J Am Cera Sac, 83 [7 1727-38(2000) urna Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F110 Turbofan Engine James M. stahl Air Force Research Laboratory, Metals, Ceramics, and NDE Division, Ceramics Development and Materials Behavior Branch, Wright-Patterson Air Force Base, Ohio 45433-7817 Four ceramic-matrix composite flap inserts were evaluated following ground testing on a General Electric F110 turbofan engine. Three of the composites accumulated -117 h of engine time. The fourth composite, a Nextel M 720 material with aluminosilicate matrix, accumulated 40 h. Large through- thickness cracks develop ed along the longitudinal edges of Nicalon /AL,O3 insert and the Nextel 720/aluminosilicate insert. The cracks developed because of high tensile stresses caused by the steep in-plane thermal gradients induced across he flap width during afterburner lights. The Nextel 720/ aluminosilicate insert also exhibited severe surface wear asso- ciated with the acoustic environment and contact with the adjacent divergent seals. Neither a Nicalon/silicon nitrocarbide insert nor a Nicalon/C insert exhibited significant signs of L. Introduction Exhaust Gases O R the past 5 to 10 years, ceramic-matrix composites(CMCs) e undergone testing in a number of military turbine engine applications. Some of the most extensive in-flight experience with CMCs has come from U.S. Navy efforts with Nicalon/C divergent flap and seal inserts for the afterburner(AB)of the General Electric F414 turbofan engine. Two such engines power the F/A-18E/F Super Hornet , In a recent Defense Advanced Navy program, four CMCs were considered for ich parallels the Seal flap flowpath elements on the ABs of General Electric F110 turbofan engines. These engines power the F16 fighter used by the U.S. Air Force and numerous other countries The AB for the F110 is comprised of a forward augmente section and a trailing variable exhaust nozzle The nozzle includes 12 divergent flaps and an equal number of divergent seals arranged an axisymmetric design which actuates in unison to change the ize of the exhaust opening. Depending on the size of the the width of the central region of the flap exposed at ar time to the hot flowpath gases can vary. When the nozzle open, the majority of the flap's surface will be exposed to hot exhaust gases as shown pictorially in Fig. 1(a). Conversely, when the nozzle is closed, as in Fig. 1(b), a comparatively narrow central strip will be When the AB is lit, a large amount of raw fuel is injected directly into the augmenter section with the combustion products expelled through the nozzle. The AB is necessary for certain flight and Exposition of the American Ceramic Society, Cocoa Beach, FL, January 15, 1997 1. Schematics of the nozzle aft looking forward, showing the overlapping of the flaps and seals in the(a) fully opened and(b) fully Systran Federal Corporation, Dayton, Ohio 45432-3068
Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts Following Ground Testing on an F110 Turbofan Engine James M. Staehler* ,† and Larry P. Zawada* Air Force Research Laboratory, Metals, Ceramics, and NDE Division, Ceramics Development and Materials Behavior Branch, Wright-Patterson Air Force Base, Ohio 45433–7817 Four ceramic-matrix composite flap inserts were evaluated following ground testing on a General Electric F110 turbofan engine. Three of the composites accumulated ;117 h of engine time. The fourth composite, a NextelTM 720 material with aluminosilicate matrix, accumulated ;40 h. Large throughthickness cracks developed along the longitudinal edges of a NicalonTM/Al2O3 insert and the Nextel 720/aluminosilicate insert. The cracks developed because of high tensile stresses caused by the steep in-plane thermal gradients induced across the flap width during afterburner lights. The Nextel 720/ aluminosilicate insert also exhibited severe surface wear associated with the acoustic environment and contact with the adjacent divergent seals. Neither a Nicalon/silicon nitrocarbide insert nor a Nicalon/C insert exhibited significant signs of distress. I. Introduction OVER the past 5 to 10 years, ceramic-matrix composites (CMCs) have undergone testing in a number of military turbine engine applications.1–3 Some of the most extensive in-flight experience with CMCs has come from U.S. Navy efforts with NicalonTM/C divergent flap and seal inserts for the afterburner (AB) of the General Electric F414 turbofan engine. Two such engines power the F/A-18E/F Super Hornet.4,5 In a recent Defense Advanced Research Project Agency (DARPA) initiative which parallels the Navy program, four CMCs were considered for use as divergent flap flowpath elements on the ABs of General Electric F110 turbofan engines. These engines power the F16 fighter used by the U.S. Air Force and numerous other countries. The AB for the F110 is comprised of a forward augmenter section and a trailing variable exhaust nozzle. The nozzle includes 12 divergent flaps and an equal number of divergent seals arranged in an axisymmetric design which actuates in unison to change the size of the exhaust opening. Depending on the size of the opening, the width of the central region of the flap exposed at any given time to the hot