Evaluation of SiC/SiC minicomposites with yttrium disilicate fiber coating

Transcription

1 Received: 22 March 2017 Accepted: 7 August 2017 DOI: /jace ORIGINAL ARTICLE Evaluation of SiC/SiC minicomposites with yttrium disilicate fiber coating Emmanuel E. Boakye 1,2 Thomas S. Key 1,2 Triplicane A. Parthasarathy 1,2 Kristin A. Keller 1,2 Samuel J. Opeka 1,2 Randall S. Hay 1 Michael K. Cinibulk 1 1 Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, OH 2 UES, Inc., Dayton, OH Correspondence Emmanuel E. Boakye, Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, OH. emmanuel.boakye.ctr@us.af.mil Abstract Hi-Nicalon-S/a-Y 2 Si 2 O 7 /SiC minicomposites were formed by polymer infiltration pyrolysis (PIP) and characterized by TEM, SEM fractography, tensile testing, and fiber push-in testing. All minicomposites with a-y 2 Si 2 O 7 fiber coatings had strengths significantly higher than the control samples without fiber coatings. Extensive fiber pullout with debonding at the coating-fiber interface or within the coating itself was observed in minicomposites with Y 2 Si 2 O 7 fiber coatings, but no debonding was observed in minicomposites without fiber coatings. Debond energies of 4.5 3, J/m 2 and average sliding stresses of 91 40, MPa were measured by fiber push-in tests. KEYWORDS ceramic matrix composites, RE 2 Si 2 O 7 oxide coating, SiC fiber, weak interface 1 INTRODUCTION The application of SiC/SiC ceramic matrix composites (CMCs) can be limited by degradation of mechanical properties in oxidizing environments, partly due to oxidation of the BN or carbon fiber coating at the fiber-matrix interface, 1 particularly at intermediate temperatures, ~700 C-800 C, and in the presence of water vapor. Fiber coatings in CMCs are engineered to provide interface debonding, crack deflection, and fiber pullout and are key to CMC toughness and composite behavior. Oxidative degradation of BN or carbon coatings negatively affects this key functionality and results in lower composite strength and toughness. 1 Although a variety of methods have been used to mitigate the oxidation of BN or carbon in SiC/SiC CMCs, 2-8 a better solution would be to replace BN or carbon with an oxidation resistant coating. Previous attempts at the development of oxidation resistant coatings for SiC-SiC composites broadly fall into two categories. The first includes works where the function of the oxidation-resistant coating was to protect the fiber itself from an attack from the service environment, e.g., oxidation. 9,10 Unlike these studies, however, the focus of the current work is not on protecting the fibers, but on the development of oxidation-resistant fiber interphase coatings to ensure that the CMC retains its composite properties after prolonged exposures in oxidizing conditions. In this regard, Shanmugham et al. showed that a mullite interphase in SiC/ SiC composites deflected cracks even after exposure in air at 1000 C for 24 hours. 11 Conversely, in this study, a SiC/ SiC composite with a carbon interphase showed brittle behavior after the same oxidative exposure. However, due to coating nonuniformity the fracture surfaces of some regions of the tested composites showed brittle behavior adjacent to regions of fiber pullout. Lee et al demonstrated the use of multilayer SiO 2 /ZrO 2 /SiO 2 oxide coatings for SiC/SiC composites. 12 Composite strength and crack deflection were retained after oxidation in air at 960 C for 10 hours. The authors concluded that the weak interface behavior may be due to a possible contribution of carbon impurities in the interphase region. Although both the works of Shanmugham and Lee are encouraging, a convincing proof of concept requires composite oxidation for longer periods of time. J Am Ceram Soc. 2018;101: wileyonlinelibrary.com/journal/jace 2017 The American Ceramic Society 91

