Nanometer-scale Surface Roughness Research Articles

Theoretical and experimental investigation of ultrasonic cutting kinematics and its effect on chip formation and surface generation in high-frequency ultrasonic vibration-assisted diamond cutting

Ultrasonic vibration-assisted cutting (UVAC) is regarded as a feasible technology to machine difficult-to-cut materials. High-frequency ultrasonic vibration-assisted cutting (HFUVAC), with a working frequency of more than low and medium frequency (20–60 kHz), has been reported to improve material machinability and prolong tool life. This paper presents a comprehensive investigation of the ultrasonic cutting kinematics and the generated chip/surface formation process in HFUVAC of a 316l stainless steel workpiece. First, the ultrasonic cutting kinematics was analyzed and verified by comparing the experimental and theoretical surface texture under different nominal cutting speeds. Based on the ultrasonic cutting kinematics and the simulated strain/stress distributions, the chip and surface formation between conventional cutting (CC) and HFUVAC was analyzed. More importantly, the incremental cutting mode was defined when the cutting stroke, namely the effective cutting length in one vibration cycle was less than 800 nm. The results show that in the incremental cutting mode, defect-free surface was achieved due to suppressed large deformation and friction action. Finally, HFUVAC of the sinusoidal microstructure was performed under the incremental cutting mode, achieving optical requirements with nanometer-scale surface roughness and submicrometric form accuracy, which validates the technical feasibility in HFUVAC of micro-structured surfaces.

Elimination of surface/subsurface defects on additively manufactured AlSi10Mg mirrors through nano-second laser irradiation.

Metal mirrors have attracted increasing attention for satisfying the growing demands for high-performance optics in airborne and spaceborne remote sensing systems. Additive manufacturing has enabled the development of metal mirrors with reduced weight and improved strength. AlSi10Mg is the most widely used metal for additive manufacturing. Diamond cutting is an effective method for obtaining nanometer-scale surface roughness. However, the surface/subsurface defects of additively manufactured AlSi10Mg deteriorate the surface roughness. Conventionally, AlSi10Mg mirrors used in near-infrared and visible systems are plated with NiP layers to improve the surface polishing performance; however, this leads to the bimetallic bending because of the different coefficients of thermal expansion between the NiP layers and AlSi10Mg blanks. In this study, a method of nanosecond-pulsed laser irradiation is proposed to eliminate the surface/subsurface defects of AlSi10Mg. The microscopic pores, unmolten particles and two-phase microstructure of the mirror surface were eliminated. The mirror surface exhibited better polishing performance, and it could be smoothly polished to a nanometer-scale surface roughness. The mirror exhibits strong temperature stability owing to the elimination of the bimetallic bending caused by the NiP layers. It is expected that the mirror surface fabricated in this study can satisfy the requirements for near-infrared or even visible applications.

Molecular Dynamics Simulations on Epoxy/Silica Interfaces Using Stable Atomic Models of Silica Surfaces

The adhesion between silica surfaces and epoxy resins was investigated via molecular dynamics (MD) simulations with stable atomic models of silica substrates prepared by density functional theory (DFT) calculations and reactive force field (ReaxFF) MD simulations. We aimed to develop reliable atomic models for evaluating the effect of nanoscale surface roughness on adhesion. Three consecutive simulations were performed: (i) stable atomic modeling of silica substrates; (ii) network modeling of epoxy resins by pseudo-reaction MD simulations; and (iii) virtual experiments via MD simulations with deformations. We prepared stable atomic models of OH- and H-terminated silica surfaces based on a dense surface model to consider the native thin oxidized layers on silicon substrates. Moreover, a stable silica surface grafted with epoxy molecules as well as nano-notched surface models were constructed. Cross-linked epoxy resin networks confined between frozen parallel graphite planes were prepared by pseudo-reaction MD simulations with three different conversion rates. Tensile tests using MD simulations indicated that the shape of the stress-strain curve was similar for all models up to near the yield point. This behavior indicated that the frictional force originated from chain-to-chain disentanglements when the adhesion between the epoxy network and silica surfaces was sufficiently strong. MD simulations for shear deformation indicated that the friction pressures in the steady state for the epoxy-grafted silica surface were higher than those for the OH- and H-terminated surfaces. The slope of the stress-displacement curve was steeper for surfaces with deeper notches (approximately 1 nm in depth), although the friction pressures for the examined notched surfaces were similar to those for the epoxy-grafted silica surface. Thus, nanometer-scale surface roughness is expected to have a large impact on the adhesion between polymeric materials and inorganic substrates.

