Highly concentrated aqueous SiC slurry containing fine SiC powder: A new approach for ultra-dense green body

SiC slurry with ultra-high concentration up to 70 vol% was prepared using oxidized fine and coarse SiC powder mixture, and dense SiC green body with a relative density of 76% was fabricated by drying the slurry at ambient condition. Three approaches were performed to prepare highly concentrated SiC slurry; preparation of SiC powder having good dispersion behavior, optimization of the oxidation condition, and optimization of bi-modal particle size distribution. An aqueous slurry with the solid loading up to 62 vol% could be prepared using fine (150 nm) SiC powder prepared by the mechanical alloying of Si and carbon. The surface property of the fine and coarse (10 μm) SiC powders was optimized using an oxidation treatment. The maximum solid loading of the fine SiC slurry prepared using oxidized powder was 66 vol%. By optimizing the mixing ratio of the oxidized fine and coarse SiC powder to 75:25, the solid loading of the SiC slurry could increase up to 70 vol%. The relative densities of the green bodies after drying 66, 68, and 70 vol% slurries were 69, 75.7, and 76.1%, respectively, which values were higher than those (58%) prepared by cold isostatic pressing under 200 MPa.     

Preparation,microstructure and ion-irradiation damage behavior of Al4SiC4-added SiC ceramics

Single-phase Al₄SiC₄ powder with a low neutron absorption cross section was synthesized and mixed with SiC powder to fabricate highly densified SiC ceramics by hot pressing. The densification of SiC ceramics was greatly improved by the decomposition of Al₄SiC₄ and the formation of aluminosilicate liquid phase during the sintering process. The resulting SiC ceramics were composed of fine equiaxed grains with an average grain size of 2.0 μm and exhibited excellent mechanical properties in terms of a high flexure strength of 593 ± 55 MPa and a fracture toughness of 6.9 ± 0.2 MPa m^1/2. Furthermore, the ion-irradiation damage in SiC ceramics was investigated by irradiating with 1.2 MeV Si⁵⁺ ions at 650 °C using a fluence of 1.1 × 10¹⁶ ions/cm², which corresponds to 6.3 displacements per atom (dpa). The evolution of the microstructure was investigated by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The breaking of Si–C bonds and the segregation of C elements on the irradiated surface was revealed by XPS, whereas the formation of Si–Si and C–C homonuclear bonds within the Si–C network of SiC grains was detected by Raman spectroscopy.     

表面处理对碳化硅浆料流变性的影响

本文通过对SiC粉体洗涤、表面氧化和聚乙二醇(PEG)包覆等处理后制得的SiC浆料的zeta电位和粘度的测试,研究了表面处理对SiC粉体流变性能的影响.研究表明:通过用酸碱洗涤过后再经过在780℃氧化处理或经PEG包覆所得的SiC粉体,都可以使其浆料的zeta电位的等电点向酸性方向偏移,从而使在pH值为11.8时的电位差大幅度增加,改善了浆料的流变性能.而且用PEG包覆过的SiC粉体浆料的电位差增幅达20左右,使其流变性改善的效果更佳.     

Influence of surface oxide layer of SiC powder on the rheological properties of its slurry

In this paper, the influence of surface oxide layer of SiC powder on the rheological behavior of its slurry was studied by acid washing, HF etching, and calcination oxidation processes. Zeta potential, particle size, inductively coupled plasma atomic emission spectrometry (ICP-AES), energy dispersive X-ray spectroscopy, and viscosity tests indicated the main factor that resulted in high viscosity of slurry is impure ions existing either on the powder surface or in the water phase, rather than the surface oxide layer. After the impure ions are removed, the surface oxide layer formed at a calcination temperature range of 400°C-750°C does not cause an increase in the slurry viscosity, but contributes to an improvement in the rheological properties of SiC slurry.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

Application of SiO2-coated SiC powder in stereolithography and sintering densification of SiC ceramic composites

