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.     

Wear characteristics of Cr3C2–NiCr and WC–Co coatings deposited by LPG fueled HVOF

Wear behavior of the HVOF deposited Cr₃C₂–NiCr and WC–Co coatings on Fe-base steels were evaluated by the pin-on-disc mechanism. The constant normal load applied to the pin was 49 N and sliding distance was 4500 m with velocity of 1 m/s, at ambient temperature and humidity. The specific wear rate of WC–Co coating was 3 mm³/N m and Cr₃C₂–NiCr coating was 5.3 mm³/N m. SEM/EDAX and XRD techniques were used to analyze the worn out surface and wear debris. The Fe₂O₃ was identified as the major phase in the wear debris. The wear mechanism is mild adhesive wear in nature.     

Microstructure and Tribology of Spark Plasma Sintered Fe–Cr–B Metamorphic Alloy Powder

The spark plasma sintering (SPS) process was used to fabricate a bulk Fe–Cr–B alloy (known as Armacor M) from gas-atomized powders. The purpose of this work is to study the microstructure, hardness and tribology of this sintered bulk alloy. Post microstructure and mechanical characterizations were performed to investigate the effects of wear on the microstructure and mechanical properties. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) showed that SPS successfully produced a fully dense bulk material containing 67 vol.% Cr_1.65Fe_0.35B_0.96 particles dispersed in 33 vol.% solid solution matrix consisting of Fe, Cr and Si. Using nanoindentation, the hardness of the Cr_1.65Fe_0.35B_0.96 particles and the matrix was found to be 24 and 6 GPa, respectively. From microindentation, the bulk hardness of the sintered alloy was 9.7 GPa (991 HV). Dry sliding wear testing under mild conditions (i.e., initial Hertzian mean contact pressure of 280 MPa) was conducted against a stainless steel pin. The steady state coefficient of friction against Armacor M was about 0.82. The wear of Armacor M proceeded primarily by adhesive and mild oxidative wear. The wear volume for Armacor M was 80% less than that of carbon steel and its wear rate was 5.53 × 10⁻⁶ mm³ N⁻¹ m⁻¹.     

The Microstructure and Abrasive Wear Resistance of Fe–Cr–C Hardfacing Alloys with the Composition of Hypoeutectic,Eutectic, and Hypereutectic at $$ \frac{Cr}{C} = 6 $$

In this investigation, three Fe–Cr–C hardfacing alloys with different carbon and chromium contents and in constant ratio of ( \fracCrC = 6 ) \left( {\frac{Cr}{C} = 6} \right) were fabricated by GTAW on AISI 1010 mild steel substrates. The OES, OM, SEM, and XRD techniques and Vickers hardness method were used for determining chemical composition, hardness, and studying the microstructure of the hardface alloys. The OES, OM, and XRD examination results indicated that different carbon and chromium contents of hardface alloys produced hypoeutectic/eutectic/hypereutectic structures. By increasing the carbon and chromium contents in the chemical composition of hardface alloys, the volume fraction of the total (Cr, Fe)₇C₃ is increased resulting to decreasing in total the austenite volume fraction and increasing the hardness of the surface. Studying the microstructure after wear test (ASTM G65) shows that at the edge of the worn surface, the transformation of austenite to martensite had occurred in all the samples. The wear test results indicate that the highest wear resistance is gained in the hypoeutectic structure with maximum hardness after the wear test. In addition, abrasive wear micromechanisms in hypoeutectic/eutectic/hypereutectic were recognized as: ploughing + cutting/ploughing + cutting + cracking/cracking + cutting, respectively.     

Friction and Wear Properties of IF–MoS2 as Additive in Paraffin Oil

Inorganic fullerene-like (IF) MoS₂ nanoparticles with diameters ranging from 70 to 120 nm were synthesized by desulphurizing the MoS₃ precursor and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Tribological properties of the IF–MoS₂, as lubricating oil additive, were evaluated using a MMW-1 four-ball tribotester. The wear scar was examined with an optical microscope and scanning electron microscopy (SEM). The wear resistance of the paraffin oil was improved and the friction coefficient of the oil was decreased by addition of the IF–MoS₂ nanoparticles. The mechanism of friction and wear of the IF–MoS₂ nanoparticles was discussed.     

Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

MoS₂–Sb₂O₃–C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a “chameleon” coating. Coatings used in this report were applied by burnishing mixed powders of MoS₂, Sb₂O₃ and graphite. The solid lubricant MoS₂ and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb₂O₃ was used as a dopant because it acts synergistically with MoS₂, improving friction and wear properties. The MoS₂–Sb₂O₃–C composite films showed lower friction and longer wear life than either single component MoS₂ or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS₂–Sb₂O₃–C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb₂O₃, which is not lubricious, was covered by the active lubricants (MoS₂ – dry, C – humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb₂O₃ particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.     

Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)–Al2O3–Ba x Sr1?x SO4 (x?=?0.25, 0.5, 0.75) Composites at Elevated Temperature

Self-lubricating ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have been fabricated by spark plasma sintering (SPS) method. The tribological properties have been evaluated using a high-temperature friction and wear tester at room temperature and 760 °C in dry sliding against alumina ball. The composites exhibit distinct improvements in effectively reducing friction and wear, as compared to the unmodified ZrO₂(Y₂O₃)–Al₂O₃ ceramics. The ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have great low and stable friction coefficients of less than 0.15 and wear rates in the order of 10^− 6mm³/Nm at 760 °C. Delamination is considered as the dominating wear mechanism of the composites at room temperature. At elevated temperature, the formation and effective spreading of Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) lubricating films during sliding play an important role in the reduction of the friction and wear.     

Effect of Tribofilm Formation on the Dry Sliding Friction and Wear Properties of Magnetron Sputtered TiAlCrYN Coatings

Nano-structured TiAlCrYN coatings, grown by unbalanced magnetron sputtering on various steel substrates, exhibited friction coefficients 0.6–0.8 and wear coefficients 10⁻¹⁶–10⁻¹⁵ m³ N⁻¹ m⁻¹ in dry sliding wear tests. This article reports comprehensive worn surface analyses using SEM, TEM, EDX, EELS and Raman spectroscopy. A ~80 nm thick tribofilm formed on the TiAlCrYN worn surface was found to have dense amorphous structure and homogeneous oxide composition of Cr_0.39Al_0.19Ti_0.20Y_0.01O_0.21. Viscous flow of the amorphous tribofilm was dominant in causing the high friction coefficient observed. The coatings showed combined wear mechanisms of tribo-oxidation and nano-scale delamination.     

Environmental Effects on the Tribology and Microstructure of MoS<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>–C Films

The tribology of molybdenum disulfide (MoS₂)–Sb₂O₃–C films was tested under a variety of environmental conditions (ambient 50% RH, 10⁻⁷ Torr vacuum, 150 Torr oxygen, and 8 Torr water) and correlated with the composition of the surface composition expressed while sliding. High friction and low friction modes of behavior were detected. The lowest coefficient of friction, 0.06, was achieved under vacuum, while sliding in 8 Torr water and ambient conditions both yielded the highest value of 0.15. Water vapor was determined to be the environmental species responsible for high friction performance. XPS evaluations revealed a preferential expression of MoS₂ at the surface of wear tracks produced under vacuum and an increase in Sb₂O₃ concentration in wear tracks produced in ambient air (50% RH). In addition, wear tracks produced by sliding in vacuum exhibited the lowest surface roughness as compared to those produced in other environments, consistent with the picture of low friction originating from well-ordered MoS₂ layers produced through sliding in vacuum.     

