Preparation and properties of NiAl(Si)/Al2O3 co-continuous composites obtained by reactive metal penetration

Reactive metal penetration was used to prepare intermetallic–ceramic composites with co-continuous structure, starting from silica glass preforms. Two subsequent metal penetrations were performed: first, the silica was immersed in a liquid Al bath, obtaining an Al(Si)/Al₂O₃ composite, then Ni was put in contact with the composite at high temperature, bringing to the substitution of Al with a Ni–Al intermetallic. The obtained composites present both phases continuous, and the whole process is a near net-shape one. The intermetallic phase is based on the Ni–Al system, with small Si content (lower than 2%) and its composition ranges from Al₃Ni₂ to a mixture of NiAl and Ni₃Al depending on the Ni content during the second penetration step.The composites present high hardness and melting point, low thermal expansion coefficient and good mechanical properties.     

High-temperature oxidation of ceramic matrix composites dispersed with metallic particles

Oxidation behavior of ceramic matrix composites dispersed with metallic particles is discussed to establish materials design for high-temperature applications. Oxidation kinetics of ceramic matrix composites dispersed with metallic particles is understood from the viewpoint of the diffusion properties and defect chemistry of matrix oxides. High-temperature oxidation of Ni(p)/partially stabilized zirconia, Ni(p)/Al₂O₃ and Ni(p)/MgO was described as examples.     

Production of a functionally graded artificial tooth root by unique sequence of processes

 A unique sequence of processes is used to produce a prototype of a functionally graded artificial tooth root: (1) Dry-jet spraying of the mixture of Ti and Al₂O₃ ultrafine particles (UFPs) produced by radio-frequency plasma onto the surface of a cylindrical Ti rod, where the composition of the UFPs is changed gradually in the outward radial direction from Ti to Al₂O₃; (2) Temperature-gradient sintering of the deposited composite, where the Ti – and the Al₂O₃– rich sides are heated simultaneously at about 1400 K and 1800 K, respectively; (3) Plasma spray coating of hydroxyapatite (HAP) onto the outermost Al₂O₃ surface of the sintered composite. The final product has compressive strength of more than 200 MPa and is durable against fatigue test of 10⁷ stress cycles at 1000 N. The adhesion strength between the Ti substrate and the Ti-Al₂O₃ functionally graded layer exceeds 65 MPa. No contamination with heavy metals is detected throughout the processes and biological cell growth is confirmed to occur on the HAP surface. With these mechanical and biochemical properties the composite produced here is considered to be highly suitable for an artificial tooth root. A series of processes developed here are expected to be applied to the production of various kinds of fine-grained functionally graded materials with complicated forms. Received: 13 October 1997 / Accepted: 27 October 1997     

In situ fabrication of α-Al2O3 and Ni2Al3 reinforced aluminum matrix composites in an Al–Ni2O3 system

α-Al₂O₃ ceramic particles and Ni₂Al₃ intermetallic compound reinforced aluminum matrix composites were successfully fabricated via exothermic dispersion (XD) reaction in an Al–Ni₂O₃ system. Thermodynamic analysis indicated that the reaction between Al and Ni₂O₃ could occur spontaneously due to its negative Gibbs free energy. The reaction characteristic was discussed by using X-ray diffraction (XRD) method and differential scanning calorimetry (DSC) analysis. The results showed that the reactions of the Al–Ni₂O₃ system consisted of two steps as following: (1) the Al firstly reacted with Ni₂O₃ to form the stable α-Al₂O₃ particles and active Ni atoms; (2) the active Ni atoms further reacted with Al to form Ni₂Al₃. The values of activation energy of the two step reactions were around 457.3 and 282.4 kJ/mol, respectively. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) revealed that the Ni₂Al₃ blocks were uniformly distributed throughout the matrix, while the α-Al₂O₃ particles were slightly segregated in the matrix. The strength of the composite is controlled by the strength of Ni₂Al₃ phase, and the tensile strength and the elongation rate of the composite with 30 vol.% reinforcement volume fraction are 210 MPa and 8%, respectively.     

