Extreme Pressure Additives Research Articles

Wear damage and effects of graphene-based lubricants/coatings during linear reciprocating sliding wear at high contact pressure

The present study reports on the ‘wear damages’ caused at inner surfaces (and sub-surfaces) of extruded zircaloy-4 tube specimens (as half cylinders) during interactive linear reciprocating sliding against outer surfaces of D2 steel half cylinders for different durations (up to 120s) at high contact pressures (of ~ 1GPa) in the absence/presence of mineral oil lubricant (having chlorinated extreme pressure additives) and graphene-based lubricants/coatings [viz., CVD-grown films of well-ordered few layers graphene (~ 7 layers) and thicker graphite (~ 1400 layers)]. The tests were conducted using a custom-made friction-and-wear tester, which also replicates the conditions (in terms of contact geometry, pressure, stroke length and frequency) relevant to the tribological phenomena occurring at the inner walls of tubes subjected to cold pilgering process. Even though none of the lubricants/coatings had any effect on coefficient of friction, presumably because of ‘boundary lubrication’ at such high contact pressures, only the graphene-based lubricants/coatings slowed down the evolution of surface roughness and wear damage. Observations of worn (top)surfaces and cross-sections indicate that ‘wear damages’ at such contact pressures are primarily caused by severe sub-surface deformation and defect/crack initiation/propagation, followed by lateral chipping/delamination; which could be suppressed by the graphene-based lubricants/coatings (but not by the oil-based lubricant).

Challenges in Formulating Vegetable Based Metalworking Lubricants: A Review

The hazardous mineral based lubricating oils are being banned worldwide due to their potential negative environmental impact. The mineral based oil does not meet the requirements of metalworking process without the support of anti-wear, extreme-pressure or extreme temperature additives. Because of additives, they become expensive to cater the needs of special requirements in industries. Besides, being environment friendly, vegetable based lubricants have exclusive properties such as high lubricity, polar nature, high viscosity index and flash point and low evaporation rate. Still they have not yet fully replaced mineral based lubricants due to issues related to their oxidation stability and inappropriate chemical structure. The challenges are to improve certain characteristics of vegetable based lubricants along-with excellent tribological properties. Advanced additive technology and chemical modifications of vegetable oils make them fit for the metalworking applications. In this review article, based on the available literature and recently published data, developments related to vegetable based lubricants and its suitability for metalworking operations are reviewed. Also, challenges in formulating vegetable based oils are put-forth.

Oil-soluble ionic liquids as antiwear and extreme pressure additives in poly-\u03b1-olefin for steel/steel contacts

To enhance the lubricating and extreme pressure (EP) performance of base oils, two types of oil-soluble ionic liquids (ILs) with similar anion albeit dissimilar cations were synthesized. The physical properties of the prepared ILs were measured. The anticorrosion properties of ILs were assessed by conducting corrosion tests on steel discs and copper strips, which revealed the remarkable anticorrosion properties of the ILs in comparison with those of the commercial additive zinc dialkyldithiophosphate (ZDDP). The tribological properties of the two ILs as additives for poly-α-olefin-10 (PAO10) with various mass concentrations were investigated. The tribological test results indicate that these ILs as additives are capable of reducing friction and wear of sliding contacts remarkably as well as enhance the EP performance of blank PAO10. Under similar test conditions, these IL additives exhibit higher lubricating and anti-wear (AW) performances than those of ZDDP based additive package in PAO10. Subsequently, X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS) were conducted to study the lubricating mechanism of the two ILs. The results indicate that the formation of tribochemical film plays the most crucial role in enhancing the lubricating and AW behavior of the mixture lubricants.

Direct Formation of Lubricious and Wear-Protective Carbon Films from Phosphorus- and Sulfur-Free Oil-Soluble Additives

Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained.

