E≈S Journal of the European Ceramic Society 20(2000)545-550 Damage tolerant oxide/oxide fiber laminate composites T. RadsickaB. SaruhanbH. Schneider, United States Air Force Research Laboratory, Wright-Patterson AFB, OH 45433-7562, US.A Institute for Materials Research, German Aerospace Center (DLR), Cologne, D-51147, Germany Accepted 10 August 1999 Abstract la order to achieve damage tolerant behavior. A fiber interface coating was not used. This technique enables damage toler- ance in materials with strong fiber-matrix bonding and under oxidizing conditions. Fabrication of composites was carried out through a slurry infiltration technique. Slurries for fiber(Nextel M 720, 3M)infiltration were prepared using a submicron a-Al, O3 owder coated with an amorphous SiOz-layer through a sol-gel process. Hot-pressing was used to densify and bond the laminate layers together, followed by pressureless heat-treatment to allow mullite to form. Room temperature three-point bending tests were performed on as-received samples and on samples which underwent long-term annealing at high temperatures(1200-1300"C)in Subsequent examination revealed that due to the lack of a fiber interface coating, matrix - infiltrated fiber layers behaved in a quasi- monolithic manner with little or no crack deflection. Layers of non-infiltrated fibers, however, provided damage tolerance by deflecting cracks in the plane of the laminate and by serving as a mechanical bond between matrix -infiltrated layers. The laminate composites demonstrate reasonable room-temperature fracture strength both in the as-received state(88 MPa) and after exposure to 1300oC air for 200 h(72 MPa)along with extensive fracture deflection through the layers of non- infiltrated fiber. Composite properties, specifically fracture strength and damage tolerance, can be tailored by varying lay-up and processing parameters such fiber-matrix ratio and type of fiber weave. C 2000 Elsevier Science Ltd. All rights reserved Keywords: Aluminosilicate fibers; Composites: Laminates: Mechanical properties: Mullite matrix 1. Introduction structures.Many other non-oxide coatings also can pro vide damage tolerance but all function at the expense of e In the most traditional sense, a fiber-reinforced cera- one of the most valued properties of oxide-oxide compo- nic-matrix composite consists of ceramic matrix mate- sites, namely oxidation resistance. Since ceramic matrix rial, fiber reinforcement and a tailored interface between composites are of particular interest for use as thermal the two. While the fiber and matrix determine for the protection tiles in the combustion chambers of jet engines, most part the strength of the material, without a sui- a sensitivity to oxidation is a distinct drawback to a mate table interface the possibility of achieving a reasonable rial with a non-oxide interface. Therefore, the develop- level of fracture toughness is limited. Such an interface ment of an all oxide-based composite system is a priority can be tailored through in-situ reactions between fiber There are some candidate oxide-based interface coat- and matrix, but normally an interface coating is depos- ings, but the selection is limited: due to high diffusion ited on the fiber before matrix infiltration. Regardless of rates of oxides and the associated reaction with fibers how it is applied, a weakened interface enables the and matrices, the deposition of such coatings on multiple necessary debonding and pullout mechanisms by filament yarns is often complicated and costly. For these deflecting cracks and dissipating crack energy Coatings reasons, it is worthwhile to explore other possible techni- such as carbon and boron nitride are quite suitable for ques to obtain damage tolerance in oxide-based ceramics this purpose because of their easy-to-cleave laminate Many studies have explored the use of novel construction techniques to achieve high strength and damage toler ance, including several papers investigating laminate con 68936. struction using monolithic and fiber-reinforced ceramic E-mail address: hartmut. schneider(@ dlr. de(H Schneider) layers.34 Of particular interest is the work of Tu et al.5 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(99)00252-6
Damage tolerant oxide/oxide ®ber laminate composites T. Radsicka , B. Saruhanb, H. Schneiderb,* a United States Air Force Research Laboratory, Wright-Patterson AFB, OH 45433-7562, USA bInstitute for Materials Research, German Aerospace Center (DLR), Cologne, D-51147, Germany Accepted 10 August 1999 Abstract Oxide-®ber/oxide-matrix composites were developed using non-in®ltrated woven ®ber layers between matrix-in®ltrated ®ber layers in order to achieve damage tolerant behavior. A ®ber interface coating was not used. This technique enables damage tolerance in materials with strong ®ber-matrix bonding and under oxidizing conditions. Fabrication of composites was carried out through a slurry in®ltration technique. Slurries for ®ber (NextelTM 720, 3M) in®ltration were prepared using a submicron a-Al2O3 powder coated with an amorphous SiO2-layer through a sol±gel process. Hot-pressing was used to densify and bond the laminate layers together, followed by pressureless heat-treatment to allow mullite to form. Room temperature three-point bending tests were performed on as-received samples and on samples which underwent long-term annealing at high temperatures (1200±1300C) in air. Subsequent examination revealed that due to the lack of a ®ber interface coating, matrix-in®ltrated ®ber layers behaved in a quasimonolithic manner with little or no crack de¯ection. Layers of non-in®ltrated ®bers, however, provided damage tolerance by de¯ecting cracks in the plane of the laminate and by serving as a mechanical bond between matrix-in®ltrated layers. The laminate composites demonstrate reasonable room-temperature fracture strength both in the as-received state (88 MPa) and after exposure to 1300C air for 200 h (72 MPa) along with extensive fracture de¯ection through the layers of non-in®ltrated ®ber. Composite properties, speci®cally fracture strength and damage tolerance, can be tailored by varying lay-up and processing parameters such as ®ber-matrix ratio and type of ®ber weave. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminosilicate ®bers; Composites; Laminates; Mechanical properties; Mullite matrix 1. Introduction In the most traditional sense, a ®ber-reinforced ceramic-matrix composite consists of ceramic matrix material, ®ber reinforcement and a tailored interface between the two. While the ®ber and matrix determine for the most part the strength of the material, without a suitable interface the possibility of achieving a reasonable level of fracture toughness is limited. Such an interface can be tailored through in-situ reactions between ®ber and matrix, but normally an interface coating is deposited on the ®ber before matrix in®ltration. Regardless of how it is applied, a weakened interface enables the necessary debonding and pullout mechanisms by de¯ecting cracks and dissipating crack energy. Coatings such as carbon and boron nitride are quite suitable for this purpose because of their easy-to-cleave laminate structures.1 Many other non-oxide coatings also can provide damage tolerance but all function at the expense of one of the most valued properties of oxide-oxide composites, namely oxidation resistance.1 Since ceramic matrix composites are of particular interest for use as thermal protection tiles in the combustion chambers of jet engines, a sensitivity to oxidation is a distinct drawback to a material with a non-oxide interface. Therefore, the development of an all oxide-based composite system is a priority. There are some candidate oxide-based interface coatings,2 but the selection is limited: due to high diusion rates of oxides and the associated reaction with ®bers and matrices, the deposition of such coatings on multiple ®lament yarns is often complicated and costly. For these reasons, it is worthwhile to explore other possible techniques to obtain damage tolerance in oxide-based ceramics. Many studies have explored the use of novel construction techniques to achieve high strength and damage tolerance, including several papers investigating laminate construction using monolithic and ®ber-reinforced ceramic layers.3,4 Of particular interest is the work of Tu et al.5 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(99)00252-6 Journal of the European Ceramic Society 20 (2000) 545±550 * Corresponding author. Tel.: +49-2203-601-2430; fax: +49-2203- 68936. E-mail address: hartmut.schneider@dlr.de (H. Schneider)
T. Radsick et al /Journal of the European Ceramic Society 20(2000)545-550 who demonstrated the concept of "H-cracking, in- microcomposite uses particles of a-alumina(AKP-50 plane crack deflection resulting from inter-laminar elastic Sumitomo Corp) which were coated with amorphous mismatches and residual stress. Also of interest is the silica through a sol-gel process, the hydrolysis of TEOS research involving porous laminate layers, again using The ratio of TEos to AKP-50 was selected so as to discontinuities in the material properties between laminate produce a final alumina/silica ratio slightly higher than layers to obtain crack deflection he stoichiometric composition of mullite(3Al2O3.2- SiO2)producing some excess a-Al2O3 after composite processing. Residual alumina was accepted in order to 2. Fabrication avoid the degradation of high-temperature properties caused by residual glassy silica This paper presents initial investigations of a concept Because the initial sintering characteristics of this based upon the use of non- infiltrated woven-fiber layers coated powder are dominated by the sintering properties in a laminate to achieve crack deflection, but with the of the outer silica coating, a compact, high-density solid added feature that the fiber layers provide a degree of can be achieved at temperatures below 1300C through crack bridging as well. In this composite, layers of transient liquid-phase sintering of SiOz. In the case of matrix-infiltrated fibers(hereafter referred to as"CMC reaction-sintered a-Al2O3 /SiO2 powder, however, mullite layers")are separated by non- infiltrated fiber cloth lay- normally begins to form in significant amounts only at ers(hereafter referred to as"fiber layers")and are hot- temperatures well above 1400C. It is known that com- fiber-laminate composites is outlined in Fig. /. process mercial alumina and aluminosilicate fibers show a severe degradation in properties due to the grain growth that occurs at these temperatures; therefore, other techniques 2.1. Synthesis of microcomposite Al2OSiOz powder need to be developed which allow mullite to form at lower temperatures. Mechnich et al. have shown that the These initial investigations of the fiber laminate com- addition of CeO2 as a sintering aid can significantly posite concept were conducted concurrently with the reduce the mullite formation temperature down to development of a mullite-based matrix material. This around 1300C. This reduction in processing tempera matrix material is based upon the microcomposite- ture enables the use of the current state-of-the-art poly- powder reaction-sintering technique developed by Sacks crystalline aluminosilicate fibers, albeit with some et al. 6 and advanced by Bartsch et al. 7 The Al,Ox-Sio reduction in fiber strength due to crystal growth and microstructural changes in the fibers at high tempera tures. o Improvements in the thermal stability of oxide- Coat alumina with silica based ceramic fibers would be an important step into the development of even stronger and more usable CMC 2. 