Effect of argon and substrate bias on diamond thin film surface morphology

Nanocrystalline materials are of high interest, because mechanical and physical properties of such materials are different from those or coarse-grained type. Continuous and smooth nanocrystalline diamond (NCD) thin films were successfully grown on mirror polished silicon substrates, using double bias plasma-enhanced hot filament chemical vapour deposition technique. A gas mixture of Ar:CH₄:H₂ and CH₄:H₂ was used as the precursor gas. The effect of the gas composition, flow rate and substrate bias during deposition on diamond crystallite size was investigated. Changing the growth parameters facilitates control of grain size of polycrystalline diamond thin films from microcrystalline to nanocrystalline. The structure of fine-grained NCD films has been studied with scanning electron microscopy and Raman spectroscopy.     

Influence of interface energy and grain boundary on the elastic modulus of nanocrystalline materials

With reducing the grain size into nanometer scale for polycrystalline materials, the influence of nonlocal interactions in grain boundaries on the mechanical properties of the material is reinforced as well as the interface energy stemming from the surfaces of grains is increased, resulting in that the mechanical properties of the polycrystalline represent size-dependence significantly. In this work, the influence of the interface energy and grain boundaries on the elastic properties of nanocrystalline materials is investigated in the framework of continuum mechanics. An analytical expression of the elastic modulus is addressed to describe the grain size effects on the Young’s modulus of nanocrystalline materials. The numerical results illustrate that the elastic modulus of nanocrystalline materials decreases with the reduction of the grain size to nanometer scale. The grain size effects become remarkable when the grain size lowers down to several tens nanometers, and the influence of the interface energy and grain boundary must be taken into account. The contribution of the density on the mechanical properties in nanocrystalline materials is analyzed by discussing the influence of the grain boundary thickness on the elastic modulus. The comparison between the proposed theoretical results and the present measurement shows that the proposed model can predict the experiments quite well.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

Friction and Wear Behaviors of Nanostructured Metals

Nanostructured （ns） materials, i.e., polycrystalline materials with grain sizes in the nanometer regime （typically below 100 nm）, have drawn considerable attention in the past decades due to their unique properties such as high strength and hardness. Wear resistance of ns materials, one of the most important properties for engineering materials, has been extensively investigated in the past decades. Obvious differences have been identified in friction and wear behaviors Between the ns materials and their corresponding coarse-grained （cg） counterparts, consistently correlating with their unique structure characteristics and mechanical properties. On the other hand, the superior tribological properties of ns materials illustrate their potential applications under contact loads. The present overview will summarize the important progresses achieved on friction and wear behaviors of ns metallic materials, including ultrafine-grained （ufg） materials in recent years. Tribological properties and effects on friction and wear behaviors of ns materials will be discussed under different wear conditions including abrasive wear, sliding wear, and fretting wear. Their correlations with mechanical properties will be analyzed. Perspectives on development of this field will be highlighted as well.     

二元互不溶体系中纳米晶过饱和固溶体机械合金化的研究进展

互不溶体系合金在机械合金化的非平衡过程中能够形成纳米晶过饱和固溶体,并显示出与其微米尺度结构合金所不同的独特性能。纳米晶过饱和固溶体不仅是互不溶体系中重要的一种亚稳态结构,也是一种重要的纳米材料体系。综述了近年来二元互不溶体系中纳米晶过饱和固溶体机械合金化的研究进展,着重介绍了纳米晶过饱和固溶体的形成机制,以及纳米晶过饱和固溶体的热稳定性、力学和物理性能。     

Structure and thermal stability of nanocrystalline materials

Nanocrystalline materials, which are expected to play a key role in the next generation of human civilization, are assembled with nanometre-sized “building blocks” consisting of the crystalline and large volume fractions of intercrystalline components. In order to predict the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to understand precisely how the structures of crystalline and intercrystalline regions vary with decrease in crystallite size. In addition, study of the thermal stability of nanocrystalline materials against significant grain growth is both scientific and technological interest. A sharp increase in grain size (to micron levels) during consolidation of nanocrystalline powders to obtain fully dense materials may consequently result in the loss of some unique properties of nanocrystalline materials. Therefore, extensive interest has been generated in exploring the size effects on the structure of crystalline and intercrystalline region of nanocrystalline materials, and the thermal stability of nanocrystalline materials against significant grain growth. The present article is aimed at understanding the structure and stability of nanocrystalline materials.     

