Growth Of SiC Grains Research Articles

Hot oscillatory pressing of SiC ceramics with B4C and C as sintering additives

SiC ceramics with 0.5 wt.% B4C and 0.5 wt.% C as sintering additives were prepared by hot oscillatory pressure (HOP) and hot pressure (HP). The mechanical properties and microstructures of the resultant ceramics were investigated. The results showed that the oscillatory pressure could effectively suppress the abnormal growth of SiC grains and promote the formation of high-density dislocation and stacking faults. The hardness of SiC ceramics made by HOP at 2000oC reached 28.9 GPa, which was much higher than that (26.8 GPa) of the sample made by HP at the same temperature. The current study suggests that HOP is a promising technique for preparing high-performance SiC ceramics.

Li2O addition as a means of achieving low-temperature densification of SiC ceramic

In order to achieve low-temperature densification of SiC ceramic, β-SiC nanopowder was doped with Li2O together with Y2O3–Al2O3–MgO as sintering aids, and then spark plasma sintered at temperatures ranging from 1450 °C to 1600 °C for 30 min under 30 MPa pressure in argon. The densities, microstructures and mechanical properties of SiC ceramics were systematically investigated, in comparison to the counterparts sintered with Y2O3–Al2O3–MgO additives. Since Li2O has a low melting point, Li2O addition effectively decreased the formation temperature of liquid secondary phase, enabling SiC ceramics to obtain higher density and lower porosity after low-temperature sintering (T ≤ 1550 °C). Besides, Li2O addition was found to decrease the viscosity of liquid phase and enhance grain growth of SiC through dissolution-reprecipitation process. When sintered at 1550 °C, Li2O addition led to a remarkable increase of density from 3.03 g/cm3 to 3.21 g/cm3 for SiC ceramic. Meanwhile, the SiC ceramic with Li2O addition exhibited a high Vickers hardness of 22.75 ± 0.35 GPa and a good fracture toughness of 3.83 ± 0.27 MPa m1/2. The research results are expected to benefit the low-cost fabrication of SiC ceramics based on pressure-assisted sintering and alleviate the thermal damage of SiC fiber in hot-pressed SiCf/SiC composites.

Effects of diamond as a main carbon source on the fabrication and mechanical properties of reaction-bonded SiC

RBSC with a low residual Si less than 5 vol% was fabricated using SiC–C preform with diamond as a main carbon source by a conventional reaction sintering process at the temperature range between 1500 °C and 1800 °C. The diamond content in the SiC–C preform varied from 10 wt% to 17 wt%. The diamond as the main carbon source used in SiC–C preform effectively minimized the residual Si in RBSC by increasing the reaction of diamond due to the enhanced Si melt infiltration even in SiC–C preform with high carbon content. The Si melt infiltration should be performed above 1600 °C for the complete reaction of diamond particles with size of 0.5 μm because of the relatively low dissolution rate of diamond in Si melt. The flexural strength of RBSC with residual Si content less than 7 vol% exceeded 400 MPa. The flexural strength of RBSC was increased with decreasing residual Si content, but sharply decreased by the residual carbon accompanying pores in RBSC. RBSC with low residual Si content displayed moderate flexural strengths around 200 MPa even above Si melt temperature because of the formation of continuous SiC structure in RBSC by the enhanced grain growth of SiC. Vickers hardness of RBSC was steadily increased from 17.7 GPa to 24.6 GPa with decreasing residual Si.

Interfacial modulation of SiC/TiC hybrid nanofibers for fabricating flexible ceramic electromagnetic wave absorbers

Titanium Carbide (TiC), characterized by its hardness, conductivity and thermal stability, can serve as a suitable additive phase for SiC nanofibers. TiC phase can improve the flexibility and electromagnetic wave absorption property of SiC nanofibers without sacrificing its high-temperature thermal stability. Herein, flexible SiC/TiC hybrid nanofibers with different Ti contents were fabricated by electrospinning and precursor infiltration pyrolysis. The introduction of Ti can not only generate conductive TiC phase, but also act as a catalyst to regulate the growth of SiC grains, so as to adjust the flexibility and electromagnetic properties of the hybrid nanofibers. At the optimal Ti precursor content of 4 wt%, the SiC/TiC-4 hybrid nanofibers exhibit a broad effective absorption bandwidth of 4.5 GHz with the thickness of 1.6 mm. The flexible SiC/TiC hybrid nanofibers This work can provide a reference for the design of flexible ceramic nanofiber absorbers and is expected to enable practical application in high temperature conditions.

