In situ Acoustic Emission for wear life detection of DLC coatings during slip-rolling friction

Wear 260 (2006) 469–478

In situ Acoustic Emission for wear life detection of DLC coatingsduring slip-rolling friction

M. Lohr, D. Spaltmann∗, S. Binkowski, E. Santner, M. WoydtLaboratory VIII. 11 Friction and Wear Test Methods, Component Tribology, Federal Institute for Materials

Research and Testing (BAM), D-12200 Berlin, Germany

Received 5 October 2004; received in revised form 23 February 2005; accepted 8 March 2005Available online 14 April 2005

Abstract

Different diamond-like carbon (DLC) coatings on a steel substrate (100Cr6) were tested under slip-rolling friction conditions againstuncoated counter bodies of the same steel. The initial maximum Hertzian pressure was varied in a range ofP0 = 1.5–2.3 GPa. The friction testswere carried out under dry conditions and with an unadditivated paraffin oil as lubricant. It could be shown that the thickness of the coatingsaffects the respective wear life. Further, a very important factor for the wear life of a coating under lubricated slip-rolling conditions is ther ssion (AE)s©

K

1

sf[miiittApcmwst

hereionsappli-AE

nglu-

plested

)nd-tion

l testne) inout

0d

oughness of the surface of the respective counterbody. The wear life tests were monitored by recording in situ the Acoustic Emiignals. Some causes for a high AE activity could be identified.2005 Elsevier B.V. All rights reserved.

eywords: Wear life; Slip-rolling friction; Diamond-like carbon (DLC); Acoustic Emission; Lubrication; Roughness

. Introduction

Diamond-like carbon (DLC) coatings show good wear re-istance and low coefficients of friction (COF) under slidingriction (as low asf < 0.01), especially under dry conditions1]. DLC coatings also lead to low wear on counterbodiesade of steel[2]. DLC coatings are already used in ball bear-

ng races or cages, as protection of hard disc drives, as well asn a number of moving parts inside automobile engines (e.g.n diesel engine injection systems)[3]. The field of applica-ions can extremely be widened, if they are fatigue resistanto slip-rolling conditions at contact pressuresP0 > 2.2 GPa.s the wear life of such DLC coatings is important for theerformance of the respective ball bearings, gears, etc., DLCoatings were tested here. In the present study, the perfor-ance of different DLC coatings under slip-rolling frictionas examined. The well-known analysis of Acoustic Emis-ion (AE) is already used successfully in tribology. Slidingests of CrN[4] and DLC coatings[5,6] were monitored in

∗ Corresponding author. Tel.: +49 30 8104 3594; fax: +49 30 8104 1817

situ by using the AE. In contrast to these sliding tests, wat least one sample is at rest, under slip-rolling conditboth samples are in motion and as a consequence, thecation of AE is more demanding. We have introduced theto monitor in situ different DLC coatings under slip-rollifriction with the intention to elucidate the origin and evotion of wear patterns during these tests.

2. Test set-up

In an Amsler type twin-disc wear tester, steel sam(100Cr6) with a DLC coating were tested under lubricaand dry slip-rolling friction. Paraffin oil (η40 = 47 mPa swithout additives was used as lubrication medium (bouary lubricated conditions). As an advantage, this lubricamedium contains no toxic substances. The tribologicaconditions are listedTable 1. In Fig. 1, the testing conditiois illustrated. Two specimens (altogether a test samplmotion rolled on each other with an applied slip of ab10%. Initially, maximum Hertzian pressures ofP0 = 1.5, 1.9

E-mail address: dirk.spaltmann@bam.de (D. Spaltmann). or 2.3 GPa could occur in the centre of the contact area

043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2005.03.009

470 M. Lohr et al. / Wear 260 (2006) 469–478

Table 1Tribological test conditions used in the Amsler type twin-disc wear tester

Specimen dimensions DiameterD: 42 mmThicknesst: 10 mm

Sample shape Basic body/substrate: cylindrical, lateral contact areaCounterbody: spherical, lateral contact area (R: 30 mm)

Tested materials Basic body/substrate: steel 100Cr6 (fine polished, HRC60) coveredwith a thin DLC coating

