Tungsten tetraboride, an inexpensive superhard material

Tungsten tetraboride (WB(4)) is an interesting candidate as a less expensive member of the growing group of superhard transition metal borides. WB(4) was successfully synthesized by arc melting from the elements. Characterization using powder X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) indicates that the as-synthesized material is phase pure. The zero-pressure bulk modulus, as measured by high-pressure X-ray diffraction for WB(4), is 339 GPa. Mechanical testing using microindentation gives a Vickers hardness of 43.3 ± 2.9 GPa under an applied load of 0.49 N. Various ratios of rhenium were added to WB(4) in an attempt to increase hardness. With the addition of 1 at.% Re, the Vickers hardness increased to approximately 50 GPa at 0.49 N. Powders of tungsten tetraboride with and without 1 at.% Re addition are thermally stable up to approximately 400 °C in air as measured by thermal gravimetric analysis.     

Acoustic Anomalies and the Critical Slowing-Down Behavior of MAPbCl3 Single Crystals Studied by Brillouin Light Scattering

Inelastic light scattering spectra of organic–inorganic halide perovskite MAPbCl₃ single crystals were investigated by using Brillouin spectroscopy. Sound velocities and acoustic absorption coefficients of longitudinal and transverse acoustic modes propagating along the cubic [100] direction were determined in a wide temperature range. The sound velocities exhibited softening upon cooling in the cubic phase, which was accompanied by the increasing acoustic damping. The obtained relaxation time showed a critical slowing-down behavior, revealing the order–disorder nature of the phase transition, which is consistent with the growth of strong central peaks upon cooling toward the phase transition point. The temperature dependences of the two elastic constants C₁₁ and C₄₄ were obtained in the cubic phase for the first time. The comparison of C₁₁ and C₄₄ with those of other halide perovskites showed that C₁₁ of MAPbCl₃ is larger and C₄₄ is slightly smaller compared to the values of MAPbBr₃ and MAPbI₃. It suggests that MAPbCl₃ has a more compact structure (smaller lattice constant) along with stronger binding forces, causing larger C₁₁ and bulk modulus in this compound, and that the shear rigidity is exceedingly small similar to other halide perovskites. The reported elastic constants in this study may serve as a testbed for theoretical and calculational approaches for MAPbCl₃.     

Microstructure,Hardness, and Elastic Modulus of a Multibeam-Sputtered Nanocrystalline Co-Cr-Fe-Ni Compositional Complex Alloy Film

A nanocrystalline Co-Cr-Ni-Fe compositional complex alloy (CCA) film with a thickness of about 1 micron was produced by a multiple-beam-sputtering physical vapor deposition (PVD) technique. The main advantage of this novel method is that it does not require alloy targets, but rather uses commercially pure metal sources. Another benefit of the application of this technique is that it produces compositional gradient samples on a disk surface with a wide range of elemental concentrations, enabling combinatorial analysis of CCA films. In this study, the variation of the phase composition, the microstructure (crystallite size and defect density), and the mechanical performance (hardness and elastic modulus) as a function of the chemical composition was studied in a combinatorial Co-Cr-Ni-Fe thin film sample that was produced on a surface of a disk with a diameter of about 10 cm. The spatial variation of the crystallite size and the density of lattice defects (e.g., dislocations and twin faults) were investigated by X-ray diffraction line profile analysis performed on the patterns taken by synchrotron radiation. The hardness and the elastic modulus were measured by the nanoindentation technique. It was found that a single-phase face-centered cubic (fcc) structure was formed for a wide range of chemical compositions. The microstructure was nanocrystalline with a crystallite size of 10–27 nm and contained a high lattice defect density. The hardness and the elastic modulus values measured for very different compositions were in the ranges of 8.4–11.8 and 182–239 GPa, respectively.     

Relationship between Young’s Modulus and Planar Density of Unit Cell,Super Cells (2 × 2 × 2), Symmetry Cells of Perovskite (CaTiO3) Lattice

