Aluminosilicate Glass-ceramic Matrix Research Articles

Influence of brick pattern interface structure on mechanical properties of continuous carbon fiber reinforced lithium aluminosilicate glass–ceramics matrix composites

Continuous carbon fiber reinforced lithium aluminosilicate glass–ceramic matrix composites have been fabricated by sol–gel process and hot pressing technique. The results show that the Cf/β-eucryptite composites hot pressed at 1300 °C and Cf/β-spodumene composites hot pressed at 1400 °C form weak interface with brick pattern characteristics, leading to high mechanical performance. The maximum flexural strength and fracture toughness reach 571 ± 32 MPa and 9.8 ± 0.6 MPa m1/2 for Cf/β-eucryptite composites and 640 ± 72 MPa and 19.9 ± 1.8 MPa m1/2 for Cf/β-spodumene composites. On increasing the hot pressing temperature, the active chemical diffusion consumes brick pattern interface layer, which leads to the formation of strong bonding between carbon fiber and the matrix. As a result, the composites exhibit brittle fracture behavior and the mechanical properties decrease significantly.

Microstructure compatibility and its effect on the mechanical properties of the α-SiC/β-Si3N4 co-reinforced barium aluminosilicate glass ceramic matrix composites

Monolithic α-SiC particles and in situ β-Si3N4-reinforced barium aluminosilicate (BAS) glass ceramic matrix composites with good mechanical properties were obtained by spark plasma sintering. However, attempts at the synthesis of ternary phase composites in order to maximize the reinforcing effects by co-adding α-SiC and β-Si3N4 were only partly successful. β-Si3N4 grain growth was remarkably hindered by the coexistent α-SiC, thus the resultant composite suffered a loss of reinforcement efficiency.

Fabrication and mechanical properties of carbon short fiber reinforced barium aluminosilicate glass–ceramic matrix composites

Dense BaSi 2Al 2O 8 (BAS) and Ba 0.75Sr 0.25Si 2Al 2O 8 (BSAS) glass–ceramic matrix composites reinforced with carbon short fibers (C sf) were fabricated by hot pressing technique. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transmission electron microscopies, and three-point bend tests. The carbon fibers had a good chemical compatibility with the glass–ceramic matrices and can effectively reinforce the BAS (or BSAS) glass–ceramic because of associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects. Doping of BAS with 25 mol% SrSi 2Al 2O 8 (SAS) can accelerate the hexacelsian to celsian transformation and result in the formation of pure monoclinic celsian in C sf/BSAS composites, which can avoid the undesirable reversible hexacelsian to orthorhombic transformation at ∼300 °C and reduce the thermal expansion mismatch between the fiber and matrix.

Thermographic Damage Detection of Ceramic Matrix Composites During Tension Testing

The fracture behavior of Nicalon fiber reinforced calcium aluminosilicate (CAS) glass-ceramic matrix composites (Nicalon/CAS) was investigated with the aid of a nondestructive evaluation (NDE) technique. Infrared (IR) thermography was employed for unidirectional Nicalon/CAS composite specimens. During tensile testing, an IR camera was used for in-situ monitoring of progressive damages of Nicalon/CAS samples. The IR camera provided the temperature changes during tensile testing. Microstructural characterization using scanning electron microscopy (SEM) was performed to investigate the fracture mechanisms of Nicalon/CAS composites. In this investigation, the thermographic NDE technique was used to facilitate a better understanding of the fracture mechanisms of the Nicalon/CAS composites during tensile testing.

Synthesis of BaAl2Si2O8 Glass‐Ceramic by a Sol‐Gel Method and the Fabrication of SiCpl/BaAl2Si2O8 Composites.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of BaAl2Si2O8 glass-ceramic by a sol-gel method and the fabrication of SiCpl/BaAl2Si2O8 composites

BaAl2Si2O8 glass ceramics were synthesized by sol-gel using partial alkoxides. The crystallization behavior of BaAl2Si2O8 gel was characterized by XRD, DTA and TGA techniques. Barium aluminosilicate glass ceramic matrix composites reinforced with SiC platelets were fabricated by hot pressing method. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transimision electron microcopies, and three point bending tests. The results show that the flexural strength and the fracture toughness of the SiCpl/BAS composites increases with increasing SiC platelet content. By incorporation of 30 vol.% SiCpl, the flexural strength and the fracture toughness of the composites increase 80% and 114% compared to the pure BAS matrix, respectively. The main toughening mechanism is crack deflection.

