Glass-ceramic Matrix Composites Research Articles

Carbon fiber-reinforced (YMAS) glass-ceramic matrix composites. III. Interfacial aspects

Unidirectional continuous carbon fiber-reinforced glass-ceramic matrix composites have been fabricated for dry sliding applications. Different fracture behaviors have been observed (three-point bend test and fracture surfaces observation). Besides, the distance between next microcracks, which have appeared in the matrix on cooling after hot-pressing of the composites, changes with the sintering temperature, suggesting fiber–matrix reactions. The nature of the fiber–matrix interface is observed in Transmission Electronic Microscopy and the interfacial shear stress is determined by push-in tests. ©

Carbon Fiber-reinforced (YMAS) Glass-Ceramic Matrix Composites. II. Structural Changes in the Matrix with Temperature

With the overall aim to study a specific 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 diffraction, and thermal analysis. Some differences between the carbon (fiber)-containing material and the carbon-free material are revealed, mainly through local oxygen-depletions due to a reduction effect 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.

Mechanical and thermal properties of graphite fiber-reinforced cordierite glass-ceramic matrix composites

Mechanical and thermal properties of graphite fiber-reinforced cordierite glass-ceramic matrix composites

SiC Nicalon fibre-glass matrix composites: interphases and their mechanism of formation

The fibre-matrix reaction that occurs during the processing of a Nicalon SiC fibre-Pyrex glass matrix composite is analysed. Interphases are characterized by means of various complementary techniques: electron diffraction, HRTEM, EDX, EELS, SIMS and XPS. Neither the results of the present study nor those previously obtained for Nicalon SiC fibre-LAS (Li2O-Al2O3-SiO2) glass or LAS + MAS (MgO-Al2O3-SiO2) glass-ceramic matrix composites support the available model of reaction. An alternative reaction mechanism is suggested whereby the dissolved and non-bridging oxygen atoms of the matrix diffuse and oxidize SiC and SiOxCy in the fibre. The reaction yields carbon and silicon oxycarbide in the fibre and SiO2 which dissolves into the matrix. When the oxygen in excess in the matrix is consumed, the reaction stops and the phases undergo a reorganization in the reaction layer which generates two interphases, one carbon rich and the other silicon oxycarbide rich. These interphases are observed at the fibre peripher...

Dispersion of Chopped Short Fibers in Production of Fibers-Reinforced Glass-Ceramic Matrix Composites

Dispersion of Chopped Short Fibers in Production of Fibers-Reinforced Glass-Ceramic Matrix Composites

Effect of cyclic loading on interfacial shear stress in SiC–CAS glass ceramic matrix composite

Mechanical fatigue has been observed to occur in the Nicalon–CAS continuous fibre reinforced glass ceramic matrix composite under cyclic loading at room temperature, and both microcrack proliferation and propagation are induced. In situ fibre push down tests within a scanning electron microscope have then been used to assess changes in interfacial properties as a result of this mechanical cyclic loading. Both the interfacial shear stress and the interfacial fracture energy decrease when specimens are subjected to mechanical cyclic loading. It is deduced that a decrease in interfacial shear stress is the most likely mechanism driving stable and progressive microcrack propagation.

Effect of cyclic loading on interfacial shear stress in SiC–CAS glass ceramic matrix composite

Effect of cyclic loading on interfacial shear stress in SiC–CAS glass ceramic matrix composite

Predicting the Thermal Shock Resistance of Fiber Reinforced Brittle Matrix Composites

The development of flaw-tolerant fiber-reinforced glass, glass-ceramic and ceramic matrix composites has resulted in a new family of high-temperature capability structural materials exhibiting quasi-ductile fracture behavior. While a great deal of research efforts have been devoted to study the mechanical behavior of these materials at both room and elevated temperatures, investigation of their behavior when subjected to abrupt thermal gradients, i.e. under thermal shock conditions, has received much less consideration. This is surprising, as the high-temperature applications for which these composite materials are candidate, e.g. in gas turbine engines, will inevitably involve some kind of thermal shock loading. It is therefore important to understand the materials` response under severe thermal transients and to be able to predict their thermal shock resistance. The topic of the present article is the development of an approach for the prediction of {Delta}{Tc} from data of the composite constituents and of the thermal shock conditions. The approach developed is compared with available experimental data on silicate matrix composites.

