Pressureless sintering of LRH nanoplates on amorphous alumina for near-infrared GAP:Mn4+ transparent ceramic film

Transparent ceramics have become a research hotspot in the preparation of fluorescent materials in recent years, because of their excellent physical and chemical properties and high transparency. Gadolinium aluminate, as a stable matrix material, is often doped with various active ions to obtain luminescence with different colors. However, it is very difficult to fabricate gadolinium aluminate transparent ceramics by a traditional method, although they are the charming solid lighting materials. Here, we developed a pressureless sintering method to prepare GdAlO₃:Mn (GAP:Mn) transparent ceramic films, which were prepared by spin coating layered rare-earth hydroxide (LRH) on amorphous alumina substrate and sintering at 1550°C for 2 h. Through the interface reaction, the Al₂O₃ reacted with Gd₂O₃ to form mesophase Gd₄Al₂O₉ below 1550°C. However, the final products are GdAlO₃ at 1550°C. The GAP:Mn⁴⁺ film exhibits a high transmittance of about 90%. Under UV excitation at 310 nm, the ceramic film outputs deep red and NIR emissions, which are both arising from the ²E_g–⁴A_2g transition of Mn⁴⁺. Due to the electron traps arising from unequal valence substitution, the ceramic film exhibits a negative thermal quenching phenomenon. The ceramic film has a good luminescence thermal stability, because its emission intensity at 150°C maintains over 72% that at room temperature. This work may pave a new way to fabricate transparent ceramics using LRHs.     

Structure characterisation and mechanical properties of crystalline alumina coatings on stainless steel fabricated via sol–gel technology and fibre laser processing

The surface modification of stainless steel by coating with alumina (Al₂O₃) was carried out using sol–gel coating technology in combination with laser processing. Alumina coatings have been synthesised via a sol–gel route and deposited on stainless steel substrates by dip coating. The coated substrates were then treated with pulsed ytterbium fibre laser radiation (λ = 1064 nm) in continuous wave mode with different specific energies. The composition and structure of the coated surfaces after laser processing were characterised by ATR-FTIR, XRD, SEM and contact angle measurements, whilst the mechanical properties of modified surfaces were determined using nano-indentation. The results showed that the alumina xerogel films coated on the substrates are successfully converted into crystalline alumina ceramic coatings by the laser irradiation, the structure of resulting coatings being dependent on the irradiation conditions, with increase of laser specific energy leading to the formation of initially γ-Al₂O₃ with increasing amounts of α-Al₂O₃ at higher energy. Nano-indentation results reveal that the laser processing results in significant improvement in hardness and Young's modulus of the alumina-coated surface and, at optimum, can achieve the mechanical properties at the same level as pure α-alumina ceramic, much higher than those of the as-dried xerogel coating.     

Effect of Y2O3 addition on tribological properties of plasma sprayed Al2O3-13% TiO2 coating

The effect of Y₂O₃ addition on structure, mechanical properties and tribological properties of Al₂O₃-13 wt% TiO₂ coating was investigated. The addition of 20 wt% Y₂O₃ resulted in better densification, stabilization of alpha (α) alumina phase and improvement in fracture toughness of Al₂O₃-13 wt% TiO₂ coating. Abrasive wear tests were performed over a range of loads and sliding speeds. The stabilization of α alumina phase further increased with an increase in severity of wear test conditions, as noted from X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analysis of worn coatings. Al₂O₃-13 wt% TiO₂-20 wt% Y₂O₃ coating displayed lower friction coefficient and lower abrasive wear rate than Al₂O₃-13 wt% TiO₂ coating, which was due to synergistic effect of α alumina phase and formation of magneli phase oxide of titanium; Ti₂O₃. Friction energy map was used to rationalize observed wear rates, to identify different regimes of wear and degradation modes of coatings.     

Direct tape casting of Al2O3/AlN slurry for AlON transparent ceramic wafers via one-step reaction sintering

In this work, transparent aluminate oxynitride (AlON) ceramic wafers were successfully fabricated by the direct non-aqueous tape casting of Al₂O₃/AlN slurry and the one-step reaction sintering for the first time. The reaction sintered AlON ceramic wafer exhibits high transmittance of 73.2 % at the wavelength of 1600 nm. This fabricating route realizes smooth and flexible tape without cracks or pinholes in Al₂O₃/AlN system and efficiently shortens the preparation cycle of transparent AlON wafers, which is a feasible way to prepare high-quality transparent AlON ceramics with large lateral sizes and thin thicknesses by reaction sintering, might also promote the application of transparent AlON ceramic wafers.     

