Fabrication and properties of three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite

In this study, a three-directional orthogonal aluminosilicate fiber fabric-reinforced mullite composite with excellent properties was prepared by the precursor impregnation and pyrolysis (PIP) method. The influence of the number of PIP cycles on the microstructure and mechanical properties of the composite was studied. The density and mechanical properties of the composite enhanced with increasing number of PIP cycles. After the 6th PIP cycle, the density of the composite increased by only 0.8%, and the flexural strength and fracture toughness of the composite reached 130.1 MPa and 4.91 MPa m^1/2, respectively. Moreover, the thermal shock resistance of the composite was investigated by the high-low temperature (from 1300 °C to room temperature) thermal treatment and water quenching method. The strength retention rate of the composite after 10 high-low temperature thermal cycles was 90.1%; the retention rate dropped to 79.5% after 15 thermal cycles. The composite with the critical thermal shock temperature difference 843.9 °C displayed excellent thermal shock resistance.     

Mullite fiber cloth-reinforced mullite composite fabricated via an optimized layer-by-layer assembly method

This article reports the fabrication of fiber cloth-reinforced mullite composite via an optimized layer-by-layer (LbL) assembly method involving the infiltration and pyrolysis of fiber cloth with mullite precursor (PIP) followed by brushing matrix onto the treated fiber cloth and hot pressing. The influence of fiber content on the microstructure and mechanical properties of composite are investigated and the crack propagation is discussed to explain the toughening mechanisms. The composite with 30?vol% fiber content exhibited a high density 2.89?g/cm³, flexural strength 135.5?MPa and fracture toughness 4.13?MPa?m^1/2. After PIP pretreatment, the gaps existed in the fiber bundle are filled with mullite matrix which transform from mullite precursor, improving the density from 2.89?g/cm³ to 2.94?g/cm³, flexural strength from 135.5?MPa to 163.2?MPa and fracture toughness from 4.13?MPa?m^1/2 to 4.55?MPa?m^1/2. The optimized LbL assembly method realizes the creation of fiber cloth-reinforced mullite composites with high density and excellent mechanical properties.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Development of mullite fibers and novel zirconia-toughened mullite fibers for high temperature applications

Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO₂ (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO₂ to higher temperatures with increasing amounts of ZrO₂ was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO₂ directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO₂.     

Fabrication of unidirectional continuous fiber-reinforced mullite matrix composite with excellent mechanical property

This article reports the fabrication of continuous fiber reinforced mullite matrix composite via layer-by-layer assembly method, involving the coating of mullite fiber with BN coating, followed by compositing the coated fibers with mullite matrix and hot pressing. The influences of the fiber coating, fiber content and sintering temperature on the microstructure and mechanical properties of the composite are investigated. By optimizing the sintering temperature and fiber content, the damage of fiber could be avoided and the microstructure and mechanical properties could be improved. The composite containing 30?vol% fibers coated by BN layer sintered at 1300?°C exhibits 90.9% theoretical density with flexural strength value of 203.2?MPa and fracture toughness value of 4.74?MPa?m^1/2. Fracture behavior is investigated to explain the toughening mechanisms. The layer-by-layer assembly method realizes the achievement of an individual architecture featuring fibers distribution and weak interfaces.     

Microstructural and mechanical characterization of fly ash cenosphere/metakaolin-based geopolymeric composites

Novel fly ash cenosphere (FAC)/metakaolin (MK)-based geopolymeric composites were prepared by adding FAC to the MK-based geopolymeric slurry. Microstructure, mechanical property, thermal conductivity, and bulk density of the FAC/MK-based geopolymeric composites were investigated. It was confirmed by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) that the FAC did not dissolve in alkaline condition, but element diffusion took place around the interface between geopolymeric matrix and FAC. The compressive strength, thermal conductivity and bulk density of FAC/MK-based geopolymeric composites decreased monotonically with the increase of the FAC content from 15 vol.% to 40 vol.%, and the minimum values for the 40 vol.% FAC/MK-based geopolymeric composite reached 36.5 MPa, 0.173 W m⁻¹ K⁻¹ and 0.82 g cm⁻³, respectively, in the range of FAC content from 15 vol.% to 40 vol.%. The results showed that the FAC could lower thermal conductivity effectively and bulk density of FAC/MK-based geopolymeric composites at a cost of slight decrease of mechanical properties. The 40 vol.% FAC/MK-based geopolymeric composite was a promising candidate material for intermediate-temperature thermal insulation applications due to its low thermal conductivity and low density.     

