Effects of lubricants on friction and wear of Ti(CN)1045 steel sliding pairs

Effects of lubricants on friction and wear of Ti(CN)1045 steel sliding pairs

Trilmlog~ ELSE\ IEK SClEN”E! c PII: Irurmnrionc~l Vol. 30. No. 3. pp. 177-182. 1997 Copyright Q 1996 Elsevier Saence Ltd Printed in Great Britain...

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Trilmlog~

ELSE\ IEK SClEN”E! c

PII:

Irurmnrionc~l

Vol. 30. No. 3. pp. 177-182. 1997 Copyright Q 1996 Elsevier Saence Ltd Printed in Great Britain. A11 rights reserved 0301-679X/96/$17.00 +O.OO

SO301-679X(96)00034-S

Effects of lubricants on friction d wear of Ti(CN)/l045 steel ding pairs Xingzhong

Zhao”,

Jiajun

Liu, Baoliang

Zhu, Zhenbi

Luo and Hezhuo

Miao

The friction and wear properties of Ti(CNV1045 steel rubbing pairs were investigated under dry and lubricated conditions by using a pin+n-disk tribometer. The selected speed range was 0.8 to 3.2 m/s and the load range was 58.8 to 235.2 N. Distilled water and a mineral oil (no additives) were used for lubrication, respectively. The wear of Ti(CN) ceramic under dry conditions was caused mainly by adhesion between the rubbing surfaces and the microfracture of Ti(CN). With the load and speed increasing, the adhesion and diffusion between rubbing surfaces increased and resulted in wear increasement of Ti(CN). Because of the brittl’eness of ceramics, the microfracture wear of Ti(CN) increased rapidly when the load was raised to some high values. The lubricating and cooling effects of the lubricants could improve the frict on and wear. Compared with water, oil was much better in imp’oving the tribological properties. The analysis results obtained from XPS and AES examinations showed that ferrous oxide was produced on the wear scars, which could reduce the adhesion between the rubbing surfaces to some extent. The lubricating effects of the oil under boundary lubrication conditions were attributed to the formation of carbon films on the rubbing surfmaces. Copyright 0 1996 Elsevier Science Ltd Keywords:

ceramic,

lubricant,

tribochemistry,

adhesive

Introduction Ceramics have attracted considerable attention in recent years because of their excellent properties such as high hardness, high resistance to chemical corrosion and thermal stability; they are being widely used for components in chemical, mechanical and other systems Generally, the friction coefficients of ceranic/ceramic rubbing pairs under dry conditions are in the range of 0.4 to 1.0, varying with the test cond.tions, such as speed, load, specimen contact models and environment situations2.3. Habig and Woydt” and !shigaki et 01.~ found that the wear rates of Si,N, for !33NJ/Si3N-l sliding pairs varied over a smaller

Tribology

wear

range of 1 X lo-” to 6 x lo-” m”/N.m with the sliding speed when compared with A1203/A120, and ZrO,/ZrO, sliding pairs. The varying ranges of wear rates of A120, and ZrO, with speed were, respectively, 5 x IO-” to 1 x 10ei5 m3/N.m and 5 x lOpI7 to 1 x IO-” m’/N.m. For many ceramic/metal pairs, significant material transfer always occurs under dry conditions6. The transfer direction was material dependent; ferrous material transferred from steel to alumina, whereas silicon-based material transferred from silicon carbide and silicon nitride to steel. With PSZ and steel the transfer layer formation was also geometry-dependent. Gates et n1.7 reported that the presence of water during the sliding of alumina against itself in a four-ball machine causes reductions in both the friction coefficient and wear. This, according to them, is due to the formation of Al(OH)3, which has a layered structure. Similar results have also been obtained by Anderson’ for water lubricated alumina/alumina sliding pairs, while for alumina/steel pairs, water is not effective as a lubricant and serious metal transfer occurred even in the presInternational

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Effects

of lubricants

on friction

and

wear:

X. Zhao

ence of water. Water and humid environments generally improve the wear and friction behaviours of Si,N3, SIC and sialon, which is thought to be attributed to the following tribochemical reactions: Si,NI