Theoretical modeling and machining experiments of cylindrical microstructure assisted by single-point diamond turning

Microstructure requires nanometer-scale surface roughness and micro- or even sub-micron form error accuracy in different applications. Two kinds of modeling theories and methods of micro-feature of rotating body and non-rotating body are studied, and the corresponding tool turning trajectory planning method is put forward. In order to process the designed micro-feature structure successfully and avoid the interference and overcutting between tool and workpiece caused by improper selection of tool parameters, the cutting parameters are analyzed and two error theories are proposed. Then, a precision-driven turning trajectory planning method is obtained, which can optimize the trajectory by adjusting turning parameters according to the setting errors. The experiments are carried out to verify the proposed theories, and the results of measurements are that the surface roughness and surface form accuracy of the cylindrical sine wave groove micro-feature surface are 0.1714 μm and 1.32 μm, respectively. The surface roughness and surface form accuracy of the cylindrical sinusoidal mesh micro-feature surface are 0.1625 μm and 1.8 μm, respectively. The results meet expectations and verify the reliability of the error theory and the trajectory optimization theory.

Diffusion bonding effects on the adhesion of tungsten dust on tungsten surfaces

Abstract High temperature excursions have the potential to strongly enhance the room temperature adhesion of tokamak dust. Planar tungsten substrates containing adhered nearly monodisperse spherical tungsten dust have been exposed to linear plasmas and vacuum furnaces. Prolonged thermal treatments of varying peak temperature and constant duration were followed by room temperature adhesion measurements with the electrostatic detachment method. Adhesive forces have been observed to strongly depend on the thermal pre-history, greatly increasing above a threshold temperature. Adhesive forces have been measured up to an order of magnitude larger than those of untreated samples. This enhancement has been attributed to atomic diffusion that slowly eliminates the omnipresent nanometer-scale surface roughness, ultimately switching the dominant interaction from long-range weak van der Waals forces to short-range strong metallic bonding.

Polishing characteristics of Nd:YAG ceramics with various Nd-dopant concentrations

Neodymium-doped polycrystalline yttrium aluminum garnet (Nd:YAG) ceramics require nanometer-scale surface roughness when used as laser gain media to minimize light scattering. Variations in grain size and crystallographic orientation can result in uneven material removal between grains and excessive material removal at the grain boundaries, hindering the smoothing of this material. The introduction of rare earth ion dopants, necessary for laser oscillation, can influence these parameters. This paper explores the effects of varying dopant concentration on grain structure and those effects on the polishing characteristics of polycrystalline Nd:YAG ceramics.

Enhanced Immunoadsorption on Imprinted Polymeric Microstructures with Nanoengineered Surface Topography for Lateral Flow Immunoassay Systems.

Lateral flow immunoassay devices have revolutionized the style of on-site disease detection and point-of-care testing in the past few decades. The surface nanotopography of a solid substrate is a dominant parameter in the efficiency of antibody immobilization, but precise control over surface roughness has not been fully investigated. Here we presented lateral flow immunoassay platforms with nanometer-scale surface roughness, reproducibly engineered using thermal nanoimprinting lithography, and investigated the effects of surface nanotopography on immunoadsorption and immunoassay performance. We fabricated three types of imprinted polycarbonate sheets with microcone array structures having different degrees of surface roughness using three types of molds fabricated by micromachining or laser ablation. The structures fabricated by laser-ablated nickel mold exhibited numerous bumps measuring several tens of nanometers, which enhanced antibody adsorption. We performed sandwich immunoassays of C-reactive protein in serum samples and achieved highly sensitive detection with a detection limit of ∼0.01 μg mL-1 and a broad dynamic range. The present results provide useful information on the remarkable effect of nanoengineered surfaces on biomolecule adsorption, and the platforms presented here will widen the applicability and versatility of lateral flow immunoassay devices.

