Volume and issue: Vol. 16 , No.1 (2021)

Page: pp. 70-80

Title: Evaluation of an Antifouling Surface Inspired by Malaysian Sharks Negaprion Brevirostris and Carcharhinus Leucas Riblets

Authors: Mohd Danial Ibrahim, Susan Philip, Su Shiung Lam, Yuta Sunami

This study uses the molecular dynamics simulations to investigate the friction between the iron plate sliding against the partially frozen graphite nanoparticle. The graphite nanoparticle is divided into the two parts along the normal direction: the part contacts to the iron plate is controlled temperature and the lower other is held in the frozen situation. We detect the friction dependent on the frozen height, the angle between the sliding direction and the graphene surfaces by rotating the graphite nanoparticle around the normal direction, the external load and the sliding speed. The friction is low in all the investigated cases, the friction coefficients ~ 0.004 to 0.066. The sliding speed slightly results in the friction, while the other factors significantly influence it.

Reduction of heat and mechanical losses is a critical factor in reducing CO₂ emissions and fuel consumption of automotive engines. Toward that end, it is necessary to investigate the lubrication conditions of engine sliding surfaces and instantaneous heat flux in the combustion chamber in order to obtain experimental data for validating computer-aided engineering (CAE) calculations. The authors used thin-film sensors to measure the distribution of the oil film pressure on the sliding surfaces of engine parts including pistons, bearings and gear tooth surfaces. This paper describes the structure and configuration of the thin-film sensors and the measurement system used for direct observation, especially of piston pins, and presents the measured results.

Tribological behaviour of sliding surfaces with star shaped micro-dimples on one of the surface is reported. One wall is smooth and sliding on the other fixed dimpled wall with constant velocity. Effect of star shaped dimple and oriented star shaped dimple has been compared with circular shaped dimple for hydrodynamic pressure generation and tribological behaviour. Effect of dimple depth, dimple area density and sliding speed on tribological behaviour were also analyzed. The results demonstrate that an unconventional star-shaped and orientated star-shaped dimple produces a larger net hydrodynamic pressure in the fluid domain and provides better stability between the sliding surfaces than a circular-shaped dimple. It is shown that geometric parameters like dimple depth and dimple area density and operation parameter like sliding speed affects the hydrodynamic average pressure and tribological behaviour of sliding surfaces significantly. The experimental findings validate the analytical and CFD findings.

Epoxy-UHMWPE composites were filled with squalane, a hydrogenated form of naturally occurring hydrocarbon lubricant, squalene. The hybrid composite composition of 15 mass% of UHMWPE, 2 mass% of squalane and the rest epoxy was found to be the optimum with the coefficient of friction of 0.15 and the specific wear rate of 3.29 × 10⁻⁶ mm³/N.m when slid in dry condition against a stainless-steel disk on a pin-on-disc apparatus. The sliding speed and the nominal pressure were kept as 0.5 m/s and 1.27 MPa, respectively, and the sliding distance was 4.5 km for each test. The FESEM (Field Emission Scanning Electron Microscope) and 3D profilometry studies showed that the formation of thin uniform transfer layers of UHMWPE on the wear track and the pin surface were responsible for low friction and high wear durability. The transfer film thickness was in the range of 0.8-1 μm. The presence of UHMWPE particles in the matrix helped improve tribology of pure epoxy even though the hardness was reduced somewhat. Squalane also helped to reduce friction. It is expected that such kind of hybrid composites may find applications as bearing materials.

Herein, cavitation erosion tests were carried out on 2205 and 2507 duplex stainless steels (DSSs) in distilled water and artificial seawater using ultrasonic cavitation erosion equipment. Optical microscope (OM), X-ray diffractometer (XRD), and scanning electron microscope (SEM) were used to observe the microstructure and cavitation erosion morphology of the specimens. Cavitation erosion and corrosion behaviors of the specimens were analyzed and the synergistic effect between cavitation erosion and corrosion in artificial seawater was also examined. The results show that the cavitation erosion resistance of 2507 DSS is better than that of 2205 DSS in all kinds of solutions. The destruction occurs first in ferrite due to its lower plasticity. The synergistic effect of cavitation erosion and corrosion accelerates the destruction of the material, and the mass loss caused by cavitation erosion-corrosion synergy for 2205 DSS is higher than that of 2507 DSS.

Due to emerging concerns for energy-efficient and environment-friendly journal bearings, efforts are being made to improve their performance behaviours. Green tribology also aims to reduce friction and wear. In this direction, surface textures are employed on the surfaces of mating parts to minimize friction and enhance load-carrying capacity. Surface textures comprising numerous micro-geometries such as dimples, grooves or pits are found to be capable of increasing bearing performance. In this article, findings of numerical and experimental investigations on the performance of textured journal bearings have been summarized. It is found that the presence of texture on the bearing surface yields lower friction and higher load-carrying capacity. This article concludes by discussing the effect of texture parameters on the performance characteristics of the journal bearing.

