Effects of surface mechanical attrition treatment (SMAT) on a rough surface of AISI 316L stainless steel

Surface mechanical attrition treatment (SMAT) improves mechanical properties of metallic materials through the formation of nanocrystallites at their surface layer. It also modifies the morphology and roughness of the work surface. Surface roughening by the SMAT has been reported previously in a smooth specimen, however in this study the starting point was a rough surface and a smoothening phenomenon is observed. In this paper, the mechanisms involved in the surface smoothening of AISI 316L stainless steel during the SMAT are elucidated. The SMAT was conducted on a specimen with a roughness of R_a = 3.98 μm for 0–20 min. The size of milling balls used in the SMAT was varied from 3.18 mm to 6.35 mm. The modification of subsurface microhardness, surface morphology, roughness and mass reduction of the specimen due to the SMAT were studied. The result shows the increasing microhardness of the surface and subsurface of the steel due to the SMAT. The impacts of milling balls deform the surface and produce a flat-like structure at this layer. Surface roughness decreases until its saturation is achieved in the SMAT. The mass reduction of the specimens is also detected and may indicate material removal or surface erosion by the SMAT. The size of milling ball is found to be the important feature determining the pattern of roughness evolution and material removal during the SMAT. From this study, two principal mechanisms in the evolution of surface morphology and roughness during the SMAT are proposed, i.e. indentation and surface erosion by the multiple impacts of milling balls. A comparative study with the results of the previous experiment indicates that the initial surface roughness has no influence in the work hardening by the SMAT but it does slightly on the saturated roughness value obtained by this treatment.     

The effect of nanostructured surface layer on the fatigue behaviors of a carbon steel

A nanostructured surface layer was formed on a carbon steel by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer of the SMATed sample was characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness and residual stress distribution along the depth from the SMATed surface layer were measured at the same time. Fatigue behaviors of the carbon steel subjected to the SMAT process were investigated. A nanostructured layer with average grains size of ∼12.7 nm was formed, of which microhardness is more than twice as high as that in matrix and residual compressive stress can reach about −400 MPa with maximum depth of ∼600 μm. The fatigue strength of as-received sample is 267 MPa and that of SMATed sample is 302 MPa based on fatigue life 5 × 10⁶ cycles. The SMAT process has improved the fatigue strength by as much as 13.1% for the carbon steel. It is shown that the SMAT is an effective method to render the material with the features, such as a nanostructured and work-hardened surface layer as well as compressive residual stresses, which can pronouncedly improve the fatigue strength of the carbon steel.     

Laser-based Surface Modifications of Aluminum and its Alloys

Aluminum (Al) and its alloys have widespread engineering applications because of their higher strength to weight ratio, ductility, and formability. However, in various applications, mechanical properties such as hardness, corrosion, wear, and fatigue resistance are prerequisite at near surface regions. Such localized modification without affecting the bulk phase can be performed by various surface-engineering approaches including electro-deposition, physical and chemical vapor depositions, thermal spraying, plasma spraying, and organic polymeric coatings. Delamination failure of such coatings from the substrate is often inevitable due to the difference in film-to-substrate elastic modulus associated with the aforementioned processes. Recently, researchers have adopted a new approach of laser surface engineering to modify the near surface regions of metallic substrate by laser beams resulting in superior mechanical properties with the formation of novel microstructures. In this article, the recent developments in the surface modification of Al and its alloy by laser treatment are reviewed. Processing parameters and resulting microstructures of Al and its alloys are briefly summarized, along with their impact on mechanical properties. Finally, this article concludes future research directions.     

