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.     

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.     

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.     

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.     

Preparation of high-porosity Al2O3 ceramic foams via selective laser sintering of Al2O3 poly-hollow microspheres

In this paper, high-porosity Al₂O₃ ceramic foams called Al₂O₃ PHM ceramics were fabricated through selective laser sintering (SLS) via Al₂O₃ poly-hollow microspheres (Al₂O₃ PHMs). SLS parameters were optimized by an orthogonal experiment as to be laser power = 6 W, scanning speed = 1800 mm/s, and scanning space = 0.15 mm. The effect of sintering temperature on microstructure, shrinkage, porosity, phase composition, mechanical properties and pore size distribution of Al₂O₃ PHM ceramics were investigated. When sintering temperature increased, Al₂O₃ PHM ceramics contained only Al₂O₃ phase and were gradually densified. With the raise of sintering temperature, the porosity of Al₂O₃ PHM ceramics decreased gradually from 77.09% to 72.41%, but shrinkage in H direction and compressive strength of Al₂O₃ PHM ceramics increased from 6.63% and 0.18 MPa to 13.10% and 0.72 MPa, respectively. Sintering temperature had little effect on pore size distribution of Al₂O₃ PHM ceramics, which only declined from 24.2 to 21.4 μm with the increase of sintering temperature from 1600 to 1650 °C. This method can not only directly prepare ceramic foams with complex shapes, but also control properties of ceramic foams. It provides a simple preparation method for many kinds of ceramic foams with complex structure and high porosity by using PHMs with different composition.     

信噪比与灰色关联分析选区激光烧结成型工艺的研究

针对选区激光烧结制件收缩率大,精度较低的问题,利用信噪比与灰色关联度相结合的选区激光烧结(SLS)工艺参数优化方法,提高尺寸精度。以制件X、Y、Z三个方向尺寸收缩率为目标函数,基于正交试验结果,采用信噪比和灰色关联分析的方法,得到工艺参数对目标函数的影响程度,同时将多目标优化问题转化为单目标优化问题,获得最佳工艺参数组合。结果表明,最佳的工艺参数组合为,激光功率为25 W,扫描速度为2200 mm/s,单层厚度为0.19 mm,扫描间距为0.31 mm。此时X/Y/Z向尺寸精度为:0.803%、0.817%、1.305%。将信噪比与灰色关联度相结合的优化方法应用到SLS工艺中,能够有效提高制件精度。     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Densification behaviour and three-dimensional printing of Y2O3 ceramic powder by selective laser sintering

The selective laser sintering (SLS) of an yttria (Y₂O₃) ceramic powder was studied to understand both the effects of i) the initial yttria particle characteristics on the powder bed behaviour and ii) the process conditions (laser power, scanning speed, hatching space) on the sintering/melting of three-dimensionally printed objects. The roughness of the powder bed, a sensitive indicator of the layer bed quality, was determined through three-dimensional optical profilometry and the powder bed packing density was modelled using the discrete-element method. Complex shaped objects including spheres and open rings were successfully fabricated by the SLS three-dimensional printing. In addition, SLS cube-shaped samples were characterized by X-ray diffraction and scanning electron microscopy. The open pore volume fraction significantly decreased from 41% without a post-SLS heat treatment to 31% with a post-SLS heat treatment at 1750 °C for 20 h under secondary vacuum. Finally, an anisotropy in elastic properties has been highlighted, Young's modulus reaches 11 GPa in the stiffest direction.     

Improvement in heat resistivity of alkaline earth silicate fiber boards by Al4SiC4 coating

Alkaline earth silicate (AES) fiber ceramic board was immersed in a novel coating slurry consisting of Al₄SiC₄ particles and silica-sol, forming a 500-μm coating layer on the AES fiber board. Linear shrinkage of the uncoated AES fiber board was over 3.0% after heating at 1100°C or higher for 8 hours, whereas the linear shrinkage of the AES fiber board with an Al₄SiC₄ coating was below 2.0%. The coating layer of Al₄SiC₄ changed to a hard shell structure consisting of cristobalite, alumina, and mullite after heating. When AES fiber board with an Al₄SiC₄ coating layer was heated at 1200°C for 8 hours, the compressive strength of the board reached 0.45 MPa, 2.5 times greater than that of the original uncoated board.     

Fabrication of Si3N4–SiC/SiO2 composites using 3D printing and infiltration processing

Si₃N₄–SiC/SiO₂ composites were prepared by employing three-dimensional (3D) printing using selective laser sintering (SLS) and infiltration processing. The process was based on the infiltration of silica sol into porous SLS parts, and silicon carbide and silicon nitride particles were bonded by melted nano-sized silica particles. To optimize the manufacturing process, the phase compositions, microstructures, porosities, and flexural strengths of the Si₃N₄–SiC/SiO₂ composites prepared at different heat-treatment temperatures and infiltration times were compared. Furthermore, the effects of the SiC mass fraction and the addition of Al₂O₃ and mullite fibers on the properties of the Si₃N₄–SiC/SiO₂ composites were investigated. After repeated infiltration and heat treatment, the flexural strength of the 3D-printed Si₃N₄–SiC/SiO₂ composite increased significantly to 76.48 MPa. Thus, a Si₃N₄–SiC/SiO₂ composite part with a complex structure was successfully manufactured by SLS and infiltration processes.     

