Studies on mechanical and dry sliding wear of Al6061–SiC composites

Particulate reinforced Al-MMCs exhibits better mechanical properties and improved wear resistance over other conventional alloys. In the present paper, the experimental results of the mechanical and tribological properties of Al6061–SiC composites are presented. The composites of Al6061 containing 2–6 wt% SiC were prepared using liquid metallurgy route. The experimental results showed that the density of the composites increase with increased SiC content and agrees with the values obtained through the rule of mixtures. The hardness and ultimate tensile strength of Al6061–SiC composites were found to increase with increased SiC content in the matrix at the cost of reduced ductility. The wear properties of the composites containing SiC were superior to that of the matrix material.     

AC electrical and optical characterization of epoxy–Al2O3 composites

The AC electrical and optical characterizations of epoxy–alumina (Al₂O₃) composites have been investigated. Sheets filled with alumina were prepared with different alumina concentrations (0, 2, 5, 8, 10, and 15 wt%). The AC electrical properties were measured by using impedance spectroscopy as a function of applied frequency in range from 50 kHz to 1 MHz and filler concentration. The results obtained showed that the applied frequency and filler concentration was found to influence the AC electrical conductivity and dielectric behavior of the prepared composites. The UV-optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The absorption coefficient and the optical energy gap (E_opt) have been obtained from the direct allowed transitions in k-space at room temperature. The tail widths (ΔE) of the localized states in the band gap were evaluated using the Urbach-edges formula. It was found that both (E_opt) and (ΔE) vary with the alumina concentration dispersed in the epoxy matrix. The refractive index (n) for the composites was determined from the collected transmittance and reflectance spectra. The dispersion behavior of the refractive index is discussed in terms of the single oscillator model.     

Effect of the SiC particle size on the dry sliding wear behavior of SiC and SiC–Gr-reinforced Al6061 composites

The effect of size of silicon carbide particles on the dry sliding wear properties of composites with three different sized SiC particles (19, 93, and 146 μm) has been studied. Wear behavior of Al6061/10 vol% SiC and Al6061/10 vol% SiC/5 vol% graphite composites processed by in situ powder metallurgy technique has been investigated using a pin-on-disk wear tester. The debris and wear surfaces of samples were identified using SEM. It was found that the porosity content and hardness of Al/10SiC composites decreased by 5 vol% graphite addition. The increased SiC particle size reduced the porosity, hardness, volume loss, and coefficient of friction of both types of composites. Moreover, the hybrid composites exhibited lower coefficient of friction and wear rates. The wear mechanism changed from mostly adhesive and micro-cutting in the Al/10SiC composite containing fine SiC particles to the prominently abrasive and delamination wear by increasing of SiC particle size. While the main wear mechanism for the unreinforced alloy was adhesive wear, all the hybrid composites were worn mainly by abrasion and delamination mechanisms.     

Development of functionally graded Cu–Sn–Ni/Al2O3 composite for bearing applications and investigation of its mechanical and wear behavior

This study investigates the mechanical and tribological properties of a functionally graded Cu–Sn–Ni/Al₂O₃ metal matrix composite, synthesized using horizontal centrifugal casting technique with dimension Φ_out100?×?Φ_in85?×?100?mm. The microstructure was examined along radial distances at 1, 8, and 13?mm from outer periphery. Specimens were tested for tensile strength from outer (1–8?mm) and inner zone (9–15?mm) of the casting and fractured surfaces were subjected to fractographic analysis. Wear resistance of inner layer was experimented using pin-on-disc tribometer based on Taguchi’s L₂₇ orthogonal array using three variable process parameters, such as applied loads (10, 20, and 30?N), sliding velocities (1, 2, and 3?m/s), and distances (500, 1000, and 1500?m). Optimum parameters were determined for wear rate on “smaller-the-better” basis using signal-to-noise ratio. Analysis of variance predicted the effect of each influential parameter and their interactions. Results depict that wear rate increased with load and distance, forming phases such as Cu₃Sn, Ni₃Sn, Cu₆Sn₅, etc. Worn surfaces analysis using scanning electron microscope predicted the formation of mechanically mixed layers, showing a V-trend influence of velocity on wear. Thus, fabricated composite shows the replaceability of conventional leaded bearing materials with superior copper functionally graded composites having better wear characteristics.     

Heat treatment effect on the microstructure,tensile properties and dry sliding wear behavior of A356–10%B4C cast composites

In present paper, an attempt was made to examine the influence of T6 heat treatment (solution treatment at 540 °C for 5 h, quenching in hot water and artificial aging at 170 °C for 8 h) on the microstructure, tensile properties and dry sliding wear behavior of A356–10%B₄C cast composites. The composite ingots were made by stir casting process. In this work, the matrix alloy and composite were characterized by optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy, tensile tests and conventional pin-on-disk experiment.     

