A comparative investigation of the wear behavior of PTFE and PI under dry sliding and simulated sand-dust conditions

The wear behavior of polytetrafluoroethylene (PTFE) and polyimide (PI) has been comparatively evaluated under dry sliding and simulated sand-dust conditions. An improved block-on-ring wear tester equipped with an attachment for simulating the sand-dust environment was used to evaluate the abrasive wear behavior of materials. The sand collected from the Yellow River of China was used to simulate the sand-dust environment, also different loads and sand-dust sizes were chosen for tribological tests. The two chosen polymers showed different wear behavior under sand-dust conditions and the wear rates of PTFE were much lower under sand-dust conditions than under dry sliding conditions. This was attributed to the formation of the tribolayer on the worn surfaces during the abrasive wear process. The sand-dust enhanced the wear resistance of PTFE, but reduced that of PI because, in contrast to PTFE, there was no tribolayer formed on the PI worn surface. The wear rate of PTFE increased under sand-dust conditions while the wear rate of PI decreased with the increase of applied load. The higher hardness of PI and fragmentation of abrasive particles under high loads accounted for the decrease in wear rate as load increased.     

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.     

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.     

Sliding wear behaviour of aluminum syntactic foam: A comparison with Al-10 wt% SiC composites

The sliding wear behaviour of cenosphere-filled aluminum syntactic foam (ASF) has been studied in comparison with that of 10 wt% SiC particle reinforced aluminum matrix composite (AMC) at a load of 3 kg and varying sliding speeds under dry and lubricated conditions using a pin-on disc test apparatus. The tribological responses such as the wear rate, the coefficient of friction and the frictional heating were investigated. The wear surfaces and subsurfaces were studied for understanding the wear mechanism. It was noted that the coefficient of friction, the wear rate, and the temperature rise for ASF are less than that for AMC in both dry and lubricated conditions. The craters (vis-à-vis exposed cenospheres) play an important role in the wear mechanism for ASF.     

Effect of Blowing Air and Floating Sand-dust Particles on the Friction and Wear Behavior of PTFE, UHMWPE and PI

The friction and wear behaviors of polytetrafluoroethylene (PTFE), ultra-high molecular weight polyethylene (UHMWPE), and polyimide (PI) have been comparatively evaluated under dry sliding, blowing air, and simulated sand-dust conditions. The tribological tests were conducted on an improved block-on-ring test rig equipped with an attachment for simulating the sand-dust environment. The reason for the difference in the tribological behavior of these polymers under the three test conditions was also comparatively discussed, based on scanning electron microscopic examination of the worn polymer specimens and counterfaces. Under blowing air conditions, the decrease of the contact temperature produced by blowing air led to the increase in the shearing strength of the sliding surface when compared with dry sliding conditions and hence to cause an increase in the friction coefficient and a remarkable decrease in the wear rate of PTFE and UHMWPE. On the contrary, blowing air produced a decrease in the friction coefficient of PI because of the formation of transfer film on the counterfaces, and an increase in the wear rate, because the blowing air considerably promoted the transfer of PI onto the counterfaces when compared with dry sliding conditions. Both PTFE and UHMWPE registered the lowest wear rate under sand-dust conditions, owing to the tribolayer formation on the worn surfaces, while PI exhibited the highest wear rate because no tribolayer was formed during the abrasive wear process.     

Tribological Behavior of Composites Based on ZA-27 Alloy Reinforced with Graphite Particles

The objective of this investigation is to assess the influence of graphite reinforcement on tribological behavior of ZA-27 alloy. The composite with 2 wt% of graphite particles was produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer, under dry and lubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of the samples were examined by the scanning electron microscopy (SEM). The obtained results revealed that ZA-27/graphite composite specimens exhibited significantly lower wear rate and coefficient of friction than the matrix alloy specimens in all the combinations of applied loads (F _n ) and sliding speeds (v) in dry and lubricated tests. The positive tribological effects of graphite reinforcement of ZA-27 in dry sliding tests were provided by the tribo-induced graphite film on the contact surface of composite. In test conditions, characterized by the small graphite content and modest sliding speeds and applied loads, nonuniform tribo-induced graphite films were formed leading to the increase of the friction coefficient and wear rate, with increase of the sliding speed and applied load. In conditions of lubricated sliding, the very fine graphite particles formed in the contact interface mix with the lubricating oil forming the emulsion with improved tribological characteristics. Smeared graphite decreased the negative influence of F _n on tribological response of composites, what is manifested by the mild regime of the boundary lubrication, as well as by realization of the mixed lubrication at lower values of the v/F _n ratio, with respect to the matrix alloy.     

