Decomposition and phase transformation in TiCrAlN thin coatingsRikard Forsén, Mats Johansson, Magnus Odén, and Naureen Ghafoor Citation: Journal of Vacuum Science & Technology A 30, 061506 (2012); doi: 10.1116/1.4757953 View online: http://dx.doi.org/10.1116/1.4757953 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/30/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Coherency strain engineered decomposition of unstable multilayer alloys for improved thermal stability J. Appl. Phys. 114, 244303 (2013); 10.1063/1.4851836 Thermally enhanced mechanical properties of arc evaporated Ti 0.34 Al 0.66 N / TiN multilayer coatings J. Appl. Phys. 108, 044312 (2010); 10.1063/1.3463422 Pressure enhancement of the isostructural cubic decomposition in Ti 1 − x Al x N Appl. Phys. Lett. 95, 181906 (2009); 10.1063/1.3256196 Influence of the bias voltage on the structure and mechanical performance of nanoscale multilayer Cr Al Y N ∕ CrN physical vapor deposition coatings J. Vac. Sci. Technol. A 27, 174 (2009); 10.1116/1.3065675 Growth and characterization of Ti Al N ∕ Cr Al N superlattices prepared by reactive direct current magnetronsputtering J. Vac. Sci. Technol. A 27, 29 (2009); 10.1116/1.3013858

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Decomposition and phase transformation in TiCrAlN thin coatings

Rikard Fors�ena)

Nanostructured Materials, Department of Physics, Chemistry and Biology (IFM), Link€oping University,581 83 Link€oping, Sweden

Mats JohanssonNanostructured Materials, Department of Physics, Chemistry and Biology (IFM), Link€oping University,581 83 Link€oping, Sweden and Seco Tools AB, 73782 Fagersta, Sweden

Magnus Od�en and Naureen GhafoorNanostructured Materials, Department of Physics, Chemistry and Biology (IFM), Link€oping University,581 83 Link€oping, Sweden

(Received 19 July 2012; accepted 24 September 2012; published 9 October 2012)

Metastable solid solutions of cubic (c)-(TixCryAlz)N coatings were grown by a reactive arc

evaporation technique to investigate the phase transformations and mechanisms that yield enhanced

high-temperature mechanical properties. Metal composition ranges of y< 17 at. % and 45< z< 62

at. % were studied and compared with the parent TiAlN material system. The coatings exhibited age

hardening up to 1000 �C, higher than the temperature observed for TiAlN. In addition, the coatings

showed a less pronounced decrease in hardness when hexagonal (h)-AlN was formed compared to

TiAlN. The improved thermal stability is attributed to lowered coherency stress and lowered

enthalpy of mixing due to the addition of Cr, which results in improved functionality in the

temperature range of 850–1000 �C. Upon annealing up to 1400 �C, the coatings decompose into

c-TiN, bcc-Cr, and h-AlN. The decomposition takes place via several intermediate phases: c-CrAlN,

c-TiCrN, and hexagonal (b)-Cr2N. The evolution in microstructure observed across different stages

of spinodal decomposition and phase transformation can be correlated to the thermal response and

mechanical hardness of the coatings. VC 2012 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4757953]