Strength degradation of alumina fiber: Irreversible phase transition after high-temperature treatment

The mechanical properties of alumina AF17-20 fiber after high-temperature treatment have been evaluated through tensile tests on single fiber and bundle. The tensile test on single fibers shows that the temperature has little effect on the elastic modulus of the fibers, which stables around 140 GPa. The test on bundles minimizes the personal errors thus giving a more reliable value of tensile strength. In general, as temperature increases, both the Weibull modulus and the tensile strength decrease gradually. De-sized fibers have the highest tensile strength, but inherent defects like pores still cause slight dispersion of the strength. Further, the strength maintains about 90% after treating below 1200 °C, and this insignificant decline is caused by the decrease of amorphous SiO₂ and the formation of aluminum silicate. In addition, the severe degradation in strength over 1200 °C is mainly attributed to the appearance and growth of mullite grains, which is only about 60% of the initial value.     

Mechanical and thermal behavior of cellular mullite materials

In this paper, the mechanical behavior and thermal properties of cellular mullite bodies obtained by thermal direct-consolidation of foamed aqueous suspensions of mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose (MC) were studied. The mechanical behavior of cellular mullite materials sintered at 1600 °C was evaluated by diametral compression at room temperature, 1000 °C and 1300 °C. The variation in the thermal diffusivity and thermal conductivity at temperatures up to 900 °C was determined using the laser-flash method. The results of the mechanical and thermal evaluation were analyzed based on the porosity features of the sintered materials, which was determined in turn by the starting system used for shaping the bodies.     

Microstructural and thermal properties of plasma sprayed mullite coatings

Thick mullite (3Al₂O₃–2SiO₂) coatings were fabricated by atmospheric plasma spraying (APS) in a mixture of crystalline and amorphous phases, as confirmed by X-ray diffraction (XRD) analysis. The coatings were isothermally heat treated in order to study recrystallization mechanism of the glassy phase. The morphology and the microstructure of both mullite feedstock and coatings were investigated by using scansion electron microscopy (SEM). The porosity of as-sprayed coating was in the range between 2 and 3% and substantially remained unchanged after thermal treatment. The thermal expansion of as-sprayed and annealed coatings was measured during heating up to the temperature of crystallization and the corresponding high-temperature extent of shrinkage was calculated. The differential scanning calorimetry (DSC) curves at different heating rates showed a sharp exothermic peak between 1243 and 1253 K, suggesting a rapid recrystallization of the amorphous phase. Finally, the heat capacity of recrystallized mullite coating was measured by DSC experiments. It was approximately 1.02 × 10³ J/kg K at 373 K and increased with increasing test temperature.     

Effect of Yb2SiO5 addition on the physical and mechanical properties of sintered mullite ceramic as an environmental barrier coating material

