Molybdenum Dialkyldithiocarbamate Research Articles

Influence of additives on the friction and wear reduction of oil-based drilling fluid

In comparison to conventional vertical hole drilling, extended reach drilling wells with long horizontal section require higher driving torque due to the increased drag on the drill string, drill column, casing and other equipment. This has led to higher friction and wear on the drilling equipment. Friction and wear preventive properties of oil-based drilling fluid is therefore important to keep oil extraction economical and sustainable. The addition of small concentrations of hydrocarbon additives to drilling fluid has shown significant positive effects on these properties. In this study, improvements in friction and wear properties of drilling fluid with the use of friction reduction additives was analyzed using a four-ball experimental configuration. Experiments resulted in wear in boundary and mixed lubrication regimes due to significant metal-metal interaction. The results showed that liquid friction reduction additives with long hydrocarbon tails and high metal affinity polar heads achieved better performance than additives that contained molybdenum dialkyldithiocarbamate. Energy dispersive spectroscopy results indicated lower levels of salts and corrosive chemicals on the surface of the wear scars for the best preforming additives.

Understanding the Friction Reduction Mechanism Based on Molybdenum Disulfide Tribofilm Formation and Removal

Among friction modifier lubricant additives, molybdenum dialkyldithiocarbamate (MoDTC) provides excellent friction behavior in boundary lubricated tribocontacts. It is well established that the low friction obtained with MoDTC is as a result of the formation of lattice structure MoS2 nanosheets. However, the relationship between the molybdenum species quantity, its distribution on the contact surface, and the friction behavior is not yet fully understood. In this work, Raman microscopy and atomic force microscopy (AFM) have been used with the aim of understanding the link between the friction behavior and the MoDTC/ZDDP tribofilm formation and removal. Tribotests were coupled with a collection of ex-situ Raman intensity maps to analyze the MoS2 tribofilm buildup. Post-test AFM analyses were implemented on the ball wear scar to acquire the average MoDTC/ZDDP tribofilm thickness. In-situ Raman spectra analyses were carried out to detect the MoS2 tribofilm removal. A good correlation was achieved between the friction coefficient measurements and Raman maps when using a linear relationship between the microscopic friction and the local amount of MoS2 tribofilm. After a rapid increase, the average MoDTC/ZDDP tribofilm thickness levels out to a steady state as the friction drop ceases. The removal rate of MoS2 from tribofilms, obtained at different temperatures, suggests that the MoS2 tribofilms are much easier to remove from tribocontacts compared to antiwear ZDDP tribofilms. This is the first study that sets out a framework to link MoS2 amount and coverage to the friction behavior, providing the basis for developing numerical models capable of predicting friction by taking into account tribochemistry processes.

The effect of clearance between tappet insert and camlobe on the tribological and tribochemical performance of cam/follower surfaces

This paper examines the effect of tappet insert clearance on the tribological and tribochemical performance of the camlobe/follower tribopair when lubricated in a fully-formulated oil containing 1 wt% of Molybdenum Dialkyl Dithiocarbamate (MoDTC). Tests were performed on a Single Cam Rig (SCR), taken from 1.25 l FORD Zetec (SE) engine. White Light Interferometry and Talysurf contact profilometry were used to characterise the wear scar on the tappet inserts and camlobes respectively. In addition, Scanning Electron Microscopy (SEM) was used on both (i.e. camlobes and tappet inserts) for wear mechanisms assessment as well as to access the durability of coatings used on tappet inserts. Energy-Dispersive X-ray (EDX) and Raman spectroscopy analyses were also used to understand the tribochemical interactions between oil additives and the cam/follower interface. Results show that the chemistry of the tribofilm derived on camlobes and tappet inserts vary as a function of tappet insert clearance and cam profile. Also, regardless of the type of coating, the smaller clearance of tappet inserts exhibited higher friction and wear. Therefore, based on this work, the use of the thicker tappet insert would be inadvisable as this possibly can cause higher fuel consumption and inefficient performance of the intake/exhaust valves of the engine.

