Two-dimensional (2D) heterostructures with outstanding frictional properties have sparked immense interest in the field of tribology. Here, graphene oxide (GO)-coated conductive probes were fabricated, and the frictional behavior of the GO/graphene heterointerface under varying bias voltages was investigated using conductive atomic force microscopy. The friction at the GO/graphene interface increases with applied bias, enabling real-time and reversible control within a low-bias regime. Owing to the dielectric property of GO and the electrical conductivity of graphene, charges accumulate at the interface. Because atomically thin thickness of 2D materials, the accumulated charges produce a large interfacial electrostatic force for stable friction control. However, the stability of this control decreases when high positive bias is applied. Scanning Kelvin probe microscopy and adhesion measurements indicate that strong electric fields enable a fraction of the accumulated electrons to tunnel through the GO barrier, thereby altering the interfacial electrostatic interactions. In contrast, high negative bias voltages induce electrochemical oxidation of graphene to varying extents, resulting in a permanent and substantial friction modulation. These findings advance the fundamental understanding of friction in 2D heterointerfaces and provide important insights for friction regulation and the development of electrically tunable smart tribological systems.

Adhesion is a fundamental property of two-dimensional (2D) materials due to their intrinsic ultrahigh surface-to-volume ratio, making adhesion very strong in many processes related to fabrication, integration, and performance of devices incorporating 2D materials. Here, time-dependent adhesion between 2D materials, including graphene, molybdenum disulfide, and hexagonal boron nitride, and silicon dioxide is investigated. The logarithmic increase in adhesion with contact time is attributed to the out-of-plane deformation of the topmost layer of 2D materials, which surpasses interlayer interactions, as revealed by van der Waals force calculations and density functional theory calculations of interfacial binding energy. Over time, the topmost layer spontaneously conforms to the contact surface, resulting in enhanced adhesion. Additionally, adhesion enhancement accelerates in high relative humidity environments, as the hydrogen bonding of water molecules strengthens the interfacial interaction. These studies based on experimental characterizations and computational analysis reveal that the adhesion of 2D materials is dynamic and can autonomously increase over time, driven by van der Waals force-induced interfacial evolution. This dynamic adhesion behavior is critical for the design and performance of 2D material-based electronic devices, lubricating coatings, sensors, and related applications.

In the chemical mechanical polishing (CMP) process for SiC substrates, enhancing the chemical oxidation rate can markedly improve processing efficiency. This study uses graphene oxide (GO) as a carrier material to prepare a novel Mn-based slurry, MnO2/GO/Al2O3, through chemical precipitation and ultrasonic impregnation methods. Chemical composition and morphological analyses revealed that MnO2 and GO are linked with Mn-O bonds, with GO significantly enhancing the dispersion of MnO2 and Al2O3 particles. The slurry's effectiveness in planarizing SiC substrates within a potassium permanganate system was systematically evaluated. CMP tests demonstrated that, compared to using Al2O3 alone, MnO2/GO/Al2O3 substantially increased the material removal rate (MRR) for SiC, achieving up to 1020 nm/h, while reducing the surface roughness (Sa) of the polished substrate to 0.664 nm. Electrochemical corrosion tests further showed that the inclusion of MnO2 and GO significantly accelerated the slurry's corrosion rate on SiC. Additionally, the physical property characterization of the polishing slurry indicated that the improvement in MRR is primarily attributed to its enhanced chemical oxidation capability. XPS analysis confirmed the formation of Mn-O-Si compounds between MnO2/ GO/Al2O3 and SiC. Based on these findings, a polishing mechanism for MnO2/GO/Al2O3 is proposed.

A three-body wear model, comprising a soft polyurethane pad, silica abrasive, and crystalline silicon substrate, was established to analyze the impact of abrasive rotation on material removal process during chemical mechanical polishing (CMP) through molecular dynamics simulation. The results revealed that abrasive rotation, facilitated by the cooling effect, would reduce the local temperature in the contact area between the abrasive and the substrate, thereby enhancing the surface quality of the substrate and decreasing the material removal volume (MRV). However, another effect of abrasive rotation, termed the "hedgehog effect", resulting in an increase in MRV. Through the trade-off between the cooling effect and the porcupine effect, the abrasive rotation can not only improve surface quality but also increase the MRV.

