Journal of the European Ceramic Society 19(1999)305-316 C)1999 Elsevier Science Limited Printed in Great Britain. All rights reserved PII:S0955-2219(98)00073-9 0955-2219/99/Ssee front matter Carbon Fiber-reinforced (YMAS) Glass Ceramic Matrix Composites. II. Structural Changes in the matrix with Temperature W. Sinkler a* M. monthioux b v. bianchi.c p goursat and e. menessierd aDepartment of Materials Science and Engineering, Northwestern University, 8225 Sheridan Road, Evanston, IL60208-3108,USA bCentre d Elaboration des Materiauxet d'Etudes Structurales. UPR A-8011 CNRS B P 4347. F31055 Toulouse Cedex 4. Laboratoire des Materiaux Ceramiques et Traitements de Surface, Universite de Limoges, UER Sciences, 123 Avenue Albert Thomas, F87060 Limoges Cedex, france Societe Ceramique and Composites, BP 7, F65460 Bazet, france (Received 14 November 1997; accepted 16 February 1998) Abstract transmission, mais aussi la microscopie electronique la microscopie optique en lumiere polar With the overall aim to study a specific family of isee, la diffraction des rayons X, et l'analyse thermi- arbon-reinforced glass-ceramic matrix composites, que Quelques differences entre le verre contenant du the structural and textural changes occurring within carbone(sous forme de fibres) et le verre quand il a glass belonging to the Mg-AlSi-Y-0 chemical n'en contient pas ont ete revelees, qui se resument system were investigated. Investigation techniques principalement a une sous-stoechiometrie locale en were mainly transmission electron microscopy, but oxygene, due a un effet reducteur du carbone. also scanning electron microscopy, polarized light L'essentiel du travail a consiste a observer les etapes optical microscopy, x-ray diffraction, and thermal de cristallisation des differentes phases pe analysis. Some differences between the carbon la stoechiometrie de depart, a savoir un silicate (fiber y-containing material and the carbon-free d' aluminium et de magnesium(indialite), un silicate material are revealed, mainly through local oxygen- d'yttrium, un oxyde aluminium et de magnesium depletions due to a reduction effect by carbon. (spinelle), en sus de corindon. Les morphologies de Mainly, the way that the various phases allowed by differentes phases, leur transformation, ainsi que leur the starting stoichiometry crystallize during an relations texturales font partie des aspects etudes increasing heat-treatment is observed, i.e. magne- C 1999 Elsevier Science Limited All rights reserved sium aluminum silicate as indialite, yttrium silicate, magnesium aluminum oxide as spinel, in addition to Keywords: composites, glass ceramics, carbon corundum. Phases transformations, crystallized fibers, microstructure- final, crystallization phase morphologies, and textural relationships between phases are among the features described 1 Introduction Resume An extensive study of carbon-reinforced YMAS Dans le but plus general d'etudier une famille parti- matrix composites has been performed from 1992 culiere de composites d matrice vitroceramique a to 1995 in our laboratories, and is now being renfort carbone, les changements structuraux et published. -The main purpose was to investigate structuraux intervenant au sein d'un verre appare- the relationships between the process conditions nant au systeme chimique Mg-Al-Si-Y-O ont ete (fiber type, densification temperature and pres- examines. Les differentes techniques d'investigation sure), and the macroscopic mechanical properties ont ete essentiellement la microscopie electronique a of the composites. -3As far as macroscopic fea- tures are due to microscopic causes, specifically in *To whom correspondence should be addressed. composites, changing the process conditions may E-mail: er(@ apollo numis. nwu.edu have induced interfacial changes at the fiber/matrix
Carbon Fiber-reinforced (YMAS) GlassCeramic Matrix Composites. II. Structural Changes in the Matrix with Temperature W. Sinkler,a * M. Monthioux,b V. Bianchi,c P. Goursatc and E. MeÂnessierd a Department of Materials Science and Engineering, Northwestern University, 8225 Sheridan Road, Evanston, IL 60208-3108, USA b Centre d'Elaboration desMateÂriaux et d'Etudes Structurales, UPR A-8011 CNRS, B.P. 4347, F31055 Toulouse Cedex 4, France c Laboratoire des MateÂriaux CeÂramiques et Traitements de Surface, Universite de Limoges, UER Sciences, 123 Avenue Albert Thomas, F87060 Limoges Cedex, France d SocieÂte CeÂramiques and Composites, BP 7, F65460 Bazet, France (Received 14 November 1997; accepted 16 February 1998) Abstract With the overall aim to study a speci®c family of carbon-reinforced glass-ceramic matrix composites, the structural and textural changes occurring within a glass belonging to the Mg±Al±Si±Y±O chemical system were investigated. Investigation techniques were mainly transmission electron microscopy, but also scanning electron microscopy, polarized light optical microscopy, X-ray diraction, and thermal analysis. Some dierences between the carbon (®ber)-containing material and the carbon-free material are revealed, mainly through local oxygendepletions due to a reduction eect by carbon. Mainly, the way that the various phases allowed by the starting stoichiometry crystallize during an increasing heat-treatment is observed, i.e. magnesium aluminum silicate as indialite, yttrium silicate, magnesium aluminum oxide as spinel, in addition to corundum. Phases transformations, crystallized phase morphologies, and textural relationships between phases are among the features described. ReÂsume Dans le but plus geÂneÂral d'eÂtudier une famille particulieÁre de composites aÁ matrice vitroceÂramique aÁ renfort carbone, les changements structuraux et structuraux intervenant au sein d'un verre appartenant au systeÁme chimique Mg±Al±Si±Y±O ont eÂte examineÂs. Les dieÂrentes techniques d'investigation ont eÂte essentiellement la microscopie eÂlectronique aÁ transmission, mais aussi la microscopie eÂlectronique aÁ balayage, la microscopie optique en lumieÁre polariseÂe, la diraction des Rayons X, et l'analyse thermique. Quelques dieÂrences entre le verre contenant du carbone (sous forme de ®bres) et le verre quand il n'en contient pas ont eÂte reÂveÂleÂes, qui se reÂsument principalement aÁ une sous-stoechiomeÂtrie locale en oxygeÁne, due aÁ un eet reÂducteur du carbone. L'essentiel du travail a consiste aÁ observer les eÂtapes de cristallisation des dieÂrentes phases permises par la stoechiomeÂtrie de deÂpart, aÁ savoir un silicate d'aluminium et de magnesium (indialite), un silicate d'yttrium, un oxyde d'aluminium et de magneÂsium (spinelle), en sus de corindon. Les morphologies de dieÂrentes phases, leur transformation, ainsi que leur relations texturales font partie des aspects eÂtudieÂs. # 1999 Elsevier Science Limited. All rights reserved Keywords: composites, glass ceramics, carbon ®bers, microstructure ± ®nal, crystallization. 1 Introduction An extensive study of carbon-reinforced YMAS matrix composites has been performed from 1992 to 1995 in our laboratories,1 and is now being published.2±5 The main purpose was to investigate the relationships between the process conditions (®ber type, densi®cation temperature and pressure), and the macroscopic mechanical properties of the composites.1±3 As far as macroscopic features are due to microscopic causes, speci®cally in composites, changing the process conditions may have induced interfacial changes at the ®ber/matrix Journal of the European Ceramic Society 19 (1999) 305±316 # 1999 Elsevier Science Limited Printed in Great Britain. All rights reserved PII: S0955-2219(98)00073-9 0955-2219/99/$Ðsee front matter 305 *To whom correspondence should be addressed. E-mail: sinkler@apollo.numis.nwu.edu
