Microstructure and mechanical properties of Al2O3–YSZ spherical polycrystalline composites

The mechanical behavior and microstructure of highly densified, spherically shaped, polycrystalline Al₂O₃–YSZ composites, processed from pseudoboehmite powders by sol–gel is reported here. Processing was carried out by combining nanometric sized α-Al₂O₃ (120 nm) seeds and YSZ particles of tetragonal structure. The YSZ particles were homogeneously distributed in a coarse-grained matrix of alumina, both inside grains and along grain boundaries. Fracture surfaces, achieved by impact tests showed toughening effects of the zirconia particles. The tetragonality of the YSZ phase stability even after fracture events and fracture toughness measurements by Vickers indentation, where the crack tip interacts with YSZ particles, are all provided and discussed. The local mechanical properties, such as elastic modulus, indentation hardness and the onset of plastic deformation or fracture contact pressure of both YSZ particles and the Al₂O₃ matrix were quantified by nanoindentation. Evidence of coercive contact pressure was observed in YSZ from indentation stress–strain curves.     

Microstructure and electrical characteristics of ZnO–B2O3–PbO–V2O5–MnO2 ceramics prepared from ZnO nanopowders

A peculiar kind of ZnO–B₂O₃–PbO–V₂O₅–MnO₂ ceramics was produced from the ZnO nanopowders directly co-doped with the oxides instead of lead zinc borate frit in this investigation. The 8 wt.% (PbO+B₂O₃) co-doped ceramics sintered at 950 °C for 2 h displayed the optimum electrical properties, that is, leakage current density J_L=6.2×10⁻⁶ A/cm², nonlinear coefficient α=22.8 and breakdown voltage V_BK=331 V/mm. The co-doping of 8 wt.% (PbO+B₂O₃) resulted in an increase in nonlinear coefficient and a decrease in leakage current density of the ZnO–V₂O₅ varistors while the sintering temperature showed no evident influence on nonlinear coefficient and leakage current density at the range of 800–950 °C.     

Effect of TiO2 on crystallization and mechanical properties of MgO–Al2O3–SiO2 glasses containing P2O5

MgO–Al₂O₃–SiO₂ glass-ceramics have been widely used in military, industrial, and construction applications. The nucleating agent is one of the most important factors in the production of glass-ceramics as it can control the crystallization temperature or the grain size. In this study, we investigated the effect of replacing P₂O₅ with different amounts of TiO₂ on the crystallization, structure, and mechanical properties of an MgO–Al₂O₃–SiO₂ system. The crystallization and microstructure were investigated by differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The mechanical properties were investigated by measuring the Vickers hardness, Young's modulus, and fracture toughness. The results showed that adding TiO₂ favored the precipitation of fine grains and significantly increased the Vickers hardness, Young's modulus, and fracture toughness of the glasses. Introducing an appropriate amount of TiO₂ can make a glass structure more compact, promote crystallization, and improve the mechanical properties of MgO–Al₂O₃–SiO₂ glass-ceramics.     

Microstructure and mechanical properties of 3-D interpenetrated network structure MoSi2–RSiC composite

In this paper, RSiC is used as matrix, organic precursor impregnation and pyrolysis – MoSi₂ melt infiltration combined processes are employed to prepare the 3-D interpenetrated network structure MoSi₂–RSiC composite. The composition, microstructure and mechanical properties of MoSi₂–RSiC composite are studied through XRD, SEM, EDS and mechanical testing. The results show that an almost fully dense MoSi₂–RSiC composite with 3-D interpenetrated network structure is obtained. The main components of the composite are SiC and MoSi₂, it also includes small amount of Mo_4.8Si₃C_0.6. The flexural strength of MoSi₂–RSiC composite is improved compared with that of RSiC, meanwhile, it inherits the high temperature strength characteristic from RSiC. The composite exhibits a typical brittle fracture behavior and the elastic modulus of it also increases with the increase of organic precursor impregnation and pyrolysis cycles.     

Microstructure and mechanical properties of Al2O3/Cu joints brazed with Ag–Cu–Ti+Zn composite fillers

In this study, Al₂O₃ ceramic and Cu bars were brazed with newly designed Ag–Cu–Ti(ABA)+Zn composite fillers. Systematic analysis of the microstructure of the brazed joints indicated that the volatilization of Zn atoms during the brazing process could promote the spreading of liquid brazing fillers on the surface of the Al₂O₃ ceramic, resulting in a uniform dendritic interfacial structure. The typical interfacial structure was an Al₂O₃/TiO/(Cu, Al)₃Ti₃O+Ag(s, s)/Cu interface. Notably, the tensile strength was improved to 20.89 MPa for Al₂O₃/Cu joint brazed with ABA+Zn composite fillers at 900 °C for 20 min, approximately 67.6% higher than the sample brazed without Zn foil. In this case, the fracture model was straight and sharp-angled inside the Al₂O₃ ceramic. In addition, the joint strength decreased with increased brazing temperatures from 900 to 940 °C.     