The formation mechanism of plug-like flow in polymer melts between smooth and rough silicon walls was investigated through molecular dynamics simulations. As the driving velocity increased, plug-like flow occurred between rough silicon walls, while smooth silicon walls exhibited Poiseuille flow. The formation process of plug-like flow involved sprouting, shaping, and forming stages. Initially, the rising driving velocity in the rough silicon channel gradually increased shear stress on confined melts above a critical value. This initiated the elongation of polymer molecules near the wall from a curled to a horizontally straightened state, reducing internal viscous resistance and creating a velocity gradient gap between the near-wall and center areas, marking the sprouting of plug-like flow. In the shaping stage, horizontally straightened polymer molecules expanded from the near-wall area to the transition area, further widening the velocity gradient gap. Finally, steady plug-like flow formed when the velocity gradient gap exceeded a critical value, namely when nearly all polymer molecules in both the near-wall and transition areas were horizontally straightened. Conversely, shear stress on melts confined between smooth walls did not reach the critical value required for plug-like flow initiation. Furthermore, simulation results showed that the elongation of polymer molecules played a more significant role than boundary slip in the formation of plug-like flow-for polymer melts.

A three-dimensional numerical simulation model of gas-liquid two-phase flow in thin-film evaporator was established by Fluent numerical simulation software to deepen the study on liquid transmission characteristics in thin-film evaporator. The radial mixing and axial transmission characteristics of liquid and material in thin-film evaporator were investigated, and the influence of viscosity on them was further discussed. The results show that the interface between the fillet and liquid film is laminar flow with a large amount of gas; the axial velocity of the fillet is faster than the liquid film, and further shear-thinning occurs in the axial transmission process, which promotes the radial mixing of liquid. The liquid gradually enters into the film and the equivalent diameter of the fillet decreases continuously. As the viscosity of the liquid increases, the amount of gas at the junction increases, and the size of fillet increases accordingly while the trend of axial decrease remains the same, but the rate of decrease becomes slower.The increasing viscosity of the liquid material causes the radial mixing between the fillet and liquid film to be suppressed and the fillet gradually becomes the main force of axial transmission.

为进一步细化熔喷纤维直径,提升熔喷布的过滤效率,在普通双槽型熔喷模头的基础上,设计了新型钝喷口结构和新型尖喷口结构来优化熔喷气流场.基于计算流体动力学的方法,采用Fluent 6.3.26 软件对熔喷气流场速度、温度和湍流强度进行数值模拟.结果表明:新型尖喷口结构可延缓气流场的速度和温度衰减,有效缓解回流区的反向速度,减小近喷口端面气流场中心线上的湍流强度,有利于获得直径更小的纤维;新型尖喷口外侧延伸长度越长,气流场速度和温度就越高,减少热量散失的作用越明显,更有利于纤维的细化;当喷口外侧延伸长度为 5 mm 时,近喷口端面气流场的湍流强度峰值最小,有利于纤维的细化.

The tuning of flexible microscale friction is desirable for the reliability of wearable electronic devices, tactile sensors, and flexible gears. Here, the tuning of friction of atomically thin graphene on a flexible polydimethylsiloxane (PDMS) substrate was obtained with the elastic modulus using a 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs)-modified microsphere probe with the diameter of 5 μm at the microscale. The friction can be tuned at a large scale with the difference in the elastic modulus of PDMS and thickness of graphene. The hydrophobic property of the FDTS SAMs-modified probe decreased friction by reducing interfacial adhesion and preventing the effect of capillary interaction; thus, the friction decreased with the increase in the elastic modulus of the PDMS substrate due to decreasing indentation depth and thus the interfacial contact area; and also, the enhanced out-of-plane stiffness effectively decreased the interfacial contact quality with the increase of the thickness of graphene. The flexible tuning of friction on graphene was further verified by the theoretical calculation from the aspects of the friction arising from the normal and lateral deformation around the contacting area. This work is meaningful for promoting the design and reliability of flexible micro-devices.

Tribological properties of graphene oxide (GO) in high-humidity environments are important for its application as a solid lubricant. Topography, friction, and adhesion of GO with water nanodroplet are investigated at high relative humidity using atomic force microscopy. The friction of GO inside the nanodroplet depends on the tip radius. For a sharp tip, the friction of GO inside the nanodroplet is higher than that outside because the resisting force of the nanodroplet to the tip is greater than the assisting force due to the mildly decreased adhesion. For a blunt tip, the significantly decreased adhesion inside the nanodroplet results in much lower friction inside the nanodroplet than that outside. These studies enhance the understanding of the friction behaviors of GO.

Current three-body abrasive wear theories are based on a macroscale abrasive indentation process,and these theories claim that material wear cannot be achieved without damaging the hard mating surface.In this study,the process of three-body nano-abrasive wear of a system including a single crystalline silicon substrate,an amorphous silica cluster,and a polyurethane pad,based on a chemical mechanical polishing(CMP)process,is investigated via molecular dynamics simulations.The cluster slid in a suspended state in smooth regions and underwent rolling impact in the asperity regions of the silicon surface,realizing non-damaging monoatomic material removal.This proves that indentation-plowing is not necessary when performing CMP material removal.Therefore,a non-indentation rolling-sliding adhesion theory for three-body nano-abrasive wear between ultrasoft/hard mating surfaces is proposed.This wear theory not only unifies current mainstream CMP material removal theories,but also clarifies that monoatomic material wear without damage can be realized when the indentation depth is less than zero,thereby perfecting the relationship between material wear and surface damage.These results provide new understanding regarding the CMP microscopic material removal mechanism as well as new research avenues for three-body abrasive wear theory at the monoatomic scale.

螺杆挤出设备因其自身结构简单、设备能耗低等优点,在橡胶塑料、纤维成型等领域被广泛使用.但实际生产中螺杆的进口总会出现进料不均匀、不连续的情况.因此,采用Fluent软件对单螺杆挤出机中聚合物的三维等温流场进行数值模拟,探究了均匀进料和不均匀进料对聚合物流场的影响,并针对不均匀进料的不良影响,对螺杆的导程和螺棱间隙进行优化改进.研究结果表明,进料的不均匀性会增大物料反应过程中的死区占比和停留时间,同时出口流量的稳定性也会遭到破坏,而适当增大螺杆导程和螺棱间隙可有效改善以上问题,加强挤出螺杆的轴向输送能力,提升产品的生产质量.

Friction properties in the electric field are important for the application of graphene as a solid lubricant in graphene-based micro/nanoelectromechanical systems. The studies based on conductive atomic force microscopy show that interfacial water between graphene and the SiO2/Si substrate affects the friction of graphene in the electric field. Friction without applying voltage remains low because the interfacial water retains a stable ice-like network. However, friction after applying voltage increases because the polar water molecules are attracted by the electric field and gather around the tip. The gathered interfacial water not only increases the deformation of graphene but is also pushed by the tip during frictional sliding, which results in the increased friction. These studies provide beneficial guidelines for the applications of graphene as a solid lubricant in the electric field.

We demonstrated that the formation and solidification of a continuous confined water film played a very important role in changing the elastic modulus of the wet polymer substrate in a nanoindentation process by a coarse-grained molecular dynamics simulation of this process. It was found that as the water content increased, the elastic modulus of the wet polymer substrate showed a non-monotonic change. Relative to the dry polymer substrate, the elastic modulus of the wet polymer first decreased. This is because the appearance of a confined water film caused the force between the polymer substrate and the indenter to change from repulsion to attraction. Subsequently, as the confined water film gradually solidified and then weakened, the elastic modulus of the wet polymer slowly increased and then rapidly increased due to a large number of interstitial water molecules gradually penetrating the polymer substrate. Therefore, it is unreasonable to explain the wet polymer degradation during nanoindentation only from the plasticization and anti-plasticization effects based on the hydrogen bond breaking and formation during stretching. The above-mentioned results will help to more comprehensively understand the degradation mechanism of the polymers' encounter with water, thus promoting further practical applications for polymers.

Up to now, the underlying mechanism of the energy dissipation process of mechanoluminescence (ML) is still dubious. It is challengeable and significant to take the initiative to control in terms of light intensity and colour. Here, by rubbing silica surface against other materials, we are able to detect a controllable luminous energy emission, the colour as well as the intensity of which can be dominated by the crystal structural state of silica. The results show that Purplish blue light is observed between quartz and alumina, while the light emitted from the interfaces of amorphous silica and alumina is orange red which is also affected by external force. The basics of such phenomenon are well explained by both experimental and theoretical simulation analysis. Our results show that the emitted light colour transition from purplish blue to orange red, is suggested to be strongly related to the increase of defect centers in amorphous silica resulting from the structural changes in the rubbing surface. Our work provides a promising way for controlling mechanoluminescence, as well as a deep view into the basic mechanism of ML light emission.

