2988 Journal of the American Ceramic Socieny'Kuo et al Vol. 80. No. 12 ecuted by dipping one fiber end and then the other in an alter- posite processing temperature to a temperature of -1000C nating manner. The coating thickness was measured from after which a temperature difference, AT, of -1000oC would lished sections of sintered samples and was uniform to develop the room-temperature thermoelastic residual ithin +20%. Two important procedures, which included fiber stresses.29 The material properties that were used in the re- sitioning within the matrix and pellet sintering, were crucial sidual-stress calculations(Youngs modulus(E), Poissons ra- to subsequent pushout testing. If the slices for fiber pushor tio(v), and the coefficient of thermal expansion, a) for both tests were not cut perpendicular to the fiber alignment, the composite systems are listed in Table I. Using these properties, misaligned fibers exhibited higher pushout resistance and af- the residual axial and radial stresses at the fiber/coating inter fected the subsequent data interpretation. In this study, five face were calculated as a function of the coating thickness(Fig oated fibers were aligned parallel to each other. To aid in fiber 1). A residual tensile axial stress was found in both composite alignment, SCS-8 fibers(Textron Specialty Materials, Lowell systems. The residual axial stress increased as the coating MA)were used as markers in the composite. These marker hickness increased in the Al2O3 fiber system; however, it de fibers were cut so that both ends were exposed after sintering creased as the coating thickness increased in the YAG fiber These exposed marker fibers revealed the relative orientation system(Fig. I(a)). A radial compressive stress that decreased of the coated fibers and greatly facilitated subsequent sample as the coating thickness increased was calculated for the Al, O3 slice cutting. As a result, the marker fibers aided fiber align- fiber system, whereas a small increase in radial tensile stress ment and positioning(before sintering)and subsequent cutting was determined for the YAG fiber system(Fig. 1(b). These of thin slices for pushout tests(after sintering) stresses had an effect on the interfacial debonding and sliding Several different matrix powders were used to fabricate the properties for each composite system, as will be demonstrated dense model specimens. Initially, an Al2O3 powder(Al6-SG, in the following sections Alcoa Aluminum Co., Pittsburgh, PA)was used, which re- uired a sintering temperature of 1600c to densify the speci- (2) Fiber Pushout Curves mens. Fiber damage that was similar to that which was ob Representative pushout curves for both AlO, and YAG fi- served by Newcomb and Tressler was noted after only 3 h at ber systems are shown in Fig. 2. These data exhibit features catey perature of 1600 C. A second set of samples was fabri that are typical of fiber pushout: the initial linear load- using a high-surface-area Al,O3 powder(75-90 m/g, as followed by a nonlinear region that Praxair Surface Technologies, Indianapolis, IN). However, was associated with progressive debonding along the interfa rge amount of sintering shrinkage and specimen bending w Debonding continued until the crack attained a critical length oted after sintering at a temperature of 1550%C, which resulted bimodal patrick s uscg derable sample distortion. Finally, a Unstable crack growth appears as a sharp load decrease in the in broken fibers and cons 40- 60 mixture of high oad-displacement curve. After complete interfacial debond- which resulted in a powder that had ng, the fiber experienced frictional sliding throughout the re- on Using the resulting bimoda mainder of the pushout test. powder, dense pellets were formed at 1550C with a moderate During frictional sliding, fibers from both systems slid at amount of sintering shrinkage, and there was no visible fiber onstant or slightly increasing loads over -20 um of damage after observation by scanning electron microscopy displacement(Fig. 2). The peak load just prior to (SEM)(Model DS-130, International Scientific Instruments fiber bonding, Pp, and the pushout load just after debonding, P,(lower case P indicates load), are identified in (2) Pushout Test Procedure Fig. 2. Representative micrographs of each pushed-out fiber are shown in Fig. 3. These micrographs indicate that debonding After sintering, pushout samples were prepared by making occurred at the fiber/coating interface for both the Yag and the slices perpendicular to the fiber orientation. Test slices were fabricated that had thicknesses in the range of 0.4-1.2 mm system(Fig 3(b) appears qualitatively rougher than that of the which provided a range of embedded fiber lengths. Following Al2 O, fiber system(Fig. 3(a). This roughness, which in- the slicing, the samples were ground and polished to a final creased the fiber sliding resistance, was responsible for the screw-driven testing machine with a I kg load cell(Model out of the YAG fibers(Fig. 2). 