Mechanical and wear properties of Mo5Si3–Mo3Si–Al2O3 composites

Mo₅Si₃ and Mo₅Si₃–Mo₃Si–Al₂O₃ composite were synthesized use MoO₃, Mo, Si and Al as raw materials by mechanically induced self propagating reaction and then consolidated by hot-pressing. The microstructure of the materials was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS). The effects of the Al₂O₃ on the mechanical and tribological properties of Mo₅Si₃–Mo₃Si–Al₂O₃ composite have been studied. It was found that benefits associated with the addition of the Al₂O₃ to Mo₅Si₃ and Mo₃Si include finer microstructure, higher strength, higher fracture toughness and higher hardness. The dry sliding wear properties of the composite were investigated using against GCr15 bearing steel in ball-on-disk system at room temperature. The results indicated that the friction coefficients and specific wear rates of Mo₅Si₃–Mo₃Si–Al₂O₃ composite were significantly reduced by the addition of Al₂O₃, and its specific wear rates decreased by an order of magnitude compare with the monophase Mo₅Si₃. The friction coefficients of test materials decrease with an increasing load. The dominant wear mechanism of the composites was interpreted by several different wear models involving plastic deformation, adhesion, brittle fracture and reaction to form a tribo-oxidation layer.     

Investigation of the effect of Al2O3–Y2O3–CaO (AYC) additives on sinterability,microstructure and mechanical properties of SiC matrix composites: A review

Appropriate properties of SiC ceramic such as high hardness, low density, high melting point and high elastic modulus make this material as a favorite candidate for different industrial applications. Although some disadvantages including high sintering temperature, low sinterability, and low fracture toughness have restricted the use of this material, previous studies showed that using Al₂O₃-Y₂O₃ additives plays an effective role in the improvement of sinterability as well as the enhancement of the properties of these composites. Moreover, the addition of CaO results in the acceleration of the formation of molten phase and the improvement of sinterability. In addition, the use of these additives cause the formation of the intermetallic phases of Al₅Y₃O₁₂ (YAG) and CaY₂O₄ and by activating the mechanisms of crack deflection, crack bridging, phase transformation, strengthening the grain boundary and changing the fracture mode from intergranular to transgranular results in improved mechanical properties. This paper attempts to investigate the effect of using Al₂O₃–Y₂O₃–CaO (AYC) additives on sinterability, microstructure, and mechanical properties of SiC matrix composites including the composites reinforced with SiC fibers and SiC matrix nano-composites. Finally, the effect of the post-sintering annealing process under two conditions i.e., with and without applying pressure (pressureless sintering) on microstructure and mechanical properties has been studied.     

Mechanical properties and chemical bonding of the Os–B system: A first-principles study

The mechanical properties of Os–B compounds containing different boron contents have been investigated systemically by first-principles calculations. Two previously unreported crystal structures of Os₂B₅ and OsB₃, crystallizing in space groups R3m and P-6m2 respectively, are determined using the ab initio evolutionary structure prediction. The calculated elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and hardness for Os–B compounds are in good agreement with the available experimental values. Our results show that the hardness of osmium borides increases with increasing boron content. Os₂B₅ and OsB₃, with hardnesses of 34.4 and 36.9 GPa respectively, can almost be considered as potential superhard materials. Further analyses on density of states, crystal orbital Hamilton population, and electron localization function demonstrate that the electronic structure of Os–B compounds is directly responsible for their particular mechanical properties. High hardness in Os₂B₅ and OsB₃ is mainly attributed to the occurrence of strong B–B covalent bonds and the disappearance of some ductile Os–Os metallic bonds.     

La2O3-Gd2O3-Mo secondary emission material

A new kind of materials La2O3-Gd2O3-Mo has been produced by powder metallurgy method.The composition and microstructure of the material were studied by XRD and SEM.It shows that no chemical reaction takes place among La2O3,Gd2O3,Mo and the rare earth oxides exist along molybdenum grain boundaries and in the pores.The emission property measurement results of this material show that adding rare earth oxide into molybdenum can improve the secondary emission coefficient of the emitter,and the emission property depends on the activation temperature.After La2O3-Gd2O3-Mo was activated at 1360℃,the maximum secondary emission coefficient can be high to 2.62,which has exceeded that for practical uses(2.0).     

Mo—La2O3阴极La2O3表面富集研究

采用俄歇电子能谱法研究了 Mo- L a2 O3阴极材料中 L a2 O3向表面的富集过程。结果表明 ,在高温下 ,L a2 O3以 L a3+ ,O2 - 离子的形式分别向表面扩散 ,然后在表面上重新结合成分子。在 112 3 K～ 142 3 K范围内 ,L a3+ ,O2 -离子的扩散系数分别为 :DL a=3.6 70 3× 10 - 1 6 exp (- 1.0 16 39× 10 5 / RT) m2 / s;DO=1.5 12 2× 10 - 1 6 exp (- 8.130 6 6× 10 4/ RT) m2 / s     

Effects of duty ratio on properties of micro-arc film on Ti–3Zr–2Sn–3Mo–25Nb

Porous titanium oxide layers, which are important features for improving the biological activity of Ti implants with bone tissues, have been obtained through the technique of micro-arc oxidation (MAO). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to evaluate the micromorphology and crystalline structure of these oxide films, and the chemical compositions were measured by electron dispersive X-ray spectroscopy (EDS). TiO₂ layers presented the crystalline phases of rutile and anatase. During the micro-arc oxidation treatment, Ca and P ions were incorporated into the oxide layer, and incorporation of Ca and P with the Ca/P content (%) of around 1.38 is similar to that in the human body. Nb₂O₅ was also identified in the treatment samples. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves. In the electrochemical corrosion tests, the treated samples presented lower values of corrosion current density than untreated Ti, indicating a better corrosion resistance. Diffusion phenomena were present in the process of corrosion.     

Annealing effect on the soft magnetic properties of high moment FeCo–O thin films

We have investigated the annealing effect on the soft magnetic properties of high magnetic moment (Fe_0.65Co_0.35)_100−xO_x (0<x<5 at.%) thin films. The coercivity of the as-deposited (Fe_0.65Co_0.35)₉₉O₁ film decreased from 120 to 5 Oe after annealing at 480 °C. The reason for this vast magnetic softening is discussed based on the microstructural observation results.     

In situ synthesis and electrical properties of CuW–La2O3 composites

CuW–La₂O₃ composites were fabricated using an in-situ synthesis. The breakdown voltage in vacuum, moving trajectory of cathode spots, electrical conductivity, and hardness of CuW–La₂O₃ composites were carefully examined. The microstructures were characterized by a scanning electron microscope (SEM). The results show that CuW–La₂O₃ composites have the maximum hardness of 220HB and the electrical conductivity of 45% IACS when the content of La₂O₃ is 0.75 wt.%. In comparison with CuW alloy, the dielectric strength, arc life and the arc mobility of the CuW–La₂O₃ composites increased by 36.9%, 9.7% and 46.6%, respectively. As a result, the addition of La₂O₃ is useful to improve the properties of CuW alloys and the in situ synthesized CuW–La₂O₃ composites should have excellent arc erosion resistance.     

Thermodynamic investigations in the solid state of the lanthanum–magnesium–zinc system

The La–Mg–Zn phase diagram is experimentally investigated at 595 K, x_La > 4% and the corresponding isothermal section is partially determined. This section includes 5 substitutional solid solutions based on the binary compounds (LaMg, LaZn, LaMg₃, LaMg_10.3 and La₂Zn₁₇) and three ternary phase (La₈(Mg,Zn)₉₂, La₃(Mg,Zn)₁₁ and La_4.27Mg_2.89Zn₃₀). The enthalpies of mixing in the ternary solid solutions are calculated at 298 K on the basis of tin solution calorimetry experiments.     

Investigation of the effect of aluminum on the phase composition of Ti–Al–Nb–Mo gamma alloys

A quantitative analysis of the influence of aluminum concentration on the phase composition of TNM-type Ti–Al–Nb–Mo γ-alloys has been carried out using the Thermo-Calc software and experimental methods. Isothermal and polythermal sections of the corresponding phase diagram have been calculated; the critical temperatures of phase transformations in the alloys of the system, and the chemical compositions of phases formed in them (β, α, α₂, γ) have been determined. The influence of the annealing temperature on the microstructure and phase composition of the alloys containing 43 and 40% Al has been studied.     

Effect of Mo2C additions on the properties of SPS manufactured WC–TiC–Ni cemented carbides

The effect of spark plasma sintering (SPS) on the microstructure and mechanical properties of WC–Co and WC–Ni cemented carbides was studied, and compared to WC–Co produced by liquid phase sintering (LPS). There were finer WC grains with larger Co pools in the spark plasma sintered WC–Co, resulting in higher hardness and slightly lower fracture toughness than the liquid phase sintered WC–Co. The influence of the addition of 0.5–5 wt.%Mo₂C to WC-based cemented carbide containing 6.25 wt.%TiC and 9.3 wt.%Ni prepared by SPS was also studied. This addition improved the wettability between WC and Ni and lead to the improvements of microstructures, resulting in good combinations of hardness, fracture toughness and modulus of elasticity that were comparable to WC–Co based cemented carbides.     

Reaction mechanism of lanthanum nitrate-doped Mo–La material during solid–liquid spray doping processing

At three critical temperatures which were obtained by thermo-gravimetry-differential thermal analysis–differential scanning calorimetry(TG/DTG/DSC)curves of lanthanum nitrate crystal, the air thermal decomposition experiments and solid–liquid spray doping simulation procedures of lanthanum nitrate crystal were carried out, and their products were analyzed by X-ray diffraction(XRD). Meanwhile, the spray doping processing of Mo O_2–50 wt% La(NO_3)_3composite powder was undergone with lanthanum nitrate solution as the dopant,and doped Mo O_2 powder was analyzed by XRD. The results demonstrate that during the traditional solid–liquid spray doping processing, lanthanum nitrate, in the form of either crystal or aqueous solution, would be converted into La(NO_3)_3á_4H_2O by the dehydration reaction, rather than be decomposed to La_2O_3 and NO or NO_2. Therefore, it is inferred that the oxynitride gas produced from the process is attributed to the decomposition of residual HNO_3 in lanthanum nitrate crystal. The source of HNO_3 is supported by the chemical composition of lanthanum nitrate crystal.     