Sliding wear behavior of planar random zinc-alloy matrix composites

The friction and wear behavior of planar random zinc-alloy matrix composites reinforced by discontinuous carbon fibres under dry sliding and lubricated sliding conditions has been investigated using a block-on-ring apparatus. The effects of fibre volume fractions and loads on the sliding wear resistance of the zinc-alloy matrix composites were studied. Experiments were performed within a load range of 50–300 N at a constant sliding velocity of 0.8 m s⁻¹. The composites with different volume fractions of carbon fibres (0–30%) were used as the block specimens, and a medium-carbon steel used as the ring specimen. Increasing the carbon fibre volume fraction significantly decreased the coefficient of friction and wear rates of both the composites and the medium-carbon steel under dry sliding conditions. Under lubricated sliding conditions, however, increasing the carbon fibre volume fraction substantially increased the coefficient of friction, and slightly increased the wear of the medium-carbon steel, while reducing the wear of the composite.Under dry sliding conditions, an increasing load increased not only the wear rates of both the composite and the unreinforced zinc alloy, but also those of their corresponding steel rings. However, the rate of increase of wear with increasing load for both the composite and its corresponding steel ring was much smaller than for the unreinforced zinc alloy and its corresponding steel ring. The coefficient of friction under dry sliding conditions appeared to be constant as load increased within a load range of 50–150 N for both the composite and the unreinforced zinc alloy, but increased at the higher loads. Under any load the coefficient of friction of the composite was lower than half that of the unreinforced zinc alloy under dry sliding conditions.     

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.     

Tribological Behaviors of Hybrid PTFE/Nomex Fabric/Phenolic Composite under Dry and Water-Bathed Sliding Conditions

The tribological behaviors of hybrid polytetrafluoroethylene (PTFE)/Nomex fabric/phenolic composite under dry sliding condition and water-bathed sliding conditions were investigated using a pin-on-disk type tribometer. The results showed that this hybrid fabric reinforced composite exhibited a higher wear rate and a lower friction coefficient under water-bathed sliding conditions compared to that measured under dry sliding condition. Scanning electron microscopy (SEM) and X-ray photoelectron microscopy (XPS) analysis demonstrated that under water-bathed sliding conditions the transfer films formed on the counterpart pins surface were of high roughness and less PTFE transferred onto the pin surface, compared to that under dry sliding condition. Moreover, the hybrid fabric composite displayed varied tribological behaviors when distilled water-bathed sliding condition and seawater-bathed sliding condition were applied separately.     

The Tribological Behaviour of Fe–28Al–5Cr/TiC Under Liquid Paraffine Lubrication

The tribological behaviour of Fe–28Al–5Cr and its composites containing 15, 25 and 50 wt% TiC (corresponding to 19.3, 31.2 and 57.6 vol%), produced by hot-pressing process, was investigated under liquid paraffine lubrication against an AISI 52100 steel ball in ambient environment at varied applied loads and sliding speeds. It was found that the wear resistance increased and friction coefficient decreased with increasing of TiC content. The coefficients of friction are in the range of 0.09–0.14 at the given testing conditions. The wear rates of all the materials except the 50% composite are on the order of 10⁻⁶–10⁻⁵ mm³ m⁻¹, the wear rate for the 50% composite is too low to quantify under the two sliding conditions, (50 N, 0.04 m/s) and (100 N, 0.02 m/s). The wear rates of all the materials increase as applied load increases and the increasing extent diminishes with the increase of TiC content, but first increase slightly and then nearly remains steadiness with increasing sliding speed. The 50 wt% composite has wear resistance about 7–20 times better than pure Fe–28Al–5Cr at different sliding parameters. The enhanced wear resistance by TiC addition is attributed to the high hardness of the composites, as well as support of the oil lubrication film/layer by the hard TiC phase. The worn surfaces of all the materials are analyzed by a scanning electron microscope. The dominant wear mechanism of the Fe–28Al–5Cr and 15% composite is grooving and flaking-off, but those of the 25 and 50% composites are mainly shallow grooving.     

