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Abstract: Fully dense ZrCcomposite was fabricated by reactive infiltration technique, where molten Zr2Cu alloy infiltrated into a porous WC preform at 1300°C in vacuum. The ZrCte exhibits a core-shell structure feature: gray WC cores were enveloped by bright tungsten shells, which are homogeneously distributed in the ZrC matrix. X-ray diffraction and transmission electron microscopy investigation show that nano-sized Zr x Cu y C z particles were formed in the ZrC matrix. The Vicker''s hardness, flexural strength and fracture toughness of the ZrCte are measured as 14.5GPa, 615MPa and 11.7MPam1/2, respectively. [Copyright &y& Elsevier]

Abstract: ZrC–W composite was produced by reactive infiltration of Zr2Cu into WC preform at 1200°C. The WC reacted completely with Zr in the alloy to form 61.8vol% ZrC and 38.2vol%W. The infiltrated composite reached a relative density of 98.3% and average grains sizes of about 0.5μm. The flexural strength and the fracture toughness of the ZrC–W composite was improved by the addition of W. [Copyright &y& Elsevier]

The genus Saussurea is distributed mainly in the temperate and subarctic regions of Eurasia and consists of about 300 species classified into six subgenera and 20 sections. Sect. Pseudoeriocoryne in the subgenus Eriocoryne comprises four species, and is delimited mainly by acaulescence and an inflorescence with congested capitula surrounded by a rosette of leaves. All of these species are endemic to the arid Qinghai–Tibet Plateau. Sequences from the chloroplast DNA trnL–F region were obtained for the four species in this section and 26 other species from four subgenera of Saussurea to resolve phylogenetic relationships among these species and to determine whether the shared characters that define sect. Pseudoeriocoryne are synapomorphic or were acquired by convergent evolution. The resulting phylogenies indicated that Saussurea sect. Pseudoeriocoryne as traditionally defined does not constitute a monophyletic group and that each of its species belongs to separate clades. Furthermore, none of these species showed a close relationship with the other species of subgenus Eriocoryne. Our results further indicated that none of the investigated subgenera are monophyletic, and that species from different subgenera clustered together. All these conclusions are provisional and their confirmation would require stronger phylogenetic support. Two possible explanations are suggested for low sequence divergence, poor resolution of internal clades and clustering of species with the rather distinct morphology of Saussurea detected in the present study. The first is rapid radiation and diversification triggered by fast habitat fragmentation due to the recent lifting of the Qinghai–Tibet Plateau and the Quaternary climate oscillations. This could have led to rapid morphological divergence while sequences diverged very little, and also caused the convergent acquisition of similar characteristics in unrelated lineages due to similar selection pressures. The second possible explanation is that both introgressive hybridization and reticulate evolution might have caused the transferring of cpDNA sequences between morphologically dissimilar species, thus leading to homogenization of sequences between lineages. [Copyright &y& Elsevier]

Mechanical activation followed by rotary heating has been successfully applied to the synthesis of Cr-doped lithium manganese oxide (LiMn2O4) powders. The precursor of LiCrxMn2−xO4 for Li-ion batteries has been synthesized from LiOH·H2O, MnO2, and Cr2O3 as starting materials by mechanical activation, which ensured a high level of homogeneity in the chemical composition of the precursor and resulted in a reduction in the synthesis temperature and heating time. Highly crystallized LiCrxMn2−xO4 (0.04≤x≤0.10) powders were obtained by subsequent rotary heating of the precursor at 550–750 °C for 2 h. Rotary heating enhanced the experimental control in chemical composition, particle size distribution and microstructure of the final products. The optimum mechanical activation condition was determined based on XRD analysis. The synthesized LiCrxMn2−xO4 powder has a narrow particle size distribution, high discharge capacities and good cyclability. The method of mechanical activation combined with rotary heating should be an excellent technique of decreasing synthesis temperature and shortening heating time, and enhancing the uniformity of particle size of the product. [Copyright &y& Elsevier]

