No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effects of Carbon as a Sintering Aid in Silicon Carbide
Journal of the American Ceramic Society
Abstract
Self-diffusion data are collected from the literature in an attempt to better understand the strong effects of carbon as a sintering aid in SiC. These data indicate that the presence of excess carbon, in addition to reducing the native SiO2 layer of the SiC, probably enhances the rate-controlling bulk self-diffusion rate of SiC by a factor of about 100. The role of carbon as a sintering aid might now be understood as being similar to that of boron in enhancing bulk diffusion while simultaneously suppressing less effective sintering mechanisms such as surface diffusion.
... The addition of boron also accelerates the formation of point defects, improving the densification kinetics of SiC ceramics [34,35]. Studies have also found that the addition of carbon cleanses the surface of the SiC particles by removing the native SiO 2 film by carbothermal reduction (SiO 2 + 3 C → SiC + 2 CO↑) and increases its surface energy, γ sv [33,36,37]. In addition, the added carbon lowers the SiO vapor pressure [36], and prevents the transport of silicon along the SiC particle surface through the presence of excess carbon, resulting in the enhancement of the bulk self-diffusion of SiC [36]. ...
... Studies have also found that the addition of carbon cleanses the surface of the SiC particles by removing the native SiO 2 film by carbothermal reduction (SiO 2 + 3 C → SiC + 2 CO↑) and increases its surface energy, γ sv [33,36,37]. In addition, the added carbon lowers the SiO vapor pressure [36], and prevents the transport of silicon along the SiC particle surface through the presence of excess carbon, resulting in the enhancement of the bulk self-diffusion of SiC [36]. It is well documented that the inability to sinter pure SiC is a consequence of its high γ gb /γ sv ratio [31,33]. ...
... Studies have also found that the addition of carbon cleanses the surface of the SiC particles by removing the native SiO 2 film by carbothermal reduction (SiO 2 + 3 C → SiC + 2 CO↑) and increases its surface energy, γ sv [33,36,37]. In addition, the added carbon lowers the SiO vapor pressure [36], and prevents the transport of silicon along the SiC particle surface through the presence of excess carbon, resulting in the enhancement of the bulk self-diffusion of SiC [36]. It is well documented that the inability to sinter pure SiC is a consequence of its high γ gb /γ sv ratio [31,33]. ...
... The most important additives used in solid state sinter are carbon and boron. [21][22][23][24][25][26][27][28][29][30][31][32][33] The main role of carbon is to remove SiO 2 from the surface of SiC granules. [21][22][23] This trend is because of the revival of SiO 2 at temperatures below 1400°C. ...
... [21][22][23][24][25][26][27][28][29][30][31][32][33] The main role of carbon is to remove SiO 2 from the surface of SiC granules. [21][22][23] This trend is because of the revival of SiO 2 at temperatures below 1400°C. It is also necessary to add a small percent of carbon to activate the SiC sintering process. ...
... It is also necessary to add a small percent of carbon to activate the SiC sintering process. 23 One hypothesis is the effect of boron on increasing density. Boron (B) is replaced by Si in SiC and increases the penetration rate in the material itself. ...
In this research, carbon sources (Resin Phenolic, Carbon Black and Graphite) and Boron Carbide (B4C) have been used to improve the mechanical properties of Silicon Carbide (SiC) composite. For this purpose, carbon sources of 0.5, 0.75, 1, 1.5, 2, 2.5, 3 and 5 wt% as well as 0.5 wt% B4C were added to SiC powder, respectively. The sample containing SiC ‐ 2.5 wt% Resin Phenolic ‐ 0.5 wt% B4C had the best properties with relative density, hardness, and fracture toughness values of 98.5%, 2820 GPa, and 3.9 MPa.√m, respectively. Examination of SEM images showed that with increasing carbon from 0.5 wt% to 2.5 wt%, the fracture changes from intergranular to transgranular.
... As far as the role of C is concerned, earlier studies suggest that in the absence of carbon the decomposition of SiC is promoted by the SiO 2 layer on SiC particles as per the following reaction. [9,10] ...
... These reactions inhibit sintering due to the formation of gaseous species and also result in particle coarsening with the development of a network of large pores through enhanced surface diffusion of free Si. Conversely, the presence of sufficient C prevents the decomposition of SiC at elevated temperature as per the following reaction [9] : ...
... Datta et al. [15] reported that B can substitute Si atom in SiC and enhances the lattice diffusion. van Rijswijk and Shanefield [9] reported that higher Si diffusion coefficient in C atmosphere favors the densification through bulk diffusion. Microanalytical analysis of sintered SiC by high-resolution Auger electron spectroscopy (HRAES) [25] indicates the absence of B enrichment at the grain boundaries that also support B incorporation in SiC lattice. ...
Effective activation energy for densification of SiC in the presence of C and B4C additives is determined by constant heating rate experiments through analysis of shrinkage in hot pressing. The activation energy (AE) for sintering increased linearly with the relative density (RD) in two different regimes. The effective AE increased from 407 ± 50 kJ/mole at 75 pct RD to 1132 ± 75 kJ/mole at 95 pct RD. Lattice diffusion is proposed as the predominant mechanism for SiC densification at higher density. This is also validated by the uniform distribution of sintering additive through electron probe microanalysis. The low AE in the regime of lower density could be attributed to the pressure-assisted particle rearrangement during hot pressing. The relative contribution of both the mechanisms between two density limits resulted in the change in AE for sintering. The mechanisms of defect generation resulting in densification are also discussed.
... One challenge with silicon carbide is that sintering is difficult due to its strong covalent bonding and low selfdiffusion coeffcient [16,17]. In order to eliminate this problem, either a sintering aid (B 4 C, B, Al, Si, TiC, ZrB 2 , TiB 2 , Al 2 O 3 , Y 2 O 3 , and C) must be used or it must be sintered at high pressure (hot press, spark plasma sintering) and/or high temperature [18][19][20][21][22][23][24][25][26][27][28]. However using oxide additives can cause the formation of a liquid phase at the sintering temperature which can decreases the mechanical properties of SiC due to the residual glassy phase [29,30]. ...
... Boron containing additives increase the densification rate of SiC by increasing the self diffusion of the system [34]. On the other hand, carbon eliminates the oxide layer on the SiC surfaces preventing excessive grain growth [20,21,23,25,28]. Stobierski et al showed that both boron and carbon should be added together to SiC in order to produce highly dense parts since glassy phase formation was observed in samples with only boron added [25,34]. ...
Due to its outstanding properties, SiC is a candidate material for use in special applications such as armor. In order to use SiC in these special applications, it is necessary to produce fully dense ceramics. The ability to produce high density materials with superior performance depends on a number of factors. One of these factors is the addition of carbon to aid sintering. In this study, the effect of different carbon sources and ratios on the elastic and mechanical properties of SiC was investigated. Two types of carbon (lamp black and phenolic resin) were added to SiC in different ratios (0-2% wt.). All samples were sintered via the spark plasma sintering (SPS) method at 1900°C for 15 minutes under 50 MPa pressure. Samples made with lamp black were reached full density at 1.0 wt.%C, and the hardness and elastic modulus values were ~22GPa and 440 GPa, respectively. While samples made with both carbon sources showed similar bulk mechanical properties, the samples made with lamp black showed more consistent microstructures. The carbon from the phonelic resin source did not appear to be as well distributed as that from the lamp black source. The results also confirmed that addition of carbon into SiC was essential to improve the density and other mechanical properties associated with it.
... Furthermore, the doping effect on crystal growth is systematically discussed. Therefore, it is of great theoretical value and practical significance to explore the deep-seated causes of the doping effect on the fabrication of dense SiC ceramics or single crystals [14][15][16]. ...
... The total energy and electronic structure calculations were carried out with the Cambridge Serial Total Energy Package (CASTEP)5 (DFT)6 code based on density function theory with the plane-wave ultrasoft pseudopotential method. [15] 3. Results and discussion Figure 1 shows the XRD patterns of SiC ceramics fabricated by HTPVT for 60 min with different contents of B4C (0 wt%, 0.4 wt%, 1 wt% and 2 wt%) in the raw materials. It could be seen that when the B4C content was 0.4 wt% or below, only two diffraction peaks were observed, located at 35.6° and 75.4°, corresponding to the (0006) and (00012) crystal faces of 6H-SiC, respectively, demonstrating that the crystal grains of 6H-SiC maintained the preferential growth characteristic and preferred to grow along the [0001] direction. ...
