Effect of surface chemistry on the tribological performance of a MEMS electrostatic lateral output motor

The effect of surface chemistry on the tribological performance and reliability of a MEMS lateral output motor is reported. Relative humidity (RH) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) coatings were used to change surface chemistry. Electrical and tribological performance of uncoated and OTS-coated motors were found to be dependent on RH. For uncoated motors, excessive wear of sliding contacts and welding (permanent adhesion) of static contacts were observed at 0.1% RH. Degradation of electrostatic force and high static friction (stiction) forces limited dynamic performance and reliability and caused device sticking at and above 70% RH. Around 50% RH, uncoated motors exhibited negligible wear, low adhesion, and a wear life at least three orders of magnitude longer than in the dry environment (experiments were stopped without failure after about one billion cycles). Water vapor behaved as a gas phase replenishable lubricant by providing a protective adsorbed film. The OTS coating broadened the operating envelope to 30–50% RH and reduced stiction, which allowed better dynamic performance at high RH. The OTS coating improved durability at 0.1% RH, but it was still poor. At high RH, stiction problems reoccurred when the OTS coating was worn away. By controlling and balancing surface chemistry (adsorbed water and OTS), excellent performance, low friction and wear, and excellent durability were attained with the lateral output motor.     

Friction and fracture properties of polysilicon coated with self-assembled monolayers

The increasing use of small micromechanical devices and advanced sensors has led to concern about the failure modes and reliability of these structures. The enormous promise will not materialize without substantial progress in overcoming the stiction, friction and wear associated with such devices and understanding the mechanical behavior of MEMS materials and structures. Self-assembled monolayers (SAMs) are release and anti-stiction coatings for MEMS. In this paper, the anti-stiction properties of octadecyltrichlorosilane (OTS) SAM were calculated. The microtribological properties of OTS SAM were investigated with a ball-on-flat microtribometer. The influence of OTS SAM on the mechanical properties of micromachined polysilicon films for MEMS was investigated with an accurate evaluation using the microtensile test device. It was concluded that the OTS SAM has good anti-stiction properties and low friction coefficients. The hydrophobic property of OTS is the main factor leading to an increase in the average fracture strength of micromachined polysilicon up to 32.46%. Thus, the operational stability and lifetime of MEMS can be raised when coated with self-assembled monolayers.     

Self-assembled monolayer on diamond-like carbon surface: formation and friction measurements

We have investigated self-assembled monolayers (SAMs) of heptadecafluoro-1,1,2,2-tetradecyltrietoxysilane (FTE) on diamond-like carbon (DLC) surfaces formed by a simple immersing process. SAM formation on DLC surfaces was verified by contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy (XPS). Water and hexadecane contact angles increased gradually with immersing time and saturated at about 110 and 70 degrees, respectively. Ellipsometric measurements showed that the film thickness was 1.4 to 1.6 nm, which corresponded reasonably to the thickness of FTE monolayer. XPS data showed the presence of FTE molecules on the DLC surface. These results ensured the SAM formation of FTE molecules on the DLC surface.We further measured and compared the friction of unlubricated, SAM coated and 2 nm thick perfluoropolyether (PFPE) coated DLC surfaces using lateral force microscopy (LFM) as functions of the applied load and the sliding velocity. The SAM coated DLC surfaces showed lower friction than the unlubricated DLC surfaces and the friction coefficient decreased by about 15% compared to the unlubricated DLC surfaces. Scratch tests revealed that the critical load of the DLC film increased due to the SAM deposition. These results are attributed to the hydrophobic nature of the SAM coated surface. On the other hand, even though the water contact angle of the SAM coated surface was larger than the 2 nm thick PFPE coated surface, the friction of the SAM coated surface was larger than that of the PFPE coated surface. Also, the critical load of the SAM coated DLC surface in scratch test was lower than the PFPE coated surface. These results indicate that the hydrophobic nature of the surface is not the only factor which determines the friction characteristics in the nano-lubricating system, and it is attributed to the mobile characteristic of PFPE lubricant.     