Physico-chemical and electrochemical characterization of Ti/RhOx–IrO2 electrodes using sol–gel technology

Sol–gel technology has been successfully used for the incorporation of RhO_x–IrO₂ on a Ti substrate. RhO_x–IrO₂ was prepared from chloride precursors of Rh and Ir, for surface studies. These metal oxides were then immobilised on solid Ti substrates via dip withdrawal coating methods to form thin films. The Ti/RhO_x–IrO₂ thin films were extensively characterized in terms of surface characterization and chemical composition and used in the oxidation of phenol. Thermo-gravimetric analysis (TGA) determined the calcination temperature at 700 °C where no further structural changes occurred due to mass loss. The rhodium oxide showed two-phase formations, RhO₂ and Rh₂O₃, which were attributed to high calcinated temperatures compare to one phase IrO₂ which was stable at lower temperatures. The scanning electron microscopy (SEM) showed that the morphology of the film was found to be rough with a grain-like appearance in the 150-nm range. The phase composition of these metal oxides was determined by X-ray diffraction (XRD) technique and found to have crystalline structures. The results obtained from Rutherford backscattering spectrometry (RBS) revealed information regarding the chemical composition of the metal oxides and confirmed the diffusion of Rh and Ir into the Ti substrate. Electrochemical characterization of the Ti/RhO_x–IrO₂ electrode, via cyclic voltammetry (CV), showed distinctive redox peaks: anodic and cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple was seen at 250 and 100 mV respectively; the peak observed at 1000 mV was associated with oxygen evolution and a broad reductive wave at −600 mV can be ascribed to the Ti/RuO_x–IrO₂ reduction, which proved that the Ti/RhO_x–IrO₂ electrode were electroactive and exhibit fast electrochemistry.     

Microwave dielectric properties of BaO–TiO2–TeO2 ternary system

Besides the applications as optical functional materials, tellurium oxides also have attracted interest as microwave dielectric materials. Most TeO₂-based binary and ternary system have large negative temperature coefficient of resonant frequency (τ_f), which is not compatible for the low-temperature cofired ceramic. To compensate τ_f close to zero, two single-phase predecessors of BaTe₄O₉ and TiTe₃O₈ are synthesized in air at 530–560 and 620–680 °C, respectively. The two predecessors show exceptional dielectric properties and their τ_f are opposite. The BaO–TiO₂–TeO₂ ternary system compounds are investigated by adjusting the ratio of BaTe₄O₉ and TiTe₃O₈ and sintered at 520–580 °C to develop the microwave properties and compensate the τ_f. After sintered at 560 °C, the ceramic sample with the composition of 0.47BaTe₄O₉–0.53TiTe₃O₈ exhibits a dielectric permittivity of 28, a Q × f-value of 12,200 GHz, and a τ_f of 4.0 ppm/°C measured at 10 GHz.     

Dielectric properties and microstructure of TiO2 modified (ZnMg)TiO3 microwave ceramics with CaO–B2O3–SiO2

The low-fired (ZnMg)TiO₃–TiO₂ (ZMT–TiO₂) microwave ceramics using low melting point CaO–B₂O₃–SiO₂ as sintering aids have been developed. The influences of Mg substituted fraction on the crystal structure and microwave properties of (Zn_1−x Mg _x )TiO₃ were investigated. The result reveals that the sufficient amount of Mg (x ≥ 0.3) could inhibit the decomposition of ZnTiO₃ effectively, and form the single-phase (ZnMg)TiO₃ at higher sintering temperatures. Due to the compensating effect of rutile TiO₂ (τ_f = 450 ppm/°C), the temperature coefficient of resonant frequency (τ_f) for (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ with biphasic structure was adjusted to near zero value. Further, CaO–B₂O₃–SiO₂ addition could reduce the sintering temperature from 1150 to 950 °C, and significantly improve the sinterability and microwave properties of ZMT–TiO₂ ceramics, which is attributed to the formation of liquid phases during the sintering process observed by SEM. The (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ dielectrics with 1 wt% CaO–B₂O₃–SiO₂ sintered at 950 °C exhibited the optimal microwave properties: ε ≈ 25, Q × f ≈ 47,000 GHz, and τ_f ≈ ± 10 ppm/°C.     