碳化硅表面硅改性层的磁介质辅助抛光

为了实现碳化硅表面硅改性层的精密抛光,获得高质量光学表面,对磁介质辅助抛光技术进行研究。设计了适合碳化硅表面硅改性层抛光的磁介质辅助抛光工具,并对抛光工具的材料去除函数进行研究。针对材料去除函数的特性,对数控磁介质辅助抛光的驻留时间算法进行了研究。采用磁介质辅助抛光技术对碳化硅表面硅改性层平面样片进行了抛光实验。经过一次抛光迭代,碳化硅样片表面硅改性层的面形精度(均方根)由0.049λ收敛到0.015λ（λ=0.6328 μm）,表面粗糙度从2 nm改善至0.64 nm。实验结果表明基于矩阵代数的驻留时间算法有效,磁介质辅助抛光适合碳化硅表面硅改性层加工。     

The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 4140 steel by means of surface mechanical attrition treatment (SMAT). Plasma nitriding of SMAT and un-SMAT AISI 4140 steel was carried out by a low-frequency pulse excited plasma unit. A series of nitriding experiments has been conducted at temperatures ranging from 380 to 500 °C for 8 h in an NH₃ gas. The samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that a much thicker compound layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples after nitriding at the low temperature. In particular, plasma nitriding SMAT AISI 4140 steel at 380 °C for 8 h can produced a compound layer of 2.5 μm thickness with very high hardness on the surface, which is similar to un-SMAT samples were plasma nitrided at approximately 430 °C within the same time.     

Nontraditional manufacturing technique-Nano machining technique based on SPM

Nano machining based on SPM is a novel, nontraditional advanced manufacturing technique. There are three main machining methods based on SPM, i.e.single atom manipulation, surface modification using physical or chemical actions and mechanical scratching. The current development of this technique is summarized. Based on the analysis of mechanical scratching mechanism, a 5 μm micro inflation hole is fabricated on the surface of inertial confinement fusion (ICF) target. The processing technique is optimized. The machining properties of brittle material, single crystal Ge, are investigated. A micro machining system combining SPM and a high accuracy stage is developed. Some 2D and 3D microstructures are fabricated using the system. This method has broad applications in the field of nano machining.     

Nontraditional manufacturing technique— Nano machining technique based on SPM

Nano machining based on SPM is a novel, nontraditional advanced manufacturing technique. There are three main machining methods based on SPM, i.e. single atom manipulation, surface modification using physical or chemical actions and mechanical scratching. The current development of this technique is summarized. Based on the analysis of mechanical scratching mechanism, a 5 μm micro inflation hole is fabricated on the surface of inertial confinement fusion (ICF) target. The processing technique is optimized. The machining properties of brittle material, single crystal Ge, are investigated. A micro machining system combining SPM and a high accuracy stage is developed. Some 2D and 3D microstructures are fabricated using the system. This method has broad applications in the field of nano machining.     

Surface treatment of aluminum by high current pulsed electron beam

The development of advanced technology based on the interaction of high current pulsed electron beam (HCPEB) with solid materials is very important at present time [R. Stark, J. Christiansen, et al., IEEE Trans. Plasma Sci. 23 (3) (1995) 258–264; T. Witke, A. Lenk, B. Schultrich, IEEE Trans. Plasma Sci. 24 (1) (1996) 61–62]. When the concentrated electron flux are acting on a material, superfast processes such as heating, melting, and solidification, as well as dynamic stresses induced in the processes impart surface layer with improved physical, chemical and mechanical properties unattainable with conventional surface treatment methods. This paper reports on an investigation on pure aluminum treated by “Nadezhda-2” HCPEB device. The extreme surface is melted to a depth of about 1 μm at the input energy density of 3 J/cm². Craters, micrometers in size, are formed in the melted surface. Vacancy and dislocations are significantly increased near the surface as a result of the electron bombardment. Based on the microhardness distribution measured on the cross-section, it has been established that the thickness of the modified layer is several hundreds of micrometers beneath the surface, significantly greater than that of the heat-affected zone. The experimental results are compared with those obtained by solving numerically the heat and stress equations, with finite difference and finite element methods, taking into account of the processes of melting and changing of physics parameters related to changing temperature. Satisfactory agreement between experimental and theoretical data is obtained.     

