Tribological behavior of RH ceramics made from rice husk sliding against stainless steel,alumina, silicon carbide,and silicon nitride

The tribological behavior of rice husk (RH) ceramics, a hard, porous carbon material made from rice husk, sliding against stainless steel, alumina, silicon carbide, and silicon nitride (Si₃N₄) under dry conditions was investigated. High hardness of RH ceramics was obtained from the polymorphic crystallinity of silica. The friction coefficients for RH ceramics disks sliding against Si₃N₄ balls were extremely low (<0.1), irrespective of contact pressure or sliding velocity. Transfer films from RH ceramics formed on Si₃N₄ balls. Wear-mode maps indicated that the wear modes were powder formation under all tested conditions, resulting in low specific wear rates (<5×10⁻⁹ mm²/N).     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 2. The analysis of wear mechanism

This paper describes the tribological performance of diamond-like carbon (DLC) coatings deposited on AISI 440C steel substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. A variety of analytic techniques were used to characterize the coatings, such as Raman spectroscopy, atomic force microscopy (AFM) and nano-indentation. The sliding wear and friction experiments were carried out by the conventional ball-on-disk tribometry against 100Cr6 steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). All the wear tests were conducted under dry sliding condition in ambient air for a total rotation cycle of 1 × 10⁵ (sliding distance ∼2.2 km). Surfaces of the coatings and the steel balls were examined before and after the sliding wear tests. The DLC coatings that had been tested all showed relatively low values of friction coefficient, in the range of 0.1-0.2 at a steady-state stage, and low specific wear rates (on the order of 10⁻⁸ mm³/Nm). It was found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Plastic deformation became more evident on the coating surface during the sliding wear test at higher contact stresses. The friction-induced transformation of the coating surface into a graphite-like phase was revealed by micro-Raman analysis, and the flash temperature of the contact asperities was estimated. It was suggested that the structural transformation taking place within the wear tracks was mainly due to the formation of compact wear debris layer rather than the frictional heating effect. On the other hand, an adherent transfer layer (tribolayer) was formed on the counterface, which was closely related to the steady-state friction during sliding and the wear mechanisms. Fundamental knowledge combined with the present tribological study led to the conclusion that adhesive wear along with abrasion was probably the dominant wear mechanism for the DLC/steel sliding systems. Additionally, fatigue processes might also be involved in the wear of the coatings.     

Effects of sliding velocity and normal load on friction and wear characteristics of multi-layered diamond-like carbon (DLC) coating prepared by reactive sputtering

Friction and wear tests were performed to investigate effects of sliding velocity and normal load on tribological characteristics of a multi-layered diamond-like carbon (DLC) coating for machine elements. The DLC coatings which consist of sequentially deposited gradient Cr/CrN, W-doped DLC (a-C:H:W) and DLC (a-C:H) layers were formed on carburized SCM 415 Cr–Mo steel disks using a reactive sputtering system. The tests against AISI 52100 steel balls were performed under various sliding velocities (0.0625, 0.125, 0.25, 0.5, 1 and 2 m/s) and normal loads (6.1, 20.7 and 49.0 N) in ambient air (relative humidity=26±2%, temperature=18±2 °C). Each test was conducted for 20 km sliding distance without lubricating oil. The results show that friction coefficients decrease with the increase in sliding velocity and normal load. Wear rates of both surfaces decrease with the increase in normal load. The increase in sliding velocity leads initially to the increase in wear rates up to the maximum value. Then, they decrease, as the sliding velocity increases above specific value that corresponds to the maximum wear rate. Through surface observation and analysis, it is confirmed that formation of transfer layers and graphitized degree of wear surfaces of DLC coatings mainly affect its tribological characteristics.     

