Wear resistance of arc-evaporated and magnetron-sputtered coatings on cemented carbides

Surface and Coatings Technology, 39/40 (1989) 445- 453

445

WEAR RESISTANCE OF ARC-EVAPORATED AND MAGNETRON-SPUTTERED COATINGS ON CEMENTED CARBIDES O. KNOTEK, M. ATZOR, F. JUNGBLUT and H.-G. PRENGEL Institut fiir Werkstoffkunde B, R W T H Aachen (F.R.G.)

(Received March 9, 1989)

Summary The refinement of the physical vapour deposition (PVD) process has created the necessary conditions for controlled production of thin film systems with defined properties. Complex film systems such as (Ti,A1)N and (Ti,A1,V)N have particularly high potential as wear resistant functional coatings for highly-stressed tools. These films are deposited using the two highly-sophisticated PVD processes of ion plating with a magnetron sputtering source and ion plating with an ARC source. Methods of attaining defined film properties by altering process parameters are indicated. Production of the films is directly linked with analysis or the development of suitable analytic techniques. Film properties s u c h as thickness, microhardness and adhesion are optimized for maximum wear resistance. X-ray microstructure analyses are used to determine the phases and their crystalline structures and orientations. Finally, scanning electron microscope images document the structure and formation of the films. The wear resistance of (Ti,A1)N- and (Ti,A1,V)N-coated indexable tips is demonstrated by means of machining tests under high stresses. The wear mechanisms are analysed and implications for further optimization of the films discussed.

1. Introduction Physical vapour deposition (PVD) coating technology is gaining increasing importance in the production of hard wear-resistant coatings on carbide tools, owing to its process advantages, e.g. flexibility in terms of coating materials and deposition at temperatures which are low by comparison with those in chemical vapour deposition (CVD) coating [1, 2]. Industrial realization of PVD coating for carbide tools is the aim [3]. In the effort to meet the constantly increasing performance specifications for coated tools, optimization of coating processes is paralleled by the development of complex hard material coatings with improved properties 0257-8972/89/$3.50

9 Elsevier Sequoia/Printed in The Netherlands

446 [4, 5]. Multicomponent TiN-based hard material compounds can be produced by substituting other metals (e.g. aluminium, vanadium) for the titanium atoms [6, 7]. The pre-conditions for the mixed phases of hard materials according to the Hume-Rothery rule are that the atomic radii are comparable and the lattice structures involved in the formation of the compounds are isotypic [8]. In the present study, complex hard material coatings in the Ti-A1-N and Ti-A1-V-N systems were deposited on carbide indexable tips, using the PVD processes of ion plating with a magnetron sputtering source (MSIP) and ion plating with an arc vaporization source (ARC). Metallographic, X-ray and electron microscope analyses of the coatings were supplemented by machining tests (turning tests) to determine the wear resistance of the (Ti,A1)N and (Ti,A1,V)N coated carbide indexable tips.

2. Experimental procedure Complex (Ti,A1)N and (Ti,A1,V)N hard material coatings were deposited on SNGN 120408 sintered carbides of the ISO M15 group (82 wt.% WC, 11 wt.% (Ti,Ta,Nb)C, 7 wt.% Co). These multicomponent hard coatings were formed using a Leybold-Heraeus Z400 coating plant (MSIP process) and an Interatom PVD 20 coating plant (ARC process), with nitrogen as a reactive atmosphere. A stable, economically-viable coating process (coating rate 6- 10 pm) was achieved in both cases. In the MSIP process, a stable plasma was invariably triggered when a voltage was applied to the target in an argon atmosphere. The ARC process is characterized by a high degree of ionization of the vaporizing metal (approximately 80-90%), so that no extraneous gas (e.g. argon) is required [2]. A stable arc at the target is achieved by a single triggering operation. In both these PVD processes, the composition of the compounds can be altered simply by changing the composition of the target and by modifying the reactive-atmosphere partial pressure and substrate bias parameters. The Ti-A1-V concentration of the targets employed (pyrometallurgical, powder metallurgical and mechanical production) is indicated in Fig. 1. In the tests, the ternary and quaternary hard material films deposited reactively from these targets are sub-divided into ARC(Ti,A1,V)N, ARC(Ti,Alx)N, aluminium-rich MSIP(Ti,A1,V)N and vanadium-rich MSIP(Ti,A1,V)N. Initial basic results are produced by metallographic analysis of the coating thickness, hardness and adhesion. X-ray microstructural analysis is then used to determine crystalline structure and orientations and to identify the phases. Scanning electron microscope (SEM) analysis is employed to document the structure and formation of the coatings. Apart from energy dispersive X-ray (EDX) analysis, quantitative electron beam microanalysis (ESMA) [9] yields data on coating composition.

