Elsevier Editorial System(tm) for Tribology International Manuscript Draft Manuscript Number: TRIBINT-D-15-00453R1 Title: Impact of ethanol on the formation of antiwear tribofilms from engine lubricants Article Type: Full Length Article Keywords: ethanol; engine lubricant; ZDDP; tribofilms Corresponding Author: Prof. H.L. Costa, PhD in Engineering Corresponding Author's Institution: Federal University of Uberlandia, Brazil First Author: H.L. Costa, PhD in Engineering Order of Authors: H.L. Costa, PhD in Engineering; Hugh A Spikes, PhD Abstract: This paper investigates the impact of ethanol fuel contamination of engine lubricants on the growth and stability of anti-wear tribofilms from ZDDP-containing lubricants. The MTM-SLIM technique was used to monitor the effects of blending 5%wt. of anhydrous and hydrated ethanol on tribofilm thickness in a fully-formulated group I oil and in a solution of ZDDP anti-wear additive dissolved in Group I base oil. Tribofilm thickness was significantly reduced by the addition of ethanol, and the reduction was more severe for hydrated than for anhydrous ethanol. When a tribofilm was allowed to form during rubbing using an ethanol-free oil, the subsequent addition of hydrated ethanol showed both the destruction of the pre-formed antiwear tribofilm and damage to the rubbed surfaces.

Universidade Federal de Uberlândia School of Mechanical Engineering

Laboratory of Tribology and Materials Campus Sta. Monica, Bl. 5K 38400-901, Uberlândia, Brazil, phone: +55 34 32394186

COVER LETTER – Tribology International

Dear Editor,

Attached is our manuscript entitled “Impact of ethanol on the formation of antiwear

tribofilms from engine lubricants”. We expect this work to be of interest to your

journal.

We believe that the topic of investigation is of high technological interest, since the use

of ethanol as vehicle engine fuel has increased for environmental reasons, both in flex-

fuel engines and as increasing amounts of ethanol are blended with gasoline in

conventional engines. In previous work we showed how contamination of the lubricant

during engine use with ethanol fuel affects friction and EHL film formation. This paper

uses a fundamental approach to investigate the impact of ethanol contamination on the

growth and stability of anti-wear tribofilms from lubricants containing ZDDP-based

anti-wear additives. For tests at 100oC, a recently-developed technique to monitor the

amount of ethanol in lubricants indicated total evaporation of ethanol at the end of these

tests. For tests at 70oC, a top-up methodology to maintain the blend at between 1% wt.

and 5% wt. ethanol was developed. In this case, the final thickness of the tribofilm was

significantly reduced by the addition of ethanol for both oils, and the reduction was

more severe for hydrated than for anhydrous ethanol.

In practical terms, our results show that the main effect of ethanol on the lubrication of

ethanol-fuelled and flex-fuelled engines might be not on friction, but on the wear of the

lubricated components and that any deleterious effects of ethanol on wear are likely to

be seen in conditions where significant levels ethanol build-up in the oil can occur, such

Cover Letter

Universidade Federal de Uberlândia School of Mechanical Engineering

Laboratory of Tribology and Materials Campus Sta. Monica, Bl. 5K 38400-901, Uberlândia, Brazil, phone: +55 34 32394186

as in stop-start city driving conditions where the oil temperature remains low.

This manuscript has been submitted solely to Tribology International, it is not

concurrently under consideration for publication in another journal, and if accepted it

will not be published elsewhere in the same form, in English or in any other language

without the written consent of the publisher. This manuscript, in its submitted form, has

been read and approved by all authors.

We will be looking forward to hearing from you. Thank you for the attention, and please

do not hesitate to contact us if you have any further queries regarding this submission.

Yours sincerely,

Henara Lillian Costa (corresponding author)

Laboratório de Tribologia e Materiais, Universidade Federal de Uberlândia, Campus

Sta. Monica, Bl. 5K 38400-901, Uberlândia, Brazil, phone: +55 34 32394036, e-mail:

ltm-henara@ufu.br.

TRIBOLOGY INTERNATIONAL

Confirmation of Authorship

As corresponding author, I, Henara Lillian Costa, hereby confirm on behalf of all

authors that:

1) The authors have obtained the necessary authority for publication.

2) The paper has not been published previously, that it is not under

consideration for publication elsewhere, and that if accepted it will not be

published elsewhere in the same form, in English or in any other language,

without the written consent of the publisher.

3) The paper does not contain material which has been published previously,

by the current authors or by others, of which the source is not explicitly cited

in the paper.

Upon acceptance of an article by the journal, the author(s) will be asked to transfer

the copyright of the article to the publisher. This transfer will ensure the widest

possible dissemination of information.

*Statement of Originality

Highlights

Contamination of ZDDP-containing lubricants with ethanol was investigated.

At 100oC, ethanol evaporated from the lubricant, not affecting the ZDDP

tribofilm.

At 70oC, a top-up methodology ensured the presence of ethanol throughout long

tests.

Ethanol reduced markedly the formation rate and final thickness of the tribofilm.

The presence of water in ethanol reduced the adherence of the tribofilm.

*Highlights (for review)

Dear editor, We thank the reviewers for their thorough work, which will certainly help to improve the quality of our contribution. We have tried to address all the comments from the three reviewers and have made changes

in the revised manuscript when appropriate. All significant changes in the manuscript have

been highlighted in red.

First of all, we apologize for the apparent lack of care when the text “reference not found” was found instead of the figure numbers. This was automatically done after the creation of the pdf file and we should have detected it. We have now entered all the references to figures manually, so the problem was solved. Below, we describe our answer to each comment in detail.

Reviewer #1: This is a well written and clear paper. The purpose and background are well stated and the data supports the conclusions. I would only suggest a few grammatical / editorial changes.

1) There are several places where the word "bi-fuels" is used and I believe this should be "bio-fuels".

The term “bi-fuel” is widely used in the United States to refer specifically to engines that use a mixture of two fuels at a constant proportion (especially 85% of ethanol and 15% of gasoline, or E85). Brazil introduced engines that can run on any proportion of ethanol and gasoline due to the lambda probe that senses the exhaust gases to detect the amount of ethanol in the fuel tank and that is the reason why they are called flex-fuel engines and not bi-fuel engines. The term “bio-fuel” is quite different and refers to a fuel originating completely from plant (or animal) origins. 2) The acronyms AE and HE are used to designate anhydrous ethanol and hydrated

ethanol. In the abstract on the cover page of the paper these terms are written out but in the asbtract that is part of the actual manuscript AE and HE are not defined. AE and HE are not used that often in the manuscript so I suggest just using the phrases anhydrous ethanol and hydrated ethanol in the entire maunscript rather than the acronyms. The authors can keep Table I as is since the acronyms are defined in the table.

We agree that the acronyms AE and HE are not widely used and we have removed them from the abstract. However, in the text, we have defined them when we first used and, after that, we decided it was better to always use AE and HE, because it was important to retain the link with figures and figure captions. Also, in a previous paper (cited) we use the same acronyms. We checked again the whole manuscript to make sure that the nomenclature used was consistent. 3) There are several times in the text where the data in a figure is about to be described

and the manuscript has a phrase "Error Reference source not found". I imagine this is a software error that has propagated into the PDF version of the manuscript. The authors need to check these errors. I found 4 of them in the Results section.

We apologize for the apparent lack of care. This was automatically done after the creation of the pdf file and we should have detected it. We have now entered all the references to figures manually, so the problem was solved.