flowpath gases can vary. When the nozzle is fully open, the majority of the flap’s surface will be exposed to hot exhaust gases as shown pictorially in Fig. 1(a). Conversely, when the nozzle is closed, as in Fig. 1(b), a comparatively narrow central strip will be exposed. When the AB is lit, a large amount of raw fuel is injected directly into the augmenter section with the combustion products expelled through the nozzle. The AB is necessary for certain flight R. J. Kerans—contributing editor Manuscript No. 189551. Received February 16, 1999; approved December 9, 1999. Portions of this paper were presented at the 21st Annual Cocoa Beach Conference and Exposition of the American Ceramic Society, Cocoa Beach, FL, January 15, 1997 (Engineering Ceramics Division). *Member, American Ceramic Society. † Systran Federal Corporation, Dayton, Ohio 45432–3068. Fig. 1. Schematics of the nozzle, aft looking forward, showing the overlapping of the flaps and seals in the (a) fully opened and (b) fully closed positions. J. Am. Ceram. Soc., 83 [7] 1727–38 (2000) 1727 journal
1728 Journal of the American Ceramic Sociery--Staehler and zawada maneuvers and to achieve supersonic flight. Because of the nature CMCs were considered in conjunction with a modified flap of the Ab, the materials of the nozzle must be able to withstand design wherein removable flowpath elements or inserts were used severe temperatures, rapid heat-ups, through-thickness and in- Each divergent flap consisted of a flat CMC insert which slid into plane thermal gradients, and acoustic loads. The relative motion of a"picture-frame"backbone structure. The new flap assembly was the flaps and seals, combined with the acoustic environment of the designed to be interchangeable with the current metal flap hard- engine, can lead to wear issues. The partial shielding by the seals ware. The intent was to improve the longevity of the flap as well so gives rise to in-plane thermal gradients across the flap inserts as provide for ease of replacement. The CMC flap inserts were which can in turn lead to high tensile stresses along the edges of tapered along their 530 mm length with the leading edge 118 the CMC flap inserts. Under extended AB lights, the maximum mm wide and the trailing edge about 140 mm. The thickness of temperatures of the exposed flap and seal surfaces can exceed 1000°C mm wide band along the longitudinal and leading edges where the The current flaps and seals on the F110 nozzle are fabricated of flowpath side was surface ground to a thickness of 1. 4-1.5 m Rene 41, a nickel-based superalloy. However, the combination of When installed, this step made the flowpath surface of high temperatures and thermal cycling on the nozzle leads to creep approximately flush with the top retaining rail of the picture cracking of the metal parts. At present, roughly 10% of the Rene General Electric on the Navy F414-GE-400 engle s that deformation which in turn causes severe warping and eventual backing structure. The basic design was similar to aps and seals must be removed or repaired after only about a third of their intended design life. As a consequence, they are a on an F110 engine by General Electric Aircraft Engines, Evendale high-maintenance, high-cost item for the Air Force. Figure 2 OH. During ground testing, the engine was run through several lows a Rene 41 flap and seal which were removed from an accelerated mission test(AMT)cycles. The purpose of the AMT ngine. Both warping and surface cracking are clearly evident. The to subject the(ground) engine to the cyclic demands of a fielded tensile response of Rene 41 at 23, 10000, and 1100C was engine but within a condensed period of time. The AMT for the measured by Finch and Zawada. The metal exhibits higher F110-powered F16 fighter is comprised of seven different cycles ultimate tensile strengths below 1000.C. However, the strength intended to simulate maneuvers such as air-to-ground gunnery and advantage that the metal holds over CMCs at low temperatures air combat. Each cycle is comprised of multiple throttle settings disappears above 1000C. The nozzle will see temperatures and specifically defines the duration and level of those throttle between 1000 and 1100 C. The metal shows significantly more settings. Many of these cycles include one or more AB lights with inelastic deformation at all temperatures compared with the CMCs durations ranging from a few seconds to a few minutes depending before failure on the simulation. The throttle settings for the cycles are derived The big advantage of the CMCs compared with Rene 41 from data recorded during F16 field and training exercises. Each of becomes apparent in creep rupture at 1000@ and 1100@C. Through the cycles to which the CMC flap inserts were subjected spanned this temperature range the CMCs are significantly more creep a period of roughly 35 min and included two AB lights. The resistant and can sustain a given static load longer before failure. desired design life for the CMc inserts was 2000 engine flight These are critical temperatures for the F110 flaps and seals. The hours. The "equivalent" AMT time based on the accumulation of clear-cut although two oxide/oxide composites stand out