Porous Alumina Preforms Research Articles

Low energy repair of co-continuous metal-ceramic composites using electrodeposition

The concept of self-healing materials, mainly inspired from biological systems, in the last several decades has been pursued toward fully autonomous materials and structures. Self-healing of polymers (extensively) and metals (to much less extent) have been demonstrated, however, there are no such reports for metal-ceramic composites. Metal-ceramic composites are technologically significant as structural and functional materials and are among the most expensive materials to manufacture and repair. Hence, technologies for self-healing metal-ceramic composites are of paramount importance. Here, we demonstrate a concept to fabricate and heal co-continuous metal-ceramic composites at room temperature. The composites are fabricated by infiltration of metal (here copper) into a porous alumina preform (fabricated by freeze-casting) through electroplating; a low-temperature and low-cost (by ∼60-times lower cost compared to traditional molten metal infiltration) process for fabrication of such composites. Additionally, the same electroplating process is demonstrated for healing damages such as grooves and cracks in the original composite, such that the healed composite recovers its strength by more than 80%. Such technology may be expanded toward fully autonomous self-healing structures.

Internal load transfer in an interpenetrating metal/ceramic composite material studied using energy dispersive synchrotron X-ray diffraction

The mechanism of internal load transfer in a newly developed interpenetrating metal/ceramic composite is studied for the first time in this work using energy dispersive synchrotron X-ray diffraction. The composite was fabricated by infiltration of Al12Si melt in an open porous alumina preform, fabricated using a mixture of two different polymer waxes as pore formers, by gas pressure infiltration. Several diffraction planes of all three crystalline phases of the composite – alumina, silicon and aluminum solid solution are investigated. Lattice strains, both along and transverse to the loading direction, in each phase are calculated from the shifts in the measured diffraction peak positions. Results show that until about 100 MPa applied compressive stress, all three phases undergo elastic deformation. At higher applied stresses aluminum undergoes plastic deformation and correspondingly load is transferred to alumina. The load carrying capability of alumina is however limited by incipient damage in this phase. At applied compressive stresses higher than 187 MPa, load carried by the alumina phase continuously decreases and correspondingly more load is again carried by the aluminum solid solution. Throughout, the fraction of load carried by silicon remains almost constant, suggesting no significant load transfer within the metallic alloy itself.

Effect of microstructure on mechanical properties and residual stresses in interpenetrating aluminum-alumina composites fabricated by squeeze casting

Aluminum-alumina composites with interpenetrating network structure are interesting structural materials due to their high resistance to elevated temperature and frictional wear, good heat conductivity, enhanced mechanical strength and fracture toughness. In this paper aluminum-alumina bulk composites and FGMs are manufactured by pressure infiltration of porous alumina preforms with molten aluminum alloy (EN AC-44200). Influence of the interpenetrating microstructure on the macroscopic bending strength, fracture toughness, hardness and heat conduction is examined. Special focus is on processing-induced thermal residual stresses in aluminum-alumina composites due to their potentially detrimental effects on material performance in structural elements under in-service conditions. The residual stresses are measured experimentally in the ceramic phase by neutron diffraction and simulated numerically using a micro-CT based Finite Element model, which takes into account the actual interpenetrating microstructure of the composite. The model predictions for two different volume fractions of alumina agree fairly well with the neutron diffraction measurements.

Processing Induced Flaws in Aluminum–Alumina Interpenetrating Phase Composites

This review paper deals with flaws in aluminum–alumina composites and FGMs induced by their manufacturing processes. Aluminum–alumina composites have been studied for many years as potentially interesting materials for applications, for example, in the automotive sector due to their enhanced mechanical strength, wear resistance, good heat conductivity and low specific weight. The focus here is on the interpenetrating phase composites (IPCs) manufactured by infiltration of porous alumina preforms with molten aluminum alloys. The primary objective is to provide an updated overview of research findings on a variety of flaws occurring at different stages of the manufacturing processes. Some precautions on how to avoid processing induced flaws in aluminum–alumina bulk composites and FGMs are mentioned.

Devitrification in SiO<sub>2</sub>-B<sub>2</sub>O<sub>3</sub>-Al<sub>2</sub>O<sub>3</sub>-La<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub> Glass during the Infiltration of Ceramic Composite

Dental prostheses made of ceramic composites infiltrated with glasses have been used due to their biocompatibility and possibility to mimic the natural teeth. In this study, the devitrification behavior of 20SiO2-25B2O3-25Al2O3-15La2O3-15TiO2 glass during the infiltration process in a porous alumina preform was investigated. Glass frits were prepared by melting the raw materials at 1500 °C for 60 min. The glass was infiltrated into the alumina preform at 1,150 or 1,200 °C for 60 min. The specimens were characterized by X-ray diffraction analysis and scanning electron microscopy. After the infiltration, it was possible to note that the devitrification process occurred in the remaining glass (excess glass that did not infiltrate in the preform), forming mostly aluminum borate and mullite crystalline phases. However, within the infiltrated composite no devitrification was noticed in the infiltrated glass. Possible explanations for this behavior are discussed.

