Microstructure and mechanical properties of hydroxyapatite ceramics with zirconia dispersion prepared by post-sintering

Microst~c~e and mecha~c~ propertiesof hydroxyapatiteceramics with zirconiadispersionprepared by pos~s~ter~g KojiIoku,~~~0

Yos~~a

and Seem

SGmiya

Research Laboratory of Engjneeriffg Materiafs, Tokyo institute of Technology. 4259 (Received 25 October 1988; accepted 5 January 7989)

~agatsuta,

Mjdor~ Yokohama 227, Japan

Hydroxyapatite ceramics with zirconia dispersion from fine powders synthesized hydrothermally were post-sintered at 1000-l 3009 under 200 NlPa of argon for 1 h without capsules, after normal sintering in air at 1200°C for 3 h. Densificatian was most significant with post-sintering at 1200%. Fracture toughness, Vickers hardness and elastic properties of these materials were investigated. Post-sintering gave twice the K,c value of transparent pure hydroxyapatite ceramics. Vickers hardness and Young’s modulus of the ceramics were increased by post-sintering. Keywords: ~ydroxyapatjte,

zirconia, mechanical propenies

Hydroxyapatite (Car0 {PO,), (OH),) is one of the materials most biocompatible with human bones and teeth, but its mechanical properties, especially toughness, are insufficient for hard tissuelm5. Recent studies have demonstrated that ceramics can be toughened byzirconia particles dispersed in them, due to transformation, microcracking, and/or crack defraction toughening mechanismse7. Zirconia-toughened hydroxyapatite ceramics prepared by hot-pressing hydroxyapatite + ZrO* powders above 1300°C were reported by Tamari et a/.*. However, the following developments are required: (1) (2)

more homogeneous dispersion than solid-solid mixing; lowering of sintering temperature to prevent the decomposition of hydroxyapatite.

Stoichiometric fine hydroxyapatite crystals can be synthesized hydrothermally’ and homogeneous dispersion of fine crystalline powders can be achieved by hydrothermal techniques”. Here we describe the preparation and characterization of fine hydroxyapatite powders dispersed with zirconia, and the phase changes of the powders with zirconia dispersion during heating. Basic information is acquired about sintering behaviour”. Hydroxyapatite ceramics with zirconia dispersion were prepared by postsintering from fine hydroxyapatite single crystals with zirconia dispersion synthesized hydrothermally. ___-._ -~ -Correspondence 0 1990

to Di K. loku.

Butterworth

Et Co (Publishers)

METHODS Powder

preparation

Coprecipitate of zirconium and yttrium hydroxides with molar ratio Y203:Zr02 = 3 : 97 was prepared with NH,OH from a solution of ZrOClz and YCI,. The coprecipitate was washed in distilled water until Cl- could not be detected in the filtrate. It was then crystallized into fine zirconia (3YZ) powder under hydrothermal conditions at 200°C under 2 MPa for 24 h. The powders obtained were dispersed in 0.1 M (NH4)2HP04 aqueous solution, then 0.167 M Ca (NOJ)2 aqueous solution was added (pH 10) to yield a white precipitate. The precipitate with weight ratio hydroxyapatite: 3YZ = 80:20 was crystallized again at 200°C under 2 MPa for 10 h in a 5 I autoclave with stirring (figure 7). The hydroxyapatite powders obtained were pressed isostatically (CIP) into discs of 6 mm diameter and 3 mm thickness. The discs were post-sintered’2,‘3 at 1 OOO1300°C under 200 MPa (Ar) for 1 h without any capsules, after normal sintering in air at 1200°C for 3 h.

Characterization

of the products

The phases produced were identified by powder X-ray diffractometry (XRD; Rigaku W-200) with Ni-filtered Cu Ka, operating at 40 kV and 80 mA. The crystalline phase of zirconia grains was identified by Raman probe microanalysis (Jobin-Yvon, U-l 000) with Ar+ laser. The morphology of the

Ltd. 0142-9612/90/010057-05$X3.00 Biomateriais

1990. Vol 1 1 January

57

Hydroxyapatite ceramics with Zr dispersion: K loku et al.

Sealed electrode

1III II

‘\*utoclave

//I:

--

Heating coil

a

Teflon stirrer

Control thermocouple .

-

Measurement thermocouple

-

Teflon b/eoker Figure 1

Reaction apparatus.

particles was observed by transmission electron microscope (TEM; Hitachi H-700). Sinterability of the powders was evaluated by dilatometry carried out in air at 1 O”C/min. The microstructures of the samples were observed with a scanning electron microscope (SEM; JEOL JSM-T200) on the polished surface after thermal etching.

