hm 37

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Plansee Seminar HM 37/1

Novel TiAlN Coating by Medium Temperature Low Pressure CVD

R. Pitonak*, A. Kpf*, R. Weienbacher*, J. Keckes**, M. Stefenelli**,

J. Todt**, I. Endler***, M. Hhn***

* BOEHLERIT GmbH & Co KG, 8605 Kapfenberg, Austria

** Department of Materials Physics, Montanuniversitt Leoben, Erich Schmid Institute of Austrian

Academy of Sciences and Materials Center Leoben GmbH, 8700 Leoben, Austria

*** Fraunhofer-Institut of Ceramic Technologies and Systems, 01277 Dresden, Germany

Abstract

Until recently Ti1-xAlxN hard coatings with NaCl structure have been prepared industrially only by PVD

processes, with a maximum x = 0.65. In this work Ti1-xAlxN coatings with x > 0.9 prepared by medium

temperature CVD are described. The structure and the properties of coatings deposited in production

scale and lab-scale are compared. There are remarkable differences. The lab-scale CVD system as well

as the modified production-scale MT-CVD plant use TiCl4, AlCl3, NH3, HCl, N2 and H2 as process gases.

If the lab-scale system is applied nearly pure fcc-Ti1-xAlxN with x=0.91 are obtained. The production scale

Ti1-xAlxN coatings contain the phases fcc-Ti(Al)N, fcc-Al(Ti)N and w-AlN. With HR-TEM the presence of

specific lamellar structures is observed which are confined in areas of a few hundred nm. The structures

consist of alternating w-AlN and fcc-Ti(Al)N lamellae. The lab-scale coatings with the highest fraction of

fcc-Ti1-xAlxN exhibit the highest hardness and Youngs modulus of 40 GPa and 537 GPa, respectively.

The production-scale coatings have also a high hardness between 32 GPa and 36 GPa regardless of

their high fraction of w-AlN. Both lab-scale and production-scale Ti1-xAlxN coatings have moderate

compressive stress. Furthermore the industrial coatings exhibit an extraordinary high oxidation

resistance up to 1.100 C (air / 1h), which is considerably higher than it is known from TiAlN- and even

AlCrN-PVD coatings. Milling tests show remarkable results in cast iron (GJS-600) and also in steel

(1.2311), but in the latter case only when lubricant was used.

Keywords

Medium Temperature Low Pressure CVD, TiAlN Coating, Nano Structure, Oxidation Resistance

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Introduction

Hard nanostructured coatings are extensively used in various engineering applications such as cutting

tools, forming tools or die casting moulds for lifetime and performance enhancement [1]. Consequently,

the protective coatings are expected to withstand a broad range of thermal, mechanical and chemical

loads. In cutting processes, for example, very high friction loads occur cyclically at the tool surface

leading to complex temperature fields with peak temperatures up to 1000C reached within milliseconds

[2,3]. Such high operating temperatures increase the demands on the resistance and the structural

stability of the coatings. One of the possible solutions is the design of complex nanocrystalline and

nanostructured coating systems with dedicated properties and novel architecture [4].

During the last 25 years Ti1-xAlxN tool-coatings, among others, have become one of the most important

wear resistant coatings, due to their outstanding performance in dry machining at high cutting speeds.

Until recently, the deposition of metastable Ti1-xAlxN with NaCl structure, which shows high hardness and

oxidation resistance, was only possible by low temperature methods like PVD [5,6] or PECVD [7,8].

While conventional thermal CVD did not allow the preparation of fcc-Ti1-xAlxN [9], it was shown by Endler

et al. that medium temperature CVD, using NH3 instead of N2 enables the preparation of metastable

Ti1-xAlxN at temperatures below 850C [10]. Recently, a new industrial CVD coating labeled as

TeraSpeed and based on TiN/TiCN/Ti1-xAlxN was introduced by Boehlerit. The coating consists of cubic

and hexagonal Ti1-xAlxN and exhibits superior oxidation resistance as well as cutting performance.