Systematic approach to preparing ceramic–glass composites with high translucency for dental restorations

ObjectiveCeramic composites are promising materials for dental restorations. However, it is difficult to prepare highly translucent composites due to the light scattering that occurs in multiphase ceramics. The objective of this work was to verify the effectiveness of a systematic approach in designing specific glass compositions with target properties in order to prepare glass infiltrated ceramic composites with high translucency. MethodsFirst it was necessary to calculate from literature data the viscosity of glass at the infiltration temperature using the SciGlass software. Then, a glass composition was designed for targeted viscosity and refractive index. The glass of the system SiO2–B2O3–Al2O3–La2O3–TiO2 prepared by melting the oxide raw materials was spontaneously infiltrated into porous alumina preforms at 1200°C. The optical properties were evaluated using a refractometer and a spectrophotometer. The absorption and scattering coefficients were calculated using the Kubelka–Munk model. ResultsThe light transmittance of prepared composite was significantly higher than a commercial ceramic–glass composite, due to the matching of glass and preform refractive indexes which decreased the scattering, and also to the decrease in absorption coefficient. SignificanceThe proposed systematic approach was efficient for development of glass infiltrated ceramic composites with high translucency, which benefits include the better aesthetic performance of the final prosthesis.

Effect of Heat Treatment Schedules and Glass Powder Particle Size on Glass Infiltration in Porous Alumina Preforms

Glass infiltrated alumina is one of the popular materials for fabrication of dental prostheses. Achieving complete and uniform glass infiltration is important to achieve desired properties in dental prostheses. This study examined the role of various factors on glass infiltration in sintered porous alumina preforms (porosity ~ 27%). Observations on infiltration in porous alumina samples at a heating rate of 3°C/min at 1100°C for 2 h showed residual glass on the surfaces of the samples indicating incomplete infiltration. Rapid heating of the samples to the peak infiltration temperature by introducing the samples into a preheated furnace resulted in complete infiltration of glass into the samples in <90 min. In addition to heating rate during infiltration, influence of soaking duration and the particle size of glass powder on glass infiltration were also examined.

Fabrication and High Temperature Creep Behaviour of Interpenetrated Nickel–Chromium/Alumina Composites

AbstractA method to manufacture unique interpenetrating 50 vol% nickel–chromium/alumina composites, namely NiCr8020/Al2O3/50pp, is reported. Key process is a high temperature squeeze casting procedure at temperatures above 1500 °C used to infiltrate alloy NiCr8020 into porous alumina preforms exhibiting a bimodal pore structure. Microstructure and mechanical properties of this new composite material are presented. Bending creep tests at 1000 and 1150 °C are performed. The obtained results are discussed in comparison to a nickel based superalloy. It is shown, that particle preform reinforcement is a promising method to improve creep resistance of nickel based alloys significantly. Due to its outstanding creep resistance, the composite material has a high potential for structural and tribological applications at high temperatures in oxidizing atmospheres.

Internal load transfer and damage evolution in a 3D interpenetrating metal/ceramic composite

The internal load transfer and compressive damage evolution in an interpenetrating Al2O3/AlSi12 composite have been studied in this work. The composite was fabricated by squeeze-casting eutectic aluminium–silicon alloy melt in a porous alumina preform. The preform was fabricated from a mixture of cellulose fibres and alumina particles via cold pressing and sintering. In an earlier work we reported the internal load transfer in the same composite material under monotonic compression and tension studied using energy dispersive synchrotron X-ray diffraction [20]. The current work is a continuation of this earlier study, aimed at obtaining further understanding about load transfer occurring during load reversal and damage behaviour during external compression. The micromechanical load partitioning between the three phases present in the composite is studied during one load cycle starting in compression followed by unloading and reloading in tension until failure. Average strain and stress value in each phase is calculated from several diffraction planes of each phase and as a result the reported strain and stress are representative of the bulk material behaviour. The load transfer results allow identifying the occurrence of a substantial Bauschinger effect in the Al solid solution phase and progressive damage evolution within the alumina phase. In situ compression test inside a scanning electron microscope showed that failure of the composite occurred by propagation of cracks through the ceramic rich regions, oriented at approximately 45° to the loading direction.

