Pressureless Sintered Silicon Carbide Research Articles

Thermal oxidation polishing of pressureless sintered silicon carbide

Pressureless sintered silicon carbide (PS–SiC) is a promising material for mirrors used in short-wavelength optical systems. However, due to its intrinsic hardness and multiphase nature, achieving atomic and close-to-atomic scale (ACS) polishing to meet the stringent requirements of the optical systems is challenging. In this study, a thermal oxidation polishing method is employed to achieve ACS polishing of PS-SiC. This method converts the hard heterogeneous SiC phase and Si phase into the “softer” homogeneous SiO2 via thermal oxidation, followed by polishing using silica sol, resulting in a finished surface roughness of 0.55 nm in Ra within 15 min. Molecular dynamics simulations are also conducted to study the thermal oxidation process of PS-SiC. The results show that SiC and Si phases oxidize at different rates, leading to roughness deterioration after thermal oxidation. This also causes unevenness in the SiC and Si phases beneath the oxidized layer. Therefore, it is necessary to control the polishing time to avoid over-polishing and potential exposure of the underlying uneven SiC and Si phases.

Joining of textured PLS-SiC ceramic with CaO-Al2O3-MgO-TiO2-SiO2 glass filler

The pressureless-sintered silicon carbide ceramic with and without surface texturing was joined with CaO-Al2O3-MgO-TiO2-SiO2 glass filler. The experimental results showed that without texturing applied to SiC substrates, the shear strength of SiC joints was 40.8 ± 6.3 MPa, with failure occurring at the interlayer. Unfilled regions in the interlayer limited the shear strength of the joints. When texturing was applied to SiC substrates, SiO2 layer was formed on the surface of substrates due to high-temperature oxidation, which subsequently promoted the wettability of filler on substrates. The textured grooves on SiC substrates could be filled with glass material during joining process. The shear strength of textured SiC joints was 69.3 ± 9.9 MPa with a mixed-mode failure when the direction of shear load was perpendicular to the texturing direction. The better wettability, larger joining area, and interlocking structure between the glass filler and substrates enhanced the shear strength of textured joints.

A Study on the Material Removal Characteristics and Damage Mechanism of Lapping for Pressureless Sintered Silicon Carbide (SSiC) Microlens Cavity.

Microlens arrays have been widely employed to control the reflection, refraction, and diffraction characteristics of light due to its distinctive surface properties. Precision glass molding (PGM) is the primary method for the mass production of microlens arrays, of which pressureless sintered silicon carbide (SSiC) is a typical mold material due to its excellent wear resistance, high thermal conductivity, high-temperature resistance, and low thermal expansion. However, the high hardness of SSiC makes it hard to be machined, especially for optical mold material that requires good surface quality. The lapping efficiency of SSiC molds is quite low. and the underlying mechanism remains insufficiently explored. In this study, an experimental study has been performed on SSiC. A spherical lapping tool and diamond abrasive slurry have been utilized and various parameters have been carried out to achieve fast material removal. The material removal characteristics and damage mechanism have been illustrated in detail. The findings reveal that the material removal mechanism involves a combination of ploughing, shearing, micro-cutting, and micro-fracturing, which aligns well with the results obtained from finite element method (FEM) simulations. This study serves as preliminary reference for the optimization of the precision machining of SSiC PGM molds with high efficiency and good surface quality.

A material point method based investigation on crack classification and transformation induced by grit geometry during scratching silicon carbide

The microcracks and residual stress are commonly regarded as non-negligible obstacles while realizing non-damage surfaces in the grinding of brittle materials. In this paper, with the combination of stress field analysis, material point method simulation and high-speed scratch experiment, a variety of cracks in the classical indentation-induced crack system were decoupled to determine the inducing conditions and coupling modes of different cracks. The tests on pressureless sintered silicon carbide (S–SiC) exhibited that the grit geometry was crucial in inducing different cracks. The Hertzian and lateral crack systems were mainly determined by the rake angle of scratching grit and can transform from one to another near the critical value of −60°. Further, a two-point stress field model was proposed to prove that large contact width is the key factor to induce radial cracks. By contrast, large contact depth is related to the appearance of bottom debris. Furthermore, Raman spectroscopy was used to investigate the velocity effect of grit on cumulative damage. Besides the stacking disorder and residual stress, even chemical disorders such as the fracture of the Si–C bond and the formation of the homonuclear bond were found to be variable at the bottom of the scratch under different contact speeds and depths. Based on the above findings, a multi-layer superposition model was proposed to re-couple the effect of different cracks and predict the workpiece's damage/force state scratched by spherical grits. It proved that blunt grit could also realize low damage machining under specific conditions.

