Tribological performance of silicone composite coatings filled with wax-containing microcapsules

The effect of wax-containing microcapsules incorporated in silicone composite coatings deposited on aluminum (Al) alloy substrates on the tribological performance of the coatings was systematically investigated. The wax-containing microcapsules were prepared via in situ polymerization. The tribological behavior of the composite coatings was evaluated using ball-on-disk tribological test. It was found that the increase in microcapsule concentration in the composite coatings apparently reduced the friction coefficient of the coatings because the lubricant released from the broken microcapsules during the tribological test of the coatings lubricated the rubbing surfaces. The results showed that the silicone composite coatings rubbed by a smaller Cr6 steel ball (3 mm diameter) under a lower normal load (100 mN) produced higher friction coefficients via reduced complication of their underlying strong substrates compared to the same coatings tested against a larger Cr6 steel ball (6 mm diameter) under a higher normal load (1 N).     

Evolution of wear,roughness, and friction of Alloy 600 superalloy surfaces in water-submersed sliding

Self-mated wear and friction of Alloy 600 superalloy was studied in a water-submersed ring-on-rod configuration, loading the side of a 6.35 mm diameter rod across the flat surface of a rotating annular ring of 100 mm outer diameter and 70 mm inner diameter producing two sliding contacts along the ring. Tests were conducted at sliding speeds of 0.178 and 0.330 m/s for sliding distances of 100 m. Normal loads of 51 and 204 N were applied, and initial R_a surface roughnesses of the rings along the sliding direction were either smooth (～0.2 μm) or rough (～7.5 μm). Increased initial ring roughness caused a ～20-fold increase in rod wear at the lighter load, whereas at the heavier load increased initial roughness only caused a ～4-fold increase in wear. At lower initial ring roughness the 4-fold decrease in normal load caused a large (one order-of-magnitude) decrease in rod wear, whereas for rings of higher initial roughness the 4-fold decrease in normal load caused only minor (2-fold or less) decreases in rod wear. Wear during this 100 m sliding distance only experienced a minor effect from the 1.8-fold change in sliding speed, as did friction. In all cases friction coefficient rapidly settled into the range 0.6–0.7, except in the cases of lower load on rings of lower initial roughness where friction coefficient remained above 1 for most of this sliding duration. At this lower load the initial ～0.2 μm rod roughnesses increased to nearly 0.8 μm by the 100 m sliding distance, whereas at the higher load this same sliding distance resulted in roughnesses returning near to the initial 0.2 μm. It was hypothesized more highly loaded cases also went through initial roughening prior to smoothening back to 0.2 μm roughness within the 100 m sliding distance, and given additional sliding the more lightly loaded cases would also experience subsequent smoothening. Increasing sliding distance to 400 m, roughnesses indicated a smoothening back to 0.2 μm level during those lightly loaded tests, with friction coefficient correspondingly dropping from 1 into the 0.6–0.7 range observed in all other cases. Extended sliding to 400 m at light loading against rings of lower initial roughness also allowed a rod wear rate which increased with increased sliding distance to be observed, approaching the same rate observed against initially rough rings within the 100 m sliding distance.     

Effect of surface laser texture on friction properties of nickel-based composite

Laser surface texturing (LST) was performed on the nickel-based composites by a Nd:YAG pulsed laser and the regular-arranged dimples with diameter of 150 μm were fabricated on their surfaces. The textured surfaces were smeared with molybdenum disulfide powder. The tribological properties of the textured and filled composites were investigated by carrying out sliding wear tests against an alumina ball as a counterface using a high temperature ball-on-disk tribometer. The tests were conducted at a sliding speed of 0.4 m/s and at normal loads ranging from 20–100 N and from room temperature to 600 °C. The friction coefficient of nickel-based composite textured and smeared with molybdenum disulfide was found to reduce from 0.18 to 0.1 at the temperature range from 200 to 400 °C. The texture with a dimple density of 7.1% was observed to prolong wear life of MoS2 film by more than four times in comparison to the texture with other dimple densities. The lubricious oxide particles stored in the dimples reduce friction coefficient at elevated temperatures and compensate for the extra lubricant owing to the degradation of MoS₂ caused by its oxidation at high temperatures.     

