Biomedical Engineering Reference
In-Depth Information
1.4.8 Tetragonal zirconia polycrystal (TZP)/molybdenum
(Mo) nanocomposites
To achieve either high strength or high toughness in composites, tetragonal
zirconia polycrystals (TZPs), stabilized with Y
2
O
3
(Y-TZP) or CeO (Ce-
TZP), have been studied (Gupta et al. 1978, Masaki 1986). The Y-TZP
ceramic containing 2-3Nmol% Y
2
O
3
exhibited enhanced strength above
1200MPa; however, toughness was approximately in the range
5-6MPam
1/2
. On the other hand, Ce-TZP ceramic containing 7-12 mol%
CeO
2
exhibited high toughness in excess of 10MPa.m
1/2
, but the strength of
600 to 800MPa was still modest in comparison with that of Y-TZP. To
improve modest strength or toughness, TZP-based composites incorporat-
ing Al
2
O
3
have been investigated. In Y-TZP containing 30 vol% Al
2
O
3
, the
strength increased in excess of 2400-3000MPa for isostatically hot-pressed
specimens, while the toughness decreased appreciably with increasing Al
2
O
3
content (Shikata et al. 1990, Tsukuma et al. 1985). In the same way, Ce-TZP
containing 50 vol% Al
2
O
3
showed an improvement in strength of 900MPa,
but the toughness decreased from 20 to 5.5MPa.m
1/2
with increasing Al
2
O
3
content (Tsukuma et al. 1988). A tradeoff between high strength and high
toughness is still unsolved for both types of monolithic TZP and TZP/Al
2
O
3
composite systems.
With regard to the strength-toughness relationship, Swain and Rose
(1986) proposed a mechanism for the limitation of strength in transforma-
tion-toughened zirconia, pointing out that maximum strength is limited by
the critical stress that induces the tetragonal to monolithic transformation.
To overcome these problems, several investigators tried another approach
for preserving high strength in the region of high toughness - using ductile
metal particles instead of brittle ceramic particles as a secondary phase. It
was expected that the metal phase would result in the inherent improvement
in toughness, but most of the ceramic matrix composites incorporating
metal dispersions such as W, Mo, Ti, Cr, Ni, etc. failed to produce the
successful results (Breval et al. 1985, Davidge 1979, Jenkins et al. 1989,
McMurtry et al. 1987, Ohji et al. 1994, Tuan and Brook 1992). This is
mainly due to the fact that the addition of second-phase dispersions
generally causes an enlargement of the flaw size in the composites and so this
system failed to improve both strength and toughness simultaneously.
Several investigators have studied nanocomposites in which nanometre-
sized particles were dispersed within ceramic matrix grains and/or at the
grain boundaries to eliminate strength-degrading flaws (Mizutani et al.
1997, Nawa et al. 1992, Siegel et al. 2001). They tried to develop ceramic/
metal nanocomposites by selecting Y
2
O
3
-stabilized TZP (3mol% Y
2
O
3
)as
the ceramic matrix material and refractory molybdenum (Mo) metal powder
as the dispersing particles. The powder mixtures, containing 10, 20, 30, 40