Tungsten and tungsten alloys are widely used in high temperature environments where arc ablation or mechanical deformation and damage are the main sources of materials failure. For high temperature critical applications in thermomechanical environments, however, the low strength limits the use of tungsten and tungsten alloys. Hence, new tungsten based materials with good high temperature thermomechanical properties need to be developed in order to extend the use of tungsten. TiC particle-reinforced tungsten based composites (TiCp/W) were fabricated by hot pressing at 2000 °C, 20 MPa in a vacuum of 1.3×10−3 Pa. The composites were examined with respect to their thermophysical and mechanical properties at room temperature and at elevated temperature. Vickers hardness and elastic modulus increased with increasing TiC content from 0 to 40 vol.%. The highest flexural strength, 843 MPa, and the highest toughness, 10.1 MPa m1/2, of the composites at room temperature were all obtained when 20 vol.% TiC particle were added. As the test temperature rose, the flexural strength of the TiCp/W composites firstly increased and then decreased, except in the monolithic tungsten. The highest strength of 1155 MPa was measured at 1000 °C in the composite containing 30 vol.% TiC particles. The strengthening effect of TiC particles on the tungsten matrix is more significant at high temperatures. With the addition of TiC particles, the thermal conduction of tungsten composites was drastically decreased from 153 W m−1 K−1 for monolithic W to 27.9 W m−1 K−1 for 40 vol.% TiCp/W composites, and the thermal expansion was also increased. The new composites are successfully used to make high temperature grips and moulds. [Copyright &y& Elsevier]

Abstract The nucleation and epitaxial growth behavior of a novel Al 2 O 3 -ZrO 2 nanoeutectic rapidly solidified from the oxyacetylene flame remelting have been investigated. The nanoeutectic ceramic consists mainly of highly textured cellular structure with an average interphase spacing of 136 nm. Especially, a singular transition mechanism associated with " one-step cellular refining " to " two-step cellular refining " is proposed in a variable region from a coarse irregular to an ultrafine lamellar structure, and the regional undercooling is estimated to be 0.57–2.86 K. The Al 2 O 3 phase nucleates as the leading phase in the rapidly solidified structure according to a quantitative calculation of nucleation rate. The cellular eutectic exhibits with a tilted feature, where its cellular growth deviates from the heat flow direction to a preferred growth orientation. [ABSTRACT FROM AUTHOR]

W-ZrC composites without residual WC have been prepared for the first time by reactive infiltration at 1300 °C for 1 h in vacuum using a molten Zr 2 Cu alloy and a newly designed partially-carburized W powder as raw materials. The as-synthesized composites consist of two major phases of W and ZrC, in which the content of W is 65 vol%. The reaction time needed to produce a fully densified W-ZrC bulk ceramic is distinctly shortened by this means, as contrasted with conventional WC/W or WC preforms. The microstructural evolution during reactive melt infiltration is investigated to obtain a better understanding of reaction mechanisms and mechanical properties of the W-ZrC composites derived by infiltrating Zr 2 Cu alloy into partially carburized W preforms. The flexural strength, Young's modulus and fracture toughness for the W-ZrC composite are 554 MPa, 339 GPa and 9.7 MPa·m 1/2 , respectively. [ABSTRACT FROM AUTHOR]

Calcia-magnesia-aluminosilicate (CMAS) corrosion behaviors of hot-pressed (Y 1- x Yb x) 2 SiO 5 ceramics at 1673 K and 1773 K have been investigated. The main corrosion product of nonstoichiometric Ca-RE-Si-O apatite phase evolves with increasing temperature or prolonging corrosion time. The growth of apatite layer at 1673 K follows approximately a linear rate law, however it follows a proximately parabolic rate law at 1773 K. The growth rate of the apatite layer at 1773 K is lower than that at 1673 K in the process of interaction between CMAS and rare-earth monosilicates. Yb-doping mediates the CMAS corrosion resistance of Y 2 SiO 5 effectively by changing the growth rate of apatite layer and the composition of formed apatite phase. This work provides new insights on compositional regulation of (Y 1- x Yb x) 2 SiO 5 EBCs materials to resist CMAS corrosion. [ABSTRACT FROM AUTHOR]