Estimated that one third of the world’s energy will be consumed by friction, as most machine elements and vehicles have failed due to excessive wear. The increase of the load-lubricant ratio has caused adverse effects on the friction and wear of the components, therefore the regulation of emissions levels. The evolution of lubricating oils and additive technologies will be a major technological effort to improve fuel economy, reduce pollutant emissions and extension of drain interval. The unique properties related to low weight, high hardness, thermal stability and chemical inertness are the divergence of ceramics particles in relation to other engineering materials, and are being considered as an alternative for future tribological applications. In this article was studied the effects by the addition of modified submicrometric particles of ZnO and Al₂O₃ separately in mineral base stock (group I) for replacing the traditional ZDDP additive. The tests were performed of according ASTM D4172 in Four Ball tribometer with three load levels, and the wear characteristics of the steel ball as well as the incorporation of particles in the wear scar were analyzed by optical and scanning electron microscopy. The function and mechanism performance of additives based submicrometric particles during operation, such as mending, rolling, film formation and polishing effect. The excellent load carrying capability of particles during lubrication evidence their outstanding performance. Increasing the loads during tribological tests, it was noted that the wear mechanisms with the ZDDP initially caused plastic deformation and corrosive wear with carbides exposure, while the addition of ZnO particles caused lightened abrasive wear with some removal of carbides and finally an intense plastic deformation. The colloidal lubricant with Al₂O₃ particles caused the same wear mechanisms, but on the wear scar presents points of surface delamination. The ceramic additives added to the mineral base stock result in a wear mechanism that benefits the tribological behavior for different lubrication regimes. Considering also that the particles on a nanometric or submicrometric scale are a type of effective additives which promising approach to enhance in the biodegradable lubricant properties.

Focusing on hydrofluoroolefin (HFO) refrigerant, which is expected to be applied as a green refrigerant, the effect of R1234yf on the tribological characteristics was compared with hydrofluorocarbon (HFC) refrigerant R32. Tribological tests were carried out with and without a model lubricant containing triethyl phosphate as an EP additive. R1234yf showed a lower friction coefficient and better wear resistance than R32. XPS analysis of lubricated tracks revealed that metal fluoride was formed from R1234yf and R32 and higher amount of metal fluoride was observed on the track formed under R1234yf than that under R32. The frictional characteristics of R1234yf were affected by refrigerant pressure and temperature of specimen. The higher the pressure of refrigerant and the temperature of the specimen, the better the tribological characteristics were observed. In order to clarify adsorption of refrigerant on metal surface, scratching tests were carried out under refrigerant atmosphere using a mass spectrometer. Although R32 did not adsorb, R1234yf adsorbed on nascent steel surface formed by scratching. The adsorption rate of R1234yf increased linearly with scratching speed. Since propylene also adsorbed on nascent steel surface, π-electrons in R1234yf play an important role on the adsorption. It can be concluded that R1234yf adsorbs easily on nascent steel surface resulting in formation of iron fluoride which acts as a boundary lubrication layer.

Following the previous paper focusing on tribological properties of hydrofluoroolefin (HFO) refrigerant, the adsorption behavior of refrigerants on the nascent iron surface was investigated experimentally and the adsorption structure, adsorption energy and dynamic process of chemical reaction of refrigerants were analyzed by a molecular simulation. The adsorption behavior on the nascent iron surface was highly dependent on the molecular structure of the refrigerant. HFO refrigerants with an unsaturated bond exhibited high adsorption activity, and halogen species also affected the adsorption activity. HFO showed higher adsorption activity than organic ester, phosphate ester and alkyl sulfide as model compounds of refrigerator oil. In adsorption simulation by neural network potential (NNP), HFO molecules showed large negative adsorption energy. The mechanism for this stronger adsorption of HFO species, density functional theory calculation was conducted, and it showed that HFO adsorbs on iron surface by electron donation from the molecule and back-donation from iron surface. There was also a good correlation between the experimental adsorption activity and the NNP-obtained adsorption energy. MD simulation of molecule adsorbed on the nascent surface at temperature of 298 K was subsequently done using the iron fluoride has been experimentally detected on friction track by using XPS in the first report of this study. It was concluded that the adsorption and tribochemical formation of iron fluoride from HFO are supported by molecular simulation performed in this study.

Zinc dithiophosphate (ZnDTP) is used in lubricating oils as an antioxidant. However, its use has been discouraged because it contains phosphorus, which has a high environmental burden. Thus, a new multifunctional additive is required, and fullerenes are attracting attention for this purpose. Although it is known that fullerenes prevent oxidation, their antioxidant performance compared to conventional lubricant antioxidants has not been evaluated, nor has their antioxidant mechanism been elucidated. In this study, we evaluated the oxidation stability of fullerenes and conventional lubricant antioxidants. In addition, model radicals were reacted with fullerenes and conventional lubricant antioxidants in oil to determine the number of radicals captured per molecule. It was shown that fullerenes can capture about five radicals per molecule in oil, which is more than can conventional lubricant antioxidants. Furthermore, it was found that the radical scavenging effect of fullerenes is correlated with the number of dissolved fullerenes.