mechanical and tribological properties of multilayered pvd tin/nbn coatings

Surface and Coatings Technology 91 (1997) 43-49

Mechanical and tribological properties of multilayered PVD TiN/NbN coatings

Mats Larsson *, Michael Bromark, Per Hedenqvist, Sture Hogmark Uppsala University, Department of Technology, Materials Science Divisioil, Box 534, S-751 21 Uppsala, Sweden

Received 11 December 1995; accepted 30 November 1996

Abstract

The tribological performance of thin hard coatings is, for a given substrate material, mainly governed by the coating hardness, coating fracture resistance, the contact temperature and chemistry (in the prevailing tribosystem). For a given application, an improved tribological performance can therefore, for example, be accomplished by increasing the coating fracture resistance while retaining the hardness, or vice versa. Four different PVD TiN/NbN multilayer coatings were deposited on high speed steel and cemented carbide substrates. Homogeneous TIN and NbN coatings were used as references. The coated composites were investigated with respect to coating thickness, morphology and microstructure, adhesion, hardness, residual stress and resistance to abrasive and erosive wear. The investigation showed that the deposition process works well for the deposition of well adhering, multilayered TiN/NbN coatings. The highest fracture resistance was found for the multilayer coating deposited with the thinnest (5-10 nm) individual layers of TiN and NbN. This implies, in combination with the high hardness of the 10/S coating, that a multilayered coating has the potential to be both tougher as well as harder than a single-layered coating provided that the multilayer consists of very thin (~5-10 nm) layers.

1. Introduction

The tribological performance of thin hard coatings is, for a given substrate material, mainly governed by the coating hardness, coating fracture resistance, the contact temperature and chemistry (in the prevailing tribosys- tern). For a given application, future improvement of tribological performance can therefore, for example, be accomplished by increasing the coating fracture resistance while retaining the hardness of today’s coatings, or vice versa.

A possible approach to obtain an increased coating fracture resistance is to introduce coatings with a multilayered structure, i.e. coatings obtained by alternately depositing two (or more) chemically and/or mechanically different materials to form a layered structure. The multilayer structure wil1 act as a crack inhibitor and thereby increase the coating fracture resistance. This effect has been suggested to result from several different mechanisms: crack deflection due to weak interfaces [ 1,2], crack tip shielding by plastic deformation in combination with strong interfaces [3,4], a favourable residual stress distribution [5] and crack deflection due to large differences in stiffness between the individual layer materials [ 61.

* Corresponding author.

Also, the coating hardness can be increased by a multilayered structure, as demonstrated by, for example, Huang et al. [7] and Chu et al. [8] for Ti/TiN and TiN/NbN multilayer coatings, respectively. To obtain this effect it is, however, necessary to keep the thickness of each individual layer below G 100 mu.

TiN is today the most widely used PVD coating for wear protection of, for example, high speed steel (HSS) cutting tools, while niobium nitride (NbN) is not avail- able on the market. NbN has, however, been used together with TiN to form so-called superlattice coatings (or “superlattices”) [&lo]. This makes the TiN/NbN system interesting for use also in multilayered coatings and this paper presents the results from the development of a process for the deposition of TiN/NbN multilayered coatings and the evaluation of some of their important mechanical and tribological properties.

2. Experimental

2.1. Substrate materials

Two different materials, one powder metallurgy (PM) HSS (ASP2030; Erasteel Kloster AB designation) and one cemented carbide (CC) grade (supplied by SEC0

0257-8972/97/$17.00 Copyright 0 1997 Elsevier Science S.A. All rights reserved PI1 SO257-8972(96)03118-O

44 M. Larsson et al.ISurface and Coatings Technology 91 (1997) 43-49

Tools AB), were used as substrates. The nominal chemi- cal compositions of the substrate materials are, for ASP2030 (wt.%): 1.28 C, 4.2 Cr, 5.0 MO, 6.4 W, 3.1 V, 8.5 Co, hardness 920 Hv (L=30 kgf), and for the CC (wt.%): 93.5 WC, 6.0 Co, 0.5 TaC, hardness 1640 H, (L= 30 kgf). Substrate dimensions were 40 x 20 x 2 and 12 x 12 x 3 mm3 for the HSS and CC, respectively.