I. INTRODUCTION

TiN coatings have been used for surface protection in var-

ious applications since the early 1980s. In the mid-1980s,

studies showed improved oxidation resistance, hardness, and

thermal stability when Al was added to TiN.1,2 Further stud-

ies of arc-evaporated TiAlN have shown that the coating can

retain its hardness up to 950 �C.3,4 The observed increase in

hardness upon annealing of TiAlN coatings has been associ-

ated with the formation of coherent nanometer-sized

domains of c-AlN and c-TiN through spinodal decomposi-

tion.3,5 The size of these domains becomes larger upon fur-

ther exposure to elevated temperatures until the c-AlN

transforms to h-AlN, at which point the coherency is lost and

the mechanical properties of the coating deteriorate.6,7 Dur-

ing high-speed metal cutting of hard materials, temperatures

may reach 1000 �C,8,9 and as the demands for better per-

forming coatings are continuously increasing, i.e., ones that

can withstand higher cutting speed, coatings that retain their

hardness at even higher temperatures are needed. Successful

attempts to increase the thermal stability of TiAlN have

recently been made by adding a third metal to the material

system.10,11 Another approach was based on a concept of

multilayer growth, where the coating architecture affects the

decomposition process and yields improved cutting perform-

ance.12,13 Previous studies indicate that coating architectures

containing TiCrAlN can outperform TiAlN coatings. For

example, multilayers of TiAlCrN with TiAlN or CrN have

been shown to have a lower wear rate during dry sliding

tests14 and to suppress the formation of h-AlN.15,16 High Cr

(40–60 at. % metal ratio)17,18 and also high Al (�70 at. %

metal ratio)19 single layer TiAlCrN coatings are known to

exhibit greater hardness and lower wear rates compared to

ternary TiAlN. For TiAlCrN coatings containing high Al

ratios (65–70 at. %), it has been suggested that the improved

mechanical properties are due to the altered bond states

caused by the addition of �20 at. % Cr.19 However, since

the TiAlCrN solid solution is unstable,20 other factors also

must be contributing to the improved properties seen at ele-

vated temperatures. We show in this paper that the micro-

structure evolution also improves the properties during

phase decomposition and that this evolution can be affected

by the degree of alloying.

Our hypothesis for this work is that by adding relatively

small amounts of Cr to c-TiAlN, the driving forces for spino-

dal decomposition and its kinetics could be favorably altered.

If the migration mechanism during the decomposition is sub-

stitutional diffusion on the metal sublattice, the presence of

Cr should obstruct the segregation of Ti and Al because of its

relatively favorable miscibility and large atomic size. Conse-

quently, the transformation of c-AlN to h-AlN would be sup-

pressed, according to previously reported theoretical work.16

We have recently published theoretical predictions support-

ing our hypothesis related to the local tendencies for spinodal

decomposition in c-TiCrAlN. The results show that the cur-

vature of the mixing free energy is altered depending on thea)Electronic mail: rikfo@ifm.liu.se

061506-1 J. Vac. Sci. Technol. A 30(6), Nov/Dec 2012 0734-2101/2012/30(6)/061506/8/$30.00 VC 2012 American Vacuum Society 061506-1

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Cr content.20 Thus, the driving force for spinodal decomposi-

tion is changed both in magnitude and direction. These calcu-

lations were also supported by some initial experimental

results. Herein, we present in a detailed manner a more sub-

stantiated verification of the nanometer-sized compositional

modulations and the route of phase transformations occurring

in TiCrAlN. We also extend our previous study to include

higher annealing temperatures beyond the spinodal decompo-

sition stage and experimentally verify the delayed formation

of h-AlN for a wider range of alloy compositions and eventu-

ally products related to the loss in hardness. We discuss the

structural evolution, mechanical properties, and the thermal

responses of as-deposited coatings and coatings annealed at

temperatures up to 1400 �C based on results obtained with

high resolution transmission electron microscopy (TEM), an-

alytical scanning transmission electron microscopy (STEM),

x-ray diffractometry (XRD), nanoindentation, and differential

scanning calorimetry (DSC).

II. EXPERIMENT DETAILS

The coatings were deposited by an industrial Sulzer/Meta-

plas MZR-323 reactive cathodic arc evaporation system using

compound cathodes in a pure N2 atmosphere onto polished

WC-Co substrates and Fe foils mounted on a rotating drum

fixture. Details about the system have been described else-

where.12 In this work, three circular cathodes, each 63 mm in

diameter, with compositions of Ti29Cr5Al66, Cr33Al67, and

Ti50Al50, respectively, were vertically mounted in one line

and equidistantly separated by 15 cm on one of the cathode

flanges of the deposition chamber. In this setup, a varying

deposition flux is obtained over the height of the drum fixture,

providing a gradient in coating composition depending on

where the substrate is placed in front of the cathodes. The

WC-Co substrates and Fe foils were equidistantly positioned

in five rows along the full height of the drum fixture at a

cathode-to-substrate distance of about 15 cm.

Before deposition, the substrates were cleaned in ultrasonic

baths of an alkali solution and alcohol. The system was then

evacuated to a pressure of less than 2.0� 10�3 Pa, after which

the substrates were sputter cleaned with Ar ions. All coatings

in this study were obtained in the same deposition run using a

cathode current of 60 A in 4.5 Pa of N2. A deposition run of

2 h using a drum rotation of 3 rpm, a fixed substrate bias of

�40 V, and a substrate temperature of approximately 500 �Cresulted in a coating thickness of �3 lm.

The compositions of the coatings were determined using

a combination of elastic recoil detection analysis (ERDA)

and energy-dispersive x-ray spectroscopy (EDX). ERDA

measurements utilized an 127I9þ ion beam accelerated to

40 MeV with an incidence angle of 22.5�. A time-of-flight

and energy detector (TOF-E ERDA) was used for detection

of the ejected species. EDX analysis was performed with a

Leo 1550 Gemini scanning electron microscope operated at

20 kV and a working distance of 10 mm. The concentration

ratio between nitrogen, aluminum, and the sum of titanium

and chromium, N:Al:(TiþCr), was obtained with ERDA.

