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Journal of Dentistry (xxxx) xx, 1–9
www.intl.elsevierhealth.com/journals/jden
The influence of simulated masticatory loading
regimes on the bi-axial flexure strength and
reliability of a Y-TZP dental ceramic
Andrew R. Curtisa, Adrian J. Wrightb, Garry J.P. Flemingc,*
a
Biomaterials unit, School of dentistry, University of Birmingham, St Chad’s Queensway, Birmingham B4
6NN, UK
b
School of Chemistry, University of Birmingham, Edgbason, Birmingham B15 2TT, UK
c
Department of Restorative Dentistry and Periodontology, Dublin Dental School and Hospital, Lincoln
Place, Dublin 2, Ireland
Received 27 July 2005; accepted 29 July 2005
KEYWORDS
Y-TZP;
Zirconia;
CAD/CAM;
Weibull analysis;
Bi-axial flexure
strength;
Simulated masticatory
loading;
Vickers hardness
Summary Objectives: The purpose of the current study was to examine the
influence of simulated masticatory loading regimes, to which all-ceramic crown or
bridge restorations will routinely be subjected during their service-life, on the
performance of a yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) dental
ceramic.
Methods: Ten sets of 30 Y-TZP ceramic discs (13 mm diameter, 1.48–1.54 mm
thickness) supplied by the manufacturer were randomly selected. Six groups were
loaded for 2000 cycles at 500 N (383–420 MPa), 700N (536–588 MPa) and 800 N (613–
672 MPa) with three groups maintained dry and the remaining three groups loaded
while immersed in water at 37G1 8C. A further two groups underwent extended
simulated masticatory loading regimes at 80 N (61–67 MPa) for 104 and 105 cycles
under dry conditions. The mean bi-axial flexure strengths, standard deviations and
associated Weibull moduli (m) were determined. The surface hardness was also
determined using the Vickers hardness indentation technique.
Results: No significant difference (PO0.05) was identified in the bi-axial flexure
strength of the simulated masticatory loading regimes and the control specimens
loaded dry or wet. A significant increase in m was identified for the Y-TZP specimens
following loading while immersed in water (8.6G1.6, 8.5G1.6 and 10.3G1.9)
compared with the control (7.1G1.3). However, the extended loading regime to
105 cycles resulted in a significant reduction in the m of the Y-TZP specimens (5.3G
1.0) compared with the control. Localised areas of increased surface hardness were
identified to occur directly beneath the spherical indenter.
Conclusions: The occurrence of localised areas of increased surface hardness could
be the result of either a transformation toughening mechanism or crushing
* Corresponding author. Tel.: C353 1 612 7371; fax: C353 1 612 7279.
E-mail address: [email protected] (G.J.P. Fleming).
0300-5712/$ - see front matter Q 2005 Published by Elsevier Ltd.
doi:10.1016/j.jdent.2005.07.009
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A.R. Curtis et al.
and densification of the material beneath the indentor manifested as the formation
of a surface layer of compressive stresses that counteracted the tensile field
generated at the tip of a propagating crack which increased the Weibull modulus of
the Y-TZP specimens. The reduced reliability of the Y-TZP specimens loaded to 80 N
for 105 cycles was associated with the accumulation of subcritical damage as a result
of the extended nature of loading.
Q 2005 Published by Elsevier Ltd.