Microstructure and mechanical properties improvement of the Nextel™ 610 fiber reinforced alumina composite

The alumina slurry with high solid content was prepared, and a rapid lamination route for fabricate the Nextel? 610 fiber reinforced alumina composite was proposed in this work. The microstructure and mechanical properties of the as-received all-oxide composite were investigated by a series of techniques. The shrinkage cracks in matrix were reduced, while porous structure in composite was maintained. The N610/alumina composite has weak matrix and weak interface, as the Young’s modulus of the alumina matrix and the interfacial shear strength of the composite are 140.8±2.5GPa and 129.1±14.6MPa. The mechanical properties of the composite are much higher than lots of oxide/oxide composites, given its flexural strength, interlaminar shear strength and the fracture toughness are 398.4±5.7MPa、27.0±0.5MPa and 14.1±0.9MPa·m^1/2, respectively. The flexural strength of the virgin composite keep stable at 25–1050 °C, while dramatically decrease at 1100–1200 °C.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Particle occlusion and mechanical properties of Ni–Al2O3 nanocomposites

Polycrystalline alumina, doped with MgO below the solubility limit, was reinforced with sub-micron particles of Ni by infiltration of Ni-nitrate into fired alumina green bodies, followed by reduction and sintering. The Ni particle size and location were monitored both after reduction and after sintering by transmission electron microscopy. Particle occlusion was found to increase with sintering time and temperature, and is correlated with experimentally detected Mg segregation to the Ni–alumina interfaces, resulting in partial depletion of Mg at the alumina grain boundaries and thus their increased mobility. Occlusion of Ni particles reduces the fracture strength and Weibull modulus of the composites, indicating that particle location is a key microstructural parameter for reaching high fracture strengths, and that this can be controlled via grain boundary and interface adsorption.     

Microstructure–thermal properties of Cu/Al2O3 bilayer prepared by direct bonding

The thermal conductivity of Cu/Al₂O₃ bilayers prepared by a direct-bonding technique was determined. The direct-bonding process started with the pre-oxidation treatment of a Cu plate at a temperature less than 600 °C. Though a thin oxide layer was located on the surface of the plate after treatment, the oxygen solutes began to diffuse into the interior of Cu plate prior to bonding. Bonding occurred by a eutectic liquid formed at 1075 °C. No reaction interphase was observed at the Cu–Al₂O₃ interface. The thermal resistance of the Cu/Al₂O₃ interface is very low. The extremely low thermal resistance can be related to the clean interface between the two materials.     

Processing and mechanical properties of Al2O3–5 vol.% Cr nanocomposites

The use of chromium (III) acetylacetonate as a source of nanometre sized chromium particles for the production of Al₂O₃–5 vol.% Cr nanocomposites has been investigated. The details of the processing procedure are crucial in determining the mechanical properties of the composite. The highest strength and fracture toughness, 736±29 MPa and 4.0±0.2 MPa m^1/2, respectively, were obtained for the nanocomposite hot pressed at 1450 °C. It is shown that the strengthening in Al₂O₃–5% Cr nanocomposites mainly results from microstructure refinement in that the mean alumina matrix grain size in the optimum composite was 0.68 μm compared with a grain size of 3.6 μm in the monolithic alumina hot pressed under identical conditions. Crack bridging and crack deflection by the nano-sized Cr particles did not occur to any significant extent. The slight improvement in fracture toughness may result from the observed change in fracture mode from intergranular fracture for monolithic alumina to transgranular failure for the nanocomposites.     

Microstructure,mechanical properties and machining performance of spark plasma sintered Al2O3–ZrO2–TiCN nanocomposites

The effect of addition of nanocrystalline ZrO₂ and TiCN to ultrafine Al₂O₃ on mechanical properties and microstructure of the composites developed by spark plasma sintering (SPS) was investigated. The distribution of the nanoparticles was dependent on their overall concentration. Maximum hardness (21 GPa) and indentation toughness (5.5 MPa m^1/2) was obtained with 23 vol% nanoparticles, which was considered as the optimum composition. The Zener pinning criteria were also satisfied at this composition with grain size of the restraining nanoparticles ～63–65 nm. Hardness of the composites follows the rule of mixtures; crack deflection and crack arrest by nanoparticles at grain boundaries along with mixed fracture mode led to high toughness in the nanocomposites. Cutting tool inserts were developed by SPS with the optimized composition and their machining performance was compared with commercial alumina based inserts. Increased toughness in the nanocomposite inserts reflects in the machining performance as the tool life improves drastically compared to that of the commercial inserts at high cutting speeds ≥500 m min^?1. This was attributed to differences in their failure modes; the commercial inserts fail catastrophically by fracture due to their low toughness whereas the nanocomposite inserts reach the tool failure criteria by crater wear at all machining conditions.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

