Al2O3 Grains Research Articles

Direct observation of atomic-scale fracture path within ceramic grain boundary core

In fracture processes, grain boundaries behave as preferential paths for crack propagation. These grain boundary fractures proceed by the atomic-bond rupture within the grain boundary cores, and thus grain boundary structures have crucial influence on the fracture properties. However, the relationship between grain boundary structures and atomic fracture processes has been a matter of conjecture, especially in the case of dopant-segregated grain boundaries which have complicated local structures and chemistries. Here, we determine the atomic-bond breaking path within a dopant-segregated Al2O3 grain boundary core, via atomic-scale observations of the as-fractured surface and the crack tip introduced by in situ nanoindentation experiments inside a transmission electron microscope. Our observations show that the atomic fracture path is selected to produce less coordination-deficient oxygen polyhedra of dopant cations, which is rationalised using first-principles calculations. The present findings indicate that the atomic coordination geometry at the grain boundary core affects the fracture processes.

Effect of nano-AlN addition on thermal diffusivity and mechanical strength of porous AL2O3 ceramic composites

Abstract In a properly made porous abrasive composite, the vitrified bond should ideally cover the grains and form a continuous network of bridges, and thus part of the heat energy from the grinding process is also transferred to the vitrified bond. Until recently, most studies on the design of composite properties have focused mainly on improving their mechanical strength and wear resistance, but increasingly the very important aspect of their thermal properties is noticed. The vitrified Al2O3 composites were made from Al2O3 grains, vitrified bond of Na2O K2O Al2O B2O3 SiO2 and AlN nanopowder. The increase in porosity in the tested composites is the effect of the AlN decomposition reaction. Crystalline phases were identified in both composites - α-Al2O3 and NaAl11O17, but with a different percentage share in individual composites. In composites doped with AlN nanopowder, the proportion of NaAl11O17 crystalline phase decreases, due to its high susceptibility to reduction by Al, obtained from the AlN decomposition reaction. The product of the redox reaction is also Na+ ions, which may participate in the formation of the glass phase and thus increasing the fraction of the residual glass phase. As a result of the partial reduction of NaAl11O17 phase, an increase in α-Al2O3 content is observed. A higher proportion of α-Al2O3 phase with high thermal conductivity can be a factor that increases the rate of heat removal from the work zone.

Insights into intragranular precipitation and strengthening effect in Al2O3/SmAlO3 ceramic with eutectic composition

This work concerns the microstructure-mechanical properties relationship in Al2O3/SmAlO3 composites prepared by hot pressing at temperatures from 1450 °C to 1650 °C. Dense samples have been prepared using co-precipitated trigonal α-Al2O3 and orthorhombic SmAlO3 at the pseudo-binary eutectic composition. The overall microstructure coarsens with increasing processing temperature, but the nanoscale precipitates within α-Al2O3 and SmAlO3 grains contribute to coherency strains and distortions in Al2O3/SmAlO3 prepared at 1650 °C. The intragranular SmAlO3 precipitates in Al2O3 grains exhibit semi-coherent interface and an orientation relationship of the intragranular phase SmAlO3 and alumina matrix close to two different zone axes, namely [23¯1]SmAlO3 and [4¯401]Al2O3. Consequently, significant strengthening effect is found from the measured hardness values on the sample sintered at 1650 °C. The measured flexural strength and single edged notched beam fracture toughness of the Al2O3/SmAlO3 ceramic are comparable to traditional secondary phases reinforced alumina composites. The sample sintered at 1500 °C exhibits a flexural strength of 558 MPa and a fracture toughness of 5.59 MPa⋅m1/2.

Effects of some Parameters on Properties of Cu-Al<sub>2</sub>O<sub>3</sub> Composite Materials Synthesized by Internal Oxidation Process

The Cu-Al2O3 composite materials applying for electrode materials were successfully synthesized by internal oxidation process. The process included steps: producing solid solution α-Cu[Al] and oxidizing completely solid solution α-Cu[Al] as powders, then adding pure Cu powders and mixing in planetary ball milling, the mixtures were compressed into cylindrical samples, reduced at 750°C in 3h and sintered at 950°C in 2h in reduction atmosphere. The XRD patterns showed that total Al in Cu-matrix were completely oxidized in range of 20¸25h. After reducing and sintering process, the final products were Cu-Al2O3 composite materials. The results showed that micro-hardness, porosity and compress strength of Cu-Al2O3 materials increased with increasing of Al2O3 content, which was the result of dispersion of the fined Al2O3 grains in Cu-matrix. When Al2O3 content was unchanged, the porosity of received materials decreased with increasing of compress pressure. Whereas, the electrical conductivity of Cu-Al2O3 composite materials decreased comparing with pure Cu and it decreased with increasing of Al2O3 content.

