Biomedical Engineering Reference
In-Depth Information
1.8.3 Optical microscopy analysis
An extensive network of the porosity of sintered samples can be obtained
using an optical microscope. Dark-filled areas reveal the pores to have a
crystalline surface texture and the denser (darker) and more porous (lighter)
regions of specimen can be readily distinguished. Also, a degree of
subsurface detail can be revealed by optical microscopy analysis. Optical
microscopy is carried out in reflected light with Nomarski differential
interference contrast (DIC) and dark-field modes on a suitable microscope.
The fibre volume fraction can be estimated for all samples. Matrix cracks
around fibres arising from residual stresses can also be observed and tend to
reach the specimen surface via dense matrix regions. Circumferential crack
patterns may indicate residual stress arising from fibre/matrix thermal
expansion mismatches.
1.8.4 Indentation techniques
The hardness and fracture toughness of materials are determined using
indentation techniques. For example, quadrant-disc specimens of Al
2
O
3
/SiC
nanocomposites and Al
2
O
3
were cut from a sintered disc (30mm in diameter
and 3.2mm thick) with a 1
m polished finish on both sides. Indentation was
performed using a Vickers hardness testing machine (Model A.V.K.-C2
Mitutoyo Corp., Kawasaki, Japan). Indentation loads of 20, 50, 100 and
200N with a holding time of 15 s for each indentation were used. The
diagonal, d, and radial crack length, c, with at least five indentations at each
load were measured using optical microscopy (Nikon, Inc., Tokyo, Japan).
The hardness, H was related to the diagonal, d, of the indentation and the
contact load, P,by:
μ
d
2
H
¼
1854
:
4
P
=
½
1
:
11
The fracture toughness, K
ic
, was determined using (Anstis et al. 1981):
Þ
1
=
2
P
c
3
=
2
K
ic
¼
0
:
0
16
ð
E
=
H
=
½
1
:
12
where E is the elastic modulus and c is the radial crack length measured
from the indentation centre.
1.8.5 Indentation thermal shock tests
The indentation thermal shock technique developed by Anderson and
Rowcliffe (1996, 1998) is used to study the thermal shock and thermal
fatigue behaviour of ceramic nanocomposites. In this technique, the thermal
shock resistance is measured by studying the propagation of median/radial
