Effect of graphite particles as additive on the curing behaviour of β-tricalcium phosphate suspensions and scaffold fabrication by digital light processing

Reducing the viscosity of high solid-loading ceramic suspensions and controlling the resolution of ceramic green parts produced by digital light processing (DLP) 3D printing are two important concerns in the ceramic additive manufacturing industry. The presence of ceramic particles within a photopolymerizable system leads to light scattering that reduces the resolution of the ceramic green scaffolds. This study introduced a graphite additive to solve these problems and focused on the effects of graphite concentration on the viscosity, curing behaviour and scaffold fabrication of β-TCP ceramic suspensions. As a result, it was found that an appropriate addition of graphite reduced the viscosity of the ceramic suspensions, and the light scattering decreased with the increase of graphite concentration. Both the cure depth (C_d) and excess width (C_ex) also decreased with the increase of graphite concentration. But, graphite has a larger effect on the width curing than depth curing.     

Fibrous Monolithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior

Monolithic ceramics have been fabricated from coated green fibers to create fibrous microstructures. The fibrous monoliths consist of high aspect ratio polycrystalline regions (cells) of a primary phase regions (cell boundaries) designed to improve fracture resistance. The cells are the remnants of the green fiber which consists of ceramic powder and a polymer binder. The coating applied on the green fiber forms the cell boundaries. Fabrication and microstructure are described for fibrous monoliths in the SiC/graphite, silicon nitride/BN, alumina/alumina–zirconia, alumina/aluminum titanate, alumina/nickel and Ce-TZP/alumina–Ce–zirconia systems. The SiC/graphite fibrous monolith displays noncatastrophic failure in flexure, with shear delamination along the weak graphite layers. Indentations in SiC/graphite cause cells to spall, with crack arrest and extrusion of graphite from the cell boundaries. Crack deflection and spalling of cells are also observed in alumina/alumina–zirconia fibrous monoliths. In the Ce-TZP/alumina system, transformed regions around indentations are significantly modified by the alumina-containing cell boundaries.     

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.     

SiC陶瓷表面增韧用环氧树脂基增韧剂的制备与性能研究

碳化硅陶瓷在磨削加工中极易产生崩碎损伤,在碳化硅陶瓷磨削层实时涂覆增韧剂是降低崩碎损伤的新方法。以E51双酚A型环氧树脂、无水乙醇、651型低相对分子质量聚酰胺树脂和1,8-二氮杂二环十一碳-7-烯(DBU)为主要成分制备了一种增韧剂,通过测量增韧剂在碳化硅陶瓷表层的接触角、浸润深度与固化时间,探究了增韧剂各组分的添加量与碳化硅陶瓷表面粗糙度对增韧剂润湿性能与固化速率的影响规律,优化出一种润湿性能好、固化速率快的增韧剂。结果表明:增韧剂的最佳质量配比为m(E51双酚A型环氧树脂)∶m(无水乙醇)∶m(651型低相对分子质量聚酰胺树脂)∶m(DBU)=1∶0.9∶0.5∶0.02,该增韧剂在碳化硅陶瓷表层的浸润时间约为160 s,浸润深度约为40 μm,可使碳化硅陶瓷的表层硬度降低约25%;增韧剂的润湿性能随着溶剂的增加或碳化硅表面粗糙度的增大而提高,促进剂添加量的改变对增韧剂的润湿性能几乎无影响;增韧剂的固化速率随溶剂的增加而降低,随促进剂的增加而提高,但当促进剂达到饱和时,固化速率不再提高。     

Curing performance and print accuracy of oxidized SiC ceramic via vat photopolymerization

To improve the print quality of SiC ceramic, an amorphous SiO₂ layer is prepared on the surface of SiC by oxidation (SiC@SiO₂). The changes in curing depth and width direction of SiC@SiO₂ curing sheets with different degrees of oxidation were investigated. The results showed that curing depth increased while curing width decreased with an increasing degree of oxidation. The edge morphologies of SiC@SiO₂ curing sheets changed from serration to flat shape with increasing degree of oxidation, and uneven overcuring regions appear in the width direction. Additionally, 12% SiC@SiO₂ ceramic was fabricated via vat photopolymerization combined with reaction sintering. The dimensional deviation ratio of printed green bodies increased with decreasing printed line width and pore diameter. The minimum printed line width of 329 μm and pore diameter of 483 μm with good print quality were obtained. Furthermore, the flexural strength of 225.4 MPa and bulk density of 2.77 g cm⁻³ for sintered 12% SiC@SiO₂ parts were obtained.     

