- Email: [email protected]
Co-sputtered films within the quasi-binary system TiN-TiB2
Surface
and Coatings
Technology
93-95
(1997)
297-302
Co-sputtered films within the quasi-binary system TIN-TiB2 P. Losbichlera,*, C. Mitterera, P.N. Gibsonb, W. Gisslerb, F. Hofer’, P. Warbichler’ bInstitute
for
“In rtitut fiir Metnllkunde md I~erkstoff~Jri~~lig, Il~onraintnii’ersitiir, Frcirl~-Josef-StrrlPe 18, A-8700 Leoben, Ausirin Adrmced Mmerials, Joint Resenrch Centre of the Cowmission of the European Cormmiries, C.P. I, I-21020 Ispro ‘ForscillctlRsinstitIlr fiir Elektronemikroskopie, Technische lJnii)ersira’r, Sreyrergnsse I7, A-8010 Groz, Ausrricr
(C’ri). Iicily
Abstract Ti-B-N films with a gradient in the chemical composition were deposited onto austenitic stainless steel and molybdenum sheets by means of unbalanced dc magnetron co-sputtering using a separxted TiN/TiB2 target. Different energetic contributions necessary for film growth were investigated adjusting suitable deposition parameters. Film microstructure was characterized by means of scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), as well as glancing angle X-ray diffraction (GAXRD) and clcctron probe microanalysis (EPMA). Coating hardness was measured using a depth sensing nanoindenter. The microstructure of the films appears featureless using SEM fracture cross sections. The grain size determined from TEM investigations-ranged between 3 and 5 nm. The composition of the films was found to lie on the quasi-binary section TiN-TiB2 within the ternary system Ti-B-N. Coatings consisted of nanocrystalline fee and hcp Ti-B-N phases. Microstructure as well as chemical composition of the films do not seem to be influenced by -- --.Influenced by varying the ion bombardvarying the deposition parameters used in this investigation. Contrarily, the hardness was strongly ment. Hardness values exceeding 50 GPa were obtained for Ti-B-N films. 0 1997 Elsevier ScienceS.A. &~~orcls:
Unbalanced
magnetron sputtering; Ultrahard
coatings; Ti-B-N
1. Introduction The development of hard coatings used for improved performance of tools is subject of numerous investigations. Growth techniques different from the state of Lhermodynamic equilibrium like physical vapour deposition are widely used to elaborate sophisticated films at low temperatures [I]. They provide the control of deposition parameters which can significantly influence the mobility of the adatoms impinging on the substrate detemlining growth mechanisms and hence the film microstructure. The nitrides, carbides and borides of titanium based alloys are mainly used for hard, protective coatings [2,3]. Besides composition-modulated layered heterostructures, coatings with nanocrystalline structures have attracted special attention for applications where ultra-hard materials are needed. Different theoretical explanations have been suggested to explain the observed strength enhancement in these nanostructures. For example, Veprek and Reiprich pointed out the importance of the structural arrangement * Corresponding author. e-mail: losbiBunileoben.ac.at
0257.8972/97/$17.00 PI2 s0257-8972(97)00440-4
Tel.: +43 3842 402449;
0 1997 Eisevier
Science
fax: +43 3842 402737;
S.A. All rights
reserved
coatings; Nanocrystalline
coatings
of individual phases to hinder dislocation movement [4]. Multiphase coatings within the ternary system Ti-B-N are anticipated by several authors as suitable candidates for tribological applications [5-121. Within this investigation, multiphase coatings have been prepared by co-sputtering from a sectioned TiN/TiB2 target using an unbalanced dc magnetron sputtering system. In the ternary system Ti-B-N the existence of a quasi-binary section of the eutectic type TIN + TiBz is claimed [ 131. The coatings deposited show a gradient in the composition depending on their position over the sectioned target. The aim of this paper is to study the formation of coatings with different chemical compositions in the quasi-binary system TiN-TiB2 as a function of different activation energies supplied for film growth.
2. Experimental
procedure
The unbalanced dc magnetron sputter system used in these experiments is shown in Fig. 1. It has been described in detail elsewhere [14], and only essential features will be presented here. A turbomolecular pumping unit was used to
298
P. Lmbichkr
et al. / Su@kce and Coatings Technology 94-95
pump the deposition chamber down to 10” Pa. TiN and TiB2 targets with diameters of 150 mm were cut into halves and one of each half was bonded onto a water-cooled backing plate. The target was positioned at a 6-cm distance from the parallel-plate substrate assembly. The horizontal distance of the substrate to the centre of the sectioned target is given by the parameter x; positive x values indicate the part close to the TiN target, negative x values the part close to the TiB2 target, 0 gives the centre of the target (compare to Fig. 1). The discharge current was maintained at I A. An external pair of I-Ielmholtz coils was used to create a uniform axial magnetic field B,,, which was varied between 0 and 80 G in the region between the target and the substrates to intensify the field from the outer magnetron pole. Plasma characteristics were determined from Langmuir probe measurements for single TiN and single TiB2 targets using a flat probe and following the procedures described in [ 1.51and [ 161. The flux ratio Jion/Jatomrwhere Jionand J,{,, are the fluxes of the ions and condensing atoms, respectively, was calculated for both targets assuming a constant film density equivalent to the bulk values. The flux ratio J& J atomis know~n as a key parameter for the formation of thin films [ 17,181. The evaluation of the atom flux for films with gradient chemical composition is not possible with suitable spatial resolution, so the measurements for TiN and TiB1, were taken as an approximation. Growth rates of about 40-55 nm min-’ for TiN and 2545 nm min-’ for TiB2 were achieved at sputtering power densities of about 2 W cm-‘. The substrates used were molybdenum sheets and metallographically polished austenitic stainless-steel discs (X 5 CrNi 18 10) precleaned with ethylene and acetone in an ultrasonic cleaner. Substrate