The process of a cluster-containing water jet impinging on a monocrystalline silicon substrate was studied by molecular dynamics simulation. The results show that as the standoff distance increases, the jet will gradually diverge. As a result, the solidified water film between the cluster and the substrate becomes "thicker" and "looser". The "thicker" and "looser" water film will then consume more input energy to achieve complete solidification, resulting in the stress region and the high-pressure region of the silicon substrate under small standoff distances to be significantly larger than those under large standoff distances. Therefore, the degree of damage sustained by the substrate will first experience a small change and then decrease quickly as the standoff distance increases. In summary, the occurrence and maintenance of complete solidification of the confined water film between the cluster and the substrate plays a decisive role in the level of damage formation on the silicon substrate. These findings are helpful for exploring the mechanism of an abrasive water jet.

The tribological performances of IL films are improved as a function of the load-carrying phase of MoS2.

The abrasive is one of the important influencing factors during the chemical mechanical polishing (CMP) process. Although alpha-alumina is one of the most commonly used sapphire polishing abrasives due to its high hardness, it often results in surface damage. To receive lower surface roughness and high material removal rate, a common approach is to modify the surface of alumina. In this work, a series of alumina/metatitanic acid composite abrasives with core-shell structure were synthesized. The CMP performances of the pure alumina and alumina/metatitanic acid core-shell abrasives on sapphire substrates were investigated after polishing under the same conditions. Experimental results indicate that the alumina/metatitanic acid core-shell abrasives can not only improve the surface quality, but also further enhance the material removal rate. Furthermore, through the X-ray photoelectron spectroscopy test, this study investigated the chemical effect mechanism of the alumina/metatitanic acid core-shell abrasives in sapphire CMP. The results show that solid-state chemical reactions occur between metatitanic acid shell and sapphire surface during CMP process. We also investigated the mechanical friction mechanism through abrasive wear and adhesive wear. (C) 2017 Elsevier Inc. All rights reserved.

Graphene is one of the key building blocks for a broad range of flexible/stretchable electronics and has great potential as a solid lubricant in flexible micro/nanoelectromechanical systems, soft robot and microfluidic devices due to their excellent electrical, mechanical and tribological properties. The nanotribological properties of graphene on soft elastic substrate were studied using the calibrated atomic force microscopy (AFM). The nanotribological properties of graphene on soft elastic substrate were enhanced by the elastic deformation compared with the hard SiO2/Si substrate. Also the friction force of graphene on soft elastic substrate decreases with the increase of the thickness and shows a sub-linear dependence on the indentation depth. A novel model of the elastic deformation enhancing puckering effect was proposed to explain the nanotribological properties of graphene on soft elastic substrate. The atomic-scale frictional behaviors of graphene on soft elastic substrate confirmed the mechanism of the elastic deformation enhanced puckering effect. These studies can provide a fundamental understanding of graphene as a solid lubricant on soft elastic substrate for graphene-based flexible MEMS/NEMS devices.

利用分子动力学模拟方法探究了外加载荷作用下纳米尺度受限水膜的流动特性.仿真结果表明:受限空间内的水膜随着载荷的增加,其出现分层现象和黏度增加,当黏度超过一个临界值后,在分层和黏度增加共同作用下,水膜的流动状态将由层流和湍流的混合状态过渡到单一的层流状态.同时,随着受限空间壁面的切向运动,受限水膜均会出现边界滑移现象,且随着载荷的增加,滑移现象越发显著.但当水膜单一层流状态形成后,受限空间壁面的滑移速度,对水膜的边界滑移长度影响并不显著.

With higher requirement setting for hard disk substrate to minimize roughness and defects of the polished surface, abrasive-free polishing (AFP) of hard disk substrate has been put forward in this paper. The effect of Zn (II) ion on the AFP of hard disk substrate in the H2O2 based slurry was investigated by AFP tests. The results indicate that the material removal rate of hard disk substrate polished in slurry with Zn (II) ion is obviously higher than that without Zn (II) ion. And surface polished by slurry containing Zn (II) ion exhibits lower surface roughness and fewer nano-asperity peaks than that without Zn (II) ion. Furthermore, the acting mechanism of Zn (II) ion in AFP of hard disk substrate was analyzed. X-ray photoelectron spectroscopy analysis shows that metal Zn appears on the polished surface, implying the tribochemistry reaction occurs during AFP. The electrochemical reaction between metal Zn and oxide film Ni2O3 on the surface of hard disk substrate during AFP can promote the chemical effect in AFP and lead to the increasing of material removal rate.