Fabrication of dense Al2O3-FeAl composites by using solid-state sintering

Highly densified alumina-iron aluminide (Al2O3-FeAl) composites consisting of ubiquitous elements were fabricated by using pulse current sintering technique under a certain uni-axial pressure. The solid-state sintering without melting FeAl was the highlight in this study. The mechanical properties of the Al2O3-FeAl composites were much greater than previously reported ones fabricated by reaction sintering technique. The poor wettability of FeAl against Al2O3 strongly influenced the mechanical properties and made it difficult to be highly densified Al2O3-FeAl composites by liquid phase sintering especially when volume fraction of FeAl to Al2O3-FeAl was high (>30.5 vol%). However, highly densified Al2O3-FeAl composites were obtained by solid-state sintering with control of Al2O3 grain size and sintering temperatures. It was concluded that highly controlled powder metallurgy made it possible to fabricate dense ceramic-metal (intermetallic) composites from the combination of materials having poor wettability.

Role of addition of La2O3 on the phase contents, microstructure and mechanical properties of ZTA-MgO ceramics

Abstract Addition of MgO into Zirconia Toughened Alumina (ZTA) enhanced the Vickers hardness (1710 HV) but reduced the fracture toughness (3.38 MPa.m1/2). The reason is attribute to the formation of MgAl2O4 phase in the ZTA-MgO system. Hence, the La2O3 was added to ZTA-MgO compounds to acquire both high hardness and fracture toughness. The mixtures were prepared with addition of La2O3 (0.1 wt.% to 10.0 wt.%) to ZTA/1.0 wt% MgO. The mixtures were pressed to a rectangular bar of (54*12*5) mm3 at 1600 oC for 4 hrs. The characterisation of phase contents, microstructure, density, porosity and mechanical properties were done through XRD, SEM, Archimedes principle and Vickers indentation technique, respectively. The optimum La2O3 addition was between (0.7 wt. % - 1.0 wt.%) for better fracture toughness of ZTA ceramics with a reasonable high hardness. The addition of La2O3 has a significant effect on the formation of finer Al2O3 grains, creating strong interface between the Al2O3 grains and LaAl11O8 grains result in denser ZTA-MgO ceramics of hardness of 2028 HV. Also, the presence of rodlike LaAl11O8 and/or LMA grains in the ZTA-MgO ceramics improved the fracture toughness up to 6.67 MPa.m1/2.

Abrasive wear behaviour of cast Al–Si–Mn alloys

In this paper, the effect of manganese (wt%) and size of the Al2O3 abrasive grains of abrasive wear behaviours of Al–Si–(0.16–0.76%) Mn alloy was examined. Abrasive wear experiments were conducted against abrasive Al2O3 paper grains with diameters in the variety of 38–250 µm, under the normal load range of 10, 20 and 30 N and the sliding velocity of 0.1 m s−1. Wear measurements have shown that the wear rate of the material is dependent on the manganese content and the abrasive grain size. The best wear resistance is in the content of 0.73 Mn, which is associated with increased toughness and strength. The Al–Si casting alloy which had 0.73% Mn content exhibited lower wear rate than the Al–Si–0.16/0.22/0.23% Mn alloys after scoured on fine abrasive Al2O3 grains (<65 µm). Wear experiments achieved on Al2O3 grains having sizes greater than 65 µm revealed that, the cast Al–Si– alloy which had 0.73% Mn content had lower wear rate than the cast Al–Si–0.16/0.22/0.23% Mn alloys.

Fabrication of Mo5SiB2-based composites by combustion synthesis involving aluminothermic reduction of MoO3

Fabrication of Mo5SiB2–Al2O3 composites with a broad range of the Mo5SiB2/Al2O3 ratio was conducted by thermite-based combustion synthesis in the SHS mode. Thermite reagents of MoO3 + 2Al were introduced into two Mo‒Si–B ternary systems adopting amorphous boron and MoB as their respective boron source. The boron-containing samples were more energetic and were applied to produce composites with Mo5SiB2/Al2O3 from 1.25 to 2.5, beyond which combustion was extinct. The composites with Mo5SiB2/Al2O3 from 0.8 to 1.3 were prepared from less exothermic MoB-based samples. Besides causing a decrease in combustion velocity and reaction temperature, the increase of the Mo5SiB2/Al2O3 proportion led to a transformation in combustion wave propagation from a steady to pulsating mode. For the samples featuring a pulsating combustion wave, the reaction time at peak combustion temperatures was extended and then the evolution of Mo5SiB2 from intermediate phases (Mo3Si and MoB) was significantly improved. The deduced activation energy suggests a lower kinetic barrier for thermite-based combustion synthesis to fabricate Mo5SiB2-based composites. The microstructure of synthesized composites indicates that quadrangular Mo5SiB2 grains with an average size of 10–15 µm are tightly packed and irregular Al2O3 grains are randomly dispersed.

