Al2O3 Ceramics Research Articles

Interfacial microstructure evolution and mechanical characterization of brazed Al2O3 joints with Ni‒Ti interlayer: An experimental and theoretical approach

Reliable brazing joints of Al2O3 ceramics were obtained using an active Ni‒Ti interlayer under vacuum conditions. The interfacial microstructure and mechanical properties of the joints were studied. The structural, electronic, and elastic properties of the primary interfacial reaction phases were determined using first–principles calculations. After brazing at 1320 °C for 30 min, Ni2Ti4O layer and columnar AlNi2Ti formed at the interface adjacent to the Al2O3 substrate. With increasing brazing temperature between 1300 °C and 1380 °C, Ni2Ti4O layer thickened gradually, and the AlNi2Ti became increasingly longer. As brazing temperature reached 1400 °C, TiO was formed at the interface, and the Ni2Ti4O content decreased significantly; moreover, bulk AlNi2Ti and TiNi3 were distributed in the brazing seam. The highest shear strength of 129 MPa was achieved when brazed at 1350 °C for 30 min. According to the first–principles calculations, Ni2Ti4O is more readily formed than AlNi2Ti, whereas AlNi2Ti exhibits greater stability than Ni2Ti4O. Both AlNi2Ti and Ni2Ti4O possess metallic bonds, contributing to the adhesion of the filler metal to the Al2O3 substrates. The calculated modulus and Poisson’s ratio indicate that both AlNi2Ti and Ni2Ti4O exhibit ductile characteristics, which assist in relieving residual stress within the joint.

Microwave thermal stress method cutting Al2O3 ceramics: modeling and experiments

Microwave thermal stress method cutting Al2O3 ceramics: modeling and experiments

The structural and mechanical properties of open-cell aluminum foams: Dependency on porosity, pore size, and ceramic particle addition

Aluminum foams are desired for lightweight, high-performance, and cost-effective materials, particularly in automotive, aerospace, and advanced power plants. Expansion of their applications depends on developing a detailed understanding of the structural and mechanical properties of aluminum foams. In this research, open-cell aluminum foams with 25.51–81.88 % porosity were manufactured through powder metallurgy using a space holder technique, with varying carbamide (urea) ratios (17–80 %) and particle sizes (1–1.4 mm, 1.7–2 mm, 2–2.4 mm). Three different ceramic particles, B4C, Al2O3, and SiC were used as reinforcement to improve the compression properties and energy absorption capacity of the foam materials. The study determined open and closed porosity ratios, spherical diameter, sphericity values, micropore sizes, mechanical properties, and energy absorption capacity of the foam samples both with and without ceramic additives. The results show that porosity ratio, pore size, and ceramic particle addition significantly affect aluminum foams' structural and mechanical properties, allowing for tailored properties for specific applications. It was observed that as porosity increased, compressive stress decreased, and the length of the plateau region and the shape change where densification began increased. However, there was no significant change in compressive stress and specific energy values with changing pore size. The optimal B4C addition was found to be 4 %, which significantly improved compressive strength to 3.75 MPa and specific energy to 4.24 MJ m−3.

Microstructural characterizations of metallized Al2O3 before/after surface treatment and Al2O3/Cu soldered joint

Joining Al2O3 ceramics to Cu heat pipes should be conducted at low temperatures since Cu heat pipes may fail at temperatures higher than 320 °C. Due to the poor wettability of low-temperature solder on Al2O3 ceramics, it is essential to metallize Al2O3 ceramics before joining Al2O3 to Cu with a low-temperature solder. Surface metallization of Al2O3 has been conducted using Ag-Cu-Ti active metal filler at 900 °C. Microanalysis indicates that an interfacial reaction occurs between the active metal filler and the Al2O3, leading to the formation of a Cu3Ti3O reaction layer. The metal layer on the surface of Al2O3 is primarily composed of Ag (s, s) (solid solution), Cu (s, s) and Ti2Cu. The polishing treatment of the surface metal layer in metallized Al2O3 results in a reduction of the oxide content and contaminants, which subsequently allows joining at low temperature. Low-temperature joining of metallized Al2O3 to Cu has been carried out using Sn-Ag-Cu solder at 280 °C. The joining area of the joint includes a Cu3Ti3O reaction layer, a metal layer and a solder layer. The solder layer mainly consists of Sn (s, s), Ag3Sn and Cu6Sn5. The Al2O3/Cu joint fractures in the solder layer after the shearing test, indicating that the strength of the solder is relatively low. However, the interfacial bonding between the metallized Al2O3, solder layer and Cu base material is satisfactory.

