Phase transition and properties of Ti±Al±N thin ®lms prepared byr.f.-plasma assisted magnetron sputtering

Min Zhoua,*, Y. Makinoa, M. Noseb, K. Nogib

aJoining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapanbTakaoka National College, Futagami 180, Takaoka City, Toyama 933, Japan

Received 11 June 1998; accepted 21 August 1998

Abstract

Pseudobinary (Ti12xAlx)N ®lms were synthesized by a new inductively combined rf-plasma assisted planar magnetron sputtering method.

From X-ray diffraction measurement, the deposited (Ti12xAlx)N ®lms were identi®ed as having the B1 structure up to 50 mol% Al (x � 0:5).

In the range from x � 0:6 to x � 0:7, two phases with the B1 and B4 structures were observed. These results suggest that the critical

composition for the phase change from B1 to B4 structure is located between 50 mol% Al and 60 mol% Al. The critical composition decided

experimentally shows a discrepancy with the theoretically predicted value (65 mol% Al), which may arise from a somewhat high substrate

temperature (4508C) in this study. Oxidation resistance increases with increasing the Al content in the (Ti12xAlx)N ®lms up to 70 mol% Al,

irrespective of coexistence of the B1 and B4 phases in the (Ti12xAlx)N ®lms with x � 0:6 and x � 0:7, while both the hardness and Young's

modulus show a maximum value, respectively. Thus, it is indicated that the existence of the (Ti12xAlx)N ®lms with the B1 structure is quite

effective for improving the oxidation resistance, and the appearance of the B4 phase in the pseudobinary nitride ®lms degrades mechanical

properties such as the hardness and Young's modulus. q 1999 Elsevier Science S.A. All rights reserved.

Keywords: (Ti12xAlx)N ®lms; Phase transitions; Sputtering; Hardness

1. Introduction

Hard material coatings are of continuously increasing

interest for wear reduction of working tools. TiN, TiC and

diamond-like carbon (DLC) ®lms are the most widely used

coatings for this purpose [1,2], especially TiN because of its

excellent properties and gold color [3,4]. However, there are

still several drawbacks such as hardness, adhesion, friction

and oxidation resistance properties that limit the practical

application of TiN. In order to improve these properties,

ternary and multilayer coatings such as (Ti,Al)N, (Ti,Zr)N

and (Ti,Nb)N [1,2,5,6] have been investigated. Among

these, the addition of aluminum in TiN to form (Ti,Al)N

ternary solid solution is attractive due to the signi®cant

enhancement of anti-oxidation and mechanical properties

in comparison with TiN [3,6,7].

In the equilibrium Ti±Al±N ternary phase diagram, Ti, Al

and N appear to have essentially no solubility in AlN, TiN

and TiAl, respectively [8]. Up to now, there have been some

reports about (Ti,Al)N thin ®lms, but most of them concen-

trate on the properties. Few reports about the phase transi-

tion and the relationship between the phase transition and

properties are found.

In this study, pseudobinary (Ti12xAlx)N ®lms were

synthesized on stainless steel 304, quartz glass and glass

ceramic substrates by a new inductively combined r.f.-

plasma assisted planar magnetron sputtering method from

pure titanium and aluminum targets in a mixture of Ar and

N2. The phase transition and the relationship between the

phase transition and properties were studied in detail.

2. Experiment

2.1. Film preparation

Fig. 1 shows the r.f.-plasma assisted planar magnetron

sputtering apparatus used in the present study (MPS-200-

HC3, ULVAC Co., Japan). In this system, both Al and Ti

are simultaneously deposited on the substrate using two

targets. An r.f. generator, operating at 13.56 MHz, powers

the two targets and two helix coils just above the two

targets. Using the helix coil, an additional inductively

coupled r.f. discharge is generated just in front of the

magnetron target. This additional r.f. discharge not only

Thin Solid Films 339 (1999) 203±208

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(98)01364-9

* Corresponding author. Tel.: 1 81-6-8798663; fax: 1 81-6-8798653;

e-mail: zhoum@jwri.osaka-u.ac.jp.

strongly increases the plasma density but also enables the

targets to be sputtered at very low pressures. The ionization

is also strongly increased, even up to 10 times compared

with normal planar magnetron sputtering [9]. The detailed

experimental conditions are given in Table 1. The composi-

tions of the pseudobinary ®lms were controlled by changing

the ratio of r.f.-power supplied to the Al cathode to that

supplied to the Ti cathode. The ®lms were deposited on

stainless steel 304, quartz glass T4040 and glass ceramic

PEG3130C substrates. Before sputtering, the stainless

steel substrates were polished to an average roughness of

about 10 nm. Cleaning was performed with acetone and

propanol in an ultrasonic bath.

