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Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS FOR BEARING APPLICATIONS. H.E. Hintermann Centre Suisse d'Electronique et de Microtechnique. CH-2007 Neuchatel, Switzerland. Abstraet Microwelds betweeri balls and raceways in conventional steel bearings when operated at extreme conditions, with or without oil/grease lubrication, can be avoided if the surfaces of the bearing elements are protected by an intercalated diffusion barrier in the form of a strongly adherent, friction and wear lowering thin film, which is insoluble in the mating materials of the bearing elements. Titanium carbide, TiC, since a long time used on cutting tools, shows excellent such combined properties and improves the tribological, the corrosion and hot behaviour as well as the anti-welding, i. e. the "emergency-lubrication" properties of the tribosystem under consideration. The thickness after polishing of TiC-coatings is of the order of one to several micrometers . Further improvement can be attained by applying an additional thin film, of molybdenum disulfide.: MoS2, in thickness one order of magnitude less than for the hard (>3000 VHN20g) TiC-coatings, on top of the highly polished TiC-coating. This solid lubricating film is very soft, comparable to the hardness of polymer-films and susceptible to humidity. Results of application with high precision bearings (e.g. for gyroscopes) in aerospace mechanisms, single TiC 01' double TiC/MoS2 coated, are presented for illustration. FRICTION/WEAR BEHA VIOUR OF HARD COA TINGS. Dry Conditions. Hard coatings deposited by evo and/or PVD have bécome known to have a low coefficient of friction (~) against themselves and against steel, and to greatIy reduce wear (W) [1]. In representing the tribological data of various materials combinations as measured on a pin-on-disk machine in an environment of 50% relative humidity (rh) and without any additional lubricant in a "FRICTION-WEAR" Diagrarn, u-w, fig. 1 [2], it can be seen, that the results obtained can be allied to one of four easily distinguishable clusters of points, 1 to IV, i. e.: 1 : High friction - high wear. Il : Low friction - high wear (e.g. plastics). Ill: Low friction - low wear (tools, machine elements). IV: High friction - low wear (break materials). The combination with the most favourable friction and wear behaviour combined is taken as unity and serves as a standard. It is the combination TiC vs. TiN. As for the interpretation of this grouping, it will suffice here to note that TiC, TiN, SiC, Al203 against each other and vs. hardened steel, respectively, form very good friction and wear combinations. Oi) Lubricated Conditions The organization for Economic Cooperation and Development (OECD), International Research Group (IRG) on Wear of Industrial Materials, has established [3] a FN-V-T diagram for lubricated sliding concentrated steel contacts. FN is the normal force, v the sliding speed, T the oil bath temperature. A cross section of such a FN-v-T data surface at constant temperature T for a completely oil-submerged ball on ring contact is shown in fig. 2. Three operating regimes with transition zones in between can be distinguished: 1 : Regime of (partial) elastohydrodynamíc (EHO) lubrication. II : Regime of boundary lubrication. III: Regime oí virtually unlubricated contact ("scuffing regime"). Latin/smerican Journal of Metallurgy ami Materials, Vo1.l3, N" 1, 2. 1993.

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Page 1: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29

WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS FOR BEARINGAPPLICATIONS.

H.E. Hintermann

Centre Suisse d'Electronique et de Microtechnique. CH-2007 Neuchatel, Switzerland.

AbstraetMicrowelds betweeri balls and raceways in conventional steel bearings when operated at extreme

conditions, with or without oil/grease lubrication, can be avoided if the surfaces of the bearing elements areprotected by an intercalated diffusion barrier in the form of a strongly adherent, friction and wear loweringthin film, which is insoluble in the mating materials of the bearing elements. Titanium carbide, TiC, since along time used on cutting tools, shows excellent such combined properties and improves the tribological, thecorrosion and hot behaviour as well as the anti-welding, i. e. the "emergency-lubrication" properties of thetribosystem under consideration. The thickness after polishing of TiC-coatings is of the order of one toseveral micrometers .