In the present work, microstructural refinement and mechanical response of Al 2 O 3 –ZrO 2 eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al 2 O 3 –ZrO 2 eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al 2 O 3 –ZrO 2 eutectics is investigated. [ABSTRACT FROM AUTHOR]

The surface microstructure of advanced tool materials is critical, as it directly affects subsequent processing or the performance in use as original condition. In this work, the microstructural evolution and formation mechanism of metal capping on the surface of Ti(C, N)-based cermets, during liquid phase sintering were investigated. Surface metal capping on Ti(C, N)-based cermets is formed during the cooling stage of liquid phase sintering, driven by the contraction pressure of ceramic particle skeleton in the middle stage, in which an adequate amount of liquid phase and significant densification are vital prerequisites. The formation of a step-like textured metal capping is attributed to volume contraction during solidification of liquid phase on the sample surface, in which directional solidification caused by carbon gradient is necessary in this process. Transport behavior of molten metal associated with metal capping on Ti(C, N)-based cermets is divided into four stages in order as infiltration, extrusion, aggregation, and evaporation. The energy of crack propagation is greatly dissipated by plastic deformation of metal capping, thereby greatly improving the fracture toughness of original surface of the sample. [Display omitted] • The formation mechanism of surface metal capping and step-like texture on Ti(C, N)-based cermets is revealed. • The transport behavior of molten metal on the surface of Ti(C, N)-based cermets is clarified. • The presence of a metal capping greatly improves the fracture toughness of the original surface of Ti(C, N)-based cermets. [ABSTRACT FROM AUTHOR]

Four kinds of quinary high-entropy rare-earth (RE) monosilicates and (Y 1- x Yb x) 2 SiO 5 have been synthesized by solid-state reaction method. Selected rare-earth oxides of Lu 2 O 3 to Gd 2 O 3 react with silica to form homogeneous monosilicate solid solutions at a sintering temperature of 1773 K for 4 h. Wherein Gd-Si-O transforms from oxy-apatite type to X2-RE 2 SiO 5 with increasing the synthesis temperature from 1573 K to 1773 K. Increasing the rare-earth ion species changes the distortion degrees of [REO m ] a little bit. However, it's more effective to improve the distortion degrees with only introducing Yb3+ into Y3+ sites of Y 2 SiO 5. Stronger distortion of [REO m ] improves the high-temperature resistance to thermal deformation, resulting in a lower coefficient of thermal expansion (CTE). Due to the highest distortion degree of [REO m ], (Y 0.2 Yb 0.8) 2 SiO 5 exhibits the lowest CTE of 6.78 × 10−6 K−1 at 1473 K. Meanwhile, RE-doping enhances the distortion and phonon scattering, contributing to a lower thermal conductivity. [ABSTRACT FROM AUTHOR]

LiMn2−xAlxO4 (0.0⩽x⩽0.10) intercalation powders have been synthesized by mechanical alloying followed by rotary heating. The precursor of LiMn2−xAlxO4 has been prepared using LiOH · H2O, MnO2, and Al2O3 by mechanical alloying, which ensured a high level of homogeneity in the chemical composition of the precursor and resulted in a reduction in the synthesis temperature and heating time. Highly crystallized LiMn2−xAlxO4 powders were obtained by subsequent rotary heating of the precursor at 600 °C for 3 h. Rotary heating enhanced the experimental control in particle size distribution of the final products. The synthesized LiMn2−xAlxO4 powder has a narrow particle size distribution, high discharge capacities and good cycleability. Compared with the conventional solid-state reaction process, the present method of mechanical alloying combined with rotary heating should be a suitable technique of decreasing synthesis temperature and shortening heating time, and enhancing the uniformity of particle size of the product. [Copyright &y& Elsevier]

