稀土在硬质合金球形粉末烧结体表面富集现象的观察

采用合金粉末烧结体表面观察法研究了稀土在WC-Co硬质合金中的作用机理。合金中的稀土分别以混合稀土(以La和Ce为主体成分)-Co预合金粉(简称RE-Co预合金粉)形式和La(NO3)3的丙酮溶液形式在湿磨时直接加入。用扫描电镜和能谱仪对平均粒径小于200μm的2种球形稀土硬质合金粉末(简称合金粉末)烧结体表面进行观察与分析,发现:以RE-Co预合金粉形式加入稀土时,在烧结过程中,合金中的稀土La和Ce以及杂质元素在合金粉末烧结体表面产生了明显的富集,形成了含La,Ce,S,Ca,W,C和O的复杂形式的化合物,在稀土富集物中没有检测到Co。以La(NO3)3形式加入稀土,在合金粉末烧结体表面没有检测到La。说明在合金粉末烧结体表面不存在La的富集或聚集。稀土的添加形式同时也影响合金粉末烧结体表面硬质相WC与粘结相的比例,以RE-Co预合金粉末形式加入稀土,合金粉末烧结体表面粘结相含量较少,因此,稀土的添加形式影响其在硬质合金中的作用机理。以RBCo预合金粉形式加入稀土,合金中的稀土不但具有较好的富集合金中杂质元素的作用,而且还可以阻止烧结体表面富粘结相结构的形成。     

From experimental data,the mechanics relationships describing the behaviour of four different low alloyed steel powders during uniaxial cold compaction

This work aims at determining the constitutive model of four commercial water atomised low alloyed steel powders during cold compaction. Single-action experiments were performed, obtaining cylindrical specimens with different H/D ratios. The distribution of axial and radial stresses was investigated, and the relationships describing both the radial stress transmission coefficient and the flow stress as functions of the relative density were determined. The radial stress transmission coefficient also confirmed the hypothesised value of Poisson’s coefficient. The friction coefficient between the powder column and the die wall was determined, also highlighting the influence of the H/D ratio. Measuring the axial and radial strains due to spring-back, the axial and radial elastic moduli were determined, as functions of the relative density. The results obtained for the four materials were compared, also highlighting both differences and similarities.     

Combined effect of internal and external factors on sintering kinetics of plasma-sprayed thermal barrier coatings

Understanding the sintering kinetics of plasma-sprayed thermal barrier coatings (PS-TBCs) is crucial to retard their performance degradation. However, under real service condition, the sintering kinetics is often affected by multiple factors. This study investigated the sintering kinetics, in a novel scale-progressive view, under the combined-effect of internal and external factors. Results show that the sintering kinetics of PS-TBCs was highly associated with their unique sintering process from nanoscale to microscale. Firstly, sintering leads to nanoscopic roughening of the pore surface. Subsequently, multiple contacts are formed between counter-surfaces. As a result, microscopic healing of pores can be finally observed. In terms of external factors, the temperature further affects the level and rate of nanoscopic roughening. This is responsible for the differences of the microscopic healing ratios, as well as the macroscopic elastic modulus.     

Tensile flow behavior of ultra low carbon,low carbon and micro alloyed steel sheets for auto application under low to intermediate strain rate

This paper is concerned with tensile characteristics of auto grade low carbon, ultra low carbon and micro alloyed steel sheets under low to intermediate strain rates ranging from 0.0007 to 250 s⁻¹. Experimental investigation reveals two important aspects of these steels under intermediate strain rate deformation. Firstly, the yield stress increases with strain rate in all these steels. Of course yield stress increment is higher for low carbon and ultra low carbon steel sheets. Secondly, the strain hardening rate drastically decreases with strain rate for low carbon and ultra low carbon steel sheets, whereas it remains steady for micro alloyed steel sheets. Based on these observations, a constitutive model has been proposed which predicts the strain rate sensitive flow behavior of all these grades within the strain rate range of automotive crash event.     

Reduced order finite element model for elastohydrodynamic lubrication: Circular contacts

This paper presents an extension of the reduced order finite element model to the case of circular elastohydrodynamic lubricated (EHL) contacts under isothermal Newtonian considerations. The line contact model was developed and validated in a previous work (Advances in Engineering Software, 2013; 56:51−62). The model is based on a finite element discretization of the EHL equations: Reynolds, linear elasticity and load balance with a reduced order model for the linear elasticity part. All equations are solved simultaneously in a fully-coupled framework using a damped-Newton procedure allowing fast convergence rates for the global solution. This model combines fast convergence rates, reduced memory requirements and negligible model reduction errors compared to the full model which makes it an attractive tool for EHL contact performance prediction.     

The role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films

This paper presents a systematic analysis of the electrode configuration influence on the electrical properties of organic semiconductor (OSC) thin-film devices. We have fabricated and electrically characterized a set of planar two-terminal devices. The differences in I-V characteristics between the top and bottom contact structures are presented and analyzed. Top-contact configurations have a linear current vs. electric field behavior, while the bottom-electrode devices display a transition from ohmic to space-charge-limited conduction regime. The transition is temperature- and thickness-dependent. Finite-element calculations show that when the OSC film is connected using top electrodes, the current flows through the OSC bulk region. On the other hand, the bottom-electrode configuration allows most of the current to flow near the OSC/substrate interface. The current probes interfacial states resulting in a space-charge conduction regime. The results shed some light on the so-called “contact effects” commonly observed in organic thin-film transistors. The findings presented here have implications for both the understanding of the charge transport in OSC films and the design of organic semiconductor devices.     

隔膜压缩机金属膜片层间接触力学行为的研究

根据隔膜压缩机膜片的受力分析,建立了三层膜片层间接触力学模型。通过理论计算获得了不同挠度情况下膜片的挠度曲线、径向应力、周向应力及主剪切应力值。根据膜片不同挠度的应力分布,求出三层膜片接触面滑动区和黏着区的分布情况,发现可以通过改变膜片材料、膜片厚度来改善膜片层间接触状态,降低膜片层间摩擦磨损,对延长膜片的使用寿命具有重要意义。