Wetting behavior and nanotribological properties of silicon nanopatterns combined with diamond-like carbon and perfluoropolyether films

A large number of silicon (Si) patterns consisting of nanopillars of varying diameter and pitch have been fabricated and further coated with diamond-like carbon (DLC) and perfluoropolyether (Z-DOL) films. The wetting behavior and nano-adhesion/friction of the patterns are investigated experimentally in relation to the nanostructures and the hydrophobicity of the materials. Measurements of water contact angle illustrate that the patterning-enhanced wettability of the Si flat surface, along with two distinct wettings which are in good agreement with the Wenzel and hemi-wicking states, depended on the value of the pitch-over-diameter ratio. In the case of the coated patterns, three wetting states are observed: the Cassie-Baxter, the Wenzel, and a transition from the Cassie-Baxter into the Wenzel, which varies with regard to the hydrophobic properties of the DLC and Z-DOL. In terms of tribological properties, it is demonstrated that a combination of the nanopatterns and the films is effective in reducing adhesive and frictional forces. In addition, the pitch and diameter of the patterns are found to significantly influence their adhesion/friction behaviors.     

Keramik-Polymer Kombinationsschichten

Ceramic Polymer Composite Coatings The tribological properties of ceramic-polymer composite coatings which consist of thermal sprayed oxide ceramics coated with polymer lacquers for numerous industrial applications are discussed in this paper. The surface of these coatings match the requirements of high wear resistance and low coefficient of friction which is desired for many industrial applications. For experimental evaluation, samples were fabricated by coating aluminum substrates with titanium dioxide by means of atmospheric plasma spraying (APS). This oxide ceramic surface was successively coated by means of air spraying with a polymer lacquer containing molybdenum disulphide (MoS₂) and polytetrafluor ethylene (PTFE) as solid lubricants. The tribological properties (coefficient of friction and wear) of this combination layer were determined using a tribometer under oscillating sliding movement.     

Nanotribology and nanomechanics in nano/biotechnology

Owing to larger surface area in micro/nanoelectromechanical systems (MEMS/NEMS), surface forces such as adhesion, friction, and meniscus and viscous drag forces become large when compared with inertial and electromagnetic forces. There is a need to develop lubricants and identify lubrication methods that are suitable for MEMS/NEMS. For BioMEMS/BioNEMS, adhesion between biological molecular layers and the substrate, and friction and wear of biological layers may be important, and methods to enhance adhesion between biomolecules and the device surface need to be developed. There is a need for development of a fundamental understanding of adhesion, friction/stiction, wear, the role of surface contamination and environment, and lubrication. MEMS/NEMS materials need to exhibit good mechanical and tribological properties on the micro/nanoscale. Most mechanical properties are known to be scale dependent. Therefore, the properties of nanoscale structures need to be measured. Component-level studies are required to provide a better understanding of the tribological phenomena occurring in MEMS/NEMS. The emergence of micro/nanotribology and atomic force microscopy-based techniques has provided researchers with a viable approach to address these problems. This paper presents an overview of micro/nanoscale adhesion, friction, and wear studies of materials and lubrication studies for MEMS/NEMS and BioMEMS/BioNEMS. It also presents a review of scale-dependent mechanical properties, and stress and deformation analysis of nanostructures.     

Superior wear resistance of diamond and DLC coatings

As the hardest known material, diamond and its coatings continue to generate significant attention for stringent applications involving extreme tribological conditions. Likewise, diamond-like carbon (DLC, especially the tetragonal amorphous carbon, ta-C) coatings have also maintained a high level interest for numerous industrial applications where efficiency, performance, and reliability are of great importance. The strong covalent bonding or sp³-hybridizaiton in diamond and ta-C coatings assures high mechanical hardness, stiffness, chemical and thermal stability that make them well-suited for harsh tribological conditions involving high-speeds, loads, and temperatures. In particular, unique chemical and mechanical nature of diamond and ta-C surfaces plays an important role in their unusual friction and wear behaviors. As with all other tribomaterials, both diamond and ta-C coatings strongly interact with the chemical species in their surroundings during sliding and hence produce a chemically passive top surface layer which ultimately determines the extent of friction and wear. Thick micro-crystalline diamond films are most preferred for tooling applications, while thinner nano/ultranano-crysalline diamond films are well-suited for mechanical devices ranging from nano- (such as NEMS) to micro- (MEMS and AFM tips) as well as macro-scale devices including mechanical pump seals. The ta-C coatings have lately become indispensable for a variety of automotive applications and are used in very large volumes in tappets, piston pins, rings, and a variety of gears and bearings, especially in the Asian market. This paper is intended to provide a comprehensive overview of the recent developments in tribology of super-hard diamond and DLC (ta-C) films with a special emphasis on their friction and wear mechanisms that are key to their extraordinary tribological performance under harsh tribological conditions. Based on the results of recent studies, the paper will also attempt to highlight what lies ahead for these films in tribology and other demanding industrial applications.     

