プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果

技術論文

Friction Reduction Effect Through Alternative Lubricants on
Aluminum Alloy Coated via Plasma Electrolytic Oxidation

プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果
Takuya NAKASE
中 瀬 拓 也

Abstract
From the view point of the environmental
protection, technical innovation for our hydrauliccomponent is required as well. Focusing onmaterial techniques, environmentally accep­ta­ble
lubricants （EALs） and light weight material
would be ap­plied for new design hydraulics, inorder to protect the environ­ment against oil spillor leakage and to reduce CO2 emissions. Many
researchers have been studying lubricants on
steel for many decades, however the lubrication
research on light weight materials（e.g., aluminiumalloy）is very limited. For example, novel surface
treatments using plasma electrolytic oxidation
せを使用した油圧機器の開発が予想されることから， （PEO）technique significantly improves wear
resistance of aluminium alloys, however systematic
これらのトライボロジー特性を把握しておくことはstudy on the interaction of EALs lubricating
意義のあることである．
aluminium alloy coated by PEO remained so far
本報では，EALs（代表的な摩耗防止剤を混合しlimited. Since a hydraulic system using PEO andたポリアルキレングリコール（PAG）やTMPエス
要旨
昨今ますます関心が高まる環境保護の観点から，油圧機器においても技術革新が要求されている．材
料技術において，有害物質による環境汚染対策には作動油を生分解性や無毒性を有する環境対応型作動油（EALs）へ代替する事，CO2排出削減対策には鉄鋼系材料から軽量材料への転換が考えられる．ところが，鉄鋼に対する潤滑油は長年研究されてきているが，軽量化が期待できるアルミニウム合金に対する研究事例は圧倒的に少ない．例えば，プラズマ電解酸化（PEO）処理をアルミニウム合金に施すと飛躍的に耐摩耗性が向上するが，PEOとEALsの相互作用に関した系統的評価はなされていない．以上のようなEALおよびアルミ摺動部材の組み合わテル）潤滑下における，PEO処理されたアルミニウム合金の摩擦摩耗特性について述べる．PEOに対するEALsの潤滑性は，ポリアルファオレフィンに最も広く使われる摩耗防止剤ZnDTPを添加した混合油より優れた結果が得られた．

EAL is expected, it is essential to investigate thetribological properties of such materialcombination.
In this paper, the friction and wear properties ofan aluminium alloy coated by PEO underlubricating with EALs（polyalkylene glycol（PAG）
or TMP-ester additivated with common anti-wearadditive）are reported. The lubricity of the EALs
on the PEO was better than polyalphaolefin withZnDTP.

―4―

KYB技報１第51号

2015―10

crystal­line alumina-type ceramic film. In general,alumina is chemically stable against most liquids,

INTRODUCTION

Hydraulic fluids/lubricants are an element ofmechanical parts and they are composed of baseoil and various additives. The lubricating
mechanisms have been studied for many decades
mainly for frequent materials used in industries
such as steel and cast iron. A well-known antiwear additive is zinc dialkyldithiophosphate, whichmost effectively prevent wear of sliding parts1）.
Recently, due to the environmental issues,
functional additives with low aquatic toxicities are
required for environmentally acceptable lubricants（EALs） in order to develop new lubricanttechnologies2）3） with equivalent performance. In
the aim of promoting and distinguishing EALs
from “classic” lubricants, environmental labels areused（Fig. 1）. In order to meet the environmental
criteria, esters are commonly used as base oils,
which are ready（ultimate）biodegradable and lesstoxic to aquatic species. Polyalkylene glycols（PAGs）are also used in specific applications andmeet the eco-tox criteria. The functional andenvironmental profile of PAGs can be individually
tailored through the backbone of the base oilmolecule4）.

Fig. 1 Environmental labels

In mechanical parts, various materials are used
like steel, aluminium alloys, titanium alloys,polymers and their composite materials, and so on.
Except for steel, alu­m inium alloys are mostcommonly used light-weight material, which
possesses excellent mechanical and physicalproperties for machine parts. However, alumi­nium

whereas it might specifically interact with polar
molecules of lubricating oils due to its ionicbonding6）.
The aim of the present study was to investigate
influence of EALs on aluminium based alloys toidentify candidates for high loaded sliding parts
under boundary lubricating condition. In thispaper, the effect of formulated oils with typicalanti-wear additives on tribo­lo­gical properties ofPEO film on A6061 alloy will be discussed.

