SiC Platelets Research Articles

Robocasting of reaction bonded silicon carbide/silicon carbide platelet composites

Pastes with a varying amount of uncoated SiC platelets (SiCplatelets) for the fabrication of reaction bonded silicon carbide (RBSC) were developed and the effect of the anisotropic particles on the microstructure and mechanical properties of the fabricated RBSC/SiCplatelets composites was studied. During printing the platelets align in printing direction due to shear forces. This texturization was confirmed by image analysis and EBSD measurement.The bending strength and the fracture toughness of about 470 MPa and 2.7 MPam1/2, respectively, did not change significantly after the addition of uncoated platelets, but showed comparable values according to the literature data for RBSC ceramics. Hardness and Young's modulus (17 GPa and 380 GPa) were also comparable to traditionally fabricated RBSC ceramics and showed a slight increase after the addition of platelets. SEM analysis of the fracture surfaces of the fabricated composites showed no pull-out and nearly no crack deflection, which may explain the constant values of the mechanical properties. Further work is therefore aimed at developing the paste filled with coated platelets.Although the influence of the uncoated platelets was small, the technological feasibility of producing textured RBSC/SiCplatelets composites using robocasting was demonstrated and the possibility to generate complex structures was successfully shown.

Microstructure and mechanical properties of hot-pressed Al2O3–mullite–ZrO2–SiC composites

In this study, the effects of SiC content, sintering temperature and Al2O3/SiO2 ratio on the microstructure and mechanical properties of hot-pressed alumina–mullite–zirconia–silicon carbide (Al2O3–mullite–ZrO2–SiC) composite ceramics were investigated systematically. The research results indicate that the composite ceramics have excellent combined properties when SiC content is 20 vol% and sintering temperature is 1530 °C, with Vickers hardness, fracture toughness and flexural strength of 17.5 GPa, 12.3 MPa m1/2 and 970 MPa, respectively. The fracture toughness and flexural strength are higher than those of monolithic Al2O3 ceramics and Al2O3-based composites, which are fairly attributed to the refined microstructures of composites and the synergistic effects of crack deflection, pull-out, branching, and phase transformation toughening mechanisms induced by short columnar Al2O3 grains and SiC platelets formed in the ceramics, as well as the ZrO2 particles existed in the ceramics.

Strength and toughness: The challenging case of TaC-based composites

This work aims at studying the relationships between strength and toughness of tantalum carbide (TaC) ceramics, a refractory ceramic used in aerospace and energy production sectors. The effect of different secondary phases was explored: (I) the addition of a transition metal silicide with suited thermo-elastic properties, TaSi2, (II) the addition of SiC particles, platelets or fibers, and (III) chopped carbon fibers. Microstructural analyses, performed by scanning and transmission electron microscopy, were essential in revealing at nanoscale level the morphological changes occurred during sintering in the reinforcing phase and its interaction with matrix and sintering additive. Mechanisms of reinforcement evolution are suggested accordingly. Fracture toughness and flexural strength were measured and the values were compared to unreinforced materials and discussed in agreement to the microstructural features. Strength approaching 1GPa was obtained upon addition of SiC particles, but residual thermal stresses prevented from notable increase of toughness, which fluctuated around 4MPa√m. A good compromise between strength and toughness was found for addition of Hi-Nicalon SiC fiber, 550MPa and 5.3MPa√m, respectively. More refractory SiC fibers resulted not effective, owing to the rising of tensional state in the matrix. On the other hand, TaSi2 led to a toughness of 4.7MPa√m and strength around 680MPa. Conversely, carbon fiber led to poor toughness due to unfavorable combination of coefficient of thermal expansion with the matrix.

Mechanical properties and in-situ toughening mechanism of pressurelessly densified ZrB2–TiB2 ceramic composites

Refractory metal diboride ceramics are promising materials for various high temperature applications. In this work, systematic studies were conducted on the mechanical properties, including the strength at elevated temperatures, of in-situ toughened ZrB2–TiB2 ceramic composites that were prepared via pressureless sintering at 2100°C for 2h. Linear shrinkage and open porosity measurements indicated that the composites were fully densified. Microstructural observations revealed intergranular cracking behaviour. Thermal residual stress along with elongated SiC platelets contributed to the improvement in toughness. The toughness was strongly affected by the ZrB2 grain size. The high temperature degradation of the flexural strength was attributed to the presence of oxide species.

