Tribological synergistic effects between plasma nitrided 52100 steeland molybdenum dithiocarbamates additive in boundarylubrication regime

Wen Yue a,n, Zhiqiang Fu a, Song Wang a, Xiaocheng Gao b, Haipeng Huang c, Jiajun Liu d

a School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, Chinab PetroChina Huabei Oilfield Company, Renqiu, Hebei 062552, Chinac SINOPEC Lubricant Company (Beijing), Beijing 100085, Chinad Mechanical Engineering Department, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 7 October 2013Received in revised form4 February 2014Accepted 13 February 2014Available online 22 February 2014

Keywords:Plasma nitridingMolybdenum dithiocarbamatesBoundary lubricationTribology

a b s t r a c t

The interactions and synergistic tribological effects between plasma nitrided 52100 steel and molybde-num dithiocarbamates (MoDTC) additive under boundary lubrication were studied in this work.The tribological behaviors of plasma nitrided and untreated steels under lubrication with differentMoDTC concentrations were examined on a four-ball friction and wear tester. The results showed anobvious synergistic effect between plasma nitrided steel and MoDTC additive. The synergistic effectindicated a lower friction coefficient without an induction phase and a higher wear resistance, which wasattributed to more amount of MoS2 and absorbed carbon were formed in the tribofilm of the nitridedsurface.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Higher work efficiency and longer service life for modernequipments are urgently required, which mainly depend on thefriction and wear characteristics of machine parts. The bettertribological behaviors are obtained by optimized mechanicaldesigns or appropriate lubrication strategies. The two contactsurfaces of rubbing pairs already could not be separated by thelubricant oil film in the boundary (or mixed) lubrication regime,therefore, new materials or surface treatments are considered tobe used [1,2]. Therefore, the research on the laws and mechanismsof synergistic effect between the modified solid surface andlubrication additives is very necessary for understanding how toachieve a lower friction coefficient and a higher wear resistance.

Plasma nitriding is an effective and widely used technique toimprove the surface hardness and tribological and anti-corrosionproperties of steels, cast irons, titanium and some alloys [3–7].Nitrided steel parts are widely applied in engineering area in theboundary (or mixed) lubrication regime. The tribological behaviorsof nitrided steel are effectively improved under lubrication withalkyl naphthalene, ionic liquid and liquid paraffin, which areattributed to the high hardness of nitrided layer and the adsorbed

boundary lubricating film [8–10]. Our previous works presentedthe synergistic effects between nitrided steel and several anti-wear additives, such as zinc dialkylithiophosphate (ZDDP), phos-phor and sulfur-free organotungsten and borate ester [11–13].However, few works have been reported on the tribochemicalinteractions between nitrided steel surfaces and friction modifieradditives.

As a friction modifier, molybdenum dithiocarbamates (MoDTC)is extensively used in the formulated oil. The effect of MoDTCcompounds on friction reduction in most investigations had beenattributed to the formation of tribofilms containing primarilyMoS2 and other molybdenum oxides [14–17]. However, the wearrates of DLC coatings lubricated with MoDTC were even higherthan those under PAO lubrication [17]. Generally, a higher MoS2/MoOx ratio is recognized as a key factor providing a lower wearrate. Therefore, synergistic effects not only depend on MoDTCitself but also depend on the modified surfaces. It is supposed thatthe tribological system could achieve a much lower frictioncoefficient and wear rate through the high wear resistance ofnitrided steel and the high friction reduction of MoDTC. Unfortu-nately, research on the synergistic effects between nitrided 52100steel and MoDTC additive under boundary lubrication have notbeen reported so far.

In this paper, the friction and wear behaviors of plasma nitrided52100 steel were compared with those of the untreated surfacelubricated with MoDTC under boundary lubrication. It attempted

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Tribology International

http://dx.doi.org/10.1016/j.triboint.2014.02.0070301-679X & 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ86 10 82320255; fax: þ86 10 82322624.E-mail address: cugbyw@163.com (W. Yue).