*Response to Reviewers

Reviewer #2: This is an excellent paper that discusses the impacts of ethanol fuels on ZDDP film formation. Authors are requested to consider the following suggested changes to improve the manuscript: 1. It was shown that the tribofilm thickness of ZDDP dropped as soon as ethanol was added in the oil (Figure 10). Authors claimed that the presence of water in ethanol reduced the "adherence" of the tribofilm. It was not clear from the paper how the adherence of ZDDP tribofilm was affected by the presence of water. The water molecules may either break the zinc polyphosphate polymeric structures of tribofilm and hence gradually remove the ZDDP tribofilm, or the water molecules probably reacted with the species (FeS/FeO) that create strong bonding between the polyphosphate layers and a substrate. Authors may want to run nano-scratch tests on ZDDP tribofilms (Baseoil +ZDDP) in the presence ethanol to prove their "adherence" hypothesis. In figure 12, it was shown that 5% AE was added in the test oil after 75 minutes and this resulted in a gradual removal of ZDDP tribofilm. In other words, there was no abrupt removal of ZDDP tribofilm; rather it took about 50-60 minutes of additional rubbing to observe an obvious removal of tribofilm. The gradual removal of ZDDP tribofilm suggests that it was probably removed by micro or nano scale chemical attacks (e.g. etching) rather than macro scale detachment of tribofilm because of poor adherence. We agree with the reviewer that probably nano-scratch tests could provide some evidence about the adherence of the tribofilms and will consider this in future works. However such tests must be carried out at the test temperature since zinc phosphate glass properties are strongly temperature dependent. In the revised manuscript, we have added the possibility of chemical etching and drawn attention to the fact that the film removal appears to be both localized full removal and also slower and more uniform removal over the whole track. 2. Authors are requested to provide page numbers in any future manuscripts. As a matter of fact, it was extremely difficult to review the paper and make recommendations for edits without page and line numbers. We have added page numbers to the text. However, figures are asked to be added separately as individual files and therefore they do not have page numbers. In relation to line numbers, the format asked by the journal does not accept them. In other journals, line numbers are created automatically when the final pdf is generated, but this does not seem to be the case for Tribology International. 3. Introduction (Section 1) and Background (Section 2) can be combined and placed under Introduction. The two sections have been combined into a single Introduction section. 4. What were the conditions (load, speed, SRR, temperature, duration) of rubbing tests used between two Stribeck runs? It was not clearly mentioned in the methodology section. The conditions for the rubbing steps between the Stribeck curves had been described in step 6:

“The glass window was withdrawn from the steel surface and a rubbing test carried out for a set duration of 3 minutes at the set temperature. In this, the steel ball was loaded against the steel disk (31 N) and the two rubbed together at SRR = 50% and a low entrainment speed of 50 mm/s.” 5. Authors claimed that the film thickness was measured "in-situ". This is not true because the film thickness was measured by breaking the contact but without removing the specimens from the holder suggesting a "semi in-situ" film thickness measurement.

This depends what you means by in situ. Wikipaedia is not clear on this saying; “in situ may describe the way a measurement is taken, that is, in the same place the phenomenon is occurring without isolating it from other systems or altering the original conditions of the test”. In the Imperial College Tribology Group where this study mainly took place the term in situ is usually used to mean within the test rig and in contact to measurements taken within the rubbing contact itself. However different authors use different wordings and there is as yet no generally recognised convention. The most important thing is to make clear what is meant when the term is used and we have revised the text to ensure this is clear. We do not like the clumsy term semi-in-situ at all. 6. Paragraph 3.2: It was claimed that the test conditions provided 8 nm EHL film thickness. Authors are requested to provide the film thickness calculation or provide a reference where it was estimated using the EHL film thickness equation. In fact, we had used in error an estimation calculated for a similar lubricant at 100oC. We have calculated the film thickness using the (referenced) Dowson and Hamrock equation and presented the results in a new Table 2. 7. What was the source of scratches after removing of ZDDP tribofilm (Figure 12)? Three body abrasive particles were generated and caused the damage on the surface? Our tests showed that the metal surface became unprotected by the tribofilm after prolonged rubbing. Without the protection conferred by the tribofilm, wear debris become more likely, probably hardened by mechanisms such as oxidation. We refer to the abrasion scratches in the revised manuscript. 8. What was the decomposition/reaction pathway of ZDDP in the presence of ethanol? Authors need to explain why ethanol negatively affected ZDDP from a chemical viewpoint. This is a very complex question, since the actual ZDDP to tribofilm reaction pathway even in the absence of ethanol has not yet been proven, despite the enormous amount of work in the literature. We had already pointed out in the original manuscript that “Our current lack of an agreed molecular reaction scheme for ZDDP tribofilm formation makes it difficult to identify the precise way that ethanol might interfere with this process.” Despite this, we tried to present some plausible alternatives. First, we suggested that ethanolysis might compete with the normal ZDDP degradation, although we point out that it is not immediately obvious why ethanolysis should so markedly reduce ZDDP film formation. Other mechanisms that we present as possible candidates are competition for the steel surface by the polar ethanol molecule or partial dissolution of the ZDDP tribofilm as it forms on the rubbing surfaces. We think these more likely. In our work, we showed very clearly that the presence of ethanol inhibits ZDDP tribofilms, but the precise mechanism by which it interferes with tribofilm formation remains as an open question that deserves further investigation. 9. Why some of the SLIM images were complete (covered the entire circle) while others were partially covering the circle (window)? In SLIM the ball is back-loaded against a coated glass window to form the circular image shown. The position of the window can be adjusted to provide an image spanning all or part of the rubbed track on the ball. All the SLIM images aimed to show part of the track on the ball and part of the non-rubbed area of the ball, in order to verify if any thermal ZDDP film was formed. Sometimes the adjustment slips slightly so that less than or more than half of the image is rubbed region. The fact that some of the images show more of the track than others is unimportant to the analysis.

10. MTM SLIM measures film thickness assuming that the ZDDP tribofilm's refractive index is same as glass. Why should we believe that ethanol and ZDDP combination did not change the refractive index of tribofilm? If ethanol changes the optical properties of ZDDP triboflm, the results presented in the manuscript can be questionable. Authors are required to justify why this important issue was not discussed or verified. The calculations of film thickness used a refractive index = 1.6, which was based on values of the literature for zinc phosphate. The refractive index of the tribofilm would have to change a lot (to values less than 1.5) to change our findings and this is very unlikely indeed. Reviewer #3: Another nice detailed organized paper from Prof. Hugh Spikes group. I have following comments.

a. Table 1, please use the same symbol for viscosity at different temperatures Corrected. b. In Experimental Methods section, it is stated that standard spacer layer

interferometry is not suitable for ZDDP films and therefore SLIM is used. Please explain the difference for better understanding of the readers.

In spacer-layer interferometry, a contact is formed between the flat surface of a glass disc and a reflective steel ball. The glass disc is sputter-coated with a thin, semi-reflecting layer of chromium, and subsequently with a thin silica spacer layer. However, ZDDP tribofilms are only generated when direct, rubbing, solid contact occurs. The rubbing process, necessary for ZDDP tribofilm formation, rapidly abrades the coated glass disk. Therefore, standard spacer-layer optical interferometry cannot be employed to study ZDDP tribofilm formation. For such films, spacer layer interferometry (SLIM) has been adapted to investigate the rubbing tracks formed within a tribometer but not within the rubbing contact itself. We have tried to explain this better in the revised manuscript, as suggested. c. Section 4.1.1, para 6, " …as indicated in Fig. 2" should be Fig. 3. Similarly in the

following paragraph, it should be Fig. 4 instead of 3. Also, on para 11, Fig. 4 instead of 3. Please check.

Corrected. d. Editorial

* Introduction section, line 12, " … vehicles with flex-fuel engines capable of using 85% ethanol ….".

We changed the wording slightly different from suggested to keep the term “bi-fuel” widely used in the United States to refer specifically to engines that use a mixture of two fuels at a constant proportion (especially 85% of ethanol and 15% of gasoline, or E85). Brazil introduced engines that can run on any proportion of ethanol and gasoline due to the lambda probe that senses the exhaust gases to detect the amount of ethanol in the fuel tank and that is the reason why they are called flex-fuel engines and not bi-fuel engines.

* Introduction section , page 2, line 6, " ….the presence of ethanol in engine oil on friction." Corrected * Background section, para 3, line 6, "..O/S exchange was for ZDDP …". Please spell out what O/S stand for.