Corrosion behavior of pressureless sintered SiC in molten LiF-NaF-KF salt: Role of metallic impurities in the salt

The corrosion of pressureless sintered silicon carbide (SiC) in molten LiF-NaF-KF (FLiNaK) salts at 700 ℃ was investigated. Results indicate that the impurities in the molten FLiNaK salt play an important role in the corrosion of SiC. The pressureless sintered SiC occurred local corrosion along the grain boundaries and the intersections of grains in the FLiNaK salt with more metallic impurities. The corrosion results in the formation of carbon, Fe3Si, and the dissolvable Si-containing products. The reaction between SiC and metallic impurities in the salt drives the corrosion of SiC by enhancing the outward diffusion of element Si.

The corrosion resistance mechanisms of the cr-coated SiC in molten Na2SO4 salt: Strengthened boundaries and protective scales

The corrosion resistance of pressureless-sintered silicon carbide (SiC) plates in the molten Na2SO4 salt was greatly enhanced by their surfaces coated with magnetron-sputtered chromium (Cr) coatings. The Cr-coated SiC plates remained good structural integrity in the 1000 °C molten Na2SO4 salt for >20 h, in contrast to the uncoated ones that completely crushed into pieces after 4 h. The corrosion-resistance mechanisms mainly depend on the strengthened boundaries by formation of the Cr-C and Cr-Si compounds to avoid the preferred corrosion of boundaries, and the barrier effect of the Cr2O3/Cr/Cr-Si-C protective scale.

A new strategy to reduce edge chipping using stress wave impedance matching

To restrain edge chipping and elucidate its mechanism during machining, the initiation of edge chipping was investigated in this study from the propagation properties of stress waves in the fractured media. Three technological principles of the support for chipping suppression were proposed to reduce the intensity of reflected extension waves, namely, the wave impedance matching, the smaller residual gap and the higher viscosity of the gap filler between the workpiece and the support. As demonstrated from the experimental results, using brass support with non-solid epoxy gap filler can significantly restrain the edge chipping of pressureless sintered silicon carbide during grinding.

Experimental and numerical analysis of the complex permittivity of open-cell ceramic foams

Open-cell ceramic foams are promising materials in the field of microwave heating. They can be manufactured from susceptor materials and can, therefore, be used as selective heating elements. In this study, the complex permittivities of ceramic foam materials, including silicon-infiltrated silicon carbide (SiSiC), pressureless sintered silicon carbide (SSiC), silicate-bonded silicon carbide (SBSiC), and cordierite were determined. The dielectric properties of the foams were determined by the cavity perturbation technique using a TE104 WR340 waveguide resonator at 2.45 GHz. Samples were preheated in a tubular furnace, enabling temperature-dependent permittivity measurements up to 200 °C. The effective dielectric constant and effective loss factor were found to depend on the porosity and material composition of the foam. The SiSiC material had a higher effective dielectric constant than the SSiC and SBSiC ceramics. The effective dielectric constant of the foams showed a trend of gradual increase with increasing temperature. Some selected dielectric mixing relations were then applied to describe the effective permittivity of the foams and compare them with predictions from finite element simulations performed using the CST Studio Suite. The foam morphologies and simple block inclusions were used in the simulations.

Material removal characteristics of ultra-precision grinding silicon carbide ceramics

ABSTRACTSilicon carbide (SiC) ceramic has been widely applied to fabricate functional components relying on their excellent physical, chemical and mechanical properties. Meanwhile, it is always highly concerned to seek suitable manufacturing technology and processes for machining such difficult-to-cut material with high-precision, high-efficiency and low-cost. In the present paper, material removal characteristics of reaction-bonded silicon carbide (RB-SiC) and pressureless sintered silicon carbide (S-SiC) were investigated by ultra-precision grinding experiments. #120, #600, #2000 and #12000 diamond cup wheel were employed for coarse, semi-finished, fine and finish grinding, respectively. The surface topographies, surface morphologies and grinding grooves were measured. The results indicated that there are distinguishing material removal characteristics between RB-SiC and S-SiC, and #2000 diamond wheel can be chosen as the final grinding wheel for most high-performance SiC ceramic products. The present study provides a technological reference to ultra-precision grinding of hard materials.