Characterization of Ni3Al Alloy Fabricated by Thermal Explosion and Hot Extrusion Process

In the present paper, the trace B doped Ni3Al alloy was prepared by the thermal explosion and hot extrusion (TH/HE) technique. Its micro-structure characterization and mechanical properties were carried out by OM, XRD, TEM, compression and micro-hardness tests. Micro-structure examinations exhibit that the TH/HE technology has transformed the powders into Ni3Al alloy with less porosity. In the synthesized part, coarse and ultrafine Ni3Al grains comprise the dual-scale grain structure with segregated Al2O3 particles along grain boundary. TEM observations reveal that γ-Ni, Al2O3 and Ni3Al phases coexist in the synthesized part. Moreover, the Al2O3 particles in the synthesized part have α-Al2O3 and γ-Al2O3 two crystal structures. In addition, there is a transition region along the interface of Al2O3 and Ni3Al grain. The subsequent hot extrusion optimize the micro-structure of the extruded part by homogenizing the Ni3Al grain, uniformly distributing Al2O3 and generating massive substructures. The great deformation results in the pile-up of dislocations, intersected dislocations and substructures. Moreover, the movement of dislocation promotes the formation of micro-twinning in the Ni3Al grain, which is accompanied with the stacking faults and nano-structure. The mechanical properties exhibit that the micro-structure optimization caused by the TE/HE technology improves the compressive ductility and strength of the Ni3Al alloy obviously. DOI: http://dx.doi.org/10.5755/j01.ms.24.4.19460

Microstructure and mechanical properties of hot-pressed Al2O3–mullite–ZrO2–SiC composites

In this study, the effects of SiC content, sintering temperature and Al2O3/SiO2 ratio on the microstructure and mechanical properties of hot-pressed alumina–mullite–zirconia–silicon carbide (Al2O3–mullite–ZrO2–SiC) composite ceramics were investigated systematically. The research results indicate that the composite ceramics have excellent combined properties when SiC content is 20 vol% and sintering temperature is 1530 °C, with Vickers hardness, fracture toughness and flexural strength of 17.5 GPa, 12.3 MPa m1/2 and 970 MPa, respectively. The fracture toughness and flexural strength are higher than those of monolithic Al2O3 ceramics and Al2O3-based composites, which are fairly attributed to the refined microstructures of composites and the synergistic effects of crack deflection, pull-out, branching, and phase transformation toughening mechanisms induced by short columnar Al2O3 grains and SiC platelets formed in the ceramics, as well as the ZrO2 particles existed in the ceramics.

Effect of silicone resin as precursor and binder on the properties of alumina-based ceramic cores using ball-shaped powders

Al2O3-based ceramic cores with uniform microstructure were fabricated by sintering ball-shaped Al2O3 powders as the matrix and silicone resin as the precursor and binder in air atmosphere. Effects of silicone resin content and sintering temperature on phase composition, microstructure and properties of the Al2O3-based ceramic cores were investigated. Results showed that a low sintering rate was beneficial for preparing high-quality Al2O3-based ceramic cores with no defects and cracks. Silicone resin was uniformly adhered on the surface of the ball-shaped Al2O3 particles, playing a well role in connecting Al2O3 particles. Sintering process made the organic-inorganic transformation process of silicone resin happen, leading to the formation of amorphous silica at 1300 °C, followed by cristobalite appeared by the crystallization of amorphous silica as the temperature was higher than 1400 °C. The sintering necks between ball-shaped Al2O3 grains were formed by the pyrolyzed products of silicone resin. As the temperature was lower than 1500 °C, the phase composition consisted of Al2O3 and silica. However, at 1600 °C, new mullite phases were in situ formed on the surface of ball-shaped Al2O3 particles. The increasing silicone resin content promoted the formation of more mullite phase. Al2O3-based ceramic cores prepared had well dimensional stability during sintering, and the linear shrinkage rate reached the minimum value of 0.23% in the ceramic cores with silicone resin content of 20 wt% at 1400 °C. The open porosities of Al2O3-based ceramic cores prepared were 24.2~30.0% and relative densities were 63.9~70.2%. The bending strength increased with the increase in the sintering temperature, and the maximum value was obtained in the ceramic cores with silicone resin content of 15 wt% at the same sintering temperature. In our research, the highest bending strength reached to 77.06 MPa in the ceramic cores with silicone resin content of 15 wt% at 1500 °C.

