influence of the substrate material on the effectiveness of coatings in metal cutting

Journal of Materials Processing Technology, 37 (1993) 65~665 655 Elsevier

Influence of the substrate material on the effectiveness of coatings in metal cutting

T.N. G o h a, M. R a h m a n b, K .H .W. S e a h b a n d C.H. Lee b

~Department of Industrial and Systems Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511 bDepartment of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511

Industrial Summary

Considerable studies concerning metal cutting in the past have been devoted to tungsten carbide tools and their coatings. As a result, a wide range of carbide substrates has been developed for more than 30 kinds of hard coatings or combination of coatings, ranging from single layer to as much as 13 layers of coatings in multi-layer coated tools. Some of these technological "spill-overs" helped in the evolution of another new generation of cutting-tool materials, namely coated cermets, which not only use less of strategic raw materials such as tungsten and cobalt, but also exhibit excellent wear properties such as good resistance to diffusion wear and the ability to retain a higher hardness at elevated temperatures.

A comparison, based on CNC lathe turning, was made of these two types of substrate materials, namely carbides and cermets, coated with some of the more common hard coatings such as .TIN, TiC, TiCN, and A120 3 using physical vapour deposition (PVD). The results are analysed and discussed.

1. Introduction

T o d a y ' s c o a t i n g t e c h n o l o g y h a s c o n t r i b u t e d s i g n i f i c a n t l y to t h e a d v a n c e - m e n t o f m a t e r i a l s for t h e m e t a l - c u t t i n g i n d u s t r i e s . V a r i o u s c o a t i n g t e c h n i q u e s , e s p e c i a l l y CVD ( c h e m i c a l v a p o u r d e p o s i t i o n ) a n d P V D ( p h y s i c a l v a p o u r depos- i t i on ) , h a v e b e e n f o u n d to e n h a n c e too l l i fe [1], a n d i n c r e a s e c u t t i n g s p e e d a n d f eed - ra t e , r e s u l t i n g in h i g h e r p r o d u c t i v i t y [2,3]. Th i s h a s b e e n p r o v e n b e y o n d d o u b t for t h e c o a t e d t u n g s t e n - c a r b i d e t o o l s a n d i t h a s s i n c e been t h e p r a c t i c e to use c o a t e d t o o l s w h e n e v e r p o s s i b l e to ge t t h e b e s t p e r f o r m a n c e f rom a c u t t i n g

Correspondence to: Dr. M. Rahman, Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511.

0924-0136/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

656 T.N. Gob et al./The effectiveness of coatings

tool [4,5]. Such a remarkable break-through by coated tungsten-carbide tools has prompted the market to be flooded with various kinds of coated tools, and amongst the newer of which is coated cermet.

Cermet itself consists of ceramic hard materials (such as nitrides, ea rbo nitrides and carbides) dispersed in a metal matrix (usually nickel). Its usage is already established in the metal-cutting industry, especially in the Asia-Pacific region and part icularly in Japan, where it has been in use successfully for the past 15 years. Currently, the increase in the demand for coated cermet ~:; i~ direct proportion to the drop in carbide usage. Its at tract iveness includes long tool-life, smooth surface finish, close tolerance, wide speed-range capabi l i ty good chip control and economy [6].

The arrival of the new generation of coated cermet may introduce a wider range of applications and improve cutt ing performance even further than for existing cermet tools: the ultimate aim is to improve productivity. Howew~r. there is still some scepticism regarding the performance of coated cerme~s a~, compared to existing uncoated cermet. This is largely because coated eermc, ts have not yet proven themselves and even if the performance is claimed to bc better, the improvement may only be marginal, which raises the questim~ of whether it is worth the trouble to coat cermet inserts.

This paper reports an investigation of the performance of' some of the more common hard coatings such as TiN, TiC, TiCN and At203 as either a single coating or multi-layer composite coatings deposited on cermet and tungsten- carbide cutting-tool inserts using the PVD technique. Turning tests were conducted on these inserts and their performances were evaluated.

