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.     

Tribological behavior of plasma-enhanced CVD a-C:H films. Part I: effect of processing parameters

A systematic study was conducted on the effect of plasma-enhanced CVD processing parameters, namely bias voltage, pressure and CH₄/Ar flow ratio, on the characteristics and tribological response of amorphous hydrogenated carbon (a-C:H) films. Film hardness, intrinsic stress, structure, composition and tribological response were characterized. Variation of processing parameters was found to produce a-C:H films with a range of characteristics with the CH₄/Ar ratio exercising a dominant effect. A low ratio produced harder films with more sp³ bonding, low hydrogen content and low wear rate; whereas a high ratio produced softer films, with more sp² bonding, higher hydrogen content and low friction. Film characteristics were found to affect the wear mechanism with softer films showing a layer-by-layer removal and harder films involving formation of fine debris. These two diverse types of films offer the opportunity to synthesize multilayered films combining desirable properties from each component.     

Testing Atmosphere Effect on Friction and Wear Behaviors of Duplex TiC/a-C(Al) Nanocomposite Carbon-Based Coating

Due to strongly tribological atmosphere sensitivity of carbon-based coatings, it is of extreme significance to investigate their friction and wear behaviors in different atmospheres. In this letter, duplex nc-TiC/a-C(Al) nanocomposite carbon-based coating coupled with high hardness, low internal stress and high adhesion strength was successfully fabricated using magnetron sputtering process. The friction and wear behaviors of as-fabricated coating were evaluated in dry N₂, humid N₂, air, dry O₂, and humid O₂ atmospheres, respectively. Results show that the as-fabricated coating possesses very high friction and wear due to the strong covalent bond interactions at the sliding interface caused by the free ??-bonds on the coating surface in dry N₂ atmosphere. Whereas the free ??-bonds can be efficiently terminated and passivated by water and/or oxygen molecules to weaken the strong covalent bond interactions to result in low friction and wear of coating in humid N₂, air, dry O₂, and humid O₂ atmospheres. The compact and homogeneous carbonaceous tribo-layer on the counterpart is mainly responsible for the lowest friction and wear of coating in humid N₂ atmosphere. Whereas the tribo-layer can be restrained to a certain extent by the tribo-chemical reaction, especially it results in a nearly negligible carbonaceous tribo-layer on the counterpart in dry O₂ atmosphere, which is mainly responsible for largely increased friction and wear of coating.     

Grey theory applied to evaluate the tribological performances of the a-C:H(N) coating films prepared by differing the nitrogen content and the film thickness

The purpose of this paper is to study the tribology performances of the aC:H(N) films by using a nanotester under different scratch loads and velocities. From the measurements of the friction coefficient and wear volume, the tribological performances including wear resistance and friction coefficients were evaluated for the hydrogenated amorphous carbon films prepared by differing film thickness and nitrogen volume friction in the gas mixture of (C₂H₂+N₂). Taguchi experimental design and the grey relational analysis were used to investigate the influence of specimen parameters (film’s thickness, nitrogen content in the film), and operating conditions in tribological tests (scratch load and scratch velocity) on the friction coefficients and the wear volume arising in the specimens with different coating films. It is found that the wear volume of thin film is increased by increasing either the nitrogen volume fraction or film thickness. Moreover, the optimal combination of the testing parameters was also determined in the use of the present model.     

Tribological Properties of Nanocomposite CrC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/a-C:H Thin Films

Recently we showed that coatings, prepared by unbalanced magnetron sputtering from a metallic Cr target in an Ar/CH₄ discharge are composed of nanocrystalline CrC _x embedded in an a-C:H matrix. This work investigates the structural correlation of such nanocomposite CrC _x /a-C:H coatings to their tribological properties. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase composition and the chemical bonding in the films deposited at different experimental conditions. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy. For CrC _x -dominated coatings deposited at CH₄ partial to total pressure ratios (p_CH4/p_t) < 0.42, only minor changes regarding the friction coefficients and the abrasive wear rates were observed although microstructural changes towards a higher degree of crystallinity were proven by transmission electron microscopy and substantiated with XPS results. For a-C:H dominated coatings deposited at p_CH4/p_t > 0.42, the friction coefficients and abrasive wear rates were shown to decrease with increasing a-C:H phase content and its more sp²-like bonding configuration. It can be concluded that the microstructural changes in terms of CrC _x crystallite coarsening and bonding configuration of the a-C:H matrix phase are responsible for the observed changes of the friction coefficients and wear rates.     

