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.     

Preparation of alumina/polystyrene core-shell composite powder via phase inversion process for indirect selective laser sintering applications

Indirect selective laser sintering (SLS) is one of the promising additive manufacturing (AM) methods that can process conventionally difficult or even impossible materials such as ceramics. In this work, an innovative phase inversion technique is used to fabricate spherical alumina particles coated with a thin layer of polystyrene (PS). Then, indirect SLS is used to fabricate green parts from the 6 wt% PS coated alumina particles via a Nd:YAG laser. The assessed SLS process parameters were the scan speed, laser power, scan spacing, pulse frequency, and pulse width. The characterization of the AL₂O₃/PS core-shell composite particles was described using techniques including SEM (for morphology), FT-IR (for chemical bonding at the interfaces), TGA (for mass loss), and DSC (for glass transition temperature, T_g). 3D green parts were then fabricated using proper process parameters as a proof of the feasibility of using SLS technique for AL₂O₃/PS core-shell composite powder. The results showed that using a Nd:YAG laser with less absorption by alumina and PS provides greater penetration through a powder bed. In addition, the possibility of sound connections among particles in every direction was observed due to the uniformity of the coating process in spite of a minimal amount of binder. In addition, green part density measurements show high values compared to previously reported results.     

氧化铝陶瓷低温烧结的研究现状和发展前景

本文综述了氧化铝陶瓷低温烧结的研究现状,包括粉体的制备,处理,成型及烧结工艺,并对此领域做了相应的设想和展望。     

Direct selective laser sintering of hexagonal barium titanate ceramics

A direct selective laser sintering (SLS) process was combined with a laser preheating procedure to decrease the temperature gradient and thermal stress, which was demonstrated as a promising approach for additive manufacturing of BaTiO₃ ceramics. The phase compositions in BaTiO₃ ceramics fabricated by SLS were investigated by X-ray and neutron diffractions. The surface morphologies and cross-section microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A dense hexagonal h-BaTiO₃ layer was formed on the surface and extended to a depth of 500 μm, with a relative density higher than 97% and absence of pores or microcracks. SLS resulted in the formation of the high-temperature phase, h-BaTiO₃, which was retained at room temperature possibly due to the high cooling rate. The grain boundaries of SLSed h-BaTiO₃ ceramics consist of a Ti-rich secondary phase. Compared with that of the pressureless sintered t-BaTiO₃ ceramics, the Vickers hardness of SLSed h-BaTiO₃ is 70% higher.     

Flash sintering of ultra-high pure alumina ceramics with fine microstructure

The nearly fully dense ultra-high pure (>99.99%) α-Al₂O₃ ceramics were prepared by flash sintering at the furnace temperature of 1300°C. Compared with the samples sintered at 1300 and 1650°C without electrical field, the flash-sintered samples exhibited remarkably improved density and finer grains. The flash-sintered samples also exhibited high hardness, which is even higher than that of the hot-pressed sample. Therefore, it is believed that flash sintering could be an effective technique for the preparation of ultra-high pure alumina ceramics.     

Strength of additively manufactured alumina with different debinding and sintering heat treatments

Lithography-based ceramic manufacturing (LCM) utilizes slurries with low solids loading, which makes pressureless sintering to full density especially difficult. A further compounding issue in sintering to full density and establishing structure–property–processing relationships for LCM alumina is the fact that the current literature lacks consensus on heat treatments to achieve full densities. Treatment specifics that are recorded are frequently ambiguous and insufficiently detailed. In this work, temperatures and times for debinding and sintering schedules were varied to characterize the influence of heat treatment parameters on density, microstructure, and flexural strength of high-purity, LCM-formed alumina. Removing the bisque-fire from debinding and fine-tuning sintering parameters produced parts with full densities, high flexural strengths, and Weibull moduli that match or exceed the values documented in a round robin study of traditionally processed alumina.     

Surface area reduction during the sintering of alumina

Surface area reduction trajectories of two well characterized calcined alumina powders were analyzed over a range of thermal profiles. Surface area reduction trajectories were confirmed to be independent of initial green density. Variations in trajectory were explained through experimental demonstrations of blended coarse and fine particle alumina systems. This work demonstrates that (1) nanoparticles do not appear to assist conventional sintering, because the nanoparticle surface area drops precipitously with minimal increase in density; and (2) the initial compact density does not contribute, nor control, surface area reduction during alumina sintering.     

