Nano-mechanical and tribological characteristics of ultra-thin amorphous carbon film investigated by afm

The mechanical as well as tribological characteristics of coating films as thin as a few nm become more crucial as applications in micro-systems grow. Especially, the amorphous carbon film has a potential to be used as a protective layer for micro-systems. In this work, quantitative evaluation of nano-indentation, scratching, and wear tests were performed on the 7nm thick amorphous carbon film using an Atomic Force Microscope (AFM). It was shown that AFM-based nano-indentation using a diamond coated tip can be feasibly utilized for mechanical characterization of ultra-thin films. Also, it was found that the critical load where the failure of the carbon film occurred was about 18μN by the ramp load scratch test. Finally, the wear experimental results showed that the quantitative wear rate of the carbon film ranged 10^-9~10^-8 mm³/N cycle. These experimental methods can be effectively utilized for a better understanding the mechanical and tribological characteristics at the nano-scale.     

Effects of Atomic Force Microscope Silicon Tip Geometry on Large-Scale Nanomechanical Modification of the Polymer Surface

The shape of an atomic force microscope (AFM) silicon tip has a significant effect on the mechanical modification of the polymer surface, especially for a longer sliding distance of from several to several hundreds of millimeters. In this work, a pyramidal silicon tip was used to cut into the polymethyl methacrylate (PMMA) surface, forming nanogrooves with a linear sliding distance of about 80 mm and wear box structures with a total tip sliding distance of 1,024 mm. The effects of the tip edges and the tip radius on the form of the wear debris chips, wear depth, and debris transferred to the tip were investigated. The experimental results showed that four sides of the tip influenced the morphology of the removed material. Adhesion appeared to play a role in the tip wear mechanism by successive removal of SiO₂ layers during transfer of adhered PMMA from the tip to the surface. The tip radius generally increased with sliding distance. Simultaneously, adhesion of the removed materials to the tip induced a larger tip radius and a sharper tip was revealed as dropping off of the materials during the test from time to time. Thus, with the same normal load the worn tip may induce failure of the machining process. The results presented in this study provide insight into long-term nanoscratch/wear and nanomechanical machining of glassy polymer surfaces with a silicon AFM tip.     

Processing outcomes of atomic force microscope tip-based nanomilling with different trajectories on single-crystal silicon

Atomic force microscope (AFM) tip-based nanomilling is an emerging technology for machining nanostructures with a high rate of material removal and slight tip wear. However, subsurface damage induced by nanomilling is poorly understood. In this study, we investigated nanomilling-induced subsurface damage of single-crystal silicon experimentally and with molecular dynamics simulations. We studied the effect of clockwise and anticlockwise trajectories on the nanochannel morphology. The clockwise trajectory resulted in a ‘U’-shaped nanochannel at a relatively low normal load. Transmission electron microscopy and Raman spectroscopy analysis of the nanochannel subsurface revealed atomic-scale defects, including dislocations, stacking faults, and amorphous silicon. Molecular dynamics simulations described the evolution of the phase transformation and subsurface damage. This work reveals the mechanism of subsurface damage of single-crystal silicon in nanomilling, which will facilitate the machining of nanostructures with minimal subsurface damage.     

Effects of Self-Assembled Monolayer and PFPE Lubricant on Wear Characteristics of Flat Silicon Tips

The effects of self-assembled monolayer (SAM) and perfluoropolyether (PFPE) lubricant on the wear characteristics of flat silicon tips were investigated. The wear test consisted of sliding the silicon tips fabricated on a flat silicon specimen against SAM and PFPE (Z-tetraol) coated silicon (100) wafer. The tips were slid at a low speed for about 15 km under an applied load of 39.2 μN. The wear volume of the tip was obtained by measuring the tip profile using an Atomic Force Microscope (AFM). It was found that the coatings were effective in reducing the wear of the tips by an order of magnitude from 10⁻⁶ to 10⁻⁷.     