2. Experimental procedure

Two types of tool inserts were prepared for the turning tests: commercially available coated cemented-carbide inserts, of grades SNUN120408, GC425, GC435, GC3015 and uncoated cermet inserts of grades SNMA120408, NX33 and SNGA120408, T110A. The commercially available NX33 and T110A uncoated cermet inserts were further coated by the PVD process with TiN, TiC, TiCN and A1203 hard coatings. The nominal coating compositions and their t h i c k nesses are as shown in Table 1.

These inserts, when at tached to a tool-holder, give the following effectiv,_, cutt ing geometry: back rake angle -6" ; end relief angle 8~'; side cutting edge angle 20 ° and end cutting edge angle 20 °. All the machining tests were performed dry using an Okuma LH35-N 22 kW CNC lathe. The work material turned was Rochling T-4 medium carbon steel (0.45%C, 0.25%Si and 0.70%Mn), its equivalent being AISI 1045 medium carbon steel.

The machining tests were carried out at three different levels of cutting speeds, feed rates, and durations of cut, modelled using a 33- ~ fractional factorial design [7], the procedures of which have been described in detail elsewhere [8]. Due to the inability of the tungsten-carbide tool to cut to the required cutt ing time at the higher cutting-speed (i.e., at 300 m/min) condition,

T.N. Goh et al./The effectiveness of coatings 657

t w o s e p a r a t e se t s of i n p u t p a r a m e t e r c o m b i n a t i o n s w e r e a d o p t e d for c e r m e t a n d t u n g s t e n - c a r b i d e too l s , as s h o w n in T a b l e 2. T h i s is n e c e s s a r y in o r d e r to o b t a i n a c o m p l e t e se t o f o u t p u t d a t a for t h e f r a c t i o n a l f a c t o r i a l d e s i g n a n a l y s i s . T h e d e p t h o f c u t was f ixed a t 2 m m t h r o u g h o u t t h e c u t t i n g tes t s .

U p o n r e a c h i n g t h e r e q u i r e d c u t t i n g in t e rva l , m a c h i n i n g was i n t e r r u p t e d to e n a b l e t he f l ank w e a r a n d t h e c r a t e r w e a r to be measu red . F l a n k wear , V B . . . . was m e a s u r e d u s i n g a n op t i ca l t o o l m a k e r ' s m i c r o s c o p e and a R a n k T a y l o ~ H o b s o n T a l y s u r f was used to m e a s u r e t he c h a r a c t e r i s t i c prof i le of the c r a t e r wear , KT.

A s c a n n i n g e l e c t r o n m i c r o s c o p e , S E M ( Jeo l JSM -T 330A ) w a s u sed to f u r t h e r s t u d y t h e w o r n i n s e r t s . P r i o r to t h e S E M e x a m i n a t i o n t h e w o r n i n s e r t s w e r e d i s s o l v e d of a d h e r i n g s t ee l u s i n g c o n c e n t r a t e d h y d r o c h l o r i d e a c i d a n d s u b s e q u e n t l y c o a t e d w i t h a t h i n l a y e r of go ld u s i n g t h e J e o l J E E 4 X v a c u u m e v a p o r a t o r to i m p r o v e t h e c o n d u c t i v i t y of t h e i n s e r t s w h i c h is c r u c i a l for t h e S E M e x a m i n a t i o n in o r d e r to o b t a i n good ima ge s .

Table 1

Nominal composition and thickness of the coating material

Insert type Nominal coating composition Nominal total coating thickness (gm)

(1) NX33 cermet + TiN 3 (2) NX33 cermet + TiCN 3 (3) Tl l0A cermet + TiC 3 (4) Tl l0A cermet + TiC + AI~O 3 3 (5) GC425 WC+TiC +TiN 8 (6) GC435 WC + TiC + A1,O 3 + TiN 8 (7) GC3015 WC + TiC + A1203 10

Table 2

Combination of input parameters for cermet and tungsten-carbide tools

Cermet tool Tungsten-carbide tool

speed feed rate cutting time speed feed rate (m/min) (mm/rev) (rain) (m/min) (mm/rev)

cutting time (min)

