Mechanical properties and sliding wear behaviour of Al2O3-SiC nanocomposites with 3–20 vol% SiC

Al₂O₃/SiC composites containing different volume fractions (3, 5, 10, 15, and 20 vol%) of SiC particles were produced by conventional mixing of alumina and silicon carbide powders, followed by hot pressing at 1740 °C for 1 h under the pressure of 30 MPa in the atmosphere of Ar. The influence of the volume fraction and size of SiC particles (two different powders with the mean size of SiC particles 40 and 200 nm were used), and final microstructure on mechanical properties and dry sliding wear behaviour in ball-on-disc arrangement were evaluated. The properties of the composites were related to a monolithic Al₂O₃ reference. Microstructure of the composites was significantly affected by the volume fraction of added SiC, with the mean size of alumina matrix grains decreasing with increasing content of SiC particles. The addition of SiC moderately improved the Vickers hardness. Fracture toughness was lower with respect to monolithic Al₂O₃, irrespective of the volume fraction and size of SiC particles. Al₂O₃/SiC nanocomposites conferred significant benefits in terms of wear behaviour under the conditions of mild dry sliding wear. Wear resistance of the alumina reference was poor, especially at the applied load of 50 N. The wear rates of composites markedly decreased with increasing volume fraction of SiC. Wear of the composites was also influenced by the material of counterparts, especially their hardness, with softer counterparts resulting in lower wear rates. All composites wore by a combination of grain pull-out with plastic deformation associated with grooving and small contribution of mechanical wear (micro-fracture). No influence of SiC particle size on wear rate or mechanism of wear was observed in the materials with identical volume fractions of SiC.     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

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.     

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.     

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.     

Mechanical properties of highly toughened ZrO2Y2O3

Y-PSZ (partially stabilized zirconia) and Y-TZP (tetragonal zirconia polycrystals) containing from 1·5 to 5·0 mol% Y₂O₃ were prepared by hot isostatic pressing. The K_IC value nonlinearly increases with the decrease from 2·5 to 2·0 mol % Y₂O₃, and has a maximum K_IC value of 20 MPa √m at 2·0 mol% Y₂O₃. In order to clarify the toughness creating mechanism of the above materials, the experimental results of K_IC, σ_f and transformation zone size were examined, with emphasis on Y₂O₃ composition, Al₂O₃ additives and grain size.     

Microwave hybrid sintering of Al2O3 and Al2O3–ZrO2 composites,and effects of ZrO2 crystal structure on mechanical properties,thermal properties,and sintering kinetics

Structural ceramics such as Al₂O₃ and Al₂O₃–ZrO₂ composites are widely used in harsh environment applications. The conventional sintering process for fabrication of these ceramics is time-consuming method that requires large amount of energy. Microwave sintering is a novel way to resolve this problem. However, to date, very limited research has been carried out to study the effects of different ZrO₂ crystal structures on Al₂O₃–ZrO₂ composites, especially on the sintering kinetics, when fabricated by microwave sintering.The microwave hybrid sintering of Al₂O₃ and Al₂O₃–ZrO₂ composites was performed in this study. Tetragonal zirconia and cubic zirconia were used as two different reinforcements for an α–alumina matrix, and the mechanical and thermal properties were studied. It was found that Al₂O₃ experienced a remarkable increase in fracture toughness of up to 42% when t-ZrO₂ was added. Al₂O₃–c-ZrO₂ also showed increased fracture toughness. The sintering kinetics were also thoroughly investigated, and the average activation energy values for the intermediate stage of sintering were estimated to be 246 ± 11 kJ/mol for pure Al₂O₃, 319 ± 71 kJ/mol for Al₂O₃–c-ZrO₂, and 342 ± 77 kJ/mol for Al₂O₃–t-ZrO₂. These values indicated that the activation energy was increased by the addition of either type of ZrO₂, with the highest value shown by Al₂O₃–t-ZrO₂.     