Effect of Fluoride Content on Friction and Wear Performance of Ni<Subscript>3</Subscript>Al Matrix High-Temperature Self-Lubricating Composites

The self-lubricating composites Ni₃Al–BaF₂–CaF₂–Ag–Cr, which have varying fluoride contents, were fabricated by the powder metallurgy technique. The effect of fluoride content on the mechanical and tribological properties of the composites was investigated. The results showed that an optimal fluoride content and a balance between lubricity and mechanical strength were obtained. The Ni₃Al–6.2BaF₂–3.8CaF₂–12.5Ag–10Cr composite showed the best friction coefficients (0.29–0.38) and wear rates (4.2 × 10⁻⁵–2.19 × 10⁻⁴ mm³ N⁻¹ m⁻¹) at a wide temperature range (room temperature to 800°C). Fluorides exhibited a good reduced friction performance at 400 and 600°C. However, at 800°C, the formation of BaCrO₄ on the worn surface due to the tribo-chemical reaction at high temperatures provided an excellent lubricating property.     

High Temperature Tribological Characteristics of Fe–Mo-based Self-Lubricating Composites

Fe–Mo-based self-lubricating composites were prepared by a powder metallurgical hot-pressing method. The tribological properties of Fe–Mo-based composites with varied CaF₂ contents at high temperature were evaluated, and the effect of glaze films on the friction and wear characteristics of composites were analyzed. The results show that the introduction of CaF₂ into Fe–Mo alloys improved the mechanical properties, and the best tribological properties of Fe–Mo–CaF₂ composites were achieved at the CaF₂ content of 8 wt% at both room temperature and 600 °C. The worn surface of Fe–Mo–CaF₂ composite at 600 °C is characterized to plastic deformation and slight scuffing, and the improved tribological properties are attributed to the formation of lubricious glaze film that composed of high-temperature lubricants CaMoO₄ and CaF₂ on the worn surface of the composites.     

Dialkyldithiophosphate Acids (HDDPs) as Effective Lubricants of Sol–Gel Titania Coatings in Technical Dry Friction Conditions

The goal of this study was the investigation of the effectiveness of dialkyldithiophosphate acids (HDDPs) films in improving the tribological properties of thin, sol–gel derived titania coatings. Amorphous, anatase, and rutile titania coatings were obtained using sol–gel dip–coating deposition after treatment at 100, 500, and 1,000 °C, respectively. Titania coatings were then modified from the liquid phase by HDDPs acids having dodecyl-(C₁₂), tetradecyl-(C₁₄), and hexadecyl-(C₁₆) alkyl chains deposited by dip–coating (DC) and Langmuir–Blodgett (LB) methods. The influence of the deposition procedure, the length of the HDDPs alkyl chain and the type of titania substrate on the surface morphology and tribological properties were studied. It was found, using wetting contact angle measurements, that these modifications of titania coatings decrease the surface free energy and increase its hydrophobicity. The surface topography imaged by Atomic force microscopy (AFM), exhibit island-like or agglomerate features for the DC deposition method, while smooth topographies were observed for LB depositions. Tribological tests were conducted by means of a microtribometer operating in the normal load range 30–100 mN. An enhancement of tribological properties was observed upon modification, as compared to unmodified titania.     

Influence of Heat Treatment on Microstructure and Sliding Wear of Thermally Sprayed Fe-Based Metallic Glass Coatings

Fe₆₂Ni₃Cr₄Mo₂W₃Si₆B₁₇C₃ amorphous coatings were thermally sprayed by a high velocity oxygen fuel spraying system (DJ-2700) and heat-treated at the temperatures ranges from 873 to 1,173 K in vacuum for 1 h. Differential scanning calorimetry, X-ray diffraction (XRD), and scanning electron microscopy were used to study the microstructural characteristics of the coatings. Vickers hardness tester was used to measure the hardness of the coatings. At the same time, the sliding wear behavior of the coatings was evaluated in a reciprocating ball-on-disk system. Within the resolution of XRD, amorphous structure without apparent crystalline phases was obtained in the as-sprayed coating. The heat treatments above 873 K led to the crystallization of amorphous phase. With the increase of heat treatment temperature, diffusion and sintering could occur between the layers of the coatings. The highest microhardness was obtained in the coating heat-treated at 973 K. When wear tested at a relative low load of 2 N, a direct correlation between the hardness and wear resistance of the coatings seems to be reasonable. However, at relative high loads, the wear resistance of the coatings is dependent on the resistance to crack initiation and growth between the layers rather than the hardness.     