Temperature dependence of the extreme-pressure behavior of CuO and TiO2 nanoparticle additives in metal-forming polymeric lubricants

PurposeThe purpose of this work is to investigate the effect of temperature on the extreme-pressure (EP) properties of CuO and TiO2 nanoparticle-filled polymeric lubricants for metal-forming processes.Design/methodology/approachThis paper studies the effect of nanoparticle additives of CuO and TiO2 on the load-carrying capacity of a metal-forming polymer lubricant used for deep-drawing at varying temperatures. EP measurements are performed with a four-ball tribotester according to the ITeE-PIB Polish method for testing lubricants under scuffing conditions. Tests are run at 25, 40, 60 and 75°C to further decrease the lubricant film thickness and determine the effect on the load-carrying capacity and the tribological mechanisms of nanoparticles. The tribological mechanisms of nanoparticles is studied using energy dispersive spectrometry (EDS).FindingsResults indicate that nanoparticle additives increase the load-carrying capacity of the polymeric lubricant at all concentrations up to 60°C attributed to a mending effect and a reduction in the area of contact of moving surfaces; at 75°C, the improvement is lowered due to nanoparticle re-agglomeration. The best results are found with TiO2 nanoparticles due to their smaller size compared to CuO.Practical implicationsNanoparticles of CuO and TiO2 are potential EP additives for metal-forming lubricants, providing protection to working components and extending tool life.Originality/valueThese results show the effectiveness and the tribological mechanisms of nanoparticle additives under EP conditions and increasing temperatures found in metal-forming processes.

Insights into the sliding wear behavior of a copper-steel pair with oils containing extreme-pressure additives

Under four typical lubricating conditions, viz.,(i) no lubrication, and lubrication with a (ii) base oil, (iii) base oil with a single extreme-pressure (EP) additive, and (iv) base oil with composite EP additives, a custom-made tribo-tester with steel on a copper ring was used to understand the friction and wear behavior of the copper-steel pair. Surface morphologies of friction-pairs were observed by scanning electron microscopy (SEM). The composition of the surface residues was analyzed using an energy dispersive spectrometer (EDS) and an X-Ray photoelectron spectrometer (XPS). The results showed three forms of wear on the copper surface in the wear process when the copper-steel pair was lubricated with base oil containing composite EP additives. During the first 500−2500s, adhesive wear appeared and then transformed into a plowing wear. After 2500s, oxidative wear in certain regions was the major force, with a concomitant drop in the relative wear rate to a value as low as 22.5%. Compared to lubrication with base oil, that with base oil containing composite EP additives significantly reduced the friction coefficient between the copper/steel pairs, with the plowings being shallow, and the morphology of the copper surface much improved. Two kinds of adsorption layers were formed on the steel surface and they played an important role in anti-wear and friction-reducing during the entire process. One is a chemical reaction layer consisting of Cu2O, CuFeS2, and FePO4 generated from the reaction of the active elements P or S with copper/iron during sliding wear, while the other is a physical adsorption layer containing decomposed borate oxides, Fe and Cu on the steel surface.

Influence of lubricant formulation on rolling contact fatigue of gears – interaction of lubricant additives with fatigue cracks

The influence of lubricant additives on rolling contact fatigue crack propagation and the mechanisms responsible for the resulting micro-scale damage, was studied via experiments conducted on complete transmissions in a test cell. Bench-scale tribological tests and the exposure of steel surfaces to two different formulated lubricants were also carried out. Scanning and Transmission Electronic Microscopy observations, Electron Dispersive Energy and X-ray Photoelectron Spectroscopy analyses indicated that the sulphur present in the extreme pressure (EP) additives has a positive impact on limiting damage propagation. Thanks to TEM observations of cracks, it was demonstrated that a sulphur rich film is formed at the crack tip. This film can act as both a barrier film towards hydrogen permeation within the metal and / or as an inhibitor of oil decomposition. The latter is associated with the nascent surface's ability to limit hydrogen generation. Without such hydrogen embrittlement, crack propagation is slowed down.

