Properties of SiCf/SiC composites fabricated by slurry infiltration and hot pressing

Abstract

Two-dimensional SiC fibre reinforced SiC ceramic matrix composites (SiC_f/SiC) were fabricated by vacuum infiltration and hot pressing using a 200 nm thick pyrolytic carbon coated Tyranno SA3 fabric and 50 nm sized β-SiC powder. Hot pressing was carried out at 1750°C for 3 h in an Ar atmosphere under a pressure of 20 MPa. Al₂O₃–Y₂O₃–MgO sintering additive (10 wt-%) and polyvinyl butyral resin (45 wt-%) with respect to the matrix SiC were found to be the optimum contents for the high density composite. Vacuum infiltration with a force gradient produced much higher amount of slurry infiltration than simple dipping. Much improved density of 3·02 g cm^?3, compared to the previous reports, was achieved for the SiC–SiC_f containing approximately 67 vol.-% of fibre. This composite showed a step increase with a stress–displacement behaviour during the three-point bending test due to the fibre reinforcement. The displacement for failure and flexural strength were 0·58 mm and 342 MPa respectively, which were much larger than those for monolithic SiC.     

Mechanical properties of an Al/Mg/Al trilaminated composite fabricated by hot rolling

An Al/Mg/Al composite with a trilaminate structure was fabricated by hot rolling and its mechanical properties at quasi-static rates of strain were investigated. The bonding strength of the trilaminated composite is about 40 MPa, mainly attributing to the mechanical bond at the interfaces. The first layer failure strength of the laminated composite increases from 305 to 372 MPa when the relative thickness of aluminium alloy layer increases from 0.235 to 0.265. The tensile and bending properties of the laminates were calculated based on the Classical Laminate Theory (CLT). The calculations of first layer failure strength based on CLT agree with the experimental data in the error of 2.9–18%. Thus, the first layer failure strength of the Al/Mg/Al trilaminated composite fabricated by hot rolling can be calculated by CLT with the maximum stress criteria. The calculations also show that the tensile modulus, the tensile rigidity, the specific tensile rigidity and the first layer failure strength of the laminated composite increase almost linearly with the relative thickness of the aluminium alloy component. The bending rigidity of the laminated composite increases with the relative thickness of aluminium alloy, and approximates to a fixed value after the relative thickness over 0.3. The specific bending rigidity increases with the relative thickness of aluminium alloy and reaches a maximum value when the relative thickness is 0.25.     

Bonding strength of Al/Mg/Al alloy tri-metallic laminates fabricated by hot rolling

One of major drawbacks of magnesium alloy is its low corrosion resistance, which can be improved by using an aluminized coating. In this paper, 7075 Al/Mg-12Gd-3Y-0·5Zr/7075 Al laminated composites were produced by a hot roll bonding method. The rolling temperature was determined based on the flow stresses of Mg-12Gd-3Y-0·5Zr magnesium alloy and 7075 Al alloy at elevated temperature. The bonding strength of the laminate composites and their mechanism were studied. The effects of the reduction ratio (single pass), the rolling temperature, and the subsequent annealing on the bonding strength were also investigated. It was observed that the bonding strength increased rapidly with the reduction ratio and slightly with the rolling temperature. The bonding strength increases with the annealing time until the annealing time reaches 2 h and then decreases. The mechanical bond plays a major role in the bonding strength.     

Characterization of conjugated polymer/anodic aluminum oxide nanocomposites fabricated via template wetting

Solution-based template wetting is demonstrated as an effective means of producing semiconductor–insulator nanocomposites. The properties of such nanocomposites formed by incorporating two commonly investigated semiconducting polymers, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT), along with their blend, into nanoporous aluminum oxide (AAO) are investigated electronically and spectroscopically. Significant improvement in hole mobility of amorphous MEH-PPV in a nanocomposite with AAO while such improvement is absent when the crystalline P3HT is utilized to fabricate the AAO nanocomposites. Spectral evidence indicates that increased molecular order is responsible for this observation. Carrier mobility intermediate to the homopolymer nanocomposites was observed in composites fabricated from an MEH-PPV:P3HT blend and an AAO membrane. Spectral evidence indicates that these two polymers phase segregate in the composite.     