Nanotribology, nanomechanics and nanomaterials characterization

Nanotribology and nanomechanics studies are needed to develop fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in magnetic storage devices, nanotechnology and other applications. Friction and wear of lightly loaded micro/nanocomponents are highly dependent on the surface interactions (a few atomic layers). These structures are generally coated with molecularly thin films. Nanotribology and nanomechanics studies are also valuable in the fundamental understanding of interfacial phenomena in macrostructures and provide a bridge between science and engineering. An atomic force microscope (AFM) tip is used to simulate a single-asperity contact with a solid or lubricated surface. AFMs are used to study the various tribological phenomena that include surface roughness, adhesion, friction, scratching, wear and boundary lubrication. In situ surface characterization of local deformation of materials and thin coatings can be carried out using a tensile stage inside an AFM. Mechanical properties such as hardness, Young's modulus of elasticity and creep/relaxation behaviour can be determined on micro- to picoscales using a depth-sensing indentation system in an AFM.     

Tribological property and drilling application of Ti-C:H and Cr-C:H coatings on high-speed steel substrates

This study investigates the tribological properties and cutting performance of Ti-DLC and Cr-DLC doped metal coatings. The tribological properties of the coatings are evaluated by testing coated disks against an AISI 1045 steel counterbody under dry conditions using an oscillating friction wear tester, and then measuring the subsequent wear depth on the coated disk, the wear width on the steel counterbody, and the friction coefficient. The cutting performance of the coatings is evaluated by using coated high-speed drills to machine stainless steel workpieces, and then measuring the resulting flank wear and hole surface roughness. The results of the wear tests show that the Ti-C:H and Ti-C:H/TiC/TiCN/TiN coatings possess excellent tribological properties, including low coefficients of friction, low wear depths, and low wear widths. Regarding the machining tests, the Ti-C:H/TiC/TiCN/TiN coating has the lowest flank wear and yields the highest hole surface quality under both dry and cutting fluid drilling conditions. The single Ti-C:H coating has excellent tribological properties, but demonstrates a relatively poorer performance in the drilling of stainless steel. Finally, the Cr-DLC coatings all exhibit a poor cutting performance under dry cutting conditions.     

An innovative and versatile approach to design highly porous, patterned, nanofibrous polymeric materials

Electrospinning technology is attracting increasing interest for the fabrication of several polymeric and inorganic nanofibrous materials useful for a wide range of applications. Although the intensive research performed to understand the physics of the electrospinning process, control of polymeric jet path and of fibre deposition still remains a challenge. In this work, micropatterned electrospun materials with tunable porosity are fabricated by using novel collector devices made up of a steel sheet coated with a vitreous enamel layer. The novel approach permits to modulate electrical properties of the collector, acting on charge distribution and charge diffusion properties of the vitreous enamel coating, without introducing substantial modification of the electrospinning apparatus. The proposed solution allows the achievement of novel electrospun products for all those advanced applications requiring materials with suited fibre deposition such as, microelectronics, optical and photonic devices, as well as scaffolds for tissue engineering.     

Diamond-like carbon coatings for orthopaedic applications: an evaluation of tribological performance

A detailed investigation of the tribological behaviour of vacuum arc diamond-like carbon coated Ti–6Al–4V against a medical grade ultra-high molecular weight polyethylene is conducted in this work in order to investigate the potential use of diamond-like carbon coatings for orthopaedic appplications. Lubricated and non-lubricated wear experiments are performed using a standard pin-on-disc wear tester. The coefficient of friction is monitored continuously during testing and wear rate calculations are performed using surface profilometry measurements of worn disc surfaces. Sliding wear tests show the existence of two distinct friction and wear regimes distinguished by physically different mechanisms. In the first stages of wear, adhesion and abrasion are the dominant mechanisms of wear while fatigue processes are activated later in the tests. The effects of diamond-like carbon coating structure, surface roughness and lubrication on tribological behaviour are presented. Optimal process–structure–property design for vacuum arc plasma deposition is utilized in order to obtain strong adhesion to the titanium alloy substrate. Diamond-like carbon coatings significantly improve the friction and wear performance of the orthopaedic bearing pair and show exceptional promise for biomedical applications. © 1999 Kluwer Academic Publishers     