Tribological performance of halloysite clay nanotubes as green lubricant additives

Nanoparticles have been recently explored as lubricant additives for improving tribological performance of metal-forming tools. Halloysite clay nanotubes (HNTs) are naturally-occurring, low-cost, and non-toxic nanoparticles, making them attractive as green lubricant additives. In this study, HNTs were dispersed at varying concentrations (0.01, 0.05, and 0.10wt%) within a polymeric lubricant for metal-forming applications. The ITeE-PIB Polish method for testing lubricants under scuffing conditions was conducted with a four-ball tribotester in order to obtain the scuffing load and load-carrying capacity (poz) under extreme pressures (EP), as this method has demonstrated to be sensitive to extreme-pressure additives; a block-on-ring test was used to obtain wear volume loss and coefficient of friction (COF). Surface roughness of worn materials was characterized with an optical 3D-surface measurement system. Results showed that 0.05wt% HNTs delayed scuffing initiation, and increased the scuffing load and load-carrying capacity by 50% and ~72%, respectively. Wear volume loss and COF were both lowered by ~70%, and surface roughness was reduced due to the formation of a tribofilm. The improved lubrication provided by environmentally-friendly HNT lubricant additives shown in this study may result in considerable cost savings through lower energy consumption and longer tool life.

Utilization of sulphurized palm oil as cutting fluid base oil for broaching process

Broaching is one of the most severe metal cutting operation that requires the use of cutting fluids formulated with extreme pressure (EP) additives to minimize metal-to-metal contact and improve tool life. Enhancement of EP performances of the cutting fluids can be achieved by addition of sulphur containing compounds that will allow the formation of metal sulfide film that has low shear strength and good antiweld properties and acts as protection layer from wear and seizure. Most of the cutting fluids are mineral oil based. However, as regards to health and environmental issues, reseach on vegetable oil based cutting fluid have been increased recently. This paper reports a study on the sulphurization of palm oil derivatives and its usage as broaching oil. Sulphurization of the palm oil derivative was conducted via non-catalytic sulphurization using elemental sulphur at various composition and under heating of 150-160°C for 3 hr. Broaching oil was made by blending the sulphurized palm oil and additive packages. The performance parameters of the broaching oil that has been observed including load carrying capacity, wear scar diameter, corrosion protection, oxidative stability, and surface finish of workpiece. From this research, it was found that sulphurized FAME based broaching oil has excellent EP properties. The optimum formulation was obtained on composition of sulphurized FAME-mineral oil with 6% wt of sulphur. The result from the test showed that kinematic viscosity of sulphurized palm oil was about 25.3 cSt (at 40 °C), load carrying capacity was 400 kgf, and wear scar diameter was 0.407 mm. In addition, it can be concluded that the class of corrosion protection of modified palm oil was 1.b (slight tarnish category), oxidative stability at 160 °C was obtained for 0.11 hr, and the surface roughness of workpiece was about 0.0418-0.0579 μm. These performances are comparable to commercial broaching oil. By this result, it indicates that sulphurized palm oil is applicable for industrial cutting fluids formulation.

Interactions between MoS2 nanotubes and conventional additives in model oils

The use of transition metal dichalcogenides nanoparticles is an emerging concept in lubrication for enhancing the tribological properties of lubricants. However, in fully-formulated products other properties are essential for providing a comprehensive protection against degradation. Therefore, the coexistence of nanoparticles with conventional additives is unavoidable. The main objective of this work is to investigate the tribological performance of MoS2 nanotubes accompanied by anti-wear, extreme pressure, detergents and dispersants. The results shown synergetic interactions between MoS2 nanotubes with anti-wear and detergents additives, slight synergy with extreme-pressure additives and antagonistic interactions with dispersants. Under extreme pressure conditions all selected additives provide synergistic effects with MoS2 nanotubes. The interaction mechanisms are investigated using several techniques and discussed in terms of tribofilm chemistry.