等离子体表面梯度硅沉积对环氧树脂电气性能的影响

环氧树脂作为常见的绝缘材料,在高压直流电场作用下易在其表面积累电荷,发生电场畸变,导致材料绝缘性能下降,影响电力系统运行可靠性。为改善气固界面的电荷特性和绝缘性能,在大气压等离子体射流技术的基础上,对环氧树脂表面进行等离子体梯度硅沉积处理。对改性前后环氧树脂表面理化特性、表面电导率、表面电荷消散和沿面耐压特性进行了多参数测量。实验结果表明,梯度改性对材料表面的物理形貌和化学组分均有明显影响,不同区域的电导率实现了梯度分布,加快了表面电荷消散速度,表面陷阱能级变浅;梯度改性后的样品沿面闪络电压提升幅度可达30.16%。通过等离子体表面梯度硅沉积处理能够改善环氧树脂表面电气性能,在高压直流设备的绝缘设计方面具有广阔的应用前景。     

In vitro and in vivo mineralization and osseointegration of nanostructured Ti6Al4V

Surface characteristics of the orthopedic implants remarkably affect the corresponding biological responses. In this study, Ti6Al4V alloy was subjected to the surface mechanical attrition treatment (SMAT) in order to achieve favorable surface properties. The SMAT substrates exhibited a highly hydrophilic surface with nanograins about 20–40 nm. Then, the in vitro and in vivo mineralizations were evaluated on the untreated and SMAT processed substrates. The in vitro experimental results showed a significant increase in the deposition of calcium-containing minerals on the SMAT surface both with and without osteoblasts. The in vivo experiments also revealed a higher bone mineral apposition on the SMAT processed implants after 8 and 12 weeks post operation. The well-organized bone formation on the SMAT substrates indicated an enhanced osseointegration on the bone-implant interface. Therefore, it was suggested that the obvious improvements of biomineralization and osseointegration were attributed to the nanostructure features on the SMAT surface, as well as the higher surface hydrophilicity.     

Study micromechanism of surface planarization in the polishing technology using numerical simulation method

With the development of semiconductor industry, the chemical mechanical polishing technology has already became the main stream method of realize the surface global flatness. In order to understanding physical essence underlying this technology, the author carried out nanometer polishing experiment of silicon wafer using molecular dynamics (MD) simulation method. The simulation result shows that larger slurry grain generate much more vacancy, dislocation, larger residual stress and intensive plastic deformation than that of small one although the larger grain acquire better surface quality.     

Structural characteristics of nanocrystalline copper after carbon ion implantation

A gradient structure was produced in a pure copper plate by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer was reduced to nanoscale and the grain size increased gradually along the depth of the treated sample. In situ transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observation was performed on the nanocrystalline copper after implantation of carbon. Carbon atoms first precipitated along the edges of the copper substrate or at the surface, then formed amorphous carbon layers. Subsequently, onion-like fullerenes were formed under electron-beam irradiation. The effects of ion implantation, electron beam irradiation, nanostructure of the substrate and interaction of C and Cu atoms on the formation of the onion-like fullerenes are discussed.     

Surface structure of alumina ceramics during irradiation by a pulsed electron beam

The structural transformations that occur in the near-surface layer in alumina ceramics during irradiation by a pulsed electron beam generated by a forevacuum plasma electron source are studied. The modification of the surface properties of the ceramics is shown to be caused by the formation of regions consisting of close-packed and identically oriented crystallites within every grain. The crystallites are elongated: their length and width are 0.5–1.5 μm and the transverse size is 0.1–0.2 μm.     