2 92 BOAKYE ET AL. In prior studies, LaPO 4 monazite was shown to be an effective interface coating in oxide-oxide CMCs. 13 However, it was found to be thermodynamically incompatible with SiC. 14 We recently showed that rare-earth disilicates are promising oxidation-resistant replacements for oxidation-prone BN and C coatings in SiC/SiC composites. 15,16 Similar to LaPO 4,Y 2 Si 2 O 7,andHo 2 Si 2 O 7 have relatively low hardness values, ~6 GPa, and deform plastically under indentation by extensive dislocation slip. 15 Crucially, unlike LaPO 4, rare-earth disilicates are thermodynamically compatible with SiC over a broad range of environmental conditions. 17 Fiber pushout studies of large diameter SCS-0 SiC fibers in Y 2 Si 2 O 7 and Ho 2 Si 2 O 7 matrices showed promising weak interface properties with debond energies of 2-4 J/m 2, 16 which is similar to values reported for BN, C, and monazite. Average sliding stresses of MPa were measured, which are even lower than the values reported for LaPO 4, 16 yet still higher than most values reported for C and BN fiber-matrix interphases. 3,18-23 However, a definitive demonstration of Y 2 Si 2 O 7 functioning as a weak fiber-matrix interface in small-diameter fiber-reinforced SiC/SiC CMCs is still lacking. The goal of the present work was to demonstrate composite behavior of SiC/SiC CMCs with a Y 2 Si 2 O 7 fibercoating interphase. YPO 4 -coated preoxidized Hi-Nicalon-S fibers were used to form minicomposites with a polymer infiltration-pyrolysis (PIP) SiC matrix. In situ yttrium disilicate coatings were formed on the fiber tows from YPO 4 coatings using the reaction: 24,25 2SiO 2 þ 2YPO 4 ¼ Y 2 Si 2 O 7 þ P 2 (g) þ 2:5O 2 (g) (1) The performance of Y 2 Si 2 O 7 as a weak fiber-matrix interface was assessed. The Hi-Nicalon-S/Y 2 Si 2 O 7 /SiC minicomposites were evaluated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), tensile testing, and fiber push-in measurements of fiber debonding and interface sliding stress. 2 EXPERIMENTAL PROCEDURES 2.1 Materials Reagent grade yttrium nitrate hexahydrate, phosphoric acid, and citric acid were obtained from Aldrich Chemical Co. (Milwaukee, WI) and used in the as-received condition. YPO 4 coatings were deposited onto Hi-Nicalon-S (Nippon Carbon Co. Ltd., Tokyo, Japan) SiC fibers. Deionized water, purified by reverse osmosis with a Barnsted Nanopure Ultrapure system (model D4744), was used in these experiments. The PIP process utilized SMP-10 (Starfire Systems, Glenville, NY), a commercially available allylhydridopolycarbosilane polymer that forms SiC on pyrolysis at temperatures over 1200 C. 26 A <1 lm SiC powder was obtained from Advanced Chemicals (Milwaukee, WI) and used as a filler in the PIP process. 2.2 Two-stage fiber-coating process Initial attempts were made to form a colloidal yttrium disilicate sol and then coat the fiber tows using a continuous vertical coater, as done previously for LaPO 4 coatings on oxide fiber tows. 17,27-30 However, particle aggregation and flocculation made it impossible to form continuous coatings. An alternative coating process was thus required. In our prior work, we showed that La 2 Si 2 O 7 formed when LaPO 4 -coated SiC fibers were heat treated in argon/low po 2 mixtures. 14,17 We also developed a process to apply continuous YPO 4 coatings on Nextel 720 fiber tows using a hetero-precipitation method Based on the formation of La 2 Si 2 O 7, 14 a thermodynamic analysis of reaction (1) was conducted and experimental conditions for the formation of a single-phase Y 2 Si 2 O 7 from YPO 4 on SiC fibers were determined. 24,25 The details of the thermodynamic analysis and relevant experiments will be published elsewhere. Briefly, it was shown that to convert YPO 4 coatings deposited onto SiC fibers to single-phase Y 2 Si 2 O 7 required first forming a SiO 2 layer on the fiber surface via controlled preoxidation of the fibers prior to application of YPO 4. Subsequent heat treatment in argon facilitates reaction (1) and the formation of Y 2 Si 2 O 7. 