Study of the tip mass and interaction force effects on the frequency response and mode shapes of the AFM cantilever

The vibrational characteristics of an atomic force microscope (AFM) cantilever beam play a key role in dynamic mode of the atomic force microscope. As the oscillating AFM cantilever tip approaches the sample, the tip–sample interaction force influences the cantilever dynamics. In this paper, we present a detailed theoretical analysis of the frequency response and mode shape behavior of a cantilever beam in the dynamic mode subject to changes in the tip mass and the interaction regime between the AFM cantilever system and the sample. We consider a distributed parameter model for AFM and use Euler–Bernoulli method to derive an expression for AFM characteristics equation contains tip mass and interaction force terms. We study the frequency response of AFM cantilever under variations of interaction force between AFM tip and sample. Also, we investigate the effect of tip mass on the frequency response and also the quality factor and spring constant of each eigenmodes of AFM micro-cantilever. In addition, the mode shape analysis of AFM cantilever under variations of tip mass and interaction force is investigated. This will incorporate the presentation of explicit analytical expressions and numerical analysis. The results show that by considering the tip mass, the resonance frequencies of the cantilever are decreased. Also, the tip mass has a significant effect on the mode shape of the higher eigenmodes of the AFM cantilever. Moreover, tip mass affects the quality factor and spring constant of each modes.     

Wear characteristics of diamond-coated atomic force microscope probe

In atomic force microscope (AFM) applications, the wear of the probe is undoubtedly a serious concern since it affects the integrity of the measurements. In this work, wear tests were performed using an AFM with lateral force monitoring capability with the aim to better understand the wear characteristics of diamond-coated probes. For the assessment of the probe wear, a transmission electron microscope (TEM) as well as a scanning electron microscope were utilized. The degree of the probe wear was quantified using the Archard's wear equation. The structure of the diamond-coated probe was analyzed by using the TEM and Raman spectroscopy. From the experimental results, two different wear characteristics, the gradual wear and the abrupt fracture of the diamond coating, were observed. In the case of gradual wear, the wear coefficient of the diamond-coated probe slid against a silicon nitride specimen was about 10(-5)-10(-6). It was also found that the wear rate significantly decreased with increase in the sliding distance. Raman spectroscopy analysis showed that the difference in the chemical structure of the diamond coating may induce the different wear phenomena. These results may be effectively utilized for fundamental understanding of nano-wear characteristics of AFM probes.     

Nano-wear of the diamond AFM probing tip under scratching of silicon, studied by AFM

In order to improve such a widely used microtribological testing procedure as surface scratching by an AFM diamond tip, an experimental study has been carried out using single-crystalline silicon as the tested material. Wear of the AFM diamond tip under scratching was observed by a decrease in the scratch depth with increasing wear cycles and by the direct imaging of the diamond tip shape using a Si₃N₄ AFM tip. It was shown that the current widely used experimental method, which assumes the diamond tip to be non-wearable, introduces uncontrollable error into the obtained values for the tested material's wear rate. The harder the tested material, the larger may be the tip wear, and, therefore, the bigger may be its effect on the obtained wear rate values. The specific wear rates for the diamond tip and a silicon wafer were estimated to be 1.4 × 10^-9 and 4.5 × 10^-4 mm³/(N m), respectively.     

Surface modification of AFM silicon probes for adhesion and wear reduction

Tip wear of silicon probes used for an atomic force microscope (AFM) is a critical issue. Wear can result in an increase of tip radius and adhesion between tip and sample, thus reducing the image resolution and introducing artifacts. In order to reduce adhesion, friction, and wear so as to reduce tip related artifacts, liquid lubricant (Z-TETRAOL), self-assembled monolayers (pentafluorophenyltriethoxysilane (PFPTES)), and fluorocarbon polymer (Fluorinert™) were applied on the silicon probe. A comprehensive investigation of adhesion, friction, and wear of the uncoated/coated tips in both ambient air and various humidity levels as well as the influence of the coatings on the image resolution was performed. Experiments showed that the coatings reduced the adhesion, friction, and wear of the silicon tip, improved the initial image resolution, and exhibited less deterioration as compared to that of uncoated tip in the long-term test.     

Characteristics of Extrusive Wear and Transition of Wear Mechanisms in Elevated-Temperature Wear of a Carbon Steel

The dry sliding wear of a medium carbon steel with different microstructures was measured under the normal load range of 50–150 N at 400°C by a pin-on-disc high-temperature wear setup. The wear behavior and wear mechanism were systematically studied; in particular, the characteristics of extrusive wear and the transition of wear mechanisms were investigated. Under low normal loads, the wear is oxidative type wear. Once the normal load reached a critical value, a mild-to-severe wear transition occurred, and subsequently an extrusive wear prevailed. The mild-to-severe wear transition depended on the microstructure of matrix; the critical normal load of the transition was 112.5 N for tempered sorbite, 125 N for lamellar pearlite, and 137.5 N for tempered martensite and tempered troostite. As oxidative wear prevailed, a thick oxide layer about 20–30 μ m and a plate-like wear debris with regular outline were recognized. However, as the extrusive wear occurred, the wear rate abruptly increased but the friction coefficient was reduced. The extrusive wear predominated due to thermal softening of the matrix and presented a superthin oxide layer (less than 0.5 μ m) and low oxide content on worn surfaces, accompanied by the appearance of ribbon-like wear debris.     