NiAl Matrix High-Temperature Self-Lubricating Composite

A high-temperature self-lubricating composite NiAl–Cr–Mo–CaF₂ was fabricated using the powder metallurgy technique, and the tribological behavior of the composite at a wide range of temperatures (room temperature to 1000 °C) was investigated. The results showed that the composite had a favorable friction coefficient of about 0.2 and an excellent wear resistance of about 1 × 10⁻⁵ mm³N⁻¹m⁻¹ at the high temperatures tested (800 and 1000 °C). The excellent self-lubricating performance was attributed to the formation of the glaze film on the worn surface consisting mainly of CaCrO₄ and CaMoO₄ as high-temperature solid lubricants.     

Semi-Empirical Molecular Modeling of Ionic Liquid Tribology: Ionic Liquid–Aluminum Oxide Surface Interactions

The interactions between the selected ionic liquids (ILs) and aluminum oxide surfaces are modeled in this report using theoretical methods. A wide range of ILs and their interactions with an aluminum oxide surface are modeled using the PM5 semi-empirical method. The ILs modeled in this study contain imidazolium (C_{3, 4, 6, 8 or 10}mim) or ammonium cations including (C₆H₁₃)₃NH⁺, (C₈H₁₇)₃NH⁺, C₈H₁₇NH₃ ⁺, (C₂H₅)₃NH⁺, and (C₈H₁₇)NH₃ ⁺. The anions include Cl⁻, Br⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, and (C₂F₅SO₂)₂N⁻. The interactions of these ILs with an Al–O surface are modeled in a stepwise manner. The lowest energy forms of the individual ILs are determined, and these ILs are allowed to form a complex with the Al–O surface. The resulting reaction enthalpies of ionic liquid-surface complex formation are seen to correlate with the tribological properties of the ILs. The strongest correlations occur within those ILs containing similar cations.     

Friction and Wear of Thermal Oxidation-Treated Ti3SiC2

Ti₃SiC₂ was thermally oxidized (TO) at 1,000 °C for 10 h. An oxide scale of ca. 25 μm was composed of rutile TiO₂ and Al₂O₃ for the outer sub-layer and mixtures of TiO₂ and SiO₂ for the inner sub-layer. The tribological behavior of Ti₃SiC₂ and TO–Ti₃SiC₂ sliding against Si₃N₄ at 25 and 600 °C was investigated. Results indicated that at both 25 and 600 °C, the oxide scale significantly improved the tribological performance of Ti₃SiC₂. The wear mechanisms of Ti₃SiC₂ and TO–Ti₃SiC₂ sliding against Si₃N₄ at 25 and 600 °C are briefly discussed.     

Ni3Al Matrix Composite with Lubricious Tungstate at High Temperatures

Ni₃Al–Ag–BaF₂/CaF₂–W composites were fabricated by the powder metallurgy route, and their tribological properties over a wide temperature range, starting from room temperature up to 800 °C, were investigated. The Ni₃Al matrix composite with 15 wt% BaF₂/CaF₂ exhibited a favorable friction coefficient (range 0.3–0.4) and wear rate (0.2–6.2 × 10⁻⁴mm³ N⁻¹ m⁻¹). The formation of BaWO₄ and CaWO₄ with lubricity on the worn surface due to a tribo-chemical reaction at high temperatures provided excellent lubricating properties. The low friction coefficient over a broad temperature range could be attributed to the synergistic effect of Ag, BaF₂/CaF₂, BaWO₄, and CaWO₄.     