The HSS was heat treated by austenitization at 1180°C followed by tempering 3 x 1 h at 560°C which resulted in a primary carbide volume fraction of 13% and a mean maximum carbide diameter of 2 ,um. The WC grains in the CC were approximately 2 pm in diameter. All substrates were polished to mirror finish, corresponding to l2, values of approximately 5 (HSS) and 10 nm VW.

2.2. Coating deposition

The coatings were deposited using BALZERS BAI 640R equipment, fitted with an electron beam evaporation source (e-gun) and a planar magnetron sputtering source. The TiN/NbN multilayer process developed in this work is based on triode ion plating (TiN deposition) and d.c. sputtering (NbN deposition) with high plasma density.

Before insertion into the vacuum chamber, all substrates were thoroughly degreased and dried with clean N, gas. The vacuum chamber was evacuated to a pressure of less than 5 x lO-‘j mbar before electron heat- ing (45 min at a substrate temperature of 450°C) commenced. Thereafter, sputter etch cleaning in argon (PAr=2 x low3 mbar) of the substrates for 15 min at 450°C with a negative substrate bias of 200 V was performed.

The coatings were deposited at 450°C using a negative substrate bias of 110 V and an Ar partial pressure of 1.2 x low3 mbar. The Nb target was tirst sputter cleaned for 3 min using a power of 1 kW. Simultaneously, a Ti interlayer (layer thickness approximately 30 m-n), followed by a TiN layer (approximately 250 nm), was deposited using the Ti source with the substrates stationary above it. The rotation of the substrate holder was then engaged, and the power of the magnetron increased to 5 kW with the evaporation source still running, i.e. deposition of the TiN/NbN multilayer commenced. Thin TIN and NbN layers (5-10 nm) were deposited using continuous substrate rotation, whereas thicker layers (100 and 500 nm) were deposited by keeping the substrates stationary above the Ti and Nb sources, respectively. For rotating substrates the deposition rates were 70 and 50 nm/min, and for stationary substrates 150 and 500 nm/min for TiN and NbN, respectively. During multilayer deposition, the e-gun emission current was regulated in order to maintain a constant pressure (0.5 x 10m3 mbar) while the magnetron power was kept constant at 5 kW. The N, was inserted at two positions:

one at the e-gun (140 seem) and one at the magnetron (15 seem). Finally, the coated substrates were cooled using pure He (PHe = 100 mbar) for 20 min.

Four different combinations of TiN/NbN coatings were deposited on both HSS and CC. The nominal thicknesses of the TiN and NbN interlayers were 500/500, lOO/lO, lo/100 and 10/5 run. In addition, homogeneous TiN and NbN coatings were deposited on both substrate materials as references. A total coating thickness of 5.0 l.un was aimed at in all cases.

2.3. Evaluation techniques

Several experimental techniques were utilised for evaluation of the fundamental, mechanical and tribological properties of the coatings and the coating/substrate composites (Table 1). These techniques are all part of the “standardised, in-house” coating characterisation line used at Uppsala and have thus been described in detail elsewhere, as indicated in Table 1.

In addition to the more or less standardized tests in Table 1, an erosion test of the coated HSS substrates was performed [21]. Erosion testing has proven to be a suitable tool for studies of crack propagation and inter- action (erosive wear of hard coatings is often of a fatigue wear type [ 111). To focus on coating erosive wear, i.e. with as little influence from the underlying substrate as possible, the following experimental parameters were chosen: an angle of impingement of 40”, a particle velocity of 20 m/s and high purity (99.7%) angular silicon carbide particles (hardness 2700 HV,, gf, size distribution 50-90 pm) as erodant. The erosive wear rate was determined by measurement of the depth of the eroded area (surface profiiometry) at regular intervals until the coating had worn through.