11997) 297-302
temperatures between 100°C and 400°C were provided by a resistance heater. Before film deposition the target was sputter-cleaned for approximately 10 min behind a shutter, before the substrates were sputter etched for 30 min at an argon pressure of 3 Pa using a dc voltage of -1500 V. The thickness of the coatings reached 2-4 pm. The morphology of the films was studied using a Cambridge Instruments Stereoscan 360 scanning electron microscope (SEM). Transmission electron microscopy (TEM) observations of cross-section foils were carried out using a Philips CM20/STEM equipped with a Gatan Imaging Filter (GIF) [19,20]. The images were recorded with a slow scan CCD camera included in the GIF with a YAG scintillator crystal and a 1024 x 1024 pixel array. The microscope was operated at a voltage of 200 kV with a LaB6 cathode. The chemical composition of the coatings was determined by wavelength dispersive electron probe microanalysis (EPMA). Quantification occurred by means of TiN and TiBp coating standards calibrated using nuclear reaction analysis (NRA). Some problems can occur in the analysis of the N content due to the overlap of the Tit, and N Ka emission line [21]. In addition, the results were crosschecked using X-ray photoelectron spectroscopy (XI’S). The X-ray diffraction (XBD) spectra were recorded with a Siemens D500 goniometer in the Bragg-Brentano mode using Cu KCYradiation. Glancing angle X-ray diffraction (GAXRD) spectra were obtained using a unmonochromated copper source at an incident angle of 1” [22]. To control the angular resolution, a high-precision variable slit was placed in front of the sample and the detector. A solid-state detector was used to isolate the Cu Ka! doublet. Coating hardness was determined from the loading and unloading curves
TO Turbomolecular Pump
SUPPlY 5kW
Fig. 1. Schematic diagram of the unbalanced dc magnetron sputter deposition unit used.
P. Losbichler
et al. 1 Su&cr
and Coatings Technology
employing an ultra-low load depth-sensing nnnoindenter (Nanoindenter II, Nano Instruments) [23]. The indenter was operated in the constant-displacement-rate mode until a depth of 200 run was reached. The indenter was kept at maximum load for 180 s, thus allowing relaxation of the induced plastic flow. Finally the unloading curve was measured by decreasing the force at constant rate, equal to the maximum loading rate during the loading curve. The measurementswere calibrated using a Si (111) wafer assuminga modulus of 157 GPa, independent of penetration depth. Under these conditions a penetration depth independent hardness of 12 GPa was obtained for the Si wafer.
94-95
(1997) 297-302
299
(a)
w
(Cl
3. Results and discussion To characterize the plasma conditions, probe measurements have been performed for non-reactive sputtering from the TiN and the TiB2 target. Detailed results have been presented in a previous paper [14]. Fig. 2 shows the ion/atom flux ratios Ji,“/J,,, incident at the substrate (position 1x1< 30 mm) as a function of B,,, for both targets demonstrating an increase of the flux ratio from 0.1 without B,,, to 0.7 at B,,, of 80 G. In this calculation, it was assumed that Jia” is composed of only single charged ions and secondary electron emission has been neglected. Applying a higher magnetic field leads to a focusing plasma and hence to a non-uniformity of the ion bombardment conditions in the vicinity of the substrate,hindering a comparison of the influence of different energies supplied for film growth. The floating potential V, decreasedwith increasing B,,, from -2 to -23 V, whereas the plasma potential VP decreasedfrom -t26 to --13 V, both with respect to the grounded anode. The energy of the ions bombarding the substrate is given by the difference of the applied bias voltage V,, and VP such that Ei,, E (e(jVb - Vpi). Within this 3
I
1
I
I ~*
0.05
I 0
I 50
/ 100
I 150
Bext PI
Fig. 2. Ion/atom flux ratio J,,,IJ,,,, incident at the substrate and growing film during sputtering of TiN and TiB2 in a 0.8 argon discharge as a function of the external magnetic field B,,, (discharge current, 1 .4; bias voltage, -80 V).
---.l Cd)
PO
30
40
diffraction
50
60
70
80
angle 28 [“I
Fig. 3. X-ray (XFD) and glancing angle X-ray diffraction (GAXRD) spectra of films deposited onto austenitic stainless steel substrates. (a) Target: TIN (XFCD). (b) Target: separated TiNiTiB2, position s = +30 mm (GAXRD). (c) Target: separated TiNITiB2, position x = +lO mm (GAXFCD). (d) Target: separated TiNiTiBz, position x = -10 mm (GAXRD). (e) Target: separated TiNffiB2, position x = -30 mm (GAXRD). (f) Target: TiBz (XRD).