Celsian formation in fiber-reinforced barium aluminosilicate glass–ceramic matrix composites

Hot pressing of barium aluminosilcate (BAS) glass or its composites reinforced with large diameter Textron chemical vapor deposited (CVD) silicon carbide SCS-6 monofilaments or small diameter multifilament Nicalon or Hi-Nicalon fibers resulted in the crystallization of both hexacelsian and monoclinic celsian phases. Effects of additions of monoclinic celsian seeds and strontium aluminosilicate (SAS) glass on crystal phase formation during hot pressing has been investigated. On doping BAS with 5 wt.% monoclinic celsian seeds or 10 wt.% SAS, only the celsian phase was formed in hot pressed monolithic specimens. However, in fiber-reinforced composites hot pressed under similar conditions, a small concentration of hexacelsian was still present as hexacelsian nucleates preferentially on surfaces and the presence of fibers provides a large surface area. When the additive concentration was increased to 10 wt.% celsian seeds or 20 wt.% SAS, celsian was the only phase detected from X-ray diffraction, with complete elimination of hexacelsian, in the hot pressed composites reinforced with large or small diameter SiC fibers.

Off-axis creep of a ceramic-matrix/continuous-ceramic-fiber composite: Experimental evaluation

The compression creep of a unidirectionally reinforced, SiC continuous fiber/calcium aluminosilicate (anorthite) glass-ceramic matrix composite was evaluated experimentally. Experiments covered the stress (−σ1) and temperature (T) ranges of 20–40 MPa and 1300–1320°C, respectively. The experiments also emphasized characterization of the rheology as a function of the angle of misorientation (ϕ) between the applied compressive load and the direction of reinforcement. For any given σ, T condition, the highest steady-state strain rate occurred for ϕ ∼ 50° (up to an order of magnitude faster than in the transverse, ϕ = 90°, case); overall composite strain in this case included a substantial contribution from displacement across the fiber-matrix interface. The data reveal that the interfacial rheology responsible for the displacement is distinctly temperature sensitive. Evaluation of the composite flow through its comparison to numerical/rheological models that scrutinize the interfacial effect implies that the interface is characterized by a non-Newtonian viscous rheology; this suggests that the interface response involves specifically the flow of the thin amorphous silica interphase that comprises a portion of the fiber-matrix interface in this material. The overall plastic response of the unidirectionally reinforced material is nevertheless rate-limited by plastic flow of the matrix and can be described by the superposition of three modes of strain, the magnitude of each being dependent specifically on ϕ.

Microstructure and mechanical properties of barium aluminosilicate glass-ceramic matrix composites reinforced with SiC whiskers

BAS glass-ceramic composites reinforced with different volume fractions (0, 10, 20, 30, 40 vol%) of SiC whiskers were successfully fabricated by a hot-pressing method. The microstructure, whisker/matrix interface structure, phase constitution and mechanical properties of the composites have been systematically studied by means of SEM, TEM, XRD techniques as well as three-point bending tests. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrixes. The celsian seeds can effectively promote the hexacelsian-to-celsian transformation in BaAl2Si2O8. The active Al2O3 added to the BAS matrix obviously reduced the amount of SiO2 in the matrix and formed needle-like mullite. The high temperature strengths of the composites were also investigated.