The surface modification of cellulose fibres for use as reinforcing elements in composite materials

The interest in using cellulose fibres as reinforcing elements in composite materials based on polymeric matrices is constantly growing, mainly because of the many advantages associated with this renewable material. However, the preparation of cellulose-based composites is perturbed by the highly hydrophilic character of the fibres, which is associated with a low interfacial compatibility with hydrophobic polymeric matrices, as well as with a loss of mechanical properties after moisture uptake. In order to reduce the hydrophilic character of cellulose fibres and to improve the strength of their adhesion to the matrix, it is necessary to undertake a structural modification of their surface. Several approaches have been studied, namely (i) physical treatments such as corona, plasma, laser, vacuum ultraviolet and γ-radiation treatments; (ii) chemical grafting by direct condensation, including surface compatibilisation with hydrophobic moieties and co-polymerisation with the matrix. The copolymerisation approach called upon different strategies: (i) The use of bi-functional molecules capable of reacting with the OH groups of the cellulose surface and leaving the second functions available for further exploitation; (ii) The direct activation of the surface and the subsequent graftingfrom polymerisation; and (iii) The condensation of organometallic compounds, followed by their coupling with suitable reactive molecules or macromolecules. The characterisation of the modified surfaces involved a variety of techniques, including elemental analysis, contact angle measurements, inverse gas chromatography, X-ray photoelectron and FTIR spectroscopy, water uptake, etc. The present survey reviews the different approaches proposed in the literature and critically assesses their respective merits and drawbacks.     

Surface elasticity and size effect on the vibrational behavior of silicon nanoresonators

Predominance of nano-scale effects observed in material behavior at small scales requires implementation of new simulation methods which are not merely based on classical continuum mechanic. On the other hand, although the atomistic modeling methods are capable of modeling nano-scale effects, due to the computational cost, they are not suitable for dynamic analysis of nano-structures. In this research, we aim to develop a continuum-based model for nano-beam vibrations which is capable of predicting the results of molecular dynamics (MD) simulations with considerably lower computational effort. In this classical-based modeling, the surface and core regions are taken to have different mechanical properties, where core atoms are assumed to have macroscale properties whereas surface layer is showing a different elastic modulus from the core components. By estimating physical parameters of proposed classical model using molecular dynamics results and the genetic algorithm, calibrated classical Euler–Bernoulli and Timoshenko beam models are developed. The results demonstrates that a Timoshenko beam model incorporating surface effects and having calibrated parameters, is able to provide almost the same results as molecular dynamics method which can be used to predict the vibrational behavior of nano-beams at different scales from nano to macro.     

Online monitoring of thermo-cycles and its correlation with microstructure in laser cladding of nickel based super alloy

Laser cladding, basically a weld deposition technique, is finding applications in many areas including surface coatings, refurbishment of worn out components and generation of functionally graded components owing to its various advantages over conventional methods like TIG, PTA etc. One of the essential requirements to adopt this technique in industrial manufacturing is to fulfil the increasing demand on product quality which could be controlled through online process monitoring and correlating the signals with the mechanical and metallurgical properties. Rapid thermo-cycle i.e. the fast heating and cooling rates involved in this process affect above properties of the deposited layer to a great extent. Therefore, the current study aims to monitor the thermo-cycles online, understand its variation with process parameters and its effect on different quality aspects of the clad layer, like microstructure, elemental segregations and mechanical properties. The effect of process parameters on clad track geometry is also studied which helps in their judicious selection to deposit a predefined thickness of coating. In this study Inconel 718, a nickel based super alloy is used as a clad material and AISI 304 austenitic steel as a substrate material. The thermo-cycles during the cladding process were recorded using a single spot monochromatic pyrometer. The heating and cooling rates were estimated from the recorded thermo-cycles and its effects on microstructures were characterised using SEM and XRD analyses. Slow thermo-cycles resulted in severe elemental segregations favouring Laves phase formation and increased γ matrix size which is found to be detrimental to the mechanical properties. Slow cooling also resulted in termination of epitaxial growth, forming equiaxed grains near the surface, which is not preferred for single crystal growth. Heat treatment is carried out and the effect of slow cooling and the increased γ matrix size on dissolution of segregated elements in metal matrix is studied.     