Tribological behavior of a new green industrial lubricant for stamping operations - Application to Stainless Steels

Thanks to a fruitful collaboration between different academic and industrial research entities, a new green lubricant solution AFULudine has been developed for stamping operations in substitution to the usual mineral oils, with the idea and the wish to combine different essential properties such as a low environmental impact and ease-of-use according to the market needs and the more and more restrictive environmental legislation (e.g. REACH). Based on the Self-Assembled-Monolayer (SAM) principle and so, on a perfect control of the chemical reaction between the solution and the substrate (grafting of molecules onto the surface), AFULudine offers an efficient technical answer for improving stamping processes. The present study, focused on stainless steel stampings (comparison between a 441-1.4509 grade and a 304-1.4301 grade), investigates the performances of this new green industrial lubricant at different levels: from laboratory tests to industrial conditions through the production of hundreds of parts. Additional results coming from tests made on a rotational pin- on-disk tribometer will allow us to appreciate and retrieve more local information about the tribofilm creation during sliding. Moreover, the comparison with different mineral oils currently used at industrial scale, will strengthen the AFULudine performances. Indeed, this new solution usually outperforms a majority of such oils whatever their viscosity and their own composition: formulation, content of extreme pressure additives (Cl, S, P)...

Tribological behavior of aeronautical steel under oil–air lubrication containing extreme-pressure and anti-wear additives

If the lubrication system of a helicopter reducer is compromised, its gears and bearings will be in a working state without lubricating oil, which causes the reducer to be damaged in a very short time. Various 2% additives of T307, T321, T202, and T391 were injected and mixed with DOD-L-85734 aeronautical oil to produce 45-min oil–air lubrication experiments performed upon 12Cr2Ni4A aeronautical steel tribo-pairs. The results show that the best anti-wear effect is produced by oil–air lubrication containing T391: its wear width under jetting oil–air just three times and quantity of oil used only 0.015 mL in 45 min was only 421.32 µm but that of dry friction for 48 s was 629.20 µm. The technology of oil–air lubrication that contains an extreme-pressure and anti-wear additive is thus a feasible way to improve the operational ability of a helicopter transmission system that is out of oil.

Performance of CrN coatings under boundary lubrication

In the case of metallic surfaces, the action of extreme-pressure (EP) and anti-wear (AW) additives is well understood and has been described in detail. However, this is not the case for coated surfaces. The aim of the present investigation was to determine the influence of additive type and concentration on the tribological behaviour of boundary-lubricated CrN coatings operating under different contact conditions. The results show that like for steel and diamond-like-carbon coatings, also in the case of CrN-coated contacts, the additive type, concentration and contact conditions have an influence on the tribological behaviour under boundary lubrication. The anti-wear additive has the smallest influence and results in complete protection of the CrN coating with respect to wear. On the other hand, the friction modifier gives the lowest friction, but high wear, while extreme-pressure additive concentrations above 1% are found to be too aggressive, even for the CrN/steel contact, and might lead to the formation of Cr–S tribofilms.

Improving the reliability of radial face seals of aircraft engine rotor supports

The article presents an advanced radial face contact seal with increased operational life. High efficiency of the radial mechanical seal is ensured by the joint application of the hydrostatic and hydrodynamic lubrication principles. The hydrodynamic effect is achieved by applying the structure of fine grooves on the rotor hub. The dependence of the flow force value in the slot for a fixed-size gap on the rotor speed is presented. A prototype has been made and tested on a dynamic test bed intended for the realization of operating conditions of support seals making up a part of an aircraft engine. The results of theoretical and experimental studies are presented. Ways of improving the seal reliability due to improving the surface mechanical properties are proposed. A pattern of the development of a functional failure of the seal assembly is presented. The most adverse operating conditions and the main reasons of increased wear of the sealing surfaces are indicated.  Methods of achieving high anti-friction characteristics of contact surfaces by applying nanostructured nonporous chrome-diamond coatings are proposed. Issues concerning the creation of advanced coatings with a positive gradient of depth mechanical properties are discussed. Anti-friction, anti-seize silver-diamond coatings and the use of diffusion molecular reinforcement technology are at the basis of the production of advanced coatings. The paper indicates the importance of anti-friction and extreme-pressure additives contained in the oil.

