Friction and wear behaviour of plasma-sprayed Cr3C2NiCr against TiO2 coating under water- and ethanol-lubricated sliding

Friction and wear behaviour of plasma-sprayed Cr3C2NiCr against TiO2 coating under water- and ethanol-lubricated sliding

g WEAR Wear214 (1998) 202-206 ELSEVIER Friction and wear behaviour of plasma-sprayed Cr3 C 2-NiCr against Ti0 2 coating under water- and ethanol-lu...

491KB Sizes 0 Downloads 12 Views

g

WEAR Wear214 (1998) 202-206

ELSEVIER

Friction and wear behaviour of plasma-sprayed Cr3 C 2-NiCr against Ti0 2 coating under water- and ethanol-lubricated sliding J.F. Li 3,*, J.Q. Huang 3, Y.F. Zhang 3, C.X. Ding 3 , P.Y. Zhang b b

a Shanghai Inslilule of Ceramics. Chinese Academy of Sciences. Shang/wi 200050. China Laboralory of Solid Lubricalion. Lan::./lOulnslilllle of Chemical Physics. Lan::./lOu 730000. China

Received 14 July 1997; accepted 16 October 1997

Abstract In order to understand the tribological properties of ceramic coatings in some fluid environments, the friction and wear behaviours of plasma-sprayed Cr3Cz-NiCr against TiO z coating under water- and ethanol-lubricated sliding were investigated with a block-on-ring arrangement. The tracks and debris were carefully collected and examined and analysed employing scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy respectively. Furthermore, the wear mechanisms of the coatings were explained in terms oftribochemical reaction and absorption-induced cracking. It was found that in comparison with dry friction condition, water deteriorated the tribological properties of both Cr3Cz-NiCr and TiO z coatings by accelerating cracking and fracturing. Ethanol reduced the friction coefficient and wear coefficient of Cr3Cz-NiCr coating, which could be attributed to the formation of a smooth sOrface film mainly consisting of CrzOJ' However, ethanol also increased the wear coefficient ofTiO z coating by absorption-induced cracking as a result oflow fracture toughness of the coating. © 1998 Elsevier Science S .A. Keywords: Plasma spraying; Friction and wear; Cr3C2-NiCr coating; TiO, coating; Water- and ethanol· lubricated sliding

1. Introduction . In today's materials design, ceramic coatings play an increasingly important role in applications where high temperatures, corrosion, oxidation and wear come into play [ I]. In the past several years, there have been many reports about the tribological properties of thermally sprayed coatings under dry friction condition [2-6], However, in some cases ceramic coatings as sliding parts must be performed in some fluid environments [7], and very little was known about their friction and wear behaviours in water [4]. Thus, it is ,,:ery important to study the friction and wear behaviours of ceramic coatings in such fluid environments in orderto extend the applications of these coatings. Cr3Cz-NiCrcoatings, consisting of hard phases of carbides and tough matrix phase of metals, have been found to be one of the most promising materials and is one of the most extensive researched wear coatings [5,6,8]. Ti0 2 coatings have been used as materials for light bearings in application such as shaft bearing sleeves and pump seals to resist wear [2]. In

* Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences. 1295 Dingxi Road, Shanghai 200050, China. Fax: + 86-2162513903. 0043·1648/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved Pll S0043-1648(97)00255-X

the author's previous work, the wear mechanism of plasmasprayed Cr3Cz-NiCr against Ti0 2 coating in air at room temperature was researched at different sliding speeds and loads [9]. Here we investigated the friction and wear behaviour of plasma-sprayed Cr3Cz-NiCr against Ti0 2 coating under water- and ethanol-lubricated sliding, and discussed the effect of fl uid media on the wear mechanism of the coatings.