+ 6 H,O-3

+ CO + Hz,

SiO, + 2 H,0-+Si(OH)3. The hydrated amorphous silica films play a role in boundary lubrication, wear rates and friction coefficients thus tend to decrease in the presence of watery-’ ‘. Lubrication by oil could also improve the tribological properties of ceramics. It has been reportedi that ZrO, and SIC mated to steel may offer greater wear resistance under oil lubrication than that of self-mated Al,O, or Al,O,-steel couples. A relatively high wear resistance with oil lubrication was also observed on ZrOz containing 10% open porosity in sliding contact with hardened steel. Uetz et &.I3 have studied the effect of different lubricants with organic and organometallic additives, and found distinct differences in friction and wear behaviour. Studt’” thought that the effects of some additives on the friction and wear of ceramics (Alz03 and SIC) were mainly due to the adsorption of the additive molecules on the ceramic surfaces, which depended largely on the lattice structures of the ceramics. In this paper, the tribological properties of Ti(CN)/l045 steel sliding pairs are investigated by using a pin-ondisk tribometer, arranged to simulate a real cutting process. The test results could provide guidance to the wear control of ceramic tools in cutting practice. The wear mechanism of Ti(CN) ceramic and the lubricating effects of the lubricants were examined by using SEM, XPS and AES. Experimental Test

machine

procedure and

the wear volume can be calculated from the following formula. wear volume = OSLB’ x sin 10” x cos 10” In this formula, L represents the length of the ceramic wear scar (5 mm in this test) and B represents the width of the wear scar (mm). The above formula can be simplified as: VW= 0.427 B’ (mm3).

SiO, + 4 NH,,

SIC + 0, + H,O;SiO,

et al.

specimens

Wear tests were carried out on a pin-on-disk tribometer. The pin specimen was fixed and the disk specimen, driven by a motor, could rotate at different speeds. The schematic diagram of the tester is shown in Fig. 1. The initial line contact model was formed between the pin and the disk, which could simulate well the contact form of cutting tool and workpiece in real cutting practice. The angle of inclination of the pin is 80”, so

The pin was made from hot pressed Ti(CN) ceramic, having.a size of 5 x 5 x 25 mm; the disk was machined from 1045 steel (oil quenched, 605 HV in hardness), 50 mm in diameter and 6 mm in thickness. The frictional surface roughness of the pin and the disk was Rn = 0.32 pm and Ra = 0.21 km, respectively. Some properties of the Ti(CN) ceramic are listed in Table 1. Test

method

Friction and wear tests were operated respectively under dry and lubricated conditions, at room temperature, about 20°C. Distilled water and a mineral oil were used for lubrication. The lubricating oil was a pure mineral oil, no additives included, with a kinetic viscosity at 25°C of 30 mm’/s. During the operating process, the water or the oil were fed into the contact point between the pin and the disk by natural falling flow from a reservoir. The average rate of flow was about 0.01 l/min. The sliding speed between the rubbing surfaces. varied over a range of 0.8 to 3.2 m/s, and a load range of 58.8 to 235.2 N was selected. Each pair had a 30 min running time under the selected speed and load. At least two tests were performed and the standard deviations were less than t 10%. Before and after testing, the specimens were cleaned ultrasonically in an acetone bath. The wear scar width of the pin was measured under an optical microscope, and then the wear volume and wear rate could be calculated. The friction force was transmitted by a transducer to a recorder continuously during the test, from which the friction coefficient could be obtained. The worn surfaces were examined by scanning electron microscopy (SEM), X-ray photo electron spectroscopy (XPS) and Auger electron spectroscopy (AES). An MgKol line was used, the pass energy was 35.75 eV, and the binding energy Cls (284.6 eV) was used as reference.