Calcium titanate-CaTiO₃ (perovskite) has been used in various industrial applications due to its dopant/doping mechanisms. Manipulation of defective grain boundaries in the structure of perovskite is essential to maximize mechanical properties and stability; therefore, the structure of perovskite has attracted attention, because without fully understanding the perovskite structure and diffracted planes, dopant/doping mechanisms cannot be understood. In this study, the areas and locations of atoms and diffracted planes were designed and investigated. In this research, the relationship between Young’s modulus and planar density of unit cell, super cells (2 × 2 × 2) and symmetry cells of nano CaTiO₃ is investigated. Elastic constant, elastic compliance and Young’s modulus value were recorded with the ultrasonic pulse-echo technique. The results were C₁₁ = 330.89 GPa, C₁₂ = 93.03 GPa, C₄₄ = 94.91 GPa and E = 153.87 GPa respectively. Young’s modulus values of CaTiO₃ extracted by planar density were calculated 162.62 GPa, 151.71 GPa and 152.21 GPa for unit cell, super cells (2 × 2 × 2) and symmetry cells, respectively. Young’s modulus value extracted by planar density of symmetry cells was in good agreement with Young’s modulus value measured via ultrasonic pulse-echo.     

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO3 orthopyroxene

Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CaIrO(3)-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO(3). Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) A(3) when K'(0) = 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to approximately 400-550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CaIrO(3)-type phases at 80-106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.     

A quenchable superhard carbon phase synthesized by cold compression of carbon nanotubes

A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of approximately 100 GPa. A hexagonal carbon phase was formed at approximately 75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp(3)-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a(0) = 2.496(4) A, c(0) = 4.123(8) A, and V(0) = 22.24(7) A (3). There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 +/- 0.2 g/cm(3), which is at least compatible to that of diamond (3.52 g/cm(3)). The bulk modulus was determined to be 447 GPa at fixed K' identical with 4, slightly greater than the reported value for diamond of approximately 440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.     

Effect of Current Density on the Microstructure and Mechanical Properties of 3YSZ/Al2O3 Composites by Flash Sintering

The 3YSZ/40 wt% Al₂O₃ composites were prepared by flash sintering at a low furnace temperature (700 °C). The effects of the current density on the relative density and Vickers hardness of the composites were systematically investigated. The results showed that the relative densities and Vickers hardness of the samples increased gradually with the increasing of the current densities, and the relative density was as high as 94.2%. The Vickers hardness of 11.3 GPa was obtained under a current density of 102 mA/mm². Joule heating and defects generation are suggested to be the main causes of rapid densification in flash sintering. The microstructure of the molten zone showed the formation of eutectic structures in the composite, suggesting that grain boundary overheating may have contributed to the formation of the molten zone.     

Structure and Properties of ZrON Coatings Synthesized by Cathodic Arc Evaporation

The transition metal oxynitrides are a coating material with decorative features due to their adjustable color and good mechanical properties. The purpose of the research was to study the effect of the relative oxygen concentration O_2(x) = O₂/(N₂ + O₂) in particular on adhesion, but also on the color, structural and mechanical properties of ZrON coatings synthesized by cathodic arc evaporation on HS6-5-2 steel substrates. The surface morphology, phase and chemical composition and mechanical properties were determined using scanning electron microscopy, X-ray diffraction, wavelength dispersive X-ray spectroscopy, nanoindentation and scratch test. It was found that color of the coatings changed from light yellow for ZrN first to gold and then to graphite for Zr-O phase with increase of oxygen concentration. X-ray diffraction patterns showed that the phase separation of ZrN and ZrO₂ occurred for about 35 at.% of oxygen in the coating. Increase in oxygen concentration in the coatings led to decrease in crystallite size from about 20 nm for ZrN to about 5 nm for coatings with about 35 at.% of oxygen and about 25 at.% of nitrogen. An increase in hardness from about 26 GPa for ZrN to about 30 GPa for coating with small concentration of oxygen (about 9 at.%) and then decrease to about 15 GPa was observed. Adhesion of Zr-O-N coatings demonstrated strong dependence on oxygen concentration. Critical load for ZrN is about 80 N and decreases with oxygen concentration increase to about 30 N for ZrO₂.     

High-Entropy Borides under Extreme Environment of Pressures and Temperatures

The high-entropy transition metal borides containing a random distribution of five or more constituent metallic elements offer novel opportunities in designing materials that show crystalline phase stability, high strength, and thermal oxidation resistance under extreme conditions. We present a comprehensive theoretical and experimental investigation of prototypical high-entropy boride (HEB) materials such as (Hf, Mo, Nb, Ta, Ti)B₂ and (Hf, Mo, Nb, Ta, Zr)B₂ under extreme environments of pressures and temperatures. The theoretical tools include modeling elastic properties by special quasi-random structures that predict a bulk modulus of 288 GPa and a shear modulus of 215 GPa at ambient conditions. HEB samples were synthesized under high pressures and high temperatures and studied to 9.5 GPa and 2273 K in a large-volume pressure cell. The thermal equation of state measurement yielded a bulk modulus of 276 GPa, in excellent agreement with theory. The measured compressive yield strength by radial X-ray diffraction technique in a diamond anvil cell was 28 GPa at a pressure of 65 GPa, which is a significant fraction of the shear modulus at high pressures. The high compressive strength and phase stability of this material under high pressures and high temperatures make it an ideal candidate for application as a structural material in nuclear and aerospace fields.     