Microstructure evolution upon annealing of a ZrB2–SiC composite containing lanthana and magnesia

Abstract The purpose of this work was to produce a dense ZrB 2 –SiC ceramic and to identify a suitable annealing cycle to crystallize the glassy phase and promote SiC growth along the c axis. The concept behind this work exploits the irreversible SiC β → α transformation occurring at temperature above 1900 °C, in such a way to have SiC platelets able to trigger effective toughening mechanisms. La 2 O 3 and MgO were used as sintering additives. The as sintered and annealed materials were examined through X-ray diffraction (XRD), to identify the crystalline phases, scanning electron microscope (SEM), to study the distribution of the secondary phases, and transmission electron microscope (TEM), to analyze the microstructure at nanoscale level, with particular attention to new crystalline phases and to the interfaces in high resolution mode. A model for the microstructure evolution during densification and upon annealing is presented.

Effects of SiC platelet and ZrSi 2 additive on sintering and mechanical properties of ZrB 2-based ceramics by hot-pressing

Fully dense Zirconium Diboride (ZrB 2)-based ceramics with Zirconium Silicide (ZrSi 2) and Silicon Carbide platelet (SiC pl) additive were fabricated by hot-press sintering in flowing argon atmosphere. Their mechanical properties and microstructure were examined. The contents of ZrSi 2 and SiC pl were optimized. The defined addition of ZrSi 2 was beneficial for densification and improving the mechanical properties of the ZrB 2-based ceramics. The addition of SiC pl obviously improved the fracture toughness of the ZrB 2-based ceramics. The range of fracture toughness of ZrB 2–SiC pl–ZrSi 2 composite was 6.7–9.0 MPa m 1/2. The flexural strength of the composites varied a little and the maximum was 597 MPa when the content of ZrSi 2 and SiC pl was both 5 vol.%. XRD results showed that a small amount of Zirconium Carbide (ZrC) formed during sintering. The microstructure of the composites was fine, compact and homogeneous.

Pressureless sintered in situ toughened ZrB 2–SiC platelets ceramics

ZrB 2–SiC composite ceramics were densified by pressureless sintering with addition of Si 3N 4 or MoSi 2 at temperatures that induced SiC anisotropic growth from particles to platelets, within a ZrB 2 matrix with rounded grains. Si 3N 4 addition resulted in the formation of large amounts of liquid phase which enhanced mass transfer mechanisms in terms of matrix grain growth and homogeneous distribution of SiC platelets having an aspect ratio of 3. On the contrary, MoSi 2 helped the densification with local formation of liquid phases leading to a finer matrix with finer SiC platelets, though more agglomerated and with a lower aspect ratio (about 2). These different microstructures had very different fracture properties values, namely a toughness of 3.8 MPa m 1/2 and a strength of 300 MPa for the Si 3N 4-doped composite; toughness of 5 MPa m 1/2 and strength of 410 MPa for the MoSi 2-doped one.

Oxidation Behavior of SiC Platelet‐Reinforced ZrB2 Ceramic Matrix Composites

Zirconium diboride (ZrB2) ceramic matrix composites reinforced by silicon carbide platelets (SiCpl) were prepared by hot pressing. Five groups of specimens were prepared, in which the contents of SiC platelets were 0, 5, 10, 15 and 20 vol%, respectively. The oxidation behaviors of specimens at different temperatures from 800° to 1600°C for 1 h and at 1200°C for different times from 0.5 to 4 h were investigated. With the increase of SiCpl content, the mass gain exhibited a decreasing trend. Oxidation thermodynamics and kinetics of the SiCpl/ZrB2 composites were discussed. A characteristic structure of hollow quadrangular prismy ZrO2 crystal emerged on the sample surface after oxidation was found and the possible formation mechanism was also presented. The surface element analysis indicated that the main products of oxidation were m‐ZrO2, SiO2, and borosilicate (aluminosilicate).

The synthesis of nanostructured SiC from waste plastics and silicon powder

Waste plastics constitute a growing environmental problem. Therefore, the treatment ofwaste plastics should be considered. Here we synthesize 3C-SiC nanomaterials coexistingwith amorphous graphite particles utilizing waste plastics and Si powder at 350–500 °C in a stainless steel autoclave. 3C-SiC could be finally obtained after refluxing with aqueousHClO4 (70 wt%)at 180 °C. X-ray powder diffraction patterns indicate that the product is 3C-SiC with the calculated latticeconstant a = 4.36 Å. Transmission electron microscopy (TEM) images show that the SiC samples presentedtwo morphologies: hexagonal platelets prepared by the waste detergent bottles or beveragebottles and nanowires prepared by waste plastic bags respectively. The correspondingselected area electron diffraction (SAED) pattern indicates that either the entire hexagonalplatelet or the nanowire is single crystalline. High-resolution TEM shows the planarsurfaces of the SiC platelet correspond to {111} planes; the lateral surfaces are {110} planesand the preferential growth direction of the nanowires is along [111]. The output of SiC was∼39% based on the amount of Si powder.