2. Composite fabrication Preparation of slurry Slurries containing microcomposite AlOx-SiO2 po der (a-Al2O3 particles coated with non-crystalline Sio2) Infiltration of cmc layers were prepared with added CeOz and mullite precursor powder(Siral, Condea, Brunnsbuttel, Germany. )CeO and mullite precursors were added in order to accelerate Drying at room temperature sintering and to improve mullite formation. Powders were ball milled for 30 min Nextel 720(3M, Minnesota, USA) woven cloth(atlas eight-harness satin weave)was cut into squares approximately 50 by 50 mm. These squares were Stack CMc and fiber layers dipped into the slurry material, manually manipulated while immersed, allowed to absorb slurry, removed and allowed to dry. Altering the matrix solids content of the Hot press(15 MPa, 1250C) slurry along with repeated dipping allowed infiltrated fiber layers of varying matrix content and distribution to be produced. This simple dipping technique added a uniform and controlled amount of matrix to the fiber cloth -the Heat treatment(1300C, 2I matrix content among individual CMC layers(prepared from a given slurry mixture)varied less than 5% Fig. 1. Flowchart of processing steps used to fabricate fiber laminate Laminate pre-forms were prepared by stacking in an alternating fashion CMC layers and fiber layers, as
who demonstrated the concept of ``H-cracking'', inplane crack de¯ection resulting from inter-laminar elastic mismatches and residual stress. Also of interest is the research involving porous laminate layers, again using discontinuities in the material properties between laminate layers to obtain crack de¯ection. 2. Fabrication This paper presents initial investigations of a concept based upon the use of non-in®ltrated woven-®ber layers in a laminate to achieve crack de¯ection, but with the added feature that the ®ber layers provide a degree of crack bridging as well. In this composite, layers of matrix-in®ltrated ®bers (hereafter referred to as ``CMC layers'') are separated by non-in®ltrated ®ber cloth layers (hereafter referred to as ``®ber layers'') and are hotpressed together. The general method used to process ®ber-laminate composites is outlined in Fig. 1. 2.1. Synthesis of microcomposite Al2O3±SiO2 powder These initial investigations of the ®ber laminate composite concept were conducted concurrently with the development of a mullite-based matrix material. This matrix material is based upon the microcompositepowder reaction-sintering technique developed by Sacks et al.6 and advanced by Bartsch et al.7 The Al2O3±SiO2 microcomposite uses particles of a-alumina (AKP-50, Sumitomo Corp) which were coated with amorphous silica through a sol±gel process, the hydrolysis of TEOS. The ratio of TEOS to AKP-50 was selected so as to produce a ®nal alumina/silica ratio slightly higher than the stoichiometric composition of mullite (3Al2O3 .2- SiO2) producing some excess a-Al2O3 after composite processing. Residual alumina was accepted in order to avoid the degradation of high-temperature properties caused by residual glassy silica.8 Because the initial sintering characteristics of this coated powder are dominated by the sintering properties of the outer silica coating, a compact, high-density solid can be achieved at temperatures below 1300C through transient liquid-phase sintering of SiO2. In the case of reaction-sintered a-Al2O3/SiO2 powder, however, mullite normally begins to form in signi®cant amounts only at temperatures well above 1400C. It is known that commercial alumina and aluminosilicate ®bers show a severe degradation in properties due to the grain growth that occurs at these temperatures; therefore, other techniques need to be developed which allow mullite to form at lower temperatures.9,10 Mechnich et al. have shown that the addition of CeO2 as a sintering aid can signi®cantly reduce the mullite formation temperature down to around 1300C.11 This reduction in processing temperature enables the use of the current state-of-the-art polycrystalline aluminosilicate ®bers, albeit with some reduction in ®ber strength due to crystal growth and microstructural changes in the ®bers at high temperatures.10 Improvements in the thermal stability of oxidebased ceramic ®bers would be an important step into the development of even stronger and more usable CMCs. 2.2. Composite fabrication Slurries containing microcomposite Al2O3±SiO2 powder (a-Al2O3 particles coated with non-crystalline SiO2) were prepared with added CeO2 and mullite precursor powder (Siral, Condea, BrunnsbuÈttel, Germany.) CeO2 and mullite precursors were added in order to accelerate sintering and to improve mullite formation. Powders were ball milled for 30 min. Nextel 720 (3M, Minnesota, USA) woven cloth (atlas eight-harness satin weave) was cut into squares approximately 50 by 50 mm. These squares were dipped into the slurry material, manually manipulated while immersed, allowed to absorb slurry, removed and allowed to dry. Altering the matrix solids content of the slurry along with repeated dipping allowed in®ltrated ®ber layers of varying matrix content and distribution to be produced. This simple dipping technique added a uniform and controlled amount of matrix to the ®ber cloth Ð the matrix content among individual CMC layers (prepared from a given slurry mixture) varied less than 5%. Laminate pre-forms were prepared by stacking in an alternating fashion CMC layers and ®ber layers, as Fig. 1. Flowchart of processing steps used to fabricate ®ber laminate composites. 546 T. Radsick et al. / Journal of the European Ceramic Society 20 (2000) 545±550