２EXPERIMENTAL

2. １ Lubricants
A polyalphaolefin（PAO）, a polyalkylene glycol（PAG）and a trimethylolpropane ester（TMP）
were used as base oils for test formulations usedin this study. The same ISO viscosity grade（ISOVG46）was selected for all base oils.
Esters and polyglycols contain molecularoxygen. TMP has three ester bonds（-COO- intriesters）, which are located mainly in the centerof the molecule. In contrast, polyglycols have an“ether” link（C-O-C）, e.g., an oxygen polarity, ineach monomer of the whole backbone.
Consequently, it is likely that the polarities of theionic bonds of the alumina will interact with thepolarities of the oxygenates in the backbones ofthe esters and polyglycols.
In order to investigate the effect of additives, awidely used anti-wear additive zincdialkyldithiophosphate（ZnDTP）and an organic
sulfur containing additive dibenzyldisulfide（DBDS）were selected and blended each by 1wt.% into the three base oils to make 6 different

alloys are soft and limited due to the poor wearresistance. When aluminium alloy is requested assliding material, anodic oxidation processes can beapplied. For further high wear resistance, thenovel plasma electrolytic oxidation （PEO）
represents an additional option. The PEO canincrease surface hardness of aluminium alloys up
to HV20005） by forming a dense and thin nano―5―

CodePAO
PAG
TMP
PAO+ZnPAG+ZnTMP+Zn
PAO+S
PAG+S
TMP+S

Table 1

Oil samples

Base oil
PAO
PAG
TMP
PAO
PAG
TMP
PAO
PAG
TMP+ZnDTP+1wt.-%+1wt.-%+1wt.-%-+DBDS+1wt.-%+1wt.-%+1wt.-%

プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果

additivated test formulations（Table 1）.

acquisition system. After the test both upper andlower specimens were cleaned with the petroleum

2. ２ Plasma electrolytic oxidation coatings
A plasma electrolytic oxidation（PEO）coating
was deposited on A6061-T6 by Keronitespirit. The wear scar on the roller was measured
with an optical microscope and the disk with aprofilometer.

International Ltd. The thickness of the asdeposited film was approximately 45 µm. The
polished disks were used for tribological tests. Thesurface roughness and thickness of the film after
the polishing are listed in Table 2.
Table 2 Plasma electrolytic oxidation coating＊

CoatingPhases

Hardness[GPa]**Thickness[µm]Roughness
Ra [µm]PEO

α/γ-Al2O3*
polished

6.932

0.30

３

RESULTS AND DISCUSSIONS

3. １ Wear
Fig. 3 shows the result of wear evaluation by
the standard inclined roller SRV® test in steel/
PEO lubricated with the various oils.
The wear rates of SUJ2 rollers were in theorder of 10－7mm3/Nm and one order of magnitude
higher than that of disks due to the contactCharacterization from XRD
by Fischerscope at 1000 mN＊＊

2. ３ Friction and wear evaluation
A roller-on-disk oscillating tribo-test（SRV ®）was made to evaluate friction and wear behavior,according to DIN 51834-4. The test condition islisted in Table 3. Specimens were ultrasonicallycleaned with petroleum spirit. A roller made ofbearing steel（SUJ2）was fixed with a holder,
where the roller was deflected by 10° to theoscillating direction（Fig 2）. Disk was made ofA6061-T6 aluminium alloy coated by the PEO.
Lubricant was dropped onto lower disk specimen
and formed a meniscus at the edge of the roller.
The normal load was controlled by electric motor.
The averaged coefficient of friction （COF）,geometry. The lowest wear rate on roller was
obtained for PAO+Zn. PAG base oil also showedcomparatively low wear on roller. DBDS, as asulfur carrier, tended to increase in all base oils
the wear on roller.
The wear on the PEO disk was very low
regardless of the lubricant type, especiallyextremely low wear rates of less than 10－8mm3/Nm were obtained when lubricated by TMP
series.

maximum COF and several friction hysteresises
were recorded during the test by a digital dataFig. 3

Table 3 Test condition
Normal load [N]50Frequency [Hz]50

Temperature [°C]80

Test duration [min]120

Wear rate of PEO coatings

3. ２ Friction behavior
Friction behavior was much more influenced bythe ad­di­tives. Fig. 4 to Fig. 6 show the evolution ofCOF as function of stroke position（as hysteresis）
over sliding time for PAO, PAG and TMP blendedoils, respectively.
The COF of PAO base oil was initiallyapproximately at 0.11 and slightly increased bysliding time. ZnDTP increased friction for PAO®

Fig. 2 Standard inclined roller in SRV tribometerand PAG（average COF of 0.12 and fluctuatedfriction hysteresis）, but did not influence friction―6―

