Lubricant Oxidation Research Articles

The effect of solid lubricants in the brake friction composite on brake emissions: A case study with graphite, Sb2S3, and MoS2

Particulate matter produced during brake application was investigated, focusing on the effect of the solid lubricants in the brake friction composite, such as graphite, antimony trisulfide, and molybdenum disulfide. Results showed that the brake emission was significantly affected by the solid lubricant type and braking temperatures. Antimony trisulfide caused high brake emissions at moderate temperatures, whereas graphite produced high emissions at elevated temperatures. The brake emission at moderate temperatures was affected by the aggressiveness of the composites and the presence of friction films on the gray iron disc surface. In contrast, the brake emissions at elevated temperatures were determined by the thermal decomposition of the composite and the oxidation of solid lubricants.

Tribochemistry of phenol and benzyl alcohol between YG8 self‐mated interfaces

In this work, the tribological performance of mixed lubricant consisting of phenol and benzyl alcohol between YG8 self‐mated interfaces was systematically investigated under 98 N and 1450 rpm. The results showed that with increasing concentration of phenol in the mixed lubricant, the friction coefficient initially decreases and then increases; however, anti‐wear performance initially was enhanced and then weaken. As the concentration of phenol was 1.0 wt.%, the tribo‐system exhibited the lowest friction coefficient and wear scar diameter. TEM, SEM and XPS techniques were employed to probe the possible tribological mechanism sliding in the mixed lubricant containing 1.0 wt.% of phenol. The phenol effectively slowed down the oxidation of the mixed lubricant, combination of friction‐induced tungsten oxides and carbon quantum dots, assuring the tribo‐system still in mixed lubrication condition, which made the tribo‐system maintain low friction coefficient and wear.

Online infrared spectra analysis of multi-phenol antioxidants in ester lubricant during friction under high-temperature oxidation

Two self-prepared multi-phenol antioxidants, 2,4,6‐tris(3,5‐di‐tert‐butyl‐4‐hydroxyphenylthio)‐1,3,5‐triazine (THA) and phloroglucinol tris(3-(3,5-di-tert-butyl-4-hydroxyphenyl)) propionate (PTP), were separately introduced into ester lubricants to investigate their antioxidative and tribological properties and mechanisms. A self-assembled platform combining a tribological tester and infrared spectrometer was employed to acquire the online infrared spectra for determining the structural variation of the lubricants with added THA/PTP during friction under high-temperature oxidation. The online infrared spectra indicated that PTP inhibited the oxidation of the ester lubricant more effectively than THA did. Moreover, the PTP antioxidant exhibited friction and wear reduction in the ester lubricant, outperforming THA. The encouraging results of the PTP suggest that multi-phenol antioxidants bridged by ester linkages can be used as multifunctional agents for ester lubricants.

Contribution Ultra Violet Radiation On Degradation Of Biodegradable Base Oil

Increasing environmental awareness led to the development of environmentally harmless lubricants made by biodegradable materials. Biodegradable lubricants are suitable for various kinds of application, one of the promising usages is for railways. Oils or greases are applied for railways or points to keep sufficient friction coefficient or to avoid wear damage. These lubricants are “once-through” lubricants, and never recovered but defused into the earth’s surface. The oxidation of lubricants on the railways will take place because of the sunlight effect. The solar radiation at the top of earth’s atmosphere contains a significant amount of UV radiation of shorter wavelength, and therefore higher energy. This study shows that the UV-B radiation can deteriorate rapeseed oil immediately. It is showed by the results of peroxide value, total acid number and molecular weight distribution are increased with oxidation time.

Online lubricant degradation monitoring using contact charging of polymers

Lubrication of machine parts is necessary to prevent friction and wear in machine operation. However, oxidation of lubricants upon operation changes their chemical and physical properties and causes lubrication performance to deteriorate. Besides, one liter of wasted/used lubricant oil can contaminate a million liters of water. Current methods for the timely detection of oil deterioration lack practicality. Here we show simple, a triboelectrification-based method for oil degradation monitoring, in which the oxidation can be monitored by the changes in the contact charging (and separation) signals of oil-dipped cellulose and a common polymer tapped to each other. The commercial and base oil samples are oxidized at 100–200 °C for up to 80 h, simultaneously monitored by ATR-FTIR and the triboelectric method. The results show that a 40–90% decrease in the triboelectric open-circuit potentials in the presence of oxidized oil can directly indicate the start of oil oxidation without other accompanying analyses.