Structure, Mechanical, and Tribological Properties of MoS2/a-C:H Composite Films

A series of hydrogenated amorphous carbon (a-C:H) films doped with molybdenum disulfide (MoS₂) were deposited by medium frequency unbalanced magnetron sputtering with mixed Ar/CH₄ gases of different volume ratios as the source gases. The effects of Ar/CH₄ ratio on morphology, microstructure, mechanical, and tribological properties of the MoS₂/a-C:H composite films were investigated. Results show that the content of MoS₂ in the as-deposited films decreases with the decreasing Ar/CH₄ ratio, and the highest Ar/CH₄ ratio favors the formation of nanostructured films. Besides, the hardness and internal stress of the composite films first decrease and then increase with decreasing Ar/CH₄ ratio. Furthermore, the film deposited at the highest Ar/CH₄ ratio exhibits excellent antiwear ability in all test environments and shows promising potential as a solid lubricating film in aviation and space industries.     

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.     

Influence of CH4 Flow Rate on Microstructure and Properties of Ti-C:H Films Deposited by DC Reactive Magnetron Sputtering

Nanocomposite Ti-containing hydrogenated carbon films (Ti-C:H) were prepared using a DC reactive magnetron sputtering system. The relationship between CH₄ flow rate and the film characterization and tribological behaviors in both ambient air and deionized water conditions were investigated. Results showed that the Ti content in the as-deposited Ti-C:H films decreased and the sp³ content increased with an increase in CH₄ flow rate. TiC nanocrystallites can be formed at a relatively low CH₄ flow rate, whereas there was almost no formation of TiC in the amorphous carbon matrix at the highest CH₄ flow rate. The hardness, elastic modulus, and internal stress of the films were decreased firstly and then increased as the CH₄ flow rate increased, whereas their adhesion presented an inversely changing trend. The friction coefficients and wear rates of Ti-C:H films in both ambient air and deionized water conditions decreased with increasing CH₄ flow rate from 8 to 12 sccm and then increased as the CH₄ flow rate continually increased. In particular, the nanocomposite Ti-C:H film deposited with a CH₄ flow rate of 12 sccm could achieve superior combining mechanical properties and low friction and high antiwear behaviors in both ambient air and deionized water conditions, indicating potential applications as a protective and lubricating film for mechanical components.     

Structure and water-lubricated tribological properties of Cr/a-C coatings with different Cr contents

The Cr containing amorphous carbon coatings (Cr/a-C) with varying Cr content were deposited using unbalanced magnetron sputtering. The results revealed that the chromium carbide nano-clusters were formed when the Cr content exceeded 4.9 at%. The critical load increased while the hardness decreased after the Cr element incorporation. Although the low Cr containing Cr/a-C coatings (≤4.9 at%) exhibited similar friction coefficient with a-C coatings, but the initial friction coefficient, running-in distance and wear rate of SUS440C balls all decreased. However, the Cr/a-C coatings with high Cr content (11.98–14.09 at%) would worsen the tribological properties because chromium carbides acted as abrasive wear particles during tribotests.     

Tribological behavior between micro- and nano-crystalline diamond films under dry sliding and water lubrication

The tribological behavior of micro- and nano-crystalline diamond films is evaluated in dry sliding and water lubricating condition. The main wear mechanism is found to be abrasive wear mode induced by self-polishing. Non-diamond components and higher compressive residual stresses are detected in flat MCD films after dry sliding, in comparison to NCD. Origin of decreased friction coefficient in CVD diamond tribosystems under water lubrication is attributed to the effect of water on the formed graphic material and the chemisorbing of diamond surface with H₂O, hydrogen or hydroxyl ions. For the MCD/NCD or NCD/MCD contact, the surface roughness of ball largely determines the stable friction coefficient in dry sliding, where NCD film usually presents higher wear rate.     

Influence of humidity on the friction of diamond and diamond-like carbon materials

The friction of diamond and diamond-like carbon (DLC) materials was evaluated in reciprocating sliding wear testing under controlled relative humidity. The testing conditions were a displacement stroke of 100 μm, an oscillatory frequency of 8 Hz and a normal load of 2 N. The coefficient of friction of diamond and hydrogen-free DLC (a-C) coatings against a corundum sphere in the steady regime decreased with an increase in relative humidity. A water layer physisorbed at the interface between the mating surfaces played two major roles: acting as a lubricant and increasing the true area of contact. However, it was noticed that the friction coefficient of the hydrogenated DLC (a-C:H) coatings first increased and then decreased with increasing relative humidity in the steady state. There appeared to be a critical relative humidity for the a-C:H coatings, at which the steady-state friction reached the maximum value. The frictional behaviour of the a-C:H coatings also showed dependence on the wear test duration. The interaction between hydrogen and oxygen at the interface between the a-C:H coating and water layer was mainly responsible for such behaviour.     