In this paper, boron-doped dense 6H-SiC ceramics was fabricated by the high-temperature physical vapor transport (HTPVT) method. The effect of B doping on the crystal structure stability of 6H-SiC was investigated based on density functional theory (DFT). The results show that B doping can be realized even under thermodynamical equilibrium conditions. Nevertheless, it is found that the B doping effects on the (0001) of Si-plane and (000-1) of C-plane are significantly different. The doping experiments demonstrated that B can observably change the crystal growth morphology, leading to the formation of elongated 6H-SiC crystals.
... Due to its strong covalent bond structure and low self-diffusion coefficient, it is quite difficult to sinter silicon carbide [2,3,16]. To obtain highly dense SiC ceramics, sintering aids are added or a pressure-assisted sintering method is used [1,[16][17][18][19][20][21][22][23][24][25][26][27][28]. To take advantage of the properties of silicon carbide, it is esential to fabricated fully dense bodies. ...
... Added carbon reacts with SiO 2 , helping to remove this oxide layer from the SiC surface. In addition, C inhibits the formation and growth of large grains growth [16][17][18]27,28]. A study showed that addition of C is essentinal to producing highly dense SiC ceramics. ...
Sintering temperature and composition are two important factors that affect the properties of ceramic materials. In this study, the effects of sintering temperature, dwelling time, intermediate dwell, and B4C content on the density, microstructure, and elastic properties of dense SiC ceramics were investigated. 1.5 wt.% C and 0.25 wt.% or 0.5 wt.% B4C were added to SiC which was spark plasma sintered (SPS) at 1900°C, 1950°C, or 2000°C under a pressure of 50 MPa in flowing argon with an intermediate dwell at 1400°C (for 1 or 30 minutes). The results of the sintering study showed that the best microstructure was obtained in the sample sintered at 1900°C for 10 minutes with an intermediate dwell at 1400°C for 30 minutes. Under these conditions, the sintered sample has reached full density (>99%) with elastic, shear and bulk modulus values of 436 GPa, 184 GPa and 233 GPa, respectively. In addition, it was observed that 0.5 wt.% B4C content gave preferred results over 0.25wt% B4C content. These conditions allow the production of fully dense silicon carbide ceramics with fine-grained microstructure.
... Nevertheless, the SiO gas makes porosities in the composite during the process, which leads to the formation of small pores and the lower density of the samples. 28 To have a precise investigation, a comparison was made between the SPS outputs of the three samples with different SiC particle sizes; 45 nm, 80 nm and 25 mm produced in 1700 C/75 MPa/10 min sintering condition. These parameters were chosen according to the previous research. ...
... The same phenomenon has been reported in another research. 28 According to the above outcomes, a model can be proposed for sintering of SiC-Si nano composite containing SiC particles with less than 45 nm particle size. ...
In this study, the effect of SiC particle size on the sintering behavior of SiC-Si nano composites fabricated by spark plasma sintering (SPS) technique was investigated and a model was proposed, accordingly. To this purpose, SiC powders with three different particle sizes of 25 µm, 80 nm and 45 nm were chosen. It was expected that hardness of the composites increase with decreasing the SiC particle size; however, the outcomes were interesting and unpredictable. The composite with 80 nm SiC particles indicated the highest hardness. Hardness of the specimen with 25 µm SiC was low because of the large particle size of its reinforcement. While 80 and 45 nm SiC particles are considered as nano particles, the composite with 45 nm SiC particles showed lower hardness due to the growth of SiC powders during sintering according to a proposed model. Two reasons for the growth of 45 nm SiC particles were defined: (i) the fineness of the SiC particles prevented the Si particles to act as a binder between them thus, they agglomerated; (ii) SiC powders were oxidized during mixing procedure and a layer of SiO 2 was formed on their surfaces. During sintering procedure, the reaction between SiC and SiO 2 was happened and as a result SiO was formed. It caused vapor transportation during sintering leading to necking between particles and in turn, grain growth.
... Thus, the main function of carbon black particles in SiC compositions is the removal of SiO 2 phase from the system [11,22,23]. The minimum amount of carbon for the removal of surface silica of SiC particles was calculated as ∼1 wt% by assuming the oxygen content of the SiC powder is 0.81 wt%. ...
... While weight loss increased drastically due to the increase of SSA from 10 to 25 m 2 g −1 , only a slight increase in weight loss were recorded when SSA increased 25 to 80 m 2 g −1 . This indicated that fine carbon particles provided densification of the samples by means of not only removal of surface silica from the system but also the activating the bulk diffusion mechanism, which is more effective on densification than surface diffusion as stated by Rijswijk et al [22]. After the exponential increase in density with SSA of carbon black powders, nearly complete densification was achieved when SSA of the carbon black powder increased to 80 m 2 g −1 . ...
... Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1][2][3][4][5][6][7][8][9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. ...
... Silicon carbide offers favourable properties owing to its covalent bond, such as high hardness and strength at high temperatures. However, such a bond causes retarding of solid-phase sintering due to slow bulk diffusion [1,2]. Prochazka et al. [3] showed that dense SiC can be sintered with the use of boron and carbon densification aids (solid state diffusion process). ...
The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1–9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.
... The presence of a native SiO 2 film on the particles reduces the driving force for sintering the addition of reducing additives, such as B, C or B 4 C is essential for solid state sintering of SiC. The effect of boron and carbon as sintering aids was exhaustively studied, [33][34][35]. As the previous studies shown, the role of boron is to increase the lattice or grain boundary diffusion coefficient [5,35], in some studies it is also suggested that this effect helps to activate densification mechanisms on later sintering stages [33,36]. ...
... The carbon that did not react with SiO 2 film tends to form localized inclusions on grain boundary which, in later sintering stages may hinders the densification by pinning mechanisms [33,35]. In this work the carbon needed as a sintering aid was calculated in order to reduce the estimated 1.5 wt% O 2 on SiC surface, and for SBC the used value was 3 wt% [2,3,34,35]. X-ray diffraction spectra, Fig. 5(a), were collected from sintered samples ground in a vibratory mill. ...
Nanosized allotropes of carbon have been attracting a lot of attention recently, but despite the steady growth of the number of scientific works on materials based on graphene family, there is still much to be explored. These two-dimensional carbon materials, such as graphene nanoplatelets, multilayer graphene or few layer graphene have emerged as a possible second phase for reinforcing ceramics, resulting in remarkable properties of these composites. Typically, graphene ceramic matrix composites are prepared by a colloidal or a powder route followed by pressure assisted sintering. Recently other traditional ceramic processes, such as tape casting, were also successfully studied. The aim of this research is to fabricate α-SiC multi-layer composites containing 2, 4 and 8 vol% of graphene nanoplatelets (GNP) by tape casting and study the effect of these additions on the mechanical behavior of the composites. In order to achieve this purpose, samples were pressureless sintered and tested for density and mechanical properties. The elastic modulus was measured by the impulse excitation of vibration method, the hardness by Vickers indentation and fracture toughness using micro Vickers indentation and by three-point bending applying the pre-cracked beam approach. Results showed that up to 4 vol%, the density and mechanical properties were directly proportional to the amount of GNP added but showed a dramatic decrease for 8 vol% of GNP. Composites with 4 vol% of GNP had a 23% increment elastic modulus, while the fracture toughness had a 34% increment compared to SiC tapes fabricated under the same conditions. Higher amounts of GNP induces porosity in the samples, thus decreasing the mechanical properties. This study, therefore, indicates that 4% is an optimal amount of GNP and suggests that excessive amounts of GNP are rather detrimental to the mechanical properties of silicon carbide ceramic materials prepared by tape casting.
... They make it possible to use these materials in many applications, such as abrasive materials, sandblasting injectors, bearings, pump components, and extrusion dies [5,6]. Unfortunately, achieving full dense ceramic sinters is difficult due to their strong covalent bond, low grain-boundary diffusion coefficient, and the inevitable presence of oxide contaminations on the raw materials [7][8][9]. Consequently, it is necessary to apply high processing temperatures, high pressure, novel sintering techniques, or sintering aids [10]. Another of their disadvantages, significantly limiting the application potential, is the low fracture toughness [11]. ...