Stereolithography additive manufacturing of SiC ceramic composites has received much attention. However, the forming efficiency and mechanical properties of their products need to be improved. This study aimed to prepare SiC ceramic composites with complex shapes and high flexural strength using a combination of digital light processing (DLP) and reactive solution infiltration process (RMI). A low-absorbance SiO₂ cladding layer was formed on the surface of SiC powder through a non-homogeneous precipitation process. With the densification of the cladding layer at high temperatures, SiO₂-coated SiC composite powder was used to formulate a photosensitive ceramic slurry with a solid content of 44 vol%. The resulting slurry exhibited a considerable improvement in curing thickness and rate and was used to mold ceramic green body with a single-layer slicing thickness of 100 μm using DLP. The ceramic blanks were then sintered and densified using a carbon thermal reduction combined with liquid silica infiltration (LSI) process, resulting in SiC ceramic composites with a density of 2.87 g/cm³ and an average flexural strength of 267.52 ± 2.5 MPa. Therefore, the proposed approach can reduce the manufacturing cycle and cost of SiC ceramic composites.     

The preparation of SiC ceramic photosensitive slurry for rapid stereolithography

Stereolithography is one of the most widely used additive manufacturing techniques for preparing high precision and complex ceramic components. Due to the high optical absorbance and refractive index of SiC powder, the rapid stereolithography of SiC ceramics components has become a key challenge. Here, we innovatively use graded silica to improve the curing thickness, rheological and settling performance of the slurry. And we presented a preparation method of SiC ceramic slurry for stereolithography with high solid content, low viscosity, low sedimentation rate and high curing thickness. The printable precision of the slurry is more than 75 μm, the dynamic viscosity is less than 2 Pa·s, and the 24 h sedimentation height is less than 5%. This strategy demonstrates a tantalizing possibility and promising prospect to rapid stereolithography of large size SiC ceramic green body.     

Improved mechanical properties of reaction-bonded SiC through in-situ formation of Ti3SiC2

Reaction-bonded SiC is a ceramic with excellent thermal properties, good corrosion resistance and the characteristic of near-net-shape manufacturing. However, the poor fracture toughness of free Si limits the applications of reaction-bonded SiC. In this study, TiC was added to reaction-bonded SiC and reacted with free Si to form Ti₃SiC₂. The effects of TiC and carbon black on the mechanical properties of reaction-bonded SiC were investigated. The results demonstrated that the in-situ formation of Ti₃SiC₂ and decrease in the content and size of free Si improved the mechanical properties of reaction-bonded SiC ceramics. The mechanical properties of TiC-added reaction-bonded SiC with 17.5 wt% carbon black were superior to those of TiC-added reaction-bonded SiC with 15 wt% carbon black. Moreover, increasing the TiC content of reaction-bonded SiC with 17.5 wt% carbon black from 0 to 7.5 wt% caused an increase in its bending strength from 183.92 to 424.43 MPa and an increase in fracture toughness from 3.7 to 5.24 MPa m^1/2.     

Dispersibility of pretreated polyacrylic acid-modified SiC powder

A slurry with high dispersibility is crucial for slip casting technology. The weakly acidic SiC powder with high dispersibility was prepared with polyacrylic acid pretreated by the sodium hydroxide (adjusted pH = 5). The pretreated polyacrylic acid can be effectively adsorbed on the surface of SiC powder, and its utilization efficiency (37%) was better than the unpretreated one (21%). The modified SiC powder with pH = 6 was obtained by polyacrylic acid modification, and the maximum solid content was 61.2 Vol%. The optimum of the pretreated polyacrylic acid addition amount was 1 wt% based on Zeta potential, solid content, and viscosity analysis data. Compared with the unmodified SiC, the Zeta potential absolute value of the modified (−63 mV) was higher due to a large amount of anionic modifier adsorbed on particle surfaces. The modified SiC slurry was more stable because the modification increases the electrostatic repulsion among the SiC molecules.     

A nitride whisker template for growth of mullite in SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer replica technique. The effects of nitride whisker template on the growth of mullite, the strut structure and mechanical properties of SiC RPCs were investigated. Prepolyurethane (PU) open-cell sponge was first coated by SiC slurry consisting of SiC, reactive Al₂O₃, microsilica and Si powder, then it was nitridized at 1400 °C in a flowing N₂ atmosphere to prepare SiC preforms. Subsequently, these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450 °C in air. The results showed that Si₂N₂O whiskers grew on the surface and in the matrix of SiC preforms after nitridation. The diameter of struts in SiC RPCs increased after vacuum infiltration process because alumina slurry was easily adhered by the surface nitride whiskers. In addition, such whiskers inside the strut of SiC preforms acted as the template to promote the growth of column-liked mullite in SiC RPCs. The mechanical properties and thermal shock resistance of SiC RPCs were greatly improved due to the special interfacial characteristics of multi-layered struts as well as better interlocked column-liked mullite in SiC skeleton.     