2. Experimental details Cr3Cz-NiCr and Ti0 2 coating were applied onto stainless steel ring and block surfaces respectively by plasma spraying using optimized spraying parameters. Some characteristics and properties of the coatings had been given in Ref. [9]. Friction and wear testes were conducted with a MM-200 wear tester using a block-on-ring arrangement in deionized water and ethanol respectively (see Refs. [9,10]). The dimension of the ring was 0 ourc AO - 0inncr 16 X 10 mm, and that of the block was 30 X 7 X 6 mm. The contact width of the block on ring was 6 mm. The surface roughness Ra of Cr3Cz-NiCr coating was less than 0.6 ~m, and that of Ti0 2 coating was less than O. I ~m after polishing. The wear tests were performed at the following conditions: a load of 800 N, a rota-

J.F. Li el al. / lVear 214 (1998) 202-206

tional speed of200 rpm, which was equal to a sliding velocity of 0.42 m . s - I at the contact surface of samples. Friction coefficient was obtained from friction torque, which was directly showed from the tester, being divided by load and radius of the ring. Wear coefficient was acquired by wear mass loss, which was measured by weighing the samples before and after each of the wear tests with a TG328B analytical balance, being divided by load, sliding distance and coating density. Prior to weighing, the samples were cleaned by an ultrasonic bath with acetone for 30 min and then kept in drying oven at 120°C for 60 min. The friction coefficients and wear coefficients were the average of three samples. The morphologies of the worn surfaces of both Cr3CZNiCr and TiO z coatings were observed with an EPMA8705QHII type scanning electron microscope (SEM). The phases of the original Cr JCz-NiCr coating and debris scraped off the surface were examined using a JAPAN-RICOH Xray diffraction spectrometer (XRD), and the element valences of the original and worn surface of CrJC2-NiCr coating were analysed employing X-ray photoelectron energy spectroscopy (XPS) of PHI550 ESCA/SAM polyfunctional electron spectrometer.

0.6

--

0.5

cQ) ·0 0.4 !E Q)

0.3

0

u

c

The friction coefficient of CrJC2-NiCr against Ti0 2 coating under different conditions is shown in Fig. I. From Fig. I, it can be seen that water somewhat increased the friction coefficient to 0.50 ± 0.04, but ethanol obviously reduced the friction coefficient to 0.15 ± 0.03 of the friction pair when compared with 0.46 ± 0.03 of the dry friction condition. Moreover, under water lubrication, friction coefficient gradually increased; however, under ethanol lubrication, friction coefficient gradually decreased during the wear-in period. These results will be discussed below. Fig. 2 shows the wear coefficient of CrJCz-NiCr against Ti0 2 coating under different conditions. From Fig. 2, it can be seen that the wear coefficients of both CrJC2-NiCr and TiO z coatings under water-lubricated sliding increased noticeably in comparison with that under dry friction. However, ethanol efficiently reduced the wear of CrJCz-NiCr coating, but accelerated the wear of Ti0 2 coating. 3.2. Wom surfaces alld debris alia lyses

The worn surface SEM micrographs of both CrJCz-NiCr and Ti0 2 coatings under water- and ethanol-lubricated sliding are shown in Fig. 3. As for the original coating and the worn surface of dry friction, the SEM micrographs were presented in a previous paper [9]. Under dry friction, melting, plastic deformation and shear fracture took place and a very thin shear film was formed on the worn surface of CrJC2-NiCr coating. Plastic deformation and fatigue pitting appeared on

I

I

-·-Dry friction --Water lubrication -'-Ethanollubrication

~-I-I-I-I--I

i3

'':;

I

0.1

LL

0.0 0

400

800

1200

1600

Sliding distance (m) Fig. I. The friction coefficient of Cr3C2-NiCr against Ti0 2 coating under different conditions at a load of 800 N and a sliding velocity of 0.42 m s-'.