Tribology International 74 (2014) 72–78

to reveal the synergistic effect between nitrided steel and MoDTC,as well as the composition of tribofilm formed on the nitridedsteel surface.

2. Experimental details

2.1. Materials and lubrication

The samples used in this work were 52100 steel balls, with adiameter of 12.7 mm, hardness of 770 HV, and surface roughnessRa 0.025 μm. The samples were nitrided in a pulsed direct currentplasma nitriding furnace (LDM2-25) at 550 1C with a voltage of700 V and a pressure of 650 Pa, for a total duration of 5 h, usingthe ammonia as a source gas. After the nitriding process, thesamples were cooled in vacuum to ambient temperature.

A synthetic oil poly-alpha-olefin (PAO) with viscosity grade4 was used as the base oil, and a friction modifier MoDTC was usedas the additive. The typical physico-chemical properties of PAOand MoDTC are listed in Tables 1 and 2. The MoDTC concentrations(mass fraction) in the base oil were 0.0%, 0.5%, 1.0%, and 1.5%,respectively.

2.2. Friction and wear test

The friction and wear tests were carried out on a MS-10JR four-ball friction and wear tester. In the present work, the tribotest wasconducted under four balls of a same material at the condition ofbath lubrication, a load of 392 N (corresponds to the initial Hertzmean contact stress of 2.293 GPa), linear speed of 0.461 m/s, andtest duration of 60 min. Before the wear test, for evaluating theExtreme-Pressure (load-carrying) capacity of lubricants, the max-imum non-seizure load (PB value) was tested. The applied normalload, 392 N, is much lower than 470 N that of the PB value of PAO[12]. It means that no seizure happened on the rubbing surfacesduring the wear test. There is a temperature probe at the bottomof the oil bath. The oil was heated by the friction heat. Thetemperatures continued increasing and the final temperatureswere mainly within the limit of 60–70 1C. The friction force andfriction coefficient were measured by a piezoelectric force trans-ducer connected to the computer. The worn scars were measuredby a microscope. The tests were replicated at least three times.A good repeatability for the friction coefficient and worn scars in

the whole process of the test was recorded and the results wereaveraged.

The minimum film thickness was determined using the Dow-son and Hamrock minimum film thickness equation for anelastohydrodynamic point contact [18]. The lambda is a ratio ofminimum lubricant film thickness to starting composite root meansquare surface roughness. The film thickness and lambda ratiowere calculated using Eqs. (1) and (2), respectively. The minimumfilm thickness,

hmin ¼ 3:63RxGn0:49Un0:68

Wn0:073 ð1�e�0:68kÞ ð1Þ

where Gn ¼ αE0; Un ¼ η0U=E0Rx; Wn ¼W=E0R2

x and η0 is dynamicviscosity; α is the viscosity-pressure coefficient; Rx is equivalentcurvature radius of ball; W is the normal pressure; E´ is theeffective modulus of elasticity and k is elliptical parameter.

Λ¼ hminffiffiffiffiffiffiffiffi2R2

q

q ð2Þ

where Rq is the roughness of the balls.The lambda ratio value is 0.81 and 0.13 for the untreated

surface and the nitrided surface, respectively, and both are below1 which means that the lubrication occurred in the boundarylubrication regime.

