Titanium boron nitride coatings of very high hardness - Science Direct

194

Surface and Coatings Technology, 68/69 (1994) 194—198

Titanium boron nitride coatings of very high hardness P. Hammer, A. Steiner, R. Villa, M. Baker, P.N. Gibson, J. Haupt, W. Gissler institute of Advanced Materials, Joint Research Centre of the Commission o[the European Communities, 1-21020 Ispra (Va). Italy

Abstract Ti—B—N coatings ofvariable composition have been sputter deposited from a heterogeneously composed Ti—BN target, consisting of a boron nitride base plate on which small Ti platelets were regularly arranged. By varying the number of platelets the concentration ratio cTI/cB ofthe target and therefore also of the coating could easily be varied. Inhomogeneities in the chemical composition were in the per cent range. Very hard coatings of up to 55 GPa were obtained by sputtering with a substrate bias of —150 V at a substrate temperature of 400 °C.The hardness maximum was found at a chemical composition where both the TiB 2 and the TiN phases coexist in equal concentrations. All coatings show very broad diffraction peaks indicating average grain sizes in the nanometre range where the Hall—Petch relation is no more valid. With coatings composed of very small grains an inverse Hall—Petch effect was observed. The tribological performance of the Ti—B—N coatings is promising; however, in general it is worse than that observed with TIN coatings. The main reason seems to be the relatively low cohesive strength of the material.

1. Introduction Boron-nitride-based transition metal coatings have attracted attention in recent years owing to their high hardness, high temperature and corrosion resistance. Ti—B—N coatings can be synthesized by almost all deposition techniques, having been initially prepared by a chemical vapour deposition (CYD) technique [1]. Following this, coatings of various compositions were synthesized by reactive sputter deposition from a TiB2 target displaying hardness values up to 4800 HV [2,3]. The tribological [4] and corrosion [5] properties of Ti—B—N coatings prepared by a similar sputter technique [6] were found to be promising. These coatings retain almost the same compositional ratio Ti: B of about 1/2 as that of the target independent of the reactive gas pressure. Recently it has been supposed [7] that optimized hardness and toughness properties might be obtained with coatings of a composition corresponding to about TiBN 05. According to the phase diagram for the system Ti—B—N [8], this chemical composition corresponds to a coating composed equally of two phases, TiB2 and TiN. In order to synthesize coatings of different Ti—B--N compositions several other methods have been employed such as plasma-assisted CVD [9], arc physical vapour deposition [10], Ti implantation into (hexagonal) BN films [11], interdiffusion of Ti/BN multilayer films [12] and cosputtering from Ti and BN targets [13,14]. In this paper we describe another more practical procedure, the sputter deposition from a target which is

0257—8972/94/$7.00 SSDI 0257-8972(94)08048-4

composed of a BN plate onto which small titanium platelets are placed at regular intervals. By varying the number of platelets the chemical composition of the deposit can be varied at will. The Ti—B—N coatings deposited by this method were characterized with respect to their structure, hardness, elasticity, and tribological performance.

2. Experimental details Film deposition was performed in Ar with a commercially available sputtering machine (Leybold Heraeus, Z 400) using the “substrate up—target down” configuration in the r.f. mode. The target diameter was 7.5 cm with a distance of 5 cm between the target and the substrate holder. Thegiving Ar gasa flow maintained at 25A 3 min1 totalwas pressure of 0.5 Pa. standard cm base pressure of 5 x iO~Pa was attained by a 4001 s_i turbomolecular pumping system. The partial pressure of the working and residual gases was measured by a quadrupole gas analyser connected to the sputter chamber by a pressure converter. Contaminating gases were present at levels of the order of 0.3%. Prior to deposition the targets were sputter cleaned for approximately 20mm. To deposit films with different Ti:B compositional ratios by sputtering, a simple method was employed, constructing a sectorized Ti—BN target as follows: onto a 99.9 at.% BN base plate small platelets of 99.9 at.% sheet titanium were placed. Tests showed that the highest

