Deformation and fracture of TiN and TiAlN coatings on a steel substrate during nanoindentation

$: Deformation and fracture of TiN and TiAlN coatings on a steel substrate during nanoindentation$
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Surface & Coatings Technolog

Deformation and fracture of TiN and TiAlN coatings on a

steel substrate during nanoindentation

L.W. Ma, J.M. Cairney, M.J. Hoffman, P.R. Munroe*

Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Received 12 August 2004; accepted in revised form 29 September 2004

Available online 11 November 2004

Abstract

The deformation mechanisms and mechanical properties of both TiN and TiAlN coatings on a V820 nitridable steel substrate have been

investigated. Deformation was induced by nanoindentation, and the microstructures of the indented regions were studied using various

techniques, including focused ion beam (FIB) and transmission electron microscopy (TEM). Two coatings deposited using a cathodic arc

process were investigated: a thicker (1.1 Am) TiAlN–TiN dual-layer coating and a slightly thinner (0.7 Am) TiN coating. Hardness was

measured using nanoindentation using a 5-Am-radius spherical indenter. A model developed by Korsunsky et al. was used to obtain the

hardness of the coatings, accounting for both the thickness of the coating and the influence of the substrate. Hardnesses of 30 and 24 GPa

were obtained for the TiAlN and TiN coatings, respectively. Both coatings exhibited broadly similar mechanisms of deformation. Columnar

cracking and shear steps at the coating–substrate interface indicated that coatings deform predominantly by shearing along the columnar grain

boundaries; however, significant lateral edge cracking was also observed, especially in the case of the TiAlN coating. The interface between

the TiAlN and TiN in the dual-layer coating did not appear to affect the deformation behaviour.

D 2004 Elsevier B.V. All rights reserved.

Keywords: TiN; TiAlN; FIB; TEM; Nanoindentation

1. Introduction

The use of physical vapour deposition (PVD) hard

coatings based on the transition metal nitride, titanium

nitride (TiN), is well established. The coatings are com-

monly applied to various kinds of steel cutting tools,

forming tools, dies, etc. [1–4], providing surfaces with

enhanced tribological properties in terms of low friction,

high hardness and improved wear resistance [1–6]. How-

ever, one disadvantage of TiN is in its application at high

temperature, since it oxidizes rapidly at temperatures above

~550 8C, to form a layer that is partially composed of the

rutile-structured TiO2 [1,2]. In order to overcome such

oxidation problems, ternary coatings, such as those based on

titanium aluminium nitride (TiAlN), are becoming increas-

ingly used because they show increased resistance to

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.09.034

* Corresponding author. Tel.: +61 2 9385 4435; fax: +61 2 9385 6400.

E-mail address: [email protected] (P.R. Munroe).

oxidation up to a temperatures of ~800 8C [7]. This is due

to a dense protective layer of an aluminium-based oxide that

is formed on the top of the (Ti,Al)N coating surface, which

acts to inhibit further oxidation and improve high-temper-

ature behaviour [7,8]. Furthermore, TiAlN is not only

effective at improving high-temperature oxidation resist-

ance, but also improves mechanical performance, through

enhanced hardness and wear resistance [7].

It has been well documented in the literature that

nanoindentation techniques can be effective in the evalua-

tion of the mechanical properties, such as hardness and

modulus, of hard coatings [9–12]. However, such techni-

ques do not provide direct observation of the coating

microstructure, in particular the mechanisms that may

operate when the coating is deformed. In recent years, the

focused ion beam (FIB) miller [13] has emerged as a

materials characterization tool that may be used to observe

the microstructure of thin films and coatings systems,

including thin films that have been locally deformed. This

y 200 (2006) 3518–3526

L.W. Ma et al. / Surface & Coatings Technology 200 (2006) 3518–3526 3519

is because the FIB can be used to prepare and image cross

sections with high positional accuracy. The FIB can also be

used for the preparation of transmission electron microscope

(TEM) specimens with submicron locational precision

[14,15]. Therefore, FIB-based techniques are highly effec-

tive in the observation of coating–substrate deformation

mechanisms following nanoindentation [16].