The Effect of the Chemical Structures of Synthetic Hydrocarbon Oils on Their Tribochemical Decomposition

Under boundary lubrication conditions, the formation of a highly active nascent surface can catalyze the decomposition of lubricant oils. Extreme pressure additives can reduce this decomposition but have undesirable environmental impacts, and so it is necessary to develop eco-friendly lubricants. The following study investigated the effects of the chemical structures of hydrocarbon oils on their tribochemical decomposition by comparing an alkyldiphenylether (ADE) to a multialkylated cyclopentane. The analysis of gaseous decomposition products via mass spectrometry during the application of friction showed that the multialkylated cyclopentane decomposed after an induction period of 1.95 km while the ADE did not degrade, even at a distance of 57.5 km, under the same mechanical contact conditions, and exhibited superior anti-wear properties. However, a high concentration of the ADE was necessary to extend the induction period for lubricant decomposition when it was used as an additive. Although the ADE showed high stability in response to friction, it was prone to decomposition once a nascent steel surface was formed. These decomposition characteristics of the ADE are closely related to its chemical structure, because the ether group in the ADE molecule preferentially adsorbs on metal oxide surfaces, forming a dense and robust film. In contrast, the weak bonding between adjacent alkyl groups reduces the shear strength at the interface. Both the benzene and alkyl groups of ADE molecules were found to detach, followed by the further rupture of C–C, C–H, and C–O bonds, catalyzed by active sites on the nascent surface such as surface defects.

QSPR Models for the Prediction of Friction Coefficient and Maximum Non-Seizure Load of Lubricants

A quantitative structure–property relationship (QSPR) model was used to study the friction coefficient and maximum non-seizure load of fatty acids, alcohols, and esters as extreme pressure and antiwear additives in lubricants on the surface of steel using several physicochemical descriptors. The tribological properties of these compounds on the steel surface were studied using a four-ball tribo-tester. Molecular refractivity and several structural descriptors were adopted in the development of the QSPR using a genetic function approximation (GFA) statistical analysis method. The results show that quantum descriptors are a better choice for predicting the friction coefficient and maximum non-seizure load of lubricants. Hydrogen bond donor, dipole, polarizability, molecular refractivity, and surface area are the most sensitive among the major contributing descriptors.

Effect of Extreme Pressure Additives on the Deformation Behavior of Oxide Scale during the Hot Rolling of Ferritic Stainless Steel Strips

High-speed steel (HSS) materials are universally used as work rolls for the hot rolling of stainless steels. Their use has increased the output of the rolling mill and decreased roll material consumption and grinding. Sticking defects often occur, however, during the hot rolling process. In this article, extreme pressure (EP) additives were dropped on the HSS samples at high temperature. Zinc dialkyl dithiophosphate (ZDDP) was chosen as the most effective EP additive by scratch tests on the HSS samples. In order to determine the optimum proportion of ZDDP in the lubricant, two reduction rates were tested on a Hille 100 experimental rolling mill by hot rolling ferritic stainless steel 445J1M at five different concentrations of ZDDP. The mechanism of EP additive action during the hot rolling process was also investigated. By analyzing the deformation behavior of the oxide scale of samples after hot rolling using different proportions of ZDDP, it was found that 20% ZDDP in the lubricant is the preferred concentration for industrial application.