Micro/nanomechanical characterization of ceramic films for microdevices

Microelectromechanical systems (MEMS) are currently fabricated using single-crystal silicon, various polysilicon films and other ceramic materials. Silicon carbide (SiC) film has recently been pursued as a material for use in MEMS devices owing to its excellent mechanical properties and high-temperature capabilities. Since physical and chemical properties, friction and wear are important issues in such small-scale devices, it is essential that the materials used in MEMS have good micro/nanomechanical and tribological properties. Micro/nanomechanical characterization of single-crystal 3C-SiC (cubic or β-SiC) films, undoped and doped (n⁺-type) polysilicon films have been carried out. For comparision, measurements on undoped single-crystal Si(100) have also been made. Hardness, elastic modulus and scratch resistance of these materials were measured by nanoindentation and microscratching using a nanoindenter. Fracture toughness was measured by microindentation using a microindenter. Friction and wear properties were measured using an accelerated ball-on-flat tribometer. It is found that the 3C-SiC film exhibits higher hardness, elastic modulus and scratch resistance as well as lower friction compared to other materials. These results show that the 3C-SiC film possesses desirable micro/nanomechanical properties that make it an ideal material for use in MEMS devices.     

Maßgeschneiderte Tribologie durch Laseroberflächenbehandlung

Tailored tribology by laser surface treatment There are quite different requirements for tribological properties of surfaces in industry. Both reduction and maximization of friction and/or wear are possible requirements. The friction and wear properties depend on the tribological system consisting of the friction partners, the medium between them and the environment around them. So for each application tailored surfaces with special tribological properties are needed. In the paper examples are presented which deal with the investigation and the implementation of laser based processes to obtain surfaces with very different tribological properties. Wavelengths, output power values, intensity distributions and beam qualities of the available lasers vary in a wide range. Also the available devices for beam formation and beam guidance enable special processes for tailoring properties for particular applications. These processes are for example the single‐layered or multi‐layered laser cladding generating homogeneous or graded claddings, the laser alloying or laser dispersing and the laser stimulated deposition of diamond layers at atmospheric pressure.     

Physical and tribological characteristics of porous NiTi SMA fabricated by powder metallurgy

This paper presents the mechanical and tribological characteristics of the NiTi shape memory alloy (SMA) fabricated by powder metallurgy. This material has prominent applications in micro-electromechanical systems, medical implants, actuator, space and aerospace industries, etc. In every field, wear characteristics plays a dominating role. In present work dry-abrasion wear behavior is determined for NiTi alloy by varying binder percentage. With increasing binder percentage from 2.5 to 15 %, density decreases from 6.5 to 5.3 g/cm3 while porosity increases from 19 % to 51 %. Increasing rotational speed and binder percentage at a constant load the wear rate increases in the NiTi alloy. Due to the presence of hard particles, NiTi exhibits a very small wear rate. The coefficient of friction is also computed for the alloys in present research work. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction were used for the investigation of surface morphology and phases in the NiTi alloy.     

影响微电子机械系统成品率和可靠性和粘合力和磨擦力

章评述了影响微电子机械系统（MEMS）成品率和可靠性的粘合力和摩擦力问题。在用氢氟酸（HF）腐蚀牺牲层、释放多晶Si微结构、干燥时,由于Si片表面薄层水的表面张力使两片亲水、间隙在微米量级的Si片粘合起来,称为“释放有关粘合”。粘合也发生在封装后器件中,当输入信号过冲时,由于Si片表面的化学状态将Si片粘合起来,称为“使用中粘合”。解决粘合的最好办法是：在MEMS微结构的表面涂以抗粘合薄膜,将成品器件在干燥气氛下封装。介绍了抗粘合薄膜的制备工艺和目前存在的问题。相比之下,具有高速运动的MEMS,其摩擦力问题更为复杂。应和抗粘合薄膜,解决了粘合,也降低了摩擦力,但摩擦依然存在。摩擦带来磨损,降低器件可靠性和寿命。寻找既抗粘合、又耐磨的薄膜,是解决高速运动MEMS可靠性和寿命的一个关键。     