Microstructure and mechanical properties of multi-walled carbon nanotubes containing Al2O3–C refractories with addition of polycarbosilane

Carbon nanotubes (CNTs) are a promising reinforcement for fabricating Al₂O₃–C refractories. However, CNTs are prone to agglomerate or react with antioxidants or reactive gaseous phases such as Al (g), Si (g) and SiO (g), etc. at high temperatures. To overcome the problems above, polycarbosilane (PCS) and multi-walled carbon nanotubes (MWCNTs) were firstly mixed with micro-alumina powder in a liquid medium and then incorporated into Al₂O₃–C refractories. Then the microstructure and mechanical properties of Al₂O₃–C refractories fired in the temperature range from 800 °C to 1400 °C were investigated in this work. The results showed that the MWCNTs were well dispersed in the specimens with addition of PCS in contrast to the specimens without PCS due to the PCS adsorption on the surface of MWCNTs during the mixing process. And the mechanical properties, such as cold modulus of rupture (CMOR), flexural modulus (FM), forces and displacements of Al₂O₃–C refractories with PCS were much higher than those without PCS, which was attributed to more homogeneous dispersion of MWCNTs, more residual MWCNTs as well as different morphologies of ceramic whiskers. Meanwhile, the oxidation resistance of Al₂O₃–C refractories with PCS was improved greatly, which was supposed that the in situ formed SiC_xO_y coating prevented the oxidation of MWCNTs to some extent.     

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (C_f/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiC_f/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in C_f/C–SiC composites. Tensile strength and interlaminar shear strength of C_f/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiC_f/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of C_f/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in C_f/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared C_f/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.     

Microstructure and mechanical properties of Ni/10 mol% Sc2O3–1 mol% CeO2–ZrO2 cermet anode for solid oxide fuel cells

The mechanical properties of anode materials play an important role in the reliability and durability of solid oxide fuel cells operating at high temperatures in a reducing environment. In this paper, we produced the results of the mechanical properties investigation of Ni/10 mol% Sc₂O₃–1 mol% CeO₂–ZrO₂ cermet anodes. Young's modulus as well as strength and fracture toughness of non-reduced and reduced anodes has been measured, both at room and at high temperatures. High temperature experiments were performed in the reducing environment of forming gas. It is shown that while at 700 °C and 800 °C, the anode specimens exhibited purely elastic deformation and brittle fracture, a brittle-to-ductile transition occurred for heating above 800 °C and the anode deformed plastically at 900 °C. Fractography of the anode specimens were performed to identify the fracture modes of anodes tested at different temperatures.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Effect of Al2O3 particles on mechanical and tribological properties of Al–Mg dual-matrix nanocomposites

This article aims to manufacture homogenous dual-matrix Al–Mg/Al₂O₃ nanocomposite from their raw materials and give insight into the correlation between powder morphology, crystallite structure and their mechanical and tribological properties. Al–Mg dual-matrix reinforced with micro/nano Al₂O₃ particles was manufactured by a novel double high-energy ball milling process followed by a cold consolidation and sintering. Microstructure and phase composition of the prepared samples were characterized using FE-SEM, EDS and XRD inspections. Mechanical and wear properties were characterized using compression and sliding wear tests. The results showed that a milling of Mg with Al₂O₃ particles in an initial step before mixing with Al has the beneficial of well dispersion of Al₂O₃ nanoparticles in Al–Mg dual matrix. The Al–Mg dual matrix reinforced with nano-size Al₂O₃ showed 3.29-times smaller crystallite size than pure Al. Moreover, the hardness and compressive strength are enhanced by adding nano-size Al₂O₃ with Al–Mg dual matrix composite while the ductility is maintained relatively high. Additionally, the wear rate of this composite was reduced by a factor of 2.7 compared to pure Al. The reduced crystallite size, the dispersion of Al₂O₃ nanoparticles and the formation of (Al–Mg)_ss were the main improvement factors for mechanical and wear properties.     

Fabrication and thermoelectric properties of SrTiO3–TiO2 composite ceramics

The paper presents the results of preparing biphase SrTiO₃–TiO₂ ceramics as a promising system for n-type thermoelectrics using the features of a two-dimensional electron gas. Ceramics was obtained by reactive spark plasma sintering of SrCO₃ and TiO₂. The dynamics of phase transformations are shown; it is clarified that phase transformations are not the driving force of sintering. The mutual stabilization of the SrTiO₃ and TiO₂ phases is shown. Unique data on the assessment of the temperature gradient in the system have been obtained. A comparison of the thermoelectric characteristics of biphasic ceramics and its constituent phases allows concluding that the role of the two-dimensional electron gas is reduced to modulating the properties of bulk phases. Clear signs of size quantization were detected by the X-ray luminescence method, which is expressed in the blueshift of the luminescence spectrum by 22.3 ± 0.8 meV.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.