Effects of magnesium oxychloride and silicon carbide additives on the foaming property during firing for porcelain ceramics and their microstructure

Porcelain green bodies that were prepared with a porcelain tile powder as a major raw material, and magnesium oxychloride (MO) and silicon carbide (SiC) as additives were fired at 1000–1200°C. Effect of the additives on the foaming property for porcelain ceramics and its microstructure was quantitatively investigated to clarify the foaming origin of polished porcelain waste during firing. The experimental results show that a small amount of the mixture of both MO and SiC added into the porcelain green body causes the body foaming during firing more significantly, compared to the addition of either SiC or MO. The foaming of porcelain green body with only SiC is more remarkable than that of the porcelain green body with only MO at the same content and firing conditions. The MO accelerates the foaming of porcelain green body with SiC during firing. In addition, the foaming mechanism was also discussed.     

Preparation of photosensitive SiO2/SiC ceramic slurry with high solid content for stereolithography

Stereolithography is a popular three-dimensional (3D) printing technology, which is widely used for manufacturing ceramic components owing to its high efficiency and precision. However, it is a big challenge to prepare SiC ceramic slurry with high solid content for stereolithography due to the strong light absorption and high refractive index of dark SiC powders. Here, we propose a novel strategy to develop photosensitive SiO₂/SiC ceramic slurry with high solid content of 50–65 vol% by adding spherical silica with low light absorbance and applying a stacking flow model to improve the solid content of the slurry. The as-prepared slurry exhibits excellent stereolithography properties with a dynamic viscosity lower than 20 Pa s and curing thickness more than 120 μm. Therefore, it can be successfully applied for stereolithography-based additive manufacturing of SiC green bodies with large size (100 mm), sub-millimeter accuracy (0.2 mm), and complex structure. The stacking flow model also shows immense potential for the stereolithography of other dark-color ceramics with high solid content.     

Mechanical Properties of Oxidation-Resistant SiC/C Compositionally Graded Graphite Materials

Mechanical properties of oxidation-resistant SiC/C compositionally graded graphite materials composed of a CVD SiC coating layer, a SiC/C graded layer, and a graphite substrate were studied by bending, compressive, and hardness tests at room temperature. CVD SiC coated graphite and graphite with a SiC/C graded layer alone were also examined. The bending strengths of oxidation–resistant SiC/C compositionally graded graphite materials as well as CVD SiC coated graphite were about twice as much as that of graphite. The bending and compressive strengths of graphite with the SiC/C graded layer alone were almost the same as those of graphite. The higher strengths of graphite with the CVD SiC layer can be mainly due to the CVD SiC strength itself, and partly due to the smooth CVD surface. No effect of the SiC/C layer on the strengths was discussed in terms of porosity, surface morphology, and the strength of SiC.     

Digital light processing of SiC ceramic from allylhydridopolycarbosilane with limited acrylate monomers

Digital light processing of ceramic precursor was used to prepare SiC rich ceramic parts in this study. In order to achieve appropriate light curing rate, the ceramic precursor allylhydropolycarbosilane (LHBPCS) was mixed with acrylate monomers tripropylene glycol diacrylate and trimethylolpropane triacrylate. The content of acrylate monomers was optimized to increase the ceramic yield and reduce the shrinkage during pyrolysis. According to the results of thermogravimetric analysis and photolithography experiment, 15 wt% acrylate monomers was appropriate. 330 mJ/cm² UV irradiation dose was selected for every layer with a thickness of 25 μm, and green bodies with different shapes were successfully printed. During pyrolysis, these printed parts changed from transparent yellow to black accompanying uniform shrinkage. At 1000 °C, the shrinkage was 24.0–26.0%, and crack-free SiC rich ceramic parts with density of 2.11 g/cm³ and chemical formula of SiC_1.31O_0.26 were obtained.     

Synthesis and characterization of non-oxide ceramic coatings on graphite substrates by solid–vapor reaction process

Silicon carbide (SiC) and silicon carbide/silicon nitride (SiC/Si₃N₄) were successfully synthesized on graphite substrates by the use of solid–vapor reaction (SVR) process. Layers of SiC and SiC/Si₃N₄ were synthesized on graphite substrate through the reaction between SiO and substrate the (SiO_(vapor) + 2C_{(from graphite)}) and N₂ and substrate the (3SiO_(vapor) + 2N_{2 (vapor)} + 3C_{(from graphite)}), respectively. With the increase of dwell time and synthesis temperature the thickness of SiC layers increases up to 1500 °C temperature. Also, with the increase of synthesis temperature hardness value of SiC coatings is increased, which is 10–15 times higher than the substrate. The critical load of SiC coatings for wear resistance is about 22 N, which was observed by scratch tests. The synthesized SiC coatings appear to consist of a β-SiC phase mixed with a minor amount of an -SiC phase, and its thickness is mainly affected by porosity of the substrate. The thickness of SiC/Si₃N₄ coatings is much thinner than that of SiC coatings, but gives higher value of surface hardness.     