Tribological studies of epoxy and its composite coatings on steel in dry and lubricated sliding

Industrial lubricants are invariably used with additives (with high sulfur and phosphorous contents) for tribological performance enhancement. However, these additives are environmentally very harmful. Hence, there is an urgent need to find alternate solutions for enhancing the tribological performance of lubricants and components without the use of harmful additives. The objective of this work is to investigate the feasibility of using polymer composite coatings in enhancing the tribological properties of steel surfaces in dry and base oil lubricated conditions. Pure epoxy and its composite (with 10?wt-% of graphene or graphite powder) films were coated onto steel substrates and tested under dry and base oil lubricated conditions. Friction and wear experiments were conducted on a ball on cylinder tribometer between polymer/composite coated cylindrical steel surface (shaft) and an uncoated steel ball as the counterface. Tests were conducted at various normal loads and speeds. In dry condition at 3 N load and 0.63?m s^??1 sliding speed, the wear life of epoxy was increased by five times and coefficient of friction was nearly the same (0.18) on inclusion of graphene nanoparticle. In lubricated case, epoxy/graphene composite coating performed eight times and more than five times better than pure epoxy and epoxy/graphite respectively.     

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.     

Carbon Nanotube Reinforced Polyimide Thin-film for High Wear Durability

In this paper, the influence of single walled carbon nano tubes (SWCNTs) addition on the tribological properties of the polyimide (PI) films on silicon substrate was studied. PI films, with and without SWCNTs, were spin coated onto the Si surface. Coefficient of friction and wear durability were characterized using a ball-on-disk tribometer by employing a 4 mm diameter Si₃N₄ ball sliding against the film, at a contact pressure of ∼370 MPa, and a sliding velocity of 0.042 ms⁻¹. Water contact angle, AFM topography, and nano-indentation tests were conducted to study the physical and mechanical properties of the films. SWCNTs marginally increased the water contact angle of PI film. The addition of SWCNTs to PI has increased the hardness and elastic modulus of pristine PI films by 60–70%. The coefficient of friction of PI films increased slightly (∼20%) after the addition of SWCNTs, whereas, there was at least two-fold increase in the wear life of the film based on the film failure condition of coefficient of friction higher than 0.3. However, the film did not show any sign of wear even after 100,000 cycles of rotation indicating its robustness. This increase in the wear durability due to the addition of the SWCNTs is believed to be because of the improvement in the load-bearing capacity of the composite film and sliding induced microstructural changes of the composite film.     

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.     

A study on tribological behaviour of transfer films of PTFE/bronze composites

Transfer films of PTFE/bronze composites with 5–30% volume content of bronze were prepared using a RFT friction and wear tester on surface of AISI-1045 steel bar by different sliding time (5–60 min). Tribological properties of these transfer films were studied using a DFPM reciprocating tribometer in a point contacting configuration under normal loads of 0.5, 1.0, 2.0 and 3.0 N. Thickness and surface morphology of the transfer films were investigated. It was found thickness of the transfer films slightly increased along with the increase of bronze content of corresponding composites. Increased sliding time of transfer film preparation is helpful to form transfer film with better ductibility and continuity, but sliding time almost has no effect on tribological properties of the transfer film. Higher bronze content in the composite improved tribological properties of the corresponding transfer film, i.e., reduced friction coefficient and prolonged wear life. All these transfer films are sensitive to load change. Their wear life becomes shorter along with the increase of load. SEM image of the worn surface show fatigue wear and adhesion wear have happened on the transfer film during the friction process. The author believe bronze in the transfer film effectively partaked in shear force applied on the transfer film and its good ductibility helped to improve tribological properties of the transfer films.     