热压烧结TiB2陶瓷的显微结构和力学性能研究

以Y₂O₃－Al₂O₃为烧结助剂,通过热压制备了TiB₂陶瓷,研究了烧结温度、烧结时间和晶化处理对材料的显微结构和力学性能的影响．实验结果表明,随着烧结温度的升高,烧结体失重增加,其抗弯强度和断裂韧性降低;烧结时间延长,其显微结构的均匀性降低,对力学性能不利．晶粒直径对TiB₂陶瓷的力学性能有重要影响．晶化处理能够导致晶界拆出YAG相,从而提高TiB₂陶瓷的高温抗弯强度．     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.     

Microstructure and fundamental properties of nanostructured gadolinium (Gd)

The nanoparticles and nanocrystalline bulk of pure gadolinium (Gd) were prepared by a novel route. The nanostructures of the single particle and the bulk of Gd were investigated, and the crystal structure was characterized. The fundamental properties, namely the physical, thermal, and mechanical characteristics, were studied for the prepared Gd bulk with an ultrafine nanograin structure. As compared with the conventional polycrystalline metal, the ultrafine nanocrystalline Gd has greatly enhanced functional and structural properties. The physical background for the changes of the fundamental properties with the reduction of the grain size to the nanoscale was analyzed.     

Influence of substrate temperature on the physical properties of thermally evaporated nanocrystalline bismuth sulfide thin films

Nanocrystalline bismuth sulfide thin films were deposited on glass substrate by thermal evaporation technique using the solvothermally synthesized nanometer-sized bismuth sulfide powder as the source material. X-ray diffraction (XRD) analysis revealed that the films are polycrystalline in nature with orthorhombic structure. The crystallinity of the thin films improved with substrate temperature, and the estimated crystallite size are in the nanometer regime. Scanning electron microscope (SEM) analysis showed homogenous distribution of grains with well defined grain boundaries. The optical transmittance of the nanocrystalline bismuth sulfide thin films increases with the increase in substrate temperature, and the optical transition was found to be direct and allowed. The estimated optical band gap energy was found to decrease with the increase in substrate temperature. The electrical resistivity of the bismuth sulfide thin films is of the order of 10⁻⁴ Ω-cm and exhibits semiconductor nature. Experimental results demonstrate that the structural, optical and electrical properties of bismuth sulfide thin films have strong dependence on the substrate temperature.     

Material Properties of Titanium Diboride

The physical, mechanical, and thermal properties of polycrystalline TiB₂ are examined with an emphasis on the significant dependence of the properties on the density and grain size of the material specimens. Using trend analysis, property relations, and interpolation methods, a coherent set of trend values for the properties of polycrystalline TiB₂ is determined for a mass fraction of TiB₂ ⩾ 98 %, a density of (4.5±0.1) g/cm³, and a mean grain size of (9±1) µm.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

大塑性变形工艺制备纳米晶过饱和固溶体的研究进展

合金在大塑性变形过程中能够形成纳米晶过饱和固溶体,呈现出不同于传统粗晶材料的微观结构和独特性能。近年来,纳米晶过饱和固溶体的形成机制及其热稳定性已成为国内外的一个研究热点。综述了大塑性变形工艺(如机械合金化法、高压扭转法等)制备纳米晶过饱和固溶体的研究概况,着重讨论分析了大塑性变形诱导纳米晶形成和固溶度扩展的几种机制及其局限性,简要介绍了纳米晶过饱和固溶体的热稳定性及其影响因素,最后对该领域今后的研究方向做出了分析和展望。     

Nanostructured materials by mechanical alloying: new results on property enhancement

Mechanical attrition—the mechanical alloying or milling of powders—is a very versatile and potent method of obtaining nanocrystalline or ultrafine grain structures with enhanced properties. This article presents three examples of enhanced properties obtained by materials in which the grain size has been reduced to the nanoscale or ultrafine scale by ball milling and consolidation of powders. Very high strength/hardness—the highest hardness yet reported for crystalline Mg alloys—for a ball milled Mg₉₇Y₂Zn₁ alloy is due in part to the nanocrystalline grain structure, along with nanoscale precipitates. A ternary Cu-base alloy with a low stacking fault energy was found to have both high strength and good ductility in a nanocrystalline material synthesized by the in situ ball milling consolidation method. This is another example that shows nanocrystalline materials need not be brittle. It is shown that bulk thermoelectric materials with superior properties can be produced by the ball milling and consolidation of powders to provide an ultrafine grain structure.     