3. Results

All coatings displayed total coating thicknesses close to 5.0 pm and the thicknesses of all individual layers were close to those aimed at (Table 2). No difference between coatings deposited on HSS and CC substrates could be discerned.

On all coating surfaces small craters and particles could be found (Fig. 1 (a)). The amount of craters was larger for coatings on CC than on HSS, and it was also noticed that the crater diameter decreased as the amount of NbN increased (Figs. 1 (a) and 1 (b)). The number of particles increased with the amount of NbN in the coating. This observation was supported by the coating surface roughness (R,) measurements, i.e. the R, value was found to increase with the amount of NbN (Fig. 2). The substrate material did not significantly influence the R, value.

A majority of the coatings displayed a very dense and

M. Larsson et al.JSurface and Coatings Technology 91 (1997) 43-49

Table 1 Methods used for evaluation of coating properties

45

Coating property Evaluation technique Ref.

Total coating and individual layer thickness

Morphology and microstructure Surface roughness Hardness Residual stress

Direct measurements on cross-sections using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) SEM Surface profilometry Microhardness measurements Beam deflection

111,121

[I31 [I41 1151 1161

Adhesion and cohesion Scratch testing Fracture resistance Scratch testing Resistance to abrasive wear Controlled microabrasion (“the dimple grinder test”)

t171 [181 119,201

Table 2 Total coating thickness (tr) and individual layer thickness (tTiN and tNbN) and the nominal relative amount of NbN in the coatings

Coating tf km) ttiN cm) tNbN (nm) Nominal relative amount of NbN

TiN NbN 500/500 lOO/lO 10/100 10/5

4.1 kO.2 4100+200 0 5.4kO.3 5400*300 1 5.2 +0.3 640 * 30 540 f 30 0.5 5.510.2 110+15 1515 0.09 6.410.3 712 95+10 0.91 4.6 +0.2 7k2 5*1 0.33

columnar microstructure (Fig. 3 (a)). The exception was the 10/5 coating, which had a more pronounced columnar microstructure, where some columns had even grown through the entire film (Fig. 3 (b)). No difference between the two substrate materials could be discerned.

For a given substrate material, the composite hardness (HV50 & was found to increase with the amount of NbN and/or the number of individual layers in the coating (Fig. 4(a)). In some cases, there was also a slight tendency for the CC substrate to yield a higher composite hardness.

In general, coatings on HSS displayed a higher residual stress than the corresponding coatings on CC. For HSS substrates, the coating residual stresses increased with the relative amount of NbN in the coatings and, in particular, the coatings with the highest amount of NbN displayed considerably higher coating residual stresses than any other coatings (Fig. 4(b)). In the case of the CC substrate, all the multilayers and the NbN coating were found to have a lower or much lower residual stress than the TiN coating (Fig. 4(b)).

The normalised critical normal load (i.e. the critical normal load, F&, divided by the substrate hardness) decreased slightly with an increasing amount of NbN for both HSS and CC substrates (Fig. 5).

FN,c corresponded to substrate exposure by adhesive failures for both substrate materials. In general, the size of the coating failures at F& increased with the amount of NbN. However, the area of exposed substrate was found to be somewhat larger for the coatings on CC than for the corresponding coatings on HSS. In addition,

some failures of TiN/NbN interfaces were found, particularly for coatings on HSS.

For both substrates, a maximum in the fracture resistance (normalised with respect to substrate hardness) was found at a relative amount of NbN of 0.33 (Fig. 6).

In the microabrasion test, the wear rates decreased with increasing amount of NbN in the coatings (Fig. 7 (a)). Only minor differences between coatings deposited on HSS and CC substrates were seen. Moreover, no spalling between substrate and coating or between coating internal layers could be found in the post-test examination of the abrasive wear scars (Fig. 7(b)).

