Vickers and Knoop hardness of electron beam deposited ZrC and HfC thin films on titanium

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Surface & Coatings Technolog

Vickers and Knoop hardness of electron beam deposited ZrC and

HfC thin films on titanium

D. Ferroa, S.M. Barinovb,T, J.V. Raua,c, A. Latinid, R. Scandurrad, B. Brunettid

aCNR Istituto per lo Studio dei Materiali Nanostrutturati, P.le Aldo Moro 7-00185 Roma, ItalybInstitute for Physical Chemistry of Ceramics RAS, Ozernaya 48, 119361 Moscow, Russia

cM.V.Lomonosov Moscow State University, 119899, Moscow, RussiadUniversita’ di Roma bLa SapienzaQ, P.le Aldo Moro 7-00185 Roma, Italy

Received 4 November 2004; accepted in revised form 17 February 2005

Available online 21 February 2006

Abstract

Submicron thickness films of zirconium and hafnium carbides were electron beam deposited on the sandblasted surface of a metallic

titanium substrate. The films were characterized by scanning electron microscopy, and the hardness of the composite film–substrate systems

was measured by both Vickers and Knoop microindentation methods in the wide indentation loading range. The intrinsic hardness of the film

was separated out from the composite hardness using an area law-of-mixtures approach and taking into account the indentation size effect.

Measured with Vickers (Hf0V) and Knoop (Hf0K) indenters, the hardness was evaluated to be of about 26 and 24 GPa, respectively, for ZrC,

and 19 and 21 GPa, respectively, for HfC films, being somewhat higher compared to published data for bulk carbides.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Thin films; Carbides; Electron beam deposition; Hardness

1. Introduction

Titanium and titanium-base alloys have found wide

applications in orthopedics due to their enhanced bio- and

mechanical compatibility with the bone tissue [1]. One of

the major problems to be solved is the protection of titanium

against oxidation at the harsh conditions of the body fluids

surrounding the implanted device. Some transient titanium

oxides can be formed on the titanium surface being low

resistant to wear. The wear debris accumulation can result in

an adverse cellular response and implant loosening. To

protect titanium against oxidation and to improve wear

resistance of implant, the surface modification by coating is

widely used. Electron beam deposited titanium carbide

coating was demonstrated to be effective to increase the

surface hardness of titanium [2]. However, diffusion of

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.02.150

T Corresponding author. Tel.: +7 95 4379892; fax: +7 95 4379893.

E-mail address: [email protected] (S.M. Barinov).

carbon from the film into pre-heated titanium substrate

during the deposition decreases the carbide film hardness

down to 9–20 GPa compared to 30 GPa for bulk TiC [3,4].

Alternatively, other bio-inactive refractory carbides might

be deposited on titanium surface, e.g. zirconium and

hafnium carbides. Hardness of bulk ZrC is about 20–22

GPa and that of HfC is about 18–20 GPa [3,4]. Thin films of

HfC deposited by pulsed laser ablation on silicon substrate

have the intrinsic hardness varying in the range from 19 to

29 GPa, being independent on the silicon substrate pre-

heating temperature [5]. The present study was aimed at the

electron beam deposition (EBD) of ZrC and HfC films on

titanium substrate and the comparative evaluation of their

hardness by Vickers and Knoop microindentation techni-

ques. In spite of the widespread use of nanoindenters to

evaluate hardness of thin solid films, the Vickers/Knoop

method has some merit due to the following reasons. The

nanoindentation contact scales are generally less than the

film thickness. The apparent hardness value varies with

contact scale in this range, and the calculated hardness

y 200 (2006) 4701–4707

D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–47074702

increases with a decrease in load. This effect is known as the

indentation size effect [6–10]. Therefore, if the hardness is

used as a material selection criterion, it is clearly insufficient

to quote a single hardness value obtained from the nano-

indentation test. Besides, it could be difficult to obtain

reproducible and reliable nanoindentation data for rough

enough films, or for the films deposited on rough substrate,

whereas the microindentation allows the averaging of the

surface roughness effect. Other problems arising with the

nanoindentation techniques are pointed out by A. Iost [10].

The Knoop indentation method is rather rarely used to

evaluate the intrinsic hardness of thin films in spite of its

advantage such as low indenter penetration depth compared

to that of the Vickers indenter, at equal loads.

