Hydroxyapatite – TZP Composites: Processing, Mechanical Properties,Microstructure and in Vitro Bioactivity

By: Nayak, YougojotiContributor(s): Bhattacharyya, Santanu [Supervisor] | Bal, Smrutisikha [Supervisor] | Department of Ceramic EngineeringMaterial type: TextTextLanguage: English Publisher: 2010Description: 196 pSubject(s): Engineering and Technology | Ceramic Engnieering | Ceramic ProcessingOnline resources: Click here to access online Dissertation note: Thesis (Ph.D)- National Institute of Technology, Rourkela Summary: The present study describes the results on hy droxyapatite (HA)-TZP (tetragonal zirconia polycrystal) composites containi ng low weight percent TZP (2 , 5, 7.5 and 10 wt%). The work could be divided into two parts. In the first part, hydroxyapatite was synthesized by two different chemical precip itation route; normal precipit ation (NP) and reverse strike precipitation (RS) using Ca(NO 3 ) 2 .4H 2 O and (NH 4 ) 2 HPO 4 as the precursor for hydroxyapatite. The as precipitated amor phous powders were studied for thermal decomposition behavior (DSC/TG), phase evol ution and phase stability of the calcined powder, particle size distri bution and non-isothermal densif ication behaviour. The TEM of the calcined hydroxyapatite powder shows that HA parti cles prepared from both the routes have elongated morphology. The particle size distribution of 850 O C calcined powder is multimodal for NP an d monomodal for RS route. The sintered density of RS route HA is higher than that of NP route for the entir e sintering range studied. The results also indicate that while RS-HA remains phase stabile until 1250 O C, NP-HA decomposes to a mixture of HA and TCP at that temperature. Based on the above observations, the RS r oute was adopted for synthesis of HA-3Y- TZP composite containing 2, 5, 7.5 and 10 wt% of TZP respectively. The composite powders were also characterized by FTIR, DSC/TG, XRD and TEM. The raw HA-TZP powders crystallized between 650 O C-1050 O C and the calcined powder (850 O C/2hrs) had phase pure HA and TZP. The composites we re sintered by pressureless sintering at 1250 O C/4hrs. XRD of the sintered composites revealed that at higher TZP content (HZ5, HZ7 and HZ10), β -TCP and α -TCP along with HA and t-ZrO 2 were present . The TEM of calcined powder showed dark col oured TZP particles (5-10 nm) uniformly distributed in light colored HA (50-150 nm) matrix. The hi ghest sintered density (> 99%) was obtained for HA- 2wt% TZP composite (HZ2). The density for 5, 7.5 and 10 wt% TZP containing composites were 96%, 92% and 90% respectively. The fracture strength of the composites appeared to be controlled by the size of t he largest flaw present in the sintered sample. The highest three poi nt bending strength was 72MPa for HZ2 composite while 30MPa was lowest for HZ5 composite. The decrease in strength at higher TZP additions could be due to the incomplete densific ation of the composite due ii to the constraining effect of TZP. The highest fracture toughness (0.97MPam 1/2 ) was recorded for HA-5 wt% TZP (HZ5). SEM micr ostructure showed a bimodal grain size distribution with presence of very fine and c oarse grains. A grain size reduction of HA was observed with TZP additio n. It may be pointed out her e that at a lower TZP calcination temperature (850 O C), the fine TZP particles forms agglomerates thereby hampering the final stage of dens ification. The Vickers hardne ss exhibited also porosity dependence. The maximum hardness (3.83GPa) wa s for HZ2 and the lowest (2.43GPa) was for HZ10 composites. Thus, it was anticipated that an effort should be made to increase the sintered density par ticularly at higher wt% TZP (i.e. 5, 7.5 and 10) which could effectively improve the strength, toughness and ha rdness of the composite. Therefore, in the next stage of work, an effort was made to observe the effect of TZP calcination temperature on the agglomeration behaviour of TZP and t hereby on sintered density, phases, mechanical properties and mi crostructure of HA-TZP composites. Therefore as discussed above, calcination temperature of TZP was increased from 850 O C to 1200 O C. It was observed that the calcinat ion temperature had a considerable effect on the densification behaviour, phase retention, strength, toughness, hardness and microstructure of composites. Two differ ent sets of the HA- TZP composite were prepared from TZP powders which have been ca lcined at two differe nt temperatures (viz 850 O C and 1200 O C). The composites prepared from both type of TZPs were sintered in air at 1250 O C/4hours. When TZP was calcined at 850 O C, the prepared HA- TZP composites had low sintered density (exc ept for 2% TZP compos ition), relatively low strength (37 MPa), toughness (0.8 MPam 1/2 ) and hardness (2.4 GPa) for HZ10 composite but significant amount of retain ed HA and TZP even at higher TZP loading such as 5, 7.5 and 10 wt%. A higher TZP ca lcination temperature c aused