Counterbody: steel 100Cr6 (rough polished, HRC60)

Applied loadsa Normal forceFN (N): 338 660 1180Hertzian pressureP0 (GPa): 1.5 1.9 2.3

Speed Basic body: 390 RPMcounterbody: 354 RPM

Tribological use Slip-rolling friction with a slide velocity of0.08 m/s (10% slip)

Number of revolutionsN Maximum 10,000,000 revolutionscorresponding to a sliding distances of121,600 m

Test atmosphere Ambient air (temperature 20◦C, relativehumidity 50%)

Lubricant Paraffin oil (without additives,η40◦C:47 mPa s), dry

a Applied loads as normal forceFN and the corresponding initial maximum Hertzian PressureP0.

according to the respective test conditions. The tests underdryslip-rolling conditions were carried out at an initial Hertzianpressure ofP0 = 1.5 GPa.

DLC coatings were tested here as a wear protection coat-ing in steel contacts. A rolling bearing steel (100Cr6) servedas the substrates of the DLC coatings and also as the mate-rial for the counterbodies. Only the cylindrical specimen wascoated with an interlayer and a DLC coating with a thicknessof about 1–10�m. The coatings of six different supplierswere monitored by using AE. The properties of the coat-ings are listed inTable 2. Their tribological behaviour in thepresent tests is only discussed here as far as it is important

for understanding of the AE signals. (More details on the onthe tribological behaviour of the coatings can be found inRefs.[8,9].) For further information about each test, refer tothe code number presented here. It consists of different lettersdenoting the respective suppliers and a number indicating thetest number of a DLC coating tested of one supplier.

The well-known cause for the failure of rolling elementsin bearings is localized defects, in which large pieces of thecontact surface are dislodged during operation. Therefore, theappearance of such a defect with the size of 1 mm2 (the size ofthe Hertzian contact area atP0 = 2.3 GPa) was chosen as thecriterion to mark the end of the wear life of the DLC coating

F ster an slip-rollf

ig. 1. Picture of a test sample in the Amsler type twin-disc wear teriction.

d sketch of the acoustic coupling of the AE sensor under lubricateding

M. Lohr et al. / Wear 260 (2006) 469–478 471

Table 2Properties of the different DLC coatings[7], which were monitored by using Acoustic Emission

Code numbers Type of DLC coating Interlayer/bondcoating

Implantation/dopant Thickness of theDLC coating (�m)

Modulus of elasticityE (GPa)

A7–A11 a-C:H:Si Si Si 10 161B5–B7 a-C:H:Me Ti, W Ti, W 2–3 No informationD1–D4 a-C:H Ti None 2 256D6–D9 a-C:H Ti None 1 256E2–E5 a-C:H:Me Cr W 1–2 150–170F1–F4 a-C:H:Si Si Si 3–5 251F5–F7 No information No information No information No information No informationG1, G3, G4 ta-C Ti None 3 500

Most of the coatings listed here were prepared using a plasma enhanced chemical vapour deposition method. (The coatings have not been specifically adjustedto the test conditions.)

tested (1 mm2-criterion[9]). If a DLC coating did not exhibita defect of such a size after about one million revolutions,it was rated as ‘resistant’ to slip-rolling fatigue. In order todetermine the end of the wear life, the tests were interruptedafter logarithmic intervals to examine the surfaces by an op-tical microscope. This procedure was time consuming andled only to a rough estimation of the wear life, because af-ter the respective test intervals the sizes of spallings largerthan 1 mm2 varied on a large scale. In order to get a finerestimation of the wear life and to save time, it was importantto introduce a technique, which was able to detect defectsin situ, while the tests were in progress. Therefore, the weartests were monitored by AE and the friction torque (COF). Ifa very intense AE activity was observed, then the respectivetest was stopped outside the defined intervals. In general, if asystem is nearing the end of its wear life, the friction torqueis dramatically increasing. However, under lubricated slip-rolling friction conditions the friction torque did not changesignificantly, even if the DLC coating was removed com-pletely from inside the wear track[9]. Under dry conditions,the COF (in averagef ∼ 0.2–0.4, depending on the type ofDLC coating) was no clear indicator either for damages tothe coatings, as it no necessarily increased due the failure(as one might expect), but in several cases even decreased.Complicated tribochemical reactions were probably the rea-

