Barium Magnesium Aluminosilicate Research Articles

Thermal cycling effect on the phase stability and fracture resistance of synthetic barium/magnesium aluminosilicate systems

This paper focuses on the high-temperature solid-state synthesis of barium-magnesium-aluminosilicate (BMAS) system and its thermal cycling stability between the room temperature and 1200 °C. The BMAS powder was synthesized by adding 0.8 mol of MgO for 1 mol of BaAl2Si2O8 (BAS system) to get a stabilized monoclinic phase (celsian-BMAS). The undoped BAS system with a metastable hexagonal phase (hexacelsian-BAS) was also produced for comparison. Both celsian-BMAS and hexacelsian-BAS powders were sintered by spark plasma sintering (SPS) technique at 1300 °C and 50 MPa. In-situ X-ray diffraction and thermogravimetric analyses together with hardness and indentation fracture resistance measurements proved the thermal stability of the celsian-BMAS system. In contrast, the hexacelsian-BAS compound suffered typical phase transformations during the thermal cycling test, causing its failure in less than 25 cycles. The presence of magnesium in the BMAS system led to the formation of secondary phases, such as Ba/Mg-rich micas and/or barium silicides, which did not affect the thermal integrity of the stable celsian phase. The celsian-BMAS system exhibited a linear thermal expansion in all unit cell directions, giving better thermal reliability to this material over the hexacelsian-BAS system. Besides, this work emphasizes the phase transformations in the studied systems that have been ignored in previous research, and it also compares several common equations for calculation of the indentation fracture resistance.

Formation of a Metastable Celsian‐Cordierite Eutectic Structure During the Crystallization of a Ba‐Osumilite Glass–Ceramic

This work addresses the formation conditions and stability of a celsian‐cordierite eutectic structure produced during the crystallization of a barium magnesium aluminosilicate (BMAS) glass–ceramic with the Ba‐osumilite stoichiometry. The melt‐derived BMAS glass–ceramic was pulverized and subsequently hot pressed to produce sintered powder compacts, some of which were subjected to post‐sintering heat treatments in air. The study of the devitrified BMAS glass revealed that a metastable celsian‐cordierite eutectic structure resulted from the prolonged annealing of the BMAS glass–ceramic between 860° and 1000°C. The celsian‐cordierite eutectic structure transformed to the thermodynamically stable Ba‐osumilite phase when annealed at temperatures >1000°C.

Study of the devitrification behaviour of a barium magnesium aluminosilicate glass-ceramic

This work presents the most important aspects of the devitrification behaviour of a barium magnesium aluminosilicate (BMAS) glass-ceramic with the Ba-osumilite stoichiometry. The melt-derived BMAS glass-ceramic was first pulverised and subsequently hot pressed to produce sintered powder compacts, some of which were subjected to post-sintering heat treatments in air. The BMAS glass devitrification behaviour was studied by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction (XRD). This study revealed that at temperatures <1000 °C, the BMAS glass crystallisation yielded several metastable crystalline phases (i.e. celsian, hexacelsian, α/β-cordierite, and β-quartz ss) instead of the thermodynamically stable Ba-osumilite. On the other hand, annealing of the BMAS glass at temperatures ≥1000 °C caused the transformation of all metastable phases to Ba-osumilite. Understanding the crystallisation behaviour of the BMAS glass-ceramic is essential in order to use this material as matrix in fibre-reinforced composites suitable for high-temperature structural applications.

Improvement of Young's modulus and tensile strength of polymer impregnation and pyrolysis processed SiC/SiC composite by improved continuity of matrix

Influences of the continuity of the matrix on Young's modulus and tensile strength of unidirectional SiC/SiC mini-composite prepared by the polymer impregnation and pyrolysis method were studied experimentally by observation of appearance of matrix and tensile test and analytically by a shear lag–Monte Carlo simulation. The continuity of the matrix was improved by the addition of particles such as ZrSiO4, barium magnesium aluminosilicate, and Pyrex (borosilicate glass) into the matrix. The improved continuity of the matrix led to the increase in stress carrying capacity of the matrix and therefore to the increase in Young's modulus and tensile strength of the composite. Such a correlation between the continuity of the matrix and the property of the composite was verified numerically by the shear lag–Monte Carlo simulation.

