BRENNAN: INTERFACIAL CHARACTERIZATION SiC/SiC composites have been presented previously frictional sliding and pull-out of the fibers-which 3-9 contribute to the composite toughness [ 121being more extensive for the hi-Nicalon fibers than for the 2.2. Composite testing Sylramic fibers. The reason for this may be related to Tensile testing was performed in accordance with the higher elastic modulus of the Sylramic fiber, or, High-Speed Research/Enabling Propulsion Materials more likely, the much higher surface roughness of the (HSR/EPM) consensus standard. The testing method Sylramic fiber compared with the Hi-Nicalon fiber,as was similar to ASTM standard C-1275-95 [10]. Ten- shown in Fig. 4. The surface roughness of the two sile specimen design was a contoured, face-loaded fibers is a reflection of the relative Sic grain size of specimen geometry with an overall length of 152 mm the fibers Fibers with higher surface roughness have and a gage-section width of 10.16 mm. The tensile been found to have onounced influence on the specimen design used a gradual radius from the tab fiber sliding behavior in ceramic-matrix composites to the gage section to reduce stress concentrations to [131 avoid grip and tab failures[11]. All of the mechanical While the Hi-Nicalon MI SiC/SiC composite sys property definitions can be found in the ASTM stan- tem appears to have better toughness than the Syl- dard. One of the most crucial and subjective measure ments is the proportional limit. The proportional limit well in high-temperature fatigue testing. As shown in was determined using the offset method. A line, run- Fig. 5, a Hi-Nicalon fiber composite failed after 38h ning parallel to the elastic modulus slope, was gener- and 19 cycles in a 2 h hold-time tensile fatigue test ated at a strain axis offset of 0.00005 mm/mm. The at 1200oC and a stress of 160 MPa, whereas a Syl proportional limit was defined as the stress level at ramic fiber composite ran out to 1000 h, 500 cycles, which point the offset line intersected the load versus under the same test conditions. The stress level during elongation curve. A servo-hydraulic machine was this test was chosen to be somewhat higher than the used for the vast majority of tensile testing. Testing proportional limit, or matrix microcrack stress of the from room temperature to elevated temperature was composites, which at this temperature is-140 MPa done in the same rig and differed only by the presence for the Sylramic fiber composite and -125 MPa for of a small furnace that heated the sample gage plus the Hi-Nicalon fiber composite. The residual 1200C radius region to within 1% of the desired temperature. tensile properties of the fatigued Sylramic fiber com Fatigue testing was conducted at elevated tempera- posite are not significantly different from those meas- ture in load control. Testing was done in a programm as-fabricated Sylramic fiber composite able servo-hydraulic machine. The testing consisted From the fracture surfaces shown in Fig. >1.fatigued an of a 2 h dwell fatigue test with the load being relieved seen that the amount of fiber pull-out for to 5% of the hold load at the end of each cycle. This Sylramic fiber composite is similar to that shown in simulated service conditions for the proposed appli- Fig 3 for the 1200C tensile sample, but the fatigued cation. Most tests were run to failure or 500 h of Hi-Nicalon composite shows a region around the fatigue exposure. All samples that made 500 h of edges of the sample that appears very brittle exposure were then tested for residual tensile proper- This embrittlement of the Hi-Nicalon fiber com- es at room temperature to note if degradation had posite is even more apparent after tensile fatigue test occurred. A selected number of fatigue tests was run ing at a lower temperature of 650%C, as shown in Fig to many thousands of hours before failure occurred, 6. While both composites ran out to the test limit of as will be described later 546h, 273 cycles, the residual 650C strength and strain-to-failure of the Hi-Nicalon fiber composite were significantly degraded, while those for the Syl- 3. RESULTS AND DISCUSSION ramic fiber composite were not 3. 1. Fracture characteristics of Hi-Nicalon versus 3. 2. Fiber/matrix interfacial characteristics of Hi- Sylramic SiC fiber MI composites Nicalon versus Sylramic SiC fiber MI composites Figure 3 shows the fracture surfaces of 1200C ten- From transmission electron microscopy (TEM) sile samples of MI SiC/SiC composites with either thin-foil analysis of the interfacial region in an as- Sylramic or Hi-Nicalon fibers. From this figure, one processed Hi-Nicalon fiber MI composite, as show can see that the fracture surface of the hi-nicalon Fig. 7, it was found that the probable reason fo fiber composite is much more fibrous in nature than the easy debonding and long fiber pull-out in these that of the Sylramic fiber composite. The measured composites, as was shown in Fig. 3, is that a thin tensile strengths of the two composites are similar, at (40 nm) carbon-rich layer has formed between th 286 MPa; however, the strain-to-failure of the Hi- BN and the Hi-Nicalon fiber during the MI composite Nicalon fiber composite(0.48%) is over twice that processing. This carbon-rich layer is probably a result of the Sylramic fiber composite(0.21%). Thus, the of interaction of the oxygen in the bn layer "toughness", or inherent matrix crack-stopping (-6 at%)with the excess carbon in the Hi-Nicalon ability, of the Hi-Nicalon fiber composite is greater fiber at the MI composite processing temperature of an that of the Sylramic fiber composite, due to the >1400C. Thicker, but similar, carbon layers haveBRENNAN: INTERFACIAL CHARACTERIZATION 4621 SiC/SiC composites have been presented previously [3–9]. 