, the

oredAE

signals were detected by means of an AE sensor (PAC-�30)with a resonant frequency of about 270 kHz. The sensor wasarranged in a distance of about 1 mm to the centre of thecontact area. The sketch inFig. 1shows that the oil served alsoas coupling medium. Under dry slip-rolling conditions, theambient air served as couplant. The output of the sensor wasamplified 40 dB (lubricated) and 60 dB (dry) by a PAC 2/4/6pre-amplifier. The height of the pre-amplification is also aquestion of the coupling medium. Paraffin oil is a much betterconductor (about 1000 times;∼60 dB) for acoustic signalsthan the surrounding air. This is why the pre-amplificationfor air as coupling medium is higher than the one for paraffinoil. Every AE signal which exceeded the threshold of 61 dB(lubricated) and 45 dB (dry) originated from the contact areaand was stored as a hit by an AE measurement system (PAC-DISP). The energy in aJ (atto = 10−18) of each hit was used.The energyE was calculated according to

E = 1

R

∫ t∗

t=0A(t)2 dt

with the resistanceR = 10 k� in the pre-amplifier,t the timein seconds, the time-dependent amplitudeA in volt duringexceeding the threshold of 61 or 45 dB and the durationt* in seconds. The function of the threshold is to suppressthe operating noise of the testing rig. Therefore, the testing

plesThe

be-d in

. InAEhere,

ty, AE0 (res

n-lubri

nt airdB

son for the different behaviours of the COF. In contrasttests were monitored successfully with AE.

3. Acoustic Emission (AE)

Sources of AE are events with a rapid release of ststrain energy like cracking and crack propagation. The

Table 3Configuration of the Acoustic Emission set-up (1 dB corresponds 1�V)

AE parameter ActiviSensor PAC-�3

Test conditions Dry/u

Coupling medium for AE AmbiePre-amplification PAC 2/4/6 (integrated filter: 100–1200 kHz) 60Threshold 45 dB

rig was started under experimental conditions with sammounted, but without bringing the samples into contact.threshold was then adjusted to prevent the noise froming detected. The configuration of the AE set-up is listeTable 3.

For monitoring the tests the AE activity was recordeddetail, the AE activity describes the number of singlehits in a number of revolutions. For the tests presented

hits (energy of each hit in aJ (atto = 10−18) in a number of revolutions)onant frequencyfR of about 270 kHz)

cated Lubricated

Paraffin oil40 dB

61 dB

472 M. Lohr et al. / Wear 260 (2006) 469–478

two extreme stages for the AE activity were observed. Oneextreme was the performance without any damage of the DLCcoating under dry and lubricated slip-rolling conditions. Theother extreme was the performance without a coating.

Fig. 2shows optical microscope images to document thedifferent tribological behaviour of DLC coated (a and b)and uncoated (c and d) test samples under lubricated anddry slip-rolling friction. Hertzian pressures ofP0 = 1.9 or2.3 GPa (lubricated) andP0 = 1.5 GPa (dry) were applied.The corresponding AE activity is shown in the AE graphs. Aslong as a DLC coating under lubricated and dry slip-rollingfriction stayed undamaged, no AE activity was recorded. Incontrast, the tribological behaviour was quite different. Un-der lubricated slip-rolling friction, the coating did not showany obvious changes. Instead, the counterbody was polishedin the wear track from rough (RZ ≈ 3�m) to fine (RZ < 1�m)at the beginning of the test. Under dry slip-rolling friction,a transfer layer from the coating to the counterbody wasformed and covered the wear track of the counterbody. Thistransfer layer reduced the COF significantly[8,10].