Friction and Wear Characteristics of C/Si Bi-layer Coatings Deposited on Silicon Substrate by DC Magnetron Sputtering

The tribological behavior of carbon/silicon bi-layer coatings deposited on a silicon substrate by DC magnetron sputtering was assessed and compared to that of amorphous carbon and silicon coatings. The motivation was to develop a wear resistant coating for silicon using thin layers of amorphous carbon and silicon. Wear tests were conducted by sliding a stainless steel ball against the coating specimens under applied normal loads in the range of 20?~?50?mN. Results showed that the wear rate of the bi-layer coating was strongly dependent on the ratio of thickness between the carbon and silicon layers. The wear rate of the bi-layer coating with 25?nm thick carbon and 102?nm thick silicon layers was about 48 and 20 times lower than that of the single-layer amorphous carbon and amorphous silicon coating, respectively. In addition, the steady-state friction coefficient of the bi-layer coating could be decreased to 0.09 by optimizing the thickness of the layer. Finally, a model for the wear reduction mechanism of the carbon/silicon bi-layer coating was proposed.     

Correlation Between Mechanical Properties and Nanofriction of [Ti–Cr/Ti–Cr–N] n and [Ti–Al/Ti–Al–N] n Multilayers

In this work, thin films deposited by pulsed DC magnetron sputtering of [Ti–Al/Ti–Al–N] _n and [Ti–Cr/Ti–Cr–N] _n multilayers of nanometric periods were analyzed by AFM in contact mode to measure values of lateral and normal forces. From these measurements, the coefficient of friction (COF) of these materials in contact with the AFM tip was calculated. Measurements were made with three types of silicon tips, diamond-coated, Pt–Cr-coated, and bare silicon. Significant differences between the tip materials in contact with the samples, which affected the COF, were observed. The effect of the environmental layer of water covering the surface sample and the tip appears as the most important factor affecting the tribology behavior of the tip-sample contact. For diamond-coated and bare silicon tips there is an additional adherence force increasing the normal load. But for tips platinum–chromium-coated there is a repulsive force due to this water layer, which behaves as a lubricant layer before a threshold load.     

Wear of silicon carbide in high speed sliding

Attritious wear of silicon carbide rubbing against a cobalt base superalloy at high speed was studied using a scanning electron microscope (SEM) and an Auger electron spectroscope (AES). The SEM study of the wear area on the silicon carbide grain showed it to be very smooth. The AES study of the groove-like marking generated by a silicon carbide grain showed a heavy concentration of carbon in areas where submicron wear debris was present. No indication of chemical reaction of the abrasive with the work material was evident. Instead, it appears that the surface atoms on the abrasive are removed preferentially, layer by layer, by oxidation under high temperature and a favorably directed shear stress.     

单相硼化物的制备及摩擦磨损性能研究

为了深入研究渗硼层中硼化物的性能,采用真空感应熔炼法制备单相硼化物材料。观察分析制备的硼化物微观组织,测试其力学性能。采用MMU-5G型销-盘式摩擦磨损试验机,在干摩擦条件下,研究了不同载荷下单相硼化物的摩擦学性能,观察其磨损表面形貌特征,探讨其磨损方式。结果表明：制备的硼化物为单一相,试样纯度高,试样的平均显微硬度为HV2065,平均断裂韧性值为1.68 MPa·m1/2;硼化物的断口处没有宏观的塑性变形,断口齐平光亮,表现为脆性断裂特征;干摩擦条件下随着载荷从10 N增加到30 N,硼化物的摩擦因数先降低后增加,20 N载荷时达到最小值,而其磨损量随着载荷的增加不断上升;随着载荷从10 N增加到30 N,磨损表面的粗糙度先逐渐上升后急剧上升; 10~20 N载荷下,硼化物的磨损以磨粒磨损为主,而25~30 N载荷下,硼化物的主要磨损方式从磨粒磨损转变为脆性破损。     

Field enhancement and apertureless near-field optical spectroscopy of single molecules

We report on fluorescence enhancement in near field optical spectroscopy by apertureless microscopy. Our apertureless microscope is designed around a confocal fluorescence microscope associated with an AFM head. First, we show that the confocal microscope alone allows single molecule imaging and single molecule fluorescence analysis. When associated with the AFM head, we demonstrate, for the first time to our knowledge, that single molecule fluorescence is enhanced under the silicon tip. We tentatively attribute this effect to field enhancement under the tip.     

