Tribology International 65 (2013) 207–214

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Tribology International

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n CorrE-m

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Effect of loading system inertia on tribological behaviour ofceramic–ceramic, ceramic–metal and metal–metal dry sliding contacts

M. Antonov n, I. Hussainova, E. AdobergDepartment of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

a r t i c l e i n f o

Article history:Received 31 July 2012Received in revised form13 March 2013Accepted 27 March 2013Available online 11 April 2013

Keywords:Wear measurementDry slidingCoatingDebris

9X/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.triboint.2013.03.025

esponding author. Tel.: +372 6203355; fax: +3ail address: Maksim.Antonov@ttu.ee (M. Anto

a b s t r a c t

Machine dynamics can contribute to a difference between results in coefficient of friction, vibrationlevel and wear rate of the same tribo-couple concerning the compliance of the support provided to asliding counterbody. The aim of the present work is revealing the effect of loading system inertiaon laboratory tests results. A developed pendulum-type tribo-tester was used for testing ceramic–ceramic, ceramic–metal and metal–metal sliding contact combinations. Diamond-like, AlTiN, TiCN andAlCrN/Si3N4 PVD coatings and bare EN X30WCrV9-3 steel disks were tested against yttria-stabilizedzirconia and EN 100Cr6 steel balls. Discussion on the mechanisms of materials degradation is based onSEM and EDS observations. It was found that inertia of loading system influences compaction ofwear debris.

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1. Introduction

Laboratory tribological testing can hardly completely simulateconditions experienced in service (real conditions) [1–7]. Whentwo rough surfaces are in sliding contact then normal displace-ments occur and excitation of vibration takes place [2]. Level ofvibration (displacement amplitude, velocity and acceleration) isaffected by load, sliding speed, motion type, contact geometry, testenvironment, sample parameters (material composition, structure,surface finish, etc.) and characteristics of tribo-device (machinedynamics) resulting from design (stiffness of frame, sample fixa-tion, etc.) and method of load application (dead weight, pneumaticor hydraulic, servo controlled, etc) [4]. A variations of nearly twoorders of magnitude for ceramic–ceramic tribopair were seen intests that were carried out under nominally identical conditionsdue to changes in the dominant wear mechanism promoted by thealteration in the machine dynamics [3]. The aim of the currentstudy is to emphasize the effect of the loading system inertia oftribotester on wear behaviour of different tribo-pair combinations(ceramic–ceramic, ceramic–metal and/or metal–metal). Differentdevices with known machine characteristics are usually applied tostudy the effect of machine dynamics [3,5] while the same deviceallowing to change the inertia of the loading system was used inthis study. The present paper is an extension of the previousworks. It was found that highest effect of inertia and rigidity of

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loading system on coefficient of friction (COF) and vibrationamplitude for TiCN–ZrO2 tribocouple is observed during run-inperiod due to initial roughness [8]. However, at the end of test thebehaviour of coating was similar with minor effect of studiedparameters. The COF of TiCN–ZrO2 tribocouple may not be stabledue to the cyclic process of formation and rupture (due to over-heating) of transfer layer on ZrO2 ball [9]. TiCN exhibited higherCOF and wear in conditions when stability of the contact betweenball and coating was improved by application of the springenabling damping of vibrations in loading system [8,9]. Coatingsobtained by physical vapour deposition (PVD) method werestudied because of their wide use nowadays due to their versatileapplicability and cost efficiency. Particularly it was important tomake assessment of coatings performance in conditions closeto stamping and punching processes with high inertia ofmoving parts.

2. Materials and methods

The range of standard Platit PVD coatings (Table 1) was studied.Coatings were deposited onto EN X30WCrV9-3 (AISI H21) hotworked tool steel substrate of HRC hardness of 4971 with thehelp of an arc ion plating technique in a Platit π-80 PVDunit [10,11]. Thickness of coatings was in the range of 2.8–3.5 μm(determined by BAQ GmbH KaloMAX ball cratering deviceaccording to EN1071-2007). Roughness of coatings was in therange Ra 0.11–0.21 μm; roughness of X30WCrV9-3 steel substratewas Ra 0.11 μm (measured by Mahr perthometer, PGK 120,contact mode).

Table 2Test conditions for MMTS and CETR.

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Ceramic balls were made of a high purity yttria-stabilizedzirconia (YSZ, 95% ZrO2, 5% Y2O3) with moderate (for ceramicmaterials) hardness of HV10 1250 and modulus of elasticity of

Fig. 1. Scheme of pendulum-type multifunctional modular tribosystem (MMTS).

Fig. 2. CETR tribometer.

