Sliding wear resistance of TiCN coatings on tool steel made by plasma-enhanced chemical vapour deposition

Wea~ 169 (1993) 195-200

195

Sliding wear resistance of TiCN coatings on tool steel made by plasma-enhanced chemical vapour deposition H.L. Wang, J.L. He and M.H. Hon Department of Materials Science and Engineering (MAT32), National Cheng Kung University, Tainan (Taiwan) (Received January 12, 1993; accepted May 27, 1993)

Abstract In order to facilitate a proper choice of coating parameters for a wear-resistant TiCN coating on tool steel by plasma-enhanced chemical vapour deposition (PECVD), the sliding wear resistance of PECV deposited TiCN on tool steel was evaluated by the washer-on-disc method. A Taguchi array was employed to reveal the influence of coating parameters on wear resistance. The results showed that the deposition temperature and the flow rates of Ar and CH4 were the dominant coating parameters for a TiCN coating for wear resistance. Optimizing the coating parameters of a TiCN coating for better load-carrying capacity and reducing the chance for deep galling resulted in the improvement of wear resistance beyond that of TiN and TiC coatings. Controlling the constituents of the TiCN coating proved to be easy, so that TiCN can be applied to meet the requirements if different wear conditions are needed.

1. Introduction The application of titanium nitride (TIN) or carbide (TIC) coatings by conventional CVD and PVD processes to extend the life and performance of tools has been demonstrated to be very successful [1-8]. Efforts have also been made using plasma-enhanced chemical vapour deposition (PECVD) to obtain TiN o r TiC coatings [9-16]. Great advantages are easily obtained from the P E C V D process, including good controllability of the constituents of the deposited films, the possibility of not needing t o rotate the substrate [15] and good throwing power [16]. The first advantage is particularly important when forming a TiCN coating which is expected to have an adjustable microstructure and mechanical properties between those of a TiN coating with high fracture toughness and high temperature stability and a TiC coating with high hardness and better adhesion [17,18]. This enables the more versatile TiCN coating to be applied on various substrates and to be used in different wear conditions. To date, few reports have paid attention to the wear behaviour of a TiCN coating formed by PECVD. A systematic evaluation of the wear resistance of TiCN by P E C V D has also not been reported. The present study evaluates the influence of coating parameters on the sliding wear resistance of TiCN coatings on tool steel made by P E C V D and also makes a comparison among TiN and TiC coatings so that a proper choice of coating parameters can be facilitated whenever TiCN

0043-1648/93/$6.00

is used as either a transition layer between TiN and TiC deposits Or as a single layer of hard coating on tool steel.

2. Experimental 2.1. Substrate preparation JIS SKD61 tool steel was u s e d as a substrate, which was shaped according to the A S T M D3702-78 washeron-disc wear test method, as shown in Fig. l(a), and then heat treated as indicated in Fig. 2. A hardness of about Hv740 was obtained. The disc substrate was ground, polished to a surface roughness of R a = 0 . 1 / z m and then the substrate was precleaned before coating.

L

-° '

-ILJ. 'L-

Fig. 1. Dimensions of (a) the disc and (b) the washer.

© 1993- Elsevier Sequoia. All rights reserved

H.L. Wang / Sliding wear resistance of TiCN coatings

196 1030 oC rain

TABLE 2. Chemical composition of the $45C washer

90 850

/

650

oy

N8 quench

5%0°C

550°C

°C/

C

Mn

Fe

0.45

0.80

Balance

Fig. 2. Heat treatment of the SKD61 specimen. --

.~-~ Rotary spindle (with drive pin)

- Gem

~

Substrate Substrat~

flow controller

~

- Washer holder

Wuher older

R.F. Generator

Fig. 3. Experimental apparatus for P E C V D coating. Load direct.ion TABLE 1. Chemical composition of the SKD61 substrate C