The high temperature ablation behavior of tungsten composites containing carbides produced by vacuum hot pressing is studied as a function of reinforcement chemistry (ZrC and TiC) and content using a self-made oxyacetylene ablation equipment. A dynamic responding multiwavelength pyrometer was employed to measure the temperature of the ablation surface, and a thermocouple was employed to measure the temperature of the back surface during the time that a specimen was being ablated. The mass and linear ablation rates are lower in composites containing ZrC, decreasing with increasing particle content in both composites system. The values of the mass and linear ablation rates were in the order from high to low: W>30TiC/W>40TiC/W>30ZrC/W>40ZrC/W (30TiC/W stands for 30 vol.% TiC particle content in the W matrix, the same below). The important temperature curves of the ablation surfaces of specimens were successfully detected online. Ablated surfaces and vertical sections of the specimens were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thermochemical oxidation of tungsten, TiC, and ZrC was the main ablation mechanism of ZrC/W and TiC/W composites. These ablation behaviors are discussed based on the thermophysical and chemical properties of both the composite systems. [Copyright &y& Elsevier]

Revealing the grain growth behavior during liquid phase sintering is of great significance for scientific application of grain growth inhibition strategies and the regulation of microstructure. In this study, the effects of both TaC addition and the particle size of starting materials on microstructure of Ti(C, N)-based cermets were investigated. Ultrafine Ti(C, N)-based cermets modified with TaC addition exhibit an obvious grain growth of grey phase during liquid phase sintering. The grain growth of grey phase is typically controlled by liquid-phase diffusion, with the grain growth index of 3 and activation energy of 48–66 kJ/mol. In particular, the increase in grain contiguity of grey phase caused by the evaporation of metal liquid phase at high temperatures, faceted transition of grey phase grains, and the loss of carbon and nitrogen are considered to be the main reasons for the decrease in the grain growth rate of grey phase at high temperatures. • The effects of TaC addition and initial particle size of raw materials on microstructure of Ti(C, N)-based cermets were elucidated. • The growth mechanism of grey-phase grains in TaC-modified ultrafine Ti(C, N)-based cermets was revealed. • The reason why the growth rate of grey phase grains in Ti(C, N)-based cermets is reduced at high temperatures was clarified. [ABSTRACT FROM AUTHOR]

This work uses in situ transmission electron microscopy (TEM) based micromechanical testing to isolate and quantify the microstructural toughening mechanisms active in nanostructured Al 2 O 3 /GdAlO 3 eutectic samples. The effect of fracture direction across orthogonal sections of the rod-like eutectic was used to reveal the influence of different fracture paths and mechanisms on toughening. The average fracture toughnesses of the rod-like structures in the longitudinal cross-section and transverse cross-section are 2.4 MPa·m1/2 and 2.7 MPa·m1/2, respectively. Multiple samples tested in the longitudinal cross-section show significant R-curve toughening response, and obtain values greatly exceeding the initial values upon crack extension. It is concluded that nanoscale crack bridging induces deflection of the crack path, which leads extrinsic energy dissipation as the crack opens. Micropillar compressions are also performed to investigate the composite's strength. Sample orientation strongly affects the deformation mode and interfacial sliding occurs when the maximal shear stress is parallel to the interface. [ABSTRACT FROM AUTHOR]

Li 7– x La 3 Zr 2– x Ta x O 12 (0 ≤ x ≤ 1) ceramics were prepared by pressureless sintering at 1230 °C in air. XRD, SEM and Impedance test were used to characterize the crystal structure, microstructure and electrical performance of Li 7– x La 3 Zr 2– x Ta x O 12 ceramics. According to XRD data, the specimens with x ≥ 0.2 sintered at 1230 °C for 1 h exhibit a cubic garnet structure without impurities. The Ta-doped specimens sintered at 1230°C for 1 h show high relative density of 91.6%–93.9%. Ta doping has a certain effect on the Li vacancy concentration, lithium arrangement, movable Li+ concentration and microstructure of the LLZO ceramic. The Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 ceramic sintered at 1230°C for 1 h has the highest total conductivity of 6.01 × 10–4 S·cm–1 at room temperature and the lowest activation energy of 0.27 eV. [ABSTRACT FROM AUTHOR]