2.3. Surface analysis

The cross-sectional hardness of the sample was measured onMH-6 Vickers0 microhardness tester at an applied load of 1.96 Nfor 5 s. Measurements were conducted for three times on eachsample and the results were averaged. The JSM-6460LV scanningelectron microscope (SEM) equipped with Oxford 7573 energydispersive spectrometer (EDS) was utilized to observe the surfacemorphologies of the balls and the worn scar of the balls. The EDSwas used to measure the elemental distribution of the wornsurface. The chemical analysis of tribofilms was performed by aPHI Quantera SXM X-ray photo-emission spectroscopy (XPS). Theinstrument was employed a high-power rotating anode andmonochromatised X-ray of Al Kα (hν¼1486.6 eV) source. Thediameter of the analyzed area was 100 μm. The residual pressurein the spectrometer was always below 6.7�10�8 Pa. The systemwas calibrated according to ISO 15472:2001 with an accuracy of70.1 eV. The analyzer was operated in the fixed-analyser-transmission mode with a pass energy of 55 eV and a step sizeof 0.2 eV (full-width-at-half-maximum (FWHM) of the peakheight for Ag 3d5/2¼0.5 eV). The position of C1s peak (284.8 eV)was considered as the reference for charge correction. The samplesfor XPS analysis were cleaned in the mixed solution of petroleumether and absolute ethanol to remove residual oil and contami-nants. Casa XPS software was used to analyze the XPS curvesobtained from long scans, and the quantitative analyses of thepeaks were performed using peak area sensitively factors. XPShandbook [19] was used to find the chemical species correspond-ing to the binding energies of the peaks.

3. Results and discussions

3.1. Characterization

The typical surface SEM morphologies of the balls are shown inFig. 1. It can be clearly seen that the spherical particles were piled upclosely with a lot of fine micro-porosities. The micro poresare distributed on the nitrided surface, which resulted in a higherroughness Ra 0.16 μm. However nothing was found on the untreated

Table 1Typical physico-chemical properties of PAO.

Item Value

Viscosity (mm2 s�1, 40 1C) 16.68Viscosity index 124Flash point (1C) 213Pour point (1C) �72Evaporation loss (%) 11.8

Table 2Typical physico-chemical properties of MoDTC.

Item Value

Appearance Black-brown liquidDensity (g cm�3) 1.01Viscosity (mm2 s�1, 40 1C) 700Flash point (1C) 135Mo content (%) 10.0S content (%) 11.0

W. Yue et al. / Tribology International 74 (2014) 72–78 73

surface. It is reported that the composition of the nitrided surfacewas mainly ε-Fe2�3N and γ0-Fe4N, while the untreated surface wasmainly martensite and ferrum [11–13]. Fig. 2 shows the hardness ofthe compound layer with a thickness about 15 μm. The hardness ofthe top layer was increased to 900–1000 HV.

3.2. Friction and wear behaviors

Fig. 3 shows the variations of friction coefficient of untreatedand nitrided surfaces with time under different concentrations ofMoDTC. It can be found that the friction coefficients of the nitridedsurface were always lower than those of the untreated surface atthe similar lubrication condition. It means that the nitrided surfacepossesses a better friction-reducing behavior under boundarylubrication. When the samples are lubricated with 0.5% MoDTC,

the untreated surface still showed a high friction coefficient, whilethe friction coefficient of nitrided surface decreased obviously.Only when the concentration of MoDTC was higher than 1.0%, thefriction coefficient of untreated surface gradually decreased to 0.05after 25 min. running; while that of the nitrided surface droppedto 0.04 quickly. It is interesting to indicate that the synergisticeffect between the nitrided surface and MoDTC was really present,and the tribofilm formed on the nitrided surface possessed betterfriction reducing properties than that on the untreated surface.

Fig. 4 shows the wear scar diameters of the balls lubricatedwith PAO containing different concentrations of MoDTC. Thenitrided surface possessed higher wear resistance than theuntreated surface at the same lubrication condition. The smallestwear scar diameter (about 0.45 mm) was recorded for the nitridedsteel balls under the lubrication with 1.5% MoDTC, which was

Fig. 1. SEM images of (a) untreated surface and (b) nitrided surface.

0 20 40 60 80 100 120 140 160 180 200 220

500

600

700

800

900

1000

Mir

coha

rdne

ss (H

v)

Distance from surface (µm)

Fig. 2. The cross-sectional microstructure and hardness of nitrided steel.

