ournal JAm.Cem.Soc,82l92465-73(1999 Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases ebastien Bertrand, Philippe Forio, Rene Pailler, and Jacques Lamon Laboratoire des Composites Thermostructuraux, UMR 5801 CNRS-SEP/SNECMA-UB1, Allee de la Boetie, 33600 Pessac, france Sic/SiC minicomposites that comprise different pyro- Japan)areas iown to be thermodynamically unstable at high The SiC-O Nicalon fibers(Nippon Carbon Co., Tokyo, and a tow of SiC fibers(Hi-Nicalon) have been prepared and SiCnpulsed chemical vapor infiltration. Pyrocarbon very low,8 xygen content (<I wt%)exhibit improved thermal P ere deposited from propane and a CH3SiCl3/H2 stability mixture, respectively. The microstructure of the inter The present paper investigates SiC/SiC minicomposites that phases has been investigated using transmission electron have been processed via P-CVI and contain nanoscale(PyC/ microscopy. The mechanical tensile behavior of the mi SiC) multilayered interphases and Hi-Nicalon fibers, Data are mposites at room t mperature exhibits the classical fea- provided on the microstructure and properties of constituents, ures of tough composites, regardless of the characteristics the features of the tensile stress-strain behavior, and matrix of the(Py C/SiC)sequences. The interfacial shear stress has cracking been determined from the width of hysteresis loops upon unloading/reloading and from the crack-spacing distance at II. Experimental Procedure saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak SiCSiC Mimicomposites Processing (50 MPa). (A) P-CVI Apparatus: In the P-CVI process, the source gases, which react to produce the matrix and the interphases L Introduction are introduced during short pulses. Each pulse involves three steps:(i)the introduction of gases, (ii)a hold-on period( during HE mechanical behavior of ceramic-matrix composites which the gases diffuse inside the fiber preform and react), and (CMCs)with continuous fibers is dependent not only on (ii) the evacuation of the gaseous reaction products. A layer of the intrinsic properties of the fiber and the matrix but also on material is deposited during each pulse. The p-CVI technique 1) ng - To control th omits the deposition of thin films the F/M bonding, an additional phase is deposited on the fiber; The P-CVI apparatus is shown in Fig. 1. The reactor is a this phase is called the interphase. The most commonly used silica glass tube(internal diameter of 25 mm, height of 50 mm) interphase material is pyrocarbon( PyC). Pyrocarbon displays a heated uniformly (5 K) in an electrical furnace (2"in the layered microtexture and has been shown to lead to high- figure). The temperature is measure trength-high-toughness SiC/SiC composites. 4 However, PyC Mass and ball flowmeters control the flow rate of the source is not stable in an oxidizing environment. Consequently, coat- gases( propane, MTS(methyltrichlorosilane, CH3 SiCl3), and ing the Py C layer with a SiC layer has been suggested as a way hydrogen(the carrier gas when using MTS).(See"3" in the to protect the pyrolytic carbon against oxidation; this procedure igure. ) First, MTS is evaporated in a heated chamber(4 " in leads to the concept of multilayered(Py C/SiC) interphases. 5. the figure), and then it is mixed with hydrogen in a pre CMCs are commonly fabricated via isothermal-isobaric regulated heated tank (5"in the figure), where it is hemical vapor infiltration(I-CVI). 7 To reduce the fabrication before admission into the reactor. During a pulse, the time, various CVI processes have been developed, -ll such as forced CVI(F-CVI), which involves steep temperature and through inlet pneumatic valves. After the hold-on time(the pressure gradients, or pressure-pulsed CVI(P-CVI residence time, t), which allows deposition of a certain thick- use of ness of material, the gases are evacuated from the reactor to the of each sublayer being up to 100 nm)in two-dimensional (2D) vacuum pump("7"in the figure)through an outlet pneumatic oven SiC/SiC composites that have been prepared according valve and liquid nitrogen trap(s)(6" in the figure).A pro- the I-CVI route has been reported by Droillard and co- grammable logic controller regulates the open and workers. 