TiAlOn black decorative coatings deposited by magnetron sputtering

Vacuum/volume 41/numbers 7-9/pages 2205 to 2208/1990

0042-207X/90$3.00 + .00 © 1990 Pergamon Press plc

Printed in Great Britain

TiAION black decorative coatings deposited by magnetron sputtering R L u t h i e r a n d F L6vy, Institut de Physique Appliqude, Ecole Polytechnique Fdddrale, 1015 Lausanne,

Switzerland Black decorative coatings with improved mechanical properties (wear resistance) were deposited from TiAION source material. A composite ceramic AI203 + 1.5TIN has been used as a target in a planar magnetron rf sputtering system (13.56 MHz, 1200 W). A non-reactive process at high argon pressure yielded black, shiny coatings with deposition rates above 0.5pro h - I. In this study we report on the structural analyses as well as on the high temperature stability of such coatings. Transmission electron microscopy studies on very thin films (5Ohm) have given evidence of a two-phase microstructure: crystallized grains of average size lOnm are embedded in an amorphous matrix. X-ray diffraction has shown that these grains had the cubic B 1 structure of TiN, with a lattice constant of 0.413 nm for as-sputtered films. Post-deposition thermal treatment under vacuum up to 900*C for 5h affected only the lattice parameter which decreased to O.409nm. At 1000°C, the amorphous matrix decomposed into two crystalline phase: =-AI20 3 and 1702 (futile). The present investigation has shown that stable black decorative coatings can be deposited by a simple PVD process from a single target.

Introduction Composite two-phase materials consisting of finely dispersed mixtures of metals and insulators (known as ceramic metals or simply cermets) have been extensively studied for their specific optical properties/. Industrial interest for decorative coatings with improved mechanical properties (wear resistance) has motivated the study of such complex structures containing a refractory compound like TiN instead of the traditional metallic element. Wear resistant T i - A I - O - N coatings have been previously investigated by Knotek e t a l 2 mainly in view of improving the high temperature stability and wear resistance of amorphous A1203 coatings. In that case a sintered Ti-AI203 target containing only 15 wt% Ti was sputtered in a reactive A r + N 2 gas mixture. The resulting amorphous coatings were reported to be stable up to 1000°C, temperature at which crystallization occurred. In our study, the composite sintered target AI203 + 1.5TiN contained 40 vol% of the metallic compound TiN. Preliminary wear tests conducted by Asulab SA indicated that the resulting material is promising for decorative coatings.

Table 1. Standard deposition conditions of 'black' TiA1ON coatings PVD technique power source

magnetron sputtering rf 13.56 MHz, 1.2 kW

Target material Target diameter Target-substrate distance Substrate electrode Substrate temperature

composite ceramic AI203+ 1.5TiN 8.5cm 7cm floating potential < lO0°C

Sputtering gas Target dc bias voltage Power density Deposition rate

pure Ar, 4 Pa -1200 V 6 W can-2 0.55 #m h -

mission electron microscopy (TEM), 50 nm TiA1ON films were deposited on carbon coated microscope grids. In addition to these black coatings, transparent films were deposited in a reactive gas Ar + 02. With an oxygen partial pressure of 2 x 10 -2 Pa and all other parameters unchanged, the deposition rate of these oxidized films was only 0.2/~m h - '.

Experimental TiAION thin films were deposited by rf magnetron sputtering in a laboratory system (Nordiko NM 1500, 13.56 MHz). The composite target A1203 + 1.5TIN was synthesized by a displacement reaction between TiO2 and AIN powder mixtures as described in ref 3. Steady-state conditions were reached after an initial pre-sputtering of 10 #m, which is several times the grain size of the composite target, allowing for selective sputtering effects. Standard deposition conditions for the black coatingsare given in Table 1. The sputter deposition process was carried out in pure argon gas at a pressure of 4Pa; a power density of 6 W cm 2 resulted in a deposition rate of 0.55/zm h-~. The water cooled substrate electrode, at a floating potential, kept the substrates at temperatures below 100°C. Sapphire discs and stainless steel plates were coated with 2/~m thick films for X-ray diffraction (XRD) and wear resistance measurements. For trans-