Structure, thermal and microwave dielectric properties of cold-sintered Li2MoO4-Al2O3 ceramic

Dielectric ceramics are essential components in communication systems that operate within the microwave frequency range. In high-density packages, dielectric substrates ceramics must possess high thermal conductivity to efficiently dissipate heat. However, achieving adequate thermal conductivity (κ) in ceramics sintered at low temperatures is challenging. In this study, we employed the cold sintering process (CSP) to fabricate Li2MoO4-x%Al2O3 (0≤x≤80, in volume) ceramics under 200 MPa pressure at 150 °C. The Li2MoO4-40%Al2O3 composite exhibited significantly enhanced κ of 5.4 W·m–1·K–1, an 80% increase compared to pure Li2MoO4 ceramic with κ of 3 W·m–1·K–1. At 40% Al2O3 content, the Li2MoO4-Al2O3 ceramic demonstrated notable microwave properties (ε ∼ 6.67, Q×f ∼ 17,846 GHz, τf ∼ −105 ×10–6/°C). Additionally, simulation of a microstrip patch antenna for 5 GHz applications using Li2MoO4-20%Al2O3 ceramic as dielectric substrate via Finite Element Simulation software showed excellent performance, with radiation efficiency exceeding 99% and low return loss (S11 < −30 dB) at both 4.9 GHz and 28.0 GHz center frequencies. These findings underscore the suitability of Li2MoO4- Al2O3 ceramics for microwave dielectric substrate. KeynotesMicrowave dielectric ceramic, Thermal conductivity, Cold sintering process, Antenna

Finite element modeling of thermal residual stresses in functionally graded aluminum-matrix composites using X-ray micro-computed tomography

Metal-ceramic composites by their nature have thermal residual stresses at the micro-level, which can compromise the integrity of structural elements made from these materials. The evaluation of thermal residual stresses is therefore of continuing research interest both experimentally and by modeling. In this study, two functionally graded aluminum alloy matrix composites, AlSi12/Al2O3 and AlSi12/SiC, each consisting of three composite layers with a stepwise gradient of ceramic content (10, 20, 30 vol%), were produced by powder metallurgy. Thermal residual stresses in the AlSi12 matrix and the ceramic reinforcement of the ungraded and graded composites were measured by neutron diffraction. Based on the X-ray micro-computed tomography (micro-XCT) images of the actual microstructure, a series of finite element models were developed to simulate the thermal residual stresses in the AlSi12 matrix and the reinforcing ceramics Al2O3 and SiC. The accuracy of the numerical predictions is high for all cases considered, with a difference of less than 5 % from the neutron diffraction measurements. It is shown numerically and validated by neutron diffraction data that the average residual stresses in the graded AlSi12/Al2O3 and AlSi12/SiC composites are lower than in the corresponding ungraded composites, which may be advantageous for engineering applications.

High dislocation-density silver particle interlayer for high-quality YSZ and Al2O3 joints

An Ag particle interlayer was adopted to join YSZ to Al2O3 ceramics in air directly. In comparison with the traditional Ag-CuO braze used in our previous work, the Ag particle interlayer prevented the formation of the brittle compound (CuAl2O4) layer along the ceramic interface. Sintering effect of particulate Ag and high-density dislocations in the interlayer are the major contributors to recrystallization of Ag and elemental interdiffusion. High Resolution Transmission Electron Microscope images confirmed that atomic bonding was achieved between the interlayer and Al2O3. Joining parameters, including temperatures and pressures, were optimized, resulting in the maximum shear strength of the joint (∼80 MPa) obtained at 950 °C/30 min with the external pressure of 0.9 MPa, which is 78 % higher than that of the joint brazed by Ag-CuO.