work, coatings were depositedusing substrate temperatures between 100and 400°C the processgaspressurewas varied between 0.4 and 1.2 Pa, different ion energieswere obtained using bias voltages from self-bias to -100 V, and the external magnetic field was varied between 0 and 80 G in order to adjust different Jion/Jatom ratios. It has been shown earlier [ 141that the microstructure and the properties of non-reactively sputtered TiN and TiB2 films are strongly affected by the ion energy and the lion/ Jatomratio. Therefore, the main emphasis of this work was laid on the influence of these parameters keeping the substrate temperature in a 0.8 Pa argon discharge constant at 300°C. XRD results of TiN and TiBz films obtained using the Bragg-Brentano mode, are shown in Fig. 3, as well as glancing angle XRD results on Ti-B-N layers. The position lines from JCPDS data [24] for TIN and TiB2 are also indicated. These layers were depositedat Eion= 87 eV and Jlonl = 0.7. While the TiN and the TiB2 layers exhibited a J ntOm single phase fee NaCl structure with a mixed crystallo-
P. Losbichirr
300
et al. /Sq+‘kce and Coatings Technology
Table 1 Chemical composition of TiN, Ti-B-N (as a function of the position x) and TiBl coatings deposited onto stainless steel substrates (E,,,, = -87 eV: J,,J J .xom= 4.7) Position .r (mm)
N/Ti
B/Ti
TiB2 -30 -10 +lO +30 TiN
0 0.5 0.6 0.7 0.8 I
2.2 1.2 0.9 0.6 0.1 0
graphic orientation and an hcp structure with a strong (001) texture, respectively, the GAXRD spectra of the Ti-B-N coatings showed very broad peaks. In addition, an overlap of peaks from different phases seems to be observed. The GAXRD spectra for films deposited close to the TiN part of the target (Fig. 3b and c> indicate an fee structure of the NaCl type exhibiting a mixed orientation. The peak positions differ slightly from the peak lines of TiN. Chupov et al. [13] claimed only a small solid solubility of boron in TiN0.96, whereas an increasing solubility in TIN,-, occurred with increasing x and temperature, An fee Ti-B-N phase has been reported by other authors [5-7,l 1,12,25]. From these considerations, one may conclude that in TiN-rich films an fee Ti-B-N phase is formed due to the incorporation of boron in the TiN lattice although the formation of a multiphase structure based on TIN and TiBa cannot completely be excluded. At the TiB2-rich part (Fig. 36 and e), a shift of the peak positions towards the line position of TiB2, and a further broadening of the peaks as well as a continuous decrease of the relative intensities are obtained. This may be due to a coexistence of a hexagonal nitrogen containing phase based on TiBz and an fee boron containing phase based on TiN both exhibiting very low average grain sizes. Additionally, the formation of an amorphous phase along the grain boundaries seems also to be possible. The XRD spectra of single phase TiN and TiB2 films exhibit high intensities (compare Fig. 3a and f) and hence these films are of high crystallinity. The spectra of Ti-B-N films basically showed low intensities accompanied by high full width at half maximum values indicating a less crystalline structure. This decrease in crystallinity can be explained by the formation of small grains in the film together with a relative large amount of disordered grain boundaries. Since the deposition of Ti-B-N coatigs occurred by means of co-sputtering from the components of the yuasibinary system TiN-TiB2, the chemical composition of the films was expected to lie near the tie line of TIN and TiB2 in the ternary system Ti-B-N. Non-reactively sputtered TiN films showed a stoichiometric composition and were not affected by changes of the deposition parameters whereas overstoichiometric films were obtained by sputtering from the TiBz target. The B/Ti ratios in this case varied from 2.12
91-95
(1997) 297-302
and 2.38. This overstoichiometry is in good agreement with other sputtered boride films [26,27]. The B/Ti ratios and the N/Ti ratios of the Ti-B-N coatings for various positions x are given in Table 1. We observed no significant influence of the deposition parameters varied in this investigation. SEM fracture cross sections of TiN and Ti-B-N coatings deposited at high ion energy and high Jion/Jar,,,,,ratio onto molybdenum sheets are shown in Fig. 4. TiN films exhibit fibrous grains with dense boundaries (Fig. 4a) which are attributable to the transition zone T of the structure zone model of Thornton [28] whereas a fracture amorphous stmcture is observed for Ti-B-N coatings (Fig. 4b). A similar featureless structure was obtained with TiBz films [14] (not shown here). Fig. 5a shows a typical TEM brightfield image of the cross-section of a Ti-B-N film deposited onto a steel substrate at the position x = 5 mm. At the film/ substrate interface only a small transition phase is formed. Adjacent to the substrate surface, the film shows a dense microstructure with very fine grains. In contrast to TiN films [29], the grain size does not change significantly with increasing layer thickness. The selected area diffraction (SAED) pattern (Fig. 5b) reveals an essential random orientation attributable to the TiN phase. The measured d-values are up to 6% higher than the values predicted from the
-
-
Fig. 4. Typical SEM fracture cross-sections of coatings deposited onto moiybdenum substrates (E,, = -87 eV; JIOn/J atom- -0.7). (a) Target: TiN. (b) Target: separated TiNRiB*, position x = -10 mm.
P. Losbichler
e! al. /Sutfke
and Coatings
JCPDS card for TiN [24]. This may be explained by the incorporation of boron in the TiN phase and hence high residual stresses.From the bright-field image and the SAED pattern, the grain size was estimated to be approximately 3-5 nm. In agreementwith the XRD results and the chemical composition, no significant influence of the deposition parameterson the microstructure was obtained for the position x = 5 mm. Contrary to these results, a strong influence of the ion energy and the JionlJarom ratio on the coating hardnesswas observed. Fig. 6 showsthe hardnessof Ti-B-N films deposited onto stainlesssteel as a function of the position x for different ion energiesand Ji,“/J;lt,, ratios. Hardnessvalues of >50 GPa have been obtained for fibs deposited with intense ion bombardment. Reducing the ion flux density leads to a distinct decreaseof the hardness.This decrease is most pronounced for coatings depositedclose to the TiN part of the target where hardnesses<30 GPa have been measured.At the TiBz part of the target, a slight increase of the hardnesswas observed especially for the coatings deposited at EionE 37 eV and J&Jat,, 3 0.7. Since the microstructure and the chemical composition seetn to be rather independent of the deposition parameters used in this investigation, the shift of the hardnessmight be attributed to the formation of different nanocrystalline phases [lo]. Furthermore, intrinsic stressescausedby ion bombardment are known to significantly influence the hardnessof a material [30]. In addition to the phasesdescribed in this paper, the additional formation of titanium or boron based compoundswith residualgaseslike oxygen or carbon seems to be possible.This will be the subject of further investigations using electron energy-lossspectroscopy.
Technology
93-95
-40
(1997)
-30
-2.0
301
297-302
-10
0
10
20
30
40
position x [mm] Fig. 6. Dependence of the hardness of Ti-B-N coatings deposited onto austenitic stainless steel subatratcs on the position s. (a) E,,, = -87 eV; = -0.7. (b) E,,,, = -126 eV; J,&I,,,, = -0.1. (c) E,,, = -37 eV; JmJJam J,,,iJ,,,,
= -0.7.