Solution precursor high-velocity oxy-fuel spray ceramic coatings

For the first time, the solution precursor high-velocity oxy-fuel spray process was used to deposit Al₂O₃–ZrO₂ ceramic coatings. X-ray diffraction analysis and transmission electron microscopy characterization show that the as-sprayed coating is composed of mixed nanocrystalline ZrO₂ and γ-Al₂O₃ as well as amorphous phases. The as-sprayed coating consists of ultrafine splats with diameters ranging from 2 to 5 μm. Few spherical particles, hollow-shell structures are also observed on the coating surface. Polished cross-section shows that the coating is quite dense with a thickness of 40 μm.     

Aerosol-deposited Al2O3/PTFE hydrophobic coatings with adjustable transparency

With the wide range of requirements for architectural glass, such as transparency, opacity, and hydrophobicity, there is a need to address the issues in the complexity of convention methods. Thus, considering functionality and applicability in various architectural windows, hydrophobic alumina/polytetrafluoroethylene (Al₂O₃/PTFE) composite layers with transparency or opacity were transferred to commercial architectural glass using a facile aerosol deposition (AD) process. We successfully fabricated hydrophobic coating layers with high transmittance (only a 0.03% difference from sheet glass) by optimizing the PTFE content in Al₂O₃ using solution-based synthesized powders to enable a uniform surface topology. The opaque hydrophobic Al₂O₃/PTFE coating layers exhibit a transmittance of approximately 0% with excellent hydrophobicity of 130°. Remarkably, this opaque film was successfully employed onto a large deposition area, curved substrate, and micro-patterned regions. It is believed that our AD-prepared composite layers have great potential for architectural glass in terms of economic feasibility and versatility.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Fabrication of Transparent Dysprosium Aluminum Garnet (Dy3Al5O12) Ceramics via a Solid‐State Reaction Method

Polycrystalline, transparent Dy₃Al₅O₁₂ ceramics were firstly fabricated by a solid‐state reaction method using high‐purity Dy₂O₃ and Al₂O₃ powders. The fully dense Dy₃Al₅O₁₂ ceramic with an average grain size of less than 10 μm was obtained by vacuum sintering at 1820°C for 6 h. The in‐line transmittance of the optimized sample reaches 80% in the visible region. Scanning electron microscopy reveals that no secondary phases and almost no pores are observed at grain boundaries or triple junctions, and the fracture mode of the ceramic is mainly transgranular. The Dy₃Al₅O₁₂ ceramic is promising for magneto‐optical applications. Verdet constant of the Dy₃Al₅O₁₂ transparent ceramic is as high as ?0.41 min·(Oe·cm)^?1.     

Anticorrosion assessment of Al2O3‐TiO2 composite thin films derived from sol‐gel dipping technique

Metal surface protection by means of nanostructured ceramic coatings is considered an issue, which is variously scrutinized, and more efforts are made to improve steel performance in accordance with this method. In this respect, this study sought to examine the protective effects of a nanostructured coating containing TiO₂ and Al₂O₃. The optimization approach for obtaining a coating without any cracks with the highest efficiency of corrosion resistance is reported. The variables under assessment were as follows: weight ratio of the oxides, calcination temperature, and preparation parameters. The synthesized samples were analyzed utilizing XRD, FESEM, AFM, Tafel test, EIS, and microhardness measurement. The results revealed that the optimized conditions were as follows: (i) Ti:Al precursor mixture with a weight ratio of 75:25, (ii) heat treatment at 1000°C. This process yielded a uniform composite coating without any cracks with 5.1 nm roughness, containing crystalline rutile phase and amorphous alumina. Not only the intended coating improves the corrosion resistance of steel up to 97%, but it also increases its surface hardness up to 10 units.     

Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al₂O₃) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al₂O₃ transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al₂O₃ ceramics for use in a wider range of optical applications.     

Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 μm and peeled away from the substrate due to the difference in thermal expansion between SiO₂ and Al₂O₃ at temperatures above 1500 °C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 μm to Ra 1.83 μm, and no peeling was observed when heated to 1550 °C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by sol-gel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 °C and withdraw rate of 5 mm/min. While a uniform, 4–5 μm thick HfO₂ reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.     

Fabrication of graphene oxide reinforced plasma sprayed Al2O3 coatings

Graphene oxide (GO) reinforced Al₂O₃ ceramic coatings were prepared on the surface of medium carbon steel by plasma spraying. The microstructure of the raw materials and coatings were characterized and analyzed by XPS, XRD, Raman and SEM. The bonding strength of the coatings was studied using a scratch method. The wear resistance of the coatings was assessed by the sliding test. The results showed that, after adding GO, the porosity of the coating reduced by about 31%, the hardness increased by approximately 10%, the bonding strength improved by 250%, and the wear rate reduced by 81% (Load: 30 N) and 84% (Load: 60 N), respectively.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Al2O3 coated glass by aerosol deposition with excellent mechanical properties for mobile electronic displays

Al₂O₃ thin film was deposited on Gorilla glass using an aerosol deposition method to improve the mechanical property of cover glass for mobile electronic device. The deposited Al₂O₃ film (approximately 1 μm thick) was a polycrystalline structure and showed a high light transmittance of approximately 90% in the visible light region. The CIE color space (L*a*b) measurement also showed a characteristic corresponding to the acceptable optical range of the cover glass. Further, it was confirmed that the bending strength improved by 10 %, as compared with bare Gorilla glass (from 6970 kgf/cm² to 7704 kgf/cm²), and the Vickers hardness increased to approximately 1700–2000 HV, as compared with that of Gorilla glass (<700 HV). Owing to the improved mechanical properties, the Al₂O₃ thin film exhibited good anti-scratch properties and is expected to be applied to the cover glass of various display products.     