as better h with 5177 AB lights for a total time under AB conditions of 36 than the rest h In the field, 2000 engine flight hours might take 3 to 5 years to acquire. By comparison, a significant amount of engine time can be accumulated in a relatively short period through AMT ground Three of the four CMCs under consideration accumulated -408 AB lights or -117 h of engine time on two different engines These included Nicalon/C. Nicalon/AlO. and Nicalon/silicon nitrocarbide (SINC)composite inserts placed in positions DFI DF3, and DF4, respectively, on the nozzle. Figure I identifies the various flap and seal locations. The fourth CMC flap insert,a Nextel 720 with aluminosilicate(AS)matrix, was added latter and ccumulated -146 AB lights or 40 h of engine time in position DFS. At about the same time that the fourth CMC flap insert was added, four CMC seal inserts made of Nextel 610/AS were placed on the engine in locations DSl through DS4. The three nicalon fiber(Nippon Carbon Co., Tokyo, Japan) containing flap inserts accumulated roughly 10%11% of the intended design life whereas the insert with Nextel 720 fibers(3M Co., St. Paul, MN between 3% and 4% The purpose of this paper is to present the post-engine test (b) evaluations made of the CMC flap inserts. This includes the results from visual examinations. material characterization. and residual strength tension tests. While Nextel 610/AS seal flowpath ele- ments were engine tested, they were not subjected to the same post-test analysis as the flap inserts and, therefore, are not I. Materials The four CMCs considered for the F110 flap of matrixes and processing routes. Three of the Nicalon fibers which consist of submicromete in an ne 41 (a)flap and(b)seal currently amorphous mixture of silicon, carbon, and oxyg fourth F110 engine. lated damage includes surface used Nextel 720 fibers which are a mixture of a-alumina and crumpling, cracking, and the sever which is apparent in the seal mullite. The suppliers of the Nicalon-based CMCs were Dow
maneuvers and to achieve supersonic flight. Because of the nature of the AB, the materials of the nozzle must be able to withstand severe temperatures, rapid heat-ups, through-thickness and inplane thermal gradients, and acoustic loads. The relative motion of the flaps and seals, combined with the acoustic environment of the engine, can lead to wear issues. The partial shielding by the seals also gives rise to in-plane thermal gradients across the flap inserts which can in turn lead to high tensile stresses along the edges of the CMC flap inserts.6 Under extended AB lights, the maximum temperatures of the exposed flap and seal surfaces can exceed 1000°C. The current flaps and seals on the F110 nozzle are fabricated of Rene´ 41, a nickel-based superalloy. However, the combination of high temperatures and thermal cycling on the nozzle leads to creep deformation which in turn causes severe warping and eventual cracking of the metal parts. At present, roughly 10% of the Rene´ flaps and seals must be removed or repaired after only about a third of their intended design life. As a consequence, they are a high-maintenance, high-cost item for the Air Force. Figure 2 shows a Rene´ 41 flap and seal which were removed from an engine. Both warping and surface cracking are clearly evident. The tensile response of Rene´ 41 at 23°, 1000°, and 1100°C was measured by Finch and Zawada.7 The metal exhibits higher ultimate tensile strengths below 1000°C. However, the strength advantage that the metal holds over CMCs at low temperatures disappears above 1000°C. The nozzle will see temperatures between 1000° and 1100°C. The metal shows significantly more inelastic deformation at all temperatures compared with the CMCs before failure. The big advantage of the CMCs compared with Rene´ 41 becomes apparent in creep rupture at 1000° and 1100°C.8 Through this temperature range the CMCs are significantly more creep resistant and can sustain a given static load longer before failure. These are critical temperatures for the F110 flaps and seals. The fatigue performance of the CMCs and Rene´ 41 at 1000°C is less clear-cut although two oxide/oxide composites stand out as better than the rest. CMCs were considered in conjunction with a modified flap design wherein removable flowpath elements or inserts were used. Each divergent flap consisted of a flat CMC insert which slid into a “picture-frame” backbone structure. The new flap assembly was designed to be interchangeable with the current metal flap hardware. The intent was to improve the longevity of the flap as well as provide for ease of replacement. The CMC flap inserts were tapered along their 530 mm length with the leading edge ;118 mm wide and the trailing edge about 140 mm. The thickness of each was fairly uniform (nominally 2.00–2.25 mm) except for a 10 mm wide band along the longitudinal and leading edges where the flowpath side was surface ground to a thickness of 1.4–1.5 mm. When installed, this step made the flowpath surface of the insert approximately flush with the top retaining rail of the picture-frame backing structure. The basic design was similar to that used by General Electric on the Navy F414-GE-400 engine.4 The CMC