Friction wear characteristics of diamond-like carbon coatings in oils containing molybdenum dialkyldithiocarbamate additive

Diamond-like carbon (DLC) coatings are typically used to reduce the friction of sliding parts in automobiles. However, the utilization of DLC coatings in oils containing molybdenum dithiocarbamate (MoDTC) additive causes severe wear of these parts. In this work, we modified the compositions of DLC coatings by varying the cathodic arc deposition process conditions and investigated the effects of the modified compositions on the coating wear properties. The results showed that the wear loss of the DLC coatings increased with the increasing hydrogen concentration or ratio between the sp2- and sp3-hybridized carbon atoms. In addition, the molybdenum oxide species produced during the MoDTC decomposition interacted with the double bonds of the DLC coatings, leading to their cleavage.

A Study on the Tribofilm Growth and Tribological Properties of Tribofilms Formed from Zinc Dialkyl Dithiophosphate (ZDDP) and Molybdenum Dialkyl Dithiocarbamate (MoDTC)

A Study on the Tribofilm Growth and Tribological Properties of Tribofilms Formed from Zinc Dialkyl Dithiophosphate (ZDDP) and Molybdenum Dialkyl Dithiocarbamate (MoDTC)

Investigation of pure sliding and sliding/rolling contacts in a DLC/Cast iron system when lubricated in oils containing MoDTC-Type friction modifier

Abstract Diamond-like carbon (DLC)/cast iron (CI) systems have been widely investigated due to their important application in engine components such as cylinders, pistons and more specifically for the cam/follower interface. The pure sliding contact of the DLC/CI system has traditionally been the focus of research; consequently less is understood about sliding/rolling contact systems. In addition, the tribological and tribochemical characteristics of the Molybdenum Dialkyl Dithiocarbamate (MoDTC) as a lubricant additive in such sliding/rolling contacts are not fully understood. In this study, a Mini Traction Machine (MTM) was used to run the experiments using alloy steel balls coated with 15 atomic percent (at. %) hydrogenated DLC (a-C: 15H) rubbing against uncoated cast iron discs. Results showed that the sliding/rolling ratio affects friction, wear and tribochemistry in CI/DLC systems; pure sliding enhances MoDTC activation. MoDTC decomposes to form MoS2, FeMoO4 and not MoO3. In addition, it was observed that MoS2/FeMoO4 ratio depends on test conditions and affects to the friction performance.

Measuring tappet rotation in a valvetrain rig when lubricated in a fully formulated oil containing MoDTC-type friction modifier

Abstract In a direct acting valve train configuration, tappet rotation plays a key role in improving lubrication, reducing wear and friction. However, to the best of the authors' knowledge, no studies were found to investigate the rotation of tappet under the effect of different coatings, thicknesses of tappets and formulations with Molybdenum Dialkyl Dithiocarbamate (MoDTC) which has been recently reported to be detrimental to Diamond-Like Carbon (DLC) wear. In this work, a new technique of measuring tappet rotation has been developed. A giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet was used. The sensor was installed very close to the tappet/bucket while the magnet was mounted into the underside of the tappet. Experiments were performed using standard production steel tappets coated with Mn-phosphate (MnPO4) and diamond-like carbon (DLC) coatings. In general, results showed that the tappet rotation is strongly dependant on oil formulation, clearance, speed/temperature, and surface roughness of the coating. MoDTC promoted the rotation of the tappet under both coatings. In addition, DLC inserts showed an increase in tappet rotation as compared to MnPO4 inserts. Nevertheless, regardless of the type of coating, the thickest tappets showed the highest rotation.

Preparation and Structure of W/Mo Films and Their Tribological Properties in the Lubrication of MoDTC

W, Mo, and W–Mo alloy films are prepared on 316L stainless steel by means of ion beam assisted deposition (IBAD). Tribological behaviors of the treated surfaces are investigated under lubrication with molybdenum dialkyldithiocarbamate (MoDTC) on a MS-T3000 friction and wear tester. The micromorphologies and chemical characteristics of the tribofilms generated on the worn surfaces are analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), respectively. The results showed that the obvious synergistic effects of better friction reduction and wear resistance between W–Mo film and MoDTC are attributed to the hard surface and the high decomposition rate of additives on the worn W–Mo surface.

