The Third Body Concept / D. Dowson et al. (Editors) (D 1996 Elsevier Science B.V. All rights reserved.
Friction and wear behaviour of plasma-sprayed Cr2O3 coatings in dry sliding against AISI D2 steel J.E. Fernandez8, Yinglong Wan&', R. Tuchoa and A. Rinconb
aDept. of MechauGd and Civil Engineering. University of Oviedo, Ctra. de Castiello, sin, 33204 Gijon, Asturias, Spain b ~ ) ~ i y s i c o - ~ ~ i e~nstitute m i c a ~ of R O G ~ S O I CSIC, ~ ~ ~ , h/iadrid, Spain
This study investigates the influence of sliding speed and norind load on the friction and bvear of plasmasprayed Cr2O3 coatings, i n dry sliding against AISI D2 steel. Friction and wear tests were perfonned i n a wide speed range 0.125-8 i d s under different noniial loads using a block-on-ring tribometer. SEM and EDS were eniployed to identify the niechrulical and clienlical changcs on the woni surfaces. A tangential impact wear inotlcl was suggested to explain the steep rising of wear froin the nliilinimn-wear to the ~iiaximuni-wear.The results show that tlie wear of Cr2O3 coatings increases with the rising of load. Secondly, tliere exist a minimum-wear sliding speed (0.5 mls) and a niilyiiiiiun-wear sliding speed (3 m l s ) to a Cr2O3 coating in dry sliding. With the iricrease of speed, tlie wear of Cr2O3 coating declines in the range 0.125 - 0.5 m i s , then rises steeply froin 0.5 niis through 3 mls, followed by a decreasing. The large variation of wear value with respect to speed can be explained by stick-slip at low speeds, tangential impact effect at median speeds and softening effect of Ilasli temperature at high speeds. I n addition, the wear niechnrusnis of a Cr2O3 coating i n dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a nlisture of abrasion and brittle fracture at lugli speeds. 1. LNTRODUCI'ION
Ceramics have received mucli attention in friction aid wear appli~itionsin industry, such as i n ceriunic engines, due to their high hardness, high chemnic;?l st abi I i t y , high anti -oxidation at high temperatures , and heat isolation properties. The high cost i n production and brittle character. however, will restrict the application of bulk ceranics in industry to a certain extent. For this reason, ceramic coatings onto materials wllich are cheap and reliable i n shock, such as steel, are more widely employed. The ceranuc coating serves as an anti-wear layer and the steel substrate acts as a shock-resistant support. Main iniportnnt thermal spray processes for ceramic coatings are plasma-spray and detonation spray, since a coating of 0.3 nun tluck a i d with about 1%S'% porosity ~ i i ibe obtained. Thermally-sprayed ceramic coatings, such as Cr2O3, WC-Co, A1203, 'I'i02, etc. have been investigated tiibologicnlly at
room and high temperatures i n dry and lubricated sliding [ 1-41 Among them, a tlieriiially-sprayed Cr 203 coating gives the highest tvenr-resistance both i n dry and lubricated cases. .4dditivcs could significantly reduce the friction aid wear of plasmasprayed C q O 3 coating iJ-61, A plasma-sprnyetl Cr2O3 coating, could have failure mcclinnisms i n sliding, such as plastic defonnation, adhesion. and brittle fracture [ 11,7,8]. The speed and load ranges employed i n all tribological studics so far are liniited. Previous studies show that sliding speed a i d load have ;I strong influence on the wear behaviours of metals [9-1I ] aid sintered ceramics [12-141. The author's recent work reveals the substantial inlluence of sliding speed and norind load on tlie wear rate of plasma-sprayed A1203 coating [ 151. Therefore. tllis work aims at investigating friction and wear behaviour of the most important ceramic coating i n industry. Cr203 by plasma spraying, i n a wide range
490 of sliding velocity (0.125-8.0 m/s) under different loads, and trying to understand tlie relationship between friction, wear and the test conditions (speed, load. temperature) froin a new point of view. 2. EXPERIMENTAL DETAILS