Effect of particle size distribution of alumina on strength of glass-infiltrated alumina

Glass-infiltrated alumina composites were prepared by infiltrating glass into a pre-sintered alumina. Three different alumina preforms were obtained from various combinations of fine and coarse alumina particles. After infiltration of glass into the porous alumina preforms, their microstructure and strength were studied. The highest bending strength of 510MPa was observed when the composite was made by mixing coarse and fine alumina powders at a ratio of 6:4. The infiltrated glass corroded the alumina preform, and the dissolved aluminum ions reprecipitated on the alumina grains during the heat-treatment for infiltration.

Analysis of the elastic properties of an interpenetrating AlSi12–Al 2O 3 composite using ultrasound phase spectroscopy

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium–silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18–65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.

Internal load transfer in a metal matrix composite with a three-dimensional interpenetrating structure

An interpenetrating composite fabricated by squeeze-casting AlSi12 melt in a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as a pore-forming agent and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Lattice strains under externally applied tension and compression in all three phases of the composite were measured using energy dispersive synchrotron X-ray diffraction. Multiple diffraction peaks in each phase were analysed, allowing information about the interplanar anisotropy of each phase to be obtained. Results show that load transfer occurs from aluminium to stiffer and stronger alumina and silicon phases in both elastic and plastic regions. Stresses as well as stress concentration factors along the loading direction in all three phases were calculated from the measured continuum mechanics equivalent lattice strain and the macroscopic elastic constants of each phase. The stress concentration factor in aluminium is less than unity throughout, and it decreases with increasing applied stress owing to load transfer. The maximum stress in alumina along the loading direction is reached in the region of plastic deformation of aluminium. Subsequently, at applied stresses higher than −300MPa in the sample under compression, stress in alumina along the loading direction decreases owing to damage.

3D printing of Al2O3/Cu–O interpenetrating phase composite

Porous alumina preforms were fabricated by indirect 3D printing using a blend of alumina and dextrin as a precursor material. The bimodal granulate powder distribution with a bed density of 0.8 g/cm3 was increased to 1.4 g/cm3 by overprinting. The porosity of the sintered bodies was controlled by adjusting the printing liquid to precursor powder ratio in the range of 33–44 vol%. The green bodies exhibited bending strengths between 4 and 55 MPa. An isotropic linear shrinkage of ~17% was obtained due to dextrin decomposition and Al2O3 sintering at 1600 °C. Post-pressureless infiltration of the sintered preforms with a Cu–O alloy at 1300 °C for 1.5 h led to the formation of a dense Al2O3/Cu–O interpenetrating phase composite (IPC). X-ray analysis of the fabricated composites showed the presence of α-Al2O3, Cu and Cu2O. CuAl2O4 spinel was not observed at the grain boundaries during HRTEM examination. The Al2O3/Cu–O interpenetrating phase composite revealed a fracture toughness of 5.5 ± 0.3 MPam1/2 and a bending strength of 236 ± 32 MPa. In order to demonstrate technological capability of this approach, complex-shaped bodies were fabricated.

Three‐Dimensional Printing of Complex‐Shaped Alumina/Glass Composites

AbstractAlumina/glass composites were fabricated by three‐dimensional printing (3DP™) and pressureless infiltration of lanthanum‐alumino‐silicate glass into sintered porous alumina preforms. The preforms were printed using an alumina/dextrin powder blend as a precursor material. They were sintered at 1600 °C for 2 h prior to glass infiltration at 1100 °C for 2 h. The influence of layer thickness and sample orientation within the building chamber of the 3D‐printer on microstructure, porosity, and mechanical properties of the preforms and final composites was investigated. The increase of the layer thickness from 90 to 150 µm resulted in an increase of the total porosity from ∼19 to ∼39 vol% and thus, in a decrease of the mechanical properties of the sintered preforms. Bending strength and elastic modulus of sintered preforms were found to attain significantly higher values for samples orientated along the Y‐axis of the 3D‐printer compared to those orientated along the X‐ or the Z‐axis, respectively. Fabricated Al2O3/glass composites exhibit improved fracture toughness, bending strength, Young's modulus, and Vickers hardness up to 3.6 MPa m1/2, 175 MPa, 228 GPa, and 12 GPa, respectively. Prototypes were fabricated on the basis of computer tomography data and computer aided design data to show geometric capability of the process.