This study aims to elucidate the impact of interfacial microstructure and properties on fracture toughness in binary Al 2 O 3 /SmAlO 3 eutectic and its polycrystalline samples via in-situ microcantilever deflection tests. Al 2 O 3 -SmAlO 3 eutectic with semi-coherent heterointerfaces exhibits superior interfacial toughness of 2.85 MPa⋅m1/2 relative to randomly oriented interfaces in Al 2 O 3 /SmAlO 3 polycrystals. SmAlO 3 grain boundaries (GBs) have the lowest toughness of 1.97 MPa⋅m1/2 among all the constituents, i.e. bulk and interfaces. The toughness of eutectic Al 2 O 3 -SmAlO 3 interfaces is intermediate between polycrystalline Al 2 O 3 -Al 2 O 3 GBs and SmAlO 3 -SmAlO 3 GBs. Fracture toughness values of both polycrystalline and eutectic Al 2 O 3 -SmAlO 3 samples obtained by mesoscopic indentation measurements are in the range of 4.2 MPa⋅m1/2to 4.7 MP⋅m1/2, which exceeds all constituent components. Much of the overall fracture toughness of the bulk samples derives from microstructural and geometric energy dissipation mechanisms. [ABSTRACT FROM AUTHOR]

The present work aims to provide fundamental insights into the grain size dependence and mechanical behavior of hot-pressed Al 2 O 3 -ZrO 2 ceramic at its eutectic composition, and further to explore the hardening effect of laser-induced surface nanoeutectic layer. The underlying correlations between densification behavior, grain size distribution and mechanical properties were elucidated. Sintering at 1550 °C promotes the densification without extensive grain growth, and in this case the sample exhibits a critical density of 99.3 %. The average grain size is tailored into a range of 0.6–0.9 μm, and the measured flexural strength and toughness reach 1100 MPa and 11 MPa·m1/2, respectively. The metastable t -ZrO 2 grains indeed play a pivotal role in energy dissipation at the crack tip through crack deflection and branching. In addition, the mechanical behavior is reasonably explained through constructing a multilevel toughening mechanism map associated with grain size distribution of ZrO 2. Particularly, surface nanocrystallized Al 2 O 3 -ZrO 2 eutectic layer with a thickness of 1000 μm free of pores and cracks is achieved by a rapid laser melting process. The outmost laser-modified nanoeutectic layer exhibits a fine cellular structure with an interphase spacing of only 105 nm and a hardness of as high as 26.1 GPa, which provides a promising potential in enhancing significantly the hardness and wear resistance for applications as sliding ceramic components. [ABSTRACT FROM AUTHOR]

Abstract Gas burst has been commonly observed during reactive sintering, especially when combustion was initialized therein. In this study, a model system (Zr + 2B = ZrB 2) was chosen to investigate this phenomenon by a combination of thermo-analysis, mass spectrum investigation and electron microscopy studies. Results showed the combustion lead to a formation of various gaseous byproducts such as CO, CO 2 , N 2 , BO, BO 2 and B 2 O 3 etc. Especially for BO(g), its generation consumed boron in the raw materials, resulting in the residual Zr-containing phase that accelerated the formation of interlocked ZrB 2 grains in the sintered ceramics, which show relatively high strength (516 ± 42 MPa). Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract This present work explores initially the feasibility of producing in-situ surface nanoeutectic Al 2 O 3 -ZrO 2 (Y 2 O 3) (SNAZ) ceramics via the oxyacetylene flame remelting (OAFR) process. Singular cellular nanoeutectics consist of nanoscale lamellae, which prevail in the samples formed under different irradiation times. Ultrafine nanoeutectics are achieved by prolonging the irradiation time, and their formation mechanisms associated with eutectic refinement are proposed to obtain a better understanding on the melt thermal history and microstructural inheritance. The enhanced nucleation undercooling is the essential mechanism associated with the microstructural refinement. SNAZ formed by the OAFR-based rapid solidification seem to offer an inexpensive route to realize in-situ microstructure tailoring or surface shaping. [ABSTRACT FROM AUTHOR]