Dry Sliding of Nitrided NiTiHf

The unlubricated friction and wear properties of the superelastic material NiTiHf, treated by either gas nitriding or plasma nitriding, have been investigated. Pin-on-disk testing of the studied materials was performed at sliding speeds from 0.01 to 1 m/s at normal loads of 1, 5, or 10 N. For all of the studied friction pairs (NiTiHf pins vs. nitrided disks) over the given parameters, the steady-state coefficients of friction varied from 0.7 to 1.6, and those of the control (NiTiHf on NiTiHf) varied from 0.5 to 1.2. Pin wear factors ranged from approximately 10^?6 against the NiTiHf and plasma-nitrided disks to approximately 10^?4 for the gas-nitrided disks. The plasma-nitrided disks provided wear protection in several cases and tended to wear by adhesion. The gas-nitrided treatment generated the most pin wear but essentially no disk wear except under the most severe of the studied conditions (1 N load and 1 m/s sliding speed). The results of this study are expected to provide guidance for design of aerospace components such as gears and fasteners.     

Tribological Properties of DLC Coatings in Helium

The aim of this research work was to investigate tribological properties of low-friction DLC coatings when operating in helium atmosphere. Two commercial DLC coatings (a-C:H and Me-C:H) were included in the investigation and compared to reference PTFE-based coatings, normally used on components operating in helium. Coatings were deposited on hardened 100Cr6 bearing steel discs and tested against uncoated steel balls in low-load pin-on-disc contact configuration. Investigation was focused on the effect of substrate roughness (R _a ?=?0.05?C0.2???m) and contact conditions, including contact pressure (150?C350?MPa) and sliding speed (0.2?C0.4?m/s) on the coefficient of friction of DLC coatings operating in helium. Results of this investigation show that for low-load sliding contact DLC coatings provide low friction in helium atmosphere, similar to soft PTFE-based coatings. At the same time DLC coatings investigated were found to substantially reduce wear of the coated surface. However, while the wear of the coated part has been more or less eliminated, application of DLC coating prolongs running-in and increases wear of the steel counter-part. Furthermore, also in helium atmosphere tribolgical behaviour of DLC coatings showed dependence on the coating type and contact conditions.     

Effect of counterface balls on the friction layer of Ni3Al matrix composites with 1.5 wt% graphene nanoplatelets

Research on the friction layer is needed to minimize friction- and wear-related mechanical failures in moving mechanical assemblies. Dry sliding tribological tests of Ni₃Al matrix composites (NMCs) with 1.5 wt% graphene nanoplatelets (GNPs) sliding against different counterface balls are undertaken at the condition of 10 N–0.234 m s^?1 in this study. When sliding against GCr15 steel, a uniform and thick friction layer is formed, resulting in a lower friction coefficient (0.29–0.31) and wear rate (2.0–3.1 × 10^?5 mm³ N^?1 m^?1). While sliding against Al₂O₃ and Si₃N₄, the formation and stability of the friction layers are restricted in the severe wear regime, and the NMCs exhibit higher friction coefficients and wear rates. Therefore, various counterface balls have a great effect on the stability and thickness of the friction layer, thus affecting the tribology performance of NMCs. The result also shows that GNPs exhibit enrichment and self-organized microstructures in the friction layer. In addition, the friction layer is also found to be divided into two layers, protecting the subsurface from further damage and reducing shear.     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

An investigation of the wear of abrasive grains by rubbing on diamond disks

In earlier wear studies¹ single abrasive grains were rubbed against the surfaces of metal disks under light loads. These studies have now been extended to include an examination of wear behaviour when using diamond-impregnated surfaces at lower sliding speeds and when using more accurately controlled test conditions. These wear tests provided values of wear resistance in the absence of chemical effects (i.e. at sufficiently low surface temperatures). The results are in excellent agreement with the conventional wear theory pertaining to lightly loaded sliders. The wear volumes are, however, two orders of magnitude greater than those obtained when rubbing tough grains against steel disks and three orders of magnitude greater than the results obtained when rubbing friable grains against similar metallic surfaces.     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

The sliding behaviors of copper alloys

Several copper alloys were rubbed against steel, chromium electroplate and tantalum at sliding speeds of 1.7 m s^?1 without lubricants. The friction, wear, metal transfer and scuffing tendencies were observed with a view to correlating these with the properties of the alloys.Except for aluminum bronze and the welded overlay rotating band materials, harder alloys showed less wear. Metal transfer, scuffing (rough deposits) and friction were not found to correlate with any property. An attempt was then made to attribute behavior to special microstructure, crystal orientation, mutual solubility and position in the periodic table but no trend was found.Heavy transfer was not usually associated with high wear and rough deposits were not usually associated with either of these phenomena.     