Fabrication of carbon nanofiber(CNF)-dispersed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites by pulsed electric-current pressure sintering and their mechanical and electrical properties

Dense Al₂O₃-based composites (≥99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300°C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al₂O₃ powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength σ_b (∼800 MPa) and fracture toughness K _IC (∼5 MPa·m^1/2) were attained at the composition of CNF/Al₂O₃ = 5:95 vol%, which σ_b and K _IC values were ∼25% and ∼5%, respectively, higher than those of monolithic Al₂O₃. This might be due to the small Al₂O₃ grains (1.6 μm) of dense sintered compacts compared with that (4.4 μm) for the pure Al₂O₃ ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al₂O₃ composites decreased rapidly from >10¹⁵ to ∼2.1 × 10⁻² Ωm (5vol%CNF addition), suggesting the machinability of Al₂O₃-based composites by electrical discharge machining.

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Fabrication of bioglass infiltrated Al2O3–(m-ZrO2) composites

Using 80 vol.% of poly methyl methacrylate (PMMA) as a pore-forming agent to obtain interconnected porous bodies, porous Al₂O₃–(m-ZrO₂) bodies were successfully fabricated. The pores were about 200 μm in diameter and were homogeneously dispersed in the Al₂O₃–25 vol.% (m-ZrO₂) matrix. To obtain Al₂O₃–(m-ZrO₂)/bioglass composites, the molten bioglass was infiltrated into porous Al₂O₃–(m-ZrO₂) bodies at 1400°C. The material properties of the Al₂O₃–(m-ZrO₂)/bioglass composites, such as relative density, hardness, compressive strength, fracture toughness and elastic modulus were investigated.     

Processing and properties of Y-TZP/Al2O3 composites

The processing and property measurement of Y-TZP/Al₂O₃ ceramic-ceramic composites was investigated. The wet chemical synthesis route was adopted for the preparation of 3Y-TZP matrix dispersed with Al₂O₃ in three different volume fractions. Characterization of the resultant powders was carried out and their densification behaviour was studied by sintering in air in the temperature range 1200–1600 °C. The role of alumina as grain-growth inhibitor for Y-TZP, and the mechanical response of these ultrafine-grain ceramic composites in terms of K_lc characteristics, have been discussed.     

Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in Ni-Al system and theα, β, γ, ɛ and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al₃Ni and NiAl were always ordered, while Ni₃Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni₃Al phase fields, particularly towards Al-rich compositions. Al₃Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni₃Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at.% Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In case of NiAl and Al₃Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔH _f) is large (> 120 kJ/mol), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases, irrespective of the magnitude of ΔH _f, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.     

Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process

Core-shell structured HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites were fabricated using a multi extrusion process. The shell of Al₂O₃-(m-ZrO₂) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO₂ dispersed in the HAp matrix. The t-ZrO₂ and m-ZrO₂ particles (< 400 nm) were homogeneously dispersed in the HAp and Al₂O₃ phases, respectively. In the HAp-(t-ZrO₂) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively.     

Binder derived semi-conductive graphite/alumina composites via novel one-step pulsed electric current sintering

Semi-conductive graphite/Al₂O₃ ceramic composites were successfully fabricated via a novel and facile approach by pulsed electric current sintering of binder embedded Al₂O₃ powders, which were prepared by a simple ball-milling assisted mixing of Al₂O₃ powder with polyvinyl alcohol solution. By altering the sintering conditions with respects to temperature and pressure controlling and concentration of binder, relative density, and microstructure of fabricated graphite/Al₂O₃ ceramic composites, and the electrical properties were schematically investigated in this study. The fabricated graphite/Al₂O₃ ceramic composites exhibit superior semi-conductive properties. Moreover, the carrier type in fabricated semi-conductive graphite/Al₂O₃ composites was successfully modified by adding polycarboxylic acid as a dispersant in binder embedded Al₂O₃ powders. The contents in this work present a new strategy for the design and development of functional ceramic composites.     