Thermal-shock behavior of a Nicalon-fiber-reinforced hybrid glass-ceramic composite

A Nicalon-fiber-reinforced hybrid composite with a matrix of barium magnesium aluminosilicate (BMAS) glass with silicon carbide whiskers was subjected to thermal shock from elevated to ambient temperatures. The combination of SiC whisker and BMAS glass resulted in a hybrid matrix with a lower thermal expansion than that of the fibers, inducing tensile stresses in the fiber upon thermal shock. This stress state resulted in microstructural damage in the form of fiber cracking and cracking along the fiber/matrix interface, as opposed to the conventional matrix cracking which is typically observed in ceramic-matrix composites. Significant damage in the composite was only observed after three thermal shock cycles. Flexural resonance measurements, used to evaluate thermal shock-induced changes in Young's modulus, showed a reduction in modulus that correlated well with the onset of microstructural damage. Finally, fiber push-out tests, performed to evaluate changes in fiber/matrix interface strength after thermal cycling, indicated a slight decrease in interfacial strength, which was attributed to recession of the carbon-rich fiber surface during thermal shock.

Fracture behavior of notched unidirectional Si-Ti-C-O/BMAS composite

Experimental and modeling studies on tensile fracture behavior of notched unidirectional Si-Ti-C-O (Tyranno fiber) reinforced BMAS (barium magnesium aluminosilicate) glass matrix composite were carried out. The longitudinal crack arose at the tip of the transverse notch before overall fracture. The critical energy release rate around at initiation of the longitudinal cracking was estimated to be nearly 100 J/m2 by application of the present model to the experimentally observed relation between the stress of the composite in the very early stage of longitudinal cracking and the notch size. The notched strength was higher than that predicted by the fracture mechanical criterion due to the blunting arising from the premature longitudinal cracking, but it was lower than that predicted by the net stress criterion due to the constraint effect arising from the bridging of the fibers and the spalling of the segmented matrix into the longitudinal crack.

Processing of platelet-reinforced glass matrix composites: effect of inclusions on sintering anisotropy

The effect of rigid, i.e. non-densifying inclusions on the densification kinetics and shrinkage anisotropy during the sintering of composite glass powder compacts is investigated. The experimental study was conducted on barium–magnesium aluminosilicate (BMAS) glass composites containing Al 2O 3-platelets as a rigid inclusion phase. Data on axial and radial shrinkage during sintering were obtained by using heating microscopy, which allows monitoring the densification process without the exertion of external loads, which could influence the sintering kinetics. The presence of rigid platelets retarded the densification of the compacts. The sintering behavior of all samples was anisotropic, with the degree of anisotropy varying with the progress of sintering and with the platelet content. A comprehensive qualitative explanation for the effect of rigid inclusions on sintering shrinkage anisotropy for both intrinsically isotropic and anisotropic powder compacts is offered. In general, the influence of rigid inclusions on shrinkage anisotropy can be explained on the basis of the retardation effect that inclusions have on the sintering and densification kinetics of porous glass compacts. The ideas advanced in this paper form the basis for the development of a predictive theoretical model for the effect of rigid inclusions on shrinkage anisotropy of glass powder compacts during sintering.

Computational analysis of mixed-mode delamination crack growth in a woven laminated ceramic-matrix composite

Mixed-mode delamination crack growth characteristics in a Nicalon/glass (barium magnesium alumino-silicate, BMAS) woven laminated ceramic-matrix composite (CMC) have been analyzed. Delamination crack growth under mixed-mode conditions was investigated by the use of a computational mechanics approach. For the purpose of this study the CMC specimen considered was in four-point bending. A T-crack configuration was investigated with particular attention to determining the influence of notch depth and delamination crack length on the effective strain-energy release rate.

In Situ Characterization of the Shrinkage Behavior of Ceramic Powder Compacts during Sintering by Using Heating Microscopy

Heating microscopy, used as optical dilatometry, provides an advantageous experimental method for characterizing in situ the sintering behavior of ceramic powder compacts. It enables the densification process to be monitored without the application of load, and thus, there is minimal interaction between the sintering sample and external constraints. In this study, the application of the technique in studing the constant-heating-rate sintering of different ceramic powder compacts is shown, focusing on assessing the isotropic or anisotropic shrinkage behavior of the powder compacts investigated. All compacts were made from powders having similar (irreguIar) particle shapes and were obtained by the same uniaxial compaction technique with the use of the same compacting pressures. However, both mullite and yttrium disilicate powder compacts showed isotropic sintering behavior, whereas barium magnesium aluminosilicate glass powder compacts showed shrinkage anisotropy. The complexities in predicting the occurrence of shrinkage anisotropy during sintering and its variation with the progress of densification are outlined briefly.

