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Advancements in the fields of biocompatible materials, manufacturing processes, computational methods and medicine have led to the emergence of a new field: micro-scale scaffolds for bone replacement and regeneration. Yet most such scaffolds produced today are characterized by very basic geometry, and their microstructure differs greatly from that of the actual tissue they are intended to replace. In this paper, we propose a novel approach for generating micro-scale scaffolds based on processing actual micro-CT images and then reconstructing a highly accurate geometrical model. This model is manufactured by means of a state-of-the-art 3D additive manufacturing process from biocompatible materials. At the micro-scale level, these scaffolds are very similar to the original tissue, thus interfacing better with the surrounding tissue and facilitating more efficient rehabilitation for the patient. Moreover, the approach facilitates the design and manufacture of patient-specific scaffolds which can copy patients exact structural and mechanical characteristics, taking into account their physical condition and medical history. By means of multi-resolution volumetric modeling methods, scaffold porosity can also be adapted according to specific mechanical requirements. The process of designing and manufacturing micro-scale scaffolds involves five major stages: (a) building a volumetric multi-resolution model from micro-CT images; (b) generation of surface geometric model in STL format; (c) additive manufacturing of the scaffold; (d) scaffold shape verification relative to the geometric design; and (e) verification of mechanical properties through finite element analysis. In this research, all the proposed stages of the approach were tested. The input included micro-CT scans of porous ceramic structure, which is quite similar to commercial porous scaffolds. The results show that the proposed method is feasible for design and manufacture of micro-scale scaffolds.
Highly porous (>60% open porosity) glass–ceramic scaffolds with remarkable mechanical properties (compression strength of ~15 MPa) were produced by indirect 3D printing. Precursor glass powders were printed into 3D ordered structures and then heat treated to sinter and develop crystalline phases. The final glass–ceramic contained a β-spodumene solid solution together with a secondary phase of lithium disilicate.
The precision of the printed geometry and the density of the struts in the scaffold depended on several processing parameters (e.g. powder size and flowability, layer thickness) and were improved by increasing the binder saturation and drying time. Two types of powders with different particle size distribution (PSD) and flowability were used. Powders with a larger PSD, could be processed within a wider range of printing parameters due to their good flowability; however, the printing precision and the struts density were lower compared to the scaffolds printed using the powder in a smaller average PSD.
Ceramic parts possessing an ordered porosity were produced for the first time by powder-based three-dimensional printing of a preceramic polymer followed by pyrolysis in an inert atmosphere. The main parameters involved in the process were investigated, and the precision of the printed and ceramized parts was assessed by means of scanning electron microscopy and micro computed tomography. The influence of two different printing solvents was investigated and the use of a mixture of 1-hexanol and hexylacetate in particular allowed the production of parts with a relative density of 80% both in the polymeric and in the ceramic state. The mixing of a cross-linking catalyst directly with the printing liquid greatly simplified the process, minimizing the necessity of preprocessing the starting powder. Three-dimensional printing of a preceramic polymer not containing any inert or active fillers was proved to be a feasible, convenient and precise process for the production of porous ceramic possessing a complex, ordered structure, such as stretch-dominated lattices.
Porous Si3N4 ceramics were prepared via partial nitridation and self-propagating high temperature synthesis (SHS) process. Raw Si and additive Y2O3 were mixed and molded under 10 MPa into a compact, the compact was partial nitridation at 1300 °C to form a porous Si/Si3N4, and then it was buried in a Si/Si3N4 bed for SHS to obtain porous Si3N4 with rod-like β-Si3N4 morphology. The processing combined the advantages of the nitridation of Si and SHS with low cost, low shrinkage and time saving. Porous Si3N4 with a porosity of 47%, a strength of 143 MPa were obtained by this method.
Tapes, cast by blade deposition of a lithium aluminosilicate glass slurry, were sintered using a YAG-fiber laser, with the aim of finding suitable parameters for an additive manufacturing process based on layer-wise slurry deposition and selective laser densification. The influence of the laser parameters (output power and scan velocity) on the sintering was evaluated, by scanning electron microscopy and by X-ray diffraction, on the basis of the quality of the processed layer. Well densified samples could be obtained only in a small window of values for the output power and the scan velocity. The measurement of the width of a set of single scanned lines allowed also to estimate the minimum resolution of the system along the layer plane.
The most successful additive manufacturing (AM) technologies are based on the layer-by-layer deposition of a flowable powder. Although considered as the third industrial revolution, one factor still limiting these processes to become completely autonomous is the often necessary build-up of support structures. Besides the prevention of lateral shifts of the part during the deposition of layers, the support assures quality and stability to the built process. The loose powder itself surrounding the built object, or so-called powder-bed, does not provide this sustenance in most existent technology available. Here we present a simple but effective and economical method for stabilizing the powder-bed, preventing distortions in the geometry with no need for support structures. This effect, achieved by applying an air flow through the powder-bed, is enabling an entirely autonomous generation of parts and is a major contribution to all powder-based additive manufacturing technologies. Moreover, it makes powder-based AM independent of gravitational forces, which will facilitate crafting items in space from a variety of powdery materials.
Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si3N4 and diameter of post-sintered ß-Si3N4 are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y2O3 as sintering additive, nano Y2O3 can promote the formation of small ß-Si3N4 nuclei, but the large amount of ß-Si3N4 (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine ß-Si3N4 grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the ß-Si3N4 grains is notable because of the pinning effect, while the effect of nano C on the refinement of the ß-Si3N4 grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.
Additive Manufacturing (AM) covers a wide range of processes, ranging from rapid prototyping technologies for polymers to directed energy deposition and powder bed fusion processes for metals and ceramics. In all cases, AM processes involve the layer-by-layer deposition of material from a digital file. Even though these processes may be known by a variety of commercial names, the general characteristics of the processes are similar, as are their impacts on different material systems.
The flexibility of the process provides unprecedented design freedom by allowing the direct fabrication of complex geometries with unique material combinations. However, the interactions between the high energy density sources (laser and electron beams) and the materials being deposited create complex processing conditions that have a significant impact on the material properties. With the wide ranging possibilities inherent in AM produced components, a fundamental understanding of the process/structure/property relationships across different material systems will allow for specific material properties to be obtained.
This Focus Issue of Journal of Materials Research is dedicated to the most recent advances in the characterization of processing/structure/property relationships in AM produced metallic, ceramic, and polymer systems. The breadth of different properties and behaviors across these different materials systems makes the characterization of AM materials extremely complex and a fertile subject for investigation. Many of these issues are addressed in this collection of papers. Such a wide range of unique research areas and the breadth of materials examined in this issue is evidence of the scope of materials issues in additive manufacturing and a glimpse into the future.
We are grateful to both the authors and reviewers of the many high-quality manuscripts submitted to this JMR Focus Issue on The Materials Science of Additive Manufacturing.