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.     

Explosive compaction and low-temperature sintering of alumina nanopowders

Physical aspects of explosive compaction of alumina nanopowders with different phase compositions are studied experimentally. Physical processes that occur during consolidation of nanoparticles under pulsed loading are considered. Conditions of retaining of the material nanostructure after compaction and subsequent low-temperature sintering are determined. Physicomechanical properties of explosive compacts and ceramics on the basis of these compacts are studied. A ceramic material characterized by a nanostructure (grain size of ≈200 nm) and high values of density (97% of the theoretical value) and microhardness (up to 23.5 GPa) is obtained. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 114–126, November–December, 2008.     

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.     

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.     

Preparation of Ti2AlC through in-situ selective laser forming and reaction sintering

This paper proposes the preparation of Ti₂AlC complex parts through the combination of in-situ selective laser forming (ISLF) and reaction sintering. By studying the impaction of scan speed and laser power on the phase composition and microstructure of the formed test specimen, it was found that the test specimen mainly consisted of Ti3Al, TiAl, TiC, and graphite reacted incompletely. When the laser power was fixed, the lower the scan speed, the higher the TiC content. The TiC phase developed from nanoparticles to coarse dendritic structures gradually. When the laser power is further increased, the dendritic structure does not increase significantly. The process optimization found that when the laser power was 40 W and the scan speed was 100 mm/s, the density of the formed test specimen was 80.308%, which was mainly because the difference between the diffusion coefficients of Ti and Al resulted in Kirkendall pores, causing the incomplete densification of the formed part.     

Selective laser sintering of porcelain

Selective Laser Sintering (SLS) technique is used to fabricate 3D porcelain products with complex shapes. Commercial powder has been studied and optimized in terms of morphology and particle size distribution in order to get a perfect powder layerwise which remains the critical step of such a technique. The influence of laser energy density (through the laser power and scan speed) and hatching space have been investigated to determine the optimized parameters that allow the greater densification of this complex multi-materials composed of kaolinite, quartz and potassium feldspar. The laser-sintered porcelain products which exhibit about 60% of porosity have been post-treated at 1350 °C under vacuum or air to further improve densification.     

反应烧结法合成镁铝尖晶石耐火材料

以合成镁铝尖晶石(分为<350μm、<177μm和<74μm三种粒级)、镁砂(粒度<88μm)和刚玉粉(粒度<44μm)为原料,配制成尖晶石粒度不同,n尖晶石:n镁砂:n刚玉粉分别为80:10:10、60:20:20、40:30:30、30:35:35、20:40:40和10:45:45的试料,分别以100MPa、150MPa和200MPa的压力压制成50mm×20mm的柱状试样,在120℃干燥24h后于1600℃2h烧成,通过反应烧结法合成镁铝尖晶石耐火材料。测量烧后试样的线变化率、显气孔率和体积密度,并利用XRD和SEM分析烧后试样的相组成和显微结构。试验结果表明:随着镁砂和刚玉粉含量的增加以及尖晶石原料粒度的增大,烧后试样的线收缩率和体积密度减小,显气孔率增大;随着成型压力的增大,试样的线收缩率和显气孔率减小,体积密度增大;采用粒度<177μm的尖晶石原料,按n尖晶石:n镁砂:n刚玉粉=20:40:40的比例配料,在200MPa压力下成型,经1600℃2h烧成,可获得显气孔率为16.3%,烧成线收缩率为0.42%的镁铝尖晶石耐火材料。     

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.     

Experimental based full process simulation of alumina selective laser processed parts densified by cold isostatic pressing and solid state sintering

The compound process of cold isostatic pressing (CIP) of alumina selective laser processed (SLP) parts and solid state sintering (SSS) and its full process simulation were realized in this paper, focusing on studying the overall deformation, relative density distribution, grain growth and sintering stress variation during the process. Especially, correlation was established between the macroscopic deformation and microscopic evolution. Model parameters for alumina are presented, which were optimized in accordance with the experimental results. CIPed part still exhibited density inhomogeneity, of which SSS tended to increase the overall density and homogenize density distribution. The sintering behavior was studied with the employment of dilatometer experiments. Furthermore, compared with conventional heating strategy, fast firing turned out to decrease sintering production time as well as drive the matter diffusion and densification process. The master sintering curve (MSC) moves upward a little under the condition of fast firing.     