Si

Mn

Cr

0.42

1.03

0.40 5.13

Mo

V

0.99 0.62

Ni

P

0.06 0.016

S

Fe

0.002

Balance

The chemical composition of the SKD61 substrate was measured, and is listed in Table 1. 2.2. Deposition process Coatings were formed in a P E C V D apparatus as shown in Fig. 3, using capacitively coupled electrodes. One of the electrodes, connected to a 13.56 MHz r.f. generator, was also used as a substrate table, while the other, with ground status, was used as a gas shower. TIC14 liquid in a vaporizer was carried by H2, N2 and CH4 as gas sources, the flow rates of which were carefully controlled. External substrate heating was supplied by a graphite heater u n d e r the substrate table, capable of heating the substrate up to 600 °C. When deposition began, reaction gases were admitted into the reaction bell jar to maintain the working pressure at about 4 Torr, and r.f. power was initiated to enhance the deposition reaction.

2.3. Wear test The wear resistance of coated specimens was evaluated by a Falex 6 multi-function wear tester using the A S T M D3702-78 washer-on-disc method. The washer was made of JIS $45C medium carbon steel with hardness Hv280, without further heat treatment. Its dimensions and chemical composition are shown in Fig. l(b) and Table 2, respectively. The washer and the

Fig. 4. Installation of the washer and disc for wear test.

disc were installed on the wear tester shown in Fig. 4 and wear test conditions were at a load of 0.38 lb mm -2 ( = 1.69 N mm-2), with a sliding speed of 42.4 m min -1 and a total distance of 800 m. Weight loss measured by a digital balance before and after the wear test was used as an indication of wear resistance. The wear scar profile was measured by a profile meter.

2.4. Evaluation method A Taguchi array was used to evaluate the influence of coating parameters on wear resistance of the coated specimens. On the basis of a survey of the reports [9-16] concerning TiN and TiC coating by PECVD, as well as our own experience [19-21], those parameters listed in Table 3 were suitably assigned to have three levels and (except for deposition time with two levels) to form an L18 orthogonal array, as shown in Table 4. The weight loss of specimens coated under the same conditions was tested twice and listed in Table 4. Being a reciprocal indicator of wear resistance, weight loss was expected to have as small a value as possible. The r/value listed in Table 4 was calculated from the weight loss (AW) using eqn. (1) and can be a positive indication of wear resistance. ~/= - 10 log(1/2)(AW12 + AW22)

(1)

where AWl is the weight loss of sample 1 and AW2 is the weight loss of sample 2. Response charts of wear resistance as a function of coating parameter were made to reveal the influence of the coating parameters.

H.L. Wang / Sliding wear resistance of TiCN coatings

197

TABLE 3. Coating parameters and the levels of the TiCN deposition process

Coating parameter

A Deposition time (h)

B Deposition temperature (°C)

C r.f. power (W)

D Vaporization temperature (°C)

E Flow rate of H2 (sccm)

F Flow rate of Ar (sccm)

G Flow rate of CH4 (sccm)

H Flow rate of N 2 (secm)

Level I Level 2 Level 3

2 3 -

500 550 600

100 200 300

30 40 45

100 150 200

50 100 150

10 20 30

40 80 160

TABLE 4. Result of the L18 orthogonal array. The symbols A, B, ..., E represent the coating as indicated in Table 3 A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2

B

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

C

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

D

1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2

E

1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1

F

1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2

G

1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3

2.5. Observation and characterization S E M was u s e d to m e a s u r e t h e film t h i c k n e s s f r o m a f r a c t u r e d m i c r o v i e w a n d to o b s e r v e t h e w e a r scar. A n X - r a y d i f f r a c t o m e t e r was e m p l o y e d to c h a r a c t e r i z e t h e m i c r o s t r u c t u r e o f t h e coating.