4. Conclusions Ti-B-N coatings have been sputtered from a sectioned TiN/TiB* target using an unbalanceddc magnetron sputtering deposition unit. The films show a gradient of the chemical composition not deviating significantly from that of the quasi-binary sectionTiN-TiB2 of the ternary systemTiB-N. The GAXRD spectraof the films with low boron content exhibited broad and low-intensity peaksslightly differing from the peak lines of TIN. Less nitrogen-containing films seemto e,xhibit a dual-phasestructure consisting of nanocrystalline fee and hexagonal Ti-B-N, respectively. Fracture amorphous microstructures were obtained by SEM fracture cross-sections. TEM bright-field images showed very fine grains in the films with a size of approximately 3-5 nm. The microstructure and chemical composition seemto be rather unaffected by the different deposition parameters used. Contrarily, the hardness was strongly affected by varying the ion energy and the ion intensity incident at the growing film. Highest values (>50 GPa) were obtained with intense ion bombardment.
Acknowledgements The authors are grateful to Dr. H. Baumann (Institut fiir Kernphysik, Johann-Wolfgang-Goethe-Universitat, Frankfurt, Germany) for nuclear reaction analysis. This work was supportedby the Fonds zur Forderung der wissenschaftlichen Forschung (Austria) under contract P10764-GTE.
References Fig. 5. TEM cross-section of a Ti-B-N film deposited onto a austenitic stainless steel substrate at the position x = 5 mm. (a) Bright-field image. (b) Corresponding selected area diffraction (SAED) pattern.
[I] H. Holleck, Sru$ Cont. Tech&, 36 (1988) 151. [2] H. Holleck, J. Vcic. Sci. Technoi., A4 (1986) 2661,
P. Losbichler
302
et al. / Swface
and Coatings
W.-D. Miinz, J. Vat. Sci. Tech&., A4 (1986) 2717. [4] S. Veprek and S. Reiprich, Thin Solid Film, 268 (1995) 64. [5] C. Mitterer, M. Rautrr and P. RGdhammer, Su$ Coclr. TrchnnE., 41 (1990) 351. [6] 0. Knotek. F. Jungblut and K. Breidenbach, Vacmw~, 7-9 (1990) 2184. 171 W. Herr, B. Matthes, E. Broszeit and K.H. Kloos, Mater. Sci. Eng., Al40 (1991) 616. [8] T. Friesen, J. Haupt, W. Gissler, A. Bama and P.B. Bama, Sq! Coat. Techno!., 48 (199 1) 169. 191 T. Friesen, J. Haupt, W. Gissler, A. Bama and P.B. Bama, Vaczarm, 43(5-7) (1992) G57. [lo] W. Gissler, Sur$ Coal. Technol., W69 (1994) 556. [l 11 J. Laimcr, H. Karner, H. Stiiri and P. RGdhammer, in J.J. Pouch and S. Alterowith (eds.), Plasma Properties, Deposition and Eiching, Materials Science Form, i’ol. 140-142, Transtech Publications, Aedermannsdorf, 1993. p. 493. [12] M. Tamura and H. Kubo, SUM Cont. Tech&., 54/55 (1992) 255. [13] V.D. Chupov. S.S. Ordan’yan and L.V. Kozlovskii, Inorg. Mater., 17 (1981) 1195. [14] P. Losbichler and C. Mitterer, SU~$ Cuor. Tech&., in press. [15] J.A. Thornton, 1. Vuc. Sci. Technol., 15 (1978) 188. [16] I. Petrov, V. Orlinov, I. Ivanov and J. Kourtev, Contrib. Plasma [33
Phys.,
28 (1988)
157.
1171 F. Adibi, I. Petrov. J.E. Greene, Appl. Phys., 73 (1993) 8580.
L. Hultman
and J.E, Sundgren,
.I.
Technology
94-95
(1997)
297-302
[ 181 J.E. Greene, in D.T. Hurle (ed.), Handbook of Cvstnl Growth, Vol. 1, Elsevier, Amsterdam, 1993, p. 640. [19] F. Hofer, P. Warbichler, P.B. Anderson and R. Pitonak, Prakt. Metallogr., 26 (1989) 506. [20] F. Hofer, P. Warbichler, P.B. Anderson and R. Pitonak, Mikrochim. Acta, II (1990) 243. 1211 G.F. Bastin and H.J.M. Heijligers, Quantitative Electron Probe Microannlysis of Nitrogen, Eindhoven University of Technology, 1988. [22] R.C. Buschert. P.N. Gibson, W. Gissler, J. Haupt and T.A. Crabb, Colloque de Physique, 10 (1989) 169. [23] M.F. Doerner and W.D. Nix, J. Mater. Res., I (1992) 397. [24] Joint Committee on Powder Diffraction Standards, Powder Diffracrion File, International Center for Diffraction Data, Swarthmore, PA, Cards 6-0642, 35-741. [25] C. Mitterer, P. Radhammer, H. Srari and F. Jeglitsch, J. Vat. Sci. Technol. A, 7 (1989) 2646. [26] C. Mitterer, J. Komenda-Stallmaier, P. Losbichler, P. Schmalz, W.S.M. Werner and H. Stari, Vaczwm, 46/11) (1995) 1281. 1271 C. Mitterer, W. Waldhauser, U. Beck and G. Reiners, .SU$ Coaf. Technol., 86/87(1996) 715. [28] J.A. Thornton, Rev. Mnfer. Sri., 7 (1977) 239. [29] G. H&ansson, L. Hultman, J.-E. Sundgren, J.E. Greene and W.-D. Miinz, Su$ Coat. Technol., 48 (1991) 51. [30] G. Hakansson, J.-E. Sundgren, D. McIntyre, J.E. Greene and W.-D. Miinz, Thin Solid Films, 1.53 (1987) 55.