-:

E

~

1:::: '0-

0 '-'

~

0

3.1. Friction coefficient alld wear coefficient

l==F:l==R=i I

.2 0.2

'u E<:J 3. Results and discussion

203

...u :::I <:J

~

18 16 14 12 10 8 6

4 2 0

Dry

Water

Ethanol

Fig. 2. The wear coefficient ofCr3C2-NiCr and TiO, coatings under different conditions at a load of 800 N and a sliding velocity of 0.42 m s - '.

the worn surface ofTi0 2 coating. From Fig. 3, it can be seen that there were many microcracks and tracks of lamella spallation and fracture on the worn surface ofthe coating in water. The wear of Ti0 2 coating in water mainly indicated flat microflaking and fracture. The wear of Ti0 2 coating in ethanol also displayed microfracture, but the density of fracture tracks was higher, and the size of single fracture track was smaller than those in water. However, except for some microcracks and dense Ti02 debris filled into pores, the worn surface of Cr3 C2 -NiCr coating in ethanol w~s very smooth. XPS and XRD were employed to make further analysis on the wear behaviours of the coatings. The analytical results of XPS are given in Table I, and XRD spectra of original CrJC2NiCr coating and debris scraped off the surface are shown in Fig. 4. There was no apparently Ti element to be detected on the worn surface ofCrJCz-NiCrcoating in water. Compared with the original state of Cr, the binding energy of 2p31Z of the worn surfaces ofCrJCz-NiCrcoating in water and ethanol had obvious shifts, indicating that there were tribochemical reactions between the coating and the fluid media, and Cr203 was formed. Furthermore, as the 576.5 eV peak of Cr 2PJ/2 intensively shaded the 574.3 eV peak of Cr 2p31Z, there was no metal Cr listed on worn surface of CrJC 2-NiCr coating in ethanol in Table I. Absorption of polar molecules such as H20, C2 H sOH on oxide solid surface induces stress corrosion [11,12]. With

J.F. Li e' a/.! Wear 214 (1')')8) 202-206

204

Fjg. 3. SE~vI rni(:[ugraphs o{ worn surf£u..:e OT' (A) Cr3C2-NiCr coating in Ti distribution of (C); (E) Ti0 2 coatiGg in eth~nol.

watcr~

the formation of Cr2 0 3 , H 2 0 and C 2 H"OH molecules can be absorbed on surfaces of both Crj C2-NiCr and Ti0 2 coating and caused stress corrosion. According to Griffith's formula about brittle fracture: iff a KlC/ C l 1 2 , where a f is fracture stress, K]c is fracture toughness of material and C is the crack length in material, the lower the fracture roughness and the longer the length of crack, the more easily fracture occurs. Under water-lubricated sliding, on the one hand, H 2 0, which has relati vely intensive polar, may pmduce higher corrosion stress. Plasma-sprayed coatings possess some pores and microcracks [9] which may reduce the fracture stress of the coalings. Thus, absorption of H2 0 on both Cr3Cr~iCr and Ti0 2 coatings intensified stress COlTosion, microcracking,

(B) TiO} coating in water: (C) Cr3 C2-NiCr coating 10 ethanol; (D)

lamella spallation and fracture, which resulted in higher wear coefficients than those under dry hiction condition for both of the two kinds of coatings. Lamella spallation and fracture resulted in that debris of Cr j C2-NiCr coating were worn off before they completely reacted with fluid media, so the peaks of Cr7 C3 clearly appeared in the XRD spectrum of debris carefully collected (Fig. 4C). Fracture wear also made the worn surfaces (Fig. 3A,B) rougher than the original surfaces of the coatings 19], and aggravated the friction force according to tribological mechanics-molecule theory [7]. Therefore, the friction coefficient of wear-in period increased with increase in sliding distance as a result that the surface roughness gradually increased during the period. On the other hanel,

Table I The XPS results of the worn surface of Cr ,C 2 -!\iCr coating Lubricant

Binding energy (e V)

Cr,CrNiCr fresh surface Water Ethanol

573.9 574.3

(en

57f>.5 (CToO,) 576.5 (Cr,O,)

~lemer:t

852.8 (Ni 1 1;52.8 (Nil 852.X (!\i)

458.5 (TiDe)

i.F. Li el af. / \Vear 214 (1998) 202-206



'8

;;l

~ ~

·L-.

