Biomedical Engineering Reference
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and 50 vol% Mo, were ball milled in acetone for 24 h. The slurries were
dried and passed through a 250
m screen. The mixtures were then hot-
pressed in a carbon die at 1400, 1500 and 1600
μ
C with an applied pressure of
30MPa for 1 h in vacuum of less than 10 4 Torr. These nanocomposites
possess a novel microstructural feature composed of a mutual intragranular
nanostructure in which either nanometre-sized Mo particles or equivalent
sized zirconia particles were located within the zirconia grains or Mo grains,
respectively. A simultaneous improvement in strength and toughness was
achieved, overcoming the strength-toughness tradeoff relation (Nawa et al.
1994b). Scanning electron micrograph (SEM) and transmission electron
micrograph (TEM) images clearly indicated improved features of the
strength properties of the nanocomposites. Submicron-sized Mo particles
were dispersed at the grain boundaries of the fine-grained/submicron-sized
zirconia matrix. In addition, a number of extremely fine Mo particles were
observed mainly within the zirconia grains. The variations of fracture
strength and toughness with Mo content for the 3Y-TZP/Mo composites
containing up to 50 vol% Mo were presented by the investigators. The
strength increased with increasing Mo content and obtained a maximum
value of 1795MPa at 40 vol% Mo. Moreover, the toughness increased
dramatically with increasing Mo content above 30 vol% and reached a
maximum value of 18.0MPa.m 1/2 at 50 vol% Mo. A summary of the
fracture strength-toughness relation for the 3Y-TZP/Mo nanocomposites
containing up to 50 vol% Mo was also presented and previous results on
various types of zirconia-toughened ceramics demonstrated by Swain and
Rose (1986) were also considered.
8
1.4.9 WC-ZrO 2 -Co nanocomposites
Tungsten carbide (WC)-based ceramic-metal systems (cermets) have been
used for decades in various engineering applications (e.g. cutting tools, rock
drill tips, tools and dies as well as general wear parts). In fact, cemented
carbides, which are usually aggregates of tungsten carbide particles bonded
with cobalt metal via liquid-phase sintering, are regarded commercially as
one of the oldest and most successful powder metallurgy products. Such
conventional two-phase (WC-Co) composite materials derive their excep-
tional combination of mechanical properties, such as elastic modulus of
550GPa, hardness of 16GPa and fracture toughness of 12MPa.m 1/2 , from
those of their components, i.e. the hard refractory WC and soft ductile
metallic Co (Berger et al. 1997, Bock et al. 1992, Cha et al. 2003a, 2003b, Jia
et al. 1998, Kim et al. 1997, 2007a, 2007b, Kolaska 1992, Lenel 1980,
Masumoto et al. 1986, Sarin 1981, Shi et al. 2005, Sivaprahasam et al. 2007,
Sutthiruangwonga and Mori 2003, Suzuki 1986). For further improvement
in the performance of such cermets in high-temperature resistant structural
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