A number of studies have evaluated the mechanical

performance of TiN-type coatings during nanoindentation

tests using both spherical and sharp indenter tips [11,17–20].

Some workers have reported the occurrence of radial and

circumferential cracks in the films and interfacial failure

during the microindentation tests with both indenter types

[17–20]. However, none of these studies have been entirely

satisfactory in the evaluation of the deformation mechanisms

of the TiN-based thin films during nanoindentation using a

spherical indenter, due to the lack of high-resolution micro-

structural data. Recently, Ma et al. [21] investigated the

deformation mechanisms in TiN coatings deposited onto a

V820 nitridable steel substrates by means of PVD coating

processes, using a combination of nanoindentation with a

spherical indenter, and microstructural analysis, including

FIB and TEM studies. Different modes of cracking, including

columnar, circumferential, edge and transgranular cracks,

and also shear steps at the base of coating, were observed. It

was concluded that deformation in TiN coating occurs

predominantly via cracking and shear sliding along the

intercolumnar grain boundaries. However, in more advanced

TiAlN-based coatings, the deformation mechanisms and

modes of cracking behaviour are yet to be investigated.

The work described in this paper compares the deforma-

tion behaviour of TiN with a dual-layer TiN–TiAlN coating.

Both coatings are deposited onto ductile V820 nitridable

steel substrates, and their mechanical performance is

assessed through nanoindentation using a spherical indenter.

The microstructures and properties of the two coatings are

compared, following nanoindentation, by use of micro-

structural analysis techniques, including FIB and TEM. FIB

cross sections were used to observe the microstructure of

TiAlN and TiN coatings in both the undeformed state and

following nanoindentation. Deformation mechanisms,

including different modes of crack propagation, will be

described. The microstructural features observed are corre-

lated with the force–displacement curves obtained during

nanoindentation. Finally, TEM cross sections of areas

locally deformed by nanoindentation, prepared using FIB-

based techniques, are used to observe these features at

higher resolution.

2. Experimental procedure

2.1. Coating process

Two coatings: (a) a binary (TiN) coating and (b) a dual-

layer TiN–TiAlN coating (which will be referred to here-

inafter as the ternary coating or simply TiAlN) were

deposited onto V820 nitridable steel substrates, using

multisource cathodic arc evaporation (CAE). Prior to the

film deposition, the substrates were initially ground and

polished to a 0.5-Am surface finish, then cleaned with

ultrasonic and mechanical agitation. This was followed by

rinsing in deionized water and drying.

For the binary coatings (TiN), steel substrates were

initially loaded into the coating chamber which was pumped

down to a base pressure of 5�10�3 Pa and heated to 250 8Cusing electric arc discharge. They were then biased at

�1 kV and cleaned by argon ion etching at a pressure of 5

Pa. The bias voltage was simultaneously reduced to �800 V

and the substrates metal-ion etched using Ti ions generated

from an arc source. Nitrogen was then introduced to a

chamber pressure of 7�10�1 Pa and a coating deposited for

about 40 min using a substrate bias of �200 V. A TiN

coating of ~0.7 Am in thickness was deposited.

For the ternary coatings (TiAlN), a base coating of TiN

was initially deposited for a period of about 20 min using

the same deposition conditions as for the single layer TiN

coating. This generated a TiN coating adjacent to the

substrate about 0.5 Am in thickness. The substrate bias was

then reduced to �100 V, and a TiAlN layer deposited for a

period of about 40 min using two Ti0.5Al0.5 cathode targets.

A 0.6-Am-thick TiAlN top layer was deposited, resulting in

a total coating thickness of 1.1 Am. Detailed microstructural

characterization of both the TiN and TiAlN coatings has

been published elsewhere [22].

