Laser stereolithography of ZrO2 toughened Al2O3

Ceramic laser stereolithography is a manufacturing process suitable candidate for the production of complex shape technical ceramics. The green ceramic is produced layer by layer through laser polymerisation of UV curable ceramic suspensions. A number of critical issues deserve attention: high solid loading and low viscosity of the suspensions, high UV reactivity, prevention of interlayer delamination in the green and in the sintered body, good mechanical performance. In this work, ZrO₂-reinforced Al₂O₃ components have been obtained from an acrylic modified zircon loaded with alumina powders. The zircon compound is effective as organic photoactivated resin and allows the dispersion of a high volume fraction of Al₂O₃ powder (up to 50 vol.%) while keeping viscosity at reasonable low values. The zircon compound also represents a liquid ceramic precursor that converts to oxide after burning out of the binder. Thank to the good dispersion of the alumina powder in the zircon acrylate, a uniform dispersion of ZrO₂ submicron particles is obtained after pyrolysis. These are located at the grain boundaries between alumina grains. Formation of both monoclinic and tetragonal ZrO₂ occurs as evidenced by XRD. No delamination occurs in bending tests as evidenced by SEM fractography, satisfactory modulus and strength values were concurrently found.     

Effect of mechanical properties on the ballistic resistance capability of Al2O3-ZrO2 functionally graded materials

Because Al₂O₃ exhibits high strength and hardness, it is prevalently used as a ceramic material. ZrO₂ is often added to increase the toughness of such a material. Therefore, this study mixed Al₂O₃ and ZrO₂ to formulate functionally graded materials (FGMs). four-layer and eleven-layer Al₂O₃-ZrO₂ FGM_S were produced from Al₂O₃ and ZrO₂ mixtures by sintering at 1500 °C. Moreover, testing sheets were created by mixing various ratios of Al₂O₃ and ZrO₂ to analyze their fracture toughness and hardness. The results revealed the 90% Al₂O₃-10% ZrO₂ sheet to exhibit a hardness of 15.12 GPa, and the 50% Al₂O₃-50% ZrO₂ sheet to attain a fracture toughness as high as 4.7 MPa m^0.5. The impact resistance test involved analyzing various types of testing sheets, including the four-layer Al₂O₃-(0%, 10%, 20%, 30%) ZrO₂ FGM, eleven-layer Al₂O₃-(0–100%) ZrO₂ FGM, 100% Al₂O₃ composite material, 90% Al₂O₃-10% ZrO₂ composite material, 70% Al₂O₃-30% ZrO₂ composite material, and 50% Al₂O₃-50% ZrO₂ composite material. The ballistic tests showed that FGMs of the same areal density (4.64 g/cm²) or thickness (11 mm) attained the highest energy absorption. The experimental results confirmed that FGMs can delay the formation and propagation of ceramic cones. Specifically, toughened alumina materials prevent the growth of radial and circumferential cracks, delay the formation of ceramic cones, decrease cones hitting against the back plane, and increase the penetrating resistant capability of the ceramic materials experiencing bullet impact, features important for applications in fields such as aerospace, aviation, automobile, the military industry, and biomedicine     

Highest UV–vis transparency of MgAl2O4 spinel ceramics prepared by hot pressing with LiF

Reaction sintering of MgO and Al₂O₃ with addition of LiF as sintering additive was used to prepare MgAl₂O₄ spinel ceramic by hot pressing. The process parameter (temperature, pressure, dwell time), the stoichiometric ratio of MgO to Al₂O₃ and the selection of the alumina raw powder are equally important for highest transparency of the spinel ceramic. With this optimization highest transparency of 86% in the visible range at λ = 640 nm together with UV transmission of 62% at 200 nm for spinel ceramic with 4 mm thickness was reached.     

Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al₂O₃/MgAl₂O₄/ZrO₂ ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al₂O₃/MgAl₂O₄/ZrO₂ eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al₂O₃, MgAl₂O₄ and ZrO₂ phases. In the eutectic microstructure, the MgAl₂O₄ and Al₂O₃ phases form the matrix, the ZrO₂ phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO₂ phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m ^1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO₂ phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.     

Aqueous colloidal processing of submicrometric SiC plus Y3Al5O12 with diamond nanoparticles

Aqueous colloidal processing was used for the environmentally friendly preparation of well-dispersed concentrated suspensions and powder mixtures of submicrometric SiC powders with submicrometric Y₃Al₅O₁₂ as sintering additive plus diamond nanoparticles as reinforcing phase. It is shown that the addition of nano-diamond markedly increases the viscosity and thixotropy of the SiC + Y₃Al₅O₁₂ suspensions, and also that, by adjusting the pH, deflocculant content, and sonication time it is possible to co-disperse these three rheologically different ceramic phases (i.e., non-oxide, oxide, and hydrophobic compounds) in aqueous media, thereby avoiding the otherwise irremediable severe hetero-aggregation. Moreover, the microstructural characterization of the powder mixtures obtained by freeze-drying the suspensions confirmed the homogeneous dispersion of the diamond nanoparticles among the submicrometric SiC and Y₃Al₅O₁₂ particles in the form of isolated or adhered nanoclusters and nanodeposits. Implications for engineering the microstructure of non-oxide ceramics with diamond nanodispersoids are discussed.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