Fretting of WC/a-C:H and Cr2N Coatings Under Grease-Lubricated and Unlubricated Conditions

The fretting phenomenon was investigated experimentally in contacts between coated and uncoated steel rod and ball specimens generating a circular Hertzian contact. A fretting wear test rig equipped with a video camera was used to observe the effects of fretting on coated steel surfaces in both grease-lubricated and unlubricated environments. Tungsten carbide reinforced amorphous hydrocarbon (WC/a-C:H) and chromium nitride (Cr₂N) coatings were tested and compared. Fretting wear volumes and surface profiles are presented for both grease-lubricated and unlubricated conditions. Videos of a coated ball fretting against a transparent sapphire flat were recorded and screen captures are presented. The role of normal load, lubrication, frequency, and amplitude of motion on the fretting wear of coatings is discussed. The lubricant released from the grease was observed to flow through channels in the stick zone of the fretting contacts. Both coatings were found to reduce fretting wear. WC/a-C:H was more effective at reducing wear under unlubricated conditions. WC/a-C:H decreased fretting wear more than Cr₂N when delamination was avoided in grease-lubricated contacts.     

Ultra-low friction of TiC/a-C composite coatings

Ultra-low coefficient of friction with a statistically averaged value of 0.0067 was observed during the tribological test on TiC/a-C composite coating synthesized by gas-solid interaction of Ti metal with CH₄ gas in a thermogravimetric analyzer. Ultra-low friction behavior appeared due to transformation of D-band associated with disordered carbon lattice obtained at lower synthesis temperatures (1050 and 1150 °C) to graphitic G-band mode, which steam from ordered atomic layering occurring at higher synthesis temperature of 1250 °C.     

Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride

Friction and wear of Si₃N₄ sliding on itself were measured at room temperature in different gaseous and liquid environments. At low sliding speed the friction coefficient ? is 0.85 in dry argon and nitrogen and 0.8 in laboratory air and oxygen. In dry gases, wear occurs by two fracture mechanisms: within 1 μm of the surface, asperity contact produces very large local stresses and cracking on a very fine scale; 3–5 μm deeper the fracture follows weaknesses of the material and is intergranular fracture with some transgranular cleavage. No evidence of plastic deformation was obtained. In water- and humidity-saturated gases wear is predominantly by a tribochemical reaction which produces an amorphous protective layer in humid gas and dissolution in liquid water. In intermediate humidity, wear is a combination of fracture and tribochemistry; the latter increases adhesion between wear particles to form a layer of compacted wear particles on the wear track. The fact that humidity decreases wear in Si₃N₄ and increases it in A1₂O₃ is explained by the differences in chemical reactivity and susceptibility to stress corrosion cracking between the two materials.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.     

Microtribological Studies of Different Nanocomposite TiC/a-C:H Coatings Using a Modified Nanoindentation Setup

The microtribological behavior of different nanocomposite TiC/a-C:H coatings against 100Cr6 (AISI 52100) balls with 250 m radius has been studied using a modified nanoindentation setup and was compared to the results of macroscopic pin-on-disc (POD) experiments. First results reveal significant differences between macroscopic friction coefficients _POD determined using POD tests and microscopic friction coefficients _micro. On the macroscopic scale low friction coefficients can be obtained for high hardness coatings. On the microscopic scale the high hardness samples induce considerable wear on the steel counterbody leading to high microscopic friction coefficients of around 0.3. For samples with lower hardness no wear has been observed and low microscopic friction coefficients (< 0.2) can be acheieved.     

碳基薄膜水润滑性能的研究进展

评述了碳基薄膜如类金刚石薄膜(DLC)和非晶氮化碳(a-CNx)薄膜水润滑的研究现状和进展。分析了第2元素加入和摩擦副材料对碳基薄膜在水中摩擦磨损特性的影响,探讨了碳基薄膜在水中的磨损机制。指出:氢化或氮化碳基薄膜的磨损率与摩擦副材料的水合反应有关,若摩擦副材料易于摩擦水合反应,碳基薄膜的磨损率很低;3种DLC薄膜在水中的磨损率与DLC的种类和对磨钢球材料无关,都在10-8mm3/(N.m)的数量级上变动;a-CNx/Si基非氧化物陶瓷摩擦副显示很低的摩擦因数和低的磨损率;在相同条件下,a-CNx薄膜比a-C薄膜更能显示优异的水润滑性能。     

Tribological characterisation of a‐C:H coatings at room temperature: Effect of counterbody material

Metal‐free amorphous carbon (a‐C:H) coatings with 15% hydrogen were deposited on bearing steel surfaces. The friction and wear performance of these specimens was characterised in oscillating sliding tests with a ball‐on‐flat geometry. Balls of four ceramic and four metallic materials were investigated in tests at room temperature. Special attention was paid to the effect of moisture by testing in dry, normal, and moist air. The effect of water vapour on the friction and wear of the a‐C:H coatings was quite different for the different counterbody materials. The wear was in all cases very low, with a coefficient of wear below 10⁻⁷ mm³/N m for most cases. The coefficient of friction was also very low, between 0.04 and 0.12 for most of the tests. The smallest wear and friction coefficients were found for oxide ceramics, while during tests against SiC and Si₃N₄ the coating was worn through during the test. The effects of counterbody material and the humidity of the surrounding air are discussed in terms of friction and wear mechanisms.     

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.