a reduction in HA and TZP phase re tention along with t he formation of CaZrO 3 and TCP. Microstructure shows a substantial increase in HA grain size with increase in TZP amount. In the HA-TZP co mposite, containing 1200 O C calcined TZP, sintered density improved for all the HA-TZP compositions along with an increase in strength toughness and hardness. The maximum bending strength, fracture toughness and hardness were 65 MPa, 1.6 MPam 1/2 and 5.25 GPa respectively obtained for HA-10 wt% TZP iii composite. Unlike the 850 O C calcined TZP batch, the str ength was independent of flaw size. It was noticed that the highes t strength also had high CaZrO 3 content thereby suggesting the presence of CaZrO 3 contributed to strength increase. The fracture toughness trend suggested that microcrack t oughening is a contri buting factor for toughness increment. It was thought the r eactions between HA and TZP could be effectively suppressed or avoided by usi ng a faster and low temperature sintering technique such as hot pre ssing. The hot pressed (950 O C/30 MPa) composites showed remarkable improvement in sintered densit y and phase retention. The XRD results revealed that HA and TZP were major phase in hot pressed HA-TZP composites which is strikingly different from the pressureless sintered composites. The densities of hot pressed composites are > 98% irrespective of their composition and TZP calcination temperature. The maximum flexural st rength (120 MPa) and hardness (5.8GPa) was obtained for HZ2 composite cont aining TZP calcined at 850 O C (H2Z8). The toughness value is maximum (1.5 MPa √ m) for HZ10 composite containing TZP calcined at 1200 O C (H10Z12). The SEM microstructure of hot pr essed HA-TZP compos ite showed the HA grain size to be in the range of 200nm. The in vitro bioactivity test was performed in our laboratory using standard SBF solution. The bioactivity tests showed apatite format ion on the surface of HA-TZP composites. The volume fraction of apatite formed its morphology and size was dependent on the HA-TZP composites as well as on aging time . The cytotoxicity tests were conducted as per ISO 10993-5 using L929 mouse fibroblast cells . No toxic effects of the specimens on the cells were observed thus confirming the nontoxic behaviour of the composites after 24 hours of exposure. The blood compatibil ity of the composite was also examined as per ISO 10993-4 by an in-vitro hemocompatibility test using human blood and none of the composite has induced haemolysi s during the scheduled exposure period.
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Thesis (Ph.D)- National Institute of Technology, Rourkela

The present study describes the results on hy
droxyapatite (HA)-TZP
(tetragonal zirconia
polycrystal) composites containi
ng low weight percent TZP (2
, 5, 7.5 and 10 wt%). The
work could be divided into two
parts. In the first part, hydroxyapatite was synthesized by
two different chemical precip
itation route; normal precipit
ation (NP) and reverse strike
precipitation (RS) using Ca(NO
3
)
2
.4H
2
O and (NH
4
)
2
HPO
4
as the precursor for
hydroxyapatite. The as precipitated amor
phous powders were studied for thermal
decomposition behavior (DSC/TG), phase evol
ution and phase stability of the calcined
powder, particle size distri
bution and non-isothermal densif
ication behaviour. The TEM
of the calcined hydroxyapatite
powder shows that HA parti
cles prepared from both the
routes have elongated morphology. The
particle size distribution of 850
O
C calcined
powder is multimodal for NP an
d monomodal for RS route.
The sintered density of RS
route HA is higher than that
of NP route for the entir
e sintering range studied. The
results also indicate that while
RS-HA remains phase stabile until 1250
O
C, NP-HA
decomposes to a mixture of HA
and TCP at that temperature.
Based on the above observations, the RS r
oute was adopted for synthesis of HA-3Y-
TZP composite containing 2,
5, 7.5 and 10 wt% of TZP
respectively. The composite
powders were also characterized by FTIR,
DSC/TG, XRD and TEM. The raw HA-TZP
powders crystallized between 650
O
C-1050
O
C and the calcined powder (850
O
C/2hrs)
had phase pure HA and TZP. The composites we
re sintered by pressureless sintering
at 1250
O
C/4hrs. XRD of the sintered composites
revealed that at higher TZP content
(HZ5, HZ7 and HZ10),
β
-TCP and
α
-TCP along with
HA and t-ZrO
2
were present
.
The
TEM of calcined powder showed dark col
oured TZP particles (5-10 nm) uniformly
distributed in light colored
HA (50-150 nm) matrix. The hi
ghest sintered density (> 99%)
was obtained for HA- 2wt% TZP composite
(HZ2). The density for 5, 7.5 and 10 wt%
TZP containing composites were 96%, 92% and 90% respectively. The fracture strength
of the composites appeared to be
controlled by the size of t
he largest flaw present in the
sintered sample. The highest three poi
nt bending strength was 72MPa for HZ2
composite while 30MPa was lowest for HZ5 composite.