150 0.06 20.0 150 0.06 20.0 150 0.12 60.0 150 0.12 60.0 150 0.25 34.6 150 0.25 34.6 212 0.06 34.6 178 0.06 34.6 212 0.12 20.0 178 0.12 20.0 212 0.25 60.0 178 0.25 60.0 300 0.06 60.0 212 0.06 60.0 300 0.12 34.6 212 0.12 34.6 300 0.25 20.0 212 0.25 20.0

658 T.N. Goh et al./The effectiveness of coatings

3. R e s u l t s a n d d i s c u s s i o n

From Figs. 1 and 2, it is observed that the coat ings on tungsten carbide and cermet have opposite effects on the crater and flank wear. Overall, it appears that as the input parameters are varied, coated tungsten-carbide tools exhibit a good crater-wear resistance (Fig. 1), whilst cermet tools demonstrate excel lenl

00

(a)

WC+'nC+TiN

-~- WC+TK::+.N ~Os+ 5N

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/ /;

o

7O

~" 60

6O

® 4O

3o

20 o

10

cutting s p e e d (m / ra in )

7 ° . . . . . . . . . . . . . . . . . .

6 0

6 0

4 0

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o

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cutting s p e e d ( r n /m in )

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lifO [ ":" . - ....

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3O ../a .............. •

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o ,6 "

50

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10 ~ J - -- . . . . . A

Durat ion o l ct~g (rain) Ouration of c u t (rain)

F i g . 1. A v e r a g e c r a t e r w e a r , KT, b a s e d o n l e v e l s £ o r : ( a ) c o a t e d t u n g s t e n c a r b i d e : a n d

( b ) c e r m e t t o o ] i n s e r t s , r e s p e c t i v e ] ) , .


(a) (b) 1 . 4 1 . 4 r

WC + TK~ + I"IN

1.2 -~ WC+nC+~p,+T~ / .

0.8

~ 0,0

X c o.41

0.2

C u t t i n g s p e e d ( m / r a i n )

0 . 8

0 . 7

E" 0 . 6 E

0.5

0.4

0 . 3

0 . 2

0.Io

F e e ~ r a t e ( m m / r e v )

1

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~ 0 .7

~ 0 .6

~ 0.5

~ 0.4

0 . 3

0 .2

D u r a t i o n o f CUT ( r a i n )

iPp o

~ _ uncoaled aomle~

1.2 I- ~1-- Cereal+]'IN C ~ t + TICN

1 ~- -A- C41rmet+TIC ~ - Ceav~+-~bC +~20 ~

0,6

0.4

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I I I I I J ¢ L L I I L ~ i 0 e e s o s ~ o o ~ o E s §

c u t t i n g s p e e d ( r n / m i n )

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0 .7

0 . 6

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F e e d - r l E i t e ( r n r n / r e v )

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0 - 1 o - - - 4o i ....

D u r a t i o n o f c u t ( r n i n )

Fig. 2. Average flank wear, VB, based on levels for: (a) coated tungsten carbide; and (b) cermet tool inserts, respectively.

f lank-wear res is tance (Fig. 2). However, this is only valid when cut t ing condi t ions are moderate: for more severe cut t ing condit ions, such as 300 m/min cut t ing speed or a feed rate of 0.25 mm/rev and cut t ing for 60 min, cermet tools out-perform carbide tools, some of the cermet tools not even reaching their tool-life cr i ter ia (based on ISO 3685). Coated tungsten-carbide tools, on the


other hand, with the except ion of the WC + TiC + A1203 coated insert, reached the end of their tool lives (based on flank wear) even when cut t ing at a moderate (212 mm/min) cu t t ing speed. It should be noted also tha t a l though the cermet-based tools had a poorer cra ter -wear resis tance as compared t(~ the coated tungsten-carbide tools, this is not significant, as all the cermet tools had not even reached their tool-life cr i ter ia (based on the average cra ter wear) even at the end of 60 min cutt ing. Accord ing to Dearnley et al., once the coat ings of the coated-carbide tools are worn through, the carbide subst ra te will wear off at a h igher ra te [9]. A coated cermet tool, on the o ther hand, may wear at a h igher ra te than most of the carbide tools, but the entire tool is worn at a cons tan t ra te and hence there is effectively a lower wear-rate tbr the ent i r i ty of its tool life.

