Biomedical Engineering Reference
In-Depth Information
Cai et al. [Cai et al., 1997a; Cai et al., 1997b] discussed the embryonic stage of
a sinter crack as observed in Figure 10.25. They found regions of low density and
cavitational defects in Al
2
O
3
/ZrO
2
/Al
2
O
3
laminates. These defects are most sus-
ceptible to residual (tensile) stresses during cooling in the core, due to the higher
CTE of zirconia. These regions of lower density (pores) must have formed as a
result of the tensile stress that develops during the differential shrinkage during
densifi cation between the Al
2
O
3
and the Al
2
O
3
/ZrO
2
layers. The pores then act as
pre - existing fl aws for the generation of thermal expansion mismatch cracks
during cooling via linkage of the pores and cavitational defects.
Eliminating transverse cracks in literature is mostly done by decreasing
the overall shrinkage of the composites. This is done in two different manners:
decreasing the composition difference between the different layers [Hillman
et al., 1996; Cai et al., 1997a], or adjusting the green density of the different
layers [Novak, 2005] Another possibility is to decrease the cooling and heating
rate during sintering [Cai et al., 1997a]. The mismatch stresses during the heating
cycle are decreased by the viscous nature of the FGM material. The sintering
mismatch stress is proportional to the mismatch sintering rate. Reduced
cracking under slow cooling rate is probably due to the relaxation of residual
stresses during the initial period of cooling. Cai et al. [Cai et al., 1997a] pro-
posed, therefore, a cooling rate of 1°C/min at high temperature. A decreased
cooling rate of 1°C/min, however, did not remove the sinter defects in the
authors' work. The cracks are also too large to remove by a hot isostatic pressing
cycle.
Therefore, a minimum Al
2
O
3
content of 75 vol. % in the homogeneous com-
posite core of the FGM discs was considered as a critical design criterion to avoid
the formation of transverse cracks.
10.8 CONCLUSIONS
Because the average life span of humans is drastically increasing and the need for
spare parts begins at about 60 years of age, biomaterials for prosthesis compo-
nents need to last for 20 years. The excellent performance of specially-designed
bioceramics that have survived the harsh clinical body environment condition,
represents one of the most remarkable accomplishments of ceramic research,
development, production, and quality assurance during the past 25 years. Of
special interest is the all-ceramic total hip joint replacement. Alumina and zirco-
nia are mainly used for the bearing components. However, zirconia can undergo
low temperature degradation in aqueous environment and alumina is brittle and
has a limited strength. Further progress has been achieved by the development of
zirconia - toughened aluminas (ZTA).
The very promising concept has been investigated in this work to combine
the best properties of ZrO
2
and Al
2
O
3
ceramics in a FGM, allowing to generate
compressive stresses at the component surfaces, thus increasing the strength
further.
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