Microwave sintering of β-SiAlON–ZrO2 composites

Densification, phase transformation, microstructure evolution and hardness of microwave sintered β-SiAlON–ZrO₂ composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed β-SiAlON, tetragonal ZrO₂ and ZrN phases, while, ZrO₂ reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered β-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Glass formation and third-order optical nonlinear characteristics of bismuthate glasses within Bi2O3–GeO2–TiO2 pseudo-ternary system

Glass-forming region of Bi₂O₃–GeO₂–TiO₂ (BGT) pseudo-ternary system was determined by using melt-quench method. A series of high transparent glass samples were selected and their structural characteristics were investigated by FT-IR and Raman spectra. By employing Z-scan and optical Kerr shutter techniques with femtosecond laser pulses as excitation source, third-order optical nonlinearities (TON) of the BGT glasses as well as the TON response time were investigated at wavelength of 800 nm. The ultrafast nonlinear response and high figure of merit suggest great potentials of BGT glasses in applications of all-optical switching or related optical devices.     

Low-temperature sintering and microwave dielectric properties of 5Li2O–0.58Nb2O5–3.23TiO2 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of the 5Li₂O–0.58Nb₂O₅–3.23TiO₂ (LNT) ceramics have been investigated. It is found that the LNT ceramics could be sintered well at ∼880 °C with low-level doping of B₂O₃ (≤2 wt.%). Only Li₂TiO₃ solid solution (Li₂TiO₃ss) crystal structure could be detected for all the ceramics with various amounts of B₂O₃ addition from the X-ray diffraction (XRD) results. And interestingly, two phases with different color in SEM images are observed in B₂O₃-doped LNT ceramics. EDS results suggest that the two different phases are two Li₂TiO₃ss phases with different amount of Nb. In addition, there is no much degradation in the microwave dielectric properties with the B₂O₃ adding. In the case of 0.5 wt.% B₂O₃-doped samples sintered at 880 °C, good microwave dielectric properties of ?_r = 22, Q × f = 32,000 GHz, τ_f = 9.5 ppm °C⁻¹ are obtained.     

Optical properties and structure of R2O–Ga2O3–SiO2 and RO–Ga2O3–SiO2 glasses

Refractive index and molar refraction of Li₂O–, Na₂O–, CaO–, and BaO–Ga₂O₃–SiO₂ glasses have been used to test the validity of a structural model of silicate glasses containing Ga₂O₃ glasses. Ga₂O₃ enters these types of glass in a similar manner as Al₂O₃. It is assumed that, for (SiO₂/Ga₂O₃) >1 and (Ga₂O₃/R₂O) ≤1, Ga₂O₃ associates primarily with modifier oxides to form GaO₄ units. The rest of modifier oxide forms silicate units with non-bridging oxygen ions. Silicate structural units have the same factors as found for binary alkali- and alkaline earth silicate glasses. Differences between experimental and model values suggest another structure for (Ga₂O₃/SiO₂) ≥1.     

Development of Al 6063–TiB2 in situ composites

In situ TiB₂ reinforced Al 6063 composites have been successfully synthesized through the chemical reaction between Al–10%Ti and Al–3%B master alloys in the Al 6063 alloy using liquid metallurgy route. The amount of TiB₂ formed in the composite is estimated using gravimetric analysis. Mechanical properties in terms of microhardness, ultimate tensile strength and modulus of elasticity have been improved by 21%, 47% and 65% respectively in comparison with matrix alloy. Further, ductility in terms of percentage elongation of the composites was found to increase by about 368% when compared with the matrix alloy. The improvement in ductility may be associated with the grain refinement of the composite with an increase in the content of Al–3%B master alloy.     

Preparation and characterization of Y2O3–Sm2O3 co-doped ceria electrolyte for IT-SOFCs

Y₂O₃–Sm₂O₃ co-doped ceria (YSDC) powder was synthesized by a gel-casting method using Ce(NO₃)₃·6H₂O, Sm₂O₃ and Y₂O₃ as raw materials. Phase structure of the synthesized powders was characterized by X-Ray diffraction analysis. Sinterability of the powders was investigated by testing the relative density and observing the microstructure of the sintered YSDC samples. Electrical conductivity of the sintered YSDC samples was measured using impedance spectra method. Single solid oxide fuel cells based on the YSDC electrolyte were also assembled and tested. The results showed that YSDC powders with single-phase fluorite structure can be obtained by calcining the dried gelcasts at temperature above 800 °C. Average particle size of the YSDC powder is 50–100 nm. Relative density of more than 95% of the theoretical can be achieved by sintering the YSDC compacts at temperature above 1400 °C. The sintered YSDC sample has an ionic conductivity of 4.74 × 10⁻² S cm⁻¹ at 800 °C in air. Single fuel cells based on the YSDC electrolyte with 50 μm in thickness were tested using humidified hydrogen as fuel and air as oxidant, and maximum power densities of about 190 and 112 mW cm⁻² were achieved at 700 and 600 °C, respectively.     