2. Materials and methods

To prepare the targets for EBD, the ZrC and HfC

powders (Aldrich, 98% pure) were uniaxially hot pressed

into 18 mm diameter and 10 mm thickness pellets. The

pellets were placed into a crucible made of titanium

diboride/boron nitride composite (Advanced Ceramics

Corp. Europe, UK). The crucible was then inserted into a

water-cooled electron beam gun (EVI-8, Ferrotec, Ger-

many), which has been positioned into a stainless-steel

chamber evacuated by a turbomolecular pump supported by

a rotary pump. Distance between the pellet and the gun was

0.25 m. The accelerating voltage in the range between

�3.05 and �10 kV, the shape, pattern and position of the

beam were controlled. The maximum operation power was

5 kW. The gun has a magnetic lens system that allows a

2708 deflection of the beam for avoiding contamination of

the evaporating material with tungsten from the emitting

filament.

The pure titanium substrates were sandblasted with a 60-

grid SiC abrasive powder to provide surface roughness of

approximately Ra 1.6 d 10�9 m necessary for best cell

adhesion to the surface of implanted device. Before the

deposition started, the substrates were heated under vacuum

of 5 d 10�5 mbar in the chamber with a high-power halogen

lamp. The deposition process was performed at an EB

accelerating voltage �3.5 kV and the emission current of

130–200 mA. The pattern of the electron beam was circular

and slow, to ensure uniform consumption of the evaporating

material. The deposition was performed at the substrate

preheating temperature either 200 8C for ZrC or 500 8C for

HfC. The temperature was measured with a thermocouple

(errorF10 8C). Deposition rate was 1–5 2/s. No significant

spitting of the target was observed, there was a light

degassing only at the beginning of the process. The coated

samples were cooled to room temperature in a nitrogen

atmosphere.

The thickness of film was evaluated by two independent

techniques. The former was scanning electron microscopy

(SEM) observation of the cross-sections of the samples (a

LEO 1450 VP SEM apparatus, Carl Zeiss, Germany). An

absolute error of the thickness measurement was F10 nm.

Alternatively, the thickness of the growing films was real-

time monitored by a quartz microbalance coupled with the

measurement of acoustic impedance of the deposited

material. The thickness measurement error in this case

was F50 nm. SEM apparatus was coupled with an energy-

dispersive X-ray (EDX) analysis system. X-ray diffraction

measurements were performed with a Philips X’Pert PRO

diffractometer (Cu Ka irradiation 1.54056 2, graphite

monochromator).

The hardness was measured with a Leica VMHT

apparatus (Leica GmbH, Germany) equipped with a stand-

ard Vickers pyramidal indenter (square-based diamond

pyramid of face angle 1368) and with a Knoop indenter of

face angles a =1720830V and b =13008. The loading and

unloading speed was 5 d 10�6 m/s, time under the peak load

being 15 s. On each sample indentations were made with 5–

7 loads ranging from 0.098 to 19.6 N. Standard deviation of

the diagonal measurements was dependent on the inden-

tation load ranging from about 17% to 4% with an increase

of the diagonal length. The measured hardness was that of

the film–substrate composite system.

To separate the hardness of the film–substrate system on

its constituents from the film and the substrate, a model

based on an area law-of-mixtures approach [11] was

applied:

Hf ¼ Hs þ Hc � Hsð Þ= 2c t=dð Þ � c2 t=dð Þ2h i

ð1Þ

where c is a constant dependent on indenter geometry and

on materials behavior (brittle or ductile); d is indentation

depth. The model is applicable at the indenter penetration

depths where the surface displacement is more than thick-

ness of the film [12]. Therefore, special attention was paid to

choose correctly the range of indentation depths where the

model is adequate. According to Ref. [12], a trend in the

composite film/substrate hardness change with an increase

of the indentation depth can schematically be represented as

is shown in Fig. 1. Region 1a corresponds to highly

localized coating elasto-plasticity and fully elastic substrate

response before the generation of an elasto-plastic enclave

in the substrate (Region 1b). In Region 2a, the substrate

deformation occurs followed by the coating deformation

sufficient to begin circumferential through-thickness coating

fracture. Region 2b corresponds to the fracture of the film

which is bent to conform to the plastically deformed

substrate. In Region 3, true-substrate-only behavior occurs

where the energy absorbed by the film is insignificant

compared to the amount dissipated by substrate. The

loading conditions corresponding to the Region 2b where

the model above is adequate have to be determined. For HfC

films deposited on silicon by pulsed laser ablation technique

this region was estimated to be in the range d / t from 1 to

about 5 [5].

Further, the indentation size effect (ISE) was taken into

account [6–10]. The reasonable expression for the Hc based

Co

mp

osi

te h

ard

nes

s

1a1b

2a

2b

3

Relative indentation depth, d/t

Fig. 1. A scheme of the composite film–substrate response in dependence

on the relative indentation depth.