Table 1 Physical Ti(CN) ceramic Properties

Base

Amount Grain size Density Hardness Bending strength Fracture toughness Impurities

plate

Rotating

shift

Fig I Scheme of contact model of the specimens 178

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and

mechanical

Unit wt.% pm g.cmp3 HV MPa MPa.m’/*

properties

Value ~80
of

Effects

I

I

I

I

I

60

80

100

120

140

0.10 40

Load

Fig 2 Variation

Results Effect

and of load

of frictiorz

160

I

I

i

I

180

200

220

240

on friction

0.5

I.0

and

1.5

Fig 4 Variation qf’ friction

load

discussion and wear

The variation of friction and wear of Ti(CN)/I045 steel pairs with load is shown in Figs 2 and 3. The sliding speed was 1.6 m/s. It can be seen that, under dry conditions, the friction coefficient remains constant as a whole when the load is raised. The wear rate of Ti(CN)), however, increases with load, and has a sudden increase when load is raised to some value (176.4 N in this test). Both the friction ,coefficient and wear rate grow gradually under the lubrication of water or oil, but their values are much lower when the oil is used as a lubricant. SEM examinations showed that adhesion and material transfer between the rubbing surfaces occur under both dry ard lubricated conditions. The most material transfer (steel to ceramic) was found under dry conditions, thus giving rise to the highest friction coefficient and wear rate of Ti(CN) among the tests. The sudden wear increase of Ti(CN) may be caused by microfracture of the Ti(CN) ceramic, which occurs more frequently under dry and relatively high load conditions. The 1ubric;ants could raise the fracture load of Ti(CN)

wear:

2.0 Speed

(N)

coejjficient with

on friction

of lubricants

X. Zhao

2.5

3.0

et al.

3.5

(m/s)

coeficient with speed

appreciably. At lower load, adhesion and material transfer were prevented by the lubricants, while at high load, steel transfer could be found on the worn ceramic surface although its amount was much less compared with unlubricated conditions. In dry conditions, the friction coefficient is independent of load, which is attributable to the adhesion occurring throughout the whole load range. This result corresponds well with Amontons First Law. Effect of speed on friction

and wear

Figures 4 and 5 show the variation of friction coefficient and wear rate with speed; the selected load was 117.6 N. It could be observed during dry tests that many sparks were produced at the contact zone, the temperature of the rubbing surfaces rising rapidly. High temperature accelerated the diffusion between the surfaces, and thus brought about very intense adhesion and increased the friction coefficient and wear rate. In lubricated conditions, both water and oil improved the friction coefficient and wear rate, especially at high speeds, the oil being more effective. The reduction of the friction coefficient and wear rate may be attribu-

8or 2 %

70 0 Oil

60 “E h-7

v water

50

A Dry

I

0

“40

60

80

100

120 Load

140

160

180

200

220

240

0.5

0

I

I

I .o

1.5

(NJ

2.0 Speed

Fig 3 Variation of wear rate of Ti(CN) with load

I

I

2.5

3.0

I

3.5

(m/s)

Fig 5 Variation of wear rate of Ti(CN) with speed Tribology

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table to both the cooling and lubricating effects of the lubricants. Wear

mechanism

of Ti(CN)

ceramic

The SEM morphologies of Ti(CN) worn surfaces are shown in Fig. 6, from which the differences in steel transfer can clearly be seen. The most severe adhesion and steel transfer occurred under dry conditions; only a little steel was transferred to the ceramic surface under oil lubrication. With the combined effects of pressure stress and frictional heating, intensive adhesion occurred between the ceramic and steel surfaces, and the adhesion junctions are continuously broken with the relative movement of the rubbing pairs. Being subjected to both compressive and shearing stresses,the adhesive junctions may break either in the steel surface or in the ceramic surface, resulting in material transfer and wear of both the sliding pairs. The steel transfer layers with ceramic particles are torn out and removed as debris, following which new adhesive junctions between the freshlyexposed ceramic and the steel surface reformed immediately. The process occurs continuously and is illustrated schematically as Fig. 7. It is thus concluded that the wear of Ti(CNj rubbing against steel is mainly caused by adhesion. Microfracture particles of Ti(CN) have also been observed by SEM. Wear-reducing

mechanism

of the

lubricants

At high temperatures, elements diffusion between the ceramic and steel surfaces occurs much more easily

et a/.