SiC Platelets Synthesized by Powder-Heating Technique

A new method for producing silicon carbide platelets with low cost and high yield was introduced. The silicon carbide platelets were synthesized by powder-heating techniques with carbon black and SiO2 powder as raw materials and CoCl2 as catalyst. The starting mixtures were heated at a temperature in the range of 1800-2000°C for the duration of about 2-4h to produce substantially completely unagglomerated silicon carbide platelets with a thickness of 5-20μm and the average diameter of 50-200μm. Compared to the conventional heating, the powder-heating technique is advantageous of less investment on equipment, easy to manufacture and convenient to operate. Furthermore, it is very suitable for realizing the scaled production because of the lower synthesizing temperature, shorter reaction time and greater output.

Mechanical Properties of Si3N4-SiC Three-Layer Composite Materials

The effects of microstructure and residual stress on the mechanical properties of Si3N4-based three-layer composite materials were investigated. The microstructure of each layer was controlled by the addition of two differently sized silicon carbides: fine SiC nanoparticles (∼200 nm) or relatively large SiC platelets (∼20 µm). When the SiC nanoparticles were added, the average grain size of Si3N4 was reduced because of the inhibition of grain growth by the particles. On the other hand, when the SiC platelets were added, the microstructure of Si3N4 was not much changed because of the large size of the platelets. Three-layer composites were fabricated by placing the Si3N4/SiC-nanoparticle layers on the surface of the Si3N4/SiC-platelet layer. The residual stress was controlled by varying the amount of SiC added. The mechanical properties of three-layer composites with various combinations of microstructure and residual stress level were investigated.

Synthesis of BaAl2Si2O8 Glass‐Ceramic by a Sol‐Gel Method and the Fabrication of SiCpl/BaAl2Si2O8 Composites.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermal conductivity of Al2O3/SiC platelet composites

Abstract The thermal conductivity of hot-pressed Al2O3/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al2O3/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselman's approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.

Synthesis of BaAl2Si2O8 glass-ceramic by a sol-gel method and the fabrication of SiCpl/BaAl2Si2O8 composites

BaAl2Si2O8 glass ceramics were synthesized by sol-gel using partial alkoxides. The crystallization behavior of BaAl2Si2O8 gel was characterized by XRD, DTA and TGA techniques. Barium aluminosilicate glass ceramic matrix composites reinforced with SiC platelets were fabricated by hot pressing method. The microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning and transimision electron microcopies, and three point bending tests. The results show that the flexural strength and the fracture toughness of the SiCpl/BAS composites increases with increasing SiC platelet content. By incorporation of 30 vol.% SiCpl, the flexural strength and the fracture toughness of the composites increase 80% and 114% compared to the pure BAS matrix, respectively. The main toughening mechanism is crack deflection.

Microstructure and fracture toughness of SiC-platelet reinforced SiC

SiC platelets into SiC starting powder (α -o rβ-SiC) followed by hot pressing. Microstructures and fracture toughness were investigated. Interaction between the introduced SiC platelets and matrix SiC powders, associated with β → α phase transformation, is discussed. α-SiC (A20 H.C. Starck GmbH, Goslar, Germany) and β-SiC powder (Betarundum, Grade UF, IBDEM, Japan) were used as the starting raw materials for the matrix phases. The SiC platelets used as reinforcement were hexagonal SiC (C-Axis Technologies, Quebec, Canada) with diameter 10‐25 µm and thickness 1‐ 6 µm (mean aspect ratio = 6). Y2O3 (Fine Grade, H.C. Starck GmbH, Goslar, Germany) and Al2O3 (AKP50, Sumitomo Chemical Co. Ltd., Tokyo, Japan) were used as sintering additives. The α-SiC powder (90 wt%) and the additives (4 wt% Y2O3 + 6 wt% Al2O3) were

A simple approach for determining the in situ fracture toughness of ceramic platelets used in composite materials by numerical simulations

Several analytical and numerical models proposed to predict the effective fracture toughness (KIC) of dispersion reinforced brittle matrix composites require the knowledge of the in situ fracture toughness of the included phase [1– 4]. These values are usually unavailable and they may differ significantly from data obtained on bulk materials. Ceramic platelets (e.g. Al2O3 and SiC platelets) have a high potential to be used as reinforcement in ceramic and glass matrices [5–13]. In order to be able to predict the effective toughness of platelet-reinforced composites it is necessary to know the value of the platelets’ intrinsic toughness. However, the data are not easily available. For example, in the case of single crystal Al2O3 platelets, many uncertainties exist in the choice of the appropriate toughness value. A very large range of data is reported in the literature, i.e. KIC = 1.7–5 MPa m1/2 [14, 15]. These two extreme values would result in a very different prediction of the fracture behavior and effective fracture toughness of the composite. Moreover, these values are often reported for bulk alumina ceramics and not for the single crystal. In fact, experimental determination of fracture toughness on loose platelets is difficult to achieve. In the present work, a parametric study using a numerical model has been carried out in order to determine a realistic value of fracture toughness for alumina platelets embedded in a borosilicate glass matrix. The method is based on numerical simulations and on the comparison between the numerical results and available experimental observations. A previous experimental study [11] has demonstrated that in borosilicate glass reinforced with alumina platelets crack deflection was more likely to occur when the platelets were inclined at certain angles with respect to the crack propagating direction. There was a critical angle between platelet and crack direction for which the crack began to deviate. From the experimental observation of several crackplatelet interactions, this angle has been found to be approximately 50◦ [11]. At greater angles of intersection, the platelet at the front of the propagating crack would be fractured, with little or no previous deflection of the crack. Typical examples of crack/platelet interactions in these composites, documenting their behavior