Wear Resistance of Aluminum Matrix Composites’ Coatings Added on AA6082 Aluminum Alloy by Laser Cladding

Ceramic-reinforced metal matrix composites are known for their high wear resistance. A coating based on these materials would be helpful to improve the wear behavior of aluminum alloys. Laser cladding has been used to deposit a coating consisting of an aluminum alloy reinforced with SiC particles on an AA6082 aluminum alloy. Laser cladding is a very energetic technique that causes the SiC particles to react with the molten aluminum to form Al4C3, which degrades the particles and reduces the properties of the coating. The formation of this detrimental compound was successfully achieved with the addition of Silicon and Titanium to the composite matrix. The microstructures of the newly developed material were characterized and the wear behavior was studied under dry sliding conditions on a pin-on-disc tribometer. The relationship between the microstructure and wear behavior was identified. The absence of Al4C3 in the Al40Si/SiC and Al12Si20Ti/SiC coatings’ microstructures resulted in an abrasion mechanism instead of a delamination mechanism. The wear behavior changed along the sliding distances. During the first 200 m of sliding distances, the wear rate of all coatings was lower than the uncoated one due to their higher microhardness. For longer sliding distances, the wear resistance of the uncoated AA6082 was higher than the coated ones due to the formation of a lubricant oxide layer on the AA6082 worn surface. For 1000 m of wear distances, the wear behavior was different for each coating. The wear rate of the Al12Si/SiC coating continued growing due to the delamination mechanism and the presence of Al4C3 that acted as starting crack points. The wear rate of the Al40Si/SiC coating decreased due to the formation of a thin, superficial oxide layer. The wear rate of the Al12SiTi/SiC progressively decreased along the sliding distance to below the substrate wear rate.

Influence of aldol condensation processes on the formation of surface films during friction in ester lubricants

The boundary lubrication mode is usually implemented in conditions of low sliding speeds and high loads. The formation of strong boundary lubricating films under this friction mode determines the operability and durability of the friction units. It is believed that the formation of surface boundary films during friction includes the stages of the lubricant oxidation, and the aldol condensation reaction of oxidized molecules. As a result, high-molecular substances called “friction polymers” are formed. The paper studies the formation of surface films in the presence of substances with different reactivity in the aldol condensation and Claisen condensation reactions. Sunflower oil, bis (2-ethylhexyl) sebacate (DEHS), triisodecyl benzene-1,2,4-tricarboxylate (TC) were used as lubricants. It is shown by ATR IR-spectroscopy of that the common thing for the studied oils is that the C=O and C-O groups participate in the formation of boundary films in these oils. The addition of substances, active in aldol condensation reactions, into lubricants does not accelerate the formation of boundary films. Additives that can chemically interact with iron contribute to the dissolution of the surface oxide film and accelerate the formation of boundary layers. The formation of “friction polymers” occurs when the lubricant molecules interact with the metal surface.

Experimental assessment of ignition characteristics of lubricating oil sprays related to low-speed pre-ignition (LSPI)

Low-speed pre-ignition (LSPI) remains one of the challenges of Direct Injection (DI) Spark Ignition (SI) engines due to its potential to induce a heavy knock. Several mechanisms have been identified in the literature as plausible causes for LSPI. The physical and chemical properties of lubricant oils play a role on some of these causes. The present work aims at getting an independent procedure to determine the proneness of lubricant oils to ignite. To this end, the ignition delay (ID) of different oil formulations is experimentally determined in a constant-pressure flow facility through two different optical techniques: Schlieren and OH* chemiluminescence imaging. The investigation explores the effect of base-stock formulation, oil specification quality level, different additive types content, aging, and oxidation on oil reactivity for several thermodynamic conditions. Differences in ignition delay were found among base stocks, correlating with the American Petroleum Institute (API) group classification. However, no significant differences were found among additive packages previously reported to yield different LSPI occurrences. Hence, differences in reactivity among lubricating oil formulations are not the determining factor explaining their different LSPI occurrences in an engine. Similarly, specific lubricant additive content, aging, and oxidation do not importantly modify the measured ignition delay.