Introduction
The use of all-ceramic restorations in aesthetic
dentistry has increased in popularity while the
use of metal-ceramic restorations have suffered a
corresponding decline as they fail to meet the
aesthetic demand of the patients and practitioners alike.1,2 The drawback of all-ceramic
restorations is their inherently brittle nature and
propensity to spontaneous failure under low
impact stresses at areas of localised high stress
concentrations located at pre-existing surface or
bulk defects.3–7 The introduction of partially
stabilised zirconia ceramics to the dental restorative armoury has increased the reliability of allceramic crown and bridge restorations due to the
occurrence of a transformation toughening mechanism.8–10 The transformation toughening mechanism occurs as a result of the allotropic nature
of zirconia, which exists in a monoclinic,
tetragonal or cubic phase.11 Transformation
toughening is induced by a martensitic phase
transformation initiated by a diffusionless shear
process at near sonic velocities with an associated volumetric increase of 4% and a shear strain
of 7%.8,11–16 The process of transformation
induces compressive stresses in the crack wake
and shear strains that counteract the tensile field
generated at the tip of a propagating crack.8,13–16
The addition of a stabiliser, namely yttria,
stabilises the transforming zirconia system in
the tetragonal phase and retains the layer of
compressive stresses, resulting in the formation
of a yttria-stabilised tetragonal zirconia polycrystalline (Y-TZP) ceramic and inducing the high
fracture strength (O1000 MPa) and fracture
toughness (O10 MPam1/2) characteristic of Y-TZP
ceramics.12,17–20
Following placement, all-ceramic crown and
bridge restorations are subjected on a daily basis
to masticatory loading which places the restoration
under repeated loading throughout a service-life of
in excess of 107 cycles.21 A dental restoration is
routinely subjected to masticatory loads in excess
of 200 N, while clenching and bruxism forces in the
most severe cases can exert loads of up to 1221 N
with biting forces routinely experienced by crown
and bridge restorations variously ranging from 150
to 665 N. 21,22 When the stresses induced by
repeated loading do not initially exceed the flexure
strength, the growth of subcritical flaws may result
in delayed catastrophic failure.6,8 The presence of
pre-existing surface or bulk defects within the
ceramic system act as stress intensifiers and
localised areas of stress concentration accelerating
the occurrence of failure at lower levels of applied
stress as a result of repeated loading.23 The use of
Y-TZP restorations, while initially possessing a high
resistance to failure and strength degradation
associated with repeated loading, are susceptible
to severe strength loss due to microcracking once a
critical load and number of cycles is exceeded.24
Lavae (3M ESPE, Seefeld, Germany) is a 5-mol%
Y-TZP ceramic produced by a CAD/CAM technique.
The aim the present study was to investigate the
influence of the simulated masticatory loading
regimes, which routinely occur in the oral environment during the service-life of a Y-TZP ceramic
restoration, on the bi-axial flexure strength,
Weibull modulus and surface hardness of discshaped Y-TZP specimens.
Materials and method
Specimens
Ten groups of specimens were selected each
consisting of 30 nominally identical disc-shaped
Y-TZP ceramic specimens (13 mm diameter, 1.48–
1.54 mm thickness) in the A4 shade, which were
supplied by the manufacturer. A contact stylus
Profilometer (Taylor Hobson, Form Talysurf Series
2, Leicester, England) was used previously to
characterize the surface roughness of the discs.25
The Ra value produced was 0.21G0.03 mm and is
universally recognized as the international parameter for roughness, namely, is a measure of the
arithmetic mean of the absolute departures of the
roughness profile from the mean (horizontal) line.25
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The effect of repeated loading on a Y-TZP ceramic
Simulated masticatory loading
Groups A and E were maintained in the ‘as received’
condition as the control groups, Groups B–D were
loaded continuously for 2000 cycles at 500 N
(383–420 MPa), 700 N (536–588 MPa) and 800 N
(613–672 MPa), respectively, to simulate loads
experienced in the oral environment. Groups F–H
were also loaded continuously while immersed in
water at 37G1 8C for 2000 cycles at 500 N (383–
420 MPa), 700 N (536–588 MPa) and 800 N (613–
672 MPa), respectively. Specimen Groups I and J
were maintained dry and loaded to a maximum
value of 80 N (61–67 MPa) for 104 and 105 cycles,
respectively, to simulate the typical level of
masticatory loading that all-ceramic crown and
bridge restorations routinely experience during
service. The simulated masticatory loading regimes
were carried out using a universal tensile-testing
instrument (Instron Ltd, Model 5544, Buckinghamshire, England). Specimen Groups B–D and F–H were
loaded to a maximum value of 500, 700 and 800 N at
a frequency of 1.8–2.7 Hz and Groups I and J to 80 N
at a frequency of 2.8 Hz on a stainless steel platen,
centrally loaded using a 3 mm spherical indenter
with a crosshead speed of 40 mm/min. Groups B–D,
I and J were loaded in air and Groups F–H were
immersed in water at 37G1 8C while centrally
loaded. Immediately following simulated masticatory loading regime each specimen was loaded to
failure in bi-axial flexure.
