Strength of additive manufactured alumina

The strength of 3D-printed alumina parts fabricated using the Lithography-based Ceramic Manufacturing (LCM) technology is investigated. The influence of the sintering parameters, printing direction, surface condition (i.e. machined or as-printed), and/or geometry on the strength distribution is studied under uniaxial and biaxial bending tests. Weibull parameters, i.e. characteristic strength and Weibull modulus, are determined and compared between the different samples. Experimental findings show that samples sintered at higher temperatures yield higher Weibull modulus, associated with a more homogeneous microstructure. Fractographic analyses reveal the influence of surface finish (as-printed or machined) on strength and show the importance of reporting testing configuration along with printing direction to assess the mechanical response of 3D-printed parts. Based on these results, manufacturing recommendations are given which shall advance the progress in optimization of alumina ceramics fabricated using the LCM technology.     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

Effect of manganese addition on sintering behavior and mechanical properties of alumina toughened zirconia

The effect of MnO₂ additives on the sintering behavior and mechanical properties of alumina-toughened zirconia (ATZ, with 10 vol% alumina) composites was investigated by incorporating different amounts of MnO₂ (0, 0.5, 1.0, and 1.5 wt%) and sintering at various temperatures ranging from 1300 to 1450 °C. The addition of MnO₂ up to 1.0 wt% improved the sintered density, hardness, flexural strength, and fracture toughness of the composite. However, the addition of 1.5 wt% MnO₂ degraded the relative density, hardness, and flexural strength of the composite due to the transformation of the ZrO₂ phase from tetragonal to monoclinic and grain coarsening. Optimal results were obtained with 1.0 wt% MnO₂ and sintering at 1450 °C, which improved the mechanical properties (hardness: 13.5 GPa, flexural strength: 1.2 GPa, fracture toughness: 8.5 MPa m^1/2) and lowered the sintering temperature compared to the conventional sintering temperature of ATZ composites (1550 °C). Thus, the ATZ composite doped with MnO₂ is a promising material for structural engineering ceramics owing to its improved mechanical properties and lower sintering temperature.     

Femtosecond laser microstructuring of alumina toughened zirconia for surface functionalization of dental implants

The continuous need for high-performance implants that can withstand mechanical loads while promoting implant integration into bone has focused recent research on the surface modification of hard ceramics. Their properties of biocompatibility, high mechanical and fatigue resistance and aesthetic color have contributed to its succefull applications in dentistry. Alumina toughened Zirconia (ATZ) has been gaining attention as a material for dental implants applications due to its advanced mechanical properties and minimal degradation at body temperature. Still, in order to improve tissue response to this bioinert material, additional modifications are desirable. Improving the surface functionality of this ceramic could lead to enhanced implant-tissue interaction and subsequently, a successful implant integration.In this work, microtopographies were developed on the surface of Alumina toughened Zirconia using an ultrafast laser methodology, aiming at improving the cellular response to this ceramic. Microscale grooves and grid-like geometries were produced on ATZ ceramics by femtosecond laser ablation, with a pulse width of 150 fs, wavelength of 800 nm and repetition rate of 1 kHz. The variation of surface topography, roughness, chemistry and wettability with different laser processing parameters was examined.Cell-surface interactions were evaluated for 7 days on both microstructured surfaces and a non-treated control with pre-osteoblasts, MC3T3-E1 cells. Both surface topographies showed to improve cell response, with increased metabolic activity when compared to the untreated control and modulating cell morphology up to 7 days.The obtained results suggest that femtosecond laser texturing may be a suitable non-contact methodology for creating tunable micro-scale surface topography on ATZ ceramics to enhance the biological response.     

