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Porous mullite ceramics with a fully closed-cell structure were fabricated by direct coagulation casting (DCC) using fly ash hollow spheres (FAHSs) as the non-sacrificial pore-forming agent. Kaolin powder was used as the filler and the suspension stabilizer, and bentonite was used as the rheological additive, respectively. The effects of bentonite on the stabilization mechanism of suspension and properties of the porous ceramics were investigated. It is found that 1 wt% additional bentonite is efficient in improving the suspension stability from 2 h to at least 24 h, due to increased suspension viscosity, thixotropy and yield stress. Densification of the kaolin powder during sintering eliminates the open pores, and the closed pores from FAHSs remain to yield a fully closed-cell structure. The porous mullite ceramics have a total porosity ranging from 44.73% to 46.12%, in which 99% of the pores are closed. Bentonite also increases the compressive strength of porous mullite ceramics by 14.4%, and compressive strength of 58.07 ± 5.44 MPa can be achieved.

The manipulation of defects during selective laser melting plays a key role in forming dense ceramic parts with complex shapes. In this study, selective laser melting of alumina with pressure assistance was performed on powder bed, and the defects during selective laser melting, including balling and cracks, were investigated. It was found that, unlike the competition mechanism between spreading and solidification in the metal system, the balling phenomenon of alumina was dominated by the spreading process. According to the calculation, the spreading time of alumina melt was less than 1/5 of the solidification time. Even the alumina powder was fully melted without splashing, the balling phenomenon could still occur. Although lower scanning speed could increase laser dwell time and improve the stability of the molten pool, the efficiency and surface roughness were sacrificed. Severe balling occurred in this condition. When pressure treatment was applied, the balling phenomenon was greatly alleviated. However, surface cracks were observed. There are mainly two reasons for the formation of surface cracks: the shrinkage of the melt and the shear stress in the edge between partially melted zone and fully melted zone. To alleviate the surface cracks, the distribution of the density of powder bed, the laser parameters, and the particle size should be well controlled. This paper provided an experimental reference for the manipulation of balling phenomenon and surface cracks of alumina melt in the SLM process performed on powder bed.

Personalized zirconium dioxide (ZrO2) all-ceramic teeth are expected to be widely used in the field of oral restoration. Here, ZrO2 all-ceramic teeth were fabricated for biological engineering by photosensitive resin based digital light processing (DLP) technology. When sintered at 1500 °C for 3 h, dense ZrO2 ceramics were obtained with relative density of 98.02 ± 0.32%, Vickers hardness of 12.62 ± 0.15 GPa and fracture toughness of 6.11 ± 0.36 MPa∙m1/2, which were close to those of the ZrO2 ceramics prepared by dry pressing. To confirm whether the prepared ZrO2 ceramics possessed good biosafety, in vitro biological experiments were further performed with rat bone marrow mesenchymal stem cells (rBMSCs) and preferable cell attachment was observed at the initial stages. Results illustrated that the compositions with cytotoxicity in green samples were completely decomposed after debinding and sintering process, and the ZrO2 ceramics could provide an ideal biocompatible environment for mesenchymal stem cells. This study shows that ZrO2 all-ceramic teeth with personalized structure prepared by DLP technology have good mechanical properties and biocompatibility, exhibiting strong potential in the field of oral restoration.

It is difficult to fabricate lightweight ceramic parts with well pore control and high structural complexity. In this paper, lightweight mullite ceramics with controlled porosity and enhanced properties were prepared via selective laser sintering (SLS) using mechanical mixed FAHSs/PA12 (Fly ash hollow spheres/polyamide 12) composites. Crack-free ceramic green bodies were prepared through SLS using the optimized process parameters: 7.2 W in laser power, 1600 mm/s in scanning velocity and 0.11 mm in hatch spacing. The influence of PA12 content on porosity, size distribution of open pore, mechanical properties and thermal property were studied. The pores in ceramic foams consisted of the closed pores from sphere core-shell structures, the open pore channels in the middle of stacking spheres and the special gaps in the middle of spheres related to SLS. It was found that the total porosity of lightweight mullite ceramics increased slightly from 85.1 ± 0.3% to 85.2 ± 0.4% with PA12 content increasing from 10 to 15 wt%, and then increased obviously to 86.7 ± 0.5% with further increasing PA12 addition to 25 wt%. This porosity increase was mainly attributed to the open pores resulted from the PA12 addition. The PA12 were firstly filled into the interspaces between stacking spheres and when the stacking volume reached the maximum, the interspaces expanded to form the special gaps between spheres with further PA12 addition. Thus, the size distribution of open pore in lightweight FAHS ceramics increased from 31.5 to 34.5 μm gradually with PA12 content increasing from 10 to 25 wt%. Finally, low thermal conductivity of 0.06 W/(m·K) was obtained, which probably resulted from the high porosity and the special pore structures of the SLS-formed ceramic foams. The understanding of the unique pore structures and microstructures will help the pore control and strength improvement of SLS-formed ceramic foams using the ceramic hollow spheres.

Isotropic Si3N4 ceramics were fabricated by low-pressure spark plasma sintering in combination with direct coagulation casting (DCC/LP-SPS). A novel phenomenon is observed that hardness and fracture toughness of Si3N4 lift simultaneously as temperature rises. It can be attributed to the enhanced densification and formation of β′-sialon. Rapid coagulation of the submicron ceramic suspension prepares green body with high relative density and offers the sufficient driving force for densification, while electric field facilitates particles rearrangement, thus Si3N4 ceramics can obtain the relative density above 97% under the axial pressure of 5 MPa at 1550 °C. As SPS temperature rises, there is the formation of β′-sialon phase in Si3N4 ceramics without AlN as sintering aid. This is most likely due to electric field promoting the motion and uniform distribution of charged species in liquid phase, which facilitates the replacement of Si4+ and N3− by Al3+ and O2− during the process of dissolution-precipitation. Additionally, compared with Si3N4 prepared by hot pressure sintering, there is no obvious directional alignment of grains in the samples that are spark plasma sintered under a low applied pressure. As a consequence, Si3N4 ceramics prepared by DCC/LP-SPS acquire complete transformation of α→β phase and full densification without abnormal grain growth, leading to ultimate formation of an isotropic self-reinforced bimodal structure containing β-Si3N4 and β′-sialon. Therefore, Si3N4 ceramics, coagulated by 45 vol% suspension and sintered at 1650 °C for 20 min with a uniaxial pressure of 5 MPa, simultaneously achieve the high Vickers hardness and fracture toughness as follows: 17.5 ± 1.0 GPa and 8.5 MPa m1/2.

Direct selective laser sintering (dSLS) is a promising method for the fabrication of complex-shaped ceramic parts. In this paper, boron carbide (B 4 C) was used as an inorganic additive to improve the laser sintering behavior of alumina. The effects of B 4 C addition on the microstructure and mechanical properties of porous alumina ceramics were investigated. Mixture of alumina powders and different amount of B 4 C were directly sintered using different SLS parameters. Results indicated that the process window of alumina could be expanded by the addition of B 4 C. Furthermore, the amount of B 4 C 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 B 4 C, which enhanced the densification process. The compressive strength of sintered samples increased with the increase of B 4 C, which reached its maximum value when the content of B 4 C was 7 wt% and the density of the samples after post treatment could reach 1.4 g/cm 3 . 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 B 4 C particles.