Parameter Description

Scheme Ball-on-disk, unidirectional sliding, disk is rotatingBall YSZ, 3 mm in diameter

EN 100Cr6, 3 mm in diameterDisk 10 mm thick and 50 mm in diameter, flat side coated,

disk plane vertical (MMTS) or horizontal (CETR)Diameter of track 12.7, 15.2, 17.7, 20.2 mmSliding distance 200–3000 m depending on resistance of coatings to

wear to produce the track of 1.5–2.0 μm deepLinear velocity 0.1 m s−1

Load 16.5 NLoading system MMTS—dead weight

CETR—servo controlledInertia of loadingsystem (MMTS)

9 and 26 kg

Atmosphere Air, relative humidity 45710%. Temperature 2572 1C

Table 1List of ceramic coatings investigated [10].

Material Type Nanohardness(GPa)

Max. usagetemperature(1C)

Diamond-like (DLC,GRADVIc)

Double nanocomposite, gradient(DLC-top layer; TiAlCN corelayer)

20 400

AlTiN Multilayer, gradient 38 900TiCN Multilayer, gradient 32 400nACRo(AlCrN/Si3N4)

Nanocomposite, gradient 40 1100

Fig. 4. Results of dry sliding testing of ceramic–ceramic, ceramic–steel and steel–steel tCETR device.

Fig. 3. Schematic representation of force variation during evaluation of inertia andrigidity of MMTS loading system. Refer to Fig. 1b for directions of movement andacceleration of platform with sensor.

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210 GPa while sufficient fracture toughness of 6.0 MPa m0.5, sup-plied by Tosoh/Nikkato corporation, Japan. Steel balls were made ofEN 100Cr6 (AISI 52100) chrome steel with hardness of HV1 800,modulus of elasticity of 207 GPa and fracture toughness of20 MPa m0.5 supplied by Redhill, Czech Republic. Roughness ofceramic and steel balls was Ra 0.08 μm (measured by Mahrperthometer, PGK 120, contact mode).

Tests were performed on multifunctional modular tribosystem(MMTS, Fig. 1) developed in Tallinn University of Technology[8,9,12] and on CETR (now Bruker) UMT-2 tribometer (Fig. 2).

MMTS enables to adjust inertia of the loading system due to itspendulum-type design (Fig. 1a). Main test parameters are given inTable 2. Every disk was tested on MMTS with inertia of 9 and 26 kg

ribopairs applying MMTS device with different inertial mass of loading system and

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and on CETR (Fig. 1c). At least 4 disks of each material were testedand the results averaged. Fresh balls were used for each new test.Disks and balls were ultrasonically cleaned with ethanol beforetesting. Six perpendicular measurements of the profile across thewear track along radii spaced equally around the disk wereperformed by Mahr perthometer PGK 120 in contact mode.Dimensional wear coefficient (for the disk) was calculated accord-ing to Eq. (1):

k¼ DALW

ð1Þ

where: D—circumference of the wear track measured in thecentre, m; A—average cross section of wear track, m2; L—totalsliding distance, m; W—applied load, N.

Scheme of MMTS is given in Fig. 1a. The ball (1) is fixed topendulum-type suspension (2) and pressed against the rotatingdisk (3). The disk is rotated by electrical motor through gearbox(4). The force required to move the suspension (2) in horizontaldirection is minimal. Normal load and friction force are continu-ously measured by sensors (5, 6) and true instantaneous coeffi-cient of friction is calculated. Level of suspension vibration(acceleration, velocity, displacement; in direction normal to thedisk (3) surface) were monitored by Monitran VM110 vibrationmeter (7) during the test. Normal load is applied by dead weightthrough loading system (8) and steel spring (9) of 3670 N m−1

constant. Inertia of the loading system (2, 5, 6, 7, 8, 9) wasmanipulated by fixing of an additional weight to the pendulumsuspension (10). Characterization of the loading system of thetribometer was done using numerically controlled motor (Fig. 1b).The motor moved the platform (11) equipped with force sensor(12). The platform (and loading system in lower position) wasaccelerated with acceleration of 0.1 m s−2 during 500 μm distanceto the right and stopped (step 1, Fig. 3). Then, after some restingperiod required for stabilization of the force value (step 2), plat-form was accelerated (step 3) to the left to the initial position with

Fig. 5. Backscattered SEM images (top view) of tested disk surfaces after dry unidirectionfrom the horizontal. Images (e) and (f) are showing PVD coating surfaces that are inclintribometer, value of inertia, number of revolutions is indicated. Ball material is indicate

acceleration of 4.0 m s−2 (moving faster than loading system, i.e.the gap between ball and disk has arisen). During the step 4 similarto step 2 the force value reached the stable value. Recorded forcevariation is shown schematically in Fig. 3 [8,9].