In this study, ytterbium monosilicate (Yb₂SiO₅)-added sintered mullite ceramics are prepared as candidate materials for environmental barrier coatings (EBCs). The effect of adding Yb₂SiO₅ on the physical and mechanical properties of the sintered mullite ceramics is investigated. The Yb₂O₃–SiO₂–Al₂O₃ ternary phase diagram indicates that adding Yb₂SiO₅ to the mullite goes beyond simply mixing; instead, liquid sintering occurs. Therefore, when we add Yb₂SiO₅ to the mullite, the sintered body possesses a denser microstructure and faster densification rate than does pure mullite. The density rapidly increases with the addition of 6 wt% Yb₂SiO₅ in the mullite, and almost full densifications are achieved with the addition of 12 wt% and 18 wt% Yb₂SiO₅. In this study, mullite ceramic that contains 12 wt% Yb₂SiO₅ exhibits the smallest plastic deformation and the highest elastic modulus among ceramics containing 6, 12, and 18 wt% Yb₂SiO₅, according to Hertzian indentation results. The results suggest that 12 wt% Yb₂SiO₅-doped mullite may be expected to act as a potential EBC material based on its excellent elastic properties, dense microstructure, and appropriate coefficient of thermal expansion.     

Low-temperature ageing of zirconia-toughened mullite composites

The change of mechanical properties of zirconia-toughened mullite composites, aged at low temperature in air, was investigated in this paper. The results indicated that the existence of microcracks, which were formed by the transformation of the zirconia tetragonal phase to the monoclinic during the cooling stage of processing was an important factor for the degradation of mechanical properties during subsequent ageing at 200–300°C. The increasing of flexural strength of the composites aged at 500–600°C was attributed to the relaxation of stress by reverse transformation of ZrO₂.     

Temperature-induced changes in morphology and microstructure of electrospun mullite nanofibers fabricated from a hybrid mullite sol

Mullite nanofibers with small diameter and high surface area are an ideal candidate as the reinforcements in composite materials, and have promising applications in the fields of catalysis, filtration, thermal storage and so forth. In this work, electrospun mullite nanofibers were successfully synthesized using a hybrid mullite sol. The morphology and microstructure of fibers calcined at different temperatures were investigated. The morphology of fibers synthesized at 900 °C is porous with coarse surface, and after crystallization it becomes compact with smooth surface. The densities of fibers increase with the increasing temperatures. At 1200 °C the surface of fibers becomes coarse again, as a result of the grain growth of mullite. The crystallization path of fibers was revealed that the Al-rich mullite (4Al₂O₃·SiO₂) together with amorphous silica formed at 1000 °C, changed into mullite with higher silica contents as temperature further increased, and finally transformed into a stable 3Al₂O₃·2SiO₂ phase at 1200 °C. During this crystallization process, the flow of amorphous silica phase and the formation of mullite crystal structure benefit the densification of fibers, leading to the resultant fibers with fine and compact microstructure. The present findings can provide a guideline for the preparation of the promising high-mechanical mullite nanofibers and the synthesized nanofibers display great potential as reinforcements in structural ceramic composites.     

Microstructural characterization and wear properties of silver and gold nanoparticle doped K-Mg-Al-Si-O-F glass-ceramics

In ascertaining the effects of silver (Ag) and gold (Au) nanoparticles on crystallization of boro-alumino-silicate system; the K₂O-MgO-Al₂O₃-SiO₂-B₂O₃-F glasses doped with/without 0.2?wt% Ag- and Au- content were melt-quenched at 1550?°C. Doping of nanoparticles considerably increased the glass-transition temperature and softening point but decreased the thermal expansion. A sharp crystallization exotherm in differential scanning calorimetry (DSC) is observed at 750?°C (?±?1?°C) for glass without nanoparticle and that broadened to 800–855?°C when contains nanoparticle. Opaque glass-ceramics were derived from the glasses by controlled heat-treatment at 1050?°C with predominant crystalline phase fluorophlogopite (KMg₃AlSi₃O₁₀F₂) mica. Traces of Ag- and Au- particles were also identified from X-ray diffraction (XRD) technique. The activation energy (E_c) of crystallization (344?±?17?kJ/mol) is decreased to 233 (?±?12) and 307 (?±?15) kJ/mol (Kissinger method) on doping with Ag- and Au- nanoparticles, respectively. Compact microstructure (FESEM) composed of rock like and plate-like mica crystals are developed in base glass-ceramic and that gets restructured to interlocked type morphology in presence of Ag- nanoparticle. Significant microstructural change induced by nanoparticle addition caused the decrease in microhardness (4.31–4.66?GPa) and increase in thermal expansion. Friction and wear testing under reciprocative sliding (using WC-Co ball) exposed that the average coefficient of friction (COF) is 0.60?±?0.2 for all glass-ceramics at 20?N load and 10?Hz frequency. At a lower load of 5?N, the average COF value is increased from 0.69 to 0.92 on use of Au-nanoparticle. A Similar trend was also observed at 10?N load as COF increased from 0.62 to 0.78.     

