Bulk titanium for structural and biomedical applicationsobtaining by spark plasma sintering (SPS) from titaniumhydride powder

Cristina Ileana Pascu • Oana Gingu •

P. Rotaru • I. Vida-Simiti • Ana Harabor •

Nicoleta Lupu

Received: 28 May 2012 / Accepted: 13 November 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract Titanium is a low density element with excel-

lent mechanical properties, and is an attractive material for

structural and biomedical applications. In recent years, a

new process technology is emerging by which titanium and

titanium alloys can be obtained by using titanium hydride

(TiH2) as a precursor for Ti and its mixture with alloying

elements. The feasibility of this manufacturing approach

has been fully demonstrated from powder to sintering and

from microstructure to mechanical properties. In this paper,

a study concerning powder metallurgy processing of Ti by

spark plasma sintering (SPS) route is presented. The

influence of the technological parameters on the hardness

and microstructures change during SPS has been studied.

The experimental results are related to microscopic, ther-

mal, and mechanical analysis.

Keywords Thermal analysis � Titanium hydride �X-ray diffraction � Spark plasma sintering � Hardness �Microstructure

Introduction

Biomaterials are processed from different biocompatible

materials (ferrous, non-ferrous, ceramics, and composites).

Titanium is a well-known metal widely used for such

applications [1–3]. One of the most cost-efficient methods

to manufacture Ti-based biomedical applications is powder

metallurgy (PM) technique [4–6]. Large spectrum of PM

processing routes provides advantages of the elaborated

Ti-products: classical sintering, laser forming, powder

injection molding, spraying, rapid solidification, and

mechanical alloying and vapor deposition [7, 8].

Recent research underlines the benefits of TiH2 used as

starting material instead of Ti powders [9–12]. Further-

more, TiH2 reacts as a foaming agent, thus the sintered Ti-

based products may present different porosity levels related

to the compaction or pressure-sintering stage. Such tech-

nologies aiming the above-mentioned materials are: pulsed

current activated sintering, replication sponge reactive

sintering [13–16]. Also, spark plasma sintering (SPS)

technique is frequently applied to obtain porous Ti prod-

ucts from titanium hydride as raw material [17–19].

All these processing routes for Ti porous products

manufacturing are based on the main advantage of TiH2

behavior, its heating/sintering, namely the dehydrogena-

tion. The releasing hydrogen determines a certain level of

product porosity. The thermal decomposition of titanium

hydride in vacuum was studied by Kovalev and peculiari-

ties of TiH2 decomposition was studied by Illekova

[20, 21]. The decomposition of TiH2 through thermal

analysis techniques has been studied by Zhang and Kisi,

using thermogravimetric analysis (TG). They compared the

dehydrogenation of nanocrystalline TiH2 produced by

reaction milling with commercially available TiH2 powder

[22].

C. I. Pascu � O. Gingu (&)

Faculty of Mechanics, University of Craiova, 107 Calea

Bucuresti Street, 200512 Craiova, Romania

e-mail: oanagingu@yahoo.com

P. Rotaru � A. Harabor

Department of Physics, Faculty of Exact Sciences, University of

Craiova, 13 A.I. Cuza Street, 200585 Craiova, Romania

I. Vida-Simiti

Faculty of Materials and Environment Engineering, Technical

University of Cluj-Napoca, 103-105 Muncii Boulevard,

Cluj-Napoca, Romania

N. Lupu

National Institute of Technical Physics, Iasi, 47 Mangeron

Boulevard, Iasi, Romania

123

J Therm Anal Calorim

DOI 10.1007/s10973-012-2824-2

The theoretical calculation which was fulfilled by

Xiangqing et al. [23] revealed that TiH2 is always stable when

the temperature is between room temperature and 450 �C, the

desorption pressure of TiH2 is only 9 Pa at 450 �C and the

desorption pressure of TiH2 is 0.15 MPa at about 700 �C.

The researchers have confirmed the possibility of con-

verting titanium hydride powder into titanium powder by

dehydrogenation process at temperatures between

450–650 �C and 700–800 �C [17, 24, 25].Also, the thermal

decomposition of titanium hydride powder (d-phase) to

titanium (a-phase) through TG and high-temperature X-ray

diffraction (HTXRD) has been studied [25].

Thus, the objective of this paper is to research a new approach

to obtain high dense Ti materials, namely by SPS using TiH2 as

raw material. The main applications of these processed PM

materials would be for structural biomedical products (femoral

stems, pivot for dental prosthesis, etc.) requiring high mechan-

ical properties. In order to underline the last features, the classic

sintering (CS) was performed too. Microstructural parameters

have been compared for both samples type.