Within the following contribution the novel Ti1-xAlxN coatings which contain a high amount of Al are

characterized in terms of microstructure and oxidation resistance and also some results of the cutting

performance are presented. The results indicate that the extraordinary performance of the coating can

be explained by its nanostructured nature.

Experimental

Coating Deposition

The basic deposition experiments have been performed in a lab-scale CVD equipment for deposition of

Al rich fcc-TiAlN layers. The film was deposited in a horizontal hot wall reactor made of Inconel, which

has an inner diameter of 79 mm and a length of 1.2 m. The CVD process was carried out at a

temperature of 800C and at a pressure of 1.0 kPa. For layer deposition a gas mixture containing 0.9

vol.% AlCl3, 0.2 vol.% TiCl4, 4.5 vol.% NH3, 9.0 vol.% N2, 18.0 vol.% Ar and H2 is used. Ammonia and

the metal chlorides are fed into the reactor by separate gas inlets. The TiAlN layers are deposited

directly on cemented carbide substrates with 6% Co and without an additional interlayer.

The depositions of the Ti1-xAlxN-layers in production-scale were performed in a commercial hot wall

LPCVD plant, using a gas mixture consisting of TiCl4, AlCl3, NH3, HCl, N2 and H2. The vertical Inconel

reactor had an inner diameter of 300 mm. The process temperature was adjusted by heating the reactor

walls using a resistance furnace. Experiments were carried out in the temperature range of 700C -

900C. The above mentioned reaction gases were used with a purity of 99.9%. The purity of the TiCl4

was 99.5% and that of the aluminum chips used for the generation of AlCl3 with HCl was 99.98%,

according to supplier analysis. The substrates used in the production-scale depositions for cutting tests

were commercial P- and K-grade cemented carbide inserts.

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The coating thickness was measured by a calotte section. The microstructure of the coatings was

characterized at a fractograph with field emission scanning electron microscopy (FESEM - NVision40

Carl Zeiss SMT GmbH). Wavelength-dispersive X-ray spectroscopy (WDX-3PC - Microspec

Corporation) was applied to determine the composition and the stoichiometric factor x in Ti1-xAlxN. The

hardness and the Youngs modulus of the layers were measured with the nanoindentation technique

using a Berkovich tip (MTS, Nano-Indenter XP). The method of Oliver and Pharr is applied for data

extraction [11]. The average over 20 measurements was formed. The X-ray diffraction (XRD)

characterization was performed using a Rigaku SmartLab diffractometer applying Cu-K radiation.

Conventional and high-resolution transmission microscopy (HR-TEM) measurements were performed

using Philips TEM CM12 and JEOL 2100F microscopes operated at 120 and 200 kV, respectively. The

JEOL microscope was equipped with an image-side Cs-corrector (CEOS), Gatan imaging filter (Tridiem)

and delivered the atomic resolution of better than 1.4 . The HR-TEM images presented here were

recorded on a 2 k 4 k pixel CCD camera at a magnification of 800.000 using an acquisition time of 1.0

second for each image and under a negative Cs imaging condition.

Results and Discussion

Structure of TiAlN Coatings by Lab-Scale CVD Process

The coating thickness of the deposited layer is about 8.4 m. X-ray diffraction shows that the coating of

the lab-scale process consists of nearly pure fcc-Ti1-xAlxN (see Figure 1(a)). The layer contains only a

small amount of AlN in the wurtzite structure (w-AlN). A SEM image of the fractograph is shown in Figure

1(b). The layer shows a distinct columnar growth.

(a) (b)

Figure 1: Lab-scale TiAlN coating (a) XRD diffractogramm and (b) SEM image of the fractograph.

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Structure of TiAlN Coatings by Production-Scale CVD Process

The coatings deposited in production scale were on one hand a Ti1-xAlxN monolayer coating, mainly for

special analyses and on the other hand a multilayer coating, consisting of a TiN/MT-TiCN ground layer

with about 3 m thickness and a Ti1-xAlxN top layer of about 5 m. Under the brand-name TeraSpeed,

this special layer sequence of about 8 m thickness in total was used for cutting tests.