The erosion rate was found to decrease with the relative amount of NbN in the coating (Fig. 8).

In the early stages of the erosive wear process, the majority of the single-particle impacts result in indent- ations (1-5 um in diameter) with extensive lip formation. These impacts were also found to introduce lateral cracks in the coating. Severe coating wear occurs as the lateral cracks propagate and interact. This was found to be the predominant wear mechanism, although some of the coating wear could be attributed to the formation of lips and their subsequent removal.

4. Discussion

The increase in surface roughness with the amount of NbN is a result of the d.c. sputtering process, which

46 M. Lawon et al,/Swface and Coatings Technology 91 (1997) 43-49

Fig. 1. Representative micrographs of the coating surface morphology. (a) lOO/lO (on HSS), (b) lO/lOO (on HSS).

50’,,,/,,, / ,,/I ,I/ / ,,,I

f 0 HSS ; ;

40 9 cc . . . . . . . . ..-..._.____.................. . . . . . . . . . . . . . . . . . . . . . . . . . .

Fig. 2. Coating surface roughness.

produces more particles (Z 1 pm) than the electron beam evaporation during deposition. These particles can be found not only on the coating surface but also within the coating. Therefore, it is believed that they may act

Fig. 3. Representative SEM fractographs of the (a) SOO/SOO/CC and (b) 10/5/HSS composites.

as stress concentrators and thereby lower the fracture resistance of the coating.

The hardness of the multilayered coating/substrate composites does not increase linearly from that of TiN to that of NbN (cf. Fig. 4(a)). This means that the hardness does not obey a simple “law of mixtures”, as is usually the case for composite materials. The increased hardnesses, particularly high in the case of the 10/S coating, must therefore be attributed to the multilayered structure. In general, the hardness of a PVD coating has been found to increase with (i) increasing compres- sive residual stress [16], (ii) decreasing grain size (i.e. a Hall-Petch relation) [7] or (iii) if the coating is a so-called “super lattice” coating [8-lo]. The causes for the high hardness of super-lattice coatings are not clear, but it is believed that the hardness increasing mechanism also in this case is a Hall-Petch related mechanism, i.e. a single TiN or NbN layer can be considered as a “subgrain” within a given columnar grain.

From complementary TEM and X-ray diffraction (XRD) studies (Figs. 9(a) and 9(b)) it is clear that the 10/5 coating is a superlattice. Because the 10/S coating

M. Larson et al./Swface and Coatings Technology 91 (1997) 43-49 47

4000

2000

4

1 i , ,i, ,, j >,,i I I 0 072 0,4 076 03 1

Relative amount NbN

g ‘:i)m 0 cc . . . . . . . . . . . . -........................; j A

k 2- E

L .._.. $ . ...,....,,.; t ~/ . . . . . . . . . . . . . . . . . . . . . . . . . . . . i $ &

s / 5 I

, i 0 !l<~I,I/,II~,!l/lll

1

0 032 024 036 03 1 b) Relative amount NbN

Fig, 4. (a) Composite hardness (load 50 gf) and (b) coating residual stress as a function of the relative NbN content in the coatings.

,// 1 III 1 #I / / I

0 HSS . . .._._.. - _... t . . . . . . . . . . . . . . . . . . . . . . . . . . . ,......................... i _.. n cc I-

8 / El w-

7 g

.Y .

.t: 6 Y q

z a- 2 .I -z E s 2

Relative amount NbN

Fig. 5. Normalised critical normal loads, i.e. the critical normal load divided by the hardness of the substrate material.

displays a relatively low residual stress (Fig. 4(b)), the high hardness can be attributed to (iii), i.e. the fact that the 10/5 coating is a “super lattice” coating.