This paper discusses the influence of Ti3AlC2—MAX phase addition, intended to act as a sintering activator and a strengthening phase, to silicon carbide. The composites are prepared via powder processing and consolidated using the spark plasma sintering (SPS) method. The effects of the Ti3AlC2 addition on microstructure, sinterability, and mechanical properties were evaluated. The addition of MAX phases allows for the production of high-density sinters containing TiC as the reinforcing phase. Titanium carbide is formed as a result of the thermal degradation of Ti3AlC2. The highest hardness of 2540 HV5 was obtained for a sample containing 15 wt. % of Ti3AlC2, while the highest fracture toughness of 4.15 MPa*m0.5 for the sample containing 20 wt. %.
... Mass transport through the gas phase is unfavorable, leading to early inhibition of the sintering process and, consequently, only partial consolidation of the material [8]. Improved sinterability of SiC is achieved through the addition of sintering aids, typically in the form of oxides (Al 2 O 3 ), their mixtures (Al 2 O 3 -Y 2 O 3 , Al 2 O 3 -AlN), or elements such as boron and carbon [9][10][11][12][13]. ...
Reduced graphene oxide, due to its structure, exhibits anisotropic properties, which are particularly evident in electrical and thermal conductivity. This study focuses on examining the influence of reduced graphene oxide in silicon carbide on these properties in directions perpendicular and parallel to the direction of the aligned rGO flakes in produced composites. Reduced graphene oxide is characterized by very high in-plane thermal and electrical conductivity. It was observed that the addition of rGO increases thermal conductivity from 64 W/mK (reference sample) up to 98 W/mK for a SiC–3 wt.% rGO composite in the direction parallel to the rGO flakes. In the perpendicular direction, the values were slightly lower, reaching up to 84 W/mK. The difference observed in electrical conductivity values is more significant and is 1–2 orders of magnitude higher for the flakes’ alignment direction. The measured electrical conductivity increased from 1.2710−8 S/m for the reference SiC sinter up to 1.55 × 10−5 S/m and 1.2410−4 S/m for the composites with 3 wt.% rGO for the perpendicular and parallel directions, respectively. This represents an enhancement of four orders of magnitude, with a clearly visible influence of the anisotropy of the rGO. The composite’s enhanced electrical and thermal conductivity make it particularly attractive for electronic devices and high-power applications.
... While sintering aids such as Al 2 O 3 , AlN, Y 2 O 3 and other rare earth oxides cause liquid phase formation [16,17,26], sintering aids such as C, B 4 C and BN provide condensation with solid-state sintering [23,[27][28][29]. The addition of C also solves the native oxide layer (SiO 2 ) problem with SiC [21,[29][30][31][32]. Using boron and carbon together as sintering aids is a highly preferred method for obtaining fully densified SiC [28,[31][32][33][34][35]. ...
In this research, SiC ceramics were densified via spark plasma sintering (SPS) with 0.5 wt.% B 4 C and 1.5 wt.% C additions at temperatures ranging from 1900 to 2000°C for 5-65 min under 10-50 MPa applied pressure with an intermediate dwell at 1400°C, and the effects of applied pressure, sintering temperature, and dwelling time were examined. The samples sintered under 50 MPa applied pressure had high density (>99%), and showed high elastic modulus (∼420 MPa). However, lower applied pressure caused a decrease in density and elastic properties. The increase of sintering temperature from 1900 to 2000°C, while sintering time and pressure remained the same, caused grains coarsening. Increasing the dwelling time for the samples sintered at 1900 and 2000°C showed that sintering at a relatively lower temperature for a longer period of time did not increase grain size significantly. On the other hand, increasing the dwelling time at 2000°C caused excessive grain growth. The results show that fine-grained highly dense SiC can be produced by spark plasma sintering at 1900°C for 5 min under 50 MPa.
... While sintering aids such as Al 2 O 3 , AlN, Y 2 O 3 and other rare earth oxides cause liquid phase formation [16,17,26], sintering aids such as C, B 4 C and BN provide condensation with solid-state sintering [23,[27][28][29]. The addition of C also solves the native oxide layer (SiO 2 ) problem with SiC [21,[29][30][31][32]. Using boron and carbon together as sintering aids is a highly preferred method for obtaining fully densified SiC [28,[31][32][33][34][35]. ...
In this research, SiC ceramics were densified via spark plasma sintering (SPS) with 0.5wt.% B4C and 1.5wt.% C additions at temperatures ranging from 1900 to 2000?C for 5-65min under 10-50MPa applied pressure with an intermediate dwell at 1400?C, and the effects of applied pressure, sintering temperature, and dwelling time were examined. The samples sintered under 50MPa applied pressure had high density (>99%), and showed high elastic modulus (~420MPa). However, lower applied pressure caused a decrease in density and elastic properties. The increase of sintering temperature from 1900 to 2000?C, while sintering time and pressure remained the same, caused grains coarsening. Increasing the dwelling time for the samples sintered at 1900 and 2000?C showed that sintering at a relatively lower temperature for a longer period of time did not increase grain size significantly. On the other hand, increasing the dwelling time at 2000?C caused excessive grain growth. The results show that fine-grained highly dense SiC can be produced by spark plasma sintering at 1900?C for 5min under 50MPa.
... First, it hindered the gaseous phase diffusion through boundary, which was a major mechanism that led to abnormal grain growth [60,61]. Second, graphite as a sintering aid could promote the solid phase diffusion, which would improve the densification behavior and pore elimination of ZrB 2 -SiC ceramics [62,63]. Both effects would lead to smaller and more uniform grain sizes after sintering. ...
Spiral fibers were considered to be an ideal toughening phase of ultra-high torsional release effect. In this work, ZrB2 (Z)–20 vol% SiC (S) spiral fiber (ZSsf) with controllable structure was prepared by a combination approach of liquid rope effect and non-solvent-induced phase separation. Dominantly depended on the kinematic viscosity (η), dropping height (H), and flow rate (Q), the geometric parameters of ZSsf involving filament diameter (d) and coil diameter (D) were followed the relationship of d ≈ 0.516×10−3Q1/2H−1/4 and D ≈ 0.25×10–3(Q/H)1/3, respectively, within the optimized η of 10–15 Pa·s. Three different microstructures of ZSsf were achieved by adjusting the polymer/solvent/non-solvent system assisted with phase diagram calculation, including dense, hollow, and hierarchical pore structures. The ZrB2–SiC with 1 wt% ZSsf composites prepared by hot isostatic pressing (HIP) exhibited a ~30% increase in fracture toughness (KIC, 4.41 MPa·m1/2) compared with the ZrB2–SiC composite, where the microscopic fracture toughness of the ZSsf was ~80% higher than that of the matrix. The fibers with a ~10 nm in-situ-synthesized graphite phase amongst grain boundaries of ZrB2 and SiC changed the fracture mode, and promoted the crack deflection and pull-out adjacent the interface of matrix and the fiber.
... Both of these reactions would be responsible for the promotion of grain growth [31]. The presence of carbon (C) is believed to avoid grain growth during the sintering of SiC through reducing SiO 2 layer by lowering the SiO vapor pressure, and prevent the surface transport of Si [32]. The β-SiC powder used in this study contained less than 0.1 wt% free C, therefore, it can be considered that the grain growth was promoted by the vapor transport and the surface diffusion of Si with the absence of C. ...
The effects of solid solution, grain size and porosity on the thermal conductivity of aluminum (Al)- and boron (B)-added porous SiC ceramics with in-situ grain growth were investigated. Al-added porous SiC ceramics showed higher porosity, higher solubility of Al in SiC lattice, and lower thermal conductivity with the range of 8.5 – 18.2 W/m•K. The higher solubility of Al in SiC lattice was considered as the reason of their low thermal conductivity. B-added porous SiC ceramics have lower porosity, lower solubility of B in SiC lattice, and higher thermal conductivity with the range of 30.6 – 79.0 W/m•K. The high thermal conductivity of B-added porous SiC ceramics would result from the low solubility of B in SiC lattice and the low porosity. The effect of grain size was found to increase the phonon mean free path for B-added SiC ceramics.
