Investigation of elemental distributions in TiAlNZrN multi-layers using analytical transmission electron microscopy

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Surface and Coatings Technology 86-87 (1996) 357-363

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Investigation of elemental distributions in TiAIN-ZrN multi-layers using analytical transmission electron microscopy J. Cawley *, J.M. Titchmarsh, L.A. Donohue .Marerials Research Institute, SheffieldHallam University, City Campus, Pond Street, Sheffield, SI I WE, UK

Abstract This microstructural characterisation is described of PVD multilayer coatings. Coatings of TiAlN-ZrN, approximately 3 urn in thickness were grown on to stainless steel substrates using a combined steered cathodic arc evaporation and unbalanced magnetron sputtering technique. The crystallography and the multilayer periodicities were investigated using X-ray diffraction. Electrontransparent cross-section samples were examined using a conventional transmission electron microscope interfaced with an imaging filter, and with a field emission gun scanning transmission microscope interfaced with an energy dispersive X-ray analyser and a parallel electron energy-loss spectrometer. Quantitative measurements of composition were acquired across individual layers of nominal 13.2nm spacing using X-ray analysis, but not from layers with periodicity 2.6 nm. However, qualitative elemental distributions from layers of periodicity 2.6 nm were easily resolved using energy-filtered images. The energy-loss spectroscopy also revealed differences in the near-edge fine structure of the nitrogen edge between the two TiAIN and ZrN. It was not possible to clarify the extent of intermixing of the metallic elements between layers.

Keywords: Elemental distributions; Tialn-zrn multi-layers; Analytical transmission electron microscopy

1. Introduction

PVD evaporation and sputtering techniques have been successfully applied to produce many different types of multi-layer coatings on a range of substrates [1-5]. Commercial utilisation and optimisation of such coatings has also increased, particularly in the areas of high speed steel cutting tool and cemented carbide performance [6,7]. More recently research into the mechanical and physical properties of such coatings has been reported by several groups [8-12], while coating architecture, and production characteristics have also been extensively studied [13,14]. An understanding of the relationship between microstructure and bulk properties is important for the optimisation of properties. Multi-layer deposition is often undertaken only on an experimental, rather than an industrial scale, using dual unbalanced magnetrons or evaporation sources in opposed cathode or multiple crucible configurations. This can limit the substrate size and geometry which can be accommodated. Systems often require complex, expensive, reactive gas partial pressure control and

* Corresponding author. 0257-8972/96/$15.00 © 1996 Elsevier Science SA All rights reserved PI! S0257-8972 (96) 03046-0

reciprocating shutters to create controlled atmosphere separation, to restrain cross-contamination, and to prevent target poisoning. In many cases ultra-high vacuum and interrupted deposition conditions must be applied and a compromise made between deposition rate and coating uniformity. Asymmetricdeposition rate distributions also commonly occur. This paper reports on the microstructural characterisation of multi-layer films deposited on an industrial scale in a common reactive gas atmosphere, using ionassisted growth, by a combined steered cathodic arc evaporation and unbalanced magnetron sputtering technique.

2. Experimental procedures

TiAIN-ZrN multi-layer coatings were grown on stainless steel substrates in a Hauser 1000-4 ABS apparatus using the process parameters listed in' Table 1. The equipment and growth procedures have been described previously [15]. Control of the multi-layer period and, hence, the mechanical and physical properties of the films was achieved by alteration of the substrate turnta-

1. Cawley et al.fSurface and Coatings Technology 86-87 ( 1996) 357-363

358 Table 1

Arc/Magnetron TiAIN-ZrN Vacuum pump down and heating Glow discharge target cleaning