Hardness and Elastic Modulus on Six-Fold Symmetry Gold Nanoparticles

The chemical synthesis of gold nanoparticles (NP) by using gold (III) chloride trihydrate (HAuCl∙3H₂O) and sodium citrate as a reducing agent in aqueous conditions at 100 °C is presented here. Gold nanoparticles areformed by a galvanic replacement mechanism as described by Lee and Messiel. Morphology of gold-NP was analyzed by way of high-resolution transmission electron microscopy; results indicate a six-fold icosahedral symmetry with an average size distribution of 22 nm. In order to understand the mechanical behaviors, like hardness and elastic moduli, gold-NP were subjected to nanoindentation measurements—obtaining a hardness value of 1.72 GPa and elastic modulus of 100 GPa in a 3–5 nm of displacement at the nanoparticle’s surface.     

Exploration of D022-Type Al3TM(TM = Sc,Ti, V,Zr, Nb,Hf, Ta): Elastic Anisotropy,Electronic Structures,Work Function and Experimental Design

The structural properties, elastic anisotropy, electronic structures and work function of D0₂₂-type Al₃TM (TM = Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta) are studied using the first-principles calculations. The results indicate that the obtained formation enthalpy and cohesive energy of these compounds are in accordance with the other calculated values. It is found that the Al₃Zr is the most thermodynamic stable compound. The mechanical property indexes, such as elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Vickers hardness are systematically explored. Moreover, the calculated universal anisotropic index, percent anisotropy and shear anisotropic factors of D0₂₂-type Al₃TM are analyzed carefully. It demonstrates that the shear modulus anisotropy of Al₃La is the strongest, while that of Al₃Ta is the weakest. In particular, the density of states at Fermi level is not zero, suggesting that these phases have metal properties and electrical conductivity. More importantly, the mechanisms of correlation between hardness and Young’s modulus are further explained by the work function. Finally, the experimental design proves that D0₂₂-Al₃Ta has an excellent strengthening effect.     

Microstructure Evolution of FeNiCoCrAl1.3Mo0.5 High Entropy Alloy during Powder Preparation,Laser Powder Bed Fusion,and Microplasma Spraying

In the present study, powder of FeCoCrNiMo_0.5Al_1.3 HEA was manufactured by gas atomization process, and then used for laser powder bed fusion (L-PBF) and microplasma spraying (MPS) technologies. The processes of phase composition and microstructure transformation during above mentioned processes and subsequent heat treatment were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and differential thermal analysis (DTA) methods. It was found that gas atomization leads to a formation of dendrites of body centered cubic (BCC) supersaturated solid solution with insignificant Mo-rich segregations on the peripheries of the dendrites. Annealing leads to an increase of element segregations till to decomposition of the BCC solid solution and formation of σ-phase and B2 phase. Microstructure and phase composition of L-PBF sample are very similar to those of the powder. The MPS coating has a little fraction of face centered cubic (FCC) phase because of Al oxidation during spraying and formation of regions depleted in Al, in which FCC structure becomes more stable. Maximum hardness (950 HV) is achieved in the powder and L-PBF samples after annealing at 600 °C. Elastic modulus of the L-PBF sample, determined by nanoindentation, is 165 GPa, that is 12% lower than that of the cast alloy (186 GPa).     

High-pressure transformations in xenon hydrates

A high-pressure investigation of the Xe*H(2)O chemical system was conducted by using diamond-anvil cell techniques combined with in situ Raman spectroscopy, synchrotron x-ray diffraction, and laser heating. Structure I xenon clathrate was observed to be stable up to 1.8 GPa, at which pressure it transforms to a new Xe clathrate phase stable up to 2.5 GPa before breaking down to ice VII plus solid xenon. The bulk modulus and structure of both phases were determined: 9 +/- 1 GPa for Xe clathrate A with structure I (cubic, a = 11.595 +/- 0.003 A, V = 1,558.9 +/- 1.2 A(3) at 1.1 GPa) and 45 +/- 5 GPa for Xe clathrate B (tetragonal, a = 8.320 +/- 0.004 A, c = 10.287 +/- 0.007 A, V = 712.1 +/- 1.2 A(3) at 2.2 GPa). The extended pressure stability field of Xe clathrate structure I (A) and the discovery of a second Xe clathrate (B) above 1.8 GPa have implications for xenon in terrestrial and planetary interiors.     

