Investigation of the Tribological Properties of Different Textured Lead Bronze Coatings Under Severe Load Conditions

lubricants Article Investigation of the Tribological Properties of Different Textured Lead Bronze Coatings under Severe Load Conditions Adolfo Senatore 1 , Giacomo Risitano 2 , Lorenzo Scappaticci 3 and Danilo D’Andrea 2,* 1 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, SA, Italy; [email protected] 2 Department of Engineering, University of Messina, Contrada di Dio (S. Agata), 98166 Messina, SA, Italy; [email protected] 3 Sustainability Engineering Department, Guglielmo Marconi University, via Plinio 44, 00193 Rome, SA, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-393-020-9246 Abstract: The purpose of this paper is to investigate the variation in the coefficient of friction (CoF) and also the wear in a lead bronze coating under different texture conditions. The tribological tests were performed using a tribometer with pin on disk configuration. Several kinds of textures, realised by a surface laser texturing, were tested by varying the diameter, depth, and density of the dimples under severe working conditions. The innovative aspect concerns the behaviour of the textured lead bronze coating and the lubrication conditions when the sample is subjected to extreme load conditions. Confocal microscopies and SEM (Scanning Electron Microscopy)/EDS (Energy Dispersive X-Ray Spectroscopy) analyses were performed to evaluate the texture behaviour and also the surface deterioration of the coating. The results show that the application of texture processing leads to an improvement in the tribological properties of the coating. By analysing separately the variation of the different geometric parameters of the dimples, it has been shown that the best results µ µ Citation: Senatore, A.; Risitano, G.; are obtained with a diameter of 50 m, a density of 5%, and a depth of 5 m. Scappaticci, L.; D’Andrea, D. Investigation of the Tribological Keywords: laser surface texturing; wear; friction Properties of Different Textured Lead Bronze Coatings under Severe Load Conditions. Lubricants 2021, 9, 34. https://doi.org/10.3390/lubricants 1. Introduction 9040034 Mechanical components, and in particular hydraulic components, are often subject to sliding contacts and high contact pressure, which cause several failure modes such as Received: 27 February 2021 delamination, seizure, noise, vibrations, and fatigue crack due to friction and wear [1–3]. Accepted: 22 March 2021 The efficiency improvement in piston pumps and motors is one of the most important Published: 26 March 2021 goals in the development of these machines [4,5]. A small improvement in pump efficiency in terms of percentage points (%) has a significant impact in terms of noise, efficiency, Publisher’s Note: MDPI stays neutral and reduced consumption in many technologies, such as CVT (Continuously Variable with regard to jurisdictional claims in Transmission) power split [6]. published maps and institutional affil- The reduction in both friction and wear is very important to ensure high efficiency and iations. extension of the component life. Typically, the problem of friction and wear is dealt with by using lubricants, but in some cases such as a low rotation speed, lubrication is insufficient. For this reason, we use coatings capable of guaranteeing dry lubrication, for example lead bronze [7–9]. However, solid lubricants such as lead are now being decommissioned Copyright: © 2021 by the authors. because of their toxicity. Licensee MDPI, Basel, Switzerland. The increasing request for high efficiency anti-friction coatings from modern industries This article is an open access article has pushed researchers towards the development of new technologies, such as PVD-TiN distributed under the terms and coating [10,11] or a super-hydrophobic/super-oleophobic nanostructured surface [12–14]. conditions of the Creative Commons Another technology studied to enhance the tribological property of materials is surface Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ texturing. Surface texturing has been known for a long time, as shown by the numerous 4.0/). studies reported in the scientific literature [15–18]. The use of texture to improve the Lubricants 2021, 9, 34. https://doi.org/10.3390/lubricants9040034 https://www.mdpi.com/journal/lubricants Lubricants 2021, 9, x 2 of 15 Lubricants 2021, 9, 34 2 of 14 surface texturing. Surface texturing has been known for a long time, as shown by the numerous studies reported in the scientific literature [15–18]. The use of texture to improve the performance of mechanical systems and reduce friction and wear is particularly used in performancethe automotive of mechanicalfield, as demonstrated systems and by reduce numerous friction scientific and wear