Microstructure and properties of alumina-silicon carbide nanocomposites fabricated by pressureless sintering and post hot-isostatic pressing

Al2O3/5%SiC nanocomposites were fabricated by pressureless sintering using MgO as a sintering aid and then post hot-isostatic pressed (HIP), which can subsequently break through the disadvantage of hot-pressing process. The MgO additive was able to promote the densification of the composites, but could not induce the grain growth of Al2O3 matrix due to the grain growth inhibition by nano-sized SiC particles. After HIP treatment, Al2O3/SiC nanocomposites achieved full densification and homogeneous distribution of nano-sized SiC particles. Moreover, the fracture morphology of HIP treated specimens was identical with that of the hot-pressed Al2O3/SiC nanocomposites showing complete transgranular fracture. Consequently, high fracture strength of 1 GPa was achieved for the Al2O3/5%SiC nanocomposites by pressureless sintering and post HIP process.     

Structure and Properties of Y2O3-Doped Al2O3-MWCNT Nanocomposites Prepared by Pressureless Sintering and Hot-Pressing

This study describes the combined effects of multi-walled carbon nanotubes (CNTs) additions and Y₂O₃ doping on the microstructures and mechanical properties of Al₂O₃-CNT nanocomposites fabricated by pressureless and hot-press sintering processes. A uniform dispersion of CNTs within the Al₂O₃ matrix was successfully attained via a combined approach using surfactant, sonication, and adequate period of incubation. Small amounts (1 wt.%) of Y₂O₃, as dopants, significantly affected the densification and properties of pressureless sintered monolithic Al₂O₃ and its nanocomposites at low CNT concentrations (<1 wt.%); however, they hardly showed any improvement at higher CNT contents. As opposed to the pressureless sintering, pressures applied during high temperature sintering in combination with the Y₂O₃ doping contributed in generating a homogenous microstructure and improved the densities (7 and 15%) and microhardness (11 and 12%) of Al₂O₃ reinforced with higher CNT contents (2 and 5 wt.%), respectively. Adding on, hot-pressed Y₂O₃-doped Al₂O₃ reinforced with 2 and 5 wt.% CNTs showed higher hardness (19 and 70%), flexural strength (10 and 5%), and fracture toughness (26 and 11%), respectively, compared to similar but CNT-free samples. These results showed that pressure-assisted sintering and Y₂O₃ are promising for the fabrication of CNT-reinforced Al₂O₃ nanocomposites, especially at higher CNT concentrations.     

Pressureless sintering of internally synthesized SiC-TiB2 composites with improved fracture strength

SiC-TiB₂ particulate composites were fabricated by converting TiO₂ to TiB₂ through the reaction between TiO₂, B₄C and C. The presence of initially very fine, in-situ created, TiB₂ particles increased driving force for sintering and enabled fabrication of a dense composite utilizing pressureless sintering and the liquid phase created between Al₂O₃ and Y₂O₃ additives. The effect of volume fraction of the in-situ formed TiB₂ on density, microstructure and flexural strength was discussed. It was found that the presence of TiB₂ particles suppressed the growth of SiC grains and enhanced fracture strength. The fracture strength of samples containing 12 vol% TiB₂ was more than 30% higher than that of the monolithic SiC. The effect of SiC grain size on fracture strength was also analyzed.     

Sintering of Al2O3–TiB2 nano-composite derived from milling assisted sol–gel method

Synthesis and sintering of an alumina /titanium diboride nano-composite have been studied as an alternative for pure titanium diboride for ceramic armor applications. Addition of TiB₂ particles to an Al₂O₃ matrix can improve its fracture toughness, hardness and flexural strength and offer advantages with respect to wear and fracture behavior. This contribution, for the first time, reports the sintering, microstructure, and properties of Al₂O₃–TiB₂ nano-composite densified with no sintering aids. Nano-composite powder was produced by combination of sol–gel and mechano-chemical methods. The densification experiments were carried out using both hot pressing and pressureless sintering routes. In the pressureless sintering route, a maximum of 92.3% of the theoretical density was achieved after sintering at 1850 °C for 2 h under vacuum. However, hot pressing at 1500 °C for 2 h under the same condition led to achieving a 99% of the theoretical density. The hot pressed Al₂O₃–TiB₂ nano-composites exhibit high Vickers hardness (16.1 GPa) and a modest indentation toughness (~ 4.2 MPa.m^1/2).     