Bi-axial flexure strength testing
The bi-axial flexure strength was determined by
centrally loading the specimens on a 10 mm knifeedge support at a crosshead speed of 1 mm/min
with a spherical indenter (3 mm diameter). A thin
sheet of rubber was placed between the specimen
and the knife-edge support to ensure uniform
loading and to accommodate any variation in the
peripheral thickness or any distortion in the specimen surfaces. The specimens were loaded to failure
using a universal tensile-testing instrument (Instron
Ltd, Model 5544, Buckinghamshire, England) in
either a dry or wet environment depending on the
simulated masticatory regime used.
The load (N) and extension (mm) at failure was
recorded and the specimen thickness at the point of
fracture of each disc-shaped specimen was
measured using a screw-gauge micrometer (Moore
and Wright, Sheffield, England) accurate to 10 mm.
The number of fragments produced was recorded to
determine the existence of a correlation between
the fracture strength and the number of fragments
3
generated at failure. The bi-axial flexure strength
was calculated according to Eq. (1)26
smax Z
h
a
i
o
P n
ð1
C
nÞ
0:485
ln
C
0:52
C
0:48
h
h2
(1)
where smax was the maximum tensile stress (MPa), P
was the measured load at fracture (N), a was the
radius of the knife-edge support (mm), h was the
specimen thickness at the fracture point (mm) and
n the Poisson’s ratio. A value of 0.23 was
substituted for the Poisson’s ratio of the Lavae
specimens under investigation (Product Specification, 3M ESPE, Seefeld, Germany).
Statistical analysis
Multiple comparisons of the group means were made
utilising a one-way analysis of variance (ANOVA) and
a paired Tukey’s multiple range test comparison at a
significance level of P!0.05. The bi-axial flexure
strength data was ranked in ascending order and
Weibull analysis27 was performed on the flexural
strength data by plotting ln ln(1/Ps) against lns
where Ps was the probability of survival and s was the
fracture stress. The gradient of the strength
distribution data (m) was determined by superimposing a regression line along the data points to
calculate m which is an inverse measure of the
strength variation and was defined by Trustrum28 as
characterising the ‘brittleness’ of a material. The
confidence limits for the test groups subjected to
Weibull analysis were considered to be significant
when the confidence intervals did not overlap.
Vickers hardness
A series of eight fragments of the ‘as received’
specimens stored dry (Groups A) and of the loaded
specimens stored and loaded in the dry condition
(Groups B–D, I and J) were randomly selected. The
diamond pyramid head of a Duramin-1 Vickers
hardness tester (Struers, Glasgow, Scotland) was
applied to the modified surface under a predetermined load (9.807 N) over 15 s to induce a diamond
shaped indent. The size of each diagonal distance as
a result of the impression of the indenter was
measured using a micrometer screw gauge to obtain
an average diagonal distance (D), an increased D
identified deeper impressions and was indicative of
decreased hardness.5 The Vickers hardness (VH)
(kgf/mm2) was obtained by calculating the surface
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A.R. Curtis et al.
area of the indent according to Eq. (2)
2P
1:854P
8
VH Z
Z
sin
68
D2
D2
(2)
where P was the predetermined load applied in
Newtons (N), D the average diagonal distance (mm),
1.854 was the Vicker’s constant and 688 was the angle
of indentation of the diamond pyramid head indenter
of the Vickers hardness tester.29 To assess the
influence of simulated masticatory loading regimes
on the surface hardness of disc-shaped specimens and
the potential generation of a localised layer (or zone)
of compressive stresses eight loaded specimens were
selected. Consequentially, the specimens were
divided into three regions and four indentation
hardness measurements were obtained from each
region and a mean VH was calculated. The three
regions were (i) directly beneath the spherical ball
indenter where loading occurred, (ii) a third of the
way from the centre to the edge of the specimens and
(iii) two-thirds of the way from the centre to the edge
of the specimens. (Fig. 1).
Results
Dry storage. Following bi-axial flexure strength
testing multiple comparisons were performed using
a one-way analysis of variance (ANOVA) and paired
Tukey test comparisons at the 95% significance level
on the resultant data which identified that there
was no significant difference (PO0.05) between the
mean bi-axial flexure strengths (Table 1) of the
control (Group A) and simulated masticatory loaded
specimens (Groups B–D). The bi-axial flexure
strength data was ranked in ascending order and
the resultant associated 95% confidence intervals of
the Weibull moduli of Groups A and B and Groups C
and D failed to overlap and were therefore
considered to be significantly different (Table 1).
