Tribology Letters最新文献

This study investigates the frictional and surface strain behaviour of silicone hemispherical finger pad simulants with different stiffness during tribological interactions with a smooth glass plate. A novel contact area and strain measurement method employing a digital image correlation technique was employed to give new understanding of the pad behaviour during sliding. The frictional behaviour of the sliding finger pad simulant is dominated by the adhesion mechanism, with a small overall contribution from deformation, as suggested by the high principal strains at the edge of the contact area. The strain behaviour is also influenced by the magnitude of the normal force and the stiffness of the samples.

Nickel acetyl acetonate (Ni(acac)₂), a metal-organic compound, was directly dispersed in base oils alkylated naphthalene (AN-5), diisooctyl sebacate (DIOS), poly-α-olefin (PAO6), and mineral oil (liquid hydrocarbon mixtures:150 N) in the presence of commercial dispersant RF1151 (monoallyl poly(isobutylene succinimide). The tribological properties of the lubricants were tested with a four-ball friction and wear tester. The friction-induced in-situ formation of carbon films on rubbed steel surfaces under the catalysis of Ni(acac)₂ was investigated, and the as-formed carbon films were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results show that Ni(acac)₂ added in the base oils can decompose to form metallic nickel to form nickel layer on the rubbed metal sub-surfaces and catalyze the degradation of the base oil molecules adsorbed to form carbon-based tribofilms. The carbon film formed from AN-5 with aromatic ring structure has a high degree of graphitization and the best friction-reducing and antiwear abilities, and those formed from PAO6 and 150 N with linear structure have a low degree of graphitization as well as good tribological properties. Under the lubrication of DIOS with Ni(acac)₂, however, there is no carbon film formation while the tribological properties of the lubricant are relatively poor, due to the absence of the catalytic metallic nickel and nickel oxide layer on the rubbed metal sub-surface. Thanks to the catalytic effect of metallic nickel released from Ni(acac)₂ for the degradation of various base oils with different molecular structure, the present approach could provide a rational pathway to tune the in-situ formation of carbon-based tribofilm on rubbed steel surfaces so as to effectively reduce the friction and wear of steel-steel sliding pair.

Previous investigation has established the formation of tribofilm is influenced by tribochemical reactions between the electrolyte and the articulating surface of cobalt alloy through sliding tests in various simulated fluids. Although it has successfully characterized the film composition via spectroscopy analysis and indicated to have impact on material loss, a comprehensive understanding of the material degradation mechanism in tribocorrosion condition was still lacking. Therefore, this study aims to investigate the role of protein in the tribocorrosive degradation of cobalt-chromium-molybdenum (CoCrMo) alloy in different simulated physiological electrolytes. Using a similar testing protocol, tribocorrosion tests were conducted with reciprocating ceramic ball against CoCrMo samples immersed in saline and culture medium, compared to both electrolytes diluted with 25% fetal bovine serum (FBS). Synergistic and mechanistic approaches were employed to model the tribocorrosive degradation. Results reveal that protein plays a beneficial role in reducing corrosive (electrochemical) surface degradation under tribocorrosion condition, whilst increasing mechanical wear degradation in the process. Despite studies have shown that tribocorrosion behavior in metal alloys is highly influenced by the presence of organic matter, this study provides a more clarity of the roles played by protein in tribocorrosive degradation on CoCrMo surface as its novel finding.

Repeated single cavitation bubble experiments were performed primarily on 316L stainless steel, and some on nickel–aluminum–bronze (NAB) and pure aluminum. The bubble dynamics were recorded with two high-speed cameras and correlated with surface images, also acquired in situ. These experiments were performed for a range of stand-off distances γ (the ratio of the distance of the solid surface from the bubble to the bubble’s maximum radius) from 0.3 to 2.15. For all stand-off distances, single pits were the only surface change detected at the beginning of damage formation. Later phases of the collapse are not axisymmetric but show regions of “stronger” collapse, and the pits occur on the material underneath those regions. For γ < 0.4, the damage is attributed to the second collapse. For γ > 0.4, the first bubble collapse is most likely responsible for pitting. Shock-wave emission was detected from the collapse regions that were linked to the damage. On 316L, the pitting rate was found to be linearly dependent on the bubble radius, indicating a non-zero lower limit for the bubble radius below which pits do not occur. In terms of stand-off distance, the pitting rate (defined here as average pits per bubble) was non-monotonic, with maxima for bubbles initiated closest to the sample (γ = 0.3) and at γ = 1.4.