... Au contraire, le carbone, réagissant préférentiellement avec SiO 2 (équation I.12), préviendrait la décomposition du SiC en espèces volatiles. Certaines études montrent également que la présence de monoxyde de carbone améliore la mobilité du silicium dans le SiC et favorise ainsi la densification du matériau [120,121,133]. ...
The development of light and high performance ballistic protections is currently a sensitive subject of research. Despite promising mechanical characteristics, the complexity of SiC shaping generally leads to the use of high content of sintering aids, favouring secondary phases formation which could weaken the material. Nevertheless, high sintering temperatures induce the presence of the α form of SiC,
conferring to the material anisotropical and lower mechanical properties than the one obtained with the cubic β phase.
The goal of this PhD work is the development of high purity cubic SiC, with density close to 100% and perfect Si:C stoichiometry to optimize the performances of this ceramic. Two kinds of precursors were considered: a commercial powder and a powder from the conversion of preceramic polymer precursor.
Firstly, the parametric study of SPS sintering allowed to reach densities of 95% for both precursors, while conserving only the cubic phase. These encouraging results being not sufficient, this study switched to the use of sintering aids. Densities close to 100% were thus reached on samples sintered with prepared mixtures from commercial powder, even for very low content of additive. The second subject of this thesis highlighted a dependence of the β --> α transition temperature of SiC as a function of sintering pressure, but also according to the kind of precursor. Indeed, the use of polymer precursor is favourable to cubic structure stability. Then, hardness measurements were performed on the most promising samples and allowed to highlight the major role of density on this property.
... [25] Scientists have tried to reduce the sintering temperature and time with the help of sinters. In addition to reducing costs, this can also control the final microstructure of the sample and thus strengthen its mechanical properties [26][27][28][29][30][31][32][33][34][35][36][37][38]. ...
In this research, thermodynamic and phase analysis of SiC-Nano/microB4C composites with different weight percentages of secondary phase including 0, 0.5, 1, and 2 wt% -nano /microB4C made by pressureless sintering method have been investigated. To this end, 0 and 1 wt% phenolic resin were added to the samples separately as a carbon source (both as a binder and as a carbon additive), respectively. The resulting compounds were milled by a planetary ball mill for 3 h at a speed of 200 rpm. The initial pressing of the samples was performed at 50 MPa and the samples were subjected to pyrolysis at 600 °C. They were then sintered at 2150 °C for 2 h under an argon atmosphere. The results showed that the composite contained SiC-0.5 wt% -nanoB4C with the main peak intensity (2θ = 35.58) in phasic analysis using XRD pattern of more than 15,000 units and FWHM equal to 0.0036 which shows that the structure is smaller than the additive mode (FWHM = 0.2210) SiC-0.5 wt% -microB4C. Also, the results of phase analysis using the XRD model show that the use of NanoB4C increases the peak intensity in the phase analysis graph compared to the MicroB4C additive mode. In addition, increasing the wt% -nanoB4C increases the amount of graphitization.
... Since SiC has remarkable properties such as low theoretical density (3.21 g/cm 3 ), a high hardness, a high elastic modulus, high thermal conductivity, good wear and oxidation resistance, and low coefficient of thermal expansion [2,3,[8][9][10][11][12][13][14][15][16][17]. The problem with SiC is that it is really difficult to achieve high dense bodies due to its strong covalent bonding and low self-diffusion coefficient without using sintering aids for instance B 4 C, B, Al, Si, TiC, ZrB 2 , TiB 2 , Al 2 O 3 , Y 2 O 3 , and C or pressure-assisted sintering method [3,6,9,13,[18][19][20][21][22][23][24][25][26][27][28]. Depending on particle size of powder, moisture in the air and additives, nonoxide high-temperature materials like B 4 C, SiC, and TiB 2 tend to have oxide layer on their surfaces. ...
High dense (>99% density) SiC ceramics were produced with addition of C and B4C by spark plasma sintering method at 1950 °C under 50 MPa applied pressure for 5 min. To remove the oxygen from the SiC, it was essential to add C. Two different mixture method were used, dry mixing (specktromill) and wet mixing (ball milling). The effect of different levels of carbon additive and mixture method on density, microstructure, elastic modulus, polytype of SiC, Vickers hardness, and fracture toughness were examined. Precisely, 1.5 wt.% C addition was sufficient to remove oxide layer from SiC and improve the properties of dense SiC ceramics. The highest hardness and elastic modulus values were 27.96 and 450 GPa, respectively. Results showed that the 4H polytype caused large elongated grains, while the 6H polytype caused small coaxial grains. It has been observed that it was important to remove oxygen to achieve high density and improve properties of SiC. Other key factor was to include sufficient amount of carbon to remove oxide layer. The results showed that excess carbon prevented to achieve high density with high elastic modulus and hardness.
... In solid state sintering of silicon carbide, boron containing species and carbon are used as sintering aids. Boron base 8 Zeynep Ayguzer Yasar and Richard Haber: Effect of Acid Etching Time and Concentration on Oxygen Content of Powder on the Microstructure and Elastic Properties of Silicon Carbide Densified by SPS additives increase the rate of self-diffusion coefficients of Si and C. Carbon removes the SiO 2 layer on the silicon carbide surface and also inhibits the grain growth by forming inclusions [19][20][21][22][23]. Magmami et. ...
In this current paper, oxygen content of a fine particle size SiC (H. C. Starck UF 25 Silicon Carbide) and coarser particle size SiC (Saint Gobain Silicon Carbide) were modified by using different concentrations of HF for etching. Fully dense silicon carbide ceramics (>99% th. density) were produced by the spark plasma sintering technique at 1950 °C under an applied pressure of 50 MPa for 5 min hold with boron carbide and carbon addition. Archimedes method, scanning electron microscopy, and the ultrasound analysis were used to examined density, microstructure, elastic (E), shear (G), and bulk (K) moduli of dense silicon carbide ceramics to investigate the effect of oxygen impurities on the densification and the properties of silicon carbide. The results showed that high oxygen content is detrimental to the final density of SPS silicon carbide. When the oxygen content increased from 0.60 to 5.92 wt.%, the relative density decreased from 99.99% to 96%. For both SiC powders, by increasing the etching time, the grain size of SiC decreased. It means that the high oxygen caused grain growth. Ultrasound analysis results showed that the high oxygen content affected the elastic properties. SiC samples with the high oxygen content had a lower elastic moduli, shear moduli and bulk moduli. It was clear that increasing the oxygen content decreased the elastic properties.
... The predominant species in the sublimated vapour were: Si, SiC2 and Si2C [20] . The equilibrium partial pressures of Si, Si2C and SiC2 were calculated at the different temperatures using [21] ： C and 2350 C. It can be seen that the pressure of three gases increased when the temperature increased, while the Si partial pressure increases at 1900 -2200 °C, and decreases above 2200 °C. the gas mixture of the Si-C in the Ar system, the mass transport was mostly driven by the temperature gradients, which lead to a difference in the vapour pressure between the source and the substrate [22] . ...
... The results pointed that the graphite powder has densification effect on the firing of the mullite precursor; it acted as a sintering aid rather than a sacrificial pore-forming agent in this study. Graphite/carbon has also been used as sintering aid to promote densification in ceramics, such as AlN [16], ZrC [17], ZrB 2 [18] and SiC [19] in other studies. (Fig. 4b) revealed that the pores were of irregular shape and were mostly interconnected. ...
Porous mullite ceramics were produced using mullite precursor and modified cenospheres as a non-sacrificial pore-forming agent. The cenospheres used are aluminosilicate hollow spheres with high silica and alumina content, which are obtained from coal-fired power plant. In this study, the cenospheres were modified using aluminum trichloride hexahydrate (AlCl3·6H2O), alkali/acid leaching and heat treatment. Various types and amounts of the modified cenospheres were mixed with mullite precursor to produce porous mullite ceramics for subsequent firing at 1500 °C. Graphite powder, as sacrificial pore-forming agent, was also used to prepare porous mullite ceramics by the same processing conditions for comparison. The study found that the use of graphite powder was unable to increase the porosity of the mullite ceramics as a result of excessive shrinkage. It acted more as a sintering aid rather than as sacrificial pore-forming agent. On the other hand, addition of modified cenospheres as non-sacrificial pore-forming agent leads to the increment of both total porosity and closed porosity, with the reduction of open porosity. The results showed that with the addition of 40 wt% of modified cenospheres to the mullite precursor, the resultant porous mullite ceramic has a total porosity of 50.2%, thermal conductivity of 1.28 Wm−1K−1, linear shrinkage of 4%, and biaxial flexural strength of 45.9 MPa. Porous mullite ceramic with majority closed pores has potential application for high temperature thermal barrier.