Rapid synthesis and consolidation of nanostuctured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgSiO<Subscript>3</Subscript> composites by high frequency induction heated sintering

Nanopowders of MgO, Al₂O₃ and SiO₂ were made by high energy ball milling. The rapid sintering of nanostuctured Al₂O₃-MgSiO₃ composites was investigated by the high-frequency induction heating sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibits grain growth. Highly dense nanostructured Al₂O₃-MgSiO₃ composites were produced with the simultaneous application of 80 MPa pressure and the induced output current of total power capacity (15 kW) within 2 min. The sintering behavior, grain size and mechanical properties of Al₂O₃-MgSiO₃ composites were investigated.     

Investigation of the effect of Al2O3–Y2O3–CaO (AYC) additives on sinterability,microstructure and mechanical properties of SiC matrix composites: A review

Appropriate properties of SiC ceramic such as high hardness, low density, high melting point and high elastic modulus make this material as a favorite candidate for different industrial applications. Although some disadvantages including high sintering temperature, low sinterability, and low fracture toughness have restricted the use of this material, previous studies showed that using Al₂O₃-Y₂O₃ additives plays an effective role in the improvement of sinterability as well as the enhancement of the properties of these composites. Moreover, the addition of CaO results in the acceleration of the formation of molten phase and the improvement of sinterability. In addition, the use of these additives cause the formation of the intermetallic phases of Al₅Y₃O₁₂ (YAG) and CaY₂O₄ and by activating the mechanisms of crack deflection, crack bridging, phase transformation, strengthening the grain boundary and changing the fracture mode from intergranular to transgranular results in improved mechanical properties. This paper attempts to investigate the effect of using Al₂O₃–Y₂O₃–CaO (AYC) additives on sinterability, microstructure, and mechanical properties of SiC matrix composites including the composites reinforced with SiC fibers and SiC matrix nano-composites. Finally, the effect of the post-sintering annealing process under two conditions i.e., with and without applying pressure (pressureless sintering) on microstructure and mechanical properties has been studied.     

Microstructure evolution and densification kinetics of Al2O3/Ti(C,N) ceramic tool material by microwave sintering

The microstructure evolution and densification kinetics of Al₂O₃/Ti(C,N) ceramic tool material during microwave sintering were studied. The density and grain growth significantly increases at the temperatures higher than 1400 °C. The calculated kinetics parameter n indicates that volume diffusion is the main densification mechanism when the sintering temperature is below 1300 °C, while grain boundary diffusion plays a leading role in the densification process when the sintering temperature is higher than 1300 °C. The grain growth activation energy of Al₂O₃/Ti(C,N) composite is 48.82 KJ/mol, which is much lower than those of monolithic Al₂O₃ in the microwave sintering and conventional sintering. The results suggested that the Al₂O₃/Ti(C,N) ceramic tool material with nearly full densification and fine grains can be prepared by two-step microwave sintering.     

Wear and oxidation resistance of Al2O3 particle dispersed Al3Ti composite with a nanostructure prepared by pulsed electric current sintering of mechanically alloyed powders

Al₃Ti-matrix composite layers containing Al₂O₃ particles were formed on Ti substrate by pulsed electric current sintering (PECS) of mechanically alloyed (MA) powders to improve the wear and oxidation properties of the Ti substrate. Reducing the grain size of each element by MA makes the combustion synthesis of Al₃Ti possible at a lower temperature. The grain size formed by the combustion synthesis of Al–Ti–Al₂O₃ powder mechanically alloyed for 720 ks was about 10 nm and its growth during sintering was suppressed by the existence of Al₂O₃. The densification behavior of the powder was investigated quantitatively. The obtained Al₃Ti/Al₂O₃ composite layer showed better wear and oxidation resistance than the monolithic Al₃Ti layer.     