3. Results and discussion T a b l e 4 shows t h e r e s u l t o f w e a r tests o f t h e c o a t e d s p e c i m e n s at t h e levels d i c t a t e d . T h e t h r e e m o s t inf l u e n t i a l p a r a m e t e r s i n d i c a t e d in T a b l e 5 by M a x - M i n a r e t h e d e p o s i t i o n t e m p e r a t u r e a n d t h e flow r a t e s o f CH4 a n d A r . R e s p o n s e c h a r t s for e a c h p a r a m e t e r as a f u n c t i o n o f t h e i r o w n level a r e s h o w n in Fig. 5, w h i c h offers an e s t i m a t i o n o f t h e i n f l u e n c e o f t h e s e p a r a m e t e r s o n t h e w e a r r e s i s t a n c e in t h e c o m m o n l y e m p l o y e d r a n g e o f P E C V D T i N o r T i C coatings. T h e S / N v a l u e s h o w e d a positive d e p e n d e n c e o n t h e d e p o s i t i o n time, v a p o r i z a t i o n t e m p e r a t u r e , a n d H2 a n d CH4 flow r a t e s , as o b s e r v e d in Fig. 5. C h a n g e s in t h e levels o f c o a t i n g p a r a m e t e r s l e a d to c h a n g e s o f film t h i c k n e s s a n d s u b s t r a t e h a r d n e s s , w h i c h

H

1 2 3 3 1 2 3 1 2 1 2 3 2 3 1 2 3 1

AWi (10 -4 g)

AW2

194 217 221 78 369 36 850 491 411 75 191 70 52 188 158 716 561 303

210 159 191 58 318 86 878 405 588 10 175 10 60 90 207 678 494 336

-q

Film thickness (/~m)

Substrate hardness (Hv)

34 34 34 43 29 44 18 27 26 45 35 40 42 37 35 23 26 30

2.41 0.83 0.36 2.22 1.88 1.89 1.67 3.53 1.16 2.58 1.44 0.59 3.04 4.34 1.16 2.79 1.9 3.69

737 749 742 684 458 555 345 325 314 623 746 730 428 540 585 284 293 336

(10 -4 g)

a r e i m p o r t a n t for w e a r r e s i s t a n c e (see T a b l e 4). R e s p o n s e c h a r t s o f film t h i c k n e s s w e r e also m a d e b u t a r e n o t s h o w n h e r e . A m o n g t h e s e r e s p o n s e charts, film t h i c k n e s s vs. d e p o s i t i o n time, a n d A r , CH4 a n d N2 flow r a t e s s h o w e d t h e s a m e t r e n d as t h e w e a r resistance. T h i s i l l u s t r a t e s t h e i m p o r t a n c e o f film thickness. R e sponse charts of substrate hardness were not made, for it is c l e a r f r o m T a b l e 4 t h a t t h e s u b s t r a t e h a r d n e s s was i n f l u e n c e d exclusively by t h e d e p o s i t i o n t e m p e r a t u r e . A h i g h e r level o f d e p o s i t i o n t e m p e r a t u r e l e a d s to s o f t e r s u b s t r a t e s . T h e h a r d n e s s fell f r o m Hv740 at level 1 d o w n to Hv300 at level 3. I n c r e a s i n g t h e d e p o s i t i o n t i m e i n c r e a s e d the film thickness. This i n c r e a s e s t h e w e a r r e s i s t a n c e a n d d e m o n s t r a t e s t h e positive d e p e n d e n c e in Fig. 5(a). T h e d e p o s i t i o n t e m p e r a t u r e n o t o n l y a f f e c t e d t h e film p r o p erties, b u t also t h e s u b s t r a t e . T h e d e p o s i t i o n t e m p e r a t u r e r e q u i r e d to o b t a i n a c r y s t a l l i n e T i N film by P E C V D is a p p r o x i m a t e l y 300 °C w h i l e it is at l e a s t 500 °C for T i C [10]; h e n c e it is b e t w e e n 300 a n d 500 °C for TiCxNl_x, d e p e n d i n g on t h e f r a c t i o n s o f C a n d N. It is t r u e t h a t a b e t t e r q u a l i t y film is o b t a i n e d w h e n

H.L. Wang / Sliding wear resistance of TiCN coatings

198

T A B L E 5. Response table for the LI8 orthogonal array

Level 1 Level 2 Level 3 Max-min

A Deposition time

B Deposition temperature

C r.f. power

(h)

(°c)

(w)

(°c)

32 35 3

37 38 25 13

34 31 35 4

33 32 35 3

40

401

35

j

\\\

\\

ao

=- 30 25 20

._____~\

35 i

J

D Vaporization temperature

\'

25

;

20

2 3 Deposition t i m e ( h r )

500 550 600 Deposition temperature

(°C)

40

40

3s

35 30

3O

25

es!