In general, tests of coated samples are compared to tests ofuncoated ones. This allows to assess the tribological perfor-mance and the acoustic behaviour of the DLC coating. Theuncoated test samples were examined after a total number of1,000,000 (lubricated) and 20,000 (dry) revolutions. A highAE activity was recorded due to the contact of steel againsts ue orf eno ands trate( heatf ionb ed byf

s andt r highA ari ant,i ife.A asa es ofs ity).I atedt ce the ityw asc

4

4

c ialH re

tested is listed in the second column of the tables. The du-ration of the respective test specified as the total number ofrevolutions is summarized in the third column. The num-ber of revolutionsN, the samples lasted till the AE criterion(high AE activity) and the 1 mm2-criterion was reached isrecorded in the fourth and fifth row. Those samples that didnot show a high AE activity or the 1 mm2-criterion after atotal of 1,000,000 revolutions were rated resistant to slip-rolling fatigue and are marked bold in the respective tables.(If there is no number in the columns of the failure criterions,the samples did not fail at all.)

In all tests, under lubricated slip-rolling friction in whichtest samples reached the 1 mm2-criterion, a high AE activitywas recorded too. In many cases, a high activity was regis-tered much earlier. Some samples, which proofed to be resis-tant to slip-rolling fatigue, were used for prolonged tests of upto 10,000,000 revolutions. The test samples B7, E3, F1 andG3 reached the 1 mm2-criterion after 1,400,000–9,600,000revolutions. However, the AE activity suggested the onset ofthe failure of these samples in the prolonged tests as early asone-tenth of the revolutions lasted.

DLC coatings of four suppliers were tested under dry slip-rolling friction. All samples were tested with a Hertzian pres-sure ofP0 = 1.5 GPa. None of these DLC coatings could berated as resistant to slip-rolling fatigue. The test samples withthe code numbers B5 and E5 failed according to the 1 mm2-c AEa ingso rlyi

in-fl oat-i Ones f thes face( ghp gf rana ndardc s, re-sTa tal of4 tingt thet n the‘ ck-n esti-g rep fei -b leD ss of2 ro-s mbero ayed

teel and due to single damage events caused by fatigretting. Under lubricated slip-rolling friction, both specimf the test sample showed slightly plastic deformationpallings in the wear track. The wear track of the subsfine polished) was blue coloured, probably due to theormation in the contact area. Under dry slip-rolling frictoth specimens showed heavy damages mainly caus

retting.As a conclusion, the direct contact of the steel surface

he resulting damage events were single sources of hits oE activity. In contrast, a DLC coating without any (we

nduced) damage showed no AE activity. This is importf the AE is to apply as an in situ detection tool for wear ln undamaged DLC-coating is able to fulfil its functionwear protection layer, i.e., it can keep the steel surfac

ubstrate and counterbody apart (resulting in no AE activn the case of coating damage, a high AE activity indiche undesirable steel-to-steel contact and as consequennd of service/lifetime of the coating. A high AE activas called AE criterion. In the following, this criterion wompared with the 1 mm2-criterion.

. Results

.1. Results of the tribological tests

The results of the tests are listed inTable 4for the lubri-ated and inTable 5for the dry experiment set. The initertzian pressureP0 at which the different samples we

e

riterion. However, this was not accompanied by a highctivity. Nevertheless, in the dry tests of the DLC coatf the other two suppliers a very high AE activity prope

ndicated the 1 mm2-criterion.One aim of the tribological tests was to examine the

uence of various parameters on the wear life of the cngs. (The respective tests were monitored by AE too.)uch parameter, which was altered, was the quality ourface finishing of the counterbody. A fine polished surRZ < 1�m) prolonged the wear life in contrast to a rouolished surface (RZ ≈ 3�m) under lubricated slip-rollin

riction. The coating in the two test samples A7 and A8gainst a rough polished surface of the counterbody (staase). They reached a total of 270 and 450 revolutionpectively, till the 1 mm2-criterion was fulfilled (seeTable 4).he corresponding test samples A9 and A10 (seeFig. 3) rangainst a fine polished surface and reached a higher to000 and 690,000 revolutions, respectively. It is interes

o note that the worst ‘polished’ sample (A9) withstandsest conditions nearly one order of magnitude longer thabest’ unpolished’ sample (A8). The influence of the thiess of the coating on the respective wear life was invated as well. InFig. 4, tests with coatings of supplier D aresented. A DLC coating of 2�m extended the wear li

n contrast to a thinner coating of 1�m. For the case of luricated slip-rolling friction inFig. 4a and b the test samp2 and D9 are presented. The sample D2 with a thickne�m was resistant to slip-rolling fatigue. The optical miccope images showed only minor defects after a total nuf 1,250,000 revolutions. The respective counterbody st