First principles calculations and synthesis of multi-phase (HfTiWZr)B2 high entropy diboride ceramics: Microstructural,mechanical and thermal characterization

First principles calculations were conducted on (HfTiWZr)B₂ high entropy diboride (HEB) composition, which indicated a low formation energy and promising mechanical properties. The (HfTiWZr)B₂ HEBs were synthesized from the constituent borides and elemental boron powders via high energy ball milling and spark plasma sintering. X-ray diffraction analyses revealed two main phases for the sintered samples: AlB₂ structured HEB phase and W-rich secondary phase. To investigate the performance of multi-phase microstructures containing a significant percentage of the HEB phase was focused in this study. The highest microhardness, nanohardness, and lowest wear volume loss were obtained for the 10 h milled and 2050 °C sintered sample as 24.34 ± 1.99 GPa, 32.8 ± 1.9 GPa and 1.41 ± 0.07 × 10^?4 mm³, respectively. Thermal conductivity measurements revealed that these multi-phase HEBs have low values varied between 15 and 23 W/mK. Thermal gravimetry measurements showed their mass gains below 2% at 1200 °C.     

Microstructural evolution and mechanical property of a SiCf/SiC composite/Ni-based superalloy joint brazed with an Au-Cu-Ti filler

A new Au-Cu-Ti filler featuring superior mechanical properties was developed to enable the brazing of a SiC_f/SiC composite (CMCs) to itself and a Ni-based superalloy (GH536). The progression of the interfacial reactions was studied using a combination of thermodynamic calculations and experimental observations. It was found that the interfacial reaction was Ti-dominant at the early brazing stage and then gradually transformed to Ni-dominant with the continuous dissolution of the GH536 substrate. Thus, the typical microstructure of GH536/Ti-Ni-Cr-Fe+(Au, Cu)ss + MoNiSi/Ni-Cr-Fe-Si-C (Ni₂Si + Fe₂Si + Cr₃C₂)+(Au, Cu)ss/Ni₂Si + TiC+(Au, Cu)ss/ Cr₃C₂+Ni₂Si + TiC + Fe₂Si/CMCs could be described by the following three stages: a Ti-dominated stage, full interdiffusion stage, and Ni-dominated stage. A maximum shear strength of 36 MPa was obtained for joints brazed at 1050℃ for 10 min, at which a failure occurred at the CMCs/brazing seam interface. The control of the interfacial reactions and the stress relaxation of (Au, Cu)ss contributed to the superior mechanical performance of the composite.     

机械法制莫来石微粉的性能与烧结

利用环缝磨、搅拌磨制得了Ｄ０．５达μｍ左右的莫来石微粉。该粉体粒度分布窄，机械力化学活性高。用此微粉与细粉进行压坯烧结试验表明，其粒径比和体积比对烧结有很大影响。在粒径比相差不大的情况下，对应每一烧结温度均存在一个微粉最低有效加入量，且烧结温度越高，该加入量越小；当粒径比相差很大时，此加入量值变得不太明显，但少量微粉引入即可显著提高坯体的烧结性能。     

Microstructural characteristics and mechanical properties of WC-Co/steel joints diffusion bonded utilizing Ni interlayer

