Research on selective laser sintering of Kaolin–epoxy resin ceramic powders combined with cold isostatic pressing and sintering

In order to fabricate traditional products with complex shapes consisting of Kaolin ceramic, selective laser sintering (SLS) combined with cold isostatic pressing (CIP) process was used to consolidate Kaolin powder with additive of epoxy resin E06. To begin preparing the material, epoxy resin (10 wt%) and Kaolin were combined through mechanical mixing, which provided a good fluidity for SLS. Investigations on the shrinkage and micro topography of Kaolin–epoxy resin SLS samples were conducted to optimize the laser sintering parameters. It was found that SLS samples represented acceptable shrinkage and high density when laser energy density was 0.3300–0.3763 J/mm². Then the SLS samples were processed by CIP to eliminate the pores in green ceramics. Finally, the optimized SLS/CIP Kaolin samples were debinded and sintered to produce crack-free Kaolin ceramics. The “Yellow Duck” Kaolin ceramic product was fabricated by combining SLS/CIP with colored glazing. The study shows a novel and promising approach to fabricate complex traditional ceramic products via SLS combined with CIP and sintering.     

Additive manufacturing of zirconia parts by indirect selective laser sintering

Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO₂–PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO₂ suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 °C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y₂O₃ stabilized ZrO₂ parts to 92 and 85%, respectively.     

Towards fabrication of high-performance Al2O3 ceramics by indirect selective laser sintering based on particle packing optimization

A novel method to fabricate high-performance Al₂O₃ Ceramics by indirect Selective Laser Sintering (idSLS) based on particle packing optimization was reported. Al₂O₃ ceramics with the particle size distribution (PSD) being adjusted based on a particle packing model was prepared with the idSLS method, and the effects of PSD on the macro-performance and microstructure of the idSLSed ceramics part was investigated. Results show that an appropriate PSD of raw material is not only essential to the good SLS formability of powder bed but also contribute to the favorable sintering characteristics of the SLSed green body through an ideal particle packing. The improvement of overall properties including mechanic strength, dimensional accuracy, and surface flatness of the SLSed green body and the final parts were achieved with a fine-tuned PSD exponent. When the raw material has a PSD exponent of 0.5–0.6, the final ceramic parts showed relatively balanced comprehensive properties of bulk density >2.0 g cm⁻³, linear change <1.4%, fracture strength >3.97 MPa, compressive strength >5.25 MPa and surface altitude difference <148 μm.     

3D打印成型陶瓷零件坯体及其致密化技术

3D打印技术在陶瓷零件成型方面具有较大应用潜力,被认为是近净尺寸成型高性能复杂结构陶瓷零件的一种新途径。本文比较了陶瓷零件或其坯体的激光选区熔化、薄材叠加制造、熔融沉积造型、光固化、三维打印和激光选区烧结等不同3D打印工艺及其致密化手段的优势和不足,认为较低的相对密度和强度是阻碍3D打印陶瓷零件实现产品应用的主要障碍。本团队近年来采用造粒混合法制备出具有良好流动性的3D打印复合陶瓷粉体,再通过激光选区烧结(SLS)和冷等静压(CIP)技术分别进行坯体成型及均匀致密化处理,制备出了高性能、复杂结构的Al_2O_3致密陶瓷零件。本文回顾了这些工作,并补充介绍了溶解沉淀和溶剂蒸发这两种制备复合陶瓷粉体的新方法,利用SLS/CIP复合工艺进一步制造了ZrO_2、SiC、高白土等其它材质的复杂陶瓷零件,为3D打印陶瓷用于航空航天、医疗、艺术等领域奠定了基础。     

Indirect selective laser sintering of epoxy resin-Al2O3 ceramic powders combined with cold isostatic pressing

