ELSEVIER Surface and Coatings Technolo gy 86-87 (1996) 351- 356

SURFACE&COARNBS

iEGHNDJDGY

Deposition and microstructure of PVD TiN-NbN multilayered coatingsby combined reactive electron beam evaporation and DC sputtering

Mats Larsson a,*, Patrik Hollman a, Per Hedenqvist a, Sture Hogmark a,Ulf Wahlstrom b, Lars Hultman C

a MaterialsScienceDivision, Department of Technology, Uppsala University, Box 534 S-751 21 Uppsala, Swedenb fMC, LinkopingUniversity, S-581 83 Linkoping, Sweden

C Thin Film Physics Division, Department of Physics, Linkoping University, S-581 83 Linktiping, Sweden

Abstract

Well-adherent TiN/NbN mult ilayer coatings were deposited on high speed steel and cemented carbide substrates in a highvacuum coating equipment using a reactive hybrid deposition process consisting of a combination of electron beam evaporation(Ti) and D.C. magnetron sputtering (Nb). Homogeneous TiN and NbN layers as well as four different multilayer TiN/NbNcoatings with periodicity 500/500 11m (TiN/NbN), 100/10, 10/100 and 10/5 nm were deposited. Analytical techniques includingX-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy and Auger electron spectroscopy wereused to characterise the phase composition, microstructure and the chemical composition of the coatings. In addition themicrohardness was determined. The phase of the homogeneous TiN and NbN coatings as well as for individual TiN and NbNlayers in the multilayer coatings was predominantly the cubic NaCl-structure. In addition the 10/5 coating was found to have asuperlattice structure. All coatings showed a dense, columnar microstructure and were slightly overstoichiometric. The highesthardness (::d400 HV) was found for the 10/5, 10/100 and the single layered NbN coating.

Keywords: Deposition; Microstructure; Multilayer; Hybrid process

1. Introduction

The tribological performance of thin hard coatings is,for a given substrate material, mainly governed by thecoating hardness, coating fracture resistance, the contacttemperature and chemistry (in the prevailing tribologicalsystem) [1]. For a given application, future improvementof tribological performance can therefore, e.g., be accom­plished by increasing the coating fracture resistancewhile retaining the hardness of today's coatings, orvice versa.

Multilayer coatings have often been found to possesssuperior mechanical and tribological properties as com­pared to single-layered coatings. High coating hardnesshas been observed for Ti/TiN multilayer [2J andTiN/NbN superlattice coatings [3 J.TijTiN multilayeredcoatings have also been found to possess high fractureresistance [4 J. Moreover, by the introduction of multi­layer coatings the wear resistance of coated components

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can be further increased. For example, WC/C multilayercoatings have been found to increase the wear resistanceof ceramics in sliding contacts [5J and TiC/TiB2 multi­layers the wear resistance of cemented carbide inserts[6]. Improved corrosion resistance and decreased ero­sive and abrasive wear rates due to multilayer coatingsare also reported [7-9].

The object of the present investigation is to deposithard and well-adhering TiN and NbN multilayer coat­ings on two different tool materials, high speed steel(HSS) and cemented carbide (CC).The TiN/NbN systemwas chosen because it has been found to possess promis­ing mechanical properties (e.g., a very high hardness)and provides a very good thermal expansion match tosteel as well as to CC. In addition, the TiN/NbN systemis a thoroughly investigated system which successfullyhas been deposited using balanced [lOJ as well asunbalanced reactive magnetron sputtering [3].

In the present investigation a reactive hybrid process,which combines reactive electron beam evaporation (Ti)and D.C. magnetron sputtering (Nb), is used for depos­ition of the TiN and NbN multilayered coatings. Plasma

352 M. Larsson et al.fSurface and Coatings Technology 86-87 ( 1996) 351-356

enhancement, obtained by a plasma beam, ensures oper­ation at conditions where a high amount of the metalconstituents .are ionised [11 J. This, in turn, improvesthe overall adatom mobility and thereby enables depos­ition at temperatures low enough to avoid softening ofthe HSS « 550°C). Multilayer coatings with differentcompositional modulation periods and thickness ratiosof the TiN and NbN layers are investigated with respectto their phase cernposition, microstructure, chemicalcomposition, surface morphology and microhardness.