T. Radsick et al. Journal of the European Ceramic Society 20(2000)545-550 illustrated in Fig. 2. A total of 13 layers(seven CMC and pressure, temperature and time parameters as the fiber six fiber layers) provided an adequate specimen thickness laminate composites of between 2.5 and 3. 8 mm, depending on the matrix for mulation. The composite lay-up was placed in a hot-press and heated to 1250C in air without applied pressure. At 3. Characterization of laminate composites 1250.C, a pressure of 15 MPa was ap perature held constant for a period of 30 min to allow the Three series of samples, each with a different matrix material to consolidate and sinter. After 30 min, pressure content as described in Table l, were tested in three was removed and the specimen was heated to a tempera- point bending at room temperature (rt) using a UTS re of 1300C. This temperature was held for a period of 10 Universal Test Machine (UTS Instruments, Inc)in 2 h to allow the components in the microcomposite pow- order to determine the effects of matrix content. The der to undergo reaction sintering to form mullite Samples effects of longterm exposure to high-temperature oxi- were subsequently cut into three-point bend samples 4.0- dizing atmospheres were also measured. For the sake of 4.5x50 mm in size. Some samples were exposed to high simplicity, only one series of samples was used for these temperatures in air for 200 h. tests. Fiber laminate composites were prepared using After hot-pressing and subsequent heat treatment at 50 wt% slurry and a double infiltration step. Three 1300.C, the laminate structure of the composite was point bend samples were cut and annealed for 200 h in readily apparent to the eye. Though the outermost fibers air at 1200, 1250 or 1300.C. Bend testing was conducted in the fiber layers were bound to the matrix phase in the at room temperature. Monolithic samples were tested CMC layers, the"uninfiltrated"nature of the fiber lay- as-received using three-point bending ers was not compromised and a clear laminar dis Mullite formation was examined by X-ray diffraction continuity in the composite was present in the 10 to 80 20 range using CuKa radiation(Siemens In order to ascertain a baseline for comparisons of D-5000 X-ray diffractometer, Siemans AG, Karlsruhe, strength values, monolithic samples were prepared by Germany). Differential scanning calorimetry was car- casting the matrix material into plaster molds. Once ried out on a Netsch 404 DSC. After three-point bend dried, the green material was processed under identical testing at RT, fracture surfaces were examined using a Philips 525M SEM 4. Results Fiber Layer 4.. Mechanical propertie Representative load-displacement curves for as- received and heat-treated samples(Fig. 3)clearly illus- trate the strength and damage-tolerance behavior of the material, even after exposure to high-temperature oxi CMC dation. The maximum strength values of the samples treated at 1200. 1250 and 1300.C were 63. 64 and 72 MPa, respectively. These values decreased slightly com pared with the 88 MPa strength of the as-received sam- dles. the area below the which corresponds to the degree of pseudoplastic beha vior (damage tolerance)significantly increased wit temperature treatment. Monolithic samples were also Fig. 2. Conceptual drawing of fiber laminate composite. Note tested in three-point bending in the as-received condi tion between CMC layers(matrix-infiltrated fiber layers)and fiber tion. Strength was determined to be 124 MPa Table l ummary of data for three slurry preparations Strength Photo infiltration, solids wt%) hickness(mm) fibre/matrix ratio (three-point bend)(MPa) I infiltration. 50% slurry 2 infiltrations. 50% slurry
illustrated in Fig. 2. A total of 13 layers (seven CMC and six ®ber layers) provided an adequate specimen thickness of between 2.5 and 3.8 mm, depending on the matrix formulation. The composite lay-up was placed in a hot-press and heated to 1250C in air without applied pressure. At 1250C, a pressure of 15 MPa was applied and the temperature held constant for a period of 30 min to allow the material to consolidate and sinter. After 30 min, pressure was removed and the specimen was heated to a temperature of 1300C. This temperature was held for a period of 2 h to allow the components in the microcomposite powder to undergo reaction sintering to form mullite. Samples were subsequently cut into three-point bend samples 4.0± 4.550 mm in size. Some samples were exposed to high temperatures in air for 200 h. After hot-pressing and subsequent heat treatment at 1300C, the laminate structure of the composite was readily apparent to the eye. Though the outermost ®bers in the ®ber layers were bound to the matrix phase in the CMC layers, the ``unin®ltrated'' nature of the ®ber layers was not compromised and a clear laminar discontinuity in the composite was present. In order to ascertain a baseline for comparisons of strength values, monolithic samples were prepared by casting the matrix material into plaster molds. Once dried, the green material was processed under identical pressure, temperature and time parameters as the ®ber laminate composites. 