Hamid Zaïdi

This study focuses on the analysis of defaults affecting rotating machines. Among the crucial elements of rotating machines, it has the hydrodynamic bearings. However, failure of these bearings can result in damage to the turbine pump. Therefore, identifying the failure at its initial stage is essential. The vibration signal generated by a bearing default is used for vibration condition and default diagnostics. The parameters extracted from the vibration data provide an indication and the appearance of the default. A vibration analysis was carried out on the efficiency of the bearings of the turbine pump of Water Benisaf Company (WBC) (pumping water for big cities), in order to characterize the effect of mechanical parameters on the proper functioning of the hydrodynamic bearings. Moreover, a numerical study of the pressure distribution in the journal bearings was done, in order to characterize the effect of pressure on vibration of hydrodynamics bearings.

The tribological behavior of molybdenum disulfide (MoS₂) coatings was systematically investigated under various controlled gas environments in a vacuum chamber. A hemispherical steel pin was slid cyclically over a MoS₂-coated steel disk, prepared via high-speed powder spraying. The study measured both dynamic and static friction coefficients under different gaseous atmospheres, including high vacuum, helium, argon, dry air, and water vapor. In high vacuum (10⁻⁵ Pa), an ultra-low dynamic friction coefficient (µ ≈ 0.01) was observed, while increasing values were recorded with helium (µ ≈ 0.03), argon (µ ≈ 0.04), dry air (µ ≈ 0.17), and water vapor (µ ≈ 0.30). Static friction coefficients followed a similar trend, decreasing significantly upon evacuation of water vapor or injection of inert gases. Surface analyses revealed that friction in vacuum or inert gases promoted smooth wear tracks and basal plane alignment of MoS₂ crystallites, while exposure to water vapor led to rougher, more disordered wear surfaces. Mass spectrometry and energetic modeling of physisorption interactions provided further insights into gas–solid interfacial mechanisms. These results demonstrate that the tribological performance of MoS₂ coatings is highly sensitive to the surrounding gas environment, with inert and vacuum conditions favoring low friction through enhanced basal plane orientation and minimal gas–surface interactions. In contrast, water vapor disrupts this structure, increasing friction and surface degradation. Understanding these interactions is crucial for optimizing MoS₂-based lubrication systems in varying atmospheric or sealed environments.

To address friction and wear challenges in dry contacts, manufacturers often employ self-lubricating materials. Graphite and its derivatives stand out as particularly suitable due to their exceptional tribological properties. However, under intense friction conditions, graphite can experience a decline in lubricating efficiency due to severe abrasive wear. This abrasive damage results in elevated activated carbon surfaces with increased surface energy, fostering greater adhesion between sliding surfaces. The low friction coefficient of graphite is not an inherent property but rather a consequence of water vapor adsorption by the material. Beyond 150 °C, desorption of the vapor occurs, leading to a transition in the friction coefficient from µ = 0.1 to µ = 0.6. To address this issue, impregnation solutions for self-lubricating materials have been developed, with various compositions tailored to specific objectives. Common types include molybdenum disulfide, soft metals and polymers. In this predominantly experimental study, the impact of polymer impregnation on the evolution of friction force and wear rate in graphite material bearings subjected to a dry fretting contact under severe thermal stresses at 270 °C was investigated. Additionally, the mechanical stresses in the bearings throughout different phases of our tests were analyzed using a numerical model.

To address friction and wear challenges in dry contacts, manufacturers often employ self-lubricating materials. Graphite and its derivatives stand out as particularly suitable due to their exceptional tribological properties. However, under intense friction conditions, graphite can experience a decline in lubricating efficiency due to severe abrasive wear. This abrasive damage results in elevated activated carbon surfaces with increased surface energy, fostering greater adhesion between sliding surfaces. The low friction coefficient of graphite is not an inherent property but rather a consequence of water vapor adsorption by the material. Beyond 150°C, desorption of the vapor occurs, leading to a transition in the friction coefficient from µ=0.1 to µ=0.6. To address this issue, impregnation solutions for self-lubricating materials have been developed, with various compositions tailored to specific objectives. Common types include molybdenum disulfide, soft metals, or polymers. In this predominantly experimental study, the impact of polymer impregnation on the evolution of friction force and wear rate in graphite material bearings subjected to a dry fretting contact under severe thermal stresses at 270°C was investigated. Additionally, the mechanical stresses in the bearings throughout different phases of our tests were analyzed using a numerical model.