Ultralow Friction Behaviors of Hydrogenated Fullerene-Like Carbon Films: Effect of Normal Load and Surface Tribochemistry

Friction and wear behaviors of hydrogenated fullerene-like (H-FLC) carbon films sliding against Si₃N₄ ceramic balls were performed at different contact loads from 1 to 20 N on a reciprocating sliding tribometer in air. It was found that the films exhibited non-Amontonian friction behaviors, the coefficient of friction (COF) decreased with normal contact load increasing: the COF was ~0.112 at 1 N contact load, and deceased to ultralow value (~0.009) at 20 N load. The main mechanism responsible for low friction and wear under varying contact pressure is governed by hydrogenated carbon transfer film that formed and resided at the sliding interfaces. In addition, the unique fullerene-like structures induce well elastic property of the H-FLC films (elastic recovery 78%), which benefits the high load tolerance and induces the low wear rate in air condition. For the film with an ultralow COF of 0.009 tested under 20 N load in air, time of flight secondary ion mass spectrometry (ToF-SIMS) signals collected inside and outside the wear tracks indicated the presence of C₂H₃ ⁻ and C₂H₅ ⁻ fragments after tribological tests on the H-FLC films surface. We think that the tribochemistry and elastic property of the H-FLC films is responsible for the observed friction behaviors, the high load tolerance, and chemical inertness of hydrogenated carbon-containing transfer films instead of the graphitization of transfer films is responsible for the steady-state low coefficients of friction, wear, and interfacial shear stress.     

Tribological Properties of Diamond-Like Carbon Films in Low and High Moist Air

Diamond-like carbon (DLC) film was deposited on Si wafer by a plasma CVD deposition system using benzene. Tribological properties of the DLC film were evaluated using a ball-on-disk tribo-meter in low (RH 1720 %) and high humidity (RH 9095 %) conditions in air. The effect of sliding speed (4.2 mm/s to 25 mm/s) and load (1.06 N to 3.08 N) on friction and wear was investigated. The friction behavior of the DLC film was obviously different in low and high humidity. When tested under low humidity conditions, the friction coefficient decreased significantly with increasing speed, and increased with load. However, under high humidity conditions, the friction coefficient increased with the speed and decreased with increasing load. The wear of the DLC film was little influenced by the sliding speed, normal load and humidity; a level of 10^-8 mm³/Nm could be obtained in all tests. The formation of a uniform transfer layer would be the main factor which controlled the friction coefficient of the DLC films. Unlike the friction, the wear resistance of the DLC film is not so easy to discuss and may be affected mainly by the tribo-chemical reaction in all the test conditions.     

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si₃N₄ sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO₂ results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si₃N₄, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si₃N₄ disks sliding against 1/4 in Si₃N₄ balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BF_x and PF_x films on the surface and creation of an electric double layer of ionic liquid.     

Tribological characterisation of hard coatings with and without DLC top layer in fretting tests

The potential of coatings to protect components against wear and to reduce friction has led to a large variety of protective coatings. In order to check the success of coating modifications and to find solutions for different purposes, initial tests with laboratory tribometers are usually done to give information about the performance of a coating. Different Ti‐based coatings (TiN, Ti(C,N), and TiAlN) and NiP were tested in comparison to coatings with an additional diamond‐like carbon (DLC) top coating. Tests were done in laboratory air at room temperature with oscillating sliding (gross slip fretting) with a ball‐on‐disc arrangement against a ceramic ball (Al₂O₃). Special attention was paid to possible effects of moisture (relative humidity). The coefficient of friction was measured on line, and the volumetric wear at the disc was determined after the test from microscopic measurements of the wear scar and additional profiles. The friction and wear behaviour is quite different for the different coatings and depends more or less on the relative humidity. The DLC coating on top of the other coatings reduces friction and wear considerably. In normal and in moist air the coefficient of wear of the DLC top‐layer coating is significantly less than 10⁻⁶ mm³/Nm and the coefficient of friction is below 0.1. In dry air, however, there is a certain tendency to high wear and high friction. Copyright © 2006 John Wiley & Sons, Ltd.     