Transparent dust removal coatings for solar cell on mars and its Anti-dust mechanism

Transparent hybrid coatings were prepared from methyltrimethoxysilane (MTMS) and colloidal silica nanoparticles (SiO2) by a sol-gel process. Influences of colloidal silica content and aging time on the surface morphology, surface composition, and the properties such as transparency, water contact angles (WCA), and pencil hardness, of the hybrid coatings were systematically investigated. Coatings with WCA as high as 110–120° and pencil hardness of 4–5 H could be achieved at appropriate colloidal silica content and aging time. The dust removal performance of the hybrid coatings was examined by vertical tilting of the dust-deposited coatings using volcanic ash as the standard dust. When the WCA was over about 100°, the hybrid coatings exhibited excellent dust removal performance under gravity, regardless of the quantity of the dust deposited. Oxygen plasma etching experiments and the dust removal tests in dry chamber indicated that the nanometer-scale surface roughness could endow the hybrid coatings with the good dust removal performance. The hydrophobic methyl groups can further enhance the dust removal performance. The hybrid coating with MTMS/SiO2 mass ratio of 1.3:1 was cast on the coverslips of the solar cell for Mars, and undergone two durability series tests, i.e. thermal shock/humidity series (THS) test and high-low temperature cycle/electron beam radiation/vacuum ultraviolet radiation series (HEVS) test. The anti-dust coverslips showed good durability in HEVS tests and dust removal percentage of 75.8% after HEVS test, implying its good service life on Mars.

Elucidating the surface geometric design of hydrophobic Australian Eucalyptus leaves: experimental and modeling studies

Three Australian native Eucalyptus species, i.e., Eucalyptus woodwardii, Eucalyptus pachyphylla and Eucalyptus dolorosa, were investigated, for the first time, with respect to the hydrophobicity of their leaves. It is well established that these leaves exhibit exceptionally high water repellency, in addition to an extraordinary ability to retain water, albeit their specific wetting mechanisms are still poorly understood. To identify the critical factors underlying this phenomenon, the surface topography of these leaves was subjected to micro-examination (SEM). Micro- and nanometer scale surface roughness was revealed, resembling that of the quintessential “lotus effect”. Surface free energy analysis was performed on two models based on the surface topographies of the study Eucalyptus species and lotus, in order to study wetting transitions on these specific microscopic surface features. The influence of surface geometrical parameters, such as edge-to-edge distance, base radius and cylindrical height, on surface free energy with different liquid penetration depths was studied with these two models. Larger energy barriers and smaller liquid-solid contact areas were more influential in the calculations for the lotus than for Eucalyptus. The information obtained from these two models may be useful for guiding the design of novel artificial surfaces in the collection and transport of micro-volume liquids.

Study on non-isothermal hot-embossing of polymer micro-prism array with efficiency and accuracy

The isothermal hot-embossing leads to inefficiency because of heating and cooling whole polymer workpiece for precision micro-forming. Hence, a non-isothermal hot-embossing is proposed by compressing and melting a micron-scale surface layer into heated core microgrooves without changing the solid-state workpiece. The objective is to understand how the upper layer micro-prism array is formed on large-size polymer surface with regard to core microgroove parameters and hot-embossing variables. First, the micro-prism shape and sizes were in-process modeled through a hybrid of mechanical compression and hot-melt flowing inside core microgrooves; then the micro-grinding with the trued wheel micro-tip was employed to fabricate various microgrooves on macro-size die cores; finally, the micro-forming accuracy and efficiency were experimentally investigated in hot-embossing. It is shown that the integrated micro-prism array with nanometer-scale surface roughness is rapidly generated inside accurate and smooth core microgrooves with 1–3 seconds. The micro-forming accuracy is dominated by the hot-embossing variables under the limitations of core microgroove parameters. Moreover, the hot-embossed surface roughness reaches 10–30 nm through the hot-melt layer flowing even if the core surface roughness is larger. It is confirmed that the rapid micro-forming sizes and surface roughness may be predicted by the micro-nano core topographies and the hot-embossing variables.