Microstructure and properties characterization of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

5at.% Yb:Lu2O3 transparent ceramics were fabricated successfully by vacuum sintering along with hot isostatic pressing posttreatment from the nanopowders. The influences of calcination temperature on morphology and microstructures of powders and ceramics were studied systematically. The optimal ceramic sample from the nanopowder calcined at 1050°C shows uniform and dense microstructure with the in-line transmittance of 81.5% at 1100 nm. The results of the thermal measurements, that is, thermal conductivity and specific heat, were related to the changes occurring in the microstructure of the ceramics studied. It was shown on this basis that appropriate control of the technological process of sintering ceramics makes it possible to obtain laser ceramics with very good thermal properties as well as maintaining their high optical quality. Concerning the laser performance, the highest-optical quality 5at.% Yb:Lu₂O₃ sample was pumped in quasi-continuous wave conditions measuring a maximum output power of 2.59 W with a corresponding slope efficiency of 32.4%.     

The effect of particle size distribution on the microstructure and properties of Al2O3 ceramics formed by stereolithography

Stereolithography (SL) shows advantages for preparing alumina-based ceramics with complex structures. The effects of the particle size distribution, which strongly influence the sintering properties in ceramic SL, have not been systematically explored until now. Herein, the influence of the particle size distribution on SL-manufactured alumina ceramics was investigated, including bending strength at room temperature, post-sintering shrinkage, porosity, and microstructural morphology. Seven particle size distributions of alumina ceramics were studied (in μm/μm: 30/5, 20/3, 10/2, 5/2, 5/0.8, 3/0.5, and 2/0.3); a coarse:fine particle ratio of 6:4 was maintained. At the same sintering temperature, the degree of sintering was greater for finer particle sizes. The particle size distribution had a larger influence on flexural strength, porosity and shrinkage than sintering temperature when the particle size distribution difference reached 10-fold but was weaker for 10 μm/2 μm, 5 μm/2 μm and 5 μm/0.8 μm. The sintering shrinkage characteristics of cuboid samples with different particle sizes were studied. The use of coarse particles influenced the accuracy of small-scale samples. When the particle size was comparable to the sample width, such as 30 μm/5 μm and 5 mm, the width shrinkage was consistent with the height shrinkage. When the particle size was much smaller than the sample width, such as 2 μm/0.3 μm and 5 mm, the width shrinkage was consistent with the length shrinkage. The results of this study provide meaningful guidance for future research on applications of SL and precise control of alumina ceramics through particle gradation.     

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.     

Formability of Fe-doped bioglass scaffold via selective laser sintering

Because of its outstanding characteristics, bioglass is considered a potential bone substitute; however, it is difficult to be fabricated into a scaffold because of insufficient strength. Although there are several methods for producing a three-dimensional ceramic scaffold, most of these methods cannot completely mimic the bone structure. In this study, a bioglass scaffold was fabricated through selective laser sintering (SLS) with the addition of the iron element that could ascend the laser absorption rate with the improvement of the formability and mechanical strength of scaffolds. As a result, the laser absorption proficiency improved with an increase in the amount of iron added; a competent bioglass scaffold could be successfully manufactured with a 5% iron element addition at the energy density of 2.5 cal/cm² (3-W power and 120 mm/s scan speed). In a comparison of scaffolds sintered with various parameters of the heat treatment, scaffolds that had favourable mechanical strength and cell survival rate could be acquired after sintering at 1100 °C. According to the result of the present study, a competent biocompatible bioglass scaffold could be obtained using the SLS process with the addition of the iron element and suitable post-processing parameters.     

Study on growth factors of SiC whisker in situ in SiCW/SiC composites based on selective laser sintering technology

In this study, SiC_W/SiC composite material was prepared through selective laser sintering (SLS), the effect of material parameters (pretreatment of PCS, content of PCS, type of catalyst and content of catalyst) on the growth number of SiC whisker was investigated. The results indicate that when the temperature variable of PCS pretreatment was 260 °C or 300 °C, the number of in situ generated SiC whisker was noticeably higher than that pretreated at 140/180/200/240 °C. The catalytic effect of ferrocene was superior than pure Fe, α-Al₂O₃ and Fe(NO₃)₃. Under this experimental condition, the best parameter was 10 wt.% of PCS and 4 wt.% of ferrocene. This find not only provides a research basis for the preparation of SiC_W/SiC composites prepared by SLS technology, but also perfects basic datas for the preparation process of SiC_W/SiC composites.     