2O

eol

t00 200 300 R.F.power (W)

30 40 45 Vaporizer temperature

4O

(°C)

E Flow rate of 1-12

(seem)

F Flow rate of Ar (seem)

G Flow rate of CH4 (seem)

H Flow rate of N2 (seem)

33 34 34 1

35 36 29 7

31 34 34 4

33 34 33 1

deposition temperature is increased. A side-effect of high deposition temperature would be substrate softening. Wear resistance therefore increased at first, as shown in Fig. 5(b). On further increasing the deposition temperature up to 600 °C at level 3, beyond the softening temperature (approximately 580 °C) of SKD61, the substrate becomes significantly softer, from Hv600 down to Hv300, and no longer has enough load-carrying capacity [22] to support the TiCN film against wear. Thus an abrupt decrease in wear resistance was observed. Micrographs of the wear scar edge of worn specimens deposited at 550 °C and 600 °C are shown in Figs. 6(a) and (b) respectively. These illustrate the result of the catastrophic wear mode after losing the load-carrying capacity of the substrate for the film deposited at 600 °C. The second important deposition parameter for wear resistance was the flow rate of Ar. The response chart of wear resistance v s . A r flow rate in Fig. 5(f) shows the same trend as the response chart of film thickness v s . A r flow rate. It is therefore suggested that the Ar

40; J,,

35

\\

30

30 I

25

20 L

2O

100 150 200 Flow r a t e of H2 ( s e e m )

50 100 150 Flow r a t e of Ar ( s e e m }

40r t[

\ (a)

30 ]am

40t I

aa~

3~p

~ 30~

[ u- 3o~

]

Sliding direction

i

10 20 30 Flow r a t e of CH 4 ( s e e m )

Sliding direction

25~ 20 l

40 80 160 Flow r a t e of Nz ( s e e m )

Fig. 5. Response charts for the coating parameters.

(b)

a0 ~-n

Fig. 6. Wear scar edge of TiCN-coated specimens deposited at

(a) 550 °C and (b) 600 °C.

H.L. Wang / Sliding wear resistance of TiCN coatings

gas influences the growth rate and the subsequent wear resistance. The effect of adding Ar during TiN coating by PECVD, investigated by Hilton et al. [23], suggests that NE-H2-based films have nothing but a decrease in growth rate, because Ar behaves as an inert gas diluting the reactive species. However, the effect of admixing Ar during TiC coating by PECVD is well known to enhance the film growth rate because of the Penning effect [17]. The response of wear resistance on TiCN growth is thus a competition between the two effects. Increasing the flow rate of Ar would cause the Penning effect at first, leading to an increase in film thickness and subsequent wear resistance. Further increasing the Ar flow rate dilutes the reactive species, causing a decrease in film thickness and wear resistance, as shown in Fig. 5(f). The ~7 value response to the C H 4 flOW rate was greater than for the Na flow rate. This might come from the difference in activity between the two gases. The reaction of the titanium source with N2 molecules with less dissociation energy is controlled by the decomposition process, leading to a lesser dependence of growth rate on flow rate. The film thickness and the subsequent wear resistance is therefore less dependent on the NE flow rate than on the CH4 flow rate. A confirmation experiment on the optimized parameters shown in Table 6 indicated a high wear resistance and thus a high ~7value. It reveals the accurate prediction of the influence of coating parameters using the Taguchi method. Figure 7 compares the apparent hardness and weight loss of blank, TIN-, TiC- and TiCN-coated specimens. The TiCN was deposited with the optimized coating parameters shown in Table 6. The deposition temperature was kept the same and the film thicknesses were all kept at 3 /xm. All the coatings promoted the apparent hardness and wear resistance as expected; nevertheless, the TiC coating, with the highest apparent hardness, had the lowest wear resistance, while the TiCN coating, with apparent hardness between that of the TiN and TiC coatings, had the highest wear resistance, which was apparently not a direct reflection of the apparent hardness as observed in this figure. Since the film thickness and deposition temperature were kept the same, the influence of the substrate on wear resistance for the three coated specimens was undoubtedly the same. It is exactly the film properties TABLE 6. Coating parameters and the wear resistance response of the conformation experiment / A

