FUNCTIONALLY GRADED ALL CERAMIC HIP JOINT - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 323

These composites are commonly known as zirconia-toughened alumina (ZTA).
In the case of a stabilized zirconia matrix reinforced with alumina particles, that
is, alumina-toughened zirconia (ATZ), the risk of hydrothermal degradation
remains.
In ZTA with eight - to - fi fteen vol. % ZrO 2 , the favorable properties of Al 2 O 3
can be maintained since ZTA is nearly as hard as pure Al 2 O 3 . The ZrO 2 may
induce extra toughening and leads to a smaller grain size in the composites since
a homogeneous distribution of small ZrO 2 particles acts as grain growth inhibitor
for Al 2 O 3 grains during sintering [Konstztowicz and Langlois, 1996]. In ZTA com-
posites, the ZrO 2 addition can take the unstabilized or the Y 2 O 3 - ZrO 2 form. The
primary difference between these two forms of the ZTA system is that the former
relies on microcrack toughening [Hannink et al., 2000; Rühle et al., 1987] whereas
the latter has transformation toughening as the primary toughening mechanism
[Hannink et al., 2000; Green et al., 1989].
The toughness of ZTA composites with unstabilized ZrO 2 addition can be
improved by the processing of Al 2 O 3 - ZrO 2 nanocomposites [De Aza et al., 2002].
In this way, a larger amount of tetragonal ZrO 2 grains is present in the alumina
matrix and contributes to the transformation toughening mechanism [De Aza
et al., 2002]. It should be clear, however, that spontaneous transformation of the
unstabilized ZrO 2 grains can only be avoided when the grain size is kept below a
critical maximum value. A colloidal processing technique to establish this was
recently developed by Schehl et al. [Schehl et al., 2002].
Another option to increase the strength of ZTA ceramics is to incorporate
platelets into the matrix. The addition of whiskers is not allowed because they
constitute a health risk, which can be avoided when platelets are grown in situ
during sintering. Belmonte et al. [Belmonte et al., 1993] developed zirconia-
toughened alumina with in situ formed plate-like shaped calcium and strontium
hexaluminates with enhanced mechanical properties. It was found that plate-like
shape hexaluminates as reinforcing phase are formed upon addition of 0.1-1 vol.
% of strontium. In this way, Burger et al. [Burger, 1998] developed a ZTA material
grade for biomedical applications, containing 25 vol. % Y-TZP. Cr 2 O 3 was added
to enhance the hardness of the alumina. The biomaterial has a tensile strength of
1150 MPa and a fracture toughness of 8.5 MPa m 1/2 [Rack and Pfaff, 2000].
10.3.4 Ceramic Coatings on Metal Implants
To avoid the brittleness of ceramic ball-heads, a metallic ball-head can be coated
with a ceramic coating like TiN or DLC (diamond-like carbon). The idea is to
combine the hardness of TiN or DLC with the good mechanical properties of the
metallic ball-heads. It was found, however, that the benefi cial properties like a
reduced wear of DLC coatings in air or in vacuum are not present in bovine
serum [Hauert, 2003]. TiN coatings have a very good biocompatibility and high
hardness, reducing wear. The bioactivity of TiN promotes the formation of calcium
phosphate phases with adhesion strength, forming a promising alternative for
plasma sprayed hydroxyapatite coatings on titanium alloy stems [Piscanec et al.,