To establish the Ti:Cr ratio, EDX was used instead of

ERDA due to the similar mass of titanium and chromium.

The yielded compositions in this study ranged from

(TixCryAlz)1N1 0.31< x< 0.54, 0.01< y< 0.17, and 0.45

< z< 0.62 (xþ yþ z¼ 1).

Postdeposition isothermal annealing was performed at

Tmax¼ 700, 800, 850, 900, 1000, 1050, and 1100 �C for 2 h

in an argon atmosphere at atmospheric pressure using a Sin-

tervac furnace from GCA Vacuum Industries. The samples

were annealed with a rate of 7 �C/min up to 40 �C below the

final annealing temperature, Tmax, and then decreased to a

rate of 5 �C/min. The samples were eventually cooled down

to 500 �C over a 1.5-h period and cooled to 100 �C over a

4-h period.

X-ray h-2h diffractograms with a 2h range from 20� to

80� were recorded with a Bruker AXS D8-advanced x-ray

diffractometer using Cu-Ka radiation.

Transmission electron microscopy, scanning transmission

electron microscopy, and x-ray energy dispersive spectros-

copy were carried out with a FEI Tecnai G2 TF 20 UT

microscope operating at 200 kV. STEM Z-contrast imaging

was conducted using a high angular annular dark field detec-

tor with a camera length of 170 mm. Cross-sectional TEM

samples were prepared by means of mechanical grinding and

polishing followed by Ar-ion beam milling until electron

transparency was achieved.

The hardness of the coatings was measured using an

UMIS nanoindenter equipped with a Berkovich diamond tip.

Indentation was performed on mechanically ground and pol-

ished tapered cross-sections of the coatings (taper angle of

�10�). The contact area of the tip versus the penetration

depth was calculated using the elastic modulus for fused

silica as reference. The average hardness 61 standard devia-

tion was extracted21 from approximately 30 indents on each

sample using a maximum load of 40–50 mN. The indentation

depths were around 200 nm (less than 10% of the coating

thickness).

The thermal response from the coatings was measured

using a Netzsch STA 410 differential scanning calorimeter.

DSC samples were prepared by removal of coated Fe foils

through mechanical grinding and subsequent dissolution in

concentrated HCl (37%). The remaining coating was filter

cleaned with acetone and crushed to a fine powder. EDX

analysis of the powder showed no compositional change after

this process. Approximately �50 mg of powder was placed

in an Al2O3 crucible, and before commencing the annealing,

it was out-gassed at 250 �C and 50 mPa for 12 h. Annealing

was conducted by heating the powder up to 1400 �C at a rate

of 20 �C/min under a He flow of 50 ml/min. As soon as

1400 �C was reached, the sample was cooled to room temper-

ature and the annealing was repeated. The second heating

cycle was used for an appropriate baseline correction of the

thermal response in the first cycle.

III. RESULTS

Here we present results from TixCryAlzN coatings cover-

ing metal composition ranges of y< 17 at. % and

45< z< 62 at. % where all samples contain 50 6 1%

061506-2 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-2

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nitrogen in their as-deposited state. In this section, we have

selected data from different compositions in order to present

both similarities and dissimilarities of coatings with high and

low Cr content.

A. Microstructure and compositional modulationsafter annealing

Figure 1 shows x-ray diffractograms in the 2h range of 32�

to 45� obtained for Ti0.31Cr0.07Al0.62N and Ti0.31Cr0.17Al0.52N

coatings in their as-deposited state, after isothermal annealing

at different temperatures for 2 h and heating to 1400 �C with a

heating rate of 20 �C/min. For the as-deposited state, two dif-

fraction peaks at 2h¼ 37.5� and 43.2� are identified as cubic

c-TiCrAlN 111 and 200, respectively, while no other phases

are observed. At 800 �C and 900 �C, the peaks observed for

Ti0.31Cr0.07Al0.62N have split into two peaks, corresponding

to Ti- and Al-rich c-TiCrAlN, respectively [c-CrN has a lat-

tice spacing of 4.196 A (Ref. 22), c-AlN 4.1 A, and c-TiN

4.24 A (Ref. 23)]. At 1000 �C, a third peak corresponding to

h-AlN 100 is detected at 33.2�, and the peak split is more

distinct. At 1100 �C, the intensity of the h-AlN 100 peak is

stronger, whereas the c-AlN peaks have almost vanished.

For Ti0.31Cr0.17Al0.52N, the peak split occurs at a higher

temperature of 1000 �C where a weak h-AlN 100 reflection

is also present. In contrast to Ti0.31Cr0.07Al0.62N, the c-AlN

peaks remain up to 1100 �C.