B

C

D

3

550 100 45

E

F

G

150 100 30

H

AW AW r/ (10 -4 g) ( 1 0 - ' g)

80

6

2

70

199

1200 [ [] Apparent hardness |I Weight lo-,-, A 1000 ~-

] 120 |_1100

1-7

oot|ll I

| _-: -t /

I BI~ir

TI_C

TiN

TLCN

Fig. 7. Apparent hardness and weight loss of blank, TIN-, TiCand TiCN-coated specimens.

,=

:~

(b)

©

(d) I

0

I

0.5 1.0 Scar profile

I

I

1.5 2.0 length: (mm)

2.5

Fig. 8. Surface profiles of the wear scars of (a) blank, (b) TIC-, (c) TiN- and (d) TiCN-coated specimens.

of the coatings that determine the wear resistance. Considering the TiC coating with the highest intrinsic hardness among the three coatings, the benefit from a harder coating, such as TiC, is a higher load-cart-ying capacity of the coating itself, but the disadvantage of a harder coating is the formation of galling wear when delametlation of the coating occurs. The delamellated aspirates of the coating layer cause weight loss and local penetration through to the substrate, as observed in Fig. 8(b). The TiN coating, with the lowest hardness and the highest toughness, released the shear, stress more easily and avoided deep galling, as shown in Fig. 8(c), resulting in better wear resistance. TiCN; with moderate hardness and brittleness (between TiC and TIN), had a better load-carrying capacity and reduced the chance for deep galling; hence the highest wear resistance was obtained. From the results and discussion above, the TiCN coating can be optimized to obtain better wear resistance

200

H.L. Wang / Sliding wear resistance of TiCN coatings

3. The constituents of TiCN coatings made by PECVD can be easily controlled to meet the requirements of different wear condition.

TIN(200)

/\

I

[ PECVD-TiN /

j/:

/

\.~CH,=4 s c e m !

I

l

Acknowledgments

~,..__CH4=8 seem! 1

I

] I

,4

The authors wish to thank the National Science Council for partial support of this study under the project NSC77-0405-E006-08, and Professor Lin who provided the wear tester.

CH4--15 s e e m

~ " ~ _ _ _ C H 4 = 30 s e e m

I

TIC(200)

L PECVD-TiC i J

i

'

i

~

,

i

i

l

37

38

3'9

40

41

42

43

44

45

Spectral angle

Fig. 9. X-ray diffraction patterns of TiCN films deposited at different CH4 gas flow rates.

than TiC or TiN coatings using the PECVD process. The constituents of TiCN coatings with properties in between those of TiC and TiN could be easily adjusted by adjusting the flow rates of CH4 and N2 to be close to TiN or TiC, as demonstrated in Fig. 9. This shows that an increase in the CH4 flow rate leads to a peak shift in the XRD pattern of the coated specimens from a 20 angle of 42.6 ° to 41.6 ° which are the (200) characteristic angles of TiN and TiC, respectively. This implies that it allowed a modulation of the TiCN film properties to meet the requirement of either a tougher more TiN-like film that can carry a heavier load or a harder more TiC-like film that can bear a higher sliding speed with a lighter load.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

4. Conclusions

16

1. The deposition temperature and the flow rates of Ar and CH4 were the major influential coating parameters for TiCN coatings for wear resistance. The deposition temperature affects the substrate hardness, while the Ar and CH4 flow rates affect the film thickness. Optimizing the coating parameters of TiCN coatings improved the wear resistance of TiCN-coated specimens beyond that of TiN- and TiC-coated specimens. 2. TiCN, with moderate hardness and brittleness (between those of TiC and TIN), had a better loadcarrying capacity and reduced the chance for deep galling; hence the highest wear resistance was obtained.

17

18 19

20

21

22 23

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