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Green Tribology: New Insights toward a Sustainable World 2023

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A special issue of Lubricants (ISSN 2075-4442).

Deadline for manuscript submissions: closed (31 July 2023) | Viewed by 24350

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Dr. Lorenza Mattei

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Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino 2, 56126 Pisa, Italy
Interests: biotribology; computational biomechanics; motion analysis; vibration analysis
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Prof. Dr. Alessandro Ruggiero

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Guest Editor

Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132-84084 Fisciano, Italy
Interests: biomechanics; (bio) tribology; human movement analysis; computational mechanics
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Special Issue Information

Dear Colleagues,

The response by the readers of the first edition of this Special Issue exceeded our expectations and, together with the Editorial Office of Lubricants, we are very pleased to propose the second edition of the Special Issue “Green Tribology: New Insights toward a Sustainable World”, aiming to cover the latest developments concerning Green Tribology.

In this particular period, we are living in the COVID-19 pandemic, and ecological issues are assuming more and more importance in the framework of a sustainable environment. In this framework, Green Tribology, defined as ‘the science and technology of the tribological aspects of ecological balance and environmental and biological impacts’, represents a modern and interesting direction in scientific research aiming to explore the role of tribological investigation regarding saving energy and the ecological environment in general.

Green Tribology is an emerging area of tribological science. It can be viewed as an interdisciplinary topic which includes classical tribology, chemical engineering, materials science, energy, green lubrication, and environmental sciences, with the purpose of improving the efficiency of processes (cleaner production) and machine components by minimizing friction, wear and dangerous pollutions, in order to protect the environment and improve the quality of life.

The main goals of Green Tribology are the minimization of friction and wear, and the reduction or complete elimination of lubrication, including self-lubrication (natural and biodegradable lubrication). Green Tribology is also devoted to the study of biomimetics, sustainable chemistry, surface texturing, environmental implications of coatings, real-time monitoring, design for degradation, and sustainable energy applications.

Recent investigations report that “…by applying for advances in Green Tribology in terms of new surfaces, materials and lubrication technologies, the total global energy loss in tribological systems could be decreased by 18% in the next 8 years and up to 40% in the next 15 years. An additional advantage of environmentally friendly Green Tribology is a significant reduction in carbon dioxide emissions and economic costs”.

This research topic aims to collect manuscripts within but not limited to the following research areas:

Contact mechanics;
Modelling and minimization of friction and wear;
Tribology of biodegradable materials;
Environmentally friendly lubrication;
Superlubricity;
Sustainable chemistry and green engineering principles;
Biomimetic and self-lubricating materials;
Surface texturing, coatings and their environmental implications;
Real-time monitoring, analysis and control of tribological systems;
Tribology of renewable and/or sustainable sources of energy;
Design for degradation.

Dr. Lorenza Mattei
Prof. Dr. Alessandro Ruggiero
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Lubricants is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

friction
wear
lubrication
biodegradable
green lubricants
superlubricity
biomimetics
surface texturing
coatings
real-time monitoring

Related Special Issue

Green Tribology: New Insights toward a Sustainable World in Lubricants (6 articles)

Published Papers (12 papers)

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Research

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20 pages, 4746 KiB

Open AccessArticle

An Investigation on the Teeth Crowning Effects on the Transient EHL Performance of Large-Scale Wind Turbine Spur Gears

by Hazim U. Jamali, H. S. S. Aljibori, Muhsin Jaber Jweeg, Oday I. Abdullah and Alessandro Ruggiero

Lubricants 2023, 11(11), 462; https://doi.org/10.3390/lubricants11110462 - 29 Oct 2023

Viewed by 1133

Abstract

Crowning is applied to wind turbine gears, including spur gears, to ensure adequate stress distribution and contact localization in wind turbine main gearbox gears to improve the gear performance in the presence of misalignments. Each gear tooth is crowned along the face width using a parabolic curve that graduates from a maximum height at the edges and vanishes at the center of the tooth flank. This crowning transfers the elastohydrodynamic contact problem from a line to a point contact case where the surface curvatures and pressure gradient are considered in both directions of the solution space. A wide range of longitudinal crowning heights is considered in this analysis under heavily loaded teeth for typical large-scale wind turbine gears. Furthermore, the variation in the velocities is considered in the analysis. The full transient elastohydrodynamic point contact solution considers the non-Newtonian oil behavior, where the numerical solution is based on the finite difference method. This work is focused on the evaluation of the effectiveness of teeth’s longitudinal crowning in terms of the consequences on the resulting pressure distribution and the corresponding film thickness. The modification of the tooth flank significantly elevates the film thickness levels over the zones close to the tooth edges without a significant increase in the pressure values. Moreover, the zone close to the tooth edges from both sides, where the pressure is expected to drop to the ambient pressure, is extended as a result of the flank modification. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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10 pages, 1116 KiB