In order to obtain representative volume-averaged powder diffraction data from the Ti0.05Al0.95N coating

produced by the production scale CVD process, a Ti0.05Al0.95N monolayer with a thickness of 5m was

deposited on steel foil which was then removed chemically. The remaining Ti0.05Al0.95N powder with

random orientation was analyzed and the diffraction data are presented in Figure 2.

The results in Figure 2 indicate a dominant amount of w-AlN and also the presence of cubic TiAlN

phase, documented especially by the (002) peak at 44.31. If the position of the fcc-AlN (002) peak is

expected at 44.4 the slight peak shift indicates that the film possesses a large amount of Ti-doped

fcc-AlN. This phase will be further denoted as fcc-Al(Ti)N. Left and right to the fcc-Al(Ti)N (002) peak,

one can observe very broadened peaks which can be at this stage attributed to fcc-Ti(Al)N and fcc-

Al(Ti)N phases which denote further fcc-TiN and fcc-AlN with unspecified amounts of Al and Ti,

respectively. The remarkable broadening of the fcc-Ti(Al)N (002) peak at 43.23 with the integral breadth

of 2.89 and approximate Scherrer size of 2.8 nm indicates that the apparent size of fcc-Ti(Al)N

coherently diffracting domains along [001] crystallographic direction is relatively small.

Figure 2: Representative powder diffraction data from Ti0.05Al0.95N powder. The vertical lines indicate the peak positions of

selected phases according the JCPDF database.

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In order to obtain more insights into the microstructure of Ti1-xAlxN top layer in the TeraSpeed multilayer

sequence, conventional TEM and HR-TEM analysis were performed. Conventional TEM images

presented in Figure 3 document a presence of very specific lamellar periodic structures which are

confined in areas of a few hundreds nm.

The lamellar structures occurred at various film cross-section positions and their mutual orientation was

obviously uncorrelated. Moreover, TEM analysis showed that a slight sample tilting resulted in the

appearance of new periodic areas and simultaneous extension of the old ones. This suggests that the

lamellar structures were highly organized with very sharp interfaces.

HR-TEM analysis revealed that the lamellae consist of alternating w-AlN and fcc-Ti(Al)N (Figure 4) with a

thickness of about 10 and 3nm, respectively. The orientation relationship between thin and thick lamellae

was determined as (110) fcc-Ti(Al)N || (100) w-AlN. In order to approximately assess the chemical

composition of the lamellae, high-angle annular dark-field imaging (HAADF) was used. The results in

Figure 4b indicate that the thin lamellae consist of heavier atoms in comparison with thicker lamellae.

This correlates well with the presence of w-AlN and fcc-Ti(Al)N identified by HR-TEM. In other words,

Figure 4b unambiguously documents that in thinner lamellae the concentration of Ti and Al is

significantly larger and smaller, respectively, as in the thick lamellae. One can moreover recognize that

the elemental distribution of Al and Ti along the lamellae is not homogeneous.

In addition to nano-lammellar structures described above, isolated regions consisting of w-AlN and fcc-

Al(Ti)N were observed in the Ti0.05Al0.95N films. The presence of fcc-Ti(Al)N was, however, practically

always correlated with the occurrence of the lamellar structure.

(a)

(b)

Figure 3: The conventional TEM patterns indicate the formation of randomly oriented lamellar structures (a) in the Ti0.05Al0.95N

coating with a period of about 13nm (b).

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(a)

(b)

Figure 4: Representative HR-TEM pattern (a) from fcc-Ti(Al)N / w-AlN lamellae with a periodicity of about 13nm and an image

from high angle angular dark field detector sensitive to Z contrast indicate the presence of fcc-Ti(Al)N or w-AlN nanolamellae

with an irregular element distribution.

Composition of TiAlN Coatings by Lab-Scale and Production-Scale CVD Processes

In Table I the results of the WDX measurement for industrial and lab-scale coatings are presented.