CoCrNi matrix high-temperature wear resistant composites with micro- and nano-Al2O3 reinforcement

The high-temperature wear resistant CoCrNi matrix composites containing micro-Al2O3 and nano-Al2O3 ceramic particles were prepared by powder metallurgy technology. The high-temperature wear mechanisms of the composites reinforced with nano-Al2O3 and micro-Al2O3 were systematically studied. The tribological performance of CoCrNi matrix composites were evaluated from 23 °C to 1000 °C sliding against Si3N4 ceramic ball. The results revealed that the CoCrNi matrix composite reinforced by nano-Al2O3 had high hardness and compactness. The wear rates of CoCrNi matrix composite with nano-Al2O3 were 0.5–5 times lower than those of the composite with micro-Al2O3 and Al2O3-free alloy. Above 800 °C, the nano-Al2O3 particles reacted with CoO and formed the high temperature solid lubricant Co(AlO2)2, resulting in low wear rate and friction coefficient. The excellent wear resistance of CoCrNi matrix composites reinforced with nano-Al2O3 was attributed to the high hardness and compactness as well as the solid lubricant oxide film on the worn surfaces, which consisted of metal oxides, NiCr2O4, Co2CrO4 and Co(AlO2)2. The wear mechanisms of CoCrNi matrix composites were established at elevated temperatures.

The Role of NOx in Engine Lubricant Oxidation

<div class="section abstract"><div class="htmlview paragraph">Fuel economy trends like engine downsizing and reduced piston ring tension have created challenging environments for modern engine oils which are expected to deliver more from less. Downsizing has led to smaller sumps meaning longer contact times of the lubricant with NOx in blow-by gasses. Fuel economy requirements have also demanded greater engine efficiency with increasing sump temperatures and blow-by rates due to reduced ring tension. These trends promote nitro-oxidation driven by the action of NOx and air on the lubricant. Nitro-oxidation has often been overlooked as a mechanism of oil oxidation in real world engines. Indeed, in bench oxidation tests purporting to protect modern engines against lubricant oxidation, the emphasis is almost exclusively on iron catalysed oxidation. This paper will show that a simple bench nitro-oxidation test is capable of reproducing trends in nitrate ester formation and consumption seen in real engines and demonstrate that lubricant oxidation and viscosity increase is accounted for by the action of NOx without the use of iron catalysts. This demonstrates that the initiation of lube oil oxidation in real engines is due to the action of NOx and it is only later in the oil life cycle that iron plays any significant role. This has important implications for the widespread use of iron-catalysed bench oxidation tests as guardians for oxidation performance in the real world. Because the oxidation observed in iron catalysed bench oxidation tests does not include NOx, the initiation and propagation of oxidation proceeds via a different mechanism and requires a different anti-oxidant solution.</div></div>

In situ Raman spectroscopy study on the effect of antioxidants on high temperature oxidation properties of TMPTO

Purpose This paper aims to investigate the thermal oxidation characteristics of the unsaturated bonds (C=C) of trimethylolpropane trioleate (TMPTO) and to reveal the high temperature oxidation decay mechanism of unsaturated esters and the nature of the anti-oxidation properties of the additives. Design/methodology/approach Using a DXR laser microscopic Raman spectrometer and Linkam FTIR600 temperature control platform, the isothermal oxidation experiments of TMPTO with or without 1.0 wt. % of different antioxidants were performed. Findings The results indicated that the Raman peaks of =C-H, C=C and -CH2- weaken gradually with prolonged oxidation time, and the corresponding Raman intensities drop rapidly at higher temperatures. The aromatic amine antioxidant can decrease the attenuation of peak intensity, as it significantly reduces the rate constant of C=C thermal oxidation. The hindered phenolic antioxidant has a protective effect during the early stages of oxidation (induction period), but it may accelerate the oxidation of C=C afterwards. Originality/value Research on the structure changes of synthetic esters during oxidation by Raman spectroscopy will be of great importance in promoting the use of Raman spectroscopy to analyze the oxidation of lubricants.

Molecular Dynamics Simulation on the Aggregation of Lubricant Oxidation Products

The aggregation phenomenon of lubricant oxidation products, fatty acids, fatty alcohols, aliphatic aldehydes and aliphatic ketones in base oil, was probed by molecular dynamics (MD) simulation. The results show that there are aggregates formed in fatty acids or fatty alcohols, and that the aggregation does not occur in aliphatic aldehydes and aliphatic ketones in single component models. Fatty acids or fatty alcohols are connected by hydrogen bonds in the form of dimers or multimers. The hydrogen bond can be formed at a higher temperature in fatty acids, but not in fatty alcohols. In the mixture models, there are dimers and/or multimers constructed by different molecules of lubricant oxidation products, and the multimers have a chain or a ring structure. The number of molecules involved in the formation of aggregates decreases with the increase of temperature. The opportunity to construct hydrogen bonds rises with the increasing concentration of oxidation products. Obvious aggregation occurs at 25 and 100 °C in conditions of high concentration, while it happens only at 25 °C with lower concentration. It is easier for fatty acids and fatty alcohols to form hydrogen bonds and the formed hydrogen bonds are more stable, which result in a higher concentration of fatty acids and fatty alcohols in the aggregates. Fatty acids and fatty alcohols are possibly more significant in the formation of aggregates and it may be beneficial to prevent the aggregation by controlling the concentration of them in lubricating oil.