Creep behaviour of Al2O3–SiC nanocomposites

Compared with monolithic fine grained Al₂O₃, Al₂O₃ nanocomposites reinforced with SiC nanoparticles display especially high modulus of rupture as well as reduced creep strain. Taking into account the fracture mode change, the morphology of ground surfaces showing plastic grooving, the low sensitivity to wear and the low dependence of erosion rate with grain size, it can be reasonably assumed that the strength improvement is associated with an increase of the interface cohesion (due to bridging by SiC particles) rather than with a grain size refinement involving substructure formation (as initially suggested by Niihara). In the present work, creep tests have been performed and the results agree with such a reinforcement of the mechanical properties by SiC particle bridging Al₂O₃–Al₂O₃ grain boundaries. Indeed, particles pinning the grain boundaries hinder grain boundary sliding resulting in a large improvement in creep resistance. In addition, SiC particles, while counteracting sliding, give rise to a recoverable viscoelastic contribution to creep. Because of the increased interface strength, the samples undergoing creep support stress levels, greater than the threshold value required to activate dislocation motion. The high stress exponent value as well as the presence of a high dislocation density in the strained materials suggests that a lattice mechanism controls the deformation process. Finally, a model is proposed which fits well with the experimental creep results.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders

In present work, ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites are fabricated through spark plasma sintering. Al₂O₃–ZrO₂ amorphous powders and polycrystal Al₂O₃ powders and are doped in the polycrystalline ZrO₂ powders, respectively. When doped with amorphous powders, the sintering of ZrO₂–Al₂O₃ nanocomposites is promoted, and ZrO₂-5 wt% Al₂O₃ and ZrO₂-10 wt% Al₂O₃ nanocomposites with relative densities of 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO₂ and polycrystalline Al₂O₃ powders, the relative densities are merely 93%. The enhanced sinterability is due to the metastability and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO₂ and Al₂O₃ grains, which further leads to excellent mechanical properties. This article provides new ideas for low-temperature sintering of nanocomposites via using doping amorphous powders.     

Phase diagram study and thermodynamic modeling of the Na2O–Al2O3–ZrO2 system

The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was experimentally studied at 1500°C–1650°C by a classical equilibration/quenching method and differential thermal analysis followed by X-ray diffraction phase analysis and electron probe micro-analysis. A sealed Pt crucible was utilized to prevent the volatile loss of Na₂O during high-temperature phase equilibrium experiments and the hydration upon quenching. The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was revealed for the first time. Based on the present experimental data and available binary modeling results in literature, the thermodynamic modeling of the ternary system was performed using the Calculation of Phase Diagram method and the phase diagram of the entire the Na₂O–Al₂O₃–ZrO₂ system was constructed and the optimized thermodynamic properties for all solids and liquid phase within the ternary system were obtained.     

Structural,mechanical and tribological properties of Cu–ZrO2/GNPs hybrid nanocomposites

Hybrid Cu–ZrO₂/GNPs nanocomposites were successfully produced using powder metallurgy technique. The effect of GNPs mass fraction, 0, 0.5, 1 and 1.5%, on mechanical and tribological properties of the produced hybrid nanocomposite was studied while maintaining ZrO₂ mass fraction constant at 5%. High energy ball milling was applied for mixing powders and compaction and sintering were applied for consolidation. The morphological analysis of the produced powder showed acceleration of Cu particles fracture during ball milling with the addition of GNPs up to 0.5% with noticeable reduction of agglomeration size. Moreover, the crystallite size of Cu–5%ZrO₂/0.5%GNPs hybrid nanocomposites revealed smaller crystallite size, 142 nm, compared to 300 nm for Cu–5%ZrO₂ nanocomposite. Additionally, the hybrid nanocomposite with 0.5% GNPs shows homogeneous distribution of both reinforcement phases in the sintered samples. This improved nano and micro structure of Cu–5%ZrO₂/0.5%GNPs nanocomposites revealed higher hardness, 169.3 HV, compared to 65.5 HV for Cu–5%ZrO₂ nanocomposite. The wear rate is decreased in this composite while it increased with increasing GNPs content more than 0.5%. The coefficient of friction is decreased as well for this hybrid nanocomposite and remain constant with increasing GNPs content more than 0.5%.     