24,25 In this work, fibers were preoxidized at 1000 C in air for 0.5 and 20 hours, forming silica layers of ~50 nm and ~250 nm thicknesses, respectively, followed by multiple coatings with YPO 4, using the hetero-precipitation method The YPO 4 -coated, preoxidized fiber tows were then used to form minicomposites (see section 2.3) YPO 4 precursor solution A precursor solution of a mixture of phosphoric acid and yttrium citrate was prepared for the fiber coating. First, a 5.6 mol L 1 stock solution of phosphoric acid was made by diluting g of concentrated phosphoric acid with 600 ml of deionized water. Next, a 1.1 mol L 1 stock solution of yttrium citrate was made by dissolving g of yttrium nitrate and g of citric acid in 600 ml of deionized water. These precursor stock solutions were then stored in a refrigerator at 0 C. A solution mixture corresponding to 20 g YPO 4 /liter of deionized water was made by mixing 45 ml of the phosphoric acid stock solution, 45 ml of yttrium citrate stock solution and 400 ml of deionized water. The YPO 4 yield was determined by drying 40 ml of the solution mixture in an alumina crucible at 120 C followed by heat treatment at 1400 C/1 h in air. The Y:citrate molar ratio was 1:2 and the Y:P molar ratio was 1:5 for all experiments. 31,35

3 BOAKYE ET AL YPO 4 coatings The preoxidized fiber tows were coated using the heterogeneous YPO 4 precipitation method previously reported for Nextel 720 fiber tows The chilled mixed phosphoric acid yttrium citrate precursor was poured into a glass cylinder and the fiber tows were hung in the solution, as shown in Figure 1. The preoxidized fibers were placed into the solution and allowed to equilibrate for 5 minutes. The cylinder was then inserted into a boiling water bath to warm the phosphoric acid yttrium citrate solution to ~35 C, which causes YPO 4 nh 2 O to precipitate onto the fiber filaments (Figure 1). The fibers were then removed from the vessel, rinsed with deionized water, dried at 120 C and heat treated at 600 C to bond the YPO 4 to the fibers. The entire coating cycle was repeated 12 times for fibers preoxidized at 1000 C for 0.5 hours and 10 times for fibers preoxidized at 1000 C for 20 hours. The multiple cycles build up the YPO 4 coating thickness and helps to ensures that all filaments in a tow bundle are coated Y 2 Si 2 O 7 -coated fibers Y 2 Si 2 O 7 forms when preoxidized YPO 4 coated fibers are heat treated at 1200 C/10 hours in argon. The X-ray diffraction and transmission electron microscopy (TEM) characterization of the YPO 4 + SiO 2 reaction to yttrium disilicate will be published in a separate paper. In the present work, the YPO 4 coated preoxidized fiber tows were used for minicomposite manufacture. 2.3 Minicomposite processing Four 30 cm lengths of the YPO 4 -coated preoxidized tows, each with 500 filaments, were infiltrated with a slurry containing SMP-10 SiC polymer precursor loaded with 30 volume percent <1 lm SiC particles. The SiC particles were used as fillers to reduce shrinkage cracking of the matrix during pyrolysis. The infiltrated tows were pulled through a heat shrink tube with a narrow tip (Figure 2A. The narrow tip of the tube helped force the matrix slurry into the tows and more uniformly infiltrate the individual filaments within the tows; the excess slurry was squeezed out in this process. The minicomposites were vacuum cured at 250 C for 3 hours with a heating rate of 1 C/min. Pyrolysis of the preceramic polymer in the minicomposite was performed in a graphite lined tube furnace under flowing argon. A heating rate of 1 C/min was used to 800 C where the sample was held for 1 hours, followed by heating at 5 C/min to 1200 C, with a 1 hours hold. The infiltration and pyrolysis process was repeated five times to improve the composite density. The minicomposite was given a final heat treatment at 1200 C for 5 hours, resulting in total heat treatment time of 10 hours at 1200 C to convert the YPO 4 into Y 2 Si 2 O 7. Minicomposites with preoxidized fibers but no YPO 4 coatings were made using the same process as a control. 2.4 Mechanical tests Fiber strength The tensile strength of yttrium disilicate coated single filaments was measured using a 2.54 cm gauge length. A minimum of 30 fibers were tested for each data point with the assumption of a uniform Hi-Nicalon-S fiber diameter of 12.4 lm. 36 As a control, preoxidized but uncoated fibers were given the same heat treatment at 1200 C/10 h and then tested under the same conditions Composite tensile strength FIGURE 1 Schematic of experimental setup for yttrium phosphate fiber coating, using hetero-precipitation process. After References [Color figure can be viewed at wileyonlinelibrary.com] Minicomposites were tested for ultimate tensile strength following the standard test procedure. 