Two-step sintering of nanocrystalline 8Y2O3 stabilized ZrO2 synthesized by glycine nitrate process

Two-step sintering was employed to consolidate nanocrystalline 8 mol% yittria stabilized zirconia processed by glycine-nitrate method. Results verified the applicability of this method to suppress the final stage of grain growth in the system. The grain size of the high density compacts (>97%) produced by two-step sintering method was seven times less than the pieces made by the conventional sintering technique. Up to ∼96% increase in the fracture toughness was observed (i.e. from 1.61 to 3.16 MPa m^1/2) with decreasing of the grain size from ∼2.15 to ∼295 nm. A better densification behavior was also observed at higher compacting pressures.     

Al2O3/Cu-O composites fabricated by pressureless infiltration of paper-derived Al2O3 porous preforms

Al₂O₃/Cu-O composites were fabricated from the paper-derived alumina matrix infiltrated with a Cu-3.2?wt% O alloy. Paper-derived alumina preforms with an open porosity ranging from ～ 14 to ～ 25?vol% were prepared by sintering of alumina-loaded preceramic papers at 1600?°C for 4?h. Pressureless infiltration at 1320?°C for 4?h of the preforms with Cu–O alloy resulted in the nearly dense materials with good mechanical and electrical properties, e.g. fracture toughness up to 6?MPa?m^0.5, four-point-bending strength up to 342?MPa, Young's modulus up to 281?GPa and electrical conductivity up to 2?MS/m depending on the volume fraction of copper alloy in the composites. The technological capability of this approach was demonstrated using prototypes in various engineering fields fabricated by lamination, corrugating and Laminated Object Manufacturing (LOM) methods.     

Fabrication and mechanical properties of Al2O3-Si3N4/ZrO2-Al2O3 laminated composites

In this paper, Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were fabricated by tape casting and hot press sintering, and the relationships between the process, microstructure, and mechanical properties of Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were determined. The SiAlON phase was found in the Al₂O₃-Si₃N₄ layer, and liquid-phase sintering was proposed. Nano-scratch tests were carried out to investigate the interface bonding strength of the laminates. The distribution of residual stresses, generated due to the different coefficients of thermal expansion between the different layers, was estimated according to lamination theory and confirmed using Vickers indentation. When the sintering temperature was 1550 °C, the sintered laminated ceramics had good mechanical properties, with a maximum strength and toughness of 413 MPa and 6.2 MPa m^1/2, respectively. The main toughness mechanics of laminated composites was residual stress.     

Effects of Pr6O11 on the microstructure and mechanical properties of Ti/Al2O3 composites prepared by pressureless sintering

Ti/Al₂O₃ composites with different volume percentages of Pr₆O₁₁ added (0–12.0  vol.%) were prepared by pressureless sintering at 1600 °C for 1.5 h. The influences of Pr₆O₁₁ on the composition, microstructure and mechanical properties of the composites were characterized and investigated. The results showed that Pr₆O₁₁ could promote the sintering of the composites by generating some new interfacial reaction products, such as AlTiO₂, Pr₂Ti₂O₇ and PrAlO₃. Pr₆O₁₁ could also inhibit the production of TiAl and Ti₃Al by the same mechanism. Additionally, Pr₆O₁₁ changed hexagonal alumina to tetragonal alumina. The latter could improve the mechanical properties of the composites by the effects of crack deflection and particle pullout when it was present in proper amounts. Composites showed satisfactory comprehensive properties when the content of Pr₆O₁₁ was no more than 3.0 vol.%.     

Infiltration of Al2O3/Y2O3 mix into SiC ceramic preforms

Silicon carbide ceramics are very interesting materials to engineering applications because of their properties. These ceramics are produced by liquid phase sintering (LPS), where elevated temperature and time are necessary, and generally form volatile products that promote defects and damage their mechanical properties. In this work was studied the infiltration process to produce SiC ceramics, using shorter time and temperature than LPS, thereby reducing the undesirable chemical reactions. SiC powder was pressed at 300 MPa and pre-sintered at 1550 °C for 30 min. Unidirectional and spontaneous infiltration of this preform by Al₂O₃/Y₂O₃ liquid was done at 1850 °C for 5, 10, 30 and 60 min. The kinetics of infiltration was studied, and the infiltration equilibrium happened when the liquid infiltrated 12 mm into perform. The microstructures show grains of the SiC surrounded by infiltrated additives. The hardness and fracture toughness are similar to conventional SiC ceramics obtained by LPS.     