Microstructure and mechanical properties of SiC composite ceramics modified by (TixZr1–x)B2 solid solution

Dense SiC-(TixZr1–x)B2 composite ceramics were prepared via solid-state spark plasma sintering (SPS) by controlling the proportion of ZrB2 (in the range of 5–25 vol%). Through data fitting, a linear functional relationship was established between x and 2θ for the (TixZr1–x)B2 solid solution, which rendered it amenable to characterization analysis. The results show that the 2-D morphology of (TixZr1–x)B2 tended to transform from angular to round as the ZrB2 proportion was increased, which was beneficial in crack deflection and increasing the fracture toughness. Moreover, the growth of SiC grains around the solid solution was inhibited, which improved its mechanical properties. The composite ceramic containing 15 vol% of ZrB2 had the best mechanical properties, with its Vickers hardness, bending strength and fracture toughness reaching 26.12 GPa, 499 MPa, and 6.45 MPa•m1/2, respectively.

Mechanical and tribological properties of SiC-GNPs composites prepared by oscillatory pressure sintering

SiC-based ceramics, as structural parts and wear-resistant parts, are required to possess outstanding mechanical properties and wear resistant. Herein, to prepare SiC-based ceramics with desirable properties, the effects of graphene nanoplatlets (GNPs) content on the densification, microstructure, mechanical and tribological properties of SiC-GNPs composites by using an oscillatory pressure sintering (OPS) process were investigated. The addition of GNPs could inhibit the growth of SiC grains, and the resultant mechanisms such as crack bridging, deflection, and GNPs pull-out also promoted the strengthening and toughening of such composites. The flexural strength and fracture toughness of the SiC-1.0 wt% GNPs composites attained the maximum values of 515 MPa (∼27% increase) and 7.11 MPa·m1/2 (∼26% increase), respectively. In addition, the friction behavior of SiC-GNPs composites was analyzed by selecting GCr15 steel ball as friction side under drying friction conditions. Compared with pure SiC ceramics, the friction coefficient and wear rate of SiC-1.0 wt% GNPs ceramics were as low as 0.65 (reduced by ∼25%) and 5.17 × 10−7 mm3·N−1·m−1 (reduced by ∼41%), respectively. This phenomenon can mainly be attributed to the lubricating film formed by GNPs during sliding and excellent mechanical properties.

Microstructure of spark plasma sintered TiC–TiB2–SiCw composite

This research intends to implement a complementary study on the consolidation and microstructure of TiC-based ceramics incorporating 30 vol% TiB2 and 10 vol% SiC whiskers (SiCw). The spark plasma sintering was employed as the manufacturing tool to produce the designed composite under the sintering pressure of 40 MPa for 7 min at 1900 °C. The X-ray diffraction study and the microstructural observations revealed the formation of graphite as an in-situ phase during the sintering process, as a result of carbon deposition from the crystalline structure of the TiC matrix. Although the SiC introduced to the composite was needle-shaped, no SiC whiskers could be found in the final microstructure due to the nucleation and growth of new SiC grains. Additionally, it was suggested that the surface oxide impurities available on the starting powders could contribute to some reactions, resulting in the in-situ formation of SiC, TiB2, and TiC ingredients. Furthermore, the grain growth phenomenon led to the formation of intragranular TiB2 grains in the TiC matrix. The plastic deformation and the applied external pressure with an inconsistency among the thermal expansion coefficients of TiB2, TiC, and SiC over the cooling stage resulted in the generation of crystalline defects, e.g., dislocations and distorted atomic planes.

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.