Mechanical properties of MgO–Al2O3–SiO2 glass-infiltrated Al2O3–ZrO2 composite

This work attempts to improve the mechanical properties of alumina-10 wt% zirconia (3 mol% yttria stabilized) composite by infiltrating a glass (magnesium aluminum silicate glass) of lower thermal expansion on the surface at high temperature. The glass improved the strength of the composite at room temperature as well as at higher temperatures. There was a significant improvement in the Weibull modulus after the glass infiltration. Glass-infiltrated samples showed better thermal shock resistance. The magnitude of strength increment was found to be in the order of the surface residual stress generated by thermo-elastic properties mismatch between the composite and the infiltrated glass.     

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo₃Si–Mo₅SiB₂ alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo₃Si (A15) and Mo₅SiB₂ (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.     

A study of the evolution of rust on Mo–Cu-bearing fire-resistant steel submitted to simulated atmospheric corrosion

The corrosion evolution of a Mo–Cu-bearing fire-resistant steel in a simulated industrial atmosphere was investigated by corrosion weight gain, XRD, EPMA, XPS, and polarization curves. The results indicate that the corrosion kinetics is closely related to the rust composition and electrochemical properties. As the corrosion proceeds, the relative content of γ-FeOOH and Fe₃O₄ decreases and α-FeOOH increases, and the rust layer becomes compact and adherent to steel substrate. Molybdenum and copper enrich in the inner rust layer, especially at the bottom of the corrosion nest, forming non-soluble molybdate and Cu(I)-bearing compounds responsible for enhanced corrosion resistance of the rust layer.     

Synthesis of ZrO2-SiC composite powder and effect of its addition on properties of Al2O3-C refractories

ZrO2-SiC composite powder was synthesized by carbothermal reduction of zircon in argon atmosphere, and it was used as the additive to prepare Al2O3-C refractories. The effects of heating temperature on the synthesis process and the addition of the synthesized composite powder on the properties of the Al2O3-C refractories were investigated. The results show that the synthesized composite powder can be easily obtained by heating the mixture of zircon and carbon black at 1 873 K for 4 h in argon atmosphere, and the relative contents of ZrO2 and SiC in sample reach about 83.7% and 16.3%, respectively. The bulk density, crushing strength and thermal shock resistance of the Al2O3-C refractories can be improved obviously by the addition of the synthesized ZrO2-SiC composite powder.     

Effect of TiO2 on thermal,structural and third-order nonlinear optical properties of Ca–La–B–O glass system

A series of calcium lanthanum metaborate glasses in the composition (wt%) of 23.88CaO–28.33La₂O₃–47.79B₂O₃ modified with TiO₂ up to 20 wt% are prepared by a melt quenching technique to study the influence of TiO₂ on their thermal, structural, linear and nonlinear optical properties. The differential thermal analysis (DTA) studies have demonstrated significant effects due to the presence of TiO₂ on the glass forming ability and crystallization situations. The glass with 15 wt% TiO₂ has achieved a eutectic composition and also exhibited a better glass forming ability among the glasses studied. The FT-IR spectra of these glasses show mainly vibration modes corresponding to stretching of BO₃ trigonal, BO₄ tetrahedral units and of B–O–B bending bonds. At higher concentrations of TiO₂, development of vibration band around 400 cm^?1 has indicated the formation of TiO₆ structural units in the glass network. The red shift of optical absorption edge (UV cutoff) shows a monotonous decrease in direct and indirect optical band gap energies (E_opt) with an increase of TiO₂ content in the glasses based on their absorption spectra. The optical transparency of these glasses is found to be varied from 64 to 87% within the wavelength range 450–1100 nm depending on the TiO₂ content. Besides these studies, linear refractive indices, the nonlinear optical properties of these glasses have also been evaluated.     

New understanding of the effect of hydrostatic pressure on the corrosion of Ni–Cr–Mo–V high strength steel

Corrosion of Ni–Cr–Mo–V high strength steel at different hydrostatic pressures is investigated by scanning electron microscopy (SEM) and finite element analysis (FEA). The results indicate that corrosion pits of Ni–Cr–Mo–V high strength steel originate from inclusions in the steel and high hydrostatic pressures accelerate pit growth rate parallel to steel and the coalescence rate of neighbouring pits, which lead to the fast formation of uniform corrosion. Corrosion of Ni–Cr–Mo–V high strength steel under high hydrostatic pressure is the interaction result between electrochemical corrosion and elastic stress.