Microstructure and mechanical properties of ZrO2-2 mol% Y2O3 ceramics

The microstructure and mechanical properties of ZrO₂-2 mol% Y₂O₃ ceramics were studied on samples prepared by cold isostatic pressing and pressureless sintering. It was shown that the density of the alloy increases with increasing sintering temperature. The Vickers hardness decreases with the appearance of the monoclinic phase and increasing its content. Compared with the single tetragonal phase, the (t + m) dual-phase structure with microcracks has a much higher fracture toughness (16·5 MPa√m) because of a complex mechanism of toughening (transformation, microcracks and residual stresses).     

多孔Al2O3-ZrO2陶瓷的微观结构

以Al₂O₃、ZrO₂陶瓷粉体为溶质,以莰烯为溶剂,以Texaphor963作为添加剂,制备出低粘度高稳定性的陶瓷浆料,采用冷冻注模工艺制备出具有较高强度的陶瓷坯体,采用无压烧结工艺,得到了多孔Al₂O₃-ZrO₂陶瓷制品,并对其微观结构进行了研究。     

Yb2O3 and Y2O3 co-doped zirconia ceramics

A suspension stabilizer-coating technique was employed to prepare x mol% Yb₂O₃ (x = 1.0, 2.0, 3.0 and 4.0) and 1.0 mol% Y₂O₃ co-doped ZrO₂ powder. A systematic study was conducted on the sintering behaviour, phase assemblage, microstructural development and mechanical properties of Yb₂O₃ and Y₂O₃ co-doped zirconia ceramics. Fully dense ZrO₂ ceramics were obtained by means of pressureless sintering in air for 1 h at 1450 °C. The phase composition of the ceramics could be controlled by tuning the Yb₂O₃ content and the sintering parameters. Polycrystalline tetragonal ZrO₂ (TZP) and fully stabilised cubic ZrO₂ (FSZ) were achieved in the 1.0 mol% Y₂O₃ stabilised ceramic, co-doped with 1.0 mol% Yb₂O₃ and 4.0 mol% Yb₂O₃, respectively. The amount of stabilizer needed to form cubic ZrO₂ phase in the Yb₂O₃ and Y₂O₃ co-doped ZrO₂ ceramics was lower than that of single phase Y₂O₃-doped materials. The indentation fracture toughness could be tailored up to 8.5 MPa m^1/2 in combination with a hardness of 12 GPa by sintering a 1.0 mol% Yb₂O₃ and 1.0 mol% Y₂O₃ ceramic at 1450 °C for 1 h.     

Microstructure and mechanical properties of gas pressure sintered Al2O3/TiCN composite

Al₂O₃–30 wt.%TiCN composites have been fabricated successfully by a two-stage gas pressure sintering schedule. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa m^1/2. The fabrication procedure involves solid state sintering of two phases without solubility to prepare Al₂O₃–TiCN composite. Little grain growth occurred for TiCN during sintering while Al₂O₃ grains grew about three times to an average size of 3–5 μm. The interface microstress arising during cooling from the processing temperature because of the thermal and/or mechanical properties mismatch between the Al₂O₃ and TiCN phase is about 50 MPa. Such a compressive microstress is not high enough to cause grain boundary cracking that may weaken the composite but it can introduce dislocations within grains, which is very good to enhance the composite properties.     

Influence of WC particles on the microstructural and mechanical properties of 3 mol% Y2O3 stabilized ZrO2 matrix composites produced by hot pressing

Yttria stabilized polycrystalline tetragonal zirconia (Y-TZP)-tungsten carbide (WC) composites were fabricated by hot pressing. Yttria (Y₂O₃) stabilizer content was kept at 3 mol% to ensure the phase structure of the Y-TZP composites to be tetragonal. To increase the moderate hardness of the 3 mol% Y₂O₃ added TZP structure, hard WC particles were added with various proportions up to 40 vol%. The TZP/WC composites were sintered at different sintering temperatures between 1450 and 1550 °C.The mechanical and microstructural properties of the resulting composites as well as the phase compositions were investigated. Reciprocating pin-on-disk tests were carried out to determine the wear behavior of the Y-TZP/WC composites. Using bi-modal WC reinforcement, the performance of the composite against wear was improved. Using dry wear sliding conditions under 55 N normal load and 45 km sliding distance, the worn volume of the 75 vol% nanosized - WC distributed 3Y-TZP/40WC composite was about 0.003 mm³.     

