Comparison of the steel-milling performance of carbide inserts with MTCVD and PVD TiCN coatings

Int. J. of Refractory

Met& & ftmi Materials 14 ( 1996) 3 l-40 Copyright 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0263-436S/96/SlS,OO

ELSEVIER

Comparison of the Steel-Milling Performance of Carbide Inserts with MTCVD and PVD TiCN Coatings A. T. Santhanam, D. T. Quint0 & G. P. Grab Kennametal Inc., PO Box 23 1, Latrobe, PA 15650-023 1, USA (Received 11 June 1993; accepted 5 August 1993) Abstract:

This study is a comparison of the performance of PVD-TiN-, PVD-TiCN-, and MTCVD-TiCN-coated milling inserts. It is shown that metal-cutting conditions and insert geometry dictate the optimum performance for each type of coating. The importance of compressive residual stress in PVD coatings in delaying the initiation of damaging cracks at the edge is evident in these results. Conditions in which thermal shock is not severe, such as dry milling or lower-speed wet milling and the use of sharp cutting edges, are shown to maximize the performance of the PVD-TiCN-coated tool. On the other hand, the MTCVD coating necessitates a strong cutting edge geometry on a tough substrate to obtain good performance, in wet milling. Practical productivity implications, including environmental concerns, are considered.

1

performance is optimum under dry conditions; wet milling is possible only at lower speeds with tough substrates. On the other hand, MTCVD-TiCN-coated inserts reportedly perform well in dry and wet milling.6~7 Such field comparisons are complicated by the effects of edge geometry because PVD-coated inserts are used with either sharp or honed edges, whereas CVD-coated inserts employ chamfers and honing to strengthen the cutting edge against chipping. This investigation is a critical assessment of MTCVD and PVD-TiCN-coated insert performance in the milling of medium-carbon-alloy steel, perhaps the most frequently machined workpiece material. The data are analyzed in the light of synergistic interaction of the properties of the coating, tool-edge geometry, cutting conditions, and the use of coolant during milling.

INTRODUCTION

Improvements in the performance of cemented carbide metal-cutting inserts have reflected the evolution of hard-coating-deposition technologies. It is noteworthy, however, that the coating technology most prevalent in turning operations, namely, the high-temperature CVD process, has been restricted to carefully designed milling tools. High-temperature CVD produces a brittle eta phase at the substrate-coating interface, which can cause chipping when subjected to the cyclic thermal and mechanical stresses characteristic of milling.1-3 The development of moderate-temperature CVD (MTCVD) proved to be advantageous in milling,4-6 which was due in part to the absence of the eta phase. The latest evolution in hard-coating technology, physical-vapor deposition (PVD), employs even lower deposition temperatures than MTCVD and has also found a niche in milling. 1-3,8Beyond PVD-TiN, secondgeneration PVD coatings such as TiCN and TiAlN have been starting to appear on the market in recent years.‘-’ 3 Feedback from industrial users of PVD-TiNcoated milling inserts has indicated that their

2

EXPERIMENTAL

METHODS

The hard-metal composition employed in this study had 1 l-5 wt% Co content, and the alloy was processed by conventional powder-metallurgical techniques. SEHW 1204 AFTN-style inserts with 31

A. T. Santhanam, D. T. Quinto, G. P. Grab

32

Fig. 1.

SEHW 1204 AFTN-style

inserts used in the present study: (a) sharp; (b) honed.

workpiece feed direction (down milling) Fig. 2.

Schematic

of the metal-cutting w= 75 mm).

test (d= 98 mm,

feedrate; the higher-speed/lower-feedrate parameters were 213 m/min and 0.18 mm/tooth feedrate. The depth of cut used for both combinations was 2.5 mm. Additional milling tests were performed without the coolant at 213 m/min. A schematic of the milling test is shown in Fig. 2. An end-of-life criterion of 0*40-mm uniform flank wear or 0.7 5-mm maximum wear (which included edge chipping) was used to determine the tool life of the test inserts.