W. Sinkler et al contact, which were able to be related to changes in powder to prevent graphite/glass reactions) under the mechanical behaviour.4 On the other hand, argon(U Grade from Air Liquide). The heating the initial chemical composition of the matrix and cooling rates were 25Cmin-l, with no pressure (within the Mg-Al-Si-YO system) was chosen applied during these steps but the atmospheric pres- not to be a parameter. Thus, because fibers were sure. The oriented pressure(10 MPa)was only exer- always carbon fibers, though from various origins ted during I h (exceptionally 0.5h)at 1050oC the overall chemical composition of the matrix did(exceptionally 1000, 1150, or 1200C), i.e. at the end not change from one composite to another. How- of the impregnation step. An annealing step at ever, structural changes were observed in the atmospheric pressure was then generally carried out, to the effect of l refore may be claimed to be due at various temperature/time conditions(Table 1) Since the type of carbon fibers was a parameter In addition, several papers have been devoted to for the overall study, results reported in this paper he study of glasses, ceramics, and glass-ceramics are concerned with composites reinforced with materials from related chemical systems(Refs 6 and T400H(SOFICAR, France) PAN-based fiber or 7, for instance, and references therein). However, Thornel (AMOCO, USA)P25 or P55 pitch-based they have mainly dealt with the thermochemical fibers. However, the influence of different fibre description of the phase diagrams, i.e. few have types on microstructural developement is expected been concerned with the microstructural character- to be negligible (all fibres being carbon), a point ization, or more specifically with imaging the tex- borne out by the results presented here. tural aspects of the structural changes. Revealing The YMAs glass powder incorporated in the how and when phases nucleate, grow, associate, and slurry was prepared by CEraMiQues COm transform within a complex system under specific POSITES Company(Bazet, France) by melting al time/pressure/temperature conditions is believed to the oxide components but Al2O3(1700C under be as important as calculating the domains of phase nitrogen in a carbon furnace), then quenching and occurrence through thermodynamics. Indeed, such grinding(< 10 um). The molar composition is 36% calculations may not consider variations in energy SiO2, 34-4% Al2O3, 20%Y203, 9.6%MgO, 0.3% due to some physical aspects(such as the effect of Fe2O. The YMas glass powder itself thus prepared nanometric sizes for crystals, for instance), and do (dark gray colored) was also investigated as a bulk not consider kinetics either. Thus, direct imaging of (i.e. not as a composite), though less extensively, in the crystallization mechanism of complex systems is order to check whether it may behave differently useful to sustain the calculations and distinguish while associated with carbon fibers or not. Various between various microstructural states sintering conditions were used on the YMAS glass This paper thus reports the structural and tex- powder for this additional investigation, the condi tural evolution of a SiO2-MgO-AlO3-Y2O3 glass tions of which will be given in the text in the 1000-1250C range, mainly through trans- mission electron microscopy (TEM), but also using 2.2 Investigation methods scanning electron microscopy(SEM), X-ray dif- ction(XRD), and thermogravimetric analysis The micro- and nanotextural and structural study was mainly performed with a transmission electron 2 Experimental 2.1 Samples and preparation procedure for the composites Composites were prepared at the Laboratoire des Materiaux Ceramique et Traitements de Surface (Limoges University). The preparation method has been extensively described in a previous paper.3 Briefly, the main steps included a liquid impregna- tion route of the fiber tows ( the slurry contained he glass powder, a solvent and some binding, wetting, etc,agents), a low temperature heat Fig. 1. YMAS glass powder. Differential thermal analysi treatment(up to 425C) in air to remove organic obtained from a thermal treatment in an AlO3 crucible from compounds without oxidizing the carbon fibers 20to1400° C with a l0°Cmin eating rate, oxygen atmo- then an oriented pressure sintering(graphite mold sphere. Peak I corresponds to the glass transition, peak 2 corresponds to the main crystallisation, peak 3 corresponds to he walls of which were coated with hexagonal BN he melting of the bulk
contact, which were able to be related to changes in the mechanical behaviour.2,4 On the other hand, the initial chemical composition of the matrix (within the Mg±Al±Si±Y±O system) was chosen not to be a parameter. Thus, because ®bers were always carbon ®bers, though from various origins,3 the overall chemical composition of the matrix did not change from one composite to another. However, structural changes were observed in the matrix, which therefore may be claimed to be due to the eect of temperature only. In addition, several papers have been devoted to the study of glasses, ceramics, and glass-ceramics materials from related chemical systems (Refs 6 and 7, for instance, and references therein). However, they have mainly dealt with the thermochemical description of the phase diagrams, i.e. few have been concerned with the microstructural characterization, or more speci®cally with imaging the textural aspects of the structural changes. Revealing how and when phases nucleate, grow, associate, and transform within a complex system under speci®c time/pressure/temperature conditions is believed to be as important as calculating the domains of phase occurrence through thermodynamics. Indeed, such calculations may not consider variations in energy due to some physical aspects (such as the eect of nanometric sizes for crystals, for instance),8 and do not consider kinetics either. Thus, direct imaging of the crystallization mechanism of complex systems is useful to sustain the calculations and distinguish between various microstructural states. This paper thus reports the structural and textural evolution of a SiO2±MgO±Al2O3±Y2O3 glass in the 1000±1250C range, mainly through transmission electron microscopy (TEM), but also using scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis. 2 Experimental 2.1 Samples and preparation procedure for the composites Composites were prepared at the Laboratoire des MateÂriaux CeÂramiques et Traitements de Surface (Limoges University). The preparation method has been extensively described in a previous paper.3 Brie¯y, the main steps included a liquid impregnation route of the ®ber tows (the slurry contained the glass powder, a solvent and some binding, wetting, etc., agents), a low temperature heat treatment (up to 425C) in air to remove organic compounds without oxidizing the carbon ®bers, then an oriented pressure sintering (graphite mold, the walls of which were coated with hexagonal BN powder to prevent graphite/glass reactions) under argon (U Grade from Air Liquide). The heating and cooling rates were 25C minÿ1 , with no pressure applied during these steps but the atmospheric pressure. The oriented pressure (10 MPa) was only exerted during 1 h (exceptionally 0.5 h) at 1050C (exceptionally 1000, 1150, or 1200C), i.e. at the end of the impregnation step. An annealing step at atmospheric pressure was then generally carried out, at various temperature/time conditions (Table 1). Since the type of carbon ®bers was a parameter for the overall study, results reported in this paper are concerned with composites reinforced with T400H (SOFICAR, France) PAN±based ®ber or Thornel (AMOCO, USA) P25 or P55 pitch-based ®bers. However, the in¯uence of dierent ®bre types on microstructural developement is expected to be negligible (all ®bres being carbon), a point borne out by the results presented here. The YMAS glass powder incorporated in the slurry was prepared by CERAMIQUES & COMPOSITES Company (Bazet, France) by melting all the oxide components but Al2O3 (1700C under nitrogen in a carbon furnace), then quenching and grinding (<10m). The molar composition is 36% SiO2, 34.4% Al2O3, 20% Y2O3, 9.6% MgO, 0.3% Fe2O. The YMAS glass powder itself thus prepared (dark gray colored) was also investigated as a bulk (i.e. not as a composite), though less extensively, in order to check whether it may behave dierently while associated with carbon ®bers or not. Various sintering conditions were used on the YMAS glass powder for this additional investigation, the conditions of which will be given in the text. 2.2 Investigation methods The micro- and nanotextural and structural study was mainly performed with a transmission electron Fig. 1. YMAS glass powder. Dierential thermal analysis obtained from a thermal treatment in an Al2O3 crucible from 20 to 1400C with a 10C minÿ1 heating rate, oxygen atmosphere. Peak 1 corresponds to the glass transition, peak 2 corresponds to the main crystallisation, peak 3 corresponds to the melting of the bulk. 306 W. Sinkler et al