~}

A

--

0

0 0

MPam l/2 , which was measured in this study employing Vickers indentation method [16], might result in that stress corrosion fractured easily even if the absorption of C2HsOH on the coating is relatively weaker. Therefore, although the wear coefficient of Ti02 coating in ethanol was lower than that in water, it was higher than that in the atmosphere. As a result, the worn surface of coating was smooth, there may be more locations to be directly contacted and induced fracture, and single location suffered lower contact stress during sliding in ethanol. So the density of fracture tracks is higher, and the size of single fracture track is smaller than those in water.

-=Cr;Cl ·=Cr ."'Ni O=CrlO:l

-

~ ~

205

o=Ti01

~

:.0 ~

CI:l

'-" ~

+J

'Vi

s:: s:: ......

d) +J

4. Conclusion 10

30

50 DiHraction angel

70

00

f)

Fig. 4. XRD spectra of (A) the original Cr3C2-NiCrcoating; (B) debris of the friction pair in cthanol; and (C) debris of the friction pair in water.

water can absorb and take away friction heat and reduce the surface temperature of friction pair [ 13], the shear strength of contact surface of materials was lower than that of dry friction which indicated melting wear and lower friction temperature [9]. Friction coefficient is directly proportional to shear strength of contact surface of materials [ 14] . Thus, the friction coefficient was somewhat higher than that under dry friction condition. The reactive activation and molecular polar of C2HsOH are weaker than those of H20. During wear-in period, the formation of Cr203 was slower and the corrosion stress of C2HsOH was weaker, it is relatively difficult to induce fracture wear for Cr3C2-NiCr coating in ethanol. Thus, the resultant of tribochemical reaction could accumulate on the surface of the coating and produce a surface film of Cr203' With the formation of the surface film of Cr203, the wear of Cr3Cr NiCr coating transferred the wear off of the surface film, so the debris of the friction pair was mainly made up of Cr20 3 and Ti0 2 (Fig. 4B). It is well-known that rougher locations have higher surface energy, so it is reasonable to think that tribochemical reaction and wear-off took place readily at these locations, and a smooth surface film ofCr20 3 was gradually formed during wear-in period. Therefore, the friction coefficient of wear-in period decreased with increase in sliding distance. The smooth surface film of Cr20 3 efficiently protected Cr3C2-NiCr coating, so the friction coefficient and wear coefficient of Cr3C 2-NiCr coating are lower than those under dry friction condition. Under dry friction, on the one hand, humid H20 in atmosphere may be absorbed on Cr3C 2-NiCr coating and aggravate corrosion fracture. On the other hand, the reaction of O2 with the coating and dispersion of O2 along pores and microcracks of the coating may accelerate the cracking and fracture of Cr3C2-NiCr coating just like the fractures of some ceramics in atmosphere [ 15]. Thus, the tribological property of Cr3C2-NiCr coating in atmosphere would not far be as well as that in ethanol. As for Ti0 2 coating, very low fracture toughness of 1.4-2.1

The. friction and wear behaviours of plasma-sprayed Cr3CrNiCr against Ti0 2 coating underdeionized-water-and ethanol-lubricated sliding were investigated with a block-onring arrangement. The main conclusions of the study are as follows. (1) The wear coefficients of both Cr3C2-NiCr and Ti0 2 coatings increased in water environment compared to dry friction condition. The wear mechanisms, which were shown in the scanning electron micrographs, indicated absorptioninduced cracking, lamella spalling and fracturing. (2) The wear coefficient ofCr3C2-NiCrcoating in ethanol environment reduced compared to dry friction condition, which may be attributed to the formation of a smooth surface film mainly consisting of Cr2 0 3. On the other hand, the wear coefficient ofTi0 2 coating increased in the sameenvironment possibly by absorption-induced cracking due to low fracture toughness of the coating. (3) From the present studies, the order of friction coefficient is as follows: friction coefficient in water environment> dry friction> friction coefficient in ethanol environment.

Acknowledgements This work was financially supported by the National Nature Sciences Fund and the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.