Because XRD results show nearly pure fcc-Ti1-xAlxN in case of the lab-scale coating the stoichiometric

factor x may be calculated from WDX. A value x = 0.91 is obtained which proofs that this fcc-Ti1-xAlxN

coating is very Al-rich (Ti0.09Al0.91N). The chlorine content is as low as 0.1 at.%. Both industrial coatings

(Samples TeraSpeed and AlTiN Boe) are also Al-rich but they are phase mixtures of fcc-Ti(Al)N, fcc-

Al(Ti)N and w-AlN and the calculated value x is a integral value.

Table I: Results of WDX measurements and x in Ti1-xAlxN coatings fabricated by lab-scale and industrial CVD processes

Sample Al [at.%] Ti [at.%] N [at.%] O [at.%] Cl [at.%] x

TeraSpeed 47.0 7.0 44.3 0.9 0.8 0.87

AlTiN Boe 48.8 6.5 43.1 1.2 0.4 0.88

TiAlN lab scale 47.4 4.7 46.6 1.2 0.1 0.91

Hardness, Youngs Modulus and Stress of TiAlN Coatings by Lab-Scale and Production-Scale CVD Processes

Both Youngs modulus and hardness measured with Nanoindenter are given in Table II. The TiAlN

coating of the lab-scale process show the highest Youngs modulus of 537 GPa and the highest

hardness of 40.8 GPa. Obviously the best mechanical properties can be achieved by both high

fcc-Ti1-xAlxN fraction and high x. These values exceed the known data of PVD-TiAlN coatings remarkably

w-AlN

fcc-Ti(Al)N

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and show the high potential of CVD-TiAlN. The production-scale coatings have also a high hardness

between 32 GPa and 36 GPa regardless of their high fraction of the softer w-AlN phase.

Table II: Youngs modulus and hardness of TiAlN coatings prepared by lab-scale and industrial CVD processes

Sample Hardness

[GPa]

Youngs modulus

[GPa]

TeraSpeed 36.1 3.7 444 52

AlTiN Boe 32.0 3.1 375 24

TiAlN lab scale 40.8 4.2 537 47

Residual stress is an important property of hard coatings affecting adhesion as well as wear behavior.

The residual stress of the industrial coating was calculated from the dependence of the lattice

parameters determined from the positions of individual diffraction lines on sin2 [12]. A Youngs modulus

of 535 GPa [13] and a Poisssons ratio of 0.3 [12] were used for the stress calculation. In Table III this

value was compared with the residual stress of a lab-scale TiAlN coating of earlier investigations [13].

Surprisingly the CVD-TiAlN coatings exhibit compressive stress which is not typical for CVD coatings.

PVD coatings usually show even higher compressive stress between -1.7 GPa and -5 GPa [14]. It is well

known that moderate compressive stress lowers the crack sensitivity and improves the wear resistance.

Table III: Residual stress of TiAlN coatings prepared by lab-scale [13] and industrial CVD processes

Sample Residual stress

[GPa]

TerraSpeed -0.91

TiAlN lab-scale -0,92 [13]

Oxidation Resistance of TiAlN Coatings by Production-Scale CVD Process

The TeraSpeed multilayer sequence deposited on cemented carbide inserts was heated in air for one

hour at temperatures up to 1100C in order to analyze their oxidation resistance. In Figure 5, optical

images of the oxidized surfaces are presented. The oxidation experiments revealed that the morphology

of the top Ti1-xAlxN coatings did not change significantly for the temperatures up to 1050C. At 1100C

however crater-like features occurred at the coating surface.

A further cross-sectional scanning electron microscopy analysis of the sample heated at 1100C (Figure

5) indicated that the craters were formed by the reaction of the substrate with air which penetrated

probably through the coating into the substrate.

In order to understand the oxidation resistance of TeraSpeed at high temperatures, a HR-TEM analysis

of the surface region of the sample heated at 1050C was performed (Figure 6). The cross-sectional

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analysis revealed a presence of an about 100 nm thick corundum layer which formed on the surface of

the Ti1-xAlxN coating during the treatment in air. The corundum layer obviously protected Ti1-xAlxN from

further degradation in a large temperature range.

As-Deposited 700C 800C

850C 900C 950C

1000 1050C 1100C

Figure 5: Surfaces of oxidized TeraSpeed structures heated for one hour in air.

