Fellow Profile: Doris Nekesa 

Project title

Raman spectroscopic studies of friction modifier Molybdenum DialkyldiThioCarbamate (MoDTC) (P11).

Project vision statement

This project aims to provide a clear understanding of the chemical processes that take place in the interfacial region of two rubbing surfaces that are in the boundary lubricated regime. The aim of this study is to develop a better understanding of the interfacial processes (additive decomposition, tribofilm formation and removal) that occur within steel/steel contacts during tests with MoDTC-containing lubricant.

Project summary

Presently, there is a strong push towards improving fuel economy in passenger cars. Poor fuel economy is attributed to high friction in various components within car engines. About 5% of the friction losses in internal combustion engines occur in the boundary lubrication regime where metal‑metal contact is present. Lubrication in boundary lubrication regime is achieved by using lubricants containing chemically active additives which react with the surfaces to form thin films known as tribofilms. The formed tribofilms provide friction reduction and wear protection due to their physicochemical properties. Molybdenum DialkyldiThioCarbamate (MoDTC) is an additive added in engine oil mainly as a friction modifier. MoDTC reduces friction by degradation of the additive to form discrete MoS₂ at the tribocontact. There is however little knowledge on the degradation process of MoDTC at the tribocontact. 

In this study, tribochemical reactions that occur in steel/steel tribocontacts in the presence of MoDTC additive have been investigated. Tribological tests were conducted using model oils comprising of MoDTC additive in mineral base oil. Tests were conducted under unidirectional linear sliding and sliding/rolling conditions. Raman spectroscopy was used to conduct chemical characterisation of the rubbed surfaces.

Results show that mechanical activation accelerates the rate of MoDTC degradation. Under tribological conditions, MoDTC decomposes to form three main compounds; MoS₂, MoS_x and FeMoO₄. MoDTC decomposition products formed at the tribocontact are dependent on test conditions. The mechanism for the degradation process has been proposed. MoDTC tribofilms were observed to grow rapidly within generated wear scars until a limiting thickness was achieved. The limiting thickness was dependent on contact parameters.

The chemical composition of MoDTC tribofilms determined the friction observed in tribotests. In sliding/rolling conditions, low friction values (µ=0.04-0.05) were obtained when the tribofilms are composed of MoS₂ while higher friction (µ=0.06-0.08) was observed when the tribofilms were composed of Fe₂O₃, Fe₃O₄, MoS_x and FeMoO₄.

MoDTC provided wear protection to the steel substrates only at test conditions which allowed MoDTC tribofilms to be present at the contact. In conditions where MoDTC tribofilms were missing from the contact, severe wear of the substrate was observed.

The durability of MoDTC tribofilms when rubbed in MoDTC‑free lubricant was observed to be dependent on the sliding configuration. MoDTC tribofilms were less durable in sliding/rolling contacts than in unidirectional sliding contacts. In unidirectional sliding conditions, it was also observed that MoDTC tribofilms formed on fresh steel samples were more durable than those formed on oxidised steel samples. This is because the adhesion of MoS₂ on iron oxide is less than on steel.

Educational Background

1. PhD Mechanical Engineering, University of Leeds (UK), 2016. Thesis title: Raman spectroscopic studies of  friction modifier Molybdenum DiThioCarbamate (MoDTC) 
2. MSc. Degree in Advanced Chemical Engineering with IT and Management.  Loughborough University (UK), 2012 .MSc Thesis: Continous microflow separation of proteins
3. BEng. Chemical Engineering, Tokyo University of Agriculture and Technology (Japan), 2011. BEng Thesis: Glucose-assisted combustion sythesis of LiMn₂O₄ powders
4. Associate Degree in Chemical and Biochemical Engineering, Toyama National College of Technology (Japan), 2009. Graduation thesis: Hydrothermal synthesis of Zeolite particles

Contacts

Doris Nekesa Khaemba
IInstitute of Functional Surfaces (IFS)
Mechanical Engineering Department
University of Leeds, LS2 9JT
E-mail:d.n.khaemba@leeds.ac.uk

Supervisors: Prof. Ardian Morina and Prof. Anne Neville. 

