Imperial College London


Faculty of EngineeringDepartment of Mechanical Engineering

Research Postgraduate







City and Guilds BuildingSouth Kensington Campus





Publication Type

10 results found

Ueda M, Wainwright B, Spikes H, Kadiric Aet al., 2022, The effect of friction on micropitting, Wear, Vol: 488-489, Pages: 1-13, ISSN: 0043-1648

Micropitting is a type of surface fatigue damage that occurs in rolling-sliding contacts operating under thin oil film conditions. It is caused by stress fluctuations, brought about by surface asperity interactions, which lead to initiation and propagation of numerous surface fatigue cracks and subsequent loss of material. Despite its increasing importance to gear and bearing reliability, the mechanisms of micropitting are poorly understood. This is particularly the case concerning the effects of friction on micropitting which are difficult to study under controlled conditions. This is because it is difficult to isolate the friction effects from other influential factors, in particular from the build-up of any anti-wear tribofilm and its subsequent effect on the running-in of counterface roughness that is known to strongly affect micropitting through its influence on severity of asperity stresses. This paper presents new data on the impact of friction on micropitting obtained using a new test methodology. Micropitting tests were conducted using a ball-on-disc MTM rig with the additional functionality to continuously monitor the growth of tribofilm during the test. Friction was varied by using custom-made oils containing different concentrations of MoDTC. Crucially, the effect of friction was isolated from the effect of counterface roughness running-in by introducing the MoDTC blend only after the running-in period was completed with a ZDDP solution alone. This approach eliminates the influence of MoDTC on ZDDP anti-wear tribofilm growth in early stages and hence ensures the same running-in takes place in each test. This gives similar asperity pressure history, regardless of the amount of MoDTC present.Resultsshow that friction has a very significant impact on micropitting; for example, the extent of micropitting was reduced by a factor of 10 when friction coefficient was reduced from about 0.1 to 0.04. Lower friction results in fewer surface cracks which grow at a s

Journal article

Ueda M, Kadiric A, Spikes H, 2021, Wear of hydrogenated DLC in MoDTC-containing oils, Wear, Vol: 474-475, Pages: 1-10, ISSN: 0043-1648

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.

Journal article

Ueda M, Kadiric A, Spikes H, 2021, Influence of steel surface composition on ZDDP tribofilm growth using Ion implantation, Tribology Letters, Vol: 69, ISSN: 1023-8883

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.

Journal article

Zhang J, Ueda M, Campen S, Spikes Het al., 2021, Boundary friction of ZDDP tribofilms, Tribology Letters, Vol: 69, Pages: 1-17, ISSN: 1023-8883

The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundar

Journal article

Garcia Gonzalez C, Ueda M, Spikes H, Wong Jet al., 2021, Temperature dependence of Molybdenum dialkyl dithiocarbamate (MoDTC) tribofilms via time-resolved Raman spectroscopy, Scientific Reports, Vol: 11, Pages: 3621-3621, ISSN: 2045-2322

Molybdenum dialkyl dithiocarbamate (MoDTC) is a friction reducing additive commonly used in lubricants. MoDTC works by forming a low-friction molybdenum disulphide (MoS<sub>2</sub>) film (tribofilm) on rubbed surfaces. MoDTC-induced MoS<sub>2</sub> 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<sub>2</sub> 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<sub>2</sub> 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<sub>c</sub>, a small amount of MoS<sub>2</sub> gives significant friction reduction. Above T<sub>c,</sub> a patchy film with more MoS<sub>2</sub>, 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.

Journal article

Zhang J, Ueda M, Campen S, Spikes Het al., 2021, Boundary friction of ZDDP tribofilms (vol 69, 8, 2021), Tribology Letters, Vol: 69, Pages: 1-1, ISSN: 1023-8883

Journal article

Ueda M, Kadiric A, Spikes H, 2020, ZDDP tribofilm formation on non-ferrous surfaces, Tribology Online, Vol: 15, Pages: 318-331, ISSN: 1881-218X