Spreading anomaly semantic segmentation and 3D reconstruction of binder jet additive manufacturing powder bed images

Variability in the inherently dynamic nature of additive manufacturing introduces imperfections that hinder the commercialization of new materials. Binder jetting produces ceramic and metallic parts, but low green densities and spreading anomalies reduce the predictability and processability of resulting geometries. In situ feedback presents a method for robust evaluation of spreading anomalies, reducing the number of required builds to refine processing parameters in a multivariate space. In this study, we report layer-wise powder bed semantic segmentation for the first time with a visually light ceramic powder, alumina, or Al2O3, leveraging an image analysis software to rapidly segment optical images acquired during the additive manufacturing process. Using preexisting image analysis tools allowed for rapid analysis of 316 stainless steel and alumina powders with small data sets by providing an accessible framework for implementing neural networks. Models trained on five build layers for each material to classify base powder, parts, streaking, short spreading, and bumps from recoater friction with testing categorical accuracies greater than 90%. Lower model performance accompanied the more subtle spreading features present in the white alumina compared to the darker steel. Applications of models to new builds demonstrated repeatability with the resulting models, and trends in classified pixels reflected corrections made to processing parameters. Through the development of robust analysis techniques and feedback for new materials, parameters can be corrected as builds progress.

Synthesis, mechanical characterization, and biocompatibility evaluation of Graphene-TiO2 reinforced alumina for biomedical applications

The study addresses the limited fracture toughness of Al2O3 ceramics by developing graphene-TiO2 reinforced alumina nanocomposites through pressing. Mechanical properties, including hardness, fracture toughness, and elasticity, were examined, alongside the structural characterization via XRD analysis and microstructure evaluation using SEM. Cytotoxicity and cell adhesion tests were conducted for in vitro biological evaluation. The inclusion of a graphene additive led to a nearly threefold increase in fracture toughness, reaching up to 8.161 MPa m1/2. Graphene doping induced a discernible shift to lower angles in the XRD pattern, revealing the presence of Al2O3, Al2TiO5, and TiO2 phases. Samples immersed in artificial body fluid for 14 days exhibited minimal ion release, with Titanium ion below 0.001 mg/L and aluminum ion below 0.05 mg/L, indicating a lack of significant ion release. The cytotoxic activity of the samples against NIH/3T3 cells was assessed through Alamar Blue analysis, revealing no significant change in cell viability after 48 h of exposure.

Study of microstructure and corrosion behavior of nano-Al2O3 coating layers on TiO2 substrate via polymeric method and microwave combustion

The study describes the successful development of a TiO2 ceramic substrate with a protective nano-Al2O3 coating using two different coating techniques: microwave combustion and polymeric methods. The coated ceramics demonstrate enhanced corrosion resistance compared to the uncoated substrate. The optimal TiO2 substrate was prepared by firing it at 1000 °C. This was done to give the desired physical properties of the TiO2 substrate for the coating procedures. Nano-Al2O3 powder was coated onto the surface of the TiO2 substrates. The TiO2 substrates with the Al2O3 coating were then calcined (heat-treated) at 800 and 1000 °C. The structures, morphology, phase composition, apparent porosity, bulk density, and compressive strength of the substrate and coated substrate were characterized. Upon firing at 1000 °C, it was discovered that the two phases of TiO2—rutile and anatase—combine in the substrate. Once the substrate has been coated with nano Al2O3 at 1000 °C, the anatase is transferred into rutile. When compared to the substrate, the coated substrate resulted in a decrease in porosity and an increase in strength. The efficiency of the ceramic metal nanoparticles Al2O3 as a good coating material to protect the TiO2 substrates against the effect of the corrosive medium 0.5 M solution of H2SO4 was measured by two methods: potentio-dynamic polarization (PDP) and the electrochemical impedance spectroscopy (EIS). The results indicated that the corrosion rate was decreased after the substrate coated with alumina from (67.71 to 16.30 C.R. mm/year) and the percentage of the inhibition efficiency recorded a high value reaching (78.56%). The surface morphology and composition after electrochemical measurements are investigated using SEM and EDX analysis. After conducting the corrosion tests and all the characterization, the results indicated that the coated TiO2 substrate prepared by the polymeric method at 800 °C displayed the best physical, mechanical, and corrosion-resistant behavior.