Following bi-axial flexure strength testing of the
Y-TZP specimens loaded to 700 N (536–588 MPa)
Region 2
Region 1
Region 3
Figure 1 A diagrammatic representation of a discshaped specimens divided into three regions where
Vickers hardness measurements were obtained by making
four indentations within each region where illustrated.
and 800 N (613–672 MPa) an increase in the number
six-, seven- and eight-fragment fractures was identified with a concomitant decrease in the number of
three-, four- and five-fragment fractures compared
with the control and the specimens loaded to 500 N
(383–420 MPa), (Table 1). The survival probability
distribution was plotted against the ranked bi-axial
flexure strength in ascending order for Group A and
the data appeared to exhibit an increased asymmetry within the lower stress range. However, the
progressively increased simulated masticatory
loads were identified to increase the symmetry
within the lower stress ranges (Fig. 2).
Wet storage. The simulated masticatory loading
regimes of the Y-TZP specimens immersed in water
(Groups F–H) were not identified following statistical analysis to be significantly different (PO0.05)
compared with the control (Group E). In addition,
the bi-axial flexure strength data of the specimens
loaded in dry conditions, or loaded while immersed
in water were not significantly different (PO0.05).
The failure of the associated confidence intervals of
Group E and Groups F–H to overlap highlighted a
significant increase in the Weibull moduli of the
loaded specimens (Table 1). The examination of
the fragment fractures identified an increase in the
number of three-, four- and five-fragment fractures
with a concomitant decrease in the number of sixand seven-fragment fractures as a result of loading
(Groups F–H) compared with the control (Group E)
and the loaded specimens maintained dry (Groups
B–D) (Table 1). The survival probability distribution
for the control (Group E) identified the occurrence
of an asymmetry within the lower stress range, in
contrast with the loaded specimens maintained dry,
the asymmetry identified to Groups F–H was not
modified as a result of the loading regimes (Fig. 3).
Extended loading. The statistical analysis of the
extended simulated masticatory loaded regimes
failed to identify a significant difference (PO0.05)
between the bi-axial flexure strengths of the
control and of Groups I and J (Table 1). The Weibull
modulus of the bi-axial flexure strengths of Groups I
and J (10.9G2.0 and 5.3G1.0, respectively), were
identified to be significantly different as the
confidence intervals failed to overlap, similarly
the confidence intervals of Group A failed to
overlap with Groups I and J (Table 1). The bi-axial
flexure strength data identified that increasing the
number of loading cycles employed from 104 to 105
(Groups I and J) resulted in an increase in the
number of three-, four- and five-fragment fractures
with a concomitant decrease in the number of six-,
seven- and eight-fragment fractures (Table 1). The
survival probability distribution of Group I was
identified to possess an increased symmetry within
Dry extended
500 N
(383–420 MPa)
700 N
(536–588 MPa)
800 N
(613–672 MPa)
Control
500 N
(383–420 MPa)
700 N
(536–588 MPa)
800 N
(613–672 MPa)
10,000
(61–67 MPa)
100,000
(61–67 MPa)
Group A
Group B
Group C
Group D
Group E
Group F
Group G
Group H
Group I
Group J
793–1451
827–1364
1039–1430
882–1398
850–1556
784–1392
844–1472
909–1467
872–1427
642–1428
1267 (161)
1216 (136)
1246 (104)
1259 (101)
1308 (188)
1216 (141)
1221 (150)
1191 (127)
1250 (115)
1195 (191)
960
1008
1112
1148
961
1003
972
1033
1072
754
7.5 (1.4)
8.4 (1.5)
13.2 (2.4)
12.0 (2.29)
7.1 (1.3)
8.6 (1.6)
8.5 (1.6)
10.3 (1.9)
10.9 (2.0)
5.3 (1.0)
6.5–8.5
7.1–9.7
12.2–14.2
10.2–13.9
6.6–7.6
7.8–9.5
7.9–9.1
9.4–11.3
9.8–12.1
4.4–6.4
0.90
3, 0 and 8
0.87
1, 6 and 13
0.96
0, 2 and 11
0.87
0, 2 and 5
0.96
0, 1 and 5
0.94
1, 4 and 12
0.97
1, 5 and 13
0.95
0, 8 and 14
0.93
2, 2 and 10
0.83
2, 5 and 12
13, 6 and 0
10, 0 and 0
13, 4 and 0
15, 8 and 0
18, 5 and 0
10, 3 and 0
7, 4 and 0
6, 2 and 0
15, 1 and 0
8, 2 and 1
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Fracture
strengths (MPa)
Mean strength
(MPa)
10% Failure
probability
Weibull
modulus
95% Confidence
intervals
R2-value
3, 4 and 5
fragment
fractures
6, 7 and 8
fragment
fractures
Wet
Control
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The effect of repeated loading on a Y-TZP ceramic
Table 1 The bi-axial flexure strengths (s) with associated Weibull moduli (m), confidence intervals, R2-values and fragment fractures of groups of 30 Lavae
specimens maintained ‘as received’ from the manufacture or loaded dry or while immersed in water at 37G1 8C at 500, 700 and 800 N for 2000 cycles or at 80 N for
104or 105 cycles (numbers in parenthesis represent the standard deviations).