Inertia (Table 2) is the tendency of an object to resist a changein motion that is numerically represented by an object's mass.Difference between highest recorded force value during accelera-tion to the left (pushing of loading system) and force duringfollowing idle time is used for calculation of effective mass withhelp of Newton's second law. Difference between highest recordedforce value after unloading (moving of platform with sensor to theleft) and force during following idle time indicate the severity ofimpact conditions (or rigidity of loading system [8,9]). The weightof the MMTS device is approximately 200 kg. During the test itwas rigidly fixed to the concrete wall of the building to improvestability. CETR device was equipped with S21M0 lower rotarydrive, DFH-20 dual friction/load sensors and suspension for max-imum load of 200 N. Details are given in Fig. 2 and Table 2.

Microstructural examination of specimens was conducted by ascanning electron microscope (SEM) Zeiss EVO MA15 suppliedwith energy dispersive X-ray spectroscopy (EDS)—INCA analyzerand Hitachi TM-1000 scanning electron microscopes with EDSmodule.

3. Results

The results of tribotesting using MMTS and CETR devices arepresented in Fig. 4 and SEM images of disk and ball surfaces areshown in Figs. 5 and 6.

Inertia affects results for all types of tribo-pairs (ceramic–ceramic, ceramic–steel and steel–steel) studied (Figs. 4 and 5).The wear rate of DLC, TiCN and nACRo coatings are almostinsensitive to inertia while in the case of AlTiN and steel thehigher value of inertia results in lower wear rate and higher peak

al sliding. Images (c) and (d) are showing PVD coating surfaces that are inclined 451ed 801 from the horizontal. Cross-cut section is visible in figure (d) and (f). Type ofd only for steel disks.

Fig. 6. Backscattered SEM images of YSZ ball after dry unidirectional sliding against TiCN coating (MMTS, inertia 26 kg). Number of revolutions is indicated.

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vibration amplitude. Results obtained by CETR device were some-where close to those obtained by MMTS. However, nACRo coatingtested by CETR had extremely low resistance to wear (Fig. 4).

Coatings damage usually occurs in the centre of the trackand at the edges and originates from the highest peaks that areprotruding the surface or from the defects (pores, inclusions, etc.)New scales are formed from the compacted entrapped debris inthe case of AlTiN coating tested by MMTS with high inertia(Fig. 5a–g). Brittle fracture with extended crack pattern isobserved for nACRo coating tested with CETR (Fig. 5h). Steel

tested with ceramic ball have well polished highly oxidizedscales when tested by MMTS with low inertia (Fig. 5i) whilewith high inertia the surface resemble the multilayer ofcompacted wear debris (Fig. 5j). There is no visible difference inbehaviour of TiCN coating tested with different value ofinertia. Some amount of wear debris collected inside thedamaged areas having low adhesion to the surface was found(Fig. 5k and l).

Ball wear is intensive in the beginning (Fig. 6a). Transfer layer isbuild up on the surface; some amount of wear debris are collected

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in front of the ball in form of ridge (Fig. 6b and c1, entrance zone).The thickness of the layer increase until some critical value andthen the layer and the ridge are removed (Fig. 6c). Roughening ofthe ball surface takes place due to removal of the layer and ruptureof ball material (Fig. 6d).

4. Discussion

As soon as the debris are formed (Fig. 7, State 1) duringtribotesting the majority of them are prone to be removed fromthe track especially when disk plane is vertical (MMTS). Debrisduring rotation of the disk can be detouched if the adhesive forcesare weak or to stay adhered to the disk and to be impacted duringthe following rotation. During the impact they may be kicked outfrom the track or to undergo the embedding into the track (Fig. 7,State 2). If they acquire required adhesion then they proceed tofulfil the role of the protective scale along with original remnants

Fig. 7. Schematic representation of processes taking place during dry sliding withloading system having inertia sufficient for compacting of wear debris on disksurface.

of the coating. This phenomenon of debris entrapment may becalled as self-healing. However, these additional scales filling theholes and/or protruding above the worn surface increase the levelof vibration (Fig. 7, State 3). The duration of the contact betweenball and surface (and COF as well) is less if the level of vibration ishigh. High COF means that more energy is dissipated in the surfacelayer. That is why the debris of steel tested with low inertia arehighly oxidised. The loading system having higher inertia cannotbe easy shifted (is more static) and thus is more suitable forembedding of wear debris. This was observed in case of AlTiNcoatings and EN X30WCrV9-3 steel (Figs. 4 and 5). Inclined AlTiNcoating surfaces are given in Fig. 5c–f to illustrate the shape of thecompacted debris.