Rheological behavior and curing deformation of paste containing 85 wt% Al2O3 ceramic during SLA-3D printing

The preparation of high solids loading Al₂O₃ paste is of great significance for improving the properties of ceramics formed by UV-curing. However, the solid contents of alumina slurry used by digital light processing (DLP) and traditional alumina paste for stereolithography (SLA) are both less than 80 wt%. With increase in solid content, the viscosity of paste increases sharply, and rheological property deteriorates. In this study, ceramic paste containing 85 wt% (62 vol%) Al₂O₃ was prepared for SLA-3D printing of ceramics, and more than 85 wt% solid content was achieved by dispersant and other additives. Effects of different dispersants on rheological and curing properties of Al₂O₃ ceramic paste were studied. At room temperature, the viscosity of 85 wt% Al₂O₃ ceramic paste was 51733 mPa s at shear rate of 30 s^?1. A novel method was proposed to control curing deformation of parts during printing. As-manufactured ceramic did not show any cracks by naked eye and exhibited excellent mechanical properties, with three-point bending strength of 540 MPa, fracture toughness of 4.19 MPa m^1/2, Vickers hardness of 16 GPa, surface roughness of 0.463 μm, and density of 3.86 g/cm³.     

Growth of Al2O3–PTFE composite film at room temperature by aerosol deposition method

To fabricate a ceramic-based substrate for 3-dimensional integration modules with a thick film coating process at room temperature, aerosol deposition method was employed. Al₂O₃ was chosen as a main coating material for the requirements of low permittivity and dielectric loss. Especially to give a functionality of plasticity, composite film with polytetrafluoroethylene (PTFE) was also studied. The effects of PTFE, which was incorporated in the film, were investigated by the microstructural characterization. It was confirmed that Al₂O₃–PTFE film with the grain size of 100–200 nm were grown at room temperature using Al₂O₃–0.5 wt% PTFE mixture powders. Dielectric constant and dielectric loss of Al₂O₃–PTFE film were 4.5 and 0.005 at 1 MHz, respectively.     

Metallization of Al2O3 ceramic by magnetron sputtering Ti/Mo bilayer thin films for robust brazing to Kovar alloy

Ti/Mo bilayer thin films were deposited onto Al₂O₃ ceramic by magnetron sputtering with a subsequent high temperature sintering to ensure the robust brazing of Al₂O₃ ceramic to Kovar (Fe–Ni–Co) alloy. The interface reaction process between Ti film and Al₂O₃ ceramic as well as the joining strength between metallized Al₂O₃ ceramic and Kovar alloy were investigated systematically using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and electronic universal testing machine. The results show that the active Ti film can react with Al₂O₃ ceramic to form Ti₃Al and TiO during high-temperature sintering process, in which the amount, size and morphology of TiO crucially depend on the sintering temperature. As the sintering temperature reaches 1200 °C, a plenty of spherical TiO nanoparticles with ~ 150 nm in diameter and metallic nature can be created across the Ti/Al₂O₃ interfaces, which can effectively act as ‘bridges’ to join Ti film to Al₂O₃ substrate firmly. Hence, the optimal joining strength of 69.6±3.1 MPa between metallized Al₂O₃ ceramic and Kovar alloy can be obtained, much better than those counterparts metallized at 900 °C and 1050 °C almost without the existence of observable TiO.     

Comparative study of the corrosion resistance of thermally sprayed ceramic coatings and their bulk ceramic counterparts

A comparative study of the corrosion properties of thermally sprayed ceramic coatings (Al₂O₃, Al₂O₃–TiO₂ with different ratios, mullite, and ZrSiO₄) and their sintered bulk ceramic counterparts was performed. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying processes. The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄ at 85 °C, respectively. The coating microstructures and phase compositions, as well as the corrosive environment were shown to have a strong effect on the corrosion resistance of the coatings. Al₂O₃–coatings were more sensitive to these factors than Al₂O₃–TiO₂ coatings were.The corrosion resistance of the bulk ceramics was superior to that of the thermally sprayed coatings. This is mainly because the coatings exhibited specific microstructure and contained amorphous and/or metastable phases not appearing in the bulk ceramics.     

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al₂O₃ coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al₂O₃ coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al₂O₃ were detected. The formation of Al₂O₃ layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al₂O₃ coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al₂O₃ coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.