3. Characterization of laminate composites Three series of samples, each with a dierent matrix content as described in Table 1, were tested in threepoint bending at room temperature (RT) using a UTS 10 Universal Test Machine (UTS Instruments, Inc) in order to determine the eects of matrix content. The eects of longterm exposure to high-temperature oxidizing atmospheres were also measured. For the sake of simplicity, only one series of samples was used for these tests. Fiber laminate composites were prepared using a 50 wt% slurry and a double in®ltration step. Threepoint bend samples were cut and annealed for 200 h in air at 1200, 1250 or 1300C. Bend testing was conducted at room temperature. Monolithic samples were tested as-received using three-point bending. Mullite formation was examined by X-ray diraction in the 10 to 80 2y range using CuKa radiation (Siemens D-5000 X-ray diractometer, Siemans AG, Karlsruhe, Germany). Dierential scanning calorimetry was carried out on a Netsch 404 DSC. After three-point bend testing at RT, fracture surfaces were examined using a Philips 525M SEM. 4. Results 4.1. Mechanical properties Representative load-displacement curves for asreceived and heat-treated samples (Fig. 3) clearly illustrate the strength and damage-tolerance behavior of the material, even after exposure to high-temperature oxidation. The maximum strength values of the samples treated at 1200, 1250 and 1300C were 63, 64 and 72 MPa, respectively. These values decreased slightly compared with the 88 MPa strength of the as-received samples. The area below the load±displacement curves which corresponds to the degree of pseudoplastic behavior (damage tolerance) signi®cantly increased with temperature treatment. Monolithic samples were also tested in three-point bending in the as-received condition. Strength was determined to be 124 MPa. Fig. 2. Conceptual drawing of ®ber laminate composite. Note connection between CMC layers (matrix-in®ltrated ®ber layers) and ®ber layers. Table 1 Summary of data for three slurry preparations Sample description (# in®ltration, solids wt%) CMC layer thickness (mm) CMC layer ®bre/matrix ratio Strength (three-point bend) (MPa) Photo of sample 1 in®ltration, 20% slurry 0.28 85/15 38 Fig. 5a 1 in®ltration, 50% slurry 0.30 62/38 73 Fig. 5b 2 in®ltrations, 50% slurry 0.38 41/59 88 Fig. 5c T. Radsick et al. / Journal of the European Ceramic Society 20 (2000) 545±550 547
T. Radsick et al /Journal of the European Ceramic Society 20(2000)545-550 50 30 20 1200 Celsius. 200 h 1250 Celsius. 200 h 1300 Celsius. 200 h 0.00 0.04 Strain(%) Fig. 3. Stress-strain curves for heat-treated samples. All samples prepared using 50 wt% solid slurry and two infiltration steps Fig 4 shows a representative three-point bend sample hot-pressed for 30 min at 1250C/15 MPa and then after testing. The extent of crack deflection through the annealed without pressure at 1300oC showed almost sample clearly shows the brittle fracture mechanism of the complete mullite formation. XRD analysis revealed that CMC layers and the crack bridging and deflection proper- less than 5% SiO2 remained unreacted with Al_O3 Matrix es of the fiber layers. The wide crack separation shown in properties are given in details elsewhere. 7.I he sample did not normally occur during testing. Rather, SEM photos of the three different series of samples the sample had to be cyclically manipulated by hand in are shown in Fig. 5(a)(c). Fig. 5(a) is a sample whose order to cause the composite to separate. This illustrates an CMC layer was prepared by a single infiltration step important advantage of fiber laminate composites, namely into a slurry with a 20 wt% solids content. CMC layers that even after failure the composite remains intact, a contain on average 14 wt% matrix and are easy to dif- desired property in a material used in aircraft engines ferentiate from fiber layers due to a somewhat planar fracture surface perpendicular to the laminate plane and 4. 2. Microstructural observations due to the absence of loose individual fibers CMc layer thickness is approximately 0.28 mm. Fig. 5(b) shows a Differential scanning calorimetry (DSC) and XRD sample where a slurry with 50 wt%solid was used In this results determined that mullite formation began to occur case, differentiation between pure fiber layers and matrix- in appreciable amounts only at 1300oC, Powder samples containing layers is easy due to a marked increase in the amount of retained matrix. Most of this matrix occupies the inter-fiber spaces, so that despite a 37% increase in matrix content, the CMC layers, with a thickness of 0.30 mm, are only 0.02 mm thicker than those prepared above with the thinner slurry Fig. 5(c)also shows a sample pre- pared with 50 wt% solid slurry, but in this case the indivi- dual fiber layers were infiltrated, allowed to dry and then infiltrated a second time. The photo shows a marked increase in CMC layer thickness; not so clearly illustrated is the increased distribution of this matrix on the surface of the layers as opposed to in the inter-fiber and inter-bundle spaces. Layer thickness was approximately 0.38 mm. Table I summarizes the data for these three preparations. 43. Discussion Fig. 4. Photo of broken three-point bend sample. Sample was prepared with a double infiltration step and using a slurry with 50% solids content Because of the strong bonding between fiber and (see Table 1). Sample was tested in as-received condition matrix and the lack of an interface coating. the cmc