2. Experimental

2.1. Substrates

Two types of substrate materials were used in thepresent experiments, one powder metallurgical (PM)HSS (20 x 40 x 2 mm), ASP2030 (Erasteel Kloster ABdesignation) and one CC (WC-6%Co, 12 x 12 x 3 mm).The chemical composition of the HSS is (wt.%): 1.3 C,4.2 Cr, 5.0 Mo, 6.4 W, 3.1 V and 8.1 Co. The HSS washeat-treated by austenitization at 1180°C followed bytempering 3 x for 1 h at 560°C. All substrates wereground and subsequently polished with 1 urn diamondsin the last step.

The substrates were cleaned with trichlorethylene,acetone and alcohol in an ultrasonic cleaner, whereuponthe substrates were blown dry with nitrogen. They werethen inserted into the vacuum chamber which wassubsequently evacuated to a base pressure of approxi­mately 10-4 Pa. Prior to deposition the substrates wereresistively heated by an electron beam to approximately450°C for 60 min to de-gas the surfaces. The substrateswere then sputter-etched in Ar at a pressure of 0.15 Paand a substrate bias of -200 V.

2.1. Deposition

All coatings were deposited in a BALZERS BAI 640 Rhigh vacuum coating equipment. The TiN was depositedusing reactive electron beam evaporation, whereas theNbN film was deposited using reactive magnetron sput­tering, In addition, a plasma beam for auxiliary ionisa­tion was utilised. A more detailed description of thecoating equipment can be found in Ref. [l1J.

For sputter deposition of stoichiometric NbN at highrates, the hysteresis curves of total pressure (Ar+ N2)

versus N2 flow were recorded. Rutherford backscatteringspectroscopy (RBS) of several preliminary coatings indi­cated that a N2 flow of 35 seem produced stoichiometricNbN coatings. The target power was kept constant at5 kW, while the N 2 was fed into the system through aseparate, f gas manifold located in the vicinity of thetarget. ·The AI' pressure was 0.30 Pa throughout thetest series.

The electron-beam evaporation of Ti is pressure con­trolled, i.e., the electron beam emission current is regu­lated to maintain a constant Nz partial pressure, whichis approximately 0.05 Pa when depositing stoichiometricTiN [9,11J. Since the N2 partial pressure is approxi­mately 0.01 Pa when depositing stoichiometric NbN[9J, the NbN layer will become somewhat metal defi­cient if the NbN process is just combined with thestandard TiN process (due to the higher N 2 partialpressure of the TiN process). However, it was decidedto accept a somewhat nitrogen rich NbN coating, dueto the difficulty in regulating the emission current of theelectron beam at very low N2 partial pressures. Thus,the N2 flow at the Ti source was held constant at 140seem while the nitrogen flow at the DC magnetron was15 seem throughout the deposition process [9].

For all coatings investigated, the deposition was initi­ated with a 10-30 nm thick Ti layer followed by a50-250 om thick TiN layer. The emission current of theelectron beam was regulated while the power of themagnetron was held constant at 5 kW throughout thedeposition. In addition, the substrate bias was -110 V,and the substrate temperature, as measured using athermocouple, increased from 400 to 450°C duringdeposition.

Thin TiN and NbN layers (5-10 nm) as well ashomogeneous TiN were deposited using continuoussubstrate rotation, whereas thicker TiN and NbN layers(100-500 nm) and homogeneous NbN were depositedby keeping the substrates alternately stationary abovethe Ti and Nb source, respectively. After deposition thecoated substrates were cooled in pure helium for 20 min.

Four combinations of TiN/NbN coatings, with nomi­nal thickness (in nm): 500/500, 100/10, 10/100 and 10/5,were deposited on both HSS and Cc. In addition,homogeneous TiN and NbN coatings were depositedon both substrate materials as references. The totalcoating thickness was in the range of 4-6 urn.