Effect of Weight Loss on Liquid-Phase-Sintered Silicon Carbide

The evaporation of silicon carbide (SiC) ceramics during sintering has been studied by thermogravimetry in a graphite furnace filled with argon. The SiC powder compacts contained 7.5 wt% eutectic composition of Y₂O₃–Al₂O₃ to promote liquid-phase sintering. A weight loss of 1–11 wt% was observed during sintering, depending on the sintering temperature and sintering time. The weight loss severely influenced the final density and the microstructure of the SiC ceramics. Particularly, the oxide sintering aids, which were homogeneously distributed in the green ceramics, were observed to segregate and form particular patterns that were dependent on the temperature, sintering time, and the total weight loss. Possible heterogeneous reactions evolving volatile species have been discussed in relation to the experimental observations.     

Oxidation bonding of porous silicon carbide ceramics with synergistic performance

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process, in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. It has been shown that a high porosity can be obtained by adding a large amount of graphite into the SiC powder compacts and that the pore diameter can be controlled by the size of graphite particles and/or SiC powders. When a 0.3-μm SiC powder was used, a high strength up to 133 MPa was achieved at a porosity of 31.5%. Moreover, oxidation-bonded SiC (OBSC) ceramics were observed to exhibit an excellent resistance to oxidation and thermal shock.     

A hybrid graphite powder PFC@G for reserving higher carbon content of self-lubrication graphite/SiC composites by RMI

The appropriate carbon content is indispensable for the application of self-lubricating graphite/SiC composites. However, it is a big challenge to retain high carbon content in the reaction-formed graphite/SiC composites because of drastic consumption of carbon by violent reaction with liquid silicon. In this study, a hybrid powder constructed by graphite particles (G) and glassy carbon derived from phenolic resin (PFC) was used as carbon sources, or PFC@G for short, to reserve higher content of carbon in the reaction-formed composites. The weight ratio of phenolic resin to graphite particles was adjusted to obtain an appropriate PFC@G with dense microstructure and close-grained surface. Compared with the graphite/SiC composites only using raw graphite particles as carbon sources, the carbon content of the composites fabricated with compact and large PFC@G has obviously increased (up to 172%). In particular, the carbon content of the composites fabricated with the weight ratio = 0.8 reached a high value of 44.26 vol.%, which exhibited outstanding self-lubrication properties among the four kinds of the composites. The mechanism of reserving higher content of carbon in the graphite/SiC composites by constructing PFC@G is investigated, revealing that a continuous SiC layer formed on the surface of the larger size PFC@G and most closely packed graphite particles inside of PFC@G were insulate from liquid silicon by the layer.     

Effect of tungsten carbide,silicon carbide and graphite particulates on the mechanical and microstructural characteristics of AA 5052 hybrid composites

Reinforced aluminium metal matrix composite materials are being used extensively in diverse fields that include aerospace and automobile. In this investigation, we introduce two distinct and novel types of aluminium hybrid composites and characterize their mechanical properties and microstructure. The first type was fabricated by reinforcing aluminium alloy (AA 5052) with tungsten carbide (WC) and graphite particulates and the second type was fabricated by reinforcing AA 5052 with silicon carbide (SiC) and graphite particulates. The composite material was processed through the melt-stir casting method and characterized by analyzing their densities, micro hardness, Charpy impact strength, tensile strength and peak elongation. Melt-stir casting method was chosen due to its cost effectiveness and productivity. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) studies were conducted to analyze thorough mixing of the reinforcements in the aluminium matrix metal. It was found that addition of tungsten carbide and graphite particulates with AA 5052 resulted in an increase in micro hardness and Charpy impact strength by 10.3% and 34.2% respectively, which are found to be better when compared to that of adding SiC and graphite particulates with AA 5052. Moreover, tensile tests revealed that there was a drop in tensile strength for the Al/SiC/graphite composites, while the peak elongation increases for both composites. On the other hand, while adding WC and graphite particulates the tensile strength of the composite improved by 15.12%. Also, the SEM fractographs taken for Al/SiC/graphite composite samples, subjected to Charpy impact and tensile tests revealed the presence of particle fractures and cracks and confirmed the possibility of plastic deformation. The results showed the Al/WC/graphite composites to be the superior among the two fabricated composites in terms of mechanical properties and therefore have good potential for structural applications.     