Steered arc metal ion etching

Magnetron sputtered base layer

Superlattice coating deposition

Base pressure <2 x 10- 5 mbar Temperature = 400°C Shutter position =in front of targets All targets in magnetron mode Power=5kW Time = 10 min Ar flow= 220 seem Pressure=2.7 x 1O-3mbar 2 TiAl targets only Arc current control 10 cycles each of I min Argon flow = 220 seem Pressure =2.7 x 10- 3 mbar Arc current = 100 A Substrate bias voltage-s -1200 V All targets in unbalanced magnetron mode 3 TiAl targets & 1 Zr target Temperature=450°C Substrate bias voltage = -80 V Total system pressure=3.5 x 10- 3 mbar 3 TiAI targets in unbalanced magnetron mode & 1 Zr target in steered arc mode Field closed around 3 magnetrons only Coil currents = 80 A for TiAl, 0 A for arc Zr Time",,2h Power to TiAI targets =8 kW Arc current = 80 A Rotation velocity=80% of maximum = ~6rpm

Substrate bias voltage> -80 V Temperature =450°C Total system pressure = 3.5 x 10- 3 mbar

ble rotation velocity, the type of rotation (single or 3-fold) and the deposition rate. Multi-layer coatings were grown either directly on to the steel substrate or with a graded base layer, grown by unbalanced magnetron, between substrate and multilayer to reduce residual stress. The multi-layer periodicities ranged between 1.9 and 13.2 nm, but only two are described in detail here: a multilayer of 2.6 nm periodicity grown directly on to the substrate, and one with 13.2nm periodicity grown on a graded layer. The coatings were all approximately 3 urn in total thickness. Microstructural characteristicswere investigated using X-ray diffraction (XRD), a conventional Philips CM20 transmission electron microscopy (TEM), and a VO HB501 scanning transmission electron microscopy (STEM). The electron microscopy was performed on thin transverse sections prepared by mechanical polishing and Ar ion beam milling. TEM was employed for imaging, selected area diffraction (SAD) and electron energy-loss filtered imaging using a Gatan GIF system

[16]. Using the OIF it was possible to record series of transmission micrographs using only those electrons which had lost energy within selected energy ranges at and close to inner shell ionisation edges of Ti and Zr. Subsequent processing of these micrographs generated elemental and jump-ratio images [17] with intensities roughly proportional to the concentrations of Ti and Zr. The field emission gun of the STEM enabled energy dispersive X-ray analysis (EDX) and parallel energyloss spectroscopy (PEELS) at high spatial resolution. The diameter of the STEM probe containing ",80% of the electron current was ",2.5 nm [18]. The probe diameter was, therefore, less than the periodicity of the sample with the largest periodicity. 3. Results 3.1. X-ray diffraction

Fig. 1 shows part of an XRD pattern from one of the multi-layer coatings, with a spacing of 13.2 urn. A peak fitting software package (PC-Identify, from Philips) was used to model the experimental spectra and isolate individual peaks within such patterns. Two distinct sets of peaks were identified, corresponding to TiAlN and ZrN, which suggested that the ZrN lattice was in tension and the TiAlN lattice in compression. The small peaks in low-angle XRD patterns, shown in Fig. 2, occurred at positions corresponding to the reciprocal lattice vectors of the multi-layers, from which measurements of the average multi-layer spacings were made. 3.2. Transmission electron microscopy

TEM was used to image the fine structure of the coatings. Fig. 3 shows the lamella nature of one of the coatings, with the lighter layers of TiAlN, ,..., 1.7 urn in thickness, alternating with the darker layers of ZrN, ",0.8 nm in thickness, consistent with the periodicity of 2.6 nm derived by XRD. The PVD films were columnar in nature, with the long axis normal to the surface of the substrate. With multi-layer coatings it is often difficult to image the columnar grain boundaries although there was clear evidence of crystallographic boundaries (arrowed in Fig. 3). SAD patterns typical of polycrystalline materials, with arcs of diameters corresponding to {Ill} and {200} planes of variable, spotty intensity, were observed in the coatings, rather than single crystal SAD spot patterns. Dark field imaging using small sections of the rings suggested that the crystallite size was typically a few nanometres. However, it was evident from Fig. 3 that the long range integrity of the individual layers was retained across columnar boundaries, implying that the growth front was extremely uniform. A enlarged section of an SAD pattern around the zero

J. Cawley et al.fSurface and Coatings Technology86-87 (1996) 357-363

359

256.,--------

--,

[counts]

(200) TiAlN

196

Substrate

(111)TiAlN

144

(111) ZrN 100

64

36

16 +------,----,....---r-------,----..,....-------r---,------,-----!