Intermediate‐Temperature Environmental Effects on Boron Nitride‐Coated Silicon Carbide‐Fiber‐Reinforced Glass‐Ceramic Composites

The environmental effects on the mechanical properties of fiber‐reinforced composites at intermediate temperatures were investigated by conducting flexural static‐fatigue experiments in air at 600° and 950°C. The material that was studied was a silicon carbide/boron nitride (SiC/BN) dual‐coated Nicalon‐fiber‐reinforced barium magnesium aluminosilicate glass‐ceramic. Comparable time‐dependent failure responses were found at 600° and 950°C when the maximum tensile stress applied in the bend bar was 60% of the room‐temperature ultimate flexural strength of as‐received materials. At both temperatures, the materials survived 500 h fatigue tests at lower stress levels. Among the samples that survived the 500 h fatigue tests, a 20% degradation in the room‐temperature flexural strength was measured in samples tested at 600°C, whereas no degradation was observed for the samples tested at 950°C. Microstructure and chemistry studies revealed interfacial oxidation in the samples that were fatigued at 600°C. The growth rate of the Si‐C‐O fiber oxidation product at 600°C was not sufficient to seal the stress‐induced cracks, so that the interior of the material was oxidized and resulted in a strength degradation and less fibrous fracture. In contrast, the interior of the material remained intact at 950°C because of crack sealing by rapid silicate formation, and strength/toughness of the composite was maintained. Also, at 600°C, BN oxidized via volatilization, because no borosilicate was formed.

Effect of preparation parameters on the properties of unidirectional SiC-fibre-reinforced MAS and BMAS glass-ceramics

The objective of this paper is to investigate the possibilities of fibre-reinforced glass-ceramics based on cordierite (MAS, magnesium alumino-silicate) and barium-osumilith (BMAS, barium magnesium alumino-silicate) and to find optimum preparation conditions with respect to mechanical properties, particularly high strength and toughness values. The prepregs were prepared by the sol-gel slurry method, the composites by application of the most effective densification conditions via the molten and high-viscosity state and by crystallisation under various temperature/pressure/time programmes. Best properties were obtained in the BMAS system with 14 wt% BaO at a relatively low SiC-fibre content of 20 to 30 vol % (strength: 534 MPa, work of fracture: 16 kJ m−2). Higher values of strength were obtained with a predominant glass phase (up to 900 MPa) but these composites have only low application temperatures. Here, the optimum fibre concentration is much higher (about 50 vol%) than in the crystallised composites. The lower optimum fibre content of the predominantly crystallised composites is a consequence of a different phase composition and microstructure of the matrix compared with that without the presence of fibres under the same thermal and pressure conditions.

Densification and crystallization of glass powder compacts during constant heating rate sintering

Heating microscopy was used to study the interaction between the processes of densification and crystallization of glass powder compacts under constant heating rate sintering conditions without the application of external loads. For barium magnesium aluminosilicate (BMAS) glass powder compacts sintered between 800–1100 °C, it has been shown that the relative rates of crystallization and densification can be controlled by changing the heating rate. Samples sintered at a high heating rate of 15 K min −1 could be fully densified in the amorphous state, delaying the onset of crystallization. In the samples sintered at a low heating rate of 1 K min −1, the onset of crystallization coincided with the termination of densification at ∼ 1000 °C. Since the experiments were performed without an application of external loads, the results are applicable for the manufacturing of dense BMAS glass-ceramics via a pressureless sintering route.

High‐Temperature Tensile Behavior of a Boron Nitride‐Coated Silicon Carbide‐Fiber Glass‐Ceramic Composite

Tensile properties of a cross‐ply glass‐ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass‐ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady‐state creep rates in the 10−9 s−1 range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 105 cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high‐temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage‐tolerant fracture behavior. However, oxidation of near‐surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro‐cracks along which oxygen could diffuse into the interior of the composite.