Carbon fiber-reinforced(YMAS) glass-ceramic matrix composites: II microscope(Philips CM12 TEM), using a 120kv indicated the following broad temperature regimes high voltage and a Super-twin objective lens. The which are summarized as follows microscope was also equipped with an energy dis- pensive X-ray analysis system(Edax), which was ·850950°C: the majority phase is amorphous, used to perform local and comparative chemical with minor amounts of pre-existing aAlO3 analysis on areas larger than 50 nm for elements (corundum) originating from the initial oxide with Z> 5. Other textural and chemical analyses constituent powder. were performed with a scanning electron micro scope (JEOL 840 SEM) both through the detection of secondary electrons, back-scattered electrons .950-1100 C: rapid crystallization followed by and X photons from fluorescence. Textural and very slow evolution (grain growth, phase structural investigations at low magnification were changes)of aMg2Al4SisO18(high temperature performed using an optical microscope(Olympus hexagonal cordierite indialite) and BH2 OM) with polarized light (for areas larger aY2Si2O,(triclinic) as main components, and than about 0.5 m. which is about the resolution MgAl2O4(spinel) as a minor component power of the OM). Standard X RD investigations were also performed Some investigations were also performed using a 1100-1200C: transformation of aY?Si,O, to thermogravimetry (TGA)and differential thermal B-Y2Si2O,(high temperature monoclinic analysis(DTA) system(Setaram TAG24, with keiviite) 9 symetrical configuration). When argon atmosphere Some discrepancies may be found relative to the was used, the furnace volume was previously literature, regarding the a to B transformation purged using argon(N56 grade from Air Liquide) temperature for Y2Si2O7 for instance, reported to at least three times occur at 1225C. This may be due to kinetic effects and differences in the sintering conditions. Other 2.3 Preparation of samples prior to TEM examples will be given later. investigation In addition to XRD, DTA(oxygen atmosphere) was used to characterize phase changes occurring Both the YMas glass-ceramic and the composite in the YMas glass powder with increasing tem materials were exclusively prepared using ion-mil- perature. A DTA trace taken at 10Cmin ling. Small pieces of composites were trimmed then shown in Fig. 1. a glass transition is seen as a mechanically thinned using diamond tools then small endothermal peak(peak 1)between 815 and diamond and Sic powder suspensions and abrasive 855C. Crystallization of the liquid is seen as a papers. The final step of mechanical thinning was large exothermal peak(peak 2)extending from performed on both sample faces using a dimple 945 to 1055 C. The point of fusion of the sub- grinder(Gatan 656) with a diamond powder sus- sequent crystallized ceramic occurs at about pension(grain sizes were in the 24 um range), in 1300%C, as indicated by the last large endother order to reach thicknesses close to about 30 um. mal peak(peak 3) Ion-thinning was then performed(Gatan duomill Density measurements of the YMas glass 600)using a 4 kv high voltage and final incidence showed a variation from 2.42 gcm for the angles of 12 amorphous to 3.1 gcm-3 for the fully crystallized Because the ceramic matrix is an insulator, it was state. Based on volume expansion measured in the usually necessary to deposit a thin(<5 nm)carbon 400-800oC temperature range, thermal expansion coating on the TEM foils by evaporation in order coefficients (CET) have been determined as to avoid problems with charging in the TEM, likely 7. 10-6 and 6.3x10-6 for the amorphous and to alter the quality of the micrographs crystalline states, respectively. 2 a peculiar experimental detail revealed that the YMAS material was able to react chemically with 3 Result carbon, which is of importance for its suitability in a carbon fiber-reinforced composite. Evidence for 3.1 Evolution of the YMAs glass-ceramic(=as a such a reaction was found in the specific optical bulk) absorption characteristics of the YMas glass- ceramic(obtained from 1050C pressureless sinter- The changes in phases occurring within the YMAs ing of the YMas glass powder in a carbon fur glass powder with an increasing sintering tempera- nace, nitrogen atmosphere). Like the starting ture(2Cmin-, 1. 5h dwell time, air atmosphere, powder, the material appeared black [Fig. 2(a)]and normal pressure)was followed by XRD. This was opaque for thicknesses exceeding x 200 um
microscope (Philips CM12 TEM), using a 120 kV high voltage and a Super-twin objective lens. The microscope was also equipped with an energy dispersive X-ray analysis system (Edax), which was used to perform local and comparative chemical analysis on areas larger than 50 nm for elements with Z>5. Other textural and chemical analyses were performed with a scanning electron microscope (JEOL 840 SEM) both through the detection of secondary electrons, back-scattered electrons and X photons from ¯uorescence. Textural and structural investigations at low magni®cation were performed using an optical microscope (Olympus BH2 OM) with polarized light (for areas larger than about 0.5 m, which is about the resolution power of the OM). Standard X RD investigations were also performed. Some investigations were also performed using a thermogravimetry (TGA) and dierential thermal analysis (DTA) system (Setaram TAG24, with symetrical con®guration). When argon atmosphere was used, the furnace volume was previously purged using argon (N56 grade from Air Liquide) at least three times. 2.3 Preparation of samples prior to TEM investigation Both the YMAS glass-ceramic and the composite materials were exclusively prepared using ion-milling. Small pieces of composites were trimmed then mechanically thinned using diamond tools then diamond and SiC powder suspensions and abrasive papers. The ®nal step of mechanical thinning was performed on both sample faces using a dimple grinder (Gatan 656) with a diamond powder suspension (grain sizes were in the 2±4m range), in order to reach thicknesses close to about 30m. Ion-thinning was then performed (Gatan Duomill 600) using a 4 kV high voltage and ®nal incidence angles of 12. Because the ceramic matrix is an insulator, it was usually necessary to deposit a thin (<5 nm) carbon coating on the TEM foils by evaporation in order to avoid problems with charging in the TEM, likely to alter the quality of the micrographs. 3 Results 3.1 Evolution of the YMAS glass-ceramic (= as a bulk) The changes in phases occurring within the YMAS glass powder with an increasing sintering temperature (2C minÿ1 , 1.5 h dwell time, air atmosphere, normal pressure) was followed by XRD. This indicated the following broad temperature regimes which are summarized as follows: . 850±950C: the majority phase is amorphous, with minor amounts of pre-existing Al2O3 (corundum) originating from the initial oxide constituent powder. . 950±1100C: rapid crystallization followed by very slow evolution (grain growth, phase changes) of Mg2Al4Si5O18 (high temperature hexagonal cordierite, = indialite) and Y2Si2O7 (triclinic)9 as main components, and MgAl2O4 (spinel) as a minor component. . 1100±1200C: transformation of Y2Si2O7 to b±Y2Si2O7 (high temperature monoclinic = keiviite).9 Some discrepancies may be found relative to the literature, regarding the to transformation temperature for Y2Si2O7 for instance, reported to occur at 1225C.9 This may be due to kinetic eects and dierences in the sintering conditions. Other examples will be given later. In addition to XRD, DTA (oxygen atmosphere) was used to characterize phase changes occurring in the YMAS glass powder with increasing temperature. A DTA trace taken at 10C minÿ1 is shown in Fig. 1. A glass transition is seen as a small endothermal peak (peak 1) between 815 and 855C. Crystallization of the liquid is seen as a large exothermal peak (peak 2) extending from 945 to 1055C. The point of fusion of the subsequent crystallized ceramic occurs at about 1300C, as indicated by the last large endothermal peak (peak 3). Density measurements of the YMAS glass showed a variation from 2.42 g cmÿ3 for the amorphous to 3.1 g cmÿ3 for the fully crystallized state.8 Based on volume expansion measured in the 400±800C temperature range, thermal expansion coecients (CET) have been determined as 7.110ÿ6 and 6.310ÿ6 for the amorphous and crystalline states, respectively.1,2 A peculiar experimental detail revealed that the YMAS material was able to react chemically with carbon, which is of importance for its suitability in a carbon ®ber-reinforced composite. Evidence for such a reaction was found in the speci®c optical absorption characteristics of the YMAS glassceramic (obtained from 1050C pressureless sintering of the YMAS glass powder in a carbon furnace, nitrogen atmosphere). Like the starting powder, the material appeared black [Fig. 2(a)] and was opaque for thicknesses exceeding &200m. Carbon ®ber-reinforced (YMAS) glass-ceramic matrix composites: II 307
