Biomedical Engineering Reference
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100
95
No cracks
90
85
80
75
70
65
60
Cracks
55
50
0
1
2
3
4
5
distance d (mm)
Figure 10.24.
Concentration profi les of graded EPD Al
2
O
3
/ZrO
2
FGM discs. When the ZrO
2
concentration of the core exceeds 25 vol. %, transverse defects are observed. [Anné, 2005a].
(a)
(b)
Figure 10.25.
Typical transverse crack in a symmetrically graded plate with 50 vol. % Al
2
O
3
in
the core (a) and a detailed micrograph of a transverse crack surface. [Anné, 2005a].
the pure Al
2
O
3
layer. The crack opening displacement of most cracks is smaller
than 5
m. From a micrograph of the crack surface (Figure 10.25b), it is clear that
the crack surface is a clear fracture surface and not a sintered surface.
Hillman et al. [Hillman et al., 1996] observed similar defects in symmetrical
laminates with Al
2
O
3
/ZrO
2
layers at the surface and a ZrO
2
central layer. They
found the coexistence of two distinct classes of defects, that is, cracks with a large
opening displacement (
μ
>
30
μ
m) and cracks with a small opening displacement
(
m). It was suggested that defects originated from differential shrinkage
during drying, and that the subsequent densifi cation opened up the defects and
produced cracks with a large opening. The surfaces of this type of crack have
smooth round grains identical to that of a free surface of a densifi ed body. Finer
cracks are thought to be caused by the sintering process. Because the crack
opening in Figure 10.25 is relatively small (
<
2
μ
m), the transverse cracks in the
FGM discs can be assigned to the sintering cycle.
<
5
μ
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