Porous Alumina Ceramics with Multimodal Pore Size Distributions

Zirconia-Toughened Alumina (ZTA) Nanoceramics with a Gradient Microstructure: A Comparative Study of ZTA Ceramics with Fibrous and Granular Morphology

ZrO2-toughened Al2O3 (ZTA) ceramic composites with a porosity gradient and with improved mechanical properties have a wide range of possible applications. We fabricated nanofibrous and nanogranular Y-ZTA and Ce-ZTA composites with a gradient microstructure by creating a temperature gradient during SPS sintering, with the use of asymmetric graphite tool arrangement (ASY). In this study, we examined the morphology effect of the starting materials on the sintering process and on the final microstructure, as well as the mechanical properties of the composites. A large temperature difference was established for both the granular and fibrous samples fabricated in the ASY configuration, which resulted in gradient porosity along the ceramics bodies: the upper part of the ceramics showed a highly porous fine microstructure, while the opposite side was highly densified. The final microstructure of the composites can be tailored by varying the morphology of the starting ceramics or the graphite configuration. A highly porous skeleton-like structure was formed by sintering fibres in the ASY configuration, whereas the granular precursors resulted in a much less porous composite. The microstructure affected the mechanical properties of the composite. Improved hardness and more than 50% higher compression strength were obtained for the granular Ce-ZTA samples as compared to the fibrous sample. Gradient porosity with fibrous or granular morphology promotes the penetration of bioactive nanosized hydroxyapatite (HAp) into the pore structure. Fibrous ZTA absorbs HAp more effectively due to its higher porosity as well as its bimodal pore structure.

Aluminium Matrix Composite Materials Reinforced by 3D-Printed Ceramic Preforms

This article employed the fused deposition modelling (FDM) method and gas-pressure infiltration to manufacture alumina/AlSi12 composites. Porous ceramic skeletons were prepared by FDM 3D printing of two different alumina powder-filed filaments. The organic component was removed using a combination of solvent and heat debinding, and the materials were then sintered at 1500 °C to complete the process. Thermogravimetric tests and DTA analysis were performed to develop an appropriate degradation and sintering program. Manufactured skeletons were subjected to microstructure analysis, porosity analysis, and bending test. The sintering process produced porous alumina ceramic samples with no residual carbon content. Open porosity could occur due to the binder's degradation. Liquid metal was infiltrated into the ceramic, efficiently filling any open pores and forming a three-dimensional network of the aluminium phase. The microstructure and characteristics of the fabricated materials were investigated using high-resolution scanning electron microscopy, computer tomography, hardness testing, and bending strength testing. The developed composite materials are characterized by the required structure-low porosity and homogenous distribution of the reinforcing phase, better mechanical properties than their matrix and more than twice as high hardness. Hence, the developed innovative technology of their manufacturing can be used in practice.

TEM Study of the Microstructure of an Alumina/Al Composite Prepared by Gas-Pressure Infiltration

Ceramic injection moulding and gas-pressure infiltration were employed for the manufacturing of alumina/AlSi₁₀Mg composites. Porous ceramic preforms were prepared by mixing alumina powder with a multi-binder system and injection moulding the powder polymer slurry. Then, the organic part was removed through a combination of solvent and thermal debinding, and, finally, the materials were sintered at different temperatures. Degrading the binder enabled open canals to form. The sintering process created a porous ceramic material consisting of alumina without any residual carbon content. During infiltration, the liquid metal filled the empty spaces (pores) effectively and formed a three-dimensional network of metal in the ceramic. The microstructure and properties of the manufactured materials were examined using high-resolution transmission electron microscopy, porosimetry, and bending strength testing. Microscopy observations showed that the fabricated composite materials are characterised by a percolation type of microstructure and a lack of unfilled pores. The research confirmed the diversified nature of the connection at the particle-matrix interface. It was observed that the interphase boundary was characterised by the lack of a transition zone between the components or a continuous transition zone, with the thickness not exceeding 30 nm. Thanks to their increased mechanical properties and low density, the obtained composites could be used in the automotive industry as a material for small piston rings and rods, connecting rods, or even gears.

Strategies to Control In Vitro Degradation of Mg Scaffolds Processed by Powder Metallurgy

Magnesium scaffolds are biodegradable, biocompatible, bioactive porous scaffolds, which find applications within tissue engineering. The presence of porosity increases surface area and enhances cell proliferation and tissue ingrowth. These characteristics make Mg scaffolds key materials to enhance the healing processes of tissues such as cartilage and bone. However, along with the increment of porosity, the corrosion of magnesium within a physiological environment occurs faster. It is, therefore, necessary to control the degradation rate of Mg scaffolds in order to maintain their mechanical properties during the healing process. Several studies have been performed to increase Mg scaffolds’ corrosion resistance. The different approaches include the modification of the Mg surface by conversion coatings or deposited coatings. The nature of the coatings varies from ceramics such as hydroxyapatite and calcium phosphates to polymers such as polycaprolactone or gelatin. In this work, we propose a novel approach to generating a protective bilayer coating on the Mg scaffold surface composed of a first layer of naturally occurring Mg corrosion products (hydroxide and phosphates) and a second layer of a homogeneous and biocompatible coating of polylactic acid. The Mg scaffolds were fabricated from Mg powder by means of powder metallurgy using ammonium bicarbonate as a space holder. The size and amount of porosity were controlled using different size distributions of space holders. We addressed the influence of scaffold pore size on the conversion and deposition processes and how the coating process influences the in vitro degradation of the scaffolds.

Effect of the Zirconia Particle Size on the Compressive Strength of Reticulated Porous Zirconia-Toughened Alumina

Reticulated porous ceramics have attracted researchers owing to the separation and collection properties of porous materials and the combined high thermal resistance and chemical stability of ceramics. Among various kinds of reticulated porous ceramics, we investigated the feasibility of using reticulated porous Zirconia-toughened Alumina as applications such as dielectric barriers, insulators, and filters with acceptable properties. An acceptable range of the compressive strength for reticulated porous ZTA applications is approximately 1 MPa. However, when the pore density of the reticulated porous ZTA specimen prepared using coarse zirconia was 60, maximum compressive strength of 1.63 MPa was obtained. To enhance the compressive strength of reticulated porous ZTA specimens, rheological control of the ZTA slurry is most important by optimizing the viscosity of the ZTA slurry, and the composition (average particle size, solid loading, organic binder, and thickener) of the ZTA slurry was controlled. The optimized processing conditions to enhance the compressive strength of reticulated porous ZTA specimens were determined. Consequently, we enhanced the compressive strength of the reticulated porous ZTA specimens from 0.37 MPa to 3.11 MPa by optimizing the ZTA slurry when the solid loading content, the pore density, the sintering temperature, the amount of PVA, and the amount of thickener were 66 wt.%, 60 PPI, 1600 °C, 2 wt.%, and 0.15 wt.%, respectively.