Reducing Friction Force of Si Material by Means of Atomic Layer-Deposited ZnO Films

With excellent lubricating property, zinc oxide (ZnO) films are promising candidates to act as protective coatings in Si-based microelectromechanical system devices for the purpose of decreasing friction forces of silicon (Si) material. In this paper, the nanotribological behavior of ZnO films prepared by atomic layer deposition on a Si (100) substrate is investigated by an atomic force microscope. The ZnO films have various thicknesses ranging from 10.0 to 182.1 nm. With the increase of film thickness, the root-mean-square roughness of the films increases, while the ratio of hardness to Young’s modulus (H/E) decreases. Due to their large surface roughness, the thick ZnO films are low in adhesion force. The friction force of the ZnO films is smaller than that of the Si (100) substrate and is greatly influenced by their adhesion force and mechanical property. In a low-load condition, the friction force is dominated by the adhesion force, and thus, the friction force of the ZnO films decreases as film thickness increases. While in a high-load condition, the friction force is dominated by plowing. Films with higher H/E possess smaller friction force, and thus, the friction force increases with the decreasing film thickness.     

Oxidation and ignition behavior of a magnesium alloy containing rare earth elements

The oxidation and ignition behavior of Mg–8 wt.%Al alloy added with rare earth (RE) was investigated. When 0.1 wt.%RE was added, the ignition-proof effect was optimum and the ignition temperature of the alloy increased from 654°C to 823°C. The oxide scales and substrates of the alloy with 0.1 wt.%RE were characterized by scanning electronic microscope, X-ray diffraction, and energy dispersive spectrometer. The results show that a layer of tightly coherent oxidation film formed on the alloy surface under high temperature mainly consists of MgO, RE₂O₃, and Al₂O₃, which is about 2.5–3.5 µm thick. The oxidation kinetics curve of Mg–8 wt.%Al–0.1 wt.%RE follows the parabola rule at 400°C and 700°C and cubic rule at 600°C, which proves that the oxidation at these temperatures is controlled by diffusion obstruction so that the oxidation film can effectively restrain the alloy from further oxidation.     

硅微光机械加速度地震检波器中M-Z光波导干涉仪结构设计

提出了一种新型的硅基M-Z干涉型光电集成加速度地震检波器。对硅基微光电机械加速度地震检波器的M-Z光波导干涉仪系统进行了系统研究和优化设计,采用K9玻璃作为基底层和包层材料,Bak7玻璃作为波导层材料,波导的宽度为4 μm,厚度为0.4 μm。M-Z光波导干涉仪的Y分支设计中采用上升反正弦S型弯曲,为适用于该干涉仪的信号臂和参考臂大跨距的特殊结构,应用波导转向镜实现了大角度光路转折,反射转向镜的内介质采用镀制金属铝膜得以实现,并利用BPM进行了仿真。为了防止TE模偏振光的偏振态经铝膜全反射后发生变化,加了4个长度为1 mm的TE模偏振器以保持其偏振态。实验结果与理论相符。     

Fabrication for multilayered composite thin films by dual-channel vacuum arc deposition

A flexible dual-channel curvilinear electromagnetic filter has been designed and constructed to fabricate multilayered composite films in vacuum arc ion plating. The filter possesses two guiding channels and one mixing unit. Multilayered TiN/AlN and TiAlN composite films can be produced by controlling the frequency or interval of the two cathodes. The x-ray photoelectron spectroscopy and low-angle x-ray diffraction results reveal the periodic Ti and Al structures in the TiN/AlN films. The TiAlN films exhibit a smooth surface morphology confirming effective filtering of macroparticles by the filter. High temperature oxidation conducted at 700 degrees C for an hour indicates that the weight increment in the TiAlN films produced by the dual filter is only half of that of the TiAlN films produced without a filter, thereby showing better resistance against surface oxidation.     

