Synthesis of nanoscale multilayered ZrN/W2N multilayered coatings by magnetron sputtering

ZrN/W₂N multilayered coatings with nanoscale modulation period in an ultra-high vacuum rf magnetron sputter chamber. XRD, SEM, Nano Indenter and profiler were employed to investigate the influence of modulation periods and working pressures on structural and mechanical properties of the coatings. The low-angle XRD pattern and cross-sectional SEM indicated a well-defined composition modulation and layer structure of the multilayered coating. All multilayered coatings revealed higher hardness than the rule-of-mixtures value of monolithic ZrN and W₂N coatings at different working pressures. The maximum hardness was up to 34 GPa. The multilayers obtained mixed polycrystalline textures of ZrN(111), W₂N(111), W₂N(200) and W₂N(311). 0.8 Pa was an optimum working pressure for mechanical property enhancement.     

Microstructural evolution of nanostructured Ti0.9Al0.1N prepared by reactive ball-milling

Nanocrystalline stoichiometric Ti_0.9Al_0.1N powder has been prepared by ball-milling the α-Ti (hcp) and aluminum (fcc) powders under N₂ at room temperature. Initially, α-Ti phase partially transformed to the transient cubic β-Ti phase and Ti_0.9Al_0.1N (fcc) phase is noticed to form after 3 h of milling. Nanocrystalline stoichiometric Ti_0.9Al_0.1N phase is formed after 7 h of milling. After 1 h of milling, all Al atoms are diffused into the α-Ti matrix. The transient β-Ti phase is noticed to form after 1 h of milling and disappears completely after 7 h of milling. Microstructure characterization of unmilled and ball-milled powders by analyzing XRD patterns employing the Rietveld structure refinement reveals the inclusion of Al and nitrogen atoms into the Ti lattice on the way to formation of Ti_0.9Al_0.1N phase. Microstructure of ball-milled samples is also characterized by HRTEM. The particle size of Ti_0.9Al_0.1N phase, as obtained from XRD method, is ∼5 nm which is very close to that obtained from HRTEM.     

Nanoindentation study of amorphous-Co79Zr13Nb8/Cr multilayers

The mechanical properties of Co₇₉Zr₁₃Nb₈/Cr multilayers were investigated using nanoindentation. The hardness is higher than the average value calculated by rule-of-mixture. The hardness and the resistance to plastic deformation characterized by the ratio of H³/E² vary similarly with periodicity (Λ). They all arrive to the maximum at Λ = 8 nm and decrease subsequently when the Λ increases. The hardness dependence on the Λ is fitted by Hall-Petch relation. The fitted index n is much lower than the normal value (~ 0.5) in many crystalline multilayers. The mutual restriction of shear band and dislocation in amorphous/crystalline structure, which is named structure barrier strengthening, should be main mechanism for the hardness enhancement. The SEM study of indents shows that the shear bands are distorted significantly at the smaller Λ (4 nm) and disappear at the larger Λ (> 20 nm). This morphology variation implies a potential improvement of plasticity caused by the restriction effect of the Cr crystalline layers on the shear bands propagation.     

Characterization of nanostructured Cr2Nx/Cu multilayers deposited by reactive DC magnetron sputtering