W. Sinkler et al b I cm Fig. 2. Aspect of the 1050C sintered(nitrogen atmosphere) YMAS glass-ceramic after a 1200 C annealing treatment (a) under argon,(b) under oxygen. The color changes from dark to clear indicating that the starting glass-ceramic material was under-stoichiometric with respect to the oxygen content As far as partial reduction of oxides can change optical absorption characteristics, it is thus likely that this unexpected color was due to such a partial reduction by the carbon of the furnace during the glass-ceramic preparation procedure. Support for a chemical reaction bertween YMaS and carbon was 盏200m found from thermogravimetric investigation. The YMAS glass-ceramic was subsequently treated for 3 h at 1200C either under oxygen or under argon (alumina furnace). This resulted in a change in color from black(YMAS glass-ceramic before and after 1200C annealing under argon)to white(after 4类A 1200.C annealing under oxygen), as illustrated in Fig 2(b), and a weight gain of 0.3%(compared to 0.1% weight gain under argon, presumably due to the me purities in the gas). This indicates that s glass-ceramic material was under-stoi- chiometric with respect to oxygen 盘 Correspondingly, Fig. 3(a) shows the 1050C sintered (nitrogen atmosphere) YMAs glass ceramic in OM using polarized light in transmis sion. The predominant color of the specimen is magenta, whichs indicates a predominantly amor phous state. Small inhomogeneities may be seen throughout the micrograph(often colored blue or yellow on the original slide) which indicates the A presence of small crystals within an amorphous Fig. 3. 1050C-sintered (nitrogen atmosphere)YMAs glass matrix. In addition, large opaque areas are present, ceramic. Black-and-white duplication of an OM image:(a) which are not voids or bubbles [since not visible in transmitted, polarised light. Black areas are assumed to be due reflected light, see Fig. 3(b)]. The strong light to oxygen-depleted zones: () same region using reflected, absorption in these regions suggests that conditions high concentrations of crystals, which are probably indialite are locally achieved which have causedan Needle-like crystals are probably corundum. enhancement of partial reduction. Figure 3(b) shows the same area using natural light in reflec- tion. Small needle-like crystals which intersect the pecimen surface are visible away from the large with the EDX measurements subsequently per opaque areas(which are no longer revealed).a formed on the matrix of the composite. The large SEM image using backscattered electrons Fig. 4 opaque areas [they can be located using Fig 3(a)] indicates that the needle-like crystals containcontain high concentrations of smaller, isometric nt elements(they appear dark), and therefore polyhedral crystals, which suggests that enhanced are presumably AlO3(corundum), consistent partial reduction is related to the phase
As far as partial reduction of oxides can change optical absorption characteristics, it is thus likely that this unexpected color was due to such a partial reduction by the carbon of the furnace during the glass-ceramic preparation procedure. Support for a chemical reaction bertween YMAS and carbon was found from thermogravimetric investigation. The YMAS glass-ceramic was subsequently treated for 3 h at 1200C either under oxygen or under argon (alumina furnace). This resulted in a change in color from black (YMAS glass-ceramic before and after 1200C annealing under argon) to white (after 1200C annealing under oxygen), as illustrated in Fig. 2(b), and a weight gain of 0.3% (compared to 0.1% weight gain under argon, presumably due to oxygen impurities in the gas). This indicates that the YMAS glass-ceramic material was under±stoichiometric with respect to oxygen. Correspondingly, Fig. 3(a) shows the 1050Csintered (nitrogen atmosphere) YMAS glass± ceramic in OM using polarized light in transmission. The predominant color of the specimen is magenta, whichs indicates a predominantly amorphous state. Small inhomogeneities may be seen throughout the micrograph (often colored blue or yellow on the original slide) which indicates the presence of small crystals within an amorphous matrix. In addition, large opaque areas are present, which are not voids or bubbles [since not visible in re¯ected light, see Fig. 3(b)]. The strong light absorption in these regions suggests that conditions are locally achieved which have caused an enhancement of partial reduction. Figure 3(b) shows the same area using natural light in re¯ection. Small needle-like crystals which intersect the specimen surface are visible away from the large opaque areas (which are no longer revealed). A SEM image using backscattered electrons Fig. 4 indicates that the needle-like crystals contain light elements (they appear dark), and therefore are presumably Al2O3 (corundum), consistent with the EDX measurements subsequently performed on the matrix of the composite. The large opaque areas [they can be located using Fig. 3(a)] contain high concentrations of smaller, isometric polyhedral crystals, which suggests that enhanced partial reduction is related to the phase Fig. 2. Aspect of the 1050C sintered (nitrogen atmosphere) YMAS glass-ceramic after a 1200C annealing treatment (a) under argon, (b) under oxygen. The color changes from dark to clear indicating that the starting glass-ceramic material was under-stoichiometric with respect to the oxygen content. Fig. 3. 1050C-sintered (nitrogen atmosphere) YMAS glassceramic. Black-and-white duplication of an OM image: (a) transmitted, polarised light. Black areas are assumed to be due to oxygen±depleted zones: (b) same region using re¯ected, natural light. The dark areas visible in (a) are associated with high concentrations of crystals, which are probably indialite. Needle-like crystals are probably corundum. 308 W. Sinkler et al
Carbon fiber-reinforced (YMAS) glass-ceramic matrix composites: II 20m Fig. 4. 1050 C-sintered (nitrogen atmosphere) YMAS glass ceramic. SEM image(back-scattered electrons). Dark crysta are either corundum or indialite, i. e. yttrium-free phases. Cor- respondingly, the bulk appears gray, due to its yttrium con- um crystallization. From the SEM image in Fig. 4, these crystals again contain light elements, which suggests that polyhedral crystals are cordierite 1g2Al4Si5O18), and that crystallization of the lat ter may be enhanced by oxygen depletion or, more probably, may have induced it. a possible mechanism is that the crystallization of cordierite has locally enhanced the reduction effect (initiated by the carbon of the furnace during the glass preparation procedure) by taking out from the matrix the amount of oxygen atoms required to reach the appropriate stoichiometry. No such effect was associated with corundum crystals since th latter did not melt and directly originate from the Fig. 5. 1000oC-sintered (air atmosphere)YMAs glass-cera- starting oxide powder mic. SEM image(back-scattered electrons): (a) low magnifi- cation;(b) higher magnification of the aggregate arrowed in In Fig. 4, beside the light elements-containing (a). Dark feather-like crystals are cordierite, white parts cordierite and corundum crystals which appear the bulk surrounding the aggregates. Arrow indicates an dark, the bulk part of the glass appears grey (i.e example of a star- like morphology typical of a primary crys- with a lighter contrast), due to the contribution of tallisation for cordierite crystals. Cordierite crystals( dark yttrium(z= 39)which did not crystallized yet as a parts)are again found associated with yttrium silicate or at silicate, or has actually crystallized but with grain least yttrium-rich areas(clear parts) sizes below the resolution power Opaque areas due to oxygen depletion are no [arrow in Fig. 5(c). It is noteworthy that cordierite longer observed in Y MAS ceramics resulting from seems to be associated with yttrium-rich areas the sintering in air of the YMAs powder. The again, though at a primary step of the crystal microstructure after such sintering at 1000 and lization. Indeed, dark parts(cordierite) are most 1200C is illustrated with backscattered electrons often surrounded by clearer parts(yttrium-bearing images from SEM in Figs 5 and 6 respectively. material) in Fig. 5(c). As soon as the temperature is Figure 5(a)shows how cordierite(dark crystals) sufficient for coarsening of the microstructure may nucleate either as spheroidal aggregates [1200C, Fig. 6(a(b)], the peculiar association [arrow in Fig. 5(a)] in which it is associated with coalescence between the cordierite and yttrium yttrium silicate [bright areas interspersed with den- silicate is lost dritic cordierite crystals in Fig. 5(b)], or as smaller It should be noted that no direct comparison is star-like associations disseminated in the bulk possible of the results of Figs 5 and 6(YMAS
