Biomedical Engineering Reference
In-Depth Information
film. This may be due to their greater mechanical stability and better
distribution throughout the microstructure. This could make pull-out and
destruction in the contact zone more difficult issues.
4.6.2 Nanocomposites with hard nanoparticles
The wear behaviour of composites reinforced with hard nanoparticles
differs in character from that of those reinforced with carbon nanofilaments.
Tatarko et al. (2010) presented a thorough study of silicon nitride based
nanocomposites sintered with a wide range of rare earth oxides that were
further doped with SiC nanoparticles. In general, the wear behaviour of
Si
3
N
4
-based materials is reported to be controlled by the following
mechanisms:
.
mechanical- or thermal-induced micro-cracking due to fatigue-assisted
stresses resulting from friction at high normal loads;
.
tribochemical reaction with water vapour at low temperatures and
normal loads (
<
400
8
C,
<
10N);
.
tribooxidation at temperatures that depend on the composition and
nature of the intergranular phases (Dong and Jahanmir, 1993; Skopp
et al., 1995; Wang and Hsu, 1996).
The introduction of SiC nanoparticles into a silicon nitride matrix improves
hardness, strength, resistance to creep, oxidation and corrosion in Si
3
N
4
ceramics. With regard to tribology, the addition of ceramic particles has two
main functions: to increase wear resistance and to improve self-lubricating
properties by selective oxidation of these compounds.
In work by Tatarko et al. (2010), the wear resistance (measured by the
ball-on-disc method of tribology) increased with decreasing size of rare
earth cations in Si
3
N
4
monoliths as well as Si
3
N
4
-SiC composite materials
sintered with different rare earth oxide additives. Figure 4.24 shows the
friction coefficient and specific wear rate of the specimens as a function of
the size of rare earth (RE) cations for such materials. The friction coefficient
of the materials slightly decreased with a decreasing ionic radius in RE
3+
from 0.74 to 0.70 in the case of monoliths, and from 0.71 to 0.64 in the case
of composites sintered with La
2
O
3
and Lu
2
O
3
, respectively. Similarly, the
specific wear rate decreased with a decreasing ionic radius in RE
3+
from
3.22
10
5
mm
3
/Nm for the monolithic Si
3
N
4
sintered with La
2
O
3
and Lu
2
O
3
, respectively, and from 1.91
10
5
mm
3
/Nm to 1.15
6
6
10
5
mm
3
/
6
10
5
mm
3
/Nm in the case of composites doped with the same
additives. As may be seen from Fig. 4.24, the friction coefficient, as well as
the specific wear rate of composites, was always lower when compared to the
monoliths with the same sintering additives, thus indicating the higher wear
resistance of Si
3
N
4
-SiC composites.
Nm to 0.89
6