In this study, diffusion bonding of WC-Co cemented carbides to a tool steel was realized utilizing Ni foils as the interlayer in a vacuum. The effects of bonding temperature and Ni interlayer thickness on interfacial microstructure and mechanical properties of the joint were studied. The research results revealed that brittle phases and cracks were suppressed due to the Ni interlayer. Moreover, the coherent relationship of [1 2 0]_WC//[1 0 1]_Ni and (0 0 1)_WC//(0 2 0)_Ni was observed at the interface of WC grains and Ni interlayer, and it greatly contributed to the bonding strength of WC-Co/steel joint. As the bonding temperature increased, the atoms diffused sufficiently, and the interfacial defect dimensions decreased. Then, the Ni interlayer was transferred to solid solutions, resulting in the high shear strength of the bonded joint. The optimum shear strength (444.7 MPa) was achieved when the bonding was carried out at 1050 °C for 1 h with a 4-μm-thick Ni interlayer. The cracks were propagated in the interlayer and the WC-Co substrate near the bonding seam.     

Development of HA-CNTs composite coating on AZ31 magnesium alloy by cathodic electrodeposition. Part 1: Microstructural and mechanical characterization

Magnesium alloys are considered as a new class of biodegradable alloys having many favorable properties to overcome the problems of currently used biomedical materials. Hydroxyapatite (HA) containing carbon nano-tubes (CNTs), with concentration ranging from 0 to 1.5?wt.%, coated on AZ31 magnesium alloy by direct and pulse cathodic electrodeposition methods. The coating properties including phase analysis, chemical bonding, morphology, and thickness were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FESEM), respectively. Nano-indentation, micro-hardness and adhesion tests were also used to evaluate the mechanical properties of the coatings. The results showed that a dense and fine coating structure was obtained by pulse deposition method. Moreover, the presence of carbon nanotubes in composite coating increased crystallization, presumably due to the increase in nucleation sites affecting the growth direction of hydroxyapatite crystals. The optimum condition having high crystallinity (71.2%) along with an uniform structure was the pulse deposited hydroxyapatite coating containing 1?wt.% CNTs. The elastic modulus and hardness of this sample increased by approximately 42% and 130% compared to the pure HA coating, respectively. Furthermore, the fracture toughness of pulse deposited HA–1wt % CNTs on AZ31 alloy reaches 1.96 ± 0.72MPa/m^0.5 which is in the range of compact bone.     

Preparation and characterization of mullite whisker reinforced ceramics made from coal fly ash

Ceramics with mullite whiskers were prepared from coal fly ash and Al₂O₃ raw materials, with AlF₃ used as an additive. The phase structures and microstructures of the ceramics were identified via X-ray diffraction and scanning electron microscopy, respectively. The results show that pickling of coal fly ash is an effective method for enhancing the flexural strength of ceramics. Sintering temperature and AlF₃ addition were also key factors influencing the creation of ideal ceramics. The ceramic made from pickled coal fly ash, 6?wt% AlF₃, and sintered at 1200?°C, exhibited the highest flexural strength of 59.1?MPa, and had a bulk density of 1.32?g/cm³ and porosity of 26.8%. The results show that ceramic materials made under these conditions are ideal candidates for manufacturing ceramic proppants for the exploitation of unconventional oil and gas resources.     

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

A new method to weaken the interfacial bonding and increase the strength of 3D mullite fiber reinforced mullite matrix (Mu_f/Mu) composites is proposed and tested in this paper. Firstly, Mu_f/Mu composites were fabricated through sol–gel process with varied sintering temperature. Then, the effects of sintering temperature on mechanical properties of the composites were tested. As sintering temperature was raised from 1000 °C to 1300 °C, the three-point flexural strength of the composites firstly decreased from 66.17 MPa to 41.83 MPa, and then increased to 63.17 MPa. In order to explain the relationship between composite strength and sintering temperature, morphology and structure of the mullite fibers and mullite matrix after the same heat-treatment as in the fabrication conditions of the composites were also investigated. Finally, it is concluded that this strength variation results from the combined effects of matrix densification, interfacial bonding and fiber degradation under different sintering temperatures.