M. Lohr et al. / Wear 260 (2006) 469–478 473

Fig. 2. Comparison of the tribological behaviour and the corresponding AE activity between DLC coated substrates and uncoated counterbodies duringlubricatedand dry slip-rolling friction. The substrates (fine polished) and the counterbodies (rough polished) are made out of steel (100Cr6). The optical microscope photoswere taken after the tests; meanwhile the AE activity was recorded during the tests (in situ). (a) Lubricated, coated,n = 0–3000,P0 = 1.9 GPa, sample withoutany damage, D2; (b) dry, coated,n = 0–3000,P0 = 1.5 GPa, sample without any damage, D4; (c) lubricated, uncoated,n = 320,000–1,000,000,P0 = 2.3 GPa,sample with slightly plastic deformation and spallings; (d) dry, uncoated,n = 1000–20,000,P0 = 1.5 GPa, sample with heavy damage.

474 M. Lohr et al. / Wear 260 (2006) 469–478

Table 4Results of the tests under lubricated slip-rolling friction

Code number (remark) Hertzian pressureP0 (GPa)

Total number ofrevolutionsN

Total number of revolutionsNtill AE criterion

Total number of revolutionsNtill 1 mm2-criterion

A7 (Cb. rough polished) 2.3 270 70 270A8 (Cb. rough polished) 2.3 450 200 450A11 (Cb. rough polished) 1.5 2,800 2,000 2,800A9 (Cb. fine polished) 2.3 4,000 2,000 4,000A10 (Cb. fine polished) 2.3 690,000 23,000 690,000B6 1.9 1,000,000 – –B7 1.9 3,000,000 180,000 3,000,000D1 (2�m DLC) 2.3 3,000 50 3,000D2 (2 �m DLC) 1.9 4,500,000 – –D3 (2 �m DLC) 1.5 2,800,000 – –D5 (2 �m DLC) 2.3 10,200,000 – –D7 (1�m DLC) 2.3 72,000 48,000 72,000D8 (1�m DLC) 2.3 63,000 11,000 63,000D9 (1�m DLC) 1.9 60,000 40,000 60,000E2 1.5 79,000 55,000 79,000E3 2.3 1,400,000 700,000 1,400,000E4 1.9 5,900,000 – –F1 1.5 2,100,000 1,100,000 2,100,000F2 1.5 48,300 220,000 483,000F4 2.3 5,000 4,500 5,000F5 1.9 78,000 30,000 78,000F6 1.9 500,000 390,000 500,000F7 2.3 5,600 4,000 5,600G1 2.3 1,000,000 – –G3 2.3 9,600,000 1,100,000 9,600,000

Bold, resistant to slip-rolling fatigue; Cb., counterbody; –, sample did not fail up to the total number of revolutions.

polished. The AE graph showed a very low AE activity dueto single cracks and very small spalling events of the coating.In contrast, the coating D9 with a thickness of only 1�mreached only a total number of 60,000 revolutions. The coat-ing showed a spalling over 1 mm2 in size and a counterbodywhich stayed polished. The AE graph of the last test intervalof sample D9 indicated the transition from a low to a highAE activity. In this case, the source of the high AE activ-ity observed was the contact steel against steel only. As thespalling (Fig. 4b) grew in size sidewise and in the directionof rotation, the steel to steel contact increased as did the AEactivity betweenn = 8000 and 24,000 revolutions.Fig. 4c andd presents images of the DLC coatings D4 and D6 of supplierD, which were tested under dry slip-rolling friction. Due tothe tribological stress, the DLC coatings of the test sampleswere partly rubbed out off the wear track. Parts of the coatingswere transferred to the respective counterbody. The removalof the coatings happened suddenly. Coinciding with this re-moval of the coating was a sharp rise in the level of the AE

activity from virtually no AE activity to a very high activitylevel. The high AE activity was mainly caused by frettingevents caused by the contact steel against steel and appearedsuddenly at a total number of 90,000 (test sample D4, 2�m)to 6500 (test sample D6, 1�m) revolutions. Considering theAE graphs of the test samples the different tribological per-formances was indicated in situ by AE activity.