Results

Sample characterization. Taking into account that the sputtering process of compounds usually produces significant dissociation, one may expect that deposited species differ from the starting compounds of TiN and Al20 3. In particular, apart from the known stable oxides and nitrides of both metallic elements, metastable compounds like (Ti, AI)N or oxinitrides may form in the films. Although a complete characterization of the observed phases is not available, electron microscopy and diffraction experiments provide some basic structural information of these TiA1ON films. The average chemical composition of both types of films ('black' and 'transparent') have been measured by electron probe microanalysis (EPMA). Trace amounts of Ar were detected in both films as shown in Table 2. For transparent films 2205

R Luthier and F L~vy: TiAION black decorative coatings

Table 2. Chemical compositions (in at%) of 'black' and 'transparent'

TiAION films as measured by EPMA. The nominal composition of the target is also given Element

Target AI203 + 1.5TiN

'black' TiA1ON

'transparent' TiA1ON

Ti A1 O N Ar

18.75 25 37.5 18.75 0

15.2 __+0.2 24.8+0.1 47.0 + 0.5 12.9 _ 0.9 0. I

11.8 + 0.2 21.1 __+0.1 66.9 __+0.5 0 0.2

deposited with a n oxygen partial pressure o f 2 x 10 - 2 Pa, absolutely no nitrogen is incorporated a n d the m e a s u r e d concentrations lead to a composition (Ti, A l ) O 2. The black coatings deposited in pure a r g o n are slightly deficient in Ti a n d N, with a l0 a t % increase in the oxygen content. F r o m published data 4.5 one can deduce that TiN has a higher total yield t h a n

Figure 1. Electron diffraction patterns of 50 nm thin TiAION films at 100keV energy: (a) 'transparent'-type deposited in a reactive gas Ar + O2, (b) 'black'-type deposited in pure Ar.

(220) (222) (400) (420) (422) Figure 2. TEM bright field micrograph of the 'black'-type TiA1ON sample (a), with the corresponding diffraction pattern (b). The overexposure allows to index the outer rings not visible in Figure l(b). 2206

R Luthier and F L~vy: TiAION black decorative coatings

Figure 3. Black TiA1ON sample (as in Figure 2): the individual crystallites contained in the grains are imaged using the (220) diffraction ring in

the dark field mode of the TEM.

A1203, which produces initially an enrichment of the target surface in the lower yield AI203 (selective sputtering). The resulting formation of a relief surface and a seeding of the low yield material onto more recessed high yield TiN grains may lead to the observed enrichment in the film. Both types of thin TiAION films, 50 nm in thickness, were examined by TEM. The electron diffraction patterns at an accelerating voltage of 100 kV are shown in Figure 1. The 'transparent' sample (a) is amorphous with a diffuse structure around the central beam. For the 'black' type (b), sharp diffraction rings appear around the diffuse structure. The ring of highest intensity corresponds to (200) planes of a BI cubic structure, with the outer rings indexed in Figure 2(b) from (220) to (422) planes. The diffracting grains appear in black on the bright field TEM micrograph of Figure 2(a). These crystallized grains have an average size of 10 nm and appear to be embedded in the amorphous matrix (in grey). Crystallites with this cubic structure are imaged in the dark field mode of the TEM using the (220) diffraction ring. Their individual average size is around 3 nm as seen in Figure 3. The grains are polycrystalline and clustering is frequent. Grazing-angle X-ray diffraction (XRD) using Cu-K~ radiation was performed on 2/~m thick samples deposited on sapphire discs. The spectra for both types of films are shown in Figure 4 for diffraction angles 2 0 between 20 and 80 °. For the 'black' TiA1ON sample, the lattice constant of the cubic structure is calculated from the position of the (200) and (220) peaks. Its value is a = 0.413 nm for the as-deposited sample. This does not correspond to any stable phase in the T i - A 1 - O N system. However, the BI cubic structure has been reported for (Ti, A1)N metastable compounds with slightly larger lattice constants. Penttinen e t a l 6 measured a value of 0.417 nm for