Fig. 4 shows a representative three-point bend sample after testing. The extent of crack de¯ection through the sample clearly shows the brittle fracture mechanism of the CMC layers and the crack bridging and de¯ection properties of the ®ber layers. The wide crack separation shown in the sample did not normally occur during testing. Rather, the sample had to be cyclically manipulated by hand in order to cause the composite to separate. This illustrates an important advantage of ®ber laminate composites, namely that even after failure the composite remains intact, a desired property in a material used in aircraft engines. 4.2. Microstructural observations Dierential scanning calorimetry (DSC) and XRD results determined that mullite formation began to occur in appreciable amounts only at 1300C, Powder samples hot-pressed for 30 min at 1250C/15 MPa and then annealed without pressure at 1300C showed almost complete mullite formation. XRD analysis revealed that less than 5% SiO2 remained unreacted with Al2O3. Matrix properties are given in details elsewhere.7,11 SEM photos of the three dierent series of samples are shown in Fig. 5(a)±(c). Fig. 5(a) is a sample whose CMC layer was prepared by a single in®ltration step into a slurry with a 20 wt% solids content. CMC layers contain on average 14 wt% matrix and are easy to differentiate from ®ber layers due to a somewhat planar fracture surface perpendicular to the laminate plane and due to the absence of loose individual ®bers. CMC layer thickness is approximately 0.28 mm. Fig. 5(b) shows a sample where a slurry with 50 wt% solid was used. In this case, dierentiation between pure ®ber layers and matrixcontaining layers is easy due to a marked increase in the amount of retained matrix. Most of this matrix occupies the inter-®ber spaces, so that despite a 37% increase in matrix content, the CMC layers, with a thickness of 0.30 mm, are only 0.02 mm thicker than those prepared above with the thinner slurry. Fig. 5(c) also shows a sample prepared with 50 wt% solid slurry, but in this case the individual ®ber layers were in®ltrated, allowed to dry and then in®ltrated a second time. The photo shows a marked increase in CMC layer thickness; not so clearly illustrated is the increased distribution of this matrix on the surface of the layers as opposed to in the inter-®ber and inter-bundle spaces. Layer thickness was approximately 0.38 mm. Table 1 summarizes the data for these three preparations. 4.3. Discussion Because of the strong bonding between ®ber and matrix and the lack of an interface coating, the CMC Fig. 3. Stress±strain curves for heat-treated samples. All samples prepared using 50 wt% solid slurry and two in®ltration steps. Fig. 4. Photo of broken three-point bend sample. Sample was prepared with a double in®ltration step and using a slurry with 50% solids content (see Table 1). Sample was tested in as-received condition. 548 T. Radsick et al. / Journal of the European Ceramic Society 20 (2000) 545±550
T. Radsick et al. Journal of the European Ceramic Society 20(2000)545-550 layers of this fiber laminate composite form a stiff material use d to assist in producing thin, quasi- monolithic layers, with a brittle fracture mechanism. Little or no damage tol- preventing excessive macro-scale cracking which can erance results from the inclusion of fibers in CMC layers, occur during the hot-pressing of thin monolithic tapes nor would any such behavior be expected in the absence of Initial attempts to produce a fiber laminate composite an interface coating. Rather, in the CMC layers, fiber is using ceramic tapes instead of CMC layers failed due to this macro-scale cracking The characteristics of the matrix in the CMC layers affect the composite properties in several ways. The amount of matrix on the surfaces of the CMC layers influences the bonding between CMC and fiber laye arising from hot-press sintering and annealing. Insuffi cient matrix on the surfaces prevents bonding between CMC layers and fiber layers. In addition, the amount of matrix and its distribution within each CMC layer affect the strength and stiffness of that layer and, as a result those of the composite. Specimens with thicker CMC layers (i.e. with higher matrix content)tended to have higher strength and a sharper drop in strength after initial matrix cracking. Samples with less matrix had a fairly flat load-displacement curve after initial matrix 1mm184kU638E18636/97sE cracking. The fiber layers function to modify the interacti between the CMC layers and to provide damage toler- ance to the material. Through hot-pressing, the outer- most fibers in the bundles of the fiber layers are bonded o the CMC layers. The woven bundles in the fiber lay- ers serve as a mechanical connection between the over- lying and underlying CMC layer, transferring loads and bridging inter-laminar and intra-laminar cracks. The ayers provide a planar discontinuity in the matrix, adding a local anisotropic weakness which serves as an a fracture path, thus improving damage tolerance Investigations to improve microcomposite-powder- derived mullite slurries need to be extended so that slurry infiltration can be optimized and RT-and high temperature(HT) strength increased. Fig. 6 illustrates several potential causes of CMC layer weakness. Clearly I mm199kU 505E Fig. 6. Typical oxide/oxide fiber laminate composite with arrows Fig. 5. Fiber laminate composites of varying matrix content and pre- showing areas of sub-optimum matrix infiltration and fiber-to-fiber paration technique:(a)2% slurry solids, I infiltration;(b)50% slurry contact. Sample was prepared with single infiltration step in slurry lids, I infiltration; (c)50% slurry solids, 2 infiltrations. with 50% solids (see Table 1)