Development of new magnesium based alloys and their nanocomposites

In the present study, 1 and 2 wt.% of aluminum were successfully incorporated into magnesium based AZ31 alloy to develop new AZ41 and AZ51 alloys using the technique of disintegrated melt deposition. AZ41-Al₂O₃ and AZ51-Al₂O₃ nanocomposites were also successfully synthesized through the simultaneous addition of aluminum (1 and 2 wt.%, respectively) and 1.5 vol.% nano-sized alumina into AZ31 magnesium following same route. Alloy and composite samples were then subsequently hot extruded at 400 °C and characterized. Microstructural characterization studies revealed equiaxed grain structure, reasonably uniform distribution of particulate and intermetallics in the matrix and minimal porosity. Physical properties characterization revealed that addition of both aluminum and nano-sized alumina reduced the coefficient of thermal expansion of monolithic AZ31. The presence of both Al and nano-sized Al₂O₃ particles also assisted in improving overall mechanical properties including microhardness, engineering and specific tensile strengths, ductility and work of fracture. The results suggest that these alloys and nanocomposites have significant potential in diverse engineering applications when compared to magnesium AZ31 alloy.     

One component metal oxide sintering additive for β-SiC based on thermodynamic calculation and experimental observations

This paper examines a range of metal oxides, including those containing relatively safe elements under neutron irradiation, such as Cr, Fe, Ta, Ti, V and W, as well as widely used oxides, Al₂O₃, MgO and Y₂O₃, as a sintering additive for β-SiC theoretically and experimentally. After selecting the most probable SiC oxidation reaction at 1973–2123 K, the condition where the metal oxide additive does not decompose SiC was calculated based on the standard Gibbs formation free energies. Thermodynamic calculations revealed that Al₂O₃, MgO and Y₂O₃ could be an effective sintering additive without decomposing SiC under hot pressing conditions, which was demonstrated experimentally. On the other hand, no one component metal oxide that contains a safe element for nuclear reactor applications was found to be an effective sintering additive due to the formation of metal carbides and/or silicides. Overall, the simulation based on thermodynamic calculations was found to be quite useful for selecting effective metal oxide additives.     

Effect of MgO particle size on the microstructure, mechanical properties and wear performance of ZTA-MgO ceramic cutting inserts

The mechanical properties, microstructure and wear performance of zirconia-toughened alumina (ZTA) cutting inserts with Magnesia (MgO) in different particle sizes as additives was investigated. The MgO particle sizes were varied from 80 nm to 7000 nm. The alumina (Al₂O₃), yittria stabilized zirconia (YSZ) and MgO powders were mixed, compacted and sintered at 1600 °C using the solid-state sintering method. The mechanical and physical properties of the samples such as wear resistance, Vickers hardness, fracture toughness, microstructure and density were analyzed. Commercially available stainless steel (316L) was used as the workpiece for the wear resistance study. It was observed that smaller MgO particle sizes induce better wear performance and mechanical properties for the cutting inserts. Wear resistance analysis showed that the cutting insert with nano-sized MgO (particle size 80 nm) had the lowest wear area of 0.019 mm². The same cutting insert also possessed the highest Vickers hardness value of 1740 Hv compared to the other samples. Furthermore, microstructural observations show that the Al₂O₃ grain size depends on the particle size of MgO, and is directly related to its hardness property.     