Further improvement can be attained by applying an additional thin film, of molybdenum disulfide.:MoS2, in thickness one order of magnitude less than for the hard (>3000 VHN20g) TiC-coatings, on top ofthe highly polished TiC-coating. This solid lubricating film is very soft, comparable to the hardness ofpolymer-films and susceptible to humidity.

Results of application with high precision bearings (e.g. for gyroscopes) in aerospace mechanisms,single TiC 01' double TiC/MoS2 coated, are presented for illustration.

FRICTION/WEAR BEHA VIOUR OFHARD COA TINGS.

Dry Conditions.Hard coatings deposited by evo and/or

PVD have bécome known to have a low coefficientof friction (~) against themselves and against steel,and to greatIy reduce wear (W) [1].

In representing the tribological data ofvarious materials combinations as measured on apin-on-disk machine in an environment of 50%relative humidity (rh) and without any additionallubricant in a "FRICTION-WEAR" Diagrarn, u-w,fig. 1 [2], it can be seen, that the results obtainedcan be allied to one of four easily distinguishableclusters of points, 1 to IV, i. e.:

1 : High friction - high wear.Il : Low friction - high wear (e.g. plastics).Ill: Low friction - low wear (tools, machineelements).IV: High friction - low wear (break materials).

The combination with the most favourablefriction and wear behaviour combined is taken as

unity and serves as a standard. It is thecombination TiC vs. TiN.

As for the interpretation of this grouping, itwill suffice here to note that TiC, TiN, SiC, Al203against each other and vs. hardened steel,respectively, form very good friction and wearcombinations.

Oi) Lubricated ConditionsThe organization for Economic Cooperation

and Development (OECD), International ResearchGroup (IRG) on Wear of Industrial Materials, hasestablished [3] a FN-V-T diagram for lubricatedsliding concentrated steel contacts. FN is thenormal force, v the sliding speed, T the oil bathtemperature. A cross section of such a FN-v-Tdata surface at constant temperature T for acompletely oil-submerged ball on ring contact isshown in fig. 2. Three operating regimes withtransition zones in between can be distinguished:

1 : Regime of (partial) elastohydrodynamíc(EHO) lubrication.

II : Regime of boundary lubrication.III: Regime oí virtually unlubricated contact

("scuffing regime").

Latin/smerican Journal of Metallurgy ami Materials, Vo1.l3, N" 1, 2. 1993.

Page 2: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

30 Revista Latinoamericana 'de Metalurgia y Materiales, Vol.l3, N° 1 Y 2, 1993.

74~

5ZJ Z06! ~ 62¡• ...WEAR rol/ iI I I

35~ ;i . @ u

.,;I

~ ~~ •. .'" u .. "" •. '-J_¡fal •• _ LL~ •.... ;.: o u.. _ :.._ ~- FNe,

~~:1: ~ ~'i=:~:

u _

- o•. •. z -.•. .•. ", .•. ~ ~ v· u.... w ~.30 / I• i

o REHRENCE ~"'., 012S • 50% r.h I<!> humld al' \• dry QIr

• •20¡- • •

Region 1: Friction low. wear nil or low. Region111: very severe wear. Intermediate region 11:characterized by transient high friction, but mildwear. Transitions 1/11, 11/111 region are the .first,second and third primary transition, respectively.Figure 3 [4] shows schematically how thecoefficient of friction, u, varíes with times in

I•

••I!

I 57••• 1 -#". t. ,1, ,1

S Pro'

FIGURE 1. FRICTION-WEAR diagram 01'various tribological materials combinations [2].

regions 1, 11 and Il, resp. as well as the wearcoefficient K [5] calculated over a period of 1 h(KIh), when a superrefinded paraffinic oil underdry air cover is used as a lubrícant.

IRG-diagrammes were obtained with hard coatedspecimens, ring or ball or both, as oil lubricatedcontacts (Re [c1a]:composite surface roughness)

S A)I v(m/s)I

10-4 10-3 : 10-2 10-1 1 : 10Vm V$

FIGURE 2. Transitiondiagram, schematic, forball-ring contact, oil-lubricated [3] [4]

Latinémerican Journal of Metallurgv and Mttterials. Vol. 13, N° 1, 2. 1993.