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A.R. Curtis et al.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
600
500 N π 700 N
800 N
Probability of Survival
Probability of Survival
Control
800
1000
1200
1400
Biaxial Flexure strength (MPa)
1600
Figure 2 The survival probability distribution plots of
the ‘as received’ Y-TZP specimens and the specimens
following the simulated masticatory loading regimes at
2000 cycles at a maximum load of 500, 700 or 800 N. The
distributions of the bi-axial flexure strength data highlighted an increased symmetry within the lower stress
range as a result of the loading regimes.
the lower stress range while the asymmetry was
identified to be increased in Group J as a result of
the simulated masticatory loading regimes (Fig. 4).
Vickers hardness. The simulated masticatory
loading regimes were highlighted to identify the
occurrence of an increased reliability of loaded
specimens which was associated with the occurrence of an increased surface hardness, directly
beneath the loading site at the centre of the Y-TZP
ceramic specimens and within two concentric
regions (Fig. 1).
500 N (383–420 MPa). The loading of the Y-TZP at
500 N for 2000 cycles was not identified to result in
Control
500 N π 700 N
800 N
1
Probability of Survival
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
600
800
1000
1200
1400
Biaxial Flexure Strength (MPa)
1600
Figure 3 The survival probability distribution plots of
the ‘as received’ Y-TZP specimens and the specimens
following the simulated masticatory loading regimes at
2000 cycles at a maximum load of 500, 700 or 800 N. The
distributions of the bi-axial flexure strength data highlighted an increased asymmetry within the lower stress
range as a result of the loading regimes.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
600
800
1000
1200
1400
Biaxial Flexure strength (MPa)
1600
Figure 4 The combined survival probability distribution
plots of the ‘as received’ Y-TZP specimens and the
specimens following the simulated masticatory loading
regimes at a maximum load of 104 or 105 cycles. The
distributions of the bi-axial flexure strength data highlighted an increased symmetry within the lower stress
range as a result of the simulated masticatory loading
regimes for 104 cycles. However, loading for 105 cycles at
80 N was identified to result in an increased asymmetry
within the lower stress range.
a statistically significant increase (PO0.05) in the
mean Vickers hardness, however, the mean surface
hardness of the area directly loaded (Region 1) was
1637G148 compared with Region 2 and 3 of 1554G
65 and 1518G77, respectively (Table 2).
700N (536–588 MPa). A statistically significant
increase in the surface hardness of specimens
loaded to 700 N (536–588 MPa) compared with
500 N (383–420 MPa), was identified to occur in
Region 1 (P!0.05), which was identified to possess
a mean VH of 1834G77 compared with Regions 2
and 3 which possessed a mean VH of 1545G70 and
1583G122, respectively (Table 2).
800N (613–672 MPa). Similarly, following the
loading of specimens to 800 N (613–672 MPa) for
2000 cycles resulted in a significant (P!0.05)
increase (compared with the 500 N (383–420 MPa)
loaded specimens) in the mean VH of Region 1,
which was 1850G112, compared with Regions 2 and
3 which were identified to be 1583G76 and 1531G
74, respectively (Table 2).
