Boundary lubrication mechanisms of carbon coatings by MoDTC and ZDDP additives

Fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like-carbon (DLC) coatings present a wide range of tribological behavior, including friction coefficients in ultra-high vacuum below 0.02. The engine oil environment which provides similar favourable air free conditions might lead to such low friction levels.In this work, the friction and wear properties of DLC coatings in boundary lubrication conditions have been investigated as a function of the hydrogen content in the carbon coating. Their interaction with ZDDP which is the exclusive antiwear agent in most automotive lubrication blends and friction-modifier additive MoDTC has been studied. Hydrogenated DLC coatings can be better lubricated in the presence of the friction-modifier additive MoDTC through the formation of MoS₂ solid lubricant material than can non-hydrogenated DLC. In contrast, the antiwear additive ZDDP does not significantly affect the wear behavior of DLC coatings. The good tribological performances of the DLC coatings suggest that they can contribute to reduce friction and wear in the engine, and so permit the significant decrease of additive concentration.     

ZDDP and MoDTC interactions in boundary lubrication—The effect of temperature and ZDDP/MoDTC ratio

Tribofilms formed under boundary lubrication from ZDDP and MoDTC additives alone or in different ratios in the lubricant have been studied. The tribological performance is linked to the tribofilm properties and consequently to the lubricating conditions. Tribofilms are formed using a reciprocating pin-on-plate tribometer. Surface sensitive analytical techniques, such as energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) have been used for tribofilm characterisation. The XPS peaks have been deconvoluted to characterise the species formed in the wear scar. The formation of species with different tribological properties, due to the decomposition of ZDDP and MoDTC molecules as a result of testing temperature, is shown. Surface analyses have shown that MoDTC decomposes, even in low-lubricant bulk temperature tests (30 °C), forming the same species as in high-lubricant bulk temperature tests (100 and 150 °C) but the tribofilms give different tribological performance. The effectiveness in friction reduction is shown to depend on the ratio between what are defined as high- and low-friction species in the tribofilm.     

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.     

The effect of ZDDP and MoDTC additives in engine oil on the friction properties of DLC‐coated and steel cam followers

Friction tests simulating cam follower sliding conditions were conducted using a pin‐on‐disc test rig. In the case of SAE 5W‐30 class engine oil, the friction coefficient of the combination of steel pins sliding on a steel disc increased from 0.11 to 0.12, while that of steel pins sliding on a diamond‐like carbon (DLC)‐coated disc decreased from 0.12 to 0.10. For 5W‐20 oil containing the friction modifier MoDTC (molybdenum dithio‐carbamates), the friction coefficient of steel pins sliding on a steel disc decreased markedly from 0.12 to 0.04. In contrast, that of steel pins sliding on a DLC‐coated disc decreased more moderately, from 0.11 to 0.08. In both cases, Zn, P, S, and Mo elements derived from ZDDP (zinc dialkyldithiophosphate) and MoDTC additives were not detected on the DLC‐coated disc, while these elements were detected on the steel disc and pins using electron probe microanalysis and X‐ray photo‐electron spectroscopy surface analysis. It appears that a tribochemical reaction film did not form on the DLC material due to its inactive chemical properties. Therefore, an increase in friction due to the formation of the film derived from ZDDP and a decrease in friction due to the formation of the film derived from MoDTC were clearly suppressed in the case of the steel pins sliding on the DLC‐coated disc. It is thought that the tribo‐chemical reaction film was instrumental in reducing friction substantially. The lateral force of the film formed on the steel disc was then measured using an atomic force microscopy lateral force microscopy test. The lateral force of the film resulting from the 5W‐30 oil was much higher than that of the film formed from the 5W‐20 oil with MoDTC. This result coincided well with the results of the friction tests. Judging from these results, it is thought that the high friction coefficient observed for the steel pins on the steel disc for the 5W‐30 oil was caused by the higher shear strength of the film formed from ZDDP. On the other hand, the very low friction coefficient observed for the steel pin‐steel disc combination for the 5W‐20 oil was presumably caused by the formation of a solid MoS₂ lubricant from the MoDTC additive.     