Bone marrow derived mesenchymal stem cell response to the RF magnetron sputter deposited hydroxyapatite coating on AZ91 magnesium alloy

The aim of this study was to investigate adhesion of bone marrow stromal cells (BMSCs) and in vitro degradation of hydroxyapatite (HA) coatings deposited on the surface of magnesium alloy (AZ91). The uniform and pore-free HA coating was successfully obtained via RF magnetron sputter deposition at the substrate bias of −25 and substrate bias of −100 V followed by post-deposition annealing. The structural features of the HA coating have been characterized and biological performance of the HA coating has been examined via in vitro test. The X-ray diffraction and Energy dispersive X-ray analysis data revealed that the phase and elemental composition of the deposited HA coatings were similar, but the surface roughness and morphology were influenced by the substrate bias voltage. The coating deposited at −25 V consists of multiple grain-boundaries. In contrast, no grain boundary but rather flat surface was observed for the sample deposited at −100 V. Atomic force microscopy (AFM) was used to obtain topographic images of the obtained coatings and quantitative assessment of the surface roughness. The roughness parameters of the deposited coatings were found to display nanometer scale surface roughness (Sa and Sq) from 92 to 130 nm over areas of 5 × 5 μm2. The in vitro direct culture experiment showed that the HA coating deposited at the substrate bias of −100 V significantly reduced the release of Mg ions in post-culture media and revealed higher BMSCs adhesion density than that on the HA coating deposited at the bias of −25 V and the uncoated AZ91 alloy. Thus, this study demonstrated that the negative pulse substrate bias applied for HA coating deposition allows to improve the surface features of the AZ91 magnesium alloy and enhance BMSCs adhesion in vitro.

3D printed optics with nanometer scale surface roughness

Complex optical devices including aspherical focusing mirrors, solar concentrator arrays, and immersion lenses were 3D printed using commercial technology and experimentally demonstrated by evaluating surface roughness and shape. The as-printed surfaces had surface roughness on the order of tens of microns. To improve this unacceptable surface quality for creating optics, a polymer smoothing technique was developed. Atomic force microscopy and optical profilometry showed that the smoothing technique reduced the surface roughness to a few nanometers, consistent with the requirements of high-quality optics, while tests of optical functionality demonstrated that the overall shapes were maintained so that near theoretically predicted operation was achieved. The optical surface smoothing technique is a promising approach towards using 3D printing as a flexible tool for prototyping and fabrication of miniaturized high-quality optics.

Three-Dimensional Integrated X-ray Diffraction Imaging of a Native Strain in Multi-Layered WSe2.

Emerging two-dimensional (2-D) materials such as transition-metal dichalcogenides show great promise as viable alternatives for semiconductor and optoelectronic devices that progress beyond silicon. Performance variability, reliability, and stochasticity in the measured transport properties represent some of the major challenges in such devices. Native strain arising from interfacial effects due to the presence of a substrate is believed to be a major contributing factor. A full three-dimensional (3-D) mapping of such native nanoscopic strain over micron length scales is highly desirable for gaining a fundamental understanding of interfacial effects but has largely remained elusive. Here, we employ coherent X-ray diffraction imaging to directly image and visualize in 3-D the native strain along the (002) direction in a typical multilayered (∼100-350 layers) 2-D dichalcogenide material (WSe2) on silicon substrate. We observe significant localized strains of ∼0.2% along the out-of-plane direction. Experimentally informed continuum models built from X-ray reconstructions trace the origin of these strains to localized nonuniform contact with the substrate (accentuated by nanometer scale asperities, i.e., surface roughness or contaminants); the mechanically exfoliated stresses and strains are localized to the contact region with the maximum strain near surface asperities being more or less independent of the number of layers. Machine-learned multimillion atomistic models show that the strain effects gain in prominence as we approach a few- to single-monolayer limit. First-principles calculations show a significant band gap shift of up to 125 meV per percent of strain. Finally, we measure the performance of multiple WSe2 transistors fabricated on the same flake; a significant variability in threshold voltage and the "off" current setting is observed among the various devices, which is attributed in part to substrate-induced localized strain. Our integrated approach has broad implications for the direct imaging and quantification of interfacial effects in devices based on layered materials or heterostructures.