Structural, optical tuning, and mechanical behavior of zirconia toughened alumina through europium substitutions.

The structural and optical features of zirconia toughened alumina (ZTA) due to the assorted range of Eu3+ substitutions are demonstrated. The characterization studies affirm the pivotal role of Eu3+ on the improved structural stability of ZTA and associated tetragonal zirconia (t-ZrO2 ) → cubic zirconia (c-ZrO2 ) transformation. Eu3+ prefers accommodation at the lattice sites of ZrO2 and their gradual accumulation induces t- → c-ZrO2 transition. Beyond the substitution limit, Eu3+ reacts with Al2 O3 to form EuAlO3 . Optical studies validate typical Eu3+ emissions, and further, the emission spectrum also predicts the symmetry of Eu3+ coordination at the ZrO2 lattice. Uniform distribution of ZrO2 and Al2 O3 grains throughout the microstructures are evident from the morphological analysis. Further, the influence of Eu3+ on the enhanced mechanical stability of ZTA is ensured from indentation technique. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1170-1179, 2019.

Fabricating a Novel Intragranular Microstructure for Al₂O₃/GdAlO₃ Ceramic Composites.

In order to make the embryonic form of intragranular structure, the Al2O3/GdAlO3 system was selected due to its excellent mechanical properties. Gd2O3 and Al(NO3)3·9H2O were used as the starting materials. A co-precipitation method was used for the preparation of fine ceramics and applied to synthesize the nano-powder of GdAlO3 firstly. Then, the nano-powder of GdAlO3 was mixed with the precipitates by the second co-precipitation method. After drying and calcination, the compound powder with eutectic composition (77 mol % Al3+—23 mol % Gd3+) was fast sintered by using the spark plasma sintering technique. The results revealed that the phases of the sintered samples were Al2O3 and GdAlO3. The phases showed a homogeneous and interlaced distribution. All the matrix grains were submicron. The sizes of the intragranular structures were between 50 nm and 150 nm. Therefore, the intragranular structure displayed a novel mixture of nanometer–submicron and submicron–submicron types. The different intragranular structures all changed the fracture modes of Al2O3 grains from intergranular fracture to transgranular fracture.

ZTA Hardness Enhancement via Hot Isostatic Press Sintering

The present work mainly deals with the influence of Y2O3 stabilize ZrO2 (YSZ) additions on the phases, microstructure and mechanical properties of Al2O3 which is known as zirconia toughened alumina (ZTA) through hot isostatic press (HIP) sintering. Different amounts of 8 mol% of yttria stabilize ZrO2 (8YSZ) were added into Al2O3 and the amount of 8YSZ was varied from 0 wt.% to 25 wt.%. There is significant evidence that microstructural coarsening within ZTA was found to produce an improvement of hardness and fracture toughness. These improvements are the evidence for the toughening mechanism through the transition of tetragonal to monoclinic (t-m) structure provide by 8YSZ which is defined to optimize the materials and mechanical properties of Al2O3 ceramics. The 8YSZ grains and Al2O3 grains are impartially distributed among each other. The XRD patterns are corresponds to yttria doped zirconia (Zr0.935Y0.065)O1.968 peaks (ICDD files No. 01-078-1808) and Al2O3 (No. 00-010-0173). The existence of (Zr0.935Y0.065)O1.968 phase relates to the metastable tetragonal form of ZrO2.

Electrostatic self-assembly preparation of reduced graphene oxide-encapsulated alumina nanoparticles with enhanced mechanical properties of alumina nanocomposites

In this study, we employed facile self-assembly methods to synthesize reduced graphene oxide-encapsulated alumina (Al2O3/rGO) nanoparticles. The Al2O3/rGO nanoparticles were subsequently incorporated into an Al2O3 matrix as filler to prepare nanocomposites. The microstructural analysis showed that relatively thin rGO sheets were homogeneously dispersed in the matrix and bonded with the Al2O3 grains forming a three-dimensional rGO network structure. The specific structure caused the rGO sheets to be anchored and bound to the matrix grains, resulting in a high contact area between the rGO sheets and the matrix, whilst the fracture mode alteration, grain refinement and improved interfacial strength of the nanocomposites were related to the unique structure. The results indicated that the nanocomposites with 2.5 vol.% rGO exhibited outstanding mechanical properties, increasing both the flexural strength by 105%, with a maximum value of 636 MPa, and the fracture toughness by 90% (5.9 MPa m1/2) when compared with monolithic Al2O3.

