788 Joumal of the American Ceramic Sociery--Bertrand et al. Vol 84. No 4 Table L. Description of Investigated Hi-Nicalon/SiC of 80N was applied using a dead weight that was progressively hung on the bottom grip via the displacement of the support at a constant speed. This force was -10% above the proportional limit (Table Ill). Acoustic emission showed that matrix cracking initi- atches Fibers, Interphase (nm) (nm) n ated below the proportional limit under a force around 50N. The first matrix cracks did not influence specimen compliance. At HN/(C/SIC)1o AR failure of the minicomposites, the chronometer was stopped by the HNT/(C/SIC)Io 50 10 falling dead weight, giving the lifetime HNT/C Il. Results and Discussion Pyc/siC b (1 Material Characterization Details on the microstructure of the nanoscale multilayered Table Il. Properties of Minicomposite Constituents (PyC/SiC)n fiber coatings have been provided. 2 AES depth profiles revealed the presence of an oxygen-enriched layer(15-50 Statistical nm)at the surface of the as-received Hi-Nicalon fibers (Table In This layer consisted of SiO, and free carbon. The first interfacial Constituents Composition (nm) (GPa) m (MPa) PyC sublayer deposited was bonded to this silicon/carbon/oxyge layer. Such sublayers have also been identified in composites Hi-Nicalon fibers Sio 15-502804.26 sponding fiber/matrix interactions were weak, and deflection of the Hi-Nicalon fibers(T) Free carbon 50-100 305 5.8 matrix cracks occurred at the fiber/interphase interfaces. 3, 10 PyC interphase Sic interphase 280 AEs depth profiles performed on treated Hi-Nicalon fibers SiC P-CVI matrix 4005.55.7 show that the surface of the fibers consists of a 50-100 nm thick TA R.= as-received, T. treated. Reference volume, I.=Im layer of free carbon(Fig. 1). The presence of such a superficial layer of free carbon increases the strength of the Py C coating/fiber interface in Nicalon/PyC/SiC composites ,,o The TEM micro- graph of Fig. I shows that the deposited Pyc is perfectly bonded ( Tensile Tests at Room Temperature to the fiber. By contrast, preexisting fiber/coating debonds are Uniaxial tension tests were performed at room temperature at a observed in the minicomposites reinforced with as-received Hi- Nicalon fibers I deformation rate of 50 um/mn using a machine and procedure metallic tubes that were then gripped into the testing machine. (2) Tensile Behavior at Room Temperature Gauge length was 20 mm. Load train compliance, Cs was Figure 2 shows that the force-deformation curves obtained for determined from tensile tests on fiber tows having various gauge HNT/(C/SiC)1o minicomposites reinforced with treated fibers engths(Cs= 0.3 um/N) exhibit typical features associated with strong fiber/matrix Unloading-reloading cycles were conducted on a few speci- bonds: ,3 mens of each batch to estimate the elastic modulus of the cracked (1) A wide curved domain up to ultimate failure attributed to minicomposites, residual strains at zero load, and T. After ultimate a high density of matrix cracks and short debonds, and failure, the test specimens were examined using SEM, and the (2) Rather narrow hysteresis loops, indicative of strong fiber/ crack spacings were measured. matrix interactions on unloading-reloading HN/(C/SiC)o minicomposites reinforced with as-received fi- (4 Static Fatigue Tests at 700C in Air bers display typical features associated with weak fiber/matrix The lifetime of the minicomposites under constant load was bonds, including measured in static fatigue at 700C in air. These temperature (1) A narrow curved domain reflecting longer debonds and a onditions were the worst, because the oxidation rate was high fo lower density of matrix cracks, and PyC and low for SiC. The minicomposite ends were glued within lumina tubes using an alumina-based ceramic adhesive. The gauge length(10 mm) was determined by the distance between the tubes. The tubes were gripped into the testing machine. The ses are used to describe the mechanical behavior of the minicomposites were positioned within the furnace hot zone where the temperature was uniform at 700C. The minicomposites were sites, as a result of the transverse cracks that locally reduce the stressed section to heated to the test temperature before loading. Then a constant force that of fibers Table Ill. Average Mechanical Properties of Minicomposites Tested Saturations FailureR Batches HN/(C/SIC)1o 351 71 0.12 135 0.30 19 0.83 HNT/(C/SiC)Io 0.79 34 750. 0.30 HNT/C 350 0.17 0.7 156 0.81 microcrack spacing in the internal matrix of the minicomposite. 