Extended loading. When the extended simulated
masticatory loading regimes were employed utilising a maximum load of 80 N (61–67 MPa) over
104 cycles a significantly higher (P!0.05) mean VH
of 1692G99 was identified to occur in Region 1,
compared with Regions 2 and 3, which were not
directly loaded and possessed a mean VH of 1472G
41 and 1475G55, respectively (Table 2). Further
increasing the maximum number of cycles
employed to 105 resulted in the occurrence of a
mean VH in Region 1 of 1643G66, while Regions 2
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Table 2 The Vickers hardness (VH) of groups of 30 Lavae specimens loaded dry at 500, 700 and 800 N for
2000 cycles or at 80 N for 104 or 105 cycles, directly beneath the indenter (Region 1) and in two concentric Regions
around the indentation site (numbers in parenthesis represent the standard deviations).
Control
Region 1
Region 2
Region 3
Group A
500 N
(383–420 MPa)
Group B
700 N
(536–588 MPa)
Group C
800 N
(613–672 MPa)
Group D
10,000
(61–67 MPa)
Group I
100,000
(61–67 MPa)
Group J
1590 (91)
1590 (91)
1590 (91)
1637 (148)
1554 (65)
1518 (77)
1834 (77)
1545 (70)
1583 (122)
1850 (112)
1583 (76)
1531 (74)
1643 (66)
1487 (36)
1457 (44)
1692 (99)
1472 (41)
1475 (55)
and 3 were revealed to posses significantly lower
(P!0.05) mean VHNs of 1487G36 and 1457G44,
respectively (Table 2).
Discussion
Dry storage. The simulated masticatory loading
regimes employed in the current study failed to
significantly alter the mean bi-axial flexure strength
of the Y-TZP ceramic specimens. Y-TZP ceramics
have previously been identified to be high strength
materials, which were not influenced by repeated
loading of up to 3000 N.6 However, when the stresses
induced by repeated loading do not initially exceed
the flexure strength, the growth of subcritical flaws
may result in the occurrence delayed catastrophic
failure following 5!105 cycles.6,8,11 Therefore, it
was suggested that the simulated masticatory loads
and stresses and the number of cycles employed in
the current study failed to induce the accumulation
of a sufficient amount of accumulated damage, nor
did the number of cycles exceed the critical number,
required to initiate a significant reduction in the biaxial flexure strengths.
The reliability of the all-ceramic dental restorations has previously been questioned due to the
intrinsically brittle nature of ceramic materials as a
result of the occurrence of surface or bulk defects
and residual stresses which are inherent to the
structure of ceramic materials.3–7 The use of
simulated masticatory loading regimes has previously been identified to result in unpredictable
rates of failure determined by the material
toughness and the resistance to crack extension.1,
30,31
In the current study the Weibull modulus of the
Y-TZP ceramic was identified to be significantly
increased as a result of the loading regimes (8.4G
1.5, 13.2G2.4 and 12.0G2.2, respectively),
compared with the control (7.5G1.4). The Vickers
indentation technique identified a localised
increase in the surface hardness of the Y-TZP
specimens (Table 2) directly beneath the spherical
indenter, which was markedly harder than the
surrounding regions possibly highlighting the occurrence of a toughening mechanism or crushing and
densification of the material beneath the indentor,
which was responsible for the increased Weibull
moduli of the Y-TZP specimens. In addition, the
improved Weibull moduli of the loaded specimens
was further highlighted by the survival probability
distribution, which identified that the asymmetry
occurring in the lower stress range was reduced as a
result of the loading regimes at 700 N (536–588 MPa)
and 800 N (613–672 MPa), compared with 500 N
(383–420 MPa) and the control.