Oxidation stability of lubricating base oils

ABSTRACTThe oxidative stability of three base lubricating oils of different grades light, medium and heavy oils (B1, B2 and B3, respectively) was studied at oxidative times 50, 100 and 150 hours. The quality of base oils under investigation were evaluated on the basis of total acid number, vaporization loss, kinematic viscosity and quantity of sludge formed. Also, the chemical composition of the studied lubricating oils was achieved via capillary gas chromatography. The comparative study of the oxidative performance of the base oils showed that the higher degrees of heavy oil than light one. The increase in the oxidation time led to increases of the evaporation loss and lubricant viscosity. Analysis of a lubricant's oxidation stability therefore contributes to a better understanding of its effectiveness under normal operating conditions and its suitability for formulating automotive lubricants. The aim of this work is the study the Potential life of lubricant oils inside engine motor at different oxidative times in view of their oxidation stability.

Elucidation of oxidation and degradation products of oxygen containing fuel components by combined use of a stable isotopic tracer and mass spectrometry

In order to reveal the degradation products of oxygen-containing fuel components, in particular fatty acid methyl esters, a novel approach was developed to characterize the oxidation behaviour. Combination of artificial alteration under pressurized oxygen atmosphere, a stable isotopic tracer, and gas chromatography electron impact mass spectrometry (GC-EI-MS) was used to obtain detailed information on the formation of oxidation products of (9Z), (12Z)-octadecadienoic acid methyl ester (C18:2 ME). Thereby, biodiesel simulating model compound C18:2 ME was oxidized in a rotating pressurized vessel standardized for lubricant oxidation tests (RPVOT), i.e., artificially altered, under 16O2 as well as 18O2 atmosphere. Identification of the formed degradation products, mainly carboxylic acids of various chain lengths, alcohols, ketones, and esters, was performed by means of GC-EI-MS. Comparison of mass spectra of compounds under both atmospheres revealed not only the degree of oxidation and the origin of oxygen atoms, but also the sites of oxidative attack and bond cleavage. Hence, the developed and outlined strategy based on a gas-phase stable isotopic tracer and mass spectrometry provides insight into the degradation of oxygen-containing fuels and fuel components by means of the accurate differentiation of oxygen origin in a degradation product.

Investigating the effect of LaF3 on the tribological performances of an environment friendly hydrophilic polyamide imide resin bonded solid lubricating coating

The aims of this study are to prepare a new environment friendly hydrophilic polyamide imide (PAI) resin bonded molybdenum disulfide (MoS2) solid lubricating coating and investigate the tribological properties and wear mechanism. It is concluded that the preparation of hydrophilic coating has greatly reduced usage rates of fossil fuel, in the meantime, the basic physical and chemical properties, mechanical and tribological behaviors of hydrophilic coatings with 3 wt.% LaF3 are comparable to that of solvent-based resin coatings, the generation of FeF2 and FeS significantly improves the adherence between friction pairs, and the degree of oxidation of MoS2 lubricant is effectively restrained on account of the attachment of nano-LaF3 to the surface of MoS2.

Theoretical Study of Epoxidation Reactions Relevant to Hydrocarbon Oxidation

Advances in computational modeling tools have allowed for the construction of detailed chemical models of oxidation processes to enhance the understanding of mechanisms and answer questions of interest in diverse fields such as DNA damage, lubricant oxidation, food chemistry, and art conservation. The construction of detailed kinetic models is facilitated by the use of kinetic correlations, such as Evans–Polanyi relationships, where the activation energy, EA, is related linearly to the heat of reaction, ΔHR: EA = E0 + α ΔHR. In this work, we present an Evans–Polanyi relationship based on properties determined from hybrid G4 quantum chemical calculations for an epoxidation reaction of peroxy species. We explore a broader chemical space at a higher level of theory than previous reports, and as a result, the importance of several key structural features, such as alkylhydroperoxy functional groups, that can strongly influence the trends observed was revealed. We suggest a subdivision of the reaction family in...