and Coatings
Technology
93-95
(1997)
297-302
Co-sputtered films within the quasi-binary system TIN-TiB2 P. Losbichlera,*, C. Mitterera, P.N. Gibsonb, W. Gisslerb, F. Hofer’, P. Warbichler’ bInstitute
for
“In rtitut fiir Metnllkunde md I~erkstoff~Jri~~lig, Il~onraintnii’ersitiir, Frcirl~-Josef-StrrlPe 18, A-8700 Leoben, Ausirin Adrmced Mmerials, Joint Resenrch Centre of the Cowmission of the European Cormmiries, C.P. I, I-21020 Ispro ‘ForscillctlRsinstitIlr fiir Elektronemikroskopie, Technische lJnii)ersira’r, Sreyrergnsse I7, A-8010 Groz, Ausrricr
(C’ri). Iicily
Abstract Ti-B-N films with a gradient in the chemical composition were deposited onto austenitic stainless steel and molybdenum sheets by means of unbalanced dc magnetron co-sputtering using a separxted TiN/TiB2 target. Different energetic contributions necessary for film growth were investigated adjusting suitable deposition parameters. Film microstructure was characterized by means of scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), as well as glancing angle X-ray diffraction (GAXRD) and clcctron probe microanalysis (EPMA). Coating hardness was measured using a depth sensing nanoindenter. The microstructure of the films appears featureless using SEM fracture cross sections. The grain size determined from TEM investigations-ranged between 3 and 5 nm. The composition of the films was found to lie on the quasi-binary section TiN-TiB2 within the ternary system Ti-B-N. Coatings consisted of nanocrystalline fee and hcp Ti-B-N phases. Microstructure as well as chemical composition of the films do not seem to be influenced by -- --.Influenced by varying the ion bombardvarying the deposition parameters used in this investigation. Contrarily, the hardness was strongly ment. Hardness values exceeding 50 GPa were obtained for Ti-B-N films. 0 1997 Elsevier ScienceS.A. &~~orcls:
Unbalanced
magnetron sputtering; Ultrahard
coatings; Ti-B-N
1. Introduction The development of hard coatings used for improved performance of tools is subject of numerous investigations. Growth techniques different from the state of Lhermodynamic equilibrium like physical vapour deposition are widely used to elaborate sophisticated films at low temperatures [I]. They provide the control of deposition parameters which can significantly influence the mobility of the adatoms impinging on the substrate detemlining growth mechanisms and hence the film microstructure. The nitrides, carbides and borides of titanium based alloys are mainly used for hard, protective coatings [2,3]. Besides composition-modulated layered heterostructures, coatings with nanocrystalline structures have attracted special attention for applications where ultra-hard materials are needed. Different theoretical explanations have been suggested to explain the observed strength enhancement in these nanostructures. For example, Veprek and Reiprich pointed out the importance of the structural arrangement * Corresponding author. e-mail: losbiBunileoben.ac.at
0257.8972/97/$17.00 PI2 s0257-8972(97)00440-4
Tel.: +43 3842 402449;
0 1997 Eisevier
Science
fax: +43 3842 402737;
S.A. All rights
reserved
coatings; Nanocrystalline
coatings
of individual phases to hinder dislocation movement [4]. Multiphase coatings within the ternary system Ti-B-N are anticipated by several authors as suitable candidates for tribological applications [5-121. Within this investigation, multiphase coatings have been prepared by co-sputtering from a sectioned TiN/TiB2 target using an unbalanced dc magnetron sputtering system. In the ternary system Ti-B-N the existence of a quasi-binary section of the eutectic type TIN + TiBz is claimed [ 131. The coatings deposited show a gradient in the composition depending on their position over the sectioned target. The aim of this paper is to study the formation of coatings with different chemical compositions in the quasi-binary system TiN-TiB2 as a function of different activation energies supplied for film growth.
2. Experimental
procedure
The unbalanced dc magnetron sputter system used in these experiments is shown in Fig. 1. It has been described in detail elsewhere [14], and only essential features will be presented here. A turbomolecular pumping unit was used to
298
P. Lmbichkr
et al. / Su@kce and Coatings Technology 94-95
pump the deposition chamber down to 10” Pa. TiN and TiB2 targets with diameters of 150 mm were cut into halves and one of each half was bonded onto a water-cooled backing plate. The target was positioned at a 6-cm distance from the parallel-plate substrate assembly. The horizontal distance of the substrate to the centre of the sectioned target is given by the parameter x; positive x values indicate the part close to the TiN target, negative x values the part close to the TiB2 target, 0 gives the centre of the target (compare to Fig. 1). The discharge current was maintained at I A. An external pair of I-Ielmholtz coils was used to create a uniform axial magnetic field B,,, which was varied between 0 and 80 G in the region between the target and the substrates to intensify the field from the outer magnetron pole. Plasma characteristics were determined from Langmuir probe measurements for single TiN and single TiB2 targets using a flat probe and following the procedures described in [ 1.51and [ 161. The flux ratio Jion/Jatomrwhere Jionand J,{,, are the fluxes of the ions and condensing atoms, respectively, was calculated for both targets assuming a constant film density equivalent to the bulk values. The flux ratio J& J atomis know~n as a key parameter for the formation of thin films [ 17,181. The evaluation of the atom flux for films with gradient chemical composition is not possible with suitable spatial resolution, so the measurements for TiN and TiB1, were taken as an approximation. Growth rates of about 40-55 nm min-’ for TiN and 2545 nm min-’ for TiB2 were achieved at sputtering power densities of about 2 W cm-‘. The substrates used were molybdenum sheets and metallographically polished austenitic stainless-steel discs (X 5 CrNi 18 10) precleaned with ethylene and acetone in an ultrasonic cleaner. Substrate
11997) 297-302
temperatures between 100°C and 400°C were provided by a resistance heater. Before film deposition the target was sputter-cleaned for approximately 10 min behind a shutter, before the substrates were sputter etched for 30 min at an argon pressure of 3 Pa using a dc voltage of -1500 V. The thickness of the coatings reached 2-4 pm. The morphology of the films was studied using a Cambridge Instruments Stereoscan 360 scanning electron microscope (SEM). Transmission electron microscopy (TEM) observations of cross-section foils were carried out using a Philips CM20/STEM equipped with a Gatan Imaging Filter (GIF) [19,20]. The images were recorded with a slow scan CCD camera included in the GIF with a YAG scintillator crystal and a 1024 x 1024 pixel array. The microscope was operated at a voltage of 200 kV with a LaB6 cathode. The chemical composition of the coatings was determined by wavelength dispersive electron probe microanalysis (EPMA). Quantification occurred by means of TiN and TiBp coating standards calibrated using nuclear reaction analysis (NRA). Some problems can occur in the analysis of the N content due to the overlap of the Tit, and N Ka emission line [21]. In addition, the results were crosschecked using X-ray photoelectron spectroscopy (XI’S). The X-ray diffraction (XBD) spectra were recorded with a Siemens D500 goniometer in the Bragg-Brentano mode using Cu KCYradiation. Glancing angle X-ray diffraction (GAXRD) spectra were obtained using a unmonochromated copper source at an incident angle of 1” [22]. To control the angular resolution, a high-precision variable slit was placed in front of the sample and the detector. A solid-state detector was used to isolate the Cu Ka! doublet. Coating hardness was determined from the loading and unloading curves
TO Turbomolecular Pump
SUPPlY 5kW
Fig. 1. Schematic diagram of the unbalanced dc magnetron sputter deposition unit used.
