Biomedical Engineering Reference
In-Depth Information
and tribological applications, the last few decades have witnessed an
increasing surge toward the development of WC-Co cermets with nanoscale
microstructure. Furthermore, with the advent of advanced electric field
assisted sintering techniques such as spark plasma sintering (SPS), the
development of dense WC-based cermets possessing submicrometre nano-
sized WC grains has been pursued in various research laboratories (Cha
et al. 2003a, 2003b, Kim et al. 2007a, 2007b, Shi et al. 2005, Sivaprahasam
et al. 2007). In order to overcome problems such as corrosion/oxidation and
softness of the metallic phase at high temperature and low wear properties
associated with WC-based cermets, researchers incorporated 6 wt% nano-
sized ZrO
2
in submicrometre WC and sintered at 1300
C for 5min via SPS
(Biswas et al. 2007, Cha and Hong 2003, Kim et al. 2004, 2006, Imasato
et al. 1995). The role of ZrO
2
in enhancing the densification kinetics was also
critically analyzed. In another investigation, it was observed that such WC-
based nanocomposites possess excellent wear resistance (Venkateswaran
et al. 2005). However, a serious drawback remained the poor indentation
toughness (6MPa.m
1/2
) of WC-6 wt% ZrO
2
spark plasma sintered at
1300
8
8
C for 5 min. To obtain a good combination of fracture toughness,
strength and hardness, slight modifications in the compositional window
were made: 3mol% yttria-stabilized ZrO
2
(3Y-TZP) powders were used as
the sintering aid and the possibility of exploiting the transformation
toughening effect of ZrO
2
was explored (Basu 2005, Garvie et al. 1975,
Hannink et al. 2000, Mukhopadhyay et al. 2007). The presence of ZrO
2
nanoparticles changes the fracture mode from intergranular fracture (for
WC-Co cermet) to nearly 100% transgranular fracture for the ZrO
2
-
containing WC-based nanocomposites. The reasons behind such a change in
fracture mode for ceramic nanocomposites in the presence of intragranular
nano-sized second-phase particles have been discussed elsewhere (Hansson
et al. 1993, Limpichaipanit and Todd 2009).
As the fracture energy for cleavage (transgranular) fracture is higher than
grain boundary (intergranular) fracture in ceramics, such a change in
fracture mode has been reported to result in an improvement of fracture
toughness, especially for ceramic nanocomposites in which the matrix grains
are equiaxed in nature (Chen and Chen 1994, Limpichaipanit and Todd
2009, Mukhopadhyay and Basu 2007). In addition to transformation
toughening and change in fracture mode in the presence of ZrO
2
, deflection
and bridging of cracks by the second-phase particles (ZrO
2
) also contribute
to the high fracture toughness of WC-6 wt% ZrO
2
(3Nmol% Y
2
O
3
) and
WC-4 wt% ZrO
2
(3mol% Y
2
O
3
)-2 wt% Co nanocomposites.
The mechanical properties of some ceramic nanocomposites are different
because of their microstructural characteristics. Such differences in
mechanical properties of various nanocomposite systems not only depend
on the microstructural scale of the nano-sized reinforcement and mechanical