Tribology and oxidation studies of fatty acid sulfide derivatives synthesized via thiol‐ene “Click” additions

The synthesis of sulfide derivatives via thiol‐ene addition reactions of methyl oleate and methyl 10‐ undecenoate as substrates is reported. The substrate fatty acids contained a terminal double bond in methyl 10‐undecenoate and an internal double bond in methyl oleate. Thiol‐ene addition of unsaturated fatty acid ester derivatives with hydroxy and amine functionality on the fatty acids was carried out under thermal conditions. The synthesized products were characterized by 1H, 13C NMR, FT‐IR, and GC‐MS analysis and evaluated for their antioxidant, extreme pressure (EP), and antiwear (AW) properties in base oil namely, epoxy jatropha fatty acid n‐butyl esters (EJB). Synthesized products exhibited comparable antioxidant efficacy to butylated hydroxyl toluene (BHT). All four synthesized sulfide derivatives significantly enhanced the weld point of the base oil EJB and exhibited comparable performance to TCP (tricresyl phosphate) at 1 wt% level. Methyl 9(10)‐(2‐hydroxyethylthio) octadecanoate at 0.5% concentration exhibited considerable improvement in the wear scar of base oil from 0.834 to 0.422 mm. The synthesized sulfide derivatives have potential as multifunctional additives for biolubricant basestocks.Practical applications: Synthesis of multifunctional additives via thiol‐ene addition under thermal conditions. The prepared sulfide derivatives were found to exhibit good antioxidant, antiwear, and extreme pressure additives. The synthesized additives exhibited comparable performance to commercial additives. The products are potential biobased alternatives for commercial additives.Sulfide derivatives as multifunctional additives.

Roughness and wettability of surfaces in boundary lubricated scuffing wear

The diversity of multidisciplinary approaches suggests that fundamentals of scuffing require systemic, complex multi-scale and multi-physics analysis of an irreversible process as it is postulated in present study. That is probably one of the reasons of lack of an unequivocal model of this irreversible transitional process from stable more or less lubricated wear to scuffing described only by one or few authors in equation(s) form. Therefore, it is useful to characterize the tribological surface properties in frame of systemic approach looking simultaneously for the optimal compromise between rheological, morphological and physicochemical features of contacting surface׳s layer. Hypothetical role connected to any group of features in the topological approach is elucidated and experimentally confirmed via the wettability, strongly combined with surface roughness and surface free energy. Due to the fact that the free energy is directly related to the surface wettability it can as well affect the scuffing activation process. For scientific and rhetoric reasons some selected results of limited boundary lubrication investigations under double blind trial conditions in case of gear oil with anti-wear (AW) and extreme pressure (EP) additives are elucidated here. The results issued from scuffing tests on AISI 4140 ground steel burnished under different forces in order to generate different surface roughness, residual stresses and surface energy are analyzed. It was stated and numerically correlated that the wettability by lubricating medium influences the scuffing resistance. Additionally, the dependence of wettability on selected parameters of roughness and a time to scuffing activation have been stated. On that basis, it is proposed to reinforce concept of “oleophilic” and “oleophobic” properties of metallic surfaces as autonomous invariants determining the activation of catastrophic wear process under boundary lubricated conditions.

New hydraulic biolubricants based on passion fruit and moringa oils and their epoxy

This study aimed to develop new hydraulic biolubricants based on vegetable oils epoxidized via performic acid, and to investigate their tribological behavior under conditions of boundary lubrication. The tribological performance of the developed lubricants was analyzed in an HFRR (high frequency reciprocating rig) apparatus. The coefficient of friction, obtained during the contact and the formation of the lubricating film, was measured. The characterizations were performed according to DIN 51524 norm (Part 2 HLP) and DIN 51517 norm (Part 3 CLP) standards. The wear was evaluated by optical and scanning electron microscopy (SEM). The results showed that adding extreme pressure (EP) and antiwear (AW) additives in fluids improves significantly the tribological properties. It was observed that the addition of EP and AW additives to in natura vegetable oils of passion fruit and moringa did not reduce the wear considerably. The biolubricants developed from passion fruit and moringa oils, modified via epoxidation, displayed satisfactory tribological properties being considered as potential lubricants, able to replace commercial mineral-based fluids.