Open AccessArticle

A BPNN-QSTR Model for Friction-Reducing Performance of Organic Liquid Lubricants on SiC/PI Friction Pair

by Tingting Wang, Liang Zhang, Hao Chen, Li Wu and Xinlei Gao

Lubricants 2023, 11(9), 387; https://doi.org/10.3390/lubricants11090387 - 10 Sep 2023

Viewed by 847

Abstract

In this study, a systematic test of 36 organic liquid compounds as lubricants in the SiC/PI friction pair was conducted to investigate their friction-reducing performance. The back propagation neural network (BPNN) method was employed to establish a quantitative structure tribo-ability relationship (QSTR) model for the friction performance of these lubricants. The developed BPNN-QSTR model exhibited excellent fitting and predictive accuracy, with R² = 0.9700, R² (LOO) = 0.6570, and q² = 0.8606. The impact of different descriptors in the model on the friction-reducing performance of the lubricants was explored. The results provide valuable guidance for the design and optimization of lubricants in SiC/PI friction systems, contributing to the development of high-performance lubrication systems. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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13 pages, 6553 KiB

Open AccessArticle

Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding

by Hong Guo, Brandon Stoyanovich, Junru Pang and Patricia Iglesias

Lubricants 2023, 11(8), 329; https://doi.org/10.3390/lubricants11080329 - 03 Aug 2023

Cited by 2 | Viewed by 1513

Abstract

Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are major limitations. In addition, environmental awareness has increased the need for more environmentally friendly lubricants. Ionic Liquids (ILs) have exhibited significant potential as lubricants and lubricant additives. One of the more interesting properties of ILs is that they can form physically-adsorbed or chemically-reacted layers that reduce friction and wear of the surfaces in contact. Among ILs, Protic Ionic Liquids (PILs) have received more attention recently because of their simple and economic synthesis route. Furthermore, the anions and cations of PILs can be selected to be considered environmentally benign. In this article, the tribological behavior of a family of six PILs are studied as additives to a biodegradable oil (BO), under aluminum-steel contact. Al2024 disks slid against AISI52100 steel balls under a normal load of 3 N and a frequency of 5 Hz at room temperature and using a ball-on-flat reciprocating tribometer. PILs used in this study, were synthesized using two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds. Results show that, although adding just 1 wt.% of any PIL to BO reduced friction and wear, the alkyl chain length influenced the lubricating ability of these ordered fluids. Wear mechanisms and surface interaction are discussed on the basis of 3D profilometry, SEM-EDX and RAMAN spectroscopy. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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19 pages, 12653 KiB

Open AccessArticle

High-Temperature Superlubricity Performance of h-BN Coating on the Textured Inconel X750 Alloy

by Qunfeng Zeng

Lubricants 2023, 11(6), 258; https://doi.org/10.3390/lubricants11060258 - 10 Jun 2023

Cited by 5 | Viewed by 1215

Abstract

The high-temperature superlubricity performances of h-BN coatings on the nontextured and textured surface of an Inconel X750 alloy is reported in the present paper. The hardness and bond strength of the h-BN coating and alloy were investigated. The tribological properties of the X750 alloy and coatings on the X750 alloy substrate were investigated at different temperatures. The surface texture was manufactured on the surface of the X750 alloy, and then coatings were deposited on the textured surface to reduce the cracking of the coating and enhance the stability of the antifriction behaviors of the h-BN coatings. The tribotest results showed that the texture is helpful to enhance the interface thermal compatibility of the coating and substrate and store the wear debris generated during sliding. Therefore, a stable superlubricity was achieved at high temperatures, and a super low friction mechanism is also discussed. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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25 pages, 8598 KiB