Nanocrystalline calcium phosphate ceramics in biomedical engineering

Nanocrystalline calcium phosphate based bioceramics are the new rage in biomaterials research. Conventionally, calcium phosphates based materials are preferred as bone grafts in hard tissue engineering because of their superior biocompatibility and bioactivity. However, this group of bioceramics exhibits poor mechanical performance, which restricts their uses in load bearing applications. The recent trend in bioceramic research is mainly concentrated on bioactive and bioresorbable ceramics, i.e. hydroxyapatite, bioactive glasses, tricalcium phosphates and biphasic calcium phosphates as they exhibit superior biological properties over other materials. In recent times, the arena of nanotechnology has been extensively studied by various researchers to overcome the existing limitations of calcium phosphates, mainly hydroxyapatite, as well as to fabricate nanostructured scaffolds to mimic structural and dimensional details of natural bone. The bone mineral consists of tiny HAp crystals in the nano-regime. It is found that nanocrystalline HAp powders improve sinterability and densification due to greater surface area, which could improve the fracture toughness and other mechanical properties. Nano-HAp is also expected to have better bioactivity than coarser crystals. Nanocrystalline calcium phosphate has the potential to revolutionize the field of hard tissue engineering from bone repair and augmentation to controlled drug delivery devices. This paper reviews the current state of knowledge and recent developments of various nanocrystalline calcium phosphate based bioceramics from synthesis to characterization.     

Mechanical properties of polycrystalline diamonds

Abstract

The room temperature mechanical properties of polycrystalline diamonds, i.e. tensile strength, transverse rupture strength, compressive strength, impact strength, fracture toughness, and elastic constants, have been determined. The applied test techniques are described and the results compared with those obtained by other authors. The fracture mode under the present experimental conditions was primarily transgranular. A grain size dependence, where strength increases with decreasing grain size, has been found. Fracture toughness was found to go through a maximum for grain sizes between 10 to 30 μm. The modulus of elasticity increases with increasing grain size. An influence of cobalt content on strength and modulus of elasticity has been found, while no significant influence on toughness could be determined. Increasing the cobalt content increases strength, but has the inverse effect on the modulus of elasticity. The results of strength, toughness, and elastic constants measurements are discussed in terms of available models and theories of polycrystalline ceramic materials. It can be seen from the results that polycrystalline diamonds behave in a manner similar to that of most engineering ceramics, but have the distinct advantage of a higher fracture toughness.

MST/596     

高性能SiC—AlN复相陶瓷

采用热压烧结工艺,通过合理的组成设计和烧结温度控制,制备出了高性能SiC-AlN复相陶瓷,在较佳条件下,复合材料的室温强度、断裂韧性、显微硬度分别高达1130MPa、6．2MPa·m^1/2、28．6GPa．显微结构研究表明,随着AlN的加入,复合材料的晶粒尺寸明显细化,并呈多层次效应,即由固溶体的形成所引起的一次晶粒细化和晶内亚晶界所引起的二次晶粒细化．     

电沉积法制备块体纳米晶材料的研究进展

综述了电沉积法制备块体纳米晶材料的原理;阐述了电流密度、电流波形、有机添加剂等工艺参数对沉积层晶粒尺寸的影响;介绍了直流电沉积、脉冲电沉积、喷射电沉积和复合电沉积等几种常见的电沉积方法;概述了电沉积法制备块体纳米晶材料的国内外研究现状;探讨了电沉积块体纳米晶材料的力学性能、磁学性能、耐蚀性能、热稳定性及其应用前景.     

Revealing the atomistic deformation mechanisms of face-centered cubic nanocrystalline metals with atomic-scale mechanical microscopy: A review

Nanocrystalline metals have many functional and structural applications due to their excellent mechanical properties compared to their coarse-grained counterparts. The atomic-scale understanding of the deformation mechanisms of nanocrystalline metals is important for designing new materials, novel structures and applications. The review presents recent developments in the methods and techniques for in situ deformation mechanism investigations on face-centered-cubic nanocrystalline metals. In the first part, we will briefly introduce some important techniques that have been used for investigating the deformation behaviors of nanomaterials. Then, the size effects and the plasticity behaviors in nanocrystalline metals are discussed as a basis for comparison with the plasticity in bulk materials. In the last part, we show the atomic-scale and time-resolved dynamic deformation processes of nanocrystalline metals using our in-lab developed deformation device.