JOURNAL OF MATERIALS SCIENCE 36 (2001) 3011 – 3025
Fracture variability and R-curve behavior
in yttria-stabilized zirconia ceramics
D. CASELLAS, J. ALCAL
´
A, L. LLANES, M. ANGLADA
Departament de Ci `encia dels Materials i Eng. Metal.l ´urgica, Universitat
Polit `ecnica de Catalunya, E.T.S. d’Enginyeria Industrial, 08028 Barcelona, Spain
E-mail: alcala@cmem.upc.es
An assessment of fracture origins is conducted in yttria-stabilized zirconia ceramics
containing different grain sizes. As the microstructure coarsens due to the application of
heat treatments, fracture origins change from single pores to transformed regions at the
free surface which are induced by the applied stress. The observation of an increasing size
of failure origins with microstructural coarsening lies as the underlying reason for the
finding that specimens containing coarser microstructures and a more pronounced R-curve
behavior do not fail at larger stresses. A fracture model is used to link the strength
variability of a fully tetragonal zirconia containing a small grain size to its pore size
distribution. The increased transformability of zirconia ceramics with coarser tetragonal
grains is evaluated by means of quantitative phase analysis, characterizations of fracture
surface morphology, and R-curve assessments. It is confirmed that tetragonal grains of up
to 4 µm may not necessarily undergo a spontaneous t–m transformation upon cooling
from sintering.
C
2001
Kluwer Academic Publishers
1. Introduction
The understanding of the fracture behavior of ceramic
materials has been the focus of intensive research for
more than 30 years. Fracture assessments in ceramics
are usually conducted considering either their strength
variability via Weibull statistics or their R-curve be-
havior. While the former is usually regarded as an ap-
proach to structural design with brittle materials, the
R-curve allows to develop a fundamental comprehen-
sion of fracture micromechanisms.
The R-curve provides a tool for the evaluation of
the crack growth resistance of ceramics using concepts
from fracture mechanics. This curve is presented as a
plot of the applied stress intensity factor in terms of
crack growth. A rising R-curve behavior is indicative
of the development of toughening mechanisms, such
as the tetragonal-to-monoclinic (t–m) phase transfor-
mation in zirconia and crack-bridging, which “shield”
the crack tip from the applied loads [1]. Thus, in the
case of monolithic zirconia ceramics, a rising R-curve
can be promoted by increasing the transformability of
small tetragonal particles. This can be achieved, for ex-
ample, by coarsening the grain size of the material by
recourse to suitable heat treatments.
A direct result of a rising R-curve behavior is the at-
tainment of stable crack growth over a range of the ap-
plied stress intensity factor for which unstable fracture
would have been triggered in the absence of toughening
mechanisms. Accordingly, a rising R-curve behavior
would be anticipated to induce an increase in fracture
strength. However, such concomitant increase of the
crack growth resistance and fracture strength is seldom
confirmed by experimental measurements in ceramic
materials. In this sense, a general trend found in brittle
materials is that the fracture strength under monotonic
loads increases as the grain size decreases [2]. For ex-
ample, yttria-stabilized tetragonal zirconia polycrystals
(Y-TZP) whose grain size lies in the submicron range
fails at∼1000 MPa, while magnesia partially-stabilized
zirconias (Mg-PSZ) whose grain size is in the 50-µm
range fail at ∼500 MPa. The general explanation ac-
counting for these observations is that ceramics with
small grain sizes contain smaller flaws than materials
with large grain sizes. An interesting aspect in the above
comparison is that the fracture strength of Mg-PSZs is
much lower than that of Y-TZPs even when the former
possess a larger crack growth resistance [3, 4].
The present work was conducted with the general
objective of assessing the interplay between fracture
strength, R-curve behavior and failure origins in yttria-
stabilized zirconia ceramics. The systematic evaluation
of fracture strength variability and failure origins in
Y-TZP materials with different grain sizes (resulting
from the application of a heat treatment) is expected
to shed light into the underlying reasons why ceramic
materials which display a more pronounced R-curve
behavior have the tendency to fail at lower stresses. A
quantification of the influence of flaw population and
R-curve on the fracture strength variability in a fine
grained Y-TZP is also attempted in this work. Such ra-
tionalization is important in bridging the gap between
statistical concepts used in structural design with ce-
ramics and more fundamental issues involving their
crack growth (R-curve) behavior.
0022–2461
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2001 Kluwer Academic Publishers
3011