Characterization and mechanical performance comparison of multiwalled carbon nanotube/polyurethane composites fabricated by electrospinning and solution casting

Multiwalled carbon nanotube/polyurethane (MWNT/PU) composites were prepared by electrospinning and solution casting. The morphological and thermal properties, and mechanical performance of the nanofiber and film composites were characterized and compared. The tensile strength of neat PU film was 9-fold higher than that of neat PU nanofibrous mat. The incorporation of MWNTs increased the tensile strength and modulus of the composite nanofibers by 69% and 140%, respectively, and 62% and 78%, respectively for composite films. The MWNT/PU composites showed an improved thermal degradation behavior, with the incorporation of low MWNT content in the composites.     

Al-Si/SiC nanoparticles composites synthesized by double stir casting

The present invention provides Al-Si/SiC nanoparticles composites with the composition of 7%Si, 15%SiC with average particle size (APS) of SiC, 20, 30, 40 nm and 65μm using a novel double stir casting method. The inventive nano-composites by double stir casting show a nearly uniform distribution and good dispersion of the nano-particles within the Al matrix, although small agglomeration was found in the matrix of the micro-composite. The enhancement in values of impact strength and tensile strength observed in this study is due to small particle size and good distribution of the nano particles, which were confirmed by SEM spectrum. Patents WO 2010135848 and WO2011/011601 have some relevant information about the topic developed in this study, because the principle in both cases relies on the interactions between metal matrix and the nano-particles. Hence, novel double stir casting method can be used to improve the properties of nano-composites.     

The texture and anisotropy of hot extruded magnesium alloys fabricated via rapid solidification powder metallurgy

Rapid solidification magnesium alloy powders produced by spinning water atomization process were hot extruded into rectangular bars, from which tensile and compression samples have been cut at 0°, 45° and 90° angles from the extrusion direction to study their anisotropy. Electron back-scattered diffraction analysis has been used to investigate the texture evolution during the extrusion process. Texture parameters like the Schmid factor and the intensity of (0 0 0 1) basal plane in the pole figure have been evaluated and correlated to the mechanical properties. Results have shown that the extruded rods exhibited high strength and relatively less anisotropy compared to other previously reported values for wrought magnesium alloys. Tensile and compression yield stresses have shown very similar values to each other at all loading directions. This limited anisotropy could be linked to both the fine grained and inter-metallic-compound-dispersed microstructure of the extruded alloys. Dynamic recrystallization behavior during hot extrusion has also been investigated in the present study.     

Preparation of SiC and SiC/ZnO nanocomposites and its properties

SiC/ZnO nanocomposites were synthesized via a two-stage process. SiC nanoparticles were synthesized by electrical explosion method, and using the SiC nanoparticles as precursor, SiC/ZnO nanocomposites were synthesized by co-precipitation processes. The SiC/ZnO nanocomposites exhibit energy tuning of band-to-band transitions. In addition to the UV emission, the green and orange emissions were also observed. To our interest the green and orange emissions decreased at first and then increased with increasing annealing temperature. The underlying mechanism is elucidated.     

Characterization of the graded distribution of primary particles and wear behavior in the A390 alloy ring with various Mg contents fabricated by centrifugal casting

Three kinds of functionally graded rings using A390 as a base alloy and containing 0, 6 and 12 wt%Mg were prepared by centrifugal casting, and their microstructure and mechanical properties were investigated and compared. It is shown that the A390 ring without Mg content takes a characteristic of the primary Si particles distribution in the inner and outer layers. The 6%Mg ring reinforced by Si/Mg₂Si particles has a different performance with its particles distribution only in the inner layer. Regarding the 12%Mg ring, the Mg₂Si reinforced particle is distributed near the entire cross-section. It is demonstrated that the existence of the light in situ Mg₂Si particles with lower density rather Al liquid makes changes to the gradient distribution of particles across the cross-section. Characteristics and distribution of the primary particles were assessed by the optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and image analyzer software. The hardness and wear rate along the thickness of samples were measured to investigate the variation in the mechanical properties corresponding to the variation in microstructure. Also, the worn surfaces of the material were examined by scanning electron microscopy (SEM) to study the dominant wear regime.     