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60

80

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Figure 4. XRD spectra (Cu-K=) of as-sputtered TiAION films deposited in pure Ar ('black') and in a reactive atmosphere Ar + 02 ('transparent').

the (Tio.aAlo.5)N compound, whereas Knotek e t a l 7 observed a decrease from 0.424 nm for pure TiN down to 0.416 nm as the atomic ratio increased to 1.2. (Ti, AI)N solid solutions retain a single phase BI structure up to a ratio AI/Ti = 1.38 and for the ratio 1.56 another phase crystallizes8. Most of the published work 9 is related to mononitride compositions, and a possible reduction of the lattice constant for lower N concentrations has never been mentioned. Supposing that all the oxygen detected in the 'black' TiAION coating is present in the amorphous matrix, and that its chemical composition is similar to that of the 'transparent' film, one would deduce the following composition for the crystallized grains: (Tio.4Alo.6)No. 8. However, this argument does not consider a possible mixed oxinitride phase in the matrix material. 2207

R Luthier and F L6vy: TiAION black decorative coatings

Table 3. Observed phases and lattice parameters calculated from the (200) peak of thermally treated 'black' TiAION films (vacuum < 10-3 Pa, duration 5 h) Annealing temperature (°C)

Observed phases

Lattice constant of B1 (nm)

as-sputtered 500 700 900 1000

B1 B1 Bl Bi ~t-AI203, TiO2 (rutile), Bl

0.4134 0.4112 0.4108 0.4094

High temperature stability. Since the observed phases in 'black' TiAION films do not correspond to stable phases, high temperature annealing experiments were performed under vacuum ( < 10 -3 Pa, duration 5 h) up to 1000°C. Modifications of the structure of thermally treated samples were measured by XRD, and are summarized in Table 3. Up to 900°C, the unique crystallized phase has the B1 structure. However, the lattice constant is reduced from 0.413 nm for the as-sputtered sample down to 0.409 nm for the sample treated at 900°C. At 1000°C, two additional phases crystallize as shown in Figure 5: ~t-Al2Oa and TiO2 (rutile), i.e. the most stable oxides of both metals. The shape of the (200) peak of the BI phase is slightly modified due to overlapping with ~t-A1203(l13 ) and TiO:(210) peaks, while the (220) peak is superimposed to weak TiO2 lines. These

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28 Figure 5. XRD spectra (Cu-K=) of annealed 'black' TiAION samples under vacuum for 5 h at 700 and 1000°C.

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Conclusions TiAION thin films have been deposited by rf sputtering using a single AI20 3 + 1.STiN target. The coatings produced in pure Ar had a shiny black colour of acute interest both for their optical and mechanical properties. The present study has shown that this two-phase composite material is stable under vacuum up to 900°C. Crystallized grains of an average size of 10 nm are embedded in an amorphous matrix. The composition of the grains is estimated to be (Tio.4Alo.6)No. s and the (Ti, Al)O2 amorphous matrix crystallizes above 1000°C to give g-A120 3 and TiO2 (futile).

Acknowledgements The targets were produced at the laboratory of ceramics, EPFL, headed by Prof A Mocellin, with great help from T Sperisen. We also wish to thank Prof O Knotek for helpful discussions. This work was financially supported by the Swiss government through a CERS funding. The participation and interest of Asulab SA and ETA SA, both members of the SMH group, are greatly appreciated.

References

oqr o

c

20

results suggest that the amorphous matrix starts to transform at 1000°C, with little effect on the Bl structure of the grains. If the amorphous matrix had the (Ti, A1)O2 composition in the as-deposited state, transformation at 1000°C into TiO2 and A120 3 would lead to an excess O concentration, that could diffuse into the embedded grains.

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