Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass was prepared from Y₂O₃, Al₂O₃, and SiO₂ micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10⁻⁶, matching-well with that of SiC_f/SiC composites. SiC_f/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiC_f/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.     

Microstructure and mechanical properties of Al2O3/Cu joints brazed with Ag–Cu–Ti+Zn composite fillers

In this study, Al₂O₃ ceramic and Cu bars were brazed with newly designed Ag–Cu–Ti(ABA)+Zn composite fillers. Systematic analysis of the microstructure of the brazed joints indicated that the volatilization of Zn atoms during the brazing process could promote the spreading of liquid brazing fillers on the surface of the Al₂O₃ ceramic, resulting in a uniform dendritic interfacial structure. The typical interfacial structure was an Al₂O₃/TiO/(Cu, Al)₃Ti₃O+Ag(s, s)/Cu interface. Notably, the tensile strength was improved to 20.89 MPa for Al₂O₃/Cu joint brazed with ABA+Zn composite fillers at 900 °C for 20 min, approximately 67.6% higher than the sample brazed without Zn foil. In this case, the fracture model was straight and sharp-angled inside the Al₂O₃ ceramic. In addition, the joint strength decreased with increased brazing temperatures from 900 to 940 °C.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Microstructure,thermophysical properties and oxidation resistance of SiCf/SiC–YSi2–Si composite fabricated through reactive melt infiltration

As an ideal component material for advanced aeroengines, SiC composite faces severe challenges of high temperatures and oxidation. Here, a high-densification SiC_f/SiC–YSi₂–Si composite was prepared through combining PIP with RMI of Si–13 at% Y alloy to achieve enhanced performance at high temperatures. Based on the analysis of the microstructure and thermophysical properties, it found that the introduction of the highly crystalline Si–Y alloy can improve the high-temperature thermal conductivity of the composite through phonon and electron conduction. In addition, Y migrates to the surface and forms yttrium silicate with increasing oxidation temperature, which facilitates the excellent long-term oxidation resistance of the composite at 1200–1300 °C. Thus, the composite retained its high strength (89.15% and 86.84%) after oxidation at 1200 °C and 1300 °C for 100 h. The experimental results clearly demonstrate that the introduction of the Si–Y alloy is an effective way of preparing high-performance SiC composites.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

High temperature sintering of SiC with oxide additives: I. Analysis in the SiC–Al2O3 and SiC–Al2O3–Y2O3 systems

The vaporization behaviour of pure Al₂O₃, Y₂O₃ and SiC as well as SiC–Al₂O₃ and SiC–Al₂O₃/Y₂O₃ mixtures has been analysed by thermodynamic calculations in an open system. Pure Al₂O₃ and Y₂O₃ evaporate congruently in the 1200–2300 K temperature range. Pure SiC vaporizes in a non-congruent manner leading to graphite formation as by-product. A SiC–Al₂O₃ mixture evaporates congruently according to the main vaporization reaction, 2 SiC(s) + Al₂O₃(s) +Al₂O(g) ⇆ 2 SiO(g) + 2 CO(g) +4 Al(g), but the overall composition changes: for SiC rich samples, the mixture tends towards pure SiC in time, and for Al₂O₃ rich samples towards pure Al₂O₃. A SiC–Al₂O₃/Y₂O₃ mixture shows similar behaviour.     

Long-sized rods of Al2O3–SiC–TiB2 ceramic composite material obtained by SHS-extrusion: Microstructure,X-ray analysis and properties

Long-sized rods of Al₂O₃–SiC–TiB₂ ceramic composite material were obtained by SHS-extrusion. The material was synthesized by self-propagating high-temperature synthesis (SHS) followed by high-temperature shear deformation. Ceramic samples app up to 465 mm in length and 5 mm in diameter were obtained. According to the results of XRD and SEM the obtained rods have a composite structure. The matrix is Al₂O₃ with distributed titanium diboride and silicon carbide particles. A uniform phase distribution was observed along the entire length of the rod. The microhardness of the matrix was 25–26 GPa, that of the dispersion-strengthening phases - 32–34 GPa. Heat resistance tests showed that during heat treatment at T = 1000 °C for 21 h, the sample specific weight gain and its real rate were 8.3 g/m² and 1 g/(m²∙h), respectively. The density, hardness and electrical resistivity of the samples obtained in this work were 3.27 g/cm³, 19.5 GPa, 3.1∙10⁻⁵ Ohm∙m, respectively.     