To fabricate Al₂O₃ ceramic components with complex shape, selective laser sintering (SLS) combined with cold isostatic pressing (CIP) process was used to consolidate Al₂O₃ powder with additive of epoxy resin E06 (ER06) and polyvinyl alcohol (PVA). The starting material preparation combined spray drying with mechanical mixing to formulate compound powder consisting of PVA (1.5 wt%), ER06 (8 wt%) and Al₂O₃ and provide a good fluidity for SLS. Experimental investigations were carried the shrinkage, relative density, bending strength of Al₂O₃-ER06 SLS specimens in order to optimize the laser sintering parameters. It was found that Al₂O₃-ER06 SLS specimens represented acceptable shrinkage, high density and bending strength when laser power, scanning speed, scanning space and layer thickness were, respectively, 21 W, 1600 mm/s, 100 μm and 150 μm. Following that, the SLS specimens were processed through CIP to eliminate the pores in green ceramics. Finally, the optimized SLS/CIP Al₂O₃ specimens were debinded, sintered to produce crack-free Al₂O₃ bodies. The final Al₂O₃ components achieved a relative high density of more than 92% after furnace sintering. The study shows a novel and promising approach to fabricate complex ceramic matrix and ceramic components via indirect SLS and CIP process.     

Effects of B4C addition on the microstructure and properties of porous alumina ceramics fabricated by direct selective laser sintering

Direct selective laser sintering (dSLS) is a promising method for the fabrication of complex-shaped ceramic parts. In this paper, boron carbide (B₄C) was used as an inorganic additive to improve the laser sintering behavior of alumina. The effects of B₄C addition on the microstructure and mechanical properties of porous alumina ceramics were investigated. Mixture of alumina powders and different amount of B₄C were directly sintered using different SLS parameters. Results indicated that the process window of alumina could be expanded by the addition of B₄C. Furthermore, the amount of B₄C played an important role in surface morphologies of alumina ceramics. It could be explained by the increase of mass transfer due to the addition of B₄C, which enhanced the densification process. The compressive strength of sintered samples increased with the increase of B₄C, which reached its maximum value when the content of B₄C was 7?wt% and the density of the samples after post treatment could reach 1.4?g/cm³. In addition, a size expansion phenomenon was observed. The size expansion could reach 5% after SLS, which could be attributed to the pin effects and oxidation behavior of B₄C particles.     

工艺参数对PS/改性滑石粉SLS烧结件力学性能的影响

利用硅烷偶联剂KH560对滑石粉进行表面改性,之后将其与聚苯乙烯(PS)粉机械共混,制备PS/改性滑石粉复合粉末。在预热温度85℃和8层网格支撑等条件下,采用选择性激光烧结(SLS)工艺将粉末制备成烧结件。采用扫描电子显微镜观察了滑石粉与PS的界面,发现通过对滑石粉进行表面改性,不仅可以使其均匀地分散于PS粉中,而且两者的相容性也得到了改善。通过正交实验法研究了SLS工艺参数对PS/改性滑石粉烧结件力学性能的影响。利用极差分析法与综合平衡法得到了多指标下的最优工艺参数组合,即单层厚度0.18 mm,扫描间距0.28 mm,激光功率27 W,扫描速度1 200 mm/s,此时PS/改性滑石粉烧结件的拉伸强度为4.29 MPa,弯曲强度为14.4 MPa,冲击强度为4.4 k J/m~2,相比纯PS粉烧结件分别提高了4.9%,10%和56%。     

Fabrication of coloured zirconia ceramics by infiltrating water debound injection moulded green body

Abstract

Abstract

The authors developed a new technique combining ceramic injection moulding and liquid precursor infiltration, presenting a new strategy for fabrication of coloured zirconia ceramics. The authors’ strategy includes ceramic injection moulding 3Y‐TZP powder using a water debinding binder system, debinding moulded parts in water and drying, immersing debound parts in solutions containing different colouring ions and then sintering and creating coloured zirconia ceramics. The fabricated coloured ceramic bars exhibit a core shell structure, and the thickness of the coloured shell can be tailored by adjusting immersion time and temperature. Using solutions containing different colouring ions, ceramic bars with various colours can be prepared.     