A Practical Procedure for Measuring Contact Angles in Wettability Studies by the Sessile Drop Method

ABSTRACTAn old procedure used to carry out a graphical derivation of curves, which is based on the optical properties of plane mirrors, has been adapted for the measurement of the contact angle (θ) formed between a liquid drop and a flat solid substrate in wettability experiments carried out by the so-called “sessile drop” method. The method was tested for mercury on soda-lime glass at room temperature in air as well as for Cusil (Ag-28wt.%Cu) and Incusil-ABA (Ag-27wt.%Cu-12wt.%In-2wt.%Ti) brazing alloys on pressureless-sintered silicon carbide (PLS-SiC) at 850 °C, under a vacuum of 10-4/10-5 Torr. The proposed method is fast, simple and accurate enough from high (∼140°) to relatively low (∼10°) contact angles. Although the proposed method has been tested for metal-ceramic systems, it is of general application, so that it would be useful for any liquid-solid system. The method is applicable for any temperature, pressure and atmospheric experimental conditions employed, as well as for any chemical composition of liquid and solid. It is also useful for both low and high contact angles, as well as for reactive and non-reactive systems, as long as a photograph of a liquid drop resting on a flat solid surface is available for the studied system.

Interfacial Reaction Mechanism between Molten Ag-Cu-Based Active Brazing Alloys and Untreated or Pre-Oxidized PLS-SiC

ABSTRACTBased on wettability and reaction interfaces previously reported, as well as on thermodynamic considerations, a likely mechanism has been proposed for the chemical interaction taking place at the metal/ceramic interface during wettability experiments carried out by the so-called “sessile drop” method. The experiments involved three Ag-Cu-based brazing alloys [Cusil (Ag-28wt.%Cu), Cusil-ABA (Ag-34.6wt.%Cu-1.58wt.%Ti) and Incusil-ABA (Ag-26.6wt.%Cu-12.4wt.%In-0.89wt.%Ti)] and as polished and pre-oxidized pressure-less sintered silicon carbide (PLS-SiC), with a total holding time of 90 minutes at 850 °C, under a Zr sponge-gettered vacuum of 10-4/10-5 Torr.

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y2O3–Sc2O3–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm−1 K−1 at room temperature.

The effect of aluminum additive on pressureless sintering of SiC

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).

Preliminary study on geo-mechanical aspects of SSiC canisters

Abstract. To meet safety requirements for underground storage of high-level nuclear waste, engineered barriers are an integral part of a modern defense-in-depth concept and therefore have to be considered in interaction with the host rock. This study presents preliminary results for the load behavior of a canister made of pressure-less sintered silicon carbide (SSiC), which forms the main retention barrier for the fission products in a new multi-layer waste package design denominated as TRIPLE C. This means a three-fold enclosure strategy, spreading the functionalities to three different ceramic barriers: first the porous potting compound surrounding each single fuel rod in the container, second the solid container wall of SSiC and third the over-pack of carbon concrete. Besides all the advantages a potential drawback of ceramics in general is their brittleness. Therefore, the behavior of SSiC structural components under static and dynamic loading has to be investigated. First results for a small model canister indicate that static loading will not create any relevant damage, even if stresses are extremely high and highly anisotropic on a canister all-around embedded. First dynamic simulations indicate that, under very unfavorable circumstances, the model canister can experience tensile stresses bigger than its tensile strength. Also, point loading may cause damage to the canister under certain conditions. Based on the performed calculations, the SSiC canister design will be optimized together with the carbon concrete over-pack, so that mechanical damage of main retention barrier can be excluded even under extreme static and dynamic conditions in a final repository.