2.1 Test Equipment Friction and wear tests were conducted on a self made block-on-ring friction aid wear tester as shown in Fig. 1. I t has a confoniid contact geoiiietry of thc specimens with a conformal contact area of 1 cm2. The ring speciinen is driven to rotate against the block specimen by a 4.3 kW DC motor. The speed of the ring can be varied from 0 rpm lo 3000 rpm (corresponding to a linear velocity r
2.2 Test Materials Tlie block specimens of steel AISI 1020 were coated with Cr2O3 by ambient plasma-spray using h4ETCO 9 ME3, 40 k W equipment and the spray parameters suggested by the manufacturer. The prolwrties of the coating are listed i n Table 1. The ring specimens were made of steel ( A N D2) hardened ,and tempered with hardness HRC 60. Table 1 Properties of used plasma-sprayed Cr2O3 coating Coniposition
99%: C q O 3 Balance: other oxides Trade inark of the powder METCO 106FH Powder size @in) 15-45 Melting point (OC) 2435 Tluchiess after polishing(mii) 0.3 fiwdness (Hv0.3) 1500 Porosity (%) 5 Rougluiess after polishing Ra ( p i n ) 0.3 Bond strength (MPa) 59-63 Density (g c111-~) 4.9
on Contact Sudace
Figure 1. The contact geometry of the used blockon-ring tribometer. The wear was measured with a Mettler AE 200 Weigher with a precision of 0. I nig. The voluine loss is then obtained by dividing tlie weight loss by the density of the specimen milIerid. Woni surfaces were examined by a JEOL-6100 SEM and analysed chemically by a Link-ESLl000
2.3 Test Conditions The tests were conducted wit11 the conditions listed in Table 2. Tlie speciiiiens were cleaned with acetone in an ultrasonic bath for 10 minutes before testing. A average value of three tests was taken for each data point. Proper test durations were chosen to make sure that tests run in the steady-wear regime for a fairly long time.
3. TRIBOLOGICAL ANALYSIS RESULTS
TEST AND SEM
3.1 Friction and Wear Results 3.1. I Friction coefficients versus speed The dry friction coefficients of Cr2O3 / steel AISI D2 pair under a nonnal load 61.3 N are plotted versus sliding velocity from 7.8 x I O - ~to 8 nils and illustrnted in Fig. 2
Table 2 Test conditions Contact Materials Outer diameters of ring specimnen,mnni Apparent contact area,mm2 Normal loads, N Apparent contact pressures&Pa Sliding speeds, ni/s Test durations, m Lubricant Eiiviroiunent temperature,OC Environment hunidity ,?6 Nuniher of tests for each data point
maximum and minimum friction coefficients was very large, indicating the existence of stick-slip phenomenon.
Block-on-Ring,conformal Block: C q Q coating Ring: AISI D2 steel, HRC60 062 100 61.3-133 0.613 -1.33 0.125 - 8.0 7500
3.1.2 Curves of wear-sliding time Figure 3 gives the wear changes with sliding time of Cr2Q I steel AISI D2 in dry sliding at a velocity of 1 mls under a normal load of 88.5 N. The wear volunies of both Cr2O3 coating and steel AISI D2 rose with the increasing of time steadily in the testing period (4x103 cycles. 7500 in).
. CrZOWrteel AISI D2 Dry rlldlng 8 - Room Temperature
10 0- 0- 0-
Sliding Velocity (mls)
Figure 2. Dry friction coefficient of the (21203 I steel AISI D2 pair versus sliding speed under a nonnal load 61.3 N The minimum and maximum friction values for each speed were obtained from a ten-minute friction test. It is noted that both the fluctuation and the average value of friction coefficient decreased withthe increasing of sliding velocity. In the speed rnnge 0.031 mls - 0.375 mls, the variation between
o^ 6 E
, I m/s
r t d AIYDZ
Slldlng Cycler (xl.E3)
Figure 3. Curve of wear-sliding time of the Cr2O3 / AISI D2 steel in dry sliding at a velocity 1 m/s under nornial load 88.5N.
492 3.1.3 Influence of load on wear The dry wear results of Cr203coating against normal load at velocities 1 mls and 3 inls are shown in Fig.4. As expected, the wear of Cr2O3 coating increases steadly with the rising of normal load.