© 1994

—

Elsevier Science S.A. All rights reserved

P. Hammer et al.

/

Very hard Ti—B—N coatings

chemical homogeneity of the coatings is obtained by arranging the platelets in a two-dimensional closepacked structure. The target composition can easily be changed by changing the number of platelets on the BN base. The concentration ratio cT~/cBof an as-deposited coating depends on the surface ratio of the respective surfaces and sputter coefficients. In the absence of sufficiently precise sputter coefficients an empirical relation between film composition and target coverage was derived. Because of preferential pumping of nitrogen the compositional ratio of N:B was found to be about 1:2. Films were deposited onto high speed steel (HSS) substrates and silicon (111) wafers. Before sputter deposition the HSS substrates were mechanically polished to an average roughness of less than 30 nm. Cleaning was performed with trichlorethylene and isopropanol in an ultrasonic bath. The film thickness as measured with a mechanical stylus device was between 1 and 3 p.m. Depending on the degree of surface coverage by the Ti sheets the deposition rate was between 1.3 and 2.5 nm mm These relatively low deposition rates could have been increased by the use of the magnetron sputter technique. However, this would have produced additional inhomogeneities in the chemical composition owing to preferential sputtering zones on the target and therefore was not employed. All films were examined by glancing angle X-ray diffractometry (GAXRD) using equipment previously described [15]. The angle of incidence for most measurements was 0.5°,giving a normal penetration depth of about 5.5 p.m in boron nitride and 0.5 p.m in titanium, titanium nitride and titanium diboride. The composition of the coatings was investigated by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) using a Perkin—Elmer spectrometer (electron spectroscopy for chemical scanning Auger microprobe analysis—(ESCA) type. The elemental concentrations were determined from XPS peak areas using the Ti 2p, N is and B is peaks and appropriate atomic sensitivity factors. Hardness and Young’s modulus were determined by an ultralow load depth-sensing nanoindenter (Nanoindenter II, Nano Instruments Inc.) from the loading and unloading curves [16]. The loading curve was measured by keeping the indentation rate constant (3 nm s_i) and measuring the displacement—load curve until a total depth of 100 nm was reached. Then a holding period of 180 s followed allowing a relaxation of the induced plastic flow. Finally the unloading curve was measured, decreasing the force at a constant rate of about 40 p.N s~.Elastic contributions were determined from the unloading curve. These were used for the calculation of Young s modulus for reducing the loading curve to solely its plastic component. The measurements were calibrated with an Si(11i) wafer assuming a modu‘.

195

lus of 157 GPa independent of penetration depth. Under these conditions also a constant hardness of 12 GPa independent of penetration was obtained for the Si wafer. The tribological behaviour of the coatings was investigated by a scratch tester (Revetest, CSEM) and a pinon-disc machine (Revetest, CSEM) using an alumina ball of 6 mm diameter as the “pin”, a normal load of 5 N and a track velocity of 0.1 m s~ over a distance of 1000 m.

3. Results Auger line scans were performed to reveal spatial chemical inhomogeneities which might have been caused by the heterogeneous composition of the target. These measurements revealed slight variations over the surface; however, the maximum deviation from the average concentration ratio cT~/cBwas only about ±4%. Fig. 1 shows the results of hardness and elasticity measurements as a function of the concentration ratio cTi/cB. The nitrogen content of these coatings was between 40% and 60% of the boron content. The coatings were sputter deposited with a bias voltage of —150 V at 400°C. The hardness H (Fig. 1(a)) displays

60.C a)’

I

I

I

I

I

I

I

I

r~.

~ 50.0 ~,,

~ 40.0 ~ 30.0 b ‘~

700

~ ~

600

8

4

-.

~

~ >•

300 06

O’8

110

1 ‘2

14

CONCENTRATION RATIO

1 ‘6

CT./CB I

Fig. 1. Hardness H and Young’s modulus E vs. cTI/cB for coatings deposited with a bias voltage of —150 V at a temperature of 400°C.