4.2. Identification of sources for a high AcousticEmission activity

Single cracks or spalling events (either in the DLC coatingor in the steel substrate/counterbody), the single plastic defor-mation of the counterbody and the contact between steel sur-faces are different sources for the single AE hits recorded inthe tests. Their influence on the level of AE will be discussedfor the lubricated slip-rolling friction tests.Fig. 3shows threetest samples, for which a high AE activity was observed. Thetest sample with the code number A10 showed a high AE

Table 5Results of the tests under dry slip-rolling friction

Code number (remark) Hertzian pressureP0 (GPa)

Total number ofrevolutionsN

Total number of revolutionsNtill AE criterion

Total number of revolutionsNtill 1 mm2-criterion

B5 1.5 3,000 – 3,000D4 (2�m DLC) 1.5 130,000 90,000 130,000DEG

6 (1�m DLC) 1.5 8,5005 1.5 100,0004 1.5 10,000

6,500 8,500– 100,000200 3,000

M. Lohr et al. / Wear 260 (2006) 469–478 475

Fig. 3. Different AE sources led to a high AE activity during lubricated slip-rolling friction. In all cases, the direct contact steel against steel was one reasonof a high AE activity. The first test sample shown with the code number A10 failed finally after a total number of 690,000 revolutions by the 1 mm2-criterion(a fine polished counterbody was used). The other test samples showed besides this “contact noise” other AE sources, like crack events in the coatingand the continuous damaging of the counterbody. (a) Lubricated, spalling < 1 mm2, n = 16,000–30,000,P0 = 2.3 GPa, A10; (b) lubricated, spalling > 1 mm2,n = 0–30,000,P0 = 2.3 GPa, D1; (c) lubricated, spalling� 1 mm2, n = 100,000–483,000,P0 = 1.5 GPa, F2.

activity much earlier as the 1 mm2-criterion was reached. Afine polished counterbody extended the wear life. Normally,a rough polished counterbody was used (RZ ≈ 3�m). In thiscase, the high AE activity originated mainly from the contactsteel against steel.Fig. 3a presents the optical microscope im-ages of the test samples A10 after a total number of 30,000revolutions and the AE graph of the interval from 16,000 to30,000. The counterbody was slightly damaged and the coat-ing showed only spallings of less than 1 mm2. After a totalnumber of about 23,000 revolutions the AE Graph indicatedthe AE criterion by a high AE activity. Spallings with an arealess than 1 mm2 could lead to a direct contact between the twosteel surfaces. This test was carried out till a total number of690,000 revolutions was reached and the surfaces in contact

were examined. The DLC coating showed a spalling largerthan 1 mm2. This test was terminated as the 1 mm2-criterionwas reached.

In contrast, the test samples with the code number D1and F2 reached the 1 mm2-criterion in the same test interval,in which the AE criterion was reached. In all these cases,the direct contact of steel against steel was one AE source.The optical microscope photos of the sample D1 taken afterabout a total number of 3000 revolutions showed a spallingmuch larger than 1 mm2 in the coating. The edges of the largespalling in the coating had dramatically damaged the respec-tive counterbody. The high AE activity, which was recordedimmediately, was due to the direct contact of steel againststeel, to the plastic deformation observed and to spallings of

476 M. Lohr et al. / Wear 260 (2006) 469–478

Fig. 4. Influence of different sizes of DLC coating thickness of supplier D and the corresponding AE activity during lubricated and dry slip-rolling friction.In the AE graphs, the AE activity is shown, means AE hits during the last test interval. With exception of the test sample with the code number D2 all failedby the 1 mm2-criterion. (a) Lubricated, 2�m DLC coating,n = 615,000–1,250,000,P0 = 1.9 GPa, D2; (b) lubricated, 1�m DLC coating,n = 30,000–60,000,P0 = 1.9 GPa, D9; (c) dry, 2�m DLC coating,n = 30,000–130,000,P0 = 1.5 GPa, D4; (d) dry, 1�m DLC coating,n = 0–8500,P0 = 1.5 GPa, D6.