Effect of O2/Ar ratio on the microstructure and tribological properties of Y2O3 films by multi-arc ion plating

Multi-Arc Ion Plating (MAIP) technology was used to deposit yttrium oxide (Y2O3) ceramic thin films on Al2O3 ceramics and silicon wafers. The effects of varying the O-to-Ar gas flow ratio (Rc: 0.5, 1.0, 1.5, 2.0) and the annealing conditions on the phase composition, microstructure, mechanical properties, and tribological performance of the Y2O3 films were assessed. The films deposited at Rc values of 1.0 and 1.5 consisted of a single cubic phase structure. In contrast, films deposited at Rc values of 0.5 and 2.0 contained a small monoclinic phase and the cubic phase structure. The film deposited at an Rc value of 1.0 (oxygen flow rate of 200 sccm) exhibited a dense microstructure, the best mechanical properties (hardness of 14.2 ± 0.191 GPa), and the lowest friction coefficient (0.142 ± 0.003). Furthermore, the colour of the film changed from grey-black to white after annealing at 800 °C for 6 h. The oxygen vacancy of the thin film increased, with improving crystallinity, decreasing hardness and elastic modulus, and increasing grain size, friction coefficient and wear rate. The film exhibited the best tribological properties at 25 °C.

A way to control sintering deformation of Al2O3 components by gradient shrinkage

Sintering deformation is a great hindrance to the development of ceramic components with complex shapes. In this work, single-face coating method has been used to prepare the special shaped Al2O3 components by taking advantage of the difference in shrinkage between substrate and coating. By adjusting the content of SiO2 in the coating material which consisted of SiO2 and Al2O3 powder, the sintering deformation of Al2O3 ceramics coated with the above material would be controlled during the sintering process. The sintering behavior of Al2O3 green body with and without single-face coating were studied through visual in-situ tests combined with photographing technology. By comparing the images of specimens under different sintering temperatures (defined this method as the image relative method), the warped deformation and the shrinkage behavior of the above specimens were discussed. Moreover, the relationship between curvature radius and shrinkage rate was deduced. Results indicate that, due to the discrepancy in shrinkage of substrate and coating, the deformation of samples occurred in the heating process. And the shrinkage, the elasticity modulus and the ratio of the sectional coating area to substrate area play a major role in determining the shape of ceramics. The way to fabricate the shape controlled ceramic components by the gradient shrinkage is a novel and effective method, which can be used for the plate and shell products with complex shapes.

Microstructure and hardness of two-step reverse-polarity flash sintered high-purity Al2O3 ceramics

In this work, high-purity alumina ceramics were prepared by using two-step reverse polarity flash sintering (PR-FS). The effects of the time distribution between the first flash and the second flash on microstructure and hardness were studied. The results show that compared to the conventional DC flash sintered alumina, the RP–FS–ed samples exhibited more uniform-microstructure, finer grains, higher relative density, and higher hardness. The underlying mechanism for such more homogeneous microstructure was related to uniform distribution of point defects under effect of reverse polarity. Two-step reverse polarity flash sintering method offers a great potential in obtaining highly densified alumina ceramics with homogeneous microstructure.

Effect of silicon carbide content on microstructure, physical and mechanical properties in vat photopolymerization of alumina

AbstractVat photopolymerization (VPP) printing of ceramic parts offers advantages such as low cost, simple operation, and short fabrication cycles. However, drawbacks include low toughness and brittleness in the printed parts. This study explores enhancing the toughness and strength of alumina (Al2O3) ceramics by incorporating silicon carbide (SiC) particles as additives. The impact of varying SiC contents on the quality of VPP‐printed Al2O3 parts is examined, encompassing microstructure, physical properties, and mechanical properties. Results indicate that optimal SiC addition reduces Al2O3 ceramics' porosity, enhances crystalline quality, and boosts mechanical properties. Excessive SiC, however, diminishes these benefits. The most significant strengthening of Al2O3 parts occurred with a 1.5 wt.% SiC content, increasing bending strength and fracture toughness by 239.7% and 564.7%, respectively. This underscore SiC's positive role in enhancing the quality of VPP‐printed Al2O3 parts.