15,16 uring the frictional push- finish of I Am. Fiber pushout tests were conducted using a increasing load that was experienced 4502, Instron Corp, Canton, MA). A diamond probe that had Five pushout tests were conducted for each coating a 95-]m-diameter flat tip was fixed onto a cylinder that was over a broad range of specimen thicknesses. Average threaded to the load cell. Pushout specimens were aligned over loads (Pp) were measured as a function of the embedded a slotted Al2 O3 substrate. Two micropositioning, stages-one length(L)for both the Al2O3 and the YAG fiber systems(Fig for a stereomicroscope and the other for the specimen stag ) In addition, pushout experiments were conducted using a were used to align the test fiber and the diamond punch. 4, 17 range of coating thicknesses. The pushout load varied with the Testing was conducted using a constant crosshead speed of 60 coating thickness in both composite systems(Fig. 5) Although it is common to assess the interface strengt for each slice thickness, coating thickness, and fiber system lag approach, this method assumes an instanta Average values of the key pushout parameters were used for along the entire length of the fiber. Mechanisti- obsequent sample comparison. In addition to mechanical char- debonding is progressive rather than instanta- acterization, SEM was used to evaluate the material micro- Residual axial and radial stresses are known to influent also is ing and sliding of fibers from a matrix. 19, 20 The coating Table I. Properties of Fibers, Matrix, and Coating Youngs modulus, Poissons the ffects26-8 In this study, an elastic shrink-fit calculation known as the bead-seal solution, 23, 24 has been used to estimate the residual thermal stresses in the model systems A2。3 sapphire 430(a),465(c)0.258.3(a),9.0(c) 3A 8.9 ApOA 0.275 ( Residual-Stress Calculation AlO, matrix 380 It was assumed that complete stress relaxation by creep pro- Al,O3, cesses occurred when the system was cooled from the com thasarathy. I Data from morgan and marshal. s"e and a data from kingery et a/. 2ecuted by dipping one fiber end and then the other in an alter￾nating manner. The coating thickness was measured from polished sections of sintered samples and was uniform to within ±20%. Two important procedures, which included fiber positioning within the matrix and pellet sintering, were crucial to subsequent pushout testing. If the slices for fiber pushout tests were not cut perpendicular to the fiber alignment, the misaligned fibers exhibited higher pushout resistance and af￾fected the subsequent data interpretation. In this study, five coated fibers were aligned parallel to each other. To aid in fiber alignment, SCS-8 fibers (Textron Specialty Materials, Lowell, MA) were used as markers in the composite. These marker fibers were cut so that both ends were exposed after sintering. These exposed marker fibers revealed the relative orientation of the coated fibers and greatly facilitated subsequent sample slice cutting. As a result, the marker fibers aided fiber align￾ment and positioning (before sintering) and subsequent cutting of thin slices for pushout tests (after sintering). Several different matrix powders were used to fabricate the dense model specimens. Initially, an Al2O3 powder (A16-SG, Alcoa Aluminum Co., Pittsburgh, PA) was used, which re￾quired a sintering temperature of 1600°C to densify the speci￾mens. Fiber damage that was similar to that which was ob￾served by Newcomb and Tressler25 was noted after only 3 h at a temperature of 1600°C. A second set of samples was fabri￾cated using a high-surface-area Al2O3 powder (75–90 m2 /g, Praxair Surface Technologies, Indianapolis, IN). However, a large amount of sintering shrinkage and specimen bending was noted after sintering at a temperature of 1550°C, which resulted in broken fibers and considerable sample distortion. Finally, a 40:60 mixture of high-surface-area powder and the A16-SG Al2O3 powder was used, which resulted in a powder that had bimodal particle-size distribution. Using the resulting bimodal powder, dense pellets were formed at 1550°C with a moderate amount of sintering shrinkage, and there was no visible fiber damage after observation by scanning electron microscopy (SEM) (Model DS-130, International Scientific Instruments, Santa Clara, CA). (2) Pushout Test Procedure After sintering, pushout samples were prepared by making slices perpendicular to the fiber orientation. Test slices were fabricated that had thicknesses in the range of 0.4–1.2 mm, which provided a range of embedded fiber lengths. Following the slicing, the samples were ground and polished to a final finish of 1 mm. Fiber pushout tests were conducted using a screw-driven testing machine with a 1 kg load cell (Model 4502, Instron Corp., Canton, MA). A diamond probe that had a 95-mm-diameter flat tip was fixed onto a cylinder that was threaded to the load cell. Pushout specimens were aligned over a slotted Al2O3 substrate. Two micropositioning