Tribological properties of asperity arrays coated with self-assembled monolayers

The effects of a self-assembled monolayer (SAM) coating on the friction and pull-off forces were determined by using two-dimensional asperity arrays on silicon wafers. The arrays were coated with SAM composed of one of five different alkylchlorsilanes. First, two-dimensional asperity arrays were created by using a focussed ion beam (FIB) system to mill patterns on silicon plates. Each silicon plate had different patterns of equally spaced asperities. Each pattern (5 × 5 μm²) had a different radius of curvature of the asperity peaks, ranging from about 200 to 2500 nm. Then, each silicon plate was immersed in a solution of a different alkylchlorsilane in hexane (either hexyltrichlorosilane, octyltrichlorosilane, dodecyltrichlorosilane, tetradecyltrichlorosilane, or octadecyltrichlorosilane), thus coating the asperity arrays with SAM. The friction and pull-off forces on the SAM-coated arrays were measured by using an atomic force microscope (AFM) that had a square flat probe. The pull-off force for SAM-coated silicon was roughly proportional to the radius of curvature of the asperity peaks. The magnitude of the pull-off force corresponded approximately to the capillary force calculated by using the contact angle of water on the surface of SAM. The friction coefficient correlated with the inverse of the alkyl-chain length of the SAM.     

CrAlTiN梯度涂层高速钢刀具的切削性能研究

用闭合场非平衡磁控溅射离子镀PVD涂层工艺在高速钢麻花钻上沉积了CrA lTiN梯度涂层。在干式切削条件下,对45#号钢和30CrMnS iA钢进行了钻削试验。通过涂层与未涂层钻头的寿命、磨损和切削力等试验比较,表明CrA lTiN梯度涂层钻头的切削性能远优于未涂层钻头,是一种极有发展前途的刀具涂层。     

Performance Evaluation of a Hard Composite Solid Lubricant Coating When Dry Machining of High-carbon Steel

A novel hard composite solid lubricant coating combining TiN and MoS_x has been developed using pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS). The tribological and mechanical properties together with their interdependencies with the coating microstructures have been assessed and reported elsewhere. This article evaluates the machining performance and correlates the underlying tribological aspects of different TiN-MoS_x coating architectures (deposited at titanium (Ti) cathode currents of 1, 3.5, and 5 A) when dry turning AISI 1080 high-carbon steel. A comparative performance study clearly established the supremacy of the composite coating (deposited at 3.5 A Ti cathode current with ～12 wt% of MoS_x) with a hard TiN underlayer over monolayer TiN, MoS_x, and other related coating architectures in terms of cutting force, tool wear, and workpiece surface roughness. The superlubricity behavior of the said composite coated tool resulted in a reduction of cutting force (by up to ～45% compared to the uncoated tool) and exhibited a tool life of 8 min, which was eight times and more than two times longer than that of the uncoated and conventional hard TiN coated counterparts, respectively. The workpiece surface roughness, R_a, also decreased by 13 to 21% when machined with the TiN-MoS_x coated tool in comparison to the uncoated cemented carbide.     

Fretting damage of TiN coated zircaloy-4 tube

Fretting has been reported and investigated for over 50 years. However, it is still one of the modern plagues for several industrial machineries. Especially, fretting of fuel rod cladding material, zircaloy-4 tube, in pressurized water reactor (PWR) must be reduced and avoided. Thin hard coatings are employed to improve the tribological properties such as friction and wear of conventional engineering materials. Among these coatings, physical vapor deposition (PVD) TiN coating is probably one of the most frequently and successfully used PVD coatings for the mitigation of fretting wear. Therefore, in this study a fretting wear experiment was performed using TiN coated zircaloy-4 tube as the fuel rod cladding material and uncoated zircaloy-4 tube as one of the grids. The fretting tester was designed and manufactured for this experiment. The number of cycles, slip amplitude and normal load were selected as main factors of fretting. The type of contact was cylinder-to-cylinder contact. The worn surface was observed by optical microscope, 3-D surface measuring instrument and scanning electron microscope (SEM). The results of this research showed that the wear volume of TiN coated zircaloy-4 tube decreased about 1.2–3 times more than uncoated tube and wear mechanisms were brittle fracture, fatigue fracture, adhesion, abrasion and oxidation.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

Development of Surface Coatings for Air-Lubricated,Compliant Journal Bearings to 650°C

Surface coatings for an air-lubricated, compliant journal for an automotive gas turbine engine were tested to find those capable of withstanding temperatures of either 540°C (1000°F) or 650°C (1200°F). Also, the coatings have to be capable of surviving the start-stop sliding contact cycles prior to rotor lift-off and at touchdown. Selected coating combinations were tested in start-stop tests at 14 kPa (2 psi) loading for 2000 cycles at room and maximum temperatures. Specific coating recommendations are: Cdo and graphite on foil versus chrome carbide on journal up to 370°C (700°F); NASA PS-120 (Tribaloy 400, silver, and CaF₂) on journal versus uncoated foil up to 540°C (1000°F); and chemcially adherent Cr₂O₃ on journal and foil up to 650°C (1200°F). The chemically adherent Cr₂O₃ coating system was further tested successfully at 35 kPa (5 psi) loading for 2000 start-stop cycles.     