The decrease in strength at
higher TZP additions could be due
to the incomplete densific
ation of the composite due
ii
to the constraining effect of TZP.
The highest fracture toughness (0.97MPam
1/2
) was
recorded for HA-5 wt% TZP (HZ5). SEM micr
ostructure showed a bimodal grain size
distribution with presence of very fine and c
oarse grains. A grain size reduction of HA
was observed with TZP additio
n. It may be pointed out her
e that at a lower TZP
calcination temperature (850
O
C), the fine TZP particles forms agglomerates thereby
hampering the final stage of dens
ification. The Vickers hardne
ss exhibited also porosity
dependence. The maximum hardness (3.83GPa) wa
s for HZ2 and the lowest (2.43GPa)
was for HZ10 composites.
Thus, it was anticipated that an effort should be made to
increase the sintered density par
ticularly at higher wt% TZP
(i.e. 5, 7.5 and 10) which
could effectively improve
the strength, toughness and ha
rdness of the composite.
Therefore, in the next stage
of work, an effort was made to
observe the effect of TZP
calcination temperature on the agglomeration
behaviour of TZP and t
hereby on sintered
density, phases, mechanical properties and mi
crostructure of HA-TZP composites.
Therefore as discussed above,
calcination temperature
of TZP was increased from
850
O
C to 1200
O
C. It was observed that the calcinat
ion temperature had a considerable
effect on the densification behaviour, phase
retention, strength,
toughness, hardness
and microstructure of composites. Two differ
ent sets of the HA-
TZP composite were
prepared from TZP powders which have been ca
lcined at two differe
nt temperatures
(viz 850
O
C and 1200
O
C). The composites prepared
from both type of TZPs were
sintered in air at 1250
O
C/4hours. When TZP
was calcined at 850
O
C, the prepared HA-
TZP composites had low sintered density (exc
ept for 2% TZP compos
ition), relatively
low strength (37 MPa), toughness (0.8 MPam
1/2
) and hardness (2.4 GPa) for HZ10
composite but significant amount of retain
ed HA and TZP even at higher TZP loading
such as 5, 7.5 and 10
wt%. A higher TZP ca
lcination temperature c
aused a reduction in
HA and TZP phase re
tention along with t
he formation of CaZrO
3
and TCP.
Microstructure shows a substantial increase
in HA grain size with increase in TZP
amount. In the HA-TZP co
mposite, containing 1200
O
C calcined TZP, sintered density
improved for all the HA-TZP compositions
along with an increase in strength toughness
and hardness. The maximum bending strength, fracture toughness and hardness were
65 MPa, 1.6 MPam
1/2
and 5.25 GPa respectively obtained for HA-10 wt% TZP
iii
composite. Unlike the 850
O
C calcined TZP batch, the str
ength was independent of flaw
size. It was noticed that the highes
t strength also had high CaZrO
3
content thereby
suggesting the presence of CaZrO
3
contributed to strength increase. The fracture
toughness trend suggested that microcrack t
oughening is a contri
buting factor for
toughness increment. It was thought the r
eactions between HA and TZP could be
effectively suppressed or avoided by usi
ng a faster and low temperature sintering
technique such as hot pre
ssing. The hot pressed (950
O
C/30 MPa) composites showed
remarkable improvement in sintered densit
y and phase retention. The XRD results
revealed that HA and TZP were major phase in
hot pressed HA-TZP composites which
is strikingly different from
the pressureless sintered composites. The densities of hot
pressed composites are > 98% irrespective
of their composition
and TZP calcination
temperature. The maximum flexural st
rength (120 MPa) and hardness (5.8GPa) was
obtained for HZ2 composite cont
aining TZP calcined at 850
O
C (H2Z8). The toughness
value is maximum (1.5 MPa

m) for HZ10 composite containing TZP calcined at 1200
O
C
(H10Z12). The SEM microstructure of hot pr
essed HA-TZP compos
ite showed the HA
grain size to be in the range of 200nm.
The in vitro bioactivity test was performed in
our laboratory using standard SBF solution.
The bioactivity tests showed apatite format
ion on the surface of
HA-TZP composites.
The volume fraction of apatite formed its
morphology and size was dependent on the
HA-TZP composites as well as on aging time
. The cytotoxicity tests were conducted as
per ISO 10993-5 using L929 mouse fibroblast cells
. No toxic effects of the specimens
on the cells were observed thus confirming
the nontoxic behaviour
of the composites
after 24 hours of exposure. The blood compatibil
ity of the composite was also examined
as per ISO 10993-4
by an in-vitro hemocompatibility test
using human blood and none
of the composite has induced haemolysi
s during the scheduled exposure period.

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