layers of this ®ber laminate composite form a sti material with a brittle fracture mechanism. Little or no damage tolerance results from the inclusion of ®bers in CMC layers, nor would any such behavior be expected in the absence of an interface coating. Rather, in the CMC layers, ®ber is used to assist in producing thin, quasi-monolithic layers, preventing excessive macro-scale cracking which can occur during the hot-pressing of thin monolithic tapes. Initial attempts to produce a ®ber laminate composite using ceramic tapes instead of CMC layers failed due to this macro-scale cracking. The characteristics of the matrix in the CMC layers aect the composite properties in several ways. The amount of matrix on the surfaces of the CMC layers in¯uences the bonding between CMC and ®ber layers arising from hot-press sintering and annealing. Insu- cient matrix on the surfaces prevents bonding between CMC layers and ®ber layers. In addition, the amount of matrix and its distribution within each CMC layer aect the strength and stiness of that layer and, as a result, those of the composite. Specimens with thicker CMC layers (i.e. with higher matrix content) tended to have higher strength and a sharper drop in strength after initial matrix cracking. Samples with less matrix had a fairly ¯at load±displacement curve after initial matrix cracking. The ®ber layers function to modify the interaction between the CMC layers and to provide damage tolerance to the material. Through hot-pressing, the outermost ®bers in the bundles of the ®ber layers are bonded to the CMC layers. The woven bundles in the ®ber layers serve as a mechanical connection between the overlying and underlying CMC layer, transferring loads and bridging inter-laminar and intra-laminar cracks. The layers provide a planar discontinuity in the matrix, adding a local anisotropic weakness which serves as an fracture path, thus improving damage tolerance. Investigations to improve microcomposite-powderderived mullite slurries need to be extended, so that slurry in®ltration can be optimized and RT-and hightemperature (HT) strength increased. Fig. 6 illustrates several potential causes of CMC layer weakness. Clearly Fig. 5. Fiber laminate composites of varying matrix content and preparation technique: (a) 2% slurry solids, 1 in®ltration; (b) 50% slurry solids, 1 in®ltration; (c) 50% slurry solids, 2 in®ltrations. Fig. 6. Typical oxide/oxide ®ber laminate composite with arrows showing areas of sub-optimum matrix in®ltration and ®ber-to-®ber contact. Sample was prepared with single in®ltration step in slurry with 50% solids (see Table 1). T. Radsick et al. / Journal of the European Ceramic Society 20 (2000) 545±550 549
T. Radsick et al. Journal of the European Ceramic Society 20(2000)545-550 resent are areas of incomplete matrix infiltration into 5. Conclusion the center of the fiber bundles and areas of fiber-to-fiber contact. not visible is the small amount of macro-scale Fiber laminate composites, a new type of laminate matrix cracking in the CMC layer green body which material which uses no fiber interface coating but display occurs on the outermost surfaces as the slurry dries. damage tolerant fracture, were fabricated. The effect that IT-test results also show that a small amount of glas he individual components of the composite had on frac containing silicates was present, somewhat affecting the ture behaviour and strength values were investigated high temperature properties of the fiber laminate com- Composites exhibited moderate strength values at RT and posites. It is likely that a better- infiltrated CMC layer at HT(up top 1300oC), retaining their strength after 200 h ill improve the properties of a fiber laminate compo- at HT in air. Properties of the fiber composites described site. Also of high importance is the weave characteristics in this paper most likely can be improved by changing the of the fiber cloth. An eight-harness satin weave was used chemical composition of microcomposite particles, by for convenience, as this is the weave provided by the improving the infiltration of the CMC layers with these manufacturer. It is in large part, however, the woven slurries prepared by using microcomposite particles and nature of the fiber cloth that provides a mechanical by changing the type of weave of the fibers connection between the adjacent CMC layers. This is easily seen by visualizing a fiber laminate composite references directional fiber layers instead of woven cloth - with 1. Chawla. K. K. Xu. Z. R. Ha. J S. Schmucker. M. and Schnel. out woven bundles, the peel strength of the fiber layers der, H, Effect of BN coating on the strength of a mullite-type ould be very low, a fact proven by experiment. fiber. App. Comp. Mater., 1997, 4, 263 2. Morgan, P. E. D. and Marshall, D. B. Functional interfaces fo Because of the strong impact of the fiber layer on total xide/oxide Mater. Sci. Eng, 1993. 162. 15-2 omposite properties, it is expected that increasing the 3. Cutler. W.A. Zok F. W. and Lange. F. F. Delamination resis. woven nature of the cloth(using four-harness, plain tance of two hybrid -composite laminates. J. Am. Ceram weave,etc. would strongly affect the material proper- oc.,1997,80(12),3029-3037 4. Cutler. W.A. Zok. F. W. and Lange. F. F. Mechanical beha- ties. This is currently being investigated lour of several hybrid cer mposite laminates. J. Nonetheless, in a straight comparison, fiber laminate Am. Ceran.Soc,1996.79(7),1825-1833. composites prepared with the current slurry formulation 5. Tu, W.C., Lange, F. F. and Evans, A. G Concept for a and dip infiltration technique and using eight-harness damage-tolerant ceramic with 'strong interfaces. Am. ceram satin woven fabric compare well with regards to the 6. Sacks. M. D. Bozkurt N. and Scheiffele G. W. Fabrication of monolithic samples. Fiber laminate composites retain mullite and mullite-matrix composites by transient viscous sin- about 70% of the baseline monolithic strength(126 MPa) ring of composite powders. J. Am. Ceram. Soc., 1991, 74(10). while adding a significant level of damage tolerance. I 28-243 applications where weight is a strong concern, fiber lami 7. Bartsch M. Saruhan B. Schmucker M. Schneider M. novel nate composites show improved performance. When mperature processing route of dense mullite ceramics by reac. tion sintering of amphorous SiO2-coated y-Al2O3 particle nano- normalized according to weight, fiber laminate compo- composites. J. Am. Ceram. Soc., 1999. 