flap inserts described in this paper were ground tested on an F110 engine by General Electric Aircraft Engines, Evendale, OH. During ground testing, the engine was run through several accelerated mission test (AMT) cycles. The purpose of the AMT is to subject the (ground) engine to the cyclic demands of a fielded engine but within a condensed period of time. The AMT for the F110-powered F16 fighter is comprised of seven different cycles intended to simulate maneuvers such as air-to-ground gunnery and air combat. Each cycle is comprised of multiple throttle settings and specifically defines the duration and level of those throttle settings. Many of these cycles include one or more AB lights with durations ranging from a few seconds to a few minutes depending on the simulation. The throttle settings for the cycles are derived from data recorded during F16 field and training exercises. Each of the cycles to which the CMC flap inserts were subjected spanned a period of roughly 35 min and included two AB lights. The desired design life for the CMC inserts was 2000 engine flight hours. The “equivalent” AMT time based on the accumulation of a comparable number of thermal cycles on the engine was ;1050 h with 5177 AB lights for a total time under AB conditions of ;36 h. In the field, 2000 engine flight hours might take 3 to 5 years to acquire. By comparison, a significant amount of engine time can be accumulated in a relatively short period through AMT ground testing. Three of the four CMCs under consideration accumulated ;408 AB lights or ;117 h of engine time on two different engines. These included Nicalon/C, Nicalon/Al2O3, and Nicalon/silicon nitrocarbide (SiNC) composite inserts placed in positions DF1, DF3, and DF4, respectively, on the nozzle. Figure 1 identifies the various flap and seal locations. The fourth CMC flap insert, a Nextel 720 with aluminosilicate (AS) matrix, was added latter and accumulated ;146 AB lights or ;40 h of engine time in position DF5. At about the same time that the fourth CMC flap insert was added, four CMC seal inserts made of Nextel 610/AS were placed on the engine in locations DS1 through DS4. The three Nicalon fiber (Nippon Carbon Co., Tokyo, Japan) containing flap inserts accumulated roughly 10%–11% of the intended design life whereas the insert with Nextel 720 fibers (3M Co., St. Paul, MN) between 3% and 4%. The purpose of this paper is to present the post-engine test evaluations made of the CMC flap inserts. This includes the results from visual examinations, material characterization, and residual strength tension tests. While Nextel 610/AS seal flowpath elements were engine tested, they were not subjected to the same post-test analysis as the flap inserts and, therefore, are not discussed here. II. Materials The four CMCs considered for the F110 flap included a range of matrixes and processing routes. Three of the composites used Nicalon fibers which consist of submicrometer b-SiC in an amorphous mixture of silicon, carbon, and oxygen. The fourth used Nextel 720 fibers which are a mixture of a-alumina and mullite. The suppliers of the Nicalon-based CMCs were Dow Fig. 2. Photographs of a Rene´ 41 divergent (a) flap and (b) seal currently used on the F110 engine. Typical service related damage includes surface crumpling, cracking, and the severe warping which is apparent in the seal. 1728 Journal of the American Ceramic Society—Staehler and Zawada Vol. 83, No. 7
Corning(Nicalon/SINC, trade name Sylramic S-200), Hitco material properties for comparison with the DC insert. This panel Technologies(Nicalon/C, trade name CeracarbMSC537EH), and was made after the DC flap insert had completed ground testing DuPont Lanxide(Nicalon/Al2O3). The Nextel 720 composite had and as such was not true witness material n aluminosilicate matrix and was fabricated by General Electric A second HT Fllo flap insert which never saw engine time was (Nextel 720/AS). Irrespective of the type of fiber, balanced used as"witness"material for this composite. A corner of the eight-harness-satin-weave(&HSW) cloths were used for the lay leading edge of this insert had been damaged during im ps In each CMC, the plies ran the full length and width of the installation on the nozzle and was never engine tested. The insert except where grinding removed surface material from along specimens used for witness purposes were cut well away from the the longitudinal and leading edges to accommodate the picture- damaged region and the test results were not affected by it. frame backing structure. Table I is a brief summary of each DuPont Lanxide made two F110 inserts of the DL compos omposite, to include compositions and processing routes. Some They performed a limited number of tension tests on witness of the details of processing and matrix fillers were omitted for specimens machined from the two panels used to fabricate the actual inserts. In one panel the room-temperature tensile strength The Nicalon 8HSW plies in the Dow Corning(DC)composite was about 154 MPa while in the second only about 97 MPa. Both were warp-aligned with the length of the flap insert. However, were significantly lower than the 200 MPa measured on material each successive ply was rotated about the warp direction relative to evaluated earlier. 