Densification and crystallisation behaviour of barium magnesium aluminosilicate glass powder compacts

The densification and crystallisation of barium magnesium aluminosilicate (BMAS) glass powder has been investigated. The aim of the study was to draw conclusions of value for the optimisation of the processing parameters for BMAS matrix ceramic composites. Pressureless sintering and hot-pressing techniques were investigated. The pressureless densification behaviour of cold-uniaxially pressed compacts was determined at isothermal and constant heating rate conditions using a high temperature microscope. The samples could be densified isothermally to full density at 930 °C prior to the onset of crystallisation. For compacts sintered at constant heating rates between 800 and 1100 °C, it was found that the simultaneous occurrence of crystallisation and densification strongly depends on the heating rate. Using hot-pressing (pressure = 20 MPa) results in full densification in the amorphous state after hour at 925°C. X-ray diffraction analysis was used to characterise the crystallinity of pressureless sintered and hot-pressed samples that were fabricated at temperatures between 850° and 1300 °C. The crystallisation behaviour did not change, in qualitative terms, with the pressure applied during hot-pressing. Combination of the densification and crystallisation results demonstrated that the BMAS glass can be densified completely at relatively low temperatures (930 °C) in the glassy state. The material can be subsequently crystallised at higher temperatures (between 1100 and 1300 °C) yielding a high-temperature-resistant microstructure consisting of Ba-osumilite, celsian and cordierite.

Fracture behavior and creep-fatigue response of fiber reinforced glass-ceramic matrix composites

The mechanical behavior of a continuous silicon carbide fiber reinforced Barium Magnesium Aluminosilicate (BMAS) glass-ceramic matrix composite is investigated at room and high temperatures. The materials were heat-treated in an oxidizing environment for 1h at 1100°C previous to mechanical testing. The static fracture behavior at room and high temperatures was analyzed in direct tension tests, while the creep-fatigue (cyclic-creep) behavior was estimated in four-point flexural tests at 1100°C. The experimental results of tensile tests have highlighted the importance of the carbon-rich layer at the fiber/matrix interface for obtaining “graceful” failures. At high temperatures (1100°C), oxidative degradation of the interface results in significant strength reduction and a transition to the brittle fracture mode. The creep-fatigue results at different stress levels are analyzed in terms of the creep-recovery behavior. Extensive viscous strain recovery was found upon the unloading period. The crept composites retained their “graceful” fracture behavior after testing, indicating that no (or limited) damage in the matrix was induced during cyclic creep at the conditions tested.

Creep and densification during anisotropic sintering of glass powders

The isothermal sintering behaviour of a barium magnesium aluminosilicate glass powder at 930°C was investigated using a heating microscope. The cylindrical samples exhibited a variable shrinkage anisotropy during sintering. The shrinkage anisotropy ratio, defined as the ratio of the relative change of height and diameter, varied linearly between ∼0.3 and ∼0.98 with the relative volume shrinkage during densification. Shrinkage anisotropy caused creep deformation of the samples. The creep rate varied exponentially with the densification rate and the ratio of creep to densification rates, \({{\dot \varepsilon _{\text{c}} } \mathord{\left/ {\vphantom {{\dot \varepsilon _{\text{c}} } {\dot \varepsilon _\rho }}} \right. \kern-\nulldelimiterspace} {\dot \varepsilon _\rho }}\), decreased as densification proceeded. This is in disagreement with most previous studies, which show a constant value of \({{\dot \varepsilon _{\text{c}} } \mathord{\left/ {\vphantom {{\dot \varepsilon _{\text{c}} } {\dot \varepsilon _\rho }}} \right. \kern-\nulldelimiterspace} {\dot \varepsilon _\rho }}\) during the densification. Overall, the study points out the relevance of variable shrinkage anisotropy and how it affects the densification behaviour of glass powders.

Creep and creep‐fatigue behaviour of continuous fibre reinforced glass‐ceramic matrix Composites

AbstractThe flexural creep and creep strain recovery behaviour during creep‐fatigue tests of a cross‐ply SiC fibre reinforced Barium Magnesium Aluminosilicate glass‐ceramic matrix composite was investigated at 1100°C in air. Only heat‐treated samples (1 h at 1100°C) were tested. Stress levels of 90, 105 and 120 MPa were examined to produce low strains („0.4”︁). A continuously decreasing creep strain rate with values between 1.6 × 10−6 s−1 to 4.7 × 10−8s−i at 120 MPa was observed with no steady‐state regime. Extensive viscous strain recovery was found upon the unloading period during the short‐duration cyclic creep (creep‐fatigue) experiments. The creep strain recovery was quantified using strain recovery ratios. These ratios showed a slight dependence on the stress and cyclic loading frequencies investigated. The crept composites retained their „graceful”︁ fracture behaviour after testing indicating that no (or limited) damage in the matrix was induced during creep and creep‐fatigue loading.