Cr20W1tccl AlSI Dry illdlng Slldlng dlrtancc 7500 m
AISI D2 steel (ring specimen) were much higher in the speed range 0.125 - 1 mls, but lower i n the speed range 2 - 8 mls. The reason that the wear of the Cr2O3 coating was even higher than that of steel at high speeds in dry sliding may be attributed LO the particle size of the used Cr2O3 powder (15-45 pm). A better iuitiwear perfonnaice of Cr2O3 coatings could be expected with powder size 5-25 pin (METCO 13GF). The minimum-wear speed and maximum-wear speed will be discussed i n the section "Disciission 'I.
sh z575zmirsl Dl Dry Wing
W a r Of 0203, 133N Wsir of s t d A N 02. 133N
Wur ot 0203.61.3 N W w of s t d *Lu Dz, 61.3 N
Figure 4. The dry wear results of Cr2O3 coatings versus nonnal load at velocities I mls and 3 nils 0
3.1.4 Influence of speed on the wear of C q O 3 coating and AISI D2 steel Tlie iilfluence of speed on the dry wear values of the Cr203 I AISI D2 pair under iioniial loads 61.3 N and 133 N are presented in F i g 5 It is seen that there exists a nlirlirnuii wear speed (0.5 mls to both Cr2O3 coating and AISI D2 steel) and a niaxiniiini-wear speed (3 nils to CqO3 coating and I i d s to AISI D2 steel). To Cr2O3 coating (block specimen). wear declined with increasing of speed in the speed range 0.125 - 0.5 nils, and rose drastically with tlie risiiig of speed in the range 0.5 3 nils, a i d then decreased with the increasing of speed in the range 3 to 8 nils. To AISI D2 steel (ring specimen), the wear values were fairly high when speeds were lower thai the minimum-wear speed (0.5 m l s ) . With the increasing of speed from 0.5 m l s , the wear value increased from 0.5 m l s to 1 inls mid reached a maximum at 1 m l s , followed by a decleming from 1 inls to 4 ids, aid then remaining nearly constait. It was also noted that coiiipiied with the wear values of the Cr2O3 coating (block specimen), tlie corresponding wear values of the
Slidina Velocity (m/r)
Figure 5. The inlluence of speed on the dry wear values of the Cr2O3 (block) l AISI D2 steel (ring) pair under normal loads 6 1.3 N and 133 N
3.2 SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Spectrum) Analysis Results As a refereiice, the surfirce of a pliisma-sprayed Cr2O3 coating after griiidiiig before wear test is demostrated in Fig.6 The surface i n Fig 6 posseses nucropores and iiucro fractures froiii the grinding. The worn surface of a Cr2O3 coating i n dry slidiiig against AISI D2 steel under 133 N normal load at speed of 0.25 inls (wear results see Fig.5) is given in Fig. 7. Figure 7 indicates the existence of a rather thick surface layer. Tlie element percentage of EDS in Fig. 7 (Fe, 77%; Cr, 21% in weight) reveals that there existed severe steel transfer. On the other hand,
493 adhesion damages were also observed. Therefore the wear mechanism in this case was adhesion damage to the CqO3 coating arid material transfer from steel to Cr2O3 coating.
The worn surface of a plasma-sprayed Cr2O3 coating in dry sliding versus AISI D2 steel under 133 N nornial load at sliding velocity of 0.5 mls (wear data shown in Fig.5) is illustrated i n Fig.8. The coverage of the transfer film of steel in Fig8 is relatively less comparated with that in Fig.7. The elements weight percentage (64%Fe, 35%Cr) i n Fig.8 from EDS verified t h s observed results. No severe adhesion damages were observed. This was the surface where a minimum-wear value was achieved. The wear mecharism in this condition was also adhesion
Iigure 6. The surface of ~1 plasma-sprayed C q 0 3 coatiag after grinding bcfore wcar lest
Figure 8. The woni surface of plasma-sprayed Cr2O3 coating in dry sliding versus AISI D2 steel under a 133 N normal load at sliding velocity 0.5 m/s (minimum wear shown in Fig.5).