37,38 Minicomposites with and without a Y 2 Si 2 O 7 interphase were affixed with epoxy to cm cm cardboard frames with a square opening in the center and with a gauge length of 2.54 cm (Figure 2B). The two arms on either side of the center opening were cut before testing, as shown in Figure 2B. Tensile tests were performed with a Synergie TM (MTS) test frame, using a crosshead speed of mm/s. The maximum load was recorded for each minicomposite and the ultimate tensile strength of the minicomposite was calculated using the load divided by the total cross-sectional area of fibers in the minicomposite. The total number of

4 94 BOAKYE ET AL. FIGURE 2 (A) YPO 4 coated and uncoated Hi-Nicalon-S tow infiltrated with SiC precursor in a shrink tube; the narrow tip of the shrink tube ensures uniform infiltration of matrix within the filaments. (B) The cured minicomposite was tested in tension using a cardboard frame to support the specimen. (After reference 45 ) fibers in a minicomposite was obtained by counting individual fibers in an SEM cross-sectional micrograph. The load train displacement was recorded but not the sample strain Fiber push in tests Fiber push-in tests were conducted with a MicroMeasure Machine (Process Equipment Inc., Tipp City, OH) that collects the load-displacement traces. Cross-sectional specimens approximately 0.4 mm thick with fiber cross-sections perpendicular to the fiber axis were prepared using tripod polishing. A flat-tipped 10 lm diameter diamond indenter was used for the fiber push-in tests. Ten fibers were pushed in each specimen. The push-in data was analyzed using the progressive roughness model of Parthasarathy et al 39,40 to obtain interface fracture energy, sliding stress and the interfacial frictional parameters of roughness amplitude and period Fractography The fracture surfaces of the minicomposites tested in tension were observed using a scanning electron microscope (Sirion, FEI, Hillsboro, Oregon). Evidence of fiber pullout and pullout lengths were evaluated. 3 RESULTS 3.1 Fiber strength The strength of minicomposites is primarily determined by the fiber strength. Processing of the minicomposites required heat treatment at 1200 C/10 h in argon. Accordingly, the strength of preoxidized YPO 4 -coated fibers after heat treatment at 1200 C/10 h in argon, during which yttrium disilicate forms, was measured as a point of reference for the strength of the minicomposites. The strength of preoxidized, but not YPO 4 -coated fibers heat treated at 1200 C/10 h in argon was also measured as a control. Table 1 summarizes the measured tensile strengths of the uncoated and yttrium disilicate coated fibers. The strength of the as-received fiber was 3.3 GPa. After preoxidation at 1000 C for 0.5 hours and 20 hours, the fiber strength dropped to GPa and GPa, respectively. However, after coating with YPO 4 and conversion to Y 2 Si 2 O 7, the strength of the fiber preoxidized for 0.5 hours dropped by ~34% (relative to the as-received strength) to GPa. In contrast, the fibers preoxidized for 20 hours retained most of their as-oxidized strength after YPO 4 was converted to Y 2 Si 2 O 7, with a final-coated fiber strength of GPa. Thus, it appears that for short preoxidation times, most of the fiber degradation occurs during the YPO 4 to Y 2 Si 2 O 7 conversion stage of the coating process. For longer preoxidation times, most of the fiber degradation is associated with the preoxidation stage. Further work is ongoing to determine the mechanisms of fiber strength degradation and to optimize the fiber-coating process to obtain the maximum-coated fiber strength. 3.2 Minicomposite characterization Figure 3 shows SEM micrographs of yttrium disilicatecoated Hi-Nicalon-S fibers in a SiC matrix, where the fibers were preoxidized for (A) 0.5 hours and (B) TABLE 1 Tensile strength of Hi-Nicalon-S fibers heat treated in argon at 1200 C for 10 h Preoxidized 0.5 h Preoxidized 20 h No coating a Yttrium Silicate Coating a As-received single filament strength ~3.3 GPa.