At any given instant, the stress state in a coating deposited on, for example, a cutting tool is affected by the (tool) geometry, residual stress, load, contact tem-

Relative amount NbN

Fig. 6. Influence of the relative amount of NbN on the normalised coating fracture resistance, i.e. the normal load corresponding to first observation of a crack running parallel to a scratch divided by the hardness of the substrate material.

o~,,,i,,,:,,,I,,,i,,,~ 0 02 0,4 0,6 03 ;

a) Relative amount NbN

Fig. 7. (a) Abrasive wear rates of the tested coatings and (b) a typical example of an abrasive wear scar (500/5OO/CC).

perature, etc. Near the cutting edge, in particular, high coating residual stresses could result in a negative influence on the performance of cutting tools or even, in extreme cases, in coating delamination. The relatively

48 M. Lam-on et aL/Surface and Coatings Technology 91 (1997) 43-49

ofj + . . . . . ..__........_...._. .._........_............ i . . . . . . . . . . . . . . . . . . . .._..... ..__.._.......___.....

0 OS

0,4

0,3

092

071

of ~(~I~~/II/‘Ill 0 02 0,4 016 C

Relative amount NbN

Fig. 8. Erosion rate versus relative amount of NbN for the coatings on HSS. Note that the erosion rate of TiN is twice the erosion rate of the other coatings.

high residual stresses (and relatively coarse surface) of the NbN and lo/100 coatings on HSS thus imply that these coating/substrate combinations might be unsuit- able for use in applications where the demand of good adhesion is crucial for tool life.

The level of the critical normal loads of the other composites is not by any means “critical”, i.e. the coatings do have sufficient adhesion for most applications. Normally, a critical normal load of 40 N (normalised critical load ~0.045 N/kg/mm’) in practice is considered to be acceptable for PVD coatings on HSS substrates.

The maximum in the coating fracture resistance (Fig. 6) is clearly a result of the large amount of interfaces in the 10/5 coating (Fig. 10). This implies, in combination with the high hardness of the 1015 coating, that the multilayered structure acts as desired. That is, an increased load carrying capacity through both improved hardness and fracture resistance (as compared to single-layer coatings) has been obtained.

The abrasion test results can be understood in terms of the composite hardness (Fig. 11(a)). It is clear that wear decreases with hardness, which is in good agreement with earlier results [ 181.

The difference in the erosion rate of homogenous TiN and NbN coatings is a result of the higher composite hardness of the latter (Fig. 11 (b)). In the case of the multilayered coatings the multilayered structure is also believed to act beneficially to reduce the erosive wear rate. This is especially pronounced for the lOO/lO coating, although it is somewhat softer than the erodant used, the wear rate is much smaller than that of the almost as hard TiN coating (Fig. 11 (b)). The small impacts on the coating surface generate vertical as well as lateral cracks. It is believed that vertical cracks are deflected at the NbN interface. This, in turn, should mean a reduction of the size of the coating fragments

i

HSS T 0 1. L ,. I j .I_ I 30 35 40 45 50

Fig. 9. (a) Cross-sectional micrograph of the 10/5 coating on HSS. (b) XRD spectra from the IO/5 coating on HSS. Note that only one (ZOO) Bragg peak can be seen. The smaller peak is the negative (200) satellite peak. The superlattice period (A) can be calculated from the Bragg peak (0,) and satellite positions (rL- O), where the j, sign refers to the positive and negative satellites, using sin(t)+_) = sin(&) +A./2A (A is the X-ray wavelength). In this case A is approximately 11+2 nm, which is in good agreement with the period in the TEM micrograph.

lb0 2bo 300 400 5&l 660 760

Total number of interfaces

Fig. 10. Normalised fracture load versus the number of interfaces in the coatings.

M. Lamon et aLlSurface and Coatings Technology 91 (1997) 43-49 49

4

b)

2000 2200 2400 2600 2800 3000 3200 3400 3600

Hvwgf [kg/mm21

2600 22bo 2400 2600 2sbo 3600 3ioo 3400 3&o

Hv50gf [kg/mm21

Fig. 11. Wear rates versus hardness. (a) The abrasive wear rate of the coatings on both substrates and (b) the erosive wear rate of the coatings on HSS.

worn away, i.e. a decreased wear rate as compared to the single-layered TiN coating.