Impact of high pressure and shear thinning on journal bearing friction

For the study of mixed lubrication in journal bearings, this paper employs a combined experimental and simulative approach. Extensive measurements on a journal bearing test rig with a low viscosity 0W20 multi-grade lubricant provide a solid basis which is complemented by experimental lubricant data that is measured under high pressure and high shear rates. In this paper, this data is used to investigate the impact of the piezoviscous effect and the non-Newtonian lubricant properties on the friction power losses in journal bearings over a wide range of dynamic loads and shaft speeds.In particular, this work seeks to predict the friction power losses for journal bearings under both moderate (50 MPa peak load) and high dynamic loads (100 MPa peak load) using the recently presented accurate numerical method (Allmaier et al., 2011 [1], Allmaier et al., 2013 [2]). From the direct comparison to the experimental data a key finding is that the simulation conforms very closely to the measured data. To be more exact, the agreement lies within the measurement uncertainty.Following this result, the influence of the often neglected piezoviscous effect and the non-Newtonian lubricant rheology is investigated. We conclude that both the piezoviscous effect and the non-Newtonian behaviour are essential to describe the lubrication with multi-grade lubricants in journal bearings. Only the consideration of both properties describes the experimental data very accurately over the entire range of operating conditions studied.     

Wear characteristics of a diffusion bonded sintered steel with short term surface treatments

A pin on disc machine was used to investigate the tribological behavior of a diffusion bonded sintered steel, with and without surface treatments of steam oxidation and manganese phosphating, over a wide range of speed (0.2–4 m/s) and applied load (4–500 N) in conditions of dry sliding and starved lubrication by oil impregnation of the porous structure of the materials. Besides the calculated wear rates, the wear mechanisms were determined by examination of the components of the rubbing system (sintered pin, disc and generated debris). A transition from a mild to a severe wear regime was identified, denoted by sharp changes of the wear rate. A transient wear regime, interposed between the mild and severe wear regimes, was detected. The rubbing surface quality degradation was in terms of material displacement around the pin circumference due to a delamination wear mechanism. Such regime was detected for the base sintered steel in dry sliding at 1 m/s for the load range 60–80 N and for both surface treatments in oil impregnated sliding at 0.5 m/s for the load range 200–300 N. Oil impregnation of the base sintered steel expanded the mild wear regime towards higher loads throughout the whole sliding speed range compared to dry sliding. For the lower speeds of 0.2 and 0.5 m/s, manganese phosphated samples in dry sliding exhibited higher transition loads compared to the base sintered steel. The lower oil impregnability of the surface treated samples, due to the sealing of porosity by steam oxidation, led to slightly lower transition loads in oil impregnated sliding, compared to the base sintered steel.     

Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces

PEEK is a high strength engineering thermoplastic that suffers from a high friction coefficient and a friction induced wear mode. Past studies with 10 μm PEEK and PTFE powders resulted in composite solid lubricant that (at the optimal composition) had a wear rate of k = 2 × 10⁻⁹ mm³/Nm with a friction coefficient of μ = 0.12. A compositional grading of PEEK and PTFE is implemented in this study to create a bulk composite with the functional requirements of component strength, stiffness and wear resistance while providing solid lubrication at the sliding interface. The tribological performances of three functionally graded PEEK components were evaluated on linear reciprocating, rotating pin-on-disk and thrust washer tribometers. Wear rates comparable to samples of the bulk solid lubricant and comparable or improved frictional performance were achieved by compositionally grading the near surface region of PEEK components.     

Analysis of load-speed sensitivity of friction composites based on various synthetic graphites

The frictional response of a multi-component phenolic-based friction material is highly complex under a set of variable loads and speeds. The present paper discusses the sensitivity of friction coefficient (μ) of friction composites containing synthetic graphite with different particle sizes (with similar crystallinity range) to braking pressure and sliding speed. The friction studies were carried out on a sub scale brake-test-rig, following 4 loads × 3 speeds experimental design. The best combination of performance properties was observed for the composite containing synthetic graphite with an average particle size of 410 μm. Other particle sizes which resulted in good performance were 38 and 169 μm. Very fine particle sizes were not beneficial for desired combination of performance properties. Regression analysis of μ following an orthogonal L₉(3 × 3) experimental design method revealed that the first order influences of sliding speed and braking pressure were significant. When all the combinatorial influences of braking pressure and sliding speed are taken into account together their simultaneous effects would be most effective in the range of graphite particle size ～80–250 μm.     