Evaluation of Local Mechanical Properties in Depth in MoDTC/ZDDP and ZDDP Tribochemical Reacted Films Using Nanoindentation

Local mechanical properties in depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using a nanoindentation technique combined with in-situ atomic force microscopy (AFM) observation. Tapping-mode AFM observation revealed that the MoDTC/ZDDP film was much rougher than the ZDDP film. Nanoindentation measurement revealed that the MoDTC/ZDDP and ZDDP tribofilms possessed different elasto-plasticities around a depth of several nanometers from the surface, although both films showed the same hardness and modulus depth distributions except in the surface area. The same mechanical depth distributions indicated that both kinds of tribofilm were functionally graded materials; that is, they consisted of a layer near the surface with lower hardness and modulus and providing lubrication and a base layer with higher hardness and modulus and serving to modify property differences at the interface. Most importantly, the different elasto-plasticities near the tribofilm surfaces revealed that the MoDTC/ZDDP tribofilm possessed lower shearing yield stress than the ZDDP tribofilm. The results of this study suggest that the presence of some solid lubricants such as MoS₂ just below the MoDTC/ZDDP film surface reduced the boundary friction coefficient.     

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.     

ZDDP and MoDTC interactions and their effect on tribological performance – tribofilm characteristics and its evolution

In this work, the interactions between two key additives in current lubricants (ZDDP and MoDTC) and the effect on tribofilm formation and tribofilm evolution under boundary lubrication are studied. The chemical and tribological characteristics of the tribofilms are probed using measurement of friction, wear and film characteristics. Tribofilms have 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, atomic force microscopy (AFM) was used. In this work, for the first time, a link between a proposed MoDTC breakdown mechanism and MoDTC tribofilm characteristics, measured on experimentally derived tribofilms, is made.     

Effects of electric field on characteristics of nano-confined lubricant films with ZDDP additive

The consequence of bearings exposed to shaft voltage is a very important tribological problem, especially with the increasing use of variable-frequency drives (VFDs) to control alternate current (AC) motors. The emerging behavior of gas micro-bubbles and the film forming characteristics between base oil (liquid paraffin) films with and without ZDDP additive under an external electric field (EEF) in a nanogap have been compared. Experimental results indicated that the micro-bubble emerging intensity increases slightly when the additive is involved in the base oil. The magnitude of the electric current flowing through the lubricant film closely related to the intensity of the micro-bubble emerging. No obvious difference in the film thickness can be found between the liquid paraffin films with and without ZDDP additive. The influence of the EEF on the film thickness of the liquid paraffin with the additive is more significant.     

Insights into the influence of additives on the thermal gradient induced migration of lubricant

Thermo‐capillary migration is a phenomenon that the thermal gradients will drive a liquid to flow from warm to cold regions. It is of great importance to prevent the lubricant migration on rubbing surfaces in the cases where the amount of lubricant is limited. In this paper, four different lubricant additives are incorporated into one base oil, and the effects of additives on the migration behaviour and surface tension coefficient are investigated. The functional mechanisms of additives are discussed. The experimental results demonstrate that the additives have remarkable influences on the migration performance of lubricant. The migration behaviour shows the relation to not only the surface tension coefficient, but also the actions between the additive and substrate. This should be considered in the designing process of an anti‐migration lubricant. Copyright © 2016 John Wiley & Sons, Ltd.     

Wet clutch transmission fluid for AWD differentials: influence of lubricant additives on friction characteristics

In recent years, several electronically controllable automotive transmission systems using wet clutches as intelligent differentials have emerged on the market. These applications place great demands on the anti‐shudder properties of the transmission fluids used. The aims of this study were (i) to investigate the influence of different additives on the friction characteristics of a transmission fluid for all‐wheel drive systems featuring wet multi‐plate clutch with a sintered brass‐based friction material and, based on this knowledge, (ii) to formulate a new transmission fluid with the desired frictional properties. In addition to excellent anti‐shudder properties, the new fluid was required to lubricate hypoid gears under high load. To meet this requirement, it is necessary to add significant amounts of extreme pressure additives to the base oil, which are known to have an unfavourable influence on anti‐shudder properties, necessitating the adoption of novel additive technologies. The additives studied include anti‐wear additives, friction modifiers, corrosion inhibitors, detergents, antioxidants and extreme pressure additives. This paper shows how different additives affect friction in different ways, and that the interactions between the different additives are important to consider. It was concluded that it is feasible to combine good anti‐shudder properties for wet clutches with good lubrication of hypoid gears. Copyright © 2006 John Wiley and Sons, Ltd.     

The mechanism of synergism between ZDDP and CeF3 additives

The interaction of zinc dialkyldithiophosphate (ZDDP) with cerium fluoride and cerium dioxide in lithium grease has been studied for friction, wear, and EP characteristics on a four-ball and SRV tester. The combination of ZDDP and cerium trifluo-ride has been shown to be beneficial in reducing wear, especially over a long period of friction and with increasing EP load. The test results show that adding cerium dioxide to lithium grease does not improve the antiwear and friction performance of the paste. The analytical results of X-ray photoelectron spectrum (ESCA, XPS) reveal that ZDDP inhibits the decomposition of cerium fluoride and improves its film-forming property. The wear scar reaches a minimum at an atomic concentration ratio of 3P:2Zn:1S:25F, and an atomic concentration ratio of 2S:2P:1Zn:4F gives the highest EP load. The scratch test results show that combining ZDDP with cerium trifluoride improves the tenacity of the surface film. The stabilisation of cerium trifluoride by ZDDP is proposed.     

Evaluation of Nanoscale Friction Depth Distribution in ZDDP and MoDTC Tribochemical Reacted Films Using a Nanoscratch Method

The distributions of local friction coefficients relative to the depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms were successfully evaluated by using a nanoscratch method combined with in situ AFM observation. It was found that both tribofilms were friction-functionally graded materials. The friction coefficients decreased from 0.35 to 0.16 with a decrease in the scratch depth from 60 to 10 nm. It was observed that the MoDTC/ZDDP and ZDDP tribofilms possessed different shear strength levels near the surface as evidenced by the different valley-shaped friction coefficient distributions they exhibited for scratch depths ranging from 2 to 10 nm. Based on our recent nanomechanical measurements, this observation indicated that both tribofilms possessed an ultra-low friction inner skin layer at a depth of about 10 nm below the surface. Most importantly, the inner skin layer of the MoDTC/ZDDP tribofilm possessed a lower friction coefficient than that of the ZDDP tribofilm (0.084 versus 0.104) and was thinner (about 3.2 nm versus 6.4 nm). These results thus revealed that the reduction in friction attributed to the MoDTC additive originates from the different friction behavior of the inner skin layers of the MoDTC/ZDDP and ZDDP tribofilms. These nanoscratch results agree with the findings of our recent work on detecting differences in mechanical properties between these tribofilms by nanoindentation measurements.     

Organoamine and organophosphate molybdenum complexes as lubricant additives

The friction and wear characteristics of molybdenum complexes, from which sulphur is absent, were examined under reciprocating sliding conditions by adding them to a paraffinic base oil with, and without, sulphur and phosphorus compounds as additives. Two kinds of molybdenum complex were used, i.e. an alkylamine complex and an alkylphosphate complex. The complexes were capable of forming MoS₂ despite the absence of sulphur in their constituents when they were added to the base oil containing sulphur compounds. The friction was reduced by the formation of a surface film comprising MoS₂ but the wear performance was little improved by combining the alkylamine complex with a sulphur compound. Both the friction and the wear performance were improved markedly by the formation of FePO₄ besides MoS₂, i.e. it was effective for the alkylamine complex and the alkylphosphate complex to be used together with sulphur and phosphorus compounds and with sulphur compounds respectively.     

The influence of soot and dispersant on ZDDP film thickness and friction

The effect of dispersed soot in engine oils is an increasingly important issue in terms of both engine durability and fuel efficiency. Using carbon black as a soot analogue, a study has been carried out to investigate the main factors that determine the impact of soot on friction and ZDDP film formation in formulated oils. It has been found that dispersed carbon black can rapidly remove ZDDP reaction films by abrasion. However, this removal can be prevented or limited by the choice of an optimal dispersant additive.     

Tribological characteristics of ashless dithiocarbamate derivatives and their combinations with ZDDP as additives in mineral oil

Two ashless dithiocarbamate derivatives, octyl 2-(dibutylcarmothioylthio) acetate (DDCO) and S-dodecyl 2-(dibutylcarbamothioylthio) ethanthioate (DDCS), were prepared. Thermal stabilities tests were conducted with a thermo-gravimetric analyzer (TGA). The tribological properties of each compound and their combinations with ZDDP in a mineral oil (HVI WH150) were evaluated using a four-ball tester. X-ray absorption near-edge structure (XANES) spectroscopy was used to characterize the chemical properties of tribofilms generated from DDCO, DDCS and their combinations with ZDDP. According to the TGA results, the synthesized compounds possess good thermal stability (initial decomposition temperatures are above 270 °C). It can be found that all the prepared compounds have better friction-reducing capacity than ZDDP, with anti-wear performance and extreme pressure property worse than ZDDP. However, their combinations with ZDDP perform better than ZDDP in tribological properties. The results of the XANES analyses indicate that the composition of the tribofilms from DDCO or DDCS is organic sulphide on the outer surface and pyrite with a little sulphite in the inner layer, which also suggests the -SC(=S)-N- part in additives structure plays key role in tribol-chemical behaviour. The XANES spectra of the combinations exhibit interestingly that the addition of DDCO or DDCS can increase the length of polyphosphate chain in the tribofilms.     

Friction property study of the surface of ZDDP and MoDTC antiwear additive films using AFM/LFM and force curve methods

The friction properties and material differences of the surface of ZDDP and MoDTC antiwear additive films, which give clear evidence of different friction coefficients in a pin-on-disc test, have been studied using atomic force microscopy (AFM)/lateral force microscopy (LFM) and force curve methods. The AFM/LFM observations show that the friction force on the surface of MoDTC additive films over the sliding area of a steel disc is lower and the friction force of ZDDP additive films is higher than that of afilmless area. Lateral force scope-trace evaluations reveal that the ratio of the friction forces on the surface of the ZDDP film, the filmless area, and the MoDTC film under the same normal force is approximately 1.5:1.0:0.7. Force curve measurements indicate that the surface materials of the ZDDP film, thefilmless area, and the MoDTC film differ according to their attractive forces, that is 29 nN for the ZDDP film, 22 nN for the filmless area, and 12 nN for the MoDTC film. These results correspond to the friction behaviour in the pin-on-disc test and also agree with the idea of the formation of solid MoS₂ lubricant from MoDTC additives on the surface of the antiwear film.     

Boric acid and borax as antiscoring additives to glycerin-based lubricant

Boric acid and borax, which are used as antiscoring additives to an environmentally friendly glycerin-based lubricant, have been studied. It is shown that the adsorption of borax obeys the Langmuir isotherm. The equilibrium constants of the adsorption of the additives and the energy of the adsorption are estimated. It has been found that borax is a more effective antiscoring additive than boric acid. For borax used as the additive, the dependence of the coefficient of friction on the load applied to a friction unit is studied and interpreted.     

The effect of lubricant additives on friction and wear studies of polyphenylene sulphide

Polyphenylene sulphide is a polymer with good thermal stability and high crystallinity. This paper summarizes the results of friction and wear studies of polyphenylene sulphide and its composites made with conventional solid lubricants to ascertain the suitability of the material as a matrix for solid lubricant additives. The polymer itself has a high coefficient of friction. Wear rate increases with load and speed. Addition of solid lubricant additives helps in improving the friction and wear of the polymer. Composites with MoS₂-Sb₂O₃ and PTFE gave better results than composites made by the addition of graphite and MoS₂ graphite. Wear rate of these composites increased with load and speed; but load and speed had little effect on friction.     

Coking and microcoking: Tools for evaluating and developing lubricant additives

Two tests for evaluating an engine lubricant's tendency to form deposits at high temperature have been developed by a working group of the GFC,

1 Groupement Français de Coordination (French Coordination Group).

set up to establish laboratory pre-selection tests. The first is a new static microcoking test where the oil is deposited on an aluminium alloy plate and submitted to thermal conditions similar to those found in the piston ring zone of an IC engine. The results are analysed to determine the temperature at the oil's thermal stability limit and to evaluate the quantity and nature of the deposits in the form of varnish, according to a grading system based on the CEC M-02-A-78 method of gauging engine cleanliness. The second is an improved panel coking test in which a thin film of oil of a given volume is fed on to a stainless steel panel, at high temperature, and the deposits formed at the end of the test are evaluated, again using the CEC rating method, in terms of quantity and type (carbon or varnish). In addition, a video system has been designed to grade the micro-coking panels through colour recognition. This not only allows visual grading to be faithfully reproduced but also eliminates the subjective element of colour grading. The results obtained with the ‘Video-Rater’ constitute a first step towards computer-assisted grading of engine parts.