Imaging out-of-plane polarized emission patterns on gap mode SERS substrates: from high molecular coverage to the single molecule regime.

Gap mode surface-enhanced Raman scattering (SERS) substrates are created when a single nanoparticle is deposited on a thin metal film, creating a region of significant electromagnetic field enhancement in the gap between the nanoparticle and the film due to excitation of a vertically-oriented, out-of-plane dipole plasmon mode, e.g. the gap plasmon. When molecules are located in the gap and couple to the gap plasmon mode, the resulting emission is polarized perpendicular to the thin film, generating SERS emission patterns that have a characteristic donut shape. We analyze these SERS emission patterns using a dipole emission model and extract out-of-plane and in-plane emission angles associated with the gap plasmon mode. Fluctuations in both of these angles reveal dynamic heterogeneity due to molecular motion within the hot spot that changes as a function of molecular coverage. We also reveal static heterogeneity associated with structural defects in the thin film component of the gap mode substrates, indicating that even nanometer-scale surface roughness can impact the quality of gap mode emission.

Effect of nanometer scale surface roughness of titanium for osteoblast function

Surface roughness is an important property for metallic materials used in medical implants or other devices. The present study investigated the effects of surface roughness on cellular function, namely cell attachment, proliferation, and differentiation potential. Titanium (Ti) discs, with a hundred nanometer- or nanometer-scale surface roughness (rough and smooth Ti surface, respectively) were prepared by polishing with silicon carbide paper. MC3T3-E1 mouse osteoblast-like cells were cultured on the discs, and their attachment, spreading area, proliferation, and calcification were analyzed. Cells cultured on rough Ti discs showed reduced attachment, proliferation, and calcification ability suggesting that the surface inhibited osteoblast function. The findings can provide a basis for improving the biocompatibility of medical devices.

Adhesion measurements for tungsten dust deposited on tungsten surfaces

The first experimental determination of the pull-off force for tungsten dust adhered to tungsten surfaces is reported. Dust deposition is conducted with gas dynamics methods in a manner that mimics sticking as it occurs in the tokamak environment. Adhesion measurements are carried out with the electrostatic detachment method. The adhesion strength is systematically characterized for spherical micron dust of different sizes and planar surfaces of varying roughness. The experimental pull-off force is nearly two orders of magnitude smaller than the predictions of contact mechanics models, but in strong agreement with the Van der Waals formula. A theoretical interpretation is provided that invokes the effects of nanometer-scale surface roughness for stiff materials such as tungsten.

Superhydrophilic TiO2 thin film by nanometer scale surface roughness and dangling bonds