4 The lower strengths were associated with the previous one such that in one case the warp-face of the cloth damage to the Nicalon fibers during fabrication which the manu- was up and in the next the fill-face was up In the DL composite facturer believed was a result of inexperience making parts to the the plies were not warp-aligned but instead each successive ply size and specifications of the flap insert. was rotated in-plane such that in one case the warp direction coincided with the length of the insert while in the next the fill direction did. This alternating warp and fill orientation cont Ill. Experimental Procedure throughout the lay-up. Despite the obvious difference in dure, the final appearances of the two lay-up schemes General Electric performed infrared (IR)ten indistinguishable ments of the CMc inserts in conjunction with the ground testing The Hitco Technologies (HT) composite, Ceracarb The IR camera was positioned in the test cell near the nozzle and SC537EH, has a fairly compliant, inhibited carbon matrix. The captured the Ir signals looking through the engine exhaust plume microstructure of this material was examined in detail by bourrat Only two of the CMC inserts the camera's field of view at et al via TEM. This composite does not employ an engineered any given time(positions DF4 and df5 in Fig. 1). This necessi- interphase per se but does develop to some extent a pyrolytic tated the repositioning of the CMC inserts during an engine down carbon fiber coating during processing. The mechanical properties time to obtain the temperature information from the remaining two of this and similar composites have been studied in detail else materials. During this repositioning the D insert was broken into where- The matrix itself contains a large number of two pieces. To keep the time on each of the Nicalon-containing rocessing-related shrinkage cracks, some of which are later filled inserts the same, all three were removed from the engine. Despite in with pyrolytic carbon via chemical vapor infiltration. While its limited engine exposure, the Nextel 720/AS insert was showing inhibitors help to suppress the oxidation of the carbon matrix, the wear and for this reason was removed as well. The IR measure- composite utilizes an overcoat for additional protection. This ments pleted using new DL and HT flap inserts in coating was a dual-layer system consisting of a CVD applied Sic positions DF4 and DF5. For practical reasons related to the environmental barrier coating( Chromalloy RT42, 100-150 um positioning of the IR camera in the test cell, only the rear thick) followed by a spray-applied seal coat( Hitco Technologies two-thirds of the CMC inserts were visible M185 glaze coating, 75-150 Hm thick). The individual plies for Initial examinations of the engine-tested Cmc inserts included the HT lay-up were alternately warp and fill oriented with the visual and low-magnification optical microscope inspections. In length of the flap insert similar to the DL composite addition, each insert was analyzed via ultrasonic C-scans. Because The General Electric(GE)composite does not use an engi- of the potential susceptibility to moisture of some of the eered fiber-matrix interphase and has a relatively strong fiber- each insert was enclosed in a sealed vacuum bag for th matrix interface. Toughness is achieved through crack deflection of the scan. The scans verified that no delaminations were within its highly porous, relatively weak matrix. This porosity in any of the inserts consists of numerous shrinkage cracks from processing and exten- Following the nondestructive evaluations(NDEs), each flap sive microporosity which is evenly dispersed throughout the insert was machined into multiple straight-sided tensile specimens matrix. The lay-up scheme used for the ge composite was similar oriented as shown in Figs. 3(a-c). The specimens were nominally to that of the dc material 10 mm wide by 90 mm in length. Specimen thickness was left as-is To supplement data collected on the DC flap insert from the parent insert Fiberglass tabs 1.6 mm thick X 10 mm X run on the engine, ning provided a 154 mm square panel 30 mm with tapered forward edges were affixed to the ends of each with the same fib composition, and processing route. specimen with epoxy for gripping during testing Specimen gauge were used to collect as-processed sections in all cases were nominally 30 mm in length. Figures 3(b) Table L. Summary of Composite Compositions and Processing Fiber-matrix interphase Matrix Processing technique Overcoat(yes/no Nicalon &HSW 0.5 um bn by CVI SiNC fillers Polymer infiltration No Midland. MI Nicalon/ cloth SING Hitco HT Nicalon/C Nicalon 8HS W None applied Inhibited carbon Pyrolysis followed Yes(see text) by CvI of carbon Gardena, CA DuPont Lanxide. DL Nicalon 8HSW-0.5 um BN, 3-5 um Alumina 240 grit Direct metal Newark DE Nicalon/ Sic filler oxidation General Electric. GE Nexto :I 720 None applied 7 wt%Al,,O3, 13 Pyrolysis Evendale. OH 720/AS 8HSW cloth
