Tribological properties of Mg/Al–CO3 layered double hydroxide as additive in base oil

Abstract

The tribological performance of Mg/Al–CO₃–layered double hydroxide (LDHs) nanoparticles in base oil was studied as a sole additive and in combination with zinc dialkyldithiophosphate (ZDDP) at 100°C. Results show that LDHs could improve antifriction and antiwear properties of base oil. The blend with LDHs alone shows better antifriction properties than that of the mixture of LDHs and ZDDP, but the addition of ZDDP helps to stabilise the friction coefficient. Surface analyses have been performed to study the morphology, nanoparticle distribution and chemical species in the tribofilm. The results show that tribofilms contain Mg, Al, C, O, Zn, P and S, and the structure of Mg/Al–CO₃–LDHs changes under shearing stress and this process can help reduce friction coefficient. Thus, LDHs could be chemically incorporated into tribofilm to reduce the polyphosphate chain length originated from ZDDP decomposition resulting in a medium chain polyphosphate, which provides very good wear protection.     

Comparing tribological behaviors of sulfur- and phosphorus-free organomolybdenum additive with ZDDP and MoDTC

A new kind of sulfur- and phosphorus-free organomolybdenum oil-based additive N, N-bis (2-hydroxyethyl)-dodecanamide molybdate (NNDM) was prepared. Its tribological performances as additive in base oil 150SN were examined on a four-ball tester, and compared with those of ZDDP and MoDTC under boundary lubrication condition. The tribofilm NNDM generated on the worn surface was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results showed that NNDM blend oil exhibited excellent load-carrying capacity, significantly reduced friction coefficient and wear rate of worn surface, which could be attributed to high amount of long-chain alkylamide and MoO_x in NNDM tribofilm.     

Action Mechanism of WS2 Nanoparticles with ZDDP Additive in Boundary Lubrication Regime

The effect in the tribological performance of WS₂ fullerene-like nanoparticles in PAO base oil when adding a ZDDP additive was studied at 100 °C in the boundary lubrication regime. The tribological properties of the dispersion surpass those obtained without one of the two additives. The friction modifier properties of the particles are improved in the presence of ZDDP, while the anti-wear properties of the ZDDP are increased when the particles are added to the dispersion. The composition of the formed tribofilm was investigated. Results show that a 50–60 nm tribofilm is formed on the steel surface composed by WS₂ mixed on the ZDDP chemical tribofilm. A WS₂-rich layer is observed at the top of the tribofilm. A correlation between the chemical composition of the tribofilm and the tribological properties of the “PAO + WS₂ + ZDDP” dispersion was made. Synergy between the two additives was proven.     

An investigation of the effects of some motor oil additives on the friction and wear behaviour of oil-soluble organomolybdenum compounds

The compatibility of oil-soluble organomolybdenum compounds with some motor oil additives (detergent, dispersant, rust inhibitor and ZDDP) has been investigated on a four-ball machine and a SRV tester. Preliminary results indicated that a combination of two agents might have either synergistic or antagonistic effects on the friction and wear performance of the organomolybdenum compounds, although many combinations were shown to be synergistic. The interactions between two agents were primarily determined by the additive types, additive concentrations and test temperatures. It was also found that the presence of calcium sulphonate detergent was particularly beneficial to molybdenum dithiophos-phate and molybdenum dithiocarbamate in terms of friction reduction and wear reduction at temperatures in a certain range. The induction period of the organomolybdenum compounds was also reduced. Surface analysis results suggested that the synergistic effects were closely related to the formation of thick films, which were rich in molybdenum and sulphur, on rubbing surfaces.     

Surface and Tribological Characteristics of Tribofilms Formed in the Boundary Lubrication Regime with Application to Internal Combustion Engines

One of the biggest challenges in engine tribology is to formulate appropriate lubricants, which will increase fuel efficiency by reducing friction, yet still provide good wear resistance. The lubricant should also be formulated to limit particulate and gaseous exhaust emissions to the levels allowed by current regulations. In real lubricant formulations there can be 10–15 additives and the interactions between additives must be taken into account. The effects of eliminating the friction modifier and friction modifier plus anti-wear additive zinc dialkyl dithiophosphate (ZDDP) from the additive package of fully formulated lubricants on friction, wear and wear film forming characteristics have been examined. Tests have been conducted under lubricated wear conditions at bulk oil temperatures of 20, 50, and 100 °C using a reciprocating pin-on-plate tribometer. Boundary lubrication conditions were varied according to the value of starting lambda ratio. The wear film has been examined by Energy Dispersive X-ray analysis (EDX) and X-ray Photoelectron Spectroscopy (XPS). In order to investigate the morphology of the reaction films formed by the additive packages of these lubricants, Atomic Force Microscopy (AFM) was used. In this paper it has been shown that tribofilms, derived from ZDDP/surface interactions, affect friction, the extent of which is determined by tribological conditions. Detergent interactions with ZDDP enhance the complexity of the tribofilm and enrich the level of C in the film whilst affecting the friction and wear response. Through integration of tribological measurements and surface analysis, progress towards improving the nature of interactions is made and forms the focus of the paper.     