Fibrous Monolithic Ceramics: II, Flexural Strength and Fracture Behavior of the Silicon Carbide/Graphite System

The fibrous monolith microstructure consists of high aspect ratio polycrystalline cells of SiC separated by thin cell boundaries containing graphite. The SiC/100% graphite fibrous monolith has noncatastrophic fracture behavior, is damage tolerant, and is notch insensitive. The failure process is characterized by fracture along weak graphite cell boundaries. The room-temperature flexural strength is 300–350 MPa. The estimated shear strength along the graphite cell boundaries is ∼ 15 MPa. Increasing the strength of the cell boundary by additions of SiC (40–60 vol%) results in a monolithic SiC material showing brittle fracture behavior but retaining damage tolerance. Strength and fracture behavior are also influenced by cell texture and orientation.     

Sliding-wear resistance of liquid-phase-sintered SiC containing graphite nanodispersoids

The influence was investigated of a graphite nanodispersoid addition on the lubricated sliding-wear behaviour of liquid-phase-sintered (LPS) SiC ceramics fabricated by spark-plasma sintering (SPS). The graphite nanodispersoids, introduced into the microstructure of the LPS SiC ceramic to act as self-lubricating phase, were obtained by graphitization of diamond nanoparticles during the SPS. It was found that the graphite nanodispersoid addition results in a lower resistance to mild wear, attributable to the lower hardness of graphite and the null lubrication it provides. Moreover, the graphite nanodispersoids, which mostly locate at grain boundaries/faces, worsen the cohesion of SiC grains, contributing together with the higher mild-wear rate to an early transition to the severe-wear stage. On the contrary, the graphite nanodispersoids were effective at improving the resistance to severe wear because they increase the fracture toughness while providing some external lubrication. Relevant implications for the microstructural design of advanced triboceramics are discussed.     

Al2O3/SiCp纳米复合材料化学制备工艺研究

化学工艺是剪裁和控制陶瓷材料显微结构最有效的手段之一。为此详细研究了Al2O3/SiCp纳米复合材料的化学制备工艺,利用非均匀成核法在纳粹SiC粒子表面包覆一层Al（OH）3后,其等电点IEP由pH=3.4移至pH7.5,表现出类似Al2O3的胶态特性,在pH＝4．5－5．0之间,包覆型SiC与Al2O3荷电性相同,通过胶态悬浮液混合,将包覆型纳米SiC均匀分散于Al2O3基体中,最后,在不加任何分散剂的条件下,制备出高固相、低粘度的包覆型SiC－Al2O3水基浆料,并成功利用凝胶注模型成型（gelcasting）工艺,制备出显微结构均匀的Al2O3/SiCp纳米复合陶瓷材料素坯。     

Protecting nuclear graphite from liquid fluoride salt and oxidation by SiC coating derived from polycarbosilane

Modification process has been conducted on commercial nuclear graphite IG-110 (Toyo Tanso Co., Ltd., Japan) by impregnation and pyrolysis of polycarbosilane (PCS) solution for getting the modified IG-110 (M-IG-110) coated by dense SiC coating for molten salt reactor. The microstructure and properties of graphite were systematically investigated and compared before and after the modification process. Results indicated that the M-IG-110 possessed of more excellent integrated properties including molten salt barrier property and oxidation resistance than bare IG-110 due to the filling effect of SiC particles in the pores of M-IG-110 and dense SiC coating adhering to the surface of M-IG-110. The fluoride salt infiltration amount of M-IG-110 under 5 atm was only 1.1 wt%, which was much less than 14.9 wt% for bare IG-110. The SiC coating derived from PCS exhibited remarkable compatibility with graphite substrate under high temperature and gave rise to excellent oxidation resistance of M-IG-110.     

Photosensitive Si3N4 slurry with combined benefits of low viscosity and large cured depth for digital light processing 3D printing

Digital light processing 3D printing can be applied to fabricate complex silicon nitride (Si₃N₄) components. However, because of the surface hydroxyl groups and large refractive index, it is still a foremost challenge to realize a stable photosensitive Si₃N₄ slurry with combined benefits of low viscosity and large curing depth. In this study, we propose a new formulation strategy to prepare Si₃N₄ slurry. Starting from the optimization of monomer ratio, we have systematically optimized powder particle size, dispersant and photoinitiator on the rheological properties and curing properties of Si₃N₄ slurry. Specifically, we have fabricated a stable photosensitive Si₃N₄ slurry (48 vol%) with a viscosity of 2.09 Pa s (30 s^?1), a critical curing energy of 126.09 mJ/cm² and a maximum curing depth of 80 µm. Finally, based on this optimized slurry, we have successfully obtained complex Si₃N₄ green body with no defect, which demonstrates great potential to fabricate arbitrary complex ceramic components for various applications.