Simultaneous synthesis and consolidation of nanocrystalline Fe2Al5 and Fe2Al5-Al2O3 by pulsed current activated sintering and their mechanical properties

Nanopowders of Fe, Al and Fe₂O₃ are fabricated by high energy ball milling. Using the pulsed current activated sintering method, the densification of nanocrystalline Fe₂Al₅ and Al₂O₃ reinforced Fe₂Al₅ composites were simultaneously synthesized and consolidated within two minutes from mechanically activated powders. The advantage of this process is that it allows very quick densification to near theoretical density and prohibition of grain growth in nanostuctured materials. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid to the application of nanomaterials. Not only the hardness but also the fracture toughness of the Fe₂Al₅-Al₂O₃ composite was higher than that of monolithic Fe₂Al₅ due to the addition of the hard phase of Al₂O₃ and the crack deflection by Al₂O₃.     

Fabrication of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> Dispersion-Strengthened Mo-Si-B Alloy by Powder Metallurgical Method

In this study, we investigate the effect of oxide dispersion strengthening on mechanical properties by dispersion of nano-sized Ta₂O₅ particles in Mo-Si-B alloy. A Mo-Si-B core-shell powder consisting of two intermetallic compounds of Mo₅SiB₂ and Mo₃Si as the core and nano-sized Mo solid solution surrounding intermetallic compounds was fabricated by chemical vapor transport. And Mo-Si-B core-shell powder with uniformly dispersed nano-sized Ta₂O₅ particles on the surface of a Mo solid solution shell was produced by a wet blending process with TaCl₅ solution and heat treatment. Then, pressureless sintering was performed at 1400°C for 3 h under a H₂ atmosphere. The hardness and fracture toughness of the Ta₂O₅-dispersed Mo-Si-B alloy were measured using Vickers hardness and 3-point bending tests, respectively. The Vickers hardness and fracture toughness of the fabricated Mo-Si-B-Ta₂O₅ alloy were more improved than that of the Mo-Si-B alloy fabricated using core-shell powder with no addition of Ta₂O₅ particles (Mo-Si-B alloy: 353 Hv, 13.5 MPa·√m, Mo-Si-B-Ta₂O₅ alloy: 509 Hv, 15.1 MPa·√m).     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

Determination of amorphous interfacial phases in Al2O3/SiC nanocomposites by computer-aided high-resolution electron microscopy

The analysis of thin amorphous layers in transmission electron microscope imaging has always been rendered difficult by the sample geometry conditions which must be met in order to single out the electronic scattering due to the amorphous phase from the overlapping crystalline diffraction fringes. This is the case of Al₂O₃/SiC nanocomposites in which spherical fine SiC particles are entrapped into the Al₂O₃-matrix grains. The spherical geometry of the SiC dispersoids involves a circular phase boundary in the thin foil which is hardly examined by conventional high-resolution techniques. In this paper an alternative technique of computer-aided high-resolution microscopy is proposed for imaging the presence of non-crystalline third phases at the interface of the alumina matrix and the silicon carbide nanosized particles in Al₂O₃/SiC nanocomposites. Implications regarding the effect of the observed layer on some toughening mechanisms proposed in literature for this materials are included.     

Investigation on Addition of Kaolinite on Sintering Behavior and Mechanical Properties of B4C

The objective of the present investigation was to study the effect of kaolinite addition on sintering behavior and mechanical properties of pressureless sintered B₄C ceramic. Different amounts of kaolinite, mainly 5 to 30 wt.%, were added to the base material. The in situ reaction of kaolinite with B₄C generates SiC and Al₂O₃, which aid the sintering process and permit pressureless sintering at temperatures between 2050 and 2150 °C. Addition of 30 wt.% kaolinite and sintering at 2150 °C resulted in improving the density of the samples to about 98.5% of the theoretical density. The composite samples exhibited very good mechanical properties (hardness, flexural strength, and fracture toughness). As wt.% Kaolinite increases, strength and toughness increase, and hardness first increases and then decreases.     