The peaks located at 2h¼ 35.1� and 40.2� belong to h-Ti

100 and 101, respectively. This phase originates from Ti-

rich droplets, which is a frequent observation in arc evapo-

rated coatings. The substrate peak at 2h¼ 34.2� (labeled S*)

is a duplicate of the substrate peak at 2h¼ 35.6� originating

from an artifact (tungsten radiation) associated with the

diffractometer.

The coated WC-Co substrates did not withstand anneal-

ing temperatures above 1100 �C without reacting with the

coating. Therefore, annealed powder samples were meas-

ured to verify the existing phases at higher temperatures

(see Sec. II regarding DSC powder sample preparation and

heating parameters). The upper part of Fig. 1 contains x-ray

diffractograms obtained from Ti0.31Cr0.07Al0.62N and

Ti0.31Cr0.17Al0.52N powder samples heated to 1400 �C. The

results show that the existing phases are h-AlN, c-TiN, and

bcc-Cr. The diffraction peak at 2h¼�36.7� obtained from

Ti0.31Cr0.17Al0.52N is slightly shifted towards higher angles

compared to Ti0.31Cr0.07Al0.62N, suggesting that the phase

is not pure c-TiN, but rather c-Ti(Cr)N where (Cr) indicates

a low Cr content.

Figure 2 shows cross-sectional TEM micrographs from

Ti0.31Cr0.07Al0.62N after annealing at different temperatures.

The micrograph obtained from the as-deposited state in Fig.

2(a) shows diffraction contrast within the columnar grains, in-

dicative of strain typically originating from crystal defects.

The high resolution image and the corresponding selected

area electron diffraction pattern obtained along the h011izone axis in Fig. 2(b) reveals a single phase coherent cubic

lattice. Figures 2(c) (800 �C) and 2(d) (900 �C) show

decreased diffraction contrasts within the grains compared to

the as-deposited state, consistent with defect annihilation

upon annealing. This is a frequent observation during anneal-

ing of arc evaporated coatings.24 Figures 2(e) (1000 �C) and

2(f) (1100 �C) show that higher annealing temperatures have

resulted in randomly oriented small crystallites. At 1100 �C of

annealing, the size of the crystallites has grown compared to

that at 1000 �C.

The compositional modulations resulting from annealing

are presented in Figs. 3(a)–3(e). Figures 3(a)–3(d) depict

100� 100 nm overlaid color-coded EDX maps of Ti, Al, and

Cr after the samples were annealed isothermally at different

temperatures for 2 h. Maps from the as-deposited state (not

shown) look similar to the sample annealed at 800 �C in Fig.

3(a). EDX results show layers containing different elemental

composition in the as-deposited state, at 800 �C and 900 �C.

The layering is an inherit artifact25 due to the sample drum

rotation and an inhomogeneous distribution of the depositing

species in the plasma. Figure 3(b) (900 �C) shows separate

Ti- and Al-rich �3 nm sized domains while the Cr-

distribution appears unaffected by the annealing. At

1000 �C, [see Fig. 3(c)], the Al- and Ti-rich domains are

FIG. 1. (Color online) X-ray diffractograms of Ti0.31Cr0.07Al0.62N (black) and

Ti0.31Cr0.17Al0.52N (red). The upper part contains diffractograms of powder

samples heated to 1400 �C (same heating procedure as for DSC). The lower

part contains diffractograms obtained from coated WC-Co substrates in as-

deposited state and after annealing at temperatures between 800 and 1100 �C.

061506-3 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-3

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enlarged to �5 nm, and Cr shows preferential location to Ti-

rich domains. At 1100 �C [see Fig. 3(d)], the domains have

grown further to �20 nm, and Cr shows stronger preferential

location to Ti-rich domains. Thus, at 1100 �C, there are two

different domains containing TiCr- and Al-enriched

TiCrAlN. The wavelength of the compositional modulation

is between 40 and 60 nm [see Fig. 3(e)].

Figure 4(a) shows an overview Z-contrast STEM micro-

graph of Ti0.31Cr0.17Al0.52N annealed at 1100 �C. The com-

positional modulations that are present within the grains and

close to the grain boundaries are depicted in the insets. Fig-

ure 4(b) contains 75� 75 nm overlaid color-coded EDX

maps of Ti, Al, and Cr. It can be seen that at this temperature

TiCr- and Al-enriched TiCrAlN are the existing phases. Fig-

ure 4(c) shows a line scan obtained from the grain interior.

The scan exhibits compositional modulation with a wave-

length between 15 and 20 nm, which is considerably shorter

than the Ti0.31Cr0.07Al0.62N alloy. Line scans across grain

boundaries (dark regions) show that they are AlN-enriched.