2.2. Nanoindentation

Nanoindentation was performed using a CSIRO UMIS-

2000 (Ultra Micro-Indentation System). For investigations

of the nature of deformation under the indented region, a

spherically tipped diamond indenter with a radius of 5 Amwas used at maximum loads ranging from 5 to 500 mN. A

spherical indenter was chosen as it provides a more uniform

stress distribution under the indenter tip compared with

sharper indenter shapes. A sharper tip changes the stress

profile, such that significant deformation of the coating

occurs at low load independently of any substrate effects.

Under such loading, the coating behaves more as a bulk

material and so the materials assessment then becomes one

of the coating material rather than the coating/substrate

system. The loading cycle was continuous with a dwell time

of 1 s at each increment. Thirty increments were used for the

lower loads (b200 mN) and 50 increments were used at

higher loads (N200 mN). For measurements of coating

hardness, a Berkovich indenter was used as the smaller

deformation zone under the indenter allows a more accurate

measurement of the hardness. The coating hardness was

determined from force–displacement curves from indenta-

tions with maximum loads ranging from 4 to 50 mN using

the Oliver–Pharr method [23]. An area function was

developed for the indenter using both sapphire and silica

Fig. 1. FIB images of cross sections of undeformed regions of (a) the binary

TiN coating deposited on a V820 nitridable steel and (b) the ternary TiAlN

coating deposited on to the same substrate.

L.W. Ma et al. / Surface & Coatings Technology 200 (2006) 3518–35263520

standards of known modulus and applied in the hardness

calculation. Furthermore, since for thin, hard coatings on

soft substrates, such as the systems investigated here, the

measured hardness decreases with indentation depth, due to

the increasing influence of the substrate. Therefore, the

hardness of the coating alone was determined by applying a

model developed by Korsunsky et al. [24], which accounts

for the effect of the substrate and the coating thickness on

hardness. The Korsunsky model has been shown to be

effective in subtracting substrate effects and obtaining

relatively accurate values for the hardness of thin coatings

[24], particularly for the case of hard coatings on soft

substrates.

2.3. Focused ion beam (FIB) analysis

Focused ion beam (FIB) milling and imaging was

performed using a FEI 200xP FIB system. Cross sections

of both TiN and TiAlN, including both undeformed and

nanoindented regions, were milled with a beam currents of

2700 pA for the initial sections, 350 pA for the final

cleaning mills and further imaged with a beam current of 11

pA. Details of the preparation of cross sections using the

FIB have been described elsewhere [14,15]. It is important

to note that images of cross sections are geometrically

distorted because of the angle at which they are viewed. The

degree of tilt from the direction of the ion beam is therefore

always specified in the FIB images.

2.4. Transmission electron microscopy (TEM)

TEM specimens of nanoindented regions in both TiN and

TiAlN coatings, after loading to either 100 or 500 mN using

a 5-Am spherical indenter, were prepared using the FIB

workstation. Details of the use of the FIB for TEM specimen

preparation are reported elsewhere [25]. Prior to the

preparation of the TEM specimens, a protective layer of

platinum was initially deposited, using FIB-assisted chem-

ical vapour deposition, along the top edge of the area from

which the specimen was to be prepared (through the centre

of the indent). The H-type technique [26] was used with an

aperture corresponding to a beam current of 2700 pA used

for the initial cuts, followed by 1000 pA for finer cuts and

finally 350 pA for the final cleaning mills. Transmission

electron microscopy was performed in a Philips CM200

field emission gun TEM operating at 200 kV.

3. Results and discussion

Fig. 1 shows FIB cross sections of the two different

materials in the undeformed state, that is, (a) the binary

coating (TiN) and (b) the ternary coating (TiAlN), deposited

onto a V820 nitridable alloy steel using cathodic arc

evaporation (CAE). The binary coating (Fig. 1a) is

approximately 0.7 Am in thickness, whilst the ternary

coating (Fig. 1b) is approximately 1.1 Am in thickness.