4,3Then, Heurtevent developed SiC/SIC micro- he valves and dictates the number of pulses composites with nanoscale(PyC/SiC), multilayered inter (B) Processing Conditions and Materials: The Hi hases( the thicknesses of the sublayers being as small as 3 Nicalon tows consist of 500 single filaments. each with an that were deposited via pulsed chemical vapor deposition(P- average diameter of 13.5+ 1.5 um. Tows with a length of 50 CVD). Other than these pioneering works, to our knowledge, mm were mounted on SiC frames(six tows per frame)to no detailed studies on nanoscale(PyC/SiC), multilayered in- infiltrate the PyC and Sic sublayers and the SiC matrix. The terphases have been reported in the literature tows were slightly twisted at a constant angle(one turn per 5 cm), to decrease the porosity in the minicomposites and to increase their fiber volume fraction, The Pyc sublayers were deposited at a temperature(r)of 1223 K, under a pressure(P of 3 kPa, with t,=5 s. 4 The SiC sublayers were deposited at T= 1223 K and P=5 kPa, with a y4 and t, =l s(a= OH/OMTS, where OH, and OMTs are the respective gas flows of hydrogen and MT The infiltration conditions for the sic script No 19031. Received March 16, 1998: approved February 13, 1999. matrix were as follows: T= 1223 K, P=3 kPa, a= 6, and 2465Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC)n Nanoscale Multilayered Interphases Se´bastien Bertrand, Philippe Forio, Rene´ Pailler,* and Jacques Lamon* Laboratoire des Composites Thermostructuraux, UMR 5801 CNRS–SEP/SNECMA-UB1, Allee de la Boetie, 33600 Pessac, France SiC/SiC minicomposites that comprise different pyrocar￾bon/silicon carbide ((PyC/SiC)n) multilayered interphases and a tow of SiC fibers (Hi-Nicalon) have been prepared via pressure-pulsed chemical vapor infiltration. Pyrocarbon and SiC were deposited from propane and a CH3SiCl3/H2 mixture, respectively. The microstructure of the inter￾phases has been investigated using transmission electron microscopy. The mechanical tensile behavior of the mini￾composites at room temperature exhibits the classical fea￾tures of tough composites, regardless of the characteristics of the (PyC/SiC) sequences. The interfacial shear stress has been determined from the width of hysteresis loops upon unloading/reloading and from the crack-spacing distance at saturation. All the experimental data indicate that the strength of the fiber/interphase interfaces is rather weak (∼50 MPa). I. Introduction THE mechanical behavior of ceramic-matrix composites (CMCs) with continuous fibers is dependent not only on the intrinsic properties of the fiber and the matrix but also on the fiber/matrix (F/M) bonding.1–3 To control the strength of the F/M bonding, an additional phase is deposited on the fiber; this phase is called the interphase. The most commonly used interphase material is pyrocarbon (PyC). Pyrocarbon displays a layered microtexture and has been shown to lead to high￾strength–high-toughness SiC/SiC composites.4 However, PyC is not stable in an oxidizing environment. Consequently, coat￾ing the PyC layer with a SiC layer has been suggested as a way to protect the pyrolytic carbon against oxidation; this procedure leads to the concept of multilayered (PyC/SiC)n interphases.5,6 CMCs are commonly fabricated via isothermal–isobaric chemical vapor infiltration (I-CVI).7 To reduce the fabrication time, various CVI processes have been developed,8–11 such as forced CVI (F-CVI),9 which involves steep temperature and pressure gradients, or pressure-pulsed CVI (P-CVI).11 The use of multilayered PyC/SiC interphases (the thickness of each