Experimental

Raw material

For this study, Ti biomaterials have been processed by SPS

technology using titanium hydride (TiH2) micronic powder

particles (particle size min. 99.9 % \63 lm, water atomised)

as raw material. Titanium hydride is produced by Chemetall

GmbH and contains the components listed in Table 1.

Also, the gram molecular weight of titanium hydride is

49.883 g mol-1, melting point is 400 �C, density is 3.91 g

cm-3, and self-ignition point at 342 �C.

Methods and techniques

SPS treatment was developed in (SPS)-FCT-(FAST) HPD5

equipment (maximum current Imax. = 20 kA; maximum

sintering temperature 2,400 �C; working temperature

2,200 �C; working in a vacuum facility; maximum operating

force F = 50 kN, graphite die). The processing parameters

were: 1000, 1050, and 1100 �C as sintering temperatures and

10, 15, and 20 min as the dwell times, respectively. The

heating rate was for 10 K min-1 and the punches load was

7 kN. The size of the cylindrical samples produced by SPS

was of 20 mm diameter and 3 mm height.

In order to point out the feasibility and advantages of

SPS route, other sintering technique is used in this study,

namely CS. Green compacts were pressed by uniaxial cold

compaction as cylindrical parts of 10 mm diameter and

5 mm height in a metallic die, at 120 MPa. For the sin-

tering treatment, a laboratory Nabertherm chamber furnace

was used (type L5/12, maximum temperature 1,200 �C. An

argon atmosphere (99.98 % purity) was delivered during

the entire CS treatment. The sintering parameters are

1,150 �C for 120 min.

Thermal analysis measurements (TG, DTA, DTG, and

DSC) of titanium hydride powders were carried out in

dynamic air and dynamic argon atmosphere (150 cm3

min-1) under non-isothermal linear regimes. A horizontal

Diamond TG/DTA analyser from PerkinElmer Instruments,

which draws simultaneously the four curves, was used for

measurements. Samples contained in alumina crucibles were

heated in the temperature range of RT–1,200 �C, each time

with a heating rate of 10 K min-1.

For XRD-characterization performed with Shimadzu

XRD-6000 X-ray diffractometer, the functioning parame-

ters of the X-ray tube (A40-Cu type) were established at a

voltage of 40 kV and a current of 30 mA. A continuous

scan measurement was chosen as operation mode in a

geometry (h/2h) setting at a scan rate of 2� min-1 and a

scan range from 10� to 100�. Divergence slit was of

1.0000�, scattering slit was of 1.0000�, and receiving slit

of 0.1500 mm. The optical evaluation of the SEM micro-

structure of Ti materials was performed on a microscope

type JEOL56LV having 3, 5 nm resolution with spec-

trometer EDX (Oxford Instruments). Micro hardness of the

final samples was determined using a micro-Vickers

hardness tester CV-400DTS.

Results and discussion

Morphological and structural characterization

of the raw material

Characterization by the scanning electron

microscopy (SEM)

In Fig. 1, SEM image of TiH2 powder is shown. From the

Fig. 1, it can be observed that TiH2 powder particles are

Table 1 Raw material components

Components Content (%)

Titanium Min. 95

Hydrogen Min. 3.8

Nitrogen Max. 0.30

Al Max. 0.13

Fe Max. 0.09

Cl Max. 0.06

Ni Max. 0.05

Si Max. 0.15

Mg Max. 0.04

C Max. 0.03

C. I. Pascu et al.

123

irregular and angular shaped which represents a great

advantage during compaction step. It is important to note

that after sintering stage, TiH2 particles change their shape

as it is visible furthermore in this research.

Characterization by the X-ray diffraction (XRD)

The precursor powder TiH2 used in the material prepara-

tion was indexed as a CFC cubic crystalline system as

given by the card no. 07-0370 (2001 JCPDS-International

Centre for Diffraction Data, PCPDFWIN v.2.2.) for

TiH1.971 having calculated cell parameter equal with

a = 4.4221 A. In the hypothesis of ellipsoidal shape of

particle, we used in the Scherrer’s equation the integral

breadth BI of the most intense peak and the obtained value

for the particle size was about 0.85 nm [26].

Figure 2 presents the RDX lines for precursor powder

TiHx (where x is 1.971) as received from the XRD mea-

surements, indexed in conformity with card no 07-0370 for

TiH1.971 phase (according to Sidhu, Argonne, National

Laboratory, Lemont, IL, USA, Private Communication).