2.2. Film analysis

The phase and crystal structure of as-deposited ®lms were

identi®ed by X-ray diffractometry using Cu Ka radiation

with a thin ®lm goniometer (M03X, Mac Science Co.,

Japan). Scans were made in different modes, namely grazing

angle mode (Seeman±Bohlin, SB, mode) and u±2u mode

(Bragg±Brentano mode). The grazing angle mode

(Seeman±Bohlin mode) was used to study the phase and

crystal structure of the thin ®lm and the u±2u mode

(Bragg±Brentano mode) was used to study the orientation

of the thin ®lm. In the present study, 48 was used as the

incident beam angle in the SB mode.

The atomic ratio of titanium to aluminum in the as-depos-

ited ®lms was determined by electron probe microanalysis

measurement (JXA-8600, JEOL, Japan) with the WDX

method. Pure TiN and AlN were used as the reference mate-

rials and the ZAF correction method was applied.

2.3. Mechanical properties

The hardness and Young's modulus of the (Ti12xAlx)N

®lms were measured by an ultra micro-indentation system

(UMIS-2000, CSIRO, Australia) which used the Swain

method [10,11]. The indenter was a Berkovich diamond

pyramid indenter. A load of 5 mN was employed so that

the penetration depth of the indenter was less than 10% of

the ®lm thickness. For each indentation, the indentation load

was incrementally increased in 40 steps and the penetration

depth was measured at each step. The hardness data and

Young's modulus data were determined from the mean

value of several measurements.

2.4. Residual stress

The evaluation of the ®lm stress was carried out with

surface pro®le and roughness measurement machine

(Form Talysurf Series S4, Rank Taylor Hobson Ltd., UK).

The change of curvature induced in the sample because of

the stress in the deposited ®lm was investigated by this

machine. A clean, undeposited blank glass ceramic

substrate was ®rst measured prior to ®lm deposition. The

data were then compared with those taken after the ®lm

deposition. The point-by-point subtraction data were ®tted

with a straight line where the slope was inversely propor-

tional to the radius of the sample. The ®lm stress s was

calculated using the following equation that was derived

from the Stoney equation [12]:

s � ET2

3 1 2 n� �L2t4d �1�

where E is Young's modulus of the substrate, n is Poisson's

ratio of the substrate, T is the thickness of the substrate, L is

the length of the substrate, t is the thickness of the ®lm

(t p T) and d is the deformation in the center of the

substrate after deposition.

2.5. Electric resistivity of the ®lms

The electric resistivity was measured by the four-probe

method [13] at room temperature.

M. Zhou et al. / Thin Solid Films 339 (1999) 203±208204

Fig. 1. Schematic diagram of the deposition apparatus.

Table 1

Experimental conditions

Apparatus

Target purity (%) Ti 99.99

Al 99.99

Target size (mm) 51

R.f. power of cathode (W) Ti 150, Al 0±150

R.f. power of coil (W) Ti 50, Al 50

Gas purity (%) Ar 99.9999

N2 99.9999

Gas ¯ow (m3/s) Ar 1:67 £ 1026

N2 8:33 £ 1027

Sputtering pressure (Pa) 0.3

Substrate temperature (8C) 450

TargetÐsubstrate distance (mm) 180

Film thickness (mm) 0.4Ð1

2.6. Oxidation of the ®lms

The weight gain due to the oxidation of the ®lms was

measured using the thermo-gravity method (TGA-50,

Shimadzu, Japan). A quartz glass substrate was used in

order to prevent reaction between the ®lm material and

the substrate and also to prevent substrate oxidation. The

samples were heated in air. The temperature was increased

at a rate of 5 K/min.

3. Results and discussion

3.1. Structure and phase transition of the (Ti12xAlx)N ®lms

Fig. 2 shows the X-ray diffraction patterns obtained from

the as-deposited (Ti12xAlx)N ®lms (x ranging from 0 to 0.8).