Adsorption structure and adhesion strength of aqueous copolymer lubricant films on Si surface

The adsorption structure of aqueous triblock copolymer polypropylene oxide‐polyethylene oxide‐polypropylene oxide, PPO‐PEO‐PPO, on the Si surface was studied using neutron reflectometer. It is found that PEO blocks formed the outer layer of the adsorbed PPO‐PEO‐PPO film, while PPO blocks formed the inner layer and served as the anchor blocks. The adhesion strength of adsorbed PPO‐PEO‐PPO copolymer film was evaluated using atomic force microscopy and scratch tests. The results revealed that the molecular structure of triblock copolymer had a considerable effect on the adhesion strength. The triblock copolymer with a longer PPO chain and a higher weight percentage of PPO exhibited stronger adhesion and better lubrication performance.     

Surface characteristics and tribology study of metal oxide thin films

Abstract

Microcrystalline high quality undoped ZnO thin films were deposited on Si(100) and Corning 1737F glass substrates by a dc magnetron sputtering system. Surface and mechanical properties of ZnO thin films deposited under different deposition conditions (thickness, deposition rate and plasma composition) were investigated. Atomic force microscopy, nanoindentation techniques and scratch tests have been carried out. The lateral grain radius was between 50 and 160 nm. Surface roughness was found to vary from 1·3 to 10·3. In order to measure the real hardness of ZnO thin films grown on Si(100) and glass Continuous Stiffness Measurement technique was used. The hardness was found to be between 11 and 13 GPa for the polycrystalline ZnO almost five times larger than for the corresponding single crystalline material, while scratch tests verified a film structure, thickness, and surface morphology dependency on the mechanical properties for these metal oxide thin films.     

Study of ss-DNA Adsorption and Nano-mechanical Properties on Mica Substrate with Surface Forces Apparatus

Many DNA-based devices need to build stable and controllable DNA films on surfaces. However, the most commonly used method of film characterization, namely, the probe-like microscopes which may destroy the sample and substrate. Surface Forces Apparatus (SFA) technique, specializing in surface interaction studies, is introduced to investigate the effects of DNA concentration on the formation of single-stranded DNA (ss-DNA) film. The result demonstrates that 50 ng/μL is the lowest concentration that ss-DNA construct a dense layer on mica. Besides, it is also indicated that at different DNA concentrations, ss-DNA exhibit diverse morphology: lying flat on surface at 50 ng/μL while forming bilayer or cross-link at 100 ng/μL, and these ss-DNA structures are stable enough due to the repeatability even under the load of 15 mN/m. At the same time, an obvious adhesion force is measured: ?6.5 mN/m at 50 ng/μL and ?5.3 mN/m at 100 ng/μL, respectively, which is attributed to the ion-correlation effect. Moreover, the atomic force microscopy (AFM) images reveal the entire surface is covered with wormlike ss-DNA and the measured surface roughness (1.8±0.2 nm) also matches well with the film thickness by SFA. The desorption behaviors of ss-DNA layer from mica surface occur by adding sodium salt into gap buffer, which is mainly ascribed to the decreased ion-ion correlation force. This paper employing SFA and AFM techniques to characterize the DNA film with flexibility and stable mechanical ability achieved by ion bridging method, is helpful to fabricate the DNA-based devices in nanoscale.     

Optical Detections From Worn and Unworn Titanium Compound Surfaces

Wear-induced roughness in terms of grooves, sharp ridges, and edges leads to scattering of the reflected light and leads unavoidably to a reduction of the optical signals in a standard specular geometry. However, by using a double-layer system consisting of titanium aluminum nitride (TiAlN) on top of a titanium nitride (TiN) layer we obtain an increase in the reflected light as a result of wear. The relative change of reflectance of light from the tribological TiAlN coated surface to the underlying layer of TiN is similar for non-worn surfaces and for surfaces exposed to an abrasive wear process. The induced roughness reduces the signals from worn samples, in a standard specular geometry, by up to 30% compared with unworn samples. Our model system of TiAlN coatings on top of ‘optical’ signal layers of TiN deposited on a 100Cr6 steel substrate, was exposed to a reciprocating wear process with up to 10⁵ repetitive cycles in a linear tribometer. The worn TiAlN layers of thicknesses up to 3 μm, with strongly developed grooves and ridges, were subsequently used for the reflectance measurements. The results show that optical reflectance monitoring is a potential technique for intelligent determination of a residual thickness of realistic tribological coatings prior to complete wear.     