Monthly reports

Monthly reports are available here

Secondments

1. Summer school at Diamond Light Source (UK) :  22-26 September 2014
2. TOTAL (France): 29 September - 24 october 2014

Presentations

1. Final meeting in University of Leeds, UK, 3-4 December 2015. Download the presentation here
2. Meeting in Diamond Light Source, UK, 27-29 April 2015. Download the presentation here
3. Meeting in SKF, Netherlands, 1-5 December 2014. Download the presentation here
   
4. Meeting in University of Leeds, UK, 30 June-4 July 2014.Download the presentation here
5. Mid-review project meeting in AC2T, Austria, 6-11January 2014. Focus group presentation (1st day) can be downloaded here. Mid-project review presentation (2nd day) can be downloaded here
6. Meeting in Ljubljana, Slovenia, 4-8 June 2013. Download the presentation here
7. Kick off meeting in Lyon, France, 3-6 December 2012. Download the presentation here
   

Conferences - Oral presentations

1. Doris N Khaemba, Anne Neville, Ardian Morina. Optimizing the friction performance of friction modifier, molybdenum dithiocarbamate (MoDTC) on ferrous surfaces. IMechE Mission of tribology, London, 2 December 2015
2. Ardian Morina, Anne Neville, Doris Khaemba, Yugal Rai. Study of MoDTC low friction tribofilm formation and removal.  International Tribology Conference (ITC), 16-20 September, 2015, Tokyo, Japan
3. Doris N Khaemba, Anne Neville, Ardian Morina. Application of Raman spectroscopy in understanding the mechanism of MoDTC decomposition under non-tribological conditions. TriboUK 16-17 April 2015, Loughborough, UK.
4. Doris N Khaemba, Anne Neville, Ardian Morina. Studying the growth of MoDTC tribofilms using micro-Raman spectroscopy. Leeds-Lyon Symposium on Tribology, 2-5 September 2014, Leeds (UK).
5. Doris N Khaemba, Anne Neville, Ardian Morina. Kinetic studies of MoDTC tribofilm formation using in-situ Raman spectroscopy. Society of Tribologists and Lubrication Engineers (STLE), Florida (USA), 18-22 May 2014.

Conferences - Poster presentations

1. Doris N Khaemba, Frederic Jarnias, Benoit Thiebaut, Anne Neville, Ardian Morina“ The influence of surface roughness and slide-roll ratio on tribological performance of MoDTC in steel/steel contacts” UK tribology Launch Event, London,  21 October 2015
2. Doris N Khaemba, Frederic Jarnias, Benoit Thiebaut, Anne Neville, Ardian Morina “Understanding the influence of tribofilm composition on friction performance of friction modifier, MoDTC” Leeds-Lyon Symposium on Tribology conference, 7-9 September, 2015, Lyon, France.
3. Doris N Khaemba, Frederic Jarnias, Benoit Thiebaut, Anne Neville, Ardian Morina. “Understanding the influence of tribofilm composition on friction performance of friction modifier, MoDTC” IMechE seminar, Leeds, June, 2015.
4. Doris N Khaemba, Anne Neville, Ardian Morina. “Micro-Raman studies on MoDTC tribofilms: the effect of rinsing samples and the size of the analyzed area” Leeds-Lyon Symposium on Tribology, 2-5 September 2014, Leeds (UK)
5. Doris N Khaemba, Anne Neville, Ardian Morina. “Application of micro-Raman spectroscopy in analysis of MoDTC tribofilms” TriboUK conference, Sheffield, 7 -8 April 2014.

Awards

• Best presentation prize. IMechE 24^th Mission of tribology. London (UK) 2^nd December 2015
• 1^st prize poster presentation. 41^st Leeds-Lyon Symposium on tribology, Leeds (UK), 2-5 September 2014
• 3^rd prize poster presentation. TriboUK 2014 Conference, Sheffield (UK), 7-8 April 2014

Publications 

1. Khaemba, D.N., Neville A., Morina, A. New insight on the decomposition of molybdenum dialkyldithiocarbamate (MoDTC): A Raman spectroscopic study.  Will soon be published in RSC advances.
2. Khaemba, D.N., Neville A., Morina, A. A methodology for Raman characterisation of MoDTC tribofilms and its application in investigating the influence of surface chemistry on friction performance of MoDTC lubricants. Tribology letters, Vol 59, pp. 1-17, 2015 

 

Extended Summary

Project title: In-situ analysis of tribofilms and interfacial process (P11)

Background

Presently, there is a strong push towards improving fuel economy in passenger cars. Poor fuel economy is attributed to high friction in various components within car engines. About 5% of the friction losses in internal combustion engines occur in the boundary lubrication regime where metal-metal contact is present. Lubrication in boundary lubrication regime is achieved by using lubricants containing chemically active additives which react with the surfaces to form thin films known as tribofilms. The formed tribofilms provide friction reduction and wear protection due to their physicochemical properties. Molybdenum DialkyldiThioCarbamate (MoDTC) is an additive added in engine oil mainly as a friction modifier. MoDTC reduces friction by degradation of the additive to form discrete MoS₂ at the tribocontact. There is however little knowledge on the degradation process of MoDTC in tribocontacts.