References [11 G. Geiger, Ceramic coatings enhance material performance, Am. Ceramic Soc. Bull. 71 (10) (1992) 1470-1481. (2) M.l. Mendelson, Theoretical evaluation of wear in plasma-sprayed Ti0 2 against grey iron, Wear 50 (1978) 71-83. [3] J.E. Fernandez, Y. Wang. R. Tucho, M.A. Martin-Luengo, R. Gancedo, A. Rincon, Friction and wear behaviour of plasma-sprayed Cr203 coatings against steel in a wide range of sliding velocities and normal loads, Tribology Int. 29 (4) (1996) 333-343. [4] J. Wei, Q. Xue, The friction and wear properties ofCr20J coating with aqueous lubrication, Wear 199 (1996) 157-159.

206

i.F. Li et aU Wear 214 (1998) 202-206

[5] M. Mohanty, R.W. Smith, M. De Bonte, J.P. Celis, E. Lugscheider, Sliding wear behavior of thermally sprayed 75/25 Cr)C2 /NiCr wear resistant coatings, Wear 198 (1996) 251-266. [6] S. Asanabe, Applications of ceramics for tribological components, Tribology Int. 20 (6) (1987) 355-364. [7] S.Z. Wen, Tribological Principle, Tsinghua University Press, Beijing, 1990. [8] Engineering Property Data on Selected Ceramics, Vo!. II, Carbides, Metal and Ceramics Information Centre, Battelle, Columbus laboratories, 505 King Avenue, Columbus, OH 43201, MCIC Report/ August, 1979,9178. [9] J.F. Li, C.X. Ding, J.Q. Huang, P.Y. Zhang, Wear mechanism of plasma-sprayed Cr)C 2-NiCr against Ti0 2 coating, Wear 2 I I (1997) 177-184. [\0] J.F. Li, J.Q. Huang, S.H. Tan, Z.M. Cheng, C.x. Ding, Tibological properties of toughened SiC ceramics under water-lubricated sliding, submitted to J. Chin. Ceramic Soc. [ I I] S. Sasaki, The effects of the surrounding atmosphere on the friction and wear of alumina, zirconia, silicon carbide and silicon nitride, Wear 134 (1989) 185-200. [12] Q. Xue, H. Liu, Friction, wear and lubrication of ceramics, Tribology (Mocaxue Xuebao) 15 (4) (1995) 376-384. [13] X. Zhao, J. Liu, B. Zhu, Z. Luo, H. Miao, Tribological characteristic of Ti(CN)-AI 2 0) composite ceramic and metal pairs lubricated with water and oil, J. Inorg. Mater. I I (4) (1996) 671-678. [14] H. Liu, M.E. Fine, H.S. Cheng, Lubricated rolling and sliding wear of a SiC-whisker-reinforced Si)N. composite against M2 tool steel, J. Am. Soc., 76( I) \05-112. [15] Q.c. Zhang, Mechanical Properties on Ceramic Materials, The Science Press, Beijing, 1987, pp. 120-144. [16] G.K. Beshish, C.W. Florey, FJ. Worzala, W.I. Lenling, Fracture toughness of thermal spray ceramic coatings detennined by the indentation technique, I. Thermal Spray Techno!. 2 (I) (1993) 35-38.

,

Biographies J.F. Li graduated from the Geological Department ofNanjing University in 1991 and obtained his M.S. in Material Sciences in Shanghai Institute of Ceramics, Chinese Academy of Sciences in September of 1996. He is now a Ph.D. student of the Institute. His research interests are friction and wear of ceramic materials and process and properties of thermalsprayed coating. J.Q. Huang has worked as an engineer at Shanghai Institute of Ceramics, Chinese Academy of Sciences since 1968. She is now an member of the Thermal Spraying Programme. • Y.F. Zhang has worked as an engineer at Shanghai Institute of Ceramics, Chinese Academy of Sciences since 1968. He is now a member of the Thermal Spraying Programme. C.X. Ding is a professor and a member of the Chinese Academy of Engineering. Since 1959 he has been manager of the Thermal Spraying Programme at Shanghai Institute of Ceramics, Chinese Academy of Sciences. P.Y. Zhang works as a associate professor in the Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.