Investigations on the thermal behaviours of SiC-ZrC continuous ceramic fibres

In this study, nanoscale composite SiC-ZrC ceramic fibres, derived from polyzirconocenecarbosilane (PZCS) via melt spinning, electron beam crosslinking, pyrolysis and sintering were investigated in detail. Compared with several commercial products of second-generation SiC fibres, the produced composite fibres exhibit improved thermal stability, mechanical properties and oxidation resistance. SiC grains in the fibre grew from 9.8 nm to 33.9 nm after annealing in an inert atmosphere at 1800 °C for 1 h, as well as decomposition of the SiCxOy phase and the growth of SiC grains affected the mechanical properties of the fibres, and the mechanical properties of the fibres were maintained at 1.1 GPa, accompanied by an increase in the modulus. After the fibres were oxidized at 1100∼1400 °C for 1 h, a dense oxide layer of SiO2-ZrO2 was formed on the surface of the fibres, which slowed down the rate of further fibre oxidation, thus, the fibres exhibited excellent oxidation resistance.

Structural evolution and mechanical properties of Cansas-III SiC fibers after thermal treatment up to 1700 °C

The effects of thermal treatment on the Cansas-Ⅲ SiC fibers were investigated via heating at temperatures from 900 to 1700 ℃ for 1–5 h in argon atmosphere. The composition and morphology of the SiC fibers were characterized and the tensile strength of the SiC fiber bundles was analyzed via two-parameter Weibull distribution analysis. The results showed that the thermal treatment has negligible influence on the microstructure of the SiC fibers at temperatures ≤ 1100 ℃. At temperatures ≥ 1300 ℃, the surface of the fibers became rough with some visible particles. Particularly, at 1700 °C, numbers of holes appeared. With the increasing of heating temperature and holding time, the average tensile strength of the SiC fibers decreased gradually from 1.81 to 1.01 GPa. The decreasing of tensile strength can be attributed to the increase of critical defect sizes, grain growth and phase transformation (β→α) of SiC.

Effect of SiC whisker on densification and mechanical properties of hot-pressed TiC-based composites

The impact of adding 40 vol% SiC whiskers on the sinterability, flexural strength, and hardness of TiC ceramics was studied. The hot-pressing route was employed to fabricate the TiC–SiC composite, as well as the monolithic TiC ceramic. The microstructural studies and X-ray diffractometry revealed the precipitation of carbon within the TiC matrix owing to the formation of non-stoichiometric TiCx during the hot-pressing. The addition of SiC whiskers noticeably improved the sinterability of TiC, attaining a fully dense composite sample. The nucleation and growth of new SiC grains were identified as the dominant factors in enhancing the relative density of the TiC-40 vol% SiC composite. Moreover, the SiC additive had an effective impact on both flexural strength and hardness of TiC, standing at 543 MPa and 2280 HV0.5 kg, respectively.

A development mechanism of graded microstructures in iron-containing SiC fibers revealed by electron microscopy

Transmission electron microscopy is utilized to investigate the microstructure development of iron-containing SiC fibers as a function of their annealing temperatures. Compositional and structural gradients along the radial directions start to form within SiC fibers fabricated at temperatures above 1200 °C. The graded microstructures are associated with the outward diffusion of iron-rich particles along the radial direction. In-situ heating TEM experiments suggest that the iron-rich particles within SiC fibers are highly mobile at elevated temperatures. Therefore, we propose that the graded microstructures along the radial directions are the synergetic effects of iron catalyzation and outgassing, namely, iron-rich particles are progressively driven to the surface of SiC fibers and in the trajectory of iron-rich particles, the decomposition of the amorphous SiCxOy phase is accelerated, promoting the growth of SiC grains. The current study emphasizes the important role of the outgassing process to the microstructure evolution of SiC fibers. Our proposed mechanism can be extended to understand the microstructural evolution of SiC fibers that use metallic additives as sintering aids.