Electrical and mechanical properties of pressureless sintered SiC-Ti2CN composites

Highly conductive SiC-Ti₂CN composites were fabricated from β-SiC and TiN powders with 10?vol% Y₂O₃-AlN additives via pressureless sintering. The effect of initial TiN content on the microstructure, and electrical and mechanical properties of the SiC-Ti₂CN composites was investigated. It was found that all specimens could be sintered to ≥98% of the theoretical density. The electrical resistivity of the SiC-Ti₂CN composites decreased with increasing initial TiN content. The SiC-Ti₂CN composites prepared from 25?vol% TiN showed the highest electrical conductivity (～1163 (Ω?cm)^?1) for any pressureless sintered SiC ceramics thus far. The high electrical conductivity of the composites was attributed to the in situ-synthesis of an electrically conductive Ti₂CN phase and the growth of N-doped SiC grains during pressureless sintering. The flexural strength, fracture toughness, and Vickers hardness of the composite fabricated with 25?vol% TiN were 430?MPa, 4.9?MPa?m^1/2, and 23.1?GPa, respectively, at room temperature.     

Effect of Al2O3 on mechanical properties of Ti3SiC2/Al2O3 composite

The effect of Al₂O₃ on mechanical properties of Ti₃SiC₂/Al₂O₃ composite fabricated by SPS was studied systematically. The results show that the hardness of the Ti₃SiC₂/Al₂O₃ composite can reach 10.28 GPa, 50% higher than that of pure Ti₃SiC₂. However, slight decrease in the other mechanical properties was observed with Al₂O₃ addition higher than 5–10 vol.%, which is believed to be due to the agglomeration of Al₂O₃ in the composite.     

Improved mechanical and wear properties of hybrid Al-Al2O3/GNPs electro-less coated Ni nanocomposite

Al was successfully reinforced with two ceramics Al₂O₃ coated Ni and graphene nanoplatelets (GNPs) coated Ni by electro-less deposition technique to form Al-Al₂O₃/$\mathit{x}$ GNPs hybrid nanocomposite ($\mathit{x}=0,0.2,0.6,1$and $1.4\%$) with improved mechanical and wear properties. Compressive strength, hardness, wear properties and coefficient of friction were investigated. The results indicated that increasing GNPs volume fraction improves compressive strength, hardness and antifriction properties of composites significantly. In comparison with pure aluminum, 1.52- fold increases in the strength, 2.45-fold increase in the hardness and 19.2-fold decreases in the wear rate of Al-10%Al₂O₃/1.4%GNPs nanocomposite are achieved. This improvement is attributed to the remarkable mechanical strength and excellent self-lubrication of grapheme, the reduction of grain size during electro-less deposition process and the increased efficient stress transfer due to the curled structure of GNPs. Additionally, coating GNPs with Ni particles prevent the formation of Al₃C₄ intermetallic phase which lead to this large improvement in the wear rate. In comparison with the available results in the literature, electro-less coating of GNPs with Ni provides 2.1 times larger hardness than composite with uncoated GNPs.     

Developments on the mechanical properties of MgO-MgAl2O4 composite refractories by ZrSiO4-3 mol% Y2O3 addition

Improvements and the effects of additions of ZrSiO₄-3 mol% Y₂O₃ into MgO-MgAl₂O₄ composite refractories on mechanical properties and thermal stress resistance parameters were investigated. Significant improvements were achieved on mechanical properties and R-R_st parameters up to ∼2 and ∼3-fold ratios. The major parameters improving mechanical properties and thermal behaviour of refractories were determined as follows: (i) the increase in resistance to crack initiation and propagation due to formation of Mg₂SiO₄ phase after decomposition of zircon; (ii) propagation of the microcracks formed in the structure for a short distance by interlinking each other; (iii) arresting or deviation of microcracks when reaching pores or ZrO₂ grains released after dissociation of zircon, located together with Y₂O₃ particles, and furthermore; (iv) co-presence of both intergranular and transgranular types of cracks, and with incorporation of zircon-Y₂O₃; (v) increase in density; and (vi) a significant reduction in MgO grain size.     

Structural, thermal and mechanical properties of transparent Yb:(Y0.97Zr0.03)2O3 ceramic

Yb doped (Y_0.97Zr_0.03)₂O₃ transparent ceramics were fabricated by solid state reaction and vacuum sintering. The microstructure, thermal and mechanical properties of Y₂O₃ ceramic, as well as the effect of Yb doping concentration on these properties were investigated in detail. The lattice parameter and unit cell volume decrease with the increasing of Yb content, whereas thermal expansive coefficient increases. With Yb content increasing from 0 to 8 at.%, the mean grain size increases from 15.82 μm to 26.54 μm, and the thermal conductivity at room temperature (RT) decreases from 11.97 to 6.39 W/m/K. The microhardness decreases with Yb content, and the microhardness and fracture toughness of (Y_0.97Zr_0.03)₂O₃ transparent ceramic is 11.11 GPa and 1.29 MPa m^1/2, respectively.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Enhanced mechanical properties of W-doped Nb4AlC3 ceramics