A remarkable enhancement in the hydrophilic nature of titanium dioxide (TiO2) films is obtained by surface modification in DC-glow discharge plasma. Thin transparent TiO2 films were coated on glass substrate by sol–gel dip coating method, and exposed in DC-glow discharge plasma. The plasma exposed TiO2 film exhibited a significant change in its wetting property contact angle, which is a representative of wetting property, has reduced to considerable limits 3.02° and 1.85° from its initial value 54.40° and 48.82° for deionized water and ethylene glycol, respectively. It is elucidated that the hydrophilic property of plasma exposed TiO2 films dependent mainly upon nanometer scale surface roughness. Variation, from 4.6nm to 19.8nm, in the film surface roughness with exposure time was observed by atomic force microscopy (AFM). Analysis of variation in the values of contact angle and surface roughness with increasing plasma exposure time reveal that the surface roughness is the main factor which makes the modified TiO2 film superhydrophilic. However, a contribution of change in the surface states, to the hydrophilic property, is also observed for small values of the plasma exposure time. Based upon nanometer scale surface roughness and dangling bonds, a variation in the surface energy of TiO2 film from 49.38 to 88.92mJ/m2 is also observed. X-ray photoelectron spectroscopy (XPS) results show change in the surface states of titanium and oxygen. The observed antifogging properties are the direct results of the development of the superhydrophilic wetting characteristics to TiO2 films.

An experimental study on the correlation of polishing force and material removal for bonnet polishing of cobalt chrome alloy

Cobalt chrome alloys are the most extensively used biomaterials for manufacturing artificial prostheses, which need nanometer scale surface roughness and micrometer scale form tolerance to extend their lifespan in the tough environment of the human body. In order to machine bearing surfaces to sufficiently high accuracy, the way in which material is removed by the final finishing, such as bonnet polishing, must be completely understood. This study has experimentally investigated the influence of process parameters (precess angle, head speed, tool offset, and tool pressure) on the polishing forces as well as the material removal in bonnet polishing of a medical grade cobalt chrome alloy. Experimental results indicate that normal force increases with the increase in the precess angle, head speed and tool offset, but varies only slightly with the variation of the tool pressure. Tangential force increases with the increase in the precess angle and tool offset while it shows little variation with the change of the head speed and tool pressure. It is concluded that both normal force and tangential force can contribute to the material removal rate, but tangential force is found to be more correlated with the width of the influence function while normal force has a stronger correlation with the maximal height of the influence function.

Osteoinduction on Acid and Heat Treated Porous Ti Metal Samples in Canine Muscle

Samples of porous Ti metal were subjected to different acid and heat treatments. Ectopic bone formation on specimens embedded in dog muscle was compared with the surface characteristics of the specimen. Treatment of the specimens by H2SO4/HCl and heating at 600°C produced micrometer-scale roughness with surface layers composed of rutile phase of titanium dioxide. The acid- and heat-treated specimens induced ectopic bone formation within 6 months of implantation. A specimen treated using NaOH followed by HCl acid and then heat treatment produced nanometer-scale surface roughness with a surface layer composed of both rutile and anatase phases of titanium dioxide. These specimens also induced bone formation after 6 months of implantation. Both these specimens featured positive surface charge and good apatite-forming abilities in a simulated body fluid. The amount of the bone induced in the porous structure increased with apatite-forming ability and higher positive surface charge. Untreated porous Ti metal samples showed no bone formation even after 12 months. Specimens that were only heat treated featured a smooth surface composed of rutile. A mixed acid treatment produced specimens with micrometer-scale rough surfaces composed of titanium hydride. Both of them also showed no bone formation after 12 months. The specimens that showed no bone formation also featured almost zero surface charge and no apatite-forming ability. These results indicate that osteoinduction of these porous Ti metal samples is directly related to positive surface charge that facilitates formation of apatite on the metal surfaces in vitro.

Micromilling characteristics and electrochemically assisted reconditioning of polycrystalline diamond tool surfaces for ultra-precision machining of high-purity SiC

The production of extremely thick silicon carbide (SiC) has recently become possible with the advent of a specific chemical vapor deposition process. Ultra-precision machining of high-purity SiC has been performed by using a polycrystalline diamond (PCD) micromilling tool to investigate the machining characteristics. Results indicate that a high-quality surface (Ra=1.7nm) can be obtained when the removed chips are thin enough to achieve ductile mode machining. Micron-sized wells and groove structures with nanometer-scale surface roughness were successfully machined by using the PCD tool. In addition, a new electrochemically assisted surface reconditioning process has been proposed to remove the contaminant material adhered onto the PCD tool surfaces after prolonged machining.