Figure 5(a) contains a lattice-resolved micrograph of

Ti0.31Cr0.07Al0.62N annealed at 1100 �C. The two upper right

insets show results from EDX mapping of the same area indi-

cating the locations of the Al- and TiCr-rich domains. From

the lattice-resolved micrograph and the elemental mapping, it

is revealed that even at a high temperature of 1100 �C the lat-

tice across the TiCr- and Al-rich TiCrAlN domains remains

mainly coherent. The lower right inset in Fig. 5(b) shows a

lattice-resolved micrograph of Ti0.31Cr0.07Al0.62N annealed at

1000 �C and its Fourier-transform. At 1000 �C, only coherent

domain boundaries were found. For Ti0.31Cr0.17Al0.52N, the

lattice remains coherent across the Al- and TiCr-rich domains

up to 1100 �C. In contrast, at the grain boundaries where

h-AlN precipitation takes place, the lattice is incoherent.

FIG. 2. TEM overview micrographs of Ti0.31Cr0.07Al0.62N in as-deposited

state and after annealing at different temperatures for 2 h.

FIG. 3. (Color online) Overlaid color-coded EDX maps of Ti0.31Cr0.07Al0.62N

obtained after annealing at 800–1100 �C (a)–(d) and a 200 nm EDX line scan

measured after annealing at 1100 �C (e).

061506-4 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-4

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B. Mechanical properties and thermal response

Figure 6 contains hardness measurements versus annealing

temperature for Ti0.52Cr0.01Al0.47N, and Ti0.31Cr0.17Al0.52N,

and published hardness data20 for the Ti0.31Cr0.07Al0.62N coat-

ing studied in this work. As a reference, the hardness of

Ti0.33Al0.67N grown under similar conditions is also plotted.12

The results reveal an age hardening3,4 effect similar to what is

reported for TiAlN.5,26 The effect is not observed for

CrAlN.27 However, the TiCrAlN coatings show a less pro-

nounced age hardening effect but also a less pronounced hard-

ness decrease at temperatures above the peak hardness

compared to the ternary system.12 Ti0.52Cr0.01Al0.47N shows

significant age hardening when annealed at 1000 �C and a

retained maximum hardness between 800 �C and 1000 �C.

Ti0.31Cr0.07Al0.62N retains its maximum hardness between

850 �C and 1000 �C. Ti0.31Cr0.17Al0.52N shows a continuous

hardness increase up to 1000 �C. At 1100 �C, the coatings

have roughly the same hardness with the exception of

Ti0.31Cr0.17Al0.52N, which is slightly harder. Above 1000 �C,

the hardness of Ti0.33Al0.67N is significantly lower compared

to the Cr-containing coatings.

Figure 7 shows the thermal response of powder

Ti0.52Cr0.01Al0.47N, Ti0.31Cr0.07Al0.62N, and Ti0.31Cr0.17Al0.52N

during heating up to 1400 �C at a rate of 20 �C/min. As a ref-

erence, the thermal response of Ti0.33Al0.67N (Ref. 12)

grown under similar conditions is also plotted. Six peaks

have been identified, where the first four peaks (denoted T1,

T2, T3, and T4) are related to exothermic reactions. Peaks T1

at �500 �C and T2 at �800 �C have previously been assigned

to defect annihilation processes, whereas T3 at �1000 �C andFIG. 4. (Color online) (a) Shows an overview Z-contrast STEM micrograph

of Ti0.31Cr0.17Al0.52N annealed at 1100 �C. (b) Contains overlaid color-

coded EDX maps and (c) a 100 nm EDX line scan obtained from a region

within a grain.

FIG. 5. (Color online) (a) TEM micrograph from Ti0.31Cr0.07Al0.62N annealed

at 1100 �C for 2 h. Two upper right insets show overlaid color-coded EDX

maps of the same area, Al (green), Ti (red), and Cr (blue). The lower right

inset (b) shows a lattice-resolved micrograph of Ti0.31Cr0.07Al0.62N annealed

at 1000 �C and its Fourier-transform.

FIG. 6. (Color online) Nanoindentation measurements from Ti0.33Al0.67N

(Ref. 12), Ti0.52Cr0.01Al0.47N, Ti0.31Cr0.07Al0.62N, and Ti0.31Cr0.17Al0.52N of

the hardness vs annealing temperature.