2.3. Coating characterisation

The coatings were structurally characterised by X-raydiffraction (XRD), scanning electron microscopy (SEM)and transmission electron microscopy (TEM). For theXRD analyses, a Philips D 5000 diffractometer withCu Kcc radiation was used. Coating microstructure wasstudied on fractured cross-sections in SEM as well ason plan-view and cross-sections in TEM. Sample prepa­ration for plan-view TEM consisted of grinding to athickness of 50 urn followed by dual gun Ar+-ion millingto obtain electron transparent specimens. The ion etch­ing was performed from the substrate side with anincident angle of 10°, the ion energy was 9 keY and thecurrent was 4 mA. The final etching was performed at3 kV and 1.5rnA in order to reduce ion induced artefactsand remove any amorphous redeposited surface layer.

M. Larsson et al.ISurface and Coatings Technology 86 -87 ( 19961 351-356 353

Specimens for cross-sectional TEM (XTEM) were pre­pared by gluing two samples film to film in a Ti grid.After grinding to a thickness of 50 11m, ion etching wasperformed at an angle of 7S (ion energy 9 kV and ioncurrent 4 rrtA). To obtain a sample with a large electron­transparent area, the sample was rocked back and forthwith the incoming ion beam, incident perpendicular tothe film/substrate interface. Also in this case, final etchingwas performed at 3 kV and 1.5 rnA. The TEM observa­tions were carried out using a Philips EM 400T electronmicroscope equipped with a LaB6 filament and operatedat 120 kV.

The chemical composition of homogeneous TiN andNbN coatings were investigated by RBS utilising2.4 MeV 4He+ ions at a scattering angle of 1680 [12].The same TiN and NbN coatings were then investigatedusing Auger electron spectroscopy (AES, JAMP 10) withan electron beam energy of 5 keY and an emissioncurrent of 5 1lA. Comparison of the RBS and the AESresults allowed calibration of the sensitivity factors ofthe AES instrument, which was used for investigation of

Fig. 1. XRD patterns of the TiN/NbN multilayered and the single­layered TiN and NbN coatings on the HSS. Note the different magnifi­cation factors of the diffraction patterns.

Fig. 2. Low-angle XRD diffraction pattern fr0111 the 10/5 coating.

2 3 428

5 6

Fig. 3. Representative SEM fractographs of different coatings' on HSS.(a) TiN, (b) NbN and (c) 10/5.

the chemical composition of the multilayered coatings.To analyse individual layers in the multilayeredcoatings, a tapered section through the coating, wasmade using the dimple grinder technique. However .thiscould only be applied on individual layer thicknesses.larger than or equal to 100 nm.

The coating hardness was obtained using a conven-

354 M. Larsson et a!'/Surface and Coatings Technology 86-87 ( 1996) 351-356

Fig. 4. Plan-view TEM micrographs from the top part of the (a) TiN on HSS. (b) NbN on HSS.

tional Vickers' microindenter and a load of 25 gf. Thisload is sufficiently low to ensure that the microhardnessvalue measured, essentially corresponds to that of thecoating material [13]. In order to ensure accuratedetermination of the indentation diagonals, the coatingsurface was polished with 1 urn diamonds.

3. Results

3.1. Phase analysis

From the XRD analysis it was found that all coatingsinvestigated had a predominantly cubic NaCl structure[14,15]. The 500/500, the NbN and possibly also the100/10 coatings displayed some hexagonal NbN [16J(Fig. 1). In addition the homogeneous TiN was foundto have a (111) preferred orientation while all othercoatings displayed a (200) preferred orientation (Fig. 1).No differences between the two substrates were observed.

Fig. 2 is a low-angle diffraction pattern from the 10/5coating where five superlattice reflections can be seen.From the diffractogram a superlattice wavelength of12 nm was calculated.

3.2. Microstructure analysis

All coatings exhibited a columnar microstructure, inwhich the size of the columns decreased with increasingfraction of NbN in the coatings (Figs. 3(a) and (b)). The10/5 coating, however, exhibited the most pronouncedand largest columns (Fig.3(c)). Similar results wereobtained for the CC substrates.

Plan-view TEM micrographs from the top region ofthe coatings revealed dense structures with high defectdensities, see Figs.4(a) and (b). The grain sizes were inthe range of 50-55 and 20-40 nm for TiN and NbN,respectively. Selected area electron diffraction (SAED)pattern insets in Fig. 4 show single-phase coatings withNaCI structure.