Characterization of the topography and thickness of gallium thin films by scanning tunneling microscopy

Scanning tunneling microscopy (STM) was employed to characterize the topography of 32 and 4 µm thick gallium films; thicknesses that have not yet been addressed. The STM images revealed submicron grains on both surfaces although a reduced grain density was observed on the thinner film. The granular structure may be explained by a thin gallium oxide layer that acted as an elastic membrane against liquid gallium underneath that expanded during freezing of the sample. We also believe that the same gallium oxide layer is also responsible for an intriguing effect that appears as soon as the tungsten tip lands on the gallium film: the onset of continuous regular z-oscillations of the tip even at rest. This is an effect that we believe has not been previously reported in the literature. We have also demonstrated, for the first time, a new thickness determination method for the gallium film based on an STM image obtained during the solid-to-liquid phase transition.     

Wear Reduction of Borosilicate Glass Microballs Using Vapor-Phase Lubrication With n-Pentanol

The effect of n-pentanol vapor-phase lubrication on the wear mechanism of borosilicate glass was investigated. Glass microballs with a diameter of ～60 μm were slid against a silicon wafer under a normal load of 100 μN up to a sliding distance of ～210 km. It was shown that wear volume of the microball could be reduced by ～18 times using n-pentanol vapor-phase lubrication. The wear mechanism was assessed using 3D laser microscopy and scanning electron microscopy. Abrasion was identified as the main mechanism of wear for the borosilicate glass under n-pentanol vapor-phase lubrication condition. In addition, despite the fact that lubrication was supplied to the sliding interface as a vapor, it was sufficient to prevent agglomeration of wear debris, which aided in decreasing abrasion.     

Surface and Tribological Chemistry of Water and Carbon Dioxide on Copper Surfaces

The tribological chemistry of carbon dioxide and water vapor is studied on copper surfaces at high pressures, with a view to understand the gas-phase lubrication of copper–copper sliding contacts. The adsorption and film formation properties are studied on vapor-deposited copper films in an ultrahigh vacuum chamber using a quartz crystal microbalance. The nature of the reactively formed film is studied after reaction by ex situ X-ray photoelectron spectroscopy (XPS). Carbon dioxide adsorbs reversibly on copper, while water vapor adsorbs both reversibly and irreversibly, where XPS reveals that the irreversibly formed film consists of a mixture of cuprous oxide/hydroxide. Measuring the thickness of the cuprous oxide/hydroxide film as a function of water vapor pressure and temperature reveals that its thickness varies between about 6 and 14 Å and is proportional to the total amount of water adsorbed on the surface. This results in a cuprous oxide/hydroxide film that is thinner at higher temperatures. Measurements of the friction coefficient as a function of temperature and pressure in the presence of water vapor shows that it correlates with film thickness, reaching a limiting value of ~0.35 for thicker films.     

High resolution observations of friction-induced oxide and its interaction with the worn surface

A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron, an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni₃Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni₃Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear.     

AZO薄膜理想表面形貌的制备与加工工艺优化

非晶硅薄膜太阳能电池主要采用掺氟氧化锡(FTO)导电玻璃作为基板,但FTO薄膜雾度较低、表面形貌无法优化,导致无法得到较优的陷光结构,从而限制了太阳能电池的转换效率。为了进一步提升太阳能电池的转换效率,探讨了替代型的掺铝氧化锌(AZO)薄膜,通过优化前段磁控溅射镀膜工艺和后段湿化学蚀刻工艺,用以平衡AZO薄膜的光电性能和雾度,从而获得具有理想表面形貌的AZO导电玻璃,使其成为理想的非晶硅薄膜太阳能电池的基板材料。实验表明,经工艺优化后制作的AZO导电玻璃可提升光电转换效率。     