M. Lohr et al. / Wear 260 (2006) 469–478 477

the counterbody. (Latest results suggest that the failure of D1is due to imperfections in the substrate rather than a failureof the coating.)

The last test sample shown inFig. 3 is F2. After a to-tal number of 483,000 revolutions, this sample reached the1 mm2-criterion by a very large spalling. Most of the coatingwas removed from inside the wear track. The counterbodywas plastically deformed. The AE graph showed the AE ac-tivity recorded during 100,000–483,000 revolutions. In thistest interval, the AE activity changed from a high AE activityto a very high AE activity. The very high AE activity was dueto the sudden contact of steel against steel and to the suddenheavy damaging of the counterbody as well as the coating.Probably the high AE activity which occurred before is dueto crack events in the coating.

5. Summary and conclusion

Wear life tests of different DLC coatings on a steel sub-strate (100Cr6) were carried out under slip-rolling frictionconditions against uncoated counter bodies of the samesteel. The initial Hertzian pressure was varied in a range ofP0 = 1.5–2.3 GPa. The friction tests were carried out underdry conditions and with an unadditivated paraffin oil as lu-b oftw et life.W pendo o bec earl ther . Im-p t ther of1 ndedb

n ex-c lifeo ighA ec-tc ctivec d‘ by:

• atingbody,

• bodyle to

• coat-indi-

After the termination of the tests, the spallings showed differ-ent sizes above 1 mm2. The total amount of coating materialremoved changed with the type and especially with the indi-vidual quality of each coating.

In the lubricated tests, the lubricant also served as cou-pling medium for the AE signals. Further tests under dryconditions have to be carried out in order to examine how thetribological behaviour influences attenuation of the recordedAE. However, a high AE activity occurred under fretting con-ditions which was induced through a large and deep spallingin a DLC coating.

Analysing the energy of the AE in addition to the AEactivity, crack and crack propagation on or in the coatingcould be identified as one source of a single AE hit[11].

The analysis of AE provides an easy and comfortable so-lution for monitoring the wear life of DLC coatings, as thetests have only to be stopped at the appearance of a high AEactivity. Therefore, using the AE has the following advan-tages:

• remote in situ monitoring of the tests;• saves time and money, especially in endurance tests (as the

tests do not have to be interrupted to check the surface ofthe coatings);

• detects the onset of undesirable wear due to the contactsteel against steel.

A

ngs-g tractD

R

gy-Ed.),tion.rak-W.po-

and434,

ted. 127

D.L.bonl. 67

ond-Solid

desd

ricant. As a criterion to mark the end of the wear lifehe coatings the appearance of a damaged area ofA > 1 mm2

as introduced (1 mm2-criterion). It could be shown that thhickness of the coatings affects the respective wear

hether it is extended or shortened seems also to den the type of the coatings. Further research will have tarried out in this matter. A very important factor for the wife of a coating under lubricated slip-rolling conditions isoughness of the surface of the respective counterbodyroving the surface finish of the counterbody such thaoughness values (Ra, RZ) are reduced by about a factor0, the wear life of the respective coating could be extey two to three orders of magnitude.

In the test presented here, the AE turned out to be aellent tool for an in situ estimation of the end of the wearf the DLC coatings. Under lubricated conditions, if a hE activity (AE criterion) was detected in situ, the resp

ive coating failed later on in the tests (1 mm2-criterion). Inases where no high AE activity was recorded, the respeoatings did not reach the 1 mm2-criterion and were labelle

resistant’ to wear. A high AE activity was mainly caused

the contact of steel against steel (spallings in the cocould lead to a contact of the substrate and the counterboth made of steel);a continuous damaging of the surface of the counter(the edges of larger spallings in the coating were abdamage the counterbody plastically);events like cracking and delamination underneath theing at the surface of the substrate (these events couldcate a sudden catastrophic failure).

cknowledgement

The authors would like to thank the Deutsche Forschuemeinschaft (DFG) for the financial support under conFG-SA 645/4.

eferences

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