Tribological behaviour and acoustic emissions of alumina, silicon nitride and SAE52100 under dry sliding

The friction, wear and acoustic emission behaviour of various combinations of alumina, silicon nitride, and SAE52100 steel, operating under dry sliding conditions, was investigated. A designed ball-on-flat-disc type of tribometer was used to conduct these experiments. This apparatus, equipped with a force sensor, using silicon strain gauges, measured simultaneously the normal load and friction force. Both forces were used to determine the real-time value of the dynamic coefficient of friction. The AE signal arising from the interaction of the surfaces in dynamic contact was also detected and a data acquisition system was used to gather this signal as well as the outputs from the force sensor, at high frequency. The effects of test duration, sliding speed and normal load on the above mentioned tribological parameters were evaluated. The interest of this study further extended to assess the correlations that may exist between the integrated rms acoustic signal (AE) and the friction mechanisms, wear volume, friction work as well as the material removal power. Under the specific conditions of the present experiments, no consistent relation was found between the variations of AE and corresponding dynamic coefficient of friction (COF) as function of time. The variation of COF and wear rate, obtained considering a fixed total sliding distance of 500 m, as function of a range of sliding speed (0.05–2.5 m/s) and normal load (5–40 N) are presented. It was found that the test duration has an important impact on wear results of the experiments conducted at different sliding speeds and fixed travelling distance. More expected behaviour was observed when the relationships between the AE and wear volume, friction work, and material removal power were investigated considering the data obtained at different loadings and fixed sliding speed. Some models representing interesting relationships which could be used for predicting tribological properties in the case of practical applications, similar to the tribo-systems investigated in this study, are proposed.     

Tribological effects of polymer surface modification through plastic deformation

The efficacy of using polymers in cylindrical applications depends closely on its surface friction and wear characteristics. In this regard, a surface modification technique through plastic deformation has been implemented. Roller burnishing is commonly used to improve the surface quality of non-ferrous surfaces, but no work showed concern about roller burnishing as a polymer surface treatment process. The objective of the present work is to investigate the influence of burnishing force and burnishing speed on the friction and wear performance of acetal homopolymer and polyurethane under dry and lubricated sliding conditions. The results reveal that the coefficient of friction and wear rate decreased to a minimum value and then increased as higher burnishing force and speed were applied. It was shown that roller burnishing had favourable prospective to be utilized as a valuable polymer surface treatment technique.     

Latest progress on tribological properties of industrial materials

Wear is closely related to friction and lubrication; the study of these three subjects is known as tribology. In science and technology it is concerned with interacting surfaces in relative motion. Soft or hard film coating, alloying and composite structuring have all been developed to control wear and friction. This is achieved by improving materials and surfaces with some characteristics that improve resistance to friction and wear. In recent years, several new solid lubricant and modern lubrication concepts have been developed to achieve better lubricity and longer wear life in demanding tribological applications. Most of the traditional solid lubricants were prepared in the form of metal, ceramic and polymer–matrix composites. They have been used successfully in various engineering applications. Recent progress in thin-film deposition technologies has led to the synthesis of new generations of self-lubricating coatings with composite or multilayered architectures, by using multiplex surface treatments. In this study, typical wear behaviors of representative materials of metallic alloys, ceramics, polymeric materials, and composites are reviewed in relation to their friction behaviors. Additionally, modeling for the wear prediction is outlined.     

Comparison of tribological behaviour for Nylon6-Al-Al2O3 and ABS parts fabricated by fused deposition modelling

In the present work, a comparison of the tribological properties of Nylon6-Al-Al₂O₃ and ABS parts fabricated using the fused deposition modelling (FDM) method is made. Under dry sliding conditions at room temperature, the sliding wear behaviour of FDM-built parts with three different proportions of Al and Al₂O₃ was investigated. Loads of 5, 10, 15 and 20 were applied at a sliding velocity of 1.36 m/s for durations of 5 and 10 minutes. The results show that all FDM-built Nylon6-Al-Al₂O₃ components have better wear resistance as compared to their FDM-built ABS counterparts. Further, the influences of filler materials on various wear mechanisms such as adhesion, abrasion, etc. are identified. It was also observed that the composite materials prepared with different proportions are more wear resistant, having less friction coefficients and friction force than the commercially used ABS material for FDM components.     