Effect of temperature and particle velocity on the erosion of a silicon carbide continuous fibre reinforced calcium aluminosilicate glass–ceramic matrix composite

A unidirectional silicon carbide continuous fibre reinforced calcium aluminosilicate (CAS/SiC) glass–ceramic matrix composite has been subjected to silica sand solid particle erosion over the temperature range 20–726°C. Initial tests were conducted with a constant mass flow rate of gas of 40 l min −1. This gave a wear rate of 0.16 mg g −1 at room temperature which increased to 0.26±0.02 mg g −1 at 400°C and above. When the increase in gas velocity, and hence particle velocity, was taken into account the wear rate was predicted to decrease with increasing temperature for a constant particle velocity. A wear rate of 0.06 mg g −1 was measured at 300°C, which showed excellent agreement with the prediction of 0.07 mg g −1. As the main mechanism of material removal is via lateral cracking, this decrease in wear rate is broadly consistent with the release of residual axial tensile stresses in the matrix resulting from mismatches in the coefficients of thermal expansion of the two phases on cooling down from the processing temperature.

Flexural mechanical properties of thermally treated unidirectional and cross-ply Nicalon-reinforced calcium aluminosilicate composites

Long, square cross-section samples of a unidirectional and a cross-ply [0/90]3s silicon carbide (Nicalon) fibre calcium aluminosilicate glass-ceramic matrix composite have been subjected to a range of thermal treatments. They were held at temperatures up to 800 °C above room temperature for 1, 6 or 24 h then slowly cooled or quenched into water. The thermal cycle was repeated up to six times for a small number of samples. The effects of these thermal regimes on Young's modulus, onset of matrix cracking (as assessed by onset of non-linearity in the load–displacement curve) and flexure strength have been monitored using three-point flexure testing. In very broad terms, where clear trends emerged, the intermediate temperature differentials, i.e. 400–650 °C, have been found to have the most detrimental effects on properties, and this has been linked to expected changes in the carboneous interphase and its subsequent replacement by silica. © 1998 Kluwer Academic Publishers

Microstructural analysis of the calcium aluminosilicate (CAS) glass-ceramic matrix in SiCf/CAS composites deformed at a high temperature

The calcium aluminosilicate (CAS: anorthite) glass-ceramic matrix materials in the SiCf/CAS composites deformed in compression at a high temperature were analyzed for microstructures. The matrix region right above the top of the SiC fiber (θ ∼ 0°) showed high density of dislocations as well as twins. On the other hand, the matrix region remote from the top of the SiC fiber (θ ∼ 90°) showed only a twinned structure. This experimental result was compared with the Meyer et al.'s mechanical modeling indicating that the interfacial traction of the matrix region right on the top of the fiber is significantly high compared to the remote stress and thus it rate limits the creep deformation of the composite.

Elevated temperature solid particle erosion of silicon carbide continuous fibre reinforced calcium aluminosilicate glass-ceramic matrix composite

A unidirectional silicon carbide continuous fibre reinforced calcium aluminusilicate (CAS/SiC) glass-ceramic matrix composite has been subjected to silica sand solid particle erosion over the temperature range 20 to 726 °C. At room temperature, tests were conducted at two velocities. The lower (7 m s −1) did not generate lateral cracks and a wear rate of 0.004 mg g −1 was observed. At the higher velocity (24 m s −1), lateral cracking was the main mechanism of material removal and an increased wear rate was found. The higher velocity wear rate of 0.16 mg g −1 at room temperature increased to 0.26 ± 0.02 mg g −1 at all other temperatures. Increasing the temperature of the test leads to smoother wear surfaces exhibiting an increasing contribution from plasticity. The reasons for the increase in wear rate from room temperature to 400 °C are unclear. They are not consistent with the release of residual axial tensile stresses in the matrix resulting from mismatches in the coefficients of thermal expansion of the two phases on cooling down from the processing temperature. Above 400 °C the changes are broadly consistent with the replacement of carbon by silica at the fibre-matrix interface, release of residual stress and softening of the glass-ceramic matrix.

Assessment of the mechanical behaviour of SiC fibre reinforced magnesium lithium aluminosilicate glass-ceramic matrix composite tested under uniaxial tensile loading

The mechanical behaviour under quasi-static uniaxial tensile loading of SiC-MAS.L glass-ceramic composite material is investigated. The accumulation of damage in unidirectional specimens is characterized by the total number of transverse matrix cracks, N, and the reduction in the longitudinal Young's modulus, E. The micromechanical parameters governing the matrix microcracking mechanism are calculated.

