Addition Of SiC Whiskers Research Articles

Effect of Combined Addition of Silicon Carbide Whiskers and Fullerene Nanoparticles on Properties of Thermoelectric Zinc Antimonide

AbstractThermoelectric power generation is a hopeful method harnessing waste thermal energy particularly covering a middle temperature range between 500 and 800K. A Zn4Sb3 compound is a promising “phonon glass electron crystal” material applicable to thermoelectric power generation around 700K. This material, however, has a problem of its brittleness. In this research, the silicon carbide whiskers were added into the Zn4Sb3 compound for overcoming the brittleness, and the fullerene nanoparticles were also added for improving the thermoelectric performance. The Zn4Sb3 compound was synthesized from mixture of pure zinc and antimony powders by a liquid-solid phase reactions method. Firstly, the synthesized compound powder was mixed with the fullerene nanoparticles. The planetary ball milling method was used in order to disentangle the fullerene agglomerate and to obtain a uniform mixture. Subsequently, the mixture was uniformly mixed with the SiC whiskers by the planetary ball milling. The final mixture was consolidated by the pulse discharge sintering. The synthesized phases were identified by XRD. The morphology of the whiskers after mixing was observed. The flexural strength and the thermoelectric properties of the sintered samples were measured. The length of SiC whiskers and the flexural strength were decreased with the mixing time. Though the addition of SiC whiskers lowered the thermoelectric performance, the combined addition of SiC whiskers and fullerene nanoparticles restored the performance by especially decrease of the thermoelectric conductivity owing to the phonon scattering.

Influence of Toughening Method on Microstructures and Mechanical Properties of Alumina-Matrix Composites

Various toughening methods, i.e. partially stabilized zirconia transformation toughening, transformation- SiC whisker reinforcing and transformation-SiC particle reinforcing were used to improve the mechanical properties of alumina ceramic. Influence of various toughening methods on microstructure and mechanical properties of the alumina-matrix composites were studied. On the basis of transformation toughening, by which the strength and toughness of Al2O3 ceramic improved simultaneously, the addition of SiC whisker substantially enhanced the toughness, whereas the addition of SiC particle increased both toughness and strength to a certain degree. Mechanical properties of the testing materials were closely related with their morphologies of fracture surfaces. Toughening mechanisms of the composites were also studied. In the transformation-whisker reinforced composite or the transformation-particle reinforced composite, the two toughening methods affected with each other and produced a cooperative toughening effect.

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It is reported that La-Al-Transition Metal alloys have high undercooled melt formation and high bulk amorphous formation ability by using rapid solidification process. We have examined the undercooling solidification and remarkable nucleation control behavior with adding SiC whisker by using containerless process. In this study, we investigated the undercooling, nucleation behavior and bulk amorphous formation for La55Al25Cu10Ni5Co5 samples and samples with SiC whisker addition which are levitated and solidified by using the new gas jet flow and high cooling type electromagnetic levitation system. Therefore, the nucleation was controlled and the high undercooling degree, ΔT(=0.22~0.28Tm), was obtained with increasing SiC whisker addition. In addition, the bulk amorphous sample, φ7 mm sphere, was obtained at the cooling rate of levitating melt and solidification, about 100 K/s, for sample with 0.2% or more SiC whisker addition.

Aging behavior of squeeze cast SiCw/AZ91 magnesium matrix composite

The aging behavior of SiC whisker reinforced AZ91 magnesium matrix composites was investigated with Vickers hardness measurement, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation process observed in the monolithic alloy was not altered by the addition of SiC whiskers. The composite exhibited an accelerated hardening response compared with the unreinforced matrix alloy. However, the addition of SiC whiskers altered the distribution of the Mg 17Al 12 precipitates. Mg 17Al 12 precipitated preferentially at the SiCw–AZ91 interface, with a definite orientation relationship with SiC whisker: (1̄10) Mg17Al12//(111) SiCw, [111] Mg17Al12//[01 1 ̄ ] SiCw. The preferential interfacial precipitate decreased the Al content in the matrix, resulting in a decrease in the amount of continuous precipitates. As a consequence, the age-hardening efficiency in the composite was lower.

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si3N4/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si3N4 matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si3N4 or Al2O3. The results showed that the addition of SiC whiskers in the Si3N4 matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al2O3 or 10 wt% Si3N4 to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si3N4/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si3N4/BN composites were present as the first‐level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull‐out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.