Preparation and characterization of Al2O3–25 mol% ZrO2 composites

Al₂O₃–ZrO₂ (AZ_x), with 25 mol% ZrO₂ content, was prepared using the co-precipitation method. Synthesized powders were characterized by thermal reaction using a differential thermal analysis technique (TG–DTA) and were investigated by phase formation using X-ray diffraction. It indicated that the reaction occurred at 850 °C; cubic (c)-ZrO₂ phase and Al₂O₃ were obtained. By increasing temperature to 1100 °C, tetragonal (t)-ZrO₂ phase was detected. The Al₂O₃–25 mol% ZrO₂ was sintered for 2 h in the temperature range of between 1300 and 1600 °C. The majority phases of ceramics were m-ZrO₂ and α-Al₂O₃, although a t-ZrO₂ phase also appeared as a minor phase and decreased with higher temperature. Moreover, morphology and particle size evolution have been determined via the SEM technique. SEM showed that the particles of powder are agglomerated and basically irregular in shape. An SEM micrograph of ceramics exhibits uniform microstructure without abnormal grain growth.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

In-situ fabrication of Al2O3–SiC nanocomposites using B2O3 as sintering aid

Al₂O₃–SiC nanocomposites with 5 and 10 vol% SiC have been in-situ fabricated by sol-gel method followed by carbothermal reduction of alumina–silica gel using B₂O₃ as sintering aid. Green bodies were formed by cold isostatic pressing of calcined gel, which was prepared by an aqueous sol-containing aluminum chloride, TEOS, sucrose and boric acid. Pressureless sintering was carried out in Ar–12%H₂ atmosphere at 1700 °C. Addition of B₂O₃ (1 or 3 wt%) was an effective densification aid in the Al₂O₃–5 vol% SiC composites, while the densification of Al₂O₃–10 vol% SiC composites was not affected by adding B₂O₃. The composite material containing 5 vol% SiC doped with 3 wt% B₂O₃ reached 98.7% of full density. Nano-sized β-SiC particles were formed in-situ by means of a reaction between mullite and carbon at 1600 °C. Scanning electron microscopy revealed that the spherical in-situ synthesized SiC nanoparticles were well distributed through the composite and located predominantly to the interior of alumina matrix grains.     

Influence of synthesis conditions on the properties of Y2O3–MgO nanopowders and sintered nanocomposites

In this study, a citrate–nitrate combustion method was applied to synthesize composite Y₂O₃–MgO nanopowders. In order to optimize the synthesis condition to support sufficient combustion, the molar ratio of citric acid to nitrate (c/n molar ratio) used in the reaction mixtures was varied between 0.17 and 0.34. Nanopowders with an average particle size of 17 nm were achieved. The properties of these nanopowders indicated that the higher molar ratios decreased the unreacted organic components and increased the amount of carbide on the surface of the oxides, which helped to inhibit the formation of carbonate groups. The amount of carbonate groups was reduced with the increasing c/n molar ratio. Y₂O₃–MgO nanocomposites fabricated through hot-isostatic-pressing sintering showed a uniform distribution of Y₂O₃ and MgO grains, which had an average size of ∼180 nm. In addition, the absorption peaks at 1410 and 1511 cm⁻¹ disappeared until the c/n molar ratio reached 0.28. A high average infrared transmittance of 83% in the range of 4000–1667 cm⁻¹ (2.5–6 μm) was obtained in the nanocomposites.     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.     

Microstructure and electrical properties of ZnO–ZrO2–Bi2O3–M3O4 (MCo,Mn) varistors

Phase evolution, microstructure and the electrical properties of ZrO₂-added pyrochlore-free ZnO–Bi₂O₃–M₃O₄ (MCo, Mn) varistors have been studied as functions of ZrO₂ content up to 10 vol% and the sintering temperature between 900 and 1300 °C. Zirconia remained as intergranular second phase particles up to 1100 °C, which retarded densification and inhibited the grain growth of ZnO. At higher temperatures, on the contrary, ZrO₂ particles began to be entrapped in ZnO grains and irreversibly transform from monoclinic to stable cubic phase dissolving transition metal ions. The grain size of ZnO decreased with increasing ZrO₂ content, and increased with the increase of the sintering temperature. Accordingly breakdown voltage changed with both ZrO₂ content and the sintering temperature as was expected. Nonlinear coefficient (α) depended primarily on the sintering temperature: it increased to >40 up to 1000 °C, and significantly decreased to <30 at higher temperatures probably due to the volatilization of Bi₂O₃. While the specimens sintered at 1200 °C or above had relatively high leakage current (I_L) and large clamping ratio (C_R), those with ZrO₂ content of 0.5–5.0 vol% and sintered below 1200 °C revealed low I_L of ⩽20 μA/cm² and C_R well below 2.0. In spite that varistor characteristics of ZrO₂-added system could not match those of commercial ZnO varistors, its low temperature sinterability and ease of breakdown voltage control via ZrO₂ content without a serious loss of its figures of merit are worth noticing, particularly for multi-layered chip varistor (MLV) application.