Corning (Nicalon/SiNC, trade name SylramicTM S-200), Hitco Technologies (Nicalon/C, trade name CeracarbTM SC537EH), and DuPont Lanxide (Nicalon/Al2O3). The Nextel 720 composite had an aluminosilicate matrix and was fabricated by General Electric (Nextel 720/AS). Irrespective of the type of fiber, balanced eight-harness-satin-weave (8HSW) cloths were used for the layups. In each CMC, the plies ran the full length and width of the insert except where grinding removed surface material from along the longitudinal and leading edges to accommodate the pictureframe backing structure. Table I is a brief summary of each composite, to include compositions and processing routes. Some of the details of processing and matrix fillers were omitted for proprietary reasons. The Nicalon 8HSW plies in the Dow Corning (DC) composite were warp-aligned with the length of the flap insert. However, each successive ply was rotated about the warp direction relative to the previous one such that in one case the warp-face of the cloth was up and in the next the fill-face was up. In the DL composite the plies were not warp-aligned but instead each successive ply was rotated in-plane such that in one case the warp direction coincided with the length of the insert while in the next the fill direction did. This alternating warp and fill orientation continued throughout the lay-up. Despite the obvious difference in procedure, the final appearances of the two lay-up schemes were indistinguishable. The Hitco Technologies (HT) composite, CeracarbTM SC537EH, has a fairly compliant, inhibited carbon matrix. The microstructure of this material was examined in detail by Bourrat et al.9 via TEM. This composite does not employ an engineered interphase per se but does develop to some extent a pyrolytic carbon fiber coating during processing. The mechanical properties of this and similar composites have been studied in detail elsewhere.9–13 The matrix itself contains a large number of processing-related shrinkage cracks, some of which are later filled in with pyrolytic carbon via chemical vapor infiltration. While inhibitors help to suppress the oxidation of the carbon matrix, the composite utilizes an overcoat for additional protection. This coating was a dual-layer system consisting of a CVD applied SiC environmental barrier coating (Chromalloy RT42, 100–150 mm thick) followed by a spray-applied seal coat (Hitco Technologies M185 glaze coating, 75–150 mm thick). The individual plies for the HT lay-up were alternately warp and fill oriented with the length of the flap insert similar to the DL composite. The General Electric (GE) composite does not use an engineered fiber–matrix interphase and has a relatively strong fiber– matrix interface. Toughness is achieved through crack deflection within its highly porous, relatively weak matrix. This porosity consists of numerous shrinkage cracks from processing and extensive microporosity which is evenly dispersed throughout the matrix. The lay-up scheme used for the GE composite was similar to that of the DC material. To supplement the tensile data collected on the DC flap insert run on the engine, Dow Corning provided a 154 mm square panel with the same fiber lay-up, composition, and processing route. Specimens from this panel were used to collect as-processed material properties for comparison with the DC insert. This panel was made after the DC flap insert had completed ground testing and as such was not true witness material. A second HT F110 flap insert which never saw engine time was used as “witness” material for this composite. A corner of the leading edge of this insert had been damaged during improper installation on the nozzle and was never engine tested. The specimens used for witness purposes were cut well away from the damaged region and the test results were not affected by it. DuPont Lanxide made two F110 inserts of the DL composite. They performed a limited number of tension tests on witness specimens machined from the two panels used to fabricate the actual inserts. In one panel the room-temperature tensile strength was about 154 MPa while in the second only about 97 MPa. Both were significantly lower than the 200 MPa measured on material evaluated earlier.14 The lower strengths were associated with damage to the Nicalon fibers during fabrication which the manufacturer believed was a result of inexperience making parts to the size and specifications of the flap insert. III. Experimental Procedure General Electric performed infrared (IR) temperature measurements of the CMC inserts in conjunction with the ground testing. The IR camera was positioned in the test cell near the nozzle and captured the IR signals looking through the engine exhaust plume. Only two of the CMC inserts were in the camera’s field of view at any given time (positions DF4 and DF5 in Fig. 1). This necessitated the repositioning of the CMC inserts during an engine down time to obtain the temperature information from the remaining two materials. During this repositioning the DL insert was broken into two pieces. To keep the time on each of the Nicalon-containing inserts the same, all three were removed from the engine. Despite its limited engine exposure, the Nextel 720/AS insert was showing wear and for this reason was removed as well. The IR measurements were completed using new DL