The ball in contact with the disk is also undergoing changes [9].The basic stages are depicted in Fig. 8. Braking and smearing of thehighest peaks of the coating as well as intensive mechano-chemical wear of YSZ ball takes place during the run-in period.Mechanical interlocking of rough peaks leads to the extremelyhigh COF recorded during Stage 2. Stage 3 is characterized by thedisruption of the first transferred layer formed on ball duringremoval of initial roughness. Steady state wear mechanism of theball comprises of cyclic formation and disruption (State 4 and 5,Figs. 8 and 6b–d) of the transferred layer. Formation of thetransferred layer is accompanied by decreasing of vibrationamplitude, rising of the COF and temperature gradient (Stage 4).Overheating results in cracking of transferred layer and rough-ening of the zirconia ball surface in exit zone (Fig. 6d). Afterdestroying of the layer the peak vibration amplitude increasewhile COF decrease (Stage 5). Then the formation of new layertakes place.

Worn areas of ball and disk are composed of the mixtures ofmaterials of both bodies with increased oxygen content (Table 3).The content of ball material in the disk wear track is expected tobe minimal due to sufficient difference in area of ball wearscar and wear track of the disk. Higher content of ball material isfound in thick compacted layers that are protruding abovethe overall surface and are severely impacted by the ball. Highoxygen content (especially in thin layers) may point to thepresence of lubricious oxides providing reduction of COF. Adhesionof transfer layer to the PVD coating is sufficiently good. In thecrater formed by loss of this layer sufficient amount of iron isfound indicating the removal of material down to the substrate(Table 3, EDS 2.5).

Processes of disk and ball wear shown in Figs. 7 and 8 are goingin parallel influencing each other. Self-healing of PVD consists ofmultiple processes taking place in different locations and runningdifferently depending on area, depth and position of the damagedzone in respect to the grinding direction. Formation and disruptionof surface layer on ball is one general process covering the wholeball surface. As soon as the disruption is initiated the rise ofstresses accompanied by the rise in temperature leads to theremoval of the whole layer (Fig. 6c and d).

Debris of nACRo coating are hard (Table 1) and do nottend to adhere back to the track resulting in extreme wear rateswhen tested by CETR with horizontal disk plane (debrismore likely remain in the track). These debris are acting asabrasive particles leading to catastrophic failure of the coating(Figs. 4 and 5h).

According to the design of the microstructure the performanceof coatings in dry sliding may be allocated as DLC–nACRo–AlTiN–TiCN ranging from the most resistant to the less resistant thattakes place when tested by MMTS with high inertia. Similarranging was observed during abrasive, erosive and impact testing[12]. However nACRo and AlTiN has lower resistance to wearcomparing to TiCN if tested by CETR that is explained by accumu-lation of debris and by generation of additional vibration caused

Fig. 8. Basic stages (1 to 5) of ceramic YSZ ball wear during dry sliding against PVD coating. Evolution of the COF is indicated by the position of the number of stage.

Table 3Results of EDS analysis of worn surfaces of YSZ ball and AlTiN coating after testing with MMTS (inertia—26 kg, duration—1000 revolutions). Locations are depicted inFigs. 7 and 8.

Material Position Elements (wt%)

Description Designation N O Al Ti Fe Zr W

YSZ ball Fresh outside the wear scar EDS 1.1 – 36.8 0.0 0.0 – 63.2 –

Transferred layer EDS 1.2 – 59.3 18.1 7.7 – 14.9 –

After removal of transferred layer EDS 1.3 – 37.1 0.4 0.1 – 62.4 –

AlTiN coating Fresh outside the wear scar EDS 2.1 26.3 1.5 35.1 37.1 0.0 0.0 0.0Worn EDS 2.2 26.0 2.2 34.9 35.4 0.9 0.1 0.5Thin transferred layer EDS 2.3 3.1 43.0 17.2 22.8 8.2 5.0 0.7Thick transferred layer EDS 2.4 8.3 31.1 18.6 23.4 5.4 12.6 0.6After removal of transferred layer EDS 2.5 4.5 16.0 10.8 20.9 41.6 1.3 4.9

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by suspension required for devices with servo controlled loadapplication.

5. Conclusions

1.

Inertia of the loading system has significant effect on perfor-mance of ceramic–ceramic, ceramic–steel and steel–steel tri-bosystems enabling self-healing.

2.

Self-healing of the tribomaterials was observed through theembedment of the entrapped wear debris to the track whenloading system inertia is high. As the result of the embedment ahigh level of vibration was detected.

3.

Wear debris accumulated in the track result in the acceleratedwear of hard nACRo coating tested by CETR with horizontaldisk plane.

Acknowledgements

Estonian Ministry of Education and Research (Targeted financeproject no. SF0140062s08) and Estonian Science Foundation

(grants ETF 8850 and 8211) are acknowledged for the financialsupport of the research.

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