Figs.5(a) and (b) are cross-sectional TEM micro­graphs from 10/5 superlattice coatings showing (a) thecentral region of a coating with corresponding SAEDpatterns and (b) a coating/HSS substrate interfaceregion. Fig. 5(a) shows a columnar microstructure seenalso in the SEM fractographs (cf. Fig. 3) and confirmsdense grain boundaries also in the multilayered coatings(cf. TiN and NbN layers in Fig. 4). The SAED patternsinset in Fig. 5(a) show that the coating had single-phase

M. Larsson et al.jSurface and Coatings Technology 86-87 (1996) 351-356 355

Fig. 5. Cross-sectional TEM micrographs from 10/5 superlattice coat­ings showing (a) the central region of a coating with correspondingSAED diffraction patterns and a higher-magnification inset of TiN andNbN layers, and (b) a coating/substrate interface region with a Tiadhesion layer and a relatively thick TiN layer indicated. Columnboundaries in (a) are indicated by arrowheads.

100 10 500 10

3600 TO 5" 500 100

[ 3200,

P t +'" +'" 28001:l] <P

::r1 2400 P•~Hsi• o CC

2000TiN 0,2 0,4 0,6 0,8 NbN

Relative amount NbN

Fig. 6. Coating hardness (HV) versus the relative amount NbN.

NaCI structure with a (200) preferred orientation inagreement with the XRD analysis (cf. Fig. 1). TiN andNbN layers are resolved in Fig.5(a) with the darkercontrast corresponding to the highest atomic numbermaterial NbN. The layer undulations seen in Fig. 5(a)

corresponded to the evolution of faceted growth surfacespresent in all coatings. The higher-magnification insetshows sharp layer interfaces. Compositional modulationwas measured to be 12 nm in agreement with the XRDresults above (cf. Fig. 2). The NbN layer thickness toperiodicity ratio was ~ 0.37. For the layer/substrateinterface region, Fig. 5(b) shows the presence of the Tiadhesion and the initial TiN layer.

3.3. Chemical composition

The RBS as well as the AES analysis showed thatboth homogeneous coatings contained small amountsof oxygen. No traces of C or Ar could, however, befound. The chemical composition of the NbN coatingwas approximately 44 at.% Nb, 54 at.% Nand 2 at.%O. The TiN coating was found to contain 48 at.% Ti,50 at.% Nand 2 at.% O. The AES analysis showed thatthe chemical composition of individual layers (2100 urn)in the multilayer coatings were the same as in the single­layered TiN and NbN coatings. No significant differencebetween coatings on the two substrate materials couldbe found.

3.4. Hardness

For all coatings, except the 10/5 multilayer, the hard­ness was found to increase with the relative amount ofNbN, see Fig. 6. The 10/5, with approximately 33%NbN and the largest number of layers in the coating,exhibited the highest hardness (~3400 HV).

4. Discussion

The present results show that the reactive hybridprocess consisting of electron beam evaporation of TiNand magnetron sputtering of NbN can be used fordeposition of cubic-phase TiN/NbN multilayered coat­ings with a dense and columnar microstructure. Inaddition, superlattice coatings with layer periodicity assmall as 12 nm can be deposited. In the following, thepresence of hexagonal NbN and its influence on thegrain size of the coatings will be discussed. Finally, thehardness of the superlattice coating will be discussedbriefly.

The Nb-N phase diagram contains both a stablehexagonal and a metastable cubic phase of NbN [17J.This, in turn, means that the deposition process of cubicNbN is more difficult to control than for stable cubicTiN [18]. The NbN phase obtained depends not onlyon the nitrogen partial pressure, deposition rate andsubstrate bias, but also on the thickness of the NbNlayer [19]. To promote formation of cubic phase layers,it was found that the thickness of the NbN layers shouldbe kept low. Nucleation of metastable cubic-phase NbN

356 M. Larsson et al.LSurface and Coatings Technology 86-87 ( 1996) 351-356

on TiN is believed to be controlled by pseudomorphicforces between the two compounds of mutual solubilityand a relatively small lattice mismatch (3.6%). Increasingthe NbN layer thickness, however, increases the relativeamount of stable hexagonal NbN.