Page 3: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

Revista Latinoamericana de Metalurgia y Materiales, Vol.I3, N° 1 Y 2, 1993. 31

O:~VlMIW",:~",'"11 .

IIIIIIIII,IIIIII, t (hrl~--------~--------~-

t0.3

0.2

""

f0.4

0.1

. 1 -8K a 2 X 100.1 ,',.

I

t (hrlU-~ ~ __

0001

0.3 .

0.2

0.1

0.3

0.2-8K s 100 X 10

t (hrl~------~~--------~--0.01

III

FIGURE 3. Friction-time diagrams, and average wear coefficients Klhin the lubrication regimes I. II, III [4] [5].

Coated RingsfUncoated balls

I[ 4].The numerical results are shown in table

TABLE 1. Critical normal forces FNcIlII and FNcJI/lII with coated rings against uncoated balls(0.5 mis, mineral oil).

Rings Balls Rc[um cla]

FNcIlII[N]

FNcII/III[N] *)

uncoateduncoated

TiCTiN

(Cr,Fe)7C3

0.12OJl80.080.080.12

40175

1200>4000

200

425425

1650>4000

450

uncoateduncoateduncoateduncoateduncoated

*) Coatings on the ring always survives FN=4000NSystem is self-healing.

Latinémerican Journal of Metallurgy and Materials, Vo1.13, N° 1, 2, 1993.

Page 4: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

32 Revista Latinoamericana de Metalurgia y Materiales, Vol.l3, N° 1 Y 2, 1993.

Coated Rings/Coated BallsThe numerical results are presented in table

2 [4].

TABLE 2. Critical normal forces FNcUIIand FNcIl/lII with coated rings against coated balls(0.5 mis, mineral oil).

Rings Balls Rc FNcI/II FNcIl/lII[um cla] [N] [N]

TiC TiC 0.08 850 850TiN TiN 0.08 1100 1100TiC TiC O.OS 650 1300

(Cr,FehC3 (Cr,FehC3 0.12 400 400

BALL BEARING APPLICATIONS

Oil LuhricatedStairiless steels (AISI 440C) oillubricated

gyro-bearings with and without TiC coatedhearings elements, rings or balls, have tested ingyroscopes over a period of several years (19/6mm precision bearings ABEC 5, contact angle17-20°, phenolic cage, oil/grease lubricated,24'000 rpm) [6], fig. 4.

STANDARD B. B.

- AFTER ~ 1000 h !

• LUB. DECOMP. - RESIN• CACE WEAR

• VIBRAT. + TORQUE INCREASE

TiC - COA TED B. B.

- AFI'ER :> 20000 h !

• LUB. REMAINS GOOD• NO CAGE WEAR VISIBLE• VIBRAT. + TORQUE LOW + CONSTo

//FIGURE 4. Gyro-bearings permanent lubrication:Rotspeed: 20'000-30'000 rpm, oil/greaselubrication, aimed lifetime: > 10'000 h [6].

Da Conditions

TiC coated unlubricated are successfullyused invacuum and outer space mechanism [1](operating satisfactorily since 1977 inMeteosatellites), as well as in the secondary circuitof He-cooled nuclear reactors, at temperatures of300 to 3500 [1]. For this type of applicationbearings elements, especially balls, are currentlyCVD coated with TiC on an industrial scale (till endof 1988: > 1'000'000 balls coated).

The AFBMA standards specify theroughness of the balls by their precision GRADEin Ra (cla) and R¡ (highest peak/lowest valley).GRADE 3 corresponds to the highest precision,see fig. 5. The superiority of the TiC-coated ballsis evident, both in the roughness level as in thescattering of the date, obtained on 10 balls each .

Fig. 6 shows the Rj-values for three seriesof balls, uncoated from two different ballproducers, and TiC-coated/polished byCSEM/SWIP. Again the indicated extremes andaverages correspond to measurements of 10 ballsof each series. .

