Citation: Bódis, E.; Jakab, M.; Bán, K.; Károly, Z. Functionally Graded Al 2 O 3 –CTZ Ceramics Fabricated by Spark Plasma Sintering. Materials 2022, 15, 1860. https://doi.org/ 10.3390/ma15051860 Academic Editor: Kenji Kaneko Received: 7 February 2022 Accepted: 28 February 2022 Published: 2 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Functionally Graded Al 2 O 3 –CTZ Ceramics Fabricated by Spark Plasma Sintering Eszter Bódis 1, * , Miklós Jakab 2 , KrisztiánBán 3 and ZoltánKároly 1 1 Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Magyar Tudósok krt. 2, H-1117 Budapest, Hungary; [email protected] 2 Research Centre of Engineering Sciences, Department of Materials Sciences and Engineering, University of Pannonia, P.O. Box 158, H-8201 Veszprém, Hungary; [email protected] 3 Department of Automobiles Technologies, Faculty of Transportation Engineering and Vehicle Engineering, Budapest University of Technology and Economics, M˝ uegyetem rkp. 3, H-1111 Budapest, Hungary; [email protected] * Correspondence: [email protected] Abstract: We studied the fabrication of functionally graded Al 2 O 3 –CeO 2 -stabilized-ZrO 2 (CTZ) ceramics by spark plasma sintering. The ceramic composite exhibits a gradual change in terms of composition and porosity in the axial direction. The composition gradient was created by layering starting powders with different Al 2 O 3 to CTZ ratios, whereas the porosity gradient was established with a large temperature difference, which was induced by an asymmetric graphite tool configuration during sintering. SEM investigations confirmed the development of a porosity gradient from the top toward the bottom side of the Al 2 O 3 –CTZ ceramic and the relative pore volume distributed in a wide range from 0.02 to 100 μm for the samples sintered in asymmetric configuration (ASY), while for the reference samples (STD), the size of pores was limited in the nanometer scale. The microhardness test exhibited a gradual change along the axis of the ASY samples, reaching 10 GPa difference between the two opposite sides of the Al 2 O 3 –CTZ ceramics without any sign of delamination or cracks between the layers. The flexural strength of the samples for both series showed an increasing tendency with higher sintering temperatures. However, the ASY samples achieved higher strength due to their lower total porosity and the newly formed elongated CeAl 11 O 18 particles. Keywords: Al 2 O 3 –ZrO 2 composite; functionally graded material; porosity 1. Introduction Functionally graded materials (FGM) have gained increasing attention over recent decades due to the diverse fields of potential applications such as in engineering [1] and biomedicine [2,3]. By definition, FGMs [4] are characterized by a gradual transition of the composition and/or physical properties of the material along the bulk in one or more directions. It results in bulky materials having different behaviors in the various parts of the material tailored to the application. FGMs can be classified mainly into continuous and step-graded types depending on the interface conditions, but definite interfaces cannot be observed in either case. In this regard, the local concentration of stress, which may induce delamination within the material, can be avoided [5]. Plenty of examples can be found for gradient structures in nature including the human body. Because of this, intensive research is carried out to mimic such structures and develop functionally graded ceramics appropriate for various biomedical applications (e.g., bones and teeth). Currently, there are numerous potential materials for biomedical applications, including hydroxyapatite (HAp), bioglass [6,7], alumina, and zirconia [2]. However, all of them suffer from some shortcomings when used as a standalone monolithic material. Hap and bioglass have very poor mechanical properties, which limit their use in replacing bones. From that point of view, Al 2 O 3 and ZrO 2 provide potential benefits because of their Materials 2022, 15, 1860. https://doi.org/10.3390/ma15051860 https://www.mdpi.com/journal/materials
Functionally Graded Al2O3–CTZ Ceramics Fabricated by Spark Plasma Sintering
May 29, 2023
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