CVD SiC fiber-reinforced barium aluminosilicate glass—ceramic matrix composites

Unidirectional CVD SiC (SCS-6) monofilament reinforced BaO Al 2O 3 2SiO 2(BAS) glass—ceramic matrix composites have been fabricated by a tape lay-up method followed by hot pressing. The glass matrix flows around fibers during hot pressing resulting in nearly fully dense (95–98%) composites. Strong and tough composites having first matrix cracking stress of 250–300 MPa and ultimate flexural strength as high as 900 MPa have been obtained. Composite fracture surfaces showed fiber pullout with no chemical reaction at the fiber/matrix interface. From fiber push out, the fiber/matrix interfacial debond strength and the sliding frictional stress were determined to be 5.9 ± 1.2 MPa and 4.8 ± 0.9 MPa, respectively. The fracture surface of an uncoated SiC (SCS-0)/BAS composite also showed fiber/matrix debonding, fiber pullout, and crack deflection around the fibers implying that the SiC fibers may need no surface coating for reinforcement of the BAS glass-ceramic. Applicability of micromechanical models in predicting the first matrix cracking stress and the ultimate strength of these composites has also been examined.

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.

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.

Hot sodium sulphate corrosion of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite

The corrosion products arising from the exposure of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite to molten sodium sulphate at 900 °C for 100 h in both oxygen and argon atmospheres were studied by X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM respectively). The microstructure of the as-received composite plates was found to be similar to that reported by other workers. The matrix consisted of grains of close to stoichiometric mullite and β-spodumene and a high silica glass with 20–50 nm wide fibre-matrix interfaces comprising a layer of turbostratic carbon and amorphous silica. The effects of hot sodium sulphate corrosion were found to be very similar in both argon and oxygen but proceeded at a much greater rate in the latter case where it had progressed 100 μm into the composite and consumed many fibres. XRD studies indicated that mullite had virtually disappeared in the corroded region and this was confirmed by SEM. TEM studies of thin sections cut from near the end of the corroded zone also showed that the matrix had become a very fine mixture of glass and β-spodumene grains and that the fibre-matrix interface region had grown to ca. 600–800 nm wide. The microstructure of this corroded interface comprised several alternating layers of turbostratic carbon, mixed carbon and amorphous silica and pure carbon, each with widths varying between ca. 100 and 200 nm. This layered structure apparently developed as a result of oxidation of the silicon carbide fibre in the presence of a gradient of oxygen partial pressure that decreased from the matrix across the interface to the fibre.

Observation of crack closure in Nicalon/CAS composites

For clarification of the issue of crack closure, an investigation was performed to draw definitive conclusions about the occurrence of crack closure and its effects on hysteresis loop shape. From the evaluation of the hysteresis loops at various minimum stresses, a regime in which crack closure operates may then be determined. The composite investigated consists of a calcium aluminosilicate (CAS) glass-ceramic matrix reinforced with 40 vol% continuous, unidirectional Nicalon silicon carbide fibres

High-Temperature Flow of SiC Continuous Fiber-Glass Ceramic Matrix Composites: The Effect of Interface/Interphase Ductility

ABSTRACTThe elevated temperature rheology of continuous SiC (Nicalon®) calcium aluminosilicate glass-ceramic matrix composites is evaluated in uniaxial compression creep experiments (-σ1 = 20-to-40 MPa; T = 1300–1320°C). The steady state strain rate is demonstrated.to be highly sensitive to the orientation of the reinforcement relative to the maximum compressional stress, with highest bulk specimen strain rates noted for conditions in vWhich the sliding between the fiber and the matrix is optimized as a kinetic flow response (i.e., a fiber orientation of approximately 40-50° from σ1). One further discovers that the temperature sensitivity (i.e., activation energy) of flow increases as the amount of interface flow/sliding increases. The experimental results suggest that the high-temperature, low-stress interface response in this composite system is related to the ductile flow of the “Planar” SiO2 reaction-layer interphase that exists (in addition to the well-recognized planar carbon interphase) in these materials. The results of these simple experiments are used to calibrate a microscale-to-macroscale rheologic model in which the fibermatrix interface is described by a viscous constitutive relationship.