Alumina-based Ceramics Research Articles

Multi-scale structure and reinforcement mechanisms of multi-element composite ceramic coatings

Ceramic coating is of significant importance for improving metallic materials in terms of mechanical properties or durability, which is commonly encountered in the field of machinery, civil engineering, and aerospace, etc. However, ceramic materials optimization is a great challenge due to the complex multi-scale design from atomic to macro level. This paper investigates the multiscale characteristics of multi-element composite ceramic coating, including electronic properties, 3D pore structure, and engineering performances and gives their bottom-up connections, via first-principles calculations and multiscale experiments. The doping of Ti, Zr, and Ce in the alumina-based ceramic crystal improves the overlap of electronic clouds between oxygen and metal atoms, which modifies the atomic charges and enhances the ionic bonding. In terms of microstructure, it reveals the mechanisms of phase transformation toughening effect of ZrO2 and the grain refinement and grain boundary purification effects of CeO2, which facilitates the amelioration of pore structure and macro mechanical properties. It provides multiscale information on the phase stability of multi-element alumina-based ceramics, shedding light on the fundamental atomic level mechanisms that play a crucial role in customizable functional designs

Contact damage response of alumina-based layered architectures under different loading configurations

Hertzian contact damage upon spherical indentation is investigated in alumina-based ceramics designed with embedded textured compressive layers. The effect of the location of the embedded layer and the testing configuration (parallel or normal to the layer orientation) on the sub-surface damage evolution is explored. An acoustic emission system for crack detection and post-mortem cross sectioning are employed to evaluate the onset and extension of damage in the different samples. Distinct acoustic signals detected at different contact loads are correlated with the onset of the ring crack, cone crack propagation and quasi-plastic deformation within the textured layer. The distinct damage mechanisms are influenced by the location of the embedded textured compressive layer and the loading orientation with respect to the layers. The design of layered ceramics for contact applications should consider the loading orientation as well as the contact stress field with respect to the “protective” layer to enhance damage tolerance.

Efficient and Controllable Preparation of Super-Hydrophobic Alumina-Based Ceramics Coating on Aviation Al-Li Alloy Surface for Corrosion Resistance and Anti-Icing Behavior

Al-Li alloys have been widely applied in aircraft structural component and shell material. However, Al-Li alloys are prone to corrosion failure, which leads to a considerable safety risk in the aerospace field and greatly limits their industrial application. Herein, a simple, low-cost, and large-scale air-spraying technique is developed for the preparation of an alumina-based ceramics coating with enhanced corrosion resistance and anti-icing behavior. The results show that the static contact angle of the as-prepared coating is 157.2 ± 0.4°, and the rolling angle is only 9.8°, suggesting a super-hydrophobic surface. Meanwhile, the electrochemical corrosion potential of the coating is 70 mV higher than that of the substrate, and the corrosion current density of the coating also decreases by 1 order of magnitude, indicating a significantly improved corrosion resistance. In addition, the fabricated super-hydrophobic coating also shows excellent anti-pollution and anti-icing characteristics. This work provides positive guidance for expanding the application of hydrophobic coating in the aerospace industry, especially in some complex corrosion, icing, and pollution environments.

Effect of ball milling conditions on the microstructure for alumina-based ceramic with nano-silica additives

Abstract Alumina-based ceramics, known for their excellent high-temperature stability and tensile strength, are utilized across various sectors like metallurgy, aerospace, and chemicals as thermal insulators and reinforcements. The mechanochemical method, which involves mechanical actions like stirring and grinding to induce chemical reactions without solvents, offers a green, time-efficient approach with minimal pollution. This technique is pivotal for nanomaterials preparation. This study successfully crafts nano-silica-enhanced alumina-based ceramics via mechanochemical methods. The impact of mechanochemical processes on ceramic properties is assessed through particle size, N2 adsorption, and various microscopic analyses.

