Al2O3 Preforms Research Articles

In-situ SEM investigation on the damage behavior of an interpenetrating metal ceramic composite

For the investigation of interpenetrating materials, the microstructure and the associated damage mechanism are crucial for the understanding of its properties. In this work, an aluminum-alumina interpenetrating composite is investigated and characterized regarding the microstructure and the tensile and compression properties during in-situ SEM experiments. The composite is fabricated by gas pressure infiltration and consists of a highly homogeneous and fine-pored Al2O3 preform and an AlSi10Mg cast alloy. Due to the fine-pored ceramic phase, building a strong contrast to the metallic phase, digital image correlation (DIC) could be applied directly on the SEM images, captured during the in-situ experiments, without the need of a speckle pattern on the surface. The microstructure analysis and DIC results were correlated with the load level, derived from stress-strain curves, to achieve deeper understanding from the damage behavior in interpenetrating metal ceramic composites.

Analysis of interpenetrating metal ceramic composite structures using an enhanced random sequential absorption microstructure generation algorithm

In this paper, we present the analysis of an interpenetrating metal ceramic composite structure. We introduce a new generation algorithm for the modeling of interpenetrating composite microstructures with connected, spherical cavities embedded into an open-porous foam structure. The method uses a geometric ansatz and is designed to create structures of special topology, as the investigated metal ceramic composite structures consisting of a connected AlSi10Mg phase showing spherical shapes embedded into an Al2O3 preform. Based on the introduced enhanced random sequential absorption approach, the generated microstructures yield numerical insights into the material that are not accessible by experimental techniques. The generated microstructures are compared to structures reconstructed from experimental CT scan data considering microstructural features and mechanical behavior. We show that the proposed method is able to generate statistically equivalent microstructures by using only a small number of statistical descriptors. The numerical formulation is validated using compression tests including plastic yielding in the aluminum and damage progression in the ceramic phase. Both the composite material and the pure ceramic preform are considered in this analysis, and good agreement is found between reconstructed and generated microstructures. Furthermore, the observations reveal the importance of the local geometrical sphere arrangement with respect to the mechanical behavior. A validation with experimental results is presented and it is shown that the model predicts microstructural properties and gives meaningful insights into the structural and material interplay. Finally, we discuss the potential of the method for the investigation of failure mechanisms.

Al2O3 Preforms Infiltrated with Poly(methyl methacrylate) for Dental Prosthesis Manufacturing

The combination of biocompatible polymers and ceramics shows great promise in the development of composites with suitable mechanical properties for dental applications. In an attempt to further expand this research line, Al2O3 commercial powders (Vitro-ceram, Alglass, In-ceram) were sintered at 1400 °C for 2 h and infiltrated with poly(methyl methacrylate) for potential use in dental prostheses. The infiltration was performed using a homemade apparatus under a pressure of 7 bar for 6 and 12 h. The microstructure (studied using a scanning electron microscope), Archimedes density, 3-point bending flexural strength and Vickers hardness of the prepared composites were assessed and quantitatively compared. In general, microstructural analyses showed ceramic- and polymer-based interpenetrating network in all materials. The preforms infiltrated for 12 h showed superior properties; among them, the Vitro-ceram-based composite also demonstrated a near-zero open porosity and optimum mechanical characteristics. Specifically, its density, strength and hardness were 2.6 ± 0.07 g/cm3, 119.3 ± 5.0 MPa and 1055.1 ± 111.0 HV, respectively, passing the acceptance criteria of ISO 6872 and making it suitable for consideration as a metal-free structure for dental crowns and fixed partial prostheses until three anterior units.

Al/Al2O3 metal matrix composites produced using magnetic field-assisted freeze-casting of porous ceramic structures

Al/Al2O3 metal matrix composites (MMCs) were produced by metal infiltration of porous ceramic preforms. The porous ceramic preforms were fabricated using the magnetic field-assisted freeze-casting method, resulting in vertically aligned porous channels. Preforms were prepared by freezing an Al2O3/Fe3O4-containing slurry within an applied magnetic field. Vertical alignment of the channels was facilitated by the magnetic response of the Fe3O4 in the slurry during the freezing process. After freezing and sublimation, the ceramic preforms were sintered and then infiltrated with molten A356 Al-based alloy. The mechanical properties of the resulting Al2O3/A356 MMCs were compared to those of bulk Al2O3, bulk Al-based alloy (A356), and porous Al2O3 preforms using micro-indentation testing. The indentation hardness and elastic moduli values of Al2O3/A356 MMCs showed good agreement with the predicted theoretical calculations. This study provides a new approach for the design of MMCs with controlled composition and improved mechanical characteristics.

