Tribological behaviour of W-C-Co coatings

Journal of Materials Processing Technology, 31 (1992) 225-234

225

Elsevier

Tribological behaviour of W-C-Co coatings A. Ramalho a, M.T. Vieira ~ and A.S. Miranda b aDepartamento de Engenharia Mecfinica, Faculdade de Ci~ncias e Tecnologia da Universidade de Coimbra, 3000 Coimbra, Portugal bArea de Metalomec~mica, Universidade do Minho, 4719 Braga, Portugal

Industrial Summary W-C-Co films were deposited on polished high-speed steel substrates by radio frequency (r.f.) magnetron sputtering with different substrate r.f. bias (17,). The variation of V~ causes a compositional variation, particularly in the cobalt content. Tribological tests carried out on films with different cobalt content have shown the effective role of this element on the wear behaviour. For a particular critical value of cobalt, W-C-Co coatings seem to be suitable for coating dies.

I. Introduction The main cause of die failure is wear: the latter makes a good surface finish and tolerance of the product impossible. Many die failures do not necessarily result from design deficiency, but arise from the wear degradation of the die in service. In fact, a fundamental requirement in die design is a suitable selection of a material that simultaneously has a high bulk mechanical resistance and a good surface wear resistance. The solution is to coat the traditional materials used in die production. By using a coating of ceramic materials toughened with metal it is possible to obtain a number of useful properties, such as good hardness, toughness and wear resistance. Hence there are advantages in coating dies that operate under severe wear conditions. Among different "cermet" coatings, ( W - C ) - C o coatings seem to be suitable for the application envisaged [1,2 ], particularly when deposited by the sputtering technique [1]. This technique preclude any adverse influence on the heat-treated substrate owing to the deposition being carried out at low temperature. The substrate softness is responsible for significantly reduced toollife in service, as coatings wear away quickly. Furthermore, after sputter-coating of the surface, the surface roughness is of the same order as that prior to coating: the roughness is not significantly increased. Thus surfaces with low roughness retain their favourable characteristics even after coating. The aim of this work is to present a new type of coating for dies using W 0924-0136/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

226

C-Co coatings on high-speed steel M2 (AISI) deposited by a r.f. magnetron with different r.f. substrate biases. Friction and wear behaviour were evaluated by pin-on-disc tests, which showed the role of the film's cobalt content on the wear characteristics of the coatings, for percentage of this element greater than those present on the films studied by Eser [ 1 ]. 2. Experimental w o r k

2. I. Materials and coatings production W-C-Co coatings were deposited on high-speed steel M2 (AISI) heat-treated (quenching and tempering) with 850 Hv30. A planar magnetron sputtering system was used to deposit the films on the surface to be tested. The system used is an Edwards ESM 100 unit with two r.f. power supplies of 1000 W and 500 W branched to the target and substrate holder, respectively. The chemical composition of the targets, as indicated by the supplier, is presented in Table 1. The deposition parameters were as follows: 0 chamber=30 cm, ¢~ cathode = 10 cm, anode/cathode distance= 6 cm, discharge power=6.25 W / c m 2, deposition pressure = 1 Pa and the negative substrate bias range Vs = 0 to 400 V. Metallographically polished specimens were ultrasonically cleaned with the usual chemical agents. The samples to be coated were previously sputter-cleaned for 15 min at a deposition pressure of 1 Pa. In all the deposition conditions studied, the films had a thickness of 3 pm or 6 ~m, measured by step profiling.

2.2. Characterization of coatings (1) Morphology. A Jeol T330 scanning electron microscope was used to characterize on one hand the film morphology on fractured specimens and on the other hand the wear mechanisms on worn surfaces. (2) Chemical composition. The chemical composition of coatings and wear surfaces was determined by EDXS. A Jeol T330 scanning electron microscope operated at 20 kV connected to an energy-dispersive X-ray detector (Tracor) allowed the determination of the chemical composition of the coatings and of TABLE1 Chemical composition of the targets Target

WC WC + 6 wt% Co C + 15 wt% C

Atomic composition (%)

Weight composition ( % )