5. Conclusions

In this work, PVD multilayered TiN/NbN coatings on HSS and CC substrates are evaluated with respect to fundamental properties such as morphology, microstructure, hardness, adhesion, fracture resistance, abrasive and erosive wear resistance. The major conclusions are: (1) the deposition process works well for the deposition

of well adhering, multilayered TiN/NbN coatings; (2) a multilayered coating has the potential to be both

tougher as well as harder than a single-layered coating provided that the multilayer consists of very thin (z 5-10 nm) layers.

Acknomledgement

Dr Leif Westin of Erasteel Kloster AB and Mr Lennart Karlsson of SEC0 Tools AB are recognised for providing the substrate materials. The hnancial support from the National Swedish Board for Technical and Industrial Development (NUTEK) is gratefully acknowledged by the authors.

References

[l] K.S. Chan, M.Y. He and J.W. Hutchinsson, J. IMater. Sci Bzg. A, 167 (1993) 57.

[2] M.Y. He, F.E. Heridia, D.J. Wissuchek, M.C. Shaw and A.G. Evans, Acta Metall. Mater., 43 (1992) 1223.

[3] Y. Huang and H.W. Zhang, Acta Metall. Mater., 41 (1993) 1523. [4] Y. Sugimura, P.G. Lim, C.F. Shih and S. Suresh, Aeta Metal/.

Mater., 43 (1995) 1157. [5] M.Y. He and A.G. Evans, Int. J Solids Struct., 31 (1994) 3443. [6] J. Gumaars, Licentiate Thesis, Uppsala University, 1995. [7] R.F. Huang, L.S. Wen, L.P. Guo, J. Gong, B.H. Yu and

H. Bangert, Surf: Coat. Technol., 50 (1992) 97. [8] X. Chu, S.A. Bamett, M.S. Wong and W.D. Sproul, Sz& Coat.

Technol., 57 (1993) 13. [9] M. Shinn, L. Hultman and S.A. Bamett, J. Mater. Res., 7

(1992) 901. [lo] X. Chu, MS. Wong, W.D. Sproul, S.L. Rohde and S.A. Bamett,

% Vat. Sci. Technol. A, 10 (1992) 1604. [ll] M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson, S. Hogmark

and E. Bergmann, ,Sztrf Eng., IO (1994) 205. [12] G. Hakansson, L. Hultman, J.-E. Sundgren, J.E. Greene and

W.-D. Mtinz, Surj Coat. Technol., 48 (1991) 51. [ 131 D.S. Rickerby and S.J. Bull, Surf Coat. TechnoL, 39/40 (1989)

315. [14] J.M. Bennet and L. Mattsson, Optical Society of America,

Washington, DC, 1989, p. 18. [15] M. Bromark, M. Larsson, P. Hedenqvist, M. Olsson and

S. Hogmark, Surx Coat. Technol., 52 (1992) 195. [16] M. Larsson, P. Hedenqvist and S. Hogmark, Surf Eng. (accepted

for publication). [17] P. Hedenqvist, M. Olsson, S. Jacobson and S. Siiderberg, Surf

Coat. Technol., 41 (1990) 31. [18] M. Larsson, M. Bromark, P. Hedenqvist and S. Hogmark, Surj

coat. Technol. (in press). [I91 A. Kassman, S. Jacobson, L. Erickson, P. Hedenqvist and

M. Olsson, Surf: Coat. Technol., 50 (1991) 75. [20] R. Gahlin, M. Larsson, P. Hedenqvist, S. Jacobson and

S. Hogmark, Surf: Coat. Technol. (submitted) [21] S. Soderberg, S. Hogmark, U. Engman and H. Swahn, Tribal.

Int., I4 (1981) 333.

mechanical and tribological properties of multilayered pvd tin/nbn coatings

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