Age-hardening behavior of a SiC w/Al-Li-Cu-Mg-Zr composite

The age-hardening behavior of a SiC w/Al–Li–Cu–Mg–Zr composite fabricated by squeeze casting method was investigated using Vickers hardness measurement and transmission electron microscopy (TEM). For comparison, the aging characteristics of the unreinforced matrix alloy with an identical processing and aging history were also examined. The results indicate that the composite exhibits an accelerated hardening response compared to the unreinforced matrix alloy at the four selected aging temperatures of 130, 160, 190 and 220 °C. Aging temperature has great influence on the hardening characteristics of the SiC w/Al–Li–Cu–Mg–Zr composite, and the suitable aging temperature range for the provided composite in this paper is 160∼190 °C. TEM observations reveal that the addition of SiC whiskers to the Al–Li–Cu–Mg–Zr alloy can speed up the growth rate of δ′ (Al 3Li) precipitates and lead to the early nucleation of S′ (Al 2CuMg) phase. The accelerated precipitation of δ′ and S′ phases is proposed to be responsible for the enhanced age-hardening of the SiC w/Al–Li–Cu–Mg–Zr composite.

Influence of SiC whisker on planar slip in Al-Li based alloys

The tensile deformation microstructure characteristics of silicon carbide whisker (SiCw) reinforced Al-Li and Al-Li-Cu-Mg-Zr composites prepared by squeeze casting technique were studied by means of transmission electron microscopy (TEM), in order to evaluate the influence of SiC whisker on planar slip in Al-Li based alloys. For the purpose of comparison, the microstructural features of the unreinforced matrix alloys with the identical fabrication, thermal processing and tensile deformation history were also investigated. Dislocation pairs and intense slip bands originated from cutting of δ′ (Al3Li) phases by moving dislocations, could be found in the specimens of Al-Li and Al-Li-Cu-Mg-Zr alloys, whereas absent in either SiCw/Al-Li or SiCw/Al-Li-Cu-Mg-Zr composites. The results demonstrate that the addition of SiC whisker to Al-Li based alloys, has a considerable effect on suppressing planar slip which is a general phenomenon in Al-Li based alloys resulting from the interaction between δ′ phase and dislocations.

Temperature effects on the tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers

In the present study, the influence of temperature on the tribological behavior of alumina reinforced with SiC whisker addition up to 20 vol.% is presented. Wear tests were performed at selected temperatures with alumina composites prepared by a modified tape casting method. Above a 673 K wear test temperature, the wear and frictional characteristics behaved differently compared with tribological behavior at 403 K. The results of this study indicate that as the temperature increased, the reinforcement effect of the unidirectionally oriented SiC whisker increased and the grain size effect was decreased. The effect of whisker orientation on friction and wear characteristics at the higher temperature was not dependent on test temperatures. The highest wear rate was obtained with a SiC whisker oriented parallel to the tape casting direction at all tested temperatures. The friction and wear mechanism of alumina composites reinforced with SiC at a high temperature is discussed based on SEM observations of worn surfaces and energy dispersion X-ray analysis.

Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics

Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.

Synthesis of a (MoSi2, Mo5Si3)/SiC Composite Using an In Situ Solid-State Displacement Reaction between Mo2C and Si.

The intermetallic compound, MoSi2, has great potential as an elevated-temperature structural material but it exhibits some undesirable properties such as low fracture toughness and low strength at elevated temperatures. Improvement of these properties can be achieved by reinforcing MoSi2 with the addition of SiC particles or whiskers, or by alloying it with Mo5Si3. The preparation of MoSi2 composites from powders gives rise to an oxidation problem since SiO2 covers the raw powder. To avoid this problem, the material must be prepared in situ. In this work, multiphase composites based on the Mo-Si-C system have been prepared using a solid-state displacement reaction between Mo2C and Si. Samples were prepared from a mixture, containing different proportions of Si and Mo2C powders, which was pelletized under vacuum at 40MPa and heat-treated in argon. The pieces obtained exhibit different proportions of SiC, MoSi2 and Mo5Si3. The microhardness obtained ranges from 12 to 14GPa, and Kc is over 8MPa·m1/2, which is higher than that of pieces made of MoSi2 only.