crystallization. From the SEM image in Fig. 4, these crystals again contain light elements, which suggests that polyhedral crystals are cordierite (Mg2Al4Si5O18), and that crystallization of the latter may be enhanced by oxygen depletion or, more probably, may have induced it. A possible mechanism is that the crystallization of cordierite has locally enhanced the reduction eect (initiated by the carbon of the furnace during the glass preparation procedure) by taking out from the matrix the amount of oxygen atoms required to reach the appropriate stoichiometry. No such eect was associated with corundum crystals since the latter did not melt and directly originate from the starting oxide powder. In Fig. 4, beside the light elements-containing cordierite and corundum crystals which appear dark, the bulk part of the glass appears grey (i.e. with a lighter contrast), due to the contribution of yttrium (Z = 39) which did not crystallized yet as a silicate, or has actually crystallized but with grain sizes below the resolution power. Opaque areas due to oxygen depletion are no longer observed in YMAS ceramics resulting from the sintering in air of the YMAS powder. The microstructure after such sintering at 1000 and 1200C is illustrated with backscattered electrons images from SEM in Figs 5 and 6 respectively. Figure 5(a) shows how cordierite (dark crystals) may nucleate either as spheroidal aggregates [arrow in Fig. 5(a)] in which it is associated with yttrium silicate [bright areas interspersed with dendritic cordierite crystals in Fig. 5(b)], or as smaller star-like associations disseminated in the bulk [arrow in Fig. 5(c)]. It is noteworthy that cordierite seems to be associated with yttrium-rich areas again, though at a primary step of the crystallization. Indeed, dark parts (cordierite) are most often surrounded by clearer parts (yttrium-bearing material) in Fig. 5(c). As soon as the temperature is sucient for coarsening of the microstructure [1200C, Fig. 6(a)±(b)], the peculiar association coalescence between the cordierite and yttrium silicate is lost. It should be noted that no direct comparison is possible of the results of Figs 5 and 6 (YMAS Fig. 5. 1000C-sintered (air atmosphere) YMAS glass-ceramic. SEM image (back-scattered electrons): (a) low magni®- cation; (b) higher magni®cation of the aggregate arrowed in (a). Dark feather-like crystals are cordierite, white parts between them are yttrium silicate; (c) higher magni®cation of the bulk surrounding the aggregates. Arrow indicates an example of a star-like morphology typical of a primary crystallisation for cordierite crystals. Cordierite crystals(dark parts) are again found associated with yttrium silicate or at least yttrium-rich areas (clear parts). Fig. 4. 1050C-sintered (nitrogen atmosphere) YMAS glassceramic. SEM image (back-scattered electrons). Dark crystals are either corundum or indialite, i.e. yttrium-free phases. Correspondingly, the bulk appears gray, due to its yttrium content. Carbon ®ber-reinforced (YMAS) glass-ceramic matrix composites: II 309
310 W. Sinkler et al powder after 1000 and 1200C sintering under air) with Figs 3 and 4(YMAs powder after 1050.C sintering under nitrogen) concerning the crystal zation progress, due to differences in the heat treatment conditions, in particular holding time and atmosphere. 3.2 Evolution of the YMAS matrix(= within carbon fiber-reinforced composites) 20m As described in detail below, four main steps in the microstructure evolution of the ymas material as a matrix in a carbon-reinforced composite may be distinguished using XRD(Fig. 7) and TEM. The various steps in the microstructural development with increasing temperature/time conditions an summarized according to the following scheme Step 1: Majority phase is amorphous, with pre-existing aAlO3(corundum) and sub- ary amounts of MgAl2O AMg2Al4SisOI8 (indialite) cordierite. This SaE5um sintering temperatures <1000°C. Fig. 6. 1200 C-sintered (air atmosphere) YMAS glass-cera mic. SEM image(back-scattered electrons ):(a)low magnifi cation;(b) higher magnification. Peculiar textures revealed in Step 2: Rapid crystallization of indialite and Fig. 5 have disappeared, due to the widespread growth and aY2Si2O7 as major components, which are subsequent coalescence of crystallized areas associated in an eutectic-type microstructure. MgAlO4 is present as a minor component. ubsidiary BY2Si2O,(keiviite) is also already present. This corresponds to a narrow tempera a: Silicate d az ture range, around 1000-1050oC. Up to B: Silicate d yttrium阝 ≈1200°C, it is followed by the domain of struc- tural stability(no change in the X-RD spectra) Step 3: Coarsening of lamellar eutectic-type L战果 microstructure, evident by distortions of lamellae. Rapid transformation of aY2 Si2O to BY2siO ·Step4: Overall a toβ transformation of Y2Si2O7, though the a structure is still pres- ent. Dramatic coarsening of all phases. Cor angle2e(°) undum or spinel with submicron grain sizes Fig. 7. YMAS matrix in composites. XRD spectra. Heat- are no longer found. Loss of lamellar cordier- tre eatment temperature increases from bottom to top Sample ite/yttrium silicate microstructure and devel code numbers refer to Table P5 and Pll correspond to opment of globular microstructure. This may follows g and 4, respectively( see text). Peaks labelling is as correspond to a 1200-1250oC range β=BY2Si2O7 The heat treatments of the composites in the study of a particular step in microstructure develop- are given, along with their degree of microstructural ment are generally consistent with the classifica development from the above classification scheme, tion in Table 1, one of the composites(P7) was in Table 1. The classification is valid for holding heat-treated(annealed) for a significantly longer time of approximately I h at the temperatures time. This causes the microstructure development to mentioned. While the temperatures characteristic correspond to a further advanced state than its heat
powder after 1000 and 1200C sintering under air) with Figs 3 and 4 (YMAS powder after 1050C sintering under nitrogen) concerning the crystallization progress, due to dierences in the heattreatment conditions, in particular holding time and atmosphere. 3.2 Evolution of the YMAS matrix (= within carbon ®ber-reinforced composites) As described in detail below, four main steps in the microstructure evolution of the YMAS material as a matrix in a carbon-reinforced composite may be distinguished using XRD (Fig. 7) and TEM. The various steps in the microstructural development with increasing temperature/time conditions are summarized according to the following scheme: . Step 1: Majority phase is amorphous, with pre-existing Al2O3 (corundum) and subsidiary amounts of MgAl2O4 (spinel) and AMg2Al4Si5O18 (indialite) cordierite. This corresponds to sintering temperatures 1000C. . Step 2: Rapid crystallization of indialite and Y2Si2O7 as major components, which are associated in an eutectic-type microstructure. MgAl2O4 is present as a minor component. Subsidiary Y2Si2O7 (keiviite) is also already present. This corresponds to a narrow temperature range, around 1000±1050C. Up to &1200C, it is followed by the domain of structural stability (no change in the X-RD spectra). . Step 3: Coarsening of lamellar eutectic-type microstructure, evident by distortions of lamellae. Rapid transformation of Y2Si2O7 to Y2Si2O7. . Step 4: Overall to transformation of Y2Si2O7, though the structure is still present. Dramatic coarsening of all phases. Corundum or spinel with submicron grain sizes are no longer found. Loss of lamellar cordierite/yttrium silicate microstructure and development of globular microstructure. This may correspond to a 1200±1250C range. The heat treatments of the composites in the study are given, along with their degree of microstructural development from the above classi®cation scheme, in Table 1. The classi®cation is valid for holding time of approximately 1 h at the temperatures mentioned. While the temperatures characteristic of a particular step in microstructure development are generally consistent with the classi®cation in Table 1, one of the composites (P7) was heat±treated (annealed) for a signi®cantly longer time. This causes the microstructure development to correspond to a further advanced state than its heat Fig. 7. YMAS matrix in composites. XRD spectra. Heattreatment temperature increases from bottom to top. Sample code numbers refer to Table 1. T1, P5 and P11 correspond to Step 1, 2 and 4, respectively (see text). Peaks labelling is as follows: A=corundum, S=spinel, I=indialite, = -Y2Si2O7, = -Y2Si2O7. Fig. 6. 1200C-sintered (air atmosphere) YMAS glass-ceramic. SEM image (back-scattered electrons): (a) low magni®- cation; (b) higher magni®cation. Peculiar textures revealed in Fig. 5 have disappeared, due to the widespread growth and subsequent coalescence of crystallized areas. 310 W. Sinkler et al