Done. * Materials section, para 3, line 3, "..fully dissolve, and tiny, ….." Corrected. * Experimental Methods section, subsection 1, line 4, "The Young's modulus …" Corrected. * Experimental Methods section, subsection 3, " ..initial image, the presence of interference colors can be used to estimate the thickness of …." Corrected. * Section 4.2.1, line 6, "…during rubbing was darker indicating a thicker film than that …" Corrected. * Conclusions section, para 3, first line is confusing. Suggested changing it to " For tests at … a methodology was employed to maintain the amount of ethanol in the lubricant at between 1 wt% and 5 wt% by topping-up the test chamber with ethanol every 30 minutes." Corrected.

1

Impact of ethanol on the formation of antiwear tribofilms from engine

lubricants

Henara L. Costa*

Laboratório de Tribologia e Materiais, Universidade Federal de Uberlândia,

Uberlândia, Brazil

Hugh Spikes

Tribology Group, Department of Mechanical Engineering, Imperial College, London,

UK

*Corresponding author, ltm-henara@ufu.br , Phone: +55 3432394036, Fax: +55 34

32394273

Abstract: This paper investigates the impact of contamination of engine lubricants with

ethanol fuel on the growth and stability of anti-wear tribofilms from ZDDP-containing

lubricants. The MTM-SLIM technique was used to monitor the effects of blending

5%wt. of both anhydrous and hydrated ethanol on tribofilm thickness in a fully-

formulated group I oil and in a solution of ZDDP anti-wear additive dissolved in Group

I base oil. Tribofilm thickness was significantly reduced by the addition of ethanol for

both oils, and the reduction was more severe for hydrated than for anhydrous ethanol.

When a tribofilm was allowed to form during rubbing using an ethanol-free oil, the

subsequent addition of hydrated ethanol showed both the destruction of the pre-formed

antiwear tribofilm and damage to the rubbed surfaces.

Keywords: ethanol; engine lubricant; ZDDP; tribofilms.

*ManuscriptClick here to view linked References

2

1. Introduction

The use of renewable fuels, in particular ethanol, has increased worldwide as an

alternative to petroleum-based gasoline and diesel derivatives [1].

In Brazil, the development of flex-fuel engines, which can run on any proportion of

ethanol and gasoline, means that ethanol fuel is in widespread use [2] and this has

helped the country to reduce carbon emissions [3], particulate mass concentration in

vehicle exhausts [3, 4] and dependence on fossil fuels. Today, half of the fuel used in

Brazilian automobiles is renewable [5] and according to the national association of

automotive vehicle manufacturers (Anfavea), over 85% of the vehicles produced in

Brazil since 2006 have been flex-fuel.

In other countries, the use of ethanol fuel has also increased in recent years. In the

United States most vehicles now use 10-15% ethanol (E10) but 11 million vehicles with

so called ―bi-fuel‖ engines capable of using 85% ethanol, 15% gasoline (E85) [6] were

sold in 2013. In 2007 the Energy Independence and Security Act established a target of

36 billion US gallons of renewable fuel use by 2022 [7] and, according to the US

Energy Information Administration (EIA), the production of ethanol in the US in 2014

was over 14 billion US gallons, which is more than twice the production in Brazil.

Sweden [8] and Belgium [9] have also adopted policies to increase the use of biofuels,

in particular of ethanol. This has encouraged considerable research to produce ethanol

from non-food sources such as cellulose.

However, the use of ethanol as fuel poses some tribological issues due to the possibility

of contamination of the lubricant with ethanol. Since ethanol has a much higher latent

heat of evaporation than gasoline, accumulation of ethanol in the lubricant can be

3

significant [10]. Considerable amounts of ethanol (between 6% and 25%) have been

measured in the sump after bench sequence tests [11, 12] and field tests [13]. Ethanol

accumulation is expected to influence lubrication and friction and has been suggested as

the main cause for severe wear that has been frequently reported by users of various

sizes and models of flex-fuel engines [14, 15].

A few studies have investigated the impact of ethanol in engine oil on friction. For a

fully formulated lubricant, although the presence of ethanol did not affect friction

measurements significantly, the combined presence of water and ethanol in the lubricant

reduced friction [10, 16].

In a previous paper [17], we reported the effects of ethanol on film formation and

friction. Elastohydrodynamic (EHL) film thicknesses were measured for lubricants

contaminated with ethanol over a wide range of speeds, to span lubrication regimes

ranging from boundary to EHL. These results were complemented with measurement of

Stribeck friction curves, to help understand the mechanisms by which the presence of

ethanol could affect friction. In order to separate the interaction of ethanol with the base

oil from that with other additives, both base oils and formulated oils without friction

modifiers were investigated.

It was shown that the addition of quite small proportions of ethanol decreased the

viscosity of both the base and formulated oils. This had the effect of slightly reducing

EHD film thickness and friction and causing the shift from full film to mixed

lubrication to occur at lower entrainment speeds. However, in slow speed, boundary

lubrication conditions, the effect of ethanol in the base oil was very different from that

of ethanol in the formulated oil. A boundary layer, which was not present in the ethanol-

free base oils, was found when the base oil was contaminated with ethanol. This

4

boundary layer may originate from oxidation of ethanol when in contact with a hot,

rubbing metal surface. In a formulated engine oil, the presence of ethanol interfered

with the formation of a thick boundary film by additives, reducing its thickness.

Consecutive Stribeck friction curves obtained for the non-contaminated formulated oil

showed a progressive shift to higher entrainment speeds, indicative of the growth of a

thick, rough boundary film, but this was supressed by the addition of ethanol [17].

These results suggested that ethanol may have a strong influence on the formation of

tribofilms.

When metal surfaces move against each other under low entrainment speeds so that

EHD films are very thin, significant rubbing contact of their asperities can occur. For

lubricants containing ZDDP antiwear additives, such rubbing has been shown to induce

the formation of thick tribofilms that protect the moving surfaces against wear [18, 19].

Zinc dialkyldithiophosphates (ZDDPs) are still used in the vast majority of commercial

lubricants, despite considerable efforts in the last two decades to replace them with

alternative antiwear additives since the presence of sulphur and phosphorus oxides and

metal salts in ZDDP is harmful to engine exhaust after-treatment devices [20].

The anti-wear performance of ZDDP appears to rely on the formation of thick anti-wear

films that act as mechanically protective barriers. Such films can be generated

thermally by immersion in heated solution (thermal films) at high temperatures

(generally above 150oC), but they can also form at much lower temperatures within a

rubbing contact (tribofilms). Actual sliding contact is necessary for the formation of

tribofilms, i.e., they do not develop in rolling contacts or when the hydrodynamic film

thickness is significantly greater than the surface roughness [21]. Comparison between

the chemistry of ZDDP thermal and tribofilms has shown that they have similar

characteristics [22], although tribofilms are mechanically stronger [23].

5

Various studies have investigated the thermal decomposition of ZDDP. In an influential

study, a wide range of species formed during ZDDP thermal decomposition were

identified, including several thionyl species, where the alkyl groups of ZDDP had

become linked to P by S atoms. This led to a proposed oxygen/sulfur (O/S) exchange

mechanism was for ZDDP reaction and film formation [24]. However the relevance of

this is questionable since it has been shown recently that zinc dialkylphosphates, that

have no S atoms in their molecules, form tribofilms very similar to those formed by

ZDDP [25].

The process of tribofilm formation is believed to be similar to the thermal degradation

process that occurs in thermal films, but driven to take place at much lower

temperatures. The drive mechanism is controversial, but the main candidates are

frictional heating due to sliding and/or pressure in the rubbing contact and mechanisms

involved in rubbing process itself (e.g., molecular strain, exoelectron or other particle

emission, free surface catalysis or molecular strain) [21, 26].

A study in 1993 [27] showed a negative impact of methanol on ZDDP tribofilms and

wear during ball-on-flat sliding tests, in particular at low temperatures. To explain the

higher wear rates for lubricants containing methanol, Olsson [28] proposed a

mechanism by which methanolysis acts directly on the ZDDP molecule, affecting the

reactions that would otherwise occur in the normal decomposition of ZDDP to supress

the formation of higher sulphides.