Grain growth and interfacial structures in SiC fibers

The understanding of the microstructures is critical to the design of SiC fibers with excellent thermal stability and mechanical properties. We introduce a density-sensitive electron microscopy method, high-angle annular dark field (HAADF) imaging to study the microstructure of SiC fibers processed at a variety of temperatures ranging from 1200 to 1800 °C. It is revealed that, irrespective of their processing temperatures, SiC grains form the skeleton of SiC fibers that are surrounded by a multiple of zones with low HAADF contrast (low-density zones, LDZs). LDZs mainly consist of amorphous SiOC phases and turbostratic graphite. As a result, three important interfaces that dictate grain growth, namely, the SiC/amorphous SiOC interface, the SiC/turbostratic graphite phase boundary and the SiC high angle grain boundary (HAGBs) emerge. We find that, the SiC/turbostratic graphite interfaces and the SiC HAGBs are more effective in suppressing the growth of SiC grains than the SiC/a-SiOC interfaces on a basis of extensive TEM characterization. Aberration corrected TEM reveals marked differences in the atomic structures of those three interfaces, shedding light on how the interfacial structures affect grain growth of SiC fibers.

The effect of nano-TiO2 additions on the densification and mechanical properties of SiC-matrix composite

This research studied the effect of adding TiO2 nanoparticles along with Al2O3 and Y2O3 additives on the physical and mechanical properties of a SiC-matrix composite. The samples were fabricated through a pressureless process at 1900 °C. The results showed that the addition of TiO2 nanoparticles up to 4.5 wt% inhibited the excessive growth of SiC grains. According to the investigations, the microstructure and final properties of composites were affected by density, synthesized phases as well as their distribution in the matrix, and grain size. The highest density, Young's modulus, hardness, indentation fracture resistance, and flexural strength, alongside with the lowest brittleness index were respectively, 98.7%, 401.2 GPa, 27.1 GPa, 6.1 MPa m1/2, 522.7 MPa and 292.19 × 10−6 m−1, attributed to sample containing 4.5 wt% TiO2. Microstructural observations showed that the stresses caused by the difference in the thermal expansion coefficients of SiC and TiC, the formation of micro-cracks, the occurrence of crack deflection and crack branching mechanisms, as well as grain pull-out were the main reasons for the improvements in indentation fracture resistance.

Microstructure and Tribological Performance of Mesocarbon Microbead-Silicon Carbide Composites.

Mesocarbon microbead–silicon carbide (MCMB–SiC) composites with 0–30 wt % MCMBs were prepared by pressureless sintering (PLS) method at 2200 °C in Ar. The microstructure and tribological properties of the prepared composites were investigated. The results show that there was a finer grain size of SiC with the increase in MCMB content because MCMBs hinder the growth of SiC grains. The hardness of the composites decreased with increasing MCMB content, whereas the fracture toughness fluctuated showing a complex trend. The tribological properties of the composites under dry friction conditions were evaluated using the pin-on-disk method against a SiC counterpart. We found that the tribological properties of the samples were influenced by the oxide film or lubricating film that formed during the wear process on wear surfaces. Different wear mechanisms were found to be associated with differing MCMB contents.

Effect of oxygen content on the sintering behaviour and mechanical properties of SiC ceramics

In the current work, SiC powders with various levels of oxygen content were obtained by two ways: acid leaching and heat treatment; and these SiC powders were densified by a pressureless sintering technique with boron carbide and carbon additives. The techniques of X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, transmission electron microscope, and mechanical testing were combined to examine the effects of oxygen impurities on the densification process of SiC ceramics. The experimental analyses indicated that high oxygen content is detrimental to the final sintering and mechanical properties of SiC ceramics. As the oxygen content increases from 0.55 to 2.10 wt%, the relative density and the hardness both decrease: the relative density drops from 98.53% to 70.55% (a 28% decrease), and the hardness drops from 27.1 ± 0.7 GPa to 6.7 ± 1.1 GPa (a 75% decrease). Furthermore, oxygen impurities can accelerate the abnormal growth and coarsening of SiC grains during sintering. In order to impede abnormal grain growth and achieve high densities of SiC ceramics, it is proposed that the oxygen content of the powders should be limited to less than 1.30 wt%.