非晶合金的高通量制备与表征

非晶合金是一种不同于传统合金材料的新型合金,其突出的机械、物理、化学等性能在工程应用领域备受关注.作为一种具有无序原子结构的新型合金,非晶合金中蕴含的丰富的物理现象在基础研究领域也备受瞩目.非晶合金往往由多个组元构成,这给成分优化和性能调制带来了巨大的挑战.材料基因组方法是最近发展起来的新方法,通过高通量制备和结构表征以及性能筛选有望加快新型非晶合金材料的探索,在高通量表征中获得的大量实验数据可以帮助人们理解非晶合金中的科学问题.本文主要介绍高通量制备和表征在非晶合金中的应用,通过列举典型案例,展示通过高通量方法探索新型非晶合金材料的作用.     

Surface characterization of collagen/elastin based biomaterials for tissue regeneration

Collagen and elastin are the main proteins of extracellular matrix. Collagen plays a crucial role in tensile strength of tissues, whereas elastin provides resilience to many organs. Both biopolymers are readily available and biocompatible. These properties point out that collagen and elastin are good components of materials for many potential medical applications. The surface properties of biomaterials play an important role in biomedicine as the majority of biological reactions occur on the surface of implanted materials. One of the methods of surface modification is UV-irradiation. The exposition of the biomaterial on ultraviolet light can alterate surface properties of the materials, their chemical stability, swelling properties and mechanical properties as well.The aim of our work was to study the surface properties and biocompatibility of new collagen/elastin based biomaterials and consideration of the influence of ultraviolet light on these properties.The surface properties of collagen/elastin based biomaterials modified by UV-irradiation were studied using the technique of atomic force microscopy (AFM) and contact angle measurements. On the basis of the results the surface free energy and its polar component was calculated using Owens-Wendt method. To assess the biological performance of films based on collagen, elastin and their blends, the response of 3T3 cell was investigated.It was found that the surface of collagen/elastin film is enriched in less polar component - collagen. Exposition on UV light increases polarity of collagen/elastin based films, due to photooxidation process. The AFM images have shown that topography and roughness of the materials had been also affected by UV-irradiation. The changes in surface properties influence on interaction between the material's surface and cells. The investigation of 3T3 cells grown on films based on collagen, elastin and their blends, leads to the conclusion that higher content of elastin in biomaterial promotes the cell adhesion and their viability on the surface. Also the suitable dose of UV light (1, 2 h) improves the biocompatibility of the materials.     

Establishing the microstructure-strengthening correlation in severely deformed surface of titanium

Surface nanostructuring of engineering materials can be utilised to enhance materials performance for various applications. The aim of this work was to investigate the evolution of microstructure and its correlation with strengthening mechanisms in nanocrystalline commercially pure titanium (cp-Ti) produced by surface mechanical attrition treatment (SMAT). The individual contributions of dislocation slip and twining as the deformation mechanisms during SMAT have been quantified using X-ray line profile analysis and corroborated with transmission electron microscopy and electron backscattered diffraction techniques. It is found that twining is operative only in the early stages of deformation. The absence of twin–twin intersections suggests that twining is not directly responsible for the initial refinement of grain size. Dislocation slip is the major deformation mode, which leads to the refinement of the microstructure by forming low-angle lamellar boundaries. Continuous dynamic recrystallisation is demonstrated to be the mechanism of nanocrystallisation in cp-Ti using detailed microscopic analysis. In contrast to previous studies, which have neglected the contribution of Taylor strengthening, it is observed that a combination of Hall–Petch and Taylor relationships can explain the strength only if separate set of parameters K (Hall–Petch constant) and α (geometrical factor in Taylor relationship) are used for the nanocrystalline surface and severely deformed sub-surface of cp-Ti. Taken together, this work provides new insights into the underlying mechanisms for engineering nanocrystalline materials.