Open AccessArticle

Analysis of Modified Finite Length Journal Bearing under Position Perturbation

by Hazim U. Jamali, Hakim S. Sultan, Oday I. Abdullah, Adnan Naji Jameel Al-Tamimi, Murtadha M. Al-Masoudy, Alessandro Ruggiero and Maria Cristina Valigi

Lubricants 2023, 11(4), 173; https://doi.org/10.3390/lubricants11040173 - 11 Apr 2023

Viewed by 1111

Abstract

The performance of journal bearings is significantly affected by the presence of misalignment, which is usually an accompanying problem for this type of bearing. This includes exceeding the design limits for the maximum pressure and the minimum film thickness levels, which affect, in other words, the load-carrying capacity of the system. In addition, it raises the possibility of increasing the wear rate at the bearing edges and increases the friction coefficient at high levels of misalignment. This paper deals with the problem of finite-length misaligned journal bearings, considering a novel comparison between two cases of misalignments: the general 3D misalignment and the vertical misalignment problems for modified bearings. The effect of introducing a variable axial bearing profile on the bearing characteristics and the time responses of the rotor bearing system under position perturbation has also been investigated. The numerical solution of this hydrodynamic problem is based on the finite difference method using Reynolds boundary conditions method. Results show that using a variable bearing profile improves bearing characteristics, such as increasing the minimum film thickness significantly and reducing the pressure levels in addition to reducing the friction coefficient. Furthermore, the modification enhances the rotor-bearing stability under position perturbation, extending the speed range for a safe operation. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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17 pages, 3253 KiB

Open AccessArticle

Viscosity and Friction Reduction of Double-End-Capped Polyalkylene Glycol Nanolubricants for Eco-Friendly Refrigerant

by Mohd Zaki Sharif, Wan Hamzah Azmi, Mohd Fairusham Ghazali, Nurul Nadia Mohd Zawawi and Hafiz Muhammad Ali

Lubricants 2023, 11(3), 129; https://doi.org/10.3390/lubricants11030129 - 12 Mar 2023

Cited by 9 | Viewed by 1288

Abstract

In sustainable tribology, researchers are investigating methods to enhance tribological performance by incorporating nanoparticles into lubricants. However, one potential drawback of this strategy is increased lubricant viscosity. The current study aimed to assess the impact of these nanoparticles on the viscosity and coefficient of friction (COF) of the nanolubricants. Three different nanolubricants were synthesized through a two-step process, including mono-nanolubricants (Al₂O₃/DEC PAG and SiO₂/DEC PAG) and hybrid nanolubricants (Al₂O₃-SiO₂/DEC PAG), at volume concentrations between 0.01% and 0.05%. The viscosity and shear flow behavior of these nanolubricants were evaluated using a digital rheometer, while the COF was measured using a Koehler four-ball tribometer. All the nanolubricants showed Newtonian behavior during the experiments. The dynamic viscosity velocity increment of SiO₂/DEC PAG was found to be the lowest (1.88%), followed by Al₂O₃-SiO₂/DEC PAG (2.74%) and Al₂O₃/DEC PAG (3.56%). The viscosity indices of all the nanolubricants were improved only at higher concentrations. At a volume concentration of 0.03%, the Al₂O₃-SiO₂/DEC PAG nanolubricant reduced the COF by up to 8.1%. The results showed that the combination of nanoparticles, temperature, and volume concentration significantly influenced the viscosity and COF of nanolubricants. This study provides essential information for developing high-performance nanolubricants with improved viscosity and COF and advancing environmentally friendly tribology solutions. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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13 pages, 4523 KiB

Open AccessArticle

Impact of Boron-Containing Lubricant Additive on Three-Way Catalyst Reactivity and Physicochemical Properties

by Daekun Kim, Todd J. Toops, Ke Nguyen, Michael J. Lance and Jun Qu

Lubricants 2023, 11(2), 53; https://doi.org/10.3390/lubricants11020053 - 31 Jan 2023