The current work aims to investigate the extent to which the tribological problems

frequently reported for flex-fuel engines may result from the effects of ethanol on the

formation and stability of protective ZDDP tribofilms. To do this, the effects on

tribofilm thickness and friction of blending 5%wt. of both anhydrous and hydrated

6

ethanol in a fully-formulated group I oil and in a solution of ZDDP anti-wear additive

dissolved in Group I base oil were monitored during prolonged rubbing tests..

2. Methodology

2.1. MATERIALS

One fully formulated oil was used in this work. The classification of this according to

the American Petroleum Institute (API) was SL and it is denoted SLB in this paper.

This lubricant is representative of a wide range of lubricants used today in Brazil but did

not contain a friction modifier additive so as to preclude the latter’s possible interactions

with ethanol. As well as containing a secondary ZDPP additive, chemical analysis of the

oil also suggested the presence of a calcium sulphonate-based detergent. In order to

separate the effects of ethanol from its interactions with other additives present in the

fully formulated oil, the corresponding base oil (Group I), to which only ZDDP was

added at a concentration of 0.08% wt. P, was also studied and it is denoted Base ZDDP

in this paper.

Small amounts of ethanol (5% wt.) were added to both lubricants to investigate the

effects of ethanol contamination on the formation of tribofilms. In Brazil, hydrated

ethanol (6.2 to 7.4 %wt. water) is used in flex-fuel engines, whereas in bi-fuel engines

in the US anhydrous ethanol is employed, since water is not soluble in gasoline. In this

study, the effects of the contamination of the lubricant with both hydrated (HE) and

anhydrous ethanol (AE) were investigated.

By heating samples of the blends on a hot plate at different temperatures, it was found

that 5%wt. AE was fully soluble in the lubricants at temperatures of 40oC and above,

but that 5% wt. HE did not fully dissolve and tiny dispersed droplets were observed

7

even when the blends were heated at 100oC. These were presumably due to the water

content. Viscosities and densities were measured for the oils and their blends with

ethanol using a SVM3000 Stabinger viscometer at 40oC, 70

oC and 100

oC. Table I

shows that the addition of ethanol reduced the viscosity of all the oils at 40oC and 70

oC,

but at 100oC it was not possible to measure the viscosity of the mixtures containing

ethanol due to the formation of a large number of bubbles in the measurement tube,

resulting from rapid evaporation of ethanol. The reduction in viscosity caused by AE

was larger than by HE, as observed in a previous study [17].

Table I. Viscosity measurements for the oils contaminated with ethanol,

where ρ is the density, ɳ is the kinematic viscosity, VI is the viscosity index, HE is

hydrated ethanol, AE is anhydrous ethanol, N.M. is not measurable.

Oil ρ at 15oC

(g/cm3)

η at 40oC

(mm2/s)

ɳ at 70oC

(mm2/s)

ɳ at 100oC

(mm2/s)

VI

Base ZDDP 0.877 30.55 10.76 5.25 102

Base ZDDP 5%HE 26.00 8.44 N.M. N.M.

Base ZDDP 5%AE 20.86 7.62 N.M. N.M.

SLB 0.870 97.16 30.81 13.621 141

SLB 5%HE 0.866 89.24 24.80 N.M. N.M.

SLB 5%AE 0.865 70.26 20.73 N.M. N.M.

2.2. EXPERIMENTAL METHODS

The growth of ZDDP tribofilms and the influence of the presence of ethanol on this

growth was monitored using optical interferometry. However standard spacer layer

interferometry [29] cannot be employed to study ZDDP tribofilm formation because

such films are generated only when direct rubbing, solid contact occurs and this process

rapidly abrades the optical coatings used for interferometry [21][30]. Spacer layer

interferometry also produces a single, averaged film thickness of a selected region of the

contact, while to study of ZDDPs it is advantageous to produce a map of film thickness

so as to identify the rubbed track region. Such maps can be produced by the technique

8

of spacer layer imaging (SLIM). To monitor ZDDP film formation, SLIM has been

adapted to investigate the rubbing tracks formed within a tribometer but not within the

rubbing contact itself [29]. In this equipment, called MTM-SLIM, a sliding/rolling

contact is generated between a steel ball and steel disk using a minitraction machine

(MTM), Figure 1. Film thickness is measured in situ within the test rig using an optical

attachment adapted to the rig. This optical attachment is a spacer layer-coated glass

window, against which the wear track on the steel ball is loaded when the latter is

stationary, without draining or cooling the system. An optical interference image is then

captured from the loaded glass window/steel ball contact and analyzed to determine the

thickness of separating film within this contact. An x-y-z stage allows the glass window

to be loaded at different positions on the wear track on the ball [30]. Tests were carried

out using the different blends at 70oC and 100

oC. The experimental sequence used in the

tests can be described as follows:

1. The test chamber, the glass disk and a new ball and disk were thoroughly

cleaned using toluene followed by analytical grade isopropanol. The balls were

19 mm AISI 52100 bearing steel with Rq of 11 ± 3 nm and the discs were AISI

52100 discs with Rq of 27 ± 3 nm. The composite surface roughness is thus 29

nm. The Young’s modulus of the balls and discs was 210 GPa

2. The glass window was loaded against the stationary steel ball. An interference

image of the window/ball contact was captured to determine the spacer layer

thickness for subsequent analysis.

3. Lubricant was added to the test chamber, which was then heated while rotating

the ball and disk in pure rolling with no applied load until the test temperature

was reached. After temperature stabilization, motion was then halted, the glass

window loaded against the track on the stationary steel ball, and an interference

9

image of the window/ball contact was captured. By comparison with the initial

image, the presence of interference colors can be used to estimate the thickness

of any solid-like tribofilm formed.

4. A Stribeck curve was obtained in which the ball was loaded against the steel

disk and friction was measured in stages, starting from an entrainment speed of

3500 mm/s, which was then continuously reduced in 25 steps down to 7 mm/s.

The normal load was 31 N, resulting in a maximum Hertz contact pressure of

0.95 GPa and a contact diameter of 250 μm. The slide-to-roll ratio (SRR) was

50%.

5. Motion was halted and the glass window was loaded against the stationary steel

ball. An interference image of the window/ball contact was captured for

subsequent analysis to determine film thickness.

6. The glass window was withdrawn from the steel surface and slow speed rubbing

carried out for a set duration of 3 minutes at the set temperature. In this, the steel

ball was loaded against the steel disk (31 N) and the two rubbed together at SRR

= 50% and a low entrainment speed of 50 mm/s.

7. Motion was halted and the glass window was loaded against the stationary steel

ball. An interference image of the window/ball contact was captured for

subsequent analysis to determine film thickness of the tribofilm.

8. A Stribeck curve was obtained as described in step 4 above.

9. Steps 6 to 8 were repeated for different rubbing times up to a few hours to obtain

a series of interference images and thus maps of the variation of ZDDP tribofilm

thickness on the ball over time, and to determine the effects of these films on

Stribeck curves.

10

EHL film thickness during the slow speed rubbing procedure described in step 6, can be

estimated from the Hamrock and Dowson equation for an EHL elliptical contact [31].

Using this equation, the test values Esteel = 210 GPa, U = 50 mm/s, W = 31 N and R =

9.5 mm, the viscosity values listed in Table 1 and taking the pressure viscosity

coefficients, estimated from [33], to be 19 GPa-1

at 70oC and 9.7 GPa

-1 at 100

oC, the

film thickness values listed in Table II were calculated. Values at 100oC are only

shown for the ethanol-free oils, since the viscosity values could not be measured for the

oils containing ethanol, as described in Section 2.1. The lambda ratios shown are the

ratios of the film thickness to the composite surface roughness. While such thin film

calculations can only be considered approximate, they do show that during the

prolonged rubbing phases of the tests, the contact operated in mixed lubrication

conditions, promoting ZDDP tribofilm formationon the rubbing surfaces.

Table II. Estimated film thickness values and corresponding λ ratios, where

HE is hydrated ethanol, AE is anhydrous ethanol, and N.E. is not estimated.