The current trend for using lower viscosity lubricants with the aim of improving fuel economy of mechanical systems means that machine components are required to operate for longer periods in thin oil film, boundary and mixed lubrication conditions, where the risk of surface damage is increased. In addition, non-ferrous materials are increasingly being introduced in machine components to reduce wear and increase efficiency. Thus, understanding of the ZDDP antiwear tribofilm formation on both ferrous and non-ferrous surfaces is increasingly important in order to formulate lubricants that give desired antiwear performance with both types of materials. In this paper the effect of ferrous and non-ferrous rubbing materials, namely, steel, Si3N4, WC, SiC and a-C:H DLC coating, on ZDDP tribofilm formation was investigated. Among non-ferrous materials, it was found that ZDDP tribofilms were formed on Si3N4 and WC in the boundary lubrication regime, but almost no tribofilms were formed on SiC and a-C:H DLC. In addition, although tribofilms formed on some non-ferrous surfaces, they were easily removed under boundary lubrication by direct asperity contact because of their weak adhesion to the substrate. This tribofilm removal makes quantification of ZDDP tribofilm formation rate on non-ferrous surfaces under boundary lubrication conditions difficult. By contrast, under high shear stress EHL conditions, thick tribofilms formed without film removal with the tribofilm thickness being the greatest for steel, followed by Si3N4 and then WC, with no tribofilm formation observed on SiC and DLC. QCM results suggest that ZDDP tribofilm formation might be considerably affected by the extent to which ZDDP adsorbs on the substrate surface. The chemical properties of tribofilms are discussed and a possible mechanism by which ZDDP forms tribofilm on non-ferrous surfaces is suggested. This study has practical implications for ways in which non-ferrous surfaces can be protected from wear via l

Journal article

Zhang J, Ewen JP, Ueda M, Wong JSS, Spikes HAet al., 2020, Mechanochemistry of zinc dialkyldithiophosphate on steel surfaces under elastohydrodynamic lubrication conditions, ACS Applied Materials & Interfaces, Vol: 12, Pages: 6662-6676, ISSN: 1944-8244

Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig capable of applying very high loads has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerisation.

Journal article

Ueda M, Kadiric A, Spikes H, 2019, On the crystallinity and durability of ZDDP tribofilm, Tribology Letters, Vol: 67, Pages: 1-13, ISSN: 1023-8883

The current trend for using lower-viscosity lubricants with the aim of improving fuel economy of mechanical systems means that machine components are required to operate for longer periods in thin oil film, mixed lubrication conditions, where the risk of surface damage is increased. Consequently, the performance and durability of the tribofilms formed by antiwear additives, and in particular zinc dialkyldithiophosphate (ZDDP), the main antiwear oil additive used in engine oils, has become an increasingly important issue. In this paper, it is confirmed that ZDDP tribofilms are initially relatively easily removed by rubbing but that they become more durable during prolonged rubbing. FIB-TEM analyses at different stages of tribofilm formation show that during the early stages of rubbing only the tribofilm close to the steel substrate is nanocrystalline, while the outer region is amorphous and easily removed. However, after prolonged rubbing all regions of the tribofilm become nanocrystalline and able to withstand rubbing in base oil without being removed. XPS analysis shows that after extended rubbing the outermost polyphosphate structures change from longer-chain structures such as metaphosphate and polyphosphate to shorter-chain structures including orthophosphate. This depolymerization of ZDDP tribofilm from long- to short-chain phosphate and consequent nanocrystallization are driven by heat and shear stress. EDX analysis shows that this conversion is promoted by diffusion of Fe cation into the bulk of the tribofilm. The finding that ZDDP tribofilms evolve during rubbing from a weaker amorphous structure to a more durable nanocrystalline one has important implications in terms of the behaviour of ZDDPs at low concentrations, on non-metallic surfaces and at very high contact pressures, as well as for the development of ZDDP tribofilm, friction and wear models.

Journal article

Ueda M, Spikes H, Kadiric A, 2019, In-situ observations of the effect of the ZDDP tribofilm growth on micropitting, Tribology International, Vol: 138, Pages: 342-352, ISSN: 0301-679X

The ongoing trend for using ever lower viscosities of lubricating oils, with the aim of improving the efficiency of mechanical systems, means that machine components are required to operate for longer periods under thin film, mixed lubrication conditions where the risk of surface damage is increased. For this reason, the role of zinc dialkyldithiophosphate (ZDDP) antiwear lubricant additive has become increasingly important in order to provide adequate surface protection. It is known that due to its exceptional effectiveness in reducing surface wear, ZDDP may promote micropitting by preventing adequate running-in of the contacting surfaces. However, the relationship between ZDDP tribofilm growth rate and the evolution of micropitting has not been directly demonstrated. To address this, we report the development of a novel technique using MTM-SLIM to obtain micropitting and observe ZDDP tribofilm growth in parallel throughout a test. This is then applied to investigate the effect of ZDDP concentration and type on micropitting.It is found that oils with higher ZDDP concentrations produce more micropitting but less surface wear and that, at a given concentration, a mixed primary-secondary ZDDP results in more severe micropitting than a primary ZDDP. Too rapid formation of a thick antiwear tribofilm early in the test serves to prevent adequate running-in of sliding parts, which subsequently leads to higher asperity stresses and more asperity stress cycles and consequently more micropitting. Therefore, any adverse effects of ZDDP on micropitting and surface fatigue in general are mechanical in nature and can be accounted for through ZDDP's influence on running-in and resulting asperity stress history. The observed correlation between antiwear film formation rate and micropitting should help in the design of oil formulations that extend component lifetime by controlling both wear and micropitting damage.

Journal article

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