Influence of NOx and Air on the Ageing Behaviour of MoDTC

Molybdenum dialkyldithiocarbamates (MoDTCs) are very effective friction modifier additives for use in engine oils and other lubricants. However, as engine oils age during extended drain intervals, MoDTCs can lose some or all of their ability to reduce friction and this is generally believed to result from their oxidative degradation. In this study, MoDTC solutions in base oil have been subjected to oil ageing in a controlled NOx/air flow rate, controlled temperature test apparatus and the effect of ageing on the ability of the MoDTC to reduce friction has been explored. As shown in previous studies, the additive’s friction-reducing properties are completely lost after a quite short period of ageing at 160 °C. However, it was found that at this temperature the presence of NOx has little if any influence on the rate of friction loss, indicating that the latter is controlled primarily by the rate of reaction of oxygen with the base oil and thus the rate of consumption of MoDTC as a peroxide decomposer. By contrast, when ageing tests are carried out at lower temperatures it is found that NOx has a very strong effect on the rate at which MoDTC loses its ability to reduce friction, so that at 100 °C NOx accelerates this rate by two orders of magnitude compared to air alone. This suggests that at low temperatures the rate at which MoDTC is consumed is controlled by its reaction as a radical inhibitor with NOx species.

The role of surface roughness and slide-roll ratio on the decomposition of MoDTC in tribological contacts

In this study, the role of surface roughness and slide-roll ratio in the decomposition and friction performance of molybdenum dialkyldithiocarbamate (MoDTC) has been investigated. Tribotests were carried out in a minitraction machine (MTM) using steel discs of varying roughness rubbing against smooth steel balls in a sliding/rolling contact. Tests were conducted at slide-roll ratio (SRR) values of SRR = 100% and 200%. Raman spectroscopy was used to perform chemical characterisation on the resulting wear scars. The friction performance of rough discs was not affected by the SRR. On the other hand, increasing the SRR from 100% to 200% in tests with smooth discs resulted in higher friction with large instabilities. Raman analysis showed significant differences in chemical composition of the wear scars generated after tests with smooth and rough discs. Wear scars generated using rough discs were mainly composed of MoS2 indicating complete MoDTC decomposition while those generated using smooth discs were composed of a mixture of MoS2, MoSx (x > 2) and FeMoO4 indicating partial MoDTC decomposition. Numerical simulation of the contact revealed that under similar loading conditions rough surfaces have higher local pressures than smoother surfaces. It is proposed that higher local pressures in rough surfaces promoted complete MoDTC decomposition. The novel finding from results presented in this study is that at similar temperature and MoDTC concentration, the degradation of MoDTC within tribocontacts is highly dependent on the roughness of the tribopair. This is because surface roughness determines the local pressure at the asperity–asperity contact.

Efficiency improvement in automobile bucket tappet/camshaft contacts by DLC coatings – Influence of engine oil, temperature and camshaft speed