Removal of impurities for the synthesis of high-property alumina-spinel ceramic from secondary aluminum dross

Alumina-based ceramics are characterized by reliable mechanical strength and chemical stability. However, the high cost and shortage of raw materials have discouraged the growing market. Secondary aluminum dross (SAD) is a hazardous waste that consists of a variety of tightly coupled aluminum phases such as metal Al, Al2O3, AlN, and MgAl2O4. In this study, a novel process to retain Al while removing impurities from SAD was developed to obtain high-purity alumina-spinel powder, which was successfully synthesized to high-property alumina-spinel ceramic by pressureless sintering. First, up to 97.6% metal Al, 99.6% AlN, 98.6% Al4C3, and 99.5% Al2S3 were oxidized and transformed to α-Al2O3. Meanwhile, 98.7% fluorine was removed from SAD by oxidizing roasting, verifying the results of thermodynamic analysis. About, 83.8% of Ca, 65.8% of Fe, 67.3% of Cu, and 44.5% of Zn were removed separately from SAD by grinding–acid leaching due to the poor stability of alumina solid solution, and 86.09% of Al was retained in SAD powder in the form of α-Al2O3. Finally, the alumina-spinel ceramics sintered at 1300 °C showed the optimum properties: the density of 3.73 g/cm3, the porosity of 1.6%, the hardness of 1703.1 Hv, and the flexural strength of 138.5 MPa, respectively, which met the standard of 95 Al2O3 ceramic. These results demonstrated that the process could realize thoroughly the normalization of complex components in SAD and the preparation of high-property ceramic, substantially promoting the consumption and high-value utilization of SAD.

Templated grain growth in rapid sintered 3D-printed alumina ceramics

Textured microstructures in ceramics have gained interest due to their beneficial effect on structural and functional properties. For instance, morphologically textured grains in alumina-based ceramics can increase their damage tolerance. Texturing of alumina ceramics may be achieved through templated grain growth (TGG) occurring during sintering at high temperatures with prolonged dwell times (hours) and may be further enhanced by the presence of a liquid phase. In this study, we demonstrate the feasibility of texturing 3D-printed alumina ceramics within minutes by combining rapid heating (∼450 °C/min) and short dwell times (<20 min). The effect of sintering temperature and dwell time is investigated on samples with and without liquid phase. Experimental findings show highest texture degree for samples rapid sintered at 1600 °C for 16 min, opening the path for tailoring the microstructure of 3D-printed ceramics using pressure-less rapid sintering protocols.

Deposition and study of plasma sprayed Al2O3-TiO2 coatings on AZ31 magnesium alloy

In this study, Al2O3, TiO2 and Al2O3+ 3wt%TiO2 coatings were deposited on AZ31 Mg alloy substrate by plasma spraying. The coatings were structurally characterized by scanning electron microscopy and X-ray diffraction. Corrosion experiments were carried out in 3.5% NaCl solution. Adhesion tests were performed according to daimler benz VDI-3198 standard. A coating layer of approximately 70 microns in thickness was deposited. High plasma enthalpy caused phase transformations in alumina-based ceramics. As a result of electrochemical corrosion study, it was determined that the coatings increased the corrosion resistance of AZ31 Mg alloy. While the most corrosion resistant coating is Al2O3+ 3wt%TiO2, the weakest coating against corrosion is TiO2. The adhesion behavior of all coatings to the substrate was at an acceptable quality levels.

Indirect forming of alumina-based ceramics by selective laser sintering combined with sol infiltration process and performance study

Indirect forming of alumina-based ceramics by selective laser sintering combined with sol infiltration process and performance study