196

/

P. Hammer et al.


a maximum of 55 GPa at cT~/cB 1. The values of Young’s modulus E (Fig. 1(b)) display strong variations (the origin of which could not be identified); however, E generally decreases with increasing cT~/cBratio. Fig. 2 shows H (Fig. 2(a)) and E (Fig. 2(b)) as function of the bias voltage Ubjas. The composition was approximately TiBNQ5, being nearly equal for all coatings, and the substrate temperature was 20°C.While E is nearly independent of Ubias, H displays a maximum at approximately —150 V. Fig. 3 displays GAXRD spectra of coatings deposited with a substrate bias voltage of —150 V at 400°Cfor three different compositions: TiB19NØ6 (spectrum a), TiBN04 (spectrum b) and TiB06NØ4 (spectrum c). The sharp peaks labelled by “S” in the spectra are due to substrate diffraction lines. The spectrum of curve a clearly reflects the TiB2 phase with its three main peaks in only slightly shifted positions. Curve b shows three very broad peaks at about superpositions of peaks from35°,43° the TiBand 61°which are 2 and TiN phases. The spectrum of curve c contains only one peak at a position near the main peak of the Ti phase. The large broadening of all peaks originates from (i) overlap of peaks originating from different phases and (ii) very small (nanometric) grain sizes. Additional structural information was obtained from XPS measurements. I

32.0

I

I

I

a)

(,,28.0

H

U’)

uJ

26.0

.

24.0

.

.4:

I

//

.

~~f4ooo~J

TIB

S

06N0~

~c) T1BN04

“1~

~

Tj8,9N06

(b)

(a)

.

I

~.

I

I

20

‘

I

40

I

60

I

80

Diffraction AngIe 2 0

100

Fig. 3. GAXRD spectra for coatings ofvarious compositions deposited with a bias voltage of

—

150 V at 400 °C.

2P3/2 line These showed for all three coatings that the Ti is split into two components at approximately 454.3 eV and 455.2 eV indicative of TiB2 and TiN respectively, showing that Ti is present in two different phases. Coatings deposited at 20°Cdisplayed hardness values which were lower by a factor of nearly 2 than those deposited at 400 °C. Therefore it was attempted to increase the hardness by a thermal post-treatment. Fig. 4 shows the change in H for coatings with different compositions, preparation techniques and annealing temperatures as a function of the annealing time t. An increase in H by about a factor of 1.5 is observed after relatively short annealing times of 30 mm. For longer times H has the tendency to decrease slowly. The hardening process is accompanied by a significant line narrowing of the two main diffraction peaks as shown in Fig. 5 for coat-

I I

I

b) 40

/

35

II

~ 320 = 0

S

C

u-I Cl)

~

-150 V

ings with a concentration ratio cTI/cB of 1/0.6 and 1/0.5 deposited at 20°C and with —30 V bias voltage. By fitting lorentzian curves to the main peaks of Fig. 5 the line widths were determined. From these a grain size I

340

=

10

30 Lf

C,) ~ 0

25

CD

I I

~

U)

~300 z =

~

lu

A

•

-

~20

--

U

800 ‘C,

-30 V, Ti B06 N04

A— •-

600 ‘C, ‘C, 600

-90 V, V, -30

-

-

Ti B28 B N05 Ti N15 -

.

,....

o >—

15~ I

I

0

-100

•

I

I

-200

-300

I

I

50

100

I

150

200

250

300

TIME t [mini

BIAS VOLTAGE [VJ Fig. 4. Hardness H as a function of annealing time at various temperFig. 2. Hardness H and Young’s modulus E vs. bias voltage for TiBN05 coatings deposited at 20°C.

atures for coatings of three different compositions deposited with bias

voltages of —30 V and —90 V at 20°C.

P. Hammer et al.

/


for coatings deposited under “non-equilibrium” conditions the experimental findings on Ti—B—N coatings are in good agreement with the phase diagram for the range). In spite of[7]. the limited validity of phase diagrams Ti—B—N Thea hardest phase in the system obtained system if the films have multiphase composition [18, are TiB2 (4000 HV) and TiN (2500 HV). Higher hardness and possibly also higher toughness might be

annealing 6 h at 800 °C U)

w

36 GPa ~

2

~

=

30 V. Td

= 20

~

w

I— 2

40 GPa

197

annealing: 6 hat 800°C

>

w

19] with the restrictive condition that soft phases such as hexagonal BN and Ti be coating absent. These condiponds to TiBN05. In the thisshould case the is composed tions are fulfilled if chemical composition corres-

27~~\::;:Ite~>TiBO.6NO4

0

I

I

I

I

20

40

60

80

100

DIFFRACTION ANGLE 20 Fig. 5. GAXRD spectra before and after annealing at 800°Cfor 6 h. Coatings of two slightly different compositions were deposited at 20°C with a bias voltage of —30 V. Table 1 Mechanical properties of Ti—B—N coatings sputter deposited with a substrate bias voltage of —150 V at 400 °Ccompared with those of a TiN coating Coating type

