Molybdenum Particles Research Articles

Reflection of a shock wave from a rigid wall in a mixture of a liquid metal and solid particles

The problem of reflection of shock waves and expansion waves from a rigid wall in a mixture of liquid iron and small molybdenum crystals is considered. The study is performed within the framework of equilibrium and nonequilibrium approximations of mechanics of heterogeneous media with different pressures of the components. The mathematical model is verified by fitting the dependence of the equilibrium-frozen velocity of sound on the initial pressure in the mixture with available experimental data. In the equilibrium approach, the dependence of the type of the reflected wave on the type of the incident wave is determined. Direct numerical calculations within the framework of the nonequilibrium model of mechanics of heterogeneous media confirmed the validity of the reflection chart obtained. The possibility of significant compacting of molybdenum particles in reflected waves is revealed.

Influence of residual and bridging stresses on the R-curve behavior of Mo- and FeAl-toughened alumina

Alumina matrix was toughened using either metal molybdenum or intermetallic FeAl particles. Mo and FeAl dispersoids were chosen because they have different thermomechanical properties (i.e. Young's modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual stresses in the matrix. The R-curve behavior of these composites was first studied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different in the two composites: 25 and 15 vol% addition led to maximum toughness in the Mo- and FeAl added composite, respectively. This difference was ascribed to residual stresses. Microscopic observation of the crack path revealed, in both composites, the systematic presence of dispersoids acting as bridging sites in the crack wake, but only a few of them were plastically stretched. Residual stresses in the Al 2O 3 matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed using microprobe fluorescence spectroscopy. Bridging microstresses were assessed in situ by a linear map along the crack profile, at the critical condition for fracture propagation. Experimentally collected residual stresses and bridging stresses were discussed to explain the different fracture behavior of the composites.

Tailoring the Microstructure of a Metal-Reinforced Ceramic Matrix Composite

Alumina matrix was prepared with varying amounts of metal molybdenum particles of two different grain sizes. R-curves were determined during stable crack propagation and a piezo-spectroscopic technique was used to assess the bridging stresses developed along the crack wake (at the metal/matrix interface) at the critical condition for crack propagation. In both kinds of composites, the toughness monotonically increases with the amount of molybdenum added. In addition, bridging stress and therefore toughness were higher when coarser particles were present. The theoretical R-curves calculated from the discrete (microscopic) bridging stress distribution obtained by in situ Raman spectroscopy were in good agreement with the experimental data. [S0094-4289(00)02003-X]

Synergistic toughening of a hybrid NiAl composite reinforced with partially stabilized zirconia and molybdenum particles

Abstract This paper presents preliminary experimental evidence of synergistic interactions between transformation toughening and crack bridging in an NiAl composite reinforced with 2 mol% yttria partially stabilized zirconia and molybdenum particles. The observed levels of synergy are shown to be consistent with predictions from a theoretical model by Amazigo and Budiansky (J.C. Amazigo, B. Budiansky, J. Mech. Phys. Solids 36 (1988) 581–595.). The implications of the results are also discussed for the potential development of synergistically toughened intermetallic and ceramic composites.

Synchrotron x-ray Study of Elastic Phase Strains in the Bulk of an Externally Loaded Cu/Mo Composite

ABSTRACTHigh-energy, high-flux x-rays from a third-generation synchrotron source were used to measure average elastic strains in the bulk of 1.5 mm thick composites consisting of a copper matrix reinforced with 7.5 vol.% molybdenum particles. From the evolution of lattice strains in both phases during uniaxial tensile deformation, the internal load transfer between phases and reinforcement damage were characterized during elastic and plastic deformation of the composite. The graininess of the diffraction rings, which is related to the Bragg peak broadening, was quantified as a function of applied stress and related to plastic deformation in the matrix.

Analysis of the Nonthermal Emission Signal Present in a Molybdenum Particle-Laden Plasma-Spray Plume

In-flight measurement of the surface temperature of plasma-sprayed particles is important for the correlation of particle characteristics to coating structure and properties. However, the use of optical pyrometry for particle surface temperature measurement has inherent uncertainties due to nonthermal emission signals in the plasma/particle plume. This nonthermal signal is especially bothersome near the torch exit and in regions of the plume where there are few particles. This work presents measurements of the nonthermal signals present when making temperature measurements of plasma-sprayed molybdenum particles. Changes in the nonthermal emission signals were found to be caused by particle vapor, the spectral plasma loading effect, and particle reflection of plasma light. Care must be taken to avoid particle temperature errors due to these effects.

Particle Temperature and Flux Measurement Utilizing a Nonthermal Signal Correction Process

In-flight measurement of the surface temperature of plasma-sprayed particles is important for the correlation of particle characteristics to coating structure and properties. However, the use of optical pyrometry for particle surface temperature measurement has inherent uncertainties due to nonthermal emission signals in the plasma/particle plume. This nonthermal signal is especially pronounced near the torch exit and in regions of the plume where there are few particles. This work shows that both plasma and particle vapor signals in the form of line emission and continuum emission can be compensated for when calculating particle temperatures from the emission of the plasma/particle plume. The nonthermal signals have been estimated and removed from the raw spectral data, producing a more accurate calculated particle temperature. Using spectral shape fitting to determine the particle temperatures allows the radiant intensity to be used for particle flux estimation, thus providing more information on the state of the particle plume. Additionally, the spectral emittance of molybdenum particles sprayed in air was found to more closely resemble the emittance of pure molybdenum than the emittance of MoO3.

Fracture energy and R-curve behavior of Al 2O 3/Mo composites

The mechanical properties of a polycrystalline Al 2O 3 containing increasing fractions of molybdenum particles are examined with particular emphasis placed on the fracture behavior. From the experimental side, both the fracture energy and the rising R-curve behaviour of the composites are measured as a function of the volume fraction of the metal dispersoid. A 20 vol.% fraction of metal dispersoid was found to markedly enhance the fracture energy of the Al 2O 3 matrix as well as to make more pronounced its rising R-curve behavior. Microscopic observation of the crack path and fracture surface revealed the presence of stretched metal particles acting as ductile bridging sites behind the crack tip. Assuming crack bridging as the main mechanism contributing to toughening, a Dugdale-like theoretical model which considers the crack faces being shielded by a constant (average) closure stress field is invoked to explain the fracture behavior of the composites.

FRACTURE CHARACTERISTICS OF BOROSILICATE GLASSES REINFORCED BY METALLIC PARTICLES

Abstract— The fracture behaviour of borosilicate glass reinforced by molybdenum and/or vanadium particles has been investigated. For the addition of 5 vol% molybdenum particles, two processing procedures have been tested and the influence of volume fraction of vanadium particles (in the range 2 to 30 vol%) on fracture resistance has been assessed. The use of chevron‐notched specimens in three‐point bending has been shown to be a reliable method for the evaluation of fracture toughness even at toughness levels of order 0.7 to 1.3 MPa √m. The existence of subtle differences in fracture behaviour of glass‐composites having comparable volume fractions of molybdenum particles but obtained by two different processing procedures has been established by statistical treatment of the fracture toughness data. An increase in the volume fraction of metallic particles results in an increase of the fracture resistance and the measured fracture toughness level. Toughening mechanisms which have been identified include both the plastic deformation of particles and the bridging of cracks by ductile particles. Some particle cleavage and debonding has been observed, which indicates that a decrease in particle plasticity, probably induced by processing or due to constraints imposed by the rigid matrix, is responsible for a smaller than expected enhancement of the fracture toughness.

A comparative study of the coated filler method and the admixture method of powder metallurgy for making metal–matrix composites

Copper–matrix composites were made by powder metallurgy (PM). The reinforcements were molybdenum particles, silicon carbide whiskers and titanium diboride platelets. The coated filler method, which involves a reinforcement coated with the matrix metal, was used. In contrast, conventional PM uses the admixture method, which involves a mixture of matrix powder and reinforcement. For all the composite systems, the coated filler method was found to be superior to the admixture method in providing composites with lower porosity, greater hardness, higher compressive yield strength, lower coefficient of thermal expansion (CTE), higher thermal conductivity and lower electrical resistivity, though the degree of superiority was greater for high than low reinforcement contents. In the coated filler method, the coating on the reinforcement separated reinforcement units from one another and provided a cleaner interface and stronger bond between reinforcement and matrix than the admixture method could provide. The highest reinforcement content attained in dense composites (<5% porosity) made by the coated filler method was 70 vol% Mo, 60 vol% TiB2 and 54 vol% SiC. The critical reinforcement volume fraction above which the porosity of composites made by the admixture method increases abruptly is 60% Mo, 42% TiB2 and 33% SiC. This fraction increases with decreasing aspect ratio of the reinforcement. Among Cu/Mo, Cu/TiB2 and Cu/SiC at the same reinforcement volume fraction (50%), Cu/Mo gave the lowest CTE, highest thermal conductivity and lowest electrical resistivity, while Cu/SiC gave the greatest hardness and Cu/TiB2 and Cu/SiC gave the highest compressive yield strength. Compared to Cu/SiC, Cu/TiB2 exhibited much higher thermal conductivity and much lower electrical resistivity.

Improving the fracture toughness of MgO–Al2O3–SiO2 glass/molybdenum composites by the microdispersion of flaky molybdenum particles

The flake-forming behaviour of powders of molybdenum, niobium, nickel, BS 316 S 12, Ni–17Cr–6Al–0.6Y, iron, titanium and Ti–6Al–4V, using a wet ball mill, was investigated. MgO–Al2O3–SiO2 (MAS) glass composites reinforced with these flaked particles were fabricated, and improvements in flexural strength evaluated. The MAS glass composites reinforced with flaky metallic particles such as molybdenum, niobium, iron, nickel and Ni–17Cr–6Al–0.6Y, showed an improvement. The effect of molybdenum particle size on the flake-forming behaviour of molybdenum, flexural strength and fracture toughness of MAS glass/molybdenum composites, were investigated. The flake-forming behaviour shows a high degree of dependence on molybdenum particle size and, upto a size of 32 μm, becomes conspicuous with increasing particle size. At 32 μm, the aspect ratio reaches a value of 17 and, above 32 μm, flake forming saturates. Fracture toughness is closely related to flake-forming behaviour and the more marked the flake forming, the greater is the increase in fracture toughness. A composite of MAS glass with flaky molybdenum particles has a greater improvement effect on fracture toughness than composites with SiC whiskers, SiC platelets or ZrO2 particles. This is closely linked to plastic deformation of the flaky metallic particles at the crack tip at the time of fracture.

Single-wall nanotubes produced by metal-catalyzed disproportionation of carbon monoxide

Isolated single-wall carbon nanotubes (SWNT) were grown by disproportionation of carbon monoxide at 1200°C, catalyzed by molybdenum particles a few nanometers in size. The tube diameters, ranging from 1 to 5 nm, closely correlated with the size of the catalytic particle found attached to the tube end. This result represents the first experimental evidence of SWNT produced by pre-formed catalytic particles. A mechanism for nucleation that is quite distinct from our recently proposed mechanism of SWNT produced by laser vaporization is advanced for formation of SWNT in the present case.