2.2. Composite testing Tensile testing was performed in accordance with High-Speed Research/Enabling Propulsion Materials (HSR/EPM) consensus standard. The testing method was similar to ASTM standard C-1275-95 [10]. Ten￾sile specimen design was a contoured, face-loaded specimen geometry with an overall length of 152 mm and a gage-section width of 10.16 mm. The tensile specimen design used a gradual radius from the tab to the gage section to reduce stress concentrations to avoid grip and tab failures [11]. All of the mechanical property definitions can be found in the ASTM stan￾dard. One of the most crucial and subjective measure￾ments is the proportional limit. The proportional limit was determined using the offset method. A line, run￾ning parallel to the elastic modulus slope, was gener￾ated at a strain axis offset of 0.00005 mm/mm. The proportional limit was defined as the stress level at which point the offset line intersected the load versus elongation curve. A servo-hydraulic machine was used for the vast majority of tensile testing. Testing from room temperature to elevated temperature was done in the same rig and differed only by the presence of a small furnace that heated the sample gage plus radius region to within 1% of the desired temperature. Fatigue testing was conducted at elevated tempera￾ture in load control. Testing was done in a programm￾able servo-hydraulic machine. The testing consisted of a 2 h dwell fatigue test with the load being relieved to 5% of the hold load at the end of each cycle. This simulated service conditions for the proposed appli￾cation. Most tests were run to failure or 500 h of fatigue exposure. All samples that made 500 h of exposure were then tested for residual tensile proper￾ties at room temperature to note if degradation had occurred. A selected number of fatigue tests was run to many thousands of hours before failure occurred, as will be described later. 3. RESULTS AND DISCUSSION 3.1. Fracture characteristics of Hi-Nicalon versus Sylramic SiC fiber MI composites Figure 3 shows the fracture surfaces of 1200°C ten￾sile samples of MI SiC/SiC composites with either Sylramic or Hi-Nicalon fibers. From this figure, one can see that the fracture surface of the Hi-Nicalon fiber composite is much more fibrous in nature than that of the Sylramic fiber composite. The measured tensile strengths of the two composites are similar, at |286 MPa; however, the strain-to-failure of the Hi￾Nicalon fiber composite (0.48%) is over twice that of the Sylramic fiber composite (0.21%). Thus, the “toughness”, or inherent matrix crack-stopping ability, of the Hi-Nicalon fiber composite is greater than that of the Sylramic fiber composite, due to the frictional sliding and pull-out of the fibers—which contribute to the composite toughness [12]—being more extensive for the Hi-Nicalon fibers than for the Sylramic fibers. The reason for this may be related to the higher elastic modulus of the Sylramic fiber, or, more likely, the much higher surface roughness of the Sylramic fiber compared with the Hi-Nicalon fiber, as shown in Fig. 4. The surface roughness of the two fibers is a reflection of the relative SiC grain size of the fibers. Fibers with higher surface roughness have been found to have a pronounced influence on the fiber sliding behavior in ceramic-matrix composites [13]. While the Hi-Nicalon MI SiC/SiC composite sys￾tem appears to have better toughness than the Syl￾ramic fiber composite system, it does not perform well in high-temperature fatigue testing. As shown in Fig. 5, a Hi-Nicalon fiber composite failed after 38 h and 19 cycles in a 2 h hold-time tensile fatigue test at 1200°C and a stress of 160 MPa, whereas a Syl￾ramic fiber composite ran out to 1000 h, 500 cycles, under the same test conditions. The stress level during this test was chosen to be somewhat higher than the proportional limit, or matrix microcrack stress of the composites, which at this temperature is |140 MPa for the Sylramic fiber composite and |125 MPa for the Hi-Nicalon fiber composite. The residual 1200°C tensile properties of the fatigued Sylramic fiber com￾posite are not significantly different from those meas￾ured for an as-fabricated Sylramic fiber composite. From the fracture surfaces shown in Fig. 5, it can be seen that the amount of fiber pull-out for the fatigued Sylramic fiber composite is similar to that shown in Fig. 3 for the 1200°C tensile sample, but the fatigued Hi-Nicalon composite shows a region around the edges of the sample that appears very brittle. This embrittlement of the Hi-Nicalon fiber com￾posite is even more apparent after tensile fatigue test￾ing at a lower temperature of 650°C, as shown in Fig. 6. While both composites ran out to the test limit of 546 h, 273 cycles, the residual 650°C strength and strain-to-failure of the Hi-Nicalon fiber composite were significantly degraded, while those for the Syl￾ramic fiber composite were not. 3.2. Fiber/matrix interfacial characteristics of Hi￾Nicalon versus Sylramic SiC fiber MI composites From transmission electron microscopy (TEM) thin-foil analysis of the interfacial region in an as￾processed Hi-Nicalon fiber MI composite, as shown in Fig. 7, it was found that the probable reason for the easy debonding and long fiber pull-out in these composites, as was shown in Fig. 3, is that a thin (|40 nm) carbon-rich layer has formed between the BN and the Hi-Nicalon fiber during the MI composite processing. This carbon-rich layer is probably a result of interaction of the oxygen in the BN layer (|6 at%) with the excess carbon in the Hi-Nicalon fiber at the MI composite processing temperature of >1400°C. Thicker, but similar, carbon layers have