and HT flap inserts in positions DF4 and DF5. For practical reasons related to the positioning of the IR camera in the test cell, only the rear two-thirds of the CMC inserts were visible. Initial examinations of the engine-tested CMC inserts included visual and low-magnification optical microscope inspections. In addition, each insert was analyzed via ultrasonic C-scans. Because of the potential susceptibility to moisture of some of the CMCs, each insert was enclosed in a sealed vacuum bag for the duration of the scan. The scans verified that no delaminations were present in any of the inserts. Following the nondestructive evaluations (NDEs), each flap insert was machined into multiple straight-sided tensile specimens oriented as shown in Figs. 3(a–c). The specimens were nominally 10 mm wide by 90 mm in length. Specimen thickness was left as-is from the parent insert. Fiberglass tabs 1.6 mm thick 3 10 mm 3 30 mm with tapered forward edges were affixed to the ends of each specimen with epoxy for gripping during testing. Specimen gauge sections in all cases were nominally 30 mm in length. Figures 3(b) Table I. Summary of Composite Compositions and Processing Supplier Composite Fiber Fiber–matrix interphase Matrix Processing technique Overcoat (yes/no) Dow Corning, Midland, MI DC Nicalon/ SiNC Nicalon 8HSW cloth ;0.5 mm BN by CVI SiNC 1 fillers Polymer infiltration and pyrolysis No Hitco Technologies, Gardena, CA HT Nicalon/C Nicalon 8HSW cloth None applied Inhibited carbon Pyrolysis followed by CVI of carbon Yes (see text) DuPont Lanxide, Newark, DE DL Nicalon/ Al2O3 Nicalon 8HSW cloth ;0.5 mm BN, 3–5 mm SiC, by CVI Alumina 1 240 grit SiC filler Direct metal oxidation No General Electric, Evendale, OH GE Nextel 720/AS Nextel 720 8HSW cloth None applied 87 wt% Al2O3, 13 wt% SiO2 Pyrolysis No July 2000 Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts 1729
Journal of the American Ceramic Sociery--Staehler and zawada ol.83.No.7 Fig 3. Schematics showing the lay-out of straight-sided specimens cut from each of the inserts: (a)the DC and HT inserts, (b)the DL insert, and(c)the GE insert and(c) include the locations of edge macrocracks observed in the load, strain, and actuator displacement data were recorded digitally GE and DL composites as a result of engine testing. Two axially using a sampling rate of 20 Hz. Specimen thickness was normally oriented tensile specimens from both of these inserts did not the average of three digital caliper measurements taken along the survive the machining process due to known cracks. length within the gauge section, one at the center and two 8-10 Straight-sided tensile specimens were also cut from the 154 mmmm each side of center. Because of significant wear to the ge square as-processed panel provided by Dow Corning as well as the insert, many of those specimens did not have a uniform cross non-engine-tested HT witness insert. In the former case a total of section. In such cases, calipers were used to measure as accuratel seven warp-aligned and seven fill-aligned specimens were pre- possible the largest and smallest thickness dimensions at each of pared. Eighteen axially oriented (two groups of nine running the three locations along the length. The high and low values at across the width) and ten transversely oriented specimens were each location were averaged and the lowest average used prepared from the HT witness insert. conjunction with the uniformly machined specimen width to All tension tests were performed using an MTS servo-hydraulic calculate a"corrected"cross-sectional area. This corrected area ad frame. Specimens were face-loaded using MTS wedge grips was used in the derivation of all subsequent stresses. with serrated wedge inserts. A clip gauge (MTS Model 632. 26 Excess material from each flap insert following machining was B-30)with a 7.56 mm gauge length was used to measure strains. used for material characterization. This included density, porosity, All testing was conducted at room temperature in ambient air and fiber volume fraction measurements. Skeletal densities under stroke control using a displacement rate of 0.05 mm/s The measured with a helium pycnometer(Micromeritics AccuPyo Table Il. Material Physical Property Summary DC Nicalon/SiNC HT Nicalon/C DL Nicalon/Al O3 GE Nextel 720/AS Skeletal density(g/cm) 2.28 32①212
and (c) include the locations of edge macrocracks observed in the GE and DL composites as a result of engine testing. Two axially oriented tensile specimens from both of these inserts did not survive the machining process due to known cracks. Straight-sided tensile specimens were also cut from the 154 mm square as-processed panel provided by Dow Corning as well as the non-engine-tested HT witness insert. In the former case a total of seven warp-aligned and seven fill-aligned specimens were prepared. Eighteen axially oriented (two groups of nine running across the width) and ten transversely oriented specimens were prepared from the HT witness insert. All tension tests were performed using an MTS servo-hydraulic load frame. Specimens were face-loaded using MTS wedge grips with serrated wedge inserts. A clip gauge (MTS Model 632.26 B-30) with a 7.56 mm gauge length was