Rheological behaviour of borosilicate composites with metallic and non-metallic dispersions

Silicate matrix composites are potential candidates for high-temperature applications. In the present investigation, the effect of metallic (Cu) and non-metallic (SiC particulates, platelets, short fibres and whiskers) additions on the rheological behaviour of borosilicate matrix composites has been evaluated. The hot-pressed composites were tested both in compression and tension in the temperature range of 625–725°C. SiC reinforced composites tested in compression exhibited varying degree of strengthening and strain rate sensitivity depending on the volume fraction and morphology of reinforcements. The degree of strengthening and strain rate sensitivity depends on the volume fraction and morphology of reinforcements. Strengthening effect increased with the volume fraction and aspect ratio of reinforcements. The flow behaviour of composites changed from Newtonian to non-Newtonian with strain rate sensitivity index value changing from unity to 0.48. A similar trend was seen in the rate sensitivity of copper composites. However, copper additions decreased the strength of the composites at lower temperatures because of the softer copper phase. Pre-oxidation of copper particles had certain strengthening effect on the composite. The apparent viscosity of SiC reinforced composites increased with volume fraction and aspect ratio of reinforcements. However, in particulate composites, the viscosity found to increase with particle size. The mechanical/hydrodynamic interactions among the particulates appeared to be responsible for such a behaviour. With increasing strain rate, the viscosity decreased progressively confirming the shear thinning of the composites. The tensile ductility of the composites with 40 vol% reinforcements was evaluated at 700°C. While 400% elongation was observed in SiC particulate, platelet and copper composites, in short fibre/whisker composites, the tensile elongation values were only 150%. Further, the elongation of SiC platelet and copper composites improved by decreasing temperature and volume fraction of reinforcements, and also elongation values >500% were recorded. The tensile ductility of borosilicate composites was limited by onset and growth of cavities nucleated at the reinforcement/matrix interfaces.

Carbon Containing Platelets in Silicon and Oriented Diamond Growth

We analyse be means of transmision electron microscopy techniques the processes during the initial stages of diamond growth by magnetron sputtering. We show that a comparatively high density of C is found as deep as 100 nm in the Si substrate. The carbon arranges in planar defects in {111} and in {001} lattice planes. Our analysis shows that these defects can best be described by coherently strained inclusions that resemble hexagonal SiC platelets. These can act as seeds for orientated growth of diamond on Si.

Microstructure and mechanical properties of SiC platelet reinforced BaOAl 2O 32SiO 2 (BAS) composites

BaOAl 2O 32SiO 2 (BAS) glass–ceramic powders were prepared by sol–gel technique. SiC platelet reinforced BAS glass–ceramic matrix composites with high density and uniform microstructure were fabricated by hot-pressing. The effect of additional crystalline seeds on hexagonal to monoclinic phase transformation of Barium aluminosilicate was studied. The effects of SiC platelet content on the microstructure and mechanical properties of the composites were also investigated. The results showed that the flexural strength and fracture toughness of the BAS glass–ceramic matrix composites can be effectively improved by the addition of silicon carbide platelets. The main toughening mechanism was crack deflection, platelets' pull-out and bridging. The increased value of flexural strength is contributed to the load transition from the matrix to SiC platelets.

Microstructure and mechanical properties of SiC-platelet reinforced Al2O3/SiC-particle hybrid composites

SiC-platelet reinforced Al2O3/SiC-particle nanocomposites were fabricated by hot-pressing the mixture through the conventional powder mixing process. The mechanical properties of Al2O3/SiC-particle/SiC-platelet hybrid composites were evaluated. Fracture toughness and work of fracture were increased by the incorporation of SiC-platelets into Al2O3/SiC-particle nanocomposites. The typical rising R-curve was shown during crack growth for these hybrid nanocomposites, whereas Al2O3/SiC-particle nanocomposites showed the constant KR value and no rising R-curve. The further improvement of Al2O3/SiC-particle nanocomposites in the creep resistance was observed by the addition of SiC platelets. The relationship between the microstructure and mechanical properties for Al2O3/SiC-particle/SiC-platelet hybrid composites was discussed.