Lithography-based ceramic manufacturing (LCM) – Viscosity and cleaning as two quality influencing steps in the process chain of printing green parts

The advantages of suspension based Additive Manufacturing (AM), e.g. the lithography-based ceramic manufacturing (LCM), are high structural resolution, and compared to other available AM techniques, the manufacturing of dense (>99%) ceramic components with high performance. This novel manufacturing technique permits innovative designs, new types of ceramic components, and offers a range of new applications; like micro reactors, catalyst supporting structures or heat exchangers, as well as cutting edge biomedical devices and personalized medical products. Some examples of personalized medical products are customised ceramic knee implants or custom spinal fusion implants. Producing these implants using LCM would allow product offerings not possible before. However, the LCM process chain includes several open points, which have to be solved, in order to get high quality end results. In this paper, the development of suspensions (curable slurries) based on different binders, and the procedures for cleaning printed parts are thoroughly considered.     

Linking flexural strength and strength-limiting flaws in additively manufactured alumina with print parameter variations

Advancing the adoption of lithography-based ceramic manufacturing (LCM) for structural applications requires understanding how processing parameters impact the printed material's flaw population and failure behavior. Exploratory studies on LCM alumina have mixed results regarding print parameter influence on strength and past fractography is insufficiently complete to guide process engineering for better performance. We perform four-point flexural strength characterization and comprehensive fractography of LCM alumina to evaluate the impact of processing parameters including orientation, layer height, and curing energy on strength-limiting defects. Characteristic strengths for fully dense, high-purity LCM alumina are near or above conventionally processed alumina within the manufacturable range of print parameters studied in this work. Conversely, the Weibull modulus varies greatly (m = 3.8–19.5) depending on print parameters. The dominant strength-controlling defects all have a printing-based origin, but vary between delamination, surface texture, and bubbles depending on print orientation. Lowest performance results from alignment of the print axis along the bar length, yielding a low Weibull modulus associated with weak delamination flaws that are not appreciably improved by varying print layer height or curing energy.     

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.     

Novel Three‐Dimensional Extrusion of Multilayered Ceramic/Camphene Mixture for Gradient Porous Ceramics

This study proposes an innovative way of creating porous ceramics with a unique gradient porous structure using three‐dimensional extrusion of a multilayered ceramic/camphene feed rod, denoted as “3D‐Ex_m”. This 3D‐Ex_m technique utilizes the wall slip phenomenon during the extrusion process, which can create a gradient core/shell structure with a gradual change in the core/shell thickness ratio. In addition, the microstructure of ceramic filaments can be tuned through the use of the camphene as a pore‐forming agent. Porous alumina ceramics produced using a bilayered feed rod comprised of the alumina/camphene mixtures with the relatively high (?_H = 40 vol%) and low ceramic contents (?_L = 10 vol%) showed a gradual change in porosity in the intermediate region between the relatively dense (porosity = ~3 vol%) and highly porous regions (porosity = ~85 vol%).     

Process development for additive manufacturing of functionally graded alumina toughened zirconia components intended for medical implant application

Additive manufacturing processes become more and more important in almost all areas, also for medical implant application. Medical implants made of ceramics are known for their excellent biological compatibility. For increasing the ingrowth of cells into an implant, a function-optimized structure with a defined porosity seems to be advantageous. Additive manufacturing is suited to realize defined and filigree structures. Lithography-based ceramic manufacturing (LCM), for example, has a high resolution, but still a limited material portfolio. Alumina toughened zirconia is a very interesting material for medical applications, but not commercially available for LCM. In this study, the development of an ATZ suspension usable in the LCM process with a bulk material density higher than 99% after sintering is presented. Furthermore, printing of lattice test structures for the evaluation of accuracy in dependence of printing dimension is demonstrated.     