Alumina powder assisted carbon nanotubes reinforced Mg matrix composites

Mg matrix composites reinforced by carbon nanotubes (CNTs)-Al₂O₃ mixture, which was synthesized by in situ growing CNTs over Al₂O₃ particles through chemical vapor deposition (CVD) using Ni catalyst, were fabricated by means of powder metallurgy process, followed by hot-extrusion. By controlling synthesis conditions, the as-grown CNTs over Al₂O₃ particles possessed high degree of graphitization, ideal morphology, higher purity and homogeneous dispersion. Due to the ‘vehicle’ carrying effect of micrometer-level A₂O₃, CNTs were easy to be homogeneously dispersed in Mg matrix under moderate ball milling. Meanwhile, Al₂O₃ particles as catalyst carriers, together with CNTs, play the roles of synergistic reinforcements in Mg matrix. Consequently, the Mg matrix composites reinforced by CNTs-Al₂O₃ mixture exhibited remarkable mechanical properties.     

Influence of SiC, Si3N4 and Al2O3 particles on the structure and properties of electrodeposited Ni

The structure and properties of electrodeposited nickel composites reinforced with inert particles like SiC, Si₃N₄ and Al₂O₃ were compared. A comparison was made with respect to structure, morphology, microhardness and tribological behaviour. The coatings were characterized with optical microscopy, Scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) technique. The cross-sectional microscopy studies revealed that the particles were uniformly distributed in all the composites. However, a difference in the surface morphology was revealed from SEM studies. The microhardness studies revealed that Si₃N₄ reinforced composite showed higher hardness compared to SiC and Al₂O₃ composite. This was attributed to the reduced crystallite size of Ni — 12 nm compared to 16 nm (SiC) and 23 nm (Al₂O₃) in the composite coating. The tribological performance of these coatings studied using a Pin-on-disk wear tester, revealed that Si₃N₄ reinforced composite exhibited better wear resistance compared to SiC and Al₂O₃ composites. However, no significant variation in the coefficient of friction was observed for all the three composites.     

Microstructure and composition of Al-Al2O3 composites made by reactive metal penetration

The microstructure of Al-Al₂O₃ composites made by reactive penetration of Al (or Al alloy) into ceramic (mullite or kaolin) preforms has been investigated using transmission electron microscopy (TEM). The Al-Al₂O₃ composites were found to contain a mutuallyinterconnected network of Al and Al₂O₃. No crystallographic orientation was observed between the Al and Al₂O₃ phase. Impurities and pores in the ceramic preforms were found to have a strong effect on the microstructure of the composites. The impurities resulted in formation of small particles in the Al₂O₃ grains of Al-Al₂O₃ composites, whereas the porosity yielded a varied ratio of Al to Al₂O₃ in the composites. The growth rate of the Al-Al₂O₃ composites was found to depend on the microstructure and composition of the ceramic preforms as well as the composition of the reactive metals. Pure aluminium penetrated into a dense mullite faster than into a porous mullite at temperatures below 1200 °C. Addition of Mg to Al reduced the growth rate, whereas a continuous phase of amorphous SiO₂ in the ceramic preforms increased the growth rate.     

Microwave processing of WC-Co composities and ferroic titanates

 The innovations in microwave processing of ceramics have been dominated to date by serendipitous discovery, because the interaction between such radiation as delivered via available tools and the materials of widely varying properties, sizes, and shapes is so complex that it has defied quantitative analysis. For over 10 years a wide variety of inorganic ceramic and semiconducting materials have been synthesized, sintered, and reacted in our own labs, including microwave hydrothermal synthesis of metals, ferrites, and electroceramic phases. These local results are summarized and used as the reference point for reporting on two different new advances: sintering of WC-Co composite tool bits and other similar objects in under 15 min, while retaining extremely fine microstructures, without any grain growth inhibitors; using reduced TiO₂ or Ta₂O₅ for the synthesis of phases such as BaTiO₃, Ba3MgTa₂O9, and Pb(Zr.Ti)O₃ in a few minutes in a 2.45 GHz field at the astonishing temperatures of 300–700 oC. Received: 2 January 1997 / Accepted: 27 March 1997     