Fig. 1. XRD pattern from a multilayercoating of 13.2nm periodicity.

[c 0 u n t 5] . . , . - - . r - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

-,

1<'1400

n=1

10000

6400

100% Rotation velocity

n=2

80% Rotation velocity

3600

60% Rotation velocity

1600

40% Rotation velocity

400 20% Rotation velocity O. O+-----.,..-----,.-----.........,r------~----_,_------,.----___i

5.0

Fig. 2. Low-angleXRD patterns from five multilayersgrownwith differentrotational speeds, showing peaksusedto measuremulti-ayer periodicities.

J. Cawley et al.ISurface and Comings Technology 86-87 ( 1996) 357-363

360

50nm Fig. 3. TEM micrograph of TiAIN (bright) and ZrN (dark) layers, periodicity=2.6 nrn.

order spot , taken from part of the region in Fig. 3, is shown in Fig. 4(a), with a schematic representation of the pattern in Fig. 4( b). Satellite spots, close to the zero order spot were observed, corresponding to the reciprocal lattice vectors of the multi-layer structure. A layer spacing of ""' 2.6 nm was measured from these spots, consistent with the XRD measurement. In the other sample, the thicker alternating layers were also imaged, but satellite spots were not observed. The lamella nature of the coating was also revealed by STEM (Fig. 5), but here the ability to enhance the contrast electronically, compared with TEM, revealed an additional contrast modulation which is probably due to a secondary beating frequency of the 3-fold rotational geometry. 3.3. Energy dispersive X-ray analysis

EDX spectra were acquired using STEM from the sample with an average periodicity of 13.2 urn. Compositional profiles of the metallic elements, normalised to 100%, across an individual layer are shown in Fig. 6. The diffuseness of the compositional interfaces was greater than those typical of equilibrium segregation at grain boundaries [19] which suggests that there could be some intermixing of the elements, rather than an abrupt change in composition at the layer interfaces. No variations in elemental concentrations were revealed by similar analysis of the coating with 2.6 urn periodicity, although increased Zr was observed in EDX spectra acquired from the darker areas, compared with the lighter areas, of the large-scale intensity variations observed in Fig. 5.

(a)

Supetlattice spots

-

Zero order spot

=26A Outer pair =52A Inner pair

(b)

Fig. 4. (a) Satellite spots in SAD pattern and (b) schematic representation of (a).

50nm Fig. 5. STEM micrograph of same sample as Fig. 3, showing additional con trust variation.

361

1. Cawley et al.ISurface and Coatings Technology 86-87 ( 1996) 357-363


.g, 30 ~ ~

20

1--0--

10

AI - x - Ti

-+--Zr

I

12

14

0 0

2

6

4

8

10

Distance (nm)

Fig. 6. Local EDX compositional profiles across layers of 13.2 nm mean periodicity.

3.4. Electron energy-loss spectroscopy

Figs. 7(a) and (b) show GIF jump-ratio maps which qualitatively display the variations in Ti and Zr concentrations close to the steel substrate on a region of the sample with a layer periodicity of 2.6 nm shown in Fig. 3. The maps clearly indicated the presence of alternating layers of ZrN and TiAlN, typically 1-2 ron thick, and locally bent. Profiles of Zr and Ti signal intensities, measured from Fig. 7 along a line normal to the film growth direction in the vicinity of the substrate, are shown in Fig. 8. These profiles confirmed the expected complimentary nature of the Ti and Zr concentrations. Of particular interest was the presence of a variable Zr concentration, with no significant Ti, at the substrate/coating interface, which suggested that any