Mechanical behavior of silicon carbide fiber-reinforced strontium aluminosilicate glass–ceramic composites

Unidirectional CVD SiC fiber-reinforced SrO·Al 2O 3·2SiO 2 (SAS) glass–ceramic matrix composites have been fabricated by hot pressing. Both carbon-rich surface coated SCS-6 and uncoated SCS-0 fibers were used as reinforcements. Monoclinic celsian, SrAl 2Si 2O 8, was the only crystalline phase observed in the matrix from X-ray diffraction. During three point flexure testing of composites, a test span to thickness ratio of ≥25 was necessary to avoid delamination. Strong and tough SCS-6/SAS composites having a first matrix cracking stress of ∼300 MPa and an ultimate strength of ∼825 MPa were fabricated. No chemical reaction between the SCS-6 fibers and the matrix was observed after high temperature processing. The SCS-0/SAS composite, having a fiber volume fraction of 0.24, exhibited a first matrix cracking stress of ∼231±20 MPa and ultimate strength of 265±17 MPa indicating a somewhat limited improvement over the monolithic material. From fiber push-out tests, the fiber/matrix debonding stress ( τ debond) and frictional sliding stress ( τ friction) in the SCS-6/SAS system were evaluated to be ∼6.7±2.3 and 4.3±0.6 MPa, respectively, indicating a weak interface. However, for the SCS-0/SAS composite, somewhat higher values of ∼17.5±2.7 MPa for τ debond and 11.3±1.6 MPa for τ friction respectively, were observed; some of the fibers were strongly bonded to the matrix and could not be pushed out. Examination of fracture surfaces revealed limited short pull-out lengths of SCS-0 fibers. The applicability of theoretical models in predicting the values of first matrix cracking stress and ultimate strength of these composites has been investigated.

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.

Chemical vapor deposited SiC (SCS-0) fiber-reinforced strontium aluminosilicate glass-ceramic composites

Unidirectional SrO · Al2O3 · 2SiO2 glass-ceramic matrix composites reinforced with uncoated chemical vapor deposited (CVD) SiC (SCS-0) fibers have been fabricated by hot-pressing under appropriate conditions using the glass-ceramic approach. Almost fully dense composites having a fiber volume fraction of 0.24 have been obtained. Monoclinic celsian, SrAl2Si2O8, was the only crystalline phase observed in the matrix by x-ray diffraction. No chemical reaction was observed between the fiber and the matrix after high temperature processing. In three-point flexure, the composite exhibited a first matrix cracking stress of ∼231 ± 20 MPa and an ultimate strength of 265 ± 17 MPa. Examination of fracture surfaces revealed limited short length fiber pull-out. From fiber push-out, the fiber/matrix interfacial debonding and frictional strengths were evaluated to be ∼17.5 ± 2.7 MPa and 11.3 ± 1.6 MPa, respectively. Some fibers were strongly bonded to the matrix and could not be pushed out. The micromechanical models were not useful in predicting values of the first matrix cracking stress as well as the ultimate strength of the composites.

Influence of interfacial strength on the fracture toughness of glass-ceramic matrix composites

Nine kinds of glass-ceramic matrix composites with different compositions and interfacial strength (τ s) were prepared. The influence of τ s on the fracture toughness(K 1c) of composites was studied. It was discoved that, for the system no chemical reaction taking place at the interface, K 1c increased proportionally with τ s increasing at the first stage, then decreased when τ s reached a certain value. According to this result, a model of relationship between τ s, thermal mismatch (Δα r) and K 1c was built up. If a chemical reaction took place and a new phase was formed in the interface, the K 1c of composite was effected by the combination of τ s, chemical bonding, radial interfacial stress and other factors.