Anisotropic shrinkage of barium-magnesium aluminosilicate glass powder compacts during sintering

Abstract The shrinkage behaviour of barium-magnesium aluminosilicate glass powder compacts has been measured at isothermal conditions using a heating microscope. The samples showed a varying shrinkage anisotropy during densification at a temperature of 930 °C. This behaviour could not be explained solely on the basis of the pore/solid interface orientation during sintering. The experimental values for the shrinkage behaviour are in very good agreement with the theoretical prediction of the stereology-based model equation of Exner and Giess.

Interfacial Microstructure and Chemistry of SiC/BN Dual‐Coated Nicalon‐Fiber‐Reinforced Glass‐Ceramic Matrix Composites

Glass‐ceramic composites with improved high‐temperature mechanical properties have been produced by incorporating continuous SiC fibers into a barium magnesium aluminosilicate matrix. Control of the fiber/matrix interface was achieved by a dual‐layer coating of SiC/BN(C) applied to the fibers by CVD. The weakly bonded interface resulted in composites with high fracture toughness and strength up to 1100°C, and the composite system was oxidatively stable during long‐term exposure to air at high temperatures. Composites with different thermal and mechanical histories were studied, and interfaces were characterized using transmission electron microscopy (TEM), Auger electron spectroscopy, and fiber pushout tests. Observations of interfacial microstructure were correlated with the mechanical properties of the composite and with interface properties determined from fiber push‐out tests.

Interfacial studies of refractory glass-ceramic-matrix/advanced-SiC-fiber-reinforced composites

The main objective of this study was to characterize the chemistry and structure of new advanced small diameter silicon based fibers and to determine how these factors influence the nature of the fiber-matrix interface in refractory glass-ceramic matrix composites. It is the nature of this interface that then determines to a great degree the thermal, environmental, and mechanical properties of the composite. The fibers to be discussed in this paper include the experimental polymer derived crystalline SiC fibers Dow Corning Corp., the SiNCO “Black” fibers from Textron Specialty Materials, and the new low oxygen radiation cured Nicalon SiC type fibers from Nippon Carbon Co. Since the availability of all of these fibers was extremely limited, emphasis was placed on the mechanical (ultimate tensile strength), chemical (scanning Auger), and microstructural (scanning and transmission electron microscopies) characterization of the fibers, as well as their fracture behavior, bonding characteristics, and interfacial compatibility with various glass-ceramic matrix materials. It was found that the Dow Corning SiC fibers exhibited a varied microstructure and chemistry that ranged from primarily large grains (300–400 nm) of β-SiC to a mixture of finer grain size (100–150 nm) SiC surrounded by extremely fine grained (less than 10 nm) graphite. When incorporated into a lithium aluminosilicate or barium-magnesium alumino-silicate (BMAS) glass-ceramic matrix composite, a very thin (approximately 12 nm) graphitic carbon-rich layer was found to have formed at the fiber-matrix interface, resulting in debonding between fiber and matrix during composite fracture. In a very refractory barium aluminosilicate matrix, the fibers did not appear to form the carbon-rich interface. The Textron “Black” fibers were found to be very similar to Dow Corning's HPZ fibers in structure and composition, except they contained more carbon (approximately 27 at.%) than HPZ (approximately 19%). From reaction couples with a BMAS glass-ceramic matrix, it was found that, like HPZ fibers, a strongly bonded reaction zone of Si 2N 2O was formed at the fiber-matrix interface. Fiber coatings would be necessary to impart a weakly bonded interface for fracture tough composites to be realized. The low oxygen Nicalon fibers were found to be about 38% stronger and 42% stiffer than comparable commercially available ceramic grade Nicalon fibers, and to be much less prone to degradation on exposure to high temperature (1300 °C). Like ceramic grade Nicalon fibers, the low oxygen fibers formed a weakly bonded carbon-rich interface when incorporated into a BMAS glass-ceramic matrix.