For both of the coatings, the microstructure consists of

columnar grains 50–100 nm in width, oriented approx-

imately parallel to the direction of growth. On closer

inspection, the ternary coating (Fig. 1b) can be seen to

consist of two layers, specifically, TiAlN and TiN, which

are, labeled x and y, respectively. These layers are

approximately 0.6 Am and 0.5 Am, respectively, in thick-

ness. The microstructures of both of these coatings have

been previously characterized and it was noted that a ~100-

nm-thick interfacial zone existed in the ternary coating at the

boundary between the TiN and TiAlN coatings, consisting

of ~10-nm-thick alternate layers of both compounds [22].

Fig. 2 shows the force–displacement curves from both

the binary and ternary coatings, subjected to nanoindenta-

tion to maximum loads of (a) 5 mN and (b) 300 mN, using a

5-Am-radius spherical indenter, while Fig. 2c is an expanded

version of the higher load region in Fig. 2b. At very low

loads (up to 5 mN, as shown in Fig. 2a) the load–

displacement behaviour of both materials is mainly elastic

and reversible, although a small amount of plastic deforma-

tion is detected for the thinner TiN coating. The change

from mainly elastic to mainly plastic deformation occurs at

low loads of the order of 5 mN in the binary coating, as

described elsewhere [21]. The depth of penetration into the

Fig. 3. The curves plotted (a) indentation depth, (b) normalized indentation

depth against measured hardness for both the coatings and the V820 steel

substrate.

Fig. 2. Force–displacement curves for both the binary and ternary coatings

deposited on a V820 nitridable steel following (a) nanoindentation to a

maximum load of 5 mN, (b) nanoindentation to a maximum load of 300

mN, (c) an expanded section of the 300-mN curve shown in panel b. All

indentations were performed using a 5-Am spherical indenter.


binary coating subjected to nanoindentation is larger than

that of the ternary coating under the same load, as shown in

Fig. 2b. This is, in part, because the addition of aluminium

to TiN to form a TiAlN ternary solid solution leads to

significant enhancement in hardness compared to the binary

coating [2,7,8], but is complicated by the fact that the

ternary coating is slightly thicker than the binary coating. A

comparison of the hardness for both coatings, where

corrections have been made for the effect of coating

thickness, is therefore necessary.

At loads greater than about 200 mN (Fig. 2b), there are

indications of pop-in events (sudden increases in indentation

depth with a small increase in load) in both coatings.

Examples are shown at points P1 (~210 mN) in the ternary

coating and P2 (~260 mN) in the binary coating, shown in

Fig. 2c, which shows an expanded portion of the curve

shown in Fig. 2b. Pop-in events are believed to be

associated with cracking events in the coatings [11], and

the pop-ins observed here will be discussed in more detail

later.

Fig. 3a shows the measured hardness plotted as a

function of the indentation depth for both coatings and for

the steel substrate (hardness data was obtained from

indentations made using a Berkovich indenter). It is noted

that even at relatively low loads, which lead to low

indentation depths, there is a strong influence from the

substrate on the hardness measured. This is evidenced by a

decrease in the measured hardness as the load is increased,

due to an increasing contribution from the substrate. At

higher loads, and therefore higher indentation depths, the

measured hardness begins to approach that of the V820 steel

Fig. 4. FIB image of the top surface of (a) the binary coating and (b) the

ternary coating following nanoindentation at a load of 100 mN using a 5-

Am spherical indenter.


substrate, which was measured to be approximately ~4.3

GPa. Clearly, the measured hardness of the ternary coating

is greater than that of the binary coating, due to both the

addition of aluminium to the coating and the increased

thickness of this coating. Therefore, the hardness of the

coatings cannot be directly compared in this simple way. In

order to compare the true hardness response of the two

coatings, a model described by Korsunsky et al. [24] is used

to analyse the data. The model uses the following equation

[24] to describe contact response between a coating/

substrate system and a Berkovich indenter.