Elastic Properties and Hardness of Mixed Alkaline Earth Silicate Oxynitride Glasses

The incorporation of nitrogen as a second anion species into oxide glasses offers unique opportunities for modifying glass properties via changes in glass polymerization and structure. In this work, the compositional dependence of elastic properties and the nanoindentation hardness of mixed alkaline-earth silicate oxynitride glasses containing a high amount of nitrogen (>15 at.%, c.a. 35 e/o) were investigated. Three series of silicon oxynitride glass compositions AE–Ca–Si–O–N glasses (where AE = Mg, Sr, and Ba) having varying amounts of modifiers were prepared using a new glass synthesis route, in which a precursor powder of metal hydrides was used. The obtained glasses contained high amounts of N (19 at.%, c.a. 43 e/o) and modifier cations (26 at.%, c.a. 39 e/o). Mg–Ca–Si–O–N glasses had high values of nanohardness (12–16 GPa), along with a reduced elastic modulus (130–153 GPa) and Young’s modulus (127–146 GPa), in comparison with the Sr–Ca- and Ba–Ca-bearing oxynitride glasses. Both the elastic modulus and the nanohardness of AE–Ca–Si–O–N glasses decreased with an increase in the atomic number of the AE element. These property changes followed a linear dependence on the effective cation field strength (ECFS) of the alkaline earth (AE) modifier, according to their valences and ionic radii. No mixed alkaline-earth effect was observed in the current investigation, indicating that the properties were more dictated by the nitrogen content.     

Experimental and Computational Studies of Compression and Deformation Behavior of Hafnium Diboride to 208 GPa

The compression behavior of the hexagonal AlB₂ phase of Hafnium Diboride (HfB₂) was studied in a diamond anvil cell to a pressure of 208 GPa by axial X-ray diffraction employing platinum as an internal pressure standard. The deformation behavior of HfB₂ was studied by radial X-ray diffraction technique to 50 GPa, which allows for measurement of maximum differential stress or compressive yield strength at high pressures. The hydrostatic compression curve deduced from radial X-ray diffraction measurements yielded an ambient-pressure volume V₀ = 29.73 Å3/atom and a bulk modulus K₀ = 282 GPa. Density functional theory calculations showed ambient-pressure volume V₀ = 29.84 Å3/atom and bulk modulus K₀ = 262 GPa, which are in good agreement with the hydrostatic experimental values. The measured compressive yield strength approaches 3% of the shear modulus at a pressure of 50 GPa. The theoretical strain-stress calculation shows a maximum shear stress τ_max~39 GPa along the (1−10) [110] direction of the hexagonal lattice of HfB₂, which thereby can be an incompressible high strength material for extreme-environment applications.     

Ultrasound elasticity of diamond at gigapascal pressures

Diamond is the hardest known material in nature and features a wide spectrum of industrial and scientific applications. The key to diamond''s outstanding properties is its elasticity, which is associated with its exceptional hardness, shear strength, and incompressibility. Despite many theoretical works, direct measurements of elastic properties are limited to only ∼1.4 kilobar (kb) pressure. Here, we report ultrasonic interferometry measurements of elasticity of void-free diamond powder in a multianvil press from 1 atmosphere up to 12.1 gigapascal (GPa). We obtained high-accuracy bulk modulus of diamond as K₀ = 439.2(9) GPa, K₀′ = 3.6(1), and shear modulus as G₀ = 533(3) GPa, G₀′ = 2.3(3), which are consistent with our first-principles simulation. In contrast to the previous experiment of isothermal equation of state, the K₀′ obtained in this work is evidently greater, indicating that the diamond is not fully described by the “n-m” Mie–Grüneisen model. The structural and elastic properties measured in this work may provide a robust primary pressure scale in extensive pressure ranges.