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T4 at �1200 �C are related to spinodal decomposition and the

transformation of c-AlN into h-AlN, respectively.5,26 The

endothermic peak T5 at �1300 �C originates from nitrogen

release,27 which is not observed for Ti0.52Cr0.01Al0.47N. The

exothermic peak T6 at �1350 �C originates from the transfor-

mation of b-Cr2N into bcc-Cr,27 and this peak also is not

observed in Ti0.52Cr0.01Al0.47N. At �1400 �C, there is an

endothermic reaction in Ti0.31Cr0.17Al0.52N, whereas for

Ti0.31Cr0.07Al0.62N and Ti0.52Cr0.01Al0.47N the trend is flat

(no reaction). On the right axis in Fig. 7, the relative mass

change is plotted versus temperature. The mass of

Ti0.52Cr0.01Al0.47N decreases �0.2% at �400 �C. Thereafter,

it is more or less stable up to �1050 �C. For all three compo-

sitions, the mass starts to decrease at �1050 �C and continues

decreasing up to 1400 �C. The mass loss rate increases for

Ti0.31Cr0.07Al0.62N and Ti0.31Cr0.17Al0.52N at �1300 �C. The

N release at 1400 �C is higher for coatings with higher Cr

content and Ti0.52Cr0.01Al0.47N has lost �1% of its initial

mass, Ti0.31Cr0.07Al0.62N �2%, and Ti0.31Cr0.17Al0.52N �3%.

It can also be seen that the transformation of c-AlN into

h-AlN, (T4), occurs earlier for the coatings with higher Al

content and the spinodal decomposition, (T3), occurs later

with higher Cr content.

IV. DISCUSSION

Based on the hardness and thermal response results, it is

clear that addition of Cr in TiAlN has a positive effect on the

high-temperature mechanical properties. The temperature

region where the coatings retain their hardness is much

wider compared to ternary TiAlN12 and CrAlN,28 which

today are the dominating nitride-based coatings used for

high-temperature wear protection. The comparable thermal

response—i.e., the signatures in terms of peaks T1–T4 of the

Cr containing coatings and ternary Ti0.33Al0.67N (Ref. 12)

grown under similar conditions—implies the same underly-

ing decompositions. Peaks T1 and T2 are related to crystal re-

covery and defect annihilation.12 For the Cr-containing

coatings, both peaks T1 and T2 are located at temperatures

roughly 100–150 �C higher compared to Ti0.33Al0.67N. Peak

T3, which is related to the spinodal decomposition, is located

at a slightly higher temperature for Ti0.52Cr0.01Al0.47N com-

pared to Ti0.33Al0.67N,12 and increasingly higher for

Ti0.31Cr0.07Al0.62N and Ti0.31Cr0.17Al0.52N. This is to be

expected based on our theoretical predictions20 that the driv-

ing force for spinodal decomposition is lowered through Cr

addition, i.e., the second derivative of the Gibbs free energy

is less negative for these compositions. Peak T4, which

marks the transformation of c-AlN into h-AlN, is both de-

pendent on Cr and Al content, making comparison more dif-

ficult. However, for Ti0.31Cr0.07Al0.62N, the peak (T4) is

located at �1250–1300 �C and the corresponding peak for

the ternary system, Ti0.33Al0.67N, with roughly the same Al

content is located at �1100 �C. Thus, an adequate addition

of Cr can be utilized for improved thermal stability of the c-

AlN phase as an alternative to lowering the Al content. One

consequence of Cr addition is the reduced coherency strain22

caused by the Ti and Al segregation, which explains the less

pronounced hardening (see Fig. 6) compared to the ternary

system TiAlN. At the onset of spinodal decomposition, pres-

ence of Cr in both Ti- and Al-rich TiCrAlN domains reduces

the lattice mismatch between the coherent domains in com-

parison to not having Cr present. This is confirmed in our

overview and elemental TEM analysis. There is an initial

reduction in lattice defect concentration when the coatings

are annealed at 900 �C. However, at the same temperature,

Ti and Al have begun to segregate, while no segregation of

Cr is observed. After annealing at 1000 �C, the domains

remain coherent, and Cr preferably relocates to Ti-rich

domains as predicted by theory.20 Despite the XRD showing

traces of h-AlN at 1000 �C, there is no dominant detrimental

effect on the hardness. This is in agreement with previous

experiments showing the stabilization of c-(Cr)AlN-phase

over h-AlN (Ref. 15) by adding Cr and that the formation of

semicoherent h-AlN and c-TiAlN domains have a positive

effect on the hardness.29 At 1100 �C, the TiCr-rich TiCrAlN

domains further increase in size accompanied by larger

grains of purer h-AlN. The transformation from c-AlN to h-

AlN occurs when Cr and Al segregate, resulting in a nearly

pure h-AlN-phase. Because the coherency is only partially

lost at this stage, the observed hardness decrease is less pro-

nounced compared to ternary TiAlN.12

We conclude that up to 1100 �C annealing temperatures

(TixCryAlz)1N1 y< 0.17 at. % and 0.45< z< 0.62 at. %

coatings undergo decompositions and phase transformations

according to these expressions:

c-TiCrAlNa:d:

! c-TiCrðAlÞNþ c-ðTiÞCrAlN900 �C

! c-TiCrðAlÞNþ c-ðTiCrÞAlNþ h-AlN1000 �C

! c-TiCrðAlÞNþ h-AlN1100 �C

:

FIG. 7. (Color online) Thermal responses measured with differential scanning

calorimetry of Ti0.31Cr0.17Al0.52N, Ti0.31Cr0.07Al0.62N, Ti0.52Cr0.01Al0.47N,

and Ti0.33Al0.67N (Ref. 12) using a heating rate of 20 �C/min. The upper part

exhibits the relative mass change (right axis) during the heating for each Cr-

containing coating, respectively.

061506-6 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-6

J. Vac. Sci. Technol. A, Vol. 30, No. 6, Nov/Dec 2012

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Our prediction20 of a lower driving force for spinodal

decomposition with higher Cr content is confirmed by the

thermal responses showing that the decomposition of

c-TiCrAlN into c-TiCrN and c-CrAlN is shifted to higher

temperatures with higher Cr content. Theoretically, a lower

driving force for spinodal decomposition leads to a larger

dominant compositional wavelength.30 However, a lower

driving force also extends the time duration of the spinodal

decomposition delaying the coarsening process. In addition,

there also is a kinetic effect in which the addition of Cr

affects the diffusion. The coarsening process taking place af-

ter the spinodal decomposition is driven by the minimization

of the gradient energy, which is rate controlled by the diffu-

sion. For higher Cr content, diffusion is expected to be

obstructed and decreased, resulting in a slower coarsening

rate. This is confirmed by the TEM analysis of annealed

Ti0.31Cr0.17Al0.52N coatings showing that the domain size at

1000 and 1100 �C is smaller compared to the coatings with

less Cr. Results obtained with XRD from the Cr-containing

coatings with higher Cr content show the presence of

c-(Cr)AlN at 1100 �C in contrast to the coatings with less Cr.

Due to the smaller domain sizes in the coatings with higher

Cr, the coherency is maintained up to higher temperatures

suppressing the transformation of c-AlN to h-AlN. The

smaller domain size also counteracts relaxation of semico-

herent h-AlN domains, which is beneficial in terms of

hardness.29

Since the WC-Co substrates did not withstand annealing

temperatures higher than 1100 �C results from TEM could

not be obtained. Thus, to establish the decomposition route

beyond 1100 �C, the conclusions are based on the results

obtained from in-situ DSC/TGA and ex-situ XRD measure-

ments using coating powder (see Sec. II). Peak identification

of the thermal response of the ternary systems CrAlN and

TiAlN has been reported previously.12,27 In our quaternary

system, TiCrAlN, traces from all the reactions occurring in

the ternary system may occur and new exothermic or endo-

thermic peaks could also arise.

In agreement with previous studies on CrAlN,27 the reason

for the endothermic reaction T5 is the release of chromium-

bonded nitrogen, and the exothermic reaction T6 is caused by

the transformation of b-Cr2N to bcc-Cr. For the coating with

low Cr content, Ti0.52Cr0.01Al0.47N, T5, and T6 are not

observed, and the minor mass loss observed at �400 �C is

most likely due to H2O out-gassing rather than nitrogen

release. For Ti0.31Cr0.07Al0.62N and Ti0.31Cr0.17Al0.52N, the

mass decrease is initiated at around 1050 �C. No endothermic

reaction is however observed until peak T5 at around

1300 �C. The relatively low Cr-concentration in the samples

results in only weak endothermic DSC reactions and instead

the co-occurring exothermic spinodal decomposition and

transformation of c-AlN into h-AlN overlap and dominate

the signals up to �1300 �C.

The nitrogen release between �1050 and �1300 �C is

�1% for both Ti0.31Cr0.07Al0.62N and Ti0.31Cr0.17Al0.52N. For

Ti0.31Cr0.07Al0.62N, this amount of N release is consistent

with a complete transformation of CrN to Cr2N. Around

1300 �C, the rate of the mass loss increases, and in agreement

with studies on CrAlN,27 this is because above this tempera-

ture more N is released from the b-Cr2N phase rather than

from the preceding c-CrN phase.