Viewed by 1786

Abstract

Boron-containing compounds are one of the lubricant additive options due to their suitable properties for additives and have been used as commercial lubricant additives. In the present study, the impact of a boron-containing lubricant oil additive, AR9100 (BR), on Pd/Rh-based three-way catalyst (TWC) performance is investigated, and the results are compared with the baseline no-additive (NA) case and the industry standard zinc dialkyl-dithiophosphate (ZDDP) results. Accelerated engine aging is performed using a genset to expose the catalysts to lubricant additives at high temperatures. All aged TWC samples are investigated for reactivity in a bench-flow reactor and characterized using a variety of analytical techniques. Compared with the no-additive case, the temperatures of 90% conversion (T₉₀) of NO, CO, C₃H₆, and C₃H₈ for the ZDDP-aged TWC sample increased by 34, 30, 37, and 48 °C. However, the T₉₀ of all gas species for the BR-aged TWC sample are similar to the NA-aged TWC sample. Additionally, a significant decrease in water–gas shift reactivity and oxygen storage capacity is observed in the ZDDP-aged sample, but not in the BR-aged sample. Inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis and electron probe microanalysis (EPMA) maps of accelerated engine aging samples show the presence of phosphorus and boron in ZDDP- and BR-aged TWC samples, respectively. However, no boron-related peaks are observed in the X-ray diffraction (XRD) pattern of the BR-aged TWC sample, which may exist in the form of an amorphous phase. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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19 pages, 10237 KiB

Open AccessArticle

Ecofriendly Protic Ionic Liquid Lubricants for Ti6Al4V

by Ana-Eva Jiménez, María-Dolores Avilés, Ramón Pamies, María-Dolores Bermúdez, Francisco-José Carrión-Vilches and José Sanes

Lubricants 2023, 11(1), 5; https://doi.org/10.3390/lubricants11010005 - 22 Dec 2022

Cited by 2 | Viewed by 2302

Abstract

Three diprotic ionic liquids (PILs) containing bis(2-hydroxyethyl) ammonium cations and citrate (DCi), lactate (DL), or salycilate (DSa) hydroxy/carboxylate anions were studied as lubricants for Ti6Al4V–sapphire contact. At room temperature, the neat PILs are non-Newtonian fluids, which show up to a 70% friction coefficient reduction with respect to water. New aqueous lubricants were developed using PILs as 1 wt.% additives in water. The new (Water + 1 wt.% PILs) lubricants showed friction reductions of higher than 50% with respect to water at room temperature. The lowest friction coefficients at room temperature were achieved with thin lubricant layers deposited on Ti6Al4V using Water + 1 wt.% PIL after water evaporation. At 100 °C, the best tribological performance, with the lowest friction coefficients and wear rates, was obtained for the PILs containing aliphatic anions: DCi, and DL. The surface layers of the sapphire balls with mild adhesion and abrasion wear mechanisms were observed via scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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17 pages, 6612 KiB

Open AccessArticle

The Use of Non-Edible Green Oils to Lubricate DC04 Steel Sheets in Sheet Metal Forming Process

by Tomasz Trzepieciński, Marek Szewczyk and Krzysztof Szwajka

Lubricants 2022, 10(9), 210; https://doi.org/10.3390/lubricants10090210 - 30 Aug 2022

Cited by 6 | Viewed by 1762

Abstract

Lubrication is a basic and relatively effective way to reduce friction in sheet metal forming operations. The drive to eliminate synthetic and mineral oils, which are difficult to recycle, from the manufacturing process has opened up opportunities for the use of vegetable-based bio-lubricants. This article presents a comparison of the lubrication performance of two non-edible oils (karanja and moringa) with the most frequently tested edible oils (sunflower and rape-seed). Deep drawing quality low-carbon steel sheets DC04, commonly used in the automotive industry, were used as the test material. Friction tests were carried out under various lubricants and normal pressures in the range between 3 and 12 MPa using the strip drawing test. Furthermore, a study was also made of the effect of a change in the surface topography and the mechanical properties of the sheet metal due to plastic deformation resulting from friction. It was found that under the most favorable lubrication conditions (sample pre-strain 21%, nominal pressure 6 MPa), karanja oil reduced the coefficient of friction by approximately 33%. Both non-edible lubricants provided the best lubrication when testing samples pre-strained at 7% under the whole range of nominal pressures. It was also revealed that in the case of the smallest pre-straining of the specimens (7%), karanja oil was the most effective within nominal pressures of 3–6 MPa, while at higher pressures (9–12 MPa), the moringa oil lowered the value of the coefficient of friction to a greater extent. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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Review