Fretting behavior of a rubber coating: Friction characteristics of rubber debris

This paper presents friction characteristics of a rubber coating undergoing fretting-like, oscillatory motion. Results are presented for rubber coated stainless steel as well as uncoated stainless steel experiencing fretting motion. Friction coefficients are experimentally investigated before and after the rubber coating is fully worn out. The effect of load, velocity and displacement amplitude on the friction coefficients of both uncoated and coated stainless steel is discussed.     

808nm含铝半导体激光器的腔面镀膜

研究了高功率808nm量子阱脊型波导结构含铝半导体激光器在空气中解理时不同镀膜方法对输出激光功率的影响,讨论了半导体激光器的灾变性光学镜面损伤机理及其腔面钝化薄膜的选择特性。对半导体激光器管芯前后腔面不镀膜,前后腔面镀上反射膜和前后腔面先镀上钝化薄膜再镀腔面反射膜方法进行了对比,测试了半导体激光器的输出功率。结果表明,先镀上钝化薄膜的器件比只镀上腔面反射膜的器件输出的激光功率高36%。只镀腔面反射膜的半导体激光器器件在电流为5A时就失效了,而镀钝化膜的器件在电流为6A时仍未失效,说明镀钝化薄膜的器件能有效地防止灾变性光学损伤和灾变性光学镜面损伤。在半导体激光器芯片腔面镀上钝化薄膜是提高大功率半导体激光器输出功率的有效方法。     

Friction and wear behaviors of MoS2/Zr coated HSS in sliding wear and in drilling processes

MoS2 metal composite coatings have been successful used in dry turning, but its suitability for dry drilling has not been yet established. Therefore, it is necessary to study the friction and wear behaviors of MoS2/Zr coated HSS in sliding wear and in drilling processes. In the present study, MoS2/Zr composite coatings are deposited on the surface of W6Mo5Cr4V2 high speed steel(HSS). Microstructural and fundamental properties of these coatings are examined. Ball-on-disc sliding wear tests on the coated discs are carried out, and the drilling performance of the coated drills is tested. Test results show that the MoS2/Zr composite coatings exhibit decreases friction coefficient to that of the uncoated HSS in sliding wear tests. Energy dispersive X-ray(EDX) analysis on the wear surface indicates that there is a transfer layer formed on the counterpart ball during sliding wear processes, which contributes to the decreasing of the friction coefficient between the sliding couple. Drilling tests indicate that the MoS2/Zr coated drills show better cutting performance compared to the uncoated HSS drills, coating delamination and abrasive are found to be the main flank and rake wear mode of the coated drills. The proposed research founds the base of the application of MoS2 metal composite coatings on dry drilling.     

Performance and Durability of High Temperature Foil Air Bearings for Oil-Free Turbomachinery

The performance and durability of advanced, high temperature foil air bearings are evaluated under a wide range (10 to 50 kPa) of loads at temperatures from 25° to 650 °C. The bearings are made from uncoated nickel based superalloy foils. The foil surface experiences sliding contact with the shaft during initial start/stop operation. To reduce friction and wear, the solid lubricant coating, PS304, is applied to the shaft by plasma spraying. PS304 is a NiCr based Cr₂O₃ coating with silver and barium fluoride/calcium fluoride solid lubricant additions.

The results show that the bearings provide lives well in excess of 30,000 cycles under all of the conditions tested. Several bearings exhibited lives in excess of 100,000 cycles. Wear is a linear function of the bearing load. The excellent performance measured in this study suggests that these bearings and the PS304 coating are well suited for advanced high temperature, oil-free turbomachinery applications.     

Tribological performance of nano-thin fluoropolymer coated sliders

With the increase in recording density, any accumulation on slider surface can cause serious problems, such as high fly–stiction and extensive slider–disk interaction. Therefore, how to mitigate the accumulation on slider surface and thus, improve the stability and reliability of head–disk interface is becoming an important issue. In this work, a nanothin fluoropolymer overcoat with very good oleophobic and hydrophobic properties was applied on slider surface with a dipping process. Tribological performance, such as fly–stiction, normal stiction, and takeoff and landing processes, of the coated sliders was studied. Test results show that although normal stiction is not lowered, normal stiction modulation is reduced obviously by the overcoat. Fly–stiction and its modulation of coated sliders are much smaller than those of uncoated sliders. Coated sliders show much better takeoff and landing performance during contact start stop tests. After tests, the surfaces of tested sliders and disks were examined with an optical microscope, surface reflectance analyzer, and TOF–SIMS to interpret the tribological performance of the coated sliders. It can be concluded that the fluoropolymer overcoat reduces the amount of accumulation on slider surface and thus, improves the tribological performance of the coated sliders.     