Mechanisms and kinetics during reactive infiltration of molten silicon in porous graphite

Liquid silicon Infiltration (LSI) is a fast and economical process to manufacture SiC-based ceramics. For a better understanding of reactive melt infiltration of liquid silicon, the wetting and infiltration of porous graphite by molten silicon were investigated at 1450, 1500 and 1550 °C for duration comprised between 10 s and 1 h. Infiltrations tests were performed in an argon atmosphere with an inductively heated furnace operating with heating and cooling rates of 300 °C.min−1. The formation and growth of SiC grains were investigated at the outer surface and within graphitic carbon substrates with 11% porosity and a narrow pore size distribution centered at 2 μm. The length of the infiltrated zone and the SiC crystals growth were determined from scanning electron microscopy. Rapid spreading and infiltration of molten silicon are observed from the first 60 s. The growth rate of the interfacial SiC layer obeys a fourth-power law with an activation energy of 260 ± 30 kJ mol−1. Pore filling by SiC is limited by volume diffusion with an activation energy equal to 320 ± 40 kJ mol−1.

Effects of aluminum, boron and carbon additives on microstructure of porous silicon carbide ceramics and their properties

Abstract Porous SiC ceramics with in-situ plate-like grain growth were fabricated by pressureless-sintering using Al, B and C additives, and the effects of Al, B and C additives on the microstructure of porous SiC ceramics and their properties were investigated. Whereas the porous SiC ceramics sintered with B-C additives mostly consisted of fine SiC grains without grain growth, Al additives with B-C system enhanced plate-like grain growth of SiC in the porous SiC ceramics. Notably, the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives consisted of plate-like SiC grains of around 10-20 μm, and the plate-like SiC grains stuck out of the walls of pores and the sintered surface in the porous SiC ceramics. XRD results suggested that the plate-like SiC grains formed in the porous SiC ceramics sintered with Al-B-C additives mainly consisted from 6H (α-SiC) phase. Compressive strength and thermal conductivity of the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives at room temperature were 50 MPa and 10 W/m·K, respectively. From these results, it is concluded that high performance porous SiC ceramics with unique morphology of entangled large plate-like SiC grains were successfully achieved based on in-situ grain growth.

Electrical and mechanical properties of pressureless sintered SiC-Ti2CN composites

Highly conductive SiC-Ti2CN composites were fabricated from β-SiC and TiN powders with 10 vol% Y2O3-AlN additives via pressureless sintering. The effect of initial TiN content on the microstructure, and electrical and mechanical properties of the SiC-Ti2CN composites was investigated. It was found that all specimens could be sintered to ≥98% of the theoretical density. The electrical resistivity of the SiC-Ti2CN composites decreased with increasing initial TiN content. The SiC-Ti2CN composites prepared from 25 vol% TiN showed the highest electrical conductivity (∼1163 (Ω cm)−1) for any pressureless sintered SiC ceramics thus far. The high electrical conductivity of the composites was attributed to the in situ-synthesis of an electrically conductive Ti2CN phase and the growth of N-doped SiC grains during pressureless sintering. The flexural strength, fracture toughness, and Vickers hardness of the composite fabricated with 25 vol% TiN were 430 MPa, 4.9 MPa m1/2, and 23.1 GPa, respectively, at room temperature.

Effects of VC additives on densification and elastic and mechanical properties of hot-pressed ZrB2–SiC composites

In this study, highly dense ZrB2-20 vol% SiC composites with 3–10 wt% VC additives were prepared by hot-pressing at 1750 °C for 1 h under a pressure of 20 MPa in a vacuum. The densification behavior and elastic and mechanical properties of the obtained composites were examined, and the effect of the VC content on the densification and the properties is analyzed. The addition of VC promotes the activation of densification mechanism at a lower temperature and inhibits the growth of ZrB2 and SiC grains during the sintering. In addition, the elastic moduli, hardness and fracture toughness that measured in the obtained composites are constant and independent of the VC content, with a shear modulus of ~ 220 GPa, Young’s modulus of ~ 500 GPa, hardness of ~ 20 GPa and fracture toughness of ~ 4.4 MPa m1/2. On the other hand, the flexural strength of the composites decreased as the VC content increased from 3 to 7 wt% and then it increased with further increasing the VC content to 10 wt%, with strength values of 620–770 MPa.