Influence of Dispersant and ZDDP on Soot Wear

Diesel engines and gasoline direct injection (GDI) engines both produce soot due to incomplete combustion of the fuel and some enters the lubricant where it accumulates between drain intervals, promoting wear of rubbing engine components. Currently the most favoured mechanism for this wear is that the anti-wear additives present in engine oils, primarily zinc dialkyldithiophosphates (ZDDPs), react very rapidly with rubbing surfaces to form relatively soft reaction products. These are easily abraded by soot, resulting in a corrosive-abrasive wear mechanism. This study has explored the impact of engine oil dispersant additives on this type of wear using combinations of dispersant, ZDDP and carbon black, a soot surrogate. It has been found that both the concentration and type of dispersant are critical in influencing wear. With most dispersants studied, wear becomes very high over an intermediate dispersant concentration range of ca 0.1–0.4 wt% N, with both lower and higher dispersant levels showing much less wear. However a few dispersants appear able to suppress high wear by ZDDP and carbon black over the whole concentration range. A series of experiments have been carried out to determine the origin of this behaviour and it is believed that high levels of dispersant, and, for a few dispersants, all concentration levels, protect the iron sulphide tribofilm initially formed by ZDDP from abrasion by carbon black.     

Lubrication of CrN Coating With Ethyl-Dimethyl-2-Methoxyethylammonium Tris(pentafluoroethyl)Trifluorophosphate Ionic Liquid as Additive to PAO 6

This paper studies ethyl-dimethyl-2-methoxyethylammonium tris(pentafluoroethyl)trifluorophosphate ionic liquid [(NEMM)MOE][FAP] as 1 wt% additive to a polyalphaolefin (PAO 6) in the lubrication of CrN PVD coating. The tribological behavior of this mixture has also been compared with a traditional oil additive, such as zinc dialkyldithiophosphate (ZDDP). Friction and wear tests were performed by means of a ball-on-plate reciprocating tribometer, and XPS was used to analyze wear surfaces. The experimental results showed that both additives substantially improve the anti-friction and anti-wear performance of the base oil. However, the tribological behavior of the ionic liquid as oil additive does not reach that of ZDDP. The interactions of each additive with the surface and tribofilm formation contributed to improve the tribological behavior of the lubricants.     

Lubrication of carbon coatings with MoS2 single sheet formed by MoDTC and ZDDP lubricants

The fuel economy and reduction of harmful elements of lubricants are becoming important issues in the automotive industry. One approach to these requirements is the potential use of low‐friction coatings in engine components exposed to boundary lubrication conditions. Diamond‐like carbon (DLC) coatings, extensively studied as ultra‐low friction films to protect ductile metals surfaces for space applications, are expected to fit the bill. The main purpose of this work is to investigate the friction and wear properties of DLC coatings lubricated with molybdenum dithiocarbamate (MoDTC) and zinc dithiophosphate (ZDDP) under boundary lubrication conditions. The mechanisms by which MoDTC reduces the friction in the centirange were studied using ultra‐high vacuum (UHV) analytical tribometer. The UHV friction tests were performed on a tribofilm previously formed on selected DLC material with MoDTC and ZDDP containing oil. Ex‐situ characterizations show that the composition of this tribofilm is similar to that of a tribofilm obtained on steel surfaces in the same lubrication conditions with MoS₂ single sheets dispersed inside zinc phosphate zones. However, analyses by X‐ray photoelectron spectroscopy (XPS) indicate that MoDTC and ZDDP additives seem to be more active on steel surfaces than carbonaceous ones. After UHV friction with the tribofilm formed on selected DLC and steel pin counterpart, the wear scars of both sliding surfaces were characterized by in‐situ analytical tools such as Auger electron spectroscopy, scanning Auger microscopy and micro‐spot XPS. Low friction is associated with the transfer of a thin MoS₂ film to the steel pin counterpart. Copyright © 2006 John Wiley & Sons, Ltd.     