KYB技報第51号

2015―10（a）
PAO（a）PAG（b）
PAO+Zn（b）PAG+Zn（c）
PAO+S（c）PAG+SFig. 4Fig. 5

Friction behavior of PAO blended oils
―7―

Friction behavior of PAG blended oils

プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果

in TMP. DBDS had a friction reduction effect forall base oils after rubbing for 2 hours, but initial
COFs were higher than that of the base oils.
The lowest COF at test end was found withTMP+S combination. TMP+S showed 45% lower
friction in comparison to PAO+Zn（reference）.
Furthermore, a positive effect with regard tofriction behavior could also be achieved by oil
formulation of PAG+S which showed 30% lower
friction than the reference oil.
3. ３

 ribochemistry of the blended EALs inT
SUJ2/PEO system
The anti-wear mechanisms of ZnDTP and（a）
TMP

DBDS have been studied by many researchers inhydrocarbon base oils, such as mineral oil andPAO for steel/steel tribosystem. Tribochemical
reaction will build tribofilms composed of complex（b）
TMP+Znreaction products from additives and surface
oxides. The tribofilm will protect the sliding
surface and improve wear resistance and may
reduce friction. In order to produce tribofilms,
additive must initially adsorb onto the surface.
When a polar base oil is used as carrier fluid, thebase oil and the additive will compete with eachother. Consequently, base oils with higher polarity
showed different wear and friction behavior byuse of steel/PEO in the present study.
Scanning Electron Microscopy/Energy
Dispersive X-ray Spectroscopy （SEM/EDX）
analysis on wear scar was conducted tounderstand the mechanism. Tribofilms fromZnDTP can be detected through zinc, phosphorus
and sulfur, whereas the metal-free DBDS onlythrough sulfur. These elements are not present
either in the tribomaterials or the base oils.
Observing the wear track of the SUJ2 roller
lubricated with PAO base oil（Fig. 7（a））, microdamage and oxidation were detected which canbe the evidence of the relative high friction andadhesive wear. This may cause seizure whenmore severe contact condition is applied. The
addition of ZnDTP into PAO prevented suchmicro-damage by tribofilm formation containing
Zn, P and O, whereby the friction value was at the（c）
TMP+S
Fig. 6

Friction behavior of TMP blended oils

highest level. Smooth sliding surfaces with sulfur
containing film were yielded by PAO+S.
Additionally, the friction was reduced.
―8―

KYB技報第51号

2015―10（a）
PAO（COF=0.112＊）（b）
PAO+Zn（COF=0.117＊）

（c）
PAO+S
（COF=0.089＊）＊

The averaged COF at test end

Fig. 7SEM image and EDX spectrum on SUJ2 roller lubricated with PAO blended oils（a）
PAG
（COF=0.094＊）（b）
PAG+Zn（COF=0.117＊）

（c）
PAG+S
（COF=0.081＊）＊

The averaged COF at test end

Fig. 8SEM image and EDX spectrum on SUJ2 roller lubricated with PAG blended oils
―9―

プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果（a）
TMP（COF=0.079＊）（b）
TMP+Zn（COF=0.076＊）

（c）
TMP+S
（COF=0.062＊）＊The averaged COF at test end

Fig. 9SEM image and EDX spectrum on SUJ2 roller lubricated with TMP blended oils

It was expected, that due to the polarity of PAGand TMP, the PEO sliding surfaces were toprotect against wear. Fig. 8（a）shows that PAG
base oil lubricated the surface better than PAO
base oil, as less or no micro-damage was observedon the wear track of the SUJ2 roller. TMP ester
provided much smoother sliding surface as shownin Fig. 9（a）
. For this reason, PAG and TMP are
also used as lubricant additives in combination
with base oils offering insufficient lubricity.
Both blended PAG oils formed tribofilms, but incontrast to blended TMP less additive elements
were found within the wear scar. In Fig. 8（b）
shows the EDX spectrum of PAG+Zn. Zinc,
phosphorus, sulfur and slight oxygen-level weredetected nearly in the same range found for
PAO+Zn. For PAG+S, a small amount of sulfur
was detected.
Considering Fig. 9（b）and（c）it would appear
that any tribofilm for two blended TMP was notobserved by EDX analysis on the sliding surfaceof the SUJ2 roller. Thus, the TMP base oilprobably has very strong interaction onto surfaceof metal and metal oxide. In terms of wear, TMP
itself proper lubricated the PEO surface resulting
in extremely low wear. It is known that esters canform anti-wear aluminium soap on alumina
rubbing surface7）.
Fig. 10 compares FE-SEM images of the wear
tracks on PEO lubricated with PAG+S andTMP+S. Sulfur and iron were detected for
PAG+S, while no sulfur but only small amount ofiron was detected for TMP+S. Pad-like tribofilm
can be observed for PAG+S as shown inFig. 10（a）（recognized as relative white contrast
arrowed in the image）. This might be compounds
containing iron and sulfur, as these elements weredetected by EDX（Fig. 10（c））. On the otherhand, TMP+S did not form such a solid tribofilm,
only sub-micron size particles（arrowed in Fig. 10（b））were observed. These particles might be thesoap-like triboreaction products mainly caused by
TMP base oil, containing no sulfur（Fig. 10（d））.
These films formed on rubbing surface probably
reduced the friction.