Understanding the effect of microstructure on the failure behavior of additively manufactured Al2O3 ceramics: 3D micromechanical modeling

Additively manufactured (AM) ceramics are gaining popularity due to improved flexibility in the design of customized structures. Microstructure-based finite element (FE) models were developed to study the strain-rate-dependent failure behavior of AM Al2O3 ceramics under uniaxial compression. Upon validation with experimental data, the model was leveraged to quantify the history of intergranular and transgranular failure mechanisms across microstructures. It was revealed that the intergranular mechanism plays a key role in the material strength across strain rates. Crystallographic orientations were found to remarkably decrease the material strength due to the earlier initiation and faster growth of the intergranular mechanism. The increase in porosity was found to amplify the transgranular mechanism resulting from a higher number of pores acting as crack nucleation sites, decreasing the material strength regardless of strain rate. The model reflected the existence of a threshold for grain boundary strength, beyond which further enhancements yield marginal improvements in material strength. Additionally, the effect of grain size on the initiation and evolution of failure mechanisms was studied and the corresponding limitations of the model were discussed. The current work correlates the microstructure of the material to its macroscale response through micromechanical models, informing the design of better-performing AM Al2O3 ceramics.

The Tribological and Adsorption Performance of Chlorophenyl Silicone Oil Using Different Ceramic Materials under High Temperature

With the development of technical requirements, the current challenges faced by bearing materials mainly revolve around high-temperature conditions and the trend towards material lightweighting. Full ceramic bearings are the new candidate due to their excellent properties. This article details the tribological and adsorption performance of chlorophenyl silicone oil (CPSO) as a high-temperature lubricant in ceramic tribological systems (ZrO2, Al2O3, and Si3N4). Among the three ceramic tribological systems, the lubrication performance can be ordered as Si3N4 &gt; Al2O3 &gt; ZrO2, as the wear rates of the ZrO2 and Al2O3 tribo-systems are almost 1135.67 and 283.33 times larger than that of the Si3N4 tribo-system, respectively. The observed results can be explained by the superior adsorption performance of CPSO on a Si3N4 ceramic surface, which was calculated by molecular dynamic simulation. The molecular dynamic simulation results show the adsorption energy of CPSO/Si3N4 is almost 54.09 and 61.18 times higher compared to that on ZrO2 and Al2O3 ceramics. These findings provide experimental and theoretical insights for understanding the lubrication performance of CPSO in a full ceramic tribo-system.

Experiment Evaluation on the Porosity of Plasma Coating from Ceramic Al2O3 – TiO2 Using the Metallography

Background: The objective of this paper is an experimental investigation of the metallographic porosity of plasma coating using standard materials from the ceramic system Al2O3-TiO2 on the substrate of steel SS400 at laboratory scale Lab at the Research Institute of Mechanical Engineering. Contribution: The main contribution of the research is the evaluation of the porosity of the coating under the influence of the main technological parameters since the porosity plays a significant role in the performance of the working surface using the built-in Image Pro-Analyzer software in the Axiovert 25 MAT microscope. Method: The spraying material is Al2O3-TiO2 powder. The input parameters are: spraying distance (Lp; current of plasma stream (Ip); powder feed flow (Gp and spraying rate (Vp). Results: The findings show that the average porosity of the Al2O3-TiO2 plasma coating is inversely decreasing in the increasing direction of the distance: spray in the range Lp = 100–200 mm, increase proportional to the increase of the current plasma flow rate in the range Ip = 400–600 A, provided that the powder feed flow Gp = 1.7 kg/h and the spraying rate Vp = 50 mm/min. Conclusion: The degree of influence of the three main technological parameters (Lp, Ip, and Gp) in the survey limit domain may be due to the selected Lp spray distance within the relatively wide recommended range of the plasma spraying equipment supplier. It is recommended to conduct the experimental planning of the L27 type to localize the optimum area.