P. Losbichler
et al. 1 Su&cr
and Coatings Technology
employing an ultra-low load depth-sensing nnnoindenter (Nanoindenter II, Nano Instruments) [23]. The indenter was operated in the constant-displacement-rate mode until a depth of 200 run was reached. The indenter was kept at maximum load for 180 s, thus allowing relaxation of the induced plastic flow. Finally the unloading curve was measured by decreasing the force at constant rate, equal to the maximum loading rate during the loading curve. The measurementswere calibrated using a Si (111) wafer assuminga modulus of 157 GPa, independent of penetration depth. Under these conditions a penetration depth independent hardness of 12 GPa was obtained for the Si wafer.
94-95
(1997) 297-302
299
(a)
w
(Cl
3. Results and discussion To characterize the plasma conditions, probe measurements have been performed for non-reactive sputtering from the TiN and the TiB2 target. Detailed results have been presented in a previous paper [14]. Fig. 2 shows the ion/atom flux ratios Ji,“/J,,, incident at the substrate (position 1x1< 30 mm) as a function of B,,, for both targets demonstrating an increase of the flux ratio from 0.1 without B,,, to 0.7 at B,,, of 80 G. In this calculation, it was assumed that Jia” is composed of only single charged ions and secondary electron emission has been neglected. Applying a higher magnetic field leads to a focusing plasma and hence to a non-uniformity of the ion bombardment conditions in the vicinity of the substrate,hindering a comparison of the influence of different energies supplied for film growth. The floating potential V, decreasedwith increasing B,,, from -2 to -23 V, whereas the plasma potential VP decreasedfrom -t26 to --13 V, both with respect to the grounded anode. The energy of the ions bombarding the substrate is given by the difference of the applied bias voltage V,, and VP such that Ei,, E (e(jVb - Vpi). Within this 3
I
1
I
I ~*
0.05
I 0
I 50
/ 100
I 150
Bext PI
Fig. 2. Ion/atom flux ratio J,,,IJ,,,, incident at the substrate and growing film during sputtering of TiN and TiB2 in a 0.8 argon discharge as a function of the external magnetic field B,,, (discharge current, 1 .4; bias voltage, -80 V).
---.l Cd)
PO
30
40
diffraction
50
60
70
80
angle 28 [“I
Fig. 3. X-ray (XFD) and glancing angle X-ray diffraction (GAXRD) spectra of films deposited onto austenitic stainless steel substrates. (a) Target: TIN (XFCD). (b) Target: separated TiNiTiB2, position s = +30 mm (GAXRD). (c) Target: separated TiNITiB2, position x = +lO mm (GAXFCD). (d) Target: separated TiNiTiBz, position x = -10 mm (GAXRD). (e) Target: separated TiNffiB2, position x = -30 mm (GAXRD). (f) Target: TiBz (XRD).
work, coatings were depositedusing substrate temperatures between 100and 400°C the processgaspressurewas varied between 0.4 and 1.2 Pa, different ion energieswere obtained using bias voltages from self-bias to -100 V, and the external magnetic field was varied between 0 and 80 G in order to adjust different Jion/Jatom ratios. It has been shown earlier [ 141that the microstructure and the properties of non-reactively sputtered TiN and TiB2 films are strongly affected by the ion energy and the lion/ Jatomratio. Therefore, the main emphasis of this work was laid on the influence of these parameters keeping the substrate temperature in a 0.8 Pa argon discharge constant at 300°C. XRD results of TiN and TiBz films obtained using the Bragg-Brentano mode, are shown in Fig. 3, as well as glancing angle XRD results on Ti-B-N layers. The position lines from JCPDS data [24] for TIN and TiB2 are also indicated. These layers were depositedat Eion= 87 eV and Jlonl = 0.7. While the TiN and the TiB2 layers exhibited a J ntOm single phase fee NaCl structure with a mixed crystallo-
P. Losbichirr
300
et al. /Sq+‘kce and Coatings Technology
Table 1 Chemical composition of TiN, Ti-B-N (as a function of the position x) and TiBl coatings deposited onto stainless steel substrates (E,,,, = -87 eV: J,,J J .xom= 4.7) Position .r (mm)
N/Ti
B/Ti
TiB2 -30 -10 +lO +30 TiN
0 0.5 0.6 0.7 0.8 I
2.2 1.2 0.9 0.6 0.1 0
graphic orientation and an hcp structure with a strong (001) texture, respectively, the GAXRD spectra of the Ti-B-N coatings showed very broad peaks. In addition, an overlap of peaks from different phases seems to be observed. The GAXRD spectra for films deposited close to the TiN part of the target (Fig. 3b and c> indicate an fee structure of the NaCl type exhibiting a mixed orientation. The peak positions differ slightly from the peak lines of TiN. Chupov et al. [13] claimed only a small solid solubility of boron in TiN0.96, whereas an increasing solubility in TIN,-, occurred with increasing x and temperature, An fee Ti-B-N phase has been reported by other authors [5-7,l 1,12,25]. From these considerations, one may conclude that in TiN-rich films an fee Ti-B-N phase is formed due to the incorporation of boron in the TiN lattice although the formation of a multiphase structure based on TIN and TiBa cannot completely be excluded. At the TiB2-rich part (Fig. 36 and e), a shift of the peak positions towards the line position of TiB2, and a further broadening of the peaks as well as a continuous decrease of the relative intensities are obtained. This may be due to a coexistence of a hexagonal nitrogen containing phase based on TiBz and an fee boron containing phase based on TiN both exhibiting very low average grain sizes. Additionally, the formation of an amorphous phase along the grain boundaries seems also to be possible. The XRD spectra of single phase TiN and TiB2 films exhibit high intensities (compare Fig. 3a and f) and hence these films are of high crystallinity. The spectra of Ti-B-N films basically showed low intensities accompanied by high full width at half maximum values indicating a less crystalline structure. This decrease in crystallinity can be explained by the formation of small grains in the film together with a relative large amount of disordered grain boundaries. Since the deposition of Ti-B-N coatigs occurred by means of co-sputtering from the components of the yuasibinary system TiN-TiB2, the chemical composition of the films was expected to lie near the tie line of TIN and TiB2 in the ternary system Ti-B-N. Non-reactively sputtered TiN films showed a stoichiometric composition and were not affected by changes of the deposition parameters whereas overstoichiometric films were obtained by sputtering from the TiBz target. The B/Ti ratios in this case varied from 2.12
91-95
(1997) 297-302
and 2.38. This overstoichiometry is in good agreement with other sputtered boride films [26,27]. The B/Ti ratios and the N/Ti ratios of the Ti-B-N coatings for various positions x are given in Table 1. We observed no significant influence of the deposition parameters varied in this investigation. SEM fracture cross sections of TiN and Ti-B-N coatings deposited at high ion energy and high Jion/Jar,,,,,ratio onto molybdenum sheets are shown in Fig. 4. TiN films exhibit fibrous grains with dense boundaries (Fig. 4a) which are attributable to the transition zone T of the structure zone model of Thornton [28] whereas a fracture amorphous stmcture is observed for Ti-B-N coatings (Fig. 4b). A similar featureless structure was obtained with TiBz films [14] (not shown here). Fig. 5a shows a typical TEM brightfield image of the cross-section of a Ti-B-N film deposited onto a steel substrate at the position x = 5 mm. At the film/ substrate interface only a small transition phase is formed. Adjacent to the substrate surface, the film shows a dense microstructure with very fine grains. In contrast to TiN films [29], the grain size does not change significantly with increasing layer thickness. The selected area diffraction (SAED) pattern (Fig. 5b) reveals an essential random orientation attributable to the TiN phase. The measured d-values are up to 6% higher than the values predicted from the
-
-
Fig. 4. Typical SEM fracture cross-sections of coatings deposited onto moiybdenum substrates (E,, = -87 eV; JIOn/J atom- -0.7). (a) Target: TiN. (b) Target: separated TiNRiB*, position x = -10 mm.
P. Losbichler
e! al. /Sutfke
and Coatings
JCPDS card for TiN [24]. This may be explained by the incorporation of boron in the TiN phase and hence high residual stresses.From the bright-field image and the SAED pattern, the grain size was estimated to be approximately 3-5 nm. In agreementwith the XRD results and the chemical composition, no significant influence of the deposition parameterson the microstructure was obtained for the position x = 5 mm. Contrary to these results, a strong influence of the ion energy and the JionlJarom ratio on the coating hardnesswas observed. Fig. 6 showsthe hardnessof Ti-B-N films deposited onto stainlesssteel as a function of the position x for different ion energiesand Ji,“/J;lt,, ratios. Hardnessvalues of >50 GPa have been obtained for fibs deposited with intense ion bombardment. Reducing the ion flux density leads to a distinct decreaseof the hardness.This decrease is most pronounced for coatings depositedclose to the TiN part of the target where hardnesses<30 GPa have been measured.At the TiBz part of the target, a slight increase of the hardnesswas observed especially for the coatings deposited at EionE 37 eV and J&Jat,, 3 0.7. Since the microstructure and the chemical composition seetn to be rather independent of the deposition parameters used in this investigation, the shift of the hardnessmight be attributed to the formation of different nanocrystalline phases [lo]. Furthermore, intrinsic stressescausedby ion bombardment are known to significantly influence the hardnessof a material [30]. In addition to the phasesdescribed in this paper, the additional formation of titanium or boron based compoundswith residualgaseslike oxygen or carbon seems to be possible.This will be the subject of further investigations using electron energy-lossspectroscopy.
Technology
93-95
-40
(1997)
-30
-2.0
301
297-302
-10
0
10
20
30
40
position x [mm] Fig. 6. Dependence of the hardness of Ti-B-N coatings deposited onto austenitic stainless steel subatratcs on the position s. (a) E,,, = -87 eV; = -0.7. (b) E,,,, = -126 eV; J,&I,,,, = -0.1. (c) E,,, = -37 eV; JmJJam J,,,iJ,,,,
= -0.7.