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Figure 6: HR-TEM image collected from a TeraSpeed sample oxidized at 1050C for one hour. The results reveal the presence

of a protective Al2O3 layer with a thickness of about 100 nm which formed at the coating surface as a consequence of the

oxidation process.

Cutting Tests

The suitability of the new coating for cutting tool applications was determined by milling experiments

using cast iron (grade GGG60) and steel (grade 1.2311). Cast iron was machined dry, while for steel

milling a lubricant was used. The results (Figures 7 & 8) clearly show that AlTiN CVD coatings in both

cases are submitted to a much slower wear than corresponding PVD and even TiCN / Al2O3 CVD

coatings. The reduced wear rate together with a higher resistance against oxidation recommend these

coatings for industrial applications. In fact two milling grades have already been launched at the market.

Ti1-xAlxN

Al2O3

80nm

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0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80

Cutting length (m)

Wid

th o

f w

ear

mark

(

m)

TiAlN PVD

AlCrN PVD

AlCrN + AlCrO PVD

TiCN / AlTiN CVD

TiCN / Al2O3 CVD

Figure 7: Wear propagation during dry milling of cast iron GGG60 using K10 grade cemented carbide inserts OCKX 0606 ADTR

with different coatings (Vc = 210 m/min, ap = 3 mm, fz = 0,3 mm)

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120

Cutting length (m)

Wid

th o

f w

ear

mark

(

m)

AlCrN + TiN PVD

TiCN / AlTiN CVD

Figure 8: Wear propagation during wet milling of 1.2311 steel using P30 grade cemented carbide inserts SNKQ 120520 SN with

different coatings (Vc = 100 m/min, ap = 1 mm, fz = 1,8 mm)

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Conclusion

As presented, it is possible to deposit Ti1-xAlxN coatings with high Al-content (0,85

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References

1. A. Cavaleiro, J. Th. M. De Hosson (Eds.), Nanostructured Coatings, Springer, New York, 2006.

2. H. Schulz, J. Dorr, I.J. Rass, M. Schulze, T. Leyendecker, G. Erkens, Surf. Coat. Technol. 146147

480-485, (2001).

3. A. Persson, S. Hogmark, J. Bergstrm, J. Mater. Process. Technol. 148, 108-118, (2004).

4. C. Mitterer, F. Holler, F. stel, D. Heim, Surf. Coat. Technol. 125, 233-239, (2000).

5. O. Knotek, M. Bhmer and T. Leyendecker, J. Vac. Sci. Technol. A 4 [6], 2695-2700, (1986)

6. W.D. Mnz, J. Vac. Sci. Technol. A 4 [6], 2717-2725, (1986)

7. A. Leonhardt, K. Bartsch and I. Endler, Surf. Coat. Technol. 76-77, 225-230, (1995)

8. S. Anderbouhr, V. Ghetta, E. Blanquet, C. Chabrol, F. Schuster, C. Bernard and R. Madar, Surf.

Coat. Technol. 115, 103-110, (1999)

9. C. Jarms, H.-R. Stock and P. Mayr, Surf. Coat. Technol. 108-109, 206-210, (1998)

10. I. Endler, M. Herrmann, M. Naupert, R. Pitonak, S. Ruppi,, M. Schneider, H. van den Berg and

H. Westphal, Proc. Euro PM2006, Gent, Belgium, 23.-25. October 2006 vol.1, 219, (2006)

11. W.C. Oliver, G.M. Pharr, J. Mater. Res. 7, 1564-1582, (1992)

12. D. Rafaja, T. Merkewitz, C. Polzer, V. Klemm, G. Schreiber, P. Polcik, M. Kathrein, Proceedings 17th

Plansee Seminar, Reutte (2009), Vol. 2, HM38/1-12

13. I. Endler, M. Hhn, M. Herrmann, R. Pitonak, S. Ruppi, M. Schneider, H. van den Berg, H. Westphal,

Surf. Coat. Technol. 203, 530-533, (2008)

14. M. Ahlgren, H. Blomqvist, Surf. Coat. Technol. 200, 157-160, (2005)

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