The “leaves on the line” problem—a study of leaf residue film formation and lubricity under laboratory test conditions

The problem of leaf residue and loss of adhesion in the wheel/rail track contact has been studied in ball-on-disc test device. The friction properties of sycamore leaf samples were measured for a range of speeds and imposed slip at a maximum contact pressure of 1.0 GPa. At the end of the test the leaf lubricant films were examined under a low-power microscope and the organic content analysed by Infra-Red (IR) Reflection-Absorption Microspectroscopy. The test samples included water-saturated leaves and the supernatant soaking water to measure the effect of water-soluble leaf components. The results were compared to pure water. During the initial film formation the leaf samples rapidly formed a slurry composed of small black particles, this dried to a thick adherent black film as the test proceeded. This suggested that the black film was due to a chemical reaction between the water-soluble leaf component and steel rather than as the result of charring of the organic material. Friction coefficients of the leaf slurry were in the range μ = 0.03–0.06 (50% slip) compared to values of μ = 0.15–0.2 for pure water. Friction was also reduced in tests with the leaf-soaking water, suggesting that water-soluble leaf components (identified as pectin) play a role in the low adhesion mechanism. IR analysis showed that the black residue films contained pectin (pectate) and cellulose derived from the leaf samples. Pectin appears to play an important role in the lubrication process. In the presence of metal ions pectin will gel and thus could form a thin but highly viscous layer on the track surface. This gel will also flocculate cellulose to create the black biomass, which provides the lubricating film.     

Intermetallic formation and its relation to interface mass loss and tribology in die casting dies

Due to high metal injection velocities typical of die casting process, oxide film on die surface breaks exposing virgin steel surface to liquid aluminum at 680 °C. This paper describes in detail the fundamental tribochemical basis for the soldering and mass loss phenomena related to this steel-cast metal interaction. Cylindrical plain H13 coupons are dipped in hot liquid aluminum, kept for predetermined times, cleaned and characterized for surface and substrate changes. The thermodynamics behind the formation of intermetallic compounds based on Gibb’s free energy calculations are presented. Adhesive strength of soldering layer is determined through pull tests on casting solidified around the test pin. From these results, a dissolution–adhesion wear model is proposed for the growth and dissolution of intermetallics and soldering. In addition, this paper includes the soldering and adhesion behavior of nitrided surfaces. It is seen that the diffusion barrier treatment prevents soldering formation and provides for reduced friction and adhesion at the interface.     

EFFECT OF SURFACE ENERGY ON THE GROWTH OF DIAMOND-LIKE CARBON/AMORPHOUS SILICON FILMS ON VARIOUS SUBSTRATES

Diamond-like carbon/amorphous silicon bilayer films were deposited on SiO₂, Ge, and Ta₂O₅ substrates using a pulsed filtered cathodic arc (PFCA) system. Amorphous silicon (a-Si) layer was firstly deposited on three substrates using DC magnetron sputtering, then diamond-like carbon (DLC) film was deposited on a-Si layer via pulsed filtered cathodic arc. The thicknesses of a-Si layer and DLC film as monitored by in-situ ellipsometry during the film deposition were 7 and 10 nm, respectively. The surface energy of SiO₂, Ge, and Ta₂O₅ substrates was determined by measuring the contact angle of water on these substrates. It was found that the contact angles of water on SiO₂, Ge, and Ta₂O₅ substrates were 53°, 63°, and 75°, respectively. This result indicates that SiO₂ has the highest surface energy while Ta₂O₅ has the lowest surface energy. The thickness of the a-Si layer and DLC film was determined from the cross-section transmission electron microscopy (TEM) images. The thinnest a-Si layer of 5.64 nm was obtained from SiO₂ substrate which has the highest surface energy. The thickest a-Si layer of 6.97 nm was obtained from Ta₂O₅ corresponding to the lowest surface energy. This study shows that the thickness of the growth film strongly depends on the surface energy of the substrate. However, the DLC films deposited on each a-Si layer of three substrates have the same thickness approximately of 9.9 nm, because all of them were deposited on a-Si layers having the same surface energy.     