Influence of whisker toughening and microstructure on the wear behavior of Si3N4- and Al2O3-matrix composites reinforced with SiC

A comparative study of the influence of randomly-oriented SiC whiskers on the abrasive wear behavior of several commercially-produced Si3N4- and Al2O3-based ceramics suggested that the residual stress states present within the materials can be important in predicting their wear resistance. The addition of SiC whiskers to the Si3N4 matrix created residual tensile stresses at the whisker-matrix interfaces which led to enhanced bulk fracture toughness, but which degraded the fracture toughness at the microstructural level, and thus the abrasive wear resistance, by promoting easier whisker debonding and removal by the abrasive particles. The addition of SiC whiskers to an alumina matrix, on the other hand, led to the creation of residual compressive stresses at whisker-matrix interfaces, producing a locally tougher interface that was more able to withstand the rigors of the abrasive wear environment. These results indicate that in brittle materials, improved bulk mechanical properties do not always translate directly to improved performance in a tribological environment.

Mechanical Properties of SiC Whisker-Reinforced Mullite Composites (Part 2). The Effects of Diametrical Change and Carbon-Coating of Whiskers.

The effects of whisker diameter and carbon-coating of whiskers on the flexural strength and the fracture toughness of SiC-whisker-reinforced mullite composites were studied, and quantification of a crack deflection was attempted. The flexural strength and the fracture toughness gradually increased with increasing the amount of SiC whiskers. The strength slightly decreased with increasing whisker diameter and after coating the SiC whiskers with carbon. In contrast, the fracture toughness slightly increased. The toughening of SiC-whisker-reinforced mullite composites is contributed to mainly by the increase of the elastic modulus by the addition of SiC whiskers with high elastic modulus and the occurrence of debond-bridging in the SiC whisker-mullite matrix interface, but is accompanied by slight crack deflection and whisker-pullout.

Synergistic strengthening and toughening of zircon ceramics by the additions of SiC whisker and 3Y-TZP simultaneously

The strengthening and toughening behaviours of SiC whiskers and tetragonal zirconia particles (3Y-TZP) in a zircon matrix were investigated. The results showed that the addition of SiC whiskers or 3Y-TZP could improve mechanical properties to some extent. The flexural strength and fracture toughness of 30 vol% SiC(w) ZrSiO 4 reached 460 MPa and 4.7 MPa m 1 2 , while the flexural strength and fracture toughness of 20 vol% 3Y-TZP ZrSiO 4 were 410 MPa and 4.3 MPa m 1 2 . Furthermore, when SiC whiskers and 3Y-TZP were introduced into the zircon matrix simultaneously, a drastic improvement in mechanical properties was achieved and a positive synergistic toughening effect was observed, showing that the combined toughening effect of SiC(w) and 3Y-TZP together was bigger than the sum of the individual toughening effects when either reinforcement acted alone. The flexural strength and fracture toughness of 30 vol% SiC(w) 20 vol% 3Y-TZP/ZrSiO 4 were 580 MPa and 7.0 MPa m 1 2 at room temperature, which were 90% and 130% higher than those of monolithic zircon and could be maintained up to 800 °C. The microstructural observations by SEM and phase analysis by XRD demonstrated that the synergistic toughening effect was engendered from the interaction between microcrack toughening mechanism of 3Y-TZP and crack deflection, interfacial debonding as well as whisker pull-out by SiC whisker.

The effect of SiC whiskers on the hot-deformation behavior of SiC w/AA2124 composites

The hot-deformation behavior of composites consisting of an AA2124 matrix reinforced with 15 vol% of SiC whiskers has been studied over a temperature range of 200 to 530 °C and a strain rate range of 1.32 × 10 −3 to 2.38 s −1. Detailed analyses of flow curves and deformed microstructures were made to identify the hot-recovery mechanism of the composites. The critical strain, ε c, for dynamic recrystallization of the composites was also evaluated by the work-hardening rate, θ, and effective stress, σ, from the flow curves. It was found that the hot-recovery mechanism of the composites was dynamic recrystallization and the addition of SiC whiskers promoted nucleation of the dynamic recrystallization. The relationship between the critical strain, ε c, and peak strain, ε P, of the composites was ε c ≈ 0.52 ε P.

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite(p) content the high‐temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite(p. Furthermore, 10 vol% SiC(w) reinforced 15 vol% mullite/TZP composites improved the high‐temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high‐temperature strengthening is attributed to load transfer from TZP matrix to SiC(w) and mullite particulates. Significant whisker pull‐out and interface debonding were also observed on the fractured surfaces when SiC(w) was coated with Al2O3 film.