D. Ferro et al. / Surface & Coatings Technology 200 (2006) 4701–4707 4703

on an area law-of-mixtures approach and accounting for

ISE is

Hc ¼ Hs0 þ Bs þ 2c1t Hf0 � Hs0ð Þ½ �=D ð2Þ

where c1i0.5 for Vickers indenter [11] and c1i2.9 for

long diagonal of Knoop indenter [10], respectively, in the

case of brittle hard film on softer substrate; Hf0 and Hs0 are

intrinsic hardness of the film and substrate, respectively; Bs

is a coefficient; D is imprint diagonal. Eq. (2) has been

widely used in experiments with Vickers pyramid inden-

tation. It is not evident either the equation results in

reasonable value for Knoop indenter geometry, or not.

According to Ref. [10], Eq. (2) holds only when 1bD c1t b2, where D =(Hf0�Hs0) /Bs (variation of film hardness

with the thickness was neglected [10]). At the first

approximation, D can be evaluated to be equal to about

0.25 d 10�6 m (Hf0i22 GPa, Hs0=2.1 GPa, Bs=76.7 d 106

GPa/m), therefore at the film thickness of about 0.5 d 10�6

m, the product D c1 t equals to approximately 1.25, being

within the Iost’s criteria range from 1 to 2. So, there is no

need to use a more complicated third order equation [10].

Fig. 2. SEM plane view of ZrC (a) and HfC (b) films; cross-sectional view

of HfC film (c).

3. Results and discussion

Scanning electron microscopy observations revealed that

the films of both ZrC and HfC carbides have a compact

microstructure, as it is shown in Fig. 2. The surface of film

is rough, partly due to the roughness of the target

sandblasted surface. There are some droplets of about 1–5

Am size on the surface resulted from the expulsion of the

target by electron beam (Fig. 2b). The film is relative

uniform in its thickness (Fig. 2c). Interface between the film

and the substrate seems to be very clear, and no diffuse

boundary was revealed by SEM even using the atomic

number contrast technique. Cross-sectional SEM measure-

ment of the film thickness resulted in its averaged value of

about 510F20 nm for ZrC and 500F34 nm for HfC films.

Independent thickness evaluation by acoustic impedance

technique gave the same result, the film thickness equals to

approximately 500 nm. Mean grain size is less than 200 nm

for ZrC and 300 nm for HfC films. Measured by energy-

dispersive X-ray analysis (EDX) metal / carbon ratio at the

surface of the film indicates the carbides retain their

stoichiometry close to Me /C=1 in the film. The content


of metals and carbon in the films was evaluated to be as

follows:

Film Hf, wt.% Zr, wt.% C, wt.%

ZrC – 76.8 23.2

HfC 94.3 – 5.8

According to these semi-quantitative data, the composi-

tion of zirconium carbide film locates in the two-phase

region ZrC+C of the phase diagram with free carbon

content of about 14 wt.%, whereas the composition of

hafnium carbide film corresponds approximately to the

HfC0.92 formula.
38
,89

53,0

3

40,0

7

38,4

1

35,0

333

,49

31,4

9

28,4

3

0

50

100

150

200

250

300

350

20 30 40 50

2Th

Inte

nsi

ty, a

.u.

b

HfC HfC

35,0

5

33,2

9

31,2

9

53,0

9

40,1

1

38,4

1

0

50

100

150

200

250

300

20 30 40 50

2Th

Inte

nsi

ty, a

.u.

a

ZrC ZrC

Fig. 3. XRD pattern of ZrC (a)

Shown in Fig. 3a,b are the XRD patterns which were

recorded from the surface of the films and compared with

the JCPDS cards #05-0682 (Ti), #11-0110 (ZrC), and #09-

0368 (HfC). Reflections due to both the titanium substrate

and the film are present. Main peaks (100) at 2Theta 33.298and (220) at 55.778 for ZrC, and (111) at 33.498 and (220) at56.218. The peaks are significantly broadened indicating the

nanocrystallinity and/or the disordering of the carbide

structure.

Fig. 4a,b show measured indentation diagonal versus

indentation load and relative indentation depth versus

indentation load, respectively, for Vickers and Knoop

indenters applied to the ZrC/Ti composite system, as an

66,9

7

56,2

1

77,5

176

,43

70,6

7

62,8

9

60 70 80 90

eta

HfC

66,3

5

55,7

7

82,3

3

77,3

7

76,1

370,6

5

63,0

9

60 70 80 90

eta

ZrC ZrC

and HfC (b) films on Ti.