... During liquidphase sintering, Y 2 O 3 and AlN react with the native SiO 2 layer which is present on the surface of SiC and MoSi 2 particles, to form Y-Al-Si-O-C-N melt with the dissolution of SiC [5]. The pyrolysis of phenolic resin yields˜60% excess carbon, which is generally believed to facilitate densification process by reacting with the SiO 2 layer and enhancing bulk self-diffusion of SiC as well [25]. Fig. 1 presents the XRD patterns of SiC-MoSi 2 composites sintered at various temperatures. ...
SiC-MoSi2 composites with low electrical resistivity and high infrared emissivity were fabricated via pressureless sintering. The relationship between microstructure evolution and electrical behaviors along with infrared emission properties of the resulting composites is investigated at various sintering temperatures. The electrical resistivity undergoes two significant drops with increasing sintering temperature. Pore elimination bears responsible for the initial decrease in electrical resistivity. Transmission electron microscopy (TEM) observation manifests that the thinned amorphous layers at SiC/MoSi2 interface decrease grain boundary resistivity and allow for electrical percolation to occur when sintering temperature further rises. Additionally, increasing sintering temperature leads to a higher infrared emissivity owing to the formation of Mo4.8Si3C0.6 and the decreased boundaries. The lowest electrical resistivity of 7.2 Ω·cm and the highest infrared emissivity of 0.721 are recorded for composite sintered at 2000 ℃. Overall, SiC-MoSi2 composites exhibit a promising prospect as infrared source elements that must endure harsh environments.
... During pressureless sintering, Y-Al-Si-O-C-N melt would be formed by the dissolution of SiC and aid in the densification based on the solution re-precipitation mechanism [12]. Besides, excess carbon yielded by pyrolysis of the phenolic resin was generally thought to promote densification by reducing the native SiO 2 layer on the surfaces of the starting SiC and MoSi 2 powders and enhancing bulk selfdiffusion of SiC [33]. The results showed that MoSi 2 did not significantly alter the densification behavior of the composites with respect to the reference SAM0 material. ...
Highly conductive SiC-MoSi2-AlN composites were fabricated by β-SiC, AlN and MoSi2 powders with Y2O3 additive via pressureless sintering. The effect of MoSi2 content on the microstructure, mechanical and electrical properties of SiC-MoSi2-AlN composites was systematically investigated. A finer microstructure was obtained and electrical conductivity was enhanced with increasing MoSi2 content. The impedance spectroscopy and potential-current measurements were implemented to figure out the electrical conduction mechanism. The introduction of MoSi2 effectively reduced the Schottky barrier height at the grain boundary, and subsequently the U-I curves changed from nonlinear to linear electrical characteristics. The notable decrease in electrical resistivity was owing to the breakdown of grain boundaries and the formation of percolation paths. The percolation threshold was in the range of 0–5.44 vol% MoSi2, much lower than the reference value. The composites with 10 wt% MoSi2 exhibited an electrical resistivity of about 60 Ω cm, suitable for infrared source element applications.
... Owing to Prochazka's work [6], carbon and boron are well-known sintering activators facilitating the process of solid state sintering. Their application allows to obtain a dense single-phase silicon carbide [7,8]. The mechanism responsible for that phenomena has been well described by Stobierski [9,10]. ...
In the present research, mechanical properties of silicon carbide sintered with four carbon forms as a sintering aid were examined. As the source of carbon, the following substrates were used: multilayer graphene (MLG), carbon black, highly oriented pyrolytic graphite (HOPG) and synthetic graphite. The sinters were prepared with the use of powder metallurgy technique and consolidated at 1900 °C using the Spark Plasma Sintering (SPS) method. The influence of carbon form on mechanical properties, such as microhardness, hardness, fracture toughness, and elastic properties such as Young's modulus was investigated. Transmission electron microscope (TEM) analysis of the obtained sinters was also performed. Differences between mechanical properties were observed for samples with varied forms of carbon. An analysis of the obtained results shows that the form of carbon can determine the mechanical properties of silicon carbide and generate significant differences in hardness and fracture toughness despite the similarities in the microstructure.
... The C-SiC exhibited an increase in relative density from 80.2 to 84.8% with the increasing amount of C addition, whereas the Si-SiC showed the maximum of 89.5% at 1 mol% Si then a slight decrease to 85.4% with the addition of more Si content. C and B additives have been commonly employed as sintering additives for pressureless sintering of SiC to achieve fully dense bodies [12][13][14]. The addition of C has been used to enhance the sinterability by reducing the SiO 2 phase on the SiC powder surface by forming CO and CO 2 gases. ...
Off-stoichiometric silicon carbide (SiC), C- and Si-added SiC (6H, α-type), with an excess amount of C or Si from 1 to 5 mol%, were fabricated by spark plasma sintering at 2373 K and 50 MPa in a vacuum. The microstructure, electrical, and thermal properties of off-stoichiometric SiC were investigated. The lattice parameters increased after the addition of C and Si, suggesting the formation of solid solutions of C and Si in SiC. The addition of C and Si increased the densification, while the addition of a small amount of Si (1 mol%) significantly improved the densification. The electrical conductivity (σ) of C-added SiC was 0.7–1.4 × 10² S m⁻¹ at 298–1150 K. The Seebeck coefficient of C-added SiC changed from n- to p-type with increasing addition of C, whereas that of Si-added SiC was almost independent of the amount of Si added. The thermal conductivity of C- and Si-added SiC was in the range of 180–250 W m⁻¹ K⁻¹, which was greater than that of pristine SiC (100 W m⁻¹ K⁻¹) at room temperature.
... Silicon carbide combines a number of unique characteristics, namely high mechanic properties within a broad scope of temperatures, resistance to the effects of acids and alkalis, as well as distinctive electrical properties [1,2]. These properties may be ascribed to strong and stiff Si-C covalent bonds, which cause tremendous problems in obtaining dense sinters due to a low self-diffusion coefficient [3][4][5]. Therefore, sintering of SiC requires applying additions facilitating the process. Dense sinters may be obtained by liquid-phase sintering with the use of oxide additions (alumina, zirconia, yttria) [6,7] and by solid state sintering, with the additions of boron and carbon [8][9][10]. ...
In this work, sintering behaviour and microstructure changes of silicon carbide sintered with different additions
(0.5, 1, 1.5, 2 wt%) of four forms of carbon as a sintering aid were examined. As the source of carbon, the
following substrates were used: Multilayer graphene (MLG), Carbon Black, Highly oriented pyrolytic graphite
(HOPG) and Synthetic graphite. In order to determine the morphology of the used carbon, the specific surface
area, porosity, and SEM observations were performed. The Raman spectra was conducted with the purpose of
determining the differences in the thickness of the analyzed platelets, as well as the physical properties of the
surface. The sinters were consolidated at 1900 °C using the Spark Plasma Sintering (SPS) method. The influence
of carbon morphology on the relative density, microstructure and phase composition of the sintered samples was
investigated.
Similar, almost non porous microstructures with equiaxed grains were obtained for carbons with markedly
different morphology (spherical particles of Carbon Black and platelets particles of synthetic graphite), whereas
significantly different microstructures were obtained for carbons with nearly identical morphology (synthetic
graphite and HOPG).
... The solid solution formation could then be proceeded directly along the TaC/HfC interphases. W. van Rijswijk and Daniel J. Shanefield [22] have ana- lyzed reported data on the SiC bulk self-diffusion in a carbon-rich en- vironment that the excess carbon can contribute to the densification of the carbide via the rapid carbon self-diffusion. Similarly, the graphi- tized PyC in our Ta(Hf)C-based composites may maintain a high ef- fective carbon vapor pressure, thereby leading to an increase in the overall self-diffusion rate of TaC/HfC. ...