Wet Storage. The influence of moisture contamination has previously been identified to be detrimental to the fracture strength of ceramic-based
dental restorative materials, routinely resulting in a
20% decrease in the mean fracture strength.32–34
The current study highlighted that the bi-axial
flexure strengths of the Y-TZP ceramic were not
detrimentally influenced by water immersion
during simulated masticatory loading. In contrast,
the repeated loading of ceramic restorations in an
aqueous environment has previously been observed
to result in an increased crack velocity, the
occurrence of stress-corrosion and the propagation
of pre-existing defects to failure at significantly
reduced bi-axial flexure strengths.33,35,36 In
addition, the accumulation of damage as a result
of loading in an active aqueous environment, such
as that encountered within the oral environment,
causes surface defects to act as stress intensifiers
and areas of local stress concentration, facilitating
the initiation of fracture at lower levels of applied
stress.24 The simulated masticatory loads employed
in the current investigation were within the upper
range to which Y-TZP crown and bridge structures
were likely to be subjected to during service.22–23
The Weibull modulus of ceramic materials has
frequently been identified to be decreased as a
result of the presence of water, whether in small
quantities such as atmospheric levels or due to
complete immersion, resulting in a reduction in the
flexural strength and Weibull modulus of ceramic
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restorations.37–39 When the Y-TZP specimens were
loaded while immersed in water during the current
study a significant increase in the Weibull moduli
were identified (8.6G1.6, 8.5G1.6 and 10.3G1.9,
respectively), compared with the unloaded specimens (7.1G1.3). The increased m was attributed to
either the formation of a layer of compressive
stresses possibly as a result of the occurrence of a
transformation toughening mechanism or crushing
and densification of the material beneath the
indentor, identified by the increased localised
surface hardness. Furthermore, there was no
significant difference between the bi-axial flexure
strength and m following the wet and dry loading
conditions. In the dental literature there is some
controversy regarding the influence of moisture on
Y-TZP ceramics.40–42 Recently Marx et al.43 evaluated the threshold intensity factors as lower
boundaries for crack propagation in ceramics
including a zirconia and the Y-TZP investigated in
the current study. The authors highlighted using
critical stress intensity values and the threshold
values that the composition of the zirconia ceramic
investigated had an influence on the crack propagation in zirconia ceramics43 which may account for
the controversy previously highlighted in the dental
literature40–42 and in the current study.
Extended loading. The extended simulated
masticatory loading regimes did not result in a
significant decrease in the bi-axial flexure strengths
of the specimens, furthermore the Weibull modulus
of the Y-TZP specimens loaded at 80 N for
104 cycles were identified to be increased following
loading, attributed to the increased VH beneath the
loading ball. The occurrence of an increased
localised surface hardness in Group I was manifested directly beneath the indenter, in Region 1
compared with Regions 2 and 3. The increased VH
was attributed to the occurrence of a localised
transformation toughening mechanism or crushing
and densification of the material beneath the
indentor which generated the formation of a layer
of compressive stress and characteristically also
gives rise to an increased Weibull modulus.12–16
However, when the Y-TZP specimens were
loaded to 105 cycles a significant reduction in the
Weibull modulus was identified (5.3G1.0)
compared with the control (7.5G1.4). In addition,
a reduction in the lower range of the bi-axial flexure
strengths (642 MPa) compared with the control
(793 MPa) and a decrease in the 10% failure
probability was identified (Table 1). In line with
the simulated masticatory loading regimes for
104 cycles a localised increase in the VH was also
identified to occur directly beneath the loading site
following loading for 105 cycles. Therefore, it was
A.R. Curtis et al.
suggested that the extended nature of loading,
even at comparatively low loads, introduced the
accumulation of subcritical microcracking, crushing
and densification of the material beneath the
indentor and high levels of tensile stresses associated with the generation, propagation and coalescence of microcracks.9,40,44–45
In summary, the Y-TZP ceramic investigated has
the potential for increasing performance when used
clinically by the generation of either a compressive
layer or crushing and densification of the material
beneath the indentor when masticatory loads in the
region commonly produced by bruxism or biting
forces are encountered in the oral environment.
Conclusions
The simulated masticatory loading regimes investigated were identified to result in an increased
surface hardness directly below the loaded area
which modified the surface defect population and
thereby the increased Weibull moduli of the Y-TZP
ceramic specimens. The increased Weibull moduli
were attributed to the formation of a localised
layer of compressive stresses or crushing and
densification of the material beneath the indentor
that counteracted the tensile field at the propagating crack-tip following loads of 700 (536–588 MPa)
and 800 N (613–672 MPa). In contrast, extending the
simulated masticatory loading regime to 105 cycles
at 80 N (61–67 MPa) reduced the Weibull moduli of
the specimens as a result of the combined influence
of the accumulation of microcrack damage from
crushing and densification of the material beneath
the indentor and the possible failure to induce a
transformation toughening mechanism to counteract the crack propagation.
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