Ano the r in teres t ing observat ion was that , unlike tungsten-carbide ~ools~ cermets do not necessar i ly perform bet ter when coated. Apar t from TiN-c~ated cermet, the TiCN-, TiC- and TiC +Al203-coa ted cermets all performed wors~. than when uncoated, based on the cu t t ing condi t ions invest igated. The reasons are discussed below.

SEM inves t iga t ion revealed tha t de laminat ion (or flaking) of the coa t ing occurs at the per iphery of the c ra ter on the rake face on the TIN-, TiCN- and

Fig. 3. SEM micrographs of: (a) a TiN coated cermet; (b) a TiCN coated cermet: (c) a TiC coated cermet: and (d) a TiC +A1203 coated cermet; showing delamination of the coating materials and/or cracking on the rake face.


TiC-coated ce rmet tools (Figs. 3 (a)-3 (c)). Cracks were also observed to run across the c ra t e r surface on TiC- and TiC +A1203-coated cermets (Figs. 3 (c) and 3 (d)). Despi te such flaws on the cra ter , the coa ted-cermet tools can still be said to per form well, judging from the i r good c ra te r -wear res is tance. This excel lent c r a t e r -wear res i s tance is an inhe ren t a t t r i bu te of the ce rmet sub- s t rate . The ser iousness of such de l amina t ion is not observed in the coa ted tungs ten-carb ide tools, a l t hough some signs of coa t ing decohes ion can be seen (Fig. 4), which has been a t t r ibu ted to the difference in the coefficients of t he rma l expans ion be tween the coa t ing and the subs t r a t e [10]. This excel len t bonding be tween the TiC coa t ing and the carbide subs t r a t e sur face leads to t he rma l c rack ing on the coa t ing surface (Fig. 5). Such the rma l c racks may be beneficial when presen t as micro-cracks , as they improve the tool t ransverse - rup tu re s t r eng th and chipping res i s t ance by re l iev ing the res idual tensi le

Fig. 4. SEM micrographs of: (a) a TiC +TiN coated carbide; (b) a TiC +AlzO 3 + TiN coated carbide; and (c) a TiC+A1203 coated carbide; showing signs of decohesion of the coating materials on the rake face.


Fig. 5. Thermal cracking on the rake face of a TiC + TiN coated carbide tool after machining for I h at 150 m/rain and 0.12 mm/rev feed rate, as observed under SEM.

stress and enhanc ing the f rac tu re toughness of the tool, as repor ted by K a t a y a m a et al. [11]. Never theless , it is suspected tha t they are responsible for the ca tas t roph ic fa i lure observed f requent ly in coa ted tools before wear has progressed to the point where the tool must be replaced [12].

On the flank face, the TIC-, TiCN- and TiC + A1203-coatings on the cermet tools were comple te ly worn off (Figs. 6 (a)-6 (c)), ending up looking similar to the uncoa ted cermet tool (Fig. 6 (e)), whils t the TiN coat ing (Fig. 6 (d)) on the TiN-coated cermet tool still r emained in tac t on the flank face when these tools had undergone the same cu t t ing condit ions. It should be noted, however , tha t the coat ings on the flank face are more evenly worn from the rake face downward as compared to the coated tungsten-carbide tools, in which the coat ings seem to have become de tached from the flank face, pul l ing along with them the subs t ra te mater ia ls (Fig. 7). This la t te r form of wear aggrava tes the flank wear of coated-carbide tools and is a resul t of poor subs t ra te proper t ies at high opera t ing t empera tu re condi t ions, i.e., at high cu t t ing speed and feed rate.

4. C o n c l u s i o n s

Results from the mach in ing tests, which were designed to s imulate typical p roduc t ion condi t ions, revea led the following:

(i) Coated-carbide tools r e t a ined a h igh res i s tance to c ra t e r wear for low to modera te cu t t ing condit ions, whilst cermet tools were able to main ta in the i r c ra te r -wear res i s tance even at h igher cu t t ing condit ions. However , on the average, cermets showed a h igher c ra t e r wear t h an coated-carbide tools, but such wear is still wi th in the tool-life cri ter ia .