Charge trapping characteristics of alumina based ceramics

The space charge dynamics is very important for electrical breakdown of alumina based ceramics. In this paper, the charge trapping/detrapping characteristics of alumina based ceramics were studied by means of isothermal surface potential decay (ISPD) method. For alumina and zirconia toughened alumina (ZTA) ceramic samples, the ISPD curves charged by corona discharge as well as microstructure characterization were carried out. For the first time, crossover phenomenon and hollow shaped potential profile were observed and reported in alumina based ceramics, indicating a surface potential decay process dominated by charge injection and volume conduction affected by the trap states in materials. In addition, the comparative trapping characteristics were evaluated based on a charge detrapping controlled decay model. The correlation between trap distribution and microstructure of alumina based ceramics was investigated. It was proposed that different charge trapping characteristics of alumina based ceramic samples was caused by varied shallow trap density of grain boundary.     

Three‐dimensional Ceramic/Camphene‐based Coextrusion for Unidirectionally Macrochanneled Alumina Ceramics with Controlled Porous Walls

We report the utility of three‐dimensional ceramic/camphene‐based coextrusion, newly developed in this study, for the production of unidirectionally macrochanneled alumina ceramics with three‐dimensionally interconnected porous alumina walls. In this technique, a continuous ceramic/camphene filament with a diameter of 1 mm, comprised of a pure camphene core and a frozen alumina/camphene shell, was produced by the coextrusion process and then deposited in a layer‐by‐layer sequence using a computer‐controlled 3‐axis moving table. Unidirectionally aligned macrochannels (~400 μm in diameter) and three‐dimensionally interconnected pores (several tens of micrometers in size) in the alumina walls were created by removing the camphene core and the camphene dendrites formed in the alumina/camphene region, respectively. The sample showed much higher compressive strength in the macrochannel direction than in the perpendicular direction. In addition, the compressive strength of the sample could increase with an increase in initial alumina content owing to a decrease in the total porosity.     

Stabilization of tricalcium phosphate slurries against sedimentation for stereolithographic additive manufacturing and influence on the final mechanical properties

Ceramic parts manufactured by lithography‐based ceramic manufacturing (LCM) excel in resolution and surface quality. The material for LCM is a photosensitive ceramic particle‐filled slurry which needs to have homogeneous properties over time and during each processing step. The goal of this study was to use “mechanical” stabilization for a tricalcium phosphate‐filled slurry done by increasing slurry viscosity, solids loading, or inducing thixotropic behavior. The modified slurries were compared with a nonstable reference slurry. While all methods lead to increased storage stability, only the stabilized slurry with 0.5 wt% fumed silica is stable during the printing process.     

Mechanical characterization of parts produced by ceramic on‐demand extrusion process

Ceramic On‐Demand Extrusion (CODE) is an additive manufacturing process recently developed to produce dense three‐dimensional ceramic components. In this paper, the properties of parts produced using this freeform extrusion fabrication process are described. High solids loading (~60 vol%) alumina paste was prepared to fabricate parts and standard test methods were employed to examine their properties including the density, strength, Young's modulus, Weibull modulus, toughness, and hardness. Microstructural evaluation was also performed to measure the grain size and critical flaw size. The results indicate that the properties of parts surpass most other ceramic additive manufacturing processes and match conventional fabrication techniques.     

Binder-free additive manufacturing of ceramics using hydrothermal-assisted jet fusion

Ceramic additive manufacturing (AM) typically uses a high fraction of organic binders to form pre-sintered green parts that require a post de-binding process to remove. The de-binding process inevitably results in severe gas expansion and residual chars, leading to structural defects, accumulated stress, and compromised material properties in the final parts. Here we report a binder-free additive manufacturing process named hydrothermal-assisted jet fusion (HJF) that utilizes a hydrothermal method to create geometrically and compositionally complex ceramics under mild temperatures. The HJF process employs a selectively deposited volatile dissolving ink, high pressure, and mild heat to strategically fuse a ceramic powder bed into complex geometries. Compared to traditional AM methods for ceramics, the HJF process eliminates the need for organic binders in green part fabrication and offers the potential to directly co-print ceramics with other dissimilar materials, such as polymers and metals, enabling the development of novel multi-functional ceramic composites.     