sublayer being up to 100 nm) in two-dimensional (2D) woven SiC/SiC composites that have been prepared according to the I-CVI route has been reported by Droillard and co￾workers.4,12,13 Then, Heurtevent14 developed SiC/SiC micro￾composites with nanoscale (PyC/SiC)n multilayered inter￾phases (the thicknesses of the sublayers being as small as 3 nm) that were deposited via pulsed chemical vapor deposition (P￾CVD). Other than these pioneering works, to our knowledge, no detailed studies on nanoscale (PyC/SiC)n multilayered in￾terphases have been reported in the literature. The Si–C–O Nicalon fibers (Nippon Carbon Co., Tokyo, Japan) are known to be thermodynamically unstable at high temperatures.15–17 Conversely, SiC Hi-Nicalon fibers with a very low oxygen content (<1 wt%) exhibit improved thermal stability.18 The present paper investigates SiC/SiC minicomposites that have been processed via P-CVI and contain nanoscale (PyC/ SiC)n multilayered interphases and Hi-Nicalon fibers. Data are provided on the microstructure and properties of constituents, the features of the tensile stress–strain behavior, and matrix cracking. II. Experimental Procedure (1) SiC/SiC Minicomposites Processing (A) P-CVI Apparatus: In the P-CVI process, the source gases, which react to produce the matrix and the interphases, are introduced during short pulses. Each pulse involves three steps: (i) the introduction of gases, (ii) a hold-on period (during which the gases diffuse inside the fiber preform and react), and (iii) the evacuation of the gaseous reaction products. A layer of material is deposited during each pulse. The P-CVI technique permits the deposition of thin films. The P-CVI apparatus is shown in Fig. 1. The reactor is a silica glass tube (internal diameter of 25 mm, height of 50 mm) heated uniformly (±5 K) in an electrical furnace (“2” in the figure). The temperature is measured with a thermocouple. Mass and ball flowmeters control the flow rate of the source gases (propane, MTS (methyltrichlorosilane, CH3SiCl3), and hydrogen (the carrier gas when using MTS)). (See “3” in the figure.) First, MTS is evaporated in a heated chamber (“4” in the figure), and then it is mixed with hydrogen in a pressure￾regulated heated tank (“5” in the figure), where it is stored before admission into the reactor. During a pulse, the source gases are introduced almost instantaneously into the reactor through inlet pneumatic valves. After the hold-on time (the residence time, tr ), which allows deposition of a certain thick￾ness of material, the gases are evacuated from the reactor to the vacuum pump (“7” in the figure) through an outlet pneumatic valve and liquid nitrogen trap(s) (“6” in the figure). A pro￾grammable logic controller regulates the open and closing of the valves and dictates the number of pulses. (B) Processing Conditions and Materials: The Hi￾Nicalon tows consist of 500 single filaments, each with an average diameter of 13.5 ± 1.5 mm. Tows with a length of 50 mm were mounted on SiC frames (six tows per frame) to infiltrate the PyC and SiC sublayers and the SiC matrix. The tows were slightly twisted at a constant angle (one turn per 5 cm), to decrease the porosity in the minicomposites and to increase their fiber volume fraction. The PyC sublayers were deposited at a temperature (T) of 1223 K, under a pressure (P) of 3 kPa, with tr 4 5 s.14 The SiC sublayers were deposited at T 4 1223 K and P 4 5 kPa, with a 4 1⁄4 and tr 4 1s(a 4 QH2 /QMTS, where QH2 and QMTS are the respective gas flows of hydrogen and MTS).14 The infiltration conditions for the SiC matrix were as follows: T 4 1223 K, P 4 3 kPa, a 4 6, and tr 4 2 s.14 T. A. Parthasarathy—contributing editor Manuscript No. 190319. Received March 16, 1998; approved February 13, 1999. Supported by SEP and CNRS, through a grant given to author SB. *Member, American Ceramic Society. J. Am. Ceram. Soc., 82 [9] 2465–73 (1999) Journal 2465