5 BOAKYE ET AL. 95 FIGURE 3 SEM micrographs of Y 2 Si 2 O 7 coating on Hi-Nicalon-S fibers showing continuous coating coverage for fibers (A) preoxidized for 0.5 hours and multiply coated 12 times, (B) preoxidized for 20 hours and multiply coated 10 times 20 hours. Fibers were coated multiple times to increase the coating thickness and to ensure virtually every filament in the tow was completely coated. The coatings were continuous with a variable thickness in the range of nm. TEM electron diffraction and EDS mapping was used to characterize the minicomposite fiber-coating interface. TEM analysis showed a-y 2 SiO 7 at the fiber-matrix interface for fibers preoxidized at 0.5 hours and 20 hours (Figures 4-6). For composites with fibers preoxidized for 0.5 hours, a thin layer of carbon was observed at the fiber/yttrium disilicate interface (Figure 4). In contrast, composites formed with fibers preoxidized for 20 hours had no carbon at the fiber-y 2 Si 2 O 7 interface. Instead, a residual silica film remained following the reaction of YPO 4 with SiO 2 (Figures 5 and 6). The thickness of the silica film varied from 5 nm to 30 nm. The yttrium disilicate coating consisted of a thin, dense inner layer of Y 2 Si 2 O 7 and a thicker, porous outer layer of Y 2 Si 2 O 7 filled with SiC matrix (Figures 4-6). 3.3 Minicomposite strength Table 2 summarizes the strengths of the minicomposites formed with and without fiber coatings. Control minicomposites without Y 2 Si 2 O 7 fiber coatings had an average strength of MPa. Although the average tensile strength of Y 2 Si 2 O 7 -coated fibers was 4% to 20% lower than uncoated preoxidized fiber (Table 1), minicomposites with Y 2 Si 2 O 7 coatings were significantly stronger than the control minicomposites. This suggests that the coating prevents fiber-matrix interaction and works as a weak interphase, deflecting cracks from the matrix and transferring load to the fibers, accompanied by fiber pullout (Figure 7). Since the minicomposite strength was calculated based FIGURE 4 TEM micrograph of Hi- Nicalon-S/yttrium-silicate/SiC composites showing the formation of a-y 2 Si 2 O 7 as the predominant phase at the fiber-matrix interface and the presence of a thin carbon film at the fiber-coating interface. Fibers in composites were preoxidized for 0.5 hours prior to YPO 4 coating [Color figure can be viewed at wileyonlinelibrary.com]

6 96 BOAKYE ET AL. FIGURE 5 TEM micrograph of Hi-Nicalon-S/yttrium-silicate/SiC composites showing the formation of a-y 2 Si 2 O 7 as the predominant phase at the fiber-matrix interface and the presence of a thin silica film at the fiber-coating interface. Fibers in composites were preoxidized for 20 hours prior to YPO 4 coating upon the total cross-sectional area of the fibers within each minicomposite, it is instructive to compare it to the tow strength. Tow strengths (r t ) were calculated from the experimentally measured single filament tensile strength (r f ) using a dry bundle failure model modified to include slack in filaments in the tows: r t ¼ f s r f ðemþ 1=m (2) where m is the measured Weibull modulus of single filaments for each fiber tow (typically about six), f s (0.75) is a factor that accounts for unknown effects with the slack in filaments playing a major role. The factor, f s, was empirically derived using extensive experimental data in our prior work and is consistent in predicting tow strength values from filament strength. 41 Compared to the calculated tow strength of coated fibers, the minicomposite strengths were lower than expected (30%-50% of the tow strength). The lower strength of the minicomposites is most likely due to a combination of fiber degradation during composite processing, filament misalignment within the tow or incomplete load sharing. A similar result was observed for minicomposites formed with Nextel 720 fibers having a monazite interphase and a Blackglas TM matrix. In this case, the asreceived tow strength was 740 MPa, while the minicomposite strength dropped to 350 MPa, representing a strength drop of ~50%. 44, Minicomposite fractography The effect of the Y 2 Si 2 O 7 interphase on crack deflection and fiber pullout was investigated, using SEM fractography. Minicomposites formed with no Y 2 Si 2 O 7 interphase were used as control samples. SEM micrographs of minicomposites after tensile testing are shown in Figure 7. The fracture surface of the control minicomposite showed no indication of crack deflection at the fiber/matrix interface and no fiber pullout (Figure 7A,B). The flat fracture surface suggests brittle failure. Figures 7C-F show