L~1(N)

L~2(N)

Wear rate H (GPa) E (GPa) (x3 N’m~’) io’~ ________________________________________________________ m TiB 2N06 35 86 4.2 36 640 TiB14NØ5 16 66 2.4 48 750 TiBN04 20 >100 9.1 55 500 TiB07N0~5 29 45 6.6 52 453 TiB0•6N0•5 20 37 1.0 27 276 TiN08 37 100 7 22 410

increase to 55 curves A for thewaslower curves and to 60 A forfrom the 35upper estimated with30the Scherrer formula [17]. However, these values should be considered approximate when dealing with such small grain sizes. The results obtained from tribological tests performed with the scratch and a pin-on-disc tester in addition to hardness H and Young’s modulus E are summarized in Table 1 and compared with results obtained for TiN coatings which were reactively sputter deposited from a Ti target at 400 °C.L~ 5is the normal force at which the onset of acoustic emission indicates a cohesive failure of the coating. L~2is the normal force at which the delamination of the coating from the substrate is completed (adhesive failure). 4. Discussion The phase diagram of the ternary Ti—B—N system [8] displays only binary phases and solid solutions of very limited solubility of boron and nitrogen (in the 1%

only of the two hard phases TiB2 and TiN in equal parts. The experimental results displayed in Fig. 1 confirm these expectations: highest hardness is actually observed at a composition corresponding to TiBN05 On the contrary, Young’s modulus E displays a cornpletely different behaviour (Fig. 1(b)), decreasing from values of the order of 600 GPa (for cTI/cB 1/2) to rather low values of about 300 GPa (for cTt/cB 1.8). Such a behaviour is understandable since at cTi/cB ~ 1/2 the coating consists mainly of the highly elastic TiB2 and TiN phases (E = 480 GPa and 590 GPa respectively), whereas at CTI/CB ~ 1.8 the low elasticity metallic Ti phase (E= 100 GPa) is already clearly present (see Fig. 3(c)). A similar behaviour as that observed for H as a function of the bias voltage (Fig. 2(a)) has also been found for other coatings [20,21]. In our case it might be mainly due to the formation of a compressive stress as a result of the bias voltage inducing ion bombardment of the growing film. A hardening due to a reduction in the average grain size2 dmight according to the the Hall—Petch be excluded: validity relationship H ccis 1/d” of this relation limited to a critical grain size dcrjt above which dislocation slip is the main plastic deformation mechanism [22]. For d < dent interparticle sliding is assumed to be the dominant plastic deformation mechanism. In this case a softening of the material with decreasing grain size is often the consequence (inverse Hall—Petch relation [23]). Our coatings seem to have such small grain sizes that the case d < dcrjt seems to apply. This can be deduced from the observation that H increases if the coatings are annealed (Fig. 4) and from the concomitant increase in the average grain size from 3 to 6 nm. For Young’s modulus a dependence on the grain size and consequently also on the bias voltage is not reported. Actually, we practically observe a bias voltage independent behaviour (Fig. 2(b)). The results of the tribological tests indicate that the performance of the Ti—B—N is inferior to that of TiN coatings (see Table 1). In particular, relatively small values for L~1were obtained, indicating a low cohesive strength, as is usually observed with covalent materials.

198

P. Hammer

et

al.

/


TiB2 has a higher metallic bonding character than TiN. Therefore an even higher cohesive strength should have been expected. Similar observations have also been reported by others [24]. The relatively good wear behaviour is probably a consequence of high hardness and adhesion.

5. Conclusions It has been shown that sputtering using a target composed of a boron nitride base plate on which small Ti platelets are regularly arranged is a valuable method to deposit simply coatings of variable Ti:B compositional ratio. Very hard coatings, up to 55 GPa, were obtained by sputtering with a substrate bias voltage of —150 V at a substrate temperature of 400 °C.The hardness maximum was found at a chemical composition at which both the TiB2 and the TiN phases coexist in equal parts. All coatings displayed very broad diffraction peaks indicating average grain sizes in the nanometre range where the Hall—Petch relation is not valid. With these coatings an inverse Hall—Petch effect was observed. The tribological performance of our Ti—B—N coatings is worse than that obtained with TiN coatings. The main reason seems to be the relatively low cohesive strength of the material.

