbon-coated ic-grade Nicalon TM(Nippon tures and are not readily applied to them. 2 Push-out behavior(and debonding in general) is affected by the topography of the sliding surfaces. The relevant surfaces are formed by the debonding crack and may differ from the surface of the fiber, these surfaces potentially can vary along A. Jagota--contributing editor he length of the debond, with variations in stress state or microstructure. The potential of significant effects due to sur- face roughness was first discussed in the context of increased adial stresses due to the geometric misfit. 4 Contradictions in measured sliding friction led to an experimental confirmation of roughness effects with the" push back seating drop"of Jere Member. American Ceramic Sociel and co-workers. 15, 16(For a brief review of subsequent workInterface Properties in High-Strength Nicalon/C/SiC Composites, As Determined by Rough Surface Analysis of Fiber Push-Out Tests Ronald J. Kerans* and Triplicane A. Parthasarathy* Wright Laboratory Materials Directorate, Wright Patterson AFB, Ohio 45433–7817 Francis Rebillat and Jacques Lamon* Laboratoire des Composites Thermostructuraux, Unite´ Mixte de Recherche, Centre National de la Recherche Scientifique–Socie´te´ Europe´ene de Propulsion–Universite´ de Bordeaux I, 33600 Pessac, France Fiber push-out curves generated on high-strength, treated￾fiber Nicalon/C/SiC composites indicate behavior that is distinctly different from other composites. These compos￾ites have been analyzed using a model that explicitly in￾cludes two sets of debond-crack-surface topographies that correspond to two stages of crack propagation. The model successfully duplicates the observed unusual push-out load–deflection behavior by adjusting interfacial toughness and topographical parameters to fit the experimental data. Although the topographical parameters of the debonding cracks are largely unconfirmed, the fitted values are con￾sistent with observations and expectations. This analysis has determined interfacial fracture energies that are un￾usually high for composites with acceptable mechanical be￾havior but are consistent with measurements on bulk py￾rocarbons. The analysis provides a unifying rationalization of the apparent inconsistencies in results reported for such materials. I. Introduction EXPERIENCE with successful composites, and subsequent theoretical treatments, has led to the prevalent assumption that good composite behavior requires that the debond crack toughness and interfacial sliding friction should both be quite low (see, for example, Cao et al.1 ). Reports of substantially improved properties of Nicalon™/C/SiC composites due to the sole processing change of pretreating the surface of the fiber2,3 have motivated reexamination of the assumptions regarding the upper limits of allowable interfacial properties.4–6 The fracture behavior of the material is similar to other tough composites, in that the interfacial region is sufficiently weak that matrix cracks deflect into debonding cracks. However, analysis indi￾cates interface property values that are significantly higher than usually assumed to be permissible. Related questions regarding details of crack deflection have been discussed recently for laminates; for a concise discussion and review, see Lee et al.7 (for a new approach to the detailed analysis of the problem in fibrous composites, see Pagano8 ). The original and the improved systems both consisted of pyrolytic carbon-coated ceramic-grade Nicalon™ (Nippon Carbon Co., Tokyo, Japan) fibers in a matrix of chemical￾vapor-deposited silicon carbide (CVI SiC). In one case, the fibers were subjected to a proprietary treatment that is reported to clean the fiber surface and remove any native oxide layer; this treatment also resulted in a somewhat different structure for the deposited carbon.4,9 The composites made with treated fibers demonstrated 30% higher strength at the same ∼1% strain-to-failure value, much-finer matrix-crack spacing, and different stress–strain behavior.2 Interface-property measurements on the treated-fiber mate￾rial have been difficult to rationalize by conventional means. Features of the unloading–reloading cycles, as well as predic￾tions of the tensile stress–strain behavior of microcomposites and minicomposites, based on a constant t approximation (t is the friction stress), imply t values that can reach 300 MPa.2,6,10 Transmission electron microscopy (TEM) analysis of tensile specimens has indicated that the debonding fracture consists of fine, diffuse multiple cracking of the carbon layer itself, as opposed to the single crack at or very near the coating/fiber interface that is common in the untreated-fiber material.2,3,11 Push-out tests have shown curves that differ from those ex￾pected from commonly used models and observed for conven￾tional composites. A combination of nanoindentor push-in tests and conventional push-out tests (Fig. 1) revealed that an initial debonding crack initiates and grows very slightly as the load increases to what, on first inspection, seems to be the initial deviation from linearity (to area ‘‘b’’ in Fig. 1), then jumps unstably to a greater length (area ‘‘b’’ to ‘‘c’’) and begins to grow with a load deflection trace that is concave upward (to area ‘‘d’’ in Fig. 1).12,13 This observation has led to a postu￾lation of two different cracking modes, with a transition from one to the other in the unstable b-to-c region in Fig. 1. As with most push-out data, there is a wide variation in behavior. Many curves indicate greater displacement between areas a and b than that shown in Fig. 1; however, for most curves, that dis￾placement remains quite small. Similarly, most curves demon￾strate upward curvature in the c-to-d region in Fig. 1, many of them to a greater degree than that shown in Fig. 1. Figure 1 seems reasonably representative, and the very small a-to-b dis￾placement provides an especially rigorous test of any model. Conventional analyses do not allow for curves with such fea￾tures and are not readily applied to them.12 Push-out behavior (and debonding in general) is affected by the topography of the sliding surfaces. The relevant surfaces are formed by the debonding crack and may differ from the surface of the fiber; these surfaces potentially can vary along the length of the debond, with variations in stress state or microstructure. The potential of significant effects due to sur￾face roughness was first discussed in the context of increased radial stresses due to the geometric misfit.14 Contradictions in measured sliding friction led to an experimental confirmation of roughness effects with the ‘‘push back seating drop’’ of Jero and co-workers.15,16 (For a brief review of subsequent work, A. Jagota—contributing editor Manuscript No. 191103. Received April 3, 1997; approved September 15, 1997. Author TAP was supported by USAF Contract No. F33615-91-C-5663, UES, Inc., Dayton, OH. *Member, American Ceramic Society. J. Am. Ceram. Soc., 81 [7] 1881–87 (1998) Journal 1881