Biomedical Engineering Reference
In-Depth Information
vol% fine ZrO
2
particles of size 1m or smaller, in a dense, fine-grained Al
2
O
3
matrix. While data in the literature are limited, hip simulator tests indicate that
the wear rate of ZTA±ZTA bearing couples is lower than that for Al
2
O
3
±Al
2
O
3
.
An Al
2
O
3
-matrix composite (AMC), manufactured by Ceram Tec, and
available under the trade name Biolox delta, has been reported to have a flexural
strength >1.2 GPa (3 times the value for Al
2
O
3
), and a fracture toughness of
6.5MPa m
1/2
(1.5 times the value for Al
2
O
3
), as well as a wear rate even lower
than the values for Al
2
O
3
-on-Al
2
O
3
or ZTA-on-ZTA (Willmann, 2001). This
AMC material is based on the ZTA composition, but also includes small
amounts of SrO and Cr
2
O
3
as additives, which react with the Al
2
O
3
matrix to
produce a small fraction of plate-like Al
2
O
3
grains during the high temperature
fabrication process. These plate-like grains provide an additional mechanism
(crack deflection at the boundaries of the plate-like grains) for enhancing the
toughness of Al
2
O
3
.
7.4.3 Hard ceramic coatings on metals
The bearing surfaces control the wear behavior, so it is not necessary to use a
monolithic bearing composed of the same material. Surface treatments such as
ion implantation and deposition of hard ceramic coatings have been widely
investigated for improving the surface hardness and, hence, the wear and
corrosion resistance of metal implants (Lappalainen and Santavirta, 2005). Many
of these treatments have not proved to be successful in clinical applications,
because of problems such as delamination of the coating, corrosion, poor
interfacial bonding, and porosity (Harman et al., 1997; Raimondi and Pietrabissa,
2000). For example, titanium nitride (TiN) coating can improve the hardness and
wear of metal (CoCr) bearings, but a key problem is that the TiN coating can
fracture and chip off when subjected to a stress concentration caused by wear
debris between the articulating surfaces, or by inadvertent contact with hard
components such as the metal rim of the acetabular cup, leading to catastrophic
third-body wear (Goldberg and Gilbert, 2004; Raimondi and Pietrabissa, 2000;
Williams et al., 2003).
Diamond-like coatings (DLCs) may represent a promising way to improve the
wear resistance of metal bearings such as CoCr. DLCs have desirable properties,
such as low friction, high wear and corrosion resistance, good surface-to-bone
bonding, and excellent resistance to surface abrasion (Lappalainen et al., 1998,
2003). Thin film diamond-coated CoCr femoral heads for use in THA are under
development by Diamicron Inc., Orem, UT.
7.4.4 Surface modification of metals
Because of the persistent concern of ceramic bearing failure in vivo, and the
unpredictable behavior of hard coatings on metals, an approach based on