and quickly’5,16. Under lubricated conditions, on the one hand, the cooling and lubricating effects of the lubricants keep the temperature of the rubbing surfaces low, the diffusion rate of the elements is reduced, and so to is the adhesion; on the other hand, the adsorption and/or tribochemical films formed on the rubbing surfaces will also prevent adhesion. Lubrication thus reduces the shearing stress on the ceramic surface and increases the critical load at which brittle fracture of the ceramic begins. Microfracture wear of ceramics occurs much more easily in dry conditions than in lubricated ones17. The X-ray energy dispersion spectrum of the waterlubricated worn surface of Ti(CN) ceramic is shown in Fig. 8. The intense Fe peak should be attributed to the transferred steel. XPS examination indicated that (see Fig. 9), Fe,O, was produced on both the worn steel and ceramic surfaces, which could prevent adhesion to some extent. Under oil lubricated conditions, a rather thick carbon film was formed on the worn Ti(CN) ceramic surface in addition to Fe,03 (see Figs 10 and 11). The carbon film was about 900 A in thickness according to the sputtering rate used. In addition, XPS analyses also show that the binding energy of Cls is changed to 284.2 eV (see Fig. 10(b)), which indicates that some graphite is produced. So the carbon film is an important factor improving the boundary lubrication situation of Ti(CN)/steel sliding pairs. The formation of the carbon film may be mainly ascribed to friction heat; the local high contact temperatures change the adsorbed oil molecules firstly into carbon-rich polymers, and

Fig 6 SEA4 morpizologies of the worn Ti(CN) swfnces: (a) dry ,friction; (b) water lubricated; (c) oil Lubricated 180

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Rubbing

surface

of lubricants

Transferred

steel

lYR.Ixl

on friction

Transferred

and

X. Zhao

et al.

steel Be subjccred to pressing stress and shearing strews

Rubbmg

Rubbing

Ti(CN)

b

(b)

(a)

New-produced ceramic surtac

wear:

New-transferred New-transferred

steel

Rubbing Ti(CN)

I

Fig 7

Schematic

description

of

Ti(CN) wear process

the

(a)

--+4

C

----J-d 01 1000

I 900

I 800

I 700

I 600

Binding VFS = 1024 10.240

0.1~00

Fig 8 X-my energy Ti( Clv,, swjace

dispersion

spectrum

of

73:

730

I 725

720 Binding

I 715 energy

energy

I-t-L+ 200 100

(eV)

worn

I 298

I 296

I 294

I 292 Binding

I

vr I 300

I 400

(b)

308

_L

I 500

I

I

T

710

705

700

(eV)

Fig 9 XPS spectrum of worn steel surface Fe,,: specsu$ace; spectrL4m 2, worn sur$ace rrLLn1 :‘. origifzal

Fig 10 XPS

spectra

formation

Fe,O,: (0) Cl,

of

of

the

I 790

I 288

energy worn

I 286

I 284

I 1X2

I 280

(eV)

Ti(CN) swface: (a)

The effect of load on the carbonization or graphitization of the oil can be found by comparing Fig. 12 with Fig. 13. Conclusions

these may then be graphitized, partially or wholly, under the high contact pressure and catalytic action of Fe, Fe2.03, Ti(CN) etc, thus improving the boundary lubricating properties of the oil’“. The difussion of Fe into a Ti(CN) surface can also be seen from Fig. 11. Tribology

(1)

The wear of Ti(CN) in Ti(CN)/1045 steel sliding pairs is mainly caused by adhesion and microfracture. High sliding speed gives rise to high temperature of the rubbing surfaces, which accelerates

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AES profile

100

and

X. Zhao

et al.

V/F

(3)

90 80 70

(4)

T12

T12

2

1

0

3

4

5

Sputter

time

(min)

6

7

References 1. Liu Jiajun Wear Tsinghun University

Fig 11 AES element distributions in depth AES survey

SE = 155.733 DAT = 1.90

06/26/95

principles and wear resistance Press, Beijing, 1993

2. Kato K. Tribology of ceramics. Vol. I, 1989, 75593

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Proc.