Alumina-based ceramics reinforced with calcium hexaaluminate

In this study, alumina ceramics containing calcium hexaaluminate are investigated. The specimens are obtained by axial pressing of granulated powders and pressureless sintering. Fracture toughness is tested using the indentation method. Microhardness is determined by the Vickers method. XRD and microstructural studies using a scanning electron microscope are carried out. It was found that the use of Ca(OH)2 leads to the formation of CaAl12O19 in sintered alumina ceramics. As the platelet content increases, the average length of the platelets increases from 2 to 3 µm. The width of the platelets is about 0.4 µm. Increasing the CaAl12O19 content decreases the size of Al2O3 grains. The average grain size of the alumina ceramics is 1.65 ± 0.02 µm. For the material containing 6 wt.% CaAl12O19 the average grain size is 0.95 ± 0.05 μm. An increase in the critical stress intensity factor for the formation of 6 wt.% CaAl12O19 in a material compared to alumina ceramics without additives has been established. The relative density of such a material is 95.3 ± 0.5 %, microhardness is 1800 ± 50 HV, fracture toughness is 5.2 ± 0.4 MPa ∙ m1 / 2. The increase in fracture toughness of composites containing CaAl12O19 platelets is due to the fracture of platelets, crack bridging and crack deflection.

The effect of particle size distribution on the microstructure and properties of Al2O3 ceramics formed by stereolithography

Stereolithography (SL) shows advantages for preparing alumina-based ceramics with complex structures. The effects of the particle size distribution, which strongly influence the sintering properties in ceramic SL, have not been systematically explored until now. Herein, the influence of the particle size distribution on SL-manufactured alumina ceramics was investigated, including bending strength at room temperature, post-sintering shrinkage, porosity, and microstructural morphology. Seven particle size distributions of alumina ceramics were studied (in μm/μm: 30/5, 20/3, 10/2, 5/2, 5/0.8, 3/0.5, and 2/0.3); a coarse:fine particle ratio of 6:4 was maintained. At the same sintering temperature, the degree of sintering was greater for finer particle sizes. The particle size distribution had a larger influence on flexural strength, porosity and shrinkage than sintering temperature when the particle size distribution difference reached 10-fold but was weaker for 10 μm/2 μm, 5 μm/2 μm and 5 μm/0.8 μm. The sintering shrinkage characteristics of cuboid samples with different particle sizes were studied. The use of coarse particles influenced the accuracy of small-scale samples. When the particle size was comparable to the sample width, such as 30 μm/5 μm and 5 mm, the width shrinkage was consistent with the height shrinkage. When the particle size was much smaller than the sample width, such as 2 μm/0.3 μm and 5 mm, the width shrinkage was consistent with the length shrinkage. The results of this study provide meaningful guidance for future research on applications of SL and precise control of alumina ceramics through particle gradation.

Development of a Transmission-Illumination-Based Crack Detection Method Using Translucent Tools for Testing of Thin-Walled Metal Sheets and Foils

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;In sheet metal testing, in-situ crack detection is either performed manually by purely visual inspection by the machine operator or automatically by a crack detection system. The automatic crack detection method, commonly integrated in sheet metal testing machines, evaluates the drawing force during forming. However, friction, vibration, and machine noise prevent reliable crack detection in thin sheets and foils. The same disturbance variables also prevent robust crack identification in thin sheets and foils by systems that analyze structure-borne sound. Crack detection systems that use reflected light methods, on the other hand, necessitate homogeneous illumination and are interfered by highly reflective as well as inhomogeneous sheet surfaces. In order to avoid the above-mentioned disadvantages of the currently existing crack detection methods, a procedure based on transmission-illumination was developed. Fundamental to this new development is the use of a translucent punch, which incorporates a light source and a camera connected to an image-processing unit. Building up on previous research performed with a 3D printed punch made from PVB, a punch made from alumina-based ceramics is investigated. The detection process works as follows: the translucent punch draws the sheet. As soon as a crack is formed, light penetrates the fracture and falls onto the image sensor of the camera. The image-processing unit detects the jump in illumination and triggers the immediate stop of the forming process. Forming tests were conducted to confirm the stable and operator independent performance of the newly developed detection process and the toughness of the translucent alumina-based ceramics punch.&lt;/div&gt;&lt;/div&gt;

Investigation of the Densification Behavior of Alumina during Spark Plasma Sintering.