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21 pages, 4638 KiB

Open AccessReview

Influence of Molecular Structure on the Physicochemical and Tribological Properties of Biolubricants: A Review

by Claudia Sanjurjo, Eduardo Rodríguez, José L. Viesca and A. Hernández Battez

Lubricants 2023, 11(9), 380; https://doi.org/10.3390/lubricants11090380 - 07 Sep 2023

Cited by 2 | Viewed by 1628

Abstract

The increase in the price of crude oil, the environmental impact, or the depletion of fossil resources has increased the need for bio-based alternatives. This has led to the search for renewable, biodegradable, and environmentally friendly raw materials to obtain lubricants that meet these characteristics. This review deals with the state of the art of biolubricants along with their most common raw materials and molecular structures, processes of chemical modification of bio-oils, as well as the relationship between their structural features and physicochemical/tribological properties. This review concludes that the production of fatty acid alkyl esters from vegetable oils is the most promising chemical route to produce a wide range of biolubricants through double transesterification reactions. It also highlights the need to explore this route for the production of microalgae-derived biolubricants due to its environmental benefits during cultivation and production processes. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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33 pages, 7132 KiB

Open AccessReview

Tribocatalysis Induced Carbon-Based Tribofilms—An Emerging Tribological Approach for Sustainable Lubrications

by Khai K. Huynh, Sang T. Pham, Kiet A. Tieu and Shanhong Wan

Lubricants 2023, 11(8), 327; https://doi.org/10.3390/lubricants11080327 - 01 Aug 2023

Cited by 2 | Viewed by 1715

Abstract

To comply with the high demand for efficient and sustainable lubrications, carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the early stages of realization and research, these tribofilms receive significant interest due to their capability to continuously and in situ repair/replenish themselves during sliding, which has been an ultimate goal of all moving mechanical systems. Structurally, these tribofilms are complex and predominantly amorphous or disordered with/without graphitic domains (e.g., graphene/graphite, onion-like carbon, etc.). Chemically, the compositions of these tribofilms vary significantly with environments, conditions, and material precursors. Yet, the structural properties of carbon-based tribofilms remain largely ambiguous, which precludes a full understanding of the mechanisms underlying the formation and lubrication performance. This review will summarize the current state-of-art research about the in situ carbon-based tribofilms that have been published since the pioneering works. Particularly, this work will highlight the recent approaches to generate these tribofilms, their associated lubrication performance, current understanding of the formation mechanics, common analytical approaches for these tribofilms, and the compatibility of these tribofilms with other additives. Together, the overall outlooks will be drawn, demonstrating the knowledge gaps and proposing further investigation tactics to tackle these emerging issues. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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37 pages, 6240 KiB

Open AccessReview

The Twelve Principles of Green Tribology: Studies, Research, and Case Studies—A Brief Anthology

by Marco Freschi, Alessandro Paniz, Elena Cerqueni, Gianmarco Colella and Giovanni Dotelli

Lubricants 2022, 10(6), 129; https://doi.org/10.3390/lubricants10060129 - 17 Jun 2022

Cited by 13 | Viewed by 4897

Abstract

Sustainability has become of paramount importance, as evidenced by the increasing number of norms and regulations concerning various sectors. Due to its intrinsic trans-sectorial nature, tribology has drawn the attention of the supporters of sustainability. This discipline allows the environmental, economic, and social impacts to be decreased in a wide range of applications following the same strategies. In 2010, Nosonovsky and Bhushan drew up 12 approaches based on the 12 principles of green chemistry and the 12 principles of green engineering, defining the “12 principles of green tribology.” This review exploits the 12 principles of green tribology to fathom the developed research related to sustainability and tribology. Different approaches and innovative studies have been proposed in this short selection as references to consider for further development, pursuing the efforts of the scientific community for a sustainable future through the contribution also of tribosystems. The manuscript aims to provide practical examples of materials, lubricants, strategies, and technologies that have contributed to the overall progress of tribology, decreasing wear and friction and increasing efficiency, and at the same time promoting sustainable development, lowering toxicity, waste production, and loss of energy and resources. Full article

(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)

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