Effect of SiC particle size on the physical and mechanical properties of extruded Al matrix nanocomposites

In this work, the effect of SiC particle size and its amount on both physical and mechanical properties of Al matrix composite were investigated. SiC of particle size 70 nm, 10 μm and 40 μm, and Al powder of particle size 60 μm were used. Composites of Al with 5 and 10 wt.% SiC were fabricated by powder metallurgy technique followed by hot extrusion. Phase composition and microstructure were characterized. Relative density, thermal conductivity, hardness and compression strength were studied. The results showed that the X-ray diffraction (XRD) analysis indicated that the dominant components were Al and SiC. Densification and thermal conductivity of the composites decreased with increasing the amount of SiC and increased with increasing SiC particle size. Scanning electron microscope (SEM) studies showed that the distribution of the reinforced particle was uniform. Increasing the amount of SiC leads to higher hardness and consequently improves the compressive strength of Al–SiC composite. Moreover, as the SiC particle size decreases, hardness and compressive strength increase. The use of fine SiC particles has a similar effect on both hardness and compressive strength.     

Oxidation of SiC powders for the preparation of SiC/mullite/alumina nanocomposites

The oxidation behaviour of two types of SiC powder of differing particle size and morphology distribution has been studied in the present work; one submicron-sized and the other micron-sized. It has been observed that the onset-temperature for significant oxidation of the SiC powder of smaller particle size is much lower than that for the SiC powder of larger particle size; namely, about 760 °C as compared with about 950 °C. Furthermore, the rate and extent of oxidation of the former SiC powder is much higher than that of the latter SiC powder. Interestingly, however, the SiC powder of smaller particle size exhibits more controllable oxidation behaviour in the context of the preparation of SiC/mullite/alumina nanocomposites, i.e., in terms of the extent of oxidation, and hence the amount of silica formed as an encapsulating outer layer and the resulting core SiC particle size, than the SiC powder of larger particle size. The SiO₂ layer formed was amorphous when the SiC powders were oxidized below 1,200 °C, but crystalline in the form of cristobalite when they were oxidized above 1,200 °C. Since the presence of amorphous silica can accelerate the sintering of the nanocomposite, oxidation of the chosen SiC powder should thus take place below 1,200 °C.     

High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion

Pure titanium matrix composite reinforced with carbon nanotubes (CNTs) was prepared by spark plasma sintering and hot extrusion via powder metallurgy process. Titanium (Ti) powders were coated with CNTs via a wet process using a zwitterionic surfactant solution containing 1.0, 2.0 and 3.0 wt.% of CNTs. In situ TiC formation via reaction of CNTs with titanium occurred during sintering, and TiC particles were uniformly dispersed in the matrix. As-extruded Ti/TiCs composite rods were annealed at 473 K for 3.6 ks to reduce the residual stress during processing. After annealing process, the tensile properties of the composites were evaluated at room temperature, 473, 573 and 673 K, respectively. Hardness test was also performed at room temperature up to 573 K with a step of 50 K. The mechanical properties of extruded Ti/CNTs composites at elevated temperature were remarkably improved by adding a small amount of CNTs, compared to extruded Ti matrix. These were due to the TiC dispersoids originated from CNTs effectively stabilized the microstructure of extruded Ti composites by their pinning effect. Moreover, the coarsening and growth of Ti grain never occurred even though they were annealed at 573, 673 K for 36 ks and 673 K for 360 ks, respectively.     

Study on W/SiC interface of SiC fiber fabricated by chemical vapor deposition on tungsten filament

SiC fiber was fabricated by chemical vapor deposition on tungsten filament heated by direct current in a CH₃SiCl₃-H₂ gas system. Microstructure of W/SiC interfacial reaction zone in the fiber was identified by means of scanning electron microscope and transmission electron microscope. Results showed that the thickness of the interfacial reaction zone is between 350 and 390 nm, and two reaction products of W₅Si₃ and WC were formed during fabricating SiC fiber. Electron diffraction analysis and composition detection indicated that W₅Si₃ is adjacent to tungsten core and WC is adjacent to SiC sheath, and the W/SiC interface can be described as W/W₅Si₃/WC/SiC. Furthermore, the formation mechanism of the interfacial reaction zone is discussed.     

Characteristics evaluation of SiC/Si nanocomposites produced by spark plasma sintering

SiC–Si composites are widely used either as a bulk material or as a matrix for fibre reinforced ceramics. In the current research, nanocomposites of SiC–Si with different volume fractions of Si were sintered by spark plasma sintering (SPS) for the first time. The effect of Si content and different sintering parameters on relative density, microstructure, hardness and fracture toughness of the sintered materials have been investigated. The relative density increased from about 83 to 99% by increasing the sintering temperature to 1700°C, sintering time to 10?min, and pressure to 70?MPa for composites containing >20?vol.-% Si. The results revealed that the full dense SiC–20?vol.-%Si composite can be obtained by SPS at 1700°C, 10?min and 70?MPa. Moreover, in this condition, the hardness and toughness of the composites reached the optimum values.     

The mechanical properties of MWNT/PMMA nanocomposites fabricated by modified injection molding

This paper established the procedure to fabricate MWNT/PMMA nanocomposite by using both the injection molding and film casting processes. The combined fabrication process could remove demerits while maintaining the merits of each process. Tensile strength of the MWNT/PMMA nanocomposite increased more than 15% and tensile stiffness also increased about 17.5%, compared to the pure PMMA. It was confirmed that this combined fabrication process efficiently dispersed MWNTs in the PMMA matrix, and also maintained the well-dispersed state more effectively. SEM images of the fractural surface show that the degree of dispersion was improved. In addition, a surfactant was used to disperse MWNTs more efficiently, and its effect on mechanical properties was also investigated.     

Microstructure and thermal shock resistance of Al2O3 fiber/ZrO2 and SiC fiber/ZrO2 composites fabricated by hot pressing

Al₂O₃ chopped fiber/ZrO₂ and SiC continuous fiber/ZrO₂ composites were fabricated by hot pressing at 1550°C and 15 MPa in vacuum. The mechanical properties of thermally shocked composites were measured at room temperature by four-point bending. The addition of Al₂O₃ fibers into ZrO₂ matrix degraded the fracture strength, but improved significantly the thermal shock resistance. In addition, the mechanical properties of SiC fiber/ZrO₂ composites were much lower than those of monolithic ZrO₂ because of the presence of microcracks on the surface. The SiC fiber/ZrO₂ composites showed an excellent thermal shock resistance.     

SiC/C nanocomposites with inverse opal structure

The synthesis, morphology, structural and optical characteristics of SiC/C nanocomposites with an inverse opal lattice have been investigated. The samples were prepared by thermochemical treatment of opal matrices filled with carbon compounds which was followed by silicon dioxide dissolution. The samples were studied by electron microscopy, x-ray diffraction, photoluminescence, IR and Raman scattering spectroscopy. The electron microscopy data revealed a highly porous periodic structure which was a three-dimensional replica of the voids of the initial opal lattice. The hexagonal silicon carbide was found to be non-uniformly distributed throughout the volume, its greater part located in the surface layer up to 50?μm deep. The data of x-ray diffraction, IR and Raman scattering spectroscopy enabled us to assume that the composite had hexagonal diamond fragments. The photoluminescence and optical reflection spectra of the composites have been measured.     

Dissimilar joining of Al/Mg light metals by compound casting process

“Compound casting” was used for production of lightweight Al/Mg couples. In order to prepare the Al/Mg couples using this process, each of the aluminum and magnesium molten metal was cast around solid cylindrical inserts of the other metal. After solidification, the interfacial microstructure and shear strength of the joint were studied. Characterization of Al/Mg interface by an optical microscope and scanning electron microscope showed that in the case of casting aluminum melt around a magnesium insert, a gap is formed at the interface, while in the process of casting magnesium melt around an aluminum insert, a relatively uniform interface composed of three different layers is formed at the interface. The results of the X-ray diffraction, energy dispersive X-ray spectroscopy, wavelength dispersive X-ray spectroscopy, and microhardness analysis of the interface showed that these three layers are mainly composed of high-hardness Al–Mg intermetallic compounds. Furthermore, it was found that the thickness of the interface is not constant throughout Al/Mg joint, and varies gradually from 190 μm at the bottom to 140 μm in the middle and 50 μm at the top of the sample. The results of shear strength tests obviously showed that the strength of the interface depends on the interface thickness and increases by decreasing the thickness of the interface.