Some industrial applications require the use of self-lubricating materials when fluid lubrication cannot be used. Carbon-based materials and in particular graphites are usually a used solution. However, the application of these materials is limited at high temperature; these materials are exposed to significant degradation and to high friction due to their high sensitivity to the air humidity and to the desorption phenomenon. This study determines the influence of a specific metallic impregnation on the graphite bearing, which is submitted to a severe thermo-vibratory loading by fretting against a stainless steel surface. The stainless steel surface has undergone a nitriding treatment by plasma. During the fretting contact, a strong transfer of the impregnant takes place from the impregnated graphite bearing to the steel conterface by adhesion; this deposit film allows a significant improvement in the tribological properties of the contact surfaces at high temperature.

In hydrodynamic lubrication, at very high rotational speed, the phenomenon of axial fluid leakage is often present. This can involve an increase of shear stress in the contact and consequently a considerable increase of the temperature. For that and in order to solve this problem, we took interest in the herringbone grooved journal bearings. The researches made before on these types of groove bearing have shown that they present a good dynamical behavior with a low eccentricity and a low axial flow. In this paper, a numerical study of a herringbone journal bearing operating behavior, under laminar and isothermal regime, is presented. The theoretical model, based on the classical Reynolds equation, is used. In order to include the film rupture and reformation, the Reynolds equation is modified using a mass conservative algorithm. To understand the behavior of these herringbone grooved journal bearings well, numerical modeling, using finite element method, has been developed. Various geometrical shapes of the herringbone grooved journal bearings have been analyzed, allowing us to limit the fluid leakage problem, by working particularly on the contact form.

In the context of a waste heat recovery application on a gasoline engine, tribological investigations have been mandatory in the design of a scroll expander. A new design has been proposed to reach higher temperature and investigations are made to ensure that the material of the parts in contact will provide good tribological performances, i.e. a low coefficient of friction and a low wear rate. This research will also prevent high mechanical losses in the machine and, therefore, increase its thermal efficiency. Some experiments have been conducted on six different materials for the tip seal of the expander and two materials for the counter face. Additionally, the experiments have been carried out for a dry and a steam-lubricated contact at several temperatures. This research does not aim to be an exhaustive tribological investigation but instead offers a first comparison between the different materials couples.

The direct metal/metal sliding contact causes a severe wear with a high friction coefficient and an important wear rate. Applying magnetic field into the rotating sliding steel Fe360 B/steel Fe360 B modifies the friction and the wear behavior of the contact. The experimental tests were conducted in three different environments such as: ambient air, pure oxygen at 10⁵ Pa and high vacuum at 5.10⁻⁵ Pa. In ambient air and without magnetic field, the metal/metal contact is ductile. When a magnetic field is applied, the formation of oxide layer evolves progressively with the number of sliding cycles. This contact has a low friction and mild wear. Under oxygen, the oxide film appears in wear track with and without applied magnetic field. The wear rate under oxygen is lower than that in ambient air with a ratio roughly equals to 10. In oxygen, the average value of friction coefficient µ decreases from µ = 0.41 to µ = 0.30. In vacuum, the increase of surface hardness causes a transfer of particles from the disc towards the pin. This transfer modifies the morphology of the wear track, increases the surfaces roughness and leads to a strong adhesive wear. The different analysis obtained with: microscopy, micro-harness, wear losses, EDX and friction evolution explains the effect of applied magnetic field on the friction behavior of Fe360 B steel.

Purpose: After more than a century of agreement with the postulate of non-slip condition (adhesion to the wall), the study of fluid-solid boundary conditions has shown renewed interest over the past two decades. Although numerous studies have not yet been arrived to a complete description of slip phenomena, however, it appears that the influence of wetting and/or surface roughness results in a weak interaction between fluid and solid; thus, the presence of the slip phenomenon is observed at the fluid-solid interface. The purpose of this paper is to highlight the presence of the slip phenomenon at the lubricated piston skirt-cylinder contact. Design/methodology/approach: For this proposal, a modified Reynolds equation and operating characteristics are determined by taking into account the slip conditions at the interface between oil-film and entire cylinder surface. Findings: The findings indicate that the operating characteristics are strongly influenced when the slip conditions are taken into account at the interface between oil-film and cylinder surface. The friction force and dissipated power might be reduced to improve diesel engine performances. Originality/value: Various research studies have been conducted to model the slip phenomenon in different lubricated contacts over the past two decades. However, there are no studies available concerning the piston-cylinder system. Peer review: The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2019-0483/

We have found the following error in the authors’ information in this article, which was recently published in Energies [1]. On page 5273, the names of the authors are incorrect; they should be changed from: