Knowledge

Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum disulphide (MoS 2 ) film (tribofilm) on rubbed surfaces. MoDTC-induced MoS 2 tribofilms have been studied extensively ex-situ; however, there is no consensus on the chemical mechanism of its formation process. By combining Raman spectroscopy with a tribometer, effects of temperature and shear stress on MoS 2 tribofilm formation in steel-steel contacts were examined. Time-resolved Raman spectra of the tribofilm were acquired , together with the instantaneous friction coefficient. The tribofilm is constantly being formed and removed mechanically during rubbing. Increasing shear stress promotes MoS 2 formation. The nature of the tribofilm is temperature-dependent, with high-temperature tribofilms giving a higher friction than lower temperature films. Below a critical temperature T c , a small amount of MoS 2 gives significant friction reduction. Above T c, a patchy film with more MoS 2 , together with a substantial amount of amorphous carbon attributed to base oil degradation, forms. The composition of this tribofilm evolves during rubbing and a temporal correlation is found between carbon signal intensity and friction. Our results highlight the mechanochemical nature of tribofilm formation process and the role of oil degradation in the effectiveness of friction modifier MoDTC.

Many concerns, such as economic and technical viability and social and ethical aspects, must be considered for a feedstock selection for advanced biofuels. Industrialized countries promote the use of industrial waste or by-products for this purpose. In particular, turpentine has several properties which make it an attractive source for biofuels, including its possible industrial waste origin. Nevertheless, turpentine has shown some disadvantages when blended directly with diesel, especially because it increases the sooting tendency. On the contrary, some derivatives of turpentine can be suitable for diesel blends. Thus, the evaluation of their properties is necessary. In the present work, the properties of hydrogenated and oxyfunctionalized turpentine have been analysed and compared with the purpose of elucidating their benefits and drawbacks in diesel fuel applications, using European standards as a reference. The results show a promising application of both hydroturpentine and oxyturpentine as diesel components. While hydroturpentine significantly improves the diesel cold flow properties, oxyturpentine noticeably reduces the sooting tendency.

The effectiveness of antiwear additives in laboratory tests is commonly evaluated using specimens made of AISI 52100 through-hardened bearing steel. However, many lubricated machine components are made of steels with significantly different material compositions, which raises an important practical question of whether the performance of antiwear additives with these other steel types is different from that established with AISI 52100. To help answer this question, this paper investigates the influence of steel composition on the formation and effectiveness of antiwear films. Four steels that are commonly used in tribological applications, namely AISI 52100 through-hardened bearing steel, 16MnCr5 case-carburised gear steel, M2 high speed steel and 440C stainless steel are tested in rolling-sliding, ball-on-disc contacts lubricated with three custom-made oils, one containing ZDDP and two containing different types of ashless antiwear additives. The relative effectiveness of their boundary films was assessed by measuring their thickness and associated wear and friction over 12 h of rubbing at two specimen roughness levels. For ZDDP it was found that the formation of antiwear film was not significantly influenced by steel composition or specimen surface roughness. A similar tribofilm thickness, final tribofilm roughness and friction was observed with all four steels. No measurable wear was observed. By contrast, for the ashless antiwear additives the thickness and effectiveness of their tribofilms was strongly influenced by steel composition, particularly at higher roughness levels. The exact trends in film thickness vs steel relationship depended on the specific chemistry of the ashless additive (ester-based or acid-based) but in general, relative to AISI 52100 steel, M2 steel promoted ashless tribofilm formation whilst 440C retarded ashless tribofilm formation. This behaviour is attributed to the presence of different alloying elements and the ability of the additives to extract metal cations from the rubbing surfaces to support the growth of a tribofilm. In all cases ZDDP films were thicker and rougher, and produced higher friction than those formed by the ashless additives. However, unlike ZDDP, ashless blends generally produced significant wear, particularly with 16MnCr5 and M2 steels. The results indicate that to ensure reliable performance of a given machine component, the chemistry of an ashless antiwear additive should be matched with the types of steel present in the lubricated machine.

This paper examines the influence of steel surface composition on antiwear tribofilm formation by ion-implanting typical steel alloying elements, Ni, Mo, Cr, V and W, into AISI 52100 bearing steel surfaces. Such implantation changes the chemical composition of the steel surface but has relatively little effect on its mechanical properties or topography. The behaviour of zinc dialkyldithiophosphate (ZDDP) antiwear additive was studied. The study employs a ball on disc tribometer with ability to monitor tribofilm development and a range of analytical tools including STEM-EDX, XPS and FIB-TEM to analyse the formed tribofilms. It was found that Ni implantation promotes ZDDP tribofilm formation while Mo and Cr implantation deters tribofilm growth. V and W implantation do not significantly change tribofilm formation. Results on the influence of ZDDP concentration on tribofilm formation rate with different implanted metals suggest that one important mechanism by which steel composition influences tribofilm formation may be by controlling the extent of ZDDP adsorption. This study shows the importance of steel surface composition on ZDDP response and also demonstrates a powerful way to study and potentially improve the tribological performance of machine components via a combination of lubricant formulation and surface modification.

Friction, wear and tribofilm growth of organic friction modifiers (glycerol monooleate and oleamide), antiwear additive (ZDDP) and binary additive system comprising the organic friction modifiers and ZDDP were studied in polyalphaolefin (PAO) and ester oil. The mechanisms underlying base oil polaritydependent frictional performance of the OFM and AW additives at high temperature (140 oC), either singly or in combination, was investigated in the light of chemical composition analysis of the tribofilms post friction measurements using energy-dispersive X-ray spectroscopy (EDX), static and dynamic timeof-flight secondary ion mass spectrometry (ToF-SIMS). Depending on the rubbing conditions, the boundary friction coefficient of the binary additive systems were found to be either lower than that of individual additives or to lay between the values for the individual additives. Chemical composition analysis of the tribofilms indicated that the nature of base oil controlled interactions between ZDDP and OFM and consequently adsorption and reactive tribofilm formation in the boundary lubrication layer. Surface roughness and wear scar width measured post tribological tests using 3D surface profiler showed improved wear performance in both PAO and ester-based additive formulations.

Two different lubricants containing zinc dialkyldithiophosphate (ZDDP) additive were tested in a rolling-sliding contact test rig (micropitting rig) at different relative humidities. The effect of relative humidity on the bulk properties (e.g. viscosity, water concentration, water saturation level) of the lubricants and their tribological performance (e.g. friction, wear, micropitting level) as well as the related tribochemistry was extensively explored. Relative humidity had a limited effect on the viscosity of the tested lubricants. However, the friction and micropitting level decreased while the wear increased at higher relative humidity. This increased wear was attributed to a thinner tribofilm and shorter chain length of the polyphosphates derived from the ZDDP additive. Hydrolysis of the ZDDP additive occurred, and the polar water molecules limited the access of the ZDDP additive to the substrate. The different polarities of the two base oils (Ester, polyalphaolefin) also led to different tribological and tribochemical performance.

Scuffing evaluation is challenging due to the catastrophic nature of this failure mode. In this paper, a new scuffing test method was designed for evaluating the scuffing capacity of fully formulated industrial oils. A barrel-on-disc technique was employed in which the specimens are moving in opposite direction under rolling–sliding conditions. The maximum Hertzian pressure could reach up to 3.06 GPa, and the test plan was a combination of increasing-sliding speed and increasing-load steps. Furthermore, the tribofilm evolution was captured using Spacer Layer Interferometry Method (SLIM), and the correlation of tribofilm and micro-scuffing/scuffing was presented. Results revealed the difference between the scuffing capacity of Environmentally Acceptable Lubricants and the Mineral oils which had the same scuffing capacity in their datasheet.

This paper describes a study of the effect on MoDTC-promoted a-C:H DLC wear of adding various surface-active additives used in engine lubricants, including ZDDP, an ashless EP additive, Ca detergents, dispersants, an OFM and a PAMA, to an MoDTC solution. Tribofilms formed on wear tracks on steel were analysed using SLIM, TEM, STEM-EDX, Raman spectroscopy and XPS. Relevant mechanisms by which these additives reduce the impact of MoDTC on DLC wear have also been suggested. DLC wear in PAO+Mo can be reduced by the presence of other surface-active additives in three ways. Firstly, asperity contact between DLC and steel can be mitigated by forming thick antiwear tribofilms. Secondly, other additives can increase the ratio of MoS₂:MoO₃, reducing the amount of wear-enhancing MoO₃ in the tribofilm. Thirdly, the amount of MoDTC tribofilm including MoO₃ can be reduced by the competitive adsorption of other surface-active additives. This study has practical implications for ways in which DLC surfaces can be protected by lubricant formulation.

In most gear drive applications mineral or synthetic oils are used as lubricants, which are made of fossil raw materials and are non-biodegradable. In applications located in critical environmental areas such as boats or harbors, eco-friendly lubricants are needed. As a result, a gear transmission fluid based on water is currently being developed in a research project supported by the Bayrische Forschungsstiftung (Bavarian Research Foundation). Results of former research showed that in general it is possible to use water-based lubricants in gear drives under certain operating conditions. Since water has a low viscosity compared to conventional used lubricants, plant extracts are added to generate higher viscosities. In order to avoid tribological influenced damages such as sliding wear and scuffing on the surface of gear flanks, adequate additives are needed. Different combinations of plant extracts and additives were investigated using the scuffing test A/8.3/RT according to DIN ISO 14635-1. The results show a surprisingly high load carrying capacity regarding scuffing. Additionally, two wear tests based on DGMK 377-01 were conducted with one sample fluid. A high risk of sliding wear was detected. Additionally, MTM and SRV measurements were conducted with different polymers to optimize the lubricant. The results of the wear tests help to define operating conditions for a future lubricant based on water and plant extracts. This paper aims to share the results of the performed experimental investigations and discusses the challenges regarding the development of such new lubricants.