Hc ¼ Hs þHf � Hsð Þ1þ kb2� � ð1Þ

The coating hardness, Hf, is expressed in terms of the

overall, composite hardness, Hc, and the substrate hardness,

Hs, where k is a constant obtained from a fitting procedure

and b=h/t (where h is the maximum indentation depth and t

is the coating thickness) which denotes the indentation

depth normalized with respect to the coating thickness.

Fig. 3b shows the normalized indentation depth (b)plotted against measured hardness for both coatings. This

figure reveals that the ternary coating is harder than the

binary coating when the effect of thickness is excluded. A fit

of the model to the range of experimental data obtained

from both ternary and binary coatings provides a hardness

of ~30 GPa for the ternary coating, which is similar to the

values reported in the literature (28–32 GPa) [7], while a fit

of the model to the hardness data for the binary coating

gives a hardness of ~24 GPa, which is again similar to the

values found in the literature (15–33 GPa) [6].

Fig. 4a and b are secondary electron FIB images of the

top surface of the binary and ternary coatings, respectively,

subjected to nanoindentation to a maximum load of 100

mN, using a 5-Am-radius spherically tipped indenter. A

large-diameter indent (~3 Am) can be clearly observed in the

binary coating (Fig. 4a) and circumferential cracks inside

the indent, also known as dnest cracksT [18,19,21] are seen,marked by an arrow. The impression of the indent appears

more pronounced in the binary coating compared to the

ternary coating indented under the same conditions (Fig.

4b), and is known from load displacement data (Fig. 2b) to

be deeper. This is because the ternary coating is both harder

and thicker than the binary coating. However, similar

circumferential cracks can nevertheless be seen in the

ternary coating, arrowed in Fig. 4b, although the density

of such cracks is lower. At this load, no outer circum-

ferential, or dedgeT cracking [21,27] outside of the inden-

tation, is evident in either of the coatings.

At higher loads, the indents are larger and a higher

density of cracks can be observed. For example, Fig. 5a and

b show the top surfaces of the binary and ternary coatings

indented to a maximum load of 500 mN using a 5-Am-radius

spherically tipped indenter. The indent is again slightly

larger for the binary coating (11 Am) than for the ternary

coating (9 Am) and again less cracks are present in the

ternary coating. Circumferential cracks, in Fig. 5a and b,

both marked by an arrow, can be seen not only inside both

of the indents, but also outside the indents, especially in the

case of the ternary coating.

Fig. 6 shows FIB cross sections of the indents in (a) the

binary and (b) ternary coating after loading to 500 mN using

a 5-Am-radius spherical indenter. Circumferential cracks

were observed in both coatings, both inside and outside of

the indented area, for example, in the regions arrowed in both

images. The outer circumferential cracks, often termed dedgecracksT [20,26], were observed in both coatings following

loads in excess of 200 mN. These edge cracks (marked T in

Fig. 6) are initiated at the surface, but are associated with

subsurface transverse (or lateral) cracking, with cracks

running parallel to the substrate–coating interface away

from the contact area. Fig. 7 shows a higher magnification

FIB image of the cross section of the indented ternary

coating shown in Fig. 6b. A number of these transverse

cracks, marked T, can be clearly observed. In contrast, a FIB

cross section of the TiN coating (Fig. 8a) contains only a

small amount of these cracks (again marked T). In addition,

columnar cracks (marked C), and shear steps at the base of

the coating (marked S) were observed in both coatings.

Fig. 6. FIB cross-sectional images through the indents in (a) the binary

coating and (b) the ternary coating, following nanoindentation at a load of

500 mN using a 5-Am spherical indenter.

Fig. 7. FIB cross-sectional image of the indented regions as shown in Fig.

6b at higher magnification.

Fig. 5. FIB images of the top surface of (a) the binary coating and (b) the

ternary coating following nanoindentation at a load of 500 mN using a 5-

Am spherical indenter.