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Corlf: on Tribal.,

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4. Habig ZrO,, 1989,

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5. Isbigaki H., Kawaguchi I., Iwasa M. and Tolbana Y. Friction and wear of hot-pressed silicon nitride and other ceramics. In Wear of Materials (Ed. K.C. Lt.&emu), American Society of Mechanical Engineers, New York, 1985, Vol. I. IS-19

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2

0

I

I

I

I

100

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300

400 Kinetic

D, 500 energy

600

I 700

I 800

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1000

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SE = 155.733 DAT=

1.90

7. Gates anism

RX, Hsu of alumina

SM. with

and water.

8. Andersson P. Water lubricated Wear 1992, 154, 37-47

Fig 12 AES spectrum of the worn steel su$ace (58.8 N, I. 6 m/s, 30 min) AES survey

K.H. and Woydt M. Sliding friction and wear of Al,O,, SIC and Si,N,. Proc. 5th Inter. Corzf: on Tribo., Vol. 3. 106-113

6. Andersson P. and Holmberg K. Limitations on the use of ceramics in unlubricated sliding applications due to transfer layer formation. Wear 1994, 175 1-8

I

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Klaus E.E. Tribochemical mechTribal. Trans. 1989, 32, 357-363 pin-on-disc

test with

9. Fischer T.E. and Tomizawa H. Interaction of tribochemistry and microfracture i‘n the friction and wear of silicon nitride. Wear of Materials (Ed. K.C. Ludema), American Society of Mechanical Engineers, New York, 1985, Vol. I, 22-32

11. Kimura Y., Okada K. and Enomoto Y. Sliding silicon nitride in plane contact. Wear of Materials, 1, 1989, 361-368 12. Zum Ghar K.-H. Sliding wear of ceramic/ceramic, and steel/steel pairs in lubricated and unlubricated 1989, 133, 1-22

3

13. Uetz layer 313 100

200

300

400 Kinetic

500 energy

Fig 13 AES spectrum of (235.2 N, 1.6 m/s, 30 min)

182

ceramics.

10. Sasaki S. The effects of surrounding atmosphere on the friction and wear of alumina. zirconia, silicon carbide and silicon nitride. Wear of Materials, ASME, Vol. I, 1989, 407-417

6

(2)

of materials.

3. Usami H., Funabashi K. and Nakamura T. Frictional properties of several kinds of ceramics against hardened carbon steel. Proc. 5th Inter. Con6 on Tribo., Vol. 3, 1989, 94-99

6

iz 3 E;

faces, but the lubricating action is most important for the wear reduction of Ti(CN). Ferrous oxide, produced on the worn surfaces, restrains adhesion between Ti(CN) and steel surfaces to some extent. However at high speed, the ferrous oxide film is too thin to prevent adhesion, so the wear of Ti(CN) was increased. When oil was used for lubrication, a rather thick carbon film (which may be graphitized partially) was formed on the rubbing surfaces. This had a good boundary lubricity, so the wear rate of Ti(CN) was much lower than that found under water lubricated conditions.

600

700

800

900

1000

(EVI

the worn

steel sur$ace

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ceramic/steel contact. Wear

H., Khosrawi M.A. and Fohl J. Mechanism of reaction formation in boundary lubrication. Wear 1984, 100, 301-

14. Studt P. Influence of lubricating oil ceramics under conditions of boundary 115, 185-191 15. Addhoum tools with 379-387

additives on friction of lubrication. Wear, 1987,

H. and Broussand D. Interaction nickel-based alloys. Mater-. Sci.

16. Wayne S.F. and Buljan S.T. Ni-based superalloy machining.

diffusion, adhesion and wear. High load frequently results in microfracture wear of the Ti(CN). The cooling and friction-reducing effects of lubricants minimize adhesion between the rubbing sur-

damage of ASME Vo/.

17. Buckley D.H. and Miyoshi Wear 1984, 100, 333-353

of ceramic cutting Eng. 1989, A109,

Wear of ceramic cutting tools Tribal. hztern. 1990, 33,4/8-426 K. Friction

and wear

in

of ceramics.

18. Zhou Chunhong Investigation of mechanism of reacted film formation under boundary lubrication at high temperature and insitu observation of adsorbed film status using infarred emission spectroscopy. Ph.D. thesis, Tsinghua UrliversiQ, I994

3 1997