― 10 ―

KYB技報2015―10（a）
SEM image of PEO lubricated with PAG+S（b）SEM image of PEO lubricated with TMP+S（c）
EDX spectra on PEO lubricated with PAG+S（d）
EDX spectra on PEO lubricated with TMP+SFig. 10４第51号

SEM image and EDX spectrum on wear track of PEO coatingfriction and wear in steel/PEO tribosystem,
while PAO has no polar part in the molecule.

CONCLUSION

In this paper, the influence on the tribologicalproperties of PEO by EALs was investigatedcompared with PAO formulations8）. The following
conclusions can be drawn.
（1）On PEO sliding against steel, the base oil typesignificantly influenced the friction and wearbehavior.
（2）P AO had the highest level of COF wheneither base oil or ZnDTP blended PAO wasused, whereas DBDS had friction reduction
effect. Hydrocarbon-base oil needs to beadditivated with anti-wear additive to protect

５ACKNOWLEDGMENTS

Deepest appreciation goes to Dr. Mathias Woydt
whose enormous support during the visiting
research at the BAM Federal Institute forMaterial Research and Testing, Berlin, for thisresearch. The author is also indebt to Prof. Shinya
Sasaki who gave the opportunity of P.h.D courseat Tokyo University of Science.
References

sliding surfaces.
（3）PAG base oil lubricated the steel well and also1） Spikes, Hn., The history and mechanisms of ZDDP,PEO comparable to PAO. Both formulations
with ZnDTP and DBDS formed tribofilms.
PAG+Zn showed similar high friction as
PAO+Zn. DBDS reduced the friction by forming

2） L ä m m l e P . , A p p l i c a t i o n o f E C L s a n d T o d a y ’ ssulphur containing solid tribofilm on PEO.
（4）T he best tribological performance on PEO

Techniques de l´Ingénieur, Paris, TRI 1 800v2, 12-2013.

was obtained with TMP regardless of theformulation.
（5）T he different influences of the tested
formulations on the tribological properties ofPEO can be interpreted by chemistry of theTribology Letters, Vol.17, No.3（2004）, pp.469-489.
Legislation, J. ASTM Int., Vol.9, No.1（2012）, Paper ID
JAI103563.
3） Woydt, M., Biolubrifiants（Biolubes）, Encyclopédie
4） Woydt, M., Polyalkylene Glycols as Next Generation
Engine Oils, J. of ASTM Int., Vol.8, No.6, paper ID
JAI103368 and ASTM STP1521, 2012, ISBN: 978-0-80317507-5.
5） Yerokhin, A. L., et al. Oxide ceramic coatings on
aluminium alloys produced by a pulsed bipolar plasma

backbone type of the base oil. The polarities

electrolytic oxidation process, Surface and Coatingsof PAG and TMP strongly influenced theTechnology, Vol.199, No.2（2005）, pp.150-157.

― 11 ―

プラズマ電解酸化処理したアルミニウム合金に対する代替潤滑油の摩擦低減効果
6） Sasaki, S., Effects of environment on friction and wearAbbreviationsof ceramics, Bulletin of Mechanical Engineering

COF（s）: Coefficient of friction（s）Laboratory Vol.58（1992）.

DBDS: Dibenzyldisulfide7） Tripathy, B. S., M. J. Furey, and C. Kajdas., Mechanism

EAL（s）: Environmentally acceptable lubricant（s）of wear reduction of alumina by tribopolymerization,

PAG（s）: Polyalkylene glycol（s）

Wear Vol.181（1995）,pp.138-147

PAO: Polyalphaolefin8） Nakase, T., Kato, S., Sasaki, S., and Woydt., M., Friction

PEO: Plasma electrolytic oxidation

reduction effect of alternative lubricants for plasma

FE-SEM: Field Emission Scanning Electron Microscopy

electrolytic oxidation coating on AA6061-T6, Proc. 55th

EDX: Energy Dispersive X-ray Spectroscopy

Tribology Conference（Tribologie-Fachtagung）, BandSRV®: Schwingung-Reibung-VerschleißII（2014）42/1-11

TMP: Trimethylolpropane（Ester）
XRD: X-ray Diffraction
ZnDTP: Zinc dialkyldithiophosphate

著者 中瀬拓也

2005年入社．技術本部基盤技術研
究所材料研究室．博士（工学）．
主にトライボロジーに係る材料技術開発に従事．

― 12 ―