With the aim to fulfill legal regulations concerning energy efficiency and greenhouse gas emissions, diamond-like carbon (DLC) coatings are increasingly used on highly stressed components of internal combustion engines to reduce friction and prolong component lifetime. The valve train is one of a key system of the combustion engine considerably contributing to frictional losses, especially under boundary and mixed lubrication conditions at lower crankshaft speeds. In this regard, the tribological contact bucket tappet/camshaft offers high potential for friction reduction but imposes high demands on DLC coatings due to highly complex kinematics and repetitive loads depending on the cam contours and the camshaft angle and speed. The aim of this work was to analyze the influence of DLC coatings on the frictional and wear behavior within the contact bucket tappet/camshaft. This tribological contact was analyzed in a one tappet/one cam friction test-rig using series-production tappets, cams and valve springs ensuring high transferability of the results into the real application. Two amorphous hydrogen containing carbon based coatings were deposited on series-production bucket tappets. The zirconium based DLC coating a-C:H/ZrCg was compared to a DLC coating system a-C:H:X deposited under industrial conditions. One mineral motor oil of SAE viscosity grade 0W20 and one synthetic motor oil polyalphaolefin (PAO) of SAE viscosity grade 5W30 formulated with different additive packages containing the anti-wear (AW)/extreme pressure (EP) additive zinc dialkyl dithiophosphate (ZDDP) and the friction modifier (FM) additives molybdenum dialkyl dithiocarbamate (MoDTC) and glycerol monooleate (GMO) were used in the test-rig. Special attention was paid to the interactions of the additive packages in the low viscosity oils with the functional surfaces of the a-C:H:ZrCg coated bucket tappets. The frictional behavior determined by time-resolved measurements of friction forces reveals that higher camshaft speeds led to increased lubrication film formation contributing to reduced frictional losses. Higher motor oil temperature resulted in temperature induced reduced viscosity of the motor oils leading to higher frictional losses. The DLC coatings a-C:H:ZrCg and a-C:H:X caused significant reduction of frictional losses under the operating conditions in the test-rig. The a-C:H:ZrCg coated bucket tappets caused higher frictional losses than uncoated bucket tappets at low camshaft in boundary friction conditions. Friction reduction at all operating points was ensured by a-C:H:X regardless of camshaft speed, motor oil temperature and the choice of motor base oil and additive package proving the great potential for application on components under challenging lubricating and loading conditions. The wear of the uncoated and a-C:H:ZrCg coated functional surfaces was analyzed by means of confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). The a-C:H:ZrCg coating contributed to wear protection in the highly loaded tribological contact despite the different lubricating and loading conditions regardless of the base oil and additive packages. The a-C:H:X coating revealed similar favorable behavior. Comprehensive surface sensitive analysis of the uncoated and a-C:H:ZrCg coated bucket tappets by means of Raman spectroscopy and XPS indicated that no tribo-chemical reaction layer was formed from the motor oil and the additive packages on a-C:H:ZrCg coated bucket tappets under the operating conditions. Moreover, the combination of additives ZDDP and GMO in PAO appears to be disadvantageous in terms of reaction layer formation since no tribo-chemical reaction layer was found on uncoated steel bucket tappets after tribological tests in contrast to the mineral oil containing ZDDP and MoDTC.

Wear Mechanisms of Hydrogenated DLC in Oils Containing MoDTC

Diamond-Like Carbon (DLC) coatings are well known for offering excellent tribological properties. They have been shown to offer low friction and outstanding wear performance in both dry and lubricated conditions. Application of these coatings for automotive components is considered as a promising strategy to cope with the emerging requirements regarding fuel economy and durability. Commercially available oils are generally optimised to work on conventional ferrous surfaces and are not necessarily effective in lubricating non-ferrous surfaces. Recently, the adverse effect of the Molybdenum DialkyldithioCarbamate (MoDTC) friction modifier additive on the wear performance of the hydrogenated DLC has been reported. However, the mechanisms by which MoDTC imposes this high wear to DLC are not yet well understood. A better understanding of DLC wear may potentially lead to better compatibility between DLC surfaces and current additive technology being achieved. In this work, the wear properties of DLC coatings in the DLC/cast iron (CI) system under boundary lubrication conditions have been investigated to try to understand what appears to be a tribocorrosion-type process. A pin-on-plate tribotester was used to run the experiments using High Speed Steel (HSS) plates coated with 15 at.% hydrogenated DLC (a-C:15H) sliding against CI pins or ceramic balls. The lubricants used in this study are typical examples of the same fully formulated oil with and without ZDDP. The friction and wear responses of the fully formulated oils are discussed in detail. Furthermore, Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray spectroscopy (EDX), Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) were used to observe the wear scar and propose wear mechanisms. The X-ray Photoelectron Spectroscopy (XPS) analysis was performed on the tribofilms to understand the tribochemical interactions between oil additives and the DLC coating. Nano-indentation analysis was conducted to assess potential structural modifications of the DLC coating. Coating hardness data could provide a better insight into the wear mode and failure mechanism of such hard coatings. Given the obtained results, the wear behaviour of the hydrogenated DLC coating was found to depend not only on the presence of ZDDP in the oil formulation but also on the counterpart type. This study revealed that the steel counterpart is a critical component of the tribocouple leading to MoDTC-induced wear of the hydrogenated DLC.