0 10 20 30 40 50 600.02

0.04

0.06

0.08

0.10

0.12

0.14

PAO PAO+0.5% MoDTCPAO+1.0% MoDTCPAO+1.5% MoDTC

Fric

tion

coef

ficie

nt

Fric

tion

coef

ficie

nt

Time (min)0 10 20 30 40 50 60

0.02

0.04

0.06

0.08

0.10

0.12

0.14 PAO PAO+0.5% MoDTC PAO+1.0% MoDTC PAO+1.5% MoDTC

Time (min)

Fig. 3. Variations of friction coefficient under different concentration of MoDTC, (a) untreated surface and (b) nitrided surface.

W. Yue et al. / Tribology International 74 (2014) 72–7874

nearly half that of the untreated surface with PAO lubrication, andalso much lower than that of the nitrided surface with PAOlubrication. It indicated that MoDTC had stronger interaction withnitrided surface exhibiting a notable wear resistance.

3.3. SEM analysis

Fig. 5 and Table 3 show the SEM images and the EDS analysisresults of the worn surfaces lubricated by PAO with 1.0% concen-tration of MoDTC. Many furrows can be found on the untreatedsurface, while the nitrided surface was smoother, which indicatedthe effective role of nitrided surface for reducing wear. Comparingwith the condition of pure PAO lubrication, both the untreated andnitrided surfaces were smoother under 1.0% MoDTC lubrication.The EDS analysis demonstrated that both two surfaces containedMo, S element coming from MoDTC, which indicated that MoDTC

could be decomposed and reacted with frictional surface duringthe friction and wear process.

3.4. XPS analysis

The XPS analysis was carried out to explore the changes of thechemical states of several typical elements on the worn surfaces.XPS spectra of the worn surfaces lubricated with PAO containing1.0% MoDTC are shown in Fig. 6, and binding energies andreference compounds are listed in Table 4. It shows that MoDTCwas decomposed under tribological contacts and tribofilms wereformed on the interacting surface. The binding energy at 709.7–710.7 eV correspond to FeO and Fe2O3 [15], and the binding energyat 712.1 eV corresponding to FeS and S 2p binding energy. Thepeaks of MoS2, FeS binding energies are close to each other, whichmakes it difficult to identify these species only by the S 2p peak[20]. MoS2 was found in both tribofilms, which is effective infriction reduction. The oxygen in both tribofilms is mainly fromoxides. N 1s peak on the tribofilm of nitrided surface correspondsto iron nitride [11–13]. The elements of C, S and Mo were detectedon both two surfaces, however, the higher C, S and Mo contentwere present on the worn nitrided surface as shown in Table 5.From these data, it can be inferred that a higher content of MoS2formed on the nitrided surface provides better tribologicalbehaviors.

0.0% 0.5% 1.0% 1.5%0.0

0.2

0.4

0.6

0.8

1.0Untreated surface Nitrided surface

Wor

n sc

ar d

iam

eter

(mm

)

MoDTC concentration

Fig. 4. Wear scar diameter of ball under MoDTC lubrication.

Fig. 5. SEM images of the worn surfaces lubricated by PAO and MoDTC, (a) Untreated surface lubricated by PAO, (b) Nitrided surface lubricated by PAO, (c) Untreated surfacelubricated with 1.0% MoDTC and (d) Nitrided surface lubricated with 1.0% MoDTC.

Table 3EDS quantification (at%) of the worn surface lubricated with 1.0% MoDTC.

C N O S Cr Fe Mo

Untreated surface 30.03 – 8.35 0.08 0.60 60.57 0.37Nitrided surface 34.55 1.02 1.12 0.20 0.82 62.21 0.18

W. Yue et al. / Tribology International 74 (2014) 72–78 75

3.5. Discussion

Based on the above results, it can be understood that MoDTC isan effective friction modifying agent, which can obviously reducethe friction coefficient of either 52100 steel untreated surface or

plasma nitrided surface. The friction coefficient of the untreatedsurface can reach 0.05, while that of the nitrided surface can reach0.04. The basic principle of the friction-reducing ability of MoDTCis the formation of tribofilm containing MoS2, which is an effectivesolid lubricant due to its layered structure [14–17]. From the XPS

538 536 534 532 530 528 538 536 534 532 530 528In

tens

ity (a

.u.)