lOnm

intermixing or surface layers with mixed composition arising from the sample preparation were minimal. Of major significance was the remarkable spatial resolution obtained with the GIF, considering that it only took a few seconds to collect the data, compared with approximately 1 h X-ray collection time to generate the EDX profiles in Fig. 6. Additional information was obtained using STEM and PEELS by positioning a stationary probe selectively on different layers of the sample with a periodicity of 13.2nm. Fig. 9 compares the N-K edge at ~ 395 eV and the Ti-L 23 edge at ~455 eV from a ZrN layer and a TiAlN layer, after removal of the background, and after scaling of the N-K edges. There was a Ti signal from the ZrN layer, which again indicated either intermixing, surface artefacts, or limited spatial reso-

(b)

Fig. 7. GIF jump-ratio maps near at stainless steel substrate, showing (a) Ti and (b) Zr distributions: 2.6 nm periodicity.

362

J. Cawley et al./Surfaceand Coatings Technology 86-87 (1996) 357-363 5000

Intensity 4500

.-.

Ti

...':

4000

3500

.'." "

3000

"

.'-

.'

"

..: .

"

'

2500

2000

1500

Zr

1000

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500 s-

o

.4~~

-' #0

.

Distance Fig, 8. Intensity profiles parallel to growth direction from GIF maps in Fig. 7.

4. Discussion

1400 CIl

C

1200

51000

o

~ 800 .2 ~ 600

..t::

a,

400

200

o

360 380 400 420 440 460 480 500 Energy Loss (eV)

Fig. 9, Comparison of N-K edge fine structure from ZrN and TiAlN layers.

lution. There was a clear difference in the N-K nearedge fine structure, i.e., electron density of states, of the two layers, which was consistently reproducible, indicative of the differences in bonding of the N atoms in the two layers, The shape of the Ti edge was insensitive to compositional changes between the two layers.

Although coatings are routinely characterised by many methods, analytical EM techniques are probable the only means of assessing the influence of growth parameters on the complex crystallography and compositional variations in PVD industrial coatings on a scale fine enough to develop an understanding of the bulk mechanical properties at a fundamental level. They provide direct means of assessing the local variability of parameters such as layer thickness and composition, crystal structure, texture, crystal defects, etc., which alternative methods such as XRD tend to average, or ignore. It has been demonstrated that a variety of EM methods is required to explore the range of available information, including the use of FEG STEM for high resolution scanning and stationary probe analysis, and the use of energy-filtered imaging on the conventional TEM. The limitations, as well as the potential, of such stateof-the-art equipment, has also been demonstrated. EDX was more useful than PEELS for quantitative analysis for all the metallic elements, but there was still inadequate resolution for meaningful analysis of the thinner

J.

Cawleyet al.ISurface and Coatings Technology 86-87 ( 1996) 357-363

layers. In addition to the limits imposed on resolution by the minimum size of the incident probe required to generate an adequate X-ray signal, there was also a fundamental geometric limitation on resolution imposed by non-planarity of the layer interfaces in the electron beam direction, which was likely to be similar to that in the plane of the image in Fig. 3, i.e., on the scale of nanometres within a foil thickness of tens ofnanometres. Apparent concentration changes are further reduced by scattering of the electron probe into adjacent layers, which increases as the layer thicknesses decrease. Both these effects increase the Zr signal when the probe is positioned on the TiAIN, and vice versa, creating the appearance of intermixing during multi-layer growth. The effect of any amorphous surface layer generated during ion-beam milling would also create the appearance of intermixing. Because of these factors it was not possible to demonstrate an abrupt change in composition, or to confirm the occurrence of intermixing during the growth of the coating, even though the profiles in Fig. 8 suggested that preparation artefacts were small in the present samples. The present study has clearly demonstrated the potential of filtered imaging for qualitative compositional mapping. The theoretical resolution in such images, typically '" 1 urn, depends on both the electron optical parameters and the particular characteristic edges of interest [16]. It should be particularly useful for mapping elemental variations in many industrial PVD multi-layer coatings when layers contain transition elements, Nb, Zr, B, C, N, and O. Near-edge fine structure in the PEELS spectra contained information related to N bonding which, presently, requires complex calculations for a full interpretation. Such spectral information might eventually be used to explain the mechanical properties of coatings, analogous to the measurement of fracture energy in NiAl [20]. In the more immediate future, further investigations will be made to see if compositional information can be extracted from the fine structure. The GIF provides a very powerful and relatively fast method for qualitative characterisation of multilayer coatings with periodicities on the nanometre scale. 5. Conclusions The microstructure and compositional variations in two PVD third generation coatings have been investigated by EDX and energy loss spectroscopy using TEM and STEM. Measurements of the multi-layer periodicities consistent with XRD data were made directly from images and from SAD patterns. EDX was used to