The results are in good agreement with those presented in

Refs. [25, 27]. It is obvious that the identification of the

titanium hydride is correct; the small difference between

the XRD lines could be due to the processing technology of

the hydride powders used in this research that can change

the spectrum.

Thermal characterization of titanium hydride

Thermal analysis (TA) is a very important technique to

study thermal stability of the materials [28–41], and is

helpful in determining the sintering conditions.

TA of titanium hydride was performed by air and argon

flow in the temperature range of RT–1,200 �C. In both

instances, the flow rate was 150 cm3 min-1. Simultaneous

curves TG, DTG, DTA, and DSC were recorded.

Figure 3 contains the four thermoanalytical curves when

determination was performed in air. For experimental data

processing, specialized software Pyris was used.

Until 342 �C partial decomposition of titanium hydride

occurred, mass loss had been compensated by binding on

the surface of the oxygen of the air flow. From the 342 �C

(accepted as auto ignition temperature), loss of remaining

hydrogen from titanium hydride occurred, unaccompanied

by thermal effect. From this temperature, mass increasing

due to formation of porous titanium occurred, which

gradually oxidizes at the amorphous titanium dioxide,

passing successively in TiO and Ti2O3 [42–44]. Consid-

ering that the initial mass of titanium hydride was 100 %

and by hydrogen loss, titanium would be representing

95.99 % of the initial compound, experimentally obtained

final mass, of 161.8 % (Fig. 3), would correspond to

reaction with oxygen and represents an experimental mass

gain of 65.81 %. Mass increasing due to oxygen binding

should be theoretically of 64.15 %. The explanation for the

excess of oxygen is avidity of titanium to gas molecules

chemisorption from work atmosphere. Titanium dioxide

formed is amorphous, passing him between 990 and

1,010 �C in anatase (maximum on DTA and DSC curves at

1,000 �C) and over 1,040 �C, with maximum on DTA and

DSC curves at 1,045 �C phase transition occurs anatase to

rutile [45].

Other manner for thermal study of titanium hydride is its

decomposition with the linear increasing of temperature in

vacuum [25, 27] or inert gas atmosphere [27]. In Fig. 4, the

decomposition of titanium hydride in argon atmosphere is

presented.

From RT to 349 �C, the sample loses 0.46 % of its mass

by desorption of adsorbed gas on the surface of titanium

hydride particles. From 470 �C, three stages ofFig. 1 SEM image on TiH2 -powder

30 40 50 60 70 802θ /°

0

20

40

60

80

100

(111

)

(200

)

(220

)

(311

)

(222

)

Inte

nsity

/n.u

.

Fig. 2 The XRD lines for TiH2 precursor powder indexed as CFC

cubic structure

Bulk titanium for structural and biomedical applications

123

decomposition of titanium hydride succeeded, especially

highlighted on DTA and DSC curves (Fig. 5).

The first stage was very weakly endothermic and was

continued from 530 �C with the second stage, stronger

endothermic. The third stage, which is weakly endother-

mic, started at 618 �C and ended at 660 �C. These stages

cannot be identified on TG and DTG curves because of the

capacity of intermediate compounds formed and, then, of

porous titanium to deposit gas from the atmosphere that

will occur decomposition processes of titanium hydride

[24, 45–49]. Only in the second stage a little loss mass of

0.15 % in temperature range 540–569 �C was observed.