'Microcrack spacing in the surface of the minicomposite. F(3) Tensile Tests at Room Temperature Uniaxial tension tests were performed at room temperature at a deformation rate of 50 mm/mn using a machine and procedure detailed elsewhere.12 The minicomposite ends were glued within metallic tubes that were then gripped into the testing machine. Gauge length was 20 mm. Load train compliance, Cs, was determined from tensile tests on fiber tows having various gauge lengths (Cs 5 0.3 mm/N). Unloading–reloading cycles were conducted on a few speci￾mens of each batch to estimate the elastic modulus of the cracked minicomposites, residual strains at zero load, and t. After ultimate failure, the test specimens were examined using SEM, and the crack spacings were measured. (4) Static Fatigue Tests at 700°C in Air The lifetime of the minicomposites under constant load was measured in static fatigue at 700°C in air. These temperature conditions were the worst, because the oxidation rate was high for PyC and low for SiC. The minicomposite ends were glued within alumina tubes using an alumina-based ceramic adhesive. The gauge length (10 mm) was determined by the distance between the tubes. The tubes were gripped into the testing machine. The minicomposites were positioned within the furnace hot zone where the temperature was uniform at 700°C. The minicomposites were heated to the test temperature before loading. Then a constant force of 80 N was applied using a dead weight that was progressively hung on the bottom grip via the displacement of the support at a constant speed. This force was ;10% above the proportional limit (Table III). Acoustic emission showed that matrix cracking initi￾ated below the proportional limit under a force around 50 N.5 The first matrix cracks did not influence specimen compliance. At failure of the minicomposites, the chronometer was stopped by the falling dead weight, giving the lifetime. III. Results and Discussion (1) Material Characterization Details on the microstructure of the nanoscale multilayered (PyC/SiC)n fiber coatings have been provided.12 AES depth profiles revealed the presence of an oxygen-enriched layer (15–50 nm) at the surface of the as-received Hi-Nicalon fibers (Table II). This layer consisted of SiO2 and free carbon. The first interfacial PyC sublayer deposited was bonded to this silicon/carbon/oxygen layer. Such sublayers have also been identified in composites reinforced with as-received Nicalon fibers (NL 202). The corre￾sponding fiber/matrix interactions were weak, and deflection of the matrix cracks occurred at the fiber/interphase interfaces.1,3,10 AES depth profiles performed on treated Hi-Nicalon fibers show that the surface of the fibers consists of a 50–100 nm thick layer of free carbon (Fig. 1). The presence of such a superficial layer of free carbon increases the strength of the PyC coating/fiber interface in Nicalon/PyC/SiC composites.1,3,10 The TEM micro￾graph of Fig. 1 shows that the deposited PyC is perfectly bonded to the fiber. By contrast, preexisting fiber/coating debonds are observed in the minicomposites reinforced with as-received Hi￾Nicalon fibers.12 (2) Tensile Behavior at Room Temperature‡ Figure 2 shows that the force–deformation curves obtained for HNT/(C/SiC)10 minicomposites reinforced with treated fibers exhibit typical features associated with strong fiber/matrix bonds:1,3 (1) A wide curved domain up to ultimate failure attributed to a high density of matrix cracks and short debonds, and (2) Rather narrow hysteresis loops, indicative of strong fiber/ matrix interactions on unloading–reloading. HN/(C/SiC)10 minicomposites reinforced with as-received fi￾bers display typical features associated with weak fiber/matrix bonds, including (1) A narrow curved domain reflecting longer debonds and a lower density of matrix cracks, and ‡ Forces instead of stresses are used to describe the mechanical behavior of the minicomposites, because derivation of a stress from the applied force is not straightforward or appropriate. The stress state is not uniform within the minicom￾posites, as a result of the transverse cracks that locally reduce the stressed section to that of fibers. Table I. Description of Investigated Hi-Nicalon/SiC Minicomposites Batches Fibers† Interphase Interphase characteristics e(PyC)‡ (nm) e(SiC)‡ (nm) n§ HN/(C/SiC)10 A.R. (20/50)10 20 50 10 HNT/(C/SiC)10 T. (20/50)10 20 50 10 HN/C A.R. (100/0)1 100 0 1 HNT/C T. (100/0)1 100 0 1 † A.R. 5 as-received, T. 5 treated. ‡ Thickness per sublayer. § Number of PyC/SiC bilayers. Table II. Properties of Minicomposite Constituents12,13 Constituents† Superficial layer Young’s Modulus (GPa) Statistical parameters Composition Thickness (nm) m so ‡ (MPa) Hi-Nicalon fibers (A.R.) SiO2 15–50 280 4.2 6 Hi-Nicalon fibers (T.) Free carbon 50–100 305 5.8 26 PyC interphase 12–80 SiC interphase 280 SiC P-CVI matrix 400 5.5 5.7 † A.R. 5 as-received, T. 5 treated. ‡ Reference volume, Vo 5 1 m3 . Table III. Average Mechanical Properties of Minicomposites Tested Batches Ec (GPa) Proportional limit§ Saturation§ Failure§ ls (mm) FE (N) εE (%) FS (N) εs (%) FR (N) εR (%) HN/(C/SiC)10 351 71 0.12 135 0.30 195 0.83 50† 175‡ HNT/(C/SiC)10 360 73 0.07 ;FR ;εR 183 0.79 35† 67‡ HN/C 342 75 0.09 100 0.30 171 0.69 40† 95‡ HNT/C 350 75 0.17 140 0.7 156 0.81 30† 100‡ † Microcrack spacing in the internal matrix of the minicomposite. ‡ Microcrack spacing in the surface of the minicomposite. § F 5 force; ε 5 deformation. 788 Journal of the American Ceramic Society—Bertrand et al. Vol. 84, No. 4