Carbon fiber-reinforced(YMAS) glass-ceramic matrix composites: II 311 Table 1. Correspondence between the sample references, the conditions they were submitted to, and the various steps as they are defined in the text for the ymas matrix evolutio Reference Sintering conditions Annealing conditions Crystallization (with pressure) (without pressure) step Paper Laboratory 1000°C/h 1000°C/h 1050°C/lh P13 1050°C/lh 1050°C/lh 1050°C/1-5h l100°C/h 1150°C/lh 1200°C/h C6-2 050°C/1/h P10 1050°C0.5h 1250°C0.5h 2222223444 C6-3 1050°C/h 1250°C/1-5h T7 C8-2 1050°C/lh 1250°C/1-5h The first column refers to the sample references as they are used in the companion articles about the work. 1. The second colum is for internal use only treatment temperature would suggest(e. g. more micrograph. These are likely of corundum, due to advanced than for the 1200@C-sintered composite the small size of all other crystalline phases at this T6). This again indicates that kinetics is very stage, as revealed by TEM below. Also, it can be important, specifically in the temperature range noticed than the matrix tends to be darker close to where the crystallization occurs. Actually, the the carbon fibers than away from them, consistent overall a to B transformation temperature for with the reduction effect by carbon revealed in the 2Si20, is now more consistent with literature% study of the yMAS glass(Section 3.1) g the sintering and heat treatment conditions Figure 9 presents a SEM image(back scattered of Table I than it was for the YMas bulk glass electrons) of a similar area in composite P3. The ( see Section 3. 1 above). However, based on the carbon fibers appear dark due to their low back- above scheme, the development of the YMAs scattering yield Crystals in the matrix are visible as material as a matrix in a composite does not differ dark areas also, indicating yttrium-free phases substantially from that observed for the YMAs (corundum, and early crystallized cordierite and material as a bulk glass, despite some evidence of spinel). The yttrium-containing gla he glass reduction by carbon. The probable reason appears an intermediate gray (yttrium is the only is that the starting glass powder was the same for heavy element in the composite). X-EDS confirmed both materials, and had already been submitted to that crystals with elongated morphologies tend to a carboreduction effect during the glass powder preparation procedure(due to the use of a carbon furnace). Discrepancies (e.g. the crystallization state at 1000 C is more advanced for the composite than for the bulk) are believed to be rather due to the differences in the thermal treatment conditions such as the absence(bulk YMAs glass) or the presence (YMAS matrix composites) of oriented pressure during sintering (likely to inhibit struc Figure 8 shows a low magnification OM image (polarized light, in transmission) of the composite 20 um P3 with a microstructure corresponding to Step 1 Due to their strong absorbance, carbon fib Fig. 8. YMAS matrix in composite P3, at Step I(see text). dark Black-and-white duplication of an OM color image(trans- rounded by the MAS-Y matrix, the overall color of appear dark because of the high absorbance of polyaromatic which was magenta (light gray in the figure) indi- carbon for visible light. The gray contrast of the matrix cor cating a main amorphous state. Some large responds to magenta, indicating amorphous state. However, (a5um) crystals were detected as blue(single gray contrast) which reveal the occurrence of crystallized arrow) or yellow(double arrows) regions in the
treatment temperature would suggest (e.g. more advanced than for the 1200C-sintered composite T6). This again indicates that kinetics is very important, speci®cally in the temperature range where the crystallization occurs. Actually, the overall to transformation temperature for Y2Si2O7 is now more consistent with literature9 using the sintering and heat treatment conditions of Table 1 than it was for the YMAS bulk glass (see Section 3.1 above). However, based on the above scheme, the development of the YMAS material as a matrix in a composite does not dier substantially from that observed for the YMAS material as a bulk glass, despite some evidence of the glass reduction by carbon. The probable reason is that the starting glass powder was the same for both materials, and had already been submitted to a carboreduction eect during the glass powder preparation procedure (due to the use of a carbon furnace). Discrepancies (e.g. the crystallization state at 1000C is more advanced for the composite than for the bulk) are believed to be rather due to the dierences in the thermal treatment conditions, such as the absence (bulk YMAS glass) or the presence (YMAS matrix composites) of oriented pressure during sintering (likely to inhibit structural rearrangements). Figure 8 shows a low magni®cation OM image (polarized light, in transmission) of the composite P3 with a microstructure corresponding to Step 1. Due to their strong absorbance, carbon ®bers appear as dark circles in the micrograph, surrounded by the MAS-Y matrix, the overall color of which was magenta (light gray in the ®gure) indicating a main amorphous state. Some large (&5m) crystals were detected as blue (single arrow) or yellow (double arrows) regions in the micrograph. These are likely of corundum, due to the small size of all other crystalline phases at this stage, as revealed by TEM below. Also, it can be noticed than the matrix tends to be darker close to the carbon ®bers than away from them, consistent with the reduction eect by carbon revealed in the study of the YMAS glass (Section 3.1). Figure 9 presents a SEM image (back scattered electrons) of a similar area in composite P3. The carbon ®bers appear dark due to their low backscattering yield. Crystals in the matrix are visible as dark areas also, indicating yttrium-free phases (corundum, and early crystallized cordierite and spinel). The yttrium-containing glassy phase appears an intermediate gray (yttrium is the only heavy element in the composite). X-EDS con®rmed that crystals with elongated morphologies tend to Table 1. Correspondence between the sample references, the conditions they were submitted to, and the various steps as they are de®ned in the text for the YMAS matrix evolution. Reference Sintering conditions Annealing conditions Crystallization Paper Laboratory (with pressure) (without pressure) step T1 C4-2 1000C/1h Ð 1 P3 C3-5 1000C/1h Ð 1 P5 C5-3 1050C/1h Ð 2 P13 C7-1 1050C/1h Ð 2 P6 C5-5 1050C/1h 1050C/1.5h 2 P14 C7-2 1100C/1h Ð 2 T5 C4-4 1150C/1h Ð 2 T6 C8-1 1200C/1h Ð 2 P7 C6-2 1050C/1/h 1050C/6h 3 P10 C10-2 1050C/0.5h 1250C/0.5h 4 P11 C6-3 1050C/1h 1250C/1.5h 4 T7 C8-2 1050C/1h 1250C/1.5h 4 The ®rst column refers to the sample references as they are used in the companion articles about the work.1,3±5 The second column is for internal use only. Fig. 8. YMAS matrix in composite P3, at Step 1 (see text). Black-and-white duplication of an OM color image (transmitted, polarized light). Round parts are carbon ®bers, which appear dark because of the high absorbance of polyaromatic carbon for visible light. The gray contrast of the matrix corresponds to magenta, indicating amorphous state. However, single or double arrows indicate blue or yellow areas (light gray contrast) which reveal the occurrence of crystallized phases (corundum). Carbon ®ber-reinforced (YMAS) glass-ceramic matrix composites: II 311