On the catalytic effect of zirconia on flash sintering of alumina

Flash sintering of alumina is more difficult than of yttria-stabilized zirconia (YSZ). Whereas (MgO doped) alumina requires fields greater than 1 kVcm^–1 and temperatures often significantly higher than 1000°C, YSZ can be flashed sintered at ∼100 Vcm^–1 at temperatures below 800°C. Mixed powders of such bi-phasic ceramics, on the other hand, can be flashed under conditions below those for alumina. This effect is usually subscribed to the influence of YSZ on the overall electrical conductivity of the composite. However, such rationalization leaves open the mechanism by which YSZ catalyzes the flash event in alumina. Here, we present results for the onset of flash in a layered structure of YSZ and alumina where both constituents extend from one electrode to the other. We find that the flash initiates, at first, exclusively in the YSZ layer, under conditions identical to those in usual voltage-to-current experiments in single phase YSZ, and then, after a brief incubation period, spreads transversely through the thickness of the alumina layer at a speed of ∼3.3 mm s^–1, while the power supply is held at constant current. This observation opens a new question as to how flash once initiated in an “easy” phase can migrate normal to itself into a second ceramic, which is nominally more-difficult-to flash. (In the present experiments, the alumina layer sintered to full density with all the shrinkage being accommodated in the thickness direction, consistent with an earlier study that articulated that flash obviates constrained sintering.) It is noteworthy that the catalytic effect depends not only on the volume fraction of YSZ, but also on the architecture of the green state (for example a two-phase powder mixture vs. layered structure), which may affect the initiation of the flash in YSZ but, likely, not its migration behavior into the second phase.     

High-performance ceramic parts with complex shape prepared by selective laser sintering: a review

ABSTRACT

Selective laser sintering (SLS) is an additive manufacturing technology which has shown great advantages in direct formation of the polymer, metal and their composites. However, ceramic parts prepared by the SLS still exhibit some fatal defects, including low density and poor mechanical properties. In this respect, recent advances for preparing ceramics have improved the final density and performance by adopting post-processing methods. In this review, three commonly used powder preparation approaches (i.e. mechanical mixing, solvent evaporation and dissolution-precipitation process) and two powder sintering mechanisms for the SLS are introduced. Porous ceramic parts are prepared directly through the SLS by virtue of their high porosity. And dense, high-performance Al₂O₃, ZrO₂, kaolin and SiC ceramic parts with complex shape are prepared by introducing CIP technology into the SLS, indicating that the hybrid technology could be the promising route for preparing high-performance ceramic parts used in various fields.     

Construction of an electric microenvironment in piezoelectric scaffolds fabricated by selective laser sintering

It is well established that the electric microenvironment plays a vital role in promoting bone regeneration and repair. In this work, a BaTiO₃ polymer scaffold was fabricated by selective laser sintering to overcome the challenges in manufacturing a 3D porous structure. More significantly, the orderly orientated dipoles of poled BaTiO₃ deflect when a force is applied to the scaffolds, which results in a large number of charges generated on the scaffolds. The results indicated that the poled scaffold with 20 wt% BaTiO₃ presented the highest electrical output performances under the same external force. The electric cues efficiently enhanced the cell viability, adhesion and proliferation under the action of ultrasound. Simultaneously, the tensile strength and modulus were significantly increased by 61.3% and 34.9%, respectively, which were attributed to the fact that crack propagation was prevented by BaTiO₃ nanoparticles. All of these positive results demonstrated that 3D piezoelectric scaffolds present great potential in bone regeneration.     

Dilatometric study of anisotropic sintering of alumina/zirconia laminates with controlled fracture behaviour

Al₂O₃ and ZrO₂ monoliths as well as layered Al₂O₃/ZrO₂ composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.