The W-doped Nb₄AlC₃ ceramics [(Nb_1-xW_x)₄AlC₃, x?=?0–0.0375] were successfully fabricated by in-situ reactive hot-press-aided method using elemental niobium, aluminum, graphite and tungsten powders. The XRD results suggest that the matrix phase (Nb_1-xW_x)₄AlC₃ and the second phase (Nb_1-xW_x)C were simultaneously formed when W was added. The SEM images show that (Nb_1-xW_x)C is dispersed in the W-doped Nb₄AlC₃ ceramics matrix. The mechanical properties of Nb₄AlC₃ were greatly enhanced by W doping. Typically, the (Nb_0.975W_0.025)₄AlC₃ exhibits the highest flexural strength (483?±?21?MPa), fracture toughness (8.5?±?0.3?MPa?m^1/2) and Young′s modulus (382?±?18?GPa) at room temperature (RT), which are increased by 59%, 15% and 30%, respectively, compared with the present Nb₄AlC₃. The Vickers hardness of (Nb_0.9625W_0.0375)₄AlC₃ (4.8?±?0.2?GPa) is 92% higher than that of Nb₄AlC₃. The (Nb_0.975W_0.025)₄AlC₃ also retains a high flexural strength of 344?±?4?MPa at 1400?°C (71% of RT value), which is much higher than the RT flexural strength (303?±?22?MPa) of the present Nb₄AlC₃. The strengthening effect is attributed to the solid solution of W and the incorporation of the second phase (Nb_1-xW_x)C. The excellent mechanical properties endow the W-doped Nb₄AlC₃ ceramics as promising high-temperature structural materials.     

Mechanical properties of Al2O3-Cr2O3/Cr3C2 nanocomposite fabricated by spark plasma sintering

The fine grains of Al₂O₃-Cr₂O₃/Cr-carbide nanocomposites were prepared by employing recently developed spark plasma sintering (SPS) technique. The initial materials were fabricated by a metal organic chemical vapor deposition (MOCVD) process, in which Cr(CO)₆ was used as a precursor and Al₂O₃ powders as matrix in a spouted chamber. The basic mechanical properties like hardness, fracture strength and toughness, and the nanoindentation characterization of nanocomposites such as Elastics modulus (E), elastic work (W_e) and plastic work (W_p) were analyzed. The microstructure of dislocation, transgranular and step-wise fracture surface were observed in the nanocomposites. The nanocomposites show fracture toughness of (4.8 MPa m^1/2) and facture strength (780 MPa), which is higher than monolithic alumina. The strengthening mechanism from the secondary phase and solid solution are also discussed in the present work. Nanoindentation characterization further illustrates the strengthening of nanocomposites.     

Design of 7 wt.% Y2O3–ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance

Plasma-sprayed stand-alone coatings of 7 wt.% Y₂O₃–ZrO₂ (YSZ), nominally 74 wt.% Al₂O₃–26 wt.% SiO₂ mullite, and a 46:54 volume ratio composite of YSZ to mullite were examined using X-ray diffraction, dilatometry, and compression creep. X-ray diffraction and dilatometer results showed that the as-sprayed predominantly amorphous mullite crystallized at 970 °C. Creep tests were conducted on all three coating types in the as-sprayed condition at stresses from 40 to 80 MPa and temperatures of 1000–1200 °C. The primary deformation mechanism in coatings made from all three materials was stress-assisted densification of the porous coating. While the creep behavior of YSZ/mullite composite specimens was between that of pure YSZ and pure mullite specimens for all combinations of temperature and stress tested, the creep response of the composite was more similar to that of pure mullite for all cases tested, consistent with mullite being the continuous phase in the composite.     

Synthesis and mechanical properties characterization of multiferroic BiFeO3-CoFe2O4 composite nanofibers

Multiferroic nanofibers with excellent mechanical properties have great potential applications in multifunctional nanodevices. BiFeO₃-CoFe₂O₄ (BFO-CFO) composite nanofibers with different molar ratios were successfully synthesized by sol-gel-based electrospinning method. The mechanical properties of BFO-CFO composite nanofibers were examined by nanoindentation technique, and further investigated by amplitude modulation-frequency modulation (AM-FM) method based on atomic force microscopy (AFM). The results of AM-FM showed that the elastic moduli of BFO-CFO composite nanofibers increased with the increase of CFO ratio, which was consistent with the results of nanoindentation. These results indicated that AFM-based AM-FM is a powerful method for nondestructively investigating the mechanical properties of materials at nanoscale, and that the results of BFO-CFO composite nanofibers are also of practical importance for the future applications of multifunctional nanodevices.