Figure 7. The worn surface of a C q O 3 coating in dry sliding against AISI D2 steel under a 133 N
riorinal load at speed 0.25 m/s (wear results see Fig.5).
Figure 9 shows the worn surface of a plasmasprayed Cr2O3 coating in dry sliding against AISI D2 steel under 133 N noriiial load at sliding velocity 3 m/s, under which a maximum-wear appeared (see Fig.5). The whole surface was full of micro brittle fractures. which was quite different froin the worn surfaces at speeds of 0.25 and 0.5 inls as shown in Figs.7 and 8. The EDS element weight in Fig.9 ( 13%Fe, 86%Cr) suggested a considerably declined transfer of steel compared with the cases at speeds
494 0.25, 0.5 and 1 mls. It is noted that the dominated wear mechanism at speed 3 mls (maximum-wear speed) appeared to be brittle fracture.
Fig.10. The worn surface of plasma-sprayed 0 2 0 3 coating in dry sliding against AISI D2 steel under 133 N normal load at sliding speed 5 mls. Figure 9. The worn surface of plasma-sprayed Cr2O3 coating in dry sliding against AISI D2 steel under 133 N normal load at sliding velocity 3 mls, under which a maximum-wear appeared (see Fig.5). The worn surface of a plasma-sprayed Cr2O3 coating in dry sliding against AISI D2 steel under 133 N normal load at a sliding speed 5 mls is shown in Fig10 (wear data given in Fig.5). The EDS element percentage of Fig. 10 is 14%Fe,85TKr. Fairly large brittle fractures and abrasive tracks were clearly observed. The dominating wear niechanisms were brittlc fracture and abrasion (possibly due to the detached C r 2 0 3 debris particles). The element weight percentage on the worn surface of plasma-sprayed Cr2O3 coatings plotted versus sliding speed is given in Fig.11. It is seen that the transfer qimatity of steel was very high at speed 0.25 m l s (77%Fe), and declined considerably with the rising of speed to 13%Fe at speed 3 mls, then remained sinall at speeds higher than3 mls.
ED9 Analyala Reaulta on Surfacer of Cr203 B l a b
Condltlona: Dry rlMlng aplnat ltecl A191 133 N load
Fe element Cr element
Figure 11. Elemental weight percentage on the worn surface of plasma-sprayed Cr2O3 coatings in dry sliding versus AISI D2 steel under 133 N load at different sliding speeds. 4. DISCUSSION
There exists a minimum-wear speed (0.5 nils for both Cr2O3 coating and steel) and a niaximun-wear
495 speed (1 mls for steel and 3 m l s for Cr203 coating) i n Fig.5 for both 133 N and 61.3 N normal loads. A minimum-wear velocity 0.5 mls was observed with Yelf-niated sintered ceramics SiAION, A1203 PSZ and SSC (sintered silicon carbide)  and with ruckel on nickel [lo]. A minimum-wear velocity 1 mis was reported with brass on steel , copper on copper mid gold on gold [lo]. By changing the stiffness in the apparatus, Soda et al. [lo] found that the high wear at speeds lower than the minimumwear speed came from the stick-slip friction process and decreased to the same value as that at the minimum-wear velocity when a much higher stiffness was employed in the apparatus. The large variation of friction coefficient in the speed range 0 03 - 0.5 mls shown in Fig. 12 verified the existence of stick-slip processes. The magnitude of the variation decreased with the rising of speed. Corresponding to the stick-slip process was and irdliesive wear mechanism with material transfer (mainly from steel surface to the Cr2O3 coating). The amount of iron transferred to Cr2O3 surface declined considerably with the increasing of speed HS can be seen in Fig.11 was consistent to the 11uctuation of friction coefficient at low speed. It is iinderstadable that higher lluctuation in friction will lead to hgher wear value. In the speed range 0.5 - 3 mls, the wear of a Cr203 coatings increased sharply with the rising of speed as demonstrated in Fig.5. The failure type of brittle fractures at speed 3 mls as shown in Fig.9 xtivates tlie authors proposing a tangentid impact wear model to explain the wear increasing from 0.5 mis through 3 nils. The suggested tangential impact near model is illustrated i n Fig. 12(a,b,c,d). When tlie two moving surface are loaded, interception in the sliding direction may happen between tlie asperities of tlie two surfaces. as demonstrated in Fig.l2(a). It should be pointed out that tlie sliding of the two surfaces is actually a discontinuous process and the moment before the IWO asperities m,&e contact is shown i n Fig. 12h). After the two asperities make contact, two lunds of results could appear depending on the speed: (1) in cxse of low speed, plastic flow may take place and no brittle fracture happening to the asperity as indicated i n Fig.l2(c), as verified by SEM
photograph in Fig.8at speed 0.5 m/s; (2)when the speed is high, the tangential impact effect will produce a brief extremely high stress inside the asperity and the asperity will be fractured as a wear debris as shown in Fig.l2(d), as supported by the brittle fractures of the worn Cr203 surface at speed 3 nils (see Fig.9).