The present work concentrated on a novel one-step pulse discharge plasma melting method for in situ fabricating Al 2 O 3 -ZrO 2 binary eutectic in a short processing time. Al 2 O 3 -ZrO 2 eutectic exhibited well-aligned fibrous and colonial structure with a minimum lamellar spacing of ∼250 nm at the growth rate of ∼176.0 μm/s. The lamellar spacing was well consistent with the inverse-square-root dependence on the growth rate as λ = 2.90 × v − 1 / 2 , and the constant of proportionality of 2.90 was close to 3.32 predicted by the JH eutectic theory. The formation mechanism of Al 2 O 3 -ZrO 2 eutectic on primary faceted-alumina crystals was also proposed. The Al 2 O 3 -ZrO 2 eutectic originated from the faceted Al 2 O 3 primary crystals, which played the leading role in the coupled growth process. The interfacial fracture behavior across the coarse intercolony regions was explored, and the fracture toughness of Al 2 O 3 -ZrO 2 eutectic ceramics was comparable to those achieved by traditional directional solidification. [ABSTRACT FROM AUTHOR]

In the present work, Al 2 O 3 -GdAlO 3 oxide nanopowders with eutectic composition of 77:23 (in molar ratio) were prepared via a chemical co-precipitation method. α-SiC particles were then incorporated with different volume fractions of 10, 15 and 20 vol% into the oxide powders. Al 2 O 3 -GdAlO 3 –SiC ceramics were hot-pressed at different sintering temperatures to obtain fully densified bulk ceramics. The as-sintered composites consist mainly of α-Al 2 O 3 , GdAlO 3 and α-SiC phases, and exhibit a dense and uniform microstructure. The GdAlO 3 grains are entangled with α-Al 2 O 3 grains as indicated by SEM observations. SiC nanoparticles are found within the interior of both α-Al 2 O 3 and GdAlO 3 grains or at the interfaces between entangled α-Al 2 O 3 and GdAlO 3 . With increasing the SiC content from 10 to 20 vol%, the high-temperature flexural strength of Al 2 O 3 -GdAlO 3 -SiC ceramics increases from 567 to 616 MPa, and the Vickers hardness increases from 17.5 to 19.5 GPa, while the fracture toughness of Al 2 O 3 -GdAlO 3 -SiC ceramics increases from 8.20 to 9.28 MPa m 1/2 , which is significantly improved with respect to that (7.5 MPa m 1/2 ) of unmodified Al 2 O 3 -GdAlO 3 ceramic. The toughening mechanisms of Al 2 O 3 -GdAlO 3 -SiC ceramics are attributed to crack deflection, pinning and bridging, and bifurcation due to the presence of SiC nanophases observed from the indentation. [ABSTRACT FROM AUTHOR]

An efficient and flexible chemical co-precipitation method has been used to synthesize nanoscale Al 2 O 3 -GdAlO 3 powders with eutectic composition. The as-synthesized powders exhibit a highly dispersive and homogeneous distribution with an average particle size of 50 nm. The phase transition in the resulting powders strongly depends upon the calcination temperature. GdAlO 3 undergoes complete crystallization after calcination at 1050 °C, however, the diffraction peaks of α-Al 2 O 3 are found at a relatively high calcination temperature of at least 1300 °C. The fully-densified Al 2 O 3 -GdAlO 3 ceramic with eutectic composition obtained by hot pressing the nanoscale powders at 1500 °C exhibits a room temperature flexural strength of 556 MPa, a Vickers hardness of 17.3 GPa and a fracture toughness of 7.5 MPa m 1/2 . The high temperature flexural strength of the as-sintered Al 2 O 3 -GdAlO 3 ceramic is measured to be 515 MPa after bending tests at 1000 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanism of phase transition from X2-RE 2 SiO 5 to apatite is revealed. • The formation of apatite in corroded Y 2 SiO 5 is inhibited by introducing Yb. • Silica-rich grain boundary is considered to reduce the corrosion degree. Environmental barrier coatings (EBCs) are usually coated on SiC ceramic or ceramic matrix composites (CMCs), in order to prevent the attack from corrosive media like water vapor and molten oxides. Solid-solutionizing strategy containing high-entropy engineering is considered as an effective way to mediate corrosion resistances of rare-earth silicates, which are promising candidates for the topcoats of EBCs. In this work, high-temperature corrosion behaviors of hot-pressed (Y 1− x Yb x) 2 SiO 5 ceramics have been investigated in a 50 %H 2 O–50 %O 2 environment at temperatures of 1573/1673/1773 K for 100 h. The main corrosion product of Y 2 SiO 5 is apatite phase, whose formation can be distinctly inhibited by introducing Yb. Consequently, introducing Yb avoids the tensile stress and cracks effectively, which are induced by the volume reduction due to the phase transition to apatite at elevated temperatures. The inhibition of apatite formation by introducing Yb is attributed to the relatively lower volatilization pressure of Yb 2 O 3 than Y 2 O 3 and the smaller cationic radius of Yb3+ than Y3+. Meanwhile, the composition deviations between grain boundary and grain surface results in the grain surface corrosion and ridge-like corrosion characteristics. [ABSTRACT FROM AUTHOR]