Tribological Behavior of TiC/a-C:H-Coated and Uncoated Steels Sliding Against Phenol–Formaldehyde Composite Reinforced with PTFE and Glass Fibers

Tribological experiments on phenol–formaldehyde composite reinforced with polytetrafluoroethylene (PTFE) and glass fibers were performed against 100Cr6 steel and TiC/a-C:H thin film-coated 100Cr6 steel. In both cases, the coefficient of friction increases with increasing sliding distance until a steady-state value is reached. Although the steady-state values of the coefficient of friction are very close and ultralow, the wear rate of the PTFE composite liner at a long sliding distance (1,000 m) is reduced when the steel ball is coated with the TiC/a-C:H coating. This behavior is mainly attributed to the smoother surface after long sliding and the improved wear resistance of TiC/a-C:H coating. PTFE transfer films are evident on the surfaces of the hard counterparts. The average thickness of the transfer film on TiC/a-C:H-coated surfaces is about 3.8 nm. On the surface of uncoated steel ball, a continuous but non-uniform transfer film of around 13.9 nm average thickness was found.     

Sensitivity of μ of Friction Materials to Load and Speed under Dry Sliding and Water Lubricated Conditions

The tribological behavior of friction materials is complex under various operating conditions. The present article discusses the sensitivity of tribological behaviors of friction materials to load and speed under dry and water-lubricated conditions qualitatively and μ of values was also analyzed quantitatively based on regression analysis. The friction and wear behaviors of friction materials were evaluated on a ring-on-block-type test rig. It was observed that the material itself was the dominant factor in μ sensitivity of friction materials, followed by sliding speed. The influence of the interaction of load and speed was negligible under water-lubricated condition. The friction coefficient of the composite was less sensitive to operating parameters under water-lubricated condition.     

Tribological Behavior of Micrometer- and Submicrometer-Size Cenosphere Particulate-Filled Glass Fiber–Reinforced Vinylester Composites Under Dry and Water-Lubricated Sliding Conditions

In this work, the tribological behavior of micrometer and submicrometer cenosphere particulate–filled E-glass fiber–reinforced vinylester composites have been investigated on a pin-on-disc tester under dry sliding and water-lubricated sliding conditions. Three different uniform sizes of cenosphere particles (2 μm, 900 nm, 400 nm) were used as fillers in the glass fiber–reinforced vinylester composites. The weight fraction of cenosphere particles has been varied in the ranges from 5, 10, 15, to 20 wt%. The experimental results show that all of the composites exhibited lower coefficient of friction and lower wear resistance under water-lubricated sliding conditions than under dry sliding. It has been noted that the submicrometer size (400 nm) cenosphere particulates as fillers contributed significantly to improve the wear resistance. It has also been noted that 10 wt% of the cenosphere particles is the most effective in reducing the wear rate and coefficient of friction. Effects of various wear parameters such as applied normal loads, sliding speeds, particle size, and particle content on the tribological behavior were also discussed. In order to understand the wear mechanism, the morphologies of the worn surface were analyzed by means of scanning electron microscopy (SEM) for composite specimens under both dry and water-lubricated sliding conditions.     

The tribological performance of DLC-based coating under the solid–liquid lubrication system with sand-dust particles

Combination of solid and liquid lubricants to meet emission or environmental requirements of future tribological systems while providing the levels of desired friction and wear performance have received considerable research attention in the near term. The aim of the present work was to investigate the tribological behavior of oil-lubricated (PAO, PFPE, SO, IL and MAC) DLC coated surfaces under the conditions without and with sand-dust particles. The effects of applied load, frequency, and sand-dust particles on the tribological performance of DLC coating were systemically studied. The analysis results showed that solid–liquid lubricating coatings including SO and IL exhibited excellent anti-friction (～0.026) but relative poor wear-resistance performances under the conditions without and with sand-dust environments. But for PFPE and PAO, they demonstrated the worst tribological behaviors with high friction coefficient and wear rates. The added sand-dust particles lead to the wear rates to the one order of magnitude large than that without sand-dust conditions for all the selected liquid lubricants. The viscosity, contact angle and work of adhesion played an important part in affecting the tribological performances. The lubrication regimes in Stribeck curve for the five kinds of liquid lubricants were affected obviously by the sand-dust particles in different way. The formed transfer films on the coating surface and pin have much influence on the tribological behavior and the transition between lubrication regimes.     

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).