stages—one for a stereomicroscope and the other for the specimen stage— were used to align the test fiber and the diamond punch.14,17 Testing was conducted using a constant crosshead speed of 60 mm/min. A minimum of four pushout tests were conducted for each slice thickness, coating thickness, and fiber system. Average values of the key pushout parameters were used for subsequent sample comparison. In addition to mechanical char￾acterization, SEM was used to evaluate the material micro￾structures. Residual axial and radial stresses are known to influence the debonding and sliding of fibers from a matrix.19,20 The coating also is known to have an important role in mitigating these effects.26–28 In this study, an elastic shrink-fit calculation, known as the bead-seal solution,23,24 has been used to estimate the residual thermal stresses in the model systems. III. Results (1) Residual-Stress Calculation It was assumed that complete stress relaxation by creep pro￾cesses occurred when the system was cooled from the com￾posite processing temperature to a temperature of ∼1000°C, after which a temperature difference, DT, of ∼1000°C would develop the room-temperature thermoelastic residual stresses.29 The material properties that were used in the re￾sidual-stress calculations (Young’s modulus (E), Poisson’s ra￾tio (n), and the coefficient of thermal expansion, a) for both composite systems are listed in Table I. Using these properties, the residual axial and radial stresses at the fiber/coating inter￾face were calculated as a function of the coating thickness (Fig. 1). A residual tensile axial stress was found in both composite systems. The residual axial stress increased as the coating thickness increased in the Al2O3 fiber system; however, it de￾creased as the coating thickness increased in the YAG fiber system (Fig. 1(a)). A radial compressive stress that decreased as the coating thickness increased was calculated for the Al2O3 fiber system, whereas a small increase in radial tensile stress was determined for the YAG fiber system (Fig. 1(b)). These stresses had an effect on the interfacial debonding and sliding properties for each composite system, as will be demonstrated in the following sections. (2) Fiber Pushout Curves Representative pushout curves for both Al2O3 and YAG fi￾ber systems are shown in Fig. 2. These data exhibit features that are typical of fiber pushout: the initial linear load– displacement region was followed by a nonlinear region that was associated with progressive debonding along the interface. Debonding continued until the crack attained a critical length and propagated unstably to complete interfacial debonding.20 Unstable crack growth appears as a sharp load decrease in the load–displacement curve. After complete interfacial debond￾ing, the fiber experienced frictional sliding throughout the re￾mainder of the pushout test. During frictional sliding, fibers from both systems slid at constant or slightly increasing loads over ∼20 mm of pushout displacement (Fig. 2). The peak load just prior to complete fiber bonding, pP, and the pushout load just after complete debonding, p1 (lower case P indicates load), are identified in Fig. 2. Representative micrographs of each pushed-out fiber are shown in Fig. 3. These micrographs indicate that debonding occurred at the fiber/coating interface for both the YAG and the Al2O3 fiber systems. The debond interface of the YAG fiber system (Fig. 3(b)) appears qualitatively rougher than that of the Al2O3 fiber system (Fig. 3(a)). This roughness, which in￾creased the fiber sliding resistance, was responsible for the increasing load that was experienced during the frictional push￾out of the YAG fibers (Fig. 2).15,16 Five pushout tests were conducted for each coating system over a broad range of specimen thicknesses. Average peak loads (pP) were measured as a function of the embedded fiber length (L) for both the Al2O3 and the YAG fiber systems (Fig. 4). In addition, pushout experiments were conducted using a range of coating thicknesses. The pushout load varied with the coating thickness in both composite systems (Fig. 5). Although it is common to assess the interface ‘‘strength’’ using a shear-lag approach, this method assumes an instanta￾neous debond along the entire length of the fiber. Mechanisti￾cally, however, debonding is progressive rather than instanta￾Table I. Properties of Fibers, Matrix, and Coating Material Young’s modulus, E (GPa) Poisson’s ratio, n Coefficient of thermal expansion, a (10−6/°C) Al2O3 (sapphire) fiber† 430 (a), 465 (c) 0.25 8.3 (a), 9.0 (c) Y3Al5O12 (YAG) fiber‡ 283 0.25 8.9 LaPO4 coating§ 133 0.275 9.6 Al2O3 matrix¶ 380 0.25 8.8 † For Al2O3, data in the a and c directions are given. E data are from Li and Bradt;30 n and a data are from the manufacturer (Saphikon, Milford, NH). ‡ Data from Par￾thasarathy.31 §Data from Morgan and Marshall.13 ¶E and a data from Kingery et al.32 2988 Journal of the American Ceramic Society—Kuo et al. Vol. 80, No. 12