Transient processes of MoS2 tribofilm formation under boundary lubrication

AbstractA tribochemistry study that involves the application of Raman spectroscopy surface analysis has been undertaken to understand the time‐dependent tribochemical reactions, for lubrication by Molybdenum dialkyl‐dithiocarbamate (MoDTC) occurring in boundary lubricated conditions. Under the conditions of rubbing and high temperature, time‐resolved Raman spectroscopy results show the intermediate steps that lead to the MoDTC additive to be tribochemically structured on the wear scar of the contacting surface. A MoS2 tribofilm with a lattice layer structure is observed on the wear scar whenever the lowest friction was achieved. An apparent shift of the A1g and E2g Raman modes, indicating qualitative and quantitative information on the MoS2 tribofilm formed, is observed to be related to low friction. Detailed analyses of Raman spectra obtained on wear scars at different test durations and temperatures indicate that both temperature and rubbing are needed for the formation of low friction MoS2 tribofilm. Copyright © 2016 John Wiley & Sons, Ltd.

Influence of MoS2 on the Rolling Contact Performance of Bearing Steels in Boundary Lubrication: A Different Approach

Effective lubrication has been achieved under vacuum conditions by employing MoS2 as a solid lubricant. The primary reason for its ability to reduce friction is attributed to its crystal structure, which allows easy shearing of MoS2 layers. This research explores the possibilities of utilizing MoS2 in soluble and insoluble forms to improve the performance of tribological systems. In this study, spherical rolling elements were coated with Ti–MoS2 and their tribological performance was evaluated in a ball-on-rod tribometer operating in boundary-lubricated (PAO ISO 10 oil) conditions. The results of these tests were compared with similar tests performed on uncoated steel specimens using PAO ISO 10 oil with and without a molybdenum dialkyldithiocarbamate (MoDTC) additive. The L50 fatigue lives of M50 rods tested against Ti–MoS2 coated balls were found to be similar to those obtained from tests performed on uncoated steel specimens using oil with MoDTC (1 wt%); that is, the L50 fatigue lives of M50 rods were improved by more than a factor of 2 in both cases. Tribofilms generated on M50 rods during testing with Ti–MoS2 coated balls in neat oil and uncoated balls in MoDTC (1 wt%) mixed oil were observed to be similar. Wear track analysis on M50 rods tested against Ti–MoS2 coated balls indicated that the tribofilms are mixtures of MoS2 and amorphous hydrocarbon (a-C:H). A possible mechanism for the formation of the a-C:H on the counterface is proposed based on the experimental investigations in this study. Further, it is concluded that the Ti–MoS2 coating on the spherical rolling elements functions as an MoS2 additive source to the lubricating oil rather than as a tribological coating.

New insights on the decomposition mechanism of Molybdenum DialkyldiThioCarbamate (MoDTC): a Raman spectroscopic study

A new decomposition mechanism for molybdenum dialkyldithiocarbamate, a friction modifier used in engine lubricants, has been proposed.

A methodology for Raman characterisation of MoDTC tribofilms and its application in investigating the influence of surface chemistry on friction performance of MoDTC lubricants

In this study, Raman spectroscopy has been employed to understand the influence of surface chemistry on friction in a tribocontact. Tribotests were conducted using molybdenum dialkyldithiocarbamate (MoDTC) lubricant in a steel/steel sliding contact. Firstly, surface chemistry in the high-friction regime, at the beginning of the test, and in the low-friction regime, after long test durations, is investigated. Secondly, the influence of temperature on the surface chemistry of the resulting wear scars was investigated. Results show that at the beginning of tribotests with MoDTC lubricant, iron oxides are formed at the tribocontact which result in high friction. At longer test durations, adsorbed MoDTC on the ferrous surface decomposes to form MoS2 and low friction is observed. Surface chemistry at the tribocontact has been found to vary depending on the test temperature. At high temperatures, MoS2 is formed which provides friction reduction, while at low temperatures, molybdenum oxide and amorphous sulphur-rich molybdenum (MoS x ) compounds are formed which do not provide friction reduction. Furthermore, it has been shown that MoS2 formed within the tribocontact at high temperatures has a slightly disordered crystal structure as a result of tribological processes.