High temperature creep behavior of SiC whisker-reinforced Al [sbnd]Fe [sbnd]V [sbnd]Si composite

Compression creep behavior of the 8009 alloy and SiC/8009 composite has been investigated at the temperatures ranging from 573 to 723 K. The addition of SiC whiskers in the 8009 alloy decreases creep rates by two orders of magnitude. The stress and temperture dependence of creep rates of the composite is similar to those of the 8009 alloy. Creep rates of the 8009 alloy and the composite are controlled by lattice diffusion of aluminum. The threshold stress for creep was used to analyze the experimental data of the 8009 alloy and the composite.

Fabrication, characterization and mechanical properties of SiC-whisker-reinforced Y-TZP composites

The microstructure and mechanical properties of hot-pressed yttria-stablized tetragonal zirconia polycrystals (Y-TZP) reinforced with up to 30 vol % SiC whiskers were investigated. The homogeneously dispersed and fully dense SiC whisker/Y-TZP composites were fabricated by wet-mixing the constitutents and uniaxially hot-pressing the resulting powder. The grain size of the matrix depended on the whisker volume fraction and the hot-pressing temperature. The significant increase of fracture toughness of about MPa m1/2 at 10 Vol % SiC and a small increase in strength were achieved by uniformly dispersing the whiskers in the Y-TZP matrix. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers and also a phase transformation of ZrO2. The observed increase in the fracture toughness of Y-TZP due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms. Good agreement was achieved between the theoretical predictions and the experimental toughness values, obtained from the Y-TZP/SiCw composites.

Improvement of the Interlaminar Properties of C/C Composites by SiC Whisker Addition.

In order to improve the interlaminar properties of C/C composites, SiC whiskers were added to the carbon matrix. C/C composites with 0, 2.5, 5.0 vol.% of SiC whiskers were examined by short beam shear, DCB and ENT tests. Other mechanical properties, such as flexural and tensile properties, were also evaluated for each specimen. Results showed that SiC whisker addition drastically improved interlaminar shear strength (ILSS) and interlaminar fracture toughness, though C/C composites with 5.0 vol.% of SiC whiskers showed a 20% decrease in tensile strength. Observation of microstructure and crack extension during fracture toughness tests revealed that main effects of SiC whisker addition were reduction in density of intrinsic cracks and change in fracture mode mainly due to reinforcement of the fiber-rich layer/matrix interface.

Thermal shock resistance of SiC whisker reinforced Si 3N 4 ceramic composites

Thermal shock resistance of hot-pressed SiCw/Si 3N 4 (w = whisker) composites was investigated by thermal shock resistance parameters and water quenching test. The results showed that the thermal shock resistance to fracture initiation and crack-stable repropagation deteriorated with increasing SiCw content in spite of the improvement of fracture toughness, K IC. This could be attributed to the great decrease of strength, σ, and the considerable increase of Young's modulus, E, and thermal expansion coefficient, α, resulting from the addition of SiC whiskers. The curves of the residual strength, σ r, after water quenching versus thermal shock temperature difference, ΔT, e.g. σ r − ΔT, showed good relationships with the calculated thermal stress fracture resistance parameter, R, and thermal stress crack stability parameter, R st. The crack propagation of SiC whisker reinforced Si 3N 4 occurred primarily in a quasi-static manner, and it could be mainly attributable to SiC whiskers' pulling-out and bridging toughening effects.

Cyclic Deformation and Low Cycle Fatigue Behavior in a 6061Al/22vol% SiC Whisker Composite

A 6061Al matrix composite reinforced with 22vol% SiC whiskers and the unreinforced matrix alloy in different aged conditions were examined at the ambient temperature. Total strain controlled cyclic deformation under fully-reversed loading and low cycle fatigue properties were measured. The mechanical test results demonstrated that the composite materials in different aged conditions cyclically hardened at the applied strain amplitudes, typically at 0.4 %, 0.6 % and 0.8 %. The underaged composite specimens showed the most pronounced cyclic hardening, while the overaged composite only hardened to a limited amount at the first a few cycles. It has been found that the addition of SiC whiskers into the 6061Al increased the cyclic stress to the largest extent in the underaged condition while to the smallest in the overaged. The low cycle fatigue resistance of the composite to cyclic straining was found to be inferior to that of the unreinforced matrix alloy. When the saturation cyclic stress amplitude is considered, however, the composite material showed superior fatigue strength to the matrix alloy in this low cycle fatigue region. The experimental phenomena are interpreted in terms of the result of the electron microscopy and the fractography of the fatigue failed samples.