Silica strengthened alumina ceramic cores prepared by 3D printing

Ceramic cores based on alumina and silica are important in the manufacturing of hollow blades. However, obtaining good properties and precision is still challenging. In this research, alumina-based ceramics cores were obtained by 3D printing technology, and the effects of silica contents on the mechanical properties of the as-obtained alumina ceramic cores were evaluated. The results showed significant improvements in flexural strengths of the ceramics from 13.3 MPa to 46.3 MPa at silica contents from 0 wt% to 30 wt% due to formation of mullite phase (Al₆Si₂O₁₃). By contrast, the flexural strengths declined as silica content further increased due to the generation of massive liquid phase. Also, porous structures and cracks were observed by scanning electron microscopy due to the removal of cured photosensitive resin and the mullitization reaction between alumina and silica, respectively. The manufacturing process of hollow blades required ceramic cores with flexural strengths greater than 20 MPa to resist the strike of metal liquid, as well as open porosity above 20 % to provide space for alkali liquor to dissolve the ceramic cores. As a result, 10 wt% silica was determined as the optimal value to yield ceramics with improved properties in terms of flexural strength (35.6 MPa) and open porosity (47.5 %), thereby satisfy the application requirement for the fabrication of ceramic cores.     

Thermoplastic 3D Printing—An Additive Manufacturing Method for Producing Dense Ceramics

In our new approach—thermoplastic 3D printing—a high‐filled ceramic suspension based on thermoplastic binder systems is used to produce dense ceramic components by additive manufacturing. Alumina (67 vol%) and zirconia (45 vol%) suspensions were prepared by ball milling at a temperature of about 100°C to adjust a low viscosity. After the preparation the suspension solidified at cooling. For the sintered samples (alumina at 1600°C, zirconia at 1500°C), a density of about 99% and higher was obtained. FESEM studies of the samples' cross section showed a homogenous microstructure and a very good bond between the single printed layers.     

Fractography of zirconia-specimens made using additive manufacturing (LCM) technology

A relatively new method to manufacture complex ceramic prototypes and components is additive manufacturing (AM). With the LCM (Lithography-based Ceramic Manufacturing)-technology the green body is manufactured layer-by-layer using selective curing of light-sensitive ceramic slurry by a mask exposure process. After curing by blue light the component is removed from the building platform and the green body is sintered to a ceramic component.The aim of this work is to investigate the influence of processing and layer architecture on the mechanical properties of an Yttria-stabilized zirconia ceramic. Strength tests were performed by uniaxial bending tests and by biaxial Ball-on-three Balls (B3B) tests. To identify typical fracture initiating flaws a systematic fractographic investigation was performed on different batches of Ball-on-three Balls-test and bending test specimens, respectively.Through additional investigations it was found that hardness and fracture toughness were independent on the layer architecture. But an extensive fractographic analysis showed that the strength was limited by flaws, which were introduced by processing and handling. If these flaws can be avoided by optimisation of the process the strength should be equal to that of conventional processed ceramics.     

Cold sintering process for ZrO2‐based ceramics: significantly enhanced densification evolution in yttria‐doped ZrO2

We recently developed a novel technique of cold sintering process (CSP) to obtain dense ceramics at extraordinarily low temperatures. In this communication, we demonstrate the feasibility of applying CSP to zirconia‐based ceramics. As exemplified by 3Y‐TZP ceramics, a significantly enhanced densification evolution is observed. Water is simply utilized as a sintering aid to assist the ceramic densification under an applied external pressure. The low‐temperature advantage of CSP outstands in contrast to the densification curves compiled from other sintering techniques. A gradual monoclinic‐to‐tetragonal phase transformation is revealed in correspondence to the densification development, as well as contributes to the mechanical hardness evolution. A Vickers Hardness reaches ~10.5 GPa after annealing the cold‐sintered ceramics at 1100°C, which is comparable to those values reported in the previous studies at higher sintering temperatures. Such a sintering methodology is of significant importance as it provides a roadmap for cost‐effective processing of zirconia‐based ceramics and composites that enable broad practical applications.     

Starch consolidation of alumina: Fabrication and mechanical properties

A consolidation technique based on gelling property of starch was used to prepare alumina ceramics. Slurry containing alumina powder, dispersant and small amount of starch (2–3.5 wt.% of powder weight) was cast into a nonporous mould and heated to gelation temperature to produce a rigid green body. A defect free green body was obtained and the total linear shrinkage during drying was 2–3% and the green density observed was 64% of theoretical value. After complete drying, ceramic compacts were sintered without debinding operation. Sintered density of 99.4% was achieved after sintering at 1600 °C for 2 h. Flexural strength values of dried and sintered alumina were ～10 and 247 MPa, respectively. The sintered ceramics showed an extremely dense microstructure.     