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder (δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy. [ABSTRACT FROM AUTHOR]

Crack-healing ability and strength recovery of different ZrO 2 (Y 2 O 3 )–Al 2 O 3 –SiC (TZ3Y20A–SiC) ceramics have been investigated by tailoring heat treatment temperature, heat treatment time, SiC content and crack size. Heat treatment in air enhances distinctly the flexural strength of TZ3Y20A–SiC ceramics. A complete crack-healing of TZ3Y20A–10 vol%SiC ceramic can be realized by optional heat treatments at 1073 K for 30 h, 1273 K for 10 h or 1373 K for 5 h, respectively. With increasing the content of nanosize SiC particles from 10 to 20 vol%, the crack-healing process of TZ3Y20A–SiC ceramics speeds up at identical temperature levels. The pre-existing cracks with a width up to 0.45 μm are healed completely under the optimum heat treatment conditions, and the flexural strengths of crack-healed TZ3Y20A–SiC ceramics are completely recovered to the value of smooth specimens. Crack closure and rebonding of crack wall due to the formation of SiO 2 glassy phase by oxidation reaction between SiC and the oxidizing atmosphere are dominant healing mechanisms in TZ3Y20A–SiC ceramics. [ABSTRACT FROM AUTHOR]

In this work, CMAS corrosion behavior of hot-pressed (Y 1- x Yb x) 2 SiO 5 ceramics at 1573 K within 32 h has been investigated. The corrosion products contain non-stoichiometric apatite and garnet, which form according to the dissolution-reprecipitation mechanism. With abundant calcia-magnesia-aluminosilicate, the growth rate of apatite layer increases rapidly with prolonging corrosion time, which results in the rapid corrosion especially after 4 h. Furthermore, the measured thickness of apatite layer in (Y 1- x Yb x) 2 SiO 5 is much lower than the predicted value based on the optical basicity (OB) theory. This work provides a new insight of solid-solutionizing strategy in the resistance of rare-earth monosilicate to CMAS corrosion. • The producing conditions of two different CMAS corrosion products are revealed. • The variation on the growth rate of apatite layer over corrosion time is discussed. • The apatite layer thickness is smaller than the predicted value based on OB theory. [ABSTRACT FROM AUTHOR]

Highlights: [•] MAO coatings on Ti2AlNb alloy consist of Al2TiO5, R-TiO2, Al2O3 and Nb2O5. [•] Two-step voltage technique enhances the densification of MAO coatings. [•] MAO coating improves the thermal radiation property of Ti2AlNb alloy. [•] MAO coatings protect Ti2AlNb substrate from the high temperature oxidation. [•] Two-step MAO coatings exhibit better high temperature properties. [ABSTRACT FROM AUTHOR]

Crack healing behavior of hot-pressed TZ3Y20A-SiC ceramics has been investigated by high-temperature oxidation. Semi-elliptical surface cracks with a length of 100μm have been set on the tensile side of each specimen using a Vickers hardness indenter. Based on flexural strength and observations of crack appearance, the effect of crack healing is finally judged. Cracks on ZrO2(Y2O3)–Al2O3 (TZ3Y20A) ceramics cannot be healed by heat treatment. However, for TZ3Y20A-SiC ceramics, complete crack-healing has been realized by heat treatment at 800°C, 1000°C and 1100°C for 30h, 10h and 5h, respectively. The crack-healing mechanism is attributed to the formation of SiO2 caused by high temperature oxidation during heat treatment. [ABSTRACT FROM AUTHOR]