Fig. 6. Flank wear of: (a) a TiC coated cermet; (b) a TiCN coated cermet tool; (c) a TiC+AI~O 3 coated cermet; (d) a TiN coated cermet; and (e) an uncoated cermet; after machining for 20 min at 212 m/min and 0.12 mm/rev feed rate, as observed under SEM.

(ii) C o a t e d - c e r m e t tools m a i n t a i n e d t h e i r e x c e l l e n t f l a n k - w e a r r e s i s t a n c e b e t t e r t h a n coa ted ca rb ide tools for all the c u t t i n g c o n d i t i o n s i n v e s t i g a t e d .

(iii) F l a n k w e a r is the m a i n tool- l i fe c o n t r o l l i n g f ac to r for bo th coa ted ca rb ide a n d c e r m e t tools.

(iv) U n i f o r m a n d g r a d u a l l y w o r n f l ank w e a r is v i t a l in p r o m o t i n g lower a f l a n k - w e a r ra te .


Fig. 7. Flank wear of: (a) a TiC + TiN coated carbide; (b) TiC + A120 3 + TiN coated cart,de: and (c) a TiC +A120 3 coated carbide; after machining for 34.6 min at 178 m/min cutting speed and 0.06 mm/rev feed rate, as observed under SEM.

(v) Coated cermets do not necessar i ly perform bet ter than uncoated ce rm~ts but have the potent ia l to do so with the r ight coa t ing technique for bet ter adherence and the r ight kind of hard coa t ing for bet ter compatibi l i ty with the cermet substrate.

Acknowledgements

The au thors wish to t hank APP TiNcoa t Technology Pte, Ltd for providing the coa t ing services and to Sumitomo Electr ic In t e rna t iona l (Singapore) ~br supplying samples of inserts.

References

[1] D.T. Quinto, A.T. Santhanam and P.C. Jindal, Mechanical properties, structure and performance of chemically vapour-deposited and physically vapour-deposited coated carbide tools, Mater. Sci. Eng., A105/106 (1988) 443 452.


[2] C, Wick, Coating improve tool life, increase productivity, Manuf. Eng., (December 1986) 26 31.

[3] C. Wick, Coated carbide tools enhance performance, Manuf. Eng., (March 1987) 45-50. [4] J.R. Coleman, Make money with multi-coats, Manuf. Eng., (January 1990) 38 42. [5] C,W. Beeghly, Get your shop out of the uncoated age, Asia-Pacific Metalwork. Equip.

News, (August 1990) 34 40. [6] C, Wick, Cermet cutting tools, Manuf. Eng., (December 1987) 35 40. [7] D.C. Montogomery, Design and Analysis of Experiments, Wiley, New York, 1976. [8] M. Rahman, K.H.W. Seah, T.N. Goh and C.H. Lee, Performance evaluation of various

tools materials used in turning, J. Inst. Eng. Singapore, 31(3) (1991) 33-38. [9] P.A. Dearnley and V. Thompson, Evaluation of failure mechanisms of ceramics and

coated carbides used for machining stainless steels, Surf. Eng., 2(3) (1986) 191 202. [10] M. Rahman, K.H.W. Seah, T.N. Goh and C.H. Lee, A study on the appropriate usage of

different tool materials, J. Mater. Process. Technol., 29 (1992) 119-132. [11] S. Katayama, M. Hasimura and T. Tanaka, Effect of micro-cracks in CVD coated layer

on transverse rupture strength and chipping resistance, Ann. CIRP, 40(1) (1991) 57 60. [12] M. Lee and M.H. Richman, Some properties of TiC-coated cemented tungsten carbides,

Met. Technol., (December 1974) 538 546. [13] P.A. Dearnley and E.M. Trent, Wear mechanisms of coated carbide tools, Met. Technol.,

(February 1982) 60 75. [14] P.A. Dearnley, Rake and flank wear mechanisms of coated cemented carbides, Surf.

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influence of the substrate material on the effectiveness of coatings in metal cutting

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