Effect of Nb2O5 doping on low-temperature sintering of TiO2-Al2O3 ceramics

This study investigates the effects of Nb2O5 as a dopant in TiO2–Al2O3 ceramics to lower sintering temperature and enhance performance, including the physical, mechanical, and dielectric properties of the ceramics at sintering temperatures 1250 °C–1350 °C. Specifically, TiO2–Al2O3 ceramics doped with 1.5 wt% Nb2O5 and sintered at 1350 °C exhibit remarkable density 94 %, compressive strength 1372 MPa, and abrasion resistance 1.36 × 10−4 mm3/N·m, surpassing those of pure alumina ceramics. This improvement is attributed to enhanced grain boundary diffusion and strain energy resulting from Nb5+ cation substitution. The incorporation of Nb2O5 in TiO2–Al2O3 ceramics reduce sintering temperatures by 150–200 °C. The dielectric constant of TiO2–Al2O3 ceramics doped with 1.5 wt% Nb2O5 sintered at 1350 °C is found to be 13.4, with a dielectric loss of 2.56 × 10−2 and insulation resistance 7.3 × 1012 Ω. Nb2O5 is an effective additive for reducing sintering temperatures in Al2O3 based ceramic.

Vat Photopolymerization of Sepiolite Fiber and 316L Stainless Steel-Reinforced Alumina with Functionally Graded Structures.

Alumina (Al2O3) ceramics are widely used in electronics, machinery, healthcare, and other fields due to their excellent hardness and high temperature stability. However, their high brittleness limits further applications, such as artificial ceramic implants and highly flexible protective gear. To address the limitations of single-phase toughening in Al2O3 ceramics, some researchers have introduced a second phase to enhance these ceramics. However, introducing a single phase still limits the range of performance improvement. Therefore, this study explores the printing of Al2O3 ceramics by adding two different phases. Additionally, a new gradient printing technique is proposed to overcome the limitations of single material homogeneity, such as uniform performance and the presence of large residual stresses. Unlike traditional vat photopolymerization printing technology, this study stands out by generating green bodies with varying second-phase particle ratios across different layers. This study investigated the effects of different contents of sepiolite fiber (SF) and 316L stainless steel (SS) on various aspects of microstructure, phase composition, physical properties, and mechanical properties of gradient-printed Al2O3. The experimental results demonstrate that compared to Al2O3 parts without added SF and 316L SS, the inclusion of these materials can significantly reduce porosity and water absorption, resulting in a denser structure. In addition, the substantial improvements, with an increase of 394.4% in flexural strength and an increase of 316.7% in toughness, of the Al2O3 components enhanced by incorporating SF and 316L SS have been obtained.

Low‐temperature sintering performance study of CCTO–Mn–Co series black Al2O3 ceramics by the one‐step method

AbstractTo confer light‐blocking properties upon the Al2O3 encapsulation material, this study employed a one‐step method using α‐Al2O3 as the main raw material to synthesize CaCu3Ti4O12(CCTO)–MnO2–Co2O3 series black Al2O3 ceramics. The research investigated their coloring effect, sintering behavior, dielectric, and mechanical properties. The results indicate that the CCTO–MnO2–Co2O3 series colorants successfully dyed the Al2O3 ceramics black, while their introduction resulted in the formation of various spinel‐type compounds and facilitated the sintering of Al2O3 ceramics. The sintering mechanism and performance effects of black Al2O3 ceramics were thoroughly investigated, revealing that with an increase in the colorant content, all properties of the samples improved. When the colorant content reached 15.0 wt.%, the coloring effect reached its optimum, with a relative density of 96.7%, a dielectric constant of 12.5, a dielectric loss of .0081, a flexural strength of 314.4 MPa, and a Vickers hardness of 1254.5 Hv.