Indentation load (N)0 5 10 15 20 25

Impr

int d

iago

nal (

10-6

m)

0

50

100

150

200

250

300

350

VickersKnoop

a ZrC/Ti

Indentation load (N)

0 5 10 15 20 25

Rel

ativ

e in

dent

atio

n de

pth,

d/t

0

5

10

15

20

25

VickersKnoop

b ZrC/Ti

Fig. 4. Imprint diagonal versus indentation load (a) and relative penetration

depth versus indentation load (b) for ZrC/Ti composite system.

Relative indentation depth, d/t0 5 10 15 20 25

Com

posi

te h

ardn

ess,

Hc

(GP

a)

2

3

4

5

6

7

8

VickersKnoop

a

Relative indentation depth, d/t

0 5 10 15 20 25 30

Com

posi

te h

ardn

ess,

Hc

(GP

a)

2

3

4

5

6

7

8

VickersKnoop

HfC/Tib

Fig. 5. Composite hardness versus indenter penetration depth for ZrC/Ti (a)

and HfC/Ti (b) systems.


example. The advantages of the Knoop indentation over the

Vickers method is that the former allows to reach the same

relative penetration depths in the much wider range of

indentation loads, and higher loads can be applied to get the

same indentation diagonal as using the Vickers technique.

Besides, the measured Knoop indentation diagonal is greater

allowing thus to decrease the relative error of the diagonal

measurement. For example, the relative error of the Knoop

indenter diagonal measurement decreases from 6.4% at the

load 0.49 N down to 4.4% at the load 9.8 N, whereas the

relative error of the Vickers diagonal measurement at the

applied load of 0.098 N was 17.2% due to the limited

optical system resolution.

Shown in Fig. 5a,b is the hardness of the composite

systems ZrC/Ti and HfC/Ti versus relative indentation depth

for both the Vickers and the Knoop indenters. The

composite hardness decreases markedly with an increase

of the d / t. The loading range corresponds to the Region 2b

of the scheme in Fig. 1.

The experimental plots Hc versus 1 /D for substrates and

all coated samples were approximated well by a linear

regression for both the Vickers and the Knoop indentations.

Fig. 6a,b show the experimental data plots, regression lines,

99% confidence intervals and correlation coefficients for

ZrC/Ti and HfC/Ti systems, respectively. Correlation

coefficients values for both the Vickers and the Knoop in-

dentations are very close each other indicating the methods

provide approximately the same accuracy of the hardness

determination. A least-squares fit of the plots to the Eq. (2)

results in the slope Bc=Bs+2c1t(Hf0�Hs0). The values of

Hs0 and Bs, which are needed to calculate the intrinsic

hardness of the film, were obtained by separate experiments

with substrate only. In the case of the Vickers method those

were estimated as equal to 1.84F0.6 GPa and 5.38 d 10�6

GPa m, respectively, whereas for the Knoop indentation Hs0

was equal to 2.10F0.4 GPa and Bs=76.69 d 10�6 GPa m.

The difference in Hs0 values is in the error range and

is supposed to result probably from different surface

conditions of the sandblasted uncoated samples. A signifi-

cant difference between values of Bs is due to indenter

geometry. Using the values of Bs, Bc and Hs0, the intrinsic

Inverse imprint diagonal, 1/D (104 m)

0 2 4 6 8 10 12 14 16

Com

posi

te h

ardn

ess,

Hc

(GP

a)

2

3

4

5

6

7

VickersKnoopRegression99% Confidencer = 0.94

r = 0.96

ZrC/Tia

Inverse indentation diagonal, 1/D (104 m-1)

0 2 4 6 8 10 12 14 16 18

Com

posi

te h

ardn

ess,

Hc

(GP

a)

1

2

3

4

5

6

7

8

VickersKnoopRegression99% Confidence

HfC/Ti

r = 0.92

r = 0.91

b

Fig. 6. Composite hardness versus inverse imprint diagonal for ZrC/Ti (a)

and HfC/Ti (b) systems; r is the correlation coefficient.

Table 1

Intrinsic hardness of refractory carbide films on Ti substrate

Film TiC [2] ZrC HfC

Thickness, nm 700F10 1500F100 510F20 500F34

Vickers

hardness, GPaT19.5 10.0 26 (r =0.96) 19 (r =0.92)

Knoop

hardness, GPaT– – 24 (r =0.94) 22 (r =0.91)

T Correlation coefficient is given in the brackets.


hardness of the films was calculated. Calculated intrinsic

hardness values for both systems under study and, also, for

electron beam deposited TiC films on titanium are give in

Table 1.