The 'surface state and interface connection tothe substrate is shown in fig. 7.

LatutAmerican .Iournat of Metallurgy and Mtiterials, Vol.l3, N° 1, 2, 1993.

Page 5: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

Revista Latinoamericana de Metalurgia y Materiales, Vol.J3, N° 1 Y 2, 1993. 33

inch.1O-6_. 02"028 5. , ~ ,o, 0,8 1

• AFBMA- GRAOE 10

• AFBMA- GRAOE S

• AFBMA - GRAOE 3

• TiC-COATED

11"'-

; '1 : ¡---4---t-·_--~- - -1- - - - - - - ~'&.~~

1, '=- -=---t¡-= --=~ :1 I I l', ,

I 1I 1I I

I,II

FIGURE 5. The surface roughness (arithmetical average: Ra in mm or c1a in millionth of an inch) specifiedby the precision GRADES 3,5 and 10 (according to AFBMA). compared to the surface roughness of a 2.381mm (3/32")-diameter TiC-coated 440C steel ball. The indicated range corresponds to the measurements on 10different balls [7].

I If- -- ~1--- ,

I I0,0060.001 0,012 0.020 0,025

inch.10-6_ 2,82,2 Ú 3 5,4 8,7 13,0

• PRODUCER1 (GR.3)

• PRODUCER 2 (GRJ )

• TiC- COATED

11 " I I 1 I I I I I

~

~ I___ II~ I

---'-,1 I~I~=~i-i~~+~~

II I I1-- - I I- - - I I

I I I I I II I I I I I I I I

IJm_ 0,06 10.083 0,137 0,221 0,3310,07

FIGURE 6. Comparison of the surface roughness (maximum roughness depth: R¡ in umor in rnillionth of aninch) of 3 series of 10 440C steel balls each-diameter 2.381 rnrn (3/32") precision GRADE 3 (AFBMA).Producer 1 and 2 correspond to uncoated balls. The arrows indicate for each series the average of themeasurements on 10 different balls [7].

LatinAmerican Journal of Metallurgy and Materials, Vol. 13, N° 1, 2, 1993.

Page 6: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

34 Revista Latinoamericana de Metalurgia y Materiales, Vo1.13, N" 1 Y 2, 1993.

FIGURE 7.. Micrograph showing a section through a TiC-coated 440C steel ball; diameter 2.381 mm(3/32") - precision GRADE 3 (AFBMA). The 'iright spots areM23C6 mixed Fe and Cr carbides; the darkregions are martensite grains [7].

TiC-coated ba11scan further be improved bythin (x l urn) sputter-deposited Mo Sj-s ol idlubricant films [8]. . .

The morphology of such films is shown ina cross section of fig. 8. It shows a columnarporous structure over -90% of its thickness andonly -10% of a randomly oriented,. densestructure, which is the tribologically active one.The columnar part can easily be removed with awooden needle, as it can be seen from fig. 9.

Fig. 10 shows the torque history of a highprecision (ABEC 7) angular contact bearing pair .ED20 CEl DD for outer space applications. CVD-TiC coated balls instead of uncoated steel ballsincrease the lifetime of the MoS2-s01id lubricant,the TiC coating provides an "emergency"lubrication, avoiding the formation of cold weldsand seizure. This situation is further illustrated infig. 11.

1-

'"eE~

''0u

triboactive

FIGURE 8. Morphology of a spuuer-deposited MOS2-s01id lubricant film [8].

Latin/unerican Journal of Metaliurgv and Mtiterials, Vol. 13, N" 1, 2, 1993.