Ta0.8Hf0.2C (TH) ceramics are desirable for applications in ultra-high temperature environments, but they are difficult to be sintered pressurelessly. TH ceramics were densified up to 98.8% from commercial powders via pressureless sintering (PLS) at 2473K. SiC was introduced as secondary phase to tailor microstructures and improve properties of TH. The influence of SiC volume fraction on the densification, microstructure evolution and room-temperature properties of TH-based ceramics were examined. Average grain size of TH was refined from 13.6 down to 2.0μm. 10vol% SiC addition contributed to densification of Ta0.8Hf0.2C-SiC (THS) composites with a relative density of 99.6%. The mechanical properties of THS were fairly good, and thermal properties of sintered THS displayed a remarkable improvement compared with TH: the coefficient of thermal expansion (CTE) showed a reduction of 8.6% while the thermal conductivity increased from 18.6 to 41.5W/mK.
... SiC has turned a special attention as advanced ceramic material recently since it offers superior properties such as high hardness, low bulk density, high oxidation resistance, thermal conductivity and thermal shock resistance [9]. Furthermore, it is an important ceramic used in structural applications, such as automotive engines, cutting tools, heat exchange and mechanical seals [2,10]. Additionally, it is used in bulk form as refractory products, as electric heating elements and resistors, as igniters for gas appliances, as ceramic burners, as mechanical seal faces, as radiation sensors, as low-weight high-strength mirrors, as high power and high temperature semiconductor devices, as radiation resistant semiconductors and as light-weight armors [2, 13]. ...
Nowadays, ceramic matrix composites are being extensively used for industries and household purposes. However, these ceramic composites materials are substantially resistant to biodegradation. In this experimental study the specimen of silicon carbide with varying percentage of titanium carbide (TiC) has been fabricated by using microwave sintering. Different samples having TiC 5%, 10%, 15% (wt%) were prepared by die pressing. Then sample were heated in microwave furnace at different range of temperatures i.e. 1150°C, 1250°C and 1350°. It was found that with increase in the weight percentage (wt %) of TiC, the porosity was maximum in 1150°C and minimum porosity achieved maximum temperature 1350°C. The maximum hardness was achieved in 1350°C when 15 wt % of TiC. Density was also increased with respect to the increase in temperature and TiC wt %.
... The relative density of the Al7 was 94.6% (Table 1), showing the clear benefit of carbon as a sintering additive. The beneficial effect of carbon has been attributed to the reduction of the native SiO 2 layers on SiC particles [27]. Because surface energy of SiO 2 (307 mJ/m 2 ) is lower than that of SiC (486-810 mJ/m 2 ), densification is inhibited by the oxide layer on the SiC particles during solid state sintering [28]. ...
This study investigates the low-temperature sintering of nano Si-C powder containing Al-C additives prepared by high-energy ball-milling. The synthesized powder was composed of ultra-fine β-SiC crystallites (~5 nm) and amorphous Si-C matrix. TEM-EDS analysis showed a relatively homogeneous distribution of Al in the synthesized powder (d50: 170 nm). The relative density of SiC containing 6.5 wt% additives was 98.1% after sintering at 1650 °C for 30 min at a pressure of 20 MPa. The SiC could be densified at 1800 °C when the additive content decreased to 3.3 wt%. The Al content in the Si-C powders changed, e.g., from 4.11 to 2.6 wt%, after sintering at 1650 °C, which value was much higher than the solubility limit (0.26 wt% at 1800 °C) due to the homogeneous distribution of Al within the powder and the low sintering temperature. Al was segregated at the grain boundary while liquid phase formation was not identified by TEM analysis, indicating that grain boundary diffusion was the main densification mechanism. The mechanical properties of the sintered specimens were similar to those of a SiC-Al4SiC4 system, which has the same chemical components but containing large amount of sintering additives (5.6 vs. 13 wt%), sintered using higher pressure (20 vs. 60 MPa) and temperature (1650 vs. 1800 °C).
... It is believed that addition of carbon during sintering of SiC activates the sintering process. Furthermore, carbon helps in eliminating the oxide impurities residing at the grain boundaries [26]. At higher sintering temperature the following reaction takes place when there is no excess carbon [27]. ...
This study investigates the effect of graphene nanoplatelets on the microstructure and mechanical properties of silicon carbide (SiC). Graphene nanoplatelets are dispersed in a liquid preceramic polymer by ball milling. Pyrolysis of the graphene nanoplatelet–preceramic polymer slurry results in near-stoichiometric SiC–graphene nanoplatelet powder. This method leads to improved dispersion of graphene in the SiC matrix as compared to conventional mechanical blending of dry powders and thereby significantly influences the resulting mechanical properties. Subsequently, spark plasma sintering (SPS) is used to consolidate dense bulk SiC–graphene composites with varying graphene content up to a maximum of 5 wt.%. X-ray diffraction (XRD) investigation reveal that inclusion of graphene restricts grain growth of SiC matrix during SPS processing. Fracture toughness of SiC-graphene composite is increased by 40% with the inclusion of 2 wt.% graphene nanoplatelets. However, for higher graphene content the change in fracture toughness is limited. Improvement in fracture toughness is due to crack bridging reinforcing mechanism provided by the graphene platelets. Finally, Raman spectroscopy is used to understand the effect of SPS processing on integrity of graphene nanoplatelets.
A pressureless two‐step sintering method was used to densify high‐entropy carbide (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C, with high density and good mechanical performance. The material was heated up to 2200°C and subsequently soaked at 1900°C for 2–8 h with the incorporation of 2–4 wt.% phenolic resin. With increasing resin content, the large‐sized pores disappeared, and the average grain size decreased. However, with increased holding time from 2 h to 8 h, the average grain and pore size did not change significantly, the density increased up to 96.2%, and evident grain growth of graphite was observed. The mechanical properties of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C with 4 wt.% phenolic resin addition and 8 h soaking at 1900°C were measured as follows: flexural strength 361±28 MPa, Vickers hardness 16.0±0.3 GPa, and elastic modulus 426±2 GPa. This article is protected by copyright. All rights reserved
A novel (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramic was successfully prepared by pressureless sintering at 2200°C. With increasing content of resin-derived-carbon, the density, and mechanical and thermal properties increased up to a maximum content of 2∼4 wt% resin addition, after which further addition was detrimental. All specimens showed high strength (≥347±36 MPa), with the highest value achieving 450±64 MPa, and fracture toughness significantly higher (>20%) than those of the corresponding monocarbides and Ta0.5Hf0.5C, (Ta1/3Zr1/3Nb1/3)C. The thermal conductivity was approximately equivalent to the lowest value of the corresponding mono-carbides, which was assumed to be due to the lattice distortion effect.
Dense silicon carbide (SiC) ceramics were prepared by the high-temperature physical vapor transport (HTPVT) method at 2300°C using three types of graphite materials, namely common graphite (CP), isostatic graphite (IG), and graphite paper (GP), as crucible lids. The results indicated that the nucleation of SiC on CG was much easier than that on GP at the initial stage, but the polycrystalline growth on GP was faster. Moreover, the polycrystalline SiC ceramics growing on CG and IG lids were smaller than that grown on GP lid because many pores appeared at the interfaces between the grown polycrystalline SiC and CG and IG lids. As a result, the strength of the grown dense SiC ceramics on GP lid was about 8%–20%, higher than the strength of ceramics grown on the CG and IG lids.
Silicon carbide ceramics were densified by spark plasma sintering (SPS) using Silicon carbide (SiC) powders with the sintering aid Boron carbide (B4C). Dense solid-phase sintered SiC ceramics were obtained by SPS with a holding time of 5 min at a temperature of ~1800 °C, which is ~350 °C lower than that required by conventional pressureless sintering (PS). The microstructure and mechanical properties of the ceramics was systematically investigated for different sintering temperatures. The sample relative density, flexural strength, hardness, elastic modulus and fracture toughness were determined to be 98.6%, 592 MPa, 28.3 GPa, 461 GPa and 3.6 MPa m1/2, respectively. The spark plasma sintered specimens showed superior mechanical properties and finer grains compared to the pressurelessly sintered specimen. The densification kinetics for the spark plasma sintered SiC ceramics and the role of B4C in the sintering process were discussed.