Fluid Film thickness (nm) λ ratio

70oC 100

oC 70

oC 100

oC

Base ZDDP 15 6 0.5 0.2

Base ZDDP 5%HE 12 N. E. 0.5 N. E.

Base ZDDP 5%AE 11 N.E. 0.4 N.E.

SLB 29 12 1.0 0.4

SLB 5%HE 25 N.E. 0.9 N. E.

SLB 5%AE 22 N.E. 0.8 N.E.

3. Results

3.1. EFFECTS OF ETHANOL ON THE FORMULATED OIL

3.1.1. Thickness of the tribofilms

Figure 2 shows interference images obtained from the wear track on the rotating ball

while rubbing a ball and steel disk together for four hours in ethanol-free formulated oil

11

SLB at 100oC. The position of the glass disk on the ball was such that the image

contained part of the wear track but also some region outside the wear track. These

images clearly show the presence of a tribofilm, even before the first rubbing step

(image corresponding to 0 minutes), just after the initial Stribeck curve was obtained.

This suggests that even the small amount of slow speed rubbing that occurs during the

measurement of a single Stribeck curve is sufficient to initiate some tribofilm growth.

Outside the wear track, no film is seen, showing that the formation of a thermal ZDDP

film is negligible at the test temperature. The thickness of the tribofilm increases with

the rubbing time and then stabilizes after around 60 minutes of rubbing. At a lower

temperature of 70oC, the interferometric images again show the formation of a tribofilm

from very early rubbing stages (Figure 3). After 30 minutes of rubbing, film thickness

reduces slightly and then seems to stabilize at a lower value than for 100oC.

From these images it is possible to estimate the mean film thickness of the tribofilms.

For this, a circular small area with a diameter of 25 µm was selected inside the wear

track and film thickness was averaged over this region. To calculate the actual film

thickness from the optical film thickness determined by optical interferometry, the

refractive index of the film is needed. As in previous work a value of 1.6 was used

based on values from the literature for zinc phosphate [22]. Clearly this is an imperfect

estimate but the actual value must lie in the range 1.55 to 1.65 depending on the precise

chemistry of the tribofilm and, even the extremes of this range of variation, would only

result in an error in the calculated value of ca 3%. The results of film thickness

measurements as a function of rubbing time are summarized in Figure 4 for all the

different test conditions using the formulated oil. This figure shows that at 100oC, the

fully-formulated SLB lubricant forms a tribofilm that grows rapidly, but then stabilizes

at around 35 nm after 1 hour of rubbing. At 70oC, the initial kinetics of film formation is

12

similar to that at 100oC, but after 1 hour of rubbing the thickness of the tribofilm

stabilizes at a significantly lower value of around 25 to 30 nm.

Initial tests showed that the addition of 5% wt. AE to the formulated oil (SLB) did not

impede the formation of the tribofilm in that similar interferometric images to those

shown in Figure 2 were observed. The mean film thickness values summarized in

Figure 4 show that the addition of AE does not change significantly either the kinetics

of formation of the tribofilm or the final film thickness.

However, since the test temperature (100oC) is above the boiling point of ethanol

(69oC), it is possible that the ethanol evaporates from the lubricant during the test. A

simple experimental technique was therefore employed to evaluate the amount of

ethanol at the end of each test. Ethanol can be extracted from hydrocarbon lubricants by

aqueous extraction [32] and after such phase separation, refractive index measurements

can be used to assess the amount of ethanol in the aqueous layer. Refractive index

measurements are very simple, fast and precise and small amounts of ethanol can

produce measurable variations in the refractive index of distilled water [33]. This

experimental technique used to measure the amount of ethanol in the lubricant has

already been described [17].

This refractive index-based technique showed that the negligible ethanol remained in

the lubricant at the end of the tests at 100oC, which suggests that the lack of effect of

ethanol on film formation at 100oC may be due to the fact the ethanol evaporates rapidly

at this temperature.

All subsequent tests were therefore carried out at 70oC, which is very close to the

boiling point of ethanol, while still being high enough for ZDDP to form tribofilms, as

indicated in Figure 3.

13

When 5%wt. AE was added to the formulated oil at 70oC, the interference images

showed that during the first10 minutes of rubbing, negligible tribofilm was formed, but

that after 15 minutes a film started to form and its thickness increased with rubbing time

to eventually approach the thickness formed by the ethanol-free lubricant (Figure 4).

Refractive index measurement of the extraction water showed that there was no ethanol

left in the oil at the end of the test at 70oC. This suggests that AE initially suppresses

ZDDP tribofilm formation but that this effect ceases when the AE evaporates.

It was thus important to measure the rate of loss of AE from solution in order to

compensate for it. To do this, a similar MTM sequence of tests was carried out, but

after every 15 minutes of rubbing the test was halted, the test chamber was drained, a

sample of the drained lubricant was collected, a fresh mixture of formulated oil + 5%wt.

AE was added, and the test resumed. This procedure was continued for the first 45

minutes of rubbing which the draining and lubricant replacement procedure was

extended to every 30 minutes.

The amount of ethanol in the sampled lubricants after each draining was measured using

refractive measurements of the extracted water and the results are shown in Table III.

After the first 15 minutes of rubbing, the amount of ethanol in the lubricant is

significantly reduced from 5% wt. to 1% wt. In the subsequent rubbing periods, the

reduction of ethanol after 15 minutes was smaller (1.7% wt. loss), but during the 30

minutes intervals it was larger. The reason that there was more evaporation of ethanol in

the first 15 minutes of rubbing is that this period actually involved initial temperature

stabilization and several stages of halting the rig, measuring film thickness and Stribeck

curve measurement, so that the actual test time (as opposed to rubbing time) was

considerably greater than 15 minutes.

14

Table III. Amounts of AE present in the drained lubricant (SLB + 5%AE)

at 70oC; new SLB + 5%AE blend was added to the test chamber after each

draining.

Rubbing time (min) 15 30 45 75 105

Refractive index, n 1.3350 1.3395 1.3390 1.3350 1.3360

Amount of ethanol (%wt.) 1 3.3 3.3 1 1.5

The rate of film build-up using this lubricant-change protocol is shown in Figure 4

(SLB 5%AE 70C change). It is evident that this approach has the effect of greatly

limiting ZDDP tribofilm formation. Unfortunately the protocol is not well-suited to

evaluate the effects of ethanol since, despite ensuring the presence of ethanol

throughout the test, it involves replacing the lubricant itself and would therefore prevent

possible chemical changes that ZDDP might undergo within the lubricant during

operation.

Therefore, a new methodology was developed based on the AE content measurements

made in the above test. In this, the amount of ethanol that was found to evaporate in the

first 15 minutes (4wt.%) was added to the test chamber after the first 15 minutes of a

test and this was then repeated after every 30 minutes of rubbing. This procedure

maintained the AE level at between 1 and 5% wt. throughout the whole test.

The interference images obtained for tests with formulated oil + 5% AE using this top-

up procedure are shown in Figure 5. They show that the intermittent addition of AE to

the test chamber to ensure that ethanol was always present prevents a thick tribofilm

from forming. Although some tribofilm is seen for rubbing times above 15 minutes, it is

much thinner than the tribofilm formed without ethanol, and is not uniform. The mean

film thickness calculation (Figure 4, SLB 5% AE top-up) shows a considerable scatter

of film thickness values after 15 minutes, with an average value of around 9 nm.

15

When HE was added using a similar top-up procedure, the interference images (Figure

6) show that although the presence of HE does not suppress completely the formation of

the tribofilm, the film formed has a patchy morphology and is again much thinner than

with the ethanol-free formulated oil. Additional tests with HE were also carried out

60oC (lower than ethanol’ boiling point) to try reducing evaporation. Those tests did not

involve the topping up procedure. The results in Figure 4 showed that although HE

supressed considerably the formation of the tribofilm for the first 60 minutes of test,

film thickness increased steadily after that, although at a very low rate. Since this could

be due to a slow evaporation of ethanol, it was decided that proceeding with tests at

70oC using the top up procedure to retain between 1 and 5% wt. of ethanol in the

lubricant was preferable to tests at 60oC without topping up.