Effect of laser surface texturing on Si3N4/TiC ceramic sliding against steel under dry friction

In efforts to investigate the influence of the surface texturing on the Si₃N₄/TiC ceramic, laser surface texturing (LST) was performed on the Si₃N₄/TiC ceramic by an Nd:YAG laser and different geometrical characteristics of regular-arranged micro-grooved textures were fabricated on the surfaces. The tribological properties of the textured and smooth samples were investigated by carrying out sliding wear tests against steel balls under dry condition using a ball-on-disk tribometer. Effect of surface texturing on the stress distribution was studied by finite element method (FEM). Results show that the textured surfaces exhibited lower friction coefficient and excellent anti-wear properties compared with smooth surfaces. The tribological characteristics depended greatly on the size and density of the micro-grooves, and the geometrical characteristics of the surface textures have a significant effect on the tribological behavior. Among the patterns investigated, the wavy-grooved samples exhibit the lowest friction coefficient and wear rate; and a large texture density may be the best for reduction of friction and wear of textured samples. While, the wear rate of balls sliding against textured surfaces is larger than that of balls sliding against smooth surfaces. FEM results show that surface texturing can improve the stress distribution of contact interfaces and reduce stress concentration.     

Influence of CaCO3 Whisker Content on Mechanical and Tribological Properties of Polyetheretherketone Composites

The mechanical and tribological properties of polyetheretherketone (PEEK) composites filled with CaCO3 whisker in various content of 0-45% (wt pct) were investigated. The composite specimens were prepared by compression molding. Tribological testing of composites in dry wear mode against carbon steel ring was carried out on a MM200 block-on-ring apparatus. Data on neat PEEK were also included for comparison. It was observed that inclusion of CaCO3 whisker affected the most mechanical properties and the friction and wear in a beneficial way. With an increase in CaCO3 whisker content, friction coefficient continuously decreased but the trends in wear performance varied. The specific wear rate showed minima as 1.28×10-6 mm3/Nm for 15% CaCO3 whisker inclusion followed by a slow increase for further CaCO3 whisker addition. In terms of friction applications, when the tribological and mechanical properties are combined, the optimal content of CaCO3 whisker in the filled PEEK should be recommended as 15% to 20%. Fairly good correlations are observed in friction coefficient vs bending modulus and wear rate vs bending strength, confirming that the bending properties prove to be the most important tribology controlling parameters in the present work.     

W6Mo5Cr4V2高速钢WC/Co激光熔覆表面的摩擦磨损性能

为了改善高速钢表面的摩擦磨损性能,应用激光熔覆技术在W6Mo5Cr4V2高速钢表面制备出WC/Co熔覆道。采用三维数码显微镜观察熔覆试件的金相组织并借助显微硬度计测试其显微硬度。采用销盘式摩擦磨损试验机分别对高速钢和WC/Co熔覆试件进行了摩擦磨损试验,并采用三维数码显微镜观察磨损形貌。结果表明:与高速钢基材相比,WC/Co熔覆道硬度提高,熔覆试件的摩擦系数和磨损量降低;WC/Co熔覆道的磨损机制以磨粒磨损为主,熔覆道间隔以磨粒磨损和黏着磨损为主;熔覆道的硬度提高、减摩效果、散热作用以及试件表面熔覆道与间隔面的软硬交替,有助于提高WC/Co熔覆试件的摩擦磨损性能。     

Tribaloy alloy reinforced tin-bronze composite coating for journal bearing applications

This article presents an experimental study of the tribological behavior of a tin/bronze-based composite coating. The improved-ductility Tribaloy alloy (T-401) particles are selected as the reinforcement. This coating is made on the bushing of planet journals used in aerospace engines, deposited with the high velocity oxygen fuel (HVOF) thermal spray technique. The tribological properties such as friction and wear resistance of the coated bushing are investigated under the WAMsc3 Sliding Contact Test, along with the leaded tin/bronze bushing tested for comparison. The testing results show that the bushing coated with the composite exhibits superior tribological properties to the leaded tin/bronze bushing in that the former runs longer before the friction coefficient reaches 0.5 and also leads a to lower wear rate than the latter. The experimental results and wear mechanisms of these two bushings are discussed with the assistance of worn surface analyses using scanning electron microscopy (SEM).