Ultra-High Tribological Performance of Magnetron Sputtered a-C:H Films in Sand-Dust Environment

The hydrogenated amorphous carbon (a-C:H) films were prepared on AISI 440C steel substrates using a RF magnetron sputtering graphite target in the CH₄ and Ar mixture atmosphere. The friction and wear behavior of a-C:H films were comparatively investigated by pin-on-disc tester under dry sliding and simulated sand-dust wear conditions. In addition, the effects of applied load, amount of sand and sand particle sizes on the tribological performance of a-C:H films were systemically studied. Results show that a-C:H films exhibited ultra-high tribological performance with low friction coefficient and ultra-low wear rate under sand-dust environments. It is very interesting to observe that the friction coefficient of a-C:H film under sand-dust conditions was relatively lower when compared with dry sliding condition, and the wear rate under sand-dust conditions kept at the same order of magnitude (×10⁻¹⁹ m³/N m) with the increase of applied load and particle size as a comparison with the dry sliding condition. Based on the formation of “ridge” layer (composite transfer layer), a transfer layer-hardening composite model was established to explain the anti-wear mechanisms and friction-reducing capacity of a-C:H solid lubrication films under sand-dust conditions.     

Wear resistant multilayer nanocomposite WC1−x/C coating on Ti–6Al–4V titanium alloy

A significant improvement of tribological properties on Ti–6Al–4V has been achieved by developed in this study multilayer treatment method for the titanium alloys. This treatment consists of an intermediate 2 μm thick TiC_xN_y layer which has been deposited by the reactive arc evaporation onto a diffusion hardened material with interstitial O or N atoms by glow discharge plasma in the atmosphere of Ar+O₂ or Ar+N₂. Subsequently, an external 0.3 μm thin nanocomposite carbon-based WC_1−x/C coating has been deposited by a reactive magnetron sputtering of graphite and tungsten targets. The morphology, microstructure, chemical and phase compositions of the substrate material after treatment and coating deposition have been investigated with use of AFM, SEM, EDX, XRD, 3D profilometry and followed by tribological investigation of wear and friction analysis. An increase of hardness in the diffusion treated near-surface zone of the Ti–6Al–4V substrate has been achieved. In addition, a good adhesion between the intermediate gradient TiC_xN_y coating and the Ti–6Al–4V substrate as well as with the external nanocomposite coating has been obtained. Significant increase in wear resistance of up to 94% when compared to uncoated Ti–6Al–4V was reported. The proposed multilayer system deposited on the Ti–6Al–4V substrate is a promising method to significantly increase wear resistance of titanium alloys.     

Nanocomposite Microstructure and Environment Self-Adapted Tribological Properties of Highly Hard Graphite-Like Film

Graphite-like carbon (GLC) nanocomposite films were fabricated by DC magnetron sputtering using high pure graphite target at ambient temperature. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) investigation showed that the as-deposited GLC films have high concentration of sp²-hybridized carbon. High-resolution transmission electron microscopy (HRTEM) images and selected area diffraction patterns (SADP) indicated a complex nanocomposite microstructure of the GLC films. As well as nanocrystalline graphite, a face-center cubic (fcc) diamond with a grain size in the range of 3–8 nm were dispersed in the amorphous carbon matrix inhomogenously and integrally. The nanocomposite GLC film had high hardness of 23 GPa, which was attributed to the mutual strengthening effect of nanoparticles and amorphous matrix. More importantly, the as-deposited nanocomposite GLC film exhibited excellent self-adapted tribological properties in different environments of ambient air, different relative humidity and water. The friction coefficients were 0.053 in ambient air and 0.046 in distilled water, while specific wear rates were 4.5 × 10⁻¹⁶ m³ N⁻¹ m⁻¹ and 1.6 × 10⁻¹⁶ m³ N⁻¹ m⁻¹, respectively. The friction regimes and mechanisms in different environments were elaborated. This film is foreseen to high potential in protecting and solid lubricating material in humidity or water environment.