Mechanical and Thermal Properties of Pressureless Sintered Silicon Carbide Ceramics with Alumina–Yttria–Calcia

The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with Al2O3–Y2O3–CaO without applied pressure was investigated. SiC ceramics containing A2O3–Y2O3–CaO as sintering additives can be sintered to >97% theoretical density at temperatures between 1750°C and 1900°C without applied pressure. A toughened microstructure, consisting of relatively large elongated grains and relatively small equiaxed grains, has been obtained when sintered at temperatures as low as 1800°C for 2 h in an argon atmosphere without applied pressure. The achievement of toughened microstructures under such mild conditions is the result of the additive composition. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature because of the decrease in the lattice oxygen content of the SiC grains. Typical sintered density, flexural strength, fracture toughness, hardness, and thermal conductivity of the 1850°C‐sintered SiC, which consisted of 62.2% 4H, 35.7% 6H, and 2.1% 3C, were 99.0%, 628 MPa, 5.3 MPa·m1/2, 29.1 GPa, and 80 W·(m·K)−1, respectively.

Effect of additive composition on mechanical properties of pressureless sintered silicon carbide ceramics sintered with alumina, aluminum nitride and yttria

Silicon carbide (SiC) ceramics were pressureless sintered with 3 vol% Al2O3-Y2O3-AlN additives with the AlN/(Al2O3+AlN) molar ratios of 0-0.75 at 1850-2000 °C for 1 hr and the effects of additive composition (i.e., changes in the AlN/(Al2O3+AlN) molar ratio while maintaining constant Y2O3 content) on the mechanical properties of the pressureless-sintered SiC ceramics were investigated. Self-reinforced microstructures consisting of relatively large platelet SiC grains and relatively small equiaxed grains have been obtained in all specimens when sintered at 1900 °C for 1 h in an argon atmosphere. The achievement of self-reinforced microstructures under such mild conditions (holding for 1 hr at 1900 °C) is caused by the beneficial effects of additive composition and the acceleration of the β→α phase transformation of SiC by seeding, i.e., the addition of 1 vol% α-SiC into β-SiC. The typical flexural strength and fracture toughness of the pressureless-sintered SiC ceramics with an AlN/(Al2O3+AlN) mole ratio of 0.5 were 433 MPa and 6.6 MPa·m1/2 at room temperature, respectively.

大型常圧焼結炭化ケイ素材料の焼却炉熱交換器への応用

大型常圧焼結炭化ケイ素材料の焼却炉熱交換器への応用

The influence of granulation on the gelcasting of pressureless-sintered silicon carbide ceramics

Gelcasting combined with pressureless sintering is a promising technological process to fabricate large scale or complex-shaped silicon carbide ceramics (SiC) products with excellent properties. To obtain high solid loading SiC slurry with low viscosity, two promising compact granulation and spray granulation methods were proposed to coarsen raw submicron SiC powders into a larger secondary granule. By compact and spray granulation processes, the average particle sizes were enlarged from 0.51μm to 40.7μm and 8.2μm, respectively. The enlarged particles decreased the volume of structural water in the slurries remarkably, resulting in the decrease in the viscosity of slurry. The viscosities of slurry prepared from spray-granulated powders and compact-granulated powders were 1/2.5 and 1/3.5 to that from submicron powder at a shear rate of 100s−1, respectively. The lower viscosities benefited to the escaped air bubble in slurries, and leaded to a dense microstructure in green and sintered bodies. The bending strengths of green bodies and sintered bodies prepared from granulated powders were about 3 times and 2.2 times to that from submicron powders. It is worth to notice that even though the dried spray-granulated powders with sphere-like appearance displayed higher tap density and well fluidence, the slurry viscosity was 58Pas higher than that of compact-granulation powders due to the hollow particle structure. As a result, compact granulation powders showed desired slurry fluidity and dense green and sintered bodies, thus high bending strength.

Pressureless sintered silicon carbide with enhanced mechanical properties obtained by the two-step sintering method

The Two-step sintering (TSS) method was applied to the pressureless sintering of commercial silicon carbide powder doped with boron and carbon. The microstructural and mechanical properties of TSS-SiC were compared to those of sintered SiC obtained with the conventional thermal cycle (CS-SiC). TSS-SiC was densified (97.7% T.D.) at 2050 °C instead of 2200 °C needed for CS-SiC (97% T.D.). Furthermore, TSS-SiC showed finer microstructure and enhanced mechanical properties. In particular, flexural strength of the TSS-SiC materials greatly increased up to 556 MPa, much higher than 341 MPa reached by CS-SiC.

大型常圧焼結炭化ケイ素材料の溶融金属浸漬管への適用

大型常圧焼結炭化ケイ素材料の溶融金属浸漬管への適用