Dispersion and stability of SiC ceramic slurry for stereolithography

Stereolithography based additive manufacturing provides an effective method to fabricate complex-shaped SiC ceramic components. The dispersion and stability of the ceramic slurry are very important for stereolithography. In this study, the dispersion and stability of SiC ceramic slurries were investigated systematically. The effects of resin monomers, dispersants, particle size, solid loading and ball milling time on the dispersion, rheological behavior and stability of SiC ceramic slurries were studied in detail. Finally, an optimal SiC ceramic slurry for stereolithography based additive manufacturing was obtained, and complex-shaped SiC ceramic architectures were fabricated.     

Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Direct ink writing of chopped carbon fibers reinforced polymer-derived SiC composites with low shrinkage and high strength

The chopped carbon fiber reinforced SiC (C_f/SiC) composite has been regarded as one of the excellent high-temperature structural materials for applications in aerospace and military fields. This paper presented a novel printing strategy using direct ink writing (DIW) of chopped fibers reinforced polymer-derived ceramics (PDCs) with polymer infiltration and pyrolysis (PIP) process for the fabrication of C_f/SiC composites with high strength and low shrinkage. Five types of PDCs printing inks with different C_f contents were prepared, their rheological properties and alignment of carbon fiber in the printing filament were studied. The 3D scaffold structures and bending test samples of C_f/SiC composites were fabricated with different C_f contents. The results found that the C_f/SiC composite with 30 wt% C_f content has high bending strength (～ 7.09 MPa) and negligible linear shrinkage (～ 0.48%). After the PIP process, the defects on the C_f/SiC composite structures were sufficiently filled, and the bending strength of C_f/SiC composite can reach up to about 100 MPa, which was about 30 times greater than that of the pure SiC matrix without C_f. This work demonstrated that the printed C_f/SiC composites by using this method is beneficial to the development of the precision and complex high-temperature structural members.     

Fabrication process and mechanical properties of C/SiC corrugated core sandwich panel

Carbon fiber reinforced SiC composite is a kind of promising high-temperature thermal protection structural material owing to the excellent oxidative resistance and superior mechanical properties at high temperatures. In this work, a novel design and fabrication process of lightweight C/SiC corrugated core sandwich panel will be proposed. The compressive and three-point bending of the C/SiC corrugated sandwich panels are conducted by experiment and numerical simulation. The relative density of as-prepared C/SiC sandwich panel and the density composite material are 1.1 and 2.1 g/cm³, respectively. As the density of the C/SiC sandwich panel is only 52.3% of the bulk C/SiC, suggesting that lightweight characteristic is realized. Moreover, the C/SiC sandwich panel manifests itself as linear-elastic behavior before failure in compression and the strength is as high as 15.1 MPa. The failure mode is governed by the core shear failure and panel interlayer cracking. The load capacity under the three-point bending C/SiC composite sandwich panel is 1947.0 N. The main failure behavior is core shear failure. The stress distribution under the compression and three-point bend was simulated by FE analysis, and the results of numerical simulations are in accordance with the experimental results.     

Mechanical properties and wear behavior of Al5083 matrix composites reinforced with high amounts of SiC particles fabricated by combined stir casting and squeeze casting; A comparative study

Combined stir casting and squeeze casting processes were used to fabricate Al5083 matrix composites reinforced with 20, 25, and 30 wt% SiC_p. The microstructure, mechanical properties and wear behavior of the composites fabricated by combined stir casting and squeeze casting were compared with those fabricated by stir casting. The results revealed that the combined casting method improved the distribution of SiC particles through the reduction of the agglomeration of SiC particle and reduced the porosities of the samples from 2.32% to 1.29% in the sample containing 30 wt% SiC. These modifications led to the enhancement of mechanical properties i.e. increased the hardness to 85 BHN and the compressive strength to 350 MPa for the sample containing 30 wt% SiC fabricated by the combined casting method. In addition, the wear resistance of the samples fabricated by the combined casting method improved because of the reduced size of the wear debris as well as the smaller worn region. The dominant wear mechanism of all the composite samples fabricated by both methods was the delamination of the tribological layer while adhesion wear was dominant in the monolithic Al alloy.     