Material

b Figure 2 Microcrack induced by Vickers indenter: (a) specimen section under indentation, (b) specimen surface.

cross-

testing

A micro-Vickers indentation method (Akashi, MVK-E II) was applied to determine the Vickers hardness and fracture toughness (K,c) of the samples at load 200 gf using the following formula, after Niihara14: K, c/Ha”2

= 0.203

(~/a)-~”

(1)

where H is the Vickers hardness, a is a characteristic dimension of the impression and c is a characteristic crack dimension (Figure 2).Young’s modulus was determined by a Knoop indentation method at load 500 gf using the following formula, after Evans et a/.15:

---___

-__

_I_,--_;----d

b/a = 0.142

- 0.45

(H/E)

(2)

where b/a is the ratio of the diagonal dimensions of the impression and E is the Young’s modulus (Figure 3). The density of the hot isostatic pressed (HIP) compacts was measured by the Archimedean method using mercury.

RESULTS Starting

AND

DISCUSSION

powders

Only hydroxyapatite and 3YZ phases were revealed by XRD in any products studied. The lattice parameters of the hydroxyapatite (Table 7) were in good agreement with stoichiometric hydroxyapatiteg’ 16.Thus, no reaction occurred between hydroxyapatite and 3YZ during the hydrothermal treatments. TEM demonstrated that the homogeneous mixture of fine hydroxyapatite single crystals (length -90 nm; aspect ratio -3.2) and fine 3YZ single crystals (-10 nm) could be obtained by the hydrothermal techniques”. The

58

Biomaterials

1990, Vol 11 January

Figure 3

Knoop indentation.

size of the crystals agreed very closely with the crystallite sizes calculated from XRD peaks (Table 2), the particles can therefore be regarded as well-crystallized single crystals”.

Normal

sintering

behaviour

According to dilatometry, shrinkage started at about 850°C then proceeded gradually with increasing temperature to Table 1

Lattice parameters

of hydroxyapatite (HA)

a0 X

(10-l

nm)

co

x

(1o-’ ml)

This work + HA)

9.418

f 0.002

6.879

+ 0.001

Previous works Ref. 9 Ref. 16

9.420 9.417

+ 0.001

6.880 6.880

f 0.0005

(3M

Hydroxyapatite ceramics with Zr dispersion: K. loku et al.

Table 2 Crystallite sizes of 3Y.Z hydroxyapatite (HA) powders synthesized hydrothermally at 200°C and 2 MPa (Ar) for 10 h

3.5 3M

HA (hexagonal)

4,,

result in linear shrinkage temperature

was

This

apatite into Ca, (PO,),: temperature

ceramics

normally

94%

density (3.49

apatite.

The

density

caused

apatite

to TCP

formation

up

to

for 20 wt% sintered H20

brought

(2.86

5), after normal

SEM

contained

g/cm3)

at 1200°C

most dense

under

microstructures

precipitates

and about

grain boundaries

at 1300°C

decomposition MPa

for

should

1 h

HIP

cRTlJ

1100 I

1200 I

1300 1

trans-

Post-sintering

temperature

(“CI

with

increasing

200

which

that MPa

grains

98%

at

in air at 1200°C

be higher than 98%.

a-TCP,

has a lower

the

(3.16

g/

post-sintered

(Ar) for

1 h had the

any pores consisted

and a-TCP

grains,

lower density

at the

ceramics

post-

due to increased

above. Post-sintering produced

of

about 0.5 pm

of cubic zirconia

6 and 7). However,

showed

as described

(Ar)

Before

1000 1

lower

by the

of about

sintering

without

4pm

(Figures

0

1 200”C’g.

density

showed

about 8 pm hydroxyapatite

200

3.1

hydroxy-

than that of hydroxyapatite

observation

HIP

of hydroxy-

also

densification

in a relative

this sample

sintered

and

After

“I

The

the calculated showed

-a

Figure 5 Density of hydroxyapatite ceramics with zirconra dispersion postsintered at temperatures indicated under 200 MPa (Ar) for 1 h, after normal sintering in air at 1200°C for 3 h.

about

to result

(Figure

ceramics

4).

wt%

decomposition

at about

for 3 h. The true relative density

cm3).