[2] C. Mitterer, M. Rauter and P. Rodhammer, Surf. Coat. Technol., 41(1990) 351. [3] 0. Knotek, R. Breidenbach, F. Jungblut and F. Löffler, Surf. Coat. Technol., 43—44 (1990) 107. [4] H. Ronkainen, I. Nieminen, K. Holmberg, A. Leyland, K. S. Fancey, A. Matthews, B. Matthes and E. Broszeit, Mater. Sci. Eng. A, 140 (1991) 602. [5] J. Aromaa, H. Ronkainen, A. Mahiout, S-P. Hannula, A. Leyland, A. Matthews, B. Matthes and E. Broszeit, Mater. Sci. Eng. A, 140 (1991) 722. [6] B. Matthes, W. Herr, F. Broszeit, K. H. Kloos, G. Nurnberger, D. Schmoeckel, F. HOhl, H.-R. Stock and P. Mayr, Mater. Sci. Eng. A, 140 (1991) 593. [7] W. Gissler, Surf. Coat. Technol., 68/69 (1994) 561. [8] H. Novotny, F. Benesovsky, C. Bruki and 0. Schob, Monatsch. Chem., 92 (1961) 403. [9] H. Karner, J. Laimer, H. Stori and P. Rodhammer, Surf. Coat. Technol., 39—40 (1989) 293. [10] M. Tamura and H. Kubo, Surf. Coat. Technol., 54—55 (1992) 255. [11] W. Gissler, J. Haupt, T. Sasaki, L. Guzman, M. Elena and L. Moro, Mater. Manuf. Proc. to be published. [12] T. Friesen, J. Haupt, W. Gissler, A. Barna and P. B. Barna, Surf. Coat. Technol., 48 (1991) 169. [13] W. Gissler, T. Friesen, J. Haupt and D. G. Rickerby, in P. K. Datta and J. S. Gray (eds.), Surface Engineering, Vol. 1,

[14]

[15] [16] [17] [18] [19]

Acknowledgement The authors are very grateful to Messrs. A. Hoffmann for sample preparation, T. Sasaki for nanoindentation measurements and L. Mammarella for ESCA—AES analysis.

[20]

[21] [22] [23]

References [1] J. L. Peytavi, A. Lebugle, G. Montel and H. Pastor, High Temp. High Pressures, 10 (1968) 341.

[24]

Fundamentals of Coatings, Royal Society of Chemistry, Cambridge 1993, p. 320. T. Friesen, J. Haupt, P. N. Gibson and W. Gissler, in M. Nastasi, D. M. Parkin and H. Gleiter (eds.), Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures, Kluwer, Dordrecht, 1993, p. 475. R. C. Buschert, P. N. Gibson, W. Gissler, J. Haupt, A. Manara, X. Jiang and K. Reichelt, Surf Interface Anal., 16 (1990) 510. M. F. Doerner and W. D. Nix, J. Mater. Res., 1(1992) 397. A. Taylor, X-ray metallography Wiley, New York, 1961. H. Holleck, J. Vac. Sci. Technol., A, 4 (1986) 2661. J. R. Weertman, M. Niedzielka and C. Youngdahl, in M. Nastasi, D. M. Parkin and H. Gleiter (eds.), Mechanical Properties and Deformation Behavior of Materials Having Ultra-Fine Microstructures, Kiuwer, Dordrecht, 1993, p. 241. J. Bull and A. M. Jones, in P. K. Datta and J. S. Gray (eds.), Surface Engineering, Vol. II, Engineering Applications, Royal Society of Chemistry, Cambridge, 1993, p. 302. J.-E. Sundgren and H. T. G. Hentzell, J. Vac. Sci. Technol. A, 4 (1986) 2259. V. G. Gryznov and L. I. Trusov, Prog. Mater. Sci., 37(1993)289. J. D. Embury and D. J. Lahaie, in M. Nastasi, D. M. Parkin and H. Gleiter (eds.), Mechanical Properties and Deformation Behaviour of Materials Having Ultra-Fine Microstructures, Kluwer, Dordrecht, 1993, p. 287. B. Matthes, F. Broszeit and K. H. Kioss, Surf. Coat. Technol., 57 (1993) 97.