10 100 200 300 400 500 600 700 800 900 1000 1100 1200

Temperature/°C

99.7

105

110

115

120

125

130

135

140

145

150

155

160–50

–40

–30

–20

–10

0

10

20

–50

–40

–30

–20

–10

0

10

20

25

0.04

0.02

0.00

–0.04

–0.02

–0.06

–0.08

–0.10

Mas

s/%

Hea

t flo

w e

ndo

up/m

W

Mic

rovo

lt en

do u

p/μV

Der

ivat

ive

mas

s/m

g m

in–1DTG

DTA

DSC

TG

Fig. 3 Thermoanalytical curves

of titanium hydride in dynamic

air atmosphere

10 100 200 300 400 500 600 700 800 900 1000 1100 1200

Temperature/°C

105

110

115

120

125–50

–40

–30

–20

–10

10

20

–50

–40

–30

–20

–10

0

10

0.04

0.02

0.00

–0.04

–0.02

–0.06

–0.08

–0.10

Mas

s/%

Hea

t flo

w e

ndo

up/m

W

Mic

rovo

lt en

do u

p/μV

Der

ivat

ive

mas

s/m

g m

in–1DTG

DTA

DSC

TG

–60

–70

3035

0

99.5

126Fig. 4 Thermoanalytical curves

of titanium hydride in dynamic

argon atmosphere

–25

–20

–10

10

15

Hea

t flo

w e

ndo

up/m

W

0

–15

5

–5

400 450 500 550 600 650 700

Temperature/°C

Area = 9240.956 mJΔH = 1466.5049 J g–1

Peak = 556.75 °CPeak Height = –22.5015 mW

X 2 = 660.00 °CY2 = –3.5169 mW

X1 = 470.00 °C

Y1 = 8.6962 mW

Onset = 504.87 °C

End = 596.00 °C

Fig. 5 Thermal effects at TiH2

decomposition in argon

C. I. Pascu et al.

123

The accumulation of argon on the surface and inside pores

of formed titanium powder was continued until 1,200 �C,

when the mass sample becomes 125.9 % of initial mass of

sample. Decomposition in more steps of titanium hydride is

presented and in other papers [27, 50–57]. In [27] is

identified the fourth stage of decomposition, when TiH0,2

passing in Ti, but this stage can be massed with the third

stage. The total endothermic effect of titanium hydride

decomposition established by DSC curve integration is

DH = 73 ± 5 kJ mol-1. This value is close to the value of

63 ± 6 kJ mol-1 calculated by Sandim et al. [25] for the

activation energy of hydrogen desorption from d-TiH2.

The TA was carried both on air and argon in order to

underline the similar behavior of the TiH2 powder during

heating, namely the mass increasing: about 60 % in air and

26 % in argon.

Thermal analysis results were used to classical sintering

regime choice.

Morphological characterization of the obtained material

The SPS samples present the classic lamellar microstruc-

ture (a ? b), finer for that sintered at 1,000 �C for 10 min

sintering time and nearly bimodal for the one processed at

1,100 �C for 20 min sintering time. That could be

explained by the temperature and time increasing for the

holding stage of SPS process. Concerning the porosity, it

can be stated that the higher the sintering parameters

(temperature and time) are, the lower the porosity level is,

as the microstructures shown in Figs. 6 and 7.

On the contrary, the TiH2 classically sintered in argon

undergoes an oxidation process as the TA proved at the

beginning of the research, Fig. 4. The heating behavior of

the TiH2 during TA in argon is confirmed by its CS in

argon, showing a conventional oxidized surface of TiO2, as

Fig. 8 presents.

Both SPS regimes allow homogeneous diffusion pro-

cesses, thus the elemental maps (windows in Figs. 6, 7)

underline the homogeneous distribution of titanium which

is confirmed by the EDX analysis presented in Figs. 9 and

10. The same Ti distribution is registered for the bulk

material elaborated by CS route (window in Fig. 8).

It can be stated, based on these results related to EDX

reports that there is no carbon diffusion into the samples

from the graphite foils usually used in SPS for punches

protection. The EDX analysis reveals the TiO2 synthesis

after CS of TiH2, Fig. 11, where minor Al as impurity

comes from the original powder, Table 2.

Comparable hardness values are mentioned in the lit-

erature for Ti sintered by different methods using TiH2 as

single raw material [10, 13] or as mechanically milled

Fig. 6 SEM microstructure aspects of Ti material processed by SPS

route at 1,000 �C for 10 min

Fig. 7 SEM microstructure aspects of Ti material processed by SPS

route at 1,100 �C for 20 min

Fig. 8 SEM microstructure of Ti material processed by classic

sintering at 1,150 �C for 120 min

Bulk titanium for structural and biomedical applications

123

powder [13] or in mixture with titanium particles [58, 59].

Based on the experimental results on microhardness, an

experimental model has been designed by software Stat-

Soft STATISTICA7 in order to monitor the influence of the

SPS parameters on material hardness. The obtained equa-

tion points out the relationship between HV and T (sin-

tering temperature, in �C) and t (dwell time, in minutes).

HV500 ¼ 2131:0185� 4:4611T þ 45:6333t þ 0:0026T2

� 0:0393Tt � 0:0844t2

ð1Þ

That is graphically represented in Fig. 12.