Experimental determination of threshold pressure and permeability based on equation-solving method for liquid metal infiltration processes

Abstract A new method for determining two key parameters (threshold pressure and permeability) for fabricating metal matrix composites was proposed based on the equation-solving method. An infiltration experimental device was devised to measure the infiltration behavior precisely with controllable infiltration velocity. Two experiments with alloy Pb-Sn infiltrating into Al2O3 preform were conducted independently under two different pressures so as to get two different infiltration curves. Two sets of coefficients which are functions of threshold pressure and permeability can be obtained through curve fitting method. By solving the two-variable equation set, two unknown variables were determined. It is shown that the determined threshold pressure and permeability are very close to the calculated ones and are also verified by another independent infiltration experiment. The proposed method is also feasible to determine the key infiltration parameters for other metal matrix composite systems.

Near-Net Shape Manufacturing of Copper-Ceramic Composites Via Gas Pressure Infiltration in Permanent Molds

Abstract Composites with a interpenetrated copper-ceramic microstructure can be customized for highest demands such as a high thermal conductivity in combination with a small thermal expansion. This article reports on the synthesis of near-net shape Cu-Al2O3 composites via gas pressure infiltration. Open-porous Al2O3 preforms are infiltrated with Cu using Si3N4 permanent molds. The infiltration quality as well as the porosity distribution within the materials are described qualitatively and quantitatively. The effects of the manufacturing process and cooling rate on the composites will be looked at.

Microchanneled biomorphic AlN-coated Al2O3 by pressureless infiltration–nitridation

Microporous Al2O3/AlN composites with aligned channels were fabricated by pressureless infiltration–nitridation of Al–Mg alloy into biomorphous Al2O3 preforms. The biomorphous Al2O3 was produced by Al-vapor infiltration in pyrolyzed rattan templates with subsequent oxidation. Infiltration–nitridation was performed using an AlMg3 alloy infiltrated into the porous Al2O3 under flowing N2. After infiltration–nitridation into the porous Al2O3 templates a composite consisting of AlN and Al2O3 as major phases was obtained. Nitridation of Al in these conditions led to hollow hexagonal AlN crystals, which coated uniformly the Al2O3 channels surface. An increase of the BET specific surface area after infiltration–nitridation of Al into the biomorphous Al2O3 was observed. Microchanneled Al2O3 coated with AlN may be used as substrates in microelectronic applications that require high thermal conductivity and low dielectric constant.

Centrifugal Castings Locally Reinforced with Porous AL2O3 Preform

Abstract The main objective of presented researches were tests of infiltration of the porous ceramic preforms at the pressure in centrifugal casting process. Make an assumption that the porous preform will be create the local reinforcement layer in specific area of the casting. For investigations the alumina porous ceramic preforms were applied (Al2O3). The pressure to infiltrate molten aluminum alloy into ceramic preforms was generated by centrifugal force. The structure of composites was examined by light and electron microscope. The investigations of composites microstructure exhibited high degree of infiltration of spherical macropores in Al2O3 ceramic preforms by the molten aluminium alloy. On the basis of structural studies has been shown that centrifugal force is effective as a driving force for the infiltration of porous preforms.

Structure of Aluminium Matrix Composite with Ceramic Preform Obtained by Centrifugal Infiltration Process

The paper presents the microstructure analyzing of composite with aluminium matrix containing porous Al2O3 preform. The AlSi12CuMgNi eutectic alloy have been modified by 1 wt% Mg and 0.03 wt% Sr additions. The magnesium to improve the wettability between matrix and ceramic reinforcement have been added. In turn 0.03 wt% strontium addition were used to changes the size and morphology of the Al-Si eutectic grains. The components and composite material obtained by the centrifugal infiltration process was characterized by means of light and scanning microscopy methods. The conducted investigations proved the large degree of infiltration of ceramic porous by the aluminium alloy and good connection in the boundary area.

Mechanical characterisation of interpenetrating network metal–ceramic composites

A variety of interpenetrating light weight metal matrix composites (porous Al2O3 preforms infiltrated with the aluminium alloy AlSi9Cu3) have been characterised under tensile und compressive loads and fractographically examined by scanning electron microscopy. Fatigue and thermophysical properties were determined. Compared to die-cast AlSi9Cu3, the mechanical properties of the interpenetrating composites were significantly improved.The elastic modulus increased more than twofold, the tensile strength increases by a factor of 2, fatigue limits increased by a factor of 2.3–2.6, density increased slightly by a factor of 1.2 and thermal conductivity is reduced by a factor of 0.5.