3.1.2. Friction tests

The Stribeck curves taken between the rubbing steps for the formulated oil at 100oC

(Figure 7) show that , as the rubbing time increases, the curves shift to the right, i.e., the

transition between full film and mixed lubrication occurs at higher and higher

entrainment speeds. After 1 hour of rubbing, no further shift occurs. In the low-speed,

boundary lubrication region friction initially increases with rubbing time but then

remains constant after 2 hours of rubbing. It is noteworthy that low speed friction

increases with speed. The addition of ethanol at 100oC does not alter the trend observed

(Figure 7.b) but this is unsurprising since it has already been shown that the ethanol will

have evaporated rapidly during the test.

Similar behaviour is seen at 70oC for the neat SLB oil (Figure 8.a), although at this

temperature there is a region at high entrainment speed where the friction remains

unchanged. This is because of the higher viscosity at 70oC, which increases the

16

thickness of the elastohydrodynamic film and means that at high speeds there is full

film lubrication with EHD friction [34, 35].

For the blend of formulated oil and HE (SLB + 5%HE, Figure 8.b) the Stribeck curves

remain unchanged during the first15 minutes of rubbing, which is consistent with the

film thickness measurements that showed negligible tribofilm formation during this

period when ethanol was still present. After this, the curves start to shift to the right, so

that after 2 hours of rubbing their shape is very similar to those obtained for the

formulated oil without ethanol. This corresponds to the loss of ethanol from solution.

The Stribeck curves for the top-up tests where ethanol is added to maintain a reasonably

constant concentration are shown in Figure 8.c for AE and in Figure 8.d for HE. No

shift of the curves to the right is observed, suggesting that negligible stable tribofilm is

growing on the metal surfaces – at least not enough to increase the friction.

Additionally, the Stribeck curves show a minor limitation of the technique used to top-

up the test chamber with ethanol: the fluctuations in friction in the full

elastohydrodynamic region (high entrainment speeds) suggest small variations in

viscosity of the lubricant, probably caused by variations of the amount of ethanol.

Therefore the refilling technique does not maintain constant the amount of ethanol. In

future it would be beneficial to use a reflux condenser system to return evaporated

ethanol to the test chamber, but this is difficult because of the need to seal around the

rotating ball shaft. However despite this limitation, the technique is able to maintain a

reasonable amount of ethanol in the lubricant throughout the tests and thus identify, if

not accurately quantify, long-lasting effects of the addition of ethanol on the formation

of anti-wear tribofilms from formulated oil.

17

3.2. EFFECTS OF ETHANOL ON BASE OIL + SECONDARY ZDDP

3.2.1. Thickness of the tribofilms

The tests carried out using the solution of secondary ZDDP additive in Group I base oil

aimed to investigate the effects of ethanol on the ZDDP tribofilm specifically and avoid

other possible tribofilms such as from the detergent. They were carried out only at

70oC, since the tests with the formulated oil showed rapid and total evaporation of

ethanol for tests at 100oC. The interference images (Figure 9) show that the rubbed

track becomes darker than with formulated oil indicating the formation of a thicker film.

The calculated film thickness from these images, summarized in Figure 10, confirms

that the thickness of the tribofilm after 1 to 2 hours of rubbing is around 41 nm for the

neat lubricant (Base + ZDDP), whereas the formulated oil (SLB) showed a final film

thickness of 28 nm at 70oC. These results agree with the trend already noted in the

literature of reduction of the tribofilm thickness when other additives, in particular

dispersants, are present in the oil formulation [21, 36].

For the tests with Group I base oil + ZDDP, the addition of ethanol (both AE and HE)

followed the procedure of topping up the test chamber with 4% of ethanol after every 30

minutes of test, in order to maintain the ethanol concentration at between 1 and 5%

throughout a test.

The interferometric images of the rubbing tracks show that the addition of AE delays

the formation of the tribofilm (Figure 11). A film of considerable thickness is formed

only after 30 minutes of rubbing. The final thickness is significantly reduced compared

with ethanol-free ZDDP. Calculation of the mean film thickness (Figure 10) shows that

the final film thickness reduces from around 41 nm for the neat lubricant to around 17

nm when AE is added.

18

The effect of the addition of HE on the formation of the tribofilm was more severe than

for AE. Repetitions of the tests resulted in slightly different behaviour, but the trend was

always the same; the HE prevented the formation of a stable ZDDP tribofilm between

the rubbing surfaces. The interferometric images (a typical example is shown in Figure

12) indicate that the initial formation of the tribofilm was also delayed, as observed for

AE. A film of considerable thickness is formed after 1 hour of rubbing but further

rubbing results in removal of this tribofilm and in the generation of quite severe

scratches in the rubbing track. It is possible that as the film starts to fail, wear debris is

generated and causes abrasive scratches. This severe removal of the tribofilm is

believed to be controlled by the water present since tests using AE + 7% of distilled

water (not presented) resulted in similar trends to those obtained with HE. The amount

of water present in HE in Brazil varies between 6.2 and 7.4 % wt.

Another question to be answered was whether the presence of HE would remove the

tribofilm in the case that it was already fully developed. To answer this, a series of

experiments were carried out at 70oC for base oil + ZDDP without ethanol for 75

minutes rubbing time. This time was found previously to be sufficient to allow the

formation a stable and thick tribofilm of around 41 nm. After this, 5% wt. HE was

added to the test chamber and topped up with 4% wt. HE every 30 minutes.

The interference images obtained using such test sequence (Figure 13) show that before

the addition of HE a thick tribofilm grows rapidly on the ball, achieving a thickness of

around 42 nm after 75 minutes of rubbing. On the addition of HE, film thickness starts

to reduce and there is strong evidence of the removal of the film due to the presence of

HE and as well as the formation of scratches in the rubbing track on the ball. Removal

appears to begin immediately and involve both localised regions where film is

completely lost as well as slower removal across the whole rubbed track.

19

3.2.2. Friction tests

Figure 14 shows Stribeck friction curves using ZDDP solution in base oil. As with the

formulated oil at 70oC, before any rubbing it was not possible to reach full boundary

lubrication since the contact remained in mixed lubrication even at the lowest

entrainment speed attainable (Figure 14.a). In the high entrainment speed region,

friction coefficient shows an approximately constant plateau, typical of full-film fluid

lubrication. As with the formulated oil, the Stribeck curves shift progressively to the

right in the test stages between 3 minutes of rubbing and 1 hour so that the boundary

friction region becomes evident. After one hour there is no significant further change in

the Stribeck curves.

For the tests with AE in which the ethanol concentration was maintained by topping up

during a test, the Stribeck curves remain unchanged for the first 15 minutes of rubbing

(Figure 14.b). The curves obtained after 30 minutes and 1 hour of rubbing show a shift

to the right but after that the curves remain unaltered and a boundary regime seems to

operate at low speeds. In the region of high speeds, where a full elastohydrodynamic

film develops, it is again possible to identify some variation in friction coefficient,

suggesting small variations in viscosity of the lubricant, probably because the refilling

technique did not maintain the amount of ethanol precisely constant. The behaviour of

the Stribeck curves when HE was present in the lubricant was much more complex

Figure 14.c). They remain largely unaltered up to 15 minutes of rubbing and then show

a shift to the right at 30 minutes, as was also observed with AE. However, after that

they show very high values of friction coefficient in the low speed region. This

behaviour was observed for the Stribeck curves corresponding to interferometric images

that showed that the tribofilm was being removed and may originate from the presence

20

in the contact of wear debris from the removed tribofilm. The final Stribeck curves,

which correspond to the images where the film was very thin and patchy, are then

similar to those obtained in the initial stages of the test where there were very thin

tribofilms. For the tests where 5% wt. of HE was added only after 75 minutes of

rubbing (Figure 14.d), the Stribeck curves show the characteristic shift to the right

before the addition of HE, corresponding to the growth of a tribofilm. When 5% wt. HE

is added, the curves start shifting back to the left, confirming the removal of the

tribofilm, particularly after 135 minutes of rubbing. After 165 of rubbing, when the film

is almost completely removed, very high friction coefficients are found for the region of

low speeds, again perhaps suggesting the presence of wear debris between the rubbing

surfaces.