W

C

Co

W

C

Co

50 45.2 38.8

50 45.2 38.8

9.6 22.4

93.9 88.2 79.8

6.1 5.8 5.2

6.0 15.0

227

the existence of adherent particles on worn surfaces. This analysis was complemented by other techniques: RBS and XPS. (3) Structure. X-ray diffractometer analysis was used for phase identification. The apparatus was a Siemens diffractometer (30 kV; 40 mA) equipped with a mono-chromator; Cu L ~ radiation was used. (4) Microhardness. The indentation hardness technique, employing a Vickers microhardness apparatus with loads in the range of 0.15 N to 10 N, was used to characterize the coatings. In order to avoid the substrate effect arising from low film thickness, the hardness values were determined using the Thomas model [3]. (5) Adhesion. The tests of coating adhesion were performed using an automatic scratch tester (CSEM). The critical load was determined using an acoustic detector and by optical microscopy. (6) Tribological behaviour. A pin-on-disc tribometer was used to characterize the tribological behaviour of an uncoated and a coated high-speed steel (pin) against a carbon steel 1045 (AISI) (disc), the contact conditions being summarized in Fig. 1. Under these test conditions the wear resistance of W C-Co films against steel surfaces subjected to high contact stresses under slide conditions can be evaluated. With the aim of determining the evolution of wear during the tribological test, the test was interrupted periodically in order to measure the wear. An optical microscope was used to measure the wear scars on the pin, these measurements, when used in the equation derived by Habig [4], enabling the determination of the pin wear volume. Concerning the wear evaluation of the discs, profilograms were obtained perpendicular to the sliding direction. The test was finished when the sliding distance was sufficiently large to define the wear evolution correctly a n d / o r when the film was completely worn.

wk

R

I ....

~

J n

Fig. 1. Schematic view of the set-up for tribilogical test: F = 2 N; r = 3 ram; R = 2 6 mm; sliding speed-- 0.5 m / s ; e n v i r o n m e n t a l conditions are n o r m a l ( R H = 50-60%, T - - 18 ° C ); disc - steel 1045 (AISI), R a = 0 . 0 4 ttm; pin - coated high-speed steel M2 (AISI), R a = 0 . 1 - 0 . 2 ttm.

228 A load cell incorporated into the pin-on-disc tribometer enabled the determination of the friction coefficient of the different sliding pairs studied. During each test the friction force was measured continuously, aided by an interfaced microcomputer. The wear surface morphologies were analysed using the same techniques as utilized in the characterization of the film morphology. 3. R e s u l t s a n d d i s c u s s i o n

3.1. Coating properties The analysis of the evolution of the physical and chemical characteristics as a function of the deposition conditions has been studied in detail elsewhere [5,6 ], these studied having made clear the role of the cobalt in the structure of the film. A critical value of the cobalt content (10 at% ) determines their amorphous or crystalline character. For all deposition conditions the examination of films by scanning electron microscopy showed they were dense and featureless (Fig. 2). Microhardness measurements on surface films deposited on M2 (AISI) substrates yielded values between 1700 and 4100 Hv (Table 2), the discrepancy in the values being attributable essentially to the residual stress induced by the different deposition conditions. The deposition conditions used in this study were selected to produce coat-

Fig. 2. Morphology of a W-C-Co r.f. sputtered coating.

229 TABLE 2 Characteristics of the films as functions of the deposition conditions Target composition (wt%)

Substrate bias (V)

Cobalt in the coating (wt%)

Hardness Hv (d = 10 ttm)

Structure

6% C o + 9 4 % WC

0 - 50 - 100

6.58 4.19 2.36

2250 3500 4100

nearly amorphous crystalline crystalline

15% C o + 8 5 % WC

0 - 100 - 200 - 400

12.08 10.125 7.85 6.58

2150 2050 2450 1700

nearly nearly nearly nearly

amorphous amorphous amorphous amorphous

The morphology of the films was featureless.

ings with different cobalt contents in the range of 2.4-12.1 wt% (Table 2 ), the study allowing the characterizations of the role of cobalt on the tribological behaviour of W-C-Co films. All the films analysed had good adhesion to the substrate. No influence of film thickness on the tribological behaviour of the films was detected for the thickness of films used (3 }~m and 6 ~tm). Thus, in this study wear and friction results were obtained from films of both thicknesses.

3.2. Tribological evaluation of the coatings Concerning the friction coefficient for the high-speed steel coated with W C-Co films (Fig. 3 ) it is verified that for all conditions this coefficient is in the range of 0.27 to 0.42: this range is significantly lower than that obtained for uncoated pins (0.58-0.65) against the same steel (1045 (AISI); Fig. 3 ). A comparison of the present results with the same kind of coatings obtained by analogous deposition techniques [1] or other techniques [2] is presented in Fig. 4. The friction coefficient of the present W-C-Co films is lower than that obtained by Garg et al. [2] using low temperature chemical vapour deposition, but higher than as presented by Eser et al. [ 1 ] using the r.f. sputtering technique. The difference between the present results and those of Eser et al. can be attributed to various factors: lower cobalt contents in their films (1-3 wt% ) compared with the cobalt content of the present coatings (2.4-12.1 wt% ); their film thicknesses ( < 1.1 ttm) were less than those used in this work; and different substrates and counterface materials were used. Ramalingham [ 7 ] has attributed an important role on the best performance of deposited films under severe loading conditions to their film thickness ( ~<1 ~m). In Fig. 4 the best friction coefficient of the W-C-Co film obtained by r.f.