Nitride/metal nanostructured multilayers of Cr₂N_x/Cu were deposited by reactive DC magnetron sputtering with various bilayer periods (2.5-30 nm) and substrate temperatures (25-400 °C). All films had a total thickness of about 470 nm and the overall chemical composition of the chromium nitride layers was close to Cr₂N_0.8. The deposited films were characterized by Rutherford Backscattering (RBS), low-angle X-ray reflectivity (XRR), high-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hardness and elastic modulus were measured by nanoindentation. The films deposited at 25 °C had a well-defined multilayer structure and the chromium nitride layers were found to crystallize into N-deficient fcc CrN_0.4 with traces of hexagonal Cr₂N_0.8. The layers were strongly textured with fcc CrN_0.4[002] and Cu[002] oriented along the growth direction — the fcc CrN_0.4 and Cu grains growing with a cube-on-cube relationship. The measured hardness values were about 8 GPa, and showed no dependence on the bilayer period. Higher deposition temperatures caused the multilayer structure to degrade, and at 400 °C the films were better described as non-textured nanocomposites with the chromium nitride crystallized entirely into the equilibrium hexagonal Cr₂N_0.8 structure. Hardness values of the high-temperature films in the range of 4-8 GPa were measured. Multilayer films deposited at 25 °C were found to be thermally stable against post-deposition annealing at temperatures up to about 400 °C. Annealing at 500 °C caused severe structural changes — the fcc CrN_0.4 phase transformed into hexagonal Cr₂N_0.8 accompanied by degradation of the periodic multilayer structure. The hardness decreased from the originally 8 GPa to about 5 GPa upon annealing.     

Effect of carbon content on the microstructure and properties of (Ti0.7W0.3)C-Ni cermet

(Ti_0.7W_0.3)C solid solution powder was synthesized by high-energy ball milling. We investigated the effect of excess carbon in this system on the microstructure, pore level, and mechanical properties of (Ti_0.7W_0.3)C?-20 wt.% Ni cermet. We also report the variations in the carbon stoichiometry of the (Ti_0.7W_0.3)C_x phase in the powder and in the (Ti_0.7W_0.3)C_x?-20 wt.% Ni cermet after carbothermal reduction and liquid phase sintering, respectively. The particle size of the solid-solution carbide decreased with increasing carbon content in the (Ti_0.7W_0.3)C-?20 wt.% Ni cermets. This occurred because the dissolution of the solid solution (Ti,W)C is hindered by the high activity of carbon. However, an increase in the carbon content generated pores and carbon segregation, resulting in poor mechanical properties, as also observed in other carbide cermets.     

多弧离子镀靶电流对膜层形貌与性能的影响

采用渗氮与多弧离子镀复合技术,在不同靶电流条件下制备Ti0.33Al0.67N膜层,分析靶电流对膜层形貌、膜层厚度、结合力及显微硬度等性能的影响.得出靶电流设计范围内存在一个最佳值,以获得膜层最优性能.     

Lateral gradients of phases,residual stress and hardness in a laser heated Ti0.52Al0.48N coating on hard metal

The influence of a local thermal treatment on the properties of Ti–Al–N coatings is not understood. In the present work, a Ti_0.52Al_0.48N coating on a WC–Co substrate was heated with a diode laser up to 900 °C for 30 s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti–Al–N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47 GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about − 200 to 500 MPa down to a depth of 6 μm is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti_0.52Al_0.48N coating on a WC–Co substrate was performed in the range of 25–1003 °C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure–property relationships in complex materials.     

Microstructure and mechanical properties of polycrystalline-Ag/amorphous-CoZrNb multilayers

Ag/CoZrNb multilayers were prepared by direct current magnetron sputtering. Their microstructure, hardness, elastic modulus and plastic deformation were investigated by X-ray diffraction, high-resolution transmission electron microscopy, X-ray absorption near-edge structure, field emission scanning electron microscopy and nanoindentation technique. The results show that the multilayers have well modulated structure. For all the multilayers, Ag layer is polycrystalline structure, while CoZrNb layer is amorphous structure. With the decrease of modulation periodicity the elastic modulus decreases due to the compliant interface. The polycrystalline Ag layer not only makes the hardness enhanced with the decrease of modulation periodicity, but also restricts the propagation of shear bands and promotes the nucleation of new shear bands, which may be of help for the plasticity enhancement of amorphous CoZrNb layer.     