Reaction process and phase formation mechanisms in reactive hot pressing (RHP) of titanium and boron nitride blend powders have been investigated in vacuum. In temperatures of up to 1300°C, chemical reactions in the Ti–BN blend powder proceed via a solid-state diffusion process. Intermediate phases of hexagonal α-TiN0.3, tetragonal Ti2N, nonstoichiometric cubic TiN0.9 and orthorhombic TiB are found before they finally transform to the stoichiometric cubic TiN and hexagonal TiB2. Both the TiN–TiB2 ceramics reactive hot-pressed at 1600 and 1800°C represent intragranular/intergranular distributions of TiB2 in TiN. In contrast with monolithic TiN and TiB2 ceramics, the reactive hot-pressed TiN–TiB2 exhibits a significant enhancement on flexural strength and fracture toughness. TiN–TiB2 ceramics reactive hot-pressed at 1800°C have a Vickers hardness of 17.8GPa, an elastic modulus of 475GPa, a flexural strength of 732MPa and a fracture toughness of 8.5MPa·m1/2. [ABSTRACT FROM AUTHOR]

In recent years, the increasing potential for the use of high temperature self-lubricating composites as sliding components at extreme environments have attracted great attention on these materials. In the present work, NiCr–BaCr2O4 self-lubricating composites have been prepared using the hot pressing method. The friction and wear properties of NiCr–BaCr2O4 composites were investigated by using a ball-on-disk friction and wear tester in sliding against alumina ball up to 800°C. At room temperature, the NiCr–20BaCr2O4 composite has a friction coefficient of 0.72, which is clearly higher than that (0.44) of NiCr alloy. However, the composites incorporated with BaCr2O4 exhibit distinct improvements in effectively reducing friction and wear in a temperature range of 400–800°C as contrasted with unmodified NiCr alloy. At 800°C, the NiCr–20BaCr2O4 composite exhibits a friction coefficient of 0.27 and a wear rate of 4.5×10–6 mm3/(Nm), while the unmodified NiCr alloy has a friction coefficient of 0.41 and a wear rate of 9.2×10–5 mm3/(Nm), respectively. At elevated temperatures, a dense and protective oxide glaze layer is generated on the worn surface of composites. The glaze layer was then investigated by means of scanning electron microscopy, laser Raman spectroscopy and nano-indentation. Tribo-oxidation reaction of BaCr2O4 during high temperature wear tests is responsible for the formation of the protective glaze layer. The newly formed BaCrO4 contains lubricious hexavalent-Cr6+, which can promote the densification of oxide glaze layer, and finally generate a dense lubricating film to effectively reduce friction and wear at elevated temperatures. [ABSTRACT FROM AUTHOR]

TiN–TiB2 composites incorporated with or without MoSi2 have been prepared by the reactive hot pressing method, using Ti, BN or MoSi2 powders as the starting materials. The influence of MoSi2 addition on tribological properties of TiN–TiB2 composites have been evaluated by using a ball-on-disk friction and wear tester in sliding against alumina ball from room temperature to 800°C. The wear performance depends mainly upon temperature as well as the applied load. At room temperature and 10N load, the TiN–TiB2 ceramic has a wear rate of 1.6×10–6 mm3 N–1 m–1, however, the TiN–TiB2–MoSi2 composite exhibits a wear rate of 1.5×10–4 mm3 N–1 m–1 under identical wear condition. At room temperature, the material removal of composites results from brittle fracture and abrasive wear during wear tests. The addition of MoSi2 can improve the oxidation resistance of TiN–TiB2 ceramic at high temperatures as it oxidizes to produce a protective SiO2 layer, which forms an effective barrier layer to restrain from further oxidation. At 800°C and 10N load, the wear rate of TiN–TiB2–MoSi2 composite is 1.7×10–6 mm3 N–1 m–1, which is only one fifth as much as the TiN–TiB2 ceramic. The low wear rate of TiN–TiB2–MoSi2 composite at 800°C is attributed to high oxidation resistance, the formation of dense SiO2 scale and self-healing of microcracks. [ABSTRACT FROM AUTHOR]