Plastic deformation of silicon during contact sliding at ambient temperature

Evidence of plastic deformation during contact sliding of silicon under relatively low loads at room temperature is presented. Sapphire spheres were slid against Si (1 0 0) under various normal loads at temperatures above and below the critical temperature. Upon chemical etching, pits that are attributed to dislocations developed along the sliding track for all experiments. This suggests that plastic deformation can readily take place in covalent solids, such as silicon, even at temperatures far below the critical temperature. The results of this work support the view that frictional force and energy dissipation are largely caused by plastic deformation of the materials near the sliding contact even under relatively low loads.     

Fatigue crack growth and damage characteristics of high-speed rail at low ambient temperature

Low temperature can be a significant problem affecting safety and maintenance of railway. In this study, the fatigue crack growth rate and rolling contact fatigue damage behaviors of high-speed rail material under different temperature conditions were investigated by a series of experiments. The results indicate that the stress and strength of rail material increase with the decrease of ambient temperature. The crack growth rate at 0 °C and − 20 °C is similar with that at 20 °C. While, when the temperature decreases to − 60 °C, the growth rate of crack increases sharply. The promotion of rail embrittlement at low temperature accompanied with the action of high stress causes the rapid failure and increase of surface crack length and subsurface crack damage. Meanwhile, three crack growth mechanism models at different temperatures can be inferred. The brittle fracture mode is increasingly apparent with the temperature decreasing.     

Time-dependent fracture of type 316L(N) steel at ambient temperature

Experimental data on tensile and compact geometry (CT) specimens of austenitic Type 316L(N) steel were obtained under sustained load conditions at room temperature. Time‐dependent crack growth, in some cases leading to failure, occurred in many of the CT specimens, dependent on the load level. However, rupture of the uniaxial specimens occurred only at stresses very close to the material's ultimate strength. The data validate the approach to assessing sustained loading effects in the R6 defect assessment procedure. In particular, sustained load effects in austenitic steel may be neglected for values of the R6 L_r parameter less than unity. Uniaxial sustained load tests were also performed at 100 °C and 200 °C. The measured strain rates decreased with increasing temperature, becoming negligible at 200 °C. This is consistent with the advice in R6 that sustained load effects in austenitic steel can be neglected at temperatures between 200 °C and the high‐temperature creep range.     

Fatigue of SIC-based ceramics at ambient temperature

The effects of static and cyclic loading on monolithic sintered silicon carbide (SiC) and SiC reinforced with 16 vol.% particulate titanium diboride (SiC/16vol.% TiB₂) have been studied. Tests were carried out at ambient temperature in air on precracked specimens loaded in three- or four-point bending. No crack growth under cyclic loading has been observed in the monolithic silicon carbide. There is an additional cyclic contribution to crack growth after static crack growth has arrested in the composite material. Observations suggest that damage to the titanium diboride particles ahead of the crack tip occurs prior to crack extension through the SiC matrix.     

Failure behavior of rocks at ambient temperature

This paper presents the results of triaxial loading experiments performed to determine the fracture strength characteristics of granite subjected to truly three-dimensional loading. These measurements were carried out on thin walled, hollow cores which were subjected to various tension-torsion, and compression-torsion loading paths in the presence of confining pressures of up to 48.3 MPa (7000 psi). The results were compared to the predicted values using the classical theories of Coulomb-Mohr and Drucker-Prager. It was observed that none of the above criteria were able to closely predict the failure over the entire range of stress states. Also, the experimental results of other investigators on the failure of Solenhofen limestone, sandstone, marble, and shale were compared with the failure criteria of Coulomb-Mohr and Drucker-Prager. It was then demonstrated that both of these criteria were effective in predicting rock failure for only lower values of . At higher values of J₁, the above criteria were relatively less effective in predicting the failure of rocks.     

Synthesis of titania-silica xerogels by co-solvent induced gelation at ambient temperature

A series of binary titania-silica mixed oxides was prepared by sol-gel synthesis at room temperature. The hydrolysis of titanium isopropoxide (Ti(ⁱOPr)₄) and tetraethylorthosilicate (TEOS) was facilitated by co-solvent induced gelation in acidic media. The resulting gels were dried, calcined, and then characterized by powder X-ray diffraction analysis, nitrogen sorption studies (at 77 K), diffuse reflectance spectroscopy, Raman microscopy, and transmission electron microscopy. The nitrogen sorption studies indicate that the porosities could be tuned when simple aromatic solvents such as toluene, p-xylene, or mesitylene were added as a co-solvent to the synthesis gel. The binary mixed metal oxide materials obtained in this study showed high activity towards the degradation of phenol, and possessed high surface areas, and large pore volumes with narrow pore size distribution without the need for additional hydrothermal synthesis or supercritical drying.     

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

Penetration of limestone targets by ogive-nosed VAR 4340 steel projectiles at oblique angles: experiments and simulations

In this paper, we document the results of a combined experimental, analytical, and computational research program that investigates the penetration of steel projectiles into limestone targets at oblique angles. We first conducted a series of depth-of-penetration experiments using 20.0 g, 7.11-mm-diameter, 71.12-mm-long, vacuum-arc-remelted (VAR) 4340 ogive-nose steel projectiles. These projectiles were launched with striking velocities between 0.4 and 1.3 km/s using a 20-mm powder gun into 0.5 m square limestone target faces with angles of obliquity of 15° and 30°. Next, we employed the initial conditions obtained from the experiments with a technique that we have developed to calculate permanent projectile deformation without erosion. With this technique we use an explicit, transient dynamic, finite element code to model the projectile and an analytical forcing function based on the dynamic expansion of a spherical cavity to represent the target. Due to angle of obliquity we developed a new free surface effect model based on the solution of a dynamically expanding spherical cavity in a finite sphere of incompressible Mohr–Coulomb target material to account for the difference in target resistance acting on the top and bottom sides of the projectile. Results from the simulations show the final projectile positions are in good agreement with the positions obtained from post-test castings of the projectile trajectories.     

Deposition of silicon oxynitride at room temperature by Inductively Coupled Plasma-CVD

Oxynitride thin films are used in important optical applications and as gate dielectric for MOS devices. Their traditional deposition processes have the drawbacks that high temperatures are needed, high mechanical stresses are induced and the deposition rate is low. Plasma assisted processes may alleviate these problems. In this study, oxynitride films were deposited at room temperature through the chemical reaction of silane, nitrogen and nitrous oxide (N₂O), in a conventional LPCVD furnace, which was modified into a high density Inductively Coupled Plasma (ICP) reactor. Deposition rates increased with applied coil power and were never lower than 10 nm/min, quite high for room temperature depositions. The films' refractive indexes and FTIR spectra indicate that for processes with low N₂O gas concentrations, when mixed together with N₂ and SiH₄, nitrogen was incorporated in the film. This incorporation increased the resistivity, which was up to 70 GΩ cm, increased the refractive index, from approximately 1.47 to approximately 1.50, and decreased the dielectric constant of these films, which varied in the 4-14 range. These characteristics are adequate for electric applications e.g. for TFT fabrication on glass or polymers which can not stand high temperature steps.     

Simple and rapid conversion of silicon carbide to nanodiamonds at ambient pressure

Nanodiamonds (NDs) were prepared under ambient pressure by sublimation decomposition of silicon carbide in an intermediate frequency furnace for only 8 min.The ...     

Erosive wear of polymer surfaces by steel ball blasting

The erosion behaviour of a variety of polymeric materials has been studied using steel balls at 57 m sec⁻¹ in an air-blast rig. It is shown that the softer polymers (polyethylene, polypropylene, polybutene-1) exhibit an incubation period prior to stabilizing to a linear erosion rate, here defined as reduction in thickness per testing time. The more brittle polymer, polystyrene, on the other hand, shows no incubation time and possesses the highest erosion rate. Further effects can arise from the morphology of semicrystalline polymers. In particular, it was found that a coarse spherulitic microstructure in polypropylene wears much faster than a fine spherulitic one. A decrease in testing temperature generally increases the wear rate. The individual mechanisms of erosive wear are illustrated by SEM micrographs of the worn surfaces. It is suggested that a “brittleness index” of the form (hardnessH/fracture energyG _Ic) is a good indicator for the erosion resistance of polymeric materials.     

制备室温纳米ZnO脱硫剂方法的研究

为了提高ZnO脱硫剂的室温硫容及脱硫精度,对直接沉淀法进行了工艺改进,并利用动态实验以及XRD和XPS等表征手段对纳米ZnO脱硫剂进行了结构和效能分析.研究表明,通过在沉淀反应后添加Na2CO3溶液,解决了直接沉淀法阴离子难洗涤、颗粒易团聚、粒径分布不均等缺点,制备出粒径只有8.00 nm、颗粒均匀分散性好的纳米ZnO脱硫剂,其室温脱硫活性是同条件下其他方法制备纳米ZnO脱硫剂脱硫活性的5～10倍,且其脱硫产物中有不同于ZnS中的晶格硫的新硫物种产生.     

Particle impact damage in silicon nitride at 1400° C

Impacts of tungsten carbide spheres on Si₃N₄ produced elastic fracture behaviour (ring and cone cracks) at room temperature, but elastic-plastic fracture behaviour (plastic impressions and radial cracks) at 1400° C. In contrast, no change in fracture pattern at the two temperatures was produced by impact with steel spheres. These results may be explained by the relative abilities of the impacting spheres to cause plastic flow at the impact site and hence to alter the stress distribution in the Si₃N₄ specimens. The type and extent of damage produced by hard particle impact at 1400° C appears to be more deleterious to structural integrity than that produced at 20° C under equivalent particle impact loading conditions.     

Study on preparation of the core-nanoshell composite absorbers by high-energy ball milling at room temperature

Electromagnetic (EM) wave pollution has become the chief physical pollution for environment. In recent years, some researches have been focused on the preparation of nano-composite absorbers at low temperatures or even at room temperature. In this letter, preparation of nanocomposite by using high-energy ball milling at room temperature is reported. The core-nanoshell composite absorbers with magnetic fly-ash hollow cenosphere (MFHC) as nuclear and nanocrystalline magnetic material as shell were prepared by high-energy ball milling and vacuum-sintering in this paper. The pre-treatment of MFHC, the sintering process and the mol ratio of starting chemicals had a significant impact for property of composite absorbers. The results of X-ray diffraction analysis (XRD), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA) analysis indicated that perfect-crystalline nanomagnetic material coating was gotten with a particle size of 12 nm after ball milling. The results show the MFHC is dielectric loss and magnetic loss too; the exchange-coupling interaction happened between ferrite of the MFHC and nanocrystalline magnetic material coating. The exchange-coupling interaction enhances magnetic loss of composite absorbers. They have a perfect EM parameters at low microwave frequency. The core-nanoshell composite absorbers have a higher magnetic loss at low frequencies, and it is consistent with requirements of the microwave absorbing material at the low-frequency absorption. The microwave absorptivity of the core-nanoshell composite absorbers is better than single material.     

高温快速退火对电子束物理气相沉积制备高硅硅钢片的影响

对电子束物理气相沉积高硅硅钢片进行了高温快速退火处理.用SEM,EDS,XRD对制备态硅钢片和热处理态组织,成分,物相进行了表征,并测试了其电阻率和磁滞回线.结果表面:高温快速退火使高硅硅钢片硅成分变均匀,靠近基板侧的相由Fe3Si变成DO3;硅钢片中孔长大,电阻率从90μΩ·m增大到160μΩ·m,矫顽力减小,磁感应强度减小.