96/04893 Carbonization behavior of hydrotreated coal tar pitch containing fine molybdenum and ruthenium particles

96/04893 Carbonization behavior of hydrotreated coal tar pitch containing fine molybdenum and ruthenium particles

Carbonization Behavior of Hydrotreated Coal Tar Pitch Containing Fine Molybdenum and Ruthenium Particles

In the carbonization of hydrotreated coal tar pitch containing fine molybdenum and ruthenium particles, a tritium tracer method revealed that prior addition of fine molybdenum and ruthenium particles into the pitch can selectively catalyze the condensation of low molecular weight hydrocarbons at temperatures below 600 °C. This is because they prevent the release of those molecules by vaporization and result in an increase in the carbonization yield of coal tar pitch and the hydrogen content of resultant cokes. The carbonization yield with ruthenium particles was higher than that with molybdenum particles. Observations by a transmission electron micrograph showed that the ruthenium particles (10 nm even at 1000 °C) were smaller than molybdenum particles and were dispersed uniformly throughout the hydrogenated pitch.

Aluminium nitride—molybdenum ceramic matrix composites: Characterization of ceramic—Metal interface

Pure aluminium nitride can be hot pressed with an addition of molybdenum powder. With this technique we obtain a ceramic matrix composite having a dispersed metallic phase. Composites produced in this manner present a homogeneous structure with very little open porosity. Electrical resistivity measurements done over a series of composites show a sharp decrease in electrical resistivity when the molybdenum volume fraction in the material increases from 0.2 to 0.22. This value corresponds to the percolation threshold of metallic phase in the ceramic matrix. The interface between molybdenum particles and aluminium nitride grains is established through a fine oxide or oxynitride layer present at the surface of AlN grains.

Copper-matrix molybdenum particle composites made from copper coated molybdenum powder

Copper-matrix molybdenum particle composites containing 33 ~73 wt.% Mo were fabricated by hot pressing copper coated molybdenum powder. For comparison, corresponding composites fabricated by hot pressing a mixture of copper and molybdenum powders were also made. The former method gave composites of lower porosity, higher hardness, higher compressive yield strength, lower coefficient of thermal expansion, lower electrical resistivity and higher thermal conductivity than the latter method. These differences are partly due to the separation of the molybdenum particles by the copper coating in the former case. In contrast, molybdenum particle clustering occurred in the latter case when the Mo content exceeded 53 wt.%.

Influence of substrate preparation on the flattening and cooling of plasma-sprayed particles

Impacts of plasma-sprayed molybdenum particles were monitored by detecting thermal radiation emitted by the hot particles when they flatten on the substrate surface. Evolution of the light intensity collected at two different wavelengths was used to obtain information about flattening time, flattening degree, and cooling time of the impinging particles. Variations of these parameters with substrate surface roughness were investigated on glass and molybdenum substrates. The substrate roughness significantly influenced the flattening degree and flattening time of the particles: the smoother the substrate, the larger the surface of the splats and the longer the flattening time. The cooling time, as determined from the decay time of the light signals after impact, was shorter on smooth substrates. In this case, the temperature of the splats was not radially uniform, with a lower cooling rate at the periphery.

Analysis of the morphology and the optical properties of sputtered molybdenum particles

The optical constants of thin sputtered molybdenum layers, embedded in a ceramic-metal composite produced by a batch sputtering deposition system were analyzed. This was accomplished by assuming a multilayer system for a tin oxide-molybdenum cermet and calculating the optical constants from angular and polarization dependent reflection and transmission spectra. These optical constants differ strongly from those for sputtered bulk material obtained ellipsometrically. A good agreement between measured effective refractive indices for cermets and effective medium calculation was found, if these optical constants for molybdenum were used in the effective medium calculations. Differences to the optical constants of the cermet determined ellipsometrically were explained by the birefringence of the cermet. The size and the shape of the embedded particles were investigated with an atomic force microscope.

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Evaluation of the optical properties of molybdenum particles in metal/ceramic composites

The optical constants of thin molybdenum layers and molybdenum/tin oxide cermets, produced by a batch sputtering deposition system, were determined by spectroscopic ellipsometry. The geometrical structure of the layers and the embedded particles was analysed with an atomic force microscope. The results were used for the investigation of a suitable effective medium theory to describe the optical constants of cermets with relatively high filling factors.