23()

!4 !0 A z

"--~ 0,

fr:

r

r.,

/ 0.5

E

©

o.4 o

g T-

4~

0 . 3 _~

0.2

in1

4~

I

.24

4.'2

!

6'.e

7.9

'

lo'.~

~2 i 1

' Uncoated

Wt% Co

Fig. 3. Friction and wear coefficients as a function of cobalt content in W - C - C o fi]ms. 1.2 u." ~= 1.0 0 u 0.8 z o0.6

i.-6,-////I

~E -I 5

~6r

7 W-C-Co

7 W-C

Sputtering LTCVD (2)

W-C-Co \ (2)

T

W-C-Co (7)

1

r

W - C - Co W - C - C o (8) / Sputtering

Cemented carbide

(1)

Fig. 4. Friction and wear coefficients of W - C - (Co) coatings and W - C - C o cemented carbides.

sputtering is shown and compared with W-C-Co sintered carbide. The friction quality of W-C-Co films can be appreciated in Table 3. The dry friction coefficients of the most common coatings tested against steel [9,10] are in general higher than those evaluated for W-C-Co films. The evolution of the wear volume as a function of the sliding distance for different cobalt contents in W-C-Co films is schematized in Fig. 5 (a). In general, after a running-in period, a steady rggime is reached where the wear rate

231 TABLE 3 Friction coefficients for several coatings u n d e r dry sliding on steel MoS2

TiC

TiN

Cr=Cy

Fe~By

AI2Oa/TiC

0.15

0.27

0.56

0.60

0.62

0.38

0,.0003

(a)

0,002

(b)

E >

~

0.0002

z K ~. 0.001

0,0001

o

0

100

200 300 400 SLIDING DISTANCE Cm)

o

100

200 3O0 SLIDING DISTANCE ( m )

4OO

Fig. 5. Evolution of the volume of: (a) coated pins; (b) coated pins compared with uncoated pins (disc - steel 1045 (AISI); × - uncoated, [] - 2.4 wt% Co, • - 4.2 wt% Co, A - 6.6 wt% Co, • 7.9 wt% Co, C) - 10.1 wt% Co, • - 12.1 wt% Co).

is constant. This allows the determination of a wear coefficient for each experiment, using the following expression K = ( V / x ) W -1

(m2/N)

where V / x is the wear rate (m3/m) and W is the normal load (N). If the wear volume of the coated and uncoated pins is compared (Fig. 5 (b)), it can be concluded that the W - C - C o films, for all cobalt contents studied, always present a significant improvement in wear resistance. The wear coefficient of W - C - C o thin films presents a maximum value for 4.2 wt% cobalt, and seems to decrease for lower and higher percentages of cobalt (Fig. 3). The inflexion point that was found in the friction coefficient can explain the lower friction and wear values obtained by Eser et el. [ 1 ]: in fact, the deposition conditions used by Eser et el. lead to cobalt percentages in the range of 1-3 wt%. For the cobalt percentages analysed in the present work the cobalt content that corresponds to the best wear resistance is 10.1 wt%, in spite of the friction coefficient not being the lowest. The W-C-Co sputtered films present, for some deposition conditions, a wear coefficient better than other W - C - C o films and W - C - C o cemented carbide

232

Fig. 6. Scanning electron micrographs of worn surfaces of: (a) an uncoated high-speed steel pin; (b) a carbon steel disc.

Fig. 7. Scanning electron micrographs of worn surfaces of: (a) a W - C - C o coated pin (2.4 wt% Co); (b) a carbon steel disc.