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demonstrate quantitative compositional variations of the metallic elements between layers with a periodicity of 13.2 nm, but the possibility of elemental intermixing during growth could neither be confirmed or discounted. The use of energy filtered imaging has been demonstrated as a rapid means of revealing variations in elemental composition with high spatial resolution, and electron energy-loss fine structure is suggested as a potential method for determining compositional variations in TiAlN/ZrN multi-layers. Acknowledgement We are grateful to Dr M Otten, of Philips NL, for generating the GIF images, and to ABA Technology for use of the VG HB501 STEM. References [1] R.F. Huang, L.S. Wen, L.P. Guo, J. Gong and B.H. Yu, Surf Coat. Technol., 50 (1992) 97-101. [2] O. Knotek, F. Leffler and G. Kramer, Surf Coat. Technol., 54/ 55, (1992) 241-255. [3] K.H. Habig, 1. Vac. Sci. Technol., A4 (1986) 2832. [4] H. Holleck, M. Lahres and P. Wall, Surf Coat. Technol., 41 (1990) 179. [5] H. Freller and H. Haessler, Surf Coat. Techno!., 36 (1988) 219. [6] J. Keem, Surface Modification Technologies: An Engineers Guide, Marcel Dekker, 1990. [7] O. Knotek and T. Leyendecker, High Technology Ceramics, Mater. Sci. Monograph 38C, 1987, Elsevier. [8] S.A. Barnett, Physics of Thin Films, Vol. 17, Academic Press, 1993, pp. 1-77. [9] H. Ljungcrantz, PhDThesis, 1995,Linkoping University, Sweden. [10] M. Shinn, L. Hultman and SA. Barnett, 1. Mater. Res., 7 (1992) 901. [11] G. Hakannson, Ph.D Thesis, 1991, 255, Linkoping University, Sweden. [12] X. Chu, M.S. Wong, W.D. Sproul, S.L. Rohde and SA. Barnett 1. Vac. Sci. Technol, A 10 (4) (1992) 1604-1609. [13] J. Vetter, W. Burgmer, H.G. Dederichs andA.J. Perry, Suri Coat. Technol., 61 (1-3) (1993) 209-214. [14] K.N. Strafford, C. Subramanian and T.P. Wilks, J. Mater. Process. Techno!., 38(1-2) (1993) 431-448. [15] L.A. Donohue, J. Cawley and J.S. Brooks, Surf Coat. Technol., 72 (1995) 128-138. [16] A.J. Gubbens andO.L. Krivanek, Ultramicroscopy, 51 (1993) 146. [17] J. Bentley, E.L. Hall and E.A. Kenik, Proc. 53rdMSA, Jones and Begell, New York, 1995. [18] J.M. Titchmarsh, in J.D. Brown and R.H. Packwood (eds.), Proc. 11th ICXOM, University of W. Ontario, Canada, 1987, p. 337. [19] I.A. Vatter and 1.M. Titchmarsh, Ultramicroscopy,28 (1989) 236. [20] D.A. Muller, S. Subramanian, S.L. Sass, 1. Silcox and P.E. Batson, in E. Etz (ed.), Microbeam Analysis 1995,VCH Inc., New York, 1995, p. 297.