The goal of achieving higher thermal efficiency in nuclear power systems, whether fission or fusion based, has invariably led to the study and development of refractory metals, ceramics, and their composites. Silicon carbide materials, owing to their favorable neutronic and high-temperature properties, have seen extensive study for over half a century in support of this goal. Currently, our community has a relatively deep understanding of the irradiation effects on this system and has developed irradiation-hardened materials that are currently in use for fission reactor fuels and available as structural composites for next generation reactors. Outside of the nuclear arena SiC has also enjoyed significant development with a wide range of ordinary and high-value product now in use including very high temperature commercial aerospace installations such as turbine engines. The paper presents a brief history of the development of SiC, focused on but not limited to irradiation applications that has led to our present understanding of the system for nuclear application.
This study was carried out to investigate the effect of Al additive amount on the microstructure development and room temperature mechanical properties of pressureless sintered silicon carbide. Different amounts of Al containing of 1, 2.5, 5, 10, and 15 wt% were added to the base material and pressureless sintering was conducted at 1950 °C for 1 h under argon atmosphere. The formation of the Al4SiC4 phase during conventional sintering was confirmed by X-ray diffraction (XRD) analysis. Microstructure, crystal phases, and density evolution were studied and correlated to Al additions and pre-sintered proceedings. The obtained results show that the addition of Al not only improves sinterability at lower temperature, but also enhances densification and hardness of SiC samples (3.05 g/cm³ and 26.9 GPa respectively).
Sintering is the key process to a pressureless sintering silicon carbide ceramic. From the perspective of industrial production, this paper discusses the influence factors of big size silicon carbide ceramic sintering from three aspects: sintering curves, the density of green body and particle size of raw material. The results were as follows: (1) Under the conditions that heating rate from 1000 to 1500 °C and from 2100 to 2150 °C is 3 °C per minute and 0.5 °C per minute respectively, a big size product of densification of the ceramics has been obtained. (2) Using dry pressing at 50 MPa, dwell time 2 min, and then with CIP at 180 MPa, dwell time 5 min, the green body densities will more uniform from center to edge. (3) Applying raw materials particle size below 0.6 μm the big size product will get more dense material. Combining with the above three methods. The final product can achieve densification. The sintered sample can been achieved with density of 3.12 g/cm³, Vickers hardness of 26 GPa, and three-point flexure strength 420 MPa.
A comparative evaluation between pressureless or self-sintered silicon carbide (SSiC), hot-pressed silicon carbide (HP-SiC), and spark plasma-sintered silicon carbide (SPS-SiC) has been carried out with emphasis on examination of their microstructures and mechanical properties. The effect of sample dimensions on density and properties of SPS-SiC has been also examined. Elastic modulus, flexural strength, and fracture toughness measured by indentation or testing of single-edge notched beam specimens have been found to follow the following trend, HP-SiC > SSiC > SPS-SiC. The SPS-SiC samples have shown size-dependent densification and mechanical properties, with the smaller sample exhibiting superior properties. The mechanical properties of sintered SiC samples appear to be influenced by relative density, grain size, and morphology, as well as the existence of intergranular glassy phase. Studies of fracture surface morphologies have revealed the mechanism of failure to be transgranular in SSiC or HP-SiC, and intergranular in case of SPS-SiC, indicating the dominating influence of grain size and α-SiC formation with high aspect ratio.
The author studied chemistry and chemical engineering in school but never actually studied ceramics as an academic discipline. Therefore this book is written from a somewhat unusual perspective, more like a chemical engineering view of processing than a typical ceramics view. The emphasis on a certain few aspects of processing is based on those serious problem areas that became apparent during the author’s several decades of industrial experience, rather than being based on theoretical considerations.
The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.
High-strength and high-modulus inorganic fibres from silicon carbide and silicon nitride of various modifications may be obtained without use of substrates by forming from melts or solutions of elemento-organic polymers with subsequent heat and chemical treatment. They have been referred to as ‘coreless’ fibres.
The dynamics of an aerosol, i.e. the evolution of the particle size distribution by particle formation and growth, diffusion, coagulation, convection and sedimentation can be described by the general dynamic or population balance equation (GDE, see for example Friedlander 1977 and Gelbard and Seinfeld 1979). Wu and Flagan (1988) have used a discrete sectional model of the GDE to describe the formation of SiO2 and Girshick and Chiu (1990) described MgO powder synthesis by thermal plasma. Landgrebe and Pratsinis (1990) used a dimensionless form of the discrete sectional model to find an universal correlation map for gas phase synthesis of powders and applied it to TiO2 synthesis from TiCl4. The GDE is a partial integro-differential equation (or a system of ordinary differential equations) which is difficult to solve. It can be replaced by a mass balance equation for monodisperse or moment models which greatly simplifies the solution (see Landgrebe and Pratsinis 1990 for a comparison of different levels of simulations). Probably, the processes best studied are flames (Biswas et al. 1997; Lindackers et al. 1997 both using coagulation models) and the mechanisms most investigated are homogeneous nucleation (e.g. Grandquist and Buhrman 1976 for Inert Gas Condensation (IGC); Panda and Pratsinis 1995 for nucleation of Al in a flow reactor or Kruis et al. 1994 for nucleation of silicon from silane) and coagulation of aerosols (e.g. Matsoukas and Friedlander 1991). Egashira et al. (1994), and Okuyama et al. (1992) described the cluster formation in the CVD production of AIN and GaAs films, respectively.
Carbon nanosheet–coated micron zirconium diboride particles were achieved by an atmospheric chemical vapor deposition in the presence of methane gas without catalyst. The microstructures and carbon structure for the novel particles were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. In addition, the growth process of hybrid particles was analyzed by thermodynamic theory. The results showed that zirconium diboride particles were coated by carbon nanosheets uniformly. The carbon nanosheets/zirconium diboride particles were employed to fabricate ceramic composite using spark plasma sintering process. The fracture toughness of the composite was improved up to 5.9 MPa·m0.5 with only 0.9 wt% of carbon nanosheets, which was 40.9% higher than that of the composite without carbon nanosheets. The toughening mechanisms were mainly known as carbon nanosheets crack bridging and pulling out which lead to the formation of crack deflection in the novel composite.
Polymethylsilane, -[MeHSi]x-, synthesized by dehydrogenative coupling of MeSiH3, will when heated to temperatures in excess of 800°C give almost pure, nanocrystalline SiC. The chemical evolution of polymer to ceramic was followed by 29Si NMR, diffuse reflectance IR and X-ray powder diffractometry. The polymer undergoes a gross structural rearrangement from -[MeHSi]x- to -[H2SiCH2]x- on heating to 400°C. Above 400°C, the resulting polycarbosilane decomposes to a hydrogenated form of SiC as evidence by spectroscopic analysis of the 600°C material. Further heating to 1000°C for 1h provides very narrow 29Si peaks indicative of ß-SiC mixed with small amounts of α-SiC. Chemical analysis, when coupled with the 29Si and the XRD results suggest that polymethylsilane produces reasonably pure, nanocrystalline SiC at temperatures much lower than previously observed for other SiC preceramic polymers.
Densification of SiC powder with total amounts of 2, 4, 8 wt% Al-B-C additive was carried out by spark plasma sintering (SPS). The unique features of the process are the possibilities of a very fast heating rate and a short soaking time to obtain fully-dense materials. The heating rate and applied pressure were kept at 100 K · minute-1 and 40 MPa, while the sintering temperature and soaking time varied from 1,700-1,800 °C for 10-40 minutes, respectively. The SPS-sintered SiC at 1,800 °C with different amounts of Al-B-C reached near-theoretical density. The sintered SiC ceramics were predominantly composed of 6H polytype with 15R and 4H polytype as minor phases. The microstructure of SiC sintered up to 1,750 °C consisted of equiaxed grains. In contrast, the growth of large elongated SiC grains in small matrix grains was shown in sintered bodies at 1800 °C, and a plate-like grains interlocking microstructure had been developed by increasing the soaking time at 1800 °C. The grain growth rate decreases with increasing amounts of Al-B-C in SiC, however, the volume fraction and the aspect ratio of large elongated SiC grains in the sintered bodies increased.
In this paper, the influence of SiO 2 on the microstructure and consolidation mechanism of recrystallized silicon carbide (RSiC) was studied by comparing the relationship of the weight losses and microstructural evolution with the SiO 2 contents at different firing temperatures. The results showed that the presence of SiO 2 resulted in a basic weight loss proportional to the SiO 2 content and an additional weight loss independent of the SiO 2 content. The consolidation mechanism of SiC was not altered by the introduction of SiO 2, involving surface diffusion at low temperatures and an evaporation-condensation process at the high temperature, while the residual ambient atmosphere primarily including SiO(g), Si 2C(g) and Si(g) inhibited the recrystallization of SiC by altering the mass transport from SiC 2(g), Si 2C(g) and Si(g) for pure SiC to that combined with the gaseous transport of SiO(g), Si 2C(g), Si(g) and SiC(g), and the surface diffusion of C(s) at the high temperature.