OxideOxide

Inte

nsity

(a.u

.)

Nitrided surface

Nitrided surface

Nitrided surface

O 1s

R-O

730 725 720 715 710 705

Inte

nsity

(a.u

.)

730 725 720 715 710 705

Fe 2p3/2

FeO 2p3/2

Fe2O

32p3/2

FeS 2p3/2

Inte

nsity

(a.u

.)

Fe 2p

FeS22p3/2

Inte

nsity

(a.u

.)

174 172 170 168 166 164 162 160 158174 172 170 168 166 164 162 160 158

Sulfide 2p

Inte

nsity

(a.u

.)

S 2p

Sulfide 2p

240 238 236 234 232 230 228 226 224 240 238 236 234 232 230 228 226 224

MoO

Inte

nsity

(a.u

.)

Mo 3d Mo 3d

MoO

Mo 3dMo 3d

MoSMoO

MoSMoO

MoS

Inte

nsity

(a.u

.)

Nitrided surfaceMo 3d

MoS

2

MoSMoS

MoS

MoSMo 3d

410 408 406 404 402 400 398 396 394 392

Inte

nsity

(a.u

.)

Binding Energy (eV)

Binding Energy (eV) Binding Energy (eV)

Binding Energy (eV)Binding Energy (eV)

Binding Energy (eV) Binding Energy (eV)

Binding Energy (eV)Binding Energy (eV)

Nitrided surfaceN 1s

Ammonium

Organics

Nitride

Oxide

Untreated surface

R-O

Oxide

O 1s

FeS

FeO

Untreated surface

Fe 2p

Fe 2p

FeO

FeSFeO

Fe 2p

Sulfide

Untreated surface

Untreated surface

S 2p

Sulfide

Fig. 6. XPS spectra of tribofilm formed on worn surface.

W. Yue et al. / Tribology International 74 (2014) 72–7876

analysis results shown in Tables 4 and 5, it is found that the highercontent of C, S and Mo were detected on the worn nitrided surfacethan that on the worn untreated surface. The lower content ofMoO3 and higher content of MoS2 were contained in the tribofilmof the nitrided surface. It implies that higher amount of MoS2promotes a lower friction coefficient for the nitrided surface.

It can be found that the reduction of friction coefficient by 1%and 1.5% MoDTC for the untreated surface showed an inductionperiod as shown in Fig. 4, i.e. the friction coefficient values couldbe separated into two distinct regions: the first stage is a highfriction induction phase, and then a second region with a lowerfriction coefficient about 0.05. The increase of MoDTC concentra-tion in the lubrication was shown to reduce the induction timeprior to friction reduction. This reduction phase is related to N andS- containing tribofilms decomposed from MoDTC performs arelatively higher friction than MoS2 [14,20]. Yamamoto and Gondoclaimed that the increase in MoDTC concentration reduces thetime required to reduce friction [21]. Morina and Neville recog-nized that the formation of a low friction MoDTC tribofilmdepends on the reaction rate which is a function of the MoDTCpercentage, temperature and contact conditions [22]. That canexplain why there are two distinct regions of friction coefficientcurve of untreated steel surfaces. However, the friction coefficientson nitrided surfaces decrease to lower values within only 5 min.The friction coefficient dropped quickly to 0.04 even lubricatingwith 0.5% MoDTC. It implies that an effective MoDTC tribofilmcontaining MoS2 is formed in a very short rubbing period.

Higher C, S and Mo content detected on the worn nitridedsurface indicated that the polar groups and carbon chains fromMoDTC may be much easier to adhere to the nitrided surfaceto form more MoS2. Due to the quick formation of tribofilmcontaining higher content of MoS2 and absorbed carbon, thenitrided surface exhibited excellent tribological behaviors throughthe subtle synergistic reaction with MoDTC additive. The induction

period for the untreated surface may be explained as weakeradhering ability to form a stable tribofilm with a lower friction.Meanwhile, the tribofilm for untreated surface with lower MoS2and higher MoO3 may promote a little higher friction.