Evaluation of a.c. conductivity of rubber ferrite composites from dielectric measurements

The effect of frequency, composition and temperature on the a.c. electrical conductivity were studied for the ceramic, Ni_1−xZn_xFe₂O₄, as well as the filler (Ni_1−xZn_xFe₂O₄) incorporated rubber ferrite composites (RFCs). Ni_1−xZn_xFe₂O₄ (where) (bix)varies from 0 to 1 in steps of 0.2 were prepared by usual ceramic techniques. They were then incorporated into a butyl rubber matrix according to a specific recipe. The a.c. electrical conductivity (σ_a.c) calculations were carried out by using the data available from dielectric measurements and by employing a simple relationship. The a.c. conductivity values were found to be of the order of 10⁻³ S/m. Analysis of the results shows that σ_a.c. increases with increase of frequency and the change is same for both ceramic Ni_1−xZn_xFe₂O₄ and RFCs. σ_a.c increases initially with the increase of zinc content and then decreases with increase of zinc. Same behaviour is observed for RFCs too. The dependence of σ_a.c on the volume fraction of the magnetic filler was also studied and it was found that the a.c. conductivity of RFCs increases with increase of volume fraction of the magnetic filler. Temperature dependence of conductivity was studied for both ceramic and rubber ferrite composites. Conductivity shows a linear dependence with temperature in the case of ceramic samples.     

Mechanical and magnetic properties of Ni-Co dispersed Al2O3 nanocomposites

Effects of the fabrication processing on the microstructure and properties of composites were investigated. High-density Ni-Co dispersed-Al₂O₃ (Al₂O₃/Ni-Co) composites were obtained by hydrogen reduction and consolidated using hot pressing and pulse electric current sintering (PECS) of Al₂O₃, Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O powder mixtures. Microstructural investigations of the hot-pressed composite fabricated using again wet/dry ball-milled powder mixture after calcination revealed that fine Ni-Co particles, about 145 nm in diameter, dispersed homogeneously at the matrix grain boundaries. In particular, fine microstructure of dispersion with the average size of 90 nm was realized for the specimen consolidated by PECS method. High strength of over 1 GPa and hardness of 19 GPa were measured for the nanocomposites prepared from the again ball-milled powder mixture. The ferromagnetism of nano-sized Ni-Co contributes to the magnetic properties of the composites. A change in the coercive force with dispersion size was observed. Also, the extent of magnetic response by an applied stress was strongly influenced by the size of Ni-Co particles. The relations between microstructure and mechanical as well as magnetic properties are discussed.     

Combustion synthesis of ferromagnetic Al2O3-based cermets in thermal explosion mode

The ferromagnetic Al₂O₃-based cermets with different ratios of Co and Co–50Ni alloys were successfully prepared by combustion synthesis in thermal explosion (TE) mode. The reaction process, microstructure, and magnetic property of cermets were investigated. The relative density of cermets can be over 95% via uniaxial loading at the time of ignition when the cermets are hot and ductile. In Al₂O₃–Co cermets, β-Co and α-Co co-exist at room temperature with average size of less than 10 μm and disperse homogeneously in the matrix, while in Al₂O₃–(Co–50Ni) cermets, the network-like Co–50Ni alloy can infiltrate into the boundary gaps of Al₂O₃ particles. The ferromagnetic Co and Co–50Ni alloys are responsible of the magnetic properties of Al₂O₃-based cermets. The saturation magnetization strongly depends on the magnetic characteristics and ratios of ferromagnetic phases. Al₂O₃–(Co–50Ni) cermets have soft magnetic properties with high magnetic susceptibility and low coercive force.     