Microscratch behavior of copper–graphite composites

Microscratch tests were carried out on Cu–graphite composites with graphite content of 0–30 vol% and normal loads of 0.5–2 N. Scratch grooves generated by the plastic deformation of surfaces were characterized for detailed friction and wear mechanisms investigation. The influence of normal load and graphite content on friction coefficient was also studied. It is found that the dominant wear mechanism transits from ploughing to micro-cutting with increasing the normal loads. A friction model for knowing the contribution of ploughing and adhesion components to friction is presented. This friction model is useful in understanding the friction mechanism of composites during scratching.     

Paraffinic mineral oil lubrication for cold forward extrusion: Effect of lubricant quantity and friction

In this paper, Finite Element (FE) and experimental analyses have been developed on the deformation of aluminium billet over a tool. Effect of friction resulted from the use of additive-free ISO460-compliant paraffinic mineral oil with kinematic viscosity of 455.192 mm²/s at 40 °C in amounts of 0.1, 1, 5, and 20 mg were examined. The time behaviour of displacements on the billet in the experiment was used as inputs for the FE model. The FE analysis results for load–displacement behaviour of the extrusion were compared with the experimental results. It was shown that significant differences exist between the four lubricant quantities on friction and contact pressure distribution.     

An investigation of friction and wear performances of bonded molybdenum disulfide solid film lubricants in fretting conditions

The friction and wear performances of bonded MoS₂ solid film lubricants with the counterpart steel ball rubbing were investigated in fretting wear conditions in order to inquire into the load-carrying capacity and wear mechanisms of bonded MoS₂ solid film lubricants under dry friction conditions. Experimental results show that the bonded MoS₂ solid film lubricants have excellent anti-friction and wear-resistance performances within a wide load range between 20 N and 800 N and within a wide oscillatory frequency range between 5 Hz and 30 Hz. It is found through analyses of the transfer films formed in the surface of the counterpart steel ball investigated by SEM, XPS and AES, that the thickness of the transfer film formed is about 38 nm and the oxidation of MoS₂ in the transfer films does not occur during dry friction process. The high load and frequency promote the formation of a compact transfer films. The compact transfer films are believed to be the predominant mechanism giving rise to high load-carrying capacity, and excellent wear-resistance performances of the bonded MoS₂ solid film lubricants.     

A study on the friction properties of poly(vinyl alcohol) hydrogel as articular cartilage against titanium alloy

Many biomaterials are being developed to repair or replace articular cartilage. One of these materials, poly(vinyl alcohol) (PVA) hydrogel prepared from aqueous solution of the polymer by freezing and thawing method may exhibit the mechanical properties required to withstand the harsh environment of diarthrodial joints. To better understand how PVA hydrogel friction is affected by different variable factors, a three-factor, three-level designed orthogonal experiment was developed. Factors include lubricant, sliding speed, and normal load. Friction coefficient of the PVA hydrogel was found to depend significantly on load and sliding speed. Lubricant had little effects on the friction coefficient. Friction coefficient of the PVA hydrogel decreased with the increase of sliding speed and the friction coefficient approximately increased linearly with the increasing load. Average friction coefficient decreased from 0.0447 to 0.0379 while the sliding speed increased from 0.06 to 0.22 m/s. Average friction coefficient increased from 0.0276 to 0.0546, almost increasing one time, while the load increased from 5 to 15 N.     

Friction and wear durability studies on the 3D negative fingerprint and honeycomb textured SU-8 surfaces

The effects of two different textures (a 3D negative fingerprint texture and a honeycomb texture) on the tribological performance of SU-8 polymer surface have been investigated with a ball-on-disc tribometer. Friction and wear behaviors of the textured surfaces are conducted against a 4 mm diameter silicon nitride (Si₃N₄) ball counterface. The coefficient of friction for the negative fingerprint textured surface (μ=∼0.08) is much lower than that of the untextured surface (∼0.2) and the honeycomb textured surface (∼0.41) under a normal load of 100 mN and a rotational speed of 2 rpm. The coefficients of friction of the textured surfaces decrease with increasing normal loads between 100 mN and 300 mN. Above the normal load of 300 mN, the coefficient of friction of the negative fingerprint textured surface increases due to the occurrence of plastic deformation. The honeycomb textured surface has shown the highest coefficient of friction. The wear durability tests are also conducted at a normal load of 100 mN and a rotational speed of 500 rpm on the untextured/textured surfaces on SU-8 in the presence of an overcoat of a nano-lubricant, perfluoropolyether(PFPE). Six samples i.e. the untextured surface (Si/SU-8 and Si/SU-8/PFPE), the 3D negative fingerprint textured surface (Si/SU-8/FP and Si/SU-8/FP/PFPE) and the honeycomb textured surface (Si/SU-8/HC and Si/SU-8/HC/PFPE), each with and without PFPE nano-lubricant, have been investigated for their tribological behaviours. The negative fingerprint pattern on SU-8 with PFPE coating has shown the highest wear life of 60,000 cycles under a normal load of 100 mN. The reasons for excellent tribological performance of 3D fingerprinted SU-8 surface are analyzed using the Hertzian contact area calculation.     