Moving velocity V
Moving velocity V
Low velocity VI
Figure 12(a,b,c,d). Tangential impact wear model: (a) idealised asperity contact under load, showing interception in the direction of moving between asperities; (b) one moving asperity and one fixed asperity before contact; (c) after contact with low speed; (d) tangential impact contact with high speed The impact stress inside the asperity and consequently the fracture rate (wear rate) of the asperity will be proportional to the sliding speed. The gradient of the wear rising with increasing of speed from mini mum - w ear to maxi mum - wear depends on the fracture tougluiess of the material. A lower fracture toughness will result in a higher gradient of wear rising. The reason that beyond speed 3 mls the wear value decreased with tlie rising of speed (see Fig.5) may be attributed to the effect of the flash
496 temperature. A high flash temperature will soften the asperities i n contact and ease the tangential impact effect. Tlie softening effect will be eillianced with tlie rising of flash temperature (namely with the increasing of speed). When a flash temperature is sufficiently high. the hardness declining and eventually plastic deformation will become a coinparable factor to tlie tangential impact effect. As a result, the wear will reduce with the increasing of speed. The plastic flow shown in Fig.10 (speed 5 ids) support tllis argument. When a flash temperature will commence to play an important role i n reducing the wear w i t h increased speed depends on the inel ling-point of the material. Since tlie melting point of steel (about 1500 OC) is lower than that of Cr2O3 coating (2435 OC), a flash temperature at speed 1 m l s might start influencing tlie wear-reducing process of steel instead of at speed 3 m l s as was tlie case for Cr203 coating (see Fig.S). 5. CONCLUSIONS The results presented above ca~ibe siiiiuiinrised as roiiows: 1) The wear of a plasma-sprayed Cr2O3 coating increases w i 111 increasing load. 2) There exist a miilimum-wear sliding speed (0.5 nils) and a maximum-wear sliding speed (3 m l s ) to the wear of a plasma-sprayed Cr2O3 coating in dry sliding against AISI D2 steel. With the increasing of speed, the wear of Cr2O3 coating declines in the range 0.125 - 0.5 mls, then rises steeply from 0.5 i d s through 3 m l s , followed by a decreasing. 3)The severe fluctuations of dry friction coefficient at speeds lower than 0.5 nils for CqO3 /steel pair, which conies from a stick-slip process, causes higher wear to Cr2O3 and steel than the minimum-wear. 4) The proposed tangential impact wear model could explain the steep rising of wear of Cr2O3 coating from 0.5 to 3 nils. The decreasing of Cr2O3 wear beyond 3 iiils may be attributed to softening effect of the llasli temperature. 5) Tlie wear mechallisins of Cr2O3 coatings i n dry sliding versus AISI D2 steel are adhesion at low
speeds (typically 0.25 mls), brittle fracture at median speeds (typically 3 m i s ) and a mixture of abrasion and brittle fracture at high speeds (typically 5-7 mls).
ACKNOWLEDGEMENT We are deeply grateful to the financial support by FICYT in Spain under the auspices of Principado de Asturias: "Tribological behaviours of plasmasprayed ceramic materials, thennoplastics and antifriction inaterials in meclirulical systems."
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