Biomedical Engineering Reference
In-Depth Information
probably due to their higher degree of hardness, the positive effect of which
was more significant than the influence of the lower fracture toughness.
The wear mechanisms and tribological characteristics were almost
analogous for monolithic Si
3
N
4
and Si
3
N
4
-SiC composite materials.
Mechanical wear (microfracture) and tribochemical reaction were found
to be the main wear mechanisms for all the studied materials. Examples of
the worn surfaces of the tested plate specimens at room temperature are
shown in Fig. 4.25. SEM observations of wear tracks revealed that with
increasing wear resistance, the worn surface becomes quite smooth, but
adherent debris is still observed. Addition of SiC into the Si
3
N
4
materials
results in a higher incidence of 'islands' of coherent debris. Moreover, the
highest amount of coherent debris was observed in material with the highest
wear resistance, i.e. for the composite sintered with Lu
2
O
3
. These 'islands' of
coherent debris, which were observed in all studied wear tracks in different
volumes, constitute a tribofilm affording some protection to the ceramic
surfaces and decreasing the wear coefficient of materials (Fig. 4.26(a) and
4.26(b)). A larger amount of coherent debris was observed in both kinds of
material in the case of composites or materials doped with smaller ionic
radius RE
3+
. The tribochemical reactions created SiO
2
-based tribofilm in
the tested samples, which was partially removed above a critical load,
resulting in microfractures in discrete regions due to the propagation of a
micro-crack (Fig. 4.26(c) and 4.26(d)). This microfracture, shown in Fig.
4.26(d), has very similar features to the fracture surface of the material after
the common bending test.
An increase in fractured areas and in the amount of silicon nitride debris
were observed in specimens with lower wear resistance. The fractured areas
show a mixture of intergranular and transgranular failure modes in the
worn surfaces. According to the mechanisms described for wear in silicon
nitride based materials (Dong and Jahanmir, 1993; Skopp et al., 1995; Wang
and Hsu, 1996), cracks initiate below the surface and their propagation is
assisted by fatigue effects in the mixed intergranular/transgranular mode,
leading to microscale delaminations and fragmentation.
Coherent layers formed on the wear surfaces contain large amounts of
oxygen (observed by EDX analyses), suggesting that the layers are
composed mainly of the oxidation products of silicon nitride. The formation
of oxide may be accelerated by the presence of small particles of silicon
nitride formed by fracture under the high contact stresses at the wear
interface. These particles have a high specific area, which would increase the
amount of reaction product formed when compared to a reaction with the
ceramic bulk. This would cause some oxidation at room temperature as the
silicon nitride surface is likely to be rapidly covered by oxide layers.
Unlike the CNTs/CNFs containing nanocomposites, where the secondary
phases reduce the coefficient of friction but also lower the wear resistance,