Synthesis and characterization of 0.3 Vf TiC–Ti3SiC2 and 0.3 Vf SiC–Ti3SiC2 composites

In this work, two composite compositions—one with 30% (v/v) SiC, the other with 30% (v/v) TiC, balance Ti₃SiC₂—were synthesized and characterized. Fully dense samples were fabricated by hot isostatically pressing Ti, SiC and C powders for 8 h at 1500 or 1600 °C and a pressure of 200 MPa. Both TiC and SiC lower grain boundary mobility in Ti₃SiC₂. Coarsening of the SiC particles was also observed. At comparable grain sizes, all composites tested were weaker in flexure than the unreinforced Ti₃SiC₂ matrix, with the reduction in strength being the worst for the SiC composites. This reduction in strength is most probably due to thermal expansion mismatches between the matrix and reinforcement phases. The composite samples were exceptionally damage tolerant; in one case a 100 N Vickers indentation (in a 1.5-mm thick bar) did not reduce the flexural strength as compared to an unindented or as-fabricated samples. The same is true for thermal shock resistance; quenching samples from 1400 °C in room temperature water, resulted in strength reductions that were 12% at best and 50% at worst. In the 25–1000 °C temperature range, the thermal expansion coefficients of the two composites were indistinguishable at 8.2×10⁻⁶ K⁻¹. The Vickers hardness values depended on load; at 100 N, the hardnesses were ≈15 GPa; at 300 N, they asymptote to 7–8 GPa. For the most part, very few cracks emanate from the corners of the Vickers indents even at loads as high as 500 N. In the few cases where cracks did initiate, fracture toughness values were crudely estimated to lie in the 5–7.5 MPa √m range.     

Microstructure and fracture toughness of Ti(C0.7N0.3)-WC-Ni cermets

Mo₂C is normally added to improve the wettability between Ti(C,N) and Ni in Ti(C,N)-based cermets. Due to the increasing price of Mo₂C, much attention has been paid to Ti(C,N)-based cermets with WC addition. In this paper, effect of WC content on the microstructure and properties of Ti(C_0.7N_0.3)-xWC-15wt.%Ni cermets free of Mo₂C was studied. The experimental results show that the microstructure is refined obviously with the increase of WC content. The fracture toughness decreases with the increase of WC content when WC content is 10-25 wt.%, and increases when WC content varies from 25 wt.% to 30 wt.%.     

Effect of various carbides on the dissolution behavior of Ti(C0.7N0.3) in a Ti(C0.7N0.3)–30Ni system

HfC, TaC, or WC were individually added to a Ti(C_0.7N_0.3)–30 wt% Ni system, in order to investigate microstructural changes and the dissolution behaviors of Ti(C_0.7N_0.3) and the carbides. Of these systems, the Ti(C_0.7N_0.3)–WC–Ni system proved to be the most favorable for the refinement of microstructure. The fraction for the cross-sectional area of the Ti(C_0.7N_0.3) cores, which is related to the amount of Ti(C_0.7N_0.3) dissolved, increases and the thickness of solid solution rims decreases in the order of HfC, TaC, and WC added. This finding indicates that the dissolution rate of Ti(C_0.7N_0.3) in a Ni melt is the lowest when WC is added. In addition, it was found that the average dissolution rate of Ti(C_0.7N_0.3) in the HfC-containing system is 1.6 and 1.9 times higher than those for Ti(C_0.7N_0.3) in the TaC- and WC-containing systems at the same sintering conditions. Further, the dissolution rates of TaC and WC were 80% of the rate found for HfC during the formation of the outer rim.     

A-site-disorder-dependent magnetocaloric properties in the mono-valent-metal doped La0.7Ca0.3MnO3 manganites

The influence of mono-valence-metal (Li, Na, and K) doping effect on the structural, resistivity, magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ polycrystalline samples is studied for a fixed (5% at Ca site) dopant concentration. All the samples crystallize in orthorhombic structure and the lattice parameters increase continuously as the dopant atoms changes from Li to Na and then K. Paramagnetic-ferromagnetic phase transition at T_C and insulator-metal phase transition at T_p are observed for all studied samples. The transition temperature decreases as Ca atoms is replaced by Li, while the transition temperature shifts to higher values as Ca is substituted by Na or K. In addition, the maximum magnetic entropy change of the K-doped sample is much smaller than that of the free- and Na-doped samples. The results are discussed according to the change of A-site-disorder effect caused by the systematic variations of A-site average ionic radius 〈r_A〉 and A-site-cation mismatch σ².     