The extraordinary mechanical properties of diamond make it an unsurpassed material in cutting, shaping, and compressing hard substances. To date, a large amount of work has aimed to remedy the drawbacks of diamond as a superhard material in the context of industrial implications (1). For instance, the thermal stability, elastic deformability, and toughness of diamond can be improved by synthesizing a nanoscale diamond crystal using high-pressure apparatus (2–6). For such implications, its high-pressure elasticity plays a critical role, and its underlying stiffening mechanism will, in turn, guide the design for better nanoscale diamond.Diamond is a high-pressure allotrope of carbon. Under high-pressure conditions, the structure of diamond is constructed through a martensitic process in which the sp³ carbon bonding is formed along a specific shear direction (7, 8). In the opposite, the procedure to break the sp³ bonding (e.g., the transformation from diamond to graphite) is usually indirect and kinetically inhibited (9). The formation and alignment of sp³ carbon bonding are the signatures of diamond, whose covalent nature governs its elastic properties. A number of theoretical works were pioneered to calculate the high-pressure elasticity of diamond. Tse et al. calculated the equation of state of diamond to terapascal pressure and temperature up to its melting curve and suggested that diamond is a well-defined covalent solid under such extreme conditions (10). Núñez Valdez et al. calculated the thermoelectricity of diamond to 500 GPa and predicted that the elastic property of diamond becomes increasingly anisotropic at high pressures (11). In stark contrast, experimental works are scarce, especially on the in situ elasticity of compressed diamond. Previously, single-crystal diamond was compressed in a diamond anvil cell (DAC) by Occelli et al., who confirmed that diamond is a Grüneisen solid up to at least 140 GPa and exhibited strengthened covalent bonding. The latest high-pressure, elasticity experiment of diamond, as far as we have investigated, was by McSkimin and Andreatch 40 y ago, who measured the ultrasonic wave velocities up to ∼0.14 GPa (12) and calculated K₀ = 442(11) GPa, K₀′ = 4.03(16). Data points on the experimental elastic modulus at gigapascal pressure range is still rare, and they are in urgent need to understand the evolution of diamond elasticity under external stress.The challenges to perform high-pressure elasticity experiment for diamond, beyond a doubt, stem from its intrinsic properties. Diamond is the key component of DAC, the main instrument for high-pressure research, and thus interferometric measurements, such as Brillouin scattering and ultrasonic interferometry, in a DAC, are impractical because of the interference of the anvils. The experiment using multianvil, high-pressure press is among the few solutions. Since the velocities of compressional wave (V_P, >18 km/s) and shear wave (V_S, >12 km/s) of diamond are very high, a large sample size is required in order to separate the echo from the reference (namely, the buffer rod) signals. The sample assembly limits the achievable pressure to ∼10 GPa. Along with the compression tool, we also developed an ultrasonic interferometry coupled with multianvil apparatus, dedicated for measuring polycrystalline elasticity under high pressure (13). The system has successfully obtained high-accuracy, elastic parameters for a variety of solids and liquids (14–17). Since diamond is extremely incompressible, such that the travel time change within this pressure range is tiny, our measurement will challenge the limit of instrumental precision using the current setups.The focus of this work is to measure the high-pressure, elastic properties of diamond. High-quality elasticity data and their pressure derivatives are the prerequisite to establish a primary pressure scale for high-pressure sciences (18). Previous experiments obtained primary scale up to 120 GPa from static compression (19, 20) and multimegabar pressures from dynamic compression (21). Since diamond is thermodynamically stable in terapascal pressure range (22), it is possible to create a diamond primary pressure scale for much extended pressures. The results will be a solid anchor for pressure gauges, for example, ruby fluorescence (23), the Raman edge of diamond anvil (24), and the X-ray diffraction of metal (25).     

Effect of Gd2O3 Addition on the Microstructure and Properties of Gd2O3-Yb2O3-Y2O3-ZrO2 (GYYZO) Ceramics