Tribological properties of tribofilms formed from ZDDP in DLC/DLC and DLC/steel contacts

Diamond-like carbon coatings (DLCs) are considered to hold great promise for improvement in friction and wear resistance of engine parts. It is hence interesting to know whether conventional engine oil additives such as ZDDP can form tribofilms and reduce friction and wear in DLC contacts as effectively as they do in steel on steel contacts. This paper compares the behaviour with ZDDP of six different DLC coatings. It is seen that ta-C gives lower boundary friction than the other types while a-C:H gives better wear prevention. A ZDDP-derived tribofilm forms on all DLCs but a pad-like structure is seen only on W-DLC in DLC/DLC tribopairs.     

Friction and wear reductions in fluorinated plain and fully formulated oils through enhanced heating time using moderate loading under boundary lubrication conditions

The use of fluorinated mix catalyst (titanium fluoride: TiF₃ + iron fluoride: FeF₃) with polytetrafluoroethylene: PTFE in engine oil is increasingly a strategy to improve fuel economy and component durability to simultaneously provide low friction and excellent wear protection. The influence of these additives shows promising results during load bearing capacity testing and long‐term durability experiments. This paper addresses the enhancement of friction and wear performances of 0.1% and 0.05% phosphorus plain zinc dialkyl dithiophosphate (ZDDP) oil and fully formulated engine oil when heated to 100°C for specific time under thermal and tribological conditions for moderate pressure loading (1.9 GPa Hertzian pressure or 180 N) which simulate the cold start of a car under 700 rpm rotational speed or the valve trains loading where most of the ZDDP are applied. Tests were performed in a ball on cylinder tribometer under boundary lubrication conditions. Surface analyses were carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results showed that the anti‐wear additives (2 wt. % PTFE + 0.5 wt. % FeF₃ + .5 wt. % TiF₃) provided excellent wear protection to 0.1% phosphorus plain and fully formulated oil when cylinder is immersed in the desired lubricant and heated for a predetermined time but performed poorly in 0.05% phosphorus fully formulated oil under the same conditions. The transform of anti‐wear coated material to the worn surfaces, which was a function of the anti‐wear performance of additives, was shown to have an influence on friction performance. The wear reducing effect, which was observed in fluorinated additives tribological tests, indicated a positive contribution and good tribofilm formations. Copyright © 2016 John Wiley & Sons, Ltd.     

Characterization of tribofilms derived from zinc dialkyl dithiophosphate and serpentine by X-ray absorption spectroscopy

The tribological performance of serpentine in combination with ZDDP as additive for base oil was investigated by a Plint high frequency friction tester at room temperature and 100 °C. The tribofilms formed by serpentine and ZDDP were analyzed using the scanning electron microscopy technique equipped with energy dispersive X-ray spectroscopy. X-ray absorption spectroscopy at phosphorus K- and L_3,2-edges, sulfur K- and L_3,2-edges, silicon K-edge, magnesium K-edge, oxygen K-edge, and zinc L_3,2-edge were recorded to determine the chemistry of the tribofilms. It is found that a combination of serpentine with ZDDP helps reduce the friction of oil blend and exhibits better antiwear properties than base oil.     

Tribochemical Transformation of Nano TiO2 to Ilmenite on the Surface of Wearing Steel Parts: Antiwear Action of Nano TiO2 as an Additive in Engine Oil

TiO₂ nanoparticles of average size about 20–30 nm were hydrothermally synthesized from TiCl₄ under mild acidic conditions. The nanoparticles were mixed with dispersant and base oil to give a partially transparent concentrate with 1.5 wt% of Ti content. The concentrate was dispersed in hexane and base oil to characterize, respectively, by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The concentrate was diluted with base oil to a parts per million level of Ti containing dispersion blends that were evaluated for wear and friction control performance. Nano TiO₂ containing fully formulated oil blend showed excellent load-bearing capability in Swingung, Reibung, Verschleiβ (SRV; oscillation, friction, wear) tests. Four-ball test results show that the wear scar diameter was considerably reduced to 0.30 mm for TiO₂-added blend compared to neat base oil (0.60 mm). The performance of TiO₂-added blend was comparable to secondary zinc dialkyl dithiophospate (ZDDP)-added blend under identical condition. Raman spectra of the worn surface on the tested ball revealed the presence of ilmenite (FeTiO₃) and no deposits of pure TiO₂.     

