MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 70

of sintering and subsequent annealing conditions. The authors' recent investiga-
tion 59,60 on Ca and Mg-doped zirconia reveals that 8 mol% CaO-doped PSZ
ceramics possess 97.5%

ρ th and 16 mol% CaO-doped FSZ possess only 91.6% ρ th ,

whereas more than 95% th in both Mg-PSZ and Mg-FSZ can be obtained, when

all are microwave (MW) sintered at 1585 °C for one hour. The microstructure of
Ca-PSZ ceramic is characterized by bimodal grain size distribution of coarser m-
ZrO 2 grains, embedded in cubic matrix. The optimized microstructure of both
Mg-PSZ and Mg-FSZ samples are characterized by the presence of coarser grains
with size in the range of 5-10

ρ

m. The obtained microstructure is superior com-
pared to conventional sintering route, which normally results in grain sizes of
20 - 50

μ

m.
SEM investigation also shows the presence of microporosity (

μ

m),
trapped at both intragranular as well as intergranular regions. XRD analysis
reveals that MW sintered Ca-FSZ contains predominantly c-ZrO 2 phase,
whereas the presence of monoclinic zirconia was recorded as a predominant
phase in Mg-PSZ samples. Although for FSZ samples, the predominant phase
is cubic, except for samples sintered at 1500 °C, for which the major phase is
m - ZrO 2 . The optimized Ca-PSZ and Ca-FSZ ceramics exhibit Vickers hardness
of around 10 GPa and 9 GPa, respectively; whereas the toughness was measured
for Ca - PSZ as 6 MPam 0.5 . Similarly, Mg - PSZ ceramics, sintered at 1585 ° C
possess a better combination of hardness (10.6 GPa) and fracture toughness
(6.8 MPa m 0.5 ).
Apart from microstructural investigation and mechanical property measure-
ment, the tribological properties in dry and simulated body fl uid were also evalu-
ated. For both Mg and Ca-doped ZrO 2 , a steady state COF of

< 10 μ

0.5 against bearing
steel ball is measured in dry condition and shows lower values (

∼

0.35 - 0.4) in SBF
lubrication contact. The wear mechanism is dominated by the formation of Fe x O y -
rich tribochemical layer, which was in contact with steel counterbody after the
steady state was attained. In the case of SBF medium, such tribochemical layer
additionally contains chloride compound. SEM images of the worn surfaces on
CaO doped zirconia are shown in Figure 3.10 and the formation of a tribochemi-
cal layer is evident.
The investigated CaO/MgO doped materials experience wear rate in the
order of 10 − 5 m m 3 /Nm (in air) and 10 − 6 to 10 − 7 m m 3 /Nm (in SBF) with the lowest
wear rate recorded with FSZ materials in SBF solution. The tribochemical layers
appear to form on abraded surface and thereby, reduce the material damage from
continued wear process. From the above, it should be clear that the optimized
MgO - doped or CaO - doped ZrO 2 exhibit much better combination of mechanical
properties, when compared to bioinert Al 2 O 3 or majority of the glass ceramics
and their composites. However, further experiments to study cytotoxicity prop-
erty and clinical trials need to be conducted on these materials to assess their
potential for biomedical applications. The above results also demonstrate the ef-
fi cacy of MW sintering as a processing tool to fabricate bioceramics with compa-
rable or better processed materials (pressureless sintering and post fabrication
annealing).

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