Lubricating properties of oil cooling fluids with tribologically active additives in the metal cutting process

The tribological characteristics (roughness of the surface, properties of shaving, residual stress in surface layers, and resistance test of cutting tools) are studied in lubricating oils with various tribologically active additives during metals cutting. Molybdenum dialkyldithiocarbamate ((n-C4H9)4N2C2Mo2S8) at its content in oil of 1–2 wt % is shown to exhibit the best properties among the studied additives.

Improving Tribological Properties of Multialkylated Cyclopentanes under Simulated Space Environment: Two Feasible Approaches.

Space mechanisms require multialkylated cyclopentanes (MACs) more lubricious, more reliable, more durable, and better adaptive to harsh space environments. In this study, two kinds of additives were added into MACs for improving the tribological properties under simulated space environments: (a) solid nanoparticles (tungsten disulfide (WS2), tungsten trioxide (WO3), lanthanum oxide (La2O3), and lanthanum trifluoride (LaF3)) for steel/steel contacts; (b) liquid additives like zinc dialkyldithiophosphate (ZDDP) and molybdenum dialkyldithiocarbamate (MoDTC) for steel/steel and steel/diamond-like carbon (DLC) contacts. The results show that, under harsh simulated space environments, addition of the solid nanoparticles into MACs allows the wear to be reduced by up to one order magnitude, while liquid additives simultaneously reduce friction and wear by 80% and 93%, respectively. Friction mechanisms were proposed according to surface/interface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The role of solid nanoparticles in reducing friction and wear mainly depends on their surface enhancement effect, and the liquid additives are attributed to the formation of tribochemical reaction film derived from ZDDP and MoDTC on the sliding surfaces.

Friction and wear performance of MoDTC‐containing and ester‐containing lubricants over steel surfaces under reciprocating conditions

AbstractLubricant oil can be regarded as a complex mixture of base oils and additives, each one with its specific functions and behaviour. In this paper, the interaction of a molybdenum dialkyldithiocarbamate (MoDTC)‐based additive and combinations of a polyalphaolefin and a synthetic ester is investigated. A reciprocating ball‐on‐disc configuration was used for tribological tests. The effect of MoDTC is seen as a sharp drop in the coefficient of friction. This friction reduction is affected by the base fluid: the effect is more intense and lasts longer when the ester content is decreased. The applied normal force also affects the MoDTC effect, which is not sustainable at higher loads. Copyright © 2014 John Wiley & Sons, Ltd.

In-situ reflection-XANES study of ZDDP and MoDTC lubricant films formed on steel and diamond like carbon (DLC) surfaces

Chemical characterisation of boundary lubricated interfaces is essential for developing mechanistic models that describe lubricant additive interactions with the surface and their effect on tribological performance. In this study the potential for using the synchrotron-based reflection mode X-ray absorption spectroscopy (XAS) technique for in-situ chemical characterisation of lubricant films has been studied. Thermal films formed from zinc dialkyl dithio phosphate (ZDDP) and molybdenum dialkyl dithio carbamate (MoDTC) lubricant additives have been formed and analysed in-situ using the X-ray absorption near edge structure (XANES) spectroscopy technique. The surface sensitivity of this approach was improved by doing the analysis in reflection mode, enabling analysis of only top layer (up to around 10nm) of the solid surface. In addition, in-lubro analysis of pre-formed tribofilms from the same additives was done using non-vacuum conditions. The results are discussed in conjunction with XANES and X-ray photoelectron spectroscopy (XPS) analysis of similar additives published in the literature. The results obtained are consistent with the existing ZDDP and MoDTC literature and provide some new insight into intermediate species not reported before. The advantages and disadvantages of the developed XANES methodology for in-situ surface chemical analysis of lubricated conditions are discussed.