4. Discussion

The tests with the ethanol-free lubricants (both formulated oil and base oil + ZDDP)

showed the formation of thick tribofilms. For the secondary ZDDP dissolved in Group I

base oil, the thickness of the tribofilm increased with rubbing time and then reached an

approximately constant level, as observed in other work [37]. For the formulated oil,

film thickness increased with rubbing time, achieved a maximum and then reduced

slightly to reach a plateau, as observed in [21] for a secondary ZDDP dissolved in

Group II base oil. The reason why the thickness of the ZDDP tribofilm reaches a

plateau value is not yet well established, but three different explanations have been

tentatively proposed. The first is that the plateau represents a balance between film

formation and film removal [38], although this is unlikely since tests where the ZDDP

solution is replaced by base oil part-way through a test indicates no loss of ZDDP film,

suggesting that under these test conditions the ZDDP film removal rate is negligible

21

[29]. A second possibility is that the diffusion of species (probably Fe atoms) drives

film growth, but when a certain film thickness is reached this diffusion becomes

negligible [37]. The third hypotheses is that the pads that form the tribofilms are

plastically squeezed between two effectively rigid steel plates, so that their final

thickness is determined by the combination of applied pressure and the yield stress of

the tribofilm.

For the formulated oil, the thickness of the tribofilm increased with temperature, as

normally reported in the literature [20, 29]. One possible explanation of this behaviour

is that viscosity increases at lower temperatures, increasing the thickness of the

elastohydrodynamic film so that less solid to solid contact occurs. Since the formation

of tribofilms requires direct solid contact to occur, the thinner tribofilms measured at

lower temperatures could be due to the presence of thicker elastohydrodynamic films

and therefore higher lambda (λ) ratios. However, Fujita [29] blended Group II base oils

to obtain the same viscosity (and thus the same λ ratio) at different temperatures and

found that there was still an increase of ZDDP tribofilm thickness with temperature.

Therefore, the difference in tribofilm thickness due to temperature is probably mainly

associated with a more rapid degradation/reaction of ZDDP at higher temperatures.

The tribofilms formed by the formulated oil were thinner than those for ZDDP

dissolved in the base oil. The presence of other additives, in particular of detergents [39,

40] and dispersants [37], has been reported to reduce the thickness of the ZDDP

tribofilms. The chemical analysis of the formulated lubricant used in the present work

suggested the presence of a calcium sulphonate-based detergent. Some researchers have

suggested that detergents interact with ZDDP in the oil before it reaches the rubbing

surfaces but it was found that this interaction was negligible at low temperatures and

only became significant at temperatures as high as 177oC [41]. During pin-on-disc tests

22

at 100oC, calcium-containing detergents retarded thermo-oxidation decomposition of

ZDDP in the lubricant [39]. The possibility was also raised that the reduction in

thickness of the tribofilm could be partially attributed to the cleaning effect of the

detergent. Other researchers have suggested that ZDDP and detergents might compete

for the rubbing surface [42]. It was proposed in [40] that calcium carbonate can

associate with the overbased calcium sulfonate in the rubbing surface along with the

ZDDP tribofilm. However, despite the reduction in the thickness of the tribofilm formed

on the rubbing surfaces, lubricants containing detergents and ZDDP result in less wear

of the rubbing surfaces than ZDDP alone dissolved in the corresponding base oil [39].

For both ethanol-free lubricants investigated in this work (base + ZDDP and SLB),

prolonged rubbing in film conditions and consequent formation of a tribofilm resulted

in Stribeck friction curves shifting to the right, i.e., the transition between boundary and

full film lubrication occurred at progressively higher entrainment speeds. The ZDDP

tribofilms had a pad-like structure, which was therefore very uneven [30] so the

tribofilm increased the effective roughness of the surfaces. It has been suggested that

this is why a higher entrainment speed is then needed to generate a fluid film between

the moving surfaces. This behaviour has been widely reported for lubricants containing

ZDDP [21, 43]. Another point is that for the formulated oil, the low-speed boundary

friction coefficient first increased and then decreased, as observed in [37] for ZDDP

dissolved in Group II based oil. These authors argued that this difference in boundary

friction may reflect changes in the predominant alkyl chain on the ZDDP film surfaces.

However, in the present work, for the tests with secondary ZDDP dissolved in Group I

base oil at 70oC, friction in the boundary regime was approximately constant. It is also

noteworthy that in Figure 7, friction increases with entrainment and thus with sliding

23

speed in the boundary lubrication regime. This behaviour probably originates from

linear alkyl sulphonate detergent molecules present in the lubricant.

The contamination of both the formulated oil and ZDDP solution with ethanol

drastically slowed the rate of formation of the tribofilm and resulted in significantly

thinner films even after long rubbing periods. This observation was in agreement with

the consecutive Stribeck curves obtained after increasing rubbing times in that the shift

of the curves to the right occurred only after much longer rubbing periods, suggesting a

delay in the formation of the tribofilm. Also, even after long rubbing periods, this shift

was less intense than for the ethanol-free lubricants, correlating with the presence of

thinner films.

One plausible explanation for this phenomenon is that ethanol might interfere with the

decomposition of ZDDP, which is essential for the generation of the tribofilm. The

presence of ethanol in the lubricant could lead to ethanolysis of ZDDP, where the alkyl

groups in the ZDDP molecule might be exchanged by ethyl groups. This ethanolysis

would then compete with the normal degradation of ZDDP. Such a mechanism has

been proposed by for the contamination of lubricant with methanol [28]. It is interesting

to note that in their study the authors found no negative interaction of methanol with

ZDDP for tests at 100oC, although at 40

oC the presence of methanol increased wear and

strongly reduced tribofilm formation. Although they raised the possibility of some

methanol evaporation at 100oC, they attributed the difference mainly to a possible

competition between methanolysis and ZDDP degradation, where methanolysis would

prevail at lower temperatures and ZDDP degradation would prevail at higher

temperatures. However, their tests used an open test chamber and had a duration of 2

hours. In the present work, where the lubricant was contained in a near fully-sealed test

chamber, it was found that ethanol completely evaporated after around 30 minutes. The

24

boiling point of methanol (65oC) is even lower than that of ethanol (69

oC) so it is

probable that evaporation of methanol was the main reason for the lack of a negative

effect of tibofilm formation by methanol at 100°C.

It is also not immediately obvious why ethanolysis should so markedly reduce ZDDP

film formation. It is more likely in the authors’ view that polar ethanol competes for the

steel surface or partial solubilises the ZDDP tribofilm as the latter forms on the rubbing

surfaces. Our current lack of an agreed molecular reaction scheme for ZDDP tribofilm

formation makes it difficult to identify the precise way that ethanol might be involved

with and inhibit the ZDDP reaction process.

The negative effect on the formation and stability of the tribofilms was more severe for

HE fuel than for AE, particularly in the absence of other additives (i.e. for ZDDP

dissolved in base oil). This must be due to the presence of water, which does not appear

to suppress completely the formation of the tribofilm, but to make it more easily

removed by rubbing. Some of the Stribeck curves for the tests containing HE showed

very high friction in the low speed region, which seemed to coincide with the stages

where the tribofilm was being removed.

It is possible that the topping up procedure increases significantly the amount of water

in the lubricant, since water will not evaporate as easily as ethanol. Addition of 5%wt.

HE to lubricant includes approximately 0.35 %wt. water, but in a three hour test with

seven 4% wt. HE top-ups but this might accumulate to reach 2.0% wt., assuming no

water at all is lost by evaporation. On the other hand, the tests where a stable tribofilm

was formed and then HE was added showed an immediate removal of the tribofilm by

rubbing due to the presence of HE, even before any topping up procedure.

25

When the composition and structure of well-developed ZDDP tribofilms were

investigated in [19], the presence of some water in the tribofilm near the rubbing

surfaces was proposed, which was associated with the great affinity of phosphate

glasses for possible contaminant water. If this is the case, such water might be

responsible for reducing adherence of the film to the surface, resulting in the localised

film removal seen, though alternatively this might result from local etching.