When the Al concentration (x) was changed from 0 to 0.6,

the ®lms were identi®ed as having the cubic B1 structure

which is the same structure as pure TiN. When x varies from

0.6 to 0.7, two phases with the cubic B1 structure and the

hexagonal B4 structure were detected. For x exceeding 0.7,

only a single phase with the hexagonal B4 structure was

observed. A summary of the phase relations in the ternary

(Ti12xAlx)N system is shown in Fig. 3. The atomic ratio of

the as-deposited ®lms was determined by EPMA with the

WDX method, combined with the ZAF correction method

using pure TiN and AlN as the reference materials. The

lattice parameter of the B1 structure, as a function of x in

the (Ti12xAlx)N thin ®lms, is shown in Fig. 4. With increas-

ing x, the lattice parameter in the B1 structure decreases

linearly from 4.2448 AÊ for TiN to 4.1462 AÊ for

(Ti0.3Al0.7)N. This suggests that titanium atoms in the TiN

lattice are substituted by aluminum atoms with smaller

atomic radius, although both titanium and nitrogen atoms

in the TiN have a coordination number of 6, while in AlN

the coordination numbers of aluminum and nitrogen atoms

are 4 [14].

The critical composition decided experimentally shows a

discrepancy with the theoretically predicted value made by

one of the authors of this paper [15,16]. In these papers, the

critical composition for B1/B4 was predicted by the two

band parameters, hybrid function H and gap reduction para-

meter S, and the crystal structure map based on these para-

meters and the composition factor finv. According to these

calculations, the critical composition for the B1(TiN)/

B4(AlN) phase transition in the Ti±Al±N system was

about 65 mol%. This suggested that the B1 phase would

transform to the B4 phase directly when the AlN content

was 65 mol% in the Ti±Al±N system.

As shown in Fig. 2 and Fig. 3, the coexistence region of

the B1 and B4 phases was observed in the composition

range from 60 mol% Al to 70 mol% Al. The coexistence

is probably attributed to high substrate temperature at

deposition because non-equilibrium B1 phase close to the

critical composition becomes unstable with increasing

substrate temperature.

3.2. Hardness and Young's modulus of the (Ti12xAlx)N ®lms

The results of Young's modulus and microhardness

measurements with a 5 mN load on the as-deposited

M. Zhou et al. / Thin Solid Films 339 (1999) 203±208 205

Fig. 2. XRD patterns of the as-deposited (Ti12xAlx)N ®lms.

Fig. 3. Crystal structure of phases in the (Ti12xAlx)N ®lms.

(Ti12xAlx )N ®lms are shown in Fig. 5. It shows that the

hardness and Young's modulus increase with increasing

Al concentration and have a maximum value at an Al

concentration of 50 mol%. These hardness values are

much higher than those of pure TiN ®lm. After the Al

concentration exceeds 50 mol%, the hardness and Young's

modulus decrease rapidly. The rapid decrease is consistent

with the appearance of the B4 phase.

Up to now, the reason why the substitution of Al atoms

for Ti atoms in the non-equilibrium (Ti12xAlx)N ®lm with

B1 structure can increase the hardness has still not been

clearly explained. However, a simple explanation can be

found on the basis of bonding characteristics. According

to Cohen [17], the bulk modulus increases with decreasing

nearest neighbor distance in AB compounds with tetrahe-

dral coordination. Assuming that the increase of bulk modu-

lus corresponds to the increase of hardness, the increase of

hardness of the compound can be explained by the decrease

of interatomic distance in the compound. As shown in Fig.

4, the interatomic distance of (Ti12xAlx)N ®lms decreases

with increasing Al content in the B1 lattice, so the increase

of hardness in these ®lms originates from the decrease of

their interatomic distance. Further, because the relation

between interatomic distance (d) and covalent band gap

(Eh) is given by the formula Eh � kd22:5 [18], the origin

of the hardness increase is probably connected with the

increase of covalent energy in these non-equilibrium

(Ti12xAlx)N ®lms

3.3. Residual stress of the (Ti12xAlx)N ®lms

Fig. 6 shows the results of the residual stress calculation

of the ®lms. It indicates that the residual stress in the

(Ti12xAlx )N ®lms is compressive, but there are two sharp

sudden changes at Al concentrations of 20 mol% and

60 mol%, respectively. The reasons for these two sudden

changes in the residual stress are different.

When the Al concentration increases from 0 mol% to

20 mol%, the compressive residual stress increases sharply

because the Al atoms bomb into the TiN lattice and generate

a very obvious atomic pinning effect.

When the Al concentration increases from 50 mol% to

60 mol%, the compressive residual stress decreases sharply

because the phase transition from the B1 to B4 structure

occurs. The AlN lattice parameter is smaller than the TiN

lattice parameter, so the volume shrinks when the B1 phase

M. Zhou et al. / Thin Solid Films 339 (1999) 203±208206

Fig. 4. The lattice parameters as a function of the Al concentration in the

(Ti12xAlx)N ®lms.