312 W. Sinkler et al be corundum(double arrow ) Cordierite crystals exhibit a star-like structure similar to that seen for with more equiaxed morphologies could occasion- cordierite, but at a larger scale. Dark crystals,cor ally also be resolved (single arrow). A few excep- responding to corundum and cordierite (spinel tional crystals of yttrium silicate were also crystals are unlikely, since too small are also visi- detected, appearing as the bright areas in Fig. 9 their compositions were checked using X-EDS They are present in very small quantity, and were thus not resolvable using XRD. The microscopic appearance of cordierite precipitates at Step 1 is illustrated using TEM in Fig. 10. The radial leaf like form is consistent with the SEM image shown as Fig. 5(c), although the size is significantly smal ler, indicating that a more advanced stage in the microstructure development was reached in crystals(single arrow) exhibiting a specific sphe- rical morphology, and a corundum crystal (large crystal, double arrow) Step 2 is illustrated in Figs 11-13. In OM, using transmitted, polarised light, an obvious color change from magenta to orange-pink was observed I um with respect to the Step I matrix. This is consistent with a fine-crystalline matrix, in which optically Fig. 10. YMAS matrix in composite TI, at Step 1(see text) anisotropic domains, in addition to some remain- Low magnification TEM image Peculiar star-like morphology ing amorphous phase (isotropic= magenta),are for cordierite at a primary crystallization step, to be compared to similar textures revealed in the MAs-Y glass by seM superimposed in the specimen cross-section. In a [Fig. 5(c). Single arrow indicates a spinel crystal with SEM image shown in Fig. 1 l(back-scattered elec haracteristic spherical morphology, double arrow indicates a trons), large crystals of yttrium silicate are now evident, and appear white in the micrograph. They ∥m um Fig. 11. YMAS matrix in composite P5, at Step 2(see text) SEM image (back-scattered electrons). Round parts are Fig 9. YMAS matrix in composite P3, at Step I(see text). carbon fibers, which appear dark because of their low Z SEM image(back-scattered electrons). Round parts are car- number. Other dark (polyhedral) parts are cordierite or cor bon fibers, which appear dark because of their very low Z undum. Bright parts are yttrium-rich crystals (probably sili- number. Other dark parts are yttrium-free crystals (single cate)exhibiting the star-like morphology specific of a primary ordierite, double arrow =corundum, as deter- crystallisation state, as already observed for cordierite mined from X-EDS). Clear parts contain Y-SiO. Straight Figs 5(c)and 10]. The gray contrast of the bulk indicates that dark lines joining fibers are cracks. most of the yttrium is still disseminated in the matrix
be corundum (double arrow). Cordierite crystals with more equiaxed morphologies could occasionally also be resolved (single arrow). A few exceptional crystals of yttrium silicate were also detected, appearing as the bright areas in Fig. 9; their compositions were checked using X-EDS. They are present in very small quantity, and were thus not resolvable using XRD. The microscopic appearance of cordierite precipitates at Step 1 is illustrated using TEM in Fig. 10. The radial leaflike form is consistent with the SEM image shown as Fig. 5(c), although the size is signi®cantly smaller, indicating that a more advanced stage in the microstructure development was reached in Fig. 5(c). In Fig. 10 are also imaged small spinel crystals (single arrow) exhibiting a speci®c spherical morphology, and a corundum crystal (large crystal, double arrow). Step 2 is illustrated in Figs 11±13. In OM, using transmitted, polarised light, an obvious color change from magenta to orange-pink was observed with respect to the Step 1 matrix. This is consistent with a ®ne-crystalline matrix, in which optically anisotropic domains, in addition to some remaining amorphous phase (isotropic = magenta), are superimposed in the specimen cross-section. In a SEM image shown in Fig. 11 (back-scattered electrons), large crystals of yttrium silicate are now evident, and appear white in the micrograph. They exhibit a star-like structure similar to that seen for cordierite, but at a larger scale. Dark crystals, corresponding to corundum and cordierite (spinel crystals are unlikely, since too small), are also visiFig. 9. YMAS matrix in composite P3, at Step 1 (see text). SEM image (back-scattered electrons). Round parts are carbon ®bers, which appear dark because of their very low Z number. Other dark parts are yttrium-free crystals (single arrow = cordierite, double arrow = corundum, as determined from X-EDS). Clear parts contain Y±Si±O. Straight dark lines joining ®bers are cracks. Fig. 10. YMAS matrix in composite T1, at Step 1 (see text). Low magni®cation TEM image. Peculiar star-like morphology for cordierite at a primary crystallization step, to be compared to similar textures revealed in the MAS-Y glass by SEM [Fig. 5(c)]. Single arrow indicates a spinel crystal with a characteristic spherical morphology, double arrow indicates a corundum crystal. Fig. 11. YMAS matrix in composite P5, at Step 2 (see text). SEM image (back-scattered electrons). Round parts are carbon ®bers, which appear dark because of their low Z number. Other dark (polyhedral) parts are cordierite or corundum. Bright parts are yttrium-rich crystals (probably silicate) exhibiting the star-like morphology speci®c of a primary crystallisation state, as already observed for cordierite [Figs 5(c) and 10]. The gray contrast of the bulk indicates that most of the yttrium is still disseminated in the matrix. 312 W. Sinkler et al
Carbon fiber-reinforced (YMAS) glass-ceramic matrix composites: II 313 ble in the matrix. The remainder of the matrix is and the small observable areas in TEM specimen now an intermediate gray, indicating that some The phase of the second yttrium silicate population yttrium remains here(by comparison to the crys- could therefore not be verified tals containing only lighter element than Si, which Electron diffraction of the eutectic bands of cor are darker). The microstructure in these inter- dierite and aY2SiO had a single-crystalline mediate gray regions could not be resolved using aspect, indicative of a structural relationship SEM. At higher magnification, in TEM, a peculiar between the two phases Based on high resolution crystalline microstructure was resolved in such micrographs such as Fig. 13, it could be deter region, which consisted of alternating bands less mined that the(002) planes of orthorhombic cor than 100 nm in width of cordierite and yttrium dierite are parallel to the interfaces of the lamellae silicate, as shown in Fig. 12. In the micrograph, the Using electron diffraction and analysis of high yttrium silicate appears dark due to yttrium's resolution TEM micrographs, a provisional orien stronger inelastic scattering. Additional crystals tation relationship between aY2Si20, a core present in Fig 12 are corundum(single arrow )and ite was determined, such that: (101a//(001) and spinel (double arrow) (001)a//(021)e where the subscripts a and c are for Figures 11 and 12 indicate the existence of two yttrium silicate and cordierite, respectively. In populations of yttrium silicate at this stage: one addition to the latter crystalline phases, large crys- population is star-shaped and has large morphol- tals of corundum(Fig. 12, single arrow), possibly ogy on the order of l um resolvable by seM already present prior to composite production, and ( Fig. 11). The other has lamellar alternating band round-shape spinel crystals exhibiting a specific microstructure(Fig. 12). Lamellar microstructures such as this are typical of eutectic decomposition ransformations, which suggest the presence of such an eutectic in the cordierite/yttrium silicate system. XRD(Fig. 7)has revealed that both aY2Si2O, and BY2Si2O7(keiviite)are present in the material at this stage. While it was possible to ver ify using electron diffraction that the yttrium sili cate of the lamellar eutectic microstructure was the a form at this stage, none of the star-like yttrium silicate population was observed in teM due to its sparse and irregular distribution in the composite 100nm Fig. 12. YMAS matrix in composite P5, at Step 2(see text) 10 nm bands are aY? Si,O, and aMg,AlgSisO(indialite), respec- Fig YMAS matrix in composite P13, at Step 2(see tively, which is a typical eutectic microstructure. Single arrow text). The lattice period is that of (002) planes from indialite at indicates a corundum crystal. Double arrows indicate spinel 0-467 nm. Though separated by aY2Si2O7 bands, the indialite crystals. The dark contrast of the yttrium silicate bands is due bands exhibit the same cristallographic orientation. The to the high Z number of yttrium. The dark contrast of the aY2Si2O7 lattice was not under the Bragg angle and therefore corundum crystals is due to excess thickne was not imaged