In previous works, when studying the frictional behaviour of the small intestine, the small intestine was always in a natural state. However, when a capsule endoscope is travelling through the small intestine, since the diameter of the capsule endoscope may larger than the small intestine, the small intestine is then expanded in circumferential direction, which implies that previous works are not sufficient. This work uses flat–flat contact to simulate a capsule travelling through the small intestine, comparing the frictional behaviour of the small intestine in its natural and circumferentially expanding states, analysing the effects of other factors such as load, velocity and lubrication, which provides a realistic basis for the optimisation and control of magnetically controlled endoscopes.

Graphical abstract

Seal wear caused by hard particles intruding into the seal surface is one of the main causes of seal failure. In this manuscript, the tribological properties of hydrogenated nitrile rubber (HNBR) in the intrusion of five different sizes of garnet particles into the friction pair were studied through the pin-disk rubbing pair. The experimental results show that the particles change the contact pattern of the friction pairs, thus changing the amount of rubber wear. Different forms of particle motion contribute differently to the friction coefficient, and the particle sliding on the rubber surface contributes the most. As the particle size decreases, the friction coefficient tends to increase and then decrease, which can be explained by energy dissipation theory.

This report investigates the kinetics of lubricant molecules in the HAMR air bearing to understand the initiation and growth of PFPE contamination on the head surface. The collisions with the air bearing induce three forces—drag, thermophoresis, and lift. Of these, we find that lift forces are negligible. Then, a sensitivity analysis of the remaining two forces reveals the conditions where they dominate. Further, a hybrid simulation strategy is utilized to track their movements. The results show that the contaminations (smear) highly depend on the interplay between the thermophoresis and drag forces. We then explain the mechanism of the formation of the various observed patterns. Finally, we offer some recommendations to exploit the air bearing to contain smear on the head.

In classical experiments, it has been found that a rigid cylinder can roll both on and under an inclined rubber plane with a friction force that depends on a power law of velocity, independent of the sign of the normal force. Further, contact area increases significantly with velocity with a related power law. We try to model qualitatively these experiments with a numerical boundary element solution with a standard linear solid and we find for sufficiently large Maugis–Tabor parameter (lambda) qualitative agreement with experiments. However, friction force increases linearly with velocity at low velocities (like in the case with no adhesive hysteresis) and then decays at large speeds. Quantitative agreement with the Persson–Brener theory of crack propagation is found for the two power law regimes, but when Maugis–Tabor parameter (lambda) is small, the cut-off stress in Persson–Brener theory depends on all the other dimensionless parameters of the problem.

The tribological property and corrosion of a steel ball sliding against anodic oxide film (AOF) on Al-Li alloy lubricated by an ionic liquid (imidazolium hexafluorophosphate, LP108) are investigated without and with electric field. Upon sliding, no bubbles are observed without electric field (0 V) and with electric field at 1 V, while bubbles are produced continually with electric field at applied voltages of 5 V and 10 V. Higher voltage produces more bubbles. It is found that bubbles accelerate the worn-out of the AOF in sliding and corrosion of the steel ball after sliding. In the absence of bubbles, friction coefficient lower than 0.1 and long wear lifetime suggest that LP108 is a good lubricant at both 0 V and 1 V. In the presence of bubbles, however, AOF is rapidly worn out, i.e., 3.3±0.2 min at 5 V and 1.6±0.3 min at 10 V. Because the bubbles are produced continually at 5 V and 10 V, there is a rapid transition of the lubricating regime from a flooded state (boundary lubrication regime) to a bubble-dominated state (dry sliding regime). By turning off the electric field after one-minute sliding (no more newly born bubbles), the effective lubrication by LP108 can be preserved. Once the tribo-test is ended, the collapse of the bubbles occurs rapidly and hence the corrosion of the steel ball is greatly accelerated when compared to that in the absence of bubbles. In addition to bubble collapse, the bubbles are responsible for the accelerated corrosion of the steel ball because they are the reactive species of electrochemical decomposition of LP108 by synergistic action of electric field and friction, which is supported by experimental evidence. In other words, neither sliding friction without electric field nor electric field without sliding friction up to 10 V produces bubbles. A friction-activated mechanism is proposed to explain the synergistic action of electric field and friction. In short, sliding friction produces wear of the steel ball and this initiates friction activation of the worn surface of the steel ball. Consequently, the generation of bubbles occurs.