4. Conclusions Ti-B-N coatings have been sputtered from a sectioned TiN/TiB* target using an unbalanceddc magnetron sputtering deposition unit. The films show a gradient of the chemical composition not deviating significantly from that of the quasi-binary sectionTiN-TiB2 of the ternary systemTiB-N. The GAXRD spectraof the films with low boron content exhibited broad and low-intensity peaksslightly differing from the peak lines of TIN. Less nitrogen-containing films seemto e,xhibit a dual-phasestructure consisting of nanocrystalline fee and hexagonal Ti-B-N, respectively. Fracture amorphous microstructures were obtained by SEM fracture cross-sections. TEM bright-field images showed very fine grains in the films with a size of approximately 3-5 nm. The microstructure and chemical composition seemto be rather unaffected by the different deposition parameters used. Contrarily, the hardness was strongly affected by varying the ion energy and the ion intensity incident at the growing film. Highest values (>50 GPa) were obtained with intense ion bombardment.
Acknowledgements The authors are grateful to Dr. H. Baumann (Institut fiir Kernphysik, Johann-Wolfgang-Goethe-Universitat, Frankfurt, Germany) for nuclear reaction analysis. This work was supportedby the Fonds zur Forderung der wissenschaftlichen Forschung (Austria) under contract P10764-GTE.
References Fig. 5. TEM cross-section of a Ti-B-N film deposited onto a austenitic stainless steel substrate at the position x = 5 mm. (a) Bright-field image. (b) Corresponding selected area diffraction (SAED) pattern.
[I] H. Holleck, Sru$ Cont. Tech&, 36 (1988) 151. [2] H. Holleck, J. Vcic. Sci. Technoi., A4 (1986) 2661,
P. Losbichler
302
et al. / Swface
and Coatings
W.-D. Miinz, J. Vat. Sci. Tech&., A4 (1986) 2717. [4] S. Veprek and S. Reiprich, Thin Solid Film, 268 (1995) 64. [5] C. Mitterer, M. Rautrr and P. RGdhammer, Su$ Coclr. TrchnnE., 41 (1990) 351. [6] 0. Knotek. F. Jungblut and K. Breidenbach, Vacmw~, 7-9 (1990) 2184. 171 W. Herr, B. Matthes, E. Broszeit and K.H. Kloos, Mater. Sci. Eng., Al40 (1991) 616. [8] T. Friesen, J. Haupt, W. Gissler, A. Bama and P.B. Bama, Sq! Coat. Techno!., 48 (199 1) 169. 191 T. Friesen, J. Haupt, W. Gissler, A. Bama and P.B. Bama, Vaczarm, 43(5-7) (1992) G57. [lo] W. Gissler, Sur$ Coal. Technol., W69 (1994) 556. [l 11 J. Laimcr, H. Karner, H. Stiiri and P. RGdhammer, in J.J. Pouch and S. Alterowith (eds.), Plasma Properties, Deposition and Eiching, Materials Science Form, i’ol. 140-142, Transtech Publications, Aedermannsdorf, 1993. p. 493. [12] M. Tamura and H. Kubo, SUM Cont. Tech&., 54/55 (1992) 255. [13] V.D. Chupov. S.S. Ordan’yan and L.V. Kozlovskii, Inorg. Mater., 17 (1981) 1195. [14] P. Losbichler and C. Mitterer, SU~$ Cuor. Tech&., in press. [15] J.A. Thornton, 1. Vuc. Sci. Technol., 15 (1978) 188. [16] I. Petrov, V. Orlinov, I. Ivanov and J. Kourtev, Contrib. Plasma [33
Phys.,
28 (1988)
157.
1171 F. Adibi, I. Petrov. J.E. Greene, Appl. Phys., 73 (1993) 8580.
L. Hultman
and J.E, Sundgren,
.I.
Technology
94-95
(1997)
297-302
[ 181 J.E. Greene, in D.T. Hurle (ed.), Handbook of Cvstnl Growth, Vol. 1, Elsevier, Amsterdam, 1993, p. 640. [19] F. Hofer, P. Warbichler, P.B. Anderson and R. Pitonak, Prakt. Metallogr., 26 (1989) 506. [20] F. Hofer, P. Warbichler, P.B. Anderson and R. Pitonak, Mikrochim. Acta, II (1990) 243. 1211 G.F. Bastin and H.J.M. Heijligers, Quantitative Electron Probe Microannlysis of Nitrogen, Eindhoven University of Technology, 1988. [22] R.C. Buschert. P.N. Gibson, W. Gissler, J. Haupt and T.A. Crabb, Colloque de Physique, 10 (1989) 169. [23] M.F. Doerner and W.D. Nix, J. Mater. Res., I (1992) 397. [24] Joint Committee on Powder Diffraction Standards, Powder Diffracrion File, International Center for Diffraction Data, Swarthmore, PA, Cards 6-0642, 35-741. [25] C. Mitterer, P. Radhammer, H. Srari and F. Jeglitsch, J. Vat. Sci. Technol. A, 7 (1989) 2646. [26] C. Mitterer, J. Komenda-Stallmaier, P. Losbichler, P. Schmalz, W.S.M. Werner and H. Stari, Vaczwm, 46/11) (1995) 1281. 1271 C. Mitterer, W. Waldhauser, U. Beck and G. Reiners, .SU$ Coaf. Technol., 86/87(1996) 715. [28] J.A. Thornton, Rev. Mnfer. Sri., 7 (1977) 239. [29] G. H&ansson, L. Hultman, J.-E. Sundgren, J.E. Greene and W.-D. Miinz, Su$ Coat. Technol., 48 (1991) 51. [30] G. Hakansson, J.-E. Sundgren, D. McIntyre, J.E. Greene and W.-D. Miinz, Thin Solid Films, 1.53 (1987) 55.