The feasibility of using electrostatic monitoring to identify diesel lubricant additives and soot contamination interactions by factorial analysis

Electrostatic monitoring is a condition monitoring technique, which has been used for monitoring lubricated sliding contacts. The electrostatic charge is dependent on material wear and charge species in the lubricant (additives and contaminants). This paper presents work carried out on a pin-on-disc (PoD) tribometer to investigate additive–additive and additive–carbon black interactions. Online electrostatic charge and coefficient of friction (CoF) measurements were recorded. Post-test electro-kinetic sonic amplitude (ESA) measurements were taken and pin and disc material loss was measured using 3D profilometry. Statistical examination of results was conducted using Analysis of Variance (ANOVA) to reveal interactions. The primary conclusions include: primary zinc dialkyldithiophosphate (ZDDP) was found to increase pin wear due to immature antiwear film formation. Interactions between carbon black and detergent, and carbon black and dispersant, were observed in electrostatic charge data and ESA measurements. A complex between ZDDP and dispersant was highlighted by measured electrostatic charge, ESA, and pin material loss.     

Antiwear Behavior of ZDDP and Fluorinated ZDDP in the Presence of Alkylated Diphenyl Amine Antioxidants

Environmental regulations have called for a reduction of phosphorus content in engine oils in recent years. The anti-wear additive zinc dialkyl dithiophosphate (ZDDP), which is also an antioxidant, is one of the most important components of engine oil additives. ZDDP is a major source of phosphorus. One way to reduce phosphorus levels is to replace ZDDP with new environmentally friendly antiwear agents that have similar or superior wear performance compared to ZDDP. Another way to address the environmental issue is to reduce the amount of ZDDP in engine oils. At the same time, it is necessary to increase the efficiency of ZDDP by finding optimum conditions that would result in improved antiwear performance. The antiwear mechanism of ZDDP involves its degradation thermally and tribologically, leading to the formation of an antiwear film that consists of polyphosphates and sulphides. The structure of the antiwear film is almost similar in both types of degradation. But the breakdown efficiency of ZDDP is diminished by the parallel reaction of ZDDP with other additives, as well as the antagonistic effects of these additives. The new fluorinated ZDDP complex developed has shown better wear performance compared to ZDDP. This would allow the possibility of further reduction of phosphorus in engine oils compared to current levels. In this paper we study the interaction of ZDDP and fluorinated ZDDP with alkylated diphenylamine. The impact of antioxidant on wear performance was examined using a ball-on-cylinder tribometer. The interactions between ZDDP and the fluorinated ZDDP with the antioxidant were studied using NMR and the surface of the tribofilm was examined using SEM, TEM, and Auger spectroscopy.     

Reduced Phosphorus Concentration Effects on Tribological Performance of Passenger Car Engine Oils

Phosphorus is present in engine oils in the form of the antiwear and antioxidation additive zinc dialkyldithiophosphate (ZDDP). Its effects on wear and friction were studied at different temperatures using a high-frequency reciprocating rig (HFRR). The electrically insulating tribofilm formation was measured using an electrical contact resistance (ECR) technique. The wear and friction performance of a fully formulated fresh oil containing 0.05 wt% phosphorus was compared with the corresponding used oil drained from a vehicle. The results show that the wear performance of fresh oils having phosphorus concentration from 0.02 to 0.1 wt% is very similar. Further reduction of phosphorus concentration below 0.02 wt% leads to high wear. The coefficient of friction increases with increased phosphorus concentration at temperatures above 80°C but decreases with increased phosphorus concentration at temperatures below 80°C. The used oil and the fresh 0 wt% P oil running on the original fresh steel surface exhibit higher wear than when both oils were evaluated on a previously formed film from a fresh oil containing 0.05 wt% phosphorus.     

Role of thermal,mechanical and oxidising treatment on structure and chemistry of carbon black and its impact on wear and friction: Part 2 – Boundary lubrication condition

Mineral oil formulations with zinc dialkyl dithiophosphate (ZDDP) and dispersant (poly isobutylene succinimide ashless dispersant or ‘PIBSA’) and fully formulated oils with and without carbon black were subjected to thermal and mechanical treatment and tribologically tested on TE 77 (high frequency reciprocating rig or ‘HFRR’) machine to examine the frictional performance during the test. These results were compared to oils without carbon black and oils with diesel soot. Results indicate that oils with just ZDDP and dispersant had the highest friction that remains constant for the duration of the test while oils with carbon black in the milled and oxidised condition had the lowest coefficient of friction and the smallest surface roughness in the tribofilm. The mechanism of wear with treated carbon black and diesel soot was found to be polishing wear as evidenced by the scanning probe microscopy images of the tribofilms. Tribofilms were analysed with X-ray absorption near edge structure (XANES) and it was seen that oils without carbon black or even with untreated carbon black had sulphates at the surface, while the oils with carbon black that were treated had a higher proportion of sulphides. A combination of both FeS and ZnS was found in the tribofilms along with short chain phosphates of Zn.     