7 BOAKYE ET AL. 97 FIGURE 6 TEM EDS maps of Hi-Nicalon-S/Y 2 Si 2 O 7 /SiC composites showing the formation of a-y 2 Si 2 O 7 as the predominant phase formed at the fiber-matrix interface and the presence of a thin silica film at the fiber-coating interface. Fibers in composites were preoxidized for 20 hours prior to YPO 4 coating [Color figure can be viewed at wileyonlinelibrary.com] TABLE 2 Strength of Hi-Nicalon-S reinforced SiC minicomposites with Y 2 Si 2 O 7 interface coatings Minicomposite strength (MPa) Calculated tow strength (MPa) Control 0.5 h 20 h 0.5 h 20 h Minicomposites with no fiber coatings served as a control. Fiber tow strength is given for reference. Uncertainty limits constitute one standard deviation of 5-8 measurements. fracture surfaces of minicomposites formed with Y 2 Si 2 O 7 fiber coatings and fibers preoxidized for 0.5 hours and 20 hours. Crack deflection at the fiber-matrix interface and fiber pullout was seen for all composites with fiber coatings. However, the extent of fiber pullout was not the same for all of the minicomposites. Typically, longer pullout lengths of ~5-30 lm were observed for composites formed with fibers preoxidized for 0.5 hours while only ~2-10 lm pullout lengths were seen for composites formed with fibers preoxidized for 20 hours. TEM characterization of the minicomposites found a thin layer of carbon, ~2-5 nm, at the fiber-coating interface for composites formed with fibers preoxidized for 0.5 hours (Figure 4). This suggests that the longer pullout for composites formed with fibers preoxidized for 0.5 hours may be due to the carbon at the fiber-coating interface, which enhances crack deflection and improves sliding (Figure 7). In tested samples, debonding was observed to occur at the fiber-coating interface and within the coating for composites formed with fibers preoxidized for both 0.5 hours and 20 hours. However, this debonding was more localized at the fiber-coating interface for composites formed with

8 98 BOAKYE ET AL. (A) (B) (C) (D) (E) (F) FIGURE 7 SEM micrographs of fracture surfaces of SiC/SiC minicomposites: (A, B) control minicomposites showing complete lack of matrix crack deflection at the fiber-matrix interfaces and (C-F) minicomposites with Y 2 Si 2 O 7 -coated fibers preoxidized for 0.5 hours (C, D) and 20 hours (E, F) showing crack deflection and fiber pullout. Longer pullout lengths were observed for composites formed with fibers preoxidized for 0.5 hours fibers preoxidized for 0.5 hours. SEM observation showed trough formation in the matrix after fiber pullout (Figure 8). SEM imaging of matrix troughs revealed striations of Y 2 Si 2 O 7, which is indicative of plastic deformation during fiber pullout. No evidence of plastic deformation at the fiber surface was observed. Similar observations were made for SCS-0 fibers pushed out from an Y 2 Si 2 O 7 matrix where evidence of extensive deformation in the Y 2 Si 2 O 7 matrix adjacent to the SCS-0 fiber was seen 16 and for LaPO 4 fiber-coating-alumina matrix interfaces, 13,23 where plastic deformation was concentrated in a band of LaPO 4 several hundred nanometers in width adjacent to fibers. 3.5 Interface parameters from fiber push-in data The results of fiber push-in tests are plotted as load vs displacement in Figure 9. The experimentally recorded displacement has been corrected by subtracting the contribution from the compliance of the test setup. Each of the data in the plots are fitted to the model of Parthasarathy et al (Figure 9D) 39,40,46 From the best fits, the progressive roughness model 39 was used to extract the interface debond energy Г i, friction coefficient l, sliding stress at the interface, and interfacial roughness amplitude (h) and period (2d). The best-fit parameters obtained for the composites formed with Y 2 Si 2 O 7 -coated fibers are shown in Table 3. The Y 2 Si 2 O 7 -coated fibers could be pushed (or displaced) and showed nonlinearity at loads below ~0.45 N (Figure 9A,B). Uncoated fibers in control minicomposites could not be displaced, and showed no nonlinearity until very high loads, ~0.9 N (Figure 9C), where the fiber itself began to crack (Figure 10). Minicomposites with Y 2 Si 2 O 7 - coated fibers had reasonably low average interfacial debond energies and average sliding stresses, although there was high variability between individual fibers.