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al2O3-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al2O3 particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al2O3 (> 1400 °C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800 °C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al2O3 grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al2O3-Ni-5 wt% SiC nanocermets sintered at 800 °C obtain the highest compressive strength of 242.5 MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04 mg/m.

Nanocrystalline Al2O3-Ni(Cr) particle synthesis by high-energy mechanical alloying method

Nanostructured feedstock material results in better surface protective coatings under high-temperature oxidation erosion environment. This research paper focuses on synthesizing ceramic-metallic (cermet) nanostructured Al2O3-Ni(Cr) feedstock particles using high-energy mechanical alloying (HE-MA) powder processing technology. The underscoring particle design objectives were twofold: (1) reduce the ceramic oxide grain size below 20 nm, and more pressingly (2) embed nano-scaled grains inside metal/alloy binder particles, hence the nanostructured cermet particles. The novelty of this research work lies in the successful embedment of nanostructured Al2O3 grains into the micron sized Ni(Cr) metallic binder matrix. In a SPEX mill with a milling speed of 1200 rpm along with variation in milling time following two separate material design compositions (design of experiments) it was possible to meet the stated objectives. Results show good conformance between the experimental and analytical data in terms of obtaining Al2O3 grain reinforced Ni(Cr) nanostructured particles with mean grain and particle sizes standing at 19 ± 1.15 nm and 15 μm ± 4.6 μm, respectively, after 12 h of milling.

Microstructure and macro properties of Al2O3 ceramics prepared by laser engineered net shaping

Laser engineered net shaping (LENS) technology, due to its ability in melting and solidification of neat ceramic materials, exhibits great potential for direct fabricating net-shaped ceramic structures. However, due to limitation of sample size, reports on the macro properties of fabricated ceramic structures are still extremely lacking. In the present work, large-sized Al2O3 ceramic samples were prepared using LENS technology to perform microstructural analysis and macro properties testing. Phase composition, grain characteristics and chemical purity of prepared samples were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence (XRF). Macro properties of large-sized samples, including the flexural strength and compressive strength, were given for the first time after being tested using a mechanical tester. Results show that microstructure of prepared Al2O3 ceramics presents typical solidification characteristics and consists of grains that grow along the deposition height direction. Primary dendrite spacing of the Al2O3 grains is about 60 µm. The phase is mainly composed of high-temperature-stable α-Al2O3 and a relative density of 99.5% is obtained. No new impurities are introduced during the depositing process. Element type and element content of fabricated samples are highly consistent with those of original powder. Property test results show that, under current process conditions, the flexural strength and compressive strength of fabricated samples reaches the level of traditional sintering method, reaching the maximum of 350 MPa and 625 MPa respectively.

Phase composition, microstructure and luminescent property evolutions in “light-scattering enhanced” Al2O3-Y3Al5O12: Ce3+ ceramic phosphors

Classic transparent ceramics for laser gain medium and window materials may benefit from their distinctive features of structural homogeneity and high transparency at large scales. However, this has restricted the ability of the ceramics for local light management. Herein, a strategy for ceramic-phosphor design was conducted in the present work via introducing light-scattering centers into single phased YAG: Ce3+ transparent ceramics, and an enhancement of light extraction was experimentally realized through the control of Al2O3 grain size. UV–vis-NIR diffuse reflectance spectra further confirmed the important roles of the excrescent Al2O3 grains. More importantly, PL characterization shows orange-white light emission with high brightness at high temperature. The results highlight that the unique configuration enables simultaneous control of light propagation, luminous efficiency and thermal stability of luminescence, and the design strategy may create great opportunities for laser lighting and displays with high laser power density.

Promote Alumina Leaching through Re-Pelleting: Effects of Directional Coating of Calcite on Silicate Grains

The contents of Fe and SiO2 in ferric bauxite are much higher than that in regular bauxite, and the leaching yield of Al2O3 from ferric bauxite cannot reach the general level of ordinary ores, even under optimal conditions. This paper introduces a pretreatment method to enhance sintering before leaching. First, mixing followed by grinding was found to be more efficient than grinding followed by mixing as a pretreatment method by forming a directional coating structure of CaCO3 on the SiO2 surface. Second, leaching yields increased grinding fineness because CaCO3 grain uniformity could be improved with the latter method. Finally, the leaching yields were increased by pulp consistency under certain conditions, as evidenced by improvements in CaCO3 uniformity, bulk density, and Al2O3 grain size, which parameters were beneficial to enhance leaching. The leaching yield of Al2O3 was increased from 75.2% to 87.15% by using the proposed mixing–grinding method with the leaching agent of water.