Synthesis and properties of glasses in the K2O–SiO2–Bi2O3–TiO2 system and bismuth titanate (Bi4Ti3O12) nano glass–ceramics thereof

Glasses were prepared by the melt-quench technique in the K₂O–SiO₂–Bi₂O₃–TiO₂ (KSBT) system and crystallized bismuth titanate, BiT (Bi₄Ti₃O₁₂) phase in it by controlled heat-treatment at various temperature and duration. Different physical, thermal, optical, and third-order susceptibility (χ³) of the glasses were evaluated and correlated with their composition. Systematic increase in refractive index (n) and χ³ with increase in BiT content is attributed to the combined effects of high polarization and ionic refraction of bismuth and titanium ions. Microstructural evaluation by FESEM shows the formation of polycrystalline spherical particles of 70–90 nm along with nano-rods of average diameter of 85–90 nm after prolonged heat treatment. A minor increase in dielectric constants (ε_r) has been observed with increase in polarizable components of BiT in the glasses, whereas a sharp increase in ε_r in glass–ceramics is found to be caused by the formation of non-centrosymmetric and ferroelectric BiT nanocrystals in the glass matrix.     

Reaction pathways,activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures

In-situ aluminum matrix composites were fabricated from Al–TiO₂–graphitic C powder mixtures using exothermic dispersion method. The effects of C/TiO₂ molar ratio on the reaction processes, activation energies and mechanical properties of the resulting materials were investigated. When the C/TiO₂ molar ratio is 0, Al reacts with TiO₂ to produce fine α-Al₂O₃ particles and Ti, which then reacts with Al to form large rod-like Al₃Ti phase. By adding graphite C into the Al–TiO₂ system, the activation energy of the first reactive step increases; in addition, the resultant Ti preferentially reacts with C to form hard TiC particles. When the C/TiO₂ molar ratio increases to 1.0, the Al₃Ti phase disappears and the reinforcements consist of nano-sized α-Al₂O₃ and TiC phases. The tensile strength of the composites increases from 239.2 MPa to 351.8 MPa and the elongation increases from 4.1% to 5.6%, suggesting a marked increase in damage tolerance (i.e., toughness).     

Slurry erosive wear behavior of Ni–P coated Si3N4 reinforced Al6061 composites

Ni–P coated Si₃N₄ reinforced Al6061 composites were fabricated by liquid metallurgy route. Percentage of reinforcement was varied from 4 wt% to 10 wt% in steps of 2. The developed composites were subjected to microstructure and sand slurry erosive wear studies. The influence of experimental parameters such as slurry concentration, rotational speed of slurry, size of impinging particles and the test duration on slurry erosive wear behavior of developed composites have been studied. Results reveals that, Al6061–Si₃N₄ composites exhibited improved wear resistance when compared with the matrix alloy under identical test conditions. With increase in slurry concentration, rotational speed of slurry, test duration, size of impinging particles, the slurry erosive wear rates of both matrix alloy and developed composites increases. However, under all the tests conditions studied, the developed composites possess higher wear resistance when compared with that of matrix alloy. Energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were used to identify the oxides/passive layer formed on the worn surfaces. Scanning Electron Microscopy (SEM) examinations were also carried out on worn surfaces to observe the possible mechanisms of material removal in the matrix alloy and developed composites.     

Sol–gel derived CaO–B2O3–SiO2 glass/CaSiO3 ceramic composites: processing and electrical properties

The CaO–B₂O₃–SiO₂ glass/CaSiO₃ ceramic (CBS/CS) composites were fabricated via sol–gel processing routes. Their densification behavior, structures and dielectric properties were investigated. The precursors of CBS glass and CS ceramic filler were firstly obtained via individual soft chemical route and then mixed together in various proportions. The results indicated that the structures of CBS/CS composites are characteristic of CS and CaB₂O₄ (CB) ceramic phases distributed in the matrix of glass phase at 800–950 °C. The CS ceramic phase not only acts as fillers, but nuclei for the crystallization of CBS glass as well such that the CS content exhibits an effect on the densification and dielectric properties of the composites. The CBS/CS composites with 10% CS sintered at 850 °C own dielectric properties of ε_r < 5 and tanδ = 6.4 × 10⁻⁴ at 1 MHz.