Tribochemical effects on the friction and wear behaviors of a-C:H and a-C films in different environment

Friction and wear behaviors of hydrogenated amorphous carbon (a-C:H) and hydrogen-free amorphous carbon (a-C) films sliding against Si₃N₄ balls were investigated in different testing environments. The result showed that two films with extreme chemical disparity (one hydrogenated, and the other hydrogen free) showed distinct different friction and wear behaviors, and the friction and wear behaviors of the both films were strongly dependent on the environment. For a-C:H films, much low friction coefficient and wear rate were obtain in dry N₂. In the water and/or oxygen containing environments, the friction coefficient and wear rate of a-C:H films were obviously increased. On the contrary, a-C films only provided low friction coefficient and wear rate in the presence of water and/or oxygen in the test chamber. In dry N₂, the highest friction coefficient and wear rate were observed for a-C films. By investigating the worn surfaces of the films using XPS, it was proposed that the environment dependence of the friction and wear behaviors of the films was closely related with the friction-induced chemical interactions between the films and water and/or oxygen molecules. The specific roles of hydrogen, water and oxygen molecules and their tribochemical effects on the friction and wear mechanism of the films are discussed.     

Rippling on Wear Scar Surfaces of Nanocrystalline Diamond Films After Reciprocating Sliding Against Ceramic Balls

The formation of nanoscopic ripple patterns on top of material surfaces has been reported for different materials and processes, such as sliding against polymers, high-force scanning in atomic force microscopy (AFM), and surface treatment by ion beam sputtering. In this work, we show that such periodic ripples can also be obtained in prolonged reciprocating sliding against nanocrystalline diamond (NCD) films. NCD films with a thickness of 0.8 µm were grown on top of silicon wafer substrates by hot-filament chemical vapor deposition using a mixture of methane and hydrogen. The chemical structure, surface morphology, and surface wear were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and AFM. The tribological properties of the NCD films were evaluated by reciprocating sliding tests against Al₂O₃, Si₃N₄, and ZrO₂ counter balls. Independent of the counter body material, clear ripple patterns with typical heights of about 30 nm induced during the sliding test are observed by means of AFM and SEM on the NCD wear scar surfaces. Although the underlying mechanisms of ripple formation are not yet fully understood, these surface corrugations could be attributed to the different wear phenomena, including a stress-induced micro-fracture and plastic deformation, a surface smoothening, and a surface rehybridization from diamond bonding to an sp ² configuration. The similarity between ripples observed in the present study and ripples reported after repeated AFM tip scanning indicates that ripple formation is a rather universal phenomenon occurring in moving tribological contacts of different materials.     

Tribological Evaluation of an Al2O3-SiO2 Ceramic Fiber Candidate for High Temperature Sliding Seals

A test program to determine the relative slitting durability of an alumina-silica candidate ceramic fiber for high temperature sliding seal applications is described. Pin-on-disk tests were used to evaluate the potential seal material by sliding a tow or bundle of the candidate ceramic fiber against a superalloy test disk. Friction was measured during the tests and fiber wear, indicated h the extent of fibers broken in the tow or bundle, was measured at the end of each test. Test variables studied included ambient temperature from 25° to 900°C, loads from 1.3 to 21.2 N, and sliding velocities from 0.025 to 0.25 m/sec. In addition, the effects of fiber diameter and elastic modulus on friction and wear were measured. Thin gold films deposited on the superalloy disk surface were evaluated in an effort to reduce friction and wear of the fibers.

In most cases, wear increased with test temperature. Friction ranged from 0.36 at 500°C and low velocity (0.025 miser) to over 1.1 at 900°C and high velocity (0.25 m/sec). The gold films resulted in satisfactory lubrication of the fibers at 25°C. At elevated temperatures diffusion of substrate elements degraded the films. These results indicate that the alumina-silica (Al₂O₃SiO₂) fiber is a good candidate material system for high temperature sliding seal applications. More work is needed to reduce friction.