Results from Cr0.30Al0.70N (Ref. 27) show that the N

release starts at a lower temperature, �925 �C, and occurs

with a higher rate compared to the quaternary coatings stud-

ied in this work. We propose that due to the presence of Ti

atoms in the close vicinity of the chromium-bonded nitrogen,

a higher energy barrier for the formation of a N-vacancy is

to be expected, which then would explain the higher temper-

ature required for Cr2N decomposition. From the XRD

results at 1400 �C, it can be seen that c-TiN is slightly shifted

toward c-Ti(Cr)N. Whether the c-TiCrN solid solution

remains stable during the N release up to 1400 �C or if it

decomposes into c-TiN and c-CrN prior to b-Cr2N formation

remains an open question.

When the temperature is increased up to 1400 �C, the

total observed mass loss is �2% for Ti0.31Cr0.07Al0.62N, sig-

nifying that the b-Cr2N phase is completely N-depleted. For

Ti0.31Cr0.17Al0.52N, the rate of the mass loss is higher above

1300 �C, but the observed mass loss of �3% suggests that

the b-Cr2N phase is not entirely N-depleted. The abrupt

shape changes of the thermal responses close to 1350 �C for

Ti0.31Cr0.07Al0.62N and 1375 �C for Ti0.31Cr0.17Al0.52N are

caused by the exothermic transformation of b-Cr2N into

bcc-Cr.27 Because the b-Cr2N phase is not yet fully

N-depleted for Ti0.31Cr0.17Al0.52N, the N release continues

and dominates the signal over the exothermic bcc-Cr forma-

tion between �1375 and 1400 �C.

The DSC powder samples are heated to 1400 �C during a

second heating cycle used for the background subtraction.

During the second heating cycle, more N was released for

Ti0.31Cr0.17Al0.52N with a total mass loss of �5% after the

second cycle corresponding to an almost completely

depleted b-Cr2N phase. The XRD measurements at 1400 �Cwere measured after the second heating cycle and therefore

show presence of bcc-C and no traces of b-Cr2N. In sum-

mary, we conclude that the decomposition continues from

1100 �C according to these expressions:

c-TiCrðAlÞNþ h� AlN1100 �C

! c-TiðCrÞNþ b-Cr2Nþ h-AlNþ N2

1300 �C

! c-TiðCrÞNþ bcc-Crþ h-AlNþ N2

1400 �C

:

V. CONCLUSIONS

The decomposition products and the intermediate states up

to 1400 �C of TixCryAlzN coatings with low Cr and high Al

content, in the range of y< 17 at. % and 45< z< 62 at. %,

were investigated and their good mechanical properties are

reported.

In agreement with our theoretical predictions,20 these coat-

ings decompose initially into Ti-rich c-TiCrAlN domains and

into metastable Al-rich c-TiCrAlN domains. The initial segre-

gation is followed by further decomposition into c-Al(Cr)N

domains with low Cr content and c-TiCr(Al)N domains with

061506-7 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-7

JVST A - Vacuum, Surfaces, and Films

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low Al. The decomposition continues into c-TiN, bcc-Cr, and

h-AlN. If the Cr concentration is increased (y¼ 17 at. %), the

domain sizes are smaller compared to compositions with

lower Cr concentrations (y< 8 at. %). The following expres-

sions are a summary of the decomposition route:

c-TiCrAlNa:d:

! c-TiCrðAlÞNþ c-ðTiÞCrAlN900 �C

! c-TiCrðAlÞNþ c-ðTiCrÞAlNþ h-AlN1000 �C

! c-TiCrðAlÞNþ h-AlN1100 �C

! c-TiðCrÞNþ b-Cr2Nþ h-AlN1300 �C

! c-TiðCrÞNþ bcc-Crþ h-AlN1400 �C

:

Through small amounts of Cr additions to ternary TiAlN,

the transformation of cubic c-AlN into hexagonal h-AlN is

delayed. Cr additions also reduce the stress associated with

domain coherency, which leads to a less pronounced age

hardening but also to a lower hardness decrease as coherency

is maintained at higher temperatures. The result is a higher

hardness of quaternary TiCrAlN coatings at elevated temper-

atures compared to ternary TiAlN coatings grown under sim-

ilar conditions and with similar Al content.

ACKNOWLEDGMENTS

This work was supported by the SSF project Designed

multicomponent coatings, MultiFilms. The authors would

like to acknowledge Professor Igor A. Abrikosov, Dr. Ferenc

Tasn�adi, and Hans Lind for useful discussions, Dr. Jens Jen-

sen for help with the compositional analysis (ERDA),

Thomas Lingefelt for technical support regarding electron

microscopy (TEM), and P€ar Fogelqvist for technical support

of the deposition system.

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061506-8 Fors�en et al.: Decomposition and phase transformation in TiCrAlN thin coatings 061506-8

J. Vac. Sci. Technol. A, Vol. 30, No. 6, Nov/Dec 2012

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