High-strength and wave-transmitting Si3N4–Si2N2O-BN composites prepared using selective laser sintering

In this study, high-strength and wave-transmission silicon nitride (Si₃N₄) composites were successfully developed via selective laser sintering (SLS) with cold isostatic pressing (CIP) after debinding and before final sintering, and the optimal moulding process parameters for the SLS Si₃N₄ ceramics were determined. The effects of the sintering aids and secondary CIP on the bulk density, porosity, flexural strength, fracture toughness, and wave-transmitting properties of the Si₃N₄ composites were studied. The results showed that the increased CIP pressure was beneficial to the densification of SLS Si₃N₄ ceramics and improved their mechanical properties. However, the wave-transmitting performance decreased as the CIP pressure increased. The Si₃N₄ ceramics prepared by the moulding of sample S₁₁ were more in line with the performance requirements of the radomes. To obtain good comprehensive performance, an additional 3% of interparticle Y₂O₃ was added to the pre-printed mixed powder of granulated Si₃N₄ particles and resin and the secondary CIP pressure was adjusted to 280 MPa. After sintering, the bending strength, fracture toughness, and dielectric constant of the Si₃N₄ ceramics were 651 MPa, 6.0 MPa m^1/2, and 3.48 respectively. This study provides an important method for preparing of Si₃N₄ composite radomes using SLS process.     

Highly dense 0.3CaCeNbWO8-0.7LaMnO3 composite ceramics fabricated by cold sintering process

The cold sintering process (CSP) has been used for fabricating functional ceramics at a low sintering temperature. In this study, highly dense 0.3CaCeNbWO₈-0.7LaMnO₃ composite ceramics have been successfully fabricated by CSP. The phase structure, microstructure, and electrical properties of composite ceramics have been investigated. The composite ceramic is mainly composed of a tetragonal CaCeNbWO₈ phase with scheelite structure and an orthorhombic LaMnO₃ phase with perovskite structure. The relative density of composite ceramic is 94.5%, and is higher than that of single phase ceramic. The resistivity of composite ceramic exhibits negative temperature coefficient characteristics, with an aging coefficient less than 2%. Such a sintering methodology is of great significance, since it provides a feasible idea for preparing composite ceramics.     

Preparation and indirect selective laser sintering of alumina/PA microspheres

Indirect selective laser sintering (SLS) is a promising additive manufacturing technique to produce ceramic parts with complex shapes in a two-step process. In the first step, the polymer phase in a deposited polymer/alumina composite microsphere layer is locally molten by a scanning laser beam, resulting in local ceramic particle bonding. In the second step, the binder is removed from the green parts by slowly heating and subsequently furnace sintered to increase the density. In this work, polyamide 12 and submicrometer sized alumina were used. Homogeneous spherical composite powders in the form of microspheres were prepared by a novel phase inversion technique. The composite powder showed good flowability and formability. Differential scanning calorimetry (DSC) was used to determine the thermal properties and laser processing window of the composite powder. The effect of the laser beam scanning parameters such as laser power, scan speed and scan spacing on the fabrication of green parts was assessed. Green parts were subsequently debinded and furnace sintered to produce crack-free alumina components. The sintered density of the parts however was limited to only 50% of the theoretical density since the intersphere space formed during microsphere deposition and SLS remained after sintering.     

选择性激光烧结用聚合物基材料制备研究进展

选择性激光烧结（selective laser sintering, SLS）是一种重要的3D打印加工技术,可制备传统加工无法制备的任意复杂形状的制件,广泛应用于航空航天、国防装备、医疗器械以及汽车等高新技术领域。本文介绍了SLS技术的加工原理和优势,综述了SLS技术加工成形用材料种类及聚合物基粉体材料的制备方法,主要包括相分离法、机械粉碎法、溶液法和喷雾干燥法。重点对SLS技术制备聚合物基压电复合材料及制品的国内外研究现状进行总结。虽然SLS打印制造技术面临聚合物原料种类少、功能缺乏、粉体生产成本高以及难以批量制备等瓶颈问题,但经过不断地创新与发展,SLS打印技术将成为高性能多功能高分子复合材料及其大型复杂制件的极佳制造方法。     

Laser additive manufacturing and homogeneous densification of complicated shape SiC ceramic parts