As concerning the dwell time, in this particular case of

SPS route, its effect is sensitive only at 1,000 �C and

0 1 20 3 4 5 6 7 8 9 10

KeVFull scale 4509 cts cursor: 0.000 KeV

Spectrum 1

Ti Ti

TiFig. 9 The EDX

characterization of bulk Ti

material processed by SPS at

1,000 �C for 10 min

0 1 20 3 4 5 6 7 8 9 10

KeVFull scale 1080 cts cursor: 0.000 KeV

Ti Ti

Ti Sum SpectrumFig. 10 The EDX

characterization of bulk Ti

material processed by SPS at

1,100 �C for 20 min

0 1 20 3 4 5 6 7 8 9 10

KeVFull scale 1561 cts cursor: 0.000 KeV

Ti

O

AI

Ti

Ti Sum SpectrumFig. 11 The EDX

characterization of bulk Ti

material processed by CS at

1,150 �C for 120 min

C. I. Pascu et al.

123

slightly visible at 1,100 �C. This means that the densifi-

cation process is almost finished at 1,100 �C for 10 min as

dwell time. It can also be observed that longer sintering

time does not improve the hardness. As far as the mi-

crohardness of the CS of TiH2 is concerned, the experi-

mental results namely HV500 579…680 validate the

presence of TiO2.

Using TiH2 as raw material, it would be expected for a

porous sintered structure due to the dehydrogenation phe-

nomenon occurring during TiH2 heating, well-known and

described in the literature [27]:

Step I: TiH1:924 ! dþ H2 " ð2ÞStep II: d! bH þ H2 " ð3ÞStep III: bH ! bH þ H2 " ð4ÞStep IV: bH ! aH þ H2 " ð5ÞaH ! aþ H2 "

During SPS, the punches operate under a constant pressure

(PSPS % 22 MPa) on the samples. In these terms, hydrogen is

released following the Eqs. (2)–(5), without swelling.

In the next figures, the surface topographies of bulk Ti

materials processed by SPS and CS are presented.

Considering the titanium hydride behavior during heating,

Fig. 13 validates the dehydrogenation of TiH2 during SPS

by the rounded and smaller Ti grains interconnected by the

sintering necks (white arrows). The grains roughness, more

visible inside the dot white circle, may also provide

information on the porous/spongy surface area of the Ti

grains after sintering and dehydrogenation.

On the contrary, for the same TiH2 processed by CS, the

dehydrogenation phenomenon was inhibited by the odd

oxidation process developed in argon atmosphere resulting

in the TiO2 formation, Fig. 14.

Conclusions

Bulk Ti materials having good densification behavior could

be successfully obtained by PM technology using SPS

Table 2 Elemental spectrum of TiH2 processed by CS

Element App

conc.

Intensity

corrn.

Mass/

%

Mass/%

sigma

Atomic/

%

O–K 43.94 0.3527 59.60 1.25 81.43

Al–K 0.52 0.6630 0.38 0.14 0.31

Ti–K 73.86 0.8831 40.02 1.24 18.26

Total 100.00

1110

Temperature/°C108010501020

440

420

400

380

360

340

20

16

12

8

Dwell time/m

in

Hardness H

V500

/daN m

m–2

Fig. 12 The hardness variation versus the SPS temperature and dwell

time

Fig. 13 Surface topography of bulk TiH2 processed by SPS at

1,100 �C for 20 min

Fig. 14 Surface topography of bulk TiH2 processed by CS at

1,150 �C for 120 min

Bulk titanium for structural and biomedical applications

123

route. The obtained experimental results underline the

advantage of SPS route versus CS route:

1. the initial TiH2 powders (\100 lm and irregular and

angular shaped) become smaller than 1 lm, almost

spherical and close to porous surface due to the

dehydrogenation effect occurred only in vacuum SPS;

2. the decomposition of titanium hydride in argon occurs in

three steps with endothermic effect of 73 ± 5 kJ mol-1;

3. the TiH2 precursor powders undergo a hydrogen

desorption phenomenon which has a great influence

on the sintered materials, especially in the CS route;

4. dense and hard Ti structures could be obtained by SPS

in 10–20 min;

5. the CS in argon atmosphere shows an odd oxidizing

phenomenon leading to the TiO2 bulk transformation

of the initial TiH2;

6. the densification process seems to be fulfilled at

1,100 �C for 10-min dwell time by SPS.

Acknowledgements The authors gratefully acknowledge to the

research groups of Material Science of University Carlos III of

Madrid, Department of Materials Science and Chemical Engineering

and Politecnico di Torino, Italy, Department of Materials Science and

Chemical Engineering for providing technical assistance on partial

SEM microstructures.

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