Approach

In this project tribochemical reactions that occur in steel/steel tribocontacts in the presence of MoDTC additive have been investigated. The main objectives of this project were:

• To study the influence of contact parameters on MoDTC decomposition, tribofilm formation and wear
• To study the relationship between the chemical composition of MoDTC tribofilms and friction performance.
• To investigate the durability of MoDTC tribofilms formed on steel substrates

Tribological tests were conducted using model oils comprising of MoDTC additive in Group III mineral base oil. Tests were conducted under unidirectional linear sliding and sliding/rolling conditions using high speed pin-on-disc (HSPOD) and MiniTraction Machine (MTM), respectively.  Friction results were obtained during tests. Tribological tests were conducted at varying temperature, MoDTC concentration, contact pressure, speed, surface roughness and slide-roll ratio. Space Layer Image Mapping (SLIM) was used to monitor the changes that occurred at the tribocontact with rubbing time. Optical microscopy and white light interferometry were used to study the wear of the tribopair after tribotests. Raman spectroscopy was used to conduct chemical characterisation of the rubbed surfaces. The potential for employing Raman spectroscopy in in-situ analysis of lubricated surfaces was investigated in detail.

Relevance to ENTICE programme

This project addressed tribochemical processes in boundary lubricated systems which was one of four key areas highlighted in the ENTICE project proposal

Main outcomes

Mechanical activation accelerates the rate of MoDTC degradation. Under tribological conditions, MoDTC decomposes to form three main compounds; MoS₂, MoS_x (x>2) and FeMoO₄.  The formation of MoS₂ in tribological contacts has been reported in literature before however presence of amorphous MoS_x and FeMoO₄ has not been reported. These findings are thus novel. MoDTC decomposition products formed at the tribocontact are dependent on the test conditions. A new mechanism for the degradation process has been proposed. This mechanism is shown in Figure 1. According to this mechanism, MoDTC decomposition process begins by rupturing of C‑S bonds in stage 1 forming molybdenum intermediate compound which undergoes intramolecular sulphonation forming amorphous MoS_x. In stage 2, MoS_x is converted to MoS₂. The conversion of MoS₂ to MoS_x is dependent on temperature, shear stress and MoDTC concentration. FeMoO₄ is formed from a side reaction of iron oxides on the steel surfaces with MoS_x.

The chemical composition of MoDTC tribofilms formed at the rubbing surfaces determine the friction observed in tribotests. In sliding/rolling conditions, low friction values (µ=0.04-0.05) were obtained when the tribofilms are composed of MoS₂ while higher friction values (µ=0.06‑0.07) was observed when the tribofilms were composed of a mixture of Fe₂O₃, Fe₃O₄, MoS_x and FeMoO₄ in varying proportions. Therefore, in order to achieve low friction ideal test conditions that ensure complete MoDTC decomposition to MoS₂ are necessary. Results from this study show that the ideal test conditions are; high temperatures (>80ºC), high MoDTC concentration (>0.3 wt%) and high contact pressure. Local contact pressures can be increased by having at least one of the tribopair having a rough finish (R_a >100 nm). Previous studies had reported that friction performance of MoDTC was dependent on contact parameters. It was however unclear how the contact parameters affected friction. In this study we have found that the link between contact parameters and friction lies on the chemical composition of MoDTC tribofilms.

MoDTC decomposition products formed at the rubbing surfaces results in formation of MoDTC tribofilms. MoDTC tribofilms grow rapidly within the generated wear scars until a limiting thickness is achieved. This limiting thickness is dependent on contact parameters. The presence of the tribofilms at the tribocontact provides wear protection to the steel substrate. There are however certain test conditions (low MoDTC concentrations, high temperatures) where MoDTC is incapable of providing wear protection. The reason for this is that MoDTC tribofilms formed at the tribocontact are removed as soon as they formed. This happens when the rate of tribofilm formation is lower than the rate of tribofilm removal. To ensure that tribofilm remains within the contact, the rate of tribofilm formation should be increased by using higher MoDTC concentrations.

The durability of MoDTC tribofilms when MoDTC additive is depleted within a tribocontact is dependent on the sliding configuration. MoDTC tribofilms were less durable in sliding/rolling contacts than in unidirectional linear sliding contacts. In unidirectional linear sliding conditions, MoDTC tribofilms formed on fresh steel samples were more durable than those formed on oxidised steel samples. This is because the adhesion of MoS₂ on iron oxide is less than on steel.