Effects of the Injection Temperature of N2 Gas on Pressureless Infiltration for Fabricating Al-Mg/Al2O3 Composites

Pressureless infiltration of molten Al−Mg alloys into particulate Al2O3 preforms has been known to occur only in a nitrogen atmosphere. In order to understand the pressureless infiltration mechanism of Al−Mg alloys into particulate Al2O3 preforms, nitrogen was injected at 25°C, 300°C, and 600°C. The higher the injection temperature of the nitrogen gas was, the lower the infiltration temperature of the molten Al−Mg alloys into the particulate Al2O3 preform was. Pressureless infiltration of the Al−6Mg alloy occurred at 700°C when the nitrogen gas was injected at 600°C. The formation of an Mg−N compound (Mg3N2) on Al2O3 particles, which improves wettability by decreasing the interfacial energy between the Al−Mg alloys and the Al2O3 particles, enabled the formation of the Al−Mg alloy/Al2O3 composite via pressureless infiltration. Increasing the injection temperature close to the melting point of the Al−Mg alloys appeared to enhance the formation of Mg3N2 on the surface of the Al2O3 particles.

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.

Microstructures and properties of Al2O3/Al–AlN composites by pressureless infiltration of Al-alloys

Al alloys were infiltrated into Al2O3 preforms in N2 and N2- 2% H2 gas mixture in the temperature regime of 900-1200°C. The kinetics of nitridation during infiltration were continuously monitored by recording weight gained during infiltration of the preform. The weight gains that are attributed to the formation of AlN in the matrix were observed to increase with processing temperature. In addition, higher weight gains were recorded in N2-H2 as compared to in N2 atmosphere. Analysis of the composites indicates that controlled matrices of AlN/Al can be formed by selection of appropriate process parameters viz. temperature and atmosphere. Based on this study it has been demonstrated that net shaped metal matrix composites (MMCs), with varying amounts of AlN in the matrix can be fabricated by infiltrating alumina preforms at $900-1000^oC$ in commercial nitrogen. By controlling the preform characteristics, it is possible to fabricate composites with significant variation in microstructural scale, and consequently matrix hardness and elastic modulus.

The displacive compensation of porosity (DCP) method for fabricating dense, shaped, high-ceramic-bearing bodies at modest temperatures

A novel process is introduced for the fabrication of dense, shaped ceramic/metal composites of high ceramic content: the Displacive Compensation of Porosity (DCP) method. In this process, a metallic liquid is allowed to infiltrate and undergo a displacement reaction with a porous oxide preform. Unlike other displacement-reaction-based processes (e.g., the C4, RMP, and AAA processes), a larger volume of oxide is generated than is consumed, so that composites with relatively high ceramic contents can be fabricated. Bar- and disk-shaped MgO/Mg-Al composites were produced by the infiltration and reaction of molten Mg with porous Al2O3 preforms at 1000 °C. By varying the relative density of the preforms (from 53.3 to 71.0% of theoretical), the magnesia content of the final composites could be adjusted from 70.4 to 85.6 vol %. Because the increase in oxide volume associated with the conversion of alumina into magnesia was accommodated by the prior pore volume of the preforms, the composites retained the shapes and dimensions (to within a few percent) of the starting preforms. The MgO/Mg-Al composites were lightweight (2.94–3.30 g/cm3), dense (97.7–99.0% of theoretical), and resistant to hydration. Bar-shaped MgO/Mg-Al composites exhibited average flexural strength and indentation toughness values of 244 MPa and 5.4 MPa · m1/2, respectively.

The fabrication of near net-shaped spinel bodies by the oxidative transformation of Mg/Al2O3 precursors

The feasibility of transforming shaped Mg–Al2O3-bearing precursors into monolithic spinel (MgAl2O4) bodies with a retention of shape and dimensions has been demonstrated.Dense, shaped precursors (disks, bars) were fabricated by the pressureless infiltration of molten Mg into porous Al2O3 preforms. After solidification (and machining, in the case of bar-shaped specimens), the Mg-bearing precursors were oxidized in flowing O2 (g) at 430–700 °C. Postoxidation annealing at 1200 °C resulted in the conversion of MgO and Al2O3 into MgAl2O4. After sintering at 1700 °C, spinel bodies that retained the precursor dimensions (to within 0.65%) were produced. Phase and microstructural analyses at various stages of processing are discussed.

Mechanical properties of composites fabricated by pressureless infiltration technique

Abstract Two-phase interpenetrating systems with residual microstresses have been shown to exhibit high fracture toughness and strength. In this work, the microstress-induced strengthening concept was applied to Al 2 O 3 Si composites. The composites were fabricated by pressureless infiltration of porous Al2O3 preforms with a molten Si. The materials obtained exhibited superior fracture toughness, strength and hardness when compared with technical aluminas having the same volume fraction of Al2O3 but containing aluminosilicate glass phases instead of silicon. For example, an Al 2 O 3 Si composite with 30 vol% Si had K1c ≈ 4.8 MPa √m and a bending strength of ~ 320 MPa compared to ~ 3.5 MPa √m and ~ 230 MPa, respectively for the technical alumina AD-85 (‘Coors’). It is believed that the strengthening effect is caused by compressive residual microstresses in an inherently weak Si phase generated as a result of the solidification-related expansion of silicon.