Microstructure evolution and mechanical properties of porous Si3N4 and dense Si3N4 joints bonded using CaO–Li2O-Al2O3–SiO2 glass-ceramic

A novel CaO-Li₂O-Al₂O₃-SiO₂ (CLAS) glass was developed for the joining of porous Si₃N₄ and dense Si₃N₄. A multiphase interlayer consisting of CaAl₂Si₂O₈, LiAlSi₂O₆ and CaSiO₃ phases was formed in joint, which possessed matched CTE with the Si₃N₄ substrates. In addition, the infiltrated layer with bilayer structure in the porous Si₃N₄ substrate was observed. The effects of joining temperature and cooling rate on microstructure, phase evolution and shear strength of joints were studied carefully. The results showed that the kinds of precipitated phases remained invariable with the joining temperature increased, but the crystallinity in the interlayer was improved remarkably as the cooling rate reduced. The maximum shear strength of 45 MPa was obtained when the joining temperature and cooling rate were 1100 °C and 5 °C/min, respectively. Moreover, fracture during the shear test occurred mainly within porous Si₃N₄ side, indicating superior joining of dense Si₃N₄/glass-ceramic/porous Si₃N₄.     

SiO2–Al2O3–CaO glass-ceramics: Effects of CaF2 on crystallization,microstructure and properties

The effects of fluorine content on the nucleation and crystallization behavior of SiO₂–Al₂O₃–CaO glass ceramics system have been investigated. The crystalline phases were determined by X-ray diffraction (XRD). The crystallization kinetics was determined by differential thermal analysis (DTA). The microstructures were examined by using scanning electron microscope (SEM). Fourier transformed infrared spectra (FTIR) analysis was used to study the glass structure. The results showed that by increasing the fluorine content, both the crystallization peak temperature (T_p) and activation energy (E) decreased. Wollastonite, anorthite and gehlenite are the main crystalline phases that exist in the glass ceramics system. The study shows that fluorine promoted initial crystallization of glass and can be used as an effective nucleating agent in the SiO₂–Al₂O₃–CaO system.     

Joining of Cf/SiC composites and Si3N4 ceramic with Y2O3–Al2O3–SiO2 glass filler for high-temperature applications

C_f/SiC composites and Si₃N₄ ceramics are candidate materials for applications in thermal protection system. This paper investigated the joining of C_f/SiC and Si₃N₄ using Y₂O₃–Al₂O₃–SiO₂ glass. The reliability of joints was evaluated by thermal shock tests. In this present work, the typical joint structure was C_f/SiC-glass-Si₃N₄. The results demonstrate that Direct bonding has been identified as the interfacial bonding mechanism at the SiC/glass and glass/Si₃N₄ interfaces. The maximum shear strength of the C_f/SiC–Si₃N₄ joint was ~34 MPa, which delivered an effective method to achieve strong, reliable bonding between the dissimilar materials. In addition, after thermal shock for 10 cycles, the residual strength remained ~13 MPa. Bubbles instead of microcracks formed in the glass filler, which was the main factor causing the degradation of the joint performance. It is suggested that improving the high temperature resistance of joining materials is the key to realize the application of this joint structure.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO₂–SiO₂ system was studied. Compositions consisting of CaO, ZrO₂, and SiO₂ were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO₃) and calcium zirconium silicate (Ca₂ZrSi₄O₁₂) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Viscosity,fragility and structure of Na2O–CaO–Al2O3–SiO2 glasses of increasing Al/Si ratio

The research and development of a new float glass with higher content of Al₂O₃ is essential for the commercial flat glass. The study on the workability and kinetic fragility of Na₂O–CaO–Al₂O₃–SiO₂ glass melts with different Al/Si ratios has been linked with the structure. The viscosities as a function of temperature for glass melts were derived on the basis of Vogel–Fulcher–Tamman (VFT) equation. Some characteristic temperatures and four characteristic temperature intervals of forming process in tin bath were estimated. The results showed that: adding 12 wt% Al₂O₃ substitute for SiO₂, the melting point (Tm) increased about 35 K, entire temperature interval in tin bath narrowed down about 20 K, the shortening of workability was mainly reflected in the viscosity range of 10^5.75–10¹⁰ Pa s, the fragility index m increased by 15%. It reveals an inverse correlation between the workability and the fragility. The structural changes on the tetrahedron structural unit Qⁿ (n=1, 2, 3, 4) were obtained by using Raman spectroscopy. Our analysis indicates that: the number of NBO reducing and a more polymerized structure with adding Al/Si ratios are responsible for the increase of viscosity; the tetrahedral distortion, a decrease of Q³/Q² in the Qⁿ species, is responsible for the increase of fragility.     