As previously mentioned, pop-ins observed in load–

displacement curves for hard coatings on ductile substrates,

such as those shown in Fig. 2, are often associated with

cracking of the coating. In a previous study [21], pop-ins in

similar load–displacement curves for TiN coatings, several

microns in thickness, were attributed to cracking and the

resulting shearing along the columnar grain boundaries,

which eventually resulted in the formation of shear steps at

the interface between the coating and the steel substrate. It is

very likely that the pop-ins labeled P1 and P2 in Fig. 2c are

associated with similar cracking events.

Fig. 8b is another higher magnification FIB cross-

section image of the binary coating, subjected to nano-

indentation to a load of 500 mN, using the 5-Am indenter.

The columnar grains within the coating can be seen to be

rotated through large angles relative to their original

orientation following heavy deformation. Moreover, in

some areas, the thickness of the coating is distorted due to

this rotation of the grains. The substrate is also heavily

deformed beneath the indenter and the steel substrate has

been driven upwards into the coating. Such deformation

was not observed in the thicker ternary coating and

demonstrates the ability of these coatings to remain intact,

even at very high levels of strain.

Cross-sectional TEM specimens were also prepared

through the centre of indented regions of both coatings,

after loading to 100 mN (Fig. 9a–c) and 500 mN (Fig. 10a–

c), using a 5-Am-radius spherical indenter [15]. Fig. 9a is a

Fig. 8. FIB cross-sectional image of (a) the indented regions as shown in

Fig. 6a at higher magnification and (b) the indented region as shown in

Fig. 6a at different position.


bright field image of the entire indent in the binary coating,

after indentation to a maximum load of 100 mN. The TiN

coating, V820 steel substrate and a protective platinum layer

are labeled in the image. Here, a number of columnar

cracks, approximately 200 nm in length, marked C, can be

Fig. 9. Bright field TEM images of (a) the binary coating and (b) the ternary coatin

spherical indenter, (c) the region in the ternary coating indicated by the white sq

clearly observed in the middle of the indent. In contrast,

only two large columnar cracks, approximately 200 and 600

nm in length, marked C, can be observed in the ternary

coating (Fig. 9b) deformed under the same conditions.

Fig. 9c is a higher magnification image of the ternary

specimen. The area from which this image was taken is

indicated by the white square in Fig. 9b. The microstructure

of the ternary coating consists of columnar grains, marked

G, ~50–100 nm wide and ranging from ~100 to 500 nm in

length, oriented approximately parallel to the direction of

growth. It is noted that a columnar crack (600 nm in length)

propagates through the ~100-nm-thick interlayer between

the TiN and TiAlN layers (marked by an arrow). It is

therefore concluded that the interface between the TiN and

TiAlN sublayers does not prevent crack propagation.

Furthermore, a transgranular crack, approximately 100 nm

in length, marked T, is observed at the end of the columnar

crack, well away from the interlayer.

Fig. 10a–c are bright field TEM images of the ternary

coating loaded to 500 mN, using a 5-Am-radius spherical

indenter. Fig. 10a shows the entire indent. A number of

columnar cracks are seen, marked C, approximately ~100–

150 nm wide and ranging from ~50 to 1000 nm in length.

The thickness of the coating is uniform across the indented

region, indicating that no plastic flow occurs in the coating

during indentation, while the substrate is heavily deformed

beneath the indent. It is again noted that the ternary coating,

although heavily deformed, has retained its structural

integrity and adhesion to the substrate. Fig. 10b is a higher

magnification image of the region indicated by the white

square in Fig. 10a. Three significant columnar cracks,

marked C, approximately ~50–100 nm wide and ~1000 nm

in length, can be clearly observed in the TiAlN coating.

Fig. 10c is a second higher magnification image of the same

specimen. The white circle, shown in Fig. 10a, indicates the

area from which this image was taken. A number of lateral

cracks, marked L, approximately ~50–100 nm wide and

ranging from ~100 to 1300 nm in length, running across the

columnar grains can be observed. Again, these cracks have

passed through the interlayer of the coating without

g, both following nanoindentation at a load of 100 mN using a 5-Am-radius

uare, shown in panel b at higher magnification.

