Biomedical Engineering Reference
In-Depth Information
10.5.4 Properties of FGM Discs
One of the main advantages of FGM materials is the infl uence of residual stresses
on the properties of the component. A correct design of a composition gradient
can generate compressive stresses at those places that are loaded under tensile
stress during operation. In this way, the strength of FGM materials can be higher
than that of the homogeneous material.
To study the FGM effect on the strength, biaxial strength measurements are
performed on homogenous and Al
2
O
3
/ZrO
2
FGM discs with a diameter of 35 mm
after sintering one hour at 1550 °C and hot isostatic pressing (20 minutes at
1390 °C at 140 MPa Ar pressure) to ensure full density. The composition profi le is
given in Figure 10.7. Hard machining is performed to obtain plan-parallel sur-
faces; 0.5 mm is removed from the pure Al
2
O
3
surface, which resulted in a com-
pressive stress of 170
30 MPa on this surface. A biaxial (ring - on - ring) test is used
according ISO 6474. Whereas an average strength of 288 MPa is reached with a
Weibull modulus of
m
= 9 for the pure Al
2
O
3
disc, the strength of the ground
Al
2
O
3
/ZrO
2
FGM discs is much higher. A mean strength of 513 MPa was reached
with a Weibull modulus of
m
= 12 (Figure 10.9 .)
The benefi cial effect of the compressive stresses in functionally graded
alumina-zirconia composites on their wear and friction behavior during sliding in
water is shown in Figure 10.10. The FGM discs in this study were prepared by
sequential slip casting [Novak and Beranič, 2005]. The results, which are com-
pared to results from homogeneous alumina, show that with increasing residual
compressive stresses in the samples of functionally graded material (FGM) the
wear is strongly reduced. As a consequence of reduced crack formation and
debris detachment from the surface (due to increased residual compressive stress)
the tribochemical layer became thinner, with fewer topographical irregularities at
the surface. This increases the role of the tribochemical actions compared to the
mechanical wear, which benefi cially affects the tribological performance in water
[Novak et al., 2007 ].
±
2
Al
2
O
3
disks
FGM disks
1
0
m = 12
m = 9
-1
-2
-3
288 MPa
513 MPa
-4
5
5.5
6
6.5
ln (Strength)
Figure 10.9.
Weibull plot of the biaxial strength of homogeneous Al
2
O
3
and Al
2
O
3
/ZrO
2
FGM
discs [Anné, 2005a]; S is the probability of survival.
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