3 chamfers or T-lands only on the cutting edges (Fig. 1) were prepared from this alloy. Half the inserts were left sharp after chamfering, and the other half were given a slight (O-04-mm radius) hone at the edge of T-lands and on the wiper edges. The inserts were coated by either the PVD (ion-plating) or MTCVD process. Three coatings were evaluated: PVD-TIN, PVD-TiCN, and MTCVD-TiCN. Coating microhardness, adhesion, and residual stress were evaluated according to previously described methods.‘4*15 Both optical-microscopy and SEM techniques were used to evaluate the surface and fracture morphology of coatings and the failure mechanisms of the inserts after metal-cutting tests. Metal-cutting evaluation consisted in face (down) milling AISI 4140 steel by employing flood coolant. Two feed/speed combinations, selected to give equivalent metal-removal rates, were used. The lower-speed/higher-feedrate combination was 152 m/min and 0.25 mm/tooth

3.1

RESULTS

Coating morphology

Figure 3 shows optical micrographs of polished cross-sections of the coated tool inserts. The PVD-TiCN coating incorporates thin inner and outer TIN layers sandwiching multilayers of TiCN with a gradient C/N concentration.‘” The dense and smooth morphology of the PVD coatings and the columnar structure of the MTCVD coating are seen in this figure. No eta phase is seen beneath any of the coatings owing to the low temperatures employed in both PVD and MTCVD coating processes. The surface morphology and fracture crosssections of the coatings are shown in the SEM micrographs of Fig. 4. The fine dimpled structure of the PVD coatings is typical of the ion-plating process. The MTCVD coating, on the other hand, shows a fine lenticular structure. The fine columnar structure of the MTCVD-TiCN coating is discernible in its fracture cross-section.

Comparison of steel-milling peg%ormanceof carbide inserts

Fig. 3.

3.2

Optical micrographs of the coated inserts used in this study. (a) PVD-TiN; (b) PVD-TiCN; (c) MTCVD-TiCN.

Coating properties

Table 1 presents the coating-thickness, microhardness, indent-adhesion, residual strain, and residual stress values for the three coatings used in this study. The PVD-TiCN coating has the highest microhardness at room temperature. The PVD-TIN and MTCVD-TiCN coatings have lower hardness values. All the three coatings exhibit adequate adhesion strength as measured by the indentation technique.14 The residual strains were measured by the The corresponding residual X-ray method. stresses were calculated by assuming elastic moduli of 640, 585, and 545 GPa, respectively, for PVD-TiN, PVD-TiCo.3No.,, and MTCVDTiC,.,N,.,. The residual stresses are compressive in the PVD coatings whereas they are slightly tensile in the MTCVD coating.

3.3 3.3.1

Metal-cutting

tests

Tool life

Tool-life data from milling tests are plotted in Fig. 5. The mean tool life along with the range is presented for each coating. At the lower metal-cutting speed (152 m/min, Fig. 5(a)), all the three coatings show tool-life improvements when the edges of the T-lands and the wiper faces are slightly honed. This improvement is greater for the MTCVD coating than for the PVD coatings. Among the three coatings, PVD-TiCN coating shows the longest tool life whether the cutting edges are sharp or honed. The PVD-coated tools are also seen to perform more consistently (a smaller variability of tool life) than the MTCVDcoated inserts at each edge condition. The consistency of the PVD-coated tools is most noticeable in comparing tools with sharp T-lands.

A. T Santhanam, D. T. Quinto, G. P. Grab

Fig. 4.

Comparison

of coating-surface morphologies (left) and their corresponding fracture cross-sections in the SEM: (a) PVD-TiN; (b) PVD-TiCN; (c) MTCVD-TiCN.

At the higher metal-cutting speed (2 13 m/mm, Fig. 5(b)), tool life decreases with edge-honing for both types of PVD coating, although the decrease in tool life for the PVD-TiCN coating is less dramatic. However, honing had a positive effect for the MTCVD coating at this speed. The MTCVD-coated tool also shows a longer tool life at the higher speed than the PVD-TiCN-coated insert. In terms of tool-life consistency, the PVDcoated inserts are again generally better than the MTCVD-coated tool. In an effort to separate the effect of additional thermal shock produced by the use of coolant,

(right) as observed

another milling test (without coolant) was performed at 2 13 m/min on the honed PVD-TiCNand MTCVD-TiCN-coated inserts. Results are presented in Fig. 6. A significantly longer tool life is obtained under dry-milling conditions (compare Fig. 6 with Fig. S(b)). The PVD-coated tool again performs more consistently, although its average tool life is lower than that of the MTCVD-coated tool. 3.3.2 Wear curves Figure 7 compares the progression of maximum wear (local wear associated with edge-chipping)

Comparison of steel-milling performance of carbide inserts

35

Table 1. Coating type and properties Coffting

PVD-TiN PVD-TiCN MTCVD-TiCN

Nominal thickness (pm)

Vickers hardness at 50-g load (kg mm-?)