studies is particularly [19–23]. Borghi used in et the al.automotive [24] have studied field, as the demonstrated effects of surface by numerous modification scientific by studieslaser texturing [19–23]. on Borghi tribological et al. [24 ] performanceshave studied of the nitriding effects steel of surface for high-perfo modificationrmance by engine laser texturing applications. on tribological Vladescu et perfor- al. [25]mances have investigated of nitriding the steel effect for high-performance of surface texture engineon friction applications. for a convergent–divergent Vladescu et al. [25 ] bearing,have investigated operating under the effect different of surface lubrication texture regimes. on friction However, for a convergent–divergent almost all the scientific bear- worksing, operatinganalysed investigate under different the tribological lubrication regimes.behaviour However, of texturing almost subjected all the scientificto loads worksvar- yinganalysed between investigate 5 and 100 theN [26–28]. tribological behaviour of texturing subjected to loads varying betweenThe novelty 5 and 100of this N [ 26paper–28]. is the use of extremal working condition to analyse the applicationsThe novelty of surface of thistexturing paper on is hydraulic the use of pumps extremal and working motors, conditionwhich represent to analyse some the applications of surface texturing on hydraulic pumps and motors, which represent some of the components most studied for the application of this technology [29–32]. Another of the components most studied for the application of this technology [29–32]. Another innovative aspect is the use of surface texturing on a lead bronze alloy, which already has innovative aspect is the use of surface texturing on a lead bronze alloy, which already has anti-friction characteristics. anti-friction characteristics. To reduce friction and wear, different shapes of texture can be used, such as dimples, To reduce friction and wear, different shapes of texture can be used, such as dimples, holes, cavities or asperities, but the most used is the micro-dimples; moreover, the effect holes, cavities or asperities, but the most used is the micro-dimples; moreover, the effect of the spatial arrangement of the dimples could be evaluated. There are many techniques of the spatial arrangement of the dimples could be evaluated. There are many techniques to achieve surface texturing including etchant, ion beam texturing, embossing, and laser to achieve surface texturing including etchant, ion beam texturing, embossing, and laser surface texturing (LST), which is the most applied technique [33] and has been used in surface texturing (LST), which is the most applied technique [33] and has been used in this work. this work. Texturised surfaces have numerous advantages in terms of mechanical and tribolog- Texturised surfaces have numerous advantages in terms of mechanical and tribological icalbehaviour. behaviour. In In fact, fact, the the presence presence of micro-incisions,of micro-incisions, allows allows an improvementan improvement of theof tribo-the tribologicallogical behavior behavior in in the the case case of of starved starved lubrication, lubrication, as as the the dimples dimples act act as as oil oil reservoir reservoir [34 ]. [34].While, While, in thein the case case of hydrodynamicof hydrodynamic or or mixed mixed lubrication, lubrication, the the micro-dimples micro-dimples behave behave like likehydrodynamic hydrodynamic bearings bearings [35 [35].]. Another Another important important feature feature of texturised of texturised surfaces surfaces is the is ability the abilityto reduce to reduce abrasive abrasive wear, wear, as micro-engravings as micro-engravings behave behave like traps like traps for debris. for debris. In Inthis this paper paper different different types types of textures of textures were were tested, tested, varying varying the diameter, the diameter, depth, depth,and densityand density of the dimples, of the dimples, using the using pin on the disk pin te onchnique. disk technique. After the Aftertribological the tribological tests, failure tests, analysisfailure was analysis performed was performed in order to in evaluate order to th evaluatee impact the of impactthe texture of the on texture the lead on bronze the lead coating.bronze coating. 2. 2.Materials Materials and and Methods Methods TheThe tribological tribological tests tests were were performed performed by by using using reciprocating reciprocating tester. tester. The The tribo-system tribo-system consistsconsists of ofthe the stationary stationary sample sample (counter-specimen) (counter-specimen) pressed pressed at atthe the required required load load against against thethe cylinder cylinder sample

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