The article presents the results of the investigation of the mechanism of the densification behavior of alumina-based ceramics during spark plasma sintering. The role of the heating rates and additives were investigated. The first (initial) stage of sintering was investigated by the Young–Cutler model. The second (intermediate) stage of sintering was investigated as a process of plastic deformation of a porous body under external pressure. It was shown that, at the initial stage, the formation of necks between the particles is controlled by grain boundary diffusion (the activation energy is Qb ≈ 20 kTm). At this stage, accommodation of the shape of the alumina particles is also occurring (an increase in the packing density). The accommodation process facilitates the shrinkage of the powder, which is reflected in a decrease in the effective activation energy of shrinkage at low heating rates (10 °C/min) to Qb ≈ 17 kTm. At heating rates exceeding 10 °C/min, the intensity of the processes of accommodation of alumina particles turns out to be much slower than the existing diffusion processes of growth of necks between the alumina particles. It was shown that the grain boundary sliding mechanism that occurs in the second stage of sintering can play a decisive role under conditions of spark plasma sintering with a high heating rate. The found value of the activation energy at the second stage of sintering is also close to the activation energy of grain–boundary diffusion of alumina (Qb ≈ 20 kTm). The influences of the second phase particles of MgO, TiO2, and ZrO2 on densification behavior of alumina-based ceramics were investigated. Since at the first stage of sintering the densification relates with the formation of necks between the particles of alumina, the additives (0.5% vol) have no noticeable effect on this process. It was also shown that the second phase particles which are located at the grain boundaries of alumina are not involved in the slip process during the second sintering stage. Analysis shows that additives act only in the final (third) stage of spark plasma sintering of alumina.

Microstructural development for optimum fracture toughness of Al2O3-3YSZ composites

ABSTRACT Alumina-based ceramics are widely used for making wear-resistant components and machine parts due to their high hardness, strength, wear-resistance, and high chemical and thermal stability. However, low fracture toughness (KIC ~ 3.0 MPa.m1/2) inhibits alumina’s usage where high toughness is a pre-requisite (e.g. ceramic engines). In this work, yttria-stabilised zirconia (3YSZ) was added to alumina and a two-step sintering process was used to make a composite with enhanced mechanical properties. The key properties – hardness, fracture toughness, and percentage theoretical density (Dth) – could be related to microstructural features. It was revealed that higher numbers of elongated grains formed with increasing additions of 3YSZ. The maximum fracture toughness and hardness achieved were 5.51 MPa.m1/2 and 10.96 GPa, respectively, for the Al2O3-30 wt% 3YSZ composite, believed mainly due to the high proportion of elongated grains. The best-performing sample had a density of 96.1% Dth and the average grain size was 0.35 µm. The thermal expansion coefficient was measured as 6.5 × 10−6 K−1.

Superplastic Deformation of Alumina Composites Reinforced with Carbon Nanofibers and with Graphene Oxide Sintered by SPS—Experimental Testing and Theoretical Interpretation

The superplastic behavior of alumina-based nanostructured ceramics (Al2O3) is an important issue in the world of materials. The main body of this paper is an analysis of the creep behavior of polycrystals, with grain boundary sliding as the main deformation mechanism at high temperatures. Concomitant accommodation of grain shapes to preserve spatial continuity has a comparatively small effect on the strain rate. The constitutive equations for small deformations, relating strain and strain rate, derived from two models for grain sliding, are compared with the experimental data with their respective uncertainties. The data follow from experiments on the plastic deformation of alumina composites reinforced, on the one hand by graphene oxide, and on the other hand by carbon nanofibers sintered by SPS. The results show good agreement between experiment and theory for these advanced ceramics, particularly for one of the assumed models. The values obtained of ξ2 for model A were in the interval 0.0002–0.1189, and for model B were in the interval 0.000001–0.0561. The values obtained of R2 for model A were in the interval 0.9122–0.9994, and for model B were in the interval 0.9586–0.9999. The threshold stress was between (3.05 · 10−15–25.68) MPa.

Contact damage tolerance of alumina-based layered ceramics with tailored microstructures.

This work demonstrates how to enhance contact damage resistance of alumina‐based ceramics combining tailored microstructures in a multilayer architecture. The multilayer system designed with textured alumina layers under compressive residual stresses embedded between alumina–zirconia layers was investigated under Hertzian contact loading and compared to the corresponding monolithic reference materials. Critical forces for crack initiation under spherical contact were detected through an acoustic emission system. Damage was assessed by combining cross‐section polishing and ion‐slicing techniques. It was found that a textured microstructure can accommodate the damage below the surface by shear‐driven, quasi‐plastic deformation instead of the classical Hertzian cone cracking observed in equiaxed alumina. In the multilayer system, a combination of both mechanisms, namely Hertzian cone cracking on the top (equiaxed) surface layer and quasi‐plastic deformation within the embedded textured layer, was identified. Further propagation of cone cracks at higher loads was hindered and/or deflected owed to the combined action of the textured microstructure and compressive residual stresses. These findings demonstrate the potential of embedding textured layers as a strategy to enhance the contact damage tolerance in alumina ceramics.

Effect of Addition of Previously-Synthesized Ce-TZP/Al2O3 Submicrometric Powder on the Properties of Al2O3-Based Ceramics

This work investigated the effect of a composite of tetragonal zirconia with alumina-reinforced Ceria (ATZ) previously synthesized, on the properties of alumina-based ceramics (Al2O3). Monolithic alumina powder and Al2O3 powder mixtures containing 5, 10, 15 and 20 wt.% of Ce-TZP/Al2O3 were processed by high energy milling, compacted and then sintered at 1600 °C - 2 h. Sintered pure alumina and composites were characterized by relative density, scanning electron microscopy, X-ray diffraction and surface roughness. Then, the elastic modulus, the Vickers hardness, the fracture toughness and the 4-point flexural strength were evaluated. The results indicated an increase in relative density as a function of the addition of ATZ, with values ​​between 94.3 ± 0.8% and 98.9 ± 0.7%. The observed microstructure after sintering showed tetragonal ZrO2 grains with average sizes of 0.6 μm well dispersed in the Al2O3 matrix, which presents average grain sizes of around 1.5 μm. The crystalline phases identified in the composites were ZrO2-tetragonal and Al2O3 hexagonal. The addition of the composite (ATZ) in the alumina matrix generates a gradual reduction in the elastic modulus (398 ± 15 GPa ~366 ± 34 GPa) and in the hardness (20.5 ± 1 GPa ~17.3 ± 0.7 GPa) of the sintered ceramics. On the other hand, the addition of this same composite (ATZ) in the alumina matrix considerably increases the materials fracture toughness, reaching values of approximately 6.7 ± 0.9 MPa.m1/2. The same trend was observed in the flexural strength results which ranged from 258 MPa (5wt.% Ce-TZP/Al2O3) to 316 MPa (20wt.% Ce,Y-TZP/Al2O3). The Ce-TZP reinforcement acts as a toughening agent of the Al2O3 matrix due to the coupled mechanisms of toughening by zirconia phase transformation, residual stresses due to the difference in thermal expansion of the crystalline phases and differences in microstructural morphologies.

Four-Point Bending Fatigue Behavior of Al2O3-ZrO2 Ceramic Biocomposites Using CeO2 as Dopant

This work investigated the effect of adding ceria-stabilized tetragonal zirconia (Ce-TZP) on the fatigue behavior of alumina-based ceramic composites. Alumina powder (control group) and mixtures containing 5 wt.% (group A) and 20 wt.% (group B) of a commercial m-ZrO2/Al2O3/CeO2 powder mixture were milled/homogenized, compacted, sintered at 1600°C-2h, and submitted to hydrothermal degradation. The samples were characterized by relative density, microstructure, crystalline phases, and static mechanical properties. The cyclic fatigue strength was determined using the modified staircase method in 4-point bending tests. The results indicate that adding the m-ZrO2/Al2O3/CeO2 powder mixture to the Al2O3-matrix increases the tetragonal-ZrO2 grains (Ce-TZP) content, presenting 2.9 wt.% of Ce-TZP and 11.9 wt.% of Ce-TZP for group A and group B, respectively. Furthermore, the addition of Ce-TZP improves densification (98.5% → 99.1%) with a slight reduction in hardness and modulus of elasticity and a significant KIC increase of the composite (KIC = 6.7 MPa.m1/2, group B) when compared to monolithic alumina (KIC=2.4 MPa.m1/2). The fatigue strength limit of the control group was around 100 MPa, while the composites (groups A and B) presented the values ​​of 279 MPa and 239 MPa, respectively. The results indicated that the incorporation of Ce-TZP significantly improves the fracture toughness of alumina-based ceramics. On the other hand, regarding the fatigue behavior, there was an increase in fatigue resistance in group A, resulting from the benefits of the t→m Ce-TZP grains transformation, which occurs during cyclic loading, producing a zone shielding that involves the tip of the crack, slowing its growth. The increase in the amount of Ce-TZP (group B) leads to an increase in the internal residual stresses between the phases due to anisotropy and difference in the thermal expansion coefficients, which accelerates the phase transformation and formation of microcracks at grain boundaries, reducing the fatigue strength of composites of group B.

A novel composite binder design for direct ink writing alumina-based ceramics with enhanced strength at low sintering temperature

Direct ink writing (DIW), an extrusion-based additive manufacturing method, has been proposed as a versatile process for preparing ceramics for structural and functional applications. However, controlling the strength of ceramics during processing remains a challenge. In this study, a novel composite binder system consisting of aqueous polyvinylpyrrolidone solution and Al(H2PO4)3 gel was adopted for the DIW of alumina ceramics. Throughout the process, the mechanical performance, dimensional shrinkage, phase compositions and microstructure of samples were evaluated. The results indicate that the composite binder improves green strength by generating hydrogen bonding, which considerably enhances the strength of alumina ceramics under low-temperature sintering due to the reaction between Al(H2PO4)3 gel and nanosized clay particles. This also promotes the high-temperature sintering performance of Al2O3-based ceramics. Overall, the current composite binder is anticipated to be promising for DIW of alumina ceramics, providing sufficient strength for post-processing and greatly improving strength after low-temperature sintering.

Characterization of a high temperature ceramics produced via two-step additive manufacturing

Selective laser sintering (SLS) is a prospective technology to manufacture ceramic of complex geometry. However it is limited by the poor sinterability of ceramics and steady cracks formation. The paper highlights a new strategy to fabricate ceramics produced via two-step procedure: metal-ceramic green body manufacturing by SLS with further conversion into the Al2O3-based ceramics by high-temperature oxidation. Via shrinkage test it was shown that the developed production process could be treated as non-shrinkable since the deviations in the samples linear dimensions are less than 0.3–0.4%. According to XRD results produced material is suitable for a long-term exploitation (>1000 h) at temperatures below 1250 °C and for a short-time (1–2 h) usage at 1300 °C. The thermal expansion of the material is close to that typical for alumina-based ceramics, it is constant up to 1000 °C with a following slight decrease up to 1400 °C due to microamounts of mullite formation.

Evolution on phase composition and properties of alumina-based ceramics fabricated from high-titania special-grade natural bauxite micropowder

In this study, Al2O3-based ceramics were fabricated with natural bauxite powder as the raw material. The phase compositions evolution behavior during the heat treatment and its influence on the properties of fabricated ceramics were investigated. With increasing heat treatment temperature, the sintering degree of high-titania special-grade bauxite became better. The samples showed decreased porosity, increased bulk density, Vickers hardness, fracture toughness and flexural strength. However, when the heat temperature increased to 1650 °C, decomposition of tieillite occurs in sample, leading to increased Al2O3 and TiO2 content in the liquid phase. Corundum and mullite grains with anisotropic growth appeared in the samples, leading to a decrease in the density, the Vickers hardness and flexural strength of samples decreased consequently. However, those anisotropic grains could prolong the crack propagation path and improve the fracture toughness of the material.