Gd and Yb elements have high chemical stability, which can stabilize the solid solution in ZrO₂. Gd₂O₃ and Yb₂O₃ have high melting points, and good oxidation resistance in extreme environments, stable chemical properties. Therefore, Gd₂O₃ and Yb₂O₃ were added to ZrO₂ to stabilize oxides, improve the high temperature stability, and effectively decrease the thermal conductivity at high temperature. In this work, 5 wt% Yb₂O₃ and 5 wt%, 10 wt%, 15 wt% Gd₂O₃ were doped into 8 wt% Y₂O₃ stabilized ZrO₂ (8YSZ) powders as thermal barrier coating materials, and sintered at 1650 °C for 6 h, 12 h, 24 h. The effects of Gd₂O₃ addition on the microstructure, density, thermal conductivity, hardness, and fracture toughness of Gd₂O₃-Yb₂O₃-Y₂O₃-ZrO₂ (GYYZO) bulk composite ceramics were investigated. It was found that the densification of the 8YSZ bulk and GYYZO bulk with 15 wt% Gd₂O₃ reached 96.89% and 96.22% sintered at 1650 °C for 24 h. With the increase of Gd₂O₃ addition, the hardness, elastic modulus and fracture toughness of the GYYZO bulk increased and the thermal conductivity and thermal expansion coefficient of the GYYZO bulk decreased. GYYZO bulk with 15 wt% Gd₂O₃ sintered at 1650 °C for 24h had the highest hardness, elastic modulus and fracture toughness of 15.61 GPa, 306.88 GPa, 7.822 MPa·m^0.5, and the lowest thermal conductivity and thermal expansion coefficient of 1.04 W/(m·k) and 7.89 × 10⁻⁶/°C at 1100 °C, respectively. The addition of Gd₂O₃ into YSZ could not only effectively reduce the thermal conductivity but also improve the mechanical properties, which would improve the thermal barrier coatings’ performances further.     

High Pressure Brillouin Spectroscopy and X-ray Diffraction of Cerium Dioxide

Simultaneous high-pressure Brillouin spectroscopy and powder X-ray diffraction of cerium dioxide powders are presented at room temperature to a pressure of 45 GPa. Micro- and nanocrystalline powders are studied and the density, acoustic velocities and elastic moduli determined. In contrast to recent reports of anomalous compressibility and strength in nanocrystalline cerium dioxide, the acoustic velocities are found to be insensitive to grain size and enhanced strength is not observed in nanocrystalline CeO2. Discrepancies in the bulk moduli derived from Brillouin and powder X-ray diffraction studies suggest that the properties of CeO2 are sensitive to the hydrostaticity of its environment. Our Brillouin data give the shear modulus, G0 = 63 (3) GPa, and adiabatic bulk modulus, KS0 = 142 (9) GPa, which is considerably lower than the isothermal bulk modulus, KT0∼ 230 GPa, determined by high-pressure X-ray diffraction experiments.     

Effect of Nb on the Microstructure,Mechanical Properties,Corrosion Behavior,and Cytotoxicity of Ti-Nb Alloys

In this paper, the effects of Nb addition (5–20 wt %) on the microstructure, mechanical properties, corrosion behavior, and cytotoxicity of Ti-Nb alloys were investigated with the aim of understanding the relationship between phase/microstructure and various properties of Ti-xNb alloys. Phase/microstructure was analyzed using X-ray diffraction (XRD), SEM, and TEM. The results indicated that the Ti-xNb alloys (x = 10, 15, and 20 wt %) were mainly composed of α + β phases with precipitation of the isothermal ω phase. The volume percentage of the ω phase increased with increasing Nb content. We also investigated the effects of the alloying element Nb on the mechanical properties (including Vickers hardness and elastic modulus), oxidation protection ability, and corrosion behavior of Ti-xNb binary alloys. The mechanical properties and corrosion behavior of Ti-xNb alloys were found to be sensitive to Nb content. These experimental results indicated that the addition of Nb contributed to the hardening of cp-Ti and to the improvement of its oxidation resistance. Electrochemical experiments showed that the Ti-xNb alloys exhibited superior corrosion resistance to that of cp-Ti. The cytotoxicities of the Ti-xNb alloys were similar to that of pure titanium.     

Influence of N2 Partial Pressure on Structure and Mechanical Properties of TiAlN/Al2O3 Multilayers

TiAlN/Al₂O₃ multilayers with different Ar/N₂ ratios were deposited on Sisubstrates in different N₂ partial pressure by magnetron sputtering. The crystalline and multilayer structures of the multilayers were determined by a glancing angle X-ray diffractometer (XRD). A nanoindenter was used to evaluate the hardness, the elastic modulus and scratch scan of the multilayers. The chemical bonding was investigated by a X-ray Photoelectron Spectroscopy (XPS). The maximum hardness (36.3 GPa) and elastic modulus (466 GPa) of the multilayers was obtained when Ar/N₂ ratio was 18:1. The TiAlN/Al₂O₃ multilayers were crystallized with orientation in the (111) and (311) crystallographic planes. The multilayers displayed stably plastic recovery in different Ar/N₂ ratios. The scratch scan and post scan surface profiles of TiAlN/Al₂O₃ multilayers showed the highest critical fracture load (L_c) of 53 mN for the multilayer of Ar/N₂ = 18:1. It indicated that the multilayer had better practical adhesion strength and fracture resistance.