To improve the density of SiC ceramic components with complicated shape built by laser sintering (LS), cold isostatic pressing (CIP) and reaction sintering (RS) were incorporated into the process. In the process of LS/CIP/RS, Phenol formaldehyde resin (PF)-SiC composite powder was prepared by mechanical mixing and cold coating methods, with an optimized content of PF at 18?wt%. For the purpose of obtaining improved density of the sintered body after final reaction sintering, carbon black was added into the initial mixed powder. The material preparation, LS forming and densification steps were optimized throughout the whole fabrication process. The final sintered SiC bodies with the bending strength of 292 ~ 348?MPa and the density of 2.94–2.98?g?cm^{? 3} were prepared using the PF coated SiC-C composite powder and the LS / CIP / RS process. The study further showed a positive and practical approach to fabricate SiC ceramic parts with complicated shape using additive manufacturing technology.     

Densification,microstructure and mechanical properties of Ta4HfC5-based ceramics obtained from synthesized nanoscale powder

Solvothermal treatment was used to synthesize nanoscale 4TaC-HfC (Ta₄HfC₅) powder at a relatively low calcination temperature and in a short period (1400 °C, 2 h). The obtained powder had uniform size distribution and dispersion. Ta₄HfC₅ ceramics were then consolidated via spark plasma sintering at 2100 °C. Ceramics had a better densification and smaller mean grain size at a shorter sintering time compared with that of materials sintered using mechanical ball milling method. The densification behavior of ceramics deriving from synthesized or ball milled powders was analyzed and the mechanical properties of different samples were investigated. To further increase the mechanical properties, a nearly fully dense Ta₄HfC₅-MoSi₂ ceramic was sintered using the synthesized powder. The mechanical properties of the ceramic composite doubled the strength values. This processing route demonstrated to be a viable approach to synthesize nanoscale Ta₄HfC₅ powder with high purity and uniformity, and obtain higher performances ceramics once sintered.     

Mechanical and dielectric properties of 3D printed highly porous ceramics fabricated via stable and durable gel ink

A novel, efficient, versatile strategy was carried out to fabricate highly porous ceramic parts based on the combination of strong colloidal gel ink fabricated with high boiling point organic solvents and DIW technique. The preparation and optimization of inks and the effect of heating temperature on the phase composition, microstructure, mechanical properties and dielectric properties of ceramic parts were systematically investigated. The strong colloidal ink exhibits excellent ambient stability and printability. The sintering temperatures bring about the evolution of phases, structural mechanical properties and dielectric properties of ceramic parts. Ultimately, Si₂N₂O single wall ceramic parts with a frame density of 1.07?1.14 g/cm³ and an apparent porosity of 53.13 ± 1.29% were successful fabricated. The dielectric constant and dielectric loss of Si₂N₂O sample (1650℃) are only 4.24 and 0.0049, respectively. This strategy provides a reference for in-situ synthesis of high-performance porous ceramic components based on the DIW.     

Plastic deformation and effects of water in room‐temperature cold sintering of NaCl microwave dielectric ceramics

NaCl ceramics were prepared by room‐temperature cold sintering using moistened NaCl powder with 4 wt% water and dry pressing using dehydrated powder. When the applied uniaxial pressure is low, the relative density of dry‐pressed NaCl ceramic is significantly lower than that of cold‐sintered ceramic, while the former is 98.5%‐99.3% and much higher than the latter (94.3%‐94.6%) for high applied pressure of 200‐300 MPa. The uniaxial pressure‐induced plastic deformation dominates the densification of dry‐pressed NaCl ceramic, and also plays a role during cold sintering as well as the dissolution‐precipitation process. The lower density of cold‐sintered NaCl ceramic under high applied pressure is attributed to the trapped water in ceramic body during cold sintering. Besides, the presence of water always promotes the microstructural homogeneity, which is responsible for the much higher Qf value of cold‐sintered NaCl ceramic. The optimal microwave dielectric properties with ε_r = 5.55, Qf = 49 600 GHz, and τ_f = ?173 ppm/°C are obtained in cold‐sintered NaCl ceramic under the applied pressure of 300 MPa, indicating that it is a promising candidate as a microwave dielectric material.     