Field enhancement and apertureless near-field optical spectroscopy of single molecules

We report on fluorescence enhancement in near field optical spectroscopy by apertureless microscopy. Our apertureless microscope is designed around a confocal fluorescence microscope associated with an AFM head. First, we show that the confocal microscope alone allows single molecule imaging and single molecule fluorescence analysis. When associated with the AFM head, we demonstrate, for the first time to our knowledge, that single molecule fluorescence is enhanced under the silicon tip. We tentatively attribute this effect to field enhancement under the tip.     

Nanotribology: tip–sample wear under adhesive contact

In this report, the irreversible variation of mass of the probe tip of an atomic force microscope (AFM) is considered from theoretical and numerical points of view through statistical methods. The tip–sample interaction due to the intermittent-contact operating mode of an AFM is modelled as a double-well potential where the wear mechanism, which reveals itself as mass sticking to the probe tip, is described as a transition between the two potential wells. We evaluate the interaction of a silicon nitride AFM/FFM tip with gold in order to compare the results with those obtained from previous experimental and numerical studies.     

碳纳米管原子力显微镜针尖的制作研究

研究在光学显微镜下，运用两个独立的三维工作台分别控制针尖和碳纳米管的位置，将碳纳米管吸附在传统的原子力显微镜针尖上。首先将碳纳米管粘附在导电的胶带上，然后用涂胶的针尖与其接触将碳纳米管粘附到针尖上，最后运用电蚀的方法优化碳纳米管针尖的长度，以达到高分辨率的要求。运用制作的碳纳米管针尖对硅表面的深槽进行成像，获得了传统针尖无法得到的信息。     

Influence of Tip Relief Modification on the Wear of Spur Gears with Asymmetric Teeth

Recently, spur gears with asymmetric teeth have been considered a way of increasing performance while maintaining the gearbox dimensions. Asymmetric teeth have different pressure angles on drive and coast sides. They provide, among other advantages, a high bending strength and low vibration. In spur gears with asymmetric teeth, wear has been observed to be a major failure mode. In this study, the impact of tip relief modification and pressure angle on the wear of spur gears with asymmetric teeth is numerically investigated. Here, the focus is on sliding wear. A wear model based on Archard's equation is employed to predict wear depth. The pressure angle and the tip relief are parameterized. In the analysis, instantaneous contact loads and Hertz pressures are used in wear depth calculations. It is shown that as the amount of the tip relief increases, the wear depth, particularly at the beginning and end of the mesh, decreases. As the number of wear cycles increases, the effect of the tip relief modification on wear depths decreases slightly. It was also shown that with an increase in tip relief, the dynamic load decreases. However, if the amount of tip relief modification increases excessively, the maximum dynamic load also increases. Therefore, an excessive increase in tip relief modification should be avoided, whereby the level of excessive increase depends on the tip relief configuration.     

Direct deformation study of AFM probe tips modified by hydrophobic alkylsilane self-assembled monolayers

The in-use wear of atomic force microscopy (AFM) probe tips is crucial for the reliability of AFM measurements. Increase of tip size for several nanometers is difficult to monitor but it can already taint subsequent AFM data. We have developed a method to study the shape evolution of AFM probe tips in nanometer scale. This approach provides direct comparison of probe shape profiles, and thus can help in evaluation of the level of tip damage and quality of acquired AFM data. Consequently, the shape degradation of probes modified by hydrophobic alkylsilane self-assembled monolayers (SAMs) was studied. The tip wear length and wear volume were adopted to quantitatively verify the effectiveness of hydrophobic coatings. When compared with their silicon counterparts, probes modified by SAM materials exhibit superior wear-resistant behavior in tapping mode scans.     

Nanometer-scale layer modification of polycarbonate surface by scratching with tip oscillation using an atomic force microscope

This paper presents a simple and reliable technique for nanometer-scale layer modification of a polycarbonate (PC) surface using an atomic force microscope (AFM). The AFM tip, coated with amorphous carbon was made to oscillate vertically at its resonance frequency. With tip oscillating in tapping mode, it scan-scratched the PC surface to make the desired modification. This action carved the PC surface without distorting it. The bottom of the depression made by scan-scratching with the oscillating tip was obviously flat in comparison with the area scan-scratched without tip oscillation in contact mode. The depth of the scan-scratched depression was controlled by adjusting the amplitude of oscillation and the scanning speed of scratching. This technique is very interesting for microtribology and surface modification.