(Fig. 4). If the values of the wear coefficient of W - C - C o films are compared with those of other coatings, it can be concluded that in the present case the wear coefficient is within the range of almost all of the better coatings used (10-16-10 -15 mZ/N) [11]. Examples of the worn surfaces of uncoated sliding pairs are shown in Figs. 6 (a) and (b). The morphology obtained with W - C - C o coatings tested against the same carbon steel is shown in Figs. 7 and 8. For the uncoated pair, the entrance- and exit-edge of the pin contact region shows the collection of wear debris particles of disc material as a lip of adherent material (Fig. 6 (a)), whilst the central region of the pin contact shows evidence of plastic deformation. There are few indications in the disc surface of scratching in the sliding direction due to abrasive action. Vestiges of abrasion have been observed, produced by primary carbides of the high-speed steel (pin), arising particularly from vanadium carbide. However, plastic deformation in the sliding direction is evident, which appears to be the principal wear mechanism of the disc. On the worn surface of the coated pins the following features were observed:

233

Fig. 8. Scanning electron micrographs of worn surfaces of: (a) a W - C - C o coated pin (I0.I wt% Co); (b) a carbon steel disc.

(i) the surface pin shows scratching in the sliding direction due to abrasive action of small particles detached from the film during the tribological test (Figs. 7(a) and 8(a); (ii) the disc always presents evidence of plastic deformation in the sliding direction, but the deformation is more uniform that that for the uncoated pair. There are also present very few indications of scratching caused by wear debris particles from the coated pin. As Eser et al. [1] found for W - C - C o coatings, adhesive wear was not observed in any of the tests, whereas wear by abrasion was present due to particles of the coatings having spilled from the pin surface. There does not seem to be any influence of the cobalt content in the W - C Co films on the wear mechanisms of worn surfaces. 4. Conclusions

This study of the dry sliding wear characteristics of different W - C - C o films deposited on M2 (AISI) steel has led to several conclusions: (1) the W - C - C o coatings offer a good dry sliding wear resistance under the conditions studied in this work compared with the same coating using other deposition techniques or with other types of coating materials; (2) the lowest wear coefficient was obtained for 10 wt% of cobalt in the film. The highest friction and wear was found when the W - C - C o film had 4.2 wt.% of cobalt; (3) a fine abrasion is the preferential wear mechanism of W - C - C o coatings sliding against carbon steel. These results suggest strongly that W - C - C o films can be applied successfully on die coatings where a high level of wear resistance will be necessary.

234

References 1

E. Eser, R.E. Ogilvie and K.A. Taylor, Friction and wear results from WC + Co coatings by d.c. - biased r.f. sputtering in a helium atmosphere, J. Vac. Sci. Technol., 15 (2) ( 1978 ) 401405. 2 D. Garg, P.N. Dyer, D.B. Dimos, S. Sunder, H.E. Hintermann and M. Maillat, Low-temperature CVD tungsten carbide coatings for wear/erosion resistance, Ceram. Eng. Sci. Process., 9 (1988) 1215-1222. 3 A. Thomas, Microhardness measurement as a quality control technique for thin, hard coatings, Surf. Eng., 3(2) (1987) 117-122. 4 K.H. Habig, On the determination of wear rates, Wear, 28 (1974) 135-139. 5 A. Cavaleiro, M.T. Vieira and G. Lempdriere, The structure of thin films deposited from a sintered tungsten carbide with a high cobalt content ( 15 wt% ), Thin Solid Films, 185 ( 1990 ) 199-217. 6 A. Cavaleiro, M.T. Vieira and G. Lemp~riere, Structure and chemical composition of W-C(Co) sputtered films, Thin Solid Films, 197 (1991) 237-255. 7 J.M.R. Gomes, Estudos do comportamento ao desgaste de materiais duros na aus~ncia de lubrificaqfio, Area de Engenharia Metalomec~nica da Universidade do Minho, Inc. Laboratory Report, 1989. 8 G. Dearnaley, Improvement of wear resistance in cemented tungsten carbide by ion implanatation, Proc. Int. Conf. on the Science of Hard Materials, Jackson, WY, USA, August 2328, 1981, Plenum Press, New York, 1983, pp. 467-484. 9 H. Boving and H.E. Hintermann, CVD and PVD low-friction/anti-wear coatings, J. Phys., C1 2(47) (1986} 111-118. 10 M. Memarian, R.C. Budhani, A.A. Karim, H.J. Doerr, C.V. Deshpandey, R.F. Bunshah and A. Doi, Friction and wear properties of TiC-A1203 composite and A12OffTiC layered coatings, J. Vac. Sci. Technol., A3 (6) (1985) 2434-2438. 11 J.R. Gomes, R.F. Silva, J.M. Vieira and A.S. Miranda, A preliminary study of the non-lubricated wear of hard materials, Int. Conf. on Tribology Trends in the 90s, Lisboa, Portugal, May 5-6, 1988.