Fig. 10. Bright field TEM images of the ternary coating following nanoindentation at a load of 500 mN using a 5-Am-radius spherical indenter. Panel a shows

the overall structure of the indentation; panel b shows, at higher magnification, the area indicated by the white square, shown in panel a and panel c shows, at

higher magnification, the area indicated by the white circle, shown in panel b.


deviation. Moreover, the shear steps that result from

columnar sliding can be observed, marked S, at the interface

between the TiAlN coating and the steel substrate.

In the present study, lateral cracking occurred more

frequently in the dual-layer ternary coating (Fig. 6b) than

the binary coating (Fig. 6a), with columnar cracking being

the more dominant fracture type in the binary coating.

Direct in situ observations of cracking in a homogeneous

stiff coating on a compliant substrate (a glass-polymer

coating–substrate system), indented with a spherical inden-

ter have shown the presence of circumferential cracks which

initiate outside the contact area at the surface of the coating,

and change direction through the thickness of the coating to

run parallel with the plane of the coating [28]. Such cracking

is quite different to the Hertzian cone cracking observed

near the contact area in monolithic materials. These cracks

are similar in appearance to the edge cracks seen in the

present study.

Previous investigations of TiN coatings have shown that

these coatings deform predominantly by shear cracking and

sliding along the columnar grain boundaries [16,21,22],

although in some cases, a limited amount of transverse

cracking is observed towards the edge of the coating. It is

thought that relatively weak columnar grain boundaries

allow this type of fracture to occur.

It is therefore suggested that in the case of the dual-

layer ternary coating, increased grain boundary strength

may hinder crack propagation through the coating,

promoting the formation of edge cracks over columnar

cracks. Increased boundary strength is possibly due to the

~100-nm-thick interfacial region, which is known to

consist of a series of compositionally distinct layers

(bilayer length ~16 nm) [13].

Despite the presence of some lateral edge cracking, it

is noted that deformation in both the binary and ternary

coating still occurs predominantly via cracking and shear

sliding along the columnar grains boundaries. This is in

agreement with our previous study of TiN coatings, and

also with prior, but less detailed studies, by Loubet et al.

[20] and Weppelmann et al. [11], who, respectively,

investigated TiN films on stainless steel substrates

deformed using a Vickers indenter, and the fracture

mechanics of TiN films on silicon substrates during

deformation using a spherical indenter. The strain

generated by nanoindentation is thought to be accom-

modated by a combination of shearing of the columnar

grains in the brittle coating and plastic flow in the

ductile steel substrate. The stress is then transferred to

the steel substrate after cracking. The onset of plastic

deformation in both coatings occurred at a similar load

and in each case involved similar deformation mecha-

nisms, although they are of different thickness and

composition.

4. Conclusions

Hardness and deformation mechanisms in both a TiN

coating and TiAlN–TiN dual-layer coating were investi-

gated using a combination of nanoindentation, FIB milling

and imaging, and transmission electron microscopy.

Even at low loads, the substrate influenced hardness

measurements in these relatively thin coatings, and so a

model developed by Korsunsky et al. [24] was used to

determine the hardness of the coating itself. Hardnesses of

30 and 24 GPa were obtained for the TiAlN and TiN

coatings, respectively.

Columnar cracking and shear steps at the coating–

substrate interface indicated that both coatings deform

predominantly by shear at the columnar grain boundaries;

however, significant lateral edge cracking was also observed

in the case of the TiAlN coating. In general terms, the

overall deformation mechanisms of the binary and ternary

coatings were similar.

Cracks passed unhindered through the interface between

the TiAlN and TiN in the dual-layer coating, and this layer

is not thought to affect the deformation behaviour of the

coatings.


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Deformation and fracture of TiN and TiAlN coatings on a steel substrate during nanoindentation

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