The wear scar diameters of both untreated and nitrided surfacesare reduced when lubricated with MoDTC. The wear of the nitridedsurface is reduced by 8.5%, 46.8%, 28.6% and 37.5% comparing with thatof the untreated surface under lubrication with PAO, 0.5% MoDTC, 1%MoDTC and 1.5% MoDTC, respectively. Obviously, the hard nitridedlayer can contribute to significantly improving the wear resistance ofthe nitrided surface compared to the untreated surface. Moreover, theMoDTC tribofilm with higher MoS2 content formed on the wornsurface could efficiently prevent the direct contact and adhesion oftwo metallic surfaces of friction pairs, and further reduce friction andwear. Therefore, nitrided surface plays an important role to providebeneficial synergistic effects with MoDTC for the 52100 steel partsworking under lubrication.

4. Conclusions

The main conclusions can be drawn from this research asfollows:

(1) Synergistic effects between plasma nitrided 52100 steel sur-face and MoDTC show an obvious improvement in friction-reduction and wear-resistance.

(2) An induction phase exhibits in friction coefficient curve foruntreated steel surfaces, whereas it does not appear in thosecurves for nitrided steel surfaces.

(3) Hard nitrided layers combining with the tribofilm of morecontent of MoS2 and carbon are the key points of thesynergistic effects between nitrided surface and MoDTCadditive.

Acknowledgements

The authors would like to thank the Beijing Natural ScienceFoundation (3132023), the National Natural Science Foundation of

Table 4Binding energy and reference compound of tribofilm formed on worn surfaces.

Elements Untreated surface Nitrided surface

B.E. (eV) FWHM (eV) Area (%) Compound B.E. (eV) FWHM (eV) Area (%) Compound

Fe 2p 709.4 2.5 65.9 FeO 708.2 2.5 30.1 Fe711.2 2.5 14.8 Fe2O3 709.9 2.5 23.6 FeO711.9 2.5 10.9 FeS 710.8 2.5 28.2 Fe2O3

714.2 2.5 5.2 FeS2 712.2 2.5 12.5 FeS716.5 2.5 3.2 Fe2O3 714.2 2.5 5.6 FeS2

N 1s – – – 397.7 2.5 59.9 Nitride400.0 2.5 25.1 Organics402 2.5 15.0 Ammonium

O 1s 530.6 1.5 57.1 Oxide 530.7 1.5 65.1 Oxide531.2 1.5 17.1 Oxide 531.3 1.5 18.2 Oxide532.1 1.5 25.8 R–O 532.3 1.5 16.7 R–O

S 2p 161.6 1.7 79.2 Sulfide 161.9 1.7 66.8 Sulfide162.7 1.7 20.8 Sulfide 163.4 1.7 33.2 Sulfide

Mo 3d 228.4 1.3 34.4 MoS2 228.1 1.3 43.7 MoS2229.2 1.3 22.2 MoS2 229.4 1.3 9.8 MoS2231.1 1.3 3.8 MoS2 231.1 1.3 23.8 MoS2231.7 1.3 26.4 MoO3 231.7 1.3 8.7 MoO3

232.5 1.3 11.1 MoS2 232.5 1.3 8.7 MoS2234.8 1.3 2.1 MoO3 234.8 1.3 5.3 MoO3

B.E., FWHM and Area correspond to binding energy, Full Width Half Maximum and the spectra area, respectively.

Table 5XPS quantification (at%) of tribofilm formed on worn surfaces.

C O N Fe S Mo

Untreated surface 20.24 38.70 – 33.29 4.51 3.26Nitrided surface 26.24 29.48 9.68 19.43 7.26 7.91

W. Yue et al. / Tribology International 74 (2014) 72–78 77

China (51275494), the Fundamental Research Funds for the CentralUniversities (2652012115) and the Tribology Science Fund of StateKey Laboratory of Tribology (SKLTKF13B10).

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