Adsorption of anionic polyelectrolyte on an SiC surface and effects on dispersion stability

Aluminum nitrate (Al(NO₃)₃) and sodium polystyrene sulfonate (PSS) were used to improve the dispersion of fine silicon carbide (SiC) powders. Effects of modification parameters on the viscosity of modified SiC slurry were studied by orthogonal experiments. Modified SiC slurry with the solid loading of 50 vol% reached the lowest viscosity of 34 MPa s. The adsorption processes of PSS on the as-received and Al(NO₃)₃ premodified SiC surface were investigated. The results indicated that the adsorption between PSS and premodified SiC surfaces was a high affinity type and was mainly controlled by active sites on an SiC surface. The Langmuir model and the pseudo-second-order model could better fit the adsorption isotherm and kinetics data, respectively. The contact angle decreased from 32.8 to 15.2° and the wettability was improved by modification. The isoelectric point of modified SiC powder shifted to the acidic region and the maximum zeta potential was obtained at pH 11. Sedimentation results also showed that a stable dispersed suspension of modified SiC was achieved at pH 11. Density–pressure curves demonstrated that the flowability and formability of SiC powder were improved by modification. The dispersion effect of PSS on SiC and Al₂O₃ composite powder was verified by viscosity and sedimentation results.     

Microstructure and mechanical properties of hot-pressed SiC nanofiber reinforced SiC composites

This study reports on the preparation and mechanical properties of a novel SiC_nf/SiC composite. The single crystal SiC nanofiber(SiC_nf) reinforced SiC ceramic matrix composites (CMC) were successfully fabricated by hot pressing the mixture of β-SiC powders, SiC_nf and Al–B–C powder. The effects of SiC_nf mass fraction as well as the hot-pressing temperature on the microstructure and mechanical properties of SiC_nf/SiC CMC were systematically investigated. The results demonstrated that the 15 wt% SiC_nf/SiC CMC obtained by hot pressing (HP) at 1850 °C with 30 MPa for 60 min possessed the maximum flexural strength and fracture toughness of 678.2 MPa and 8.33 MPa m^1/2, respectively. The nanofibers pull out, nanofibers bridging and cracks deflection were found by scanning electron microscopy, which are believed can strengthen and toughen the SiC_nf/SiC CMC via consuming plenty of the fracture energy. Besides, although the relative density of the prepared SiC_nf/SiC CMC further increased with the sintering temperature rose to 1900 °C, the further coarsend composites grains results in the deterioration of the mechanical properties for the obtained composites compared to 1850 °C.     

Microstructure and properties of porous SiC ceramics by LPCVI technique regulation

A new gradient pore structure in porous SiC ceramics was fabricated by low pressure chemical vapor infiltration (LPCVI). Effects of deposition duration on the mechanical properties and permeability of porous SiC ceramics were investigated. Results demonstrated that pore diameter and shapes decreased from the surface to the interior along with LPCVI duration. Porous SiC ceramics with deposition duration of 160 h exhibited flexural strength of 48.05 MPa and fracture toughness of 1.30 MPa m^1/2, where 221% and 189% improvements were obtained compared to porous SiC ceramics without LPCVI, due to CVI-SiC layer strengthening effect. Additionally, at the same gas velocity, pressure drop increase rate was faster due to apparent porosity and pore size change.     

The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP

Traditionally, SiC components with complex shapes are very difficult or even impossible to fabricate. This paper aims to develop a new manufacturing process, combining selective laser sintering (SLS), cold isostatic pressing (CIP) and polymer infiltration pyrolysis (PIP), to manufacture complex silicon carbide parts and improve the mechanical properties of silicon carbide ceramic parts. The density and porosity of SiC/SiC composites were measured. Furthermore, the mechanical properties of the specimens with cold isostatic pressing and the specimens without cold isostatic pressing were compared. The bending strength of the specimens with cold isostatic pressing was 201?MPa, and the elastic modulus was 1.27?GPa. And, the bending strength of the specimens without cold isostatic pressing was 142?MPa, and the elastic modulus was 0.88?GPa. Increasing the density of SiC/SiC can enhance the mechanical properties of SiC/SiC composites.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.