Characterization of ceramic components fabricated using binder jetting additive manufacturing technology

Binder jetting additive manufacturing is an emerging technology with capability of processing a wide range of commercial materials, including metals and ceramics (316 SS, 420 SS, Inconel 625, Iron, Silica). In this project, aluminum oxide (Al₂O₃) powder was used for part fabrication. Various build parameters (e.g. layer thickness, saturation, particle size) were modified and different sintering profiles were investigated to achieve nearly full-density parts (~96%). The material's microstructure and physical properties were characterized. Full XRD, compression testing, and dielectric testing were conducted on all parts. Sintered alumina parts were achieved with an average compressive strength of 131.86 MPa (16 h sintering profile) and a dielectric constant of 9.47–5.65 for a frequency range of 20 Hz to 1 MHz. The complexity offered by additive processing aluminum oxide can be extended to the manufacturing of high value energy and environmental components for environmental systems (e.g. filters and membranes) or biomedical implants with integrated reticulated structures for improved osseointegration.     

Coupling additive manufacturing and microwave sintering: A fast processing route of alumina ceramics

In this paper, a quick and efficient route to produce complex-shape alumina is reported. Alumina pieces are shaped by additive manufacturing (stereolithography) and densified by microwave sintering. Two raw powders are investigated in terms of both printing and microwave sintering: one alumina grade appropriate for additive manufacturing but not for microwave sintering, and vice-versa for the other grade. The mixture of the two raw powders allows both stereolithography printing and microwave sintering. Alumina parts processed by additive manufacturing followed by microwave sintering were successfully prepared and exhibited relative densities of about 93%, elastic modulus up to 236 GPa and Vickers hardness up to 12 GPa. Notwithstanding the part properties, the as-proposed coupling resulted in a time saving of 30%.     

Development of UV-curable ZrO2 slurries for additive manufacturing (LCM-DLP) technology

Digital Light Processing (DLP) is a powerful technique for the preparation of ceramic parts with high resolution and complex shapes. In the last years, the development of photosensitive slurries for the production of ceramics with good mechanical properties has received much attention. In this work, ZrO₂ UV-curable slurries were prepared in two steps for their application in DLP. Firstly, the surface modification of the ZrO₂ particles was carried out using a dispersing agent and secondly, the modified powder was dispersed in an acrylate based mixture. Parts with different geometries were printed and a resolution experiment was also carried out in order to determine the limitations of the slurry. Finally, 30 bars were produced to study the mechanical properties of the sintered parts (ρ = 6.00 ± 0.01 g/mL) by 4-point bending tests and Weibull analysis, obtaining a flexural strength σ₀ = 741 (718–765) MPa with a Weibull coefficient of 11.4.     

Influence of resin composition on rheology and polymerization kinetics of alumina ceramic suspensions for digital light processing (DLP) additive manufacturing

Alumina ceramics with optimized microstructures and mechanical properties were obtained by the attractive digital lighting processing (DLP) additive manufacturing methodology in the present study. A acrylate-based resin system was designed for the alumina powders with a mean particle size of 0.5 μm. The influence of oligomer on the viscosity and polymerization kinetics of the ceramic suspensions has been elaborately discussed by rheology, curing depth and photo-DSC characterizations. The results indicated that the introduction of oligomer has improved the cross-linking density of resins and decreased the critical dose of energy for resin polymerization, which contributed to a tougher ceramic-resin slice with higher dimensional accuracy. Densifying processes including debinding and high temperature sintering of the ceramic parts were conducted according to the TG-DTA characterizations, alumina ceramics with uniform microstructures and eliminated delamination or intralaminar cracks were finally obtained. The flexural strength was 471 MPa for the ceramics obtained from the resin composition containing 20 wt% oligomer, Weibull modulus for the ceramics were determined to be 17.31 by evaluating thirty all sides polished ceramics, indicating the highly uniform property of the ceramics fabricated by DLP additive manufacturing.     