Process−microstructure−properties relationship in Al−CNTs−Al2O3 nanocomposites manufactured by hybrid powder metallurgy and microwave sintering process

Al−2CNTs−xAl₂O₃ nanocomposites were manufactured by a hybrid powder metallurgy and microwave sintering process. The correlation between process-induced microstructural features and the material properties including physical and mechanical properties as well as ultrasonic parameters was measured. It was found that physical properties including densification and physical dimensional changes were closely associated with the morphology and particle size of nanocomposite powders. The maximum density was obtained by extensive particle refinement at milling time longer than 8 h and Al₂O₃ content of 10 wt.%. Mechanical properties were controlled by Al₂O₃ content, dispersion of nano reinforcements and grain size. The optimum hardness and strength properties were achieved through incorporation of 10 wt.% Al₂O₃ and homogenous dispersion of CNTs and Al₂O₃ nanoparticles (NPs) at 12 h of milling which resulted in the formation of high density of dislocations and extensive grain size refinement. Also both longitudinal and shear velocities and attenuation increase linearly by increasing Al₂O₃ content and milling time. The variation of ultrasonic velocity and attenuation was attributed to the degree of dispersion of CNTs and Al₂O₃ and also less inter-particle spacing in the matrix. The larger Al₂O₃ content and more homogenous dispersion of CNTs and Al₂O₃ NPs at longer milling time exerted higher velocity and attenuation of ultrasonic wave.     

Properties and pulsed current activated consolidation of nanostuctured MgSiO3-MgAl2O4 composites

Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. Nanopowders of MgO, Al₂O₃ and SiO₂ were made by high energy ball milling. The simultaneous synthesis and consolidation of nanostuctured MgAl₂O₄-MgSiO₃ composites from milled 2MgO, Al₂O₃ and SiO₂ powders was investigated by the pulsed current activated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured MgAl₂O₄-MgSiO₃ composites were produced with a simultaneous application of 80 MPa pressure and a pulsed current of 2000A within 1min. The fracture toughness of MgAl₂O₄-Mg₂SiO₄ composites sintered from 60 mol%MgO-20 mol%Al₂O₃-20mol%SiO₂ powders milled for 4 h was 3.2MPa·m^1/2. The fracture toughness of MgAl₂O₄-MgSiO₃ composite is higher than that of monolithic MgAl₂O₄.     

Processing and microstructure of alumina-based composites

A study of powder structure and its effect on the sintering tendency of certain alumina-based ceramic systems, that is, Al₂O₃-SiO₂ and Al₂O₃-ZrO₂, was carried out to improve their mechanical strength and fracture toughness. The compacting behavior and the sintering characteristics were optimized through control of various parameters such as composition, compaction pressure, sintering temperature, and time. Best densification was obtained for mixtures prepared using very fine and deagglomerated alumina powders.     

Bulk alumina support with high tolerant strain and its reinforcing mechanisms

Bulk porous Al₂O₃ support was fabricated using a mixture of fine α-Al₂O₃ powder and Al(OH)₃ particles, followed by pressureless sintering at temperatures between 1100°C and 1300°C. Al₂O₃ support with high surface area was obtained, due to the presence of transitional Al₂O₃ phases that were produced by the decomposition of Al(OH)₃ even after sintering. The Al₂O₃ support exhibited superior mechanical properties and high strain to failure, compared with those fabricated by traditional methods. The strain to failure increased with the amount of Al(OH)₃ in the mixture, but decreased with increasing sintering temperature. A conceptual model for the microstructural evolution and reinforcing mechanisms, based on transmission electron microscopy (TEM) observations, is proposed. It reveals that the interface bonding of the Al₂O₃ boundaries formed at the initial nucleation stage during θ- to α-Al₂O₃ transformation is stronger than that formed by subsequent grain growth. The interface bonding between the Al₂O₃ grains, which came from the decomposition of Al(OH)₃, is stronger than that between the original Al₂O₃ grains in the starting mixture.