9 BOAKYE ET AL. 99 FIGURE 8 SEM micrographs of minicomposite after tensile testing showing fiber pullout and the presence of trough. Note the striations in the matrix trough, indicative of plastic deformation similar to observation made for monazite [Color figure can be viewed at wileyonlinelibrary.com] The high variability in the debond energy and sliding stress between different fibers may be due to a number of factors. First, the push-in tests are highly sensitive to even small deviations of the tested fibers from exact axial orientation, 16 which is difficult to control and assure in minicomposites containing tows of small diameter fibers. Other factors potentially contributing to the high variability in the test results include potentially incomplete coating coverage of individual filaments and the possible presence of fibers partially debonded prior to the fiber push-in test. There were no significant differences in the interfacial roughness and friction coefficients for minicomposites with fibers preoxidized for 0.5 hours and 20 hours (Table 3) despite the noticeable difference in pullout lengths, Figure 7. The roughness amplitude of ~30 nm is most likely inherited from the roughness of the fiber and sliding occurring at the fiber-coating interface. In the case where debonding occurred within the coating, the roughness is more likely to be due to the grain size of the coating. The difference in the roughness period may be related to the difference in the original YPO 4 -coating thickness, Figure 3. The average roughness period was nm and nm for fibers preoxidized for 0.5 hours and 20 hours, respectively. The average sliding stress at the fiber-matrix interface for composites formed with coated fibers that were preoxidized for 0.5 hours and 20 hours was about the same, MPa and MPa, respectively. However the scatter in the data was high (see reasons given above) making it difficult to make accurate conclusions on the effect of carbon on the sliding stress for composites formed with fibers preoxidized for 0.5 hours. 4 DISCUSSION The results presented can be summarized by two major observations that suggest that Y 2 Si 2 O 7 coatings performed mechanically as a weak interphase in SiC/SiC minicomposites: (1). Composites formed with Y 2 Si 2 O 7 fiber coatings showed fiber pullout (Figure 7C-F) and had significantly higher strengths than the control sample (Table 2), which had brittle failure with flat fracture surfaces with neither crack deflection nor fiber pullout (Figure 7A,B). (2). Push-in tests confirmed that fibers in composites with Y 2 Si 2 O 7 -coated fibers pushed, whereas fibers in composites with no Y 2 Si 2 O 7 interphase did not (Figure 10). The possibility of debonding at the fiber-coating interface depends on the critical value of the ratio ᴦ c /ᴦ f, where ᴦ c is the interfacial debond energy and ᴦ f is the fracture energy of the fiber. Using the elastic mismatch (a) of 0.44 from our prior work, 16 the critical values for ᴦ c /ᴦ f calculated by He and Hutchinson, plotted as a function of elastic mismatch, were obtained from reference. 47 The ᴦ c /ᴦ f value corresponding to a = 0.44 is 0.4. The criteria for crack deflection at the fiber-coating interface is 48 C c =C f \0:4: (3) From our prior work, the fracture energy of the fiber was 22.5 J/m The condition for crack arrest at the fibercoating interface and debonding is as follows:

10 100 BOAKYE ET AL. FIGURE 9 Fiber push-in curves for SiC/SiC minicomposites formed with and without Y 2 Si 2 O 7 interfaces. Composites with Y 2 Si 2 O 7 interfaces were formed with fibers preoxidized for: (A) 0.5 hours, and (B) 20 hours. Control (C) composites formed without Y 2 Si 2 O 7 interface showed no nonlinearity until very high loads, where the fiber itself began to crack. (D) An example of push-in data from this work, analyzed using the progressive roughness model. 39 The solid line is a fit to the data from which the interface parameters were deduced [Color figure can be viewed at wileyonlinelibrary.com] TABLE 3 Interface properties measured from push-in experiments Preoxidation time, h Roughness amp, h (nm) Roughness period, 2d (nm) Г (J/m 2 ) Friction coefficient, l Sliding stress (MPa) ᴦ Re /ᴦ f (<0.4) C c \C f a or C c \9 J/m 2 (4) The debond energies for the minicomposites are listed in Table 3. The average debond energy of J/m 2 for the minicomposites formed with Y 2 Si 2 O 7 -coated fibers is below the critical value of 9 J/m 2, suggesting debonding and fiber pullout as expected, and as indeed observed in Figures 7, 8 and 10. Due to compliance of the sample tab and potential slippage in the grips, strain measurements were not obtained. Full composites will be made in the near future and tested to measure strain to failure and also to study oxidation behavior. 