Functionalization of Scanning Force Microscopy Cantilevers via Galvanic Displacement Technique

A galvanic displacement technique is used to coat silicon scanning force microscopy cantilevers with copper. The copper coating is characterized using X-ray photoelectron spectroscopy, scanning force microscopy, contact angle measurements, scanning electron microscopy and energy-dispersive X-ray spectroscopy. This coating technique results in uniform, reflective and conformal films and hence, no stress-induced bending of the cantilever is observed. To demonstrate the effectiveness of this approach for tribological studies, the coated cantilevers are chemically modified with alkanethiol monolayers in order to functionalize the cantilevers. The effect of changed surface energy are detected with adhesion measurements in water and ethanol.     

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.     

Scuffing Performance of Amorphous Carbon During Dry-Sliding Contact

Scuffing is a major problem that limits the life and reliability of sliding tribo-components. When scuffing occurs, friction force rises sharply and is accompanied by an increase in noise and vibration; severe wear and plastic deformation also occur on the damaged surface. Attempts have been made over the years to combat scuffing by enhancing the surface properties of the machine elements, and by methods involving lubricant formulation and coating application.

In this study, the authors evaluated the scuffing performance of an amorphous, near-frictionless carbon (NFC) coating that provides super-low friction under dry sliding conditions. The test configuration used a ball-on-flat contact in reciprocating sliding. The coating was deposited on HI3 steel. An uncoated 52100 steel ball was tested against various coated flats in room air. Compared to uncoated surfaces, the carbon coating increased the scuffing resistance of the sliding surfaces by two orders of magnitude. Microscopic analysis shows that scuffing occurred on coaled surfaces only if the coating had been completely removed. It appears that depending on coating type, the authors observed that coating failure occurs before scuffing failure by one of two distinct mechanisms: the coating failed in a brittle manner and by spoiling, or by gradual wear.     

Wear and friction characteristics of atmospheric plasma sprayed Cr3C2–NiCr coatings

ABSTRACT

The present work focuses on investigating the wear and friction characteristics of the Atmospheric Plasma Sprayed Cr₃C₂-NiCr coatings deposited onto the surface of die steel material. The as-sprayed specimens were characterized. The coating porosity, bond strength and microhardness values were evaluated. Wear tests were performed on the high-temperature pin-on-disc tribometer at room temperatures, 400°C and 800°C under two loads as 25N and 50N in the laboratory. The wear mechanisms of all the worn-out samples were studied by scanning electron microscopy (SEM) technique. The specific wear rates and the coefficient of friction values were analyzed. The developed coating showed better wear resistance than its uncoated counterpart. The coefficient of friction values for coated specimens decreased at elevated temperatures. At room temperatures, the wear mode was observed to be adhesive and further at elevated temperatures of testing, the wear mode was observed to be the combination of oxidative, adhesive and abrasive.     

Effect of feldspar porcelain coating upon the wear behavior of zirconia dental crowns

Opposing dental surfaces, both natural teeth and restoration materials, are submitted to wear. The effect of the presence of feldspar porcelain coating upon the wear properties of dental zirconia opposing human teeth was evaluated using pin-on-plate test geometry. Human molar cusps performed as pins, coated and uncoated commercial zirconia performed as plates. Tests were carried out at room temperature in citric acid solution during 21,600 cycles, using 1 Hz, 1.96 N and 5 mm amplitude. Wear loss was measured by weighing the cusps before and after testing. The material loss of the plates was assessed by profilometry. Surface roughness and hydrophilicity of the plates' surfaces were evaluated by roughness and contact angle measurements.Results show higher mass loss for teeth tested against feldspar veneered plates, together with higher friction coefficient. No wear was detected on unveneered zirconia surfaces. Contact angle results show distinct affinity of veneered (25°) and unveneered zirconia (70°) surfaces towards distilled water.Porcelain coating of zirconia dental crowns affects tooth/crown wear behavior, resulting in increased wear of both the artificial crown and the opposing natural teeth. Coating should therefore be avoided in occlusal crown surfaces.