82(6), 1388-1392. ite samples retain 83% of the baseline monolithic 8. Schneider. H. Okada. K. and Pask. J. A. Mullite and Mullite strength. This is because the low density fiber layers add ceramics. John Wiley and Sons, Chichester, 1994 to the bulk of the material, but add little to the weight 9. Anon Prospectus data. Minneapolis (MN): 3M Compa Weight is usually considered to be more important than 10. Milz C. Ph. D. research German Aerospace Center, in press. 11. Mechnich P Schneider H. Schmucker M. Saruhan B. Accelerated size in the specific application of jet engine combustion reaction bonding of mullite, J. Am. Ceram. Soc.. 1998, 81(7). cham 1931-1937
present are areas of incomplete matrix in®ltration into the center of the ®ber bundles and areas of ®ber-to-®ber contact. Not visible is the small amount of macro-scale matrix cracking in the CMC layer green body which occurs on the outermost surfaces as the slurry dries. HT-test results also show that a small amount of glass containing silicates was present, somewhat aecting the high temperature properties of the ®ber laminate composites. It is likely that a better-in®ltrated CMC layer will improve the properties of a ®ber laminate composite. Also of high importance is the weave characteristics of the ®ber cloth. An eight-harness satin weave was used for convenience, as this is the weave provided by the manufacturer. It is in large part, however, the woven nature of the ®ber cloth that provides a mechanical connection between the adjacent CMC layers. This is easily seen by visualizing a ®ber laminate composite with ®ber layers comprised of matrix-free 0/90 unidirectional ®ber layers instead of woven cloth Ð without woven bundles, the peel strength of the ®ber layers would be very low, a fact proven by experiment. Because of the strong impact of the ®ber layer on total composite properties, it is expected that increasing the woven nature of the cloth (using four-harness, plain weave, etc.) would strongly aect the material properties. This is currently being investigated. Nonetheless, in a straight comparison, ®ber laminate composites prepared with the current slurry formulation and dip in®ltration technique and using eight-harness satin woven fabric compare well with regards to the monolithic samples. Fiber laminate composites retain about 70% of the baseline monolithic strength (126 MPa) while adding a signi®cant level of damage tolerance. In applications where weight is a strong concern, ®ber laminate composites show improved performance. When normalized according to weight, ®ber laminate composite samples retain 83% of the baseline monolithic strength. This is because the low density ®ber layers add to the bulk of the material, but add little to the weight. Weight is usually considered to be more important than size in the speci®c application of jet engine combustion chamber. 5. Conclusion Fiber laminate composites, a new type of laminate material which uses no ®ber interface coating but displays damage tolerant fracture, were fabricated. The eect that the individual components of the composite had on fracture behaviour and strength values were investigated. Composites exhibited moderate strength values at RT and at HT (up top 1300C), retaining their strength after 200 h at HT in air. Properties of the ®ber composites described in this paper most likely can be improved by changing the chemical composition of microcomposite particles, by improving the in®ltration of the CMC layers with these slurries prepared by using microcomposite particles and by changing the type of weave of the ®bers. References 1. Chawla, K. K., Xu, Z. R., Ha, J. S., SchmuÈcker, M. and Schneider, H., Eect of BN coating on the strength of a mullite-type ®ber. App. Comp. Mater., 1997, 4, 263. 2. Morgan, P. E. D. and Marshall, D. B., Functional interfaces for oxide/oxide composites. Mater. Sci. Eng., 1993, 162, 15±25. 3. Cutler, W. A., Zok, F. W. and Lange, F. F., Delamination resistance of two hybrid ceramic-composite laminates. J. Am. Ceram. Soc., 1997, 80(12), 3029±3037. 4. Cutler, W. A., Zok, F. W. and Lange, F. F., Mechanical behaviour of several hybrid ceramic-matrix-composite laminates. J. Am. Ceram. Soc., 1996, 79(7), 1825±1833. 5. Tu, W. C., Lange, F. F. and Evans, A. G., Concept for a damage-tolerant ceramic with `strong' interfaces. J. Am. Ceram. Soc., 1996, 79(2), 417±424. 6. Sacks, M. D., Bozkurt, N. and Scheiele, G. W., Fabrication of mullite and mullite-matrix composites by transient viscous sintering of composite powders. J. Am. Ceram. Soc., 1991, 74(10), 2428±2437. 7. Bartsch M, Saruhan B, SchmuÈcker M, Schneider M. Novel lowtemperature processing route of dense mullite ceramics by reaction sintering of amphorous SiO2-coated g-Al2O3 particle nanocomposites. J. Am. Ceram. Soc., 1999, 82(6), 1388±1392. 8. Schneider, H., Okada, K. and Pask, J. A., Mullite and Mullite ceramics. John Wiley and Sons, Chichester, 1994. 9. Anon. Prospectus data. Minneapolis (MN): 3M Company. 10. Milz C. Ph.D. research German Aerospace Center, in press. 11. Mechnich P, Schneider H, SchmuÈcker M, Saruhan B. Accelerated reaction bonding of mullite, J. Am. Ceram. Soc., 1998, 81(7), 1931±1937. 550 T. Radsick et al. / Journal of the European Ceramic Society 20 (2000) 545±550