Page 7: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

Revista Latinoamericana de Metalurgia y Materiales, Vol.l3, N" 1 Y 2. 1993. 35

FIGURE 9. MoS2-films scratched with a wooden needle. It remains 0.1 to 0.2 um oftribologically active film [8].

dimenslons : 42~ 20~12mm'

vacu

: AISI 52100/M"1I:5¡[iH'J

: Pb·bronc~DiN1716(CuPb15Sn8)1MoSl 'TiC-: AISI4t.OC/TiC/MoS,: 40N sott (s pr in q O,12mm/Kp)

: 750 rpm. 2'10.7 mbar

: average lorque: zero lo peak lorque

r inqs,

cageballsaxial pr etoa d

speed

60

ErnerqenctRunnilJg

50

...--->(

'-- •••• + •. _. . t -~---------------t

m fu ~ ~ WREVOLUTlON5 x 106

o

FIGURE 10. Torque bearing pair E020 7CE 100 with MoS2 01' TiC/MoS2-coatedbearings parts. respectively [8] [10].

Fig. 12 finally shows the state of the art1987 of TiC/MoS2 coated bearings for outer spacemechanism. Both processes are specified and

space qualified and are regularly applied by theAerospace industry in Europe and the USA.

Latinámerican Journal of Metallurgy and Mtuerials, Vol.l3, N° 1, 2, 1993.

Page 8: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

36 Revista Latinoamericana de Metalurgia y Materiales. Vol.J3. N° 1 Y 2. 1993.

60

50

40

E 30 »::z STEEL BALLS I~

I.~ 20>( Iw 10

,;:)oa:o~

o 10

,___r-" .TIC-COATEO I TiC-EMERGENCVSTEEL BALL S 1 RUNNING BARRIER,,===-- ....J1

20 30 40 50NUMBER OF REVOLUTIONS 11 HY'

FIGURE 11. Schernatic-of "emergency" action of TiC intermediate coatings forMoS2-coated steel balls [8][ 10].

Vacuum : 2 ' 10.7 mbor~ : average lorque0--0 : zero lo peak lorque

Ring5 : AISI 521 00/MoS2~ 40 Coge Pb·bronce DIN 1716 (CuPb 15Sn8)fMoS2E :

Z 80115 : A1SI440C/TIC/MoS21 30,

Axiol preload 40 N son (spring 0.12 mm/k p)o :..- .~ speed : 750 rpmw 20::>aQ! ~2 10 ~ ~ ••- - -~

105 106 101 108 109REVOLUTIONS

FIGURE 12. Torque-lifetime behavior of TiC/MoS2 coated bearing.

Latin/vnerican Journal of Metallurgy and Materials, Vol.J3. N° 1. 2. 1993.

Page 9: WEAR REDUCING, FRICTION LOWERING CVD AND PVD … Revista Latinoamericana de Metalurgia y Materiales. Vo1.l3. N° 1 Y 2. 1993. 29 WEAR REDUCING, FRICTION LOWERING CVD AND PVD COATINGS

Revista Latinoamericana de Metalurgia y Materiales, Vol. 13, N° 1 Y 2, 1993. 37

REFERENCES1. H.E. Hintermann, Tribology International 1.3.(1980) 267.

2. H.E. Hintermann, Wear lQQ (1984) 381.

3. A.W.J. de Gee, A. Begelinger and G. Salomon,Proc. Internat. Wear Conf. ASME, Reston VA(1983).

4. H. Boving, H.E. Hintermann, A Begelinger andA.W.J. de Gee, Wear ss (1983) 13.

5. C.N. Rowe, in "Wear Control Handbook"ASME, New York (1980) 143.

6. H. Boving, H.E. Hinterrnann and Stehlé,Lubrication Engineering si (1981) 534; ibid}2(1983) 209.

7. H. Boving and H. E. Hintermann, Thin SolidFilms 151 (1987) 253.

8. C. Menoud, H. Kocher and H.E. Hintermann,Proc. 5th Internat. Conf. on Ion & PlasmaAssisted Techniques, IPAT '85, Munich (1985)277.

9. C. Menoud, G. Zeming, H. Kocher, and H.E.Hintermann, Proc. 6th Internat. Conf on Ion &Plasma Techniques, IPAT '87, Brighton (1987)214.

10. H.E. Hintermann, Technishe Rundschau Nr25 (1987) 22.

11. To be published elsewhere.

Latiu American Journal of Metallurgy and Materials, Vo1.l3, N" 1, 2, 1993.