Correlation Studies of WS 2 Fullerene-Like Nanoparticles Enhanced Tribofilms: A Scanning Electron Microscopy Analysis

The beneficiary effects of tungsten disulphide (WS₂) inorganic fullerene-like nanoparticles (IFLNPs) in the lubrication industry were shown in recent years. However, their successful incorporation into lubricants (oils, greases) is not straightforward. In practice, the lubricant contains several components for different purposes, e.g. reducing the oxidization of the oil (antioxidant), minimizing the wear rate (anti-wear additive), dispersants, etc. These additives can contain chemically active compounds, which under the lubrication process (where locally extreme conditions can develop: high pressure and flash temperatures) can change the chemistry in the contact zone and block the beneficial effects of the inorganic nanoparticles. In this investigation, poly-alpha-olefin (PAO) is being used as base oil in which the WS₂ nanoparticles and different additives are mixed. A ball-on-disc sliding test revealed that certain additives inhibit the nanoparticles to reduce friction (less than 5 % decrease in friction coefficient), while in other cases, the friction reduction was above 50 %. The comparison is being made between PAO + additive and PAO + additive + IFLNPs. Scanning electron microscope and energy dispersive X-ray spectroscopy were used to investigate the elemental composition of the tribofilms formed on the wear marks. Further analysis was made in order to reveal correlations between elemental compositions of the tribofilms and external parameters such as the friction coefficient and wear rate. For instance, a strong correlation between tungsten content of the tribofilm and the friction coefficient was found.     

Interaction of ZDDP with Borated Dispersant Using XANES and XPS

The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms.

The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and the formation of phosphate in tribofilms and thermal films. B K-edge XANES shows that boron has a trigonal coordination in the untreated additive, but the coordination changes partially to a tetrahedral coordination in the tribofilm upon rubbing. No BN was detected in the film analyzed by B K-edge or N K-edge. Boron 1s XPS also did not show the presence of BN in the film.     

Friction Reduction and Antiwear Capacity of Engine Oil Blends Containing Zinc Dialkyl Dithiophosphate and Molybdenum-Complex Additives

The efficacy of oil blends containing zinc dialkyl dithiophosphate (ZnDTP) and molybdenum (Mo)-complex additives to improve the tribological properties of boundary-lubricated steel surfaces was investigated experimentally. The performance of oil blends containing three different types of Mo-complex additives of varying Mo and S contents with or without primary/secondary ZnDTP additions were investigated at 100°C. The formation of antiwear tribofilms was detected in situ by observing the friction force and contact voltage responses. Wear volume and surface topography measurements obtained from surface profilometry and scanning electron microscopy studies were used to quantify the antiwear capacity of the formed tribofilms. The tribological properties are interpreted in terms of the tribofilm chemical composition studied by X-ray photoelectron spectroscopy. The results demonstrate that blending the base oil only with the Mo-compound additives did not improve the friction characteristics. However, an optimum mixture of Mo complexes and ZnDTP additive provided sufficient amounts of S and Mo for the formation of antiwear tribofilms containing low-shear strength MoS ₂ that reduces sliding friction. In addition, the formation of a glassy phosphate phase due to the synergistic effect of the ZnDTP additive enhances the wear resistance of the tribofilm. This study shows that ZnDTP- and Mo-containing additives incorporated in oil blends at optimum proportions improve significantly the tribological properties of boundary-lubricated steel surfaces sliding at elevated temperatures.     

Studies On Competitive Interactions and Blending Order of Engine Oil Additives by Variable Temperature 31P-NMR and IR Spectroscopy

Interactions between various engine oil additives including dispersant, detergent, antiwear/antioxidant, and viscosity index improvers have been investigated through changes in the ³¹P-NMR and IR spectra. Significant changes in the ³¹P-NMR and IR spectra of zinc dialkyldithiophosphate (ZDDP) have been observed by varying the blending order of additives. Dispersant-detergent (polyisobutylene succinimide-calcium overbased sulfonate) and viscosity index improver-detergent (dispersant olefin copolymer-calcium overbased sulphonate) interactions were found to be strong and irreversible in nature. Variable temperature ³¹P-NMR spectra have provided enough evidence to suggest that the actual performance of an additive may not be decided by its bulk interactions with other additives at room temperature, and that tribochemical reactions occurring at higher temperature play a greater role.