Near-zero-shrinkage Al2O3 ceramic foams with coral-like and hollow-sphere structures via selective laser sintering and reaction bonding

The large shrinkage that ceramics undergo during sintering is a severe challenge for high-performance porous ceramics. In this study, we report a powder-based selective laser sintering (SLS) approach to prepare Al₂O₃ ceramic foams with near-zero shrinkage, high porosity, and outstanding strength. The ceramic foams consist of specific coral-like and hollow-sphere structures derived from the raw Al₂O₃/Al composite powders via reaction bonding (RB). A near-zero shrinkage of 0.91 ± 0.15 % and a high porosity of 73.7 ± 0.2 % can be achieved based on the Kirkendall effect during the oxidation of Al particles. Meanwhile, the reinforced sintering necks and robust bond-bridge connections between hollow-sphere and coral-like structures result in a remarkable bending strength of 7.37 ± 0.37 MPa. This measured strength is more than six times higher than other fabricated samples from spherical Al₂O₃ powders, and the comprehensive performance of ceramic foams prepared by this novel SLS/RB strategy is exceptionally remarkable versus that via conventional forming methods.     

Dense and strong calcite ceramics prepared by room-temperature cold sintering based on high-pressure-enhanced solubility

Dense and strong calcite (CaCO₃) ceramics were prepared by room-temperature cold sintering with the aid of water and high pressure of up to 900 MPa. Under atmospheric pressure, calcite is barely soluble in water. However, the microstructure evolution and stress-strain analysis during cold sintering revealed that the dissolution-precipitation, plastic deformation, and pressure-solution-creep mechanisms played a crucial role in the densification and mechanical robustness of calcite ceramics, which was attributed to the significantly enhanced solubility of calcite in water under high pressure. The calcite ceramic cold sintered under 900 MPa from micron powder exhibited the highest relative density of 92.1% and best mechanical properties with compressive strength, flexural strength, hardness, and Young's modulus of 276.5 MPa, 52.5 MPa, 1.64 GPa, and 53.7 GPa, respectively. The as-prepared calcite ceramic was stronger and harder than most stones and cement, indicating its promising application as novel building and biomimetic materials. The present study also provides a new strategy for densifying ceramics with low solubility by cold sintering.     

Preparation of machinable bioactive mica diopside-fluoroapatite glass-ceramics

Abstract

A fluorphlogopite based glass powder was added to a bioactive glass-ceramic in the system CaO-MgO-P₂O₅ (Fe₂O₃)-SiO₂ in order to improve its machinability. The sinterability of compacted mixtures, their crystallisation behaviour during sintering and their mechanical properties were evaluated and compared with the extreme ends compounds. According to results, fluoroapatite crystallised as a new phase only in the mixtures of the two glasses, its constituents originated by counter diffusion of two glasses components. Fracture toughness and microhardness of the best sample were higher than twice of the fluorphlogopite glass-ceramic composition. The machinability of samples was evaluated by naked eye observation and determination of brittleness factor.     

Ultrasonic-assisted preparation of complex-shaped ceramic cutting tools by microwave sintering

Complex-shaped ceramic tools were prepared by ultrasonic-assisted molding technology and microwave sintering. A uniaxial ultrasonic vibration compaction system was designed. The effects of ultrasonic vibration and molding pressures on mechanical properties and microstructures of A₂O₃ and Al₂O₃/SiC complex-shaped milling cutters were investigated by simulations and experiments. The results indicated that the density was distributed unevenly within the green ceramic tool samples, especially poor in the tool edges. These drawbacks were well improved by ultrasonic-assisted molding technology and microwave sintering. Applying ultrasonic vibration during the compaction process of green ceramic tool samples could improve the densification, hardness and reduce the randomness of mechanical properties of the sintered ceramic tool materials, especially for the difficult-to-sinter composite ceramics (with poor density). For A₂O₃ ceramic tool, fully dense microstructure was achieved, and for Al₂O₃/SiC ceramic tool, the cutting edges showed better density than that of the interior body, which was beneficial to improve the abrasion resistance of cutting tool.