ble in the matrix. The remainder of the matrix is now an intermediate gray, indicating that some yttrium remains here (by comparison to the crystals containing only lighter element than Si, which are darker). The microstructure in these intermediate gray regions could not be resolved using SEM. At higher magni®cation, in TEM, a peculiar crystalline microstructure was resolved in such region, which consisted of alternating bands less than 100 nm in width of cordierite and yttrium silicate, as shown in Fig. 12. In the micrograph, the yttrium silicate appears dark due to yttrium's stronger inelastic scattering. Additional crystals present in Fig. 12 are corundum (single arrow) and spinel (double arrow). Figures 11 and 12 indicate the existence of two populations of yttrium silicate at this stage: one population is star-shaped and has large morphology on the order of 1m resolvable by SEM (Fig. 11). The other has lamellar alternating band microstructure (Fig. 12). Lamellar microstructures such as this are typical of eutectic decomposition transformations, which suggest the presence of such an eutectic in the cordierite/yttrium silicate system. XRD (Fig. 7) has revealed that both Y2Si2O7 and Y2Si2O7 (keiviite) are present in the material at this stage. While it was possible to verify using electron diraction that the yttrium silicate of the lamellar eutectic microstructure was the form at this stage, none of the star-like yttrium silicate population was observed in TEM due to its sparse and irregular distribution in the composite and the small observable areas in TEM specimen. The phase of the second yttrium silicate population could therefore not be veri®ed. Electron diraction of the eutectic bands of cordierite and Y2Si2O7 had a single-crystalline aspect, indicative of a structural relationship between the two phases. Based on high resolution micrographs such as Fig. 13, it could be determined that the (002) planes of orthorhombic cordierite are parallel to the interfaces of the lamellae. Using electron diraction and analysis of high resolution TEM micrographs, a provisional orientation relationship between Y2Si2O7 and cordierite was determined, such that: ( 101) //(001)c and (001) //(021)c where the subscripts and c are for yttrium silicate and cordierite, respectively. In addition to the latter crystalline phases, large crystals of corundum (Fig. 12, single arrow), possibly already present prior to composite production, and round-shape spinel crystals exhibiting a speci®c Fig. 12. YMAS matrix in composite P5, at Step 2 (see text). Low magni®cation TEM image. Alternate dark and clear bands are Y2Si2O7 and Mg2Al4Si5O18 (indialite), respectively, which is a typical eutectic microstructure. Single arrow indicates a corundum crystal. Double arrows indicate spinel crystals. The dark contrast of the yttrium silicate bands is due to the high Z number of yttrium. The dark contrast of the corundum crystals is due to excess thickness. Fig. 13. YMAS matrix in composite P13, at Step 2 (see text).The lattice period is that of (002) planes from indialite at 0.467 nm. Though separated by Y2Si2O7 bands, the indialite bands exhibit the same cristallographic orientation. The Y2Si2O7 lattice was not under the Bragg angle and therefore was not imaged. Carbon ®ber-reinforced (YMAS) glass-ceramic matrix composites: II 313
314 W. Sinkler et al spherulitic aspect(Fig. 12, double arrow) were also of the aY Si2O, structure reported by Liddell and Thompson. It is likely that the change in mor- The composite representative of Step 3(P7)was phology and the structural aB transformation annealed for an extended time of 6 h at 1050 C(see are related as the developement of BY2Si2O7 would Table 1). The XRD spectrum did not differ at this change the cordierite/yttrium silicate interface point from that after Step 2, and no significant structure. This may result in a loss of coherency differences were detected in SEM. Figure 14 shows and an increase in the surface energy, which would a TEM image of the lamellar cordierite/yttrium increase the driving force for morphological silicate microstructure corresponding to that dis- change and/or coarsenin cussed above for Step 2. The wavy form of th The final step in the microstructure evolution of lamellae is indicative of the beginning of a mor- the YMAS matrix, Step 4, is represented by phology change for the lamellar microstructure due to the beginning of coarsening, and possibly driven by a minimization of the surface area asso- ciated with the microstructure. In addition the Y2Si2O7 crystals exhibit numerous stacking faults at this stage(Fig. 15) which are not observed at Step 2. This is presumably related to the gradual a-B transformation of Y2Si2O7. The funda mental planes parallel to the stacking disorder evi dent in Fig. 16 are consistent with the(011)planes nm Fig. 15. YMAS matrix in composite P7, at Step 3(see High resolution TEM image. The aY2Si2O7 lattice now bits a peculiar contrast revealing stacking faults. This :85m Fig. 16. YMAS matrix in composite Pll, at Step 4(see text) SEM image(back-scattered electrons). Both carbon fibers and 50 nm the remaining bulk have now a similar dark contrast, due to the widespread concentration of yttrium into large keiviite crystals. Most of the keiviite crystals now exhibit a round Fig. 14. YMAS matrix in composite P7, at Step 3(see text). Low magnification TEM image. The lamellar eutectic microstructure morphology, the sizes of which are large enough to be imaged by SEM. Elongated crystals are inherited from the previous deforms, and lamellar crystals tend to become spherical
spherulitic aspect (Fig. 12, double arrow) were also found. The composite representative of Step 3 (P7) was annealed for an extended time of 6 h at 1050C (see Table 1). The XRD spectrum did not dier at this point from that after Step 2, and no signi®cant dierences were detected in SEM. Figure 14 shows a TEM image of the lamellar cordierite/yttrium silicate microstructure corresponding to that discussed above for Step 2. The wavy form of the lamellae is indicative of the beginning of a morphology change for the lamellar microstructure, due to the beginning of coarsening, and possibly driven by a minimization of the surface area associated with the microstructure. In addition, the Y2Si2O7 crystals exhibit numerous stacking faults at this stage (Fig. 15) which are not observed at Step 2. This is presumably related to the gradual ! transformation of Y2Si2O7. The fundamental planes parallel to the stacking disorder evident in Fig. 16 are consistent with the (0 11) planes of the Y2Si2O7 structure reported by Liddell and Thompson.9 It is likely that the change in morphology and the structural ! transformation are related as the developement of Y2Si2O7 would change the cordierite/yttrium silicate interface structure. This may result in a loss of coherency and an increase in the surface energy, which would increase the driving force for morphological change and/or coarsening. The ®nal step in the microstructure evolution of the YMAS matrix, Step 4, is represented by Fig. 14. YMAS matrix in composite P7, at Step 3 (see text). Low magni®cation TEM image. The lamellar eutectic microstructure deforms, and lamellar crystals tend to become spherical. Fig. 15. YMAS matrix in composite P7, at Step 3 (see text). High resolution TEM image. The Y2Si2O7 lattice now exhibits a peculiar contrast revealing stacking faults. This might precede the ! transformation. Fig. 16. YMAS matrix in composite P11, at Step 4 (see text). SEM image (back-scattered electrons). Both carbon ®bers and the remaining bulk have now a similar dark contrast, due to the widespread concentration of yttrium into large keiviite crystals. Most of the keiviite crystals now exhibit a round morphology, the sizes of which are large enough to be imaged by SEM. Elongated crystals are inherited from the previous lamellar microstructure. 314 W. Sinkler et al