Effects of ball milling and high-pressure torsion for improving mechanical properties of Al–Al2O3 nanocomposites

Pure Al powders were mixed with a 30 % volume fraction of Al₂O₃ powders having particle sizes of ~30 nm. The mixed powders were first subjected to ball milling (BM) and thereafter consolidated by high-pressure torsion (HPT) at room temperature under a pressure of 3 GPa for 10 turns. The Al–Al₂O₃ composite produced by BM and HPT (BM + HPT) had a more uniform dispersion of the nano-sized Al₂O₃ particles in the Al matrix. Hardness values of the BM + HPT composites were higher than those of the composites without BM. It is shown that the use of BM powders for HPT is more effective in achieving a uniform dispersion of the nano-sized Al₂O₃ particles and in improving mechanical properties of the Al–Al₂O₃ nanocomposites.     

Microstructure and mechanical properties of chromium and chromium/nickel particulate reinforced alumina ceramics

A range of Al₂O₃-Cr and Al₂O₃-Cr/Ni composites have been made using either pressureless sintering in the presence of a graphite bed or hot pressing. Examination of the microstructures shows that they are fully dense (typically 98–99% of the theoretical density) and that the micrometre-scale metallic particles remain discrete and homogeneously dispersed in all composites. All of the hot pressed specimens had higher flexural strengths than the sintered materials. Within each processing route, the composites had slightly lower strength values than the equivalent monolithic alumina specimens. This was attributed to weak interfacial bonding. Fracture toughness behaviour was investigated using indentation and double cantilever beam methods. All of the composites were found to be tougher than the parent alumina and to show resistance-curve behaviour. For the composites, maximum fracture toughness values were 5–6 MPa m^1/2 (about double the value for alumina) for process zone sizes of a few millimetres, although steady state was not reached in the limited number of specimens tested. Examination of fracture surfaces and indentation cracks showed that the toughening potential of the metal particles was not exploited to any significant extent. This was mainly due to weak metal-Al₂O₃ interfaces, but also because of carbon embrittlement of the metallic particles in which chromium was the major constituent.     

Magnetoelectric and electrical properties of WO3-doped (Ni0.8Zn0.1Cu0.1)Fe2O4/[Pb(Ni1/3Nb2/3)O3–Pb(Zn1/3Nb2/3)O3–PbTiO3] composites

A total of 5 mol% WO₃-doped (1−x)(Ni_0.8Zn_0.1Cu_0.1)Fe₂O₄/xPb(Ni_1/3Nb_2/3)O₃–Pb(Zn_1/3Nb_2/3)O₃–PbTiO₃ ((1−x)NZCF/xPNN-PZN-PT) magnetoelectric particulate ceramic composites were prepared by conventional solid-state reaction method via low-temperature sintering process. X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM) observation indicate that piezoelectric phase and ferrite phase coexist in the sintered particulate ceramic composites. Dielectric property of the (1−x)NZCF/x0.53PNN–0.02PZN–0.05Pb(Ni_1/2W_1/2)O₃–0.40PT ((1−x)NZCF/xPNN-PZN-PNW-PT, nominal composition) composites is improved greatly as compared to that of the undoped (1−x)NZCF/xPNN-PZN-PT composites. The WO₃-doped (1−x)NZCF/xPNN-PZN-PT composites exhibit typical P–E hysteresis loops at room temperature accompanied by the decrease of saturation polarization (P _s) and remnant polarization (P _r). At the same time, piezoelectric property of the composites deteriorates greatly with the increase of ferrite content. The (1−x)NZCF/xPNN-PZN-PNW-PT composites can be electrically and magnetically poled and exhibit apparent magnetoelectric (ME) effect. A maximum ME voltage coefficient of 13.1 mV/(cm Oe) is obtained in the 0.1NZCF/0.9PNN-PZN-PNW-PT composite at 400 Oe d.c. magnetic bias field superimposed 1 kHz a.c. magnetic field with 5 Oe amplitude. The addition of WO₃ in the piezoelectric phase decreases sintering temperature greatly from 1180 °C to 950 °C and decreases dielectric loss sharply of the composites, thus the ME voltage coefficient increases. Such ceramic processing is valuable for the preparation of magnetoelectric particulate ceramic composites with excellent ME effect.