5 CONCLUSIONS The results of this work confirm the efficacy of yttrium disilicate as a weak interface in SiC/SiC composites. Preoxidized Hi-Nicalon-S fibers were coated with YPO 4 and infiltrated with a slurry consisting of a SiC forming preceramic polymer (SMP-10) and <1 lm SiC particles to form SiC/SiC minicomposites. Curing of the minicomposites was performed in argon at 1200 C for a total time of 10 hours to densify the matrix and to convert the SiO 2 /YPO 4 coatings to yttrium disilicate. TEM observations showed a dense layer of a-y 2 Si 2 O 7 formed near the fiber surface, along with a porous outer layer (~500 nm

11 BOAKYE ET AL. 101 FIGURE 10 SEM micrographs of push-in SiC/SiC composites. Composites with Y 2 Si 2 O 7 interfaces pushed whereas the control with no Y 2 Si 2 O 7 interphase did not. Attempts to push the control at very high loads caused the fiber to crack thick) infiltrated with SiC. For fibers preoxidized for 0.5 hours, a thin carbon layer was present between the fiber and coating, while for fibers preoxidized for 20 hours, a thin silica layer was present. In both cases the Y 2 Si 2 O 7 grain size was ~50 nm and the morphology was equiaxed. Tension tests were performed on individual fibers and minicomposites where the individually tested fibers and minicomposites experienced the same thermal processing history. Although the average tensile strength of the coated fibers was lower than the uncoated ones, the average tensile strength of the minicomposites made with the coated fibers was 2-3 times higher than control minicomposites formed with uncoated fibers. SEM fractography showed neither crack deflection nor fiber pullout for minicomposites with uncoated fibers, while crack deflection and fiber pullout were observed at the interface between the a-y 2 Si 2 O 7 coating and the fiber. SEM observation of matrix troughs after fiber pullout showed striations in the a-y 2 Si 2 O 7 coating, which is indicative of plastic deformation during fiber push-out. ACKNOWLEDGMENTS This work was completed under Air Force Contracts #: FA D-5226 and FA D ORCID Emmanuel E. Boakye Triplicane A. Parthasarathy Randall S. Hay REFERENCES 1. Kerans RJ, Hay RS, Parthasarathy TA, et al. Interface design for oxidation resistant ceramic composites. J Am Ceram Soc. 2002;85: Shi J. CMC-Design A Brite Euram Program to Develop Design Methodologies for CMC Components for Gas Turbine Engines. In: Roode MV, Ferber MK, Richerson DW, eds. Ceramic Gas Turbine Design and Test Experience: Progress in Ceramic Gas Turbine Development, vol. 1. New York: ASME Press; 2002: Rebillat F, Lamon J, Naslain R, et al. Properties of multilayered interphases in SiC/SiC chemical-vapor-infiltrated composites with weak and strong interfaces. J Am Ceram Soc. 1998;81: Sun EY, Nutt SR, Brennan JJ. Interfacial microstructure and chemistry of SiC/BN dual-coated nicalon-fiber-reinforced glass-ceramic matrix composites. J Am Ceram Soc. 1994;77: Sun EY, Lin H-T, Brennan JJ. Intermediate-temperature environmental effects on boron nitride-coated silicon carbide-fiber-reinforced glass-ceramic composites. J Am Ceram Soc. 1997;80: Brennan JJ. Interfacial characterization of a slurry-cast melt-infiltrated SiC/SiC ceramic-matrix composite. Acta Mater. 2000;48: Sun EY, Nutt SR, Brennan JJ. Flexural creep of coated SiC-fiberreinforced glass-ceramic composites. J Am Ceram Soc. 1995;78: Sun EY, Nutt SR, Brennan JJ. High-temperature tensile behavior of a boron nitride-coated silicon carbide-fiber glass-ceramic composite. J Am Ceram Soc. 1996;79: Walukas DM. A Study of the Mechanical Properties and Oxidation of Nicalon SiC Composites with Sol-gel derived Oxide Interfacial Coatings. M.S. Thesis, The University of Tennessee; Luthra KL. Oxidation-resistant fiber coatings for non-oxide ceramic composites. J Am Ceram Soc. 1997;80: Shanmugham S, Stinton DP, Rebillat F, et al. Oxidation-resistant interfacial coatings for continuous fiber ceramic composites. Ceram Eng Sci Proc. 1995;16: Lee WY, Lara-Curzio E, More KL. Multilayered oxide interphase concept for ceramic-matrix composites. J Am Ceram Soc. 1998;81: Keller KA, Mah T, Parthasarathy TA, et al. Effectiveness of monazite coatings in oxide/oxide composites after long term exposure at high temperature. J Am Ceram Soc. 2003;86: Cinibulk MK, Fair GE, Kerans RJ. High-temperature stability of lanthanum orthophosphate (monazite) on silicon carbide at low oxygen partial pressures. J Am Ceram Soc. 2008;91:

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Evaluation of SiC/SiC minicomposites with yttrium disilicate fiber coating. J Am Ceram Soc. 2018;101: