Materials Science Forum Vols. 706-709

Abstract: Microalloyed medium carbon steels have been applied for auto forging products at the aim of lower cost since 1980’s in China. Without quenching and tempering processes, the cost of heat treatment was greatly decreased, so they are widely applied in car forgings such as crankshaft, connecting rod. Recently, with the aim of improving performance and reducing production cost, new technologies, such as: reductions of microalloyed elements, tailored components, and mechanical properties forecast, have been developed in China auto industry. The designation, processing, properties prediction and microstructure of auto components have been controlled comprehensively. With the development of new technologies, microalloyed medium carbon steel gradually began to be applied to product important parts, and replace Quenching and Tempering steels. Both steel makers and end product users are expressed their desire to share benefits of technological innovation. The technologies will have a huge developing space and very bright developing prospects in motor parts industry in future.

Abstract: The relatively higher alloy content of AHSS leads to complex transformation behavior after finishing rolling and the transformation is unable to complete just after coiling, which causes some extent of deformation due to the transformation dilation. In the present study, low strain rate deformation was performed at γ/α temperature zone after the sample was deformed to simulate the deformation and transformation interaction after coiling by using Gleeble 3800 thermo-mechanical simulator. All the flow stress curves display a sharp decrease after certain strain when deformation temperatures below 750°C. The interaction between deformation and transformation is used to explain the phenomena.

Abstract: The texture of thin films, originating from a solid state reaction between a deposited film and a single crystal substrate is investigated. The relation between the phase formation and texture is analyzed for a number of these systems, such as Co/Si, Ni/Si or Co/Ge, where a metal film is allowed to react with a semiconductor single crystal substrate during heating and a summary of these results in presented in this article. It was found that the texture of the resulting films can be very complex, consisting of a variety of simultaneously occurring texture components such as epitaxy, fiber and axiotaxy texture. The close connection between the phase formation and texture is demonstrated by the fact that even a small intervention in either one, can have a huge effect on the resulting phase and/or its texture. From this, we show that the effect of the addition of ternary elements (e.g. Pt, W, C) to the thin films can only be understood if one considers its effects on both the kinetics and the thermodynamics of the reactions, as well as on the texture of the phases. We show how this can be used to influence technologically important properties of the films, such of formation temperature or stability.

Abstract: Multilayer thin-film structures are widely used in microelectronic, optoelectronic and micro-/nanoelectromechanical devices. Mechanical properties and mechanical stresses are very critical for such devices. Structural stresses are composed of an intrinsic part resulting from growth defects and a thermal part caused by the mismatch of the thermal expansion coefficients of the different materials in the multilayer system and between coating and substrate. Excessive tensile stresses result in cracking of the film and/or substrate, and compressive stress can cause buckling. Determination and control of stress is an important scientific issue for both functional thin films and engineering coatings from the viewpoint of performance and integrity. In the present study the in situ stress measurements for thin films of Ti (300nm thick) and Cu (1000nm thick) sputtered on Cu substrates were performed using a method proposed in [6]. The stresses that occurred in the Ti film were tensile, stresses that occurred in the Cu film were compressive. The continuous measurements were performed both during the process of the deposition and after the magnetron cut off. The film delamination from a substrate occurred in both cases. The analysis of obtained curves (the specific force versus time) was performed. Analysis revealed that the arising stresses are not caused by any nonstationary heating process of the material surface. Local buckling due to interatomic binding weakening can lead to Cu film delamination. The major cause of Ti film delamination can be a microcracking due to substrate’s thin surface layer yielding.

Abstract: The intrinsic n-type (II-VI) semiconductor ZnO may become ferromagnetic at room temperature, by small additions of magnetic ions, resulting in what is called a Diluted Magnetic Semiconductors (DMS). The potential application of DMS in spintronic devices of is driving the research effort to dope magnetic elements into this semiconductors with a depth distribution as uniform as possible. The doping levels and the depth distribution of dopants are critical parameters for the magnetic properties of this material and the possible clustering of dopants can play a significant negative role in its macroscopic magnetic properties. Thin ZnO (0001) films of between 100nm and 500nm, grown on c-Al₂O₃ by MOCVD were implanted with Co, Eu and Co+Eu by ion irradiation at low energies. In order to improve the depth distribution of dopants, the ion implantation was carried out through a number of appropriately chosen range foils. The results show an increase in the level of dopant homogeneity throughout the entire thickness of the film, and a ferromagnetic behavior above room temperature for Zn_0.96Co_0.04O, Zn_0.96Eu_0.04O and Zn_0.92Co_0.04Eu_0.04O.