Synthesis of boride coatings on steel using plasma transferred arc (PTA) process and its wear performance

Boriding of the surface of a tool steel using boron powder and the plasma transferred arc process was investigated. It was shown that this method is an easy and effective technique in producing uniform alloyed layers with a thickness of about 1.5 mm and a hardness between 1000 and 1300 HV.The microstructure of the borided surfaces consists of primary Fe₂B-type borides and a eutectic mixture of borides and martensite. Some cracks are observed in the eutectic regions but they do not seem to critically affect the behaviour of the coatings in sliding wear.The wear rate of pin on disc tests is primarily affected by the applied load and it lies between 10⁻⁵ mm³/m for low loads and 10⁻² mm³/m for high loads. Two distinct regimes of mild and severe wear are obtained separated by a critical load. Mild wear is due to the load supporting effect of borides and severe wear is due to their breakage above a critical load. The wear rate is not significantly affected by the sliding velocity and is consistent with the friction coefficient.The friction coefficient varies from 0.13 to 0.23 and depends strongly on the oxidation status of the wear track. The sliding velocity affects the sliding distance where the coefficient of friction reaches equilibrium.     

Comparisons of dry sliding tribological behaviors between coarse-grained and nanocrystalline copper

Dry sliding tribological behaviors of nanocrystalline (NC) and coarse grained (CG) Cu were studied by using a ball-on-plate tribometer with a counterface ball of cemented tungsten carbide. The results showed that prior to oxidation and delamination, the steady-state friction coefficients (FCs) of NC and CG Cu are comparable (～0.35). As oxidation with delamination of wear surface occur, the FC for either CG or NC Cu increases gradually, approaching a steady-state FC (～0.63). The wear resistance of the NC Cu was enhanced by at least one order of magnitude under the measured loads ranging from 5 N to 25 N in comparison with the CG counterpart, which is mainly attributed to the higher hardness of the NC layer.     

Effect of pH on wear and friction of silicon nitride sliding against alumina in water

The influence of the pH in water lubricated sliding contacts was evaluated in terms of friction and wear. The experiments were carried out using a ball-on-disc setup. Si₃N₄ balls and Al₂O₃ discs were tested at temperature of (22±2) °C, sliding speed of (1.00±0.03) m/s and normal load of (54.25±0.17) N. Eight types of water with pHs varying from 3 to 12 were used as lubricant. The running-in period, friction coefficient and wear-volume were shown to be nearly independent of the initial pH values within the DLVO range (4≤pH≤10), since at these range the water׳s pH tends to the same value (7.6±0.3) during the test. Superlubricity could be reached with negligible wear by properly setting the electrochemical properties and operating conditions of the tribosystem.     

Microstructure and wear behaviour of Ni-based surface coating on copper substrate

The Ni-based surface coatings were prepared by a vacuum infiltration casting technique on copper substrate. The surface coatings were fabricated through copper melt penetrating into thin preforms whose thickness could change. By optimizing the processing parameters, compact surface coatings were achievable as confirmed through SEM observation. The surface coating was mainly composed of solid solution of Ni, solid solution of Cu and CrB. The macro-hardness of the coating was about HRC 58, and the micro-hardness of the coating shows a gradient distribution. The average micro-hardness of the coating was about HV450. Wear behaviour was investigated by using block-on-ring dry sliding linear contact at several loads (50 N–300 N) and two different sliding speeds (0.424 m/s and 0.848 m/s). Wear rate and friction coefficient were estimated using a method founded upon the PV factor theory. The surface oxidation predominated as the principle wear mechanism at low load. Meanwhile, adhesion and oxidation mechanism were observed when the coatings were tested at higher load more than 200 N. Friction coefficient decreased with increasing load and sliding speed.     