Additive Manufacturing of Ceramics: Issues,Potentialities, and Opportunities

Additive manufacturing (AM) is a technology which has the potential not only to change the way of conventional industrial manufacturing processes, adding material instead of subtracting, but also to create entirely new production and business strategies. Since about three decades, AM technologies have been used to fabricate prototypes or models mostly from polymeric or metallic materials. Recently, products have been introduced into the market that cannot be produced in another way than additively. Ceramic materials are, however, not easy to process by AM technologies, as their processing requirements (in terms of feedstock and/or sintering) are very challenging. On the other hand, it can be expected that AM technologies, once successful, will have an extraordinary impact on the industrial production of ceramic components and, moreover, will open for ceramics new uses and new markets.     

Non-destructive optical second harmonic generation imaging of 3D printed aluminum nitride ceramics

Aluminum nitride (AlN) ceramics have attracted broad interest due to their potential applications in electronics. Additive manufacturing of ceramic components are rapidly advancing, could provide a new way of manufacturing over conventional methods. Non-destructive testing of 3D printed ceramic samples is an important step for quality control in manufacturing. Here we show that AlN ceramics show strong optical second harmonic generation (SHG) signals due to its wurtzite crystal structure. Microscopic SHG imaging can also examine the microscopy domains in AlN ceramics with submicron spatial resolution. This technique has the potential to be applied as a non-destructive testing method for examining 3D printed AlN ceramic components.     

Gelcasting of alumina foams using agarose solutions

Aqueous gelcasting of dense or cellular ceramics by using biopolymers as gel-formers, instead of monomers, is a promising technology mainly in terms of environmental aspects. The main difficulty of using biopolymer solutions in processing of cellular ceramics by foaming method is their high viscosity, which prevents the foaming capacity of the ceramic suspension. In this work, the procedure for preparing concentrated agarose solutions (4 wt.%) by dissolving under overpressure conditions was evaluated for the gelcasting of alumina foams, and the rheological behaviour of alumina suspensions containing agarose was studied. The viscosity of the gelling solution obtained under overpressure conditions was lower than that prepared by simply heating at 90 °C, thus providing high foaming capacity of the alumina suspensions and consequently manufacturing of highly porous ceramics (86–90%). The microstructure of alumina foams was typically composed of approximately spherical cells interconnected by circular windows. The use of different agarose concentrations in alumina suspensions effected the rheological conditions, which resulted in changes in the pore and window sizes of the resulting ceramics. Depending on agarose concentration (0.50–1.0 wt.% on a dry solids basis) in the starting (35 vol.%) alumina slurry, the mean pore size ranged from 529 to 375 μm, while the mean window size varied from 113 to 77 μm.     

Processing of dense bioinspired ceramics with deliberate microstructure

The architectures of biological hard materials reveal finely tailored complex assemblies of mineral crystals. Numerous recent studies associate the design of these local assemblies with impressive macroscopic response. Reproducing such exquisite control in technical ceramics conflicts with commonly used processing methods. Here, we circumvent this issue by combining the recently developed Magnetically Assisted Slip Casting (MASC) technique with the well-established process of templated grain growth (TGG). MASC enables the local control over the orientation of platelets dispersed among smaller isotropic particles. After a high-temperature pressureless heat treatment, the grains of the final ceramic follow the same orientation as of the initial platelets. This combination allows us to produce 95% dense alumina part with a grain orientation following any deliberate orientation. We successfully fabricated microstructures inspired from biological materials with ceramics that present periodically varying patterns with a programmable pitch down to a few tens of micrometers. The periodically textured dense ceramics exhibit matching variation of local hardness, confirming the capacity of the process to tailor local properties. This unique micrometer scale control over the local mechanical properties could be applied to adapt ceramic structures to complex loads using this inexpensive and scalable process. In systems where functional properties also depend on anisotropic grain orientation, the principle presented here could enable the creation of new multifunctional ceramics.