Fabrication of lightweight ZrO2 fiberboards using hollow ZrO2 fibers

ZrO₂ fiberboards with ultra-low densities (0.34–0.40 g/cm³) were fabricated using biomorphic ZrO₂ hollow fibers, which have a lower density and better thermal insulation than traditional ZrO₂ solid fibers. The effects of sol binder content, sintering temperature, and proportion of solid fibers on the density, microstructure, compressive strength, linear shrinkage, and thermal conductivity of lightweight ZrO₂ fiberboards were investigated. The results showed that the hollow features of biomorphic ZrO₂ fibers were successfully maintained after they were made into ZrO₂ fiberboards, which made them less dense and thermally conductive. The best conditions were found to be a sol binder content of 30 vol%, sintering temperature of 1400 °C, and 20 wt% sintered solid fibers to balance thermal insulation and compressive strength. The results show that the density and thermal conductivity of lightweight ZrO₂ fiberboard gives it obvious advantages as a heat-insulating ceramic. Specifically, when the sintering temperature was 1400 °C, the sample had an ultra-low density of 0.34–0.40 g/cm³, a thermal conductivity of 0.101–0.116 W/(m·K) (at 500 °C), a compressive strength of 0.05–0.24 MPa, and a linear shrinkage of 9.4–13%.     

Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics

Cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor ceramics were fabricated by CO₂ laser sintering under various powers. X-ray diffraction results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. The reaction profile caused by laser ablation increased from 0.28 mm (15 W) to 2.68 mm (40 W), and it appeared deeper in the scanning sequence because of the low rigidity of YAG melts compared with solid precursors. Photoluminescence (PL) measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation presented more intense yellow emission (enhancement of 54%) as compared with YAG:Ce by solid-state reaction (SSR, 1500 °C). The laser beams were believed to distribute activators (Ce) more homogeneously compared with SSR. Additionally, a rougher surface of YAG:Ce phosphor ceramics with periodic holes caused by laser ablation resulted in more excitation and emissivity of PL.     

Suppressed atomic diffusion in flash sintering of bismuth telluride

Bi₂Te₃ materials were synthesised by the recently developed flash sintering (FS) method, and the rapid densification effect was studied. Whereas a fully densified sample can be obtained with a feeding current of 1.2 kA for 1 s, the limited heating effect reduces grain growth and atomic diffusion. Interestingly, the significant chemical reaction suppression with oxygen contaminants, which induces electron doping, has a meaningful electronic properties impact. The large negative Seebeck coefficient of ? 138.9 μV/K in the sample prepared by conventional current sintering of 773 K for 3 min, in which the oxygen diffuses into the Bi₂Te₃ phase, is significantly reduced to ? 8.5 μV/K in the FS sample, much closer to the intrinsic p-type conduction in Bi₂Te₃ raw powder. These results suggest that the limited FS heating effect may contribute to preserving the intrinsic raw powder properties in the sintered material by avoiding excess atomic diffusion and undesirable reactions.     

Fabrication and laser operation of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

In this article, 5 at.% Yb:Lu₂O₃ transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) posttreatment using co-precipitated nano-powders. The influence of precipitant molar ratio, ammonium hydrogen carbonate, to metal ions (AHC/M³⁺, R value) on the properties of Yb:Lu₂O₃ precursors and calcined powders was investigated systematically. It was found that the powders with different R value calcined at 1100°C for 4 hours were pure cubic Lu₂O₃ but the morphologies of precursors and powders behaved differently. The opaque samples pre-sintered at 1500°C for 2 hours grew into transparent ceramics after HIP posttreatment at 1750°C for 1 hour. The final ceramic with R = 4.8 showed the best optical quality with the in-line transmittance of 79.7% at 1100 nm. The quasi-CW laser operation was performed at 1034 nm and 1080 nm with a maximum output power up to 8.15 W as well as a corresponding slope efficiency of 58.4%.     

Effects of short carbon fiber on the macro-properties,mechanical performance and microstructure of SiSiC composite fabricated by selective laser sintering

Selective laser sintering was combined with reactive melt infiltration to fabricate SiSiC part, and the effects of carbon fiber (C_f) on the properties of the SLS green body, the carbonized and final SiSiC sample were investigated. Results show that the addition of an appropriate amount of C_f (1.59 wt%~2.97 wt%) can increase the bulk density and geometric precision of the sample at all stages, and improve the mechanical properties of green and carbonized samples. The main phases composed of the SiSiC composite were free Si, a-SiC, β-SiC, plus a very small amount of Al–Si alloy. With 1.59 wt% C_f addition, a relatively comprehensive favorable macro-properties of both the green sample and carbonized sample was achieved, and the homogeneous microstructure of the latter favored the decreased free Si content and increased β-SiC content of the final composite. The evolution mechanism of C_f added to the raw material is inferred to be the mutual diffusion of [C] and [Si] that occurred at the C_f/Si melt boundary leading to the formation of the siliconized C_f with relatively large diameter size (24.3 μm) and high aspect ratio (>30). Amorphous C, which derived from the pyrolysis of epoxy resin E12, undergone a dissolution-precipitation mechanism with the formation of fine-grain β-SiC.