Carbon-fibre-reinforced (YMAS) glass-ceramic matrix composites. I. Preparation, structure and fracture strength

Unidirectional continuous carbon-fibre-reinforced glass-ceramic matrix composites are fabricated for dry sliding applications. The microstructure of the matrix has been characterized by X-ray diffraction and the microtexture and structure of the fibres (pitch-based and PAN-based) have been studied by transmission electronic microscopy. The different fracture behaviours of the composites are described (three-point bend test and fracture surface observation by scanning electronic microscopy). Due to the thermal expansion mismatch between the fibres and the matrix, a network of microcracks appears in the composites on cooling after hot-pressing. The relationship between the microcrack spacing and the fracture behaviour suggests modifications of the fibre-matrix bond.

Fiber Coatings for SiC‐Fiber‐Reinforced BMAS Glass‐Ceramic Composites

Interfaces in SK‐fiber‐reinforced glass‐ceramic matrix composites were modified by applying different coatings to the fibers and varying the coating thickness. Coatings of SiC/BN, Si3N4/BN, and BN were applied to the fibers by CVD prior to composite fabrication. Interfacial microstructures were characterized using high‐resolution and analytical transmission electron microscopy. Oxidation of the SiC fibers during composite fabrication was suppressed by the fiber coatings, provided that the coatings were sufficiently thick to prevent oxygen diffusion from the matrix. The SiC/RN and BN coatings were stable during high‐temperature exposures, while the Si3N4/BN coating underwent chemical reactions.

Calcia-alumina fibre reinforced β-spodumene glass-ceramic matrix composites

The purpose of this study is to develop the IMS (Inviscid Melt Spinning) CA (46.5 wt% CaO-53.5 wt% Al 2 O 3 ) fibre reinforced β-spodumene glass-ceramic matrix composites for relatively low temperature application. Mechanical properties of the composite were examined using four-point bend tests.

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.

The processing of a magnesium-alumino-silicate matrix, SiC fibre glass-ceramic matrix composite using a pulsed Nd-YAG laser

The first part outlined the findings of trials investigating the Nd-YAG pulsed laser processing of a magnesium alumino-silicate matrix, SiC fibre glass-ceramic matrix composite. The tests studied the influence of the laser pulse parameters, principally upon the material removal rate. In Part II, associated trials are reported, in which the laser process variable's influence upon the quality of the processed surface is the most significant criterion for assessment. The effect of different types and pressures of assist gas and point of focus have been examined by electron microscopy, and the maximum speed of cut for various thicknesses of the composite material has been identified. As with the work on the pulse parameters, the findings have been compared with those obtained in previous work on a borosilicate glass matrix, SiC fibre composite.

Influence of Fiber-Matrix Bond Strength on the Performance of Nicalon/CAS-II Composite

The influence of the fiber-matrix interphase on the tensile behavior of unidirectional Nicalon fiber-reinforced CAS-II glass-ceramic matrix composite has been investigated. The failure strain decreases at high temperatures (1000°C) to a value comparable to the matrix cracking strain. The embrittlement process appears to be the result of oxidation of the carbon interphase as the matrix crack encounters fibers (given sufficient strain and/or time), the interphase subsequently fuses with a high bond strength and the crack grows through the fibers. In the absence of matrix cracking (at high temperatures) the oxidation process does not penetrate more than a few fiber diameters in from exposed surfaces transverse to the fiber direction. The contribution of the degradation of the fibers to the embrittlement appears to be less important.

Oxidation of BN‐Coated SiC Fibers in Ceramic Matrix Composites

Thermodynamic calculations were performed to analyze the simultaneous oxidation of BN and SiC. The results show that, with limited amounts of oxygen present, the formation of SiO2 should occur prior to the formation of B2O3. This agrees with experimental observations of oxidation in glassceramic matrix composites with BN‐coated SiC fibers, where a solid SiO2 reaction product containing little or no boron has been observed. The thermodynamic calculations suggest that this will occur when the amount of oxygen available is restricted. One possible explanation for this behavior is that SiO2 formation near the external surfaces of the composite closes off cracks or pores, such that vapor phase O2 diffusion into the composite occurs only for a limited time. This indicates that BN‐coated SiC fibers will not always oxidize to form significant amounts of a lowmelting, borosilicate glass.

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.