Indentadhesion critical load, P,, (k@

Residual strain in coating 6)

Residual stress in coating (MPa)

3-4 3-4 5.0

2150+ 100 2750 -1:150 214Ok60

45 45 45

-057 - 066 + @09

- 3.580 - 3775 + 490

sharp T-land

n

honed T-land

m

0 PVD TiN

PVD TiCN

MTCVD

TiCN

ia)

Fig. 5.

PVD TIN

PVD TiCN

MTCVD

TiCN

(13)

Tool life of PVD- and MTCVD-coated inserts in down milling (with coolant) of AISI 4140 steel. End-of-life 0.75 mm maximum wear (which includes chipping).

1zoo0

Comparative wear curves higher-speed/lower-feedrate similar pattern. honed T-land

MTCVD

obtained under the condition showed a

m

0 PVD TiCN

criterion:

TiCN

Tool life of honed PVD- and MTCVD-TiCNcoated inserts in milling (no coolant) of AISI 4140 steel.

Fig. 6.

for the various test inserts at 152 m/mm. The ability of a honed edge to resist chipping is clear, and the effect is most dramatic with the MTCVD coating. At this metal-cutting speed, wear is lowest for the PVD-TiCN coating, which leads to long tool life, as was seen earlier in Fig. 5(a).

3.3.3 Failure mode In this investigation, as in most milling applications, edge-chipping was invariably the determining factor for the end of tool life. It was observed that the number and severity of the thermal cracks increased with increasing machining time, and localized chipping at the insert edge followed. The use of flood coolant significantly increased thermal cracking and consequent edge-chipping. In addition to chipping, localized wear also occurred and progressed with increasing milling time. However, even at the end of tool life, uniform flank wear was relatively small. Figure 8 shows tool edges after milling a length of 1800 mm on AISI 4140 steel. This figure prompts a number of observations. First, edge-chipping is associated with the interaction of thermal cracks perpendicular to the cutting edge, with lateral cracks along the base of the T-land. The number

36

A. T. Santhanam, D. T. Quinto, G. P. Grab

milling

4140 steel.

MTCVD TiCN -BPVD TIN ” a PVD TiCN - .A -

(honed T-land)

length of cut (mm)

Fig. 7.

Fig. 8.

length of cut (mm)

Progress of maximum wear and the effect of tool-edge-honing

on the maximum wear in wet milling.

A comparison of tool edges after 1800 mm of down milling (same machining conditions AFTN inserts, T-land, no hone (left) and T-land, honed (right). (a) PVD-TIN; (b) PVD-TiCN;

as in Fig. 7) SEHW 1204 (c) MTCVD-TiCN.

Comparison of steel-millingperformance of carbide inserts

of thermal cracks increases from MTCVD-TiCN to PVD-TiCN and PVD-TiN. Further, the tendency for lateral chipping is greater for the inserts with no hone and is particularly severe for the MTCVD-TiCN-coated tool.

TRS (WC-co)-

DISCUSSION

The primary tool-failure mechanism in the milling of steel workpieces is insert-edge-chipping. The factors that control chipping are: substrate, coating, insert-edge geometry, and metal-cutting conditions. One has to consider the effect of each of the above variables as well as their synergistic interaction. 4.1

Substrate/coating requirements for milling

Interrupted cutting subjects the milling insert edge to cyclic mechanical and thermal stresses, the magnitude of which is determined by the speed, feedrate, and use of coolants. The tool material and geometry should therefore be designed to maximize impact-fracture strength and thermalshock resistance sufficiently to ensure useful tool life. In addition, the presence of a protective hard coating should lower friction stresses and delay the onset of abrasive and chemical wear. The coated WC-Co inserts selected for this study meet the basic requirements for milling tools. The substrate, by virtue of its composition and microstructure, has high values of transverse rupture strength (2.6 GPa) and fracture toughness (K,, = 12.7 MPa,/m). Conventional (high-temperature) CVD coatings feature varying degrees of inter-facial embrittling eta phase and pre-existing thermal cracks, which limit their effectiveness in milling applications. Whereas MTCVD and PVD coatings are, respectively, partially or completely free of those limitations, they differ from each other in the type and level of residual stress. The PVD-TiCN coating employed in this study had a high compressive residual stress, whereas the MTCVD coating possessed a slightly tensile residual stress. It is known that high compressive stresses resist crack initiation at the coatl2 and improve tool-life consistency.’ ing 1-3*7~8, A useful framework for the discussion of the edge-chipping mechanism of coated inserts is shown in Fig. 9, after Suzuki.” For simplicity, we assume that the elastic moduli of both MTCVD

-

W)--

‘J,iA) -_

c3

4

37

I

&AMTCVD)

Ef(PVD ) EAWC-Co) .

8

Fig. 9. Elastic-deformation model for crack initiation in MTCVD and PVD-TiCN-coated cemented carbides, after Suzuki et al.”

and PVD coatings are the same and that the modulus of the substrate is unchanged as its transverse rupture strength is varied. The fracture strain of the MTCVD-TiCN coating is ef (MTCVD). If we neglect the small residual stress and its associated strain in the MTCVD coating, we see that the PVD fracture strain Ef (PVD) = cf (MTCVD) f E, where E, is the residual compressive strain associated with the compressive residual stress (- a,). The fracture strain of the substrate ef (WC-Co) is dictated by its transverserupture-strength (TRS) value, which is likely to be higher than the respective values of either coating (which are not measurable). Edge-chipping is governed by the fracture of the coated-alloy system (coating and substrate) in response to applied stresses. In this model, the coating cracks are initiated at a lower stress than that needed to initiate cracking of the substrate, so that the effective rupture strength of the system is decreased relative to the TRS of the uncoated substrate. However, the MTCVD-coated alloy always ruptures at a lower stress (a,(A)) than the PVD-coated alloy (o,(B)). (When the TRS of the uncoated substrate corresponds to a fracture strain er (WC-Co) which is less than cr (PVD), u~B) = TRS of the substrate, which is rate-limiting for chipping.) Since fatigue-cracking induced by mechanical and thermal stresses is a statistical process, the total number of cracks and the cracklength distribution are important factors; however, these considerations are beyond the scope of this study. it can be safely surmized that, in the MTCVD coating, which has a slight residual tensile stress, there are already some pre-existing thermal cracks that add to those induced during

A. T. Santhanam, D. T. Quinto, G. P. Grab

38

Fig. 10.

SPG 120412 style inserts (sharp) used in down milling (no coolant) of AISI 4140 steel in another study.‘”

interrupted cutting; cracks present are This may account sistency observed inserts. 4.2

8000

in the PVD coating, the only those created during cutting. for the better tool-life confor the PVD-TiCN-coated

3

Edge geometry

2 1

the cutting edge minimizes and therefore strengthens the milling applications. l* Honing the a similar (although not 5(a) 5(b) show that honing the edge improves tool life dramatically for the MTCVD-coated tool at both low and high speeds. However, tool life for coated only at lower speeds. cite here our laboratory I9 on an insert-style SPG 120412 (no chamfer), shown in Fig. 10. Figure 11 shows the milling performance of PVD- and MTCVD-TiCN-coated inserts with a less tough substrate (TRS = 2.2 GPa, K1, = 10.8 MPa,/m). The edge of the milling insert was sharp and relatively prone to edge-chipping. At essentially the same high speed/low feedrate, with the dry condition used in this study (the operation described in Fig. 6 employed a chamfered and honed edge), the PVD-TiCN-coated insert exhibited a 2.5 X -longer tool life and better consistency than the MTCVD-coated insert. The beneficial effect of PVD-TiCN coating is attributed to its high compressive residual stress. When this stress was annealed out by heat treatment at 850°C for 1 h, the propensity to edgechipping increase dramatically (Fig. 12), and

6000

e YI BQ 5000

4000

;j s

3000

1000

0

PVD TiCN

MTCVD TiCN

Fig. 11. Tool life of PVD- and MTCVD-TiCN-coated SPG 120412 style inserts in down milling (no coolant) of AISI 4 140 steel.

100 -

q n

v) 75 % B %

annealed 8.5OWl hr. as-coated

-

50-’

.s & u xi-k?

6dO

length

Fig. 12.

1800

12bO

A0

30bo

of cut (mm)

Effect of annealing PVD-TiCN-coated

on chipping tendency milling insert.

of

Comparison of steel-millingpetiormance

milling performance dropped to the level of the MTVCD-coated insert. These results indicate that, when chipping dominates tool life, the compressive residual stress in the coating is beneficial to the extent that the number of supercritical cracks is suppressed in the alloy system. As the propensity to chipping is reduced, e.g. by the use of a tougher substrate or a more tolerant edge geometry, the role of compressive residual stress is not as pronounced. 4.3

Cutting speed/feedrate

A combination of low cutting speed and high feedrate resulted in a longer tool life for each of the tools used in this study (Fig. S(a)) as compared with higher speeds and lower feedrates (Fig. 5(b)). The shorter tool lives at the higher speed cannot be ascribed to cyclic mechanical stresses alone, because the cutting forces are actually lower at higher speeds and lower feedrates. Thermal shock resulting from the use of higher speeds and coolant must therefore be responsible for increased damage to the insert edge and the attendant reduced tool life. The ability of the coated alloy to resist thermal shock varies directly with the coating thermal conductivity,13 which in turn is related to the carbon content of the coating composition. The carbon/nitrogen ratio of the coatings employed in this study increases in the PVD-TiCN, and following order: PVD-TiN, MTCVD-TiCN. (It may be noted that the decrease in the number of observed thermal cracks follows the same sequence in Fig. 8.) Figure 5(b) confirms that the tool lives follow the same order. These considerations, however, are not controlling under the lower-speed/high-feedrate condition since thermal stresses are not rate-limiting. In this case, the mechanical stresses play a dominant role, which can be countered by residual stress in the coating.

of carbide inserts

39

strong factor, the PVD-TiCN coating becomes significantly more capable in suppressing insertedge-chipping, which thereby increases tool-life consistency. Because elimination of coolant decreases the severity of thermal stresses, it can be deduced that the chipping mechanism must be governed by cyclic mechanical stresses. The high residual compressive stresses within the PVD coating would be expected to minimize edgecracking, whereas the tensile stress in the MTCVD coating would make it susceptible to fatigue-induced cracking, resulting in a larger variation in tool life. 4.5

Productivity

implications

On the shop floor, reliability of metal-cutting performance is more valuable than a long tool life alone. Consistency and predictability are paramount considerations in untended machining operations, where tool-indexing is dictated by minimum tool life. In steel-milling with coatedcarbide inserts, tool-life consistency can be promoted by selecting a tough substrate, strong edge geometry, appropriate hard coating, and a correct choice of milling parameters. Dry cutting conditions, with their environmental advantages, are also recommended because they reduce thermal stress, which is much more damaging for the tool edge than cyclic mechanical stress. Although metal-removal rates are the same at the two combinations of speed and feed studied here, the longer tool lives obtained at the lower speed/ higher feedrate translate to higher productivity. The PVD-TiCN coating is seen to provide maximum benefit, in terms of both longer tool life and higher consistency under these conditions.

5

CONCLUSIONS

5.1 Second-generation

4.4

Wet versus dry conditions

A comparison of Fig. 5(b) and Fig. 6 shows that, as conditions are switched from wet (with coolant) to dry milling, tool life tripled. The PVD-TiCNcoated tools exhibit less variation in tool life than the MTCVD-TiCN-coated inserts, although the former show a lower mean tool life than the latter, in a similar manner to previous findings on PVDand CVD-coated tools.’ Apparently, under drymilling conditions, where thermal shock is not a

PVD-TiCN-coated inserts show a tool-life advantage over PVD-TiN in wet milling of alloy steel under the conditions employed in this investigation. The high residual stress in the PVD-TiCN coating accounts for improved performance by inhibiting the early initiation of surface cracks that lead to edge-chipping.

5.2 PVD-TiCN coating has a decided tool-life coating advantage over MTCVD-TiCN under dry milling conditions, but only a

A. II? Santhanam,

40

D. 11 Quinto, G. P. Grab

qualified advantage during wet milling, e.g. lower-speed/higher-feedrate conditions. 5.3 A cutting-edge

geometry along with a substrate material designed for high fracture strength is necessary for optimized MTCVDcoated-insert milling performance. A chamfered and honed edge increases the tool life of the MTCVD-TiCN-coated inserts but benefits the PVD-coated inserts only under the lower-speed/high-feedrate condition. A sharp cutting edge, even with a less tough substrate, however, can be tolerated by the PVD-TiCN-coated insert when the thermalstress component is not severe.

5.4 Both

PVD-TiCNand MTCVD-TiCNcoated inserts show dramatically longer tool life in dry, compared with wet (with coolant), milling operations.

5.5 Tool-performance

consistency is generally better for PVD-coated inserts in comparison with the MTCVD-coated tools. By combining this feature with the environmentally advantageous aspect of dry milling, better productivity may be realized. ACKNOWLEDGEMENTS

The authors wish to thank F. B. Battaglia for his help in conducting the metal-cutting tests and M. F. Beblo for residual-stress measurements. Stimulating discussions with Dr P. C. Jindal are gratefully acknowledged.

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2. Kamachi, K., Ito, T. & Yamamoto. T., .%rrj: J. Inr., I ( 1986) 82. 3. Quinto, D. T.. Santhanam. A. T. & Jindal, P. C.. Marer. _ Sci. Engng, A105/106 (lY88) 443. 4. Honetti-Lang, M., Bonetti. R., Hintermann, H. E. & Lohmann. D.. Inr. J. Kejkuct. & flurd Metnls. 1 i lY82) 161. 5. Wahl. H.. Wohlfarth. 11. & Bonetti. R.. Metnl /‘on&>r Kf,/).. 39 (10X4) 220. 6. Kubcl, E., Surf: Cbaf. Technol., 49 ( I Y9 1) 268. 7. Kubcl, E., In Proceedings ofInternational Conference on Advances in Hard-metal Producriun, Bonn, Germany, 19Y2. 8. Tsukamoto, T., Sasaki, K., Shibuki, K., Momma, H. & Takatsu. S., In Advances in IIard Metal Production. Vol. I. Luzern. Switzerland. MPR Publishing Services. Hellstone. UK. 1YX3. 0. Bergmann, E., Kaufmann, H.. Schmid, R. & Vogel, J., .I‘@ Co& Tech&., 42 ( 1990) 237. 0. Knotek, O., Munz, W. D. & Leycndecker, T., J. VW. Sci. Technol., A5 (1987) 2 173. 1. Coil, B. F.. Fontana, R. & Sathrum. P., Mater. Sci. Engq, A140(1991)816. 2. Yamagata. K., Nomura. T.. Tobioka. M. & Kawai, C.. Iw. J. Hefrcrcr. & lfurd Me/al.s, 7 ( 1988) 140. 3. Konig, W., Fritsch, R. & Kammermeier. I)., Sccrj C’uur. kchnol.. 49 ( 1Y9 1) 3 16. 14. Mehrotra, P. K. & Quinto, D. T., J. Vat. Sci. ‘ltchnol., A3 (1985) 2401. 1s. Quinto. D. T., Wolfe, G. J. & Jindal, P. C.. Thin Solid Films, 153 (1987) IY. 16. Quinto, D. T. & Kaufmann, A., U.S. P. 5.075.181 (24 Dec. 1991). 17. Suzuki, H., Matsubara, H., Matsuo, A. 6i Shibuki, K.. J. Japan Sot. Powder & Powder Metall., 33 ( 1086) 36. 1X. Shaw, M. C., Metulcutting Principles, Oxford University Press, New York, NY, USA. 1984, p. 457. 19. Jindal, P. C., Private communication, Kennametal Inc.