Biomedical Engineering Reference
In-Depth Information
4.7
Linearised Weibull plots for two materials. The parameters of the
two two-parameter distributions are shown in the figure.
which is known as the fracture origin. In ceramics, the fracture origin usually
has two sources: (i) microstructure imperfections that arise during processing,
such as pores, inner residual cracks, non-sintered clusters of secondary
phases, impurities and unwanted inclusions, large grains, etc. and (ii) defects
from machining and handling (usually surface cracks). While the latter may
be quite easily seen and identified, it is necessary to know and minimise the
former type of defect to ensure the reliability of the component. These defects
are therefore the subject of extensive studies. This is even more important in
composite materials, as the processes of milling and mixing two or more
starting powder phases leads to various technological defects (e.g. Dusza and
Kromp, 2003; Bala´ zsi et al., 2006; Dusza et al., 2009; Csehova´ et al., 2011).
Pores are one of the most frequently occurring technological flaws,
especially macropores and clusters of pores that remain after non-optimal
sintering. Typical examples are shown in Fig. 4.8, where macropores in
various ceramic and/or non-metallic materials are illustrated and identified
as the most common causes of failure in their respective materials. These
macropores can be simply void areas where the matrix grains have not
bonded adequately (Fig. 4.8(a)), impurities remaining from processing (Fig.
4.8(b)), empty spaces connected to clusters of very large grains in bimodal
structures (Fig. 4.8(c)) or other types of macropores (Fig. 4.8(d)). However,
residual microporosity may also have a significant influence on structural
integrity. An example is illustrated in Fig. 4.9 in which fast spark plasma
sintering of monolithic alumina resulted in incomplete bonding of the grains