Fig. 5. The hardness and Young's modulus as a function of the Al concen-

tration in the (Ti12xAlx)N ®lms.

Fig. 6. The residual stress as a function of the Al concentration in the

(Ti12xAlx)N ®lms.

transforms to the B4 phase. The shrinking of the volume

generates tensile stress, so the compressive residual stress of

the ®lms decreases sharply.

3.4. Electric resistivity of the (Ti12xAlx)N ®lms

TiN conducts electricity similar to the metal, but AlN is a

very good insulator. The electric resistivity of (Ti12xAlx)N

®lms deposited on the silica glass substrates was measured

at room temperature. The results are shown in Fig. 7. The

electric resistivity of (Ti12xAlx)N ®lms in the B1 structure

increases linearly. However, when the Al concentration

exceeds 50 mol%, the phase transition from B1 to B4

occurs. Because AlN is a very good insulator, the electric

resistivity of the (Ti12xAlx)N ®lms increases sharply from

23.5 V cm for (Ti0.5Al0.5)N to 136 V cm for (Ti0.4Al0.6)N.

3.5. Anti-oxidation property of the (Ti12xAlx)N ®lms

One of the most signi®cant disadvantages of TiN thin ®lm

is that the oxidation of TiN is initiated at as low as 5508C,

which is considerably lower than the typical working

temperature for high speed cutting tools (up to 7008C) [2].

But (Ti12xAlx)N ®lms have much higher thermal stability

than TiN ®lms[6,7]. Fig. 8 shows the oxidation curves of

(Ti12xAlx)N ®lms compared with a TiN ®lm measured by

the TG method. The initiation of oxidation of pure TiN ®lm

occurred at 5508C in air, while the (Ti12xAlx)N ®lms with a

cubic B1 structure began to oxidize at higher temperature

than 5508C. These temperatures for the initiation of oxida-

tion become higher with increasing x value and the

(Ti12xAlx)N ®lms with x � 0:6 and x � 0:7 show high stabi-

lity for oxidation in air up to 9508C, irrespective of the

coexistence of the B4 phase. Thus, it is expected that the

B1-type metastable (Ti12xAlx)N ®lms, especially with

higher AlN content, show quite high stability for oxidation.

The reason for the high oxidation resistance of the B1-type

metastable (Ti12xAlx)N ®lms remains unclear, although it

has been suggested by theoretical calculation and XPS

measurement that the oxidation of titanium in these meta-

stable (Ti12xAlx)N ®lms is suppressed because the electric

energy of titanium is stabilized in the B1-type lattice [19].

4. Summary

Using a new inductively combined r.f.-plasma assisted

planar magnetron sputtering method, pseudobinary

(Ti12xAlx)N ®lms were synthesized on stainless steel 304,

quartz glass and glass ceramic substrates. Pure titanium and

aluminum targets were used to deposit the ®lm simulta-

neously.

Up to 50 mol% Al, the formation of (Ti12xAlx)N ®lms

with the B1 structure was identi®ed by XRD, and the coex-

istence of the B1 and B4 phases was observed in the range of

aluminum content from 60 mol% to 70 mol%. The experi-

mentally decided critical composition (around 55 mol% Al)

for the phase transition from B1 to B4 was found at a lower

Al content than the theoretically predicted value (about

65 mol% Al). The discrepancy is attributed to the somewhat

higher substrate temperature.

Irrespective of coexistence of the B1 and B4 phases, the

synthesized pseudobinary (Ti12xAlx)N ®lms containing

60 mol% Al and 70 mol% Al showed quite excellent resis-

tance to oxidation in air, resulting in proof of the excellence

of the metastable (Ti12xAlx)N ®lms with the B1 structure for

anti-oxidation. Improvement of the hardness and Young's

M. Zhou et al. / Thin Solid Films 339 (1999) 203±208 207

Fig. 7. The electric resistivity as a function of the Al concentration in the

(Ti12xAlx)N ®lms.

Fig. 8. Oxidation curves as a function of the Al concentration in the

(Ti12xAlx)N ®lms.

modulus were also observed for these pseudobinary nitride

®lms, but it is suggested that the coexistence of the B1 and

B4 phases is not suitable for mechanical properties such as

hardness and Young's modulus.

Acknowledgements

The authors would like to thank Mr Nakatsuka for his

help in EPMA.

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