‘. The normal

behaviour

Post-sintering

density

pure

E 3.3 zw -% E 0 3.2

for 3 h had about with

3YZ-80

vapour,

to a-TCP

(Figure

at 1300°C

by the significant with

increasing

1200°C

compared

ceramics

temperature

because

with

at 1200°C

g/cm3)

Post-sintering

1200°C

increased

g/cm3)

of p-TCP

with

of hydroxy-

,,.--

A

I%‘E

A higher

compared

decomposition

sintered

(3.28

density

at 1300°C.

(TCP) by reaction with zirconia’

of the compacts

sintering

16%

to sintering

caused

3.4

11.1 * 2.0

of about

needed

hydroxyapatitet8. density

D IO1 (nm)

(nm)

25.4 + 3.0

80.7 ‘r 10.0

(tetragonal)

ceramics

at 1200°C. of

highest

density. Figure 6 SEM of the polished and thermally etched surface of the postsintered hydroxyapatite ceramics with zirconia dispersion by (HIP at 1200°C under 200 MPa (ArJ for 1 h, after normal sintering in air at 1200°C for 3 h. (White particles are zirconia.)

3.5

p

Mechanical

3.0

0 CT

The ceramics

post-sintered

at 12OO”C,

for 1 h had the highest Vickers

ZI c E

properties under 200

hardness

MPa (Ar)

and the highest K, c

value (Figure 8). The K, c value was twice that of transparent pure hydroxyapatite

2.5

cracking The

induced

ceramic

particles ism6,7.

must

900

1000

1100 Temperature

1200

1300

(“C)

Figure 4 Density of hydroxyapatite ceramics with zirconia dispersion sintered in air at temperatures indicated for 3 h.

of the

only

by dispersed

deflection

cubic

of micro-

in Figure

zirconia

toughening was

mechan-

detected (Figure

zirconia was not detected

10.

zirconia on the

7). Further-

on the fractured

by XRD. The Vickers hardness and Young’s modulus ceramics

significant

were

densification

of the hydroxyapatite ing

toughened

9)13. SEM is shown

surface by Raman spectroscopy

more, monoclinic surface

be

(Figure indenter

due to the crack In fact,

polished

2.0

ceramics

byvickers

zirconia

toughened

by post-sintering

(Table 3). Toughness

ceramics

content’.

byzirconia

increased

due to

and elasticity

should increase with increas-

Dense

hydroxyapatite

must be applicable

Eiomaterials

ceramics

to the tooth root.

1990, Vol I 1 January

59

Hydroxyapatite ceramics with Zr dispersion: K loku et al.

7 6Ar+

/,

F

/ 25-=-

LA

-

I

Post -sintering temperature (“C) Figure 9 Mechanical properties of the post-sintered pure hydroxyapatite ceramics by HIP at temperatures indicated under 200 MPa (Ar) for 1 h, after normal sintering in air at 1050°C for 3 h.

I

1

I

800

I

L

600

1

I

400

I

200

Raman shift (cm-‘) Figure 7 Raman spectra of different grain sizes of zirconia particles of the post-sintered ceramics shown in Figure 6: (a) IOpm (h) 2 pm (c) 1 pm.

7 6if iz,\ E 0

ik -

4 i-

?

z

2 1

5Y.m I

I

1000

1100

0

I

1200

Post -sinterinq temperature

I

1300 (“C)

Figure 8 Mechanical properties of the post-sintered hydroxyapatite ceramics with zirconia dispersion by HIP at temperatures indicated under 200 MPa (Ar) for 1 h, after normal sintering in air at 1200°C for 3 h.

SUMMARY Fine hydroxyapatite single crystals dispersed with fine 3YZ single crystals were prepared by hydrothermal techniques of 200°C under 2 MPa for 10 h. Densification of the hydroxy-

60

Biomaterials

1990, Vol 11 January

Figure 10 SEM of an indent and surface crack on hydroxyapatite ceramics with zirconia dispersion post-sintered at 1200°C under200 MPa (Ar) for 1 h, after normal sintering in air at 1200°C for 3 h. Crack deflection is clearly observed.

apatite compact was not improved by zirconia dispersion. It caused decomposition of hydroxyapatite into TCP by reaction with zirconia at over 850°C. Post-sintering brought about densification with increasing temperature, i.e. relative density -98% at 1200°C under 200 MPa (Ar) for 1 h, after

Hydroxyapafite

Tab/e 3

Mechanical

properties

of

several

hydroxyapatite

apatite

Akao.

(HA)

M.,

Mwa.

92, Normal

Post-

Relative

sintering

sintermg

denstty

(96)

Hv*

K,c

(GPa)

(MPa

ml”)

HA

E**

Evans,

(GPa)

Technology

1050=c,

-

90.0

4.1

1.2

7

+ HA

12OO’C.