Preparation of Sorbitol Palmitate by Organic Catalysis and Its Application for Base Oil Stabilization

Esters are excellent lubricants and high performance industrial fluids, but they are often costly. We prepared the ester sorbitol palmitate via an inexpensive phase-transfer catalysis method as an additive for the retardation of oil oxidation. The effects of the sorbitan palmitate content on the lubricant properties and oxidation stability of a base oil were determined. The addition of sorbitan palmitate to the oil retarded oxidation and enhanced the pour point depression. A novel method for inhibiting oxidation through the action of micellar cores was suggested. This micellar inhibition offers a new concept for the protection of lubricants against oxidative degradation.

Effect of Calix[n]Arene Derivatives on Oxidation Resistance of Plastic Lubricants

The effect of the nature of calix[n]arenes [n = 4, 6, 8] added to a plastic lubricant on the oxidation resistance of the lubricant at high temperatures is studied. Such characteristics as the number of phenolic fragments in the calixarene structure, the presence of tert-butyl groups on the upper rim of the calix[n]arene, and presence of an O-alkyl substituent in the lower rim are examined. It is shown that calix[8]arenes are capable of inhibiting high-temperature oxidation of plastic lubricants, regardless of the presence of a tert-butyl substituent on the upper rim and an alkyl substituent on the lower rim.

Role of atomic layer deposited solid lubricants in the sliding wear reduction of carbon–carbon composites at room and higher temperatures

High frequency reciprocating sliding wear behavior and mechanisms were investigated at 25, 150 and 400°C for solid lubricant oxide coated carbon–carbon composites (CCC). Atomic layer deposition (ALD) was employed to infiltrate a trilayer nanolaminate coating of ZnO/Al2O3/ZrO2 into the porous CCC. The ALD coatings exhibited excellent conformality and pore-filling down to ~55μm into the CCC, which cannot be achieved by other coating techniques. Compared to uncoated CCC, ALD infiltration resulted in a 64% improvement in the wear rate to 1.5×10−6mm3/N·m at 25°C, while a 31% improvement was determined during 150°C sliding. Conversely, higher temperature sliding at 400°C resulted in increasing wear rates by an order of magnitude due to combined oxidation of the steel counterface and exposed CCC. Cross-sectional focused ion beam-transmission electron microscopy (FIB-TEM) studies of the worn surfaces were performed to elucidate the solid lubrication mechanisms. It was determined that a highly deformed composite surface tribolayer of nanolaminate coating and amorphous C was responsible for improved sliding wear compared to the uncoated CCC that just exhibited an amorphous C tribolayer. Inside the deformed composite tribolayer, a high density of near surface ZnO (0002) basal plane sliding/shear-induced stacking faults was observed in comparison to unworn ALD nanolaminates. Activating subsurface basal stacking faults in ALD ZnO promotes intrafilm shear/slip and hence improves wear resistance. The slip of partial dislocations results from a dislocation glide process along the ZnO (0002) basal planes. Thus, ALD ZnO/Al2O3/ZrO2 nanolaminate coatings are potential candidates for providing wear reduction from low to intermediate temperatures in moving mechanical assembles, such as CCC bushings that experience fretting-type wear.

Oxidation and low temperature stability of polymerized soybean oil-based lubricants

Oxidation and low temperature stability of polymerized soybean oil (PSO)-based lubricants have been investigated by the pressurized differential scanning calorimetry (PDSC) method. It was found that PSO samples have lower oxidative stability than their precursor, soybean oil. The main reason for the decreased stability is the generation of tertiary carbons during polymerization. By using antioxidant additives, the PSO samples responded very well and increased their onset temperature (OT) by 70–80°C. Thermogravimetric analyses have been performed on the PSO samples and it was observed that they are thermally stable at temperatures up to 250°C. Cold flow property data shows that the PSO samples are good for use in formulating high temperature lubricants. Two model compounds which have terminal double bonds, triacyl-10-undecenoate and allyl 10-undecenoate, have been polymerized. The oxidation stability of these two compounds was investigated by the PDSC method and compared to that of the PSO samples. This study will help us understand the relationships between oxidation stability and molecular structure of PSO-based lubricants. In addition, oxidation stability data of polyethylene (PE) and polypropylene (PP) were used in helping to explain the oxidation stability results of the PSO samples.