Direct additive manufacturing of melt growth Al2O3-ZrO2 functionally graded ceramics by laser directed energy deposition

Functionally graded ceramics (FGC), which combine properties of different ceramics in one part, usually have better comprehensive function and structural efficiency. In this study, four different gradient transition Al₂O₃-ZrO₂ FGC samples were prepared by laser directed energy deposition (LDED) method. The results show that there is an obvious interface in direct transition sample. The transition section bears tensile stress caused by difference of thermophysical properties of materials, resulting in significant longitudinal cracks. Element transition in interface region shows a step sharp transition. The direct transition sample shows intergranular fracture and the bonding strength is very low. Gradient transition mode can effectively suppress cracks, and avoid the step transition of microstructure and elements. Elements, microhardness of 25, 20 wt% FGC samples realized a nearly linear smooth transition. The interface fracture of FGC samples changed to transgranular fracture, bonding strength was significantly improved, and the maximum flexural strength reached 160.19 MPa.     

Direct additive manufacturing of TiCp reinforced Al2O3-ZrO2 eutectic functionally graded ceramics by laser directed energy deposition

Laser directed energy deposition (LDED) provides an ideal manufacturing technique to produce ceramic matrix composites, which are endows with superior and customizable mechanical properties by the reinforcement of gradient distribution of rigid particles. In this paper, we for the first time manufactured TiCp reinforced Al₂O₃-ZrO₂ eutectic functionally graded ceramics with two different transition modes using LDED. The results show that the gradient transition realizes gradually increase of TiCp particles in Al₂O₃-ZrO₂ eutectic matrix. With the increase of TiCp content, the morphology of Al₂O₃-ZrO₂ eutectic matrix changes from lamellar or rod-shaped to irregular shape. Meanwhile, LDED realizes controllable fabrication of properties in different positions of gradient materials, and the wear resistance of TiCp rich regions has increased by 43.4% compared with pure Al₂O₃-ZrO₂ region.     

The role of plasticizer in optimizing the rheological behavior of ceramic pastes intended for stereolithography-based additive manufacturing

Adding plasticizer is an efficient way to regulate the rheological behavior of ceramic paste and quality of green body in stereolithography-based additive manufacturing. The type and content of plasticizers (polyethylene glycol (PEG) and dibutyl phthalate (DBP)) had substantial effects on the rheological behavior and solid loading of ceramic paste, leading to varied macro / micro structure and strength of the green and sintered parts. DBP significantly reduced the viscosity and increased solid loading, and could adjust the flexibility of the green body by reducing the crosslinking density of UV curing system. PEG could inhibit crack initiation to some extent, but it was less effective for preventing cracking than DBP on ceramic parts with large-sized cross sections. It was concluded that DBP was more suitable as a plasticizer in alumina paste for SL additive manufacturing to form dense parts without defects.     

Direct laser melting of Al2O3 ceramic paste for application in ceramic additive manufacturing

We develop the direct laser melting of ceramic paste technology for application in ceramic additive manufacturing (AM). The Al₂O₃ ceramic paste, which is a homogeneous mixture of DI-water and Al₂O₃ ceramic powders, was deposited on an Al₂O₃ substrate using free-forming extrusion (FFE), and subsequently melted by a CO₂ laser. To better control the laser melting process, the flow behavior of the laser-melted Al₂O₃ was investigated by evaluating the microstructure of the laser-melted Al₂O₃ single tracks. When the laser scanning speed increased from 1 to 3.5 mm/s at a fixed laser power, the permeation of the molten Al₂O₃ into the surrounding porous paste was reduced, resulting in the improvement of the surface uniformity of the laser-melted Al₂O₃ tracks. Through optimizing the laser scanning strategy, a fully-dense Al₂O₃ layer with smooth surface was achieved. The phase composition and density of the laser-melted Al₂O₃ layers were evaluated to study their properties. The thickness of the dense Al₂O₃ layer varied from ～90 μm to ～120 μm periodically due to the line-by-line scanning of the Gaussian laser beam. In addition, the relationship between the melting thickness and the laser scanning speed was also investigated to further improve the controllability of the laser melting process. This direct laser melting of ceramic paste technology is promising for applications in ceramic AM, such as 3D printing of ceramic components and high-temperature ceramic welding.