Fabrication and abrasive wear behavior of ZrO2-SiC-Al2O3 ceramic

In this paper, ZrO₂-SiC-Al₂O₃ ceramic was fabricated by as-prepared ZrO₂-SiC powders and commercial α-Al₂O₃ powders, sintered at 1450 °C for 1 h. ZrO₂-SiC composite powders were synthesized through carbothermal reduction with zircon as raw material and carbon black as the reductant. Through ZrO₂-SiC-Al₂O₃ ceramic mechanical properties measurements it was indicated that this ceramic had excellent properties in both bending strength and hardness. In addition, ZrO₂-SiC-Al₂O₃ ceramic abrasive wear property was measured. ZrO₂-SiC-Al₂O₃ ceramic mass loss increased along with the increase in both wheel rate and applied load. The type of wear particles had an important effect on abrasive wear resistance. ZrO₂-SiC-Al₂O₃ ceramic displayed excellent abrasive wear resistance in bentonite and quartz slurries in comparison with SiC slurry.     

Electrophoretic shaping of sub-micron alumina in ethanol

The electrophoretic deposition (EPD) method was used to shape sub-micron Al₂O₃ green body in ethanol. The uniformity of the final deposit was affected by the colloidal properties of the suspension. Therefore, the stability of Al₂O₃ suspension in ethanol was studied in terms of electrophoretic mobility, viscosity and conductivity. The EPD kinetics were further investigated with different electrical conditions. The effect of cellulose acetate membrane on deposit was also studied. The green body with a relative density of about 60 %TD was associated with a narrow pore size distribution, indicating a high homogeneity of particle coordination. After presintering and HIPing at 1250 °C, a fully dense alumina ceramic was obtained with an average grain size of 0.65 μm.     

Nickel–alumina graded coatings obtained by dipping and EPD on nickel substrates

Nickel substrates have been coated by Ni/Al₂O₃ composite films by a dipping process using aqueous suspensions that contain a temporary binder. Two-layer and three-layer graded coatings have been produced, consisting of pure Ni powder and Ni/Al₂O₃ composites with Al₂O₃ contents of 15 and 30 vol.% as intermediate layers to release sintering and thermal stresses. The laminates were further coated with a ceramic layer of Al₂O₃/ZrO₂ that was deposited by electrophoretic deposition using a non-aqueous suspension. A continuous, thin Al₂O₃ layer surrounding Ni grains developed at the intermediate composite layer of Ni/Al₂O₃ allows the ceramic coating to maintain strongly adhered to the nickel substrate by means of a porous substrate/coating interface.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.     

New laser machining technology of Al2O3 ceramic with complex shape

A new method was developed for the fabrication of complex-shaped Al₂O₃ ceramic parts by combining laser machining and gelcasting technique. The unwanted ceramic powders parts were selectively removed by laser machining specified by a computer program, and the gelcast Al₂O₃ green bodies were machined to a designed shape by a CO₂ laser at a wavelength of 10.6 μm. The influences of solid loading, laser output power, scanning speed and nitrogen purge on the machining of green Al₂O₃ ceramic bodies were studied. The experimental parameters were optimized, the green Al₂O₃ bodies with solid loading of 40 vol% or below were easier to be machined, while the green bodies with solid loading of 45 vol% or above were hard to be further machined due to the surface sintering. Better machining quality and deeper machining depth could be obtained when the laser power is 30 W. The green Al₂O₃ bodies with complex shape were obtained by the laser machining.     

Hydrolysis-induced aqueous gelcasting for near-net shape forming of ZTA ceramic composites

A new near-net shape forming process called “hydrolysis-induced aqueous gelcasting” (GCHAS) is reported in this paper for the consolidation of ZTA composites, ZTA-30 (70 wt.% Al₂O₃ + 30 wt.% ZrO₂) and ZTA-60 (40 wt.% Al₂O₃ + 60 wt.% ZrO₂). For comparison purposes, ceramics having the same chemical compositions were also consolidated by hydrolysis-assisted solidification (HAS). All the starting suspensions contained a solids loading of 50 vol.%. In the precursor powder mixtures, 1–5 wt.% of Al₂O₃ was replaced by equivalent amounts of AlN to enhance or promote or co-promote the consolidation of suspensions by HAS or by GCHAS, respectively. The suspensions for GCHAS were prepared by dispersing the ZTA powder precursor mixtures in a premix solution of 20 wt.% MAM (methacrylamide), MBAM (methylenebisacrylamide) and NVP (n-vinylpyrrolidinone) in 3:1:3 ratio in de-ionized water. Ceramics consolidated via GCHAS exhibited superior mechanical properties after consolidation and after sintering for 1 h at 1600 °C in comparison to those consolidated by HAS.     

Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Mn2+ activated MgAlON transparent ceramic: A new green-emitting transparent ceramic phosphor for high-power white LED

A novel Mn²⁺ activated green-emitting MgAlON transparent ceramic phosphor was synthesized from Mg_0.21Al_2.57O_3.80N_0.20:0.03Mn²⁺ (MgAlON:Mn) phosphor powder by pressureless sintering combining with hot isostatic pressing. By crystalline structure refinement and cathodoluminescence (CL) characterization, it is demonstrated that Mn²⁺ was dissolved in the spinel lattice and occupied the tetrahedral site. The ceramic, retaining high transmittance in UV–vis region (up to 82% at 800 nm) and excellent thermal-mechanical properties of MgAlON transparent ceramic-matrix, shows a strong green emission at 513 nm under 445 nm light excitation. Compared with its powder counterpart, the ceramic phosphor exhibits higher green color purity, higher internal quantum efficiency (47%) and lower thermal quenching. It is suggested that this novel green solid phosphor could be applied in high color rendering and high-power white light-emitting diodes when combined with a red solid phosphor and a blue LED chip.     

Mechanical properties of melt-grown Al2O3–ZrO2(Y2O3) eutectics with different microstructure

The effect of the microstructure on the mechanical properties of Al₂O₃–ZrO₂(Y₂O₃) eutectic ceramic oxides was studied. Rods processed by the laser-heated floating zone method with three different microstructures were obtained as the growth rate increased: a homogeneous dispersion of irregular ZrO₂ lamellae within the Al₂O₃ matrix, colonies with a core containing a dispersion of submicron ZrO₂ lamellae or rods surrounded by a thick intercolony region, and elongated cells formed by a dispersion of very fine ZrO₂ lamellae and separated by thin intercellular boundaries. The average flexure strength (close to 1.6 GPa) of the eutectics made up of a homogeneous dispersion of ZrO₂ lamellae was outstanding, and they also presented an excellent Weibull modulus (12.9) when the microstructure was homogeneous throughout the sample. Banding did not affect the average strength but degraded the Weibull modulus. In general, the flexure strength decreased as the size of the main morphological features of the microstructure (colony or cell diameter) increased. The thickness of the intercellular boundaries increased with the Y₂O₃ content and, above a critical value, reduced dramatically the strength by activating a new failure mechanism based on the coalescence of the pores and shrinkage cavities concentrated at the intercellular boundaries.     

Rheological properties of aqueous alumina–alumina-doped Y-PSZ suspensions

The effect of the substitution of alumina (Al₂O₃) by 0.3 wt% Al₂O₃-doped 3 mol% yttria-partially stabilized zirconia (Y-PSZ) on the rheological properties of concentrated aqueous slips was studied. Al₂O₃–Al₂O₃-doped Y-PSZ aqueous suspensions with different Al₂O₃-doped Y-PSZ contents: 0, 22 and 50 vol% were prepared using ammonium polyacrylate (NH₄PA) as dispersant. The particle size distributions of Al₂O₃ and Al₂O₃-doped Y-PSZ powders were similar; however, the particle shape and the surface coating of alumina conferred a markedly higher specific surface area to the Al₂O₃-doped Y-PSZ powder. The substitution of Al₂O₃ by Al₂O₃-doped Y-PSZ in the mixtures decreased the negative surface charge of the powders at pH 9, thereby increasing the amount of NH₄PA adsorbed and consequently the electrosteric repulsion between particles. However, the viscosity and yield stress values increased with increasing Al₂O₃-doped Y-PSZ content for all the solid loading studied. This could be explained by a larger interaction size of the Al₂O₃-doped Y-PSZ particles which resulted in a higher effective volume solid fraction and a lower amount of free-liquid available for flow.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Investigation of high-temperature reactions within the ZrSiO4–Al2O3 system

Solid state reactions between ZrSiO₄ and αAl₂O₃ in powders of stoichiometric composition 3Al₂O₃·2SiO₂ were studied by X-ray diffraction and electron scanning microscopy with energy dispersive X-ray analysis (SEM + EDX). Data were obtained at temperature ranging from 1400 °C to 1600 °C for a period of time ranging from 30 min to 60 h. The results indicate that ZrSiO₄ and αAl₂O₃ react and form crystalline ZrO₂, crystalline mullite (almost 3Al₂O₃·2SiO₂ composition) and non-crystalline silicon–alumina phase (pre-mullite). At the temperature of 1600 °C the fastest stage of reaction is dissociation of ZrSiO₄. Obtained results show that dissociation of zircon is a first-order reaction. The dissolution of Al₂O₃ particles and diffusion of Al into non-crystalline phase seem to be the slowest step of the reaction.