FUNCTIONALLY GRADED ALL CERAMIC HIP JOINT - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 322

TABLE 10.2. Overview of Mechanical Properties of Femur Bone, Al 2 O 3 and ZrO 2

Properties Femur [Currey, 1984 ] Alumina Zirconia

Density (g/cm 3 )

≥ 3.97 > 6.00

Tensile strength (MPa) 121 > 500 900 - 1200

Hardness (GPa, HV 10 ) 0.37 22 12

m 1/2 ) Toughness (MPa 4 5 - 10

Compressive strength (MPa) 167 4100 2000

E - modulus (GPa) 17 - 18 380 210

( 1500 MPa) [Clarke et al., 2003], also the fracture toughness is much higher

(Table 10.2). The higher fracture toughness is of importance in femoral heads due
to the tensile stress induced by the taper fi t of the femoral stem [Cordingley et al.,
2003]. Therefore, smaller ball-heads are possible with TZP compared to Al 2 O 3 .
Most of the zirconia implants are stabilized with two to three mol % Y 2 O 3
[Cales et al., 1994], in order to stabilize the tetragonal ZrO 2 phase at room
temperature. The high toughness of these ceramics is related to transformation
toughening, that is, the stress-induced martensitic transformation of the tetrago-
nal to monoclinic ZrO 2 in the vicinity of a propagating crack tip, resulting in a
local volume expansion of three to four percent that results in compressive
stresses inhibiting crack propagation [Hannink et al., 2000].
However, there is a major concern about the hydrothermal degradation of
ZrO 2 at room temperature. The low temperature degradation (LTD) of Y-TZP
involves a spontaneous transformation of the tetragonal phase when ageing in
water, resulting in the formation of microcracks at the surface and a decrease of
the mechanical properties [Sato et al., 1985]. Whether LTD will occur or not
depends on the environment with which the material is in contact. The presence
of water is an essential prerequisite. Also the composition, the microstructure,
and the processing route of the material as well as any possible thermal activation
play a role. Therefore, the grain size of the t-ZrO 2 grains, the Y 2 O 3 content, the
Y 2 O 3 distribution, density, surface roughness and purity are important parame-
ters in the possible degradation process [Piconi and Maccauro, 1999]. It has to be
noted that ageing effects of ZrO 2 in a water or salt solution can differ from ageing
in bovine serum. To avoid ageing of ZrO 2 , several approaches have been pro-
posed such as reducing the initial grain size, coating the implant surface with a
stable layer of cubic ZrO 2 , replacing the Y 2 O 3 stabilizer by CeO 2 , the addition of
small amounts of SiO 2 , the addition of small amounts of alumina, and the use of
ZrO 2 composites [Clarke et al., 2003].

>

10.3.3 Zirconia Toughened Alumina
ZrO 2 and Al 2 O 3 have both disadvantages, that is, the brittleness of Al 2 O 3 and the
risk of hydrothermal degradation in ZrO 2 . To avoid these problems, a good alter-
native is composites in which ZrO 2 particles are embedded in an Al 2 O 3 matrix.