Y2O3–Al2O3–SiO2-based glass-ceramic fillers for the laser-supported joining of SiC

Four glass-ceramic filler compositions within the Y₂O₃–Al₂O₃–SiO₂ system were tested for their suitability in laser-supported joining of SiC materials. The compositions differed in terms of their primary crystallization behavior. Joint zone microstructures were investigated and joint tightness was determined using helium leak rate measurements after joining and subsequent annealing at 900 °C and 1050 °C.Yttria- and silica-rich compositions showed a partial crystallization of yttrium silicates during the short laser processing. Subsequent heat treatment effected further crystallization toward equilibrium conditions. Despite the strong change in the degree of crystallization no reduction of the tightness was observed for the best compositions; after 500 h annealing at 1050 °C tightness values of less than 10⁻⁸ mbar l s⁻¹ were measured. These results demonstrate the potential of the investigated filler system for high temperature stable hermetic sealing. At the same time the creation of homogeneously structured joints from glass-ceramic fillers using a laser-supported technology needs the understanding of the crystallization kinetics.     

Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Crystallization behavior and melt structure of typical CaO–SiO2 and CaO–Al2O3-Based mold fluxes

Crystallization behavior and melt structure of two typical mold fluxes A (CaO–SiO₂-based) and B (CaO–Al₂O₃-based) for casting high-aluminum steel were investigated using double hot thermocouple technology (DHTT), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results suggest that the crystallization temperature of Flux B is higher, and its crystallization incubation time is shorter compared with Flux A. The precipitated phase in Flux A is CaSiO₃, whereas BaAl₂O₄ and Ca₂Al₂SiO₇ form in Flux B. The structure analyses suggest that the degree of polymerization of Flux A is larger than that of Flux B. In addition, the major structural units of Flux A are Si–O–Si, Q₀^Si, Q₁^Si, Q₂^Si and Q₃^Si, but those of Flux B are mainly aluminate (Al–O–Al, Al–O^-), aluminosilicate (Al–O–Si) and silicate units (Q₀^Si, Q₁^Si, Q₂^Si and Q₃^Si). These different melt structures are the main reasons why the precipitated phases in these two mold fluxes are different, and the crystallization ability of Flux A is weaker than Flux B.     

Enhanced mechanical and microwave-absorption properties of SiCf/AlPO4 composite with PIP–SiC interphase and the MWCNTs filler

A single-layer radar-absorbing structure in the X-band (8.2 GHz to 12.4 GHz) was designed and fabricated by blending multi-walled carbon nanotubes (MWCNTs) with the binder matrix of SiC fiber/aluminum phosphate matrix (SiC_f/AlPO₄) composite. The SiC interphase was successfully prepared on SiC fibers by a precursor infiltration and pyrolysis (PIP) method. The morphology of as-received interphase was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The effects of PIP–SiC interphase on the mechanical and dielectric properties of the composite were investigated. The influence of MWCNTs content on the dielectric and microwave-absorption properties of coated SiC_f/AlPO₄ composite was discussed. When the content of MWCNTs was between 1.5 wt% and 3.5 wt% and the composite thickness is in the range of 2.5–3.5 mm, the SiC_f/AlPO₄ composite achieved excellent absorbing wave property in X-band.     

Crystallization characteristics of CaO–Al2O3–SiO2–Li2O–B2O3 mold flux with different values of w(CaO)/w(Al2O3)

The effect of C/A ratio (abbreviation of w(CaO)/w(Al₂O₃)) on the crystallization characteristics was investigated. With an increase in C/A ratio from 1.1 to 1.8, the crystallization ability first decreased and then increased; the crystallization ability is weakest and strongest with C/A ratios of 1.5 and 1.8, respectively. Increasing C/A ratio, the crystalline phase changed from LiAlO₂ and CaO·Al₂O₃ to LiAlO₂ and 3CaO·Al₂O₃. The Li⁺ ions in the slag took precedence over Ca²⁺ ions to participate in charge compensation because the mold flux contains Al3+ which is more advantageous for a monovalent cation, and LiAlO₂ formed preferentially over CaO·Al₂O₃. With a further increase in C/A ratio, 3CaO·Al₂O₃ formed from the combination of Ca²⁺ ions and Q_Al² units, and the precipitated amount of 3CaO·Al₂O₃ increased.