The effect of oxide-forming alloying elements on the high temperature wear of a hot work steel

A great deal of research has been conducted to clarify the role of oxide films in the wear of metals. Oxides formed during dry sliding of steels at high temperatures determine their tribological behavior. The present work deals with the influence of the oxide-forming alloying elements aluminum and silicon on the oxidation and wear of three selected hot work steels. For this investigation, ball-on-disc experiments were carried out in ambient air and 500 °C. Wear tracks on the disks and balls were characterized using both a scanning electron microscope and an optical profiler. The oxidation products were characterized using X-ray photoelectron spectroscopy and energy-dispersive electron probe microanalysis. The results show that the alloying elements aluminum and silicon yield a reduction of the oxide film thickness and thus lead to an increase in mechanical wear as temperature rises.     

Monitoring of Wear Characteristics of TiN and TiAlN Coatings at Long Sliding Distances

TiN and TiAlN thin hard coatings have been widely applied on machine components and cutting tools to increase their wear resistance. These coatings have different wear behaviors, and determination of their wear characteristics in high-temperature and high-speed applications has great importance in the selection of suitable coating material to application. In this article, the wear behavior of single-layer TiN and TiAlN coatings was investigated at higher sliding speed and higher sliding distances than those in the literature. The coatings were deposited on AISI D2 cold-worked tool steel substrates using a magnetron sputtering system. The wear tests were performed at a sliding speed of 45 cm/s using a ball-on-disc method, and the wear area was investigated at seven different sliding distances (36–1,416 m). An Al₂O₃ ball was used as the counterpart material. The wear evolution was monitored using a confocal optical microscope and surface profilometer after each sliding test. The coefficient of friction and coefficient of wear were recorded with increasing sliding distance. It was found that the wear rate of the TiAlN coating decreases with sliding distance and it is much lower than that of TiN coating at longer sliding distance. This is due to the Al₂O₃ film formation at high temperature in the contact zone. Both coatings give similar coefficient of friction data during sliding with a slight increase in that of the TiAlN coating at high sliding distances due to the increasing alumina formation. When considering all results, the TiAlN coating is more suitable for hard machining applications.     

钛合金微弧氧化层的摩擦学性能研究

利用微弧氧化技术在Ti6Al4V合金表面制备了富含钙、磷的多孔氧化陶瓷层,研究了微弧氧化层表面形貌、组成及摩擦学性能。研究结果表明,随着电压的升高,氧化层表面微孔孔径、粗糙度和Ca、P元素含量增大,显微硬度增大。25%小牛血清润滑条件下的微弧氧化层与ZrO2陶瓷球的摩擦学实验表明,微弧氧化层的摩擦因数高于Ti6Al4V钛合金,但磨损率明显降低,表明微弧氧化Ti6Al4V合金具有良好的耐磨性能。     

Equipment for deposition of thin metallic films bombarded by fast argon atoms

Deposition of thin metallic films on dielectric substrates using a source of metal atom flow combined with a flow fast argon atoms has been investigated and the investigation results are presented. The fast atoms are produced due to charge-exchange collisions in a vacuum chamber of argon ions, which are accelerated by potential difference between the hollow-cathode glow-discharge plasma and an emissive grid and enter the chamber through the grid. The metal atoms produced due to ion sputtering of a metallic foil placed on the inner surface of the hollow cathode enter the chamber through the same grid. Substrate pretreatment and pulse-periodic bombardment of the growing film by ～1-keV argon atoms both ensure adhesion of copper to glass up to 2 × 10⁷ Pa. The use of a hollow substrate holder, whose inner surface is also covered with the same foil, makes it possible to exclude losses of the depositing metal and allows recommendation of the equipment for beam-assisted deposition of precious metal films.