It could be pointed out that the difference between

Vickers and Knoop hardness evaluations is small being

within the range of the measurement error. The ZrC film is

somewhat harder compared to the HfC film, at equal

thickness. This is in accordance with the behavior of bulk

carbides hardness, which decreases with an increase of

valence electrons number and, respectively, of molar

weight of the carbide [3]. A reduced intrinsic hardness

of TiC film compared to that for both the bulk titanium

carbide (about 30 GPa [4]) and the pulsed laser ablation

(PLA) deposited on silicon TiC film of about 700 nm

thickness (32 GPa [13]) has been explained as the result of

the diffusion of carbon into the metallic titanium substrate

during the deposition process, which has been carried out

at the substrate pre-heating temperature as high as 800 8C[2]. A decrease in carbon content is known to lower the

hardness of refractory carbides. Particularly, a decrease in

carbon content in titanium carbide lowers the hardness

down to 11.76 GPa in the bulk sintered ceramics of TiC0.58

composition [3]. Vickers hardness values for bulk Zr and

Hf carbides are 20–22 and 18–20 GPa, respectively [3].

Therefore, the diffusion of carbon from the ZrC and HfC

films is expected to be not so significant as it was revealed

for TiC, probably due to the activation energy of carbon

self-diffusion is higher in these carbides than in titanium

carbide: 399 kJ/mol for TiC, 473 kJ/mol for ZrC and 545

kJ/mol for HfC [3]. Besides, the substrate pre-heating

temperature to deposit low-hardness, 1500 nm thickness

TiC film has been 800 8C, much higher than in the case of

ZrC and HfC deposition (200 and 500 8C, respectively),furthering the carbon diffusion into the substrate. Hardness

of HfC film deposited on silicon by pulsed laser ablation

(PLA) method, i.e. in the case when diffusion of carbon

from the film is negligible, has been evaluated to be of

about 29 GPa at the film thickness of 650 nm [5,14].

Hardness of ZrC PLA-deposited film on Si substrate was

evaluated to be of 31 GPa at the film thickness of about

500 nm [15]. Enhanced hardness of PLA-deposited

refractory carbides films on Si has been related to their

ultrafine nanocrystalline structure: evaluated from X-ray

diffraction data, the grain size in the TiC films was in the

range 25 to approximately 30 nm, as an example [16].

Hardness of the electron beam deposited thin films is

somewhat higher than that of bulk carbides. The hardness

of very thin films is known to rise generally with

decreasing the thickness due to reduced degrees of

freedom, which results in lowered stress relaxation by

means of plastic deformation [15]. This behavior is similar

to the yield stress variation with the thickness of very thin

crystals or fibers [17]. The following reasons may be

responsible for such lowered stress relaxation capacity in

thin films: confined dislocation mobility and/or crack

propagation by the film–substrate interface, and nano-

crystallinity of the film resulting in effective dislocation

pinning by the crystallite boundaries [18]. It could be

keeping in mind that the structure of the films which is

forming during the EB deposition is in non-equilibrium

state. Deposition of the films by evaporation in vacuum

results always in tensile stress imposed on the film due to

the densification of the disordered to some extent structure

that is bfrozingQ during the deposition [19]. These tensile

stress reduce the measured hardness promoting cracking

under the indentation.


Thus, submicron thickness films of refractory Zr and Hf

carbides were successfully deposited on titanium substrate

by electron beam deposition method and their intrinsic

hardness was revealed to exceed that of respective bulk

carbides, as evaluated independently by both Vickers and

Knoop microindentation techniques.

4. Conclusions

The following conclusions can be drawn from the results

of the study.

1. Electron beam deposition method allows the fabrication

of submicron thickness, dense continuous ZrC and HfC

films on rough sandblasted surface of metallic titanium

substrate.

2. The intrinsic film hardness was separated out from the

composite film–substrate hardness measured by both the

Vickers and the Knoop methods. The calculated

intrinsic hardness values for both the ZrC film (Hf0V

26 GPa and Hf0K 24 GPa) and the HfC film (Hf0V 19

GPa and Hf0K 21 GPa) are somewhat higher compared

to those reported for bulk carbides. Vickers and Knoop

methods resulted in similar hardness value estimations,

however an advantage of the Knoop method is the

extended indentation loading range, that is useful in

practice.

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Vickers and Knoop hardness of electron beam deposited ZrC and HfC thin films on titanium

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Transcript of Vickers and Knoop hardness of electron beam deposited ZrC and HfC thin films on titanium