used to measure strains. All testing was conducted at room temperature in ambient air under stroke control using a displacement rate of 0.05 mm/s. The load, strain, and actuator displacement data were recorded digitally using a sampling rate of 20 Hz. Specimen thickness was normally the average of three digital caliper measurements taken along the length within the gauge section, one at the center and two 8–10 mm each side of center. Because of significant wear to the GE insert, many of those specimens did not have a uniform cross section. In such cases, calipers were used to measure as accurately as possible the largest and smallest thickness dimensions at each of the three locations along the length. The high and low values at each location were averaged and the lowest average used in conjunction with the uniformly machined specimen width to calculate a “corrected” cross-sectional area. This corrected area was used in the derivation of all subsequent stresses. Excess material from each flap insert following machining was used for material characterization. This included density, porosity, and fiber volume fraction measurements. Skeletal densities were measured with a helium pycnometer (Micromeritics AccuPyc Table II. Material Physical Property Summary DC Nicalon/SiNC HT Nicalon/C DL Nicalon/Al2O3 GE Nextel 720/AS Skeletal density (g/cm3 ) 2.38 2.28 3.08 3.26 Vf (%) 38.3 38.2 (43.6)† 37.3 37.1 † Fiber volume fraction excluding volume associated with composite overcoat. Fig. 3. Schematics showing the lay-out of straight-sided specimens cut from each of the inserts: (a) the DC and HT inserts, (b) the DL insert, and (c) the GE insert. 1730 Journal of the American Ceramic Society—Staehler and Zawada Vol. 83, No. 7
Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts 1731 A B ig. 4. Surface temperature contour maps, inC, of the exposed portions of (A)the DC and()the Ge inserts derived from an infrared image taken 25-30 maximum AB light. The aft or trailing edge of each insert is to the right. The forward third of each insert was not visible outline of the GE temperature map was a consequence of the IR-camera position relative to the nozzle Model 1330). Fiber volume fractions were obtained from digital IV. Results image analyses of polished surfaces using Olympus CUE-4 Image Analyzer software(version 3.2), a video monitor(Sony Trinitron Figure 4 shows IR temperature maps of the dC and ge inserts Model PVM-1271Q) CCD video camera(Sony Galai Model recorded near the end of an AB light. Because of the size of the XC-57), and optical microscope(Leitz Metallovert Model 090- nozzle opening at the time of the image, approximately half of 124.012). To eliminate fiber pullout during preparat each inserts surface was masked by the adjacent seals. The highest ished surfaces were oriented 45 to both the 0 and 90 fiber temperatures in the dC insert, -8500875C, were recorded near directions. Table II summarizes the skeletal densities and fiber the contact regions with the adjacent seals. In contrast, the center volume fractions for each composite strip of the dC insert was nearly 200C cooler. This was due in 000k Est IR Data 000k 800 g 800 600 (No AB 400 DC F110 Flap IT F110 Flap calon/SINc Nicalon/C Normalized Location Across width Normalized Location Across width IR Data IR Data 800 DL F 200 GE F110 Flap 0.8 formalized Location Across width Fig. 5. Thermal profiles across the midsection of each CMC insert at maximum AB and at IRP based on Ir data and estimated edge temperatures: (a)DC, (b)HT,(c)DL, and (d)GE
Model 1330). Fiber volume fractions were obtained from digital image analyses of polished surfaces using Olympus CUE-4 Image Analyzer software (version 3.2), a video monitor (Sony Trinitron Model PVM-1271Q), CCD video camera (Sony Galai Model XC-57), and optical microscope (Leitz Metallovert Model 090– 124.012). To eliminate fiber pullout during preparation, the polished surfaces were oriented ;45° to both the 0° and 90° fiber directions. Table II summarizes the skeletal densities and fiber volume fractions for each composite. IV. Results Figure 4 shows IR temperature maps of the DC and GE inserts recorded near the end of an AB light. Because of the size of the nozzle opening at the time of the image, approximately half of each insert’s surface was masked by the adjacent seals. The highest temperatures in the DC insert, ;850°–875°C, were recorded near the contact regions with the adjacent seals. In contrast, the center strip of the DC insert was nearly 200°C cooler. This was due in Fig. 4. Surface temperature contour maps, in °C, of the exposed portions of (A) the DC and (B) the GE inserts derived from an infrared image taken 25–30 s into a maximum AB light. The aft or trailing edge of each insert is to the right. The forward third of each insert was not visible to the camera. The trapezoidal outline of the GE temperature map was a consequence of the IR-camera position relative to the nozzle. Fig. 5. Thermal profiles across the midsection of each CMC insert at maximum AB and at IRP based on IR data and estimated edge temperatures: (a) DC, (b) HT, (c) DL, and (d) GE. July 2000 Performance of Four Ceramic-Matrix Composite Divergent Flap Inserts 1731