Carbogran UF-05, UF-10 and UF-15 were investigated. Sinterability was determined depending on powder particle size, sintering temperature, sintering atmosphere and additives. Correlations of sintered density and microstructure with these parameters are presented.
UHV-fracture surfaces of different (B, C)- and (Al, C)-doped pressureless sintered SiC materials are investigated by using high resolution Auger electron spectroscopy (HRAES). The grain boundaries at intergranular fractured regions of (B, C)-doped materials are evidently free of any enrichment of impurity elements or sintering additives. In contrast, the (Al, C)-doped materials reveal the existence of thin Al-containing grain boundary films. Sputtering indicates their thickness to be less than 1 nm. The concentrations of the elements segregated in the grain boundary films are determined.
The 30Si lattice self-diffusion coefficients in high-purity β-SiC are reported for the temperature range 2283 to 2547 K and may be represented by the expression {Mathematical expression}. Decomposition of the sample at the grain boundaries prevented detection of diffusion along these paths of fast transport. Lattice diffusion of Si was concluded to occur by a mechanism involving a direct jump to the nearest Si vacancy without the previous occupation of a normally unfilled position. A comparison of C and Si diffusion in this material is also given.
Pressureless sintering of SiC was accomplished at 2100°C with oxide additives. These additives were the products of the reaction of Al(OH)3 with HCl and of Y(OH)3 with HCOOH. These reaction products were dissolved in water and mixed with submicrometer β-SiC. A mixture of equal weights of these additives was effective for the sintering of SiC.
The14C self-diffusion coefficients for both lattice (D
lc*) and grain boundary (D
bc*) transport in high purity CVDβ-SiC are reported for the range 2128 to 2374 K. The Suzuoka analysis technique revealed thatD
bc* is 105 to 106 faster thanD
bc*; the respective equations are given by $$\begin{gathered} D_{I c}^* = (2.62 \pm 1.83) \times 10^8 exp\left\{ { - \frac{{(8.72 \pm 0.14)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ D_{b c}^* = (4.44 \pm 2.03) \times 10^7 exp\left\{ { - \frac{{(5.84 \pm 0.09)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ \end{gathered} $$
A vacancy mechanism is assumed to be operative for lattice transport. From the standpoint of crystallography and energetics, reasons are given in support of a path of transport which involves an initial jump to a vacant tetrahedral site succeeded by a jump to a normally occupied C vacancy.
In available crystals, the self-diffusivity of carbon has been found to
be extrinsic (experimental range 1853 to 2088°C), the carbon
self-diffusivity being higher in p-type than in n-type material. The
self-diffusion coefficient for Si is approximately ten times higher than
that for carbon between 1927 and 2017°C.
The sintering behavior of primarily covalently bonded β-SiC, Si, and Si3N4 was studied using surface area and densification measurements as well as observations of microstructures developed during firing. The existence of highly dense, microscopic regions and large (≥100°) dihedral angles in fired compacts of β-SiC and Si which experience little macroscopic densification suggests that macroscopic densification is not intrinsically limited by the effects of surface energy. The mechanism proposed to explain the microstructure that develops in unsinterable covalent solids which do not undergo a phase change is based on the existence of a high ratio of surface and/or vapor-phase matter transport-to-volume and/or grain-boundary transport. The addition of boron to both β-SiC- and Si-containing carbon retards surface and/or vapor-phase transport and grain growth at lower temperatures, which results in enhanced densification at high temperatures. Macroscopic densification of β-SiC and α-Si3N4 can also be retarded by the formation of a continuous network of high-aspect-ratio grains of the polymorphic form that rigidities the sintering body. Finally, the sintering of pure Si depends sensitively on particle size in the submicron range. Nearly theoretical density is achieved in Si powder of ∼0.06-μm size. This result suggests that other pure covalently bonded solids can also be sintered to high density without applied pressure.
The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B4C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.
The self-diffusion of30Si in high purity and N-doped α-SiC single crystals has been measured in the temperature range 2273 to 2573 K. The diffusion (D
Si*) in N-doped crystals exceeds that in the pure crystals because of the increase in the concentration of the charged acceptor-type Si vacancies in the presence of the N species. A comparison ofD
C* andD
Si* shows that the former exceeds the latter by approximately 102, primarily because of the greater entropy of migration of C. Possible crystallographic paths of transport for both species are also discussed.
The microstructures and compositions of Al- and B-doped pressureless sintered SiC-materials from four different sources were investigated by the combined usage of several microanalytical techniques. Besides the fundamental ceramography and chemical analysis the methods of Auger electron spectroscopy (AES), wavelength dispersive analysis of X-rays (WDX), microautoradiography and scanning transmission electron microscopy (STEM) were used. In transgranular fractured surface regions the stoichiometric Si/C relation was found by AES. The grain boundaries, however, are enriched with C and O, and also partly with B and Al. Additives and impurities are distributed in an inhomogeneous manner; the heterogeneous inclusions are very differently sized from less than 0.1m (STEM) to 10 to 20m (WDX on polished specimens) and 200m (AES on fracture surfaces). These results reveal the need for improving the production process.
Silicon Carbide Family of Structural Ceramics " ; pp. 99-148 in Structural CeramicsRtmosphere Effects in Sintering of Silicon Carbide
- M Srinivasan
- L Hoffman
- Gauker
M. Srinivasan, " Silicon Carbide Family of Structural Ceramics " ; pp. 99-148 in Structural Ceramics, Val. 29, Treatise on Materials Science and Technology. Edited by J. B. Wachtman. Academic Press, New York, 1989. 5S. Prochazka et ul., 'Rtmosphere Effects in Sintering of Silicon Carbide " ; pp. 366-80 in Proceedings of the International Symposium on Densification of Oxide and Non-Oxide Ceramics, Hakone, Japan, 1978. Edited by S. Somiya. Gakufutsu Bunken Fukyu-kai, Tokyo, Japan, 1979. 'T. Mizrah, M. Hoffman, and L. Gauker, " Pressureless Sintering of Alpha-Sic, " Powder Metull. Int., 16 [5] 217 (1984).
American Elsevier Self-Diffusion of Carbon-14 in High-Purity and N-Doped u-SiC Single Crystals Self-Diffusion of Silicon-30 in Alpha-Sic Single Crystals
- J D Hong
- M H Hon
- R F J D Davis
- R F Hong
- J D Davies
- R F Hong
- D E Davis
- Newbury
'J. D. Hong, M. H. Hon, and R. F. Davis, " Self-Diffusion in Alpha and Beta SIC " ; p. 409 in Materials Science Monographs, Vol. 6. American Elsevier, New York, 1980. " J.D. Hong and R.F. Davies, " Self-Diffusion of Carbon-14 in High-Purity and N-Doped u-SiC Single Crystals, " J: Am. Cerum Soc., 63 [9-101 -. 546-;2 (1980).. " J. D. Hong, R. F. Davis, and D. E. Newbury, " Self-Diffusion of Silicon-30 in Alpha-Sic Single Crystals, " J. Muter. Sci., 16 [9] 2485 (1981).
The Role of Boron and Carbon in the Sintering of Sic " ; pp. 171-82 in Special Ceramics
- S Prochazka
References 'S. Prochazka, " The Role of Boron and Carbon in the Sintering of Sic " ; pp. 171-82 in Special Ceramics, No.
Method of Fabricating Silicon Car-bide Articles Self-Diffusion in Silicon Carbide
- g W Hollenberg
- R L N Craner
- R L Ghostagore
- Coble
7G.W. Hollenberg and R. L. Crane, " Method of Fabricating Silicon Car-bide Articles, " U.S. Pat. No. 3966855, 1976. 'R. N. Ghostagore and R. L. Coble, " Self-Diffusion in Silicon Carbide, " Phys. Rev., 143 [2] 623 (1966).
- Greskovich C.
Method of Fabricating Silicon Carbide Articles
- G W L Hollenbergandr
- Crane