Micro-structure, nano-property and nano-tribological behaviour of the permalloy/copper multilayers

The micro-structure, nano-mechanical behaviour of permalloy/copper magnetic multilayers with various permalloy layer thickness were investigated using scanning probe microscope, X-ray diffraction and nano-mechanical measuring system. Experimental results show that both copper and permalloy layers of these multilayers grow as a meta-stable phase. The multilayers are a good combination of toughness and hardness in the nano-mechanical properties compared with permalloy and copper single layer films. The permalloy layer thickness has a great effect on the micro-tribological behaviour of the permalloy/copper multilayers. When the permalloy layer thickness is reduced to ≤ 1.6 nm, the multilayers exhibit a large critical load, a small scratch width and a low residual scratch depth. When the permalloy layer thickness is reduced from 2.8 nm to 1.2 nm, the surface topography of the multilayers becomes smooth and the particle size decreases from about 120 nm to 25 nm, which will be beneficial to the applications that require a smooth surface.     

Amorphous-CuTa/amorphous-CoZrNb multilayers: Structure, mechanical properties and thermal stability

CuTa/CoZrNb multilayers were prepared by direct current magnetron sputtering. The structure, mechanical properties and thermal stability were investigated by X-ray diffraction, high-resolution transmission electron microscopy, vacuum annealing and nanoindentation. The results show that the as-deposited multilayers possess amorphous/amorphous structure. The amorphous feature leads to the lack of hardness enhancement with decreasing modulation periodicity, which can be attributed to the absence of the dislocation movement. After thermal annealing, the density increases due to the decrease of the bond distance and the annihilation of defects with increasing annealing temperature. The densification results in the enhancement of the elastic modulus and hardness. The enhancement for the multilayers with smaller modulation periodicity is more sensitive to thermal annealing because of larger volume fraction of interfaces.     

Preparation of ultra-fine TiC0.7N0.3-based cermet

Since ultra-fine Ti(C, N) has large surface and high activity, preparation of high performance cermets using ultra-fine Ti(C, N) powders is very difficult at the present. In the paper, deoxidation process of ultra-fine TiC_0.7N_0.3 powder is carried out firstly, and the oxygen content of ultra-fine TiC_0.7N_0.3 powder can be decreased from more than 1 wt% to 0.06 wt%; milling technology of ultra-fine TiC_0.7N_0.3-based cermet is studied in the paper, the results show that the optimum milling time is 45 h and the ball to powder weight ratio is 15:1, and the dispersant helps to achieve a homogeneous distribution of the ultra-fine powder; during vacuum sintering of ultra-fine cermet, pores tend to form, hence NT6B shows relatively lower properties than NT6A. After HIP process (1350 °C, 90 min, 70 MPa), the porosity can be largely decreased. The prepared ultra-fine cermet has typical core–rim microstructure, finer grain size and enhanced properties.     

La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-xFex合金的相结构和电化学储氢性能（英文）

采用感应熔炼和热处理的方法制备La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75?xFex(x=0~0.20)合金,并研究合金的相结构和电化学储氢性能。全部合金均为单一的具有CaCu5结构的LaNi5相,LaNi5相的晶格常数a和晶胞体积随着x值的增加而增大。最大放电容量随着x值的增加从319.0mA·h/g(x=0)降低到291.9mA·h/g(x=0.20)。在1200mA/g的电流密度下HRD值从53.1%(x=0)降低到44.2%(x=0.20)。合金电极的循环稳定性随着x值的增加而增强,这主要归因于合金抗粉化能力的增强。     