-

>94+

4.6

1.9

149

Vol.

3h 12OO’C.

1200°C.

3h

3h

>98+

5.5

2.1

171

8

load

200

gf

** Knoop:

load

500

gf

9

10

mechamsms

In zrcoma

II, Advances

in Ceramics,

Ruble

OH,

in air at 1200°C

ceramics

with zirconia

had the

highest modulus

because

were

(5.5

and

ceramics

The Vickers

of the ceramics

post-sintered

hardness

(1 71 GPa) The

zirconia dispersion

for 3 h. The hydroxyapatite

dispersion

Vickers

MPa ml”).

mechanism.

12

and

(Eds

N.

Ceramic

Society.

Tamarl,

N.,

exlstmg

phases

loku.

the

at 1200°C

GPa),

the

design

M. OH,

M.

alloys,

smtered 1984,

Science

Vol.

American

12

and

(Eds

Ceramic

N.

Society,

of zirconla-toughened

of Zirconia

Claussen,

and

Ruble

AH

1984. I.,

in Ceramics,

Heuer),

American

pp 325-351

Mechamcal

ceramics

propenles

obtamed

and zwconla,

ceramics

II, Advances

and

USA,

Kondo.

of composite

K.. Yoshlmura,

highest

could

be

K,c

1565-l

570

Somiya,

S., Yoshtmura.

increased

wth

and

by smtermg

Yogyo-Kyokai-Shi

of a

1987,

USA,

modulus

Columbus,

12

14

gratefully

thank

Toray

industries

1988,

Fujwara,

95,

15

spectroscopy.

and

Yoshimura,

and

Ceramic

Meeting

synthesis

dlsperslon, of MRS,

Research

Vol

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Biomedical 1 10)

Society.

(Eds

J.S.

PennsylvanIa,

M.,

Hattori.

of

T..

apatlte

Aoki.

H..

ceramics,

Uchlda,

M.

and

Yogyo-Kyokai-Shi

753-755 Yoshlmura,

M.

fme

condltlons.

Seramikkusu

96,

10

109-l

Marshall,

Somlya.

S.,

Japan T.

and

20.

J.

Am.

12--l

Evans,

A.G..

A

Sot.

apattte

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-

1988,

Its application

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ratios

Ceram

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hydrothermal

Sot.

of ceramics

1985,

elastic-modulus-to-hardness

measurements,

under

(J, Ceram.

mwofracture

Noma,

Post-stnterlng

synthesized

Ronbunshi

Ceramics

D.B..

determining

and

powders

K.. Indentation

problems,

tatlon

S., Hydrothermal

Materials

Post-smtering

from

Nilhara.

Science

American

pp 445-450

S..

K.,

T.,

and hafnta

pp 43-55

zrconla

(Proc.

Powder

Vol. 2 1

of

1988,

Kumakl.

zirconla

Ceramic

Somlya, wth

Kaishl

K. and

smgle-crystal

1987,

synthesis

Kagaku

2.. Hishmuma.

Ceramics,

USA,

M.

Devices

S.,

95,

loku,

and

for analysis

OH,

Njppon

dopants.

in

and B.L. Glammara),

Somiya,

by post-sintering.

Nakal.

powders

and

S., Hydrothermal

crystals,

of ultrafIne

homogeneous

K., Yoshimura.

Hanker

Somlya,

M..

Advances

Materials

by

toughening

and the Young’s

loku,

and single

processing

hydroxyapatite

value

toughened

of a crack deflection

hardness

11

M.

hydroxyapatlte

Society,

highest

ACKNOWLEDGEMENTS

Raman

of

pp 193-212

Columbus,

ultrafine

ceramics

with

Heuer).

Technology

of hydroxyapatite

1987,

authors

A.H.

1984,

Mourl,

Technology,

13

The

and

USA,

Science

powders

(2.1

toughness

et al.

Yogyo-Kyokai-Sh!

Toughening

Hydrothermal

Young’s

K. loku

806-809

+ See text.

normal sintering

Fracture

phosphate,

N., Microstructure

mixture *Vlckers:

H.,

Zr dlspersw?:

of Zirconia M.

Claussen, (ZTC),

3YZ

Aokl.

and fl-trIcalcwm

A.G.,

Columbus,

110

3h

and

wrth

672-674

Claussen, Pure

N.

hydroxyapatite

ceramics

ceramics

usmg

1982.

method Knoop

65,

for

Inden-

C-175-C-

176 16

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