The presence of a competition between nucleationand growth of hexagonal NbN and cubic NbN on TiNlayers and in the NbN layers will, in effect, lead torenucIeation during growth, as can be inferred from theTEM (and SEM) results. Smaller columnar grains wereobserved in the coatings with thick NbN layers (e.g., the500/500 and the homogeneous NbN) compared to thosewith thin layers of NbN (e.g., the 10/5 and the 100/10).

In the case of the superlattice coatings, i.e., the 10/5coating, the relatively high hardness is, however, not ashigh as the maximum hardness values reported forTiN/NbN superlattice coatings with similar phase andperiodicity [10]. This is probably a consequence of thefact that the deposition process not is optimised, withrespect to the substrate bias as well as the partialpressure of nitrogen. Especially the effect of ion bom­bardment through substrate bias on grain boundarystrength is believed to be important in order to obtaina high hardness [19].

5. Conclusion

The present work shows that the reactive hybridprocess consisting of electron beam evaporation of TiNand magnetron sputtering of NbN works well for depos­ition of cubic (NaCI-structure) multilayered TiN/NbNcoatings with a dense columnar microstructure.Deposition of superIattice coatings with a layer periodic­ity as small as 12 nm was demonstrated.

Acknowledgement

Dr LeifWestin of Erasteel Kloster AB and Mr LennartKarlsson of SECO Tools AB are recognised for provid-

ing the substrate materials. In addition, Dr NilsLundberg, Royal Institute of Technology, Sweden, isacknowledged for his assistance with the RBS analysis.The financial support from the National Swedish Boardfor Technical and Industrial Development (NUTEK) isgratefully acknowledged by the authors.

References

[lJ P. Hedenqvist, Ph.D. thesis No. 360, Uppsala University,Sweden, 1991.

[2J R.F. Huang, 1.S. Wen, L.P. Guo, J. Gong and B.H. Yu, Surf.Coat. Tee/mol., 50 (1992) 97.

[3J X. Chu, SA Barnett, M.S. Wong and W.D. Sproul, Surf. Coat.Technol.; 57 (1993) 13.

[4J M. Larsson, M. Bromark, P. Hedenqvist S. and Hogmark, SIIIf.Coat. Technol., 76/77 (1995) 202.

[5J A. Blomberg, 1. Erickson, J. Lu, S. Hogmark and M. Olsson,Surf. Coat. Technol., 57 (1993) 51.

[6J H. Holleck, M. Lahres and P. Woll, SlIIf. Coat. Technol., 41(1990) 179.

[7J R. HUbler, A. Schroer, W. Ensinger, G.K. Wolf, W.H. Schreinerand I.J.R. Baumvol, Smf. Coat. Techno!., 60 (1993) 561.

[8J E. Quesnel, Y. Paule au, P. Monge-Cadet and M. Brun, SIIlf. Coat.Technol.; 62 (1993) 474.

[9J M. Larsson, M. Bromark, P. Hedenqvist and S. Hogmark, Sub­mitted to Surf. Coat. Tee/mol.

[10J M. Shinn, 1. Hultman and S.A. Barnett, J. Mater. Res., 7(4)(1992) 901.

[llJ E. Bergmann, Surf. Coat. Technol.; 57 (1993) 133.[12] M.J. Vasile, A.B. Emerson and F.A. Baiocchi, J. Vac. Sci. Technol.,

A 8 (1990) 99.[13J A.L. Ilinski and G.B. Lyack, Ind. Lab. (USSR), 44 (1978) 1719.[14] Powder Diffraction File no. 38-1420 (JCPDS-ICDD, 1993).[15J Powder DiITraction File no. 38-1155 (JCPDS-ICDD, 1993).[16] Powder Diffraction File no. 20-801 (JCPDS-ICDD, 1993).[17J P. Ettmayer and W. Lengauer, Encyclopaedia ofInorganic chemis­

try, John Wiley, West Sussex, 1994, p. 2498.[18J M.S. Wong, W.D. Sproul X. Chu and S.A. Barnett, J. Vac. Sci.

Technol.; A 11 (1993) 1528.[19J H. Ljungcrantz, Ph.D. thesis No. 408, Linkoping University,

Sweden, 1995.