Activation Mechanism and Infiltration Kinetic for Pressureless Melt Infiltration of Ti Activated Al2O3 Preforms by High Melting Alloy

The infiltration mechanism of X3CrNi13-4 in titanium activated porous alumina preforms has been studied. Investigations revealed isolated steel-covered titanium particles beyond the infiltration front. The only transport path possible for the steel to form such wetted islands is through the gas phase. Supersaturation due to the mixing of the steel gas phase with the titanium rich gas phase over the activator particle surfaces is proposed as condensation mechanism. Progressive condensation leads to the formation of a melt network, which serves as pathway for the original steel melt to infiltrate the preforms and to fill the remaining pore space in the non-wetting X3CrNi13-4/Ti-Al2O3 sytem.

Experimental Models for Activtation Mechanism of Pressureless Infiltration in the Non‐Wetting Steel‐Alumina MMC System

Common activation mechanisms, which are responsible for infiltration in other non-wetting systems, play only a secondary role in the Ti activated Al2O3/X3CrNiMo13-4 system for pressureless alloy infiltration into porous ceramic preforms. Experiments with a model pore consisting of two juxtaposed parallel alumina plates with lithographically imprinted arrays of Ti-dots show that transport through the vapour phase and subsequent condensation of the steel components onto the activator are the key steps of the activation mechanism. The mechanism was additionally confirmed by infiltration experiments with porous alumina preforms consisting of alternating layers with and without Ti parallel to the infiltration direction. Only layers with activator particles were infiltrated.

Kinetics of Pressureless Infiltration of Al-Mg Melts into Porous Alumina Preforms

Effective “hydrodynamic” radii governing infiltration kinetics of reactive Al-Mg melts into alumina preforms were found to be three orders of magnitude smaller than the average pore size of the packed bed and also smaller compared with the kinetics for a nonreactive system. A sinusoidal capillary model was developed to predict flow kinetics within the packed bed. For the reactive system, two factors were ascribed for additional melt retardation: (1) different intrinsic wettabilities of the two liquids on alumina, thereby leading to significantly different “effective” local contact angles; and (2) local solute depletion from the meniscus, which was incorporated as a time-dependent contact angle.

Fabrication of Al2O3-based composites by indirect 3D-printing

A powder mixture of alumina and dextrin was used as a precursor material for fabrication of porous alumina preforms by indirect three-dimensional printing. Post-pressureless infiltration of the fabricated preforms with copper alloys resulted in dense composites with interpenetrating microstructure. The fabrication procedure involves four steps: a) freeze-drying of Al2O3/dextrin blends, b) three-dimensional printing of the green bodies, c) drying, dextrin decomposition and sintering of the printed bodies and d) post-pressureless infiltration of Cu-alloy into as-fabricated Al2O3 porous preforms. As result of the dextrin decomposition and Al2O3 sintering an average linear shrinkage of 17.7% was measured. After sintering the Al2O3 preforms with ∼36 vol.% porosity were obtained. A post-infiltration with copper alloy at 1300 °C for 1.5 h led to formation of dense Al2O3/Cu parts. X-ray analysis showed the presence of α-Al2O3, Cu and Cu2O only. Al2O3/Cu composite exhibits a fracture toughness of ∼5.5 MPa m1/2 and bending strength of ∼236 MPa. Fractographic analysis showed that crack bridging by plastically deformed metal phase may control the fracture toughness of this composite.

Infiltration and directional solidification of CMSX-4 through a particulate ceramic preform

A potential method for producing selectively reinforced, single-crystal superalloy castings was investigated. Porous particulate alumina preforms were infiltrated with the liquid Ni-based superalloy CMSX-4 under a gas pressure of about 1 atm, and then the alloy was solidified directionally through the preform at different rates. Optical microscopy and Laue X-ray diffraction showed that the orientation of the metallic grains was unaffected by the preform and that no stray grains were nucleated there. In the particular experimental configuration, single grain orientations (i.e., single crystals) were achieved at cooling rates ≤15 °C/min. Furthermore, the microsegregation pattern in the composite region showed that the alloy solidified from the center of the interstices toward the alumina particles, further indicating that nucleation did not occur on the preform. Microsegregation in the composite region is lower than in the unreinforced regions due to geometric confinement of solidification in the narrow spaces between the ceramic particles.

Nickel–alumina nanocomposite powders prepared by novel in situchemical reduction

Nanocomposite powders of Ni–Al2O3 (5 and 10 vol% Ni) were prepared from porous alumina preforms with high specific area (100 m2 g?1) impregnated with nickel nitrate. Samples were obtained by reduction under a controlled oxygen partial pressure either directly or through an intermediate step, giving nickel aluminum spinel. In both cases, homogeneous dispersions of nickel particles in the alumina matrix were achieved. The Ni particle size ranged from 10 to 100 nm, depending on the preparation conditions and temperature.