Mechanical alloying and spark plasma sintering of the intermetallic compound Ti50Al50

This study investigates the microstructures and mechanical properties of Ti₅₀Al₅₀ alloys prepared via mechanical alloying (MA) starting from elemental powders. The process of the spark plasma sintering (SPS) has also been studied. It is found that the nanocrystallization process of the Ti–Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100–1200 °C with a compaction pressure of 30 MPa and a heating rate of 30 °C min⁻¹. Specimens with high densities and approaching the equilibrium state can be obtained in short time by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth.     

High tunability of pulsed laser deposited Ba0.7Sr0.3TiO3 thin films on perovskite oxide electrode

Ferroelectric thin films such as BST, PZT and PLZT are extensively being studied for the fabrication of DRAMS since they have high dielectric constant. The large and reversible remnant polarization of these materials makes it attractive for nonvolatile ferroelectric RAM application. In this paper we report the characterization of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown by pulsed laser ablation on oxide electrodes. The structural and electrical properties of the fabricated devices were studied. Growth of crystalline BST films was observed on La_0.5Sr_0.5CoO₃ (LSCO) thin film electrodes at relatively low substrate temperature compared to BST grown on PtSi substrates. Electrical characterization was carried out by fabricating PtSi/LSCO/BST/LSCO heterostructures. The leakage current of the heterostructure is studied and a band structure is modeled based on the transport properties of the heterostructure. The dielectric constant of the BST film is found to be 630 at 100 kHz with a loss tangent of 0.04. The capacitance voltage characteristics show high tunability for BST thin films.     

The mechanical properties of single phase γ Ti47Al51Mn2 polycrystals

The mechanical properties of polycrystalline samples of the single phase γ-Ti₄₇Al₅₁Mn₂ alloy have been studied during compression tests in a wide range of temperatures (120–1270 K). The flow stress and the work hardening rate are measured during imposed strain rate tests, while the strain rate sensitivity of the stress is examined using both strain rate jumps and stress relaxation experiments. From the temperature, strain and stress dependence of these parameters, it is shown that the investigated temperature domain can be divided into three régimes corresponding to different deformation mechanisms. The results are compared to the data available in the literature and are found to be in good agreement with the dislocation structures and dislocation motion mechanisms that we have previously reported.     

On the synthesis and characterization of a new manganite type CMR material: La0.7Hg0.3MnO3

A new manganite type CMR material, La_0.7Hg_0.3MnO₃ has been successfully synthesized and has been found to exhibit magnetoresistance (≈9%) at low fields (≈1.5 kG). The synthesis has been carried out through a solid state reaction route consisting of the formation of La_0.7MnO₃ followed by diffusion of Hg leading to La_0.7Hg_0.3MnO₃. The as grown samples are polycrystalline and correspond to an orthorhombic unit cell with the lattice parameters; a=5.5183 Å, b=5.6383 Å and c=7.5368 Å. The typical grain size as revealed by scanning electron microscopy is in the range of 0.5–2 μm. The ρ–T behaviour shows a peak at T_IM=227 K. The ρ–T behaviour above this temperature corresponds to that of an insulator and below this to that of a metal. The ρ–T behaviour remains unaltered when a magnetic field (H_dc=1.5 kG) is applied. However, with this magnetic field a drop in the resistivity is observed up to 77 K. At room temperature the magnetoresistance ratio (MRR) is too small but it steadily increases as the temperature is decreased. Thus, MRRs at 227.13 and 77 K are 3.41 and 9.05%, respectively, in an applied field of H_dc=1.5 kG. At a given temperature the variation in MRR with field H_dc is rapid at lower field values (H_dc<1.2 kG) and scales linearly for higher field values (H_dc>1.2 kG). It may be mentioned that the present work on the synthesis and magnetoresistance behaviour of La_0.7Hg_0.3MnO₃, is the first of its type.