Improving tribological behaviour of piston ring–cylinder liner frictional pair by liner surface texturing

Experiments were carried out on a reciprocating tester. The lubricant was supplied into the inlet side of the contact zone. The construction of tribological tester allows to measure the friction force between specimen and counter-specimen. Tribological behaviours of cylinder liners with and without oil pockets were compared. Specimens were cut from plateau honed cylinder liners made of grey cast iron. Counter-specimens were cut from grey cast iron piston rings. A special tool acted as a hammer to form additional dimples on the liner surfaces. The area density of oil pockets was about 13%. Specimen surfaces had dimples with average depths about 5 μm and diameters in the range 0.15–0.2 mm. Two batches of tribological tests were carried out, in regimes of full and starved lubrication. Friction tests were conducted at three mean sliding speeds: 0.44, 0.66 and 0.88 m/s. Experiments were performed with normal load in the range 50–300 N, starting from the lowest load. Normal load increased in a stepwise manner after 2 min at each load, until the maximum load was reached. Areal surface topographies of specimens and counter-specimens were measured before and after two batches of tests by white light interferometer.     

Tribology characteristics of magnesium alloy AZ31B and its composites

In this paper, wear characteristics of magnesium alloy, AZ31B, and its nano-composites, AZ31B/nano-Al₂O_3, processed by the disintegrated melt deposition technique are investigated. The experiments were carried out using a pin-on-disk configuration against a steel disk counterface under different sliding speeds of 1, 3, 5, 7 and 10 m/s for 10 N normal load, and 1, 3 and 5 m/s for 30 N normal load. The worn samples and wear debris were then examined under a field emission scanning electron microscopy equipped with an energy dispersive spectrometer to reveal its wear features. The wear test results show that the wear rates of the composites are gradually reduced over the sliding speed range for both normal loads. The composite wear rates are higher than that of the alloy at low speeds and lower when sliding speed further increased. The coefficient of friction results of both the alloy and composites are in the range of 0.25–0.45 and reaches minimums at 5 m/s under 10 N and 3 m/s under 30 N load. Microstructural characterization results established different dominant mechanisms at different sliding speeds, namely, abrasion, delamination, oxidation, adhesion and thermal softening and melting. An experimental wear map was then constructed.     

Friction behaviour of chemical vapor deposited self-assembled monolayers on silicon wafer

Friction characteristics of self-assembled monolayers (SAMs) coated on Si-wafer (1 0 0) by chemical vapor deposition technique were studied experimentally at nano and micro-scales. Four self-assembled monolayers, such as dimethyldichlorosilane (DMDC), diphenyldichlorosilane (DPDC), perfluorooctyltrichlorosilane (PFOTS) and perfluorodecanoicacid (PFDA) coated on Si-wafer (1 0 0) were used as test materials. Nano-scale friction was measured using atomic force microscopy (AFM) in the range of 0–40 nN normal loads, in LFM (lateral force microscopy) mode, using a contact mode type Si₃N₄ tip. Results showed that the friction of SAMs at this scale was influenced by their physical/chemical properties, while that of Si-wafer by its inherent adhesion. Further, micro-scale friction tests were also performed with a ball-on-flat type micro-tribotester under reciprocating motion. Friction was measured in the range of 1500–4800 μN applied normal loads using glass balls of varying radii, viz., 0.25, 0.5 and 1 mm. It was observed that the performance of SAMs was more superior to Si-wafer even at micro-scale, except for PFDA. Evidences obtained using scanning electron microscope showed that Si-wafer and PFDA exhibited wear at this scale. Wear in the case of Si-wafer was due to solid–solid adhesion and that in the case of PFDA due to the influence of humidity (moisture). The micro-scale friction in both these materials was severely influenced by their wear.     

Effect of three nitriding treatments on tribological performance of 42CrAlMo7 steel in boundary lubrication

Evolution of friction and wear of 42CrAlMo7 steels with different nitriding processes was investigated during boundary-lubricated rolling–sliding tests. The wear behaviour of nitrided steel with a thin compound layer (produced by plasma nitriding and by gas nitriding followed by oxidation) was characterised by the early removal of the compound layer, and the wear resistance was thus, given by the underlying diffusion layer. In the case of the material with a thick compound layer (produced by gas nitriding) wear was restricted to the compound layer. In this material, at low applied load (300 N, i.e. 485 MPa of Hertzian pressure, in this work), after the removal of the external porous layer wear tended to be negligible. At high applied load (1000 N, 890 MPa), on the other hand, the wear rate became higher than that of the diffusion layer. The friction behaviour was followed by determining the λ-factor evolution during each test. For a given λ-factor, the friction coefficients at 300 N were lower than at 1000 N.