Another point to be considered is hydrolysis. (Spedding and Watkins 1982) showed that

the decomposition of ZDDP in solution and ZDDP tribofilm formation is an hydrolytic

process, so it might be inferred that at least traces of water are likely to be beneficial.

However the use of SEM and XPS to analyse how the addition of 2% wt. of water

affected the wear tracks produced in AISI 52100 steel after four-ball tests [44] showed

that water inhibited the growth of the ZDDP tribofilm and reduced the length of poly-

phosphate chains. In further work [45], the use of a ball-on-disc tribometer under

boundary lubrication/extreme pressure conditions assessed the effects of added water

and environmental humidity on the performance of ZDDP dissolved in polyalphaolefin

base oil. The effect of water on friction was very small, but it increased wear

considerably. SEM and XPS analysis of the tribofilms formed in the wear tracks

showed that the presence of 1% wt. water inhibited the growth of the ZDDP tribofilm

and caused only phosphate chains to form. The authors claimed that hydrolysis led to

the depolymerisation of longer chain phosphates, resulting in phosphates with shorter

chain lengths.

Whatever the precise mechanism, results here presented clearly show that the presence

of both anhydrous ethanol and ethanol with water are deleterious to the formation and

stability of a thick ZDDP tribofilm. Our previous results have shown that the presence

of ethanol does not have a negative effect on EHD fluid film and friction. It seems

26

therefore that in practical terms the main effect of ethanol on the lubrication of ethanol-

fuelled and flex-fuelled engines might be not on friction, but on the wear or seizure of

the lubricated components. However, wear tests need to be carried out to confirm the

negative impact of ethanol on wear of parts using ZDDP-containing lubricants. It

should also be noted that, based on this work, any deleterious effects of ethanol on wear

are more likely to be seen in conditions where significant levels ethanol build-up in the

oil can occur, such as in stop-start city driving conditions where the oil temperature

remains low than in long distance, high speed driving.

5. Conclusions

The effects of ethanol contamination on the thickness of the tribofilms and on Stribeck

curves have been investigated for fully formulated oil and for a simple solution of

ZDDP in Group I base oil. For both lubricants, thick anti-wear tribofilms were

developed due to rubbing in the absence of added ethanol. The tribofilms were thicker

at higher temperatures and for ZDDP dissolved in the base oil compared to the

formulated oil. The formation of the tribofilms caused Stribeck curves to shift to the

right, i.e. to higher entrainment speed, as widely reported in the literature for tribofilms

formed from ZDDP additives.

The addition of ethanol to formulated oil at 100oC did not affect the formation of the

antiwear tribofilm, but it was shown that the ethanol rapidly and fully evaporated from

the lubricant at this temperature. This emphasises the importance when studying ethanol

in lubricants of monitoring and maintaining the appropriate ethanol concentration

throughout testing.

27

For tests at 70oC, a methodology was employed to maintain the amount of ethanol in the

lubricant at between 1 wt.% and 5 wt.% by topping-up the test chamber with ethanol

every 30 minutes. Using this methodology, ethanol reduced the rate of formation of the

tribofilm very markedly and the final thickness was much less than for ethanol-free

lubricants. These observations of the tribofilm behavior were corroborated by Stribeck

friction curves, where the shift to high entrainment speeds was delayed and less intense

when ethanol was present.

The presence of HE (as used in Brazil) resulted in a less stable tribofilm, which was

more easily removed than when AE was present, and also evidence of surface damage.

Tests where a stable tribofilm was formed in the absence of ethanol and then HE was

added showed strong evidence of the removal of the tribofilm by rubbing in the

presence of HE.

Acknowledgements

The authors are grateful to Petrobras, in particular to Dr. Luis Fernando Lastres, for

providing lubricant and hydrated ethanol fuel samples and to CNPq and Fapemig

(Brazil) for financial support to H.L.Costa.

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31

Figure Captions

Figure 1. Scheme of the MTM-SLIM test.

Figure 2. Sequence of interference images from wear track for SLB at

100oC during increasing rubbing times.

Figure 3. Sequence of interference images from wear track for SLB at 70oC

during increasing rubbing times.

Figure 4. Calculated average film thicknesses for the Group I formulated oil

(SLB) at different temperatures and amounts of ethanol as a function of rubbing

time.

Figure 5. Sequence of interference images from wear track for SLB +5%AE

at 70oC after increasing rubbing times; the lubricant was topped up with 4% wt. of

ethanol every 30 minutes of test to maintain its level.

Figure 6. Sequence of interference images from wear track for SLB +5%HE

at 70oC after increasing rubbing times; the lubricant was topped up with 4% of

ethanol every 30 minutes of test to maintain its level.

Figure 7. Consecutive Stribeck curves after increasing rubbing times for the

formulated oil at 100oC: (a) neat SLB; (b) SLB + 5%AE.

Figure 8. Variation of Stribeck curve with rubbing time for the formulated

oil at 70oC: (a) neat SLB; (b) SLB + 5%HE, no change in lubricant; (c) SLB +

5%AE; lubricant topped up with 4%wt of AE during test; (d) SLB + 5%HE;

lubricant topped up with 4%wt of HE during test.

Figure 9. Sequence of interference images from wear track for Base +

ZDDP at 70oC after increasing rubbing times.

Figure 10. Effect of the addition of ethanol on the calculated average film

thicknesses for Base + ZDDP at 70oC as a function of rubbing time.

Figure 11. Sequence of interference images from wear track for Base +

ZDDP + 5%AE at 70oC after increasing rubbing times; lubricant topped up with

4%wt of AE every 30 minutes of test to maintain its level.

Figure 12. Sequence of interference images from wear track for Base +

ZDDP + 5%HE at 70oC after increasing rubbing times; lubricant was topped up

with 4% of HE every 30 minutes of test to maintain its level.

Figure 13. Sequence of interference images from wear track for Base +

ZDDP at 70oC; 5%HE was added after 75 minutes of rubbing and then lubricant

was topped up with 4% of HE every 30 minutes of test to maintain its level.

Figure 14. Variation of Stribeck curve with rubbing time for Base + ZDDP

at 70oC: (a) neat Base + ZDDP; (b) Base + ZDDP + 5%AE, lubricant topped up

with 4%wt of AE during test; (c) Base + ZDDP + 5%HE, lubricant topped up with

4%wt of HE during test; (d) Base + ZDDP; 5%HE was added after 75 minutes of

rubbing.

Figure 1. Scheme of the MTM-SLIM test.

Figure 2. Interference images, SLB, 100oC

Figure 3. Interference images, SLB, 70oC

Figure 4. Calculated average film thicknesses, SLB

Figure 5. Interference images, SLB +5%AE at 70oC

Figure 6. Interference images, SLB +5%HE at 70oC, top-up.

Figure 7.a Stribeck curves, 100oC. neat SLB

Figure 7.b. Stribeck curves, 100oC, SLB + 5%AE

Figure 8.a Stribeck curves, 70oC, neat SLB

Figure 8.b Stribeck curves, 70oC, neat SLB+5%HE

Figure 8.c Stribeck curves, 70oC, neat SLB+5%AE, top-up

Figure 8.d Stribeck curves, 70oC, neat SLB+5%HE, top-up

Figure 9. Interference images, 70oC, Base+ZDDP.

Figure 10. Calculated average film thicknesses, 70oC Base + ZDDP

Figure 11. Interference images, 70oC, Base+ZDDP+5%AE

Figure 12. Interference images, 70oC, Base+ZDDP+5%HE

Figure 13. Interference images, 5%HE added after tribofilm forms

Figure 14.a Stribeck curves, 70oC, Base+ZDDP

Figure 14.b Stribeck curves, 70oC, Base+ZDDP+5%AE

Figure 14.c Stribeck curves, 70oC, Base+ZDDP+5%HE

Figure 14.d Stribeck curves, 5%HE added after 75 min. rubbing