Abstract: Superhydrophobic surfaces were prepared using a very simple and low-cost method by spray coating. A high static water contact angle of about 154° was obtained by deposition of stearic acid on an aluminium alloy. However, this coating demonstrated a high contact angle hysteresis (~ 30º). On the other hand, superhydrophobic surfaces with a static contact angle of about 162º and 158º, and a low contact angle hysteresis of about 3º and 5º were respectively obtained by incorporating nanoparticles of SiO₂ and CaCO₃ in stearic acid. The excellent resulting hydrophobicity is attributed to the synergistic effects of micro/nanoroughness and low surface energy. A study of the wettability of these surfaces at temperatures ranging from 20 to-10 °C showed that the superhydrophobic surface becomes rather hydrophobic at supercooled temperatures.

Abstract: The evolution of grain boundary microstructures in gold thin films during annealing was investigated in order to find a clue to the development of high performance thin films by grain boundary engineering. The {111} oriented grains with the lowest surface energy were preferentially grown by surface energy-driven grain growth during annealing. The sharp {111} texture was developed by annealing at the temperature more than 873K. The remarkably high fraction of low-Σ coincidence site lattice (CSL) boundaries occurred when the area fraction of {111} texture increased to more than 95%. In particular, the fraction of some low-Σ CSL boundaries (Σ1,Σ3,Σ7) for the most sharply {111} textured specimen was found to be one order higher than those predicted for a random polycrystal. The utility of grain boundary engineering is discussed for controlling the performance degradation caused by the percolation phenomena of grain boundary diffusion in gold thin films.

Abstract: Stress induced grain boundary migration was experimentally investigated in aluminum bicrystals. Migration of planar symmetrical <100> and <111> tilt boundaries under a shear stress was observed to be accompanied by a lateral translation of the adjacent grains. This coupling proved to be the typical migration mode for all investigated boundaries, no matter whether low-or high angle, low Σ CSL coincidence or non-coincidence boundary. The migration-shear coupling was also observed for asymmetrical tilt boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of the migration activation parameters. Contrary to expectations, a high angle Σ7 tilt boundary moved under an applied stress, but produced practically no shear during its migration.

Abstract: The size dependent hardening in Al-4wt%Cu thin film on Si substrate has been investigated by the numerical calculation of indentation stress field and the observation of plastic zone microstructure of indented film. Distribution of internal stress predicted by triangular dislocation loop (TDL) model shows no size dependency with the different number of dislocations when the constant line density is assumed at the contact surface. TEM cross-sectional observation reveals that the plastic deformation is dominantly induced inside the film, and the growth of plastic zone is restricted at the interface of hard Si substrate. The size dependent hardening in soft film and hard substrate system is discussed from the change in dislocation density with respect to the plastic zone microstructure.

Abstract: Diamond films grown by Chemical Vapor Deposition (CVD) are widely used as surface overlay coating onto WC-Co cutting tools to develop their performances. To improve the diamond adhesion with the substrate, many pre-treatment methods have been applied such as giving a suitable diffusion barrier layer on the substrates. Single zirconium and tantalum nitride layers have been sputtered onto WC-Co substrates as diffusion barriers and buffer layers under specific reactive sputtering conditions. Microstructure of the tantalum nitride layer has been controlled using process parameters in order to understand mechanisms occurring during Co diffusion in correlation with microstructure. Moreover, a multilayer coating composed of TaN and ZrN thin film sequences as diffusion barrier is also evaluated. To improve the nuclei density of diamond during CVD processing, a thin Mo extra layer has been added (500 nm) whatever the diffusion barrier used. In this study, bilayer and multilayer systems have been tested to optimize nanocrystalline CVD diamond deposition grown with negative biased substrates. For all systems, after diamond deposition, a massive carburization of molybdenum and tantalum nitride is observable whereas zirconium nitride is not. A TEM study is done to improve understanding of phenomena occurring at the interfaces during the process. The key role played by each layer microstructure is discussed.