447 target material ARC - coating ARC (Ti,AI,'

Ti

MSIP - coating

VlNl-~o/k~~J MSlP (Ti,AI,ViN I ,.L.

AI

~,I I ~ ] ~

v o ~ ~ ~[um - [ ~~~ j

W

Fig. 1. Target compositionsfor the production of complex (Ti,A1)N and (Ti,A1,V)Nhard material coatings. The wear resistance of MSIP(Ti,A1,V)N- ARC(Ti,A1,V)N- and ARC (Ti,Alx)N-coated carbide indexable tips is determined in machining tests under high stresses.

3. R e s u l t s

3.1. MSIP(Ti,AI, V)N-coated sintered carbide Aluminium-rich MSIP(Ti,A1,V)N coatings exhibit the typical violet colouring of (Ti,A1)N, whereas vanadium-rich MSIP(Ti,A1,V)N coatings have a yellow-gold colour resembling that of TiN. Coating adhesion analysis using the scratch test indicated critical loads in excess of 7 0 N for all MSIP(Ti,A1,V)N coatings. There were no noticeable differences in coating adhesion for different coating compositions in the ultra-hard range of interest for wear resistance. Peak hardnesses of 3000 HV 0.05 are measured. If aluminium-rich MSIP(Ti,A1,V)N coatings are deposited sub-stoichiometrically, there is a distinct drop in hardness to 1500 HV 0.05 for these aluminium-rich films. Vanadium-rich MSIP(Ti,A1,V)N coatings achieve their greatest hardness (3000 HV 0.05) at 4 at.% V in the coating. Particularly noticeable here are the flat curves with no pronounced hardness maximum, indicating non-critical stoichiometric behaviour. X-ray structural analyses of the complex MSIP(Ti,A1,V)N coatings reveal the face-centered cubic (f.c.c.) lattice structure of TiN. Influenced by the alloying elements aluminium and vanadium, the X-ray spectrum is displaced towards the higher angles, greatly reducing the size of the MSIP(Ti,A1,V)N cell as compared to the TiN cell. The deviation of the TiN cell lattice parameter is plotted against aluminium and vanadium content in Fig. 2. The contraction of the cell (lattice parameter difference Ad) is limited to a maximum alloying-elements/titanium ratio of 1.1 in the case of aluminium-rich MSIP(Ti,A1,V)N. The lattice parameter difference is then approximately 0.008 nm. If this saturation limit of the cubic cell is exceeded by

448

0.010 JO

<3

I

I

I

[

I

IMSlP(Ti,AI)N]

[ aluminiumrichi MSIP(Ti,AI,V}NI

0.008

~

~ ~ ;

/

0.006

-"

// b=,

I

nm

o.oo4

/

CI=aTiN-(I(Ti,AI,V)N

~

= 0.002 0

J

0

0.2

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]

I

]

l

0.4 0.6 0.8 1.0 at.-% (AI +V)/at.-% Ti

l

1.2

1.4

Fig. 2. Lattice parameter difference of complex MSIP(Ti,A1,V)N and MSIP(Ti,A1)N coatings as a function of the (A1 + V)/Ti ratio.

further increasing aluminium content, no further reduction of the lattice parameter is observable. Under X-ray analysis, no second phase apart from the cubic (Ti,A1,V)N mixed phase is demonstrable at these high aluminium contents of 30 at. %. The wear resistance of MSIP(Ti,A1,V)N-coated carbide indexable tips is investigated in machining tests (Fig. 3). Aluminium-rich MSIP(Ti,A1,V)N coatings display good wear behaviour at low aluminium contents. Excessive aluminium content (greater than 30 at.%) leads to partially lower crater wear resistance despite the good results obtained in metallographic analyses. This result may be ascribed to possible reaching or crossing of the solubility limit for aluminium and vanadium in the TiN lattice. This confirms the metastable character of aluminium-rich MSIP(Ti,A1,V)N coatings already observed in vacuum annealing tests at roughly 1000 ~ The performances of vanadium-rich MSIP(Ti,A1,V)N-coated carbide indexable tips are clearly documented in the machining test. Crater wear is less than on the CVD TiC-TiN reference specimen. MSIP(Ti,A1,V)N with a high vanadium content exhibits particularly good thermal stability and diffusion resistance under the high temperature loads of the machining process, with correspondingly little crater wear, but an arbitrary increase in vanadium content of MSIP(Ti,A1,V)N coatings is limited by the associated increase in flank wear.

3.2. ARC(Ti,AI, V)N- and ARC(Ti,Alx)N-coated sintered carbide No problems are presented by ARC reactive processing of the complex hard material coatings of the Ti-A1-V-N and Ti-A1-N systems under investigation. ESMA analysis indicated that stoichiometric hard material

449

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I

I

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i

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-

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mm

CVD- TiC/TiN

0.2

--

z

0.1

4 cL-~ V

1

2

3 rnin

1

2

3 rain

culling time i c Fig. 3. Crater and flank wear on MSIP(Ti,A1,V)N-coated carbide indexable tips.

compounds are deposited across a wide range of reactive gas volume settings, virtually irrespective of N 2 partial pressure. ARC(Ti,A1,V)N and ARC(Ti,Alx)N coatings have good adhesive properties on carbide substrates. Critical loads of more than 75 N were measured in the scratch test. The ARC coatings in the test were produced to a coating thickness of 6 - 1 4 # m . A hardness increase for ARC(Ti,A1,V)N and ARC(Ti,Alx)N coatings is observable in microhardness measurements when the bias voltage is raised (Fig. 4). Above a bias voltage of 200 V, ARC(Ti,A1,V)N coatings achieve coating hardnesses of 3000HV 0.05. ARC(Ti,AI~)N coatings show comparatively lower hardnesses. A decline in microhardness is observable when the aluminium content in the TiA1 target is increased from 25 to 50 at.%. Whereas ARC(Ti,AI~)N coatings produced with a Tio.7~Alo.25 target attain a hardness of 2700 HV 0.05 at high bias voltages, microhardness values ranging from 1800 to 2200 HV 0.05 are measured for similar coatings deposited with a Ti0.sAlo.5 target. X-ray analyses confirm the cubic lattice structure of the TiN cell, with little change in the size of the cell and the (111) plane as the preferred growth plane, for both ARC(Ti,A1,V)N and ARC(Ti,AI~)N coatings. Figure 5 shows the lattice parameters as a function of the bias voltage. The lattice parameters of the ARC(Ti,A1,V)N mixed phase correspond to that of TiN in the ASTM register, while the lattice parameter of the ARC(Ti,A1DN mixed phase tends towards lower values, especially when coatings are deposited at low bias voltages. This reduced cubic cell in ARC(Ti,AI~)N coatings produced

450

5000

I

I

4000

I

ARC (Ti'AI'VJNl

~ t

CD :~

_ 3000

~ 2000 9-~ E 1000

x TiAI 6V4 z~ Tio.TsAIo,2s 9 Tio.sAIo.s

0

0

f

1

1

100

200

300

bias

V

400

voLIoge

Fig. 4. Microhardness of ARC(Ti,A1,V)N and ARC(Ti,A1,)N coatings as a function of bias

voltage.

with a Tio.sAlo. 5 target corresponds to the lattice parameter size of MSIP(Ti,A1)N and aluminium rich MSIP(Ti,A1,V)N coatings (cf. Fig. 2). The increase in the lattice parameter of the (Ti,Alx)N mixed phase when the bias voltage is raised is based on a reduction in the aluminium content of the coating (Fig. 6). Since titanium has a higher ionization level (roughly 80%) than aluminium (roughly 50%), more titanium ions condense on the substrate at higher bias voltages, altering the Ti/A1 ratio in

0.430

I

I

I

nm

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--'-"

C

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0.414

targets •

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TiO.TsAIo.2s 9 Tio.s AIo,s I

0

100

J

I

200 bias voltage

300

V

400

Fig. 5. Lattice parameters of ARC(Ti,A1,V)N and ARC(Ti,Alx)N coatings as a function of bias voltage.

451

25 at-%

~ 2=

g E <.9

:D

I

A

I

I

A~,,~

20

9

15i I

Targets : x TiAI6VL A Tio.TsAio.25 9 Tio.s AIo.s

~

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ARC (TiA V}N ,

,

2r P

100

I

200

I

300

V

&O0

bias voltage Fig. 6. h l u m i n i u m content of ARC(Ti,AI,V)N and ARC(Ti,Alx)N coatings as a function of bias voltage.

uninfluenced stoichiometric deposition. ESMA analyses document this stoichiometric metal-metalloid composition and simultaneously confirm EDX studies used to confirm the titanium, vanadium and aluminium concentrations in the coating. The aluminium concentration in the complex ARC(Ti,Alx)N coatings is particularly easy to modify, and can be adjusted within a concentration range from 22 to 8 at.% in the coating, depending on the bias voltage, if a Tio.~Alo.5 target is employed. Where a b i a s voltage is applied, ARC(Ti,A1,V)N and ARC(Ti,Alx)N coatings grow in a dense, compact structure, as indicated by the SEM images in Fig. 7. The performance and wear resistance of ARC(Ti,A1,V)N- and ARC(Ti,Al~)N-coated carbide indexable tips were examined in a tool life

ARC(Ti,AI,V)N

5pro

ARC(Ti,Alx)N }

Fig. 7. Rupture structure of ARC(Ti,A1,V)N and ARC(Ti,AI~)N coatings on carbide.

452

12 min

9:27 9

8

~- ~-----:----t

E

,::,-,

~

C 60 v c = 220 m/rnin ap = 2.5 mm s = OJ, mm/U

I HMM15

6

c

,...,

4

ARC(Ti,AIx) N'1 ICVD-stondord O/

U

,

,

1000 IJ m

Fig. 8. Edge-holding of ARC(Ti,A1,V)N- and ARC(Ti,Al~)N-coated carbide indexable tips (coating thickness: 10 - 12 ~m). ARC(Ti,Alx)Nz with Tio.75Alo.25,ARC(Ti,Alx)N2 with Tio.6Alo.5.

t u r n i n g test. T h e tool life of the c o a t e d tool up to a p o i n t s h o r t l y p r e c e d i n g c u t t i n g edge f r a c t u r e was selected as a c r i t e r i o n for c o a t i n g q u a l i t y (Fig. 8). In terms of w e a r resistance, the p e r f o r m a n c e of complex ARC coatings on carbide tools is c o m p a r a b l e to t h a t of m o d e r n CVD coatings. ARC(Ti,Alx)N 1 coatings p r o d u c e d with a Ti0.75Alo.25 t a r g e t display the best w e a r b e h a v i o u r in the tool life t u r n i n g test. Like the ARC(Ti,A1,V)N and ARC(Ti,Alx)N 2 coatings, these coatings m e e t m o d e r n w e a r p r o t e c t i o n req u i r e m e n t s for highly-stressed c u t t i n g tools. U n c o a t e d M15 carbide i n d e x a b l e tips, by comparison, r e a c h t h e i r p e r f o r m a n c e limit u n d e r these h i g h stresses after a m a c h i n i n g time of as little as 30 min. T h e w e a r images of the tool c u t t i n g edge exhibit p r o n o u n c e d c r a t e r w e a r in addition to w e a r on the flank (VB = 0.2- 0.3 mm). M a x i m u m c r a t e r depths of 60 - 80 tLm were measured.

453

4. C o n c l u s i o n s Complex hard material coatings of the Ti-A1-N and Ti-A1-V-N systems possess considerable potential as wear-resistant functional coatings on highly-stressed carbide tools. These multicomponent hard material coatings are deposited on carbide indexable tips using the two modern PVC processes of ion plating with a magnetron sputtering source (MSIP) and with an arc source (ARC). The influence of aluminium and vanadium on the properties of MSIP coatings was examined. Machining tests (turning tests) with MSIP(Ti,A1,V)N coated carbide indexable tips confirm the differing influences of vanadium and aluminium. It was demonstrated t ha t a higher vanadium content clearly promotes stability and diffusion resistance, increasing the wear resistance of the face. Aluminium-rich MSIP(Ti,A1,V)N coatings possess high abrasion resistance, as documented by the good flank wear results. The development of complex coating systems was confirmed by the performance and wear resistance of ARC(Ti,A1,V)N and ARC(Ti,A1)N coatings on carbide tools. Coating properties can be modified according to requirements via the bias voltage applied to the substrate. The edge-holding of ARC(Ti,A1,V)N- and ARC(Ti,A1)N-coated indexable tips is comparable to t h a t of modern CVD coated indexable tips, as demonstrated under high stresses in the tool life turning test.

References 1 2 3 4 5 6 7 8

O. Knotek, W. D. Mfinz and T. Leyendecker, J. Vac. Sci. Technol., A 5 (1987) 2173-2179. E. Ertfirk and H.-J. Heuvel, Thin Solid Films, 153 (1987) 135- 147. P. RSdhammer, K. Kailer and A. Mairhofer, SURTEC Berlin, 1987. H. Holleck, Z. Werkstofftech., 17 (1986) 334-341. U. KSnig, Surf. Coat. Technol., 33 (1987) 91- 103. O. Knotek, T. Leyendecker and F. Jungblut, Thin Solid Films, 153 (1987) 83-90. H. Freller and H. Haessler, Thin Solid Films, 153 (1987) 67 - 74. W. Hume-Rotheryand G